GENERAL BIOLOGY

Boundless
 Boundless
General Biology

Boundless
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 TABLE OF CONTENTS

https://courses.lumenlearning.com/boundless-biology/ 3.2: Carbohydrates - 4.11: The Endomembrane

Importance of Carbohydrates System and Proteins - Vesicles
2.8: Atoms, Isotopes, Ions,
1: THE STUDY OF LIFE 3.3: Lipid Molecules - and Vacuoles
and Molecules - Ions and
Introduction 4.12: The Endomembrane
1.1: The Science of Biology - Ionic Bonds
3.4: Lipid Molecules - Waxes System and Proteins - The
Introduction to the Study of 2.9: Atoms, Isotopes, Ions,
3.5: Lipid Molecules - Endoplasmic Reticulum
Biology and Molecules - Covalent
Bonds and Other Bonds and Phospholipids 4.13: The Endomembrane
1.2: The Science of Biology -
3.6: Lipid Molecules - System and Proteins - The
Scientific Reasoning Interactions
Steroids Golgi Apparatus
1.3: The Science of Biology - 2.10: Atoms, Isotopes, Ions,
and Molecules - Hydrogen 3.7: Proteins - Types and 4.14: The Endomembrane
The Scientific Method
Bonding and Van der Waals Functions of Proteins System and Proteins -
1.4: The Science of Biology - Lysosomes
Basic and Applied Science Forces 3.8: Proteins - Amino Acids
2.11: Water - Water’s Polarity 3.9: Proteins - Protein 4.15: The Endomembrane
1.5: The Science of Biology - System and Proteins -
Publishing Scientific Work 2.12: Water - Gas, Liquid, and Structure
Peroxisomes
1.6: The Science of Biology - Solid Water 3.10: Proteins - Denaturation
and Protein Folding 4.16: The Cytoskeleton -
Branches and Subdisciplines 2.13: Water - Heat of
Microfilaments
of Biology Vaporization 3.11: Nucleic Acids - DNA
4.17: The Cytoskeleton -
1.7: Themes and Concepts of 2.14: Water - High Heat and RNA
Intermediate Filaments and
Biology - Properties of Life Capacity 3.12: Nucleic Acids - The
Microtubules
1.8: Themes and Concepts of 2.15: Water - Water’s Solvent DNA Double Helix
4.18: Connections between
Biology - Levels of Properties 3.13: Nucleic Acids - DNA
Cells and Cellular Activities -
Organization of Living Things 2.16: Water - Cohesive and Packaging
Extracellular Matrix of
1.9: Themes and Concepts of Adhesive Properties 3.14: Nucleic Acids - Types of Animal Cells
Biology - The Diversity of 2.17: Water - pH, Buffers, RNA
4.19: Connections between
Life Acids, and Bases Cells and Cellular Activities -
2.18: Carbon - The Chemical 4: CELL STRUCTURE Intercellular Junctions
2: THE CHEMICAL Basis for Life 4.1: Studying Cells - Cells as
FOUNDATION OF LIFE 2.19: Carbon - Hydrocarbons the Basic Unit of Life 5: STRUCTURE AND
2.1: Atoms, Isotopes, Ions, 2.20: Carbon - Organic 4.2: Studying Cells - FUNCTION OF PLASMA
and Molecules - Overview of Isomers Microscopy MEMBRANES
Atomic Structure 2.21: Carbon - Organic 4.3: Studying Cells - Cell 5.1: Components and
2.2: Atoms, Isotopes, Ions, Enantiomers Theory Structure - Components of
and Molecules - Atomic 2.22: Carbon - Organic 4.4: Studying Cells - Cell Size Plasma Membranes
Number and Mass Number Molecules and Functional 4.5: Prokaryotic Cells - 5.2: Components and
2.3: Atoms, Isotopes, Ions, Groups Characteristics of Prokaryotic Structure - Fluid Mosaic
and Molecules - Isotopes 2.23: Synthesis of Biological Cells Model
2.4: Atoms, Isotopes, Ions, Macromolecules - Types of 4.6: Eukaryotic Cells - 5.3: Components and
and Molecules - The Periodic Biological Macromolecules Characteristics of Eukaryotic Structure - Membrane Fluidity
Table 2.24: Synthesis of Biological Cells 5.4: Passive Transport - The
2.5: Atoms, Isotopes, Ions, Macromolecules - 4.7: Eukaryotic Cells - The Role of Passive Transport
and Molecules - Electron Dehydration Synthesis Plasma Membrane and the 5.5: Passive Transport -
Shells and the Bohr Model 2.25: Synthesis of Biological Cytoplasm Selective Permeability
2.6: Atoms, Isotopes, Ions, Macromolecules - Hydrolysis 4.8: Eukaryotic Cells - The 5.6: Passive Transport -
and Molecules - Electron Nucleus and Ribosomes Diffusion
Orbitals 3: BIOLOGICAL 4.9: Eukaryotic Cells - 5.7: Passive Transport -
2.7: Atoms, Isotopes, Ions, MACROMOLECULES Mitochondria Facilitated Transport
and Molecules - Chemical 3.1: Carbohydrates - 4.10: Eukaryotic Cells - 5.8: Passive Transport -
Reactions and Molecules Carbohydrate Molecules Comparing Plant and Animal Osmosis
Cells

1
 5.9: Passive Transport - 7.4: Glycolysis - Importance 8: PHOTOSYNTHESIS 9.7: Response to the Cellular
Tonicity of Glycolysis Signal - Termination of the
8.1: Overview of
5.10: Active Transport - 7.5: Glycolysis - The Energy- Signal Cascade
Photosynthesis - The Purpose
Electrochemical Gradient Requiring Steps of Glycolysis 9.8: Response to the Cellular
and Process of Photosynthesis
5.11: Active Transport - 7.6: Glycolysis - The Energy- Signal - Cell Signaling and
8.2: Overview of
Primary Active Transport Releasing Steps of Glycolysis Gene Expression
Photosynthesis - Main
5.12: Active Transport - 7.7: Glycolysis - Outcomes of Structures and Summary of 9.9: Response to the Cellular
Secondary Active Transport Glycolysis Signal - Cell Signaling and
Photosynthesis
5.13: Bulk Transport - 7.8: Oxidation of Pyruvate Cellular Metabolism
8.3: Overview of
Endocytosis and the Citric Acid Cycle - 9.10: Response to the Cellular
Photosynthesis - The Two
5.14: Bulk Transport - Breakdown of Pyruvate Parts of Photosynthesis Signal - Cell Signaling and
Exocytosis 7.9: Oxidation of Pyruvate Cell Growth
8.4: The Light-Dependent
and the Citric Acid Cycle - 9.11: Response to the Cellular
Reactions of Photosynthesis -
6: METABOLISM Acetyl CoA to CO₂ Introduction to Light Energy Signal - Cell Signaling and
7.10: Oxidation of Pyruvate Cell Death
6.1: Energy and Metabolism - 8.5: The Light-Dependent
and the Citric Acid Cycle - Reactions of Photosynthesis - 9.12: Signaling in Single-
The Role of Energy and
Citric Acid Cycle Celled Organisms - Signaling
Metabolism Absorption of Light
7.11: Oxidative in Yeast
6.2: Energy and Metabolism - 8.6: The Light-Dependent
Phosphorylation - Electron Reactions of Photosynthesis - 9.13: Signaling in Single-
Types of Energy
Transport Chain Celled Organisms - Signaling
6.3: Energy and Metabolism - Processes of the Light-
7.12: Oxidative Dependent Reactions in Bacteria
Metabolic Pathways
Phosphorylation - 8.7: The Light-Independent
6.4: Energy and Metabolism - 10: CELL
Chemiosmosis and Oxidative Reactions of Photosynthesis -
Metabolism of Carbohydrates REPRODUCTION
Phosphorylation AM and C4 Photosynthesis
6.5: Potential, Kinetic, Free,
7.13: Oxidative 8.8: The Light-Independent 10.1: Cell Division
and Activation Energy - Free
Phosphorylation - ATP Yield Reactions of Photosynthesis - 10.1A: The Role of the Cell
Energy
7.14: Metabolism without The Calvin Cycle Cycle
6.6: Potential, Kinetic, Free,
Oxygen - Anaerobic Cellular 8.9: The Light-Independent 10.1B: Genomic DNA and
and Activation Energy - The
Respiration Reactions of Photosynthesis - Chromosomes
First Law of Thermodynamics
7.15: Connections of The Carbon Cycle 10.1C: Eukaryotic
6.7: Potential, Kinetic, Free,
Carbohydrate, Protein, and Chromosomal Structure and
and Activation Energy - The
Lipid Metabolic Pathways - 9: CELL Compaction
Second Law of
Connecting Other Sugars to COMMUNICATION
Thermodynamics 10.2: The Cell Cycle
Glucose Metabolism
6.8: Potential, Kinetic, Free, 9.1: Signaling Molecules and 10.2A: Interphase
7.16: Connections of
and Activation Energy - Cellular Receptors - Signaling
Carbohydrate, Protein, and 10.2B: The Mitotic Phase
Activation Energy Molecules and Cellular and the G0 Phase
Lipid Metabolic Pathways -
6.9: ATP - Adenosine Receptors
Connecting Proteins to 10.3: Control of the Cell
Triphosphate 9.2: Signaling Molecules and
Glucose Metabolism Cycle
6.10: Enzymes - Active Site Cellular Receptors - Forms of
7.17: Connections of 10.3A: Regulation of the
and Substrate Specificity Signaling
Carbohydrate, Protein, and Cell Cycle by External
6.11: Enzymes - Control of 9.3: Signaling Molecules and
Lipid Metabolic Pathways - Events
Metabolism Through Enzyme Cellular Receptors - Types of
Connecting Lipids to Glucose 10.3B: Regulation of the Cell
Regulation Receptors
Metabolism Cycle at Internal
9.4: Signaling Molecules and
7.18: Regulation of Cellular Checkpoints
7: CELLULAR Cellular Receptors - Signaling
Respiration - Regulatory 10.3C: Regulator Molecules
RESPIRATION Mechanisms for Cellular
Molecules
9.5: Propagation of the of the Cell Cycle
7.1: Energy in Living Systems Respiration
Cellular Signal - Binding 10.4: Cancer and the Cell
- Transforming Chemical 7.19: Regulation of Cellular
Initiates a Signaling Pathway Cycle
Energy Respiration - Control of
7.2: Energy in Living Systems Catabolic Pathways 9.6: Propagation of the 10.4A: Proto-oncogenes
- Electrons and Energy Cellular Signal - Methods of 10.4B: Tumor Suppressor
Intracellular Signaling Genes
7.3: Energy in Living Systems
- ATP in Metabolism

2
 10.5: Prokaryotic Cell 12.2F: Lethal Inheritance 14.3A: Basics of DNA 16: GENE EXPRESSION
Division Patterns Replication
16.1: Regulation of Gene
10.5A: Binary Fission 12.3: Laws of Inheritance 14.3B: DNA Replication in
Expression - The Process and
12.3A: Mendel’s Laws of Prokaryotes Purpose of Gene Expression
11: MEIOSIS AND Heredity 14.3C: DNA Replication in Regulation
SEXUAL 12.3B: Mendel’s Law of Eukaryotes 16.2: Regulation of Gene
REPRODUCTION Dominance 14.3D: Telomere Replication Expression - Prokaryotic
11.1: The Process of Meiosis - 12.3C: Mendel’s Law of 14.4: DNA Repair versus Eukaryotic Gene
Introduction to Meiosis Segregation 14.4A: DNA Repair Expression
11.2: The Process of Meiosis - 12.3D: Mendel’s Law of 16.3: Prokaryotic Gene
Meiosis I Independent Assortment 15: GENES AND Regulation - The trp Operon-
11.3: The Process of Meiosis - 12.3E: Genetic Linkage and PROTEINS A Repressor Operon
Meiosis II Violation of the Law of 16.4: Prokaryotic Gene
15.1: The Genetic Code - The
11.4: The Process of Meiosis - Independent Assortment Regulation - Catabolite
Relationship Between Genes
Comparing Meiosis and 12.3F: Epistasis Activator Protein (CAP)- An
and Proteins
Mitosis Activator Regulator
15.2: The Genetic Code - The
11.5: Sexual Reproduction - 13: MODERN 16.5: Prokaryotic Gene
Central Dogma- DNA
Advantages and UNDERSTANDINGS OF Regulation - The lac Operon-
Encodes RNA and RNA
Disadvantages of Sexual INHERITANCE An Inducer Operon
Encodes Protein
Reproduction 16.6: Eukaryotic Gene
13.1: Chromosomal Theory 15.3: Prokaryotic
11.6: Sexual Reproduction - Regulation - The Promoter
and Genetic Linkage Transcription - Transcription
Life Cycles of Sexually and the Transcription
13.1A: Chromosomal Theory in Prokaryotes
Reproducing Organisms Machinery
of Inheritance 15.4: Prokaryotic
16.7: Eukaryotic Gene
13.1B: Genetic Linkage and Transcription - Initiation of
12: MENDEL'S Regulation - Transcriptional
Distances Transcription in Prokaryotes
EXPERIMENTS AND Enhancers and Repressors
13.1C: Identification of 15.5: Prokaryotic
HEREDITY 16.8: Eukaryotic Gene
Chromosomes and Transcription - Elongation and
Regulation - Epigenetic
12.1: Mendels Experiments Karyotypes Termination in Prokaryotes
Control- Regulating Access to
and the Laws of Probability 15.6: Eukaryotic Transcription
13.2: Chromosomal Basis of Genes within the
12.1A: Introduction to Inherited Disorders - Initiation of Transcription in Chromosome
Mendelian Inheritance Eukaryotes
13.2A: Disorders in 16.9: Eukaryotic Gene
12.1B: Mendel’s Model 15.7: Eukaryotic Transcription Regulation - RNA Splicing
Chromosome Number
System - Elongation and Termination
13.2B: Chromosomal 16.10: Eukaryotic Gene
12.1C: Mendelian Crosses in Eukaryotes
Structural Rearrangements Regulation - The Initiation
12.1D: Garden Pea 15.8: RNA Processing in Complex and Translation Rate
13.2C: X-Inactivation Eukaryotes - mRNA
Characteristics Revealed the 16.11: Eukaryotic Gene
Basics of Heredity Processing
14: DNA STRUCTURE Regulation - Regulating
12.1E: Rules of Probability 15.9: RNA Processing in Protein Activity and
AND FUNCTION Eukaryotes - Processing of
for Mendelian Inheritance Longevity
14.1: Historical Basis of tRNAs and rRNAs
12.2: Patterns of Inheritance 16.12: Regulating Gene
Modern Understanding 15.10: Ribosomes and Protein Expression in Cell
12.2A: Genes as the Unit of
14.1A: Discovery of DNA Synthesis - The Protein Development - Gene
Heredity
14.1B: Modern Applications Synthesis Machinery Expression in Stem Cells
12.2B: Phenotypes and
of DNA 15.11: Ribosomes and Protein 16.13: Regulating Gene
Genotypes
Synthesis - The Mechanism of Expression in Cell
12.2C: The Punnett Square 14.2: DNA Structure and
Protein Synthesis Development - Cellular
Approach for a Monohybrid Sequencing
15.12: Ribosomes and Protein Differentiation
Cross 14.2A: The Structure and
Synthesis - Protein Folding, 16.14: Regulating Gene
12.2D: Alternatives to Sequence of DNA
Modification, and Targeting Expression in Cell
Dominance and 14.2B: DNA Sequencing
Recessiveness Techniques Development - Mechanics of
Cellular Differentation
12.2E: Sex-Linked Traits 14.3: DNA Replication

3
 16.15: Regulating Gene 17.2: Mapping Genomes 18.2B: Reproductive 19.2B: Genetic Drift
Expression in Cell 17.2A: Genetic Maps Isolation 19.2C: Gene Flow and
Development - Establishing 17.2B: Physical Maps and 18.2C: Speciation Mutation
Body Axes during Integration with Genetic 18.2D: Allopatric Speciation 19.2D: Nonrandom Mating
Development Maps 18.2E: Sympatric Speciation and Environmental Variance
16.16: Regulating Gene 19.3: Adaptive Evolution
17.3: Whole-Genome 18.3: Hybrid Zones and Rates
Expression in Cell
Sequencing of Speciation 19.3A: Natural Selection and
Development - Gene
Expression for Spatial 17.3A: Strategies Used in 18.3A: Hybrid Zones Adaptive Evolution
Positioning Sequencing Projects 18.3B: Varying Rates of 19.3B: Stabilizing,
16.17: Regulating Gene 17.3B: Use of Whole- Speciation Directional, and Diversifying
Expression in Cell Genome Sequences of Model 18.4: Evolution of Genomes Selection
Development - Cell Migration Organisms 19.3C: Frequency-
18.4A: Genomic Similiarities
in Multicellular Organisms 17.3C: Uses of Genome Dependent Selection
between Distant Species
16.18: Regulating Gene Sequences 19.3D: Sexual Selection
18.4B: Genome Evolution
Expression in Cell 17.4: Applying Genomics 19.3E: No Perfect Organism
18.4C: Whole-Genome
Development - Programmed 17.4A: Predicting Disease Duplication
Cell Death Risk at the Individual Level 20: PHYLOGENIES AND
18.4D: Gene Duplications
16.19: Cancer and Gene 17.4B: Pharmacogenomics, THE HISTORY OF LIFE
and Divergence
Regulation - Altered Gene Toxicogenomics, and 20.1: Organizing Life on
18.4E: Noncoding DNA
Expression in Cancer Metagenomics Earth
18.4F: Variations in Size and
16.20: Cancer and Gene 17.4C: Genomics and Number of Genes 20.1A: Phylogenetic Trees
Regulation - Epigenetic Biofuels
18.5: Evidence of Evolution 20.1B: Limitations of
Alterations in Cancer
17.5: Genomics and Phylogenetic Trees
16.21: Cancer and Gene 18.5A: The Fossil Record as
Proteomics 20.1C: The Levels of
Regulation - Cancer and Evidence for Evolution
17.5A: Genomics and Classification
Transcriptional Control 18.5B: Fossil Formation
Proteomics 20.2: Determining
16.22: Cancer and Gene 18.5C: Gaps in the Fossil
Regulation - Cancer and Post- 17.5B: Basic Techniques in Record Evolutionary Relationships
Transcriptional Control Protein Analysis 20.2A: Distinguishing
18.5D: Carbon Dating and
16.23: Cancer and Gene 17.5C: Cancer Proteomics Estimating Fossil Age between Similar Traits
Regulation - Cancer and 18.5E: The Fossil Record 20.2B: Building
Translational Control 18: EVOLUTION AND and the Evolution of the Phylogenetic Trees
THE ORIGIN OF Modern Horse 20.3: Perspectives on the
17: BIOTECHNOLOGY SPECIES
18.5F: Homologous Phylogenetic Tree
AND GENOMICS 18.1: Understanding Structures 20.3A: Limitations to the
17.1: Biotechnology Evolution 18.5G: Convergent Classic Model of
17.1A: Biotechnology 18.1A: What is Evolution? Evolution Phylogenetic Trees
17.1B: Basic Techniques to 18.1B: Charles Darwin and 18.5H: Vestigial Structures 20.3B: Horizontal Gene
Manipulate Genetic Material Natural Selection 18.5I: Biogeography and the Transfer
(DNA and RNA) 18.1C: The Galapagos Distribution of Species 20.3C: Endosymbiotic
17.1C: Molecular and Finches and Natural Theory and the Evolution of
Cellular Cloning Selection 19: THE EVOLUTION OF Eukaryotes
17.1D: Reproductive 18.1D: Processes and POPULATIONS 20.3D: Web, Network, and
Cloning Patterns of Evolution 19.1: Population Evolution Ring of Life Models
17.1E: Genetic Engineering 18.1E: Evidence of
19.1A: Defining Population
17.1F: Genetically Modified Evolution 21: VIRUSES
Evolution
Organisms (GMOs) 18.1F: Misconceptions of 21.1: Viral Evolution,
19.1B: Population Genetics
17.1G: Biotechnology in Evolution Morphology, and
19.1C: Hardy-Weinberg
Medicine 18.2: Formation of New Principle of Equilibrium Classification
17.1H: Production of Species 21.1A: Discovery and
19.2: Population Genetics
Vaccines, Antibiotics, and 18.2A: The Biological Detection of Viruses
Hormones Species Concept 19.2A: Genetic Variation
21.1B: Evolution of Viruses

4
 21.1C: Viral Morphology 22.4D: Bacterial Foodborne 24: FUNGI 25.1F: The Major Divisions
21.1D: Virus Classification Diseases of Land Plants
24.1: Characteristics of Fungi
21.2: Virus Infections and 22.5: Beneficial Prokaryotes 25.2: Green Algae- Precursors
24.1A: Characteristics of
Hosts 22.5A: Symbiosis between of Land Plants
Fungi
21.2A: Steps of Virus Bacteria and Eukaryotes 24.1B: Fungi Cell Structure 25.2A: Streptophytes and
Infections 22.5B: Early Biotechnology- Reproduction of Green
and Function
21.2B: The Lytic and Cheese, Bread, Wine, Beer, Algae
24.1C: Fungi Reproduction
Lysogenic Cycles of and Yogurt 25.2B: Charales
24.2: Ecology of Fungi
Bacteriophages 22.5C: Prokaryotes and 25.3: Bryophytes
21.2C: Animal Viruses Environmental 24.2A: Fungi Habitat,
Decomposition, and 25.3A: Bryophytes
21.2D: Plant Viruses Bioremediation
Recycling 25.3B: Liverworts and
21.3: Prevention and 24.2B: Mutualistic Hornworts
Treatment of Viral Infections 23: PROTISTS
Relationships with Fungi and 25.3C: Mosses
21.3A: Vaccines and 23.1: Eukaryotic Origins Fungivores 25.4: Seedless Vascular Plants
Immunity 23.1A: Early Eukaryotes 24.3: Classifications of Fungi 25.4A: Seedless Vascular
21.3B: Vaccines and Anti- 23.1B: Characteristics of Plants
24.3A: Chytridiomycota-
Viral Drugs for Treatment Eukaryotic DNA 25.4B: Vascular Tissue-
The Chytrids
21.4: Prions and Viroids 23.1C: Endosymbiosis and Xylem and Phloem
24.3B: Zygomycota - The
21.4.1: 21-4A- Prions and the Evolution of Eukaryotes 25.4C: The Evolution of
Conjugated Fungi
Viroids 23.1D: The Evolution of Roots in Seedless Plants
24.3C: Ascomycota - The
Mitochondria 25.4D: Ferns and Other
Sac Fungi
22: PROKARYOTES- 23.1E: The Evolution of Seedless Vascular Plants
24.3D: Basidiomycota- The
BACTERIA AND Plastids 25.4E: The Importance of
Club Fungi
ARCHAEA 23.2: Characteristics of 24.3E: Deuteromycota - The Seedless Vascular Plants
22.1: Prokaryotic Diversity Protists Imperfect Fungi
22.1A: Classification of 23.2A: Cell Structure, 24.3F: Glomeromycota 26: SEED PLANTS
Prokaryotes Metabolism, and Motility 26.1: Evolution of Seed Plants
24.4: Fungal Parasites and
22.1B: The Origins of 23.2B: Protist Life Cycles Pathogens 26.1A: The Evolution of
and Habitats
Archaea and Bacteria 24.4A: Fungi as Plant, Seed Plants and Adaptations
22.1C: Extremophiles and 23.3: Groups of Protists Animal, and Human for Land
Biofilms 23.3A: Excavata Pathogens 26.1B: Evolution of
22.2: Structure of Prokaryotes 23.3B: Chromalveolata- 24.5: Importance of Fungi in Gymnosperms
22.2A: Basic Structures of Alveolates Human Life 26.1C: Evolution of
Prokaryotic Cells 23.3C: Chromalveolata- 24.5A: Importance of Fungi Angiosperms
22.2B: Prokaryotic Stramenopiles in Human Life 26.2: Gymnosperms
Reproduction 23.3D: Rhizaria
26.2A: Characteristics of
22.3: Prokaryotic Metabolism 23.3E: Archaeplastida 25: SEEDLESS PLANTS Gymnosperms
23.3F: Amoebozoa and 26.2B: Life Cycle of a
22.3A: Energy and Nutrient 25.1: Early Plant Life
Opisthokonta Conifer
Requirements for 25.1A: Early Plant Life
Prokaryotes 23.4: Ecology of Protists 26.2C: Diversity of
25.1B: Evolution of Land
22.3B: The Role of 23.4A: Protists as Primary Gymnosperms
Plants
Prokaryotes in Ecosystems Producers, Food Sources, 26.3: Angiosperms
25.1C: Plant Adaptations to
and Symbionts
22.4: Bacterial Diseases in Life on Land 26.3A: Angiosperm Flowers
Humans 23.4B: Protists as Human 26.3B: Angsiosperm Fruit
25.1D: Sporophytes and
Pathogens
22.4A: History of Bacterial Gametophytes in Seedless 26.3C: The Life Cycle of an
Diseases 23.4C: Protists as Plant Plants Angiosperm
Pathogens
22.4B: Biofilms and Disease 25.1E: Structural 26.3D: Diversity of
22.4C: Antibiotics- Are We Adaptations for Land in Angiosperms
Facing a Crisis? Seedless Plants 26.4: The Role of Seed Plants

5
 26.4A: Herbivory and 28.2B: Class Anthozoa 29.3C: Evolution of 30.11: Plant Development -
Pollination 28.2C: Class Scyphozoa Amniotes Meristems
26.4B: The Importance of 28.2D: Class Cubozoa and 29.4: Reptiles 30.12: Plant Development -
Seed Plants in Human Life Class Hydrozoa 29.4A: Characteristics of Genetic Control of Flowers
26.4C: Biodiversity of Plants 28.3: Superphylum Amniotes 30.13: Transport of Water and
Lophotrochozoa 29.4B: Characteristics of Solutes in Plants - Water and
27: INTRODUCTION TO Reptiles Solute Potential
28.3A: Superphylum
ANIMAL DIVERSITY Lophotrochozoa 29.4C: Evolution of Reptiles
30.14: Transport of Water and
Solutes in Plants - Pressure,
27.1: Features of the Animal 28.3B: Phylum 29.4D: Modern Reptiles
Gravity, and Matric Potential
Kingdom Platyhelminthes 29.5: Birds
30.15: Transport of Water and
27.1A: Characteristics of the 28.3C: Phylum Rotifera 29.5A: Characteristics of Solutes in Plants - Movement
Animal Kingdom 28.3D: Phylum Nemertea Birds of Water and Minerals in the
27.1B: Complex Tissue 28.3E: Phylum Mollusca 29.5B: Evolution of Birds Xylem
Structure 28.3F: Classification of 29.6: Mammals 30.16: Transport of Water and
27.1C: Animal Reproduction Phylum Mollusca Solutes in Plants -
and Development 29.6A: Characteristics of
28.3G: Phylum Annelida Transportation of
Mammals
27.2: Features Used to 28.4: Superphylum Ecdysozoa Photosynthates in the Phloem
29.6B: Evolution of
Classify Animals 30.17: Plant Sensory Systems
28.4A: Superphylum Mammals
27.2A: Animal Ecdysozoa and Responses - Plant
29.6C: Living Mammals
Characterization Based on Responses to Light
28.4B: Phylum Nematoda 29.7: The Evolution of
Body Symmetry 30.18: Plant Sensory Systems
28.4C: Phylum Arthropoda Primates
27.2B: Animal and Responses - The
28.4D: Subphyla of
Characterization Based on 29.7A: Characteristics and Phytochrome System and Red
Arthropoda Evolution of Primates
Features of Embryological Light Response
Development 28.5: Superphylum 29.7B: Early Human 30.19: Plant Sensory Systems
Deuterostomia Evolution and Responses - Blue Light
27.3: Animal Phylogeny
28.5A: Phylum 29.7C: Early Hominins Response
27.3A: Constructing an
Echinodermata 29.7D: Genus Homo 30.20: Plant Sensory Systems
Animal Phylogenetic Tree
28.5B: Classes of and Responses - Plant
27.3B: Molecular Analyses
Echinoderms 30: PLANT FORM AND Responses to Gravity
and Modern Phylogenetic
28.5C: Phylum Chordata PHYSIOLOGY 30.21: Plant Sensory Systems
Trees
and Responses - Auxins,
27.4: The Evolutionary 30.1: The Plant Body - Plant
29: VERTEBRATES Tissues and Organ Systems
Cytokinins, and Gibberellins
History of the Animal
29.1: Chordates 30.22: Plant Sensory Systems
Kingdom 30.2: Stems - Functions of
and Responses - Abscisic
27.4A: Pre-Cambrian 29.1A: Characteristics of Stems
Acid, Ethylene, and
Animal Life Chordata 30.3: Stems - Stem Anatomy
Nontraditional Hormones
27.4B: The Cambrian 29.1B: Chordates and the 30.4: Stems - Primary and
30.23: Plant Sensory Systems
Explosion of Animal Life Evolution of Vertebrates Secondary Growth in Stems
and Responses - Plant
27.4C: Post-Cambrian 29.1C: The Evolution of 30.5: Stems - Stem Responses to Wind and Touch
Evolution and Mass Craniata and Vertebrata Modifications
30.24: Plant Defense
Extinctions 29.1D: Characteristics of 30.6: Roots - Types of Root Mechanisms - Against
Vertebrates Systems and Zones of Growth Herbivores
28: INVERTEBRATES 29.2: Fishes 30.7: Roots - Root 30.25: Plant Defense
29.2A: Agnathans- Jawless Modifications Mechanisms - Against
28.1: Phylum Porifera
Fishes 30.8: Leaves - Leaf Structure Pathogens
28.1A: Phylum Porifera
29.2B: Gnathostomes - and Arrangment
28.1B: Morphology of
Jawed Fishes 30.9: Leaves - Types of Leaf 31: SOIL AND PLANT
Sponges
Forms NUTRITION
28.1C: Physiological 29.3: Amphibians
30.10: Leaves - Leaf 31.1: Nutritional
Processes in Sponges 29.3A: Characteristics and Structure, Function, and Requirements of Plants
28.2: Phylum Cnidaria Evolution of Amphibians
Adaptation
29.3B: Modern Amphibians 31.1A: Plant Nutrition
28.2A: Phylum Cnidaria

6
 31.1B: The Chemical 32.9: Pollination and 33.13: Homeostasis - 35.4: How Neurons
Composition of Plants Fertilization - Development of Thermoregulation Communicate - Nerve
31.1C: Essential Nutrients Fruit and Fruit Types 33.14: Homeostasis - Heat Impulse Transmission within
for Plants 32.10: Pollination and Conservation and Dissipation a Neuron- Resting Potential
31.2: The Soil Fertilization - Fruit and Seed 35.5: How Neurons
Dispersal 34: ANIMAL NUTRITION Communicate - Nerve
31.2A: Soil Composition
32.11: Asexual Reproduction - AND THE DIGESTIVE Impulse Transmission within
31.2B: Soil Formation
Asexual Reproduction in SYSTEM a Neuron- Action Potential
31.2C: Physical Properties of
Plants 34.1: Digestive Systems - 35.6: How Neurons
Soil
32.12: Asexual Reproduction Introduction Communicate - Synaptic
31.3: Nutritional Adaptations - Natural and Artificial Transmission
34.2: Digestive Systems -
of Plants Methods of Asexual 35.7: How Neurons
Herbivores, Omnivores, and
31.3A: Nitrogen Fixation- Reproduction in Plants Carnivores Communicate - Signal
Root and Bacteria 32.13: Asexual Reproduction Summation
34.3: Digestive Systems -
Interactions - Plant Life Spans 35.8: How Neurons
Invertebrate Digestive
31.3B: Mycorrhizae- The Systems Communicate - Synaptic
Symbiotic Relationship 33: THE ANIMAL BODY- Plasticity
34.4: Digestive Systems -
between Fungi and Roots BASIC FORM AND 35.9: The Nervous System
Vertebrate Digestive Systems
31.3C: Nutrients from Other FUNCTION 35.10: The Central Nervous
34.5: Digestive Systems -
Sources 33.1: Animal Form and System - Cerebral Cortex and
Digestive System- Mouth and
Function - Characteristics of Stomach Brain Lobes
32: PLANT the Animal Body 35.11: The Central Nervous
34.6: Digestive Systems -
REPRODUCTIVE 33.2: Animal Form and System - Midbrain and Brain
Digestive System- Small and
DEVELOPMENT AND Function - Body Plans Stem
Large Intestines
STRUCTURE 33.3: Animal Form and 35.12: The Central Nervous
34.7: Nutrition and Energy
32.1: Plant Reproductive Function - Limits on Animal Production - Food System - Spinal Cord
Development and Structure - Size and Shape Requirements and Essential 35.13: The Peripheral
Plant Reproductive 33.4: Animal Form and Nutrients Nervous System - Autonomic
Development and Structure Function - Limiting Effects of 34.8: Nutrition and Energy Nervous System
32.2: Plant Reproductive Diffusion on Size and Production - Food Energy and 35.14: The Peripheral
Development and Structure - Development ATP Nervous System - The
Sexual Reproduction in 33.5: Animal Form and 34.9: Digestive System Sensory-Somatic Nervous
Gymnosperms Function - Animal Processes - Ingestion System
32.3: Plant Reproductive Bioenergetics 34.10: Digestive System 35.15: Neurodegenerative
Development and Structure - 33.6: Animal Form and Processes - Digestion and Disorders - Introduction
Sexual Reproduction in Function - Animal Body Absorption 35.16: Nervous System
Angiosperms Planes and Cavities 34.11: Digestive System Disorders -
32.4: Pollination and 33.7: Animal Primary Tissues Processes - Elimination Neurodevelopmental
Fertilization - Introduction - Epithelial Tissues Disorders - Autism and
34.12: Digestive System
32.5: Pollination and 33.8: Animal Primary Tissues ADHD
Regulation - Neural
Fertilization - Pollination by - Loose, Fibrous, and Responses to Food 35.17: Nervous System
Insects Cartilage Connective Tissues Disorders -
34.13: Digestive System
32.6: Pollination and 33.9: Animal Primary Tissues Neurodevelopmental
Regulation - Hormonal
Fertilization - Pollination by - Bone, Adipose, and Blood Disorders - Mental Illnesses
Responses to Food
Bats, Birds, Wind, and Water Connective Tissues 35.18: Nervous System
32.7: Pollination and 33.10: Animal Primary 35: THE NERVOUS Disorders - Other
Fertilization - Double Tissues - Muscle Tissues and SYSTEM Neurological Disorders
Fertilization in Plants Nervous Tissues
35.1: Neurons and Glial Cells 36: SENSORY
32.8: Pollination and 33.11: Homeostasis - - Introduction SYSTEMS
Fertilization - Development of Homeostatic Process
35.2: Neurons and Glial Cells
the Seed 33.12: Homeostasis - Control 36.1: Sensory Processes -
- Neurons
of Homeostasis Reception
35.3: Neurons and Glial Cells
- Glia

7
 36.2: Sensory Processes - 37.7: Regulation of Body 38.14: Muscle Contraction 39.9: Breathing - The
Transduction and Perception Processes - Hormonal and Locomotion - Structure Mechanics of Human
36.3: Somatosensation - Regulation of the and Function of the Muscular Breathing
Somatosensory Receptors Reproductive System System 39.10: Breathing - Types of
36.4: Somatosensation - 37.8: Regulation of Body 38.15: Muscle Contraction Breathing
Integration of Signals from Processes - Hormonal and Locomotion - Skeletal 39.11: Breathing - The Work
Mechanoreceptors Regulation of Metabolism Muscle Fibers of Breathing
36.5: Somatosensation - 37.9: Regulation of Body 38.16: Muscle Contraction 39.12: Breathing - Dead
Thermoreception Processes - Hormonal Control and Locomotion - Sliding Space- V/Q Mismatch
36.6: Taste and Smell - Tastes of Blood Calcium Levels Filament Model of 39.13: Transport of Gases in
and Odors 37.10: Regulation of Body Contraction Human Bodily Fluids -
36.7: Taste and Smell - Processes - Hormonal 38.17: Muscle Contraction Transport of Oxygen in the
Reception and Transduction Regulation of Growth and Locomotion - ATP and Blood
37.11: Regulation of Body Muscle Contraction 39.14: Transport of Gases in
36.8: Hearing and Vestibular
Sensation - Sound Processes - Hormonal 38.18: Muscle Contraction Human Bodily Fluids -
Regulation of Stress and Locomotion - Regulatory Transport of Carbon Dioxide
36.9: Hearing and Vestibular
Proteins in the Blood
Sensation - Reception of
Sound
38: THE 38.19: Muscle Contraction
MUSCULOSKELETAL and Locomotion - Excitation– 40: THE CIRCULATORY
36.10: Hearing and Vestibular
SYSTEM Contraction Coupling SYSTEM
Sensation - The Vestibular
System 38.1: Types of Skeletal 38.20: Muscle Contraction 40.1: Overview of the
Systems - Functions of the and Locomotion - Control of Circulatory System - The Role
36.11: Hearing and Vestibular
Musculoskeletal System Muscle Tension of the Circulatory System
Sensation - Balance and
Determining Equilibrium 38.2: Types of Skeletal 40.2: Overview of the
Systems - Types of Skeletal 39: THE RESPIRATORY
36.12: Vision - Light Circulatory System - Open
Systems SYSTEM
36.13: Vision - Anatomy of and Closed Circulatory
the Eye 38.3: Types of Skeletal 39.1: Systems of Gas Systems
36.14: Vision - Transduction Systems - Human Axial Exchange - The Respiratory 40.3: Overview of the
of Light Skeleton System and Direct Diffusion Circulatory System - Types of
36.15: Vision - Visual 38.4: Types of Skeletal 39.2: Systems of Gas Circulatory Systems in
Processing Systems - Human Exchange - Skin, Gills, and Animals
Appendicular Skeleton Tracheal Systems 40.4: Components of the
37: THE ENDOCRINE 38.5: Bone - Introduction 39.3: Systems of Gas Blood - The Role of Blood in
SYSTEM 38.6: Bone - Cell Types in Exchange - Amphibian and the Body
Bones Bird Respiratory Systems 40.5: Components of the
37.1: Types of Hormones -
38.7: Bone - Bone 39.4: Systems of Gas Blood - Red Blood Cells
Hormone Functions
Development Exchange - Mammalian 40.6: Components of the
37.2: Types of Hormones -
38.8: Bone - Growth of Bone Systems and Protective Blood - White Blood Cells
Lipid-Derived, Amino Acid-
38.9: Bone - Bone Mechanisms 40.7: Components of the
Derived, and Peptide
Remodeling and Repair 39.5: Gas Exchange across Blood - Platelets and
Hormones
38.10: Joints and Skeletal Respiratory Surfaces - Gas Coagulation Factors
37.3: How Hormones Work -
Movement - Classification of Pressure and Respiration 40.8: Components of the
Introduction
Joints on the Basis of 39.6: Gas Exchange across Blood - Plasma and Serum
37.4: How Hormones Work -
Structure and Function Respiratory Surfaces - Basic 40.9: Mammalian Heart and
Intracellular Hormone
Principles of Gas Exchange Blood Vessels - Structures of
Receptors 38.11: Joints and Skeletal
Movement - Movement at 39.7: Gas Exchange across the Heart
37.5: How Hormones Work -
Synovial Joints Respiratory Surfaces - Lung 40.10: Mammalian Heart and
Plasma Membrane Hormone Volumes and Capacities
38.12: Joints and Skeletal Blood Vessels - Arteries,
Receptors
Movement - Types of 39.8: Gas Exchange across Veins, and Capillaries
37.6: Regulation of Body
Synovial Joints Respiratory Surfaces - Gas 40.11: Mammalian Heart and
Processes - Hormonal Exchange across the Alveoli
38.13: Joints and Skeletal Blood Vessels - The Cardiac
Regulation of the Excretory
Movement - Bone and Joint Cycle
System
Disorders

8
 40.12: Blood Flow and Blood 41.13: Hormonal Control of 42.15: Disruptions in the 44: ECOLOGY AND THE
Pressure Regulation - Blood Osmoregulatory Functions - Immune System - BIOSPHERE
Flow Through the Body Epinephrine and Hypersensitivities
44.1: The Scope of Ecology
40.13: Blood Flow and Blood Norepinephrine
Pressure Regulation - Blood 41.14: Hormonal Control of 43: ANIMAL 44.1A: Introduction to
REPRODUCTION AND Ecology
Pressure Osmoregulatory Functions -
Other Hormonal Controls for DEVELOPMENT 44.1B: Organismal Ecology
41: OSMOTIC Osmoregulation and Population Ecology
43.1: Reproduction Methods
REGULATION AND THE 44.1C: Community Ecology
43.1A: Methods of and Ecosystem Ecology
EXCRETORY SYSTEM 42: THE IMMUNE
Reproducing
41.1: Osmoregulation and
SYSTEM 44.2: Biogeography
43.1B: Types of Sexual and
Osmotic Balance - 42.1: Innate Immune Asexual Reproduction 44.2A: Biogeography
Introduction Response - Innate Immune 43.1C: Sex Determination 44.2B: Energy Sources
41.2: Osmoregulation and Response 44.2C: Temperature and
43.2: Fertilization
Osmotic Balance - Transport 42.2: Innate Immune Water
of Electrolytes across Cell Response - Physical and 43.2A: External and Internal
44.2D: Inorganic Nutrients
Membranes Chemical Barriers Fertilization
and Other Factors
41.3: Osmoregulation and 42.3: Innate Immune 43.2B: The Evolution of
44.2E: Abiotic Factors
Osmotic Balance - Concept of Response - Pathogen Reproduction
Influencing Plant Growth
Osmolality and Recognition 43.3: Human Reproductive
44.3: Terrestrial Biomes
Milliequivalent 42.4: Innate Immune Anatomy and Gametogenesis
44.3A: What constitutes a
41.4: Osmoregulation and Response - Natural Killer 43.3A: Male Reproductive
biome?
Osmotic Balance - Cells Anatomy
Osmoregulators and 44.3B: Tropical Wet Forest
42.5: Innate Immune 43.3B: Female Reproductive
Osmoconformers and Savannas
Response - The Complement Anatomy
41.5: Nitrogenous Wastes - System 44.3C: Subtropical Deserts
43.3C: Gametogenesis
Nitrogenous Waste in and Chaparral
42.6: Adaptive Immune (Spermatogenesis and
Terrestrial Animals- The Urea Response - Antigen- 44.3D: Temperate
Oogenesis)
Cycle presenting Cells- B and T Grasslands
43.4: Hormonal Control of
41.6: Nitrogenous Wastes - cells 44.3E: Temperate Forests
Human Reproduction
Nitrogenous Waste in Birds 42.7: Adaptive Immune 44.3F: Boreal Forests and
43.4A: Male Hormones Arctic Tundra
and Reptiles- Uric Acid Response - Humoral Immune
43.4B: Female Hormones
41.7: Excretion Systems - Response 44.4: Aquatic Biomes
Contractile Vacuoles in 42.8: Adaptive Immune 43.5: Fertilization and Early
44.4A: Abiotic Factors
Microorganisms Response - Cell-Mediated Embryonic Development
Influencing Aquatic Biomes
41.8: Excretion Systems - Immunity 43.5A: Fertilization 44.4B: Marine Biomes
Flame Cells of Planaria and 42.9: Adaptive Immune 43.5B: Cleavage, the 44.4C: Estuaries- Where the
Nephridia of Worms Response - Cytotoxic T Blastula Stage, and Ocean Meets Fresh Water
41.9: Excretion Systems - Lymphocytes and Mucosal Gastrulation
44.4D: Freshwater Biomes
Malpighian Tubules of Insects Surfaces 43.6: Organogenesis and
44.5: Climate and the Effects
41.10: Human 42.10: Adaptive Immune Vertebrate Formation
of Global Climate Change
Osmoregulatory and Response - Immunological
43.6A: Organogenesis
Excretory Systems - Kidney Memory 44.5A: Climate and Weather
43.6B: Vertebrate Axis
Structure 42.11: Adaptive Immune 44.5B: Causes of Global
Formation
41.11: Human Response - Regulating Climate Change
Osmoregulatory and Immune Tolerance 43.7: Human Pregnancy and 44.5C: Evidence of Global
Birth Climate Change
Excretory Systems - Nephron- 42.12: Antibodies - Antibody
The Functional Unit of the Structure 43.7A: Human Gestation 44.5D: Past and Present
Kidney 42.13: Antibodies - Antibody 43.7B: Labor and Birth Effects of Climate Change
41.12: Human Functions 43.7C: Contraception and
Osmoregulatory and 42.14: Disruptions in the Birth Control
Excretory Systems - Kidney Immune System - 43.7D: Infertility
Function and Physiology Immunodeficiency

9
45: POPULATION AND 45.5A: The Role of Species 46.1D: Modeling Ecosystem 47.1C: Biodiversity Change
COMMUNITY ECOLOGY within Communities Dynamics through Geological Time
45.5B: Predation, Herbivory, 46.2: Energy Flow through 47.1D: The Pleistocene
45.1: Population Demography
and the Competitive Ecosystems Extinction
45.1A: Population
Exclusion Principle 46.2A: Strategies for 47.1E: Present-Time
Demography
45.5C: Symbiosis Acquiring Energy Extinctions
45.1B: Population Size and
45.5D: Ecological 46.2B: Productivity within 47.2: The Importance of
Density
Succession Trophic Levels Biodiversity to Human Life
45.1C: Species Distribution
45.6: Innate Animal Behavior 46.2C: Transfer of Energy 47.2A: Human Health and
45.1D: The Study of
45.6A: Introduction to between Trophic Levels Biodiversity
Population Dynamics
Animal Behavior 46.2D: Ecological Pyramids 47.2B: Agricultural Diversity
45.2: Environmental Limits to
Population Growth 45.6B: Movement and 46.2E: Biological 47.2C: Managing Fisheries
Migration Magnification 47.3: Threats to Biodiversity
45.2A: Exponential
45.6C: Animal 46.3: Biogeochemical Cycles 47.3A: Habitat Loss and
Population Growth
Communication and Living 46.3A: Biogeochemical Sustainability
45.2B: Logistic Population
in Groups Cycles
Growth 47.3B: Overharvesting
45.6D: Altruism and 46.3B: The Water
45.2C: Density-Dependent 47.3C: Exotic Species
Populations (Hydrologic) Cycle
and Density-Independent 47.3D: Climate Change and
45.6E: Mating Systems and 46.3C: The Carbon Cycle
Population Regulation Biodiversity
Sexual Selection
45.3: Life History Patterns 46.3D: The Nitrogen Cycle 47.4: Preserving Biodiversity
45.7: Learned Animal
45.3A: Life History Patterns 46.3E: The Phosphorus 47.4A: Measuring
Behavior
and Energy Budgets Cycle Biodiversity
45.7A: Simple Learned 46.3E: The Sulfur Cycle
45.3B: Theories of Life 47.4B: Changing Human
Behaviors
History Behavior in Response to
45.7B: Conditioned Behavior 47: CONSERVATION
45.4: Human Population Biodiversity Loss
45.7C: Cognitive Learning BIOLOGY AND
Growth 47.4C: Ecological
and Sociobiology BIODIVERSITY Restoration
45.4A: Human Population
Growth 47.1: The Biodiversity Crisis
46: ECOSYSTEMS
45.4B: Overcoming Density- 47.1A: Loss of Biodiversity INDEX
46.1: Ecology of Ecosystems
Dependent Regulation 47.1B: Types of Biodiversity
45.4C: Age Structure, 46.1A: Ecosystem Dynamics
Population Growth, and 46.1B: Food Chains and
Food Webs Thumbnail: A tigress having a bath in Ranthambhore Tiger Reserve,
Economic Development
Rajasthan (CC BY 2.0; Koshy Koshy via Wikipedia)
45.5: Community Ecology 46.1C: Studying Ecosystem
Dynamics
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BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

10
 CHAPTER OVERVIEW

1: THE STUDY OF LIFE

1.1: The Science of Biology - Introduction to the Study of Biology
1.2: The Science of Biology - Scientific Reasoning
1.3: The Science of Biology - The Scientific Method
1.4: The Science of Biology - Basic and Applied Science
1.5: The Science of Biology - Publishing Scientific Work
1.6: The Science of Biology - Branches and Subdisciplines of Biology
1.7: Themes and Concepts of Biology - Properties of Life
1.8: Themes and Concepts of Biology - Levels of Organization of Living Things
1.9: Themes and Concepts of Biology - The Diversity of Life

This page titled 1: The Study of Life is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

1
1.1: THE SCIENCE OF BIOLOGY - INTRODUCTION TO THE STUDY OF
BIOLOGY
relatively new species, having inhabited this planet for only the last
 LEARNING OBJECTIVES 200,000 years (approximately).

Describe the field of biological science

THE STUDY OF LIFE

Biology is a natural science concerned with the study of life and
living organisms. Modern biology is a vast and eclectic field
composed of many specialized disciplines that study the structure,
function, growth, distribution, evolution, or other features of living
organisms. However, despite the broad scope of biology, there are
certain general and unifying concepts that govern all study and
research:
the cell is the basic unit of life
genes (consisting of DNA or RNA) are the basic unit of heredity
evolution accounts for the unity and diversity seen among living
organisms
all organisms survive by consuming and transforming energy
Figure 1.1.1: Stromatolites: Stromatolites, sedimentary accretions
all organisms maintain a stable internal environment formed by the actions of cyanobacteria, provide fossil evidence of
life on Earth about 3.5 billion years ago.

HISTORY OF BIOLOGICAL SCIENCE

Although modern biology is a relatively recent development,
sciences related to and included within it have been studied since
ancient times. Natural philosophy was studied as early as the ancient
civilizations of Mesopotamia, Egypt, the Indian subcontinent, and
China. However, the origins of modern biology and its approach to
the study of nature are most often traced back to ancient Greece.
(Biology is derived from the Greek word “bio” meaning “life” and
the suffix “ology” meaning “study of.”)
Advances in microscopy also had a profound impact on biological
thinking. In the early 19th century, a number of biologists pointed to
the central importance of the cell and in 1838, Schleiden and
Schwann began promoting the now universal ideas of the cell theory.
Jean-Baptiste Lamarck was the first to present a coherent theory of
evolution, although it was the British naturalist Charles Darwin who
spread the theory of natural selection throughout the scientific
community. In 1953, the discovery of the double helical structure of
DNA marked the transition to the era of molecular genetics.

Figure 1.1.1: Biology: The Study of Life: A collection of organisms

clockwise from top left: bacteria, koala, fern, toadstool, tree frog,
tarantula.
Biological research indicates the first forms of life on Earth were
microorganisms that existed for billions of years before the
evolution of larger organisms. The mammals, birds, and flowers so
familiar to us are all relatively recent, originating within the last 200
million years. Modern-appearing humans, Homo sapiens, are a

1.1.1 https://bio.libretexts.org/@go/page/12643
 information is available for anyone to read, learn from, or even
question/dispute. This makes science an iterative, or cumulative,
process, where previous research is used as the foundation for new
research. Our current understanding of any issue in the sciences is
the culmination of all previous work.
Pseudoscience is a belief presented as scientific although it is not a
product of scientific investigation. Pseudoscience is often known as
fringe or alternative science. It usually lacks the carefully-controlled
and thoughtfully-interpreted experiments which provide the
foundation of the natural sciences and which contribute to their
advancement.

KEY POINTS
Biology has evolved as a field of science since it was first
studied in ancient civilizations, although modern biology is a
relatively recent field.
Science is a process that requires the testing of ideas using
evidence gathered from the natural world. Science is iterative in
nature and involves critical thinking, careful data collection,
rigorous peer review, and the communication of results.
Science also refers to the body of knowledge produced by
scientific investigation.
Figure 1.1.1: Phrenology: Dr. Spurzheim’s divisions of the organs of Pseudoscience is a belief presented as scientific although it is not
phrenology marked externally: Phrenology is a pseudoscience that a product of scientific investigation.
attempted to determine brain function and personality by analyzing
an individual’s skull. KEY TERMS
SCIENCE AND PSEUDOSCIENCE pseudoscience: Any belief purported to be scientific or
supported by science that is not a product of scientific
Science is a process for learning about the natural world. Most
investigation.
scientific investigations involve the testing of potential answers to
important research questions. For example, oncologists ( cancer science: A process for learning about the natural world that tests
doctors) are interested in finding out why some cancers respond well ideas using evidence gathered from nature.
to chemotherapy while others are unaffected. Based on their Biology: A natural science concerned with the study of life and
living organisms.
growing knowledge of molecular biology, some doctors suspect a
connection between a patient’s genetics and their response to
This page titled 1.1: The Science of Biology - Introduction to the Study of
chemotherapy. Many years of research have produced numerous Biology is shared under a CC BY-SA 4.0 license and was authored,
scientific papers documenting the evidence for a connection between remixed, and/or curated by Boundless.
cancer, genetics, and treatment response. Once published, scientific

1.1.2 https://bio.libretexts.org/@go/page/12643
1.2: THE SCIENCE OF BIOLOGY - SCIENTIFIC REASONING

 LEARNING OBJECTIVES

Compare and contrast theories and hypotheses

THE PROCESS OF SCIENCE

Science (from the Latin scientia, meaning “knowledge”) can be
defined as knowledge that covers general truths or the operation of
general laws, especially when acquired and tested by the scientific
method. The steps of the scientific method will be examined in
detail later, but one of the most important aspects of this method is
the testing of hypotheses (testable statements) by means of
repeatable experiments. Although using the scientific method is
inherent to science, it is inadequate in determining what science is.
This is because it is relatively easy to apply the scientific method to
disciplines such as physics and chemistry, but when it comes to
disciplines like archaeology, paleoanthropology, psychology, and
geology, the scientific method becomes less applicable as it becomes
more difficult to repeat experiments.
These areas of study are still sciences, however. Consider
archaeology: even though one cannot perform repeatable
experiments, hypotheses may still be supported. For instance, an Figure 1.2.1: Scientific Reasoning: Scientists use two types of
reasoning, inductive and deductive, to advance scientific knowledge.
archaeologist can hypothesize that an ancient culture existed based
on finding a piece of pottery. Further hypotheses could be made Inductive reasoning is a form of logical thinking that uses related
about various characteristics of this culture. These hypotheses may observations to arrive at a general conclusion. This type of reasoning
be found to be plausible (supported by data) and tentatively is common in descriptive science. A life scientist such as a biologist
accepted, or may be falsified and rejected altogether (due to makes observations and records them. These data can be qualitative
contradictions from data and other findings). A group of related or quantitative and the raw data can be supplemented with drawings,
hypotheses, that have not been disproven, may eventually lead to the pictures, photos, or videos. From many observations, the scientist
development of a verified theory. A theory is a tested and confirmed can infer conclusions (inductions) based on evidence. Inductive
explanation for observations or phenomena that is supported by a reasoning involves formulating generalizations inferred from careful
large body of evidence. Science may be better defined as fields of observation and the analysis of a large amount of data. Brain studies
study that attempt to comprehend the nature of the universe. provide an example. In this type of research, many live brains are
observed while people are doing a specific activity, such as viewing
SCIENTIFIC REASONING images of food. The part of the brain that “lights up” during this
One thing is common to all forms of science: an ultimate goal “to activity is then predicted to be the part controlling the response to
know.” Curiosity and inquiry are the driving forces for the the selected stimulus; in this case, images of food. The “lighting up”
development of science. Scientists seek to understand the world and of the various areas of the brain is caused by excess absorption of
the way it operates. To do this, they use two methods of logical radioactive sugar derivatives by active areas of the brain. The
thinking: inductive reasoning and deductive reasoning. resultant increase in radioactivity is observed by a scanner. Then
researchers can stimulate that part of the brain to see if similar
responses result.
Deductive reasoning or deduction is the type of logic used in
hypothesis-based science. In deductive reason, the pattern of
thinking moves in the opposite direction as compared to inductive
reasoning. Deductive reasoning is a form of logical thinking that
uses a general principle or law to forecast specific results. From
those general principles, a scientist can extrapolate and predict the
specific results that would be valid as long as the general principles
are valid. Studies in climate change can illustrate this type of
reasoning. For example, scientists may predict that if the climate
becomes warmer in a particular region, then the distribution of
plants and animals should change. These predictions have been

1.2.1 https://bio.libretexts.org/@go/page/12644
written and tested, and many such predicted changes have been KEY POINTS
observed, such as the modification of arable areas for agriculture A hypothesis is a statement/prediction that can be tested by
correlated with changes in the average temperatures. experimentation.
Both types of logical thinking are related to the two main pathways A theory is an explanation for a set of observations or
of scientific study: descriptive science and hypothesis-based science. phenomena that is supported by extensive research and that can
Descriptive (or discovery) science, which is usually inductive, aims be used as the basis for further research.
to observe, explore, and discover, while hypothesis-based science, Inductive reasoning draws on observations to infer logical
which is usually deductive, begins with a specific question or conclusions based on the evidence.
problem and a potential answer or solution that can be tested. The Deductive reasoning is hypothesis-based logical reasoning that
boundary between these two forms of study is often blurred and deduces conclusions from test results.
most scientific endeavors combine both approaches. The fuzzy
boundary becomes apparent when thinking about how easily KEY TERMS
observation can lead to specific questions. For example, a gentleman theory: a well-substantiated explanation of some aspect of the
in the 1940s observed that the burr seeds that stuck to his clothes and natural world based on knowledge that has been repeatedly
his dog’s fur had a tiny hook structure. Upon closer inspection, he confirmed through observation and experimentation
discovered that the burrs’ gripping device was more reliable than a hypothesis: a tentative conjecture explaining an observation,
zipper. He eventually developed a company and produced the hook- phenomenon, or scientific problem that can be tested by further
and-loop fastener popularly known today as Velcro. Descriptive observation, investigation, and/or experimentation
science and hypothesis-based science are in continuous dialogue.
This page titled 1.2: The Science of Biology - Scientific Reasoning is shared
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

Figure 1.2.1: A Burr: This fruit attaches to animal fur via the hooks
on its surface to improve distribution. Velcro is an example of a
biomimetic invention which has copied burrs and uses small flexible
hooks to reversibly attach to fluffy surfaces.

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1.3: THE SCIENCE OF BIOLOGY - THE SCIENTIFIC METHOD

 LEARNING OBJECTIVES

Discuss hypotheses and the components of a scientific

experiment as part of the scientific method

THE SCIENTIFIC METHOD

Biologists study the living world by posing questions about it and
seeking science -based responses. This approach is common to other
sciences as well and is often referred to as the scientific method. The
scientific method was used even in ancient times, but it was first
documented by England’s Sir Francis Bacon (1561–1626) who set
up inductive methods for scientific inquiry. The scientific method
can be applied to almost all fields of study as a logical, rational,
problem-solving method.

Figure 1.3.1: The Scientific Method: The scientific method consists

of a series of well-defined steps. If a hypothesis is not supported by
experimental data, a new hypothesis can be proposed.

PROPOSING A HYPOTHESIS
Recall that a hypothesis is an educated guess that can be tested.
Hypotheses often also include an explanation for the educated guess.
To solve one problem, several hypotheses may be proposed. For
example, the student might believe that his friend is tall because he
drinks a lot of milk. So his hypothesis might be “If a person drinks a
Figure 1.3.1: Sir Francis Bacon: Sir Francis Bacon (1561–1626) is
lot of milk, then they will grow to be very tall because milk is good
credited with being the first to define the scientific method. for your bones.” Generally, hypotheses have the format “If…
The scientific process typically starts with an observation (often a then…” Keep in mind that there could be other responses to the
problem to be solved) that leads to a question. Let’s think about a question; therefore, other hypotheses may be proposed. A second
simple problem that starts with an observation and apply the hypothesis might be, “If a person has tall parents, then they will also
scientific method to solve the problem. A teenager notices that his be tall, because they have the genes to be tall. ”
friend is really tall and wonders why. So his question might be, Once a hypothesis has been selected, the student can make a
“Why is my friend so tall? ” prediction. A prediction is similar to a hypothesis but it is truly a
guess. For instance, they might predict that their friend is tall
because he drinks a lot of milk.

TESTING A HYPOTHESIS
A valid hypothesis must be testable. It should also be falsifiable,
meaning that it can be disproven by experimental results.
Importantly, science does not claim to “prove” anything because
scientific understandings are always subject to modification with
further information. This step—openness to disproving ideas—is
what distinguishes sciences from non-sciences. The presence of the
supernatural, for instance, is neither testable nor falsifiable. To test a
hypothesis, a researcher will conduct one or more experiments

1.3.1 https://bio.libretexts.org/@go/page/12645
designed to eliminate one or more of the hypotheses. Each sequence, there is flexibility. Many times, science does not operate
experiment will have one or more variables and one or more in a linear fashion. Instead, scientists continually draw inferences
controls. A variable is any part of the experiment that can vary or and make generalizations, finding patterns as their research
change during the experiment. The control group contains every proceeds. Scientific reasoning is more complex than the scientific
feature of the experimental group except it is not given the method alone suggests.
manipulation that is hypothesized. For example, a control group
could be a group of varied teenagers that did not drink milk and they KEY POINTS
could be compared to the experimental group, a group of varied In the scientific method, observations lead to questions that
teenagers that did drink milk. Thus, if the results of the experimental require answers.
group differ from the control group, the difference must be due to In the scientific method, the hypothesis is a testable statement
the hypothesized manipulation rather than some outside factor. To proposed to answer a question.
test the first hypothesis, the student would find out if drinking milk In the scientific method, experiments (often with controls and
affects height. If drinking milk has no affect on height, then there variables) are devised to test hypotheses.
must be another reason for the height of the friend. To test the In the scientific method, analysis of the results of an experiment
second hypothesis, the student could check whether or not his friend will lead to the hypothesis being accepted or rejected.
has tall parents. Each hypothesis should be tested by carrying out
appropriate experiments. Be aware that rejecting one hypothesis KEY TERMS
does not determine whether or not the other hypotheses can be scientific method: a way of discovering knowledge based on
accepted. It simply eliminates one hypothesis that is not valid. Using making falsifiable predictions (hypotheses), testing them, and
the scientific method, the hypotheses that are inconsistent with developing theories based on collected data
experimental data are rejected. hypothesis: an educated guess that usually is found in an “if…
While this “tallness” example is based on observational results, then…” format
control group: a group that contains every feature of the
other hypotheses and experiments might have clearer controls. For
instance, a student might attend class on Monday and realize she had experimental group except it is not given the manipulation that is
hypothesized
difficulty concentrating on the lecture. One hypothesis to explain
this occurrence might be, “If I eat breakfast before class, then I am
This page titled 1.3: The Science of Biology - The Scientific Method is
better able to pay attention.” The student could then design an shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
experiment with a control to test this hypothesis. curated by Boundless.
The scientific method may seem too rigid and structured. It is
important to keep in mind that although scientists often follow this

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1.4: THE SCIENCE OF BIOLOGY - BASIC AND APPLIED SCIENCE
valid. It is true that there are problems that demand immediate
 LEARNING OBJECTIVES attention; however, few solutions would be found without the help
of the wide knowledge foundation generated through basic science.
Differentiate between basic and applied science

TWO TYPES OF SCIENCE: BASIC SCIENCE AND

APPLIED SCIENCE
The scientific community has been debating for the last few decades
about the value of different types of science. Is it valuable to pursue
science for the sake of simply gaining knowledge, or does scientific
knowledge only have worth if we can apply it to solving a specific
problem or to bettering our lives? This question focuses on the
differences between two types of science: basic science and applied Figure 1.4.1: A Link Between Basic and Applied Science: The
Human Genome Project was a 13-year collaborative effort among
science. researchers working in several different fields of science. The
Basic science or “pure” science seeks to expand knowledge project, which sequenced the entire human genome, was completed
in 2003.
regardless of the short-term application of that knowledge. It is not
focused on developing a product or a service of immediate public or One example of how basic and applied science can work together to
commercial value. The goal of basic science is knowledge for solve practical problems occurred after the discovery of DNA
knowledge’s sake; though this does not mean that, in the end, it may structure led to an understanding of the molecular mechanisms
governing DNA replication. Strands of DNA, unique in every
not result in a practical application.
human, are found in our cells where they provide the instructions
In contrast, applied science or “technology” aims to use science to
necessary for life. During DNA replication, DNA makes new copies
solve real-world problems such as improving crop yields, finding a
of itself shortly before a cell divides. Understanding the mechanisms
cure for a particular disease, or saving animals threatened by a
of DNA replication enabled scientists to develop laboratory
natural disaster. In applied science, the problem is usually defined
techniques that are now used to identify genetic diseases, pinpoint
for the researcher.
individuals who were at a crime scene, and determine paternity.
Without basic science, it is unlikely that applied science would exist.
Another example of the link between basic and applied research is
the Human Genome Project, a study in which each human
chromosome was analyzed and mapped to determine the precise
sequence of DNA subunits and the exact location of each gene. (The
gene is the basic unit of heredity; an individual’s complete collection
of genes is his or her genome. ) Other less complex organisms have
also been studied as part of this project in order to gain a better
understanding of human chromosomes. The Human Genome Project
relied on basic research carried out with simple organisms and, later,
with the human genome. An important end goal eventually became
using the data for applied research to seek cures and early diagnoses
for genetically-related diseases.
Figure 1.4.1: Example of Applied Science: After Hurricane Ike While research efforts in both basic science and applied science are
struck the Gulf Coast in 2008, the U.S. Fish and Wildlife Service usually carefully planned, it is important to note that some
rescued this brown pelican. Thanks to applied science, scientists discoveries are made by serendipity; that is, by means of a fortunate
knew how to rehabilitate the bird.
accident or a lucky surprise. Penicillin was discovered when
Some individuals may perceive applied science as “useful” and basic biologist Alexander Fleming accidentally left a petri dish of
science as “useless.” A question these people might pose to a
Staphylococcus bacteria open. An unwanted mold grew on the dish,
scientist advocating knowledge acquisition would be, “What for?” A killing the bacteria. The mold turned out to be Penicillium and a new
careful look at the history of science, however, reveals that basic
antibiotic was discovered. Even in the highly organized world of
knowledge has resulted in many remarkable applications of great science, luck, when combined with an observant, curious mind, can
value. Many scientists think that a basic understanding of science is
lead to unexpected breakthroughs.
necessary before an application is developed; therefore, applied
science relies on the results generated through basic science. Other KEY POINTS
scientists think that it is time to move on from basic science and The only goal of basic science research is to increase the
instead to find solutions to actual problems. Both approaches are knowledge base of a particular field of study.

1.4.1 https://bio.libretexts.org/@go/page/12646
Applied science uses the knowledge base supplied by basic KEY TERMS
science to devise solutions, often technological, to specific basic science: research done solely to expand the knowledge
problems. base
The basic science involved in mapping the human genome is applied science: The discipline dealing with the art or science of
leading to applied science techniques that will diagnose and treat applying scientific knowledge to practical problems.
genetic diseases.
This page titled 1.4: The Science of Biology - Basic and Applied Science is
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

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1.5: THE SCIENCE OF BIOLOGY - PUBLISHING SCIENTIFIC WORK
The introduction starts with brief, but broad, background
 LEARNING OBJECTIVES information about what is known in the field. A good introduction
also gives the rationale and justification for the work. The
Describe the role played by peer-reviewed scientific articles
introduction refers to the published scientific work of others and,
therefore, requires citations following the style of the journal. Using
REPORTING SCIENTIFIC WORK the work or ideas of others without proper citation is considered
Scientists must share their findings in order for other researchers to plagiarism.
expand and build upon their discoveries. Collaboration with other The materials and methods section includes a complete and accurate
scientists—when planning, conducting, and analyzing results—are description of the substances and the techniques used by the
all important for scientific research. For this reason, a major aspect researchers to gather data. The description should be thorough, yet
of a scientist’s work is communicating with peers and disseminating concise, while providing enough information to allow another
results to peers. Scientists can share results by presenting them at a researcher to repeat the experiment and obtain similar results. This
scientific meeting or conference, but this approach can reach only section will also include information on how measurements were
the select few who are present. Instead, most scientists present their made and what types of calculations and statistical analyses were
results in peer-reviewed manuscripts that are published in scientific used to examine raw data. Although the materials and methods
journals. Peer-reviewed manuscripts are scientific papers that are section gives an accurate description of the experiments, it does not
reviewed by a scientist’s colleagues or peers. These colleagues are discuss them.
qualified individuals, often experts in the same research area, who
Journals may require separate results and discussion sections, or it
judge whether or not the scientist’s work is suitable for publication.
may combine them in one section. If the journal does not allow the
The process of peer review helps to ensure that the research
combination of both sections, the results section simply narrates the
described in a scientific paper or grant proposal is original,
findings without any further interpretation. The results are presented
significant, logical, and thorough. Grant proposals, which are
by means of tables or graphs, but no duplicate information should be
requests for research funding, are also subject to peer review.
presented. In the discussion section, the researcher will interpret the
Scientists publish their work so other scientists can reproduce their
results, describe how variables may be related, and attempt to
experiments under similar or different conditions to expand on the
explain the observations. It is indispensable to conduct an extensive
findings. The experimental results must be consistent with the
literature search to put the results in the context of previously-
findings of other scientists.
published scientific research. Therefore, proper citations are
included in this section as well.
Finally, the conclusion section summarizes the importance of the
experimental findings. While the scientific paper almost certainly
answered one or more scientific questions that were stated, any good
research should lead to more questions. A well-written scientific
paper leaves doors open for the researcher and others to continue
and expand on the findings.
Review articles do not follow the IMRAD format because they do
not present original scientific findings or primary literature. Instead,
Figure 1.5.1: Scientific Journal: Scientific research is published in they summarize and comment on findings that were published as
peer-reviewed scientific journals. primary literature. They typically include extensive reference
A scientific paper is very different from creative writing. Although sections.
creativity is required to design experiments, there are fixed
guidelines when it comes to presenting scientific results. Scientific
KEY POINTS
writing must be brief, concise, and accurate. It needs to be succinct The body of scientific knowledge is recorded in peer-reviewed
but detailed-enough to allow peers to reproduce the experiments. science journals which allow other scientists to determine what
has been done previously and where their own research fits in the
The scientific paper consists of several specific sections:
larger field of study.
introduction, materials and methods, results, and discussion. This
A scientific article generally follows the steps of the scientific
structure is sometimes called the “IMRaD” format. There are usually
method: introduction (background, observations, question),
acknowledgment and reference sections, as well as an abstract (a
materials and methods (hypothesis and experimental plan),
concise summary) at the beginning of the paper. There might be
results (analysis of collected data), and discussion (conclusions
additional sections depending on the type of paper and the journal
drawn from analysis).
where it will be published; for example, some review papers require
Peer reviewers are other researchers in that field of study who
an outline.
carefully dissect, analyze, and critique a research article

1.5.1 https://bio.libretexts.org/@go/page/12647
 submitted for publication. independent researchers to evaluate the contribution, importance,
Review articles (summaries and commentaries on prior research and accuracy of the manuscript’s contents.
in a field of study) also go through the peer-review process.
This page titled 1.5: The Science of Biology - Publishing Scientific Work is
KEY TERMS shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
peer review: The scholarly process whereby manuscripts curated by Boundless.
intended to be published in an academic journal are reviewed by

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1.6: THE SCIENCE OF BIOLOGY - BRANCHES AND SUBDISCIPLINES OF
BIOLOGY
Another field of biological study, neurobiology, is the study of the
 LEARNING OBJECTIVES nervous system, and although it is considered a branch of biology, it
is also recognized as an interdisciplinary field of study known as
Recognize the various subfields of biology; e.g.
neuroscience. Because of its interdisciplinary nature, this
microbiology, genetics, evolutionary, etc.
subdiscipline focuses on different functions of the nervous system
using molecular, cellular, developmental, medical, and
BRANCHES OF BIOLOGICAL STUDY computational approaches.
The scope of biology is broad and therefore contains many branches Additional branches of biology include paleontology, which uses
and subdisciplines. Biologists may pursue one of those fossils to study life’s history; zoology, which studies animals; and
subdisciplines and work in a more focused field. The biological botany, which studies plants. Biologists can also specialize as
branches are divided according to the focus of the discipline and can biotechnologists, ecologists, or physiologists. This is just a small
even be divided based on the types of techniques and tools used to sample of the many fields that biologists can pursue.
study that specific focus. However, with the increasing amount of
basic biological information growing due to advances in technology
and databases, there is often cross-discipline and collaboration
between branches. For instance, molecular biology and biochemistry
study biological processes at the molecular and chemical level,
respectively, including interactions among molecules such as DNA,
RNA, and proteins, as well as the way they are regulated.
Microbiology, the study of microorganisms, is the study of the
structure and function of single-celled organisms. It is quite a broad
branch itself, and depending on the subject of study, there are also
microbial physiologists, ecologists, and geneticists, among others.

BIOLOGICAL DISCIPLINES AND CAREERS

Forensic science is the application of science to answer questions
related to the law. Biologists as well as chemists and biochemists
can be forensic scientists. Forensic scientists provide scientific
evidence for use in courts, and their job involves examining trace
Figure 1.6.1: Paleontology: Researchers work on excavating
materials associated with crimes.Their job activities are primarily dinosaur fossils at a site in Castellón, Spain.
related to crimes against people such as murder, rape, and assault.
Biology is the culmination of the achievements of the natural
Their work involves analyzing samples such as hair, blood, and
sciences from their inception to today. Excitingly, it is the cradle of
other body fluids, including the processing of DNA found in many
emerging sciences such as the biology of brain activity, genetic
different environments and materials associated with the crime
engineering of custom organisms, and the biology of evolution that
scenes.
uses the laboratory tools of molecular biology to retrace the earliest
stages of life on earth. A scan of news headlines—whether reporting
on immunizations, a newly discovered species, sports doping, or a
genetically-modified food—demonstrates the way biology is active
in and important to our everyday world.

KEY POINTS
Biology is broad and focuses on the study of life from various
perspectives.
The branches and subdisciplines of biology, which are highly
focused areas, have resulted in the development of careers that
are specific to these branches and subdisciplines.
Branches of biological study include microbiology, physiology,
ecology and genetics; subdisciplines within these branches can
include: microbial physiology, microbial ecology and microbial
genetics.
Figure 1.6.1: Forensic Science: This forensic scientist works in a
DNA extraction room at the U.S. Army Criminal Investigation
Laboratory at Fort Gillem, GA.

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1.7: THEMES AND CONCEPTS OF BIOLOGY - PROPERTIES OF LIFE
Movement toward a stimulus is considered a positive response,
 LEARNING OBJECTIVES while movement away from a stimulus is considered a negative
response.
Describe the properties of life
REPRODUCTION
All living organisms share several key characteristics or functions: Single-celled organisms reproduce by first duplicating their DNA.
order, sensitivity or response to the environment, reproduction, They then divide it equally as the cell prepares to divide to form two
growth and development, regulation, homeostasis, and energy new cells. Multicellular organisms often produce specialized
processing. When viewed together, these eight characteristics serve reproductive germline cells that will form new individuals. When
to define life. reproduction occurs, genes containing DNA are passed along to an
organism’s offspring. These genes ensure that the offspring will
belong to the same species and will have similar characteristics, such
as size and shape.

Figure 1.7.1: Multicellular Organisms: A toad represents a highly

organized structure consisting of cells, tissues, organs, and organ
systems. Figure 1.7.1: Reproduction: Although no two look alike, these
kittens have inherited genes from both parents and share many of the
ORDER same characteristics.

Organisms are highly organized, coordinated structures that consist GROWTH AND DEVELOPMENT
of one or more cells. Even very simple, single-celled organisms are
All organisms grow and develop following specific instructions
remarkably complex: inside each cell, atoms make up molecules;
coded for by their genes. These genes provide instructions that will
these in turn make up cell organelles and other cellular inclusions. In
direct cellular growth and development, ensuring that a species’
multicellular organisms, similar cells form tissues. Tissues, in turn,
young will grow up to exhibit many of the same characteristics as its
collaborate to create organs (body structures with a distinct
parents.
function). Organs work together to form organ systems.
REGULATION
Even the smallest organisms are complex and require multiple
regulatory mechanisms to coordinate internal functions, respond to
stimuli, and cope with environmental stresses. Two examples of
internal functions regulated in an organism are nutrient transport and
blood flow. Organs (groups of tissues working together) perform
specific functions, such as carrying oxygen throughout the body,
removing wastes, delivering nutrients to every cell, and cooling the
body.

Figure 1.7.1: Response to Stimuli: The leaves of this sensitive plant

(Mimosa pudica) will instantly droop and fold when touched. After a
few minutes, the plant returns to normal.

SENSITIVITY OR RESPONSE TO STIMULI

Organisms can respond to diverse stimuli. For example, plants can
grow toward a source of light, climb on fences and walls, or respond
to touch. Even tiny bacteria can move toward or away from
chemicals (a process called chemotaxis) or light (phototaxis).

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 ENERGY PROCESSING
All organisms use a source of energy for their metabolic activities.
Some organisms capture energy from the sun and convert it into
chemical energy in food; others use chemical energy in molecules
they take in as food.

Figure 1.7.1: Homeostasis: Polar bears (Ursus maritimus) and other

mammals living in ice-covered regions maintain their body
temperature by generating heat and reducing heat loss through thick
fur and a dense layer of fat under their skin.
Figure 1.7.1: Adaptation in the flat-tailed horned lizard: This lizard
HOMEOSTASIS exhibits a flattened body and coloring that helps camouflage it, both
of which are adaptive traits that help it avoid predators.
In order to function properly, cells need to have appropriate
conditions such as proper temperature, pH, and appropriate EVOLUTION
concentration of diverse chemicals. These conditions may, however, As a population of organisms interacts with the environment,
change from one moment to the next. Organisms are able to individuals with traits that contribute to reproduction and survival in
maintain internal conditions within a narrow range almost that particular environment will leave more offspring. Over time
constantly, despite environmental changes, through homeostasis those advantageous traits (called adaptations ) will become more
(literally, “steady state”)—the ability of an organism to maintain common in the population. This process, change over time, is called
constant internal conditions. For example, an organism needs to evolution, and it is one of the processes that explain the diverse
regulate body temperature through a process known as species seen in biology. Adaptations help organisms survive in their
thermoregulation. Organisms that live in cold climates, such as the ecological niches, and adaptive traits may be structural, behavioral,
polar bear, have body structures that help them withstand low or physiological; as such, adaptations frequently involve other
temperatures and conserve body heat. Structures that aid in this type properties of organisms such as homeostasis, reproduction, and
of insulation include fur, feathers, blubber, and fat. In hot climates, growth and development.
organisms have methods (such as perspiration in humans or panting
in dogs) that help them to shed excess body heat. KEY POINTS
Order can include highly organized structures such as cells,
tissues, organs, and organ systems.
Interaction with the environment is shown by response to stimuli.
The ability to reproduce, grow and develop are defining features
of life.
The concepts of biological regulation and maintenance of
homeostasis are key to survival and define major properties of
life.
Organisms use energy to maintain their metabolic processes.
Populations of organisms evolve to produce individuals that are
adapted to their specific environment.

KEY TERMS
phototaxis: The movement of an organism either towards or
away from a source of light
gene: a unit of heredity; the functional units of chromosomes that
determine specific characteristics by coding for specific proteins
Figure 1.7.1: Energy Processing: The California condor
chemotaxis: the movement of a cell or an organism in response
(Gymnogyps californianus) uses chemical energy derived from food
to power flight. to a chemical stimulant

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1.8: THEMES AND CONCEPTS OF BIOLOGY - LEVELS OF ORGANIZATION OF
LIVING THINGS
considered living: they are not made of cells. To make new viruses,
 LEARNING OBJECTIVES they have to invade and hijack the reproductive mechanism of a
living cell; only then can they obtain the materials they need to
Describe the biological levels of organization from the
reproduce. ) Some organisms consist of a single cell and others are
smallest to highest level
multicellular. Cells are classified as prokaryotic or eukaryotic.
Prokaryotes are single-celled or colonial organisms that do not have
Living things are highly organized and structured, following a
membrane-bound nuclei; in contrast, the cells of eukaryotes do have
hierarchy that can be examined on a scale from small to large. The membrane-bound organelles and a membrane-bound nucleus.
atom is the smallest and most fundamental unit of matter. It consists
In larger organisms, cells combine to make tissues, which are groups
of a nucleus surrounded by electrons. Atoms form molecules which
of similar cells carrying out similar or related functions. Organs are
are chemical structures consisting of at least two atoms held together
collections of tissues grouped together performing a common
by one or more chemical bonds. Many molecules that are
function. Organs are present not only in animals but also in plants.
biologically important are macromolecules, large molecules that are
An organ system is a higher level of organization that consists of
typically formed by polymerization (a polymer is a large molecule
functionally related organs. Mammals have many organ systems. For
that is made by combining smaller units called monomers, which are
instance, the circulatory system transports blood through the body
simpler than macromolecules). An example of a macromolecule is
and to and from the lungs; it includes organs such as the heart and
deoxyribonucleic acid (DNA), which contains the instructions for
blood vessels. Furthermore, organisms are individual living entities.
the structure and functioning of all living organisms.
For example, each tree in a forest is an organism. Single-celled
prokaryotes and single-celled eukaryotes are also considered
organisms and are typically referred to as microorganisms.
All the individuals of a species living within a specific area are
collectively called a population. For example, a forest may include
many pine trees. All of these pine trees represent the population of
pine trees in this forest. Different populations may live in the same
specific area. For example, the forest with the pine trees includes
populations of flowering plants and also insects and microbial
populations. A community is the sum of populations inhabiting a
particular area. For instance, all of the trees, flowers, insects, and
other populations in a forest form the forest’s community. The forest
itself is an ecosystem. An ecosystem consists of all the living things
in a particular area together with the abiotic, non-living parts of that
environment such as nitrogen in the soil or rain water. At the highest
level of organization, the biosphere is the collection of all
ecosystems, and it represents the zones of life on earth. It includes
land, water, and even the atmosphere to a certain extent. Taken
together, all of these levels comprise the biological levels of
organization, which range from organelles to the biosphere.

Figure 1.8.1: DNA: All molecules, including this DNA molecule,

are composed of atoms.

FROM ORGANELLES TO BIOSPHERES

Macromolecules can form aggregates within a cell that are
surrounded by membranes; these are called organelles. Organelles
are small structures that exist within cells. Examples of these
include: mitochondria and chloroplasts, which carry out
indispensable functions. Mitochondria produce energy to power the
cell while chloroplasts enable green plants to utilize the energy in
sunlight to make sugars. All living things are made of cells, and the
cell itself is the smallest fundamental unit of structure and function
in living organisms. (This requirement is why viruses are not

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 Figure 1.8.1: Biological Levels of Organization: The biological
levels of organization of living things follow a hierarchy, such as the
one shown. From a single organelle to the entire biosphere, living
organisms are part of a highly structured hierarchy.

KEY POINTS
The atom is the smallest and most fundamental unit of matter.
The bonding of at least two atoms or more form molecules.
The simplest level of organization for living things is a single
organelle, which is composed of aggregates of macromolecules.
The highest level of organization for living things is the
biosphere; it encompasses all other levels.
The biological levels of organization of living things arranged
from the simplest to most complex are: organelle, cells, tissues,
organs, organ systems, organisms, populations, communities,
ecosystem, and biosphere.

KEY TERMS
molecule: The smallest particle of a specific compound that
retains the chemical properties of that compound; two or more
atoms held together by chemical bonds.
macromolecule: a very large molecule, especially used in
reference to large biological polymers (e.g. nucleic acids and
proteins)
polymerization: The chemical process, normally with the aid of
a catalyst, to form a polymer by bonding together multiple
identical units (monomers).

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1.9: THEMES AND CONCEPTS OF BIOLOGY - THE DIVERSITY OF LIFE
domain, and this resulted in a new taxonomic tree. Many organisms
 LEARNING OBJECTIVES belonging to the Archaea domain live under extreme conditions and
are called extremophiles. To construct his tree, Woese used genetic
Recognize the three major domains used for classification
relationships rather than similarities based on morphology (shape).
Woese’s tree was constructed from comparative sequencing of the
The fact that biology has such a broad scope as a science has to do
genes that are universally distributed, present in every organism, and
with the tremendous diversity of life on Earth. The source of this
conserved (meaning that these genes have remained essentially
diversity is evolution, the process of gradual change during which
unchanged throughout evolution). Woese’s approach was
new species arise from older species. Evolutionary biologists study
revolutionary because comparisons of physical features are
the evolution of living things in everything from the microscopic
insufficient to differentiate between the prokaryotes that appear
world to ecosystems.
fairly similar in spite of their tremendous biochemical diversity and
The evolution of various life forms on Earth can be summarized in a
genetic variability. The comparison of homologous DNA and RNA
phylogenetic tree using phylogeny. A phylogenetic tree is a diagram sequences provided Woese with a sensitive device that revealed the
showing the evolutionary relationships among biological species extensive variability of prokaryotes, and which justified the
based on similarities and differences in genetic or physical traits or separation of the prokaryotes into two domains: bacteria and
both. A phylogenetic tree is composed of nodes and branches. The archaea. DNA, the universal genetic material, contains the
internal nodes represent ancestors and are points in evolution when, instructions for the structure and function of all living organisms and
based on scientific evidence, an ancestor is thought to have diverged can be divided into genes whose expression varies between
to form two new species. The length of each branch is proportional organisms. The RNA, which is transcribed from DNA, varies
to the time elapsed since the split.
between organisms as well based on the expression of specific
genes. Thus, to examine differences at this molecular level provides
a more accurate depiction of the diversity which exists.

KEY POINTS
The three major Domains of Life include: Domain Bacteria,
Domain Eukarya and Domain Archaea.
Domain Bacteria and Domain Archaea include prokaryotic cells
that lack membrane-enclosed nuclei and organelles.
Domain Eukarya include eukaryotes and more complex
organisms that contain membrane-bound nuclei and organelles.
Carl Woese defined Archaea as a new domain and constructed
the phylogentic tree of life which shows separation of all living
organisms.
The phylogenetic tree of life was constructed by Carl Woese
Figure 1.9.1: Phylogenetic Tree of Life: This phylogenetic tree was using sequencing data of ribosomal RNA genes. Therefore,
constructed by microbiologist Carl Woese using data obtained from genetics classification surpassed morphological cataloguing,
sequencing ribosomal RNA genes. The tree shows the separation of
living organisms into three domains: Bacteria, Archaea, and which was the traditional way of organizing living beings.
Eukarya. Bacteria and Archaea are prokaryotes, single-celled
organisms lacking intracellular organelles. KEY TERMS
phylogeny: the evolutionary history of an organism
CARL WOESE AND THE PHYLOGENETIC TREE
extremophile: an organism that lives under extreme conditions
of temperature, salinity, etc; commercially important as a source
In the past, biologists grouped living organisms into five kingdoms:
of enzymes that operate under similar conditions
animals, plants, fungi, protists, and bacteria. The organizational
DNA: a biopolymer of deoxyribonucleic acids (a type of nucleic
scheme was based mainly on physical features, as opposed to
acid) that has four different chemical groups, called bases:
physiology, biochemistry, or molecular biology, all of which are
adenine, guanine, cytosine, and thymine
used by modern systematics. The pioneering work of American
microbiologist Carl Woese in the early 1970s has shown, however, CONTRIBUTIONS AND ATTRIBUTIONS
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 CHAPTER OVERVIEW

2: THE CHEMICAL FOUNDATION OF LIFE

2.1: Atoms, Isotopes, Ions, and Molecules - Overview of Atomic Structure
2.2: Atoms, Isotopes, Ions, and Molecules - Atomic Number and Mass Number
2.3: Atoms, Isotopes, Ions, and Molecules - Isotopes
2.4: Atoms, Isotopes, Ions, and Molecules - The Periodic Table
2.5: Atoms, Isotopes, Ions, and Molecules - Electron Shells and the Bohr Model
2.6: Atoms, Isotopes, Ions, and Molecules - Electron Orbitals
2.7: Atoms, Isotopes, Ions, and Molecules - Chemical Reactions and Molecules
2.8: Atoms, Isotopes, Ions, and Molecules - Ions and Ionic Bonds
2.9: Atoms, Isotopes, Ions, and Molecules - Covalent Bonds and Other Bonds and Interactions
2.10: Atoms, Isotopes, Ions, and Molecules - Hydrogen Bonding and Van der Waals Forces
2.11: Water - Water’s Polarity
2.12: Water - Gas, Liquid, and Solid Water
2.13: Water - Heat of Vaporization
2.14: Water - High Heat Capacity
2.15: Water - Water’s Solvent Properties
2.16: Water - Cohesive and Adhesive Properties
2.17: Water - pH, Buffers, Acids, and Bases
2.18: Carbon - The Chemical Basis for Life
2.19: Carbon - Hydrocarbons
2.20: Carbon - Organic Isomers
2.21: Carbon - Organic Enantiomers
2.22: Carbon - Organic Molecules and Functional Groups
2.23: Synthesis of Biological Macromolecules - Types of Biological Macromolecules
2.24: Synthesis of Biological Macromolecules - Dehydration Synthesis
2.25: Synthesis of Biological Macromolecules - Hydrolysis

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1
2.1: ATOMS, ISOTOPES, IONS, AND MOLECULES - OVERVIEW OF ATOMIC
STRUCTURE
any electrons and calculate the atom’s mass based on the number of
 LEARNING OBJECTIVES protons and neutrons alone.

Discuss the electronic and structural properties of an atom Electrons contribute greatly to the atom’s charge, as each electron
has a negative charge equal to the positive charge of a proton.
Scientists define these charges as “+1” and “-1. ” In an uncharged,
An atom is the smallest unit of matter that retains all of the chemical
neutral atom, the number of electrons orbiting the nucleus is equal to
properties of an element. Atoms combine to form molecules, which
the number of protons inside the nucleus. In these atoms, the
then interact to form solids, gases, or liquids. For example, water is
positive and negative charges cancel each other out, leading to an
composed of hydrogen and oxygen atoms that have combined to
atom with no net charge.
form water molecules. Many biological processes are devoted to
breaking down molecules into their component atoms so they can be Table 2.1.1: Protons, neutrons, and electrons: Both protons and neutrons
have a mass of 1 amu and are found in the nucleus. However, protons have a
reassembled into a more useful molecule. charge of +1, and neutrons are uncharged. Electrons have a mass of
approximately 0 amu, orbit the nucleus, and have a charge of -1.
ATOMIC PARTICLES Charge Mass (amu) Location
Atoms consist of three basic particles: protons, electrons, and proton +1 1 nucleus
neutrons. The nucleus (center) of the atom contains the protons neutron 0 1 nucles
(positively charged) and the neutrons (no charge). The outermost electron -1 0 orbitals
regions of the atom are called electron shells and contain the
electrons (negatively charged). Atoms have different properties Exploring Electron Properties: Compare the behavior of electrons
based on the arrangement and number of their basic particles. to that of other charged particles to discover properties of electrons
such as charge and mass.
The hydrogen atom (H) contains only one proton, one electron, and
no neutrons. This can be determined using the atomic number and VOLUME OF ATOMS
the mass number of the element (see the concept on atomic numbers Accounting for the sizes of protons, neutrons, and electrons, most of
and mass numbers). the volume of an atom—greater than 99 percent—is, in fact, empty
space. Despite all this empty space, solid objects do not just pass
through one another. The electrons that surround all atoms are
negatively charged and cause atoms to repel one another, preventing
atoms from occupying the same space. These intermolecular forces
prevent you from falling through an object like your chair.

KEY POINTS
An atom is composed of two regions: the nucleus, which is in the
center of the atom and contains protons and neutrons, and the
outer region of the atom, which holds its electrons in orbit
Figure 2.1.1: Structure of an atom: Elements, such as helium, around the nucleus.
depicted here, are made up of atoms. Atoms are made up of protons Protons and neutrons have approximately the same mass, about
and neutrons located within the nucleus, with electrons in orbitals
surrounding the nucleus. 1.67 × 10-24 grams, which scientists define as one atomic mass
unit (amu) or one Dalton.
ATOMIC MASS Each electron has a negative charge (-1) equal to the positive
Protons and neutrons have approximately the same mass, about 1.67 charge of a proton (+1).
× 10-24 grams. Scientists define this amount of mass as one atomic Neutrons are uncharged particles found within the nucleus.
mass unit (amu) or one Dalton. Although similar in mass, protons
are positively charged, while neutrons have no charge. Therefore, KEY TERMS
the number of neutrons in an atom contributes significantly to its atom: The smallest possible amount of matter which still retains
mass, but not to its charge. its identity as a chemical element, consisting of a nucleus
Electrons are much smaller in mass than protons, weighing only 9.11 surrounded by electrons.
proton: Positively charged subatomic particle forming part of the
× 10-28 grams, or about 1/1800 of an atomic mass unit. Therefore,
they do not contribute much to an element’s overall atomic mass. nucleus of an atom and determining the atomic number of an
element. It weighs 1 amu.
When considering atomic mass, it is customary to ignore the mass of

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neutron: A subatomic particle forming part of the nucleus of an This page titled 2.1: Atoms, Isotopes, Ions, and Molecules - Overview of
atom. It has no charge. It is equal in mass to a proton or it weighs Atomic Structure is shared under a CC BY-SA 4.0 license and was authored,
1 amu. remixed, and/or curated by Boundless.

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2.2: ATOMS, ISOTOPES, IONS, AND MOLECULES - ATOMIC NUMBER AND
MASS NUMBER
Scientists determine the atomic mass by calculating the mean of the
 LEARNING OBJECTIVES mass numbers for its naturally-occurring isotopes. Often, the
resulting number contains a decimal. For example, the atomic mass
Determine the relationship between the mass number of an
of chlorine (Cl) is 35.45 amu because chlorine is composed of
atom, its atomic number, its atomic mass, and its number of
several isotopes, some (the majority) with an atomic mass of 35 amu
subatomic particles
(17 protons and 18 neutrons) and some with an atomic mass of 37
amu (17 protons and 20 neutrons).
ATOMIC NUMBER Given an atomic number (Z) and mass number (A), you can find the
Neutral atoms of an element contain an equal number of protons and number of protons, neutrons, and electrons in a neutral atom. For
electrons. The number of protons determines an element’s atomic example, a lithium atom (Z=3, A=7 amu) contains three protons
number (Z) and distinguishes one element from another. For (found from Z), three electrons (as the number of protons is equal to
example, carbon’s atomic number (Z) is 6 because it has 6 protons. the number of electrons in an atom), and four neutrons (7 – 3 = 4).
The number of neutrons can vary to produce isotopes, which are
atoms of the same element that have different numbers of neutrons. KEY POINTS
The number of electrons can also be different in atoms of the same Neutral atoms of each element contain an equal number of
element, thus producing ions (charged atoms). For instance, iron, Fe, protons and electrons.
can exist in its neutral state, or in the +2 and +3 ionic states. The number of protons determines an element’s atomic number
and is used to distinguish one element from another.
MASS NUMBER The number of neutrons is variable, resulting in isotopes, which
An element’s mass number (A) is the sum of the number of protons are different forms of the same atom that vary only in the number
and the number of neutrons. The small contribution of mass from of neutrons they possess.
electrons is disregarded in calculating the mass number. This Together, the number of protons and the number of neutrons
approximation of mass can be used to easily calculate how many determine an element’s mass number.
neutrons an element has by simply subtracting the number of Since an element’s isotopes have slightly different mass
protons from the mass number. Protons and neutrons both weigh numbers, the atomic mass is calculated by obtaining the mean of
about one atomic mass unit or amu. Isotopes of the same element the mass numbers for its isotopes.
will have the same atomic number but different mass numbers.
KEY TERMS
mass number: The sum of the number of protons and the
number of neutrons in an atom.
atomic number: The number of protons in an atom.
atomic mass: The average mass of an atom, taking into account
all its naturally occurring isotopes.

This page titled 2.2: Atoms, Isotopes, Ions, and Molecules - Atomic Number
and Mass Number is shared under a CC BY-SA 4.0 license and was
Figure 2.2.1: Atomic number, chemical symbol, and mass number: authored, remixed, and/or curated by Boundless.
Carbon has an atomic number of six, and two stable isotopes with
mass numbers of twelve and thirteen, respectively. Its average
atomic mass is 12.11.

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2.3: ATOMS, ISOTOPES, IONS, AND MOLECULES - ISOTOPES
half-life, or the time it takes for half of the original concentration of
 LEARNING OBJECTIVES an isotope to decay back to its more stable form. Because the half-
life of 14C is long, it is used to date formerly-living objects such as
Discuss the properties of isotopes and their use in
old bones or wood. Comparing the ratio of the 14C concentration
radiometric dating
found in an object to the amount of 14C in the atmosphere, the
amount of the isotope that has not yet decayed can be determined.
WHAT IS AN ISOTOPE? On the basis of this amount, the age of the material can be accurately
Isotopes are various forms of an element that have the same number calculated, as long as the material is believed to be less than 50,000
of protons but a different number of neutrons. Some elements, such years old. This technique is called radiocarbon dating, or carbon
as carbon, potassium, and uranium, have multiple naturally- dating for short.
occurring isotopes. Isotopes are defined first by their element and
then by the sum of the protons and neutrons present.
Carbon-12 (or 12C) contains six protons, six neutrons, and six
electrons; therefore, it has a mass number of 12 amu (six protons
and six neutrons).
Carbon-14 (or 14C) contains six protons, eight neutrons, and six
electrons; its atomic mass is 14 amu (six protons and eight
neutrons).
While the mass of individual isotopes is different, their physical and
chemical properties remain mostly unchanged.
Isotopes do differ in their stability. Carbon-12 (12C) is the most
abundant of the carbon isotopes, accounting for 98.89% of carbon
on Earth. Carbon-14 (14C) is unstable and only occurs in trace Figure 2.3.1: Application of carbon dating: The age of carbon-
amounts. Unstable isotopes most commonly emit alpha particles containing remains less than 50,000 years old, such as this pygmy
mammoth, can be determined using carbon dating.
(He2+) and electrons. Neutrons, protons, and positrons can also be
Other elements have isotopes with different half lives. For example,
emitted and electrons can be captured to attain a more stable atomic 40
K (potassium-40) has a half-life of 1.25 billion years, and 235U
configuration (lower level of potential energy ) through a process
(uranium-235) has a half-life of about 700 million years. Scientists
called radioactive decay. The new atoms created may be in a high
often use these other radioactive elements to date objects that are
energy state and emit gamma rays which lowers the energy but alone
does not change the atom into another isotope. These atoms are older than 50,000 years (the limit of carbon dating). Through the use
called radioactive isotopes or radioisotopes. of radiometric dating, scientists can study the age of fossils or other
remains of extinct organisms.
RADIOCARBON DATING
KEY POINTS
Carbon is normally present in the atmosphere in the form of gaseous
Isotopes are atoms of the same element that contain an identical
compounds like carbon dioxide and methane. Carbon-14 (14C) is a
naturally-occurring radioisotope that is created from atmospheric number of protons, but a different number of neutrons.
14N (nitrogen) by the addition of a neutron and the loss of a proton, Despite having different numbers of neutrons, isotopes of the
which is caused by cosmic rays. This is a continuous process so same element have very similar physical properties.
Some isotopes are unstable and will undergo radioactive decay to
more 14C is always being created in the atmosphere. Once produced,
the 14C often combines with the oxygen in the atmosphere to form become other elements.
carbon dioxide. Carbon dioxide produced in this way diffuses in the The predictable half-life of different decaying isotopes allows
scientists to date material based on its isotopic composition, such
atmosphere, is dissolved in the ocean, and is incorporated by plants
via photosynthesis. Animals eat the plants and, ultimately, the as with Carbon-14 dating.
radiocarbon is distributed throughout the biosphere.
KEY TERMS
In living organisms, the relative amount of 14C in their body is isotope: Any of two or more forms of an element where the
approximately equal to the concentration of 14C in the atmosphere. atoms have the same number of protons, but a different number
When an organism dies, it is no longer ingesting 14C, so the ratio of neutrons within their nuclei.
between 14C and 12C will decline as 14C gradually decays back to half-life: The time it takes for half of the original concentration
14
N. This slow process, which is called beta decay, releases energy of an isotope to decay back to its more stable form.
through the emission of electrons from the nucleus or positrons. radioactive isotopes: an atom with an unstable nucleus,
After approximately 5,730 years, half of the starting concentration of characterized by excess energy available that undergoes
14
C will have been converted back to 14N. This is referred to as its

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radioactive decay and creates most commonly gamma rays, amount of 14C in the atmosphere.
alpha or beta particles.
radiocarbon dating: Determining the age of an object by This page titled 2.3: Atoms, Isotopes, Ions, and Molecules - Isotopes is
comparing the ratio of the 14C concentration found in it to the shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
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2.4: ATOMS, ISOTOPES, IONS, AND MOLECULES - THE PERIODIC TABLE

 LEARNING OBJECTIVES

Discuss the organization of the periodic table

MATTER AND ELEMENTS

Matter comprises all of the physical objects in the universe, those
that take up space and have mass. All matter is composed of atoms
of one or more elements, pure substances with specific chemical and
physical properties. There are 98 elements that naturally occur on
earth, yet living systems use a relatively small number of these.
Living creatures are composed mainly of just four elements: carbon,
hydrogen, oxygen, and nitrogen (often remembered by the acronym
CHON). As elements are bonded together they form compounds that
often have new emergent properties that are different from the
properties of the individual elements. Life is an example of an
emergent property that arises from the specific collection of
molecules found in cells. Figure 2.4.1: The periodic table: The periodic table shows the
atomic mass and atomic number of each element. The atomic
number appears above the symbol for the element and the
approximate atomic mass appears below it.
The arrangement of the periodic table allows the elements to be
grouped according to their chemical properties. Within the main
group elements ( Groups 1-2, 13-18), there are some general trends
that we can observe. The further down a given group, the elements
have an increased metallic character: they are good conductors of
both heat and electricity, solids at room temperature, and shiny in
appearance. Moving from left to right across a period, the elements
have greater non-metallic character. These elements are insulators,
poor heat conductors, and can exist in different phases at room
temperature (brittle solid, liquid, or gas). The elements at the
Elements of the human body arranged by percent of total mass: boundary between the metallic elements (grey elements) and
There are 25 elements believed to play an active role in human nonmetal elements (green elements) are metalloid in character (pink
health. Carbon, hydrogen, oxygen, and nitrogen make up elements). They have low electrical conductivity that increases with
approximately 96% of the mass in a human body. temperature. They also share properties with both the metals and the
nonmetals.
THE PERIODIC TABLE
The different elements are organized and displayed in the periodic
table. Devised by Russian chemist Dmitri Mendeleev (1834–1907)
in 1869, the table groups elements that, although unique, share
certain chemical properties with other elements. In the periodic table
the elements are organized and displayed according to their atomic
number and are arranged in a series of rows (periods) and columns
(groups) based on shared chemical and physical properties. If you
look at a periodic table, you will see the groups numbered at the top
of each column from left to right starting with 1 and ending with 18.
In addition to providing the atomic number for each element, the
periodic table also displays the element’s atomic mass. Looking at
carbon, for example, its symbol (C) and name appear, as well as its
atomic number of six (in the upper left-hand corner) and its atomic Figure 2.4.1: The main group elements: Within the p-block at the
boundary between the metallic elements (grey elements) and
mass of 12.11. nonmetal elements (green elements) there is positioned boron and
silicon that are metalloid in character (pink elements), i.e., they have
low electrical conductivity that increases with temperature.

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Today, the periodic table continues to expand as heavier and heavier The periodic table continues to expand today as heavier and
elements are synthesized in laboratories. These large elements are heavier elements are created in laboratories around the world.
extremely unstable and, as such, are very difficult to detect; but their
continued creation is an ongoing challenge undertaken by scientists KEY TERMS
around the world. element: Pure chemical substances consisting of only one type
of atom with a defined set of chemical and physical properties.
KEY POINTS emergent properties: Properties found in compound structures
All matter is made from atoms of one or more elements. Living that are different from those of the individual components and
creatures consist mainly of carbon, hydrogen, oxygen, and would not be predicted based on the properties of the individual
nitrogen (CHON). components.
Combining elements creates compounds that may have emergent periodic table: A tabular chart of the chemical elements
properties. according to their atomic numbers so that elements with similar
The periodic table is a listing of the elements according to properties are in the same column.
increasing atomic number that is further organized into columns
based on similar physical and chemical properties and electron This page titled 2.4: Atoms, Isotopes, Ions, and Molecules - The Periodic
configuration. Table is shared under a CC BY-SA 4.0 license and was authored, remixed,
and/or curated by Boundless.
As one moves down a column or across a row, there are some
general trends for the properties of the elements.

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2.5: ATOMS, ISOTOPES, IONS, AND MOLECULES - ELECTRON SHELLS AND
THE BOHR MODEL
electrons in their outer shells, respectively. Theoretically, they would
 LEARNING OBJECTIVES be more energetically stable if they followed the octet rule and had
eight.
Construct an atom according to the Bohr model

ELECTRON SHELLS AND THE BOHR MODEL

Figure 2.5.1: Bohr diagrams: Bohr diagrams indicate how many

electrons fill each principal shell. Group 18 elements (helium, neon,
Figure 2.5.1: Orbitals in the Bohr model: The Bohr model was and argon are shown) have a full outer, or valence, shell. A full
developed by Niels Bohr in 1913. In this model, electrons exist valence shell is the most stable electron configuration. Elements in
within principal shells. An electron normally exists in the lowest other groups have partially-filled valence shells and gain or lose
energy shell available, which is the one closest to the nucleus. electrons to achieve a stable electron configuration.
Energy from a photon of light can bump it up to a higher energy An atom may gain or lose electrons to achieve a full valence shell,
shell, but this situation is unstable and the electron quickly decays
back to the ground state. In the process, a photon of light is released. the most stable electron configuration. The periodic table is arranged
As previously discussed, there is a connection between the number in columns and rows based on the number of electrons and where
of protons in an element, the atomic number that distinguishes one these electrons are located, providing a tool to understand how
element from another, and the number of electrons it has. In all electrons are distributed in the outer shell of an atom. As shown in,
electrically-neutral atoms, the number of electrons is the same as the the group 18 atoms helium (He), neon (Ne), and argon (Ar) all have
number of protons. Each element, when electrically neutral, has a filled outer electron shells, making it unnecessary for them to gain or
number of electrons equal to its atomic number. lose electrons to attain stability; they are highly stable as single
atoms. Their non-reactivity has resulted in their being named the
An early model of the atom was developed in 1913 by Danish
inert gases (or noble gases). In comparison, the group 1 elements,
scientist Niels Bohr (1885–1962). The Bohr model shows the atom
including hydrogen (H), lithium (Li), and sodium (Na), all have one
as a central nucleus containing protons and neutrons with the
electron in their outermost shells. This means that they can achieve a
electrons in circular orbitals at specific distances from the nucleus.
stable configuration and a filled outer shell by donating or losing an
These orbits form electron shells or energy levels, which are a way
electron. As a result of losing a negatively-charged electron, they
of visualizing the number of electrons in the various shells. These
become positively-charged ions. When an atom loses an electron to
energy levels are designated by a number and the symbol “n.” For
become a positively-charged ion, this is indicated by a plus sign
example, 1n represents the first energy level located closest to the
after the element symbol; for example, Na+. Group 17 elements,
nucleus.
including fluorine and chlorine, have seven electrons in their
Electrons fill orbit shells in a consistent order. Under standard outermost shells; they tend to fill this shell by gaining an electron
conditions, atoms fill the inner shells (closer to the nucleus) first, from other atoms, making them negatively-charged ions. When an
often resulting in a variable number of electrons in the outermost atom gains an electron to become a negatively-charged ion this is
shell. The innermost shell has a maximum of two electrons, but the indicated by a minus sign after the element symbol; for example, F-.
next two electron shells can each have a maximum of eight Thus, the columns of the periodic table represent the potential shared
electrons. This is known as the octet rule which states that, with the state of these elements’ outer electron shells that is responsible for
exception of the innermost shell, atoms are more stable energetically their similar chemical characteristics.
when they have eight electrons in their valence shell, the outermost
electron shell. Examples of some neutral atoms and their electron KEY POINTS
configurations are shown in. As shown, helium has a complete outer In the Bohr model of the atom, the nucleus contains the majority
electron shell, with two electrons filling its first and only shell. of the mass of the atom in its protons and neutrons.
Similarly, neon has a complete outer 2n shell containing eight Orbiting the positively-charged core are the negatively charged
electrons. In contrast, chlorine and sodium have seven and one electrons, which contribute little in terms of mass, but are

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electrically equivalent to the protons in the nucleus. KEY TERMS
In most cases, electrons fill the lower- energy orbitals first, octet rule: A rule stating that atoms lose, gain, or share electrons
followed by the next higher energy orbital until it is full, and so in order to have a full valence shell of 8 electrons. (Hydrogen is
on until all electrons have been placed. excluded because it can hold a maximum of 2 electrons in its
Atoms tend to be most stable with a full outer shell (one which, valence shell. )
after the first, contains 8 electrons), leading to what is commonly electron shell: The collective states of all electrons in an atom
called the ” octet rule “. having the same principal quantum number (visualized as an
The properties of an element are determined by its outermost orbit in which the electrons move).
electrons, or those in the highest energy orbital.
Atoms that do not have full outer shells will tend to gain or lose This page titled 2.5: Atoms, Isotopes, Ions, and Molecules - Electron Shells
electrons, resulting in a full outer shell and, therefore, stability. and the Bohr Model is shared under a CC BY-SA 4.0 license and was
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2.6: ATOMS, ISOTOPES, IONS, AND MOLECULES - ELECTRON ORBITALS
p, and d subshells and can hold 18 electrons. Principal shell 4n has s,
 LEARNING OBJECTIVES p, d, and f orbitals and can hold 32 electrons. Moving away from the
nucleus, the number of electrons and orbitals found in the energy
Distinguish between electron orbitals in the Bohr model
levels increases. Progressing from one atom to the next in the
versus the quantum mechanical orbitals
periodic table, the electron structure can be worked out by fitting an
extra electron into the next available orbital. While the concepts of
Although useful to explain the reactivity and chemical bonding of
electron shells and orbitals are closely related, orbitals provide a
certain elements, the Bohr model of the atom does not accurately
more accurate depiction of the electron configuration of an atom
reflect how electrons are spatially distributed surrounding the because the orbital model specifies the different shapes and special
nucleus. They do not circle the nucleus like the earth orbits the sun,
orientations of all the places that electrons may occupy.
but are rather found in electron orbitals. These relatively complex
shapes result from the fact that electrons behave not just like
particles, but also like waves. Mathematical equations from quantum
mechanics known as wave functions can predict within a certain
level of probability where an electron might be at any given time.
The area where an electron is most likely to be found is called its
orbital.

FIRST ELECTRON SHELL

The closest orbital to the nucleus, called the 1s orbital, can hold up
to two electrons. This orbital is equivalent to the innermost electron
shell of the Bohr model of the atom. It is called the 1s orbital
because it is spherical around the nucleus. The 1s orbital is always
filled before any other orbital. Hydrogen has one electron; therefore,
it has only one spot within the 1s orbital occupied. This is designated Figure 2.6.1: Diagram of the S and P orbitals: The s subshells are
as 1s1, where the superscripted 1 refers to the one electron within the shaped like spheres. Both the 1n and 2n principal shells have an s
1s orbital. Helium has two electrons; therefore, it can completely fill orbital, but the size of the sphere is larger in the 2n orbital. Each
sphere is a single orbital. p subshells are made up of three dumbbell-
the 1s orbital with its two electrons. This is designated as 1s2, shaped orbitals. Principal shell 2n has a p subshell, but shell 1 does
referring to the two electrons of helium in the 1s orbital. On the not.
periodic table, hydrogen and helium are the only two elements in the
first row (period); this is because they are the sole elements to have KEY POINTS
electrons only in their first shell, the 1s orbital. The Bohr model of the atom does not accurately reflect how
electrons are spatially distributed around the nucleus as they do
SECOND ELECTRON SHELL not circle the nucleus like the earth orbits the sun.
The second electron shell may contain eight electrons. This shell The electron orbitals are the result of mathematical equations
contains another spherical s orbital and three “dumbbell” shaped p from quantum mechanics known as wave functions and can
orbitals, each of which can hold two electrons. After the 1s orbital is predict within a certain level of probability where an electron
filled, the second electron shell is filled, first filling its 2s orbital and might be at any given time.
then its three p orbitals. When filling the p orbitals, each takes a The number and type of orbitals increases with increasing atomic
single electron; once each p orbital has an electron, a second may be number, filling in various electron shells.
added. Lithium (Li) contains three electrons that occupy the first and The area where an electron is most likely to be found is called its
second shells. Two electrons fill the 1s orbital, and the third electron orbital.
then fills the 2s orbital. Its electron configuration is 1s22s1. Neon
(Ne), on the other hand, has a total of ten electrons: two are in its KEY TERMS
innermost 1s orbital, and eight fill its second shell (two each in the electron shell: The collective states of all electrons in an atom
2s and three p orbitals). Thus, it is an inert gas and energetically having the same principal quantum number (visualized as an
stable: it rarely forms a chemical bond with other atoms. orbit in which the electrons move).
orbital: A specification of the energy and probability density of
THIRD ELECTRON SHELL an electron at any point in an atom or molecule.
Larger elements have additional orbitals, making up the third
electron shell. Subshells d and f have more complex shapes and This page titled 2.6: Atoms, Isotopes, Ions, and Molecules - Electron
contain five and seven orbitals, respectively. Principal shell 3n has s, Orbitals is shared under a CC BY-SA 4.0 license and was authored,
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2.7: ATOMS, ISOTOPES, IONS, AND MOLECULES - CHEMICAL REACTIONS
AND MOLECULES
should be equal, such that no atoms are, under normal
 LEARNING OBJECTIVES circumstances, created or destroyed.
+
Describe the properties of chemical reactions and 2H O
2 2
→ 2H O
2
O
2
(2.7.2)

compounds
Even though all of the reactants and products of this reaction are
molecules (each atom remains bonded to at least one other atom), in
According to the octet rule, elements are most stable when their
this reaction only hydrogen peroxide and water are representative of
outermost shell is filled with electrons. This is because it is
a subclass of molecules known as compounds: they contain atoms of
energetically favorable for atoms to be in that configuration.
more than one type of element. Molecular oxygen, on the other
However, since not all elements have enough electrons to fill their
hand, consists of two doubly bonded oxygen atoms and is not
outermost shells, atoms form chemical bonds with other atoms,
classified as a compound but as an element.
which helps them obtain the electrons they need to attain a stable
electron configuration. When two or more atoms chemically bond IRREVERSIBLE AND REVERSIBLE REACTIONS
with each other, the resultant chemical structure is a molecule. The
familiar water molecule, H2O, consists of two hydrogen atoms and Some chemical reactions, such as the one shown above, can proceed
one oxygen atom, which bond together to form water. Atoms can in one direction until the reactants are all used up. The equations that
form molecules by donating, accepting, or sharing electrons to fill describe these reactions contain a unidirectional arrow and are
their outer shells. irreversible. Reversible reactions are those that can go in either
direction. In reversible reactions, reactants are turned into products,
but when the concentration of product goes beyond a certain
threshold, some of these products will be converted back into
reactants; at this point, the designations of products and reactants are
reversed. This back and forth continues until a certain relative
balance between reactants and products occurs: a state called
equilibrium. These situations of reversible reactions are often
denoted by a chemical equation with a double headed arrow pointing
towards both the reactants and products.
For example, in human blood, excess hydrogen ions (H+) bind to
Figure 2.7.1: Atoms bond to form molecules: Two or more atoms
may bond with each other to form a molecule. When two hydrogens bicarbonate ions (HCO3–) forming an equilibrium state with
and an oxygen share electrons via covalent bonds, a water molecule carbonic acid (H2CO3). If carbonic acid were added to this system,
is formed. some of it would be converted to bicarbonate and hydrogen ions.
Chemical reactions occur when two or more atoms bond together to In biological reactions, however, equilibrium is rarely obtained
form molecules or when bonded atoms are broken apart. The because the concentrations of the reactants or products or both are
substances used in the beginning of a chemical reaction are called constantly changing, often with a product of one reaction being a
the reactants (usually found on the left side of a chemical equation), reactant for another. To return to the example of excess hydrogen
and the substances found at the end of the reaction are known as the ions in the blood, the formation of carbonic acid will be the major
products (usually found on the right side of a chemical equation). An direction of the reaction. However, the carbonic acid can also leave
arrow is typically drawn between the reactants and products to the body as carbon dioxide gas (via exhalation) instead of being
indicate the direction of the chemical reaction. For the creation of converted back to bicarbonate ion, thus driving the reaction to the
the water molecule shown above, the chemical equation would be: right by the chemical law known as law of mass action. These
2H
+
O → 2H O (2.7.1) reactions are important for maintaining the homeostasis of our
2 2 2

blood.
An example of a simple chemical reaction is the breaking down of
hydrogen peroxide molecules, each of which consists of two KEY POINTS
hydrogen atoms bonded to two oxygen atoms (H2O2). The reactant Atoms form chemical bonds with other atoms thereby obtaining
hydrogen peroxide is broken down into water (H2O), and oxygen, the electrons they need to attain a stable electron configuration.
which consists of two bonded oxygen atoms (O2). In the equation The substances used in the beginning of a chemical reaction are
below, the reaction includes two hydrogen peroxide molecules and called the reactants and the substances found at the end of the
two water molecules. This is an example of a balanced chemical reaction are known as the products.
equation, wherein the number of atoms of each element is the same Some reactions are reversible and will reach a relative balance
on each side of the equation. According to the law of conservation of between reactants and products: a state called equilibrium.
matter, the number of atoms before and after a chemical reaction

2.7.1 https://bio.libretexts.org/@go/page/12662
 An arrow is typically drawn between the reactants and products atoms held together by chemical bonds.
to indicate the direction of the chemical reaction. reaction: A transformation in which one or more substances is
converted into another by combination or decomposition
KEY TERMS
reactant: Any of the participants present at the start of a This page titled 2.7: Atoms, Isotopes, Ions, and Molecules - Chemical
chemical reaction. Reactions and Molecules is shared under a CC BY-SA 4.0 license and was
molecule: The smallest particle of a specific compound that authored, remixed, and/or curated by Boundless.
retains the chemical properties of that compound; two or more

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2.8: ATOMS, ISOTOPES, IONS, AND MOLECULES - IONS AND IONIC BONDS

 LEARNING OBJECTIVES

Predict whether a given element will more likely form a

cation or an anion
Figure 2.8.1: Electron Transfer Between Na and Cl: In the formation
of an ionic compound, metals lose electrons and nonmetals gain
IONS AND IONIC BONDS electrons to achieve an octet. In this example, sodium loses one
Some atoms are more stable when they gain or lose an electron (or electron to empty its shell and chlorine accepts that electron to fill its
possibly two) and form ions. This results in a full outermost electron shell.
shell and makes them energetically more stable. Now, because the Ionic bonds are formed between ions with opposite charges. For
number of electrons does not equal the number of protons, each ion instance, positively charged sodium ions and negatively charged
has a net charge. Cations are positive ions that are formed by losing chloride ions bond together to form sodium chloride, or table salt, a
electrons (as the number of protons is now greater than the number crystalline molecule with zero net charge. The attractive force
of electrons). Negative ions are formed by gaining electrons and are holding the two atoms together is called the electromagnetic force
called anions (wherein there are more electrons than protons in a and is responsible for the attraction between oppositely charged
molecule ). Anions are designated by their elemental name being ions.
altered to end in “-ide”. For example, the anion of chlorine is called Certain salts are referred to in physiology as electrolytes (including
chloride, and the anion of sulfur is called sulfide. sodium, potassium, and calcium). Electrolytes are ions necessary for
This movement of electrons from one element to another is referred nerve impulse conduction, muscle contractions, and water balance.
to as electron transfer. As illustrated, sodium (Na) only has one Many sports drinks and dietary supplements provide these ions to
electron in its outer electron shell. It takes less energy for sodium to replace those lost from the body via sweating during exercise.
donate that one electron than it does to accept seven more electrons
KEY POINTS
to fill the outer shell. When sodium loses an electron, it will have 11
protons, 11 neutrons, and only 10 electrons. This leaves it with an Ions form from elements when they gain or lose an electron
overall charge of +1 since there are now more protons than causing the number of protons to be unequal to the number of
electrons. It is now referred to as a sodium ion. Chlorine (Cl) in its electrons, resulting in a net charge.
lowest energy state (called the ground state) has seven electrons in If there are more electrons than protons (from an element gaining
its outer shell. Again, it is more energy efficient for chlorine to gain one or more electrons), the ion is negatively charged and called
one electron than to lose seven. Therefore, it tends to gain an an anion.
electron to create an ion with 17 protons, 17 neutrons, and 18 If there are more protons than electrons (via loss of electrons),
electrons. This gives it a net charge of -1 since there are now more the ion is positively charged and is called a cation.
electrons than protons. It is now referred to as a chloride ion. In this Ionic bonds result from the interaction between a positively
example, sodium will donate its one electron to empty its shell, and charged cation and a negatively charged anion.
chlorine will accept that electron to fill its shell. Both ions now
KEY TERMS
satisfy the octet rule and have complete outer shells. These
transactions can normally only take place simultaneously; in order ion: An atom, or group of atoms, bearing an electrical charge,
for a sodium atom to lose an electron, it must be in the presence of a such as the sodium and chlorine atoms in a salt solution.
suitable recipient like a chlorine atom. ionic bond: A strong chemical bond caused by the electrostatic
attraction between two oppositely charged ions.

This page titled 2.8: Atoms, Isotopes, Ions, and Molecules - Ions and Ionic
Bonds is shared under a CC BY-SA 4.0 license and was authored, remixed,
and/or curated by Boundless.

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2.9: ATOMS, ISOTOPES, IONS, AND MOLECULES - COVALENT BONDS AND
OTHER BONDS AND INTERACTIONS
the vicinity of the oxygen nucleus than they do near the nucleus of
 LEARNING OBJECTIVES the hydrogen atoms.

Compare the relative strength of different types of bonding

interactions

The octet rule can be satisfied by the sharing of electrons between

atoms to form covalent bonds. These bonds are stronger and much
more common than are ionic bonds in the molecules of living
organisms. Covalent bonds are commonly found in carbon-based
organic molecules, such as DNA and proteins. Covalent bonds are
also found in inorganic molecules such as H2O, CO2, and O2. One,
two, or three pairs of electrons may be shared between two atoms,
making single, double, and triple bonds, respectively. The more
covalent bonds between two atoms, the stronger their connection.
Thus, triple bonds are the strongest.
The strength of different levels of covalent bonding is one of the
main reasons living organisms have a difficult time in acquiring
nitrogen for use in constructing nitrogenous molecules, even though
molecular nitrogen, N2, is the most abundant gas in the atmosphere.
Molecular nitrogen consists of two nitrogen atoms triple bonded to Figure 2.9.1: Polar and Nonpolar Covalent Bonds: Whether a
each other. The resulting strong triple bond makes it difficult for molecule is polar or nonpolar depends both on bond type and
molecular shape. Both water and carbon dioxide have polar covalent
living systems to break apart this nitrogen in order to use it as bonds, but carbon dioxide is linear, so the partial charges on the
constituents of biomolecules, such as proteins, DNA, and RNA. molecule cancel each other out.
The formation of water molecules is an example of covalent
NONPOLAR COVALENT BONDS
bonding. The hydrogen and oxygen atoms that combine to form
Nonpolar covalent bonds form between two atoms of the same
water molecules are bound together by covalent bonds. The electron
element or between different elements that share electrons equally.
from the hydrogen splits its time between the incomplete outer shell
For example, molecular oxygen (O2) is nonpolar because the
of the hydrogen atom and the incomplete outer shell of the oxygen
electrons will be equally distributed between the two oxygen atoms.
atom. In return, the oxygen atom shares one of its electrons with the
The four bonds of methane are also considered to be nonpolar
hydrogen atom, creating a two-electron single covalent bond. To
because the electronegativies of carbon and hydrogen are nearly
completely fill the outer shell of oxygen, which has six electrons in
identical.
its outer shell, two electrons (one from each hydrogen atom) are
needed. Each hydrogen atom needs only a single electron to fill its HYDROGEN BONDS AND VAN DER WAALS
outer shell, hence the well-known formula H2O. The electrons that INTERACTIONS
are shared between the two elements fill the outer shell of each,
Not all bonds are ionic or covalent; weaker bonds can also form
making both elements more stable.
between molecules. Two types of weak bonds that frequently occur
POLAR COVALENT BONDS are hydrogen bonds and van der Waals interactions. Without these
two types of bonds, life as we know it would not exist.
There are two types of covalent bonds: polar and nonpolar. In a polar
covalent bond, the electrons are unequally shared by the atoms Hydrogen bonds provide many of the critical, life-sustaining
because they are more attracted to one nucleus than the other. The properties of water and also stabilize the structures of proteins and
relative attraction of an atom to an electron is known as its DNA, the building block of cells. When polar covalent bonds
electronegativity: atoms that are more attracted to an electron are containing hydrogen are formed, the hydrogen atom in that bond has
considered to be more electronegative. Because of the unequal a slightly positive charge (δ+) because the shared electrons are
distribution of electrons between the atoms of different elements, a pulled more strongly toward the other element and away from the
slightly positive (δ+) or slightly negative (δ-) charge develops. This hydrogen atom. Because the hydrogen has a slightly positive charge,
partial charge is known as a dipole; this is an important property of it’s attracted to neighboring negative charges. The weak interaction
water and accounts for many of its characteristics. The dipole in between the δ+ charge of a hydrogen atom from one molecule and
water occurs because oxygen has a higher electronegativity than the δ- charge of a more electronegative atom is called a hydrogen
hydrogen, which means that the shared electrons spend more time in bond. Individual hydrogen bonds are weak and easily broken;
however, they occur in very large numbers in water and in organic

2.9.1 https://bio.libretexts.org/@go/page/12664
polymers, and the additive force can be very strong. For example, bonds, along with hydrogen bonds, help form the three-dimensional
hydrogen bonds are responsible for zipping together the DNA structures of the proteins in our cells that are required for their
double helix. proper function.

KEY POINTS
A polar covalent bond arises when two atoms of different
electronegativity share two electrons unequally.
A non-polar covalent bond is one in which the electrons are
shared equally between two atoms.
Hydrogen bonds and Van Der Waals are responsible for the
folding of proteins, the binding of ligands to proteins, and many
other processes between molecules.

KEY TERMS
hydrogen bond: A weak bond in which a hydrogen atom in one
Figure 2.9.1: Adenosine Triphosphate, ATP: Adenosine molecule is attracted to an electronegative atom (usually nitrogen
Triphosphate, or ATP, is the most commonly used cofactor in nature. or oxygen) in the same or different molecule.
Its biosynthesis involves the fixation of nitrogen to provide
feedstocks that eventually produce the carbon-nitrogen bonds it covalent bond: A type of chemical bond where two atoms are
contains. connected to each other by the sharing of two or more electrons.
Like hydrogen bonds, van der Waals interactions are weak dipole: Any object (such as a magnet, polar molecule or
interactions between molecules. Van der Waals attractions can occur antenna), that is oppositely charged at two points (or poles).
between any two or more molecules and are dependent on slight
This page titled 2.9: Atoms, Isotopes, Ions, and Molecules - Covalent Bonds
fluctuations of the electron densities, which can lead to slight
and Other Bonds and Interactions is shared under a CC BY-SA 4.0 license
temporary dipoles around a molecule. For these attractions to
and was authored, remixed, and/or curated by Boundless.
happen, the molecules need to be very close to one another. These

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2.10: ATOMS, ISOTOPES, IONS, AND MOLECULES - HYDROGEN BONDING
AND VAN DER WAALS FORCES
APPLICATIONS FOR HYDROGEN BONDS
 LEARNING OBJECTIVES Hydrogen bonds occur in inorganic molecules, such as water, and
Describe how hydrogen bonds and van der Waals organic molecules, such as DNA and proteins. The two
interactions occur complementary strands of DNA are held together by hydrogen
bonds between complementary nucleotides (A&T, C&G). Hydrogen
Ionic and covalent bonds between elements require energy to break. bonding in water contributes to its unique properties, including its
Ionic bonds are not as strong as covalent, which determines their high boiling point (100 °C) and surface tension.
behavior in biological systems. However, not all bonds are ionic or
covalent bonds. Weaker bonds can also form between molecules.
Two weak bonds that occur frequently are hydrogen bonds and van
der Waals interactions.

δ−

δ+ 1
H Figure 2.10.1: Water droplets on a leaf: The hydrogen bonds formed
between water molecules in water droplets are stronger than the
δ− O other intermolecular forces between the water molecules and the
δ+
H leaf, contributing to high surface tension and distinct water droplets.
δ−
δ+ In biology, intramolecular hydrogen bonding is partly responsible
δ+
δ−
for the secondary, tertiary, and quaternary structures of proteins and
nucleic acids. The hydrogen bonds help the proteins and nucleic
acids form and maintain specific shapes.

VAN DER WAALS INTERACTIONS

Figure 2.10.1: Hydrogen bonds between water molecules: The Like hydrogen bonds, van der Waals interactions are weak
slightly negative oxygen side of the water molecule and the slightly
positive hydrogen side of the water molecule are attracted to each attractions or interactions between molecules. Van der Waals
other and form a hydrogen bond. (Public Domain; Magasjukur2 via attractions can occur between any two or more molecules and are
Wikipedia) dependent on slight fluctuations of the electron densities, which are
not always symmetrical around an atom. For these attractions to
HYDROGEN BONDING
happen, the molecules need to be very close to one another. These
Hydrogen bonds provide many of the critical, life-sustaining
bonds—along with ionic, covalent, and hydrogen bonds—contribute
properties of water and also stabilize the structures of proteins and to the three-dimensional structure of proteins that is necessary for
DNA, the building block of cells. When polar covalent bonds
their proper function.
containing hydrogen form, the hydrogen in that bond has a slightly
positive charge because hydrogen’s one electron is pulled more KEY POINTS
strongly toward the other element and away from the hydrogen. Hydrogen bonds provide many of the critical, life-sustaining
Because the hydrogen is slightly positive, it will be attracted to properties of water and also stabilize the structures of proteins
neighboring negative charges. When this happens, an interaction and DNA, the building block of cells.
occurs between the δ+of the hydrogen from one molecule and the δ– Hydrogen bonds occur in inorganic molecules, such as water,
charge on the more electronegative atoms of another molecule, and organic molecules, such as DNA and proteins.
usually oxygen or nitrogen, or within the same molecule. This Van der Waals attractions can occur between any two or more
interaction is called a hydrogen bond. This type of bond is common molecules and are dependent on slight fluctuations of the
and occurs regularly between water molecules. Individual hydrogen electron densities.
bonds are weak and easily broken; however, they occur in very large While hydrogen bonds and van der Waals interactions are weak
numbers in water and in organic polymers, creating a major force in individually, they are strong combined in vast numbers.
combination. Hydrogen bonds are also responsible for zipping
together the DNA double helix.

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2.11: WATER - WATER’S POLARITY
As a result of water’s polarity, each water molecule attracts other
 LEARNING OBJECTIVES water molecules because of the opposite charges between them,
forming hydrogen bonds. Water also attracts, or is attracted to, other
Describe the actions that occur due to water’s polarity
polar molecules and ions, including many biomolecules, such as
sugars, nucleic acids, and some amino acids. A polar substance that
One of water’s important properties is that it is composed of polar
interacts readily with or dissolves in water is referred to as
molecules. The two hydrogen atoms and one oxygen atom within
hydrophilic (hydro- = “water”; -philic = “loving”). In contrast,
water molecules (H2O) form polar covalent bonds. While there is no
nonpolar molecules, such as oils and fats, do not interact well with
net charge to a water molecule, the polarity of water creates a water, as shown in. These molecules separate from it rather than
slightly positive charge on hydrogen and a slightly negative charge
dissolve in it, as we see in salad dressings containing oil and vinegar
on oxygen, contributing to water’s properties of attraction. Water’s
(an acidic water solution). These nonpolar compounds are called
charges are generated because oxygen is more electronegative, or
hydrophobic (hydro- = “water”; -phobic = “fearing”).
electron loving, than hydrogen. Thus, it is more likely that a shared
electron would be found near the oxygen nucleus than the hydrogen KEY POINTS
nucleus. Since water is a nonlinear, or bent, molecule, the difference The difference in electronegativities between oxygen and
in electronegativities between the oxygen and hydrogen atoms hydrogen atoms creates partial negative and positive charges,
generates the partial negative charge near the oxygen and partial respectively, on the atoms.
positive charges near both hydrogens. Water molecules attract or are attracted to other polar molecules.
Molecules that do not dissolve in water are known as
hydrophobic (water fearing) molecules.

KEY TERMS
hydrophilic: having an affinity for water; able to absorb, or be
wetted by water
hydrophobic: lacking an affinity for water; unable to absorb, or
be wetted by water
polarity: The intermolecular forces between the slightly
positively-charged end of one molecule to the negative end of
another or the same molecule.

This page titled 2.11: Water - Water’s Polarity is shared under a CC BY-SA
4.0 license and was authored, remixed, and/or curated by Boundless.
Figure 2.11.1: Nonpolar Molecules: Oil and water do not mix. As
this macro image of oil and water shows, oil does not dissolve in
water but forms droplets instead. This is due to it being a nonpolar
compound.

2.11.1 https://bio.libretexts.org/@go/page/12667
2.12: WATER - GAS, LIQUID, AND SOLID WATER
The ice crystals that form upon freezing rupture the delicate
 LEARNING OBJECTIVES membranes essential for the function of living cells, irreversibly
damaging them. Cells can only survive freezing if the water in them
Explain the biological significance of ice’s ability to float on
is temporarily replaced by another liquid like glycerol.
water

WATER’S STATES: GAS, LIQUID, AND SOLID

The formation of hydrogen bonds is an important quality of liquid
water that is crucial to life as we know it. As water molecules make
hydrogen bonds with each other, water takes on some unique
chemical characteristics compared to other liquids, and since living
things have a high water content, understanding these chemical
features is key to understanding life. In liquid water, hydrogen bonds
are constantly formed and broken as the water molecules slide past Figure 2.12.1: Ice Density: Hydrogen bonding makes ice less dense
each other. The breaking of these bonds is caused by the motion than liquid water. The (a) lattice structure of ice makes it less dense
than the freely flowing molecules of liquid water, enabling it to (b)
(kinetic energy) of the water molecules due to the heat contained in float on water.
the system. When the heat is raised as water is boiled, the higher
kinetic energy of the water molecules causes the hydrogen bonds to KEY POINTS
break completely and allows water molecules to escape into the air As water is boiled, kinetic energy causes the hydrogen bonds to
as gas (steam or water vapor). On the other hand, when the break completely and allows water molecules to escape into the
temperature of water is reduced and water freezes, the water air as gas (steam or water vapor).
molecules form a crystalline structure maintained by hydrogen When water freezes, water molecules form a crystalline structure
bonding (there is not enough energy to break the hydrogen bonds). maintained by hydrogen bonding.
This makes ice less dense than liquid water, a phenomenon not seen Solid water, or ice, is less dense than liquid water.
in the solidification of other liquids. Ice is less dense than water because the orientation of hydrogen
Water’s lower density in its solid form is due to the way hydrogen bonds causes molecules to push farther apart, which lowers the
bonds are oriented as it freezes: the water molecules are pushed density.
farther apart compared to liquid water. With most other liquids, For other liquids, solidification when the temperature drops
solidification when the temperature drops includes the lowering of includes the lowering of kinetic energy, which allows molecules
kinetic energy between molecules, allowing them to pack even more to pack more tightly and makes the solid denser than its liquid
tightly than in liquid form and giving the solid a greater density than form.
the liquid. Because ice is less dense than water, it is able to float at the
The low density of ice, an anomaly, causes it to float at the surface surface of water.
of liquid water, such as an iceberg or the ice cubes in a glass of
KEY TERMS
water. In lakes and ponds, ice forms on the surface of the water
density: A measure of the amount of matter contained by a given
creating an insulating barrier that protects the animals and plant life
in the pond from freezing. Without this layer of insulating ice, plants volume.
and animals living in the pond would freeze in the solid block of ice This page titled 2.12: Water - Gas, Liquid, and Solid Water is shared under a
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2.13: WATER - HEAT OF VAPORIZATION

 LEARNING OBJECTIVES

Explain how heat of vaporization is related to the boiling

point of water

Water in its liquid form has an unusually high boiling point

temperature, a value close to 100°C. As a result of the network of
hydrogen bonding present between water molecules, a high input of
energy is required to transform one gram of liquid water into water
vapor, an energy requirement called the heat of vaporization. Water
has a heat of vaporization value of 40.65 kJ/mol. A considerable
amount of heat energy (586 calories) is required to accomplish this
change in water. This process occurs on the surface of water. As
liquid water heats up, hydrogen bonding makes it difficult to
separate the water molecules from each other, which is required for Figure 2.13.1: Humidity, Evaporation, and Boiling: (a) Because of
the distribution of speeds and kinetic energies, some water
it to enter its gaseous phase (steam). As a result, water acts as a heat molecules can break away to the vapor phase even at temperatures
sink, or heat reservoir, and requires much more heat to boil than below the ordinary boiling point. (b) If the container is sealed,
does a liquid such as ethanol (grain alcohol), whose hydrogen evaporation will continue until there is enough vapor density for the
condensation rate to equal the evaporation rate. This vapor density
bonding with other ethanol molecules is weaker than water’s and the partial pressure it creates are the saturation values. They
hydrogen bonding. Eventually, as water reaches its boiling point of increase with temperature and are independent of the presence of
100° Celsius (212° Fahrenheit), the heat is able to break the other gases, such as air. They depend only on the vapor pressure of
water.
hydrogen bonds between the water molecules, and the kinetic energy
The fact that hydrogen bonds need to be broken for water to
(motion) between the water molecules allows them to escape from
evaporate means that a substantial amount of energy is used in the
the liquid as a gas. Even when below its boiling point, water’s
process. As the water evaporates, energy is taken up by the process,
individual molecules acquire enough energy from each other such
cooling the environment where the evaporation is taking place. In
that some surface water molecules can escape and vaporize; this
many living organisms, including humans, the evaporation of sweat,
process is known as evaporation.
which is 90 percent water, allows the organism to cool so that
homeostasis of body temperature can be maintained.

KEY POINTS
The dissociation of liquid water molecules, which changes the
substance to a gas, requires a lot of energy.
The boiling point of water is the temperature in which there is
enough energy to break the hydrogen bonds between water
molecules.
Water is converted from its liquid form to its gaseous form
(steam) when the heat of vaporization is reached.
Evaporation of sweat (mostly water) removes heat from the
surface of skin, cooling the body.

KEY TERMS
heat of vaporization: The energy required to transform a given
quantity of a substance from a liquid into a gas at a given
pressure (often atmospheric pressure).

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2.14: WATER - HIGH HEAT CAPACITY
many organisms are mainly composed of water, the property of high
 LEARNING OBJECTIVES heat capacity allows highly regulated internal body temperatures.
For example, the temperature of your body does not drastically drop
Explain the biological significance of water’s high specific
to the same temperature as the outside temperature while you are
heat
skiing or playing in the snow. Due to its high heat capacity, water is
used by warm blooded animals to more evenly disperse heat in their
WATER’S HIGH HEAT CAPACITY bodies; it acts in a similar manner to a car’s cooling system,
The capability for a molecule to absorb heat energy is called heat transporting heat from warm places to cool places, causing the body
capacity, which can be calculated by the equation shown in the to maintain a more even temperature.
figure. Water’s high heat capacity is a property caused by hydrogen
bonding among water molecules. When heat is absorbed, hydrogen KEY POINTS
bonds are broken and water molecules can move freely. When the Water has the highest heat capacity of all liquids.
temperature of water decreases, the hydrogen bonds are formed and Oceans cool slower than the land due to the high heat capacity of
release a considerable amount of energy. Water has the highest water.
specific heat capacity of any liquid. Specific heat is defined as the To change the temperature of 1 gram of water by 1 degree
amount of heat one gram of a substance must absorb or lose to Celsius, it takes 1.00 calorie.
change its temperature by one degree Celsius. For water, this amount
is one calorie, or 4.184 Joules. As a result, it takes water a long time KEY TERMS
to heat and a long time to cool. In fact, the specific heat capacity of heat capacity: The capability of a substance to absorb heat
water is about five times more than that of sand. This explains why energy
the land cools faster than the sea. specific heat: the amount of heat, in calories, needed to raise the
C=QΔT.C=QΔT. temperature of 1 gram of water by 1 degree Celsius
The resistance to sudden temperature changes makes water an This page titled 2.14: Water - High Heat Capacity is shared under a CC BY-
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experiencing wide temperature fluctuation. Furthermore, because

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2.15: WATER - WATER’S SOLVENT PROPERTIES
Since many biomolecules are either polar or charged, water readily
 LEARNING OBJECTIVES dissolves these hydrophilic compounds. Water is a poor solvent,
however, for hydrophobic molecules such as lipids. Nonpolar
Explain why some molecules do not dissolve in water.
molecules experience hydrophobic interactions in water: the water
changes its hydrogen bonding patterns around the hydrophobic
WATER’S SOLVENT PROPERTIES molecules to produce a cage-like structure called a clathrate. This
Water, which not only dissolves many compounds but also dissolves change in the hydrogen-bonding pattern of the water solvent causes
more substances than any other liquid, is considered the universal the system’s overall entropy to greatly decrease, as the molecules
solvent. A polar molecule with partially-positive and negative become more ordered than in liquid water. Thermodynamically, such
charges, it readily dissolves ions and polar molecules. Water is a large decrease in entropy is not spontaneous, and the hydrophobic
therefore referred to as a solvent: a substance capable of dissolving molecule will not dissolve.
other polar molecules and ionic compounds. The charges associated
with these molecules form hydrogen bonds with water, surrounding KEY POINTS
the particle with water molecules. This is referred to as a sphere of Water dissociates salts by separating the cations and anions and
hydration, or a hydration shell, and serves to keep the particles forming new interactions between the water and ions.
separated or dispersed in the water. Water dissolves many biomolecules, because they are polar and
When ionic compounds are added to water, individual ions interact therefore hydrophilic.
with the polar regions of the water molecules during the dissociation
KEY TERMS
process, disrupting their ionic bonds. Dissociation occurs when
atoms or groups of atoms break off from molecules and form ions. dissociation: The process by which a compound or complex
Consider table salt (NaCl, or sodium chloride): when NaCl crystals body breaks up into simpler constituents such as atoms or ions,
+
are added to water, the molecules of NaCl dissociate into Na and usually reversibly.
–
Cl ions, and spheres of hydration form around the ions. The hydration shell: The term given to a solvation shell (a structure
positively-charged sodium ion is surrounded by the partially- composed of a chemical that acts as a solvent and surrounds a
negative charge of the water molecule’s oxygen; the negatively- solute species) with a water solvent; also referred to as a
charged chloride ion is surrounded by the partially-positive charge hydration sphere.
of the hydrogen in the water molecule.
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Figure 2.15.1: Dissociation of NaCl in water: When table salt

(NaCl) is mixed in water, spheres of hydration form around the ions.

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2.16: WATER - COHESIVE AND ADHESIVE PROPERTIES

 LEARNING OBJECTIVES

Describe the cohesive and adhesive properties of water.

WATER’S COHESIVE AND ADHESIVE

PROPERTIES
Have you ever filled a glass of water to the very top and then slowly
added a few more drops? Before it overflows, the water forms a
dome-like shape above the rim of the glass. This water can stay
above the glass because of the property of cohesion. In cohesion,
water molecules are attracted to each other (because of hydrogen
bonding), keeping the molecules together at the liquid-gas (water-
air) interface, although there is no more room in the glass.
Cohesion allows for the development of surface tension, the capacity
of a substance to withstand being ruptured when placed under
tension or stress. This is also why water forms droplets when placed
Figure 2.16.1: Adhesion: Capillary action in a glass tube is caused
on a dry surface rather than being flattened out by gravity. When a by the adhesive forces exerted by the internal surface of the glass
small scrap of paper is placed onto the droplet of water, the paper exceeding the cohesive forces between the water molecules
floats on top of the water droplet even though paper is denser (the themselves.
mass per unit volume) than the water. Cohesion and surface tension Why are cohesive and adhesive forces important for life? Cohesive
keep the hydrogen bonds of water molecules intact and support the and adhesive forces are important for the transport of water from the
item floating on the top. It’s even possible to “float” a needle on top roots to the leaves in plants. These forces create a “pull” on the
of a glass of water if it is placed gently without breaking the surface water column. This pull results from the tendency of water
tension. molecules being evaporated on the surface of the plant to stay
connected to water molecules below them, and so they are pulled
along. Plants use this natural phenomenon to help transport water
from their roots to their leaves. Without these properties of water,
plants would be unable to receive the water and the dissolved
minerals they require. In another example, insects such as the water
strider use the surface tension of water to stay afloat on the surface
layer of water and even mate there.

Figure 2.16.1: Surface Tension: The weight of the needle is pulling

the surface downward; at the same time, the surface tension is
pulling it up, suspending it on the surface of the water and keeping it
from sinking. Notice the indentation in the water around the needle.
These cohesive forces are related to water’s property of adhesion, or
the attraction between water molecules and other molecules. This
attraction is sometimes stronger than water’s cohesive forces, Figure 2.16.1: Cohesion & Adhesion: Water’s cohesive and
adhesive properties allow this water strider (Gerris sp.) to stay
especially when the water is exposed to charged surfaces such as
afloat.
those found on the inside of thin glass tubes known as capillary
tubes. Adhesion is observed when water “climbs” up the tube placed KEY POINTS
in a glass of water: notice that the water appears to be higher on the Cohesion holds hydrogen bonds together to create surface
sides of the tube than in the middle. This is because the water tension on water.
molecules are attracted to the charged glass walls of the capillary Since water is attracted to other molecules, adhesive forces pull
more than they are to each other and therefore adhere to it. This type the water toward other molecules.
of adhesion is called capillary action. Water is transported in plants through both cohesive and
adhesive forces; these forces pull water and the dissolved

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 minerals from the roots to the leaves and other parts of the plant. cohesion: Various intermolecular forces that hold solids and
liquids together; attraction between like molecules
KEY TERMS
adhesion: The ability of a substance to stick to an unlike This page titled 2.16: Water - Cohesive and Adhesive Properties is shared
substance; attraction between unlike molecules under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

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2.17: WATER - PH, BUFFERS, ACIDS, AND BASES

 LEARNING OBJECTIVES

Explain the composition of buffer solutions and how they

maintain a steady pH

SELF-IONIZATION OF WATER
Hydrogen ions are spontaneously generated in pure water by the
dissociation (ionization) of a small percentage of water molecules
into equal numbers of hydrogen (H+) ions and hydroxide (OH–)
ions. The hydroxide ions remain in solution because of their
hydrogen bonds with other water molecules; the hydrogen ions,
consisting of naked protons, are immediately attracted to un-ionized
water molecules and form hydronium ions (H O ). By convention,
3
+

scientists refer to hydrogen ions and their concentration as if they

were free in this state in liquid water.
+ −
2H O ⇋ H O + OH (2.17.1)
2 3

The concentration of hydrogen ions dissociating from pure water is

1 × 10-7 moles H+ ions per liter of water. The pH is calculated as the
negative of the base 10 logarithm of this concentration:
+ Figure 2.17.1: The pH scale: The pH scale measures the
pH = − log 10 [H ] (2.17.2)
concentration of hydrogen ions (H+) in a solution.
The negative log of 1 × 10-7 is equal to 7.0, which is also known as Non-neutral pH readings result from dissolving acids or bases in
neutral pH. Human cells and blood each maintain near-neutral pH. water. Using the negative logarithm to generate positive integers,
high concentrations of hydrogen ions yield a low pH, and low
PH SCALE concentrations a high pH.
The pH of a solution indicates its acidity or basicity (alkalinity). The An acid is a substance that increases the concentration of hydrogen
pH scale is an inverse logarithm that ranges from 0 to 14: anything ions (H+) in a solution, usually by dissociating one of its hydrogen
below 7.0 (ranging from 0.0 to 6.9) is acidic, and anything above 7.0 atoms. A base provides either hydroxide ions (OH–) or other
(from 7.1 to 14.0) is basic (or alkaline ). Extremes in pH in either negatively-charged ions that react with hydrogen ions in solution,
direction from 7.0 are usually considered inhospitable to life. The thereby reducing the concentration of H+ and raising the pH.
pH in cells (6.8) and the blood (7.4) are both very close to neutral,
whereas the environment in the stomach is highly acidic, with a pH STRONG ACIDS AND STRONG BASES
of 1 to 2. The stronger the acid, the more readily it donates H+. For example,
hydrochloric acid (HCl) is highly acidic and completely dissociates
into hydrogen and chloride ions, whereas the acids in tomato juice or
vinegar do not completely dissociate and are considered weak acids;
conversely, strong bases readily donate OH– and/or react with
hydrogen ions. Sodium hydroxide (NaOH) and many household
cleaners are highly basic and give up OH– rapidly when placed in
water; the OH– ions react with H+ in solution, creating new water
molecules and lowering the amount of free H+ in the system, thereby
raising the overall pH. An example of a weak basic solution is
seawater, which has a pH near 8.0, close enough to neutral that well-
adapted marine organisms thrive in this alkaline environment.

BUFFERS
How can organisms whose bodies require a near-neutral pH ingest
acidic and basic substances (a human drinking orange juice, for
example) and survive? Buffers are the key. Buffers usually consist of
a weak acid and its conjugate base; this enables them to readily

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Maintaining a constant blood pH is critical to a person’s well-being. Located at: http://cnx.org/content/m44392/latest...e_02_02_02.jpg. License:
The buffer that maintains the pH of human blood involves carbonic CC BY: Attribution
heat capacity. Provided by: Wiktionary. Located at:
acid (H2CO3), bicarbonate ion (HCO3–), and carbon dioxide (CO2). en.wiktionary.org/wiki/heat_capacity. License: CC BY-SA: Attribution-
When bicarbonate ions combine with free hydrogen ions and ShareAlike
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dangerously reducing its pH; likewise, if too much OH– is Structural Biochemistry/Water. Provided by: Wikibooks. Located at:
en.wikibooks.org/wiki/Structural_Biochemistry/Water%23High_Heat_Capa
introduced into the system, carbonic acid will combine with it to city. License: CC BY-SA: Attribution-ShareAlike
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buffering of blood pH levels: the blue arrows show the process of License: CC BY: Attribution
raising pH as more CO2 is made; the purple arrows indicate the Structural Biochemistry/Unique Properties/Cohesive Behavior/Melting Point and
reverse process, lowering pH as more bicarbonate is created. Boiling Point. Provided by: Wikibooks. Located at:
en.wikibooks.org/wiki/Structural_Biochemistry/Unique_Properties/Cohesive
Antacids, which combat excess stomach acid, are another example _Behavior/Melting_Point_and_Boiling_Point. License: CC BY-SA:
of buffers. Many over-the-counter medications work similarly to Attribution-ShareAlike
heat of vaporization. Provided by: Wikipedia. Located at:
blood buffers, often with at least one ion (usually carbonate) capable en.Wikipedia.org/wiki/heat%20of%20vaporization. License: CC BY-SA:
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License: CC BY: Attribution
KEY POINTS OpenStax College, Water. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44392/latest/Figure_02_02_02.jpg.
A basic solution will have a pH above 7.0, while an acidic License: CC BY: Attribution
solution will have a pH below 7.0. OpenStax College, Humidity, Evaporation, and Boiling. October 25, 2013.
Provided by: OpenStax CNX. Located at: http://cnx.org/content/m42219/1.5/.
Buffers are solutions that contain a weak acid and its a conjugate License: CC BY: Attribution
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Located at: http://cnx.org/content/m44392/latest/?collection=col11448/latest.
thereby maintaining an overall steady pH in the solution. License: CC BY: Attribution
pH is equal to the negative logarithm of the concentration of H+ Structural Biochemistry/Unique Properties/Versatility as a Solvent. Provided by:
ions in solution: pH = – log[H+]. Wikibooks. Located at:
en.wikibooks.org/wiki/Structural_Biochemistry/Unique_Properties/Versatilit
y_as_a_Solvent. License: CC BY-SA: Attribution-ShareAlike
KEY TERMS Structural Biochemistry/Water. Provided by: Wikibooks. Located at:
en.wikibooks.org/wiki/Structural_Biochemistry/Water%23Universal_Solven
alkaline: having a pH greater than 7; basic t. License: CC BY-SA: Attribution-ShareAlike
acidic: having a pH less than 7 hydration shell. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/hydration%20shell. License: CC BY-SA: Attribution-
buffer: a solution composed of a weak acid and its conjugate ShareAlike
base that can be used to stabilize the pH of a solution dissociation. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/dissociation. License: CC BY-SA: Attribution-
ShareAlike
CONTRIBUTIONS AND ATTRIBUTIONS Clathrate compound. Provided by: Wikipedia. Located at:
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. en.Wikipedia.org/wiki/Clathrate_compound. License: CC BY-SA:
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BY: Attribution OpenStax College, Water. October 16, 2013. Provided by: OpenStax CNX.
polarity. Provided by: Wiktionary. Located at: Located at: http://cnx.org/content/m44392/latest/Figure_02_02_01.jpg.
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hydrophilic. Provided by: Wiktionary. Located at: Located at: http://cnx.org/content/m44392/latest/Figure_02_02_02.jpg.
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Located at: http://cnx.org/content/m44392/latest/?collection=col11448/latest. License: CC BY: Attribution
License: CC BY: Attribution cohesion. Provided by: Wiktionary. Located at:
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2.18: CARBON - THE CHEMICAL BASIS FOR LIFE
properties that allow it to form covalent bonds to as many as four
 LEARNING OBJECTIVES different atoms, making this versatile element ideal to serve as the
basic structural component, or “backbone,” of the macromolecules.
Explain the properties of carbon that allow it to serve as a
building block for biomolecules STRUCTURE OF CARBON
Individual carbon atoms have an incomplete outermost electron
Carbon is the fourth most abundant element in the universe and is shell. With an atomic number of 6 (six electrons and six protons),
the building block of life on earth. On earth, carbon circulates the first two electrons fill the inner shell, leaving four in the second
through the land, ocean, and atmosphere, creating what is known as shell. Therefore, carbon atoms can form four covalent bonds with
the Carbon Cycle. This global carbon cycle can be divided further other atoms to satisfy the octet rule. The methane molecule provides
into two separate cycles: the geological carbon cycles takes place an example: it has the chemical formula CH4. Each of its four
over millions of years, whereas the biological or physical carbon hydrogen atoms forms a single covalent bond with the carbon atom
cycle takes place from days to thousands of years. In a nonliving by sharing a pair of electrons. This results in a filled outermost shell.
environment, carbon can exist as carbon dioxide (CO2), carbonate
rocks, coal, petroleum, natural gas, and dead organic matter. Plants
and algae convert carbon dioxide to organic matter through the
process of photosynthesis, the energy of light.

Figure 2.18.1: Structure of Methane: Methane has a tetrahedral

geometry, with each of the four hydrogen atoms spaced 109.5° apart.

KEY POINTS
Figure 2.18.1: Carbon is present in all life: All living things contain All living things contain carbon in some form.
carbon in some form, and carbon is the primary component of
macromolecules, including proteins, lipids, nucleic acids, and Carbon is the primary component of macromolecules, including
carbohydrates. Carbon exists in many forms in this leaf, including in proteins, lipids, nucleic acids, and carbohydrates.
the cellulose to form the leaf’s structure and in chlorophyll, the Carbon’s molecular structure allows it to bond in many different
pigment which makes the leaf green.
ways and with many different elements.
CARBON IS IMPORTANT TO LIFE The carbon cycle shows how carbon moves through the living
In its metabolism of food and respiration, an animal consumes and non-living parts of the environment.
glucose (C6H12O6), which combines with oxygen (O2) to produce
KEY TERMS
carbon dioxide (CO2), water (H2O), and energy, which is given off
as heat. The animal has no need for the carbon dioxide and releases octet rule: A rule stating that atoms lose, gain, or share electrons
it into the atmosphere. A plant, on the other hand, uses the opposite in order to have a full valence shell of 8 electrons (has some
reaction of an animal through photosynthesis. It intakes carbon exceptions).
dioxide, water, and energy from sunlight to make its own glucose carbon cycle: the physical cycle of carbon through the earth’s
and oxygen gas. The glucose is used for chemical energy, which the biosphere, geosphere, hydrosphere, and atmosphere; includes
plant metabolizes in a similar way to an animal. The plant then emits such processes as photosynthesis, decomposition, respiration and
the remaining oxygen into the environment. carbonification
macromolecule: a very large molecule, especially used in
Cells are made of many complex molecules called macromolecules,
reference to large biological polymers (e.g., nucleic acids and
which include proteins, nucleic acids (RNA and DNA),
proteins)
carbohydrates, and lipids. The macromolecules are a subset of
organic molecules (any carbon-containing liquid, solid, or gas) that This page titled 2.18: Carbon - The Chemical Basis for Life is shared under
are especially important for life. The fundamental component for all a CC BY-SA 4.0 license and was authored, remixed, and/or curated by
of these macromolecules is carbon. The carbon atom has unique Boundless.

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2.19: CARBON - HYDROCARBONS
have a significant impact on the shape a particular molecule can
 LEARNING OBJECTIVES assume.

Discuss the role of hydrocarbons in biomacromolecules

HYDROCARBONS
Hydrocarbons are organic molecules consisting entirely of carbon
and hydrogen, such as methane (CH4). Hydrocarbons are often used
as fuels: the propane in a gas grill or the butane in a lighter. The
many covalent bonds between the atoms in hydrocarbons store a
great amount of energy, which is released when these molecules are Figure 2.19.1: Hydrocarbon Chains: When carbon forms single
bonds with other atoms, the shape is tetrahedral. When two carbon
burned (oxidized). Methane, an excellent fuel, is the simplest atoms form a double bond, the shape is planar, or flat. Single bonds,
hydrocarbon molecule, with a central carbon atom bonded to four like those found in ethane, are able to rotate. Double bonds, like
different hydrogen atoms. The geometry of the methane molecule, those found in ethene cannot rotate, so the atoms on either side are
locked in place.
where the atoms reside in three dimensions, is determined by the
shape of its electron orbitals. The carbon and the four hydrogen HYDROCARBON RINGS
atoms form a shape known as a tetrahedron, with four triangular The hydrocarbons discussed so far have been aliphatic
faces; for this reason, methane is described as having tetrahedral hydrocarbons, which consist of linear chains of carbon atoms.
geometry. Another type of hydrocarbon, aromatic hydrocarbons, consists of
closed rings of carbon atoms. Ring structures are found in
hydrocarbons, sometimes with the presence of double bonds, which
can be seen by comparing the structure of cyclohexane to benzene.
The benzene ring is present in many biological molecules including
some amino acids and most steroids, which includes cholesterol and
the hormones estrogen and testosterone. The benzene ring is also
found in the herbicide 2,4-D. Benzene is a natural component of
crude oil and has been classified as a carcinogen. Some
hydrocarbons have both aliphatic and aromatic portions; beta-
Figure 2.19.1: Methane: Methane has a tetrahedral geometry, with carotene is an example of such a hydrocarbon.
each of the four hydrogen atoms spaced 109.5° apart.
As the backbone of the large molecules of living things,
hydrocarbons may exist as linear carbon chains, carbon rings, or
combinations of both. Furthermore, individual carbon-to-carbon
bonds may be single, double, or triple covalent bonds; each type of
bond affects the geometry of the molecule in a specific way. This
three-dimensional shape or conformation of the large molecules of
life (macromolecules) is critical to how they function. Figure 2.19.1: Hydrocarbon Rings: Carbon can form five-and six
membered rings. Single or double bonds may connect the carbons in
the ring, and nitrogen may be substituted for carbon.
HYDROCARBON CHAINS
Hydrocarbon chains are formed by successive bonds between carbon KEY POINTS
atoms and may be branched or unbranched. The overall geometry of Hydrocarbons are molecules that contain only carbon and
the molecule is altered by the different geometries of single, double, hydrogen.
and triple covalent bonds. The hydrocarbons ethane, ethene, and Due to carbon’s unique bonding patterns, hydrocarbons can have
ethyne serve as examples of how different carbon-to-carbon bonds single, double, or triple bonds between the carbon atoms.
affect the geometry of the molecule. The names of all three The names of hydrocarbons with single bonds end in “-ane,”
molecules start with the prefix “eth-,” which is the prefix for two those with double bonds end in “-ene,” and those with triple
carbon hydrocarbons. The suffixes “-ane,” “-ene,” and “-yne” refer bonds end in “-yne”.
to the presence of single, double, or triple carbon-carbon bonds, The bonding of hydrocarbons allows them to form rings or
respectively. Thus, propane, propene, and propyne follow the same chains.
pattern with three carbon molecules, butane, butene, and butyne for
four carbon molecules, and so on. Double and triple bonds change KEY TERMS
the geometry of the molecule: single bonds allow rotation along the covalent bond: A type of chemical bond where two atoms are
axis of the bond, whereas double bonds lead to a planar connected to each other by the sharing of two or more electrons.
configuration and triple bonds to a linear one. These geometries

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aliphatic: Of a class of organic compounds in which the carbon
This page titled 2.19: Carbon - Hydrocarbons is shared under a CC BY-SA
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aromatic: Having a closed ring of alternate single and double
bonds with delocalized electrons.

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2.20: CARBON - ORGANIC ISOMERS
GEOMETRIC ISOMERS
 LEARNING OBJECTIVES Geometric isomers, on the other hand, have similar placements of
Give examples of isomers their covalent bonds but differ in how these bonds are made to the
surrounding atoms, especially in carbon-to-carbon double bonds. In
The three-dimensional placement of atoms and chemical bonds the simple molecule butene (C4H8), the two methyl groups (CH3)
within organic molecules is central to understanding their chemistry. can be on either side of the double covalent bond central to the
Molecules that share the same chemical formula but differ in the molecule. When the carbons are bound on the same side of the
placement (structure) of their atoms and/or chemical bonds are double bond, this is the cis configuration; if they are on opposite
known as isomers. sides of the double bond, it is a trans configuration. In the trans
configuration, the carbons form a more or less linear structure,
STRUCTURAL ISOMERS whereas the carbons in the cis configuration make a bend (change in
Structural isomers (such as butane and isobutane ) differ in the direction) of the carbon backbone.
placement of their covalent bonds. Both molecules have four
CIS OR TRANS CONFIGURATIONS
carbons and ten hydrogens (C4H10), but the different arrangement of
the atoms within the molecules leads to differences in their chemical In triglycerides (fats and oils), long carbon chains known as fatty
properties. For example, due to their different chemical properties, acids may contain double bonds, which can be in either the cis or
butane is suited for use as a fuel for cigarette lighters and torches, trans configuration. Fats with at least one double bond between
whereas isobutane is suited for use as a refrigerant and a propellant carbon atoms are unsaturated fats. When some of these bonds are in
in spray cans. the cis configuration, the resulting bend in the carbon backbone of
the chain means that triglyceride molecules cannot pack tightly, so
they remain liquid (oil) at room temperature. On the other hand,
triglycerides with trans double bonds (popularly called trans fats),
have relatively linear fatty acids that are able to pack tightly together
at room temperature and form solid fats.

Figure 2.20.1: Cis and Trans Fatty Acids: These space-filling

models show a cis (oleic acid) and a trans (eliadic acid) fatty acid.
Notice the bend in the molecule cause by the cis configuration.
In the human diet, trans fats are linked to an increased risk of
cardiovascular disease, so many food manufacturers have reduced or
eliminated their use in recent years. In contrast to unsaturated fats,
triglycerides without double bonds between carbon atoms are called
saturated fats, meaning that they contain all the hydrogen atoms
available. Saturated fats are a solid at room temperature and usually
of animal origin.
Figure 2.20.1: Isomers: Molecules that have the same number and
type of atoms arranged differently are called isomers. (a) Structural KEY POINTS
isomers have a different covalent arrangement of atoms. (b) Isomers are molecules with the same chemical formula but have
Geometric isomers have a different arrangement of atoms around a
double bond. (c) Enantiomers are mirror images of each other. different structures.
Isomers differ in how their bonds are positioned to surrounding
atoms.

2.20.1 https://bio.libretexts.org/@go/page/12678
When the carbons are bound on the same side of the double KEY TERMS
bond, this is the cis configuration; if they are on opposite sides of fatty acid: Any of a class of aliphatic carboxylic acids, of
the double bond, it is a trans configuration. general formula CnH2n+1COOH, that occur combined with
Triglycerides, which show both cis and trans configurations, can glycerol as animal or vegetable oils and fats.
occur as either saturated or unsaturated, depending upon how isomer: Any of two or more compounds with the same
many hydrogen atoms they have attached to them. molecular formula but with different structure.

This page titled 2.20: Carbon - Organic Isomers is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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2.21: CARBON - ORGANIC ENANTIOMERS
of each other and are, thus, commonly called enantiomorphs; hence,
 LEARNING OBJECTIVES this structural property is now commonly referred to as
enantiomerism. Enantiopure compounds refer to samples having,
Give examples of enantiomers
within the limits of detection, molecules of only one chirality.
Enantiomers of each other often show different chemical reactions
Stereoisomers are a type of isomer where the order of the atoms in
with other substances that are also enantiomers. Since many
the two molecules is the same but their arrangement in space is
molecules in the bodies of living beings are enantiomers themselves,
different. The two main types of stereoisomerism are
there is sometimes a marked difference in the effects of two
diastereomerism (including ‘cis-trans isomerism’) and optical
enantiomers on living beings. In drugs, for example, often only one
isomerism (also known as ‘enantiomerism’ and ‘chirality’). Optical
of a drug’s enantiomers is responsible for the desired physiologic
isomers are stereoisomers formed when asymmetric centers are
effects, while the other enantiomer is less active, inactive, or
present; for example, a carbon with four different groups bonded to
sometimes even responsible for adverse effects. Owing to this
it. Enantiomers are two optical isomers (i.e. isomers that are
discovery, drugs composed of only one enantiomer (“enantiopure”)
reflections of each other). Every stereocenter in one isomer has the
can be developed to enhance the pharmacological efficacy and
opposite configuration in the other. They share the same chemical
sometimes do away with some side effects.
structure and chemical bonds, but differ in the three-dimensional
placement of atoms so that they are mirror images, much as a KEY POINTS
person’s left and right hands are. Compounds that are enantiomers of
Enantiomers are stereoisomers, a type of isomer where the order
each other have the same physical properties except for the direction
of the atoms in the two molecules is the same but their
in which they rotate polarized light and how they interact with
arrangement in space is different.
different optical isomers of other compounds.
Many molecules in the bodies of living beings are enantiomers;
The amino acid alanine is example of an entantiomer. The two there is sometimes a large difference in the effects of two
structures, D-alanine and L-alanine, are non-superimposable. In enantiomers on organisms.
nature, only the L-forms of amino acids are used to make proteins. Enantiopure compounds refer to samples having, within the
Some D forms of amino acids are seen in the cell walls of bacteria, limits of detection, molecules of only one chirality.
but never in their proteins. Similarly, the D-form of glucose is the Compounds that are enantiomers of each other have the same
main product of photosynthesis and the L-form of the molecule is physical properties except for the direction in which they rotate
rarely seen in nature. polarized light and how they interact with different optical
isomers of other compounds.

KEY TERMS
enantiomer: One of a pair of stereoisomers that is the mirror
image of the other, but may not be superimposed on this other
stereoisomer.
chirality: The phenomenon in chemistry, physics, and
mathematics in which objects are mirror images of each other,
but are not identical.
stereoisomer: one of a set of the isomers of a compound in
which atoms are arranged differently about a chiral center; they
Figure 2.21.1: Enantiomers: D-alanine and L-alanine are examples exhibit optical activity
of enantiomers or mirror images. Only the L-forms of amino acids
are used to make proteins. This page titled 2.21: Carbon - Organic Enantiomers is shared under a CC
Organic compounds that contain a chiral carbon usually have two BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
non-superposable structures. These two structures are mirror images

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2.22: CARBON - ORGANIC MOLECULES AND FUNCTIONAL GROUPS

 LEARNING OBJECTIVES

Describe the importance of functional groups to organic

molecules

LOCATION OF FUNCTIONAL GROUPS

Functional groups are groups of atoms that occur within organic
molecules and confer specific chemical properties to those
molecules. When functional groups are shown, the organic molecule
is sometimes denoted as “R.” Functional groups are found along the
“carbon backbone” of macromolecules which is formed by chains
and/or rings of carbon atoms with the occasional substitution of an
element such as nitrogen or oxygen. Molecules with other elements
in their carbon backbone are substituted hydrocarbons. Each of the
four types of macromolecules—proteins, lipids, carbohydrates, and
nucleic acids—has its own characteristic set of functional groups
that contributes greatly to its differing chemical properties and its
function in living organisms.

PROPERTIES OF FUNCTIONAL GROUPS

A functional group can participate in specific chemical reactions.
Some of the important functional groups in biological molecules
include: hydroxyl, methyl, carbonyl, carboxyl, amino, phosphate,
and sulfhydryl groups. These groups play an important role in the
formation of molecules like DNA, proteins, carbohydrates, and
Figure 2.22.1: Examples of functional groups: The functional
lipids. groups shown here are found in many different biological molecules,
where “R” is the organic molecule.
CLASSIFYING FUNCTIONAL GROUPS
Functional groups are usually classified as hydrophobic or HYDROGEN BONDS BETWEEN FUNCTIONAL
hydrophilic depending on their charge or polarity. An example of a GROUPS
hydrophobic group is the non-polar methane molecule. Among the Hydrogen bonds between functional groups (within the same
hydrophilic functional groups is the carboxyl group found in amino molecule or between different molecules) are important to the
acids, some amino acid side chains, and the fatty acid heads that function of many macromolecules and help them to fold properly
form triglycerides and phospholipids. This carboxyl group ionizes to and maintain the appropriate shape needed to function correctly.
release hydrogen ions (H+) from the COOH group resulting in the Hydrogen bonds are also involved in various recognition processes,
negatively charged COO– group; this contributes to the hydrophilic such as DNA complementary base pairing and the binding of an
nature of whatever molecule it is found on. Other functional groups, enzyme to its substrate.
such as the carbonyl group, have a partially negatively charged
oxygen atom that may form hydrogen bonds with water molecules,
again making the molecule more hydrophilic.

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2.23: SYNTHESIS OF BIOLOGICAL MACROMOLECULES - TYPES OF
BIOLOGICAL MACROMOLECULES
smaller molecules, called monomers. Typically all the monomers in
 LEARNING OBJECTIVES a polymer tend to be the same, or at least very similar to each other,
linked over and over again to build up the larger macromolecule.
Identify the four major classes of biological macromolecules
These simple monomers can be linked in many different
combinations to produce complex biological polymers, just as a few
Nutrients are the molecules that living organisms require for survival types of Lego blocks can build anything from a house to a car.
and growth but that animals and plants cannot synthesize
themselves. Animals obtain nutrients by consuming food, while
plants pull nutrients from soil.

Monomers and polymers: Many small monomer subunits combine

to form this carbohydrate polymer.
Examples of these monomers and polymers can be found in the
sugar you might put in your coffee or tea. Regular table sugar is the
disaccharide sucrose (a polymer), which is composed of the
monosaccharides fructose and glucose (which are monomers). If we
were to string many carbohydrate monomers together we could
Figure 2.23.1: Sources of biological macromolecules: Foods such as make a polysaccharide like starch. The prefixes “mono-” (one), “di-”
bread, fruit, and cheese are rich sources of biological
macromolecules.
(two),and “poly-” (many) will tell you how many of the monomers
have been joined together in a molecule.
Many critical nutrients are biological macromolecules. The term
“macromolecule” was first coined in the 1920s by Nobel laureate
Hermann Staudinger. Staudinger was the first to propose that many
large biological molecules are built by covalently linking smaller
biological molecules together.

Figure 2.23.3: The molecule sucrose (common table sugar): The

carbohydrate monosaccharides (fructose and glucose) are joined to
make the disaccharide sucrose.
Biological macromolecules all contain carbon in ring or chain form,
which means they are classified as organic molecules. They usually
also contain hydrogen and oxygen, as well as nitrogen and additional
minor elements.

FOUR CLASSES OF BIOLOGICAL

MACROMOLECULES
There are four major classes of biological macromolecules:
Figure 2.23.2: Living organisms are made up of chemical building
blocks: All organisms are composed of a variety of these biological 1. carbohydrates
macromolecules.
2. lipids
MONOMERS AND POLYMERS 3. proteins
4. nucleic acids
Biological macromolecules play a critical role in cell structure and
function. Most (but not all) biological macromolecules are polymers, Each of these types of macromolecules performs a wide array of
which are any molecules constructed by linking together many important functions within the cell; a cell cannot perform its role

2.23.1 https://bio.libretexts.org/@go/page/12682
within the body without many different types of these crucial The four major classes of biological macromolecules are
molecules. In combination, these biological macromolecules make carbohydrates, lipids, proteins, and nucleic acids.
up the majority of a cell’s dry mass. (Water molecules make up the
majority of a cell’s total mass.) All the molecules both inside and KEY TERMS
outside of cells are situated in a water-based (i.e., aqueous) polymer: A relatively large molecule consisting of a chain or
environment, and all the reactions of biological systems are network of many identical or similar monomers chemically
occurring in that same environment. bonded to each other.
monomer: A relatively small molecule that can form covalent
KEY POINTS bonds with other molecules of this type to form a polymer.
Biological macromolecules are important cellular components
and perform a wide array of functions necessary for the survival This page titled 2.23: Synthesis of Biological Macromolecules - Types of
and growth of living organisms. Biological Macromolecules is shared under a CC BY-SA 4.0 license and
was authored, remixed, and/or curated by Boundless.

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2.24: SYNTHESIS OF BIOLOGICAL MACROMOLECULES - DEHYDRATION
SYNTHESIS
image

 LEARNING OBJECTIVES

Explain dehydration (or condensation) reactions

DEHYDRATION SYNTHESIS
Most macromolecules are made from single subunits, or building
blocks, called monomers. The monomers combine with each other
via covalent bonds to form larger molecules known as polymers. In
doing so, monomers release water molecules as byproducts. This
type of reaction is known as dehydration synthesis, which means “to
put together while losing water. ” It is also considered to be a
condensation reaction since two molecules are condensed into one
larger molecule with the loss of a smaller molecule (the water.)
In a dehydration synthesis reaction between two un-ionized
monomers, such as monosaccharide sugars, the hydrogen of one
monomer combines with the hydroxyl group of another monomer, Figure 2.24.1: A dehydration synthesis reaction involving ionized
monomers.: In the dehydration synthesis reaction between two
releasing a molecule of water in the process. The removal of a amino acids, with are ionized in aqueous environments like the cell,
hydrogen from one monomer and the removal of a hydroxyl group an oxygen from the first amino acid is combined with two hydrogens
from the other monomer allows the monomers to share electrons and from the second amino acid, creating a covalent bond that links the
two monomers together to form a dipeptide. In the process a water
form a covalent bond. Thus, the monomers that are joined together molecule is formed.
are being dehydrated to allow for synthesis of a larger molecule.
As additional monomers join via multiple dehydration synthesis
reactions, the chain of repeating monomers begins to form a
polymer. Different types of monomers can combine in many
configurations, giving rise to a diverse group of macromolecules.
Three of the four major classes of biological macromolecules
(complex carbohydrates, nucleic acids, and proteins), are composed
Figure 2.24.1: A dehydration synthesis reaction involving un-
ionized moners..: In the dehydration synthesis reaction between two of monomers that join together via dehydration synthesis reactions.
molecules of glucose, a hydroxyl group from the first glucose is Complex carbohydrates are formed from monosaccharides, nucleic
combined with a hydrogen from the second glucose, creating a acids are formed from mononucleotides, and proteins are formed
covalent bond that links the two monomeric sugars
(monosaccharides) together to form the dissacharide maltose. In the from amino acids.
process, a water molecule is formed. There is great diversity in the manner by which monomers can
When the monomers are ionized, such as is the case with amino combine to form polymers. For example, glucose monomers are the
acids in an aqueous environment like cytoplasm, two hydrogens constituents of starch, glycogen, and cellulose. These three are
from the positively-charged end of one monomer are combined with polysaccharides, classified as carbohydrates, that have formed as a
an oxygen from the negatively-charged end of another monomer, result of multiple dehydration synthesis reactions between glucose
again forming water, which is released as a side-product, and again monomers. However, the manner by which glucose monomers join
joining the two monomers with a covalent bond. together, specifically locations of the covalent bonds between
connected monomers and the orientation (stereochemistry) of the
covalent bonds, results in these three different polysaccharides with
varying properties and functions. In nucleic acids and proteins, the
location and stereochemistry of the covalent linkages connecting the
monomers do not vary from molecule to molecule, but instead the
multiple kinds of monomers (five different monomers in nucleic
acids, A, G, C, T, and U mononucleotides; 21 different amino acids
monomers in proteins) are combined in a huge variety of sequences.
Each protein or nucleic acid with a different sequence is a different
molecule with different properties.

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KEY POINTS Monomers like glucose can join together in different ways and
During dehydration synthesis, either the hydrogen of one produce a variety of polymers. Monomers like mononucleotides
monomer combines with the hydroxyl group of another and amino acids join together in different sequences to produce a
monomer releasing a molecule of water, or two hydrogens from variety of polymers.
one monomer combine with one oxygen from the other monomer
KEY TERMS
releasing a molecule of water.
The monomers that are joined via dehydration synthesis covalent bond: A type of chemical bond where two atoms are
reactions share electrons and form covalent bonds with each connected to each other by the sharing of two or more electrons.
other. monomer: A relatively small molecule which can be covalently
As additional monomers join via multiple dehydration synthesis bonded to other monomers to form a polymer.
reactions, this chain of repeating monomers begins to form a
This page titled 2.24: Synthesis of Biological Macromolecules -
polymer.
Dehydration Synthesis is shared under a CC BY-SA 4.0 license and was
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examples of polymers that are formed by dehydration synthesis.

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2.25: SYNTHESIS OF BIOLOGICAL MACROMOLECULES - HYDROLYSIS
carbohydrates are broken down by amylase, sucrase, lactase, or
 LEARNING OBJECTIVES maltase. Proteins are broken down by the enzymes trypsin, pepsin,
peptidase and others. Lipids are broken down by lipases. Once the
Explain hydrolysis reactions
smaller metabolites that result from these hydrolytic enzymezes are
absorbed by cells in the body, they are further broken down by other
Polymers are broken down into monomers in a process known as
enzymes. The breakdown of these macromolecules is an overall
hydrolysis, which means “to split water,” a reaction in which a water
energy-releasing process and provides energy for cellular activities.
molecule is used during the breakdown. During these reactions, the
polymer is broken into two components. If the components are un- KEY POINTS
ionized, one part gains a hydrogen atom (H-) and the other gains a Hydrolysis reactions use water to breakdown polymers into
hydroxyl group (OH–) from a split water molecule. This is what monomers and is the opposite of dehydration synthesis, which
happens when monosaccharides are released from complex forms water when synthesizing a polymer from monomers.
carbohydrates via hydrolysis. Hydrolysis reactions break bonds and release energy.
Biological macromolecules are ingested and hydrolyzed in the
digestive tract to form smaller molecules that can be absorbed by
cells and then further broken down to release energy.

KEY TERMS
Figure 2.25.1: Hydrolysis reaction generating un-ionized products.: enzyme: a globular protein that catalyses a biological chemical
In the hydrolysis reaction shown here, the disaccharide maltose is
broken down to form two glucose monomers with the addition of a reaction
water molecule. One glucose gets a hydroxyl group at the site of the hydrolysis: A chemical process of decomposition involving the
former covalent bond, the other glucose gets a hydrogen atom. This splitting of a bond by the addition of water.
is the reverse of the dehydration synthesis reaction joining these two
monomers.
 EXERCISE 2.25.1
If the components are ionized after the split, one part gains two
hydrogen atoms and a positive charge, the other part gains an 1. What are biological macromolecules? Name the four major
oxygen atom and a negative charge. This is what happens when classes.
amino acids are released from protein chains via hydrolysis. 2. Biological macromolecules are organic. What does that
mean?
3. What are monomers? What are polymer?
4. Explain the process “dehydration synthesis.” Is there another
name for this process? Explain.
5. Explain Figure 1 in your own words.
Figure 2.25.1: Hydrolysis reaction generating ionized products.: In 6. Give an example of how condensation can form different
the hydrolysis reaction shown here, the dipeptide is broken down to
form two ionized amino acids with the addition of a water molecule.
carbohydrates.
One amino acid gets an oxygen atom and a negative charge, the 7. Explain the process of Hydrolysis.
other amino acid gets two hydrogen atoms and a positive charge. 8. Explain Figure 2 in your own words.
This is the reverse of the dehydration synthesis reaction joining
9. What role do enzymes play in hydrolysis and condensation?
these two monomers.
Explain.
These reactions are in contrast to dehydration synthesis (also known
10. In our bodies, food is hydrolyzed, or broken down into
as condensation) reactions. In dehydration synthesis reactions, a
smaller molecules. Explain why.
water molecule is formed as a result of generating a covalent bond
11. The breakdown of macromolecules provides...
between two monomeric components in a larger polymer. In
12. Create a comparison chart to indicate the enzymes that break
hydrolysis reactions, a water molecule is consumed as a result of
down carbohydrates, proteins, and lipids.
breaking the covalent bond holding together two components of a
polymer.
CONTRIBUTIONS AND ATTRIBUTIONS
Dehydration and hydrolysis reactions are chemical reactions that are
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In our bodies, food is first hydrolyzed, or broken down, into smaller An Introduction to Molecular Biology/Macromolecules and Cells. Provided by:
molecules by catalytic enzymes in the digestive tract. This allows for Wikibooks. Located at:
easy absorption of nutrients by cells in the intestine. Each en.wikibooks.org/wiki/An_Introduction_to_Molecular_Biology/Macromolec
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 CHAPTER OVERVIEW

3: BIOLOGICAL MACROMOLECULES
3.1: Carbohydrates - Carbohydrate Molecules
3.2: Carbohydrates - Importance of Carbohydrates
3.3: Lipid Molecules - Introduction
3.4: Lipid Molecules - Waxes
3.5: Lipid Molecules - Phospholipids
3.6: Lipid Molecules - Steroids
3.7: Proteins - Types and Functions of Proteins
3.8: Proteins - Amino Acids
3.9: Proteins - Protein Structure
3.10: Proteins - Denaturation and Protein Folding
3.11: Nucleic Acids - DNA and RNA
3.12: Nucleic Acids - The DNA Double Helix
3.13: Nucleic Acids - DNA Packaging
3.14: Nucleic Acids - Types of RNA

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1
3.1: CARBOHYDRATES - CARBOHYDRATE MOLECULES

 LEARNING OBJECTIVES

Describe the structure of mono-, di-, and poly-saccharides

Carbohydrates can be represented by the stoichiometric formula

(CH2O)n, where n is the number of carbons in the molecule.
Therefore, the ratio of carbon to hydrogen to oxygen is 1:2:1 in
carbohydrate molecules. The origin of the term “carbohydrate” is
based on its components: carbon (“carbo”) and water (“hydrate”).
Carbohydrates are classified into three subtypes: monosaccharides,
disaccharides, and polysaccharides.

MONOSACCHARIDES
Monosaccharides (mono- = “one”; sacchar- = “sweet”) are simple
sugars. In monosaccharides, the number of carbons usually ranges
from three to seven. If the sugar has an aldehyde group (the
functional group with the structure R-CHO), it is known as an
aldose, and if it has a ketone group (the functional group with the
structure RC(=O)R’), it is known as a ketose. Depending on the
number of carbons in the sugar, they also may be known as trioses
(three carbons), pentoses (five carbons), and or hexoses (six
carbons). Monosaccharides can exist as a linear chain or as ring-
shaped molecules; in aqueous solutions they are usually found in
ring forms.

Figure 3.1.1: Monosaccharides: Monosaccharides are classified

based on the position of their carbonyl group and the number of
carbons in the backbone. Aldoses have a carbonyl group (indicated
in green) at the end of the carbon chain, and ketoses have a carbonyl
group in the middle of the carbon chain. Trioses, pentoses, and
hexoses have three, five, and six carbon backbones, respectively.

COMMON MONOSACCHARIDES
Glucose (C6H12O6) is a common monosaccharide and an important
source of energy. During cellular respiration, energy is released from
glucose and that energy is used to help make adenosine triphosphate
(ATP). Plants synthesize glucose using carbon dioxide and water,
and glucose, in turn, is used for energy requirements for the plant.
Galactose (a milk sugar) and fructose (found in fruit) are other
common monosaccharides. Although glucose, galactose, and
fructose all have the same chemical formula (C6H12O6), they differ
structurally and stereochemically. This makes them different
molecules despite sharing the same atoms in the same proportions,
and they are all isomers of one another, or isomeric
monosaccharides. Glucose and galactose are aldoses, and fructose is
a ketose.

DISACCHARIDES
Disaccharides (di- = “two”) form when two monosaccharides
undergo a dehydration reaction (also known as a condensation
reaction or dehydration synthesis). During this process, the hydroxyl
group of one monosaccharide combines with the hydrogen of

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another monosaccharide, releasing a molecule of water and forming COMMON POLYSACCHARIDES
a covalent bond. A covalent bond formed between a carbohydrate Glycogen is the storage form of glucose in humans and other
molecule and another molecule (in this case, between two vertebrates. It is made up of monomers of glucose. Glycogen is the
monosaccharides) is known as a glycosidic bond. Glycosidic bonds animal equivalent of starch and is a highly branched molecule
(also called glycosidic linkages) can be of the alpha or the beta type. usually stored in liver and muscle cells. Whenever blood glucose
levels decrease, glycogen is broken down to release glucose in a
process known as glycogenolysis.
Cellulose is the most abundant natural biopolymer. The cell wall of
plants is mostly made of cellulose and provides structural support to
the cell. Cellulose is made up of glucose monomers that are linked
by β 1-4 glycosidic bonds. Every other glucose monomer in
cellulose is flipped over, and the monomers are packed tightly as
extended long chains. This gives cellulose its rigidity and high
tensile strength—which is so important to plant cells.

Figure 3.1.1: Disaccharides: Sucrose is formed when a monomer of Figure 3.1.1: Polysaccharides: In cellulose, glucose monomers are
glucose and a monomer of fructose are joined in a dehydration linked in unbranched chains by β 1-4 glycosidic linkages. Because
reaction to form a glycosidic bond. In the process, a water molecule of the way the glucose subunits are joined, every glucose monomer
is lost. By convention, the carbon atoms in a monosaccharide are is flipped relative to the next one resulting in a linear, fibrous
numbered from the terminal carbon closest to the carbonyl group. In structure.
sucrose, a glycosidic linkage is formed between carbon 1 in glucose
and carbon 2 in fructose. CARBOHYDRATE FUNCTION
Carbohydrates serve various functions in different animals.
COMMON DISACCHARIDES
Arthropods have an outer skeleton, the exoskeleton, which protects
Common disaccharides include lactose, maltose, and sucrose.
their internal body parts. This exoskeleton is made of chitin, which
Lactose is a disaccharide consisting of the monomers glucose and
is a polysaccharide-containing nitrogen. It is made of repeating units
galactose. It is found naturally in milk. Maltose, or malt sugar, is a
of N-acetyl-β-d-glucosamine, a modified sugar. Chitin is also a
disaccharide formed by a dehydration reaction between two glucose
major component of fungal cell walls.
molecules. The most common disaccharide is sucrose, or table sugar,
which is composed of the monomers glucose and fructose. KEY POINTS
Monosaccharides are simple sugars made up of three to seven
POLYSACCHARIDES
carbons, and they can exist as a linear chain or as ring-shaped
A long chain of monosaccharides linked by glycosidic bonds is
molecules.
known as a polysaccharide (poly- = “many”). The chain may be
Glucose, galactose, and fructose are monosaccharide isomers,
branched or unbranched, and it may contain different types of
which means they all have the same chemical formula but differ
monosaccharides. Starch, glycogen, cellulose, and chitin are primary
structurally and chemically.
examples of polysaccharides.
Disaccharides form when two monosaccharides undergo a
Plants are able to synthesize glucose, and the excess glucose is dehydration reaction (a condensation reaction); they are held
stored as starch in different plant parts, including roots and seeds. together by a covalent bond.
Starch is the stored form of sugars in plants and is made up of Sucrose (table sugar) is the most common disaccharide, which is
glucose monomers that are joined by α1-4 or 1-6 glycosidic bonds. composed of the monomers glucose and fructose.
The starch in the seeds provides food for the embryo as it germinates A polysaccharide is a long chain of monosaccharides linked by
while the starch that is consumed by humans is broken down by glycosidic bonds; the chain may be branched or unbranched and
enzymes into smaller molecules, such as maltose and glucose. The can contain many types of monosaccharides.
cells can then absorb the glucose.

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KEY TERMS biopolymer: Any macromolecule of a living organism that is
isomer: Any of two or more compounds with the same formed from the polymerization of smaller entities; a polymer
molecular formula but with different structure. that occurs in a living organism or results from life.
dehydration reaction: A chemical reaction in which two
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3.2: CARBOHYDRATES - IMPORTANCE OF CARBOHYDRATES
rich diets also have a protective role in reducing the occurrence of
 LEARNING OBJECTIVES colon cancer. In addition, a meal containing whole grains and
vegetables gives a feeling of fullness. As an immediate source of
Describe the benefits provided to organisms by
energy, glucose is broken down during the process of cellular
carbohydrates
respiration, which produces adenosine triphosphate (ATP), the
energy currency of the cell. Without the consumption of
BENEFITS OF CARBOHYDRATES carbohydrates, the availability of “instant energy” would be reduced.
Biological macromolecules are large molecules that are necessary Eliminating carbohydrates from the diet is not the best way to lose
for life and are built from smaller organic molecules. One major weight. A low-calorie diet that is rich in whole grains, fruits,
class of biological macromolecules are carbohydrates, which are vegetables, and lean meat, together with plenty of exercise and
further divided into three subtypes: monosaccharides, disaccharides, plenty of water, is the more sensible way to lose weight.
and polysaccharides. Carbohydrates are, in fact, an essential part of
our diet; grains, fruits, and vegetables are all natural sources of CONTRIBUTIONS AND ATTRIBUTIONS
carbohydrates. Importantly, carbohydrates provide energy to the dehydration reaction. Provided by: Wiktionary. Located at:
http://en.wiktionary.org/wiki/dehydration_reaction. License: CC BY-SA:
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Figure 3.2.1: Carbohydrates: Carbohydrates are biological Located at: http://cnx.org/content/m44400/latest...ol11448/latest. License: CC
macromolecules that are further divided into three subtypes: BY: Attribution
monosaccharides, disaccharides, and polysaccharides. Like all ATP. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/ATP.
License: CC BY-SA: Attribution-ShareAlike
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CARBOHYDRATES IN NUTRITION glucose. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/glucose.
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have been an important part of the human diet for thousands of
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fats to be parts of a well-balanced diet. Calorie-wise, a gram of
KEY POINTS
carbohydrate provides 4.3 Kcal. In comparison, fats provide 9
Carbohydrates provide energy to the body, particularly through glucose, a simple
Kcal/g, a less desirable ratio. Carbohydrates contain soluble and sugar that is found in many basic foods.
insoluble elements; the insoluble part is known as fiber, which is Carbohydrates contain soluble and insoluble elements; the insoluble part is
known as fiber, which promotes regular bowel movement, regulates the rate of
mostly cellulose. Fiber has many uses; it promotes regular bowel consumption of blood glucose, and also helps to remove excess cholesterol from
movement by adding bulk, and it regulates the rate of consumption the body.
As an immediate source of energy, glucose is broken down during the process of
of blood glucose. Fiber also helps to remove excess cholesterol from cellular respiration, which produces ATP, the energy currency of the cell.
the body. Fiber binds and attaches to the cholesterol in the small Since carbohydrates are an important part of the human nutrition, eliminating
them from the diet is not the best way to lose weight.
intestine and prevents the cholesterol particles from entering the
bloodstream. Then cholesterol exits the body via the feces. Fiber-

3.2.1 https://bio.libretexts.org/@go/page/12687
KEY TERMS for adenosine triphosphate.

carbohydrate: A sugar, starch, or cellulose that is a food source of energy for an

animal or plant; a saccharide.
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C6H12O6; it is a principal source of energy for cellular metabolism by Boundless.
ATP: A nucleotide that occurs in muscle tissue, and is used as a source of energy
in cellular reactions, and in the synthesis of nucleic acids. ATP is the abbreviation

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3.3: LIPID MOLECULES - INTRODUCTION
the number of hydrogens on each carbon. Stearic acid and palmitic
 LEARNING OBJECTIVES acid, which are commonly found in meat, are examples of saturated
fats.
Differentiate between saturated and unsaturated fatty acids
When the hydrocarbon chain contains a double bond, the fatty acid
is said to be unsaturated. Oleic acid is an example of an unsaturated
GLYCEROL AND FATTY ACIDS
fatty acid. Most unsaturated fats are liquid at room temperature and
A fat molecule consists of two main components: glycerol and fatty are called oils. If there is only one double bond in the molecule, then
acids. Glycerol is an alcohol with three carbons, five hydrogens, and it is known as a monounsaturated fat; e.g. olive oil. If there is more
three hydroxyl (OH) groups. Fatty acids have a long chain of than one double bond, then it is known as a polyunsaturated fat; e.g.
hydrocarbons with a carboxyl group attached and may have 4-36 canola oil. Unsaturated fats help to lower blood cholesterol levels
carbons; however, most of them have 12-18. In a fat molecule, the whereas saturated fats contribute to plaque formation in the arteries.
fatty acids are attached to each of the three carbons of the glycerol
Unsaturated fats or oils are usually of plant origin and contain cis
molecule with an ester bond through the oxygen atom. During the
unsaturated fatty acids. Cis and trans indicate the configuration of
ester bond formation, three molecules are released. Since fats consist
the molecule around the double bond. If hydrogens are present in the
of three fatty acids and a glycerol, they are also called
same plane, it is referred to as a cis fat; if the hydrogen atoms are on
triacylglycerols or triglycerides.
two different planes, it is referred to as a trans fat. The cis double
bond causes a bend or a “kink” that prevents the fatty acids from
packing tightly, keeping them liquid at room temperature.

Figure 3.3.1: Fatty Acids: Saturated fatty acids have hydrocarbon

chains connected by single bonds only. Unsaturated fatty acids have
one or more double bonds. Each double bond may be in a cis or
trans configuration. In the cis configuration, both hydrogens are on
the same side of the hydrocarbon chain. In the trans configuration,
Figure 3.3.1: Triacylglycerols: Triacylglycerol is formed by the the hydrogens are on opposite sides. A cis double bond causes a kink
joining of three fatty acids to a glycerol backbone in a dehydration in the chain.
reaction. Three molecules of water are released in the process.
TRANS FATS
SATURATED VS. UNSATURATED FATTY ACIDS In the food industry, oils are artificially hydrogenated to make them
semi-solid and of a consistency desirable for many processed food
Fatty acids may be saturated or unsaturated. In a fatty acid chain, if products. During this hydrogenation process, gas is bubbled through
there are only single bonds between neighboring carbons in the oils to solidify them, and the double bonds of the cis-conformation
hydrocarbon chain, the fatty acid is said to be saturated. Saturated in the hydrocarbon chain may be converted to double bonds in the
fatty acids are saturated with hydrogen since single bonds increase trans-conformation.

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Margarine, some types of peanut butter, and shortening are examples KEY POINTS
of artificially-hydrogenated trans fats. Recent studies have shown Fats provide energy, insulation, and storage of fatty acids for
that an increase in trans fats in the human diet may lead to an many organisms.
increase in levels of low-density lipoproteins (LDL), or “bad” Fats may be saturated (having single bonds) or unsaturated
cholesterol, which in turn may lead to plaque deposition in the (having double bonds).
arteries, resulting in heart disease. Many fast food restaurants have Unsaturated fats may be cis (hydrogens in same plane) or trans
recently banned the use of trans fats, and food labels are required to (hydrogens in two different planes).
display the trans fat content. Olive oil, a monounsaturated fat, has a single double bond
whereas canola oil, a polyunsaturated fat, has more than one
ESSENTIAL FATTY ACIDS
double bond.
Essential fatty acids are fatty acids required for biological processes, Omega-3 fatty acid and omega-6 fatty acid are essential for
but not synthesized by the human body. Consequently, they have to human biological processes, but they must be ingested in the diet
be supplemented through ingestion via the diet and are nutritionally because they cannot be synthesized.
very important. Omega-3 fatty acid, or alpha-linoleic acid (ALA),
falls into this category and is one of only two fatty acids known to KEY TERMS
be essential for humans (the other being omega-6 fatty acid, or hydrogenation: The chemical reaction of hydrogen with another
linoleic acid). These polyunsaturated fatty acids are called omega-3 substance, especially with an unsaturated organic compound, and
because the third carbon from the end of the hydrocarbon chain is usually under the influence of temperature, pressure and
connected to its neighboring carbon by a double bond. Salmon, catalysts.
trout, and tuna are good sources of omega-3 fatty acids. ester: Compound most often formed by the condensation of an
Research indicates that omega-3 fatty acids reduce the risk of alcohol and an acid, by removing water. It contains the functional
sudden death from heart attacks, reduce triglycerides in the blood, group carbon-oxygen double bond joined via carbon to another
lower blood pressure, and prevent thrombosis by inhibiting blood oxygen atom.
clotting. They also reduce inflammation and may help reduce the carboxyl: A univalent functional group consisting of a carbonyl
risk of some cancers in animals. and a hydroxyl functional group (-CO.OH); characteristic of
carboxylic acids.
Fats have important functions, and many vitamins are fat soluble.
Fats serve as a long-term storage form of fatty acids and act as a
source of energy. They also provide insulation for the body.

This page titled 3.3: Lipid Molecules - Introduction is shared under a CC

BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

Figure 3.3.1: Omega Fatty Acids: Alpha-linolenic acid is an

example of an omega-3 fatty acid. It has three cis double bonds and,
as a result, a curved shape. For clarity, the carbons are not shown.
Each singly bonded carbon has two hydrogens associated with it,
also not shown.

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3.4: LIPID MOLECULES - WAXES
Unlike most natural waxes, which are esters, synthetic waxes consist
 LEARNING OBJECTIVES of long-chain hydrocarbons lacking functional groups. Paraffin wax
is a type of synthetic wax derived from petroleum and refined by
Describe the roles played by waxes
vacuum distillation. Synthetic waxes may also be obtained from
polyethylene. Millions of of these waxes are produced annually, and
WAXES they are used in adhesives, cosmetics, sealants and lubricants,
Waxes are a type of long chain nonpolar lipid. Natural waxes are insecticides, and UV protection. They are also used in foods like
typically esters of fatty acids and long chain alcohols. Waxes are chewing gum.
synthesized by many animals and plants. Animal wax esters are
typically derived from a variety of carboxylic acids and fatty
alcohols. The composition of a wax depends not only on the species,
but also on the geographic location of the organism. The best known
animal wax is beeswax, but other insects secrete waxes as well. A Figure 3.4.1: Generic structure formula of bee waxes: Ester myricyl
palmitate is a major component of beeswax.
major component of beeswax is the ester myricyl palmitate, which
bees use for constructing honeycombs. Spermaceti is also a wax that KEY POINTS
occurs in large amounts in the oil of a sperm whale’s head. One of
Natural waxes are typically esters of fatty acids and long chain
its main constituents is cetyl palmitate, an ester of a fatty acid and
alcohols.
fatty alcohol. Plant waxes are derived from mixtures of long-chain
Animal wax esters are derived from a variety of carboxylic acids
hydrocarbons containing functional groups such as alkanes, fatty
and fatty alcohols.
acids, alcohols, diols, ketones, and aldehydes. Plants also use waxes
Plant waxes are derived from mixtures of long-chain
as a protective coating to control evaporation and hydration and to
hydrocarbons containing functional groups.
prevent them from drying out. Waxes are valuable to both plants and
Because of their hydrophobic nature, waxes prevent water from
animals because of their hydrophobic nature. This makes them water
sticking on plants and animals.
resistant, which prevents water from sticking on surfaces.
Synthetic waxes are derived from petroleum or polyethylene and
consist of long-chain hydrocarbons that lack functional groups.
Synthetic and waxes are used in adhesives, cosmetics, food, and
many other commercial products.

KEY TERMS
paraffin wax: A waxy white solid hydrocarbon mixture used to
make candles, wax paper, lubricants, and sealing materials.
polyethylene: A polymer consisting of many ethylene monomers
bonded together; used for kitchenware, containers etc.

This page titled 3.4: Lipid Molecules - Waxes is shared under a CC BY-SA
4.0 license and was authored, remixed, and/or curated by Boundless.

Figure 3.4.1: Plant Waxes: Waxy coverings on some leaves are used
as protective coatings.

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3.5: LIPID MOLECULES - PHOSPHOLIPIDS
The lipid tails, on the other hand, are uncharged, nonpolar, and
 LEARNING OBJECTIVES hydrophobic, or “water fearing.” A hydrophobic molecule repels and
is repelled by water. Some lipid tails consist of saturated fatty acids
Describe phospholipids and their role in cells
and some contain unsaturated fatty acids. This combination adds to
the fluidity of the tails that are constantly in motion.
DEFINING CHARACTERISTICS OF
PHOSPHOLIPIDS PHOSPHOLIPIDS AND BIOLOGICAL MEMBRANES
Phospholipids are major components of the plasma membrane, the
outermost layer of animal cells. Like fats, they are composed of fatty The cell membrane consists of two adjacent layers of phospholipids,
acid chains attached to a glycerol backbone. Unlike triglycerides, which form a bilayer. The fatty acid tails of phospholipids face
which have three fatty acids, phospholipids have two fatty acids that inside, away from water, whereas the phosphate heads face the
help form a diacylglycerol. The third carbon of the glycerol outward aqueous side. Since the heads face outward, one layer is
backbone is also occupied by a modified phosphate group. However, exposed to the interior of the cell and one layer is exposed to the
just a phosphate group attached to a diacylglycerol does not qualify exterior. As the phosphate groups are polar and hydrophilic, they are
as a phospholipid. This would be considered a phosphatidate attracted to water in the intracellular fluid.
(diacylglycerol 3-phosphate), the precursor to phospholipids. To
qualify as a phospholipid, the phosphate group should be modified
by an alcohol. Phosphatidylcholine and phosphatidylserine are
examples of two important phospholipids that are found in plasma
membranes.

Figure 3.5.1: Phospholipid Bilayer: The phospholipid bilayer

consists of two adjacent sheets of phospholipids, arranged tail to tail.
The hydrophobic tails associate with one another, forming the
interior of the membrane. The polar heads contact the fluid inside
and outside of the cell.
Because of the phospholipds’ chemical and physical characteristics,
the lipid bilayer acts as a semipermeable membrane; only lipophilic
solutes can easily pass the phospholipd bilayer. As a result, there are
two distinct aqueous compartments on each side of the membrane.
This separation is essential for many biological functions, including
cell communication and metabolism.

MEMBRANE FLUIDITY
A cell’s plasma membrane contain proteins and other lipids (such as
cholesterol) within the phospholipid bilayer. Biological membranes
Figure 3.5.1: Phospholipid Molecule: A phospholipid is a molecule
with two fatty acids and a modified phosphate group attached to a remain fluid because of the unsaturated hydrophobic tails, which
glycerol backbone. The phosphate may be modified by the addition prevent phospholipid molecules from packing together and forming
of charged or polar chemical groups. Two chemical groups that may a solid.
modify the phosphate, choline and serine, are shown here. Both
choline and serine attach to the phosphate group at the position If a drop of phospholipids is placed in water, the phospholipids
labeled R via the hydroxyl group indicated in green. spontaneously form a structure known as a micelle, with their
hydrophilic heads oriented toward the water. Micelles are lipid
STRUCTURE OF A PHOSPHOLIPID MOLECULE
molecules that arrange themselves in a spherical form in aqueous
solution. The formation of a micelle is a response to the amphipathic
A phospholipid is an amphipathic molecule which means it has both
nature of fatty acids, meaning that they contain both hydrophilic and
a hydrophobic and a hydrophilic component. A single phospholipid hydrophobic regions.
molecule has a phosphate group on one end, called the “head,” and
two side-by-side chains of fatty acids that make up the lipid “tails. ”
The phosphate group is negatively charged, making the head polar
and hydrophilic, or “water loving.” The phosphate heads are thus
attracted to the water molecules in their environment.

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 The fatty acid chains are the uncharged, nonpolar tails, which are
hydrophobic.
Since the tails are hydrophobic, they face the inside, away from
the water and meet in the inner region of the membrane.
Since the heads are hydrophilic, they face outward and are
attracted to the intracellular and extracellular fluid.
If phospholipids are placed in water, they form into micelles,
which are lipid molecules that arrange themselves in a spherical
form in aqueous solutions.
Figure 3.5.1: Micelles: An example of micelles in water.
KEY TERMS
KEY POINTS
micelle: Lipid molecules that arrange themselves in a spherical
Phospholipids consist of a glycerol molecule, two fatty acids, form in aqueous solutions.
and a phosphate group that is modified by an alcohol. amphipathic: Describing a molecule, such as a detergent, which
The phosphate group is the negatively-charged polar head, which has both hydrophobic and hydrophilic groups.
is hydrophilic.
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BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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3.6: LIPID MOLECULES - STEROIDS
It has also been discovered that steroids can be active in the brain
 LEARNING OBJECTIVES where they affect the nervous system, These neurosteroids alter
electrical activity in the brain. They can either activate or tone down
Describe some functions of steroids
receptors that communicate messages from neurotransmitters. Since
these neurosteroids can tone down receptors and decrease brain
STRUCTURE OF STEROID MOLECULES activity, steroids are often used in anesthetic medicines.
Unlike phospholipids and fats, steroids have a fused ring structure.
Although they do not resemble the other lipids, they are grouped CONTRIBUTIONS AND ATTRIBUTIONS
with them because they are also hydrophobic and insoluble in water. OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44401/latest...ol11448/latest. License: CC
All steroids have four linked carbon rings, and many of them, like BY: Attribution
cholesterol, have a short tail. Many steroids also have the –OH hydrogenation. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/hydrogenation. License: CC BY-SA: Attribution-
functional group, and these steroids are classified as alcohols called ShareAlike
sterols. ester. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/ester.
License: CC BY-SA: Attribution-ShareAlike
carboxyl. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/carboxyl. License: CC BY-SA: Attribution-ShareAlike
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CC BY: Attribution
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44401/latest...ol11448/latest. License: CC
BY: Attribution
Principles of Biochemistry/Lipids. Provided by: Wikibooks. Located at:
en.wikibooks.org/wiki/Princip...y/Lipids%23WAX. License: CC BY-SA:
Attribution-ShareAlike
Structural Biochemistry/Lipids/Waxes, Soaps, and Detergents. Provided by:
Wikibooks. Located at: en.wikibooks.org/wiki/Structu...rgents%23Waxes.
License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Biological Molecules. October 22, 2013. Provided by:
OpenStax CNX. Located at: http://cnx.org/content/m45426/latest/. License:
CC BY: Attribution
polyethylene. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/polyethylene. License: CC BY-SA: Attribution-
ShareAlike
paraffin wax. Provided by: Wiktionary. Located at:
Figure 3.6.1: Steroid Structures: Steroids, such as cholesterol and en.wiktionary.org/wiki/paraffin_wax. License: CC BY-SA: Attribution-
ShareAlike
cortisol, are composed of four fused hydrocarbon rings.
OpenStax College, Lipids. October 16, 2013. Provided by: OpenStax CNX.
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CHOLESTEROL CC BY: Attribution
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Cholesterol is the most common steroid and is mainly synthesized in Located at: http://cnx.org/content/m44401/latest...e_03_03_07.jpg. License:
the liver; it is the precursor to vitamin D. Cholesterol is also a CC BY: Attribution
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precursor to many important steroid hormones like estrogen,
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testosterone, and progesterone, which are secreted by the gonads and CC BY: Attribution
endocrine glands. Therefore, steroids play very important roles in OpenStax College, Lipids. October 16, 2013. Provided by: OpenStax CNX.
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the body’s reproductive system. Cholesterol also plays a role in CC BY: Attribution
synthesizing the steroid hormones aldosterone, which is used for Wachs - Wax. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/Fi...achs_-_Wax.svg. License: Public Domain:
osmoregulation, and cortisol, which plays a role in metabolism. No Known Copyright
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Cholesterol is also the precursor to bile salts, which help in the Located at: http://cnx.org/content/m44401/latest...ol11448/latest. License: CC
emulsification of fats and their absorption by cells. It is a component BY: Attribution
OpenStax College, The Cell Membrane. October 22, 2013. Provided by:
of the plasma membrane of animal cells and the phospholipid
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bilayer. Being the outermost structure in animal cells, the plasma CC BY: Attribution
membrane is responsible for the transport of materials and cellular Structural Biochemistry/Lipids/Micelles. Provided by: Wikibooks. Located at:
en.wikibooks.org/wiki/Structu...ipids/Micelles. License: CC BY-SA:
recognition; and it is involved in cell-to-cell communication. Thus, Attribution-ShareAlike
steroids also play an important role in the structure and function of Boundless. Provided by: Boundless Learning. Located at:
www.boundless.com//biology/definition/micelle. License: CC BY-SA:
membranes. Attribution-ShareAlike
amphipathic. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/amphipathic. License: CC BY-SA: Attribution-

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CC BY: Attribution License: CC BY: Attribution
OpenStax College, Lipids. October 16, 2013. Provided by: OpenStax CNX. Wachs - Wax. Provided by: Wikimedia. Located at:
Located at: http://cnx.org/content/m44401/latest...e_03_03_05.jpg. License: commons.wikimedia.org/wiki/File:Wachs_-_Wax.svg. License: Public
CC BY: Attribution Domain: No Known Copyright
OpenStax College, Lipids. October 16, 2013. Provided by: OpenStax CNX. Structural Biochemistry/Lipids/Micelles. Provided by: Wikibooks. Located at:
Located at: http://cnx.org/content/m44401/latest...e_03_03_11.jpg. License: en.wikibooks.org/wiki/Structural_Biochemistry/Lipids/Micelles. License: CC
CC BY: Attribution BY-SA: Attribution-ShareAlike
Wachs - Wax. Provided by: Wikimedia. Located at: OpenStax College, Lipids. October 16, 2013. Provided by: OpenStax CNX.
commons.wikimedia.org/wiki/Fi...achs_-_Wax.svg. License: Public Domain: Located at: http://cnx.org/content/m44401/latest/Figure_05_01_03a.jpg.
No Known Copyright License: CC BY: Attribution
Structural Biochemistry/Lipids/Micelles. Provided by: Wikibooks. Located at: OpenStax College, The Cell Membrane. October 22, 2013. Provided by:
en.wikibooks.org/wiki/Structural_Biochemistry/Lipids/Micelles. License: CC OpenStax CNX. Located at: http://cnx.org/content/m46021/latest/. License:
BY-SA: Attribution-ShareAlike CC BY: Attribution
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Located at: http://cnx.org/content/m44401/latest..._05_01_03a.jpg. License: Located at: http://cnx.org/content/m44401/latest/Figure_03_03_10.jpg.
CC BY: Attribution License: CC BY: Attribution
OpenStax College, The Cell Membrane. October 22, 2013. Provided by:
OpenStax CNX. Located at: http://cnx.org/content/m46021/latest/. License: KEY POINTS
CC BY: Attribution
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. Steroids are lipids because they are hydrophobic and insoluble in water, but they
Located at: http://cnx.org/content/m44401/latest...ol11448/latest. License: CC do not resemble lipids since they have a structure composed of four fused rings.
BY: Attribution Cholesterol is the most common steroid and is the precursor to vitamin D,
OpenStax College, Hormones. October 22, 2013. Provided by: OpenStax CNX. testosterone, estrogen, progesterone, aldosterone, cortisol, and bile salts.
Located at: http://cnx.org/content/m46667/latest/. License: CC BY: Cholesterol is a component of the phospholipid bilayer and plays a role in the
Attribution structure and function of membranes.
Metabolomics/Metabolites/Lipids/Steroids. Provided by: Wikibooks. Located Steroids are found in the brain and alter electrical activity in the brain.
at: en.wikibooks.org/wiki/Metabol...ipids/Steroids. License: CC BY-SA: Because they can tone down receptors that communicate messages from
Attribution-ShareAlike neurotransmitters, steroids are often used in anesthetic medicines.
hormone. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/hormone. License: CC BY-SA: Attribution-ShareAlike KEY TERMS
osmoregulation. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/osmoregulation. License: CC BY-SA: Attribution- neurotransmitter: any substance, such as acetylcholine or dopamine, responsible
ShareAlike for sending nerve signals across a synapse between two neurons
neurotransmitter. Provided by: Wiktionary. Located at: osmoregulation: the homeostatic regulation of osmotic pressure in the body in
en.wiktionary.org/wiki/neurotransmitter. License: CC BY-SA: Attribution- order to maintain a constant water content
ShareAlike hormone: any substance produced by one tissue and conveyed by the
OpenStax College, Lipids. October 16, 2013. Provided by: OpenStax CNX. bloodstream to another to affect physiological activity
Located at: http://cnx.org/content/m44401/latest...e_03_03_02.jpg. License:
CC BY: Attribution This page titled 3.6: Lipid Molecules - Steroids is shared under a CC BY-SA
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3.7: PROTEINS - TYPES AND FUNCTIONS OF PROTEINS
Because form determines function, any slight change to a protein’s
 LEARNING OBJECTIVES shape may cause the protein to become dysfunctional. Small
changes in the amino acid sequence of a protein can cause
Differentiate among the types and functions of proteins
devastating genetic diseases such as Huntington’s disease or sickle
cell anemia.
TYPES AND FUNCTIONS OF PROTEINS
Proteins perform essential functions throughout the systems of the ENZYMES
human body. These long chains of amino acids are critically Enzymes are proteins that catalyze biochemical reactions, which
important for: otherwise would not take place. These enzymes are essential for
catalyzing chemical reactions chemical processes like digestion and cellular metabolism. Without
synthesizing and repairing DNA enzymes, most physiological processes would proceed so slowly (or
transporting materials across the cell not at all) that life could not exist.
receiving and sending chemical signals Because form determines function, each enzyme is specific to its
responding to stimuli substrates. The substrates are the reactants that undergo the chemical
providing structural support reaction catalyzed by the enzyme. The location where substrates
bind to or interact with the enzyme is known as the active site,
Proteins (a polymer) are macromolecules composed of amino acid
because that is the site where the chemistry occurs. When the
subunits (the monomers ). These amino acids are covalently attached
substrate binds to its active site at the enzyme, the enzyme may help
to one another to form long linear chains called polypeptides, which
in its breakdown, rearrangement, or synthesis. By placing the
then fold into a specific three-dimensional shape. Sometimes these
substrate into a specific shape and microenvironment in the active
folded polypeptide chains are functional by themselves. Other times
site, the enzyme encourages the chemical reaction to occur. There
they combine with additional polypeptide chains to form the final
are two basic classes of enzymes:
protein structure. Sometimes non-polypeptide groups are also
required in the final protein. For instance, the blood protein Substrate
Enzyme changes shape Products
slightly as substrate binds
hemogobin is made up of four polypeptide chains, each of which Active site

also contains a heme molecule, which is ring structure with an iron

atom in its center.
Proteins have different shapes and molecular weights, depending on
the amino acid sequence. For example, hemoglobin is a globular
protein, which means it folds into a compact globe-like structure, but Substrate entering Enzyme/substrate Enzyme/products Products leaving
active site of enzyme complex complex active site of enzyme
collagen, found in our skin, is a fibrous protein, which means it folds
into a long extended fiber-like chain. You probably look similar to Figure 3.7.1: Enzyme reaction: A catabolic enzyme reaction
showing the substrate matching the exact shape of the active site.
your family members because you share similar proteins, but you
look different from strangers because the proteins in your eyes, hair, Catabolic enzymes: enzymes that break down their substrate
and the rest of your body are different. Anabolic enzymes: enzymes that build more complex molecules
from their substrates
Enzymes are essential for digestion: the process of breaking larger
food molecules down into subunits small enough to diffuse through
a cell membrane and to be used by the cell. These enzymes include
amylase, which catalyzes the digestion carbohydrates in the mouth
and small intestine; pepsin, which catalyzes the digestion of proteins
in the stomach; lipase, which catalyzes reactions need to emulsify
fats in the small intestine; and trypsin, which catalyzes the further
digestion of proteins in the small intestine.
Enzymes are also essential for biosynthesis: the process of making
new, complex molecules from the smaller subunits that are provided
to or generated by the cell. These biosynthetic enzymes include
DNA Polymerase, which catalyzes the synthesis of new strands of
the genetic material before cell division; fatty acid synthetase, which
the synthesis of new fatty acids for fat or membrane lipid formation;
and components of the ribosome, which catalyzes the formation of
Figure 3.7.1: Human Hemoglobin: Structure of human hemoglobin.
The proteins’ α and β subunits are in red and blue, and the iron- new polypeptides from amino acid monomers.
containing heme groups in green. From the protein data base.

3.7.1 https://bio.libretexts.org/@go/page/12698
HORMONES KEY POINTS
Some proteins function as chemical-signaling molecules called Proteins are essential for the main physiological processes of life
hormones. These proteins are secreted by endocrine cells that act to and perform functions in every system of the human body.
control or regulate specific physiological processes, which include A protein’s shape determines its function.
growth, development, metabolism, and reproduction. For example, Proteins are composed of amino acid subunits that form
insulin is a protein hormone that helps to regulate blood glucose polypeptide chains.
levels. Other proteins act as receptors to detect the concentrations of Enzymes catalyze biochemical reactions by speeding up
chemicals and send signals to respond. Some types of hormones, chemical reactions, and can either break down their substrate or
such as estrogen and testosterone, are lipid steroids, not proteins. build larger molecules from their substrate.
The shape of an enzyme’s active site matches the shape of the
OTHER PROTEIN FUNCTIONS substrate.
Proteins perform essential functions throughout the systems of the Hormones are a type of protein used for cell signaling and
human body. In the respiratory system, hemoglobin (composed of communication.
four protein subunits) transports oxygen for use in cellular
metabolism. Additional proteins in the blood plasma and lymph KEY TERMS
carry nutrients and metabolic waste products throughout the body. amino acid: Any of 20 naturally occurring α-amino acids
The proteins actin and tubulin form cellular structures, while keratin (having the amino, and carboxylic acid groups on the same
forms the structural support for the dead cells that become carbon atom), and a variety of side chains, that combine, via
fingernails and hair. Antibodies, also called immunoglobins, help peptide bonds, to form proteins.
recognize and destroy foreign pathogens in the immune system. polypeptide: Any polymer of (same or different) amino acids
Actin and myosin allow muscles to contract, while albumin joined via peptide bonds.
nourishes the early development of an embryo or a seedling. catalyze: To accelerate a process.

This page titled 3.7: Proteins - Types and Functions of Proteins is shared
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

Figure 3.7.1: Tubulin: The structural protein tubulin stained red in

mouse cells.

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3.8: PROTEINS - AMINO ACIDS

 LEARNING OBJECTIVES

Describe the structure of an amino acid and the features that

confer its specific properties

STRUCTURE OF AN AMINO ACID

Amino acids are the monomers that make up proteins. Each amino
acid has the same fundamental structure, which consists of a central
carbon atom, also known as the alpha (α) carbon, bonded to an
amino group (NH2), a carboxyl group (COOH), and to a hydrogen
atom. In the aqueous environment of the cell, the both the amino
group and the carboxyl group are ionized under physiological
conditions, and so have the structures -NH3+ and -COO–,
respectively. Every amino acid also has another atom or group of
atoms bonded to the central atom known as the R group. This R
group, or side chain, gives each amino acid proteins specific
characteristics, including size, polarity, and pH.

Figure 3.8.1: Types of amino acids: There are 21 common amino

acids commonly found in proteins, each with a different R group
(variant group) that determines its chemical nature. The 21st amino
acid, not shown here, is selenocysteine, with an R group of -CH2-
SeH.

CHARACTERISTICS OF AMINO ACIDS

Which categories of amino acid would you expect to find on the
surface of a soluble protein, and which would you expect to find in
the interior? What distribution of amino acids would you expect to
find in a protein embedded in a lipid bilayer?
The chemical composition of the side chain determines the
characteristics of the amino acid. Amino acids such as valine,
Figure 3.8.1: Amino acid structure: Amino acids have a central methionine, and alanine are nonpolar (hydrophobic), while amino
asymmetric carbon to which an amino group, a carboxyl group, a acids such as serine, threonine, and cysteine are polar (hydrophilic).
hydrogen atom, and a side chain (R group) are attached. This amino
acid is unionized, but if it were placed in water at pH 7, its amino The side chains of lysine and arginine are positively charged so
group would pick up another hydrogen and a positive charge, and these amino acids are also known as basic (high pH) amino acids.
the hydroxyl in its carboxyl group would lose and a hydrogen and Proline is an exception to the standard structure of an amino acid
gain a negative charge.
because its R group is linked to the amino group, forming a ring-like
TYPES OF AMINO ACIDS structure.
The name “amino acid” is derived from the amino group and Amino acids are represented by a single upper case letter or a three-
carboxyl-acid-group in their basic structure. There are 21 amino letter abbreviation. For example, valine is known by the letter V or
acids present in proteins, each with a specific R group or side chain. the three-letter symbol val.
Ten of these are considered essential amino acids in humans because
the human body cannot produce them and they must be obtained PEPTIDE BONDS
from the diet. All organisms have different essential amino acids The sequence and the number of amino acids ultimately determine
based on their physiology. the protein’s shape, size, and function. Each amino acid is attached
to another amino acid by a covalent bond, known as a peptide bond.
When two amino acids are covalently attached by a peptide bond,
the carboxyl group of one amino acid and the amino group of the
incoming amino acid combine and release a molecule of water. Any
reaction that combines two monomers in a reaction that generates
H2O as one of the products is known as a dehydration reaction, so
peptide bond formation is an example of a dehydration reaction.

3.8.1 https://bio.libretexts.org/@go/page/12699
 acids, whereas the term protein is used for a polypeptide or
polypeptides that have folded properly, combined with any
additional components needed for proper functioning, and is now
functional.

KEY POINTS
Each amino acid contains a central C atom, an amino group
(NH2), a carboxyl group (COOH), and a specific R group.
The R group determines the characteristics (size, polarity, and
pH) for each type of amino acid.
Peptide bonds form between the carboxyl group of one amino
acid and the amino group of another through dehydration
Figure 3.8.1: Peptide bond formation: Peptide bond formation is a synthesis.
dehydration synthesis reaction. The carboxyl group of one amino A chain of amino acids is a polypeptide.
acid is linked to the amino group of the incoming amino acid. In the
process, a molecule of water is released.
KEY TERMS
POLYPEPTIDE CHAINS amino acid: Any of 20 naturally occurring α-amino acids
The resulting chain of amino acids is called a polypeptide chain. (having the amino, and carboxylic acid groups on the same
Each polypeptide has a free amino group at one end. This end is carbon atom), and a variety of side chains, that combine, via
called the N terminal, or the amino terminal, and the other end has a peptide bonds, to form proteins.
free carboxyl group, also known as the C or carboxyl terminal. R group: The R group is a side chain specific to each amino acid
When reading or reporting the amino acid sequence of a protein or that confers particular chemical properties to that amino acid.
polypeptide, the convention is to use the N-to-C direction. That is, polypeptide: Any polymer of (same or different) amino acids
the first amino acid in the sequence is assumed to the be one at the N joined via peptide bonds.
terminal and the last amino acid is assumed to be the one at the C
This page titled 3.8: Proteins - Amino Acids is shared under a CC BY-SA
terminal.
4.0 license and was authored, remixed, and/or curated by Boundless.
Although the terms polypeptide and protein are sometimes used
interchangeably, a polypeptide is technically any polymer of amino

3.8.2 https://bio.libretexts.org/@go/page/12699
3.9: PROTEINS - PROTEIN STRUCTURE
with one another, forming long fibers made from millions of
 LEARNING OBJECTIVES aggregated hemoglobins that distort the red blood cells into crescent
or “sickle” shapes, which clog arteries. People affected by the
Summarize the four levels of protein structure
disease often experience breathlessness, dizziness, headaches, and
abdominal pain.
The shape of a protein is critical to its function because it determines
whether the protein can interact with other molecules. Protein
structures are very complex, and researchers have only very recently
been able to easily and quickly determine the structure of complete
proteins down to the atomic level. (The techniques used date back to
the 1950s, but until recently they were very slow and laborious to
use, so complete protein structures were very slow to be solved.)
Early structural biochemists conceptually divided protein structures
into four “levels” to make it easier to get a handle on the complexity
of the overall structures. To determine how the protein gets its final
shape or conformation, we need to understand these four levels of
protein structure: primary, secondary, tertiary, and quaternary.

PRIMARY STRUCTURE
Figure 3.9.1: Sickle cell disease: Sickle cells are crescent shaped,
A protein’s primary structure is the unique sequence of amino acids while normal cells are disc-shaped.
in each polypeptide chain that makes up the protein. Really, this is
just a list of which amino acids appear in which order in a SECONDARY STRUCTURE
polypeptide chain, not really a structure. But, because the final A protein’s secondary structure is whatever regular structures arise
protein structure ultimately depends on this sequence, this was from interactions between neighboring or near-by amino acids as the
called the primary structure of the polypeptide chain. For example, polypeptide starts to fold into its functional three-dimensional form.
the pancreatic hormone insulin has two polypeptide chains, A and B. Secondary structures arise as H bonds form between local groups of
amino acids in a region of the polypeptide chain. Rarely does a
single secondary structure extend throughout the polypeptide chain.
It is usually just in a section of the chain. The most common forms
of secondary structure are the α-helix and β-pleated sheet structures
and they play an important structural role in most globular and
fibrous proteins.

Figure 3.9.1: Primary structure: The A chain of insulin is 21 amino

acids long and the B chain is 30 amino acids long, and each
sequence is unique to the insulin protein.
The gene, or sequence of DNA, ultimately determines the unique
sequence of amino acids in each peptide chain. A change in
nucleotide sequence of the gene’s coding region may lead to a
different amino acid being added to the growing polypeptide chain,
causing a change in protein structure and therefore function.
The oxygen-transport protein hemoglobin consists of four
polypeptide chains, two identical α chains and two identical β
chains. In sickle cell anemia, a single amino substitution in the
hemoglobin β chain causes a change the structure of the entire
Figure 3.9.1: Secondary structure: The α-helix and β-pleated sheet
protein. When the amino acid glutamic acid is replaced by valine in
form because of hydrogen bonding between carbonyl and amino
the β chain, the polypeptide folds into an slightly-different shape that groups in the peptide backbone. Certain amino acids have a
creates a dysfunctional hemoglobin protein. So, just one amino acid propensity to form an α-helix, while others have a propensity to
form a β-pleated sheet.
substitution can cause dramatic changes. These dysfunctional
hemoglobin proteins, under low-oxygen conditions, start associating In the α-helix chain, the hydrogen bond forms between the oxygen
atom in the polypeptide backbone carbonyl group in one amino acid

3.9.1 https://bio.libretexts.org/@go/page/12700
and the hydrogen atom in the polypeptide backbone amino group of made from more than one polypeptide chain. Proteins made from a
another amino acid that is four amino acids farther along the chain. single polypeptide will not have a quaternary structure.
This holds the stretch of amino acids in a right-handed coil. Every In proteins with more than one subunit, weak interactions between
helical turn in an alpha helix has 3.6 amino acid residues. The R the subunits help to stabilize the overall structure. Enzymes often
groups (the side chains) of the polypeptide protrude out from the α- play key roles in bonding subunits to form the final, functioning
helix chain and are not involved in the H bonds that maintain the α- protein.
helix structure.
For example, insulin is a ball-shaped, globular protein that contains
In β-pleated sheets, stretches of amino acids are held in an almost both hydrogen bonds and disulfide bonds that hold its two
fully-extended conformation that “pleats” or zig-zags due to the non- polypeptide chains together. Silk is a fibrous protein that results
linear nature of single C-C and C-N covalent bonds. β-pleated sheets from hydrogen bonding between different β-pleated chains.
never occur alone. They have to held in place by other β-pleated
sheets. The stretches of amino acids in β-pleated sheets are held in
their pleated sheet structure because hydrogen bonds form between
the oxygen atom in a polypeptide backbone carbonyl group of one
β-pleated sheet and the hydrogen atom in a polypeptide backbone
amino group of another β-pleated sheet. The β-pleated sheets which
hold each other together align parallel or antiparallel to each other.
The R groups of the amino acids in a β-pleated sheet point out
perpendicular to the hydrogen bonds holding the β-pleated sheets
together, and are not involved in maintaining the β-pleated sheet
structure.

TERTIARY STRUCTURE
The tertiary structure of a polypeptide chain is its overall three-
dimensional shape, once all the secondary structure elements have
folded together among each other. Interactions between polar,
nonpolar, acidic, and basic R group within the polypeptide chain
create the complex three-dimensional tertiary structure of a protein.
When protein folding takes place in the aqueous environment of the
body, the hydrophobic R groups of nonpolar amino acids mostly lie
in the interior of the protein, while the hydrophilic R groups lie
mostly on the outside. Cysteine side chains form disulfide linkages
in the presence of oxygen, the only covalent bond forming during
protein folding. All of these interactions, weak and strong, determine
the final three-dimensional shape of the protein. When a protein
loses its three-dimensional shape, it will no longer be functional.

Figure 3.9.1: Four levels of protein structure: The four levels of

protein structure can be observed in these illustrations.

KEY POINTS
Protein structure depends on its amino acid sequence and local,
low-energy chemical bonds between atoms in both the
polypeptide backbone and in amino acid side chains.
Protein structure plays a key role in its function; if a protein loses
its shape at any structural level, it may no longer be functional.
Primary structure is the amino acid sequence.
Secondary structure is local interactions between stretches of a
Figure 3.9.1: Tertiary structure: The tertiary structure of proteins is
determined by hydrophobic interactions, ionic bonding, hydrogen polypeptide chain and includes α-helix and β-pleated sheet
bonding, and disulfide linkages. structures.
Tertiary structure is the overall the three-dimension folding
QUATERNARY STRUCTURE driven largely by interactions between R groups.
The quaternary structure of a protein is how its subunits are oriented Quarternary structures is the orientation and arrangement of
and arranged with respect to one another. As a result, quaternary subunits in a multi-subunit protein.
structure only applies to multi-subunit proteins; that is, proteins

3.9.2 https://bio.libretexts.org/@go/page/12700
KEY TERMS hydrogen bonds with C=O groups in the backbone of an adjacent
antiparallel: The nature of the opposite orientations of the two fully-extended strand
strands of DNA or two beta strands that comprise a protein’s α-helix: secondary structure of proteins where every backbone
secondary structure N-H creates a hydrogen bond with the C=O group of the amino
disulfide bond: A bond, consisting of a covalent bond between acid four residues earlier in the same helix.
two sulfur atoms, formed by the reaction of two thiol groups,
This page titled 3.9: Proteins - Protein Structure is shared under a CC BY-
especially between the thiol groups of two proteins
SA 4.0 license and was authored, remixed, and/or curated by Boundless.
β-pleated sheet: secondary structure of proteins where N-H
groups in the backbone of one fully-extended strand establish

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3.10: PROTEINS - DENATURATION AND PROTEIN FOLDING

 LEARNING OBJECTIVES

Discuss the process of protein denaturation

Each protein has its own unique sequence of amino acids and the
interactions between these amino acids create a specify shape. This
shape determines the protein’s function, from digesting protein in
the stomach to carrying oxygen in the blood.

CHANGING THE SHAPE OF A PROTEIN

If the protein is subject to changes in temperature, pH, or exposure
to chemicals, the internal interactions between the protein’s amino
acids can be altered, which in turn may alter the shape of the protein.
Although the amino acid sequence (also known as the protein’s
primary structure) does not change, the protein’s shape may change
so much that it becomes dysfunctional, in which case the protein is
considered denatured. Pepsin, the enzyme that breaks down protein
in the stomach, only operates at a very low pH. At higher pHs
pepsin’s conformation, the way its polypeptide chain is folded up in Figure 3.10.1: Denaturing a protein is occasionally irreversible:
(Top) The protein albumin in raw and cooked egg white. (Bottom) A
three dimensions, begins to change. The stomach maintains a very paperclip analogy visualizes the process: when cross-linked,
low pH to ensure that pepsin continues to digest protein and does not paperclips (‘amino acids’) no longer move freely; their structure is
denature. rearranged and ‘denatured’.

Because almost all biochemical reactions require enzymes, and Chaperone proteins (or chaperonins ) are helper proteins that provide
because almost all enzymes only work optimally within relatively favorable conditions for protein folding to take place. The
narrow temperature and pH ranges, many homeostatic mechanisms chaperonins clump around the forming protein and prevent other
polypeptide chains from aggregating. Once the target protein folds,
regulate appropriate temperatures and pH so that the enzymes can
the chaperonins disassociate.
maintain the shape of their active site.

REVERSING DENATURATION KEY POINTS

It is often possible to reverse denaturation because the primary Proteins change their shape when exposed to different pH or
structure of the polypeptide, the covalent bonds holding the amino temperatures.
The body strictly regulates pH and temperature to prevent
acids in their correct sequence, is intact. Once the denaturing agent
proteins such as enzymes from denaturing.
is removed, the original interactions between amino acids return the
Some proteins can refold after denaturation while others cannot.
protein to its original conformation and it can resume its function.
Chaperone proteins help some proteins fold into the correct
However, denaturation can be irreversible in extreme situations, like
frying an egg. The heat from a pan denatures the albumin protein in shape.
the liquid egg white and it becomes insoluble. The protein in meat
KEY TERMS
also denatures and becomes firm when cooked.
chaperonin: proteins that provide favorable conditions for the
correct folding of other proteins, thus preventing aggregation
denaturation: the change of folding structure of a protein (and
thus of physical properties) caused by heating, changes in pH, or
exposure to certain chemicals

CONTRIBUTIONS AND ATTRIBUTIONS

amino acid. Provided by: Wiktionary. Located at:
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3.11: NUCLEIC ACIDS - DNA AND RNA

 LEARNING OBJECTIVES

Describe the structure of nucleic acids and the types of

molecules that contain them

TYPES OF NUCLEIC ACIDS

The two main types of nucleic acids are deoxyribonucleic acid
(DNA) and ribonucleic acid (RNA). DNA is the genetic material
found in all living organisms, ranging from single-celled bacteria to
multicellular mammals. It is found in the nucleus of eukaryotes and
in the chloroplasts and mitochondria. In prokaryotes, the DNA is not
enclosed in a membranous envelope, but rather free-floating within
the cytoplasm.
The entire genetic content of a cell is known as its genome and the
study of genomes is genomics. In eukaryotic cells, but not in
prokaryotes, DNA forms a complex with histone proteins to form
chromatin, the substance of eukaryotic chromosomes. A
chromosome may contain tens of thousands of genes. Many genes
contain the information to make protein products; other genes code
for RNA products. DNA controls all of the cellular activities by Figure 3.11.1: DNA and RNA: A nucleotide is made up of three
turning the genes “on” or “off. ” components: a nitrogenous base, a pentose sugar, and one or more
The other type of nucleic acid, RNA, is mostly involved in protein phosphate groups. Carbon residues in the pentose are numbered 1′
through 5′ (the prime distinguishes these residues from those in the
synthesis. In eukaryotes, the DNA molecules never leave the nucleus base, which are numbered without using a prime notation). The base
but instead use an intermediary to communicate with the rest of the is attached to the 1′ position of the ribose, and the phosphate is
cell. This intermediary is the messenger RNA (mRNA). Other types attached to the 5′ position. When a polynucleotide is formed, the 5′
phosphate of the incoming nucleotide attaches to the 3′ hydroxyl
of RNA—like rRNA, tRNA, and microRNA—are involved in group at the end of the growing chain. Two types of pentose are
protein synthesis and its regulation. found in nucleotides, deoxyribose (found in DNA) and ribose (found
in RNA). Deoxyribose is similar in structure to ribose, but it has an
H instead of an OH at the 2′ position. Bases can be divided into two
NUCLEOTIDES categories: purines and pyrimidines. Purines have a double ring
DNA and RNA are made up of monomers known as nucleotides. structure, and pyrimidines have a single ring.
The nucleotides combine with each other to form a polynucleotide:
DNA or RNA. Each nucleotide is made up of three components: NITROGENOUS BASE
The nitrogenous bases are organic molecules and are so named
1. a nitrogenous base
because they contain carbon and nitrogen. They are bases because
2. a pentose (five-carbon) sugar
they contain an amino group that has the potential of binding an
3. a phosphate group
extra hydrogen, and thus, decreasing the hydrogen ion concentration
Each nitrogenous base in a nucleotide is attached to a sugar in its environment, making it more basic. Each nucleotide in DNA
molecule, which is attached to one or more phosphate groups. contains one of four possible nitrogenous bases: adenine (A),
guanine (G) cytosine (C), and thymine (T).
Adenine and guanine are classified as purines. The primary structure
of a purine consists of two carbon-nitrogen rings. Cytosine, thymine,
and uracil are classified as pyrimidines which have a single carbon-
nitrogen ring as their primary structure. Each of these basic carbon-
nitrogen rings has different functional groups attached to it. In
molecular biology shorthand, the nitrogenous bases are simply
known by their symbols A, T, G, C, and U. DNA contains A, T, G,
and C whereas RNA contains A, U, G, and C.

FIVE-CARBON SUGAR
The pentose sugar in DNA is deoxyribose and in RNA it is ribose.
The difference between the sugars is the presence of the hydroxyl
group on the second carbon of the ribose and hydrogen on the

3.11.1 https://bio.libretexts.org/@go/page/12703
second carbon of the deoxyribose. The carbon atoms of the sugar DNA provides the code for the cell ‘s activities, while RNA
molecule are numbered as 1′, 2′, 3′, 4′, and 5′ (1′ is read as “one converts that code into proteins to carry out cellular functions.
prime”). The sequence of nitrogen bases (A, T, C, G) in DNA is what
forms an organism’s traits.
PHOSPHATE GROUP The nitrogen bases A and T (or U in RNA) always go together
The phosphate residue is attached to the hydroxyl group of the 5′ and C and G always go together, forming the 5′-3′
carbon of one sugar and the hydroxyl group of the 3′ carbon of the phosphodiester linkage found in the nucleic acid molecules.
sugar of the next nucleotide, which forms a 5′3′ phosphodiester
linkage. The phosphodiester linkage is not formed by simple KEY TERMS
dehydration reaction like the other linkages connecting monomers in nucleotide: the monomer comprising DNA or RNA molecules;
macromolecules: its formation involves the removal of two consists of a nitrogenous heterocyclic base that can be a purine
phosphate groups. A polynucleotide may have thousands of such or pyrimidine, a five-carbon pentose sugar, and a phosphate
phosphodiester linkages. group
genome: the cell’s complete genetic information packaged as a
KEY POINTS double-stranded DNA molecule
The two main types of nucleic acids are DNA and RNA. monomer: A relatively small molecule which can be covalently
Both DNA and RNA are made from nucleotides, each containing bonded to other monomers to form a polymer.
a five-carbon sugar backbone, a phosphate group, and a nitrogen
base. This page titled 3.11: Nucleic Acids - DNA and RNA is shared under a CC
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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3.12: NUCLEIC ACIDS - THE DNA DOUBLE HELIX

 LEARNING OBJECTIVES

Describe the structure of DNA

A DOUBLE-HELIX STRUCTURE
DNA has a double-helix structure, with sugar and phosphate on the
outside of the helix, forming the sugar-phosphate backbone of the
DNA. The nitrogenous bases are stacked in the interior in pairs, like
the steps of a staircase; the pairs are bound to each other by
Figure 3.12.1: Antiparallel Strands: In a double stranded DNA
hydrogen bonds. The two strands of the helix run in opposite molecule, the two strands run antiparallel to one another so one is
directions. This antiparallel orientation is important to DNA upside down compared to the other. The phosphate backbone is
replication and in many nucleic acid interactions. located on the outside, and the bases are in the middle. Adenine
forms hydrogen bonds (or base pairs) with thymine, and guanine
base pairs with cytosine.

DNA REPLICATION
During DNA replication, each strand is copied, resulting in a
daughter DNA double helix containing one parental DNA strand and
a newly synthesized strand. At this time it is possible a mutation
may occur. A mutation is a change in the sequence of the nitrogen
bases. For example, in the sequence AATTGGCC, a mutation may
cause the second T to change to a G. Most of the time when this
happens the DNA is able to fix itself and return the original base to
the sequence. However, sometimes the repair is unsuccessful,
resulting in different proteins being created.

KEY POINTS
The structure of DNA is called a double helix, which looks like a
twisted staircase.
Figure 3.12.1: DNA is a Double Helix: Native DNA is an The sugar and phosphate make up the backbone, while the
antiparallel double helix. The phosphate backbone (indicated by the nitrogen bases are found in the center and hold the two strands
curvy lines) is on the outside, and the bases are on the inside. Each
base from one strand interacts via hydrogen bonding with a base
together.
from the opposing strand. The nitrogen bases can only pair in a certain way: A pairing with
T and C pairing with G. This is called base pairing.
BASE PAIRS Due to the base pairing, the DNA strands are complementary to
Only certain types of base pairing are allowed. This means Adenine each other, run in opposite directions, and are called antiparallel
pairs with Thymine, and Guanine pairs with Cytosine. This is known strands.
as the base complementary rule because the DNA strands are
complementary to each other. If the sequence of one strand is KEY TERMS
AATTGGCC, the complementary strand would have the sequence mutation: any error in base pairing during the replication of
TTAACCGG. DNA
sugar-phosphate backbone: The outer support of the ladder,
forming strong covalent bonds between monomers of DNA.
base pairing: The specific way in which bases of DNA line up
and bond to one another; A always with T and G always with C.

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3.13: NUCLEIC ACIDS - DNA PACKAGING

 LEARNING OBJECTIVES

Describe how DNA is packaged differently in prokaryotes

and eukaryotes

A eukaryote contains a well-defined nucleus, whereas in prokaryotes

the chromosome lies in the cytoplasm in an area called the nucleoid.
In eukaryotic cells, DNA and RNA synthesis occur in a separate
compartment from protein synthesis. In prokaryotic cells, both
processes occur together. What advantages might there be to
separating the processes? What advantages might there be to having
them occur together?

Figure 3.13.1: Eukaryotic and prokaryotic cells: A eukaryote

contains a well-defined nucleus, whereas in prokaryotes, the
chromosome lies in the cytoplasm in an area called the nucleoid.
The size of the genome in one of the most well-studied prokaryotes,
Figure 3.13.1: Eukaryotic chromosomes: These figures illustrate the
E.coli, is 4.6 million base pairs (approximately 1.1 mm, if cut and compaction of the eukaryotic chromosome.
stretched out). So how does this fit inside a small bacterial cell? The In interphase, eukaryotic chromosomes have two distinct regions
DNA is twisted by what is known as supercoiling. Supercoiling
that can be distinguished by staining. The tightly packaged region is
means that DNA is either under-wound (less than one turn of the
known as heterochromatin, and the less dense region is known as
helix per 10 base pairs) or over-wound (more than 1 turn per 10 base
euchromatin.
pairs) from its normal relaxed state. Some proteins are known to be
Heterochromatin usually contains genes that are not expressed, and
involved in the supercoiling; other proteins and enzymes such as
is found in the regions of the centromere and telomeres. The
DNA gyrase help in maintaining the supercoiled structure.
euchromatin usually contains genes that are transcribed, with DNA
Eukaryotes, whose chromosomes each consist of a linear DNA
packaged around nucleosomes but not further compacted.
molecule, employ a different type of packing strategy to fit their
DNA inside the nucleus. At the most basic level, DNA is wrapped KEY POINTS
around proteins known as histones to form structures called In eukaryotic cells, DNA and RNA synthesis occur in a different
nucleosomes. The histones are evolutionarily conserved proteins that location than protein synthesis; in prokaryotic cells, both these
are rich in basic amino acids and form an octamer. The DNA (which processes occur together.
is negatively charged because of the phosphate groups) is wrapped DNA is “supercoiled” in prokaryotic cells, meaning that the
tightly around the histone core. This nucleosome is linked to the DNA is either under-wound or over-wound from its normal
next one with the help of a linker DNA. This is also known as the relaxed state.
“beads on a string” structure. This is further compacted into a 30 nm In eukaryotic cells, DNA is wrapped around proteins known as
fiber, which is the diameter of the structure. At the metaphase stage histones to form structures called nucleosomes.
the chromosomes are at their most compact, approximately 700 nm
in width, and are found in association with scaffold proteins. KEY TERMS
nucleosomes: The fundamental subunit of chromatin, composed
of a little less than two turns of DNA wrapped around a set of
eight proteins called histones.
histones: The chief protein components of chromatin, which act
as spools around which DNA winds.

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3.14: NUCLEIC ACIDS - TYPES OF RNA
RNA is a single stranded molecule, compared to the double helix of
 LEARNING OBJECTIVES DNA.
The DNA molecules never leave the nucleus but instead use an
Describe the structure and function of RNA
intermediary to communicate with the rest of the cell. This
intermediary is the messenger RNA (mRNA). When proteins need
RNA STRUCTURE AND FUNCTION
to be made, the mRNA enters the nucleus and attaches itself to one
The two main types of nucleic acids are deoxyribonucleic acid of the DNA strands. Being complementary, the sequence of nitrogen
(DNA) and ribonucleic acid (RNA). DNA is the genetic material bases of the RNA is opposite that of the DNA. This is called
found in all living organisms and is found in the nucleus of transcription. For example, if the DNA strand reads TCCAAGTC,
eukaryotes and in the chloroplasts and mitochondria. In prokaryotes, then the mRNA strand would read AGGUUCAG. The mRNA then
the DNA is not enclosed in a membranous envelope. carries the code out of the nucleus to organelles called ribosomes for
The other type of nucleic acid, RNA, is mostly involved in protein the assembly of proteins.
synthesis. Just like in DNA, RNA is made of monomers called Once the mRNA has reached the ribosomes, they do not read the
nucleotides. Each nucleotide is made up of three components: a instructions directly. Instead, another type of RNA called transfer
nitrogenous base, a pentose (five-carbon) sugar called ribose, and a RNA (tRNA) needs to translate the information from the mRNA
phosphate group. Each nitrogenous base in a nucleotide is attached into a usable form. The tRNA attaches to the mRNA, but with the
to a sugar molecule, which is attached to one or more phosphate opposite base pairings. It then reads the sequence in sets of three
groups. bases called codons. Each possible three letter arrangement of
A,C,U,G (e.g., AAA, AAU, GGC, etc) is a specific instruction, and
the correspondence of these instructions and the amino acids is
known as the “genetic code.” Though exceptions to or variations on
the code exist, the standard genetic code holds true in most
organisms.
The ribosome acts like a giant clamp, holding all of the players in
position, and facilitating both the pairing of bases between the
messenger and transfer RNAs, and the chemical bonding between
the amino acids. The ribosome has special subunits known as
ribosomal RNAs (rRNA) because they function in the ribosome.
These subunits do not carry instructions for making a specific
proteins (i.e., they are not messenger RNAs) but instead are an
integral part of the ribosome machinery that is used to make proteins
from mRNAs. The making of proteins by reading instructions in
mRNA is generally known as ” translation.”

CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44403/latest...ol11448/latest. License: CC
BY: Attribution
genome. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/genome. License: CC BY-SA: Attribution-ShareAlike
Figure 3.14.1: RNA Structure: A nucleotide is made up of three nucleotide. Provided by: Wiktionary. Located at:
components: a nitrogenous base, a pentose sugar, and one or more en.wiktionary.org/wiki/nucleotide. License: CC BY-SA: Attribution-
phosphate groups. Carbon residues in the pentose are numbered 1′ ShareAlike
monomer. Provided by: Wiktionary. Located at:
through 5′ (the prime distinguishes these residues from those in the
en.wiktionary.org/wiki/monomer. License: CC BY-SA: Attribution-ShareAlike
base, which are numbered without using a prime notation). The base OpenStax College, Nucleic Acids. October 16, 2013. Provided by: OpenStax
is attached to the 1′ position of the ribose, and the phosphate is CNX. Located at: http://cnx.org/content/m44403/latest...e_03_05_01.jpg.
attached to the 5′ position. When a polynucleotide is formed, the 5′ License: CC BY: Attribution
phosphate of the incoming nucleotide attaches to the 3′ hydroxyl OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
group at the end of the growing chain. Two types of pentose are Located at: http://cnx.org/content/m44403/latest...ol11448/latest. License: CC
found in nucleotides, deoxyribose (found in DNA) and ribose (found BY: Attribution
in RNA). Deoxyribose is similar in structure to ribose, but it has an mutation. Provided by: Wiktionary. Located at:
H instead of an OH at the 2′ position. Bases can be divided into two en.wiktionary.org/wiki/mutation. License: CC BY-SA: Attribution-ShareAlike
categories: purines and pyrimidines. Purines have a double ring Boundless. Provided by: Boundless Learning. Located at:
structure, and pyrimidines have a single ring. www.boundless.com//biology/de...n/antiparallel. License: CC BY-SA:
Attribution-ShareAlike
In RNA, the nitrogenous bases vary slightly from those of DNA. OpenStax College, Nucleic Acids. October 16, 2013. Provided by: OpenStax
Adenine (A), guanine (G), and cytosine (C) are present, but instead CNX. Located at: http://cnx.org/content/m44403/latest...e_03_05_01.jpg.
License: CC BY: Attribution
of thymine (T), a pyrimidine called uracil (U) pairs with adenine. OpenStax College, Nucleic Acids. October 16, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44403/latest...e_03_05_03.png.

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CNX. Located at: http://cnx.org/content/m44403/latest...e_03_05_02.jpg. CNX. Located at: http://cnx.org/content/m44403/latest...e_03_05_03.png.
License: CC BY: Attribution License: CC BY: Attribution
DNA structure. Provided by: OpenStax CNX. Located at: OpenStax College, Nucleic Acids. October 16, 2013. Provided by: OpenStax
http://cnx.org/contents/GFy_h8cu@9.8...and-Sequencing. License: CC BY- CNX. Located at: http://cnx.org/content/m44403/latest...e_03_05_02.jpg.
SA: Attribution-ShareAlike License: CC BY: Attribution
Histones. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Histone. figure7.jpg. Provided by: OpenStax CNX. Located at:
License: CC BY-SA: Attribution-ShareAlike http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.87.
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CNX. Located at: http://cnx.org/content/m44403/latest...e_03_05_01.jpg. figure6.jpg. Provided by: OpenStax CNX. Located at:
License: CC BY: Attribution http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.87.
OpenStax College, Nucleic Acids. October 16, 2013. Provided by: OpenStax License: CC BY-SA: Attribution-ShareAlike
CNX. Located at: http://cnx.org/content/m44403/latest...e_03_05_03.png. OpenStax College, Nucleic Acids. May 20, 2015. Provided by: OpenStax CNX.
License: CC BY: Attribution Located at: http://cnx.org/contents/cb178029-ce1.../Nucleic_Acids. License:
OpenStax College, Nucleic Acids. October 16, 2013. Provided by: OpenStax CC BY-SA: Attribution-ShareAlike
CNX. Located at: http://cnx.org/content/m44403/latest...e_03_05_02.jpg.
License: CC BY: Attribution KEY POINTS
figure7.jpg. Provided by: OpenStax CNX. Located at:
http://cnx.org/contents/185cbf87-c72...f21b5eabd@9.87. License: CC BY-SA: The nitrogen bases in RNA include adenine (A), guanine (G), cytosine (C), and
Attribution-ShareAlike uracil (U).
figure6.jpg. Provided by: OpenStax CNX. Located at: Messenger RNA (mRNA) carries the code from the DNA to the ribosomes, while
http://cnx.org/contents/185cbf87-c72...f21b5eabd@9.87. License: CC BY-SA: transfer RNA (tRNA) converts that code into a usable form.
Attribution-ShareAlike Ribosomes are the sites where tRNA and rRNA assemble proteins.
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. RNA differs from DNA in that it is single stranded, has uracil instead of thymine,
Located at: http://cnx.org/content/m44403/latest...ol11448/latest. License: CC carries the code for making proteins instead of directing all of the cell ‘s
BY: Attribution functions, and has ribose as its five-carbon sugar instead of deoxyribose.
Fact Sheet: DNA-RNA-Protein. Provided by: microBEnet. Located at:
http://microbe.net/simple-guides/fact-sheet-dna-rna-protein/. License: CC KEY TERMS
BY-SA: Attribution-ShareAlike
codon. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/codon. codon: a sequence of three adjacent nucleotides, which encode for a specific
License: CC BY-SA: Attribution-ShareAlike amino acid during protein synthesis or translation
transcription. Provided by: Wiktionary. Located at: transcription: the synthesis of RNA under the direction of DNA
en.wiktionary.org/wiki/transcription. License: CC BY-SA: Attribution-
ShareAlike This page titled 3.14: Nucleic Acids - Types of RNA is shared under a CC
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 CHAPTER OVERVIEW

4: CELL STRUCTURE
4.1: Studying Cells - Cells as the Basic Unit of Life
4.2: Studying Cells - Microscopy
4.3: Studying Cells - Cell Theory
4.4: Studying Cells - Cell Size
4.5: Prokaryotic Cells - Characteristics of Prokaryotic Cells
4.6: Eukaryotic Cells - Characteristics of Eukaryotic Cells
4.7: Eukaryotic Cells - The Plasma Membrane and the Cytoplasm
4.8: Eukaryotic Cells - The Nucleus and Ribosomes
4.9: Eukaryotic Cells - Mitochondria
4.10: Eukaryotic Cells - Comparing Plant and Animal Cells
4.11: The Endomembrane System and Proteins - Vesicles and Vacuoles
4.12: The Endomembrane System and Proteins - The Endoplasmic Reticulum
4.13: The Endomembrane System and Proteins - The Golgi Apparatus
4.14: The Endomembrane System and Proteins - Lysosomes
4.15: The Endomembrane System and Proteins - Peroxisomes
4.16: The Cytoskeleton - Microfilaments
4.17: The Cytoskeleton - Intermediate Filaments and Microtubules
4.18: Connections between Cells and Cellular Activities - Extracellular Matrix of Animal Cells
4.19: Connections between Cells and Cellular Activities - Intercellular Junctions

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1
4.1: STUDYING CELLS - CELLS AS THE BASIC UNIT OF LIFE
variety, however, cells from all organisms—even ones as diverse as
 LEARNING OBJECTIVES bacteria, onion, and human—share certain fundamental
characteristics.
State the general characteristics of a cell

Close your eyes and picture a brick wall. What is the basic building
block of that wall? A single brick, of course. Like a brick wall, your
body is composed of basic building blocks, and the building blocks
of your body are cells.

CELLS AS BUILDING BLOCKS

A cell is the smallest unit of a living thing. A living thing, whether Figure 4.1.1: Various Cell Types: (a) Nasal sinus cells (viewed with
a light microscope), (b) onion cells (viewed with a light
made of one cell (like bacteria) or many cells (like a human), is microscope), and (c) Vibrio tasmaniensis bacterial cells (seen
called an organism. Thus, cells are the basic building blocks of all through a scanning electron microscope) are from very different
organisms. Several cells of one kind that interconnect with each organisms, yet all share certain characteristics of basic cell structure.
other and perform a shared function form tissues; several tissues KEY POINTS
combine to form an organ (your stomach, heart, or brain); and
A living thing can be composed of either one cell or many cells.
several organs make up an organ system (such as the digestive
There are two broad categories of cells: prokaryotic and
system, circulatory system, or nervous system). Several systems that
eukaryotic cells.
function together form an organism (like a human being). There are
Cells can be highly specialized with specific functions and
many types of cells all grouped into one of two broad categories:
characteristics.
prokaryotic and eukaryotic. For example, both animal and plant cells
are classified as eukaryotic cells, whereas bacterial cells are KEY TERMS
classified as prokaryotic.
prokaryotic: Small cells in the domains Bacteria and Archaea
TYPES OF SPECIALIZED CELLS that do not contain a membrane-bound nucleus or other
membrane-bound organelles.
Your body has many kinds of cells, each specialized for a specific
eukaryotic: Having complex cells in which the genetic material
purpose. Just as a home is made from a variety of building materials,
is contained within membrane-bound nuclei.
the human body is constructed from many cell types. For example,
cell: The basic unit of a living organism, consisting of a quantity
epithelial cells protect the surface of the body and cover the organs
of protoplasm surrounded by a cell membrane, which is able to
and body cavities within. Bone cells help to support and protect the
synthesize proteins and replicate itself.
body. Cells of the immune system fight invading bacteria.
Additionally, blood and blood cells carry nutrients and oxygen This page titled 4.1: Studying Cells - Cells as the Basic Unit of Life is
throughout the body while removing carbon dioxide. Each of these shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
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4.2: STUDYING CELLS - MICROSCOPY

 LEARNING OBJECTIVES

Compare and contrast light and electron microscopy.

MICROSCOPY
Cells vary in size. With few exceptions, individual cells cannot be
seen with the naked eye, so scientists use microscopes (micro- =
“small”; -scope = “to look at”) to study them. A microscope is an
instrument that magnifies an object. Most photographs of cells are
taken with a microscope; these images can also be called
micrographs.
The optics of a microscope’s lenses change the orientation of the
image that the user sees. A specimen that is right-side up and facing
right on the microscope slide will appear upside-down and facing
left when viewed through a microscope, and vice versa. Similarly, if
the slide is moved left while looking through the microscope, it will Figure 4.2.1: Light and Electron Microscopes: (a) Most light
microscopes used in a college biology lab can magnify cells up to
appear to move right, and if moved down, it will seem to move up. approximately 400 times and have a resolution of about 200
This occurs because microscopes use two sets of lenses to magnify nanometers. (b) Electron microscopes provide a much higher
the image. Because of the manner by which light travels through the magnification, 100,000x, and a have a resolution of 50 picometers.
lenses, this system of two lenses produces an inverted image Light microscopes, commonly used in undergraduate college
(binocular, or dissecting microscopes, work in a similar manner, but laboratories, magnify up to approximately 400 times. Two
they include an additional magnification system that makes the final parameters that are important in microscopy are magnification and
image appear to be upright). resolving power. Magnification is the process of enlarging an object
in appearance. Resolving power is the ability of a microscope to
LIGHT MICROSCOPES distinguish two adjacent structures as separate: the higher the
To give you a sense of cell size, a typical human red blood cell is resolution, the better the clarity and detail of the image. When oil
about eight millionths of a meter or eight micrometers (abbreviated immersion lenses are used for the study of small objects,
as eight μm) in diameter; the head of a pin of is about two magnification is usually increased to 1,000 times. In order to gain a
thousandths of a meter (two mm) in diameter. That means about 250 better understanding of cellular structure and function, scientists
red blood cells could fit on the head of a pin. typically use electron microscopes.
Most student microscopes are classified as light microscopes.
Visible light passes and is bent through the lens system to enable the
ELECTRON MICROSCOPES
user to see the specimen. Light microscopes are advantageous for In contrast to light microscopes, electron microscopes use a beam of
viewing living organisms, but since individual cells are generally electrons instead of a beam of light. Not only does this allow for
transparent, their components are not distinguishable unless they are higher magnification and, thus, more detail, it also provides higher
colored with special stains. Staining, however, usually kills the cells. resolving power. The method used to prepare the specimen for
viewing with an electron microscope kills the specimen. Electrons
have short wavelengths (shorter than photons) that move best in a
vacuum, so living cells cannot be viewed with an electron
microscope.
In a scanning electron microscope, a beam of electrons moves back
and forth across a cell’s surface, creating details of cell surface
characteristics. In a transmission electron microscope, the electron
beam penetrates the cell and provides details of a cell’s internal
structures. As you might imagine, electron microscopes are
significantly more bulky and expensive than light microscopes.

KEY POINTS
Light microscopes allow for magnification of an object
approximately up to 400-1000 times depending on whether the
high power or oil immersion objective is used.

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Light microscopes use visible light which passes and bends KEY TERMS
through the lens system. resolution: The degree of fineness with which an image can be
Electron microscopes use a beam of electrons, opposed to visible recorded or produced, often expressed as the number of pixels
light, for magnification. per unit of length (typically an inch).
Electron microscopes allow for higher magnification in electron: The subatomic particle having a negative charge and
comparison to a light microscope thus, allowing for visualization orbiting the nucleus; the flow of electrons in a conductor
of cell internal structures. constitutes electricity.

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4.3: STUDYING CELLS - CELL THEORY
composed of one or more cells; the cell is the basic unit of life; and
 LEARNING OBJECTIVES new cells arise from existing cells. Rudolf Virchow later made
important contributions to this theory.
Identify the components of cell theory
Schleiden and Schwann proposed spontaneous generation as the
method for cell origination, but spontaneous generation (also called
CELL THEORY
abiogenesis) was later disproven. Rudolf Virchow famously stated
The microscopes we use today are far more complex than those used “Omnis cellula e cellula”… “All cells only arise from pre-existing
in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper who cells. “The parts of the theory that did not have to do with the origin
had great skill in crafting lenses. Despite the limitations of his now- of cells, however, held up to scientific scrutiny and are widely
ancient lenses, van Leeuwenhoek observed the movements of agreed upon by the scientific community today. The generally
protista (a type of single-celled organism) and sperm, which he accepted portions of the modern Cell Theory are as follows:
collectively termed “animalcules. ”
1. The cell is the fundamental unit of structure and function in
In a 1665 publication called Micrographia, experimental scientist living things.
Robert Hooke coined the term “cell” for the box-like structures he 2. All organisms are made up of one or more cells.
observed when viewing cork tissue through a lens. In the 1670s, van 3. Cells arise from other cells through cellular division.
Leeuwenhoek discovered bacteria and protozoa. Later advances in
lenses, microscope construction, and staining techniques enabled The expanded version of the cell theory can also include:
other scientists to see some components inside cells. Cells carry genetic material passed to daughter cells during
cellular division
All cells are essentially the same in chemical composition
Energy flow (metabolism and biochemistry) occurs within cells

KEY POINTS
The cell theory describes the basic properties of all cells.
The three scientists that contributed to the development of cell
theory are Matthias Schleiden, Theodor Schwann, and Rudolf
Virchow.
A component of the cell theory is that all living things are
composed of one or more cells.
A component of the cell theory is that the cell is the basic unit of
life.
A component of the cell theory is that all new cells arise from
existing cells.

KEY TERMS
Figure 4.3.1: Structure of an Animal Cell: The cell is the basic unit cell theory: The scientific theory that all living organisms are
of life and the study of the cell led to the development of the cell
made of cells as the smallest functional unit.
theory.
By the late 1830s, botanist Matthias Schleiden and zoologist This page titled 4.3: Studying Cells - Cell Theory is shared under a CC BY-
Theodor Schwann were studying tissues and proposed the unified SA 4.0 license and was authored, remixed, and/or curated by Boundless.
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4.4: STUDYING CELLS - CELL SIZE

 LEARNING OBJECTIVES

Describe the factors limiting cell size and the adaptations

cells make to overcome the surface area to volume issue

At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly

Figure 4.4.1: Surface Area to Volume Ratios: Notice that as a cell
smaller than eukaryotic cells, which have diameters ranging from 10 increases in size, its surface area-to-volume ratio decreases. When
to 100 μm. The small size of prokaryotes allows ions and organic there is insufficient surface area to support a cell’s increasing
molecules that enter them to quickly diffuse to other parts of the cell. volume, a cell will either divide or die. The cell on the left has a
volume of 1 mm3 and a surface area of 6 mm2, with a surface area-
Similarly, any wastes produced within a prokaryotic cell can quickly to-volume ratio of 6 to 1, whereas the cell on the right has a volume
diffuse out. This is not the case in eukaryotic cells, which have of 8 mm3 and a surface area of 24 mm2, with a surface area-to-
developed different structural adaptations to enhance intracellular volume ratio of 3 to 1.
transport. Smaller single-celled organisms have a high surface area to volume
ratio, which allows them to rely on oxygen and material diffusing
into the cell (and wastes diffusing out) in order to survive. The
higher the surface area to volume ratio they have, the more effective
this process can be. Larger animals require specialized organs
(lungs, kidneys, intestines, etc.) that effectively increase the surface
area available for exchange processes, and a circulatory system to
move material and heat energy between the surface and the core of
the organism.
Increased volume can lead to biological problems. King Kong, the
fictional giant gorilla, would have insufficient lung surface area to
meet his oxygen needs, and could not survive. For small organisms
with their high surface area to volume ratio, friction and fluid
dynamics (wind, water flow) are relatively much more important,
Figure 4.4.1: Relative Size of Atoms to Humans: This figure shows and gravity much less important, than for large animals.
relative sizes on a logarithmic scale (recall that each unit of increase
in a logarithmic scale represents a 10-fold increase in the quantity
However, increased surface area can cause problems as well. More
being measured). contact with the environment through the surface of a cell or an
In general, small size is necessary for all cells, whether prokaryotic organ (relative to its volume) increases loss of water and dissolved
or eukaryotic. Consider the area and volume of a typical cell. Not all substances. High surface area to volume ratios also present problems
cells are spherical in shape, but most tend to approximate a sphere. of temperature control in unfavorable environments.
The formula for the surface area of a sphere is 4πr2, while the
formula for its volume is 4πr3/3. As the radius of a cell increases, its
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increases as the cube of its radius (much more rapidly). BY: Attribution
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OpenStax College, Introduction. October 16, 2013. Provided by: OpenStax ShareAlike
CNX. Located at: http://cnx.org/content/m44404/latest...e_04_00_00.jpg. OpenStax College, Introduction. October 16, 2013. Provided by: OpenStax
License: CC BY: Attribution CNX. Located at: http://cnx.org/content/m44404/latest...e_04_00_00.jpg.
OpenStax College, Studying Cells. October 16, 2013. Provided by: OpenStax License: CC BY: Attribution
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License: CC BY: Attribution CNX. Located at: http://cnx.org/content/m44405/latest...1_01ab_new.jpg.
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. License: CC BY: Attribution
Located at: http://cnx.org/content/m44405/latest...ol11448/latest. License: CC Diagram of an animal cell in three dimensions. Provided by: Wikimedia.
BY: Attribution Located at: commons.wikimedia.org/wiki/Fi...dimensions.png. License:
General Biology/Cells/Cell Structure. Provided by: Wikibooks. Located at: Public Domain: No Known Copyright
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Attribution-ShareAlike http://cnx.org/contents/GFy_h8cu@9.8...karyotic-Cells. License: CC BY-SA:
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CNX. Located at: http://cnx.org/content/m44404/latest...e_04_00_00.jpg.
License: CC BY: Attribution KEY POINTS
OpenStax College, Studying Cells. October 16, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44405/latest...1_01ab_new.jpg. As a cell grows, its volume increases much more rapidly than its surface area.
License: CC BY: Attribution Since the surface of the cell is what allows the entry of oxygen, large cells cannot
Diagram of an animal cell in three dimensions. Provided by: Wikimedia. get as much oxygen as they would need to support themselves.
Located at: commons.wikimedia.org/wiki/Fi...dimensions.png. License: As animals increase in size they require specialized organs that effectively
Public Domain: No Known Copyright increase the surface area available for exchange processes.
Cell Size. Provided by: OpenStax CNX. Located at:
http://cnx.org/contents/GFy_h8cu@9.8...karyotic-Cells. License: CC BY-SA: KEY TERMS
Attribution-ShareAlike
Surface Area to Volume Ratios. Provided by: Wikipedia. Located at: surface area: The total area on the surface of an object.
en.Wikipedia.org/wiki/Surface-area-to-volume_ratio. License: CC BY-SA:
Attribution-ShareAlike This page titled 4.4: Studying Cells - Cell Size is shared under a CC BY-SA
Surface area. Provided by: Wiktionary. Located at: 4.0 license and was authored, remixed, and/or curated by Boundless.
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4.5: PROKARYOTIC CELLS - CHARACTERISTICS OF PROKARYOTIC CELLS
developed different structural adaptations to enhance intracellular
 LEARNING OBJECTIVES transport.

Describe the structure of prokaryotic cells

COMPONENTS OF PROKARYOTIC CELLS

All cells share four common components:
1. a plasma membrane: an outer covering that separates the cell’s
interior from its surrounding environment.
2. cytoplasm: a jelly-like cytosol within the cell in which other
cellular components are found
3. DNA: the genetic material of the cell
4. ribosomes: where protein synthesis occurs
However, prokaryotes differ from eukaryotic cells in several ways.
A prokaryote is a simple, single-celled (unicellular) organism that
Figure 4.5.1: Microbial Size: This figure shows relative sizes of
lacks an organized nucleus or any other membrane-bound organelle. microbes on a logarithmic scale (recall that each unit of increase in a
We will shortly come to see that this is significantly different in logarithmic scale represents a 10-fold increase in the quantity being
eukaryotes. Prokaryotic DNA is found in a central part of the cell: measured).
the nucleoid. Small size, in general, is necessary for all cells, whether prokaryotic
or eukaryotic. Let’s examine why that is so. First, we’ll consider the
Most prokaryotes have a peptidoglycan cell wall and many have a
area and volume of a typical cell. Not all cells are spherical in shape,
polysaccharide capsule. The cell wall acts as an extra layer of
but most tend to approximate a sphere. You may remember from
protection, helps the cell maintain its shape, and prevents
your high school geometry course that the formula for the surface
dehydration. The capsule enables the cell to attach to surfaces in its
area of a sphere is 4πr2, while the formula for its volume is 4/3πr3.
environment. Some prokaryotes have flagella, pili, or fimbriae.
Thus, as the radius of a cell increases, its surface area increases as
Flagella are used for locomotion. Pili are used to exchange genetic
the square of its radius, but its volume increases as the cube of its
material during a type of reproduction called conjugation. Fimbriae
radius (much more rapidly). Therefore, as a cell increases in size, its
are used by bacteria to attach to a host cell.
surface area-to-volume ratio decreases. This same principle would
apply if the cell had the shape of a cube. If the cell grows too large,
the plasma membrane will not have sufficient surface area to support
the rate of diffusion required for the increased volume. In other
words, as a cell grows, it becomes less efficient. One way to become
more efficient is to divide; another way is to develop organelles that
perform specific tasks. These adaptations led to the development of
more sophisticated cells called eukaryotic cells.

Figure 4.5.1: General Structure of a Prokaryotic Cell: This figure

shows the generalized structure of a prokaryotic cell.All prokaryotes
have chromosomal DNA localized in a nucleoid, ribosomes, a cell Figure 4.5.1: Cell Surface Size: Notice that as a cell increases in
membrane, and a cell wall.The other structures shown are present in size, its surface area-to-volume ratio decreases.When there is
some, but not all, bacteria. insufficient surface area to support a cell’s increasing volume, a cell
will either divide or die.The cell on the left has a volume of 1 mm3
CELL SIZE and a surface area of 6 mm2, with a surface area-to-volume ratio of
6 to 1, whereas the cell on the right has a volume of 8 mm3 and a
At 0.1 to 5.0 μm in diameter, prokaryotic cells are significantly surface area of 24 mm2, with a surface area-to-volume ratio of 3 to
smaller than eukaryotic cells, which have diameters ranging from 10 1.
to 100 μm. The small size of prokaryotes allows ions and organic
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molecules that enter them to quickly diffuse to other parts of the cell.
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Similarly, any wastes produced within a prokaryotic cell can quickly Located at: http://cnx.org/content/m44406/latest...ol11448/latest. License: CC
diffuse out. This is not the case in eukaryotic cells, which have BY: Attribution
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 ShareAlike The cell wall of a prokaryote acts as an extra layer of protection, helps maintain
eukaryotic. Provided by: Wiktionary. Located at: cell shape, and prevents dehydration.
http://en.wiktionary.org/wiki/eukaryotic. License: CC BY-SA: Attribution- Prokaryotic cell size ranges from 0.1 to 5.0 μm in diameter.
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KEY TERMS
License: CC BY: Attribution eukaryotic: Having complex cells in which the genetic material is organized into
OpenStax College, Prokaryotic Cells. October 16, 2013. Provided by: OpenStax membrane-bound nuclei.
CNX. Located at: http://cnx.org/content/m44406/latest...e_04_02_01.jpg. prokaryotic: Of cells, lacking a nucleus.
License: CC BY: Attribution nucleoid: the irregularly-shaped region within a prokaryote cell where the
OpenStax College, Prokaryotic Cells. October 16, 2013. Provided by: OpenStax genetic material is localized
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License: CC BY: Attribution This page titled 4.5: Prokaryotic Cells - Characteristics of Prokaryotic Cells
is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
KEY POINTS curated by Boundless.
Prokaryotes lack an organized nucleus and other membrane-bound organelles.
Prokaryotic DNA is found in a central part of the cell called the nucleoid.

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4.6: EUKARYOTIC CELLS - CHARACTERISTICS OF EUKARYOTIC CELLS

 LEARNING OBJECTIVES

Describe the structure of eukaryotic cells

EUKARYOTIC CELL STRUCTURE

Like a prokaryotic cell, a eukaryotic cell has a plasma membrane,
cytoplasm, and ribosomes. However, unlike prokaryotic cells,
eukaryotic cells have:
1. a membrane-bound nucleus
2. numerous membrane-bound organelles (including the
endoplasmic reticulum, Golgi apparatus, chloroplasts, and
mitochondria)
3. several rod-shaped chromosomes Figure 4.6.1: Eukaryotic Nucleus: The nucleus stores chromatin
(DNA plus proteins) in a gel-like substance called the
Because a eukaryotic cell’s nucleus is surrounded by a membrane, it nucleoplasm.The nucleolus is a condensed region of chromatin
is often said to have a “true nucleus. ” Organelles (meaning “little where ribosome synthesis occurs.The boundary of the nucleus is
called the nuclear envelope.It consists of two phospholipid bilayers:
organ”) have specialized cellular roles, just as the organs of your an outer membrane and an inner membrane.The nuclear membrane
body have specialized roles. They allow different functions to be is continuous with the endoplasmic reticulum.Nuclear pores allow
compartmentalized in different areas of the cell. substances to enter and exit the nucleus.

THE NUCLEUS & ITS STRUCTURES OTHER MEMBRANE-BOUND ORGANELLES

Typically, the nucleus is the most prominent organelle in a cell. Mitochondria are oval-shaped, double membrane organelles that
Eukaryotic cells have a true nucleus, which means the cell’s DNA is have their own ribosomes and DNA. These organelles are often
surrounded by a membrane. Therefore, the nucleus houses the cell’s called the “energy factories” of a cell because they are responsible
DNA and directs the synthesis of proteins and ribosomes, the for making adenosine triphosphate (ATP), the cell’s main energy-
cellular organelles responsible for protein synthesis. The nuclear carrying molecule, by conducting cellular respiration. The
envelope is a double-membrane structure that constitutes the endoplasmic reticulum modifies proteins and synthesizes lipids,
outermost portion of the nucleus. Both the inner and outer while the golgi apparatus is where the sorting, tagging, packaging,
membranes of the nuclear envelope are phospholipid bilayers. The and distribution of lipids and proteins takes place. Peroxisomes are
nuclear envelope is punctuated with pores that control the passage of small, round organelles enclosed by single membranes; they carry
ions, molecules, and RNA between the nucleoplasm and cytoplasm. out oxidation reactions that break down fatty acids and amino acids.
The nucleoplasm is the semi-solid fluid inside the nucleus where we Peroxisomes also detoxify many poisons that may enter the body.
find the chromatin and the nucleolus. Furthermore, chromosomes Vesicles and vacuoles are membrane-bound sacs that function in
are structures within the nucleus that are made up of DNA, the storage and transport. Other than the fact that vacuoles are somewhat
genetic material. In prokaryotes, DNA is organized into a single larger than vesicles, there is a very subtle distinction between them:
circular chromosome. In eukaryotes, chromosomes are linear the membranes of vesicles can fuse with either the plasma
structures. membrane or other membrane systems within the cell. All of these
organelles are found in each and every eukaryotic cell.

ANIMAL CELLS VERSUS PLANT CELLS

While all eukaryotic cells contain the aforementioned organelles and
structures, there are some striking differences between animal and
plant cells. Animal cells have a centrosome and lysosomes, whereas
plant cells do not. The centrosome is a microtubule-organizing
center found near the nuclei of animal cells while lysosomes take
care of the cell’s digestive process.

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 KEY POINTS
Eukaryotic cells are larger than prokaryotic cells and have a
“true” nucleus, membrane-bound organelles, and rod-shaped
chromosomes.
The nucleus houses the cell’s DNA and directs the synthesis of
proteins and ribosomes.
Mitochondria are responsible for ATP production; the
endoplasmic reticulum modifies proteins and synthesizes lipids;
and the golgi apparatus is where the sorting of lipids and proteins
takes place.
Peroxisomes carry out oxidation reactions that break down fatty
acids and amino acids and detoxify poisons; vesicles and
vacuoles function in storage and transport.
Animal cells have a centrosome and lysosomes while plant cells
do not.
Plant cells have a cell wall, a large central vacuole, chloroplasts,
and other specialized plastids, whereas animal cells do not.

Figure 4.6.1: Animal Cells: Despite their fundamental similarities, KEY TERMS
there are some striking differences between animal and plant eukaryotic: Having complex cells in which the genetic material
cells.Animal cells have centrioles, centrosomes, and lysosomes,
whereas plant cells do not.
is organized into membrane-bound nuclei.
organelle: A specialized structure found inside cells that carries
In addition, plant cells have a cell wall, a large central vacuole,
out a specific life process (e.g. ribosomes, vacuoles).
chloroplasts, and other specialized plastids, whereas animal cells do
photosynthesis: the process by which plants and other
not. The cell wall protects the cell, provides structural support, and
photoautotrophs generate carbohydrates and oxygen from carbon
gives shape to the cell while the central vacuole plays a key role in
dioxide, water, and light energy in chloroplasts
regulating the cell’s concentration of water in changing
environmental conditions. Chloroplasts are the organelles that carry This page titled 4.6: Eukaryotic Cells - Characteristics of Eukaryotic Cells is
out photosynthesis. shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

Figure 4.6.1: Plant Cells: Plant cells have a cell wall, chloroplasts,
plasmodesmata, and plastids used for storage, and a large central
vacuole, whereas animal cells do not.

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4.7: EUKARYOTIC CELLS - THE PLASMA MEMBRANE AND THE CYTOPLASM

 LEARNING OBJECTIVES

Explain the structure and purpose of the plasma membrane

of a cell

THE PLASMA MEMBRANE

Despite differences in structure and function, all living cells in
multicellular organisms have a surrounding plasma membrane (also
known as the cell membrane). As the outer layer of your skin
separates your body from its environment, the plasma membrane
separates the inner contents of a cell from its exterior environment. Figure 4.7.1: Phospholipid Bilayer: The phospholipid bilayer
The plasma membrane can be described as a phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail.
The hydrophobic tails associate with one another, forming the
with embedded proteins that controls the passage of organic
interior of the membrane. The polar heads contact the fluid inside
molecules, ions, water, and oxygen into and out of the cell. Wastes and outside of the cell.
(such as carbon dioxide and ammonia) also leave the cell by passing The plasma membrane’s main function is to regulate the
through the membrane. concentration of substances inside the cell. These substances include
ions such as Ca++, Na+, K+, and Cl–; nutrients including sugars, fatty
acids, and amino acids; and waste products, particularly carbon
dioxide (CO2), which must leave the cell.
The membrane’s lipid bilayer structure provides the cell with access
control through permeability. The phospholipids are tightly packed
together, while the membrane has a hydrophobic interior. This
structure causes the membrane to be selectively permeable. A
membrane that has selective permeability allows only substances
meeting certain criteria to pass through it unaided. In the case of the
plasma membrane, only relatively small, non-polar materials can
Figure 4.7.1: Eukaryotic Plasma Membrane: The eukaryotic plasma move through the lipid bilayer (remember, the lipid tails of the
membrane is a phospholipid bilayer with proteins and cholesterol membrane are nonpolar). Some examples of these materials are
embedded in it.
other lipids, oxygen and carbon dioxide gases, and alcohol.
The cell membrane is an extremely pliable structure composed
However, water-soluble materials—such as glucose, amino acids,
primarily of two adjacent sheets of phospholipids. Cholesterol, also
and electrolytes—need some assistance to cross the membrane
present, contributes to the fluidity of the membrane. A single
because they are repelled by the hydrophobic tails of the
phospholipid molecule consists of a polar phosphate “head,” which
phospholipid bilayer.
is hydrophilic, and a non-polar lipid “tail,” which is hydrophobic.
Unsaturated fatty acids result in kinks in the hydrophobic tails. The TRANSPORT ACROSS THE MEMBRANE
phospholipid bilayer consists of two phospholipids arranged tail to
All substances that move through the membrane do so by one of two
tail. The hydrophobic tails associate with one another, forming the
general methods, which are categorized based on whether or not
interior of the membrane. The polar heads contact the fluid inside
energy is required. Passive (non-energy requiring) transport is the
and outside of the cell.
movement of substances across the membrane without the
expenditure of cellular energy. During this type of transport,
materials move by simple diffusion or by facilitated diffusion
through the membrane, down their concentration gradient. Water
passes through the membrane in a diffusion process called osmosis.
Osmosis is the diffusion of water through a semi-permeable
membrane down its concentration gradient. It occurs when there is
an imbalance of solutes outside of a cell versus inside the cell. The
solution that has the higher concentration of solutes is said to be
hypertonic and the solution that has the lower concentration of
solutes is said to be hypotonic. Water molecules will diffuse out of
the hypotonic solution and into the hypertonic solution (unless acted
upon by hydrostatic forces).

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 cytoplasm is the location for most cellular processes, including
metabolism, protein folding, and internal transportation.

KEY POINTS
All eukaryotic cells have a surrounding plasma membrane,
which is also known as the cell membrane.
The plasma membrane is made up by a phospholipid bilayer with
embedded proteins that separates the internal contents of the cell
from its surrounding environment.
Only relatively small, non- polar materials can easily move
through the lipid bilayer of the plasma membrane.
Passive transport is the movement of substances across the
Figure 4.7.1: Osmosis: Osmosis is the diffusion of water through a
membrane that does not require the use of energy while active
semipermeable membrane down its concentration gradient. If a
membrane is permeable to water, though not to a solute, water will transport is the movement of substances across the membrane
equalize its own concentration by diffusing to the side of lower using energy.
water concentration (and thus the side of higher solute
concentration). In the beaker on the left, the solution on the right
Osmosis is the diffusion of water through a semi- permeable
side of the membrane is hypertonic. membrane down its concentration gradient; this occurs when
there is an imbalance of solutes outside of a cell compared to the
In contrast to passive transport, active (energy-requiring) transport is
inside the cell.
the movement of substances across the membrane using energy from
adenosine triphosphate (ATP). The energy is expended to assist KEY TERMS
material movement across the membrane in a direction against their
phospholipid: Any lipid consisting of a diglyceride combined
concentration gradient. Active transport may take place with the
with a phosphate group and a simple organic molecule such as
help of protein pumps or through the use of vesicles. Another form
choline or ethanolamine; they are important constituents of
of this type of transport is endocytosis, where a cell envelopes
biological membranes
extracellular materials using its cell membrane. The opposite process
hypertonic: having a greater osmotic pressure than another
is known as exocytosis. This is where a cell exports material using
hypotonic: Having a lower osmotic pressure than another; a cell
vesicular transport.
in this environment causes water to enter the cell, causing it to
CYTOPLASM swell.
The cell’s plasma membrane also helps contain the cell’s cytoplasm, This page titled 4.7: Eukaryotic Cells - The Plasma Membrane and the
which provides a gel-like environment for the cell’s organelles. The Cytoplasm is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.

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4.8: EUKARYOTIC CELLS - THE NUCLEUS AND RIBOSOMES

 LEARNING OBJECTIVES

Explain the purpose of the nucleus in eukaryotic cells

THE NUCLEUS
One of the main differences between prokaryotic and eukaryotic
cells is the nucleus. As previously discussed, prokaryotic cells lack
an organized nucleus while eukaryotic cells contain membrane-
bound nuclei (and organelles ) that house the cell’s DNA and direct
the synthesis of ribosomes and proteins.
The nucleus stores chromatin (DNA plus proteins) in a gel-like
substance called the nucleoplasm. To understand chromatin, it is
helpful to first consider chromosomes. Chromatin describes the
material that makes up chromosomes, which are structures within
the nucleus that are made up of DNA, the hereditary material. You Figure 4.8.1: The nucleus stores the hereditary material of the cell:
The nucleus is the control center of the cell. The nucleus of living
may remember that in prokaryotes, DNA is organized into a single cells contains the genetic material that determines the entire
circular chromosome. In eukaryotes, chromosomes are linear structure and function of that cell.
structures. Every eukaryotic species has a specific number of The nucleoplasm is also where we find the nucleolus. The nucleolus
chromosomes in the nuclei of its body’s cells. For example, in is a condensed region of chromatin where ribosome synthesis
humans, the chromosome number is 46, while in fruit flies, it is occurs. Ribosomes, large complexes of protein and ribonucleic acid
eight. Chromosomes are only visible and distinguishable from one (RNA), are the cellular organelles responsible for protein synthesis.
another when the cell is getting ready to divide. In order to organize They receive their “orders” for protein synthesis from the nucleus
the large amount of DNA within the nucleus, proteins called where the DNA is transcribed into messenger RNA (mRNA). This
histones are attached to chromosomes; the DNA is wrapped around mRNA travels to the ribosomes, which translate the code provided
these histones to form a structure resembling beads on a string. by the sequence of the nitrogenous bases in the mRNA into a
These protein-chromosome complexes are called chromatin. specific order of amino acids in a protein.

Figure 4.8.1: Ribosomes are responsible for protein synthesis:

Ribosomes are made up of a large subunit (top) and a small subunit
(bottom). During protein synthesis, ribosomes assemble amino acids
Figure 4.8.1: DNA is highly organized: This image shows various into proteins.
levels of the organization of chromatin (DNA and protein). Along Lastly, the boundary of the nucleus is called the nuclear envelope. It
the chromatin threads, unwound protein-chromosome complexes,
we find DNA wrapped around a set of histone proteins. consists of two phospholipid bilayers: an outer membrane and an
inner membrane. The nuclear membrane is continuous with the
endoplasmic reticulum, while nuclear pores allow substances to
enter and exit the nucleus.

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KEY POINTS KEY TERMS
The nucleus contains the cell ‘s DNA and directs the synthesis of histone: any of various simple water-soluble proteins that are
ribosomes and proteins. rich in the basic amino acids lysine and arginine and are
Found within the nucleoplasm, the nucleolus is a condensed complexed with DNA in the nucleosomes of eukaryotic
region of chromatin where ribosome synthesis occurs. chromatin
Chromatin consists of DNA wrapped around histone proteins and nucleolus: a conspicuous, rounded, non-membrane bound body
is stored within the nucleoplasm. within the nucleus of a cell
Ribosomes are large complexes of protein and ribonucleic acid chromatin: a complex of DNA, RNA, and proteins within the
(RNA) responsible for protein synthesis when DNA from the cell nucleus out of which chromosomes condense during cell
nucleus is transcribed. division

This page titled 4.8: Eukaryotic Cells - The Nucleus and Ribosomes is
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

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4.9: EUKARYOTIC CELLS - MITOCHONDRIA
Cellular respiration is the process of making ATP using the chemical
 LEARNING OBJECTIVES energy found in glucose and other nutrients. In mitochondria, this
process uses oxygen and produces carbon dioxide as a waste
Explain the role of the mitochondria.
product. In fact, the carbon dioxide that you exhale with every
breath comes from the cellular reactions that produce carbon dioxide
One of the major features distinguishing prokaryotes from
as a by-product.
eukaryotes is the presence of mitochondria. Mitochondria are
It is important to point out that muscle cells have a very high
double-membraned organelles that contain their own ribosomes and
concentration of mitochondria that produce ATP. Your muscle cells
DNA. Each membrane is a phospholipid bilayer embedded with
need a lot of energy to keep your body moving. When your cells
proteins. Eukaryotic cells may contain anywhere from one to several
don’t get enough oxygen, they do not make a lot of ATP. Instead, the
thousand mitochondria, depending on the cell’s level of energy
small amount of ATP they make in the absence of oxygen is
consumption. Each mitochondrion measures 1 to 10 micrometers (or
accompanied by the production of lactic acid.
greater) in length and exists in the cell as an organelle that can be
ovoid to worm-shaped to intricately branched. In addition to the aerobic generation of ATP, mitochondria have
several other metabolic functions. One of these functions is to
MITOCHONDRIA STRUCTURE generate clusters of iron and sulfur that are important cofactors of
Most mitochondria are surrounded by two membranes, which would many enzymes. Such functions are often associated with the reduced
result when one membrane-bound organism was engulfed into a mitochondrion-derived organelles of anaerobic eukaryotes.
vacuole by another membrane-bound organism. The mitochondrial
inner membrane is extensive and involves substantial infoldings ORIGINS OF MITOCHONDRIA
called cristae that resemble the textured, outer surface of alpha- There are two hypotheses about the origin of mitochondria:
proteobacteria. The matrix and inner membrane are rich with the endosymbiotic and autogenous, but the most accredited theory at
enzymes necessary for aerobic respiration. present is endosymbiosis. The endosymbiotic hypothesis suggests
mitochondria were originally prokaryotic cells, capable of
implementing oxidative mechanisms. These prokaryotic cells may
have been engulfed by a eukaryote and became endosymbionts
living inside the eukaryote.

KEY POINTS
Mitochondria contain their own ribosomes and DNA; combined
with their double membrane, these features suggest that they
might have once been free-living prokaryotes that were engulfed
Figure 4.9.1: Mitochondrial structure: This electron micrograph by a larger cell.
shows a mitochondrion as viewed with a transmission electron
microscope. This organelle has an outer membrane and an inner Mitochondria have an important role in cellular respiration
membrane. The inner membrane contains folds, called cristae, which through the production of ATP, using chemical energy found in
increase its surface area. The space between the two membranes is glucose and other nutrients.
called the intermembrane space, and the space inside the inner
membrane is called the mitochondrial matrix. ATP synthesis takes Mitochondria are also responsible for generating clusters of iron
place on the inner membrane. and sulfur, which are important cofactors of many enzymes.
Mitochondria have their own (usually) circular DNA chromosome
that is stabilized by attachments to the inner membrane and carries KEY TERMS
genes similar to genes expressed by alpha-proteobacteria. alpha-proteobacteria: A taxonomic class within the phylum
Mitochondria also have special ribosomes and transfer RNAs that Proteobacteria — the phototropic proteobacteria.
resemble these components in prokaryotes. These features all adenosine triphosphate: a multifunctional nucleoside
support the hypothesis that mitochondria were once free-living triphosphate used in cells as a coenzyme, often called the
prokaryotes. “molecular unit of energy currency” in intracellular energy
transfer
MITOCHONDRIA FUNCTION cofactor: an inorganic molecule that is necessary for an enzyme
Mitochondria are often called the “powerhouses” or “energy to function
factories” of a cell because they are responsible for making
This page titled 4.9: Eukaryotic Cells - Mitochondria is shared under a CC
adenosine triphosphate (ATP), the cell’s main energy-carrying
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
molecule. ATP represents the short-term stored energy of the cell.

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4.10: EUKARYOTIC CELLS - COMPARING PLANT AND ANIMAL CELLS
the pH within lysosomes is more acidic than the pH of the
 LEARNING OBJECTIVES cytoplasm. Many reactions that take place in the cytoplasm could
not occur at a low pH, so the advantage of compartmentalizing the
Differentiate between the structures found in animal and
eukaryotic cell into organelles is apparent.
plant cells
THE CELL WALL
ANIMAL CELLS VERSUS PLANT CELLS The cell wall is a rigid covering that protects the cell, provides
Each eukaryotic cell has a plasma membrane, cytoplasm, a nucleus, structural support, and gives shape to the cell. Fungal and protistan
ribosomes, mitochondria, peroxisomes, and in some, vacuoles; cells also have cell walls. While the chief component of prokaryotic
however, there are some striking differences between animal and cell walls is peptidoglycan, the major organic molecule in the plant
plant cells. While both animal and plant cells have microtubule cell wall is cellulose, a polysaccharide comprised of glucose units.
organizing centers (MTOCs), animal cells also have centrioles When you bite into a raw vegetable, like celery, it crunches. That’s
associated with the MTOC: a complex called the centrosome. because you are tearing the rigid cell walls of the celery cells with
Animal cells each have a centrosome and lysosomes, whereas plant your teeth.
cells do not. Plant cells have a cell wall, chloroplasts and other
specialized plastids, and a large central vacuole, whereas animal
cells do not.

THE CENTROSOME
The centrosome is a microtubule-organizing center found near the Figure 4.10.1: Cellulose: Cellulose is a long chain of β-glucose
nuclei of animal cells. It contains a pair of centrioles, two structures molecules connected by a 1-4 linkage. The dashed lines at each end
of the figure indicate a series of many more glucose units. The size
that lie perpendicular to each other. Each centriole is a cylinder of of the page makes it impossible to portray an entire cellulose
nine triplets of microtubules. The centrosome (the organelle where molecule.
all microtubules originate) replicates itself before a cell divides, and
the centrioles appear to have some role in pulling the duplicated CHLOROPLASTS
chromosomes to opposite ends of the dividing cell. However, the Like mitochondria, chloroplasts have their own DNA and
exact function of the centrioles in cell division isn’t clear, because ribosomes, but chloroplasts have an entirely different function.
cells that have had the centrosome removed can still divide; and Chloroplasts are plant cell organelles that carry out photosynthesis.
plant cells, which lack centrosomes, are capable of cell division. Photosynthesis is the series of reactions that use carbon dioxide,
water, and light energy to make glucose and oxygen. This is a major
difference between plants and animals; plants (autotrophs) are able
to make their own food, like sugars, while animals (heterotrophs)
must ingest their food.
Like mitochondria, chloroplasts have outer and inner membranes,
but within the space enclosed by a chloroplast’s inner membrane is a
set of interconnected and stacked fluid-filled membrane sacs called
thylakoids. Each stack of thylakoids is called a granum (plural =
grana). The fluid enclosed by the inner membrane that surrounds the
grana is called the stroma.

Figure 4.10.1: The Centrosome Structure: The centrosome consists

of two centrioles that lie at right angles to each other. Each centriole
is a cylinder made up of nine triplets of microtubules. Nontubulin
proteins (indicated by the green lines) hold the microtubule triplets
together.

LYSOSOMES
Animal cells have another set of organelles not found in plant cells:
lysosomes. The lysosomes are the cell’s “garbage disposal.” In plant
cells, the digestive processes take place in vacuoles. Enzymes within
the lysosomes aid the breakdown of proteins, polysaccharides,
lipids, nucleic acids, and even worn-out organelles. These enzymes
are active at a much lower pH than that of the cytoplasm. Therefore,

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Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m46073/latest/. License: CC BY: Attribution KEY POINTS
OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44407/latest...ol11448/latest. License: CC Centrosomes and lysosomes are found in animal cells, but do not exist within
BY: Attribution plant cells.
OpenStax College, Eukaryotic Cells. October 16, 2013. Provided by: OpenStax The lysosomes are the animal cell’s “garbage disposal”, while in plant cells the
CNX. Located at: http://cnx.org/content/m44407/latest...e_04_03_07.jpg. same function takes place in vacuoles.
License: CC BY: Attribution Plant cells have a cell wall, chloroplasts and other specialized plastids, and a
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. large central vacuole, which are not found within animal cells.
Located at: http://cnx.org/content/m44407/latest...ol11448/latest. License: CC The cell wall is a rigid covering that protects the cell, provides structural support,
BY: Attribution and gives shape to the cell.
protist. Provided by: Wiktionary. Located at: The chloroplasts, found in plant cells, contain a green pigment called chlorophyll,
http://en.wiktionary.org/wiki/protist. License: CC BY-SA: Attribution- which captures the light energy that drives the reactions of plant photosynthesis.
ShareAlike The central vacuole plays a key role in regulating a plant cell’s concentration of
heterotroph. Provided by: Wiktionary. Located at: water in changing environmental conditions.
en.wiktionary.org/wiki/heterotroph. License: CC BY-SA: Attribution-
ShareAlike KEY TERMS
autotroph. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/autotroph. License: CC BY-SA: Attribution- protist: Any of the eukaryotic unicellular organisms including protozoans, slime
ShareAlike molds and some algae; historically grouped into the kingdom Protoctista.
OpenStax College, Eukaryotic Cells. October 22, 2013. Provided by: OpenStax autotroph: Any organism that can synthesize its food from inorganic substances,
CNX. Located at: http://cnx.org/content/m45432/latest/. License: CC BY: using heat or light as a source of energy
Attribution heterotroph: an organism that requires an external supply of energy in the form
OpenStax College, Eukaryotic Cells. October 16, 2013. Provided by: OpenStax of food, as it cannot synthesize its own
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4.11: THE ENDOMEMBRANE SYSTEM AND PROTEINS - VESICLES AND
VACUOLES
cytoplasm could not occur at a low pH, so again, the advantage of
 LEARNING OBJECTIVES compartmentalizing the eukaryotic cell into organelles is apparent.

Summarize the functions of vesicles and vacuoles in cells VACUOLES

Vacuoles are an essential component of plant cells. If you look at the
Vesicles and vacuoles are membrane-bound sacs that function in figure below, you will see that plant cells each have a large central
storage and transport. Other than the fact that vacuoles are somewhat vacuole that occupies most of the area of the cell. The central
larger than vesicles, there is a very subtle distinction between them: vacuole plays a key role in regulating the cell’s concentration of
the membranes of vesicles can fuse with either the plasma water in changing environmental conditions, and houses the
membrane or other membrane systems within the cell. The digestive processes.
membrane of a vacuole does not fuse with the membranes of other
Have you ever noticed that if you forget to water a plant for a few
cellular components. Additionally, some agents within plant
days, it wilts? That’s because as the water concentration in the soil
vacuoles, such as enzymes, break down macromolecules.
becomes lower than the water concentration in the plant, water
VESICLES moves out of the central vacuoles and cytoplasm. As the central
vacuole shrinks, it leaves the cell wall unsupported. This loss of
A vesicle is a small structure within a cell, consisting of fluid
support to the cell walls of plant cells results in the wilted
enclosed by a lipid bilayer. Vesicles form naturally during the
appearance of the plant.
processes of secretion (exocytosis), uptake (phagocytosis) and
transport of materials within the cytoplasm. Alternatively, they may
be prepared artificially, in which case they are called liposomes.
Vesicles can fuse with the plasma membrane to release their contents
outside the cell. Vesicles can also fuse with other organelles within
the cell.

Figure 4.11.1: Plant Cell Structure: The Central Vacuole in a typical

plant cell is quite large and is surrounded by the tonoplast or
vacuolar membrane.

Figure 4.11.1: Animal Cell: In this animal cell illustration #4 The central vacuole also supports the expansion of the cell. When
denotes a vacuole. the central vacuole holds more water, the cell gets larger without
Vesicles perform a variety of functions. Because they are separated having to invest a lot of energy in synthesizing new cytoplasm.
from the cytosol, the inside of a vesicle can be different from the Contractile vacuoles are found in certain protists, especially those in
cytosolic environment. For this reason, vesicles are a basic tool used Phylum Ciliophora. These vacuoles take water from the cytoplasm
by the cell for organizing cellular substances. Vesicles are involved and excrete it from the cell to avoid bursting due to osmotic
in metabolism, transport, buoyancy control, and enzyme storage. pressure.
They can also act as chemical reaction chambers.
KEY POINTS
LYSOSOMES Vesicles are small structures within a cell, consisting of fluid
Animal cells have a set of organelles not found in plant cells: enclosed by a lipid bilayer involved in transport, buoyancy
lysosomes. Lysosomes are a cell’s “garbage disposal.” Enzymes control, and enzyme storage.
within the lysosomes aid the breakdown of proteins, Lysosomes, which are found in animal cells, are the cell’s
polysaccharides, lipids, nucleic acids, and worn-out organelles. “garbage disposal.” The digestive processes take place in these,
These enzymes are active at a much lower pH than that of the and enzymes within them aid in the breakdown of proteins,
cytoplasm. Therefore, the pH within lysosomes is more acidic than polysaccharides, lipids, nucleic acids, and worn-out organelles.
the pH of the cytoplasm. Many reactions that take place in the Central vacuoles, which are found in plants, play a key role in
regulating the cell’s concentration of water in changing

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4.12: THE ENDOMEMBRANE SYSTEM AND PROTEINS - THE ENDOPLASMIC
RETICULUM

 LEARNING OBJECTIVES

Describe the structure of the endoplasmic reticulum and its

role in synthesis and metabolism

THE ENDOPLASMIC RETICULUM

The endoplasmic reticulum (ER) is a series of interconnected
membranous sacs and tubules that collectively modifies proteins and
synthesizes lipids. However, these two functions are performed in
separate areas of the ER: the rough ER and the smooth ER. The Figure 4.12.1: Rough Endoplasmic Reticulum: This transmission
hollow portion of the ER tubules is called the lumen or cisternal electron micrograph shows the rough endoplasmic reticulum and
other organelles in a pancreatic cell.
space. The membrane of the ER, which is a phospholipid bilayer
embedded with proteins, is continuous with the nuclear envelope. SMOOTH ER
The smooth endoplasmic reticulum (SER) is continuous with the
ROUGH ER
RER but has few or no ribosomes on its cytoplasmic surface.
The rough endoplasmic reticulum (RER) is so named because the Functions of the SER include synthesis of carbohydrates, lipids, and
ribosomes attached to its cytoplasmic surface give it a studded steroid hormones; detoxification of medications and poisons; and
appearance when viewed through an electron microscope. storage of calcium ions. In muscle cells, a specialized SER called the
Ribosomes transfer their newly synthesized proteins into the lumen sarcoplasmic reticulum is responsible for storage of the calcium ions
of the RER where they undergo structural modifications, such as that are needed to trigger the coordinated contractions of the muscle
folding or the acquisition of side chains. These modified proteins cells.
will be incorporated into cellular membranes—the membrane of the
ER or those of other organelles —or secreted from the cell (such as KEY POINTS
protein hormones, enzymes ). The RER also makes phospholipids If the endoplasmic reticulum (ER) has ribosomes attached to it, it
for cellular membranes. If the phospholipids or modified proteins is called rough ER; if it does not, then it is called smooth ER.
are not destined to stay in the RER, they will reach their destinations The proteins made by the rough endoplasmic reticulum are for
via transport vesicles that bud from the RER’s membrane. Since the use outside of the cell.
RER is engaged in modifying proteins (such as enzymes, for Functions of the smooth endoplasmic reticulum include synthesis
example) that will be secreted from the cell, the RER is abundant in of carbohydrates, lipids, and steroid hormones; detoxification of
cells that secrete proteins. This is the case with cells of the liver, for medications and poisons; and storage of calcium ions.
example.
KEY TERMS
lumen: The cavity or channel within a tube or tubular organ.
reticulum: A network

This page titled 4.12: The Endomembrane System and Proteins - The
Endoplasmic Reticulum is shared under a CC BY-SA 4.0 license and was
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4.13: THE ENDOMEMBRANE SYSTEM AND PROTEINS - THE GOLGI
APPARATUS
modification is the addition of short chains of sugar molecules.
 LEARNING OBJECTIVES These newly-modified proteins and lipids are then tagged with
phosphate groups or other small molecules so that they can be routed
Describe the structure of the Golgi apparatus and its role in
to their proper destinations.
protein modification and secretion
Finally, the modified and tagged proteins are packaged into
secretory vesicles that bud from the trans face of the Golgi. While
We have already mentioned that vesicles can bud from the ER and
some of these vesicles deposit their contents into other parts of the
transport their contents elsewhere, but where do the vesicles go?
cell where they will be used, other secretory vesicles fuse with the
Before reaching their final destination, the lipids or proteins within
plasma membrane and release their contents outside the cell.
the transport vesicles still need to be sorted, packaged, and tagged so
that they wind up in the right place. Sorting, tagging, packaging, and In another example of form following function, cells that engage in a
distribution of lipids and proteins takes place in the Golgi apparatus great deal of secretory activity (such as cells of the salivary glands
(also called the Golgi body), a series of flattened membranes. that secrete digestive enzymes or cells of the immune system that
secrete antibodies) have an abundance of Golgi. In plant cells, the
Golgi apparatus has the additional role of synthesizing
polysaccharides, some of which are incorporated into the cell wall
and some of which are used in other parts of the cell.

KEY POINTS
The Golgi apparatus is a series of flattened sacs that sort and
package cellular materials.
The Golgi apparatus has a cis face on the ER side and a trans
face opposite of the ER.
Figure 4.13.1: The Golgi apparatus sorts and packages cellular The trans face secretes the materials into vesicles, which then
products: The Golgi apparatus in this white blood cell is visible as a fuse with the cell membrane for release from the cell.
stack of semicircular, flattened rings in the lower portion of the
image. Several vesicles can be seen near the Golgi apparatus.
KEY TERMS
The receiving side of the Golgi apparatus is called the cis face. The
vesicle: A membrane-bound compartment found in a cell.
opposite side is called the trans face. The transport vesicles that
formed from the ER travel to the cis face, fuse with it, and empty This page titled 4.13: The Endomembrane System and Proteins - The Golgi
their contents into the lumen of the Golgi apparatus. As the proteins Apparatus is shared under a CC BY-SA 4.0 license and was authored,
and lipids travel through the Golgi, they undergo further remixed, and/or curated by Boundless.
modifications that allow them to be sorted. The most frequent

4.13.1 https://bio.libretexts.org/@go/page/12723
4.14: THE ENDOMEMBRANE SYSTEM AND PROTEINS - LYSOSOMES
A lysosome is composed of lipids, which make up the membrane,
 LEARNING OBJECTIVES and proteins, which make up the enzymes within the membrane.
Usually, lysosomes are between 0.1 to 1.2μm, but the size varies
Describe how lysosomes function as the cell’s waste
based on the cell type. The general structure of a lysosome consists
disposal system
of a collection of enzymes surrounded by a single-layer membrane.
The membrane is a crucial aspect of its structure because without it
A lysosome has three main functions: the breakdown/digestion of
the enzymes within the lysosome that are used to breakdown foreign
macromolecules (carbohydrates, lipids, proteins, and nucleic acids),
substances would leak out and digest the entire cell, causing it to die.
cell membrane repairs, and responses against foreign substances
Lysosomes are found in nearly every animal-like eukaryotic cell.
such as bacteria, viruses and other antigens. When food is eaten or
They are so common in animal cells because, when animal cells take
absorbed by the cell, the lysosome releases its enzymes to break
in or absorb food, they need the enzymes found in lysosomes in
down complex molecules including sugars and proteins into usable
order to digest and use the food for energy. On the other hand,
energy needed by the cell to survive. If no food is provided, the
lysosomes are not commonly-found in plant cells. Lysosomes are
lysosome’s enzymes digest other organelles within the cell in order
not needed in plant cells because they have cell walls that are tough
to obtain the necessary nutrients.
enough to keep the large/foreign substances that lysosomes would
In addition to their role as the digestive component and organelle- usually digest out of the cell.
recycling facility of animal cells, lysosomes are considered to be
parts of the endomembrane system. Lysosomes also use their KEY POINTS
hydrolytic enzymes to destroy pathogens (disease-causing Lysosomes breakdown/digest macromolecules (carbohydrates,
organisms) that might enter the cell. A good example of this occurs lipids, proteins, and nucleic acids), repair cell membranes, and
in a group of white blood cells called macrophages, which are part respond against foreign substances such as bacteria, viruses and
of your body’s immune system. In a process known as phagocytosis other antigens.
or endocytosis, a section of the plasma membrane of the macrophage Lysosomes contain enzymes that break down the
invaginates (folds in) and engulfs a pathogen. The invaginated macromolecules and foreign invaders.
section, with the pathogen inside, then pinches itself off from the Lysosomes are composed of lipids and proteins, with a single
plasma membrane and becomes a vesicle. The vesicle fuses with a membrane covering the internal enzymes to prevent the
lysosome. The lysosome’s hydrolytic enzymes then destroy the lysosome from digesting the cell itself.
pathogen. Lysosomes are found in all animal cells, but are rarely found
within plant cells due to the tough cell wall surrounding a plant
cell that keeps out foreign substances.

KEY TERMS
enzyme: a globular protein that catalyses a biological chemical
reaction
lysosome: An organelle found in all types of animal cells which
contains a large range of digestive enzymes capable of splitting
most biological macromolecules.

This page titled 4.14: The Endomembrane System and Proteins - Lysosomes
is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

Figure 4.14.1: Lysosomes digest foreign substances that might harm

the cell: A macrophage has engulfed (phagocytized) a potentially
pathogenic bacterium and then fuses with a lysosomes within the
cell to destroy the pathogen. Other organelles are present in the cell
but for simplicity are not shown.

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4.15: THE ENDOMEMBRANE SYSTEM AND PROTEINS - PEROXISOMES
products are then safely released into the cytoplasm. Like miniature
 LEARNING OBJECTIVES sewage treatment plants, peroxisomes neutralize harmful toxins so
that they do not cause damage in the cells. The liver is the organ
Name the various functions that peroxisomes perform inside
primarily responsible for detoxifying the blood before it travels
the cell
throughout the body; liver cells contain an exceptionally high
number of peroxisomes.
PEROXISOMES
A type of organelle found in both animal cells and plant cells, a KEY POINTS
peroxisome is a membrane-bound cellular organelle that contains Lipid metabolism and chemical detoxification are important
mostly enzymes. Peroxisomes perform important functions, functions of peroxisomes.
including lipid metabolism and chemical detoxification. They also Peroxisomes are responsible for oxidation reactions that break
carry out oxidation reactions that break down fatty acids and amino down fatty acids and amino acids.
acids. Peroxisomes oversee reactions that neutralize free radicals,
which cause cellular damage and cell death.
Peroxisomes chemically neutralize poisons through a process
that produces large amounts of toxic H2O2, which is then
converted into water and oxygen.
The liver is the organ primarily responsible for detoxifying the
blood before it travels throughout the body; as a result, liver cells
contain large amounts of peroxisomes.

KEY TERMS
enzyme: a globular protein that catalyses a biological chemical
reaction
free radical: Any molecule, ion or atom that has one or more
unpaired electrons; they are generally highly reactive and often
only occur as transient species.

CONTRIBUTIONS AND ATTRIBUTIONS

Vacuole. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Vacuole.
Figure 4.15.1: Peroxisomes: Peroxisomes are membrane-bound License: CC BY-SA: Attribution-ShareAlike
organelles that contain an abundance of enzymes for detoxifying Eukaryotic Cells. Provided by: OpenStax CNX. Located at:
harmful substances and lipid metabolism. http://cnx.org/contents/GFy_h8cu@9.8...karyotic-Cells. License: CC BY:
Attribution
In contrast to the digestive enzymes found in lysosomes, the Vesicles. Provided by: Wikipedia. Located at:
enzymes within peroxisomes serve to transfer hydrogen atoms from en.Wikipedia.org/wiki/Vesicl...and_chemistry). License: CC BY-SA:
Attribution-ShareAlike
various molecules to oxygen, producing hydrogen peroxide (H2O2). Lysosome. Provided by: Wikipedia. Located at:
In this way, peroxisomes neutralize poisons, such as alcohol, that en.Wikipedia.org/wiki/Lysosome. License: CC BY: Attribution
649px-Plant_cell_structure_svg_vacuole.svg.png. Provided by: Wikipedia.
enter the body. In order to appreciate the importance of peroxisomes,
Located at: commons.wikimedia.org/wiki/F...vg_vacuole.svg. License: Public
it is necessary to understand the concept of reactive oxygen species. Domain: No Known Copyright
Animal Cell. Provided by: Wikipedia. Located at:
Reactive oxygen species (ROS), such as peroxides and free radicals, commons.wikimedia.org/wiki/F...nimal_Cell.svg. License: Public Domain: No
are the highly-reactive products of many normal cellular processes, Known Copyright
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including the mitochondrial reactions that produce ATP and oxygen Located at: http://cnx.org/content/m44435/latest...ol11448/latest. License: CC
metabolism. Examples of ROS include the hydroxyl radical OH, BY: Attribution
reticulum. Provided by: Wiktionary. Located at:
H2O2, and superoxide (O−2). Some ROS are important for certain en.wiktionary.org/wiki/reticulum. License: CC BY-SA: Attribution-ShareAlike
cellular functions, such as cell signaling processes and immune lumen. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/lumen.
License: CC BY-SA: Attribution-ShareAlike
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harmful to the body. Free radicals are reactive because they contain Located at: commons.wikimedia.org/wiki/F...vg_vacuole.svg. License: Public
free unpaired electrons; they can easily oxidize other molecules Domain: No Known Copyright
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throughout the cell, causing cellular damage and even cell death. commons.wikimedia.org/wiki/F...nimal_Cell.svg. License: Public Domain: No
Free radicals are thought to play a role in many destructive processes Known Copyright
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Located at: http://cnx.org/content/m44435/latest...ol11448/latest. License: CC
enzymes that convert H2O2 into water and oxygen. These by-
BY: Attribution

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vesicle. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/vesicle. http://cnx.org/content/m44435/latest...e_04_04_04.jpg. License: CC BY:
License: CC BY-SA: Attribution-ShareAlike Attribution
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Attribution This page titled 4.15: The Endomembrane System and Proteins -
OpenStax College, The Endomembrane System and Proteins. October 16, 2013.
Provided by: OpenStax CNX. Located at: Peroxisomes is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.

4.15.2 https://bio.libretexts.org/@go/page/12725
4.16: THE CYTOSKELETON - MICROFILAMENTS
Actin is powered by ATP to assemble its filamentous form, which
 LEARNING OBJECTIVES serves as a track for the movement of a motor protein called myosin.
This enables actin to engage in cellular events requiring motion such
Describe the structure and function of microfilaments
as cell division in animal cells and cytoplasmic streaming, which is
the circular movement of the cell cytoplasm in plant cells. Actin and
MICROFILAMENTS myosin are plentiful in muscle cells. When your actin and myosin
If all the organelles were removed from a cell, the plasma membrane filaments slide past each other, your muscles contract.
and the cytoplasm would not be the only components left. Within the Microfilaments also provide some rigidity and shape to the cell.
cytoplasm there would still be ions and organic molecules, plus a They can depolymerize (disassemble) and reform quickly, thus
network of protein fibers that help maintain the shape of the cell, enabling a cell to change its shape and move. White blood cells
secure some organelles in specific positions, allow cytoplasm and (your body’s infection-fighting cells) make good use of this ability.
vesicles to move within the cell, and enable unicellular organisms to They can move to the site of an infection and engulf the pathogen.
move independently. This network of protein fibers is known as the
cytoskeleton. There are three types of fibers within the cytoskeleton: KEY POINTS
microfilaments, intermediate filaments, and microtubules. Of the Microfilaments assist with cell movement and are made of a
three types of protein fibers in the cytoskeleton, microfilaments are protein called actin.
the narrowest. They function in cellular movement, have a diameter Actin works with another protein called myosin to produce
of about 7 nm, and are made of two intertwined strands of a globular muscle movements, cell division, and cytoplasmic streaming.
protein called actin. For this reason, microfilaments are also known Microfilaments keep organelles in place within the cell.
as actin filaments.
KEY TERMS
actin: A globular structural protein that polymerizes in a helical
fashion to form an actin filament (or microfilament).
filamentous: Having the form of threads or filaments
myosin: a large family of motor proteins found in eukaryotic
tissues, allowing mobility in muscles

This page titled 4.16: The Cytoskeleton - Microfilaments is shared under a

CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Boundless.

Figure 4.16.1: Microfilaments are the thinnest component of the

cytoskeleton.: Microfilaments are made of two intertwined strands
of actin.

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4.17: THE CYTOSKELETON - INTERMEDIATE FILAMENTS AND
MICROTUBULES
INTERMEDIATE FILAMENTS
 LEARNING OBJECTIVES Intermediate filaments (IFs) are cytoskeletal components found in
Describe the roles of microtubules as part of the cell’s animal cells. They are composed of a family of related proteins
cytoskeleton sharing common structural and sequence features. Intermediate
filaments have an average diameter of 10 nanometers, which is
between that of 7 nm actin (microfilaments), and that of 25 nm
MICROTUBULES
microtubules, although they were initially designated ‘intermediate’
As their name implies, microtubules are small hollow tubes. because their average diameter is between those of narrower
Microtubules, along with microfilaments and intermediate filaments, microfilaments (actin) and wider myosin filaments found in muscle
come under the class of organelles known as the cytoskeleton. The cells. Intermediate filaments contribute to cellular structural
cytoskeleton is the framework of the cell which forms the structural elements and are often crucial in holding together tissues like skin.
supporting component. Microtubules are the largest element of the
cytoskeleton. The walls of the microtubule are made of polymerized FLAGELLA AND CILIA
dimers of α-tubulin and β-tubulin, two globular proteins. With a Flagella (singular = flagellum ) are long, hair-like structures that
diameter of about 25 nm, microtubules are the widest components of extend from the plasma membrane and are used to move an entire
the cytoskeleton. They help the cell resist compression, provide a cell (for example, sperm, Euglena). When present, the cell has just
track along which vesicles move through the cell, and pull replicated one flagellum or a few flagella. When cilia (singular = cilium) are
chromosomes to opposite ends of a dividing cell. Like present, however, many of them extend along the entire surface of
microfilaments, microtubules can dissolve and reform quickly. the plasma membrane. They are short, hair-like structures that are
used to move entire cells (such as paramecia) or substances along
the outer surface of the cell (for example, the cilia of cells lining the
Fallopian tubes that move the ovum toward the uterus, or cilia lining
the cells of the respiratory tract that trap particulate matter and move
it toward your nostrils).
Despite their differences in length and number, flagella and cilia
share a common structural arrangement of microtubules called a “9
+ 2 array.” This is an appropriate name because a single flagellum or
cilium is made of a ring of nine microtubule doublets surrounding a
single microtubule doublet in the center.

Figure 4.17.1: Micrtubule Structure: Microtubules are hollow, with

walls consisting of 13 polymerized dimers of α-tubulin and β-tubulin
(right image). The left image shows the molecular structure of the
tube.
Microtubules are also the structural elements of flagella, cilia, and
centrioles (the latter are the two perpendicular bodies of the
centrosome ). In animal cells, the centrosome is the microtubule-
organizing center. In eukaryotic cells, flagella and cilia are quite
different structurally from their counterparts in prokaryotes.

Figure 4.17.1: Microtubules are the structural component of flagella:

This transmission electron micrograph of two flagella shows the 9 +
2 array of microtubules: nine microtubule doublets surround a single
microtubule doublet.

KEY POINTS
Microtubules help the cell resist compression, provide a track
Figure 4.17.1: Stained Keratin Intermediate filaments: Keratin
cytoskeletal intermediate filaments are concentrated around the edge along which vesicles can move throughout the cell, and are the
of the cells and merge into the surface membrane. This network of components of cilia and flagella.
intermediate filaments from cell to cell holds together tissues like Cilia and flagella are hair-like structures that assist with
skin.
locomotion in some cells, as well as line various structures to

4.17.1 https://bio.libretexts.org/@go/page/12730
trap particles. KEY TERMS
The structures of cilia and flagella are a “9+2 array,” meaning microtubule: Small tubes made of protein and found in cells;
that a ring of nine microtubules is surrounded by two more part of the cytoskeleton
microtubules. flagellum: a flagellum is a lash-like appendage that protrudes
Microtubules attach to replicated chromosomes during cell from the cell body of certain prokaryotic and eukaryotic cells
division and pull them apart to opposite ends of the pole, cytoskeleton: A cellular structure like a skeleton, contained
allowing the cell to divide with a complete set of chromosomes within the cytoplasm.
in each daughter cell.
This page titled 4.17: The Cytoskeleton - Intermediate Filaments and
Microtubules is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.

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4.18: CONNECTIONS BETWEEN CELLS AND CELLULAR ACTIVITIES -
EXTRACELLULAR MATRIX OF ANIMAL CELLS
the molecular structure of the receptor. The receptor, in turn,
 LEARNING OBJECTIVES changes the conformation of the microfilaments positioned just
inside the plasma membrane. These conformational changes induce
Explain the role of the extracellular matrix in animal cells
chemical signals inside the cell that reach the nucleus and turn “on”
or “off” the transcription of specific sections of DNA. This affects
EXTRACELLULAR MATRIX OF ANIMAL CELLS the production of associated proteins, thus changing the activities
Most animal cells release materials into the extracellular space. The within the cell.
primary components of these materials are proteins. Collagen is the An example of the role of the extracellular matrix in cell
most abundant of the proteins. Its fibers are interwoven with communication can be seen in blood clotting. When the cells lining
carbohydrate-containing protein molecules called proteoglycans. a blood vessel are damaged, they display a protein receptor called
Collectively, these materials are called the extracellular matrix. Not tissue factor. When a tissue factor binds with another factor in the
only does the extracellular matrix hold the cells together to form a extracellular matrix, it causes platelets to adhere to the wall of the
tissue, but it also allows the cells within the tissue to communicate damaged blood vessel and stimulates the adjacent smooth muscle
with each other. cells in the blood vessel to contract (thus constricting the blood
vessel). Subsequently, a series of steps are initiated which then
prompt the platelets to produce clotting factors.

KEY POINTS
The extracellular matrix of animal cells is made up of proteins
and carbohydrates.
Cell communication within tissue and tissue formation are main
functions of the extracellular matrix of animal cells.
Tissue communication is kick-started when a molecule within the
matrix binds a receptor; the end results are conformational
changes that induce chemical signals that ultimately change
activities within the cell.

KEY TERMS
collagen: Any of more than 28 types of glycoprotein that forms
elongated fibers, usually found in the extracellular matrix of
connective tissue.
proteoglycan: Any of many glycoproteins that have
heteropolysaccharide side chains
extracellular matrix: All the connective tissues and fibres that
Figure 4.18.1: The Extracellular Matrix: The extracellular matrix are not part of a cell, but rather provide support.
consists of a network of proteins and carbohydrates.
How does this cell communication occur? Cells have protein This page titled 4.18: Connections between Cells and Cellular Activities -
receptors on the extracellular surfaces of their plasma membranes. Extracellular Matrix of Animal Cells is shared under a CC BY-SA 4.0
When a molecule within the matrix binds to the receptor, it changes license and was authored, remixed, and/or curated by Boundless.

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4.19: CONNECTIONS BETWEEN CELLS AND CELLULAR ACTIVITIES -
INTERCELLULAR JUNCTIONS
in epithelial tissues that line internal organs and cavities and
 LEARNING OBJECTIVES comprise most of the skin. For example, the tight junctions of the
epithelial cells lining your urinary bladder prevent urine from
Describe the purpose of intercellular junctions in the
leaking out into the extracellular space.
structure of cells

INTERCELLULAR JUNCTIONS
The extracellular matrix allows cellular communication within
tissues through conformational changes that induce chemical
signals, which ultimately transform activities within the cell.
However, cells are also capable of communicating with each other
via direct contact through intercellular junctions.
There are some differences in the ways that plant and animal cells
communicate directly. Plasmodesmata are junctions between plant
cells, whereas animal cell contacts are carried out through tight
junctions, gap junctions, and desmosomes.

JUNCTIONS IN PLANT CELLS

In general, long stretches of the plasma membranes of neighboring
plant cells cannot touch one another because they are separated by
the cell wall that surrounds each cell. How then can a plant transfer
Figure 4.19.1: Tight Junctions: Tight junctions form watertight
water and other soil nutrients from its roots, through its stems, and to connections between adjacent animal cells. Proteins create tight
its leaves? This transport primarily uses the vascular tissues (xylem junction adherence.
and phloem); however, there are also structural modifications called Also found only in animal cells are desmosomes, the second type of
plasmodesmata (singular: plasmodesma) that facilitate direct intercellular junctions in these cell types. Desmosomes act like spot
communication in plant cells. Plasmodesmata are numerous welds between adjacent epithelial cells, connecting them. Short
channels that pass between cell walls of adjacent plant cells and proteins called cadherins in the plasma membrane connect to
connect their cytoplasm; thereby, enabling materials to be intermediate filaments to create desmosomes. The cadherins join
transported from cell to cell, and thus throughout the plant. two adjacent cells together and maintain the cells in a sheet-like
formation in organs and tissues that stretch, such as the skin, heart,
and muscles.

Figure 4.19.1: Plasmodesmata: A plasmodesma is a channel

between the cell walls of two adjacent plant cells. Plasmodesmata
allow materials to pass from the cytoplasm of one plant cell to the
cytoplasm of an adjacent cell.

JUNCTIONS IN ANIMAL CELLS

Communication between animal cells can be carried out through Figure 4.19.1: Desmosomes: A desmosome forms a very strong spot
weld between cells. It is created by the linkage of cadherins and
three types of junctions. The first, a tight junction, is a watertight intermediate filaments.
seal between two adjacent animal cells. The cells are held tightly Lastly, similar to plasmodesmata in plant cells, gap junctions are the
against each other by proteins (predominantly two proteins called third type of direct junction found within animal cells. These
claudins and occludins). This tight adherence prevents materials junctions are channels between adjacent cells that allow for the
from leaking between the cells. These junctions are typically found transport of ions, nutrients, and other substances that enable cells to

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http://en.wiktionary.org/wiki/collagen. License: CC BY-SA: Attribution-
adjacent animal cells align, a channel between the two cells forms. ShareAlike
Gap junctions are particularly important in cardiac muscle. The OpenStax College, Connections between Cells and Cellular Activities. October
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contract in tandem. OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
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KEY POINTS OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44413/latest/?collection=col11448/latest.
Plasmodesmata are intercellular junctions between plant cells License: CC BY: Attribution
that enable the transportation of materials between cells. plasmodesma. Provided by: Wiktionary. Located at:
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A tight junction is a watertight seal between two adjacent animal ShareAlike
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Desmosomes connect adjacent cells when cadherins in the
connexon. Provided by: Wiktionary. Located at:
plasma membrane connect to intermediate filaments. en.wiktionary.org/wiki/connexon. License: CC BY-SA: Attribution-ShareAlike
Similar to plasmodesmata, gap junctions are channels between OpenStax College, Connections between Cells and Cellular Activities. October
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other substances. Attribution
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Attribution
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of plant cells and some algal cells, enabling transport and 16, 2013. Provided by: OpenStax CNX. Located at:
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communication between them. Attribution
connexon: An assembly of six connexins forming a bridge called OpenStax College, Connections between Cells and Cellular Activities. October
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occludin: Together with the claudin group of proteins, it is the Attribution
main component of the tight junctions.

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 CHAPTER OVERVIEW

5: STRUCTURE AND FUNCTION OF PLASMA MEMBRANES

5.1: Components and Structure - Components of Plasma Membranes
5.2: Components and Structure - Fluid Mosaic Model
5.3: Components and Structure - Membrane Fluidity
5.4: Passive Transport - The Role of Passive Transport
5.5: Passive Transport - Selective Permeability
5.6: Passive Transport - Diffusion
5.7: Passive Transport - Facilitated Transport
5.8: Passive Transport - Osmosis
5.9: Passive Transport - Tonicity
5.10: Active Transport - Electrochemical Gradient
5.11: Active Transport - Primary Active Transport
5.12: Active Transport - Secondary Active Transport
5.13: Bulk Transport - Endocytosis
5.14: Bulk Transport - Exocytosis

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1
5.1: COMPONENTS AND STRUCTURE - COMPONENTS OF PLASMA
MEMBRANES
Cell
 LEARNING OBJECTIVES
Extracellular fluid
Nucleus
Describe the function and components of the plasma Cytoplasm

membrane

STRUCTURE OF PLASMA MEMBRANES

Cell membrane
The plasma membrane (also known as the cell membrane or Carbohydrate

cytoplasmic membrane) is a biological membrane that separates the Glycoprotein

interior of a cell from its outside environment. Globular protein

Protein Channel
(Transport protein)
The primary function of the plasma membrane is to protect the cell
from its surroundings. Composed of a phospholipid bilayer with Cholesterol

embedded proteins, the plasma membrane is selectively permeable

Glycolipid
to ions and organic molecules and regulates the movement of
Surface protein Alpha-helix protein
substances in and out of cells. Plasma membranes must be very Globular protein Filaments of (Integral protein)
(Integral) cytoskeleton Peripheral protein
flexible in order to allow certain cells, such as red blood cells and
white blood cells, to change shape as they pass through narrow
Phospholipid bilayer Phospholipid
capillaries. (Phosphatidylcholine)

The plasma membrane also plays a role in anchoring the Hydrophilic head

cytoskeleton to provide shape to the cell, and in attaching to the

extracellular matrix and other cells to help group cells together to
form tissues. The membrane also maintains the cell potential. Hydrophobic tail

In short, if the cell is represented by a castle, the plasma membrane

is the wall that provides structure for the buildings inside the wall,
regulates which people leave and enter the castle, and conveys Figure 5.1.1: The plasma membrane: The plasma membrane is
composed of phospholipids and proteins that provide a barrier
messages to and from neighboring castles. Just as a hole in the wall between the external environment and the cell, regulate the
can be a disaster for the castle, a rupture in the plasma membrane transportation of molecules across the membrane, and communicate
causes the cell to lyse and die. with other cells via protein receptors.

THE PLASMA MEMBRANE AND CELLULAR

TRANSPORT
The movement of a substance across the selectively permeable
plasma membrane can be either “passive”—i.e., occurring without
the input of cellular energy —or “active”—i.e., its transport requires
the cell to expend energy.
The cell employs a number of transport mechanisms that involve
biological membranes:
1. Passive osmosis and diffusion: transports gases (such as O2 and
CO2) and other small molecules and ions
2. Transmembrane protein channels and transporters: transports
small organic molecules such as sugars or amino acids
3. Endocytosis: transports large molecules (or even whole cells) by
engulfing them
4. Exocytosis: removes or secretes substances such as hormones or
enzymes

THE PLASMA MEMBRANE AND CELLULAR

SIGNALING
Among the most sophisticated functions of the plasma membrane is
its ability to transmit signals via complex proteins. These proteins
can be receptors, which work as receivers of extracellular inputs and

5.1.1 https://bio.libretexts.org/@go/page/12738
as activators of intracellular processes, or markers, which allow cells The plasma membrane protects intracellular components from
to recognize each other. the extracellular environment.
Membrane receptors provide extracellular attachment sites for The plasma membrane mediates cellular processes by regulating
effectors like hormones and growth factors, which then trigger the materials that enter and exit the cell.
intracellular responses. Some viruses, such as Human The plasma membrane carries markers that allow cells to
Immunodeficiency Virus (HIV), can hijack these receptors to gain recognize one another and can transmit signals to other cells via
entry into the cells, causing infections. receptors.
Membrane markers allow cells to recognize one another, which is KEY TERMS
vital for cellular signaling processes that influence tissue and organ
plasma membrane: The semipermeable barrier that surrounds
formation during early development. This marking function also
the cytoplasm of a cell.
plays a later role in the “self”-versus-“non-self” distinction of the
receptor: A protein on a cell wall that binds with specific
immune response. Marker proteins on human red blood cells, for
molecules so that they can be absorbed into the cell.
example, determine blood type (A, B, AB, or O).
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KEY POINTS Membranes is shared under a CC BY-SA 4.0 license and was authored,
The principal components of the plasma membrane are lipids ( remixed, and/or curated by Boundless.
phospholipids and cholesterol), proteins, and carbohydrates.

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5.2: COMPONENTS AND STRUCTURE - FLUID MOSAIC MODEL
with other non-polar molecules in chemical reactions, but generally
 LEARNING OBJECTIVES do not interact with polar molecules. When placed in water,
hydrophobic molecules tend to form a ball or cluster. The
Describe the fluid mosaic model of cell membranes
hydrophilic regions of the phospholipids tend to form hydrogen
bonds with water and other polar molecules on both the exterior and
The fluid mosaic model was first proposed by S.J. Singer and Garth
interior of the cell. Thus, the membrane surfaces that face the
L. Nicolson in 1972 to explain the structure of the plasma
interior and exterior of the cell are hydrophilic. In contrast, the
membrane. The model has evolved somewhat over time, but it still
middle of the cell membrane is hydrophobic and will not interact
best accounts for the structure and functions of the plasma with water. Therefore, phospholipids form an excellent lipid bilayer
membrane as we now understand them. The fluid mosaic model
cell membrane that separates fluid within the cell from the fluid
describes the structure of the plasma membrane as a mosaic of
outside of the cell.
components —including phospholipids, cholesterol, proteins, and
carbohydrates—that gives the membrane a fluid character. Plasma
membranes range from 5 to 10 nm in thickness. For comparison,
human red blood cells, visible via light microscopy, are
approximately 8 µm wide, or approximately 1,000 times wider than
a plasma membrane. The proportions of proteins, lipids, and
carbohydrates in the plasma membrane vary with cell type. For
example, myelin contains 18% protein and 76% lipid. The
mitochondrial inner membrane contains 76% protein and 24% lipid.

Figure 5.2.1: The Components and functions of the Plasma

Membrane: The principal components of a plasma membrane are
lipids (phospholipids and cholesterol), proteins, and carbohydrates
attached to some of the lipids and some of the proteins.

Figure 5.2.1: Phospholipid aggregation: In an aqueous solution,

Figure 5.2.1: The fluid mosaic model of the plasma membrane: The
phospholipids tend to arrange themselves with their polar heads
fluid mosaic model of the plasma membrane describes the plasma
facing outward and their hydrophobic tails facing inward.
membrane as a fluid combination of phospholipids, cholesterol, and
proteins. Carbohydrates attached to lipids (glycolipids) and to
proteins (glycoproteins) extend from the outward-facing surface of
the membrane.
The main fabric of the membrane is composed of amphiphilic or
dual-loving, phospholipid molecules. The hydrophilic or water-
loving areas of these molecules are in contact with the aqueous fluid
both inside and outside the cell. Hydrophobic, or water-hating
molecules, tend to be non- polar. A phospholipid molecule consists
of a three-carbon glycerol backbone with two fatty acid molecules
attached to carbons 1 and 2, and a phosphate-containing group
attached to the third carbon. This arrangement gives the overall
molecule an area described as its head (the phosphate-containing
group), which has a polar character or negative charge, and an area
called the tail (the fatty acids), which has no charge. They interact

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 Figure 5.2.1: Structure of integral membrane proteins: Integral
membrane proteins may have one or more alpha-helices that span
the membrane (examples 1 and 2), or they may have beta-sheets that
span the membrane (example 3).
Carbohydrates are the third major component of plasma membranes.
They are always found on the exterior surface of cells and are bound
either to proteins (forming glycoproteins) or to lipids (forming
glycolipids). These carbohydrate chains may consist of 2–60
monosaccharide units and can be either straight or branched. Along
with peripheral proteins, carbohydrates form specialized sites on the
cell surface that allow cells to recognize each other. This recognition
Figure 5.2.1: The structure of a phospholipid molecule: This
function is very important to cells, as it allows the immune system to
phospholipid molecule is composed of a hydrophilic head and two
hydrophobic tails. The hydrophilic head group consists of a differentiate between body cells (called “self”) and foreign cells or
phosphate-containing group attached to a glycerol molecule. The tissues (called “non-self”). Similar types of glycoproteins and
hydrophobic tails, each containing either a saturated or an
unsaturated fatty acid, are long hydrocarbon chains.
glycolipids are found on the surfaces of viruses and may change
frequently, preventing immune cells from recognizing and attacking
Proteins make up the second major component of plasma
them. These carbohydrates on the exterior surface of the cell—the
membranes. Integral proteins (some specialized types are called
carbohydrate components of both glycoproteins and glycolipids—
integrins) are, as their name suggests, integrated completely into the
are collectively referred to as the glycocalyx (meaning “sugar
membrane structure, and their hydrophobic membrane-spanning
coating”). The glycocalyx is highly hydrophilic and attracts large
regions interact with the hydrophobic region of the the phospholipid
amounts of water to the surface of the cell. This aids in the
bilayer. Single-pass integral membrane proteins usually have a
interaction of the cell with its watery environment and in the cell’s
hydrophobic transmembrane segment that consists of 20–25 amino
ability to obtain substances dissolved in the water.
acids. Some span only part of the membrane—associating with a
single layer—while others stretch from one side of the membrane to KEY POINTS
the other, and are exposed on either side. Some complex proteins are
The main fabric of the membrane is composed of amphiphilic or
composed of up to 12 segments of a single protein, which are
dual-loving, phospholipid molecules.
extensively folded and embedded in the membrane. This type of
Integral proteins, the second major component of plasma
protein has a hydrophilic region or regions, and one or several
membranes, are integrated completely into the membrane
mildly hydrophobic regions. This arrangement of regions of the
structure with their hydrophobic membrane-spanning regions
protein tends to orient the protein alongside the phospholipids, with
interacting with the hydrophobic region of the phospholipid
the hydrophobic region of the protein adjacent to the tails of the
bilayer.
phospholipids and the hydrophilic region or regions of the protein
Carbohydrates, the third major component of plasma
protruding from the membrane and in contact with the cytosol or
membranes, are always found on the exterior surface of cells
extracellular fluid.
where they are bound either to proteins (forming glycoproteins )
or to lipids (forming glycolipids).

KEY TERMS
amphiphilic: Having one surface consisting of hydrophilic
amino acids and the opposite surface consisting of hydrophobic
(or lipophilic) ones.
hydrophilic: Having an affinity for water; able to absorb, or be
wetted by water, “water-loving.”

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5.3: COMPONENTS AND STRUCTURE - MEMBRANE FLUIDITY
In animals, the third factor that keeps the membrane fluid is
 LEARNING OBJECTIVES cholesterol. It lies alongside the phospholipids in the membrane and
tends to dampen the effects of temperature on the membrane. Thus,
Explain the function of membrane fluidity in the structure of
cholesterol functions as a buffer, preventing lower temperatures
cells
from inhibiting fluidity and preventing higher temperatures from
increasing fluidity too much. Cholesterol extends in both directions
MEMBRANE FLUIDITY the range of temperature in which the membrane is appropriately
There are multiple factors that lead to membrane fluidity. First, the fluid and, consequently, functional. Cholesterol also serves other
mosaic characteristic of the membrane helps the plasma membrane functions, such as organizing clusters of transmembrane proteins
remain fluid. The integral proteins and lipids exist in the membrane into lipid rafts.
as separate but loosely-attached molecules. The membrane is not
like a balloon that can expand and contract; rather, it is fairly rigid KEY POINTS
and can burst if penetrated or if a cell takes in too much water. The membrane is fluid but also fairly rigid and can burst if
However, because of its mosaic nature, a very fine needle can easily penetrated or if a cell takes in too much water.
penetrate a plasma membrane without causing it to burst; the The mosaic nature of the plasma membrane allows a very fine
membrane will flow and self-seal when the needle is extracted. needle to easily penetrate it without causing it to burst and allows
it to self-seal when the needle is extracted.
If saturated fatty acids are compressed by decreasing
temperatures, they press in on each other, making a dense and
fairly rigid membrane.
If unsaturated fatty acids are compressed, the “kinks” in their
tails push adjacent phospholipid molecules away, which helps
maintain fluidity in the membrane.
The ratio of saturated and unsaturated fatty acids determines the
fluidity in the membrane at cold temperatures.
Cholesterol functions as a buffer, preventing lower temperatures
Figure 5.3.1: Membrane Fluidity: The plasma membrane is a fluid
from inhibiting fluidity and preventing higher temperatures from
combination of phospholipids, cholesterol, and proteins. increasing fluidity.
Carbohydrates attached to lipids (glycolipids) and to proteins
(glycoproteins) extend from the outward-facing surface of the KEY TERMS
membrane.
phospholipid: Any lipid consisting of a diglyceride combined
The second factor that leads to fluidity is the nature of the
with a phosphate group and a simple organic molecule such as
phospholipids themselves. In their saturated form, the fatty acids in
choline or ethanolamine; they are important constituents of
phospholipid tails are saturated with bound hydrogen atoms; there
biological membranes
are no double bonds between adjacent carbon atoms. This results in
fluidity: A measure of the extent to which something is fluid.
tails that are relatively straight. In contrast, unsaturated fatty acids
The reciprocal of its viscosity.
do not contain a maximal number of hydrogen atoms, although they
do contain some double bonds between adjacent carbon atoms; a CONTRIBUTIONS AND ATTRIBUTIONS
double bond results in a bend of approximately 30 degrees in the OpenStax College, Components and Structure. October 28, 2013. Provided by:
string of carbons. Thus, if saturated fatty acids, with their straight OpenStax CNX. Located at: http://cnx.org/content/m44416/latest/#tab-
ch05_01_01. License: CC BY: Attribution
tails, are compressed by decreasing temperatures, they press in on OpenStax College, Components and Structure. October 28, 2013. Provided by:
each other, making a dense and fairly rigid membrane. If unsaturated OpenStax CNX. Located at: http://cnx.org/content/m44416/latest/#tab-
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fatty acids are compressed, the “kinks” in their tails elbow adjacent OpenStax College, Components and Structure. October 28, 2013. Provided by:
phospholipid molecules away, maintaining some space between the OpenStax CNX. Located at: http://cnx.org/content/m44416/latest/#tab-
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phospholipid molecules. This “elbow room” helps to maintain OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
fluidity in the membrane at temperatures at which membranes with Located at: http://cnx.org/content/m44415/latest...ol11448/latest. License: CC
saturated fatty acid tails in their phospholipids would “freeze” or BY: Attribution
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solidify. The relative fluidity of the membrane is particularly en.wiktionary.org/wiki/receptor. License: CC BY-SA: Attribution-ShareAlike
important in a cold environment. A cold environment tends to plasma membrane. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/plasma_membrane. License: CC BY-SA: Attribution-
compress membranes composed largely of saturated fatty acids, ShareAlike
making them less fluid and more susceptible to rupturing. Many Illustration of au00a0Eukaryoticu00a0cell membrane. Provided by: Wikipedia.
Located at: en.Wikipedia.org/wiki/Cell_me..._diagram_4.svg. License: CC
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hydrophobic. Provided by: Wiktionary. Located at: phospholipid. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/hydrophobic. License: CC BY-SA: Attribution- http://en.wiktionary.org/wiki/phospholipid. License: CC BY-SA: Attribution-
ShareAlike ShareAlike
amphiphilic. Provided by: Wiktionary. Located at: Illustration of au00a0Eukaryoticu00a0cell membrane. Provided by: Wikipedia.
en.wiktionary.org/wiki/amphiphilic. License: CC BY-SA: Attribution- Located at: en.Wikipedia.org/wiki/Cell_me..._diagram_4.svg. License: CC
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hydrophilic. Provided by: Wiktionary. Located at: OpenStax College, Components and Structure. October 16, 2013. Provided by:
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Illustration of au00a0Eukaryoticu00a0cell membrane. Provided by: Wikipedia. Attribution
Located at: en.Wikipedia.org/wiki/Cell_me..._diagram_4.svg. License: CC OpenStax College, Biology. October 29, 2013. Provided by: OpenStax CNX.
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5.4: PASSIVE TRANSPORT - THE ROLE OF PASSIVE TRANSPORT
concentration; this process continues until the substance is evenly
 LEARNING OBJECTIVES distributed in a system. In solutions containing more than one
substance, each type of molecule diffuses according to its own
Indicate the manner in which various materials cross the cell
concentration gradient, independent of the diffusion of other
membrane
substances. Many factors can affect the rate of diffusion, including,
but not limited to, concentration gradient, size of the particles that
Plasma membranes must allow or prevent certain substances from
are diffusing, and temperature of the system.
entering or leaving a cell. In other words, plasma membranes are
In living systems, diffusion of substances in and out of cells is
selectively permeable; they allow some substances to pass through,
mediated by the plasma membrane. Some materials diffuse readily
but not others. If they were to lose this selectivity, the cell would no
through the membrane, but others are hindered; their passage is
longer be able to sustain itself, and it would be destroyed. Some
made possible by specialized proteins, such as channels and
cells require larger amounts of specific substances than other cells;
transporters. The chemistry of living things occurs in aqueous
they must have a way of obtaining these materials from extracellular
solutions; balancing the concentrations of those solutions is an
fluids. This may happen passively, as certain materials move back
ongoing problem. In living systems, diffusion of some substances
and forth, or the cell may have special mechanisms that facilitate
would be slow or difficult without membrane proteins that facilitate
transport. Some materials are so important to a cell that it spends
transport.
some of its energy (hydrolyzing adenosine triphosphate (ATP)) to
obtain these materials. Red blood cells use some of their energy to KEY POINTS
do this. All cells spend the majority of their energy to maintain an
Plasma membranes are selectively permeable; if they were to
imbalance of sodium and potassium ions between the interior and
lose this selectivity, the cell would no longer be able to sustain
exterior of the cell.
itself.
The most direct forms of membrane transport are passive. Passive In passive transport, substances simply move from an area of
transport is a naturally-occurring phenomenon and does not require higher concentration to an area of lower concentration, which
the cell to exert any of its energy to accomplish the movement. In does not require the input of energy.
passive transport, substances move from an area of higher Concentration gradient, size of the particles that are diffusing,
concentration to an area of lower concentration. A physical space in and temperature of the system affect the rate of diffusion.
which there is a range of concentrations of a single substance is said Some materials diffuse readily through the membrane, but others
to have a concentration gradient. require specialized proteins, such as channels and transporters, to
carry them into or out of the cell.

KEY TERMS
concentration gradient: A concentration gradient is present
when a membrane separates two different concentrations of
molecules.
passive transport: A movement of biochemicals and other
atomic or molecular substances across membranes that does not
Figure 5.4.1: Passive Transport: Diffusion is a type of passive
transport. Diffusion through a permeable membrane moves a require an input of chemical energy.
substance from an area of high concentration (extracellular fluid, in permeable: Of or relating to substance, substrate, membrane or
this case) down its concentration gradient (into the cytoplasm). material that absorbs or allows the passage of fluids.
The passive forms of transport, diffusion and osmosis, move
materials of small molecular weight across membranes. Substances This page titled 5.4: Passive Transport - The Role of Passive Transport is
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5.5: PASSIVE TRANSPORT - SELECTIVE PERMEABILITY

 LEARNING OBJECTIVES 
Describe how membrane permeability, concentration
gradient, and molecular properties affect biological diffusion
rates.

SELECTIVE PERMEABILITY
Plasma membranes are asymmetric: the interior of the membrane is
not identical to the exterior of the membrane. In fact, there is a
considerable difference between the array of phospholipids and
proteins between the two leaflets that form a membrane. On the
interior of the membrane, some proteins serve to anchor the
membrane to fibers of the cytoskeleton. There are peripheral
proteins on the exterior of the membrane that bind elements of the
extracellular matrix. Carbohydrates, attached to lipids or proteins,
are also found on the exterior surface of the plasma membrane.
These carbohydrate complexes help the cell bind substances that the
cell needs in the extracellular fluid. This adds considerably to the
selective nature of plasma membranes.

Pore size
Figure 5.5.1: Asymmetry in Plasma Membranes: The exterior
surface of the plasma membrane is not identical to the interior Trace m
surface of the same membrane.
Recall that plasma membranes are amphiphilic; that is, they have
hydrophilic and hydrophobic regions. This characteristic helps the
movement of some materials through the membrane and hinders the
movement of others. Lipid-soluble material with a low molecular
weight can easily slip through the hydrophobic lipid core of the
membrane. Substances such as the fat-soluble vitamins A, D, E, and Diffusion across a semipermeable membrane: This interactive
K readily pass through the plasma membranes in the digestive tract shows that smaller molecules have an easier time making it across a
and other tissues. Fat-soluble drugs and hormones also gain easy semipermeable membrane. Adjust the pore size so the larger
entry into cells and are readily transported into the body’s tissues molecules can make it through!
and organs. Molecules of oxygen and carbon dioxide have no charge
and so pass through membranes by simple diffusion. KEY POINTS
The interior and exterior surfaces of the plasma membrane are
Polar substances present problems for the membrane. While some
polar molecules connect easily with the outside of a cell, they cannot not identical, which adds to the selective permeability of the
membrane.
readily pass through the lipid core of the plasma membrane.
Additionally, while small ions could easily slip through the spaces in Fat soluble substances are able to pass easily to the hydrophobic
interior of the plasma membrane and diffuse into the cell.
the mosaic of the membrane, their charge prevents them from doing
so. Ions such as sodium, potassium, calcium, and chloride must have Polar molecules and charged molecules do not diffuse easily
through the lipid core of the plasma membrane and must be
special means of penetrating plasma membranes. Simple sugars and
amino acids also need help with transport across plasma membranes, transported across by proteins, sugars, or amino acids.
achieved by various transmembrane proteins (channels).

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KEY TERMS This page titled 5.5: Passive Transport - Selective Permeability is shared
polar: a separation of electric charge leading to a molecule or its under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
chemical groups having an electric dipole by Boundless.
amphiphilic: Having one surface consisting of hydrophilic
amino acids and the opposite surface consisting of hydrophobic
(or lipophilic) ones.

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5.6: PASSIVE TRANSPORT - DIFFUSION
Extent of the concentration gradient: The greater the difference
 LEARNING OBJECTIVES in concentration, the more rapid the diffusion. The closer the
distribution of the material gets to equilibrium, the slower the
Describe diffusion and the factors that affect how materials
rate of diffusion becomes.
move across the cell membrane.
Mass of the molecules diffusing: Heavier molecules move more
slowly; therefore, they diffuse more slowly. The reverse is true
DIFFUSION for lighter molecules.
Diffusion is a passive process of transport. A single substance tends Temperature: Higher temperatures increase the energy and
to move from an area of high concentration to an area of low therefore the movement of the molecules, increasing the rate of
concentration until the concentration is equal across a space. You are diffusion. Lower temperatures decrease the energy of the
familiar with diffusion of substances through the air. For example, molecules, thus decreasing the rate of diffusion.
think about someone opening a bottle of ammonia in a room filled Solvent density: As the density of a solvent increases, the rate of
with people. The ammonia gas is at its highest concentration in the diffusion decreases. The molecules slow down because they have
bottle; its lowest concentration is at the edges of the room. The a more difficult time getting through the denser medium. If the
ammonia vapor will diffuse, or spread away, from the bottle; medium is less dense, diffusion increases. Because cells
gradually, more and more people will smell the ammonia as it primarily use diffusion to move materials within the cytoplasm,
spreads. Materials move within the cell ‘s cytosol by diffusion, and any increase in the cytoplasm’s density will inhibit the
certain materials move through the plasma membrane by diffusion. movement of the materials. An example of this is a person
Diffusion expends no energy. On the contrary, concentration experiencing dehydration. As the body’s cells lose water, the rate
gradients are a form of potential energy, dissipated as the gradient is of diffusion decreases in the cytoplasm, and the cells’ functions
eliminated. deteriorate. Neurons tend to be very sensitive to this effect.
Dehydration frequently leads to unconsciousness and possibly
coma because of the decrease in diffusion rate within the cells.
Solubility: As discussed earlier, nonpolar or lipid-soluble
materials pass through plasma membranes more easily than polar
materials, allowing a faster rate of diffusion.
Surface area and thickness of the plasma membrane: Increased
surface area increases the rate of diffusion, whereas a thicker
membrane reduces it.
Figure 5.6.1: Diffusion: Diffusion through a permeable membrane
moves a substance from an area of high concentration (extracellular Distance travelled: The greater the distance that a substance must
fluid, in this case) down its concentration gradient (into the travel, the slower the rate of diffusion. This places an upper
cytoplasm). limitation on cell size. A large, spherical cell will die because
Each separate substance in a medium, such as the extracellular fluid, nutrients or waste cannot reach or leave the center of the cell.
has its own concentration gradient independent of the concentration Therefore, cells must either be small in size, as in the case of
gradients of other materials. In addition, each substance will diffuse many prokaryotes, or be flattened, as with many single-celled
according to that gradient. Within a system, there will be different eukaryotes.
rates of diffusion of the different substances in the medium.
A variation of diffusion is the process of filtration. In filtration,
FACTORS THAT AFFECT DIFFUSION material moves according to its concentration gradient through a
membrane; sometimes the rate of diffusion is enhanced by pressure,
Molecules move constantly in a random manner at a rate that
causing the substances to filter more rapidly. This occurs in the
depends on their mass, their environment, and the amount of thermal
kidney where blood pressure forces large amounts of water and
energy they possess, which in turn is a function of temperature. This
accompanying dissolved substances, or solutes, out of the blood and
movement accounts for the diffusion of molecules through whatever
into the renal tubules. The rate of diffusion in this instance is almost
medium in which they are localized. A substance will tend to move
totally dependent on pressure. One of the effects of high blood
into any space available to it until it is evenly distributed throughout
pressure is the appearance of protein in the urine, which is
it. After a substance has diffused completely through a space
“squeezed through” by the abnormally high pressure.
removing its concentration gradient, molecules will still move
around in the space, but there will be no net movement of the KEY POINTS
number of molecules from one area to another. This lack of a
Substances diffuse according to their concentration gradient;
concentration gradient in which there is no net movement of a
within a system, different substances in the medium will each
substance is known as dynamic equilibrium. While diffusion will go
diffuse at different rates according to their individual gradients.
forward in the presence of a concentration gradient of a substance,
After a substance has diffused completely through a space,
several factors affect the rate of diffusion:
removing its concentration gradient, molecules will still move

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around in the space, but there will be no net movement of the KEY TERMS
number of molecules from one area to another, a state known as diffusion: The passive movement of a solute across a permeable
dynamic equilibrium. membrane
Several factors affect the rate of diffusion of a solute including concentration gradient: A concentration gradient is present
the mass of the solute, the temperature of the environment, the when a membrane separates two different concentrations of
solvent density, and the distance traveled. molecules.

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5.7: PASSIVE TRANSPORT - FACILITATED TRANSPORT

 LEARNING OBJECTIVES

Explain why and how passive transport occurs

Facilitated transport is a type of passive transport. Unlike simple

diffusion where materials pass through a membrane without the help
of proteins, in facilitated transport, also called facilitated diffusion,
materials diffuse across the plasma membrane with the help of
membrane proteins. A concentration gradient exists that would allow
these materials to diffuse into the cell without expending cellular
energy. However, these materials are ions or polar molecules that are
repelled by the hydrophobic parts of the cell membrane. Facilitated
transport proteins shield these materials from the repulsive force of
the membrane, allowing them to diffuse into the cell.
The material being transported is first attached to protein or
glycoprotein receptors on the exterior surface of the plasma
Figure 5.7.1: Channel Proteins in Facilitated Transport: Facilitated
membrane. This allows the material that is needed by the cell to be transport moves substances down their concentration gradients.
removed from the extracellular fluid. The substances are then passed They may cross the plasma membrane with the aid of channel
to specific integral proteins that facilitate their passage. Some of proteins.
these integral proteins are collections of beta-pleated sheets that Channel proteins are either open at all times or they are “gated,”
form a channel through the phospholipid bilayer. Others are carrier which controls the opening of the channel. The attachment of a
proteins which bind with the substance and aid its diffusion through particular ion to the channel protein may control the opening or
the membrane. other mechanisms or substances may be involved. In some tissues,
sodium and chloride ions pass freely through open channels,
CHANNELS whereas in other tissues, a gate must be opened to allow passage. An
The integral proteins involved in facilitated transport are collectively example of this occurs in the kidney, where both forms of channels
referred to as transport proteins; they function as either channels for are found in different parts of the renal tubules. Cells involved in the
the material or carriers. In both cases, they are transmembrane transmission of electrical impulses, such as nerve and muscle cells,
proteins. Channels are specific for the substance that is being have gated channels for sodium, potassium, and calcium in their
transported. Channel proteins have hydrophilic domains exposed to membranes. Opening and closing of these channels changes the
the intracellular and extracellular fluids; they additionally have a relative concentrations on opposing sides of the membrane of these
hydrophilic channel through their core that provides a hydrated ions, resulting in the facilitation of electrical transmission along
opening through the membrane layers. Passage through the channel membranes (in the case of nerve cells) or in muscle contraction (in
allows polar compounds to avoid the nonpolar central layer of the the case of muscle cells).
plasma membrane that would otherwise slow or prevent their entry
into the cell. Aquaporins are channel proteins that allow water to CARRIER PROTEINS
pass through the membrane at a very high rate. Another type of protein embedded in the plasma membrane is a
carrier protein. This protein binds a substance and, in doing so,
triggers a change of its own shape, moving the bound molecule from
the outside of the cell to its interior; depending on the gradient, the
material may move in the opposite direction. Carrier proteins are
typically specific for a single substance. This adds to the overall
selectivity of the plasma membrane. The exact mechanism for the
change of shape is poorly understood. Proteins can change shape
when their hydrogen bonds are affected, but this may not fully
explain this mechanism. Each carrier protein is specific to one
substance, and there are a finite number of these proteins in any
membrane. This can cause problems in transporting enough of the
material for the cell to function properly.

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 KEY POINTS
A concentration gradient exists that would allow ions and polar
molecules to diffuse into the cell, but these materials are repelled
by the hydrophobic parts of the cell membrane.
Facilitated diffusion uses integral membrane proteins to move
polar or charged substances across the hydrophobic regions of
the membrane.
Channel proteins can aid in the facilitated diffusion of substances
by forming a hydrophilic passage through the plasma membrane
through which polar and charged substances can pass.
Figure 5.7.1: Carrier Proteins: Some substances are able to move Channel proteins can be open at all times, constantly allowing a
down their concentration gradient across the plasma membrane with
the aid of carrier proteins. Carrier proteins change shape as they particular substance into or out of the cell, depending on the
move molecules across the membrane. concentration gradient; or they can be gated and can only be
An example of this process occurs in the kidney. Glucose, water, opened by a particular biological signal.
salts, ions, and amino acids needed by the body are filtered in one Carrier proteins aid in facilitated diffusion by binding a particular
part of the kidney. This filtrate, which includes glucose, is then substance, then altering their shape to bring that substance into or
reabsorbed in another part of the kidney. Because there are only a out of the cell.
finite number of carrier proteins for glucose, if more glucose is
present than the proteins can handle, the excess is not transported; it
KEY TERMS
is excreted from the body in the urine. In a diabetic individual, this is facilitated diffusion: The spontaneous passage of molecules or
described as “spilling glucose into the urine.” A different group of ions across a biological membrane passing through specific
carrier proteins called glucose transport proteins, or GLUTs, are transmembrane integral proteins.
involved in transporting glucose and other hexose sugars through membrane protein: Proteins that are attached to, or associated
plasma membranes within the body. with the membrane of a cell or an organelle.
Channel and carrier proteins transport material at different rates.
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Channel proteins transport much more quickly than do carrier under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
proteins. Channel proteins facilitate diffusion at a rate of tens of by Boundless.
millions of molecules per second, whereas carrier proteins work at a
rate of a thousand to a million molecules per second.

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5.8: PASSIVE TRANSPORT - OSMOSIS

 LEARNING OBJECTIVES

Describe the process of osmosis and explain how

concentration gradient affects osmosis

OSMOSIS AND SEMIPERMEABLE MEMBRANES

Osmosis is the movement of water through a semipermeable

membrane according to the concentration gradient of water across
the membrane, which is inversely proportional to the concentration
of solutes. Semipermeable membranes, also termed selectively
permeable membranes or partially permeable membranes, allow Figure 5.8.1: Osmosis: In osmosis, water always moves from an
area of higher water concentration to one of lower concentration. In
certain molecules or ions to pass through by diffusion.
the diagram shown, the solute cannot pass through the selectively
While diffusion transports materials across membranes and within permeable membrane, but the water can.
cells, osmosis transports only water across a membrane. The Returning to the beaker example, recall that it has a mixture of
semipermeable membrane limits the diffusion of solutes in the solutes on either side of the membrane. A principle of diffusion is
water. Not surprisingly, the aquaporin proteins that facilitate water that the molecules move around and will spread evenly throughout
movement play a large role in osmosis, most prominently in red the medium if they can. However, only the material capable of
blood cells and the membranes of kidney tubules. passing through the membrane will diffuse through it. In this
example, the solute cannot diffuse through the membrane, but the
MECHANISM OF OSMOSIS water can. Water has a concentration gradient in this system. Thus,
Osmosis is a special case of diffusion. Water, like other substances, water will diffuse down its concentration gradient, crossing the
moves from an area of high concentration to one of low membrane to the side where it is less concentrated. This diffusion of
concentration. An obvious question is what makes water move at water through the membrane—osmosis—will continue until the
all? Imagine a beaker with a semipermeable membrane separating concentration gradient of water goes to zero or until the hydrostatic
the two sides or halves. On both sides of the membrane the water pressure of the water balances the osmotic pressure. In the beaker
level is the same, but there are different concentrations of a example, this means that the level of fluid in the side with a higher
dissolved substance, or solute, that cannot cross the membrane solute concentration will go up.
(otherwise the concentrations on each side would be balanced by the
solute crossing the membrane). If the volume of the solution on both KEY POINTS
sides of the membrane is the same but the concentrations of solute Osmosis occurs according to the concentration gradient of water
are different, then there are different amounts of water, the solvent, across the membrane, which is inversely proportional to the
on either side of the membrane. If there is more solute in one area, concentration of solutes.
then there is less water; if there is less solute in one area, then there Osmosis occurs until the concentration gradient of water goes to
must be more water. zero or until the hydrostatic pressure of the water balances the
To illustrate this, imagine two full glasses of water. One has a single osmotic pressure.
teaspoon of sugar in it, whereas the second one contains one-quarter Osmosis occurs when there is a concentration gradient of a solute
cup of sugar. If the total volume of the solutions in both cups is the within a solution, but the membrane does not allow diffusion of
same, which cup contains more water? Because the large amount of the solute.
sugar in the second cup takes up much more space than the teaspoon
of sugar in the first cup, the first cup has more water in it.
KEY TERMS
solute: Any substance that is dissolved in a liquid solvent to
create a solution
osmosis: The net movement of solvent molecules from a region
of high solvent potential to a region of lower solvent potential
through a partially permeable membrane
semipermeable membrane: A type of biological membrane that
will allow certain molecules or ions to pass through it by
diffusion and occasionally by specialized facilitated diffusion

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5.9: PASSIVE TRANSPORT - TONICITY
HYPERTONIC SOLUTIONS
 LEARNING OBJECTIVES As for a hypertonic solution, the prefix hyper- refers to the
Define tonicity and describe its relevance to osmosis extracellular fluid having a higher osmolarity than the cell’s
cytoplasm; therefore, the fluid contains less water than the cell does.
Tonicity describes how an extracellular solution can change the Because the cell has a relatively higher concentration of water, water
volume of a cell by affecting osmosis. A solution’s tonicity often will leave the cell, and the cell will shrink.
directly correlates with the osmolarity of the solution. Osmolarity
ISOTONIC SOLUTIONS
describes the total solute concentration of the solution. A solution
In an isotonic solution, the extracellular fluid has the same
with low osmolarity has a greater number of water molecules
osmolarity as the cell. If the osmolarity of the cell matches that of
relative to the number of solute particles; a solution with high
the extracellular fluid, there will be no net movement of water into
osmolarity has fewer water molecules with respect to solute
or out of the cell, although water will still move in and out.
particles. In a situation in which solutions of two different
osmolarities are separated by a membrane permeable to water, Blood cells and plant cells in hypertonic, isotonic, and hypotonic
though not to the solute, water will move from the side of the solutions take on characteristic appearances. Cells in an isotonic
membrane with lower osmolarity (and more water) to the side with solution retain their shape. Cells in a hypotonic solution swell as
higher osmolarity (and less water). This effect makes sense if you water enters the cell, and may burst if the concentration gradient is
remember that the solute cannot move across the membrane, and large enough between the inside and outside of the cell. Cells in a
thus the only component in the system that can move—the water— hypertonic solution shrink as water exits the cell, becoming
moves along its own concentration gradient. An important shriveled.
distinction that concerns living systems is that osmolarity measures
the number of particles (which may be molecules) in a solution.
KEY POINTS
Therefore, a solution that is cloudy with cells may have a lower Osmolarity describes the total solute concentration of a solution;
osmolarity than a solution that is clear if the second solution solutions with a low solute concentration have a low osmolarity,
contains more dissolved molecules than there are cells. while those with a high osmolarity have a high solute
concentration.
HYPOTONIC SOLUTIONS Water moves from the side of the membrane with lower
Three terms—hypotonic, isotonic, and hypertonic—are used to osmolarity (and more water) to the side with higher osmolarity
relate the osmolarity of a cell to the osmolarity of the extracellular (and less water).
fluid that contains the cells. In a hypotonic situation, the In a hypotonic solution, the extracellular fluid has a lower
extracellular fluid has lower osmolarity than the fluid inside the cell, osmolarity than the fluid inside the cell; water enters the cell.
and water enters the cell. (In living systems, the point of reference is In a hypertonic solution, the extracellular fluid has a higher
always the cytoplasm, so the prefix hypo- means that the osmolarity than the fluid inside the cell; water leaves the cell.
extracellular fluid has a lower concentration of solutes, or a lower In an isotonic solution, the extracellular fluid has the same
osmolarity, than the cell cytoplasm. ) It also means that the osmolarity as the cell; there will be no net movement of water
extracellular fluid has a higher concentration of water in the solution into or out of the cell.
than does the cell. In this situation, water will follow its
concentration gradient and enter the cell, causing the cell to expand.
KEY TERMS
osmolarity: The osmotic concentration of a solution, normally
expressed as osmoles of solute per litre of solution.
hypotonic: Having a lower osmotic pressure than another; a cell
in this environment causes water to enter the cell, causing it to
swell.
hypertonic: having a greater osmotic pressure than another
isotonic: having the same osmotic pressure

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Figure 5.9.1: Changes in Cell Shape Due to Dissolved Solutes:
Osmotic pressure changes the shape of red blood cells in hypertonic,
isotonic, and hypotonic solutions.

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5.10: ACTIVE TRANSPORT - ELECTROCHEMICAL GRADIENT
supply of metabolic energy may be spent maintaining these
 LEARNING OBJECTIVES processes. For example, most of a red blood cell’s metabolic energy
is used to maintain the imbalance between exterior and interior
Define an electrochemical gradient and describe how a cell
sodium and potassium levels required by the cell. Because active
moves substances against this gradient
transport mechanisms depend on a cell’s metabolism for energy, they
are sensitive to many metabolic poisons that interfere with the
ELECTROCHEMICAL GRADIENTS supply of ATP.
Simple concentration gradients are differential concentrations of a Two mechanisms exist for the transport of small-molecular weight
substance across a space or a membrane, but in living systems, material and small molecules. Primary active transport moves ions
gradients are more complex. Because ions move into and out of cells across a membrane and creates a difference in charge across that
and because cells contain proteins that do not move across the membrane, which is directly dependent on ATP. Secondary active
membrane and are mostly negatively charged, there is also an transport describes the movement of material that is due to the
electrical gradient, a difference of charge, across the plasma electrochemical gradient established by primary active transport that
membrane. The interior of living cells is electrically negative with does not directly require ATP.
respect to the extracellular fluid in which they are bathed. At the
same time, cells have higher concentrations of potassium (K+) and CARRIER PROTEINS FOR ACTIVE TRANSPORT
lower concentrations of sodium (Na+) than does the extracellular
fluid. In a living cell, the concentration gradient of Na+ tends to An important membrane adaption for active transport is the presence
drive it into the cell, and the electrical gradient of Na+ (a positive of specific carrier proteins or pumps to facilitate movement. There
ion) also tends to drive it inward to the negatively-charged interior. are three types of these proteins or transporters: uniporters,
The situation is more complex, however, for other elements such as symporters, and antiporters. A uniporter carries one specific ion or
potassium. The electrical gradient of K+, a positive ion, also tends to molecule. A symporter carries two different ions or molecules, both
drive it into the cell, but the concentration gradient of K+ tends to in the same direction. An antiporter also carries two different ions or
drive K+ out of the cell. The combined gradient of concentration and molecules, but in different directions. All of these transporters can
electrical charge that affects an ion is called its electrochemical also transport small, uncharged organic molecules like glucose.
gradient. These three types of carrier proteins are also found in facilitated
diffusion, but they do not require ATP to work in that process. Some
examples of pumps for active transport are Na+-K+ ATPase, which
carries sodium and potassium ions, and H+-K+ ATPase, which
carries hydrogen and potassium ions. Both of these are antiporter
carrier proteins. Two other carrier protein pumps are Ca2+ ATPase
and H+ATPase, which carry only calcium and only hydrogen ions,
respectively.

Figure 5.10.1: Electrochemical Gradient: Electrochemical gradients

arise from the combined effects of concentration gradients and Figure 5.10.1: Uniporters, Symporters, and Antiporters: A uniporter
electrical gradients. carries one molecule or ion. A symporter carries two different
molecules or ions, both in the same direction. An antiporter also
MOVING AGAINST A GRADIENT carries two different molecules or ions, but in different directions.
To move substances against a concentration or electrochemical
KEY POINTS
gradient, the cell must use energy. This energy is harvested from
adenosine triphosphate (ATP) generated through the cell’s The electrical and concentration gradients of a membrane tend to
metabolism. Active transport mechanisms, collectively called drive sodium into and potassium out of the cell, and active
pumps, work against electrochemical gradients. Small substances transport works against these gradients.
constantly pass through plasma membranes. Active transport To move substances against a concentration or electrochemical
maintains concentrations of ions and other substances needed by gradient, the cell must utilize energy in the form of ATP during
living cells in the face of these passive movements. Much of a cell’s active transport.

5.10.1 https://bio.libretexts.org/@go/page/13092
 Primary active transport, which is directly dependent on ATP, “molecular unit of energy currency” in intracellular energy
moves ions across a membrane and creates a difference in charge transfer
across that membrane. active transport: movement of a substance across a cell
Secondary active transport, created by primary active transport, membrane against its concentration gradient (from low to high
is the transport of a solute in the direction of its electrochemical concentration) facilitated by ATP conversion
gradient and does not directly require ATP. electrochemical gradient: The difference in charge and
Carrier proteins such as uniporters, symporters, and antiporters chemical concentration across a membrane.
perform primary active transport and facilitate the movement of
solutes across the cell’s membrane. This page titled 5.10: Active Transport - Electrochemical Gradient is shared
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
KEY TERMS by Boundless.
adenosine triphosphate: a multifunctional nucleoside
triphosphate used in cells as a coenzyme, often called the

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5.11: ACTIVE TRANSPORT - PRIMARY ACTIVE TRANSPORT
With the phosphate group removed and potassium ions attached,
 LEARNING OBJECTIVES the carrier protein repositions itself towards the interior of the
cell.
Describe how a cell moves sodium and potassium out of and
The carrier protein, in its new configuration, has a decreased
into the cell against its electrochemical gradient
affinity for potassium, and the two ions are released into the
cytoplasm. The protein now has a higher affinity for sodium
PRIMARY ACTIVE TRANSPORT ions, and the process starts again.
The primary active transport that functions with the active transport
Several things have happened as a result of this process. At this
of sodium and potassium allows secondary active transport to occur.
point, there are more sodium ions outside of the cell than inside and
The secondary transport method is still considered active because it
more potassium ions inside than out. For every three ions of sodium
depends on the use of energy as does primary transport.
that move out, two ions of potassium move in. This results in the
interior being slightly more negative relative to the exterior. This
difference in charge is important in creating the conditions necessary
for the secondary process. The sodium-potassium pump is, therefore,
an electrogenic pump (a pump that creates a charge imbalance),
creating an electrical imbalance across the membrane and
contributing to the membrane potential.

KEY POINTS
The sodium-potassium pump moves K+ into the cell while
moving Na+ at a ratio of three Na+ for every two K+ ions.
Figure 5.11.1: Active Transport of Sodium and Potassium: Primary
active transport moves ions across a membrane, creating an When the sodium-potassium- ATPase enzyme points into the
electrochemical gradient (electrogenic transport). cell, it has a high affinity for sodium ions and binds three of
One of the most important pumps in animals cells is the sodium- them, hydrolyzing ATP and changing shape.
potassium pump ( Na+-K+ ATPase ), which maintains the As the enzyme changes shape, it reorients itself towards the
electrochemical gradient (and the correct concentrations of Na+ and outside of the cell, and the three sodium ions are released.
K+) in living cells. The sodium-potassium pump moves two K+ into The enzyme’s new shape allows two potassium to bind and the
the cell while moving three Na+ out of the cell. The Na+-K+ ATPase phosphate group to detach, and the carrier protein repositions
exists in two forms, depending on its orientation to the interior or itself towards the interior of the cell.
exterior of the cell and its affinity for either sodium or potassium The enzyme changes shape again, releasing the potassium ions
ions. The process consists of the following six steps: into the cell.
With the enzyme oriented towards the interior of the cell, the After potassium is released into the cell, the enzyme binds three
carrier has a high affinity for sodium ions. Three sodium ions sodium ions, which starts the process over again.
bind to the protein.
KEY TERMS
ATP is hydrolyzed by the protein carrier, and a low-energy
phosphate group attaches to it. electrogenic pump: An ion pump that generates a net charge
As a result, the carrier changes shape and re-orients itself flow as a result of its activity.
towards the exterior of the membrane. The protein’s affinity for Na+-K+ ATPase: An enzyme located in the plasma membrane
sodium decreases, and the three sodium ions leave the carrier. of all animal cells that pumps sodium out of cells while pumping
The shape change increases the carrier’s affinity for potassium potassium into cells.
ions, and two such ions attach to the protein. Subsequently, the
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by Boundless.

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5.12: ACTIVE TRANSPORT - SECONDARY ACTIVE TRANSPORT
directly used to move the molecule across the membrane.
 LEARNING OBJECTIVES Both antiporters and symporters are used in secondary active
transport.
Differentiate between primary and secondary active
Secondary active transport brings sodium ions into the cell, and
transport
as sodium ion concentrations build outside the plasma
membrane, an electrochemical gradient is created.
SECONDARY ACTIVE TRANSPORT (CO- If a channel protein is open via primary active transport, the ions
TRANSPORT) will be pulled through the membrane along with other substances
Unlike in primary active transport, in secondary active transport, that can attach themselves to the transport protein through the
ATP is not directly coupled to the molecule of interest. Instead, membrane.
another molecule is moved up its concentration gradient, which Secondary active transport is used to store high-energy hydrogen
generates an electrochemical gradient. The molecule of interest is ions in the mitochondria of plant and animal cells for the
then transported down the electrochemical gradient. While this production of ATP.
process still consumes ATP to generate that gradient, the energy is The potential energy in the hydrogen ions is translated into
not directly used to move the molecule across the membrane, hence kinetic energy as the ions surge through the channel protein ATP
it is known as secondary active transport. Both antiporters and synthase, and that energy is used to convert ADP into ATP.
symporters are used in secondary active transport. Co-transporters
can be classified as symporters and antiporters depending on KEY TERMS
whether the substances move in the same or opposite directions secondary active transport: A method of transport in which the
across the cell membrane. electrochemical potential difference created by pumping ions out
Secondary active transport brings sodium ions, and possibly other of the cell is used to transport molecules across a membrane.
compounds, into the cell. As sodium ion concentrations build
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outside the plasma membrane because of the action of the primary
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active transport process, an electrochemical gradient is created. If a Located at: http://cnx.org/content/m44418/latest...ol11448/latest. License: CC
channel protein exists and is open, the sodium ions will be pulled BY: Attribution
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through the membrane. Many amino acids, as well as glucose, enter en.Wikipedia.org/wiki/electro...cal%20gradient. License: CC BY-SA:
a cell this way. This secondary process is also used to store high- Attribution-ShareAlike
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surge through the channel protein ATP synthase, and that energy is License: CC BY: Attribution
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License: CC BY: Attribution
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Located at: http://cnx.org/content/m44418/latest...ol11448/latest. License: CC
BY: Attribution
Boundless. Provided by: Boundless Learning. Located at:
www.boundless.com//biology/de...ctrogenic-pump. License: CC BY-SA:
Attribution-ShareAlike
Na -K ATPase. Provided by: Wikipedia. Located at:
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Figure 5.12.1: Secondary Active Transport: An electrochemical CNX. Located at: http://cnx.org/content/m44418/latest...e_05_03_03.jpg.
gradient, created by primary active transport, can move other License: CC BY: Attribution
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transport or secondary active transport. Located at: http://cnx.org/content/m44418/latest...ol11448/latest. License: CC
BY: Attribution
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5.13: BULK TRANSPORT - ENDOCYTOSIS
releases its contents into the extracellular fluid. The endosomal
 LEARNING OBJECTIVES membrane again becomes part of the plasma membrane.

Describe endocytosis, including phagocytosis, pinocytosis, PINOCYTOSIS

and receptor-mediated endocytosis. A variation of endocytosis is called pinocytosis. This literally means
“cell drinking” and was named at a time when the assumption was
Endocytosis is a type of active transport that moves particles, such that the cell was purposefully taking in extracellular fluid. In reality,
as large molecules, parts of cells, and even whole cells, into a cell. this is a process that takes in molecules, including water, which the
There are different variations of endocytosis, but all share a common cell needs from the extracellular fluid. Pinocytosis results in a much
characteristic: the plasma membrane of the cell invaginates, forming smaller vesicle than does phagocytosis, and the vesicle does not
a pocket around the target particle. The pocket pinches off, resulting need to merge with a lysosome.
in the particle being contained in a newly-created intracellular
vesicle formed from the plasma membrane.

PHAGOCYTOSIS
Phagocytosis (the condition of “cell eating”) is the process by which
large particles, such as cells or relatively large particles, are taken in
by a cell. For example, when microorganisms invade the human
body, a type of white blood cell called a neutrophil will remove the
invaders through this process, surrounding and engulfing the
microorganism, which is then destroyed by the neutrophil.

Figure 5.13.1: Pinocytosis: In pinocytosis, the cell membrane

invaginates, surrounds a small volume of fluid, and pinches off.
Potocytosis, a variant of pinocytosis, is a process that uses a coating
protein, called caveolin, on the cytoplasmic side of the plasma
membrane, which performs a similar function to clathrin. The
cavities in the plasma membrane that form the vacuoles have
membrane receptors and lipid rafts in addition to caveolin. The
vacuoles or vesicles formed in caveolae (singular caveola) are
smaller than those in pinocytosis. Potocytosis is used to bring small
molecules into the cell and to transport these molecules through the
Figure 5.13.1: Phagocytosis: In phagocytosis, the cell membrane
cell for their release on the other side of the cell, a process called
surrounds the particle and engulfs it. transcytosis.
In preparation for phagocytosis, a portion of the inward-facing
surface of the plasma membrane becomes coated with a protein
RECEPTOR-MEDIATED ENDOCYTOSIS
called clathrin, which stabilizes this section of the membrane. The A targeted variation of endocytosis, known as receptor-mediated
coated portion of the membrane then extends from the body of the endocytosis, employs receptor proteins in the plasma membrane that
cell and surrounds the particle, eventually enclosing it. Once the have a specific binding affinity for certain substances. In receptor-
vesicle containing the particle is enclosed within the cell, the clathrin mediated endocytosis, as in phagocytosis, clathrin is attached to the
disengages from the membrane and the vesicle merges with a cytoplasmic side of the plasma membrane. If uptake of a compound
lysosome for the breakdown of the material in the newly-formed is dependent on receptor-mediated endocytosis and the process is
compartment ( endosome ). When accessible nutrients from the ineffective, the material will not be removed from the tissue fluids or
degradation of the vesicular contents have been extracted, the blood. Instead, it will stay in those fluids and increase in
newly-formed endosome merges with the plasma membrane and concentration. Some human diseases are caused by the failure of
receptor-mediated endocytosis. For example, the form of cholesterol

5.13.1 https://bio.libretexts.org/@go/page/13097
termed low-density lipoprotein or LDL (also referred to as “bad” Although receptor-mediated endocytosis is designed to bring
cholesterol) is removed from the blood by receptor-mediated specific substances that are normally found in the extracellular fluid
endocytosis. In the human genetic disease familial into the cell, other substances may gain entry into the cell at the
hypercholesterolemia, the LDL receptors are defective or missing same site. Flu viruses, diphtheria, and cholera toxin all have sites
entirely. People with this condition have life-threatening levels of that cross-react with normal receptor-binding sites and gain entry
cholesterol in their blood, because their cells cannot clear LDL into cells.
particles from their blood.
KEY POINTS
Endocytosis consists of phagocytosis, pinocytosis, and receptor -
mediated endocytosis.
Endocytosis takes particles into the cell that are too large to
passively cross the cell membrane.
Phagocytosis is the taking in of large food particles, while
pinocytosis takes in liquid particles.
Receptor-mediated endocytosis uses special receptor proteins to
help carry large particles across the cell membrane.

KEY TERMS
endosome: An endocytic vacuole through which molecules
internalized during endocytosis pass en route to lysosomes
neutrophil: A cell, especially a white blood cell that consumes
foreign invaders in the blood.

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Figure 5.13.1: Receptor-Mediated Endocytosis: In receptor-

mediated endocytosis, uptake of substances by the cell is targeted to
a single type of substance that binds to the receptor on the external
surface of the cell membrane.

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5.14: BULK TRANSPORT - EXOCYTOSIS
like enzymes, peptide hormones and antibodies from different cells,
 LEARNING OBJECTIVES the flipping of the plasma membrane, the placement of integral
membrane proteins(IMPs) or proteins that are attached biologically
Describe exocytosis and the processes used to release
to the cell, and the recycling of plasma membrane bound receptors
materials from the cell.
(molecules on the cell membrane that intercept signals).

Exocytosis’ main purpose is to expel material from the cell into the KEY POINTS
extracellular fluid; this is the opposite of what occurs in endocytosis. Exocytosis is the opposite of endocytosis as it involves releasing
In exocytosis, waste material is enveloped in a membrane and fuses materials from the cell.
with the interior of the plasma membrane. This fusion opens the Exocytosis has five stages, each leading up to the vesicle binding
membranous envelope on the exterior of the cell and the waste with the cell membrane.
material is expelled into the extracellular space. Exocytosis is used Many bodily functions include the use of exocytosis, such as the
continuously by plant and animal cells to excrete waste from the release of neurotransmitters into the synaptic cleft and the release
cells. of enzymes into the blood.

KEY TERMS
secretion: The act of secreting (producing and discharging) a
substance, especially from a gland.
vesicle: A membrane-bound compartment found in a cell.
membrane that intercept signals).

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Figure 5.14.1: Exocytosis: In exocytosis, vesicles containing
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substances fuse with the plasma membrane. The contents are then CNX. Located at: http://cnx.org/content/m44419/latest...e_05_04_03.jpg.
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Exocytosis is composed of five main stages. The first stage is called OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
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significant distance, the vesicle containing the material that is to be Exocytosis. Provided by: epiehonorsbiology Wikispace. Located at:
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Some examples of cells releasing molecules via exocytosis include License: CC BY: Attribution
the secretion of proteins of the extracellular matrix and secretion of
neurotransmitters into the synaptic cleft by synaptic vesicles. Some This page titled 5.14: Bulk Transport - Exocytosis is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.
examples of cells using exocytosis include: the secretion of proteins

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 CHAPTER OVERVIEW

6: METABOLISM
6.1: Energy and Metabolism - The Role of Energy and Metabolism
6.2: Energy and Metabolism - Types of Energy
6.3: Energy and Metabolism - Metabolic Pathways
6.4: Energy and Metabolism - Metabolism of Carbohydrates
6.5: Potential, Kinetic, Free, and Activation Energy - Free Energy
6.6: Potential, Kinetic, Free, and Activation Energy - The First Law of Thermodynamics
6.7: Potential, Kinetic, Free, and Activation Energy - The Second Law of Thermodynamics
6.8: Potential, Kinetic, Free, and Activation Energy - Activation Energy
6.9: ATP - Adenosine Triphosphate
6.10: Enzymes - Active Site and Substrate Specificity
6.11: Enzymes - Control of Metabolism Through Enzyme Regulation

Thumbnail: Metabolic Metro Map. (CC BY-SA 4.0; Chakazul).

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1
6.1: ENERGY AND METABOLISM - THE ROLE OF ENERGY AND METABOLISM

 LEARNING OBJECTIVES

Explain the importance of metabolism

ENERGY AND METABOLISM

All living organisms need energy to grow and reproduce, maintain
their structures, and respond to their environments. Metabolism is
the set of life-sustaining chemical processes that enables organisms
transform the chemical energy stored in molecules into energy that
can be used for cellular processes. Animals consume food to
replenish energy; their metabolism breaks down the carbohydrates,
lipids, proteins, and nucleic acids to provide chemical energy for
these processes. Plants convert light energy from the sun into
chemical energy stored in molecules during the process of
photosynthesis.

BIOENERGETICS AND CHEMICAL REACTIONS

Scientists use the term bioenergetics to discuss the concept of energy

flow through living systems such as cells. Cellular processes such as
the building and breaking down of complex molecules occur through
step-by-step chemical reactions. Some of these chemical reactions
are spontaneous and release energy, whereas others require energy to
proceed. All of the chemical reactions that take place inside cells, Figure 6.1.1: Most energy comes from the sun, either directly or
including those that use energy and those that release energy, are the indirectly: Most life forms on earth get their energy from the sun.
Plants use photosynthesis to capture sunlight, and herbivores eat
cell’s metabolism. those plants to obtain energy. Carnivores eat the herbivores, and
decomposers digest plant and animal matter.

CELLULAR METABOLISM
Every task performed by living organisms requires energy. Energy is
needed to perform heavy labor and exercise, but humans also use a
great deal of energy while thinking and even while sleeping. For
every action that requires energy, many chemical reactions take
place to provide chemical energy to the systems of the body,
including muscles, nerves, heart, lungs, and brain.
The living cells of every organism constantly use energy to survive
and grow. Cells break down complex carbohydrates into simple
sugars that the cell can use for energy. Muscle cells may consumer
energy to build long muscle proteins from small amino acid
molecules. Molecules can be modified and transported around the
cell or may be distributed to the entire organism. Just as energy is
required to both build and demolish a building, energy is required
for both the synthesis and breakdown of molecules.
Many cellular process require a steady supply of energy provided by
the cell’s metabolism. Signaling molecules such as hormones and
neurotransmitters must be synthesized and then transported between
cells. Pathogenic bacteria and viruses are ingested and broken down
by cells. Cells must also export waste and toxins to stay healthy, and
many cells must swim or move surrounding materials via the beating
motion of cellular appendages like cilia and flagella.

6.1.1 https://bio.libretexts.org/@go/page/13100
 metabolism is the set of the processes that makes energy
available for cellular processes.
Metabolism is a combination of chemical reactions that are
spontaneous and release energy and chemical reactions that are
non-spontaneous and require energy in order to proceed.
Living organisms must take in energy via food, nutrients, or
sunlight in order to carry out cellular processes.
The transport, synthesis, and breakdown of nutrients and
molecules in a cell require the use of energy.

KEY TERMS
Figure 6.1.1: Eating provides energy for activities like flight: A metabolism: the complete set of chemical reactions that occur in
hummingbird needs energy to maintain prolonged periods of flight. living cells
The hummingbird obtains its energy from taking in food and bioenergetics: the study of the energy transformations that take
transforming the nutrients into energy through a series of
biochemical reactions. The flight muscles in birds are extremely place in living organisms
efficient in energy production. energy: the capacity to do work

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KEY POINTS Metabolism is shared under a CC BY-SA 4.0 license and was authored,
All living organisms need energy to grow and reproduce, remixed, and/or curated by Boundless.
maintain their structures, and respond to their environments;

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6.2: ENERGY AND METABOLISM - TYPES OF ENERGY
CHEMICAL ENERGY
 LEARNING OBJECTIVES Potential energy is not only associated with the location of matter,
Differentiate between types of energy but also with the structure of matter. A spring on the ground has
potential energy if it is compressed, as does a rubber band that is
Energy is a property of objects which can be transferred to other pulled taut. The same principle applies to molecules. On a chemical
objects or converted into different forms, but cannot be created or level, the bonds that hold the atoms of molecules together have
destroyed. Organisms use energy to survive, grow, respond to potential energy. This type of potential energy is called chemical
stimuli, reproduce, and for every type of biological process. The energy, and like all potential energy, it can be used to do work.
potential energy stored in molecules can be converted to chemical For example, chemical energy is contained in the gasoline molecules
energy, which can ultimately be converted to kinetic energy, that are used to power cars. When gas ignites in the engine, the
enabling an organism to move. Eventually, most of energy used by bonds within its molecules are broken, and the energy released is
organisms is transformed into heat and dissipated. used to drive the pistons. The potential energy stored within
chemical bonds can be harnessed to perform work for biological
KINETIC ENERGY processes. Different metabolic processes break down organic
Energy associated with objects in motion is called kinetic energy. molecules to release the energy for an organism to grow and survive.
For example, when an airplane is in flight, the airplane is moving
through air very quickly—doing work to enact change on its
surroundings. The jet engines are converting potential energy in fuel
to the kinetic energy of movement. A wrecking ball can perform a
large amount of damage, even when moving slowly. However, a still
wrecking ball cannot perform any work and therefore has no kinetic
energy. A speeding bullet, a walking person, the rapid movement of
molecules in the air that produces heat, and electromagnetic
radiation, such as sunlight, all have kinetic energy.

POTENTIAL ENERGY
What if that same motionless wrecking ball is lifted two stories
above a car with a crane? If the suspended wrecking ball is not
moving, is there energy associated with it? Yes, the wrecking ball
has energy because the wrecking ball has the potential to do work.
This form of energy is called potential energy because it is possible
for that object to do work in a given state.
Objects transfer their energy between potential and kinetic states. As
the wrecking ball hangs motionlessly, it has 0%0% kinetic and
100%100%potential energy. Once the ball is released, its kinetic
energy increases as the ball picks up speed. At the same time, the
ball loses potential energy as it nears the ground. Other examples of Figure 6.2.1: Chemical energy: The molecules in gasoline (octane,
potential energy include the energy of water held behind a dam or a the chemical formula shown) contain chemical energy. This energy
person about to skydive out of an airplane. is transformed into kinetic energy that allows a car to race on a
racetrack.

KEY POINTS
All organisms use different forms of energy to power the
biological processes that allow them to grow and survive.
Kinetic energy is the energy associated with objects in motion.
Potential energy is the type of energy associated with an object’s
potential to do work.
Chemical energy is the type of energy released from the
breakdown of chemical bonds and can be harnessed for
Figure 6.2.1: Potential energy vs. kinetic energy: Water behind a metabolic processes.
dam has potential energy. Moving water, such as in a waterfall or a
rapidly flowing river, has kinetic energy.

6.2.1 https://bio.libretexts.org/@go/page/13101
KEY TERMS kinetic energy: The energy possessed by an object because of its
chemical energy: The net potential energy liberated or absorbed motion, equal to one half the mass of the body times the square
during the course of a chemical reaction. of its velocity.
potential energy: Energy possessed by an object because of its
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under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
stretched or compressed spring, as a chemical reactant, or by
by Boundless.
having rest mass).

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6.3: ENERGY AND METABOLISM - METABOLIC PATHWAYS
constantly, and demand energy provided by ATP and other high-
 LEARNING OBJECTIVES energy molecules like NADH (nicotinamide adenine dinucleotide)
and NADPH.
Describe the two major types of metabolic pathways
CATABOLIC PATHWAYS
METABOLIC PATHWAYS Catabolic pathways involve the degradation of complex molecules
The processes of making and breaking down carbohydrate into simpler ones, releasing the chemical energy stored in the bonds
molecules illustrate two types of metabolic pathways. A metabolic of those molecules. Some catabolic pathways can capture that
pathway is a step-by-step series of interconnected biochemical energy to produce ATP, the molecule used to power all cellular
reactions that convert a substrate molecule or molecules through a processes. Other energy-storing molecules, such as lipids, are also
series of metabolic intermediates, eventually yielding a final product broken down through similar catabolic reactions to release energy
or products. For example, one metabolic pathway for carbohydrates and make ATP.
breaks large molecules down into glucose. Another metabolic
pathway might build glucose into large carbohydrate molecules for IMPORTANCE OF ENZYMES
storage. The first of these processes requires energy and is referred Chemical reactions in metabolic pathways rarely take place
to as anabolic. The second process produces energy and is referred spontaneously. Each reaction step is facilitated, or catalyzed, by a
to as catabolic. Consequently, metabolism is composed of these two protein called an enzyme. Enzymes are important for catalyzing all
opposite pathways: types of biological reactions: those that require energy as well as
1. Anabolism (building molecules) those that release energy.
2. Catabolism (breaking down molecules)
KEY POINTS
A metabolic pathway is a series of chemical reactions in a cell
that build and breakdown molecules for cellular processes.
Anabolic pathways synthesize molecules and require energy.
Catabolic pathways break down molecules and produce energy.
Because almost all metabolic reactions take place non-
spontaneously, proteins called enzymes help facilitate those
chemical reactions.
Figure 6.3.1: Anabolic and catabolic pathways: Anabolic pathways
are those that require energy to synthesize larger molecules.
Catabolic pathways are those that generate energy by breaking down KEY TERMS
larger molecules. Both types of pathways are required for catabolism: destructive metabolism, usually including the
maintaining the cell’s energy balance.
release of energy and breakdown of materials
ANABOLIC PATHWAYS enzyme: a globular protein that catalyses a biological chemical
Anabolic pathways require an input of energy to synthesize complex reaction
molecules from simpler ones. One example of an anabolic pathway anabolism: the constructive metabolism of the body, as
is the synthesis of sugar from CO2. Other examples include the distinguished from catabolism
synthesis of large proteins from amino acid building blocks and the
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These processes are critical to the life of the cell, take place by Boundless.

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6.4: ENERGY AND METABOLISM - METABOLISM OF CARBOHYDRATES
PRODUCING CARBOHYDRATES
 LEARNING OBJECTIVES (PHOTOSYNTHESIS)
Analyze the importance of carbohydrate metabolism to Plants and some other types of organisms produce carbohydrates
energy production through the process called photosynthesis. During photosynthesis,
plants convert light energy into chemical energy by building carbon
dioxide gas molecules (CO2) into sugar molecules like glucose.
METABOLISM OF CARBOHYDRATES
Because this process involves building bonds to synthesize a large
Carbohydrates are one of the major forms of energy for animals and molecule, it requires an input of energy (light) to proceed. The
plants. Plants build carbohydrates using light energy from the sun synthesis of glucose by photosynthesis is described by this equation
(during the process of photosynthesis), while animals eat plants or (notice that it is the reverse of the previous equation):
other animals to obtain carbohydrates. Plants store carbohydrates in
6CO2+6H2O+energy→ C6H12O6+6O2
long polysaccharides chains called starch, while animals store
carbohydrates as the molecule glycogen. These large As part of plants’ chemical processes, glucose molecules can be
polysaccharides contain many chemical bonds and therefore store a combined with and converted into other types of sugars. In plants,
lot of chemical energy. When these molecules are broken down glucose is stored in the form of starch, which can be broken down
during metabolism, the energy in the chemical bonds is released and back into glucose via cellular respiration in order to supply ATP.
can be harnessed for cellular processes.
KEY POINTS
The breakdown of glucose living organisms utilize to produce
energy is described by the equation:
C6H12O6+6O2→6CO2+6H2O+energy.
The photosynthetic process plants utilize to synthesize glucose is
described by the equation:6CO2+6H2O+energy→ C6H12O6+6O2
Glucose that is consumed is used to make energy in the form of
ATP, which is used to perform work and power chemical
reactions in the cell.
During photosynthesis, plants convert light energy into chemical
energy that is used to build molecules of glucose.
Figure 6.4.1: All living things use carbohydrates as a form of
energy.: Plants, like this oak tree and acorn, use energy from sunlight KEY TERMS
to make sugar and other organic molecules. Both plants and animals
(like this squirrel) use cellular respiration to derive energy from the
adenosine triphosphate: a multifunctional nucleoside
organic molecules originally produced by plants triphosphate used in cells as a coenzyme, often called the
“molecular unit of energy currency” in intracellular energy
ENERGY PRODUCTION FROM CARBOHYDRATES transfer
(CELLULAR RESPIRATION ) glucose: a simple monosaccharide (sugar) with a molecular
The metabolism of any monosaccharide (simple sugar) can produce formula of C6H12O6; it is a principal source of energy for
energy for the cell to use. Excess carbohydrates are stored as starch cellular metabolism
in plants and as glycogen in animals, ready for metabolism if the
energy demands of the organism suddenly increase. When those CONTRIBUTIONS AND ATTRIBUTIONS
energy demands increase, carbohydrates are broken down into OpenStax College, Energy and Metabolism. October 26, 2013. Provided by:
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constituent monosaccharides, which are then distributed to all the CC BY: Attribution
living cells of an organism. Glucose (C6H12O6) is a common OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44421/latest...ol11448/latest. License: CC
example of the monosaccharides used for energy production. BY: Attribution
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Inside the cell, each sugar molecule is broken down through a License: CC BY-SA: Attribution-ShareAlike
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released from the bonds in the monosaccharide, it is harnessed to en.wiktionary.org/wiki/metabolism. License: CC BY-SA: Attribution-
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synthesize high-energy adenosine triphosphate (ATP) molecules. bioenergetics. Provided by: Wiktionary. Located at:
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Attribution curated by Boundless.

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6.5: POTENTIAL, KINETIC, FREE, AND ACTIVATION ENERGY - FREE ENERGY
biologists because these reactions can be harnessed to perform work
 LEARNING OBJECTIVES inside the cell. An important distinction must be drawn between the
term spontaneous and the idea of a chemical reaction that occurs
Discuss the concept of free energy.
immediately. Contrary to the everyday use of the term, a
spontaneous reaction is not one that suddenly or quickly occurs. The
FREE ENERGY rusting of iron is an example of a spontaneous reaction that occurs
Since chemical reactions release energy when energy-storing bonds slowly, little by little, over time.
are broken, how is the energy associated with chemical reactions If a chemical reaction requires an input of energy rather than
quantified and expressed? How can the energy released from one releasing energy, then the ∆G for that reaction will be a positive
reaction be compared to that of another reaction? value. In this case, the products have more free energy than the
A measurement of free energy is used to quantitate these energy reactants. Thus, the products of these reactions can be thought of as
transfers. Free energy is called Gibbs free energy (G) after Josiah energy-storing molecules. These chemical reactions are called
Willard Gibbs, the scientist who developed the measurement. Recall endergonic reactions; they are non-spontaneous. An endergonic
that according to the second law of thermodynamics, all energy reaction will not take place on its own without the addition of free
transfers involve the loss of some amount of energy in an unusable energy.
form such as heat, resulting in entropy. Gibbs free energy
specifically refers to the energy associated with a chemical reaction
that is available after accounting for entropy. In other words, Gibbs
free energy is usable energy or energy that is available to do work.

CALCULATING ∆G
Every chemical reaction involves a change in free energy, called
delta G (∆G). The change in free energy can be calculated for any
system that undergoes a change, such as a chemical reaction. To Figure 6.5.1: Exergonic and Endergonic Reactions: Exergonic and
calculate ∆G, subtract the amount of energy lost to entropy (denoted endergonic reactions result in changes in Gibbs free energy.
Exergonic reactions release energy; endergonic reactions require
as ∆S) from the total energy change of the system. This total energy energy to proceed.
change in the system is called enthalpy and is denoted as ∆H. The
formula for calculating ∆G is as follows, where the symbol T refers FREE ENERGY AND BIOLOGICAL PROCESSES
to absolute temperature in Kelvin (degrees Celsius + 273):
G=ΔH−TΔS. In a living cell, chemical reactions are constantly moving towards
The standard free energy change of a chemical reaction is expressed equilibrium, but never reach it. A living cell is an open system:
as an amount of energy per mole of the reaction product (either in materials pass in and out, the cell recycles the products of certain
kilojoules or kilocalories, kJ/mol or kcal/mol; 1 kJ = 0.239 kcal) chemical reactions into other reactions, and chemical equilibrium is
under standard pH, temperature, and pressure conditions. Standard never reached. In this way, living organisms are in a constant
pH, temperature, and pressure conditions are generally calculated at energy-requiring, uphill battle against equilibrium and entropy.
pH 7.0 in biological systems, 25 degrees Celsius, and 100 When complex molecules, such as starches, are built from simpler
kilopascals (1 atm pressure), respectively. It is important to note that molecules, such as sugars, the anabolic process requires energy.
cellular conditions vary considerably from these standard conditions; Therefore, the chemical reactions involved in anabolic processes are
therefore, standard calculated ∆G values for biological reactions will endergonic reactions. On the other hand, the catabolic process of
be different inside the cell. breaking sugar down into simpler molecules releases energy in a
series of exergonic reactions. As in the example of rust above, the
ENDERGONIC AND EXERGONIC REACTIONS breakdown of sugar involves spontaneous reactions, but these
If energy is released during a chemical reaction, then the resulting reactions don’t occur instantaneously. An important concept in the
value from the above equation will be a negative number. In other study of metabolism and energy is that of chemical equilibrium.
words, reactions that release energy have a ∆G < 0. A negative ∆G Most chemical reactions are reversible. They can proceed in both
also means that the products of the reaction have less free energy directions, releasing energy into their environment in one direction,
than the reactants because they gave off some free energy during the and absorbing it from the environment in the other direction.
reaction. Reactions that have a negative ∆G and, consequently,
release free energy, are called exergonic reactions. Exergonic means
energy is exiting the system. These reactions are also referred to as
spontaneous reactions because they can occur without the addition
of energy into the system. Understanding which chemical reactions
are spontaneous and release free energy is extremely useful for

6.5.1 https://bio.libretexts.org/@go/page/13105
 KEY POINTS
Every chemical reaction involves a change in free energy, called
delta G (∆G).
To calculate ∆G, subtract the amount of energy lost to entropy
(∆S) from the total energy change of the system; this total energy
change in the system is called enthalpy (∆H ): ΔG=ΔH−TΔS.
Endergonic reactions require an input of energy; the ∆G for that
reaction will be a positive value.
Exergonic reactions release free energy; the ∆G for that reaction
will be a negative value.

KEY TERMS
exergonic reaction: A chemical reaction where the change in the
Gibbs free energy is negative, indicating a spontaneous reaction
endergonic reaction: A chemical reaction in which the standard
change in free energy is positive, and energy is absorbed
Gibbs free energy: The difference between the enthalpy of a
Figure 6.5.1: Endergonic and Exergonic Processes: Shown are some
examples of endergonic processes (ones that require energy) and system and the product of its entropy and absolute temperature
exergonic processes (ones that release energy). These include (a) a
compost pile decomposing, (b) a chick hatching from a fertilized This page titled 6.5: Potential, Kinetic, Free, and Activation Energy - Free
egg, (c) sand art being destroyed, and (d) a ball rolling down a hill. Energy is shared under a CC BY-SA 4.0 license and was authored, remixed,
and/or curated by Boundless.

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6.6: POTENTIAL, KINETIC, FREE, AND ACTIVATION ENERGY - THE FIRST LAW
OF THERMODYNAMICS

 LEARNING OBJECTIVES

Describe the first law of thermodynamics

Thermodynamics is the study of heat energy and other types of

energy, such as work, and the various ways energy is transferred
within chemical systems. “Thermo-” refers to heat, while
“dynamics” refers to motion.

THE FIRST LAW OF THERMODYNAMICS

The first law of thermodynamics deals with the total amount of
energy in the universe. The law states that this total amount of
energy is constant. In other words, there has always been, and
always will be, exactly the same amount of energy in the universe.
Energy exists in many different forms. According to the first law of
thermodynamics, energy can be transferred from place to place or
changed between different forms, but it cannot be created or
destroyed. The transfers and transformations of energy take place
around us all the time. For instance, light bulbs transform electrical
energy into light energy, and gas stoves transform chemical energy
from natural gas into heat energy. Plants perform one of the most
biologically useful transformations of energy on Earth: they convert
the energy of sunlight into the chemical energy stored within organic
molecules.

Figure 6.6.1: The first law of thermodynamics: Shown are two

examples of energy being transferred from one system to another
and transformed from one form to another. Humans can convert the
chemical energy in food, like this ice cream cone, into kinetic energy
by riding a bicycle. Plants can convert electromagnetic radiation
(light energy) from the sun into chemical energy.

THE SYSTEM AND SURROUNDINGS

Thermodynamics often divides the universe into two categories: the
system and its surroundings. In chemistry, the system almost always
refers to a given chemical reaction and the container in which it
takes place. The first law of thermodynamics tells us that energy can
neither be created nor destroyed, so we know that the energy that is
absorbed in an endothermic chemical reaction must have been lost
from the surroundings. Conversely, in an exothermic reaction, the
heat that is released in the reaction is given off and absorbed by the
surroundings. Stated mathematically, we have:
ΔE=ΔEsys+ΔEsurr=0

6.6.1 https://bio.libretexts.org/@go/page/13106
 Figure 6.6.1: Rocket launch: The powerful chemical reaction
propelling the rocket lets off tremendous heat to the surroundings
and does work on the surroundings (the rocket) as well.

KEY POINTS
Figure 6.6.1: The system and surroundings: A basic diagram According to the first law of thermodynamics, the total amount
showing the fundamental distinction between the system and its
surroundings in thermodynamics. of energy in the universe is constant.
Energy can be transferred from place to place or transformed into
HEAT AND WORK different forms, but it cannot be created or destroyed.
We know that chemical systems can either absorb heat from their Living organisms have evolved to obtain energy from their
surroundings, if the reaction is endothermic, or release heat to their surroundings in forms that they can transfer or transform into
surroundings, if the reaction is exothermic. However, chemical usable energy to do work.
reactions are often used to do work instead of just exchanging heat.
For instance, when rocket fuel burns and causes a space shuttle to KEY TERMS
lift off from the ground, the chemical reaction, by propelling the first law of thermodynamics: A version of the law of
rocket, is doing work by applying a force over a distance. conservation of energy, specialized for thermodynamical
systems, that states that the energy of an isolated system is
If you’ve ever witnessed a video of a space shuttle lifting off, the
constant and can neither be created nor destroyed.
chemical reaction that occurs also releases tremendous amounts of
work: A measure of energy expended by moving an object,
heat and light. Another useful form of the first law of
usually considered to be force times distance. No work is done if
thermodynamics relates heat and work for the change in energy of
the object does not move.
the internal system:
ΔEsys=Q+W This page titled 6.6: Potential, Kinetic, Free, and Activation Energy - The
While this formulation is more commonly used in physics, it is still First Law of Thermodynamics is shared under a CC BY-SA 4.0 license and
important to know for chemistry. was authored, remixed, and/or curated by Boundless.

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6.7: POTENTIAL, KINETIC, FREE, AND ACTIVATION ENERGY - THE SECOND
LAW OF THERMODYNAMICS

 LEARNING OBJECTIVES

Explain how living organisms can increase their order

despite the second law of thermodynamics

THE SECOND LAW OF THERMODYNAMICS

A living cell ‘s primary tasks of obtaining, transforming, and using
energy to do work may seem simple enough, but they are more
problematic than they appear. The second law of thermodynamics
explains why: No energy transfers or transformations in the universe
are completely efficient. In every energy transfer, some amount of
energy is lost in a form that is unusable. In most cases, this energy is
in the form of heat. Thermodynamically, heat energy is defined as
the energy transferred from one system to another that is not doing
work. For example, when an airplane flies through the air, some of Figure 6.7.1: Entropy: Entropy is a measure of randomness or
the energy of the flying plane is lost as heat energy due to friction disorder in a system. Gases have higher entropy than liquids, and
with the surrounding air. This friction heats the air by temporarily liquids have higher entropy than solids.
increasing the speed of air molecules. Likewise, some energy is lost Entropy changes also occur in chemical reactions. In an exergonic
in the form of heat during cellular metabolic reactions. This is good chemical reaction where energy is released, entropy increases
for warm-blooded creatures like us because heat energy helps to because the final products have less energy inside them holding their
maintain our body temperature. Strictly speaking, no energy transfer chemical bonds together. That energy has been lost to the
is completely efficient because some energy is lost in an unusable environment, usually in the form of heat.
form. All physical systems can be thought of in this way. Living things are
highly ordered, requiring constant energy input to be maintained in a
ENTROPY state of low entropy. As living systems take in energy-storing
An important concept in physical systems is disorder (also known as molecules and transform them through chemical reactions, they lose
randomness). The more energy that is lost by a system to its some amount of usable energy in the process because no reaction is
surroundings, the less ordered and more random the system is. completely efficient. They also produce waste and by-products that
Scientists define the measure of randomness or disorder within a are not useful energy sources. This process increases the entropy of
system as entropy. High entropy means high disorder and low the system’s surroundings. Since all energy transfers result in the
energy. To better understand entropy, remember that it requires loss of some usable energy, the second law of thermodynamics states
energy to maintain structure. For example, think about an ice cube. that every energy transfer or transformation increases the entropy of
It is made of water molecules bound together in an orderly lattice. the universe. Even though living things are highly ordered and
This arrangement takes energy to maintain. When the ice cube melts maintain a state of low entropy, the entropy of the universe in total is
and becomes water, its molecules are more disordered, in a random constantly increasing due to the loss of usable energy with each
arrangement as opposed to a structure. Overall, there is less energy energy transfer that occurs. Essentially, living things are in a
in the system inside the molecular bonds. Therefore, water can be continuous uphill battle against this constant increase in universal
said to have greater entropy than ice. entropy.
This holds true for solids, liquids, and gases in general. Solids have
the highest internal energy holding them together and therefore the
KEY POINTS
lowest entropy. Liquids are more disordered and it takes less energy During energy transfer, some amount of energy is lost in the
to hold them together. Therefore they are higher in entropy than form of unusable heat energy.
solids, but lower than gases, which are so disordered that they have Because energy is lost in an unusable form, no energy transfer is
the highest entropy and lowest amount of energy spent holding them completely efficient.
together. The more energy that is lost by a system to its surroundings, the
less ordered and more random the system is.
Entropy is a measure of randomness and disorder; high entropy
means high disorder and low energy.
As chemical reactions reach a state of equilibrium, entropy
increases; and as molecules at a high concentration in one place
diffuse and spread out, entropy also increases.

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KEY TERMS entropy: A measure of randomness and disorder in a system.
second law of thermodynamics: Every energy transfer or
This page titled 6.7: Potential, Kinetic, Free, and Activation Energy - The
transformation increases the entropy of the universe since all
Second Law of Thermodynamics is shared under a CC BY-SA 4.0 license
energy transfers result in the loss of some usable energy.
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6.8: POTENTIAL, KINETIC, FREE, AND ACTIVATION ENERGY - ACTIVATION
ENERGY
involves a transmembrane ion pump that is extremely important for
 LEARNING OBJECTIVES cellular function.

Discuss the concept of activation energy FREE ENERGY DIAGRAMS

Free energy diagrams illustrate the energy profiles for a given
Many chemical reactions, and almost all biochemical reactions do reaction. Whether the reaction is exergonic (ΔG<0) or endergonic
not occur spontaneously and must have an initial input of energy (ΔG>0) determines whether the products in the diagram will exist at
(called the activation energy) to get started. Activation energy must a lower or higher energy state than the reactants. However, the
be considered when analyzing both endergonic and exergonic measure of the activation energy is independent of the reaction’s
reactions. Exergonic reactions have a net release of energy, but they ΔG. In other words, at a given temperature, the activation energy
still require a small amount of energy input before they can proceed depends on the nature of the chemical transformation that takes
with their energy-releasing steps. This small amount of energy input place, but not on the relative energy state of the reactants and
necessary for all chemical reactions to occur is called the activation products.
energy (or free energy of activation) and is abbreviated EA.
Although the image above discusses the concept of activation energy
within the context of the exergonic forward reaction, the same
principles apply to the reverse reaction, which must be endergonic.
Notice that the activation energy for the reverse reaction is larger
than for the forward reaction.

Figure 6.8.1: Activation energy: Activation energy is the energy

required for a reaction to proceed; it is lower if the reaction is
catalyzed. The horizontal axis of this diagram describes the
sequence of events in time.

ACTIVATION ENERGY IN CHEMICAL REACTIONS

Why would an energy-releasing, negative ∆G reaction actually

require some energy to proceed? The reason lies in the steps that Figure 6.8.1: Activation energy in an endergonic reaction: In this
take place during a chemical reaction. During chemical reactions, endergonic reaction, activation energy is still required to transform
the reactants A + B into the product C. This figure implies that the
certain chemical bonds are broken and new ones are formed. For activation energy is in the form of heat energy.
example, when a glucose molecule is broken down, bonds between
the carbon atoms of the molecule are broken. Since these are energy- HEAT ENERGY
storing bonds, they release energy when broken. However, to get The source of the activation energy needed to push reactions
them into a state that allows the bonds to break, the molecule must forward is typically heat energy from the surroundings. Heat energy
be somewhat contorted. A small energy input is required to achieve (the total bond energy of reactants or products in a chemical
this contorted state, which is called the transition state: it is a high- reaction) speeds up the motion of molecules, increasing the
energy, unstable state. For this reason, reactant molecules don’t last frequency and force with which they collide. It also moves atoms
long in their transition state, but very quickly proceed to the next and bonds within the molecule slightly, helping them reach their
steps of the chemical reaction. transition state. For this reason, heating up a system will cause
Cells will at times couple an exergonic reaction (ΔG<0) with chemical reactants within that system to react more frequently.
endergonic reactions (ΔG>0), allowing them to proceed. This Increasing the pressure on a system has the same effect. Once
spontaneous shift from one reaction to another is called energy reactants have absorbed enough heat energy from their surroundings
coupling. The free energy released from the exergonic reaction is to reach the transition state, the reaction will proceed.
absorbed by the The activation energy of a particular reaction determines the rate at
endergonic reaction. One example of energy coupling using ATP which it will proceed. The higher the activation energy, the slower
the chemical reaction will be. The example of iron rusting illustrates

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rates. In other words, in order for important cellular reactions to Attribution
occur at significant rates (number of reactions per unit time), their OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
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activation energies must be lowered; this is referred to as catalysis. BY: Attribution
This is a very good thing as far as living cells are concerned. first law of thermodynamics. Provided by: Wikipedia. Located at:
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temperatures alone provided enough heat energy for these exergonic
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k=AeEa/RT Attribution
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lowering its activation energy. Attribution
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6.9: ATP - ADENOSINE TRIPHOSPHATE
ATP from ADP. Since ATP hydrolysis releases energy, ATP
 LEARNING OBJECTIVES synthesis must require an input of free energy.
ADP is combined with a phosphate to form ATP in the following
Explain the role of ATP as the currency of cellular energy
reaction:

ATP: ADENOSINE TRIPHOSPHATE ADP+Pi+free energy→ATP+H2O

Adenosine triphosphate (ATP) is the energy currency for cellular ATP AND ENERGY COUPLING
processes. ATP provides the energy for both energy-consuming
Exactly how much free energy (∆G) is released with the hydrolysis
endergonic reactions and energy-releasing exergonic reactions,
of ATP, and how is that free energy used to do cellular work? The
which require a small input of activation energy. When the chemical
calculated ∆G for the hydrolysis of one mole of ATP into ADP and
bonds within ATP are broken, energy is released and can be
Pi is −7.3 kcal/mole (−30.5 kJ/mol). However, this is only true under
harnessed for cellular work. The more bonds in a molecule, the more
standard conditions, and the ∆G for the hydrolysis of one mole of
potential energy it contains. Because the bond in ATP is so easily
ATP in a living cell is almost double the value at standard
broken and reformed, ATP is like a rechargeable battery that powers
conditions: 14 kcal/mol (−57 kJ/mol).
cellular process ranging from DNA replication to protein synthesis.
ATP is a highly unstable molecule. Unless quickly used to perform
MOLECULAR STRUCTURE work, ATP spontaneously dissociates into ADP + Pi, and the free
Adenosine triphosphate (ATP) is comprised of the molecule energy released during this process is lost as heat. To harness the
adenosine bound to three phosphate groups. Adenosine is a energy within the bonds of ATP, cells use a strategy called energy
nucleoside consisting of the nitrogenous base adenine and the five- coupling.
carbon sugar ribose. The three phosphate groups, in order of closest
ENERGY COUPLING IN SODIUM-POTASSIUM
to furthest from the ribose sugar, are labeled alpha, beta, and
PUMPS
gamma. Together, these chemical groups constitute an energy
powerhouse. The two bonds between the phosphates are equal high-
energy bonds (phosphoanhydride bonds) that, when broken, release
sufficient energy to power a variety of cellular reactions and
processes. The bond between the beta and gamma phosphate is
considered “high-energy” because when the bond breaks, the
products [adenosine diphosphate (ADP) and one inorganic
phosphate group (Pi)] have a lower free energy than the reactants
(ATP and a water molecule). ATP breakdown into ADP and Pi is
called hydrolysis because it consumes a water molecule (hydro-,
meaning “water”, and lysis, meaning “separation”).

Figure 6.9.1: Energy Coupling: Sodium-potassium pumps use the

energy derived from exergonic ATP hydrolysis to pump sodium and
potassium ions across the cell membrane.
Cells couple the exergonic reaction of ATP hydrolysis with the
endergonic reactions of cellular processes. For example,
transmembrane ion pumps in nerve cells use the energy from ATP to
pump ions across the cell membrane and generate an action
potential. The sodium-potassium pump (Na+/K+pump) drives
sodium out of the cell and potassium into the cell. When ATP is
Figure 6.9.1: Adenosine Triphosphate (ATP): ATP is the primary hydrolyzed, it transfers its gamma phosphate to the pump protein in
energy currency of the cell. It has an adenosine backbone with three
phosphate groups attached.
a process called phosphorylation. The Na+/K+ pump gains the free
energy and undergoes a conformational change, allowing it to
ATP HYDROLYSIS AND SYNTHESIS release three Na+ to the outside of the cell. Two extracellular K+ ions
ATP is hydrolyzed into ADP in the following reaction: bind to the protein, causing the protein to change shape again and
discharge the phosphate. By donating free energy to the Na+/K+
ATP+H2O→ADP+Pi+free energy
pump, phosphorylation drives the endergonic reaction.
Like most chemical reactions, the hydrolysis of ATP to ADP is
reversible. The reverse reaction combines ADP + Pi to regenerate

6.9.1 https://bio.libretexts.org/@go/page/13120
ENERGY COUPLING IN METABOLISM reaction or system.
During cellular metabolic reactions, or the synthesis and breakdown endergonic: Describing a reaction that absorbs (heat) energy
of nutrients, certain molecules must be altered slightly in their from its environment.
conformation to become substrates for the next step in the reaction exergonic: Describing a reaction that releases energy (heat) into
series. In the very first steps of cellular respiration, glucose is broken its environment.
down through the process of glycolysis. ATP is required for the free energy: Gibbs free energy is a thermodynamic potential that
phosphorylation of glucose, creating a high-energy but unstable measures the useful or process-initiating work obtainable from a
intermediate. This phosphorylation reaction causes a conformational thermodynamic system at a constant temperature and pressure
change that allows enzymes to convert the phosphorylated glucose (isothermal, isobaric).
molecule to the phosphorylated sugar fructose. Fructose is a hydrolysis: A chemical process of decomposition involving the
necessary intermediate for glycolysis to move forward. In this splitting of a bond by the addition of water.
example, the exergonic reaction of ATP hydrolysis is coupled with
CONTRIBUTIONS AND ATTRIBUTIONS
the endergonic reaction of converting glucose for use in the
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KEY TERMS
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energy coupling: Energy coupling occurs when the energy
produced by one reaction or system is used to drive another

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6.10: ENZYMES - ACTIVE SITE AND SUBSTRATE SPECIFICITY

 LEARNING OBJECTIVES

Describe models of substrate binding to an enzyme’s active

site.

ENZYME ACTIVE SITE AND SUBSTRATE

SPECIFICITY
Enzymes bind with chemical reactants called substrates. There may
Figure 6.10.1: Induced Fit: According to the induced fit model, both
be one or more substrates for each type of enzyme, depending on the enzyme and substrate undergo dynamic conformational changes
particular chemical reaction. In some reactions, a single-reactant upon binding. The enzyme contorts the substrate into its transition
substrate is broken down into multiple products. In others, two state, thereby increasing the rate of the reaction.
substrates may come together to create one larger molecule. Two
ENZYME-SUBSTRATE COMPLEX
reactants might also enter a reaction, both become modified, and
When an enzyme binds its substrate, it forms an enzyme-substrate
leave the reaction as two products.
complex. This complex lowers the activation energy of the reaction
The enzyme’s active site binds to the substrate. Since enzymes are and promotes its rapid progression by providing certain ions or
proteins, this site is composed of a unique combination of amino chemical groups that actually form covalent bonds with molecules as
acid residues (side chains or R groups). Each amino acid residue can a necessary step of the reaction process. Enzymes also promote
be large or small; weakly acidic or basic; hydrophilic or chemical reactions by bringing substrates together in an optimal
hydrophobic; and positively-charged, negatively-charged, or neutral. orientation, lining up the atoms and bonds of one molecule with the
The positions, sequences, structures, and properties of these residues atoms and bonds of the other molecule. This can contort the
create a very specific chemical environment within the active site. A substrate molecules and facilitate bond-breaking. The active site of
specific chemical substrate matches this site like a jigsaw puzzle an enzyme also creates an ideal environment, such as a slightly
piece and makes the enzyme specific to its substrate. acidic or non-polar environment, for the reaction to occur. The
enzyme will always return to its original state at the completion of
ACTIVE SITES AND ENVIRONMENTAL
the reaction. One of the important properties of enzymes is that they
CONDITIONS
remain ultimately unchanged by the reactions they catalyze. After an
Environmental conditions can affect an enzyme’s active site and, enzyme is done catalyzing a reaction, it releases its products
therefore, the rate at which a chemical reaction can proceed. (substrates).
Increasing the environmental temperature generally increases
reaction rates because the molecules are moving more quickly and KEY POINTS
are more likely to come into contact with each other. The enzyme ‘s active site binds to the substrate.
However, increasing or decreasing the temperature outside of an Increasing the temperature generally increases the rate of a
optimal range can affect chemical bonds within the enzyme and reaction, but dramatic changes in temperature and pH can
change its shape. If the enzyme changes shape, the active site may denature an enzyme, thereby abolishing its action as a catalyst.
no longer bind to the appropriate substrate and the rate of reaction The induced fit model states an substrate binds to an active site
will decrease. Dramatic changes to the temperature and pH will and both change shape slightly, creating an ideal fit for catalysis.
eventually cause enzymes to denature. When an enzyme binds its substrate it forms an enzyme-substrate
complex.
INDUCED FIT AND ENZYME FUNCTION Enzymes promote chemical reactions by bringing substrates
For many years, scientists thought that enzyme-substrate binding together in an optimal orientation, thus creating an ideal
took place in a simple “lock-and-key” fashion. This model asserted chemical environment for the reaction to occur.
that the enzyme and substrate fit together perfectly in one The enzyme will always return to its original state at the
instantaneous step. However, current research supports a more completion of the reaction.
refined view called induced fit. As the enzyme and substrate come
together, their interaction causes a mild shift in the enzyme’s KEY TERMS
structure that confirms an ideal binding arrangement between the substrate: A reactant in a chemical reaction is called a substrate
enzyme and the substrate. This dynamic binding maximizes the when acted upon by an enzyme.
enzyme’s ability to catalyze its reaction. induced fit: Proposes that the initial interaction between enzyme
and substrate is relatively weak, but that these weak interactions
rapidly induce conformational changes in the enzyme that
strengthen binding.

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6.11: ENZYMES - CONTROL OF METABOLISM THROUGH ENZYME
REGULATION

 LEARNING OBJECTIVES

Explain the effect of an enzyme on chemical equilibrium

CONTROL OF METABOLISM THROUGH ENZYME

REGULATION
Cellular needs and conditions vary from cell to cell and change
within individual cells over time. For example, a stomach cell
requires a different amount of energy than a skin cell, fat storage
cell, blood cell, or nerve cell. The same stomach cell may also need
more energy immediately after a meal and less energy between
meals.
A cell’s function is encapsulated by the chemical reactions it can
carry out. Enzymes lower the activation energies of chemical
reactions; in cells, they promote those reactions that are specific to Figure 6.11.1: Enzyme inhibition: Competitive and noncompetitive
inhibition affect the rate of reaction differently. Competitive
the cell’s function. Because enzymes ultimately determine which inhibitors affect the initial rate, but do not affect the maximal rate,
chemical reactions a cell can carry out and the rate at which they can whereas noncompetitive inhibitors affect the maximal rate.
proceed, they are key to cell functionality.
ALLOSTERIC INHIBITION AND ACTIVATION
COMPETITIVE AND NONCOMPETITIVE In noncompetitive allosteric inhibition, inhibitor molecules bind to
INHIBITION an enzyme at the allosteric site. Their binding induces a
The cell uses specific molecules to regulate enzymes in order to conformational change that reduces the affinity of the enzyme’s
promote or inhibit certain chemical reactions. Sometimes it is active site for its substrate. The binding of this allosteric inhibitor
necessary to inhibit an enzyme to reduce a reaction rate, and there is changes the conformation of the enzyme and its active site, so the
more than one way for this inhibition to occur. In competitive substrate is not able to bind. This prevents the enzyme from
inhibition, an inhibitor molecule is similar enough to a substrate that lowering the activation energy of the reaction, and the reaction rate
it can bind to the enzyme’s active site to stop it from binding to the is reduced.
substrate. It “competes” with the substrate to bind to the enzyme. However, allosteric inhibitors are not the only molecules that bind to
In noncompetitive inhibition, an inhibitor molecule binds to the allosteric sites. Allosteric activators can increase reaction rates. They
enzyme at a location other than the active site (an allosteric site). bind to an allosteric site which induces a conformational change that
The substrate can still bind to the enzyme, but the inhibitor changes increases the affinity of the enzyme’s active site for its substrate.
the shape of the enzyme so it is no longer in optimal position to This increases the reaction rate.
catalyze the reaction.

Figure 6.11.1: Allosteric inhibitors and activators: Allosteric

inhibitors modify the active site of the enzyme so that substrate
binding is reduced or prevented. In contrast, allosteric activators
modify the active site of the enzyme so that the affinity for the
substrate increases.

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COFACTORS AND COENZYMES FEEDBACK INHIBITION IN METABOLIC
Many enzymes only work if bound to non-protein helper molecules PATHWAYS
called cofactors and coenzymes. Binding to these molecules Feedback inhibition is when a reaction product is used to regulate its
promotes optimal conformation and function for their respective own further production. Cells have evolved to use feedback
enzymes. These molecules bind temporarily through ionic or inhibition to regulate enzyme activity in metabolism, by using the
hydrogen bonds or permanently through stronger covalent bonds. products of the enzymatic reactions to inhibit further enzyme
Cofactors are inorganic ions such as iron (Fe2+) and magnesium activity. Metabolic reactions, such as anabolic and catabolic
(Mg2+). For example, DNA polymerase requires a zinc ion (Zn2+) to processes, must proceed according to the demands of the cell. In
build DNA molecules. Coenzymes are organic helper molecules order to maintain chemical equilibrium and meet the needs of the
with a basic atomic structure made up of carbon and hydrogen. The cell, some metabolic products inhibit the enzymes in the chemical
most common coenzymes are dietary vitamins. Vitamin C is a pathway while some reactants activate them.
coenzyme for multiple enzymes that take part in building collagen,
an important component of connective tissue. Pyruvate
dehydrogenase is a complex of several enzymes that requires one
cofactor and five different organic coenzymes to catalyze its
chemical reaction. The availability of various cofactors and
coenzymes regulates enzyme function.

Figure 6.11.1: Feedback inhibition: Metabolic pathways are a series

of reactions catalyzed by multiple enzymes. Feedback inhibition,
where the end product of the pathway inhibits an earlier step, is an
important regulatory mechanism in cells.
The production of both amino acids and nucleotides is controlled
through feedback inhibition. For an example of feedback inhibition,
consider ATP. It is the product of the catabolic metabolism of sugar
(cellular respiration), but it also acts as an allosteric regulator for the
same enzymes that produced it. ATP is an unstable molecule that can
spontaneously dissociate into ADP; if too much ATP were present,
most of it would go to waste. This feedback inhibition prevents the
production of additional ATP if it is already abundant. However,
while ATP is an inhibitor, ADP is an allosteric activator. When
levels of ADP are high compared to ATP levels, ADP triggers the
catabolism of sugar to produce more ATP.

KEY POINTS
In competitive inhibition, an inhibitor molecule competes with a
substrate by binding to the enzyme ‘s active site so the substrate
is blocked.
In noncompetitive inhibition (also known as allosteric
inhibition), an inhibitor binds to an allosteric site; the substrate
Figure 6.11.1: Vitamins: Vitamins are important coenzymes or can still bind to the enzyme, but the enzyme is no longer in
precursors of coenzymes and are required for enzymes to function optimal position to catalyze the reaction.
properly. Multivitamin capsules usually contain mixtures of all the
vitamins at different percentages. Allosteric inhibitors induce a conformational change that
changes the shape of the active site and reduces the affinity of
ENZYME COMPARTMENTALIZATION the enzyme’s active site for its substrate.
In eukaryotic cells, molecules such as enzymes are usually Allosteric activators induce a conformational change that
compartmentalized into different organelles. This organization changes the shape of the active site and increases the affinity of
contributes to enzyme regulation because certain cellular processes the enzyme’s active site for its substrate.
are contained in separate organelles. For example, the enzymes Feedback inhibition involves the use of a reaction product to
involved in the later stages of cellular respiration carry out reactions regulate its own further production.
exclusively in the mitochondria. The enzymes involved in the Inorganic cofactors and organic coenzymes promote optimal
digestion of cellular debris and foreign materials are located within enzyme orientation and function.
lysosomes. Vitamins act as coenzymes (or precursors to coenzymes) and are
necessary for enzymes to function.

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KEY TERMS License: CC BY: Attribution
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coenzyme. Provided by: Wiktionary. Located at:
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to function. OpenStax College, Enzymes. October 16, 2013. Provided by: OpenStax CNX.
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 CHAPTER OVERVIEW

7: CELLULAR RESPIRATION
Cellular respiration is a set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from
nutrients into adenosine triphosphate (ATP), and then release waste products.
7.1: Energy in Living Systems - Transforming Chemical Energy
7.2: Energy in Living Systems - Electrons and Energy
7.3: Energy in Living Systems - ATP in Metabolism
7.4: Glycolysis - Importance of Glycolysis
7.5: Glycolysis - The Energy-Requiring Steps of Glycolysis
7.6: Glycolysis - The Energy-Releasing Steps of Glycolysis
7.7: Glycolysis - Outcomes of Glycolysis
7.8: Oxidation of Pyruvate and the Citric Acid Cycle - Breakdown of Pyruvate
7.9: Oxidation of Pyruvate and the Citric Acid Cycle - Acetyl CoA to CO₂
7.10: Oxidation of Pyruvate and the Citric Acid Cycle - Citric Acid Cycle
7.11: Oxidative Phosphorylation - Electron Transport Chain
7.12: Oxidative Phosphorylation - Chemiosmosis and Oxidative Phosphorylation
7.13: Oxidative Phosphorylation - ATP Yield
7.14: Metabolism without Oxygen - Anaerobic Cellular Respiration
7.15: Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways - Connecting Other Sugars to Glucose Metabolism
7.16: Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways - Connecting Proteins to Glucose Metabolism
7.17: Connections of Carbohydrate, Protein, and Lipid Metabolic Pathways - Connecting Lipids to Glucose Metabolism
7.18: Regulation of Cellular Respiration - Regulatory Mechanisms for Cellular Respiration
7.19: Regulation of Cellular Respiration - Control of Catabolic Pathways

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1
7.1: ENERGY IN LIVING SYSTEMS - TRANSFORMING CHEMICAL ENERGY
eukaryotes, including humans, and takes place mostly within the
 LEARNING OBJECTIVES mitochondria. Respiration occurs within the cytoplasm of
prokaryotes. Several prokaryotes and a few eukaryotes use an
Discuss the importance of cellular respiration
inorganic molecule other than oxygen to drive the oxidation of their
nutrients in a process called anaerobic respiration. Electron
INTRODUCTION: CELLULAR RESPIRATION acceptors for anaerobic respiration include nitrate, sulfate, carbon
An electrical energy plant converts energy from one form to another dioxide, and several metal ions.
form that can be more easily used. For example, geothermal energy The energy released during cellular respiration is then used in other
plants start with underground thermal energy (heat) and transform it biological processes. These processes build larger molecules that are
into electrical energy that will be transported to homes and factories. essential to an organism’s survival, such as amino acids, DNA, and
proteins. Because they synthesize new molecules, these processes
are examples of anabolism.

KEY POINTS
Organisms ingest organic molecules like the carbohydrate
glucose to obtain the energy needed for cellular functions.
The energy in glucose can be extracted in a series of chemical
reactions known as cellular respiration.
Cellular respiration produces energy in the form of ATP, which is
the universal energy currency for cells.
Figure 7.1.1: Energy Plant: This geothermal energy plant transforms
thermal energy from deep in the ground into electrical energy, which KEY TERMS
can be easily used. aerobic respiration: the process of converting the biochemical
Like a generating plant, living organisms must take in energy from energy in nutrients to ATP in the presence of oxygen
their environment and convert it into to a form their cells can use. adenosine triphosphate: a multifunctional nucleoside
Organisms ingest large molecules, like carbohydrates, proteins, and triphosphate used in cells as a coenzyme, often called the
fats, and convert them into smaller molecules like carbon dioxide “molecular unit of energy currency” in intracellular energy
and water. This process is called cellular respiration, a form of transfer
catabolism, and makes energy available for the cell to use. The cellular respiration: the set of the metabolic reactions and
energy released by cellular respiration is temporarily captured by the processes that take place in the cells of organisms to convert
formation of adenosine triphosphate (ATP) within the cell. ATP is biochemical energy from nutrients into adenosine triphosphate
the principle form of stored energy used for cellular functions and is (ATP)
frequently referred to as the energy currency of the cell. catabolism: the breakdown of large molecules into smaller ones
The nutrients broken down through cellular respiration lose usually accompanied by the release of energy
electrons throughout the process and are said to be oxidized. When
This page titled 7.1: Energy in Living Systems - Transforming Chemical
oxygen is used to help drive the oxidation of nutrients the process is
Energy is shared under a CC BY-SA 4.0 license and was authored, remixed,
called aerobic respiration. Aerobic respiration is common among the
and/or curated by Boundless.

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7.2: ENERGY IN LIVING SYSTEMS - ELECTRONS AND ENERGY

 LEARNING OBJECTIVES

Describe the role played by electrons in energy production

and storage

ELECTRONS AND ENERGY

The removal of an electron from a molecule via a process called
oxidation results in a decrease in the potential energy stored in the
oxidized compound. When oxidation occurs in the cell, the electron
(sometimes as part of a hydrogen atom) does not remain un-bonded
in the cytoplasm. Instead, the electron shifts to a second compound,
reducing the second compound (oxidation of one species always
occurs in tandem with reduction of another).
Figure 7.2.1: The structure of NADH and NAD+: The oxidized
The shift of an electron from one compound to another removes form of the electron carrier (NAD+) is shown on the left and the
some potential energy from the first compound (the oxidized reduced form (NADH) is shown on the right. The nitrogenous base
compound) and increases the potential energy of the second in NADH has one more hydrogen ion and two more electrons than
in NAD+.
compound (the reduced compound). The transfer of electrons
between molecules via oxidation and reduction is important because NAD+ can accept electrons from an organic molecule according to
most of the energy stored in atoms is in the form of high-energy the general equation:
electrons; it is this energy that is used to fuel cellular functions. The RH (Reducing agent) + NAD+ (Oxidizing agent) → NADH
transfer of energy in the form of electrons allows the cell to transfer (Reduced) + R (Oxidized)
and use energy in an incremental fashion: in small packages rather When electrons are added to a compound, the compound is reduced.
than as a single, destructive burst. A compound that reduces another is called a reducing agent. In the
above equation, RH is a reducing agent and NAD+ is reduced to
ELECTRON CARRIERS NADH. When electrons are removed from a compound, the
In living systems, a small class of molecules functions as electron compound is oxidized. In the above equation, NAD+ is an oxidizing
shuttles: they bind and carry high-energy electrons between agent and RH is oxidized to R. The molecule NADH is critical for
compounds in cellular pathways. The principal electron carriers we cellular respiration and other metabolic pathways.
will consider are derived from the vitamin B group, which are
Similarly, flavin adenine dinucleotide (FAD+) is derived from
derivatives of nucleotides. These compounds can be easily reduced
vitamin B2, also called riboflavin. Its reduced form is FADH2. A
(that is, they accept electrons) or oxidized (they lose electrons).
second variation of NAD, NADP, contains an extra phosphate group.
Nicotinamide adenine dinucleotide (NAD) is derived from vitamin
Both NAD+ and FAD+ are extensively used in energy extraction
B3, niacin. NAD+ is the oxidized form of niacin; NADH is the
from sugars, and NADP plays an important role in anabolic
reduced form after it has accepted two electrons and a proton (which
reactions and photosynthesis.
together are the equivalent of a hydrogen atom with an extra
electron). It is noteworthy that NAD+must accept two electrons at KEY POINTS
once; it cannot serve as a one-electron carrier.
When electrons are added to a compound, the compound is
reduced; a compound that reduces another is called a reducing
agent.
When electrons are removed from a compound, the compound is
considered oxidized; a compound that oxidizes another is called
an oxidizing agent.
The transfer of energy in the form of electrons allows the cell to
transfer and use energy in an incremental fashion.
The principle electron carriers are NAD+ and NADH because
they can be easily oxidized and reduced, respectively.
NAD+ is the oxidized form of the niacin and NADH is the
reduced form after it has accepted two electrons and a proton.

KEY TERMS
oxidation: A reaction in which the atoms of an element lose
electrons and the valence of the element increases.

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reduction: A reaction in which electrons are gained and valence electron shuttle: molecules that bind and carry high-energy
is reduced; often by the removal of oxygen or the addition of electrons between compounds in cellular pathways
hydrogen.
nicotinamide adenine dinucleotide: (NAD) An organic This page titled 7.2: Energy in Living Systems - Electrons and Energy is
coenzyme involved in biological oxidation and reduction shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
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reactions.

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7.3: ENERGY IN LIVING SYSTEMS - ATP IN METABOLISM
an inorganic phosphate ion (Pi), and the release of free energy. To
 LEARNING OBJECTIVES carry out life processes, ATP is continuously broken down into ADP,
and, like a rechargeable battery, ADP is continuously regenerated
Compare the two methods by which cells utilize ATP for
into ATP by the reattachment of a third phosphate group. Water,
energy.
which was broken down into its hydrogen atom and hydroxyl group
during ATP hydrolysis, is regenerated when a third phosphate is
ATP IN LIVING SYSTEMS added to the ADP molecule, reforming ATP.
A living cell cannot store significant amounts of free energy. Excess Obviously, energy must be infused into the system to regenerate
free energy would result in an increase of heat in the cell, which ATP. In nearly every living thing on earth, the energy comes from
would lead to excessive thermal motion that could damage and then the metabolism of glucose. In this way, ATP is a direct link between
destroy the cell. Rather, a cell must be able to handle that energy in a the limited set of exergonic pathways of glucose catabolism and the
way that enables the cell to store energy safely and release it for use multitude of endergonic pathways that power living cells.
as needed. Living cells accomplish this by using the compound
adenosine triphosphate (ATP). ATP is often called the “energy PHOSPHORYLATION
currency” of the cell and can be used to fill any energy need of the When ATP is broken down by the removal of its terminal phosphate
cell. group, energy is released and can be used to do work by the cell.
Often the released phosphate is directly transferred to another
molecule, such as a protein, activating it. For example, ATP supplies
the energy to move the contractile muscle proteins during the
mechanical work of muscle contraction. Recall the active transport
work of the sodium-potassium pump in cell membranes.
Phosphorylation by ATP alters the structure of the integral protein
that functions as the pump, changing its affinity for sodium and
potassium. In this way, the cell performs work, using energy from
ATP to pump ions against their electrochemical gradients.
Sometimes phosphorylation of an enzyme leads to its inhibition. For
Figure 7.3.1: Adenosine triphosphate.: ATP (adenosine example, the pyruvate dehydrogenase (PDH) complex could be
triphosphate) has three phosphate groups that can be removed by phosphorylated by pyruvate dehydrogenase kinase (PDHK). This
hydrolysis to form ADP (adenosine diphosphate) or AMP
(adenosine monophosphate).The negative charges on the phosphate reaction leads to inhibition of PDH and its inability to convert
group naturally repel each other, requiring energy to bond them pyruvate into acetyl-CoA.
together and releasing energy when these bonds are broken.

ATP STRUCTURE AND FUNCTION

The core of ATP is a molecule of adenosine monophosphate (AMP),
which is composed of an adenine molecule bonded to a ribose
molecule and to a single phosphate group. Ribose is a five-carbon
sugar found in RNA, and AMP is one of the nucleotides in RNA.
Figure 7.3.1: Protein phosphorylation: In phosphorylation reactions,
The addition of a second phosphate group to this core molecule the gamma phosphate of ATP is attached to a protein.
results in the formation of adenosine diphosphate (ADP); the
addition of a third phosphate group forms adenosine triphosphate ENERGY FROM ATP HYDROLYSIS
(ATP). The energy from ATP can also be used to drive chemical reactions
The addition of a phosphate group to a molecule requires energy. by coupling ATP hydrolysis with another reaction process in an
Phosphate groups are negatively charged and, thus, repel one enzyme. In many cellular chemical reactions, enzymes bind to
another when they are arranged in a series, as they are in ADP and several substrates or reactants to form a temporary intermediate
ATP. This repulsion makes the ADP and ATP molecules inherently complex that allow the substrates and reactants to more readily react
unstable. The release of one or two phosphate groups from ATP, a with each other. In reactions where ATP is involved, ATP is one of
process called dephosphorylation, releases energy. the substrates and ADP is a product. During an endergonic chemical
reaction, ATP forms an intermediate complex with the substrate and
ENERGY FROM ATP enzyme in the reaction. This intermediate complex allows the ATP
Hydrolysis is the process of breaking complex macromolecules to transfer its third phosphate group, with its energy, to the substrate,
apart. During hydrolysis, water is split, or lysed, and the resulting a process called phosphorylation. Phosphorylation refers to the
hydrogen atom (H+) and a hydroxyl group (OH–) are added to the addition of the phosphate (~P). When the intermediate complex
larger molecule. The hydrolysis of ATP produces ADP, together with breaks apart, the energy is used to modify the substrate and convert

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phosphate ion oxidation. Provided by: Wiktionary. Located at:
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7.4: GLYCOLYSIS - IMPORTANCE OF GLYCOLYSIS

 LEARNING OBJECTIVES

Explain the importance of glycolysis to cells

Nearly all of the energy used by living cells comes to them from the
energy in the bonds of the sugar glucose. Glucose enters
heterotrophic cells in two ways. One method is through secondary
active transport in which the transport takes place against the
glucose concentration gradient. The other mechanism uses a group
of integral proteins called GLUT proteins, also known as glucose
transporter proteins. These transporters assist in the facilitated
diffusion of glucose. Glycolysis is the first pathway used in the
breakdown of glucose to extract energy. It takes place in the
cytoplasm of both prokaryotic and eukaryotic cells. It was probably
one of the earliest metabolic pathways to evolve since it is used by
nearly all of the organisms on earth. The process does not use
oxygen and is, therefore, anaerobic.
Glycolysis is the first of the main metabolic pathways of cellular
respiration to produce energy in the form of ATP. Through two Figure 7.4.1: Cellular Respiration: Glycolysis is the first pathway of
cellular respiration that oxidizes glucose molecules. It is followed by
distinct phases, the six-carbon ring of glucose is cleaved into two the Krebs cycle and oxidative phosphorylation to produce ATP.
three-carbon sugars of pyruvate through a series of enzymatic
reactions. The first phase of glycolysis requires energy, while the KEY POINTS
second phase completes the conversion to pyruvate and produces Glycolysis is present in nearly all living organisms.
ATP and NADH for the cell to use for energy. Overall, the process Glucose is the source of almost all energy used by cells.
of glycolysis produces a net gain of two pyruvate molecules, two Overall, glycolysis produces two pyruvate molecules, a net gain
ATP molecules, and two NADH molecules for the cell to use for of two ATP molecules, and two NADH molecules.
energy. Following the conversion of glucose to pyruvate, the
glycolytic pathway is linked to the Krebs Cycle, where further ATP KEY TERMS
will be produced for the cell’s energy needs. glycolysis: the cellular metabolic pathway of the simple sugar
glucose to yield pyruvic acid and ATP as an energy source
heterotroph: an organism that requires an external supply of
energy in the form of food, as it cannot synthesize its own

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7.5: GLYCOLYSIS - THE ENERGY-REQUIRING STEPS OF GLYCOLYSIS
low and the concentration of ATP is high. Thus, if there is
 LEARNING OBJECTIVES “sufficient” ATP in the system, the pathway slows down. This is a
type of end-product inhibition, since ATP is the end product of
Outline the energy-requiring steps of glycolysis
glucose catabolism.
Step 4. The newly-added high-energy phosphates further destabilize
FIRST HALF OF GLYCOLYSIS (ENERGY-
fructose-1,6-bisphosphate. The fourth step in glycolysis employs an
REQUIRING STEPS)
enzyme, aldolase, to cleave 1,6-bisphosphate into two three-carbon
In the first half of glycolysis, two adenosine triphosphate (ATP) isomers: dihydroxyacetone-phosphate and glyceraldehyde-3-
molecules are used in the phosphorylation of glucose, which is then phosphate.
split into two three-carbon molecules as described in the following
Step 5. In the fifth step, an isomerase transforms the
steps.
dihydroxyacetone-phosphate into its isomer, glyceraldehyde-3-
phosphate. Thus, the pathway will continue with two molecules of a
single isomer. At this point in the pathway, there is a net investment
of energy from two ATP molecules in the breakdown of one glucose
molecule.

KEY POINTS
Figure 7.5.1: The first half of glycolysis: investment: The first half ATP molecules donate high energy phosphate groups during the
of glycolysis uses two ATP molecules in the phosphorylation of
glucose, which is then split into two three-carbon molecules. two phosphorylation steps, step 1 with hexokinase and step 3
Step 1. The first step in glycolysis is catalyzed by hexokinase, an with phosphofructokinase, in the first half of glycolysis.
enzyme with broad specificity that catalyzes the phosphorylation of In steps 2 and 5, isomerases convert molecules into their isomers
six-carbon sugars. Hexokinase phosphorylates glucose using ATP as to allow glucose to be split eventually into two molecules of
the source of the phosphate, producing glucose-6-phosphate, a more glyceraldehyde-3-phosphate, which continues into the second
reactive form of glucose. This reaction prevents the phosphorylated half of glycolysis.
glucose molecule from continuing to interact with the GLUT The enzyme aldolase in step 4 of glycolysis cleaves the six-
proteins. It can no longer leave the cell because the negatively- carbon sugar 1,6-bisphosphate into two three-carbon sugar
charged phosphate will not allow it to cross the hydrophobic interior isomers, dihydroxyacetone-phosphate and glyceraldehyde-3-
of the plasma membrane. phosphate.

Step 2. In the second step of glycolysis, an isomerase converts KEY TERMS

glucose-6-phosphate into one of its isomers, fructose-6-phosphate.
glucose: a simple monosaccharide (sugar) with a molecular
An enzyme that catalyzes the conversion of a molecule into one of
formula of C6H12O6; it is a principal source of energy for
its isomers is an isomerase. (This change from phosphoglucose to
cellular metabolism
phosphofructose allows the eventual split of the sugar into two
adenosine triphosphate: a multifunctional nucleoside
three-carbon molecules).
triphosphate used in cells as a coenzyme, often called the
Step 3. The third step is the phosphorylation of fructose-6- “molecular unit of energy currency” in intracellular energy
phosphate, catalyzed by the enzyme phosphofructokinase. A second transfer
ATP molecule donates a high-energy phosphate to fructose-6-
phosphate, producing fructose-1,6-bisphosphate. In this pathway, This page titled 7.5: Glycolysis - The Energy-Requiring Steps of Glycolysis
phosphofructokinase is a rate-limiting enzyme. It is active when the is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
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7.6: GLYCOLYSIS - THE ENERGY-RELEASING STEPS OF GLYCOLYSIS
Step 7. In the seventh step, catalyzed by phosphoglycerate kinase
 LEARNING OBJECTIVES (an enzyme named for the reverse reaction), 1,3-
bisphosphoglycerate donates a high-energy phosphate to ADP,
Outline the energy-releasing steps of glycolysis
forming one molecule of ATP. (This is an example of substrate-level
phosphorylation. ) A carbonyl group on the 1,3-bisphosphoglycerate
SECOND HALF OF GLYCOLYSIS (ENERGY- is oxidized to a carboxyl group, and 3-phosphoglycerate is formed.
RELEASING STEPS)
Step 8. In the eighth step, the remaining phosphate group in 3-
So far, glycolysis has cost the cell two ATP molecules and produced phosphoglycerate moves from the third carbon to the second carbon,
two small, three-carbon sugar molecules. Both of these molecules producing 2-phosphoglycerate (an isomer of 3-phosphoglycerate).
will proceed through the second half of the pathway where sufficient The enzyme catalyzing this step is a mutase (isomerase).
energy will be extracted to pay back the two ATP molecules used as
Step 9. Enolase catalyzes the ninth step. This enzyme causes 2-
an initial investment while also producing a profit for the cell of two
phosphoglycerate to lose water from its structure; this is a
additional ATP molecules and two even higher-energy NADH
dehydration reaction, resulting in the formation of a double bond
molecules.
that increases the potential energy in the remaining phosphate bond
and produces phosphoenolpyruvate (PEP).
Step 10. The last step in glycolysis is catalyzed by the enzyme
pyruvate kinase (the enzyme in this case is named for the reverse
reaction of pyruvate’s conversion into PEP) and results in the
production of a second ATP molecule by substrate-level
phosphorylation and the compound pyruvic acid (or its salt form,
pyruvate). Many enzymes in enzymatic pathways are named for the
reverse reactions since the enzyme can catalyze both forward and
Figure 7.6.1: The second half of glycolysis: return on investment: reverse reactions (these may have been described initially by the
The second half of glycolysis involves phosphorylation without ATP reverse reaction that takes place in vitro, under non-physiological
investment (step 6) and produces two NADH and four ATP conditions).
molecules per glucose.
Step 6. The sixth step in glycolysis oxidizes the sugar KEY POINTS
(glyceraldehyde-3-phosphate), extracting high-energy electrons, The net energy release in glycolysis is a result of two molecules
which are picked up by the electron carrier NAD+, producing of glyceraldehyde-3- phosphate entering the second half of
NADH. The sugar is then phosphorylated by the addition of a glycolysis where they are converted to pyruvic acid.
second phosphate group, producing 1,3-bisphosphoglycerate. Note Substrate -level phosphorylation, where a substrate of glycolysis
that the second phosphate group does not require another ATP donates a phosphate to ADP, occurs in two steps of the second-
molecule. half of glycolysis to produce ATP.
Here, again, there is a potential limiting factor for this pathway. The The availability of NAD+ is a limiting factor for the steps of
continuation of the reaction depends upon the availability of the glycolysis; when it is unavailable, the second half of glycolysis
oxidized form of the electron carrier NAD+. Thus, NADH must be slows or shuts down.
continuously oxidized back into NAD+ in order to keep this step
going. If NAD+ is not available, the second half of glycolysis slows KEY TERMS
down or stops. If oxygen is available in the system, the NADH will NADH: nicotinamide adenine dinucleotide (NAD) carrying two
be oxidized readily, though indirectly, and the high-energy electrons electrons and bonded with a hydrogen (H) ion; the reduced form
from the hydrogen released in this process will be used to produce of NAD
ATP. In an environment without oxygen, an alternate pathway
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7.7: GLYCOLYSIS - OUTCOMES OF GLYCOLYSIS
KEY POINTS
 LEARNING OBJECTIVES Although four ATP molecules are produced in the second half,
Describe the energy obtained from one molecule of glucose the net gain of glycolysis is only two ATP because two ATP
going through glycolysis molecules are used in the first half of glycolysis.
Enzymes that catalyze the reactions that produce ATP are rate-
limiting steps of glycolysis and must be present in sufficient
OUTCOMES OF GLYCOLYSIS
quantities for glycolysis to complete the production of four ATP,
Glycolysis starts with one molecule of glucose and ends with two two NADH, and two pyruvate molecules for each glucose
pyruvate (pyruvic acid) molecules, a total of four ATP molecules, molecule that enters the pathway.
and two molecules of NADH. Two ATP molecules were used in the Red blood cells require glycolysis as their sole source of ATP in
first half of the pathway to prepare the six-carbon ring for cleavage, order to survive, because they do not have mitochondria.
so the cell has a net gain of two ATP molecules and 2 NADH Cancer cells and stem cells also use glycolysis as the main
molecules for its use. If the cell cannot catabolize the pyruvate source of ATP (process known as aerobic glycolysis, or Warburg
molecules further (via the citric acid cycle or Krebs cycle), it will effect).
harvest only two ATP molecules from one molecule of glucose.
KEY TERMS
pyruvate: any salt or ester of pyruvic acid; the end product of
glycolysis before entering the TCA cycle

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heterotroph. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/heterotroph. License: CC BY-SA: Attribution-
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glycolysis. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/glycolysis. License: CC BY-SA: Attribution-ShareAlike
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Located at: http://cnx.org/content/m44432/latest...ol11448/latest. License: CC
BY: Attribution
glucose. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/glucose.
License: CC BY-SA: Attribution-ShareAlike
Figure 7.7.1: Glycolysis produces 2 ATP, 2 NADH, and 2 pyruvate adenosine triphosphate. Provided by: Wikipedia. Located at:
molecules: Glycolysis, or the aerobic catabolic breakdown of en.Wikipedia.org/wiki/adenosine%20triphosphate. License: CC BY-SA:
glucose, produces energy in the form of ATP, NADH, and pyruvate, Attribution-ShareAlike
which itself enters the citric acid cycle to produce more energy. OpenStax College, Carbohydrate Metabolism. November 10, 2013. Provided by:
OpenStax CNX. Located at: http://cnx.org/content/m46451/latest/. License:
Mature mammalian red blood cells do not have mitochondria and CC BY: Attribution
are not capable of aerobic respiration, the process in which OpenStax College, Glycolysis. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44432/latest...e_07_02_01.jpg. License:
organisms convert energy in the presence of oxygen. Instead, CC BY: Attribution
glycolysis is their sole source of ATP. Therefore, if glycolysis is OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44432/latest/?collection=col11448/latest.
interrupted, the red blood cells lose their ability to maintain their License: CC BY: Attribution
sodium-potassium pumps, which require ATP to function, and NADH. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/NADH.
eventually, they die. For example, since the second half of glycolysis License: CC BY-SA: Attribution-ShareAlike
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(which produces the energy molecules) slows or stops in the absence OpenStax CNX. Located at: http://cnx.org/content/m46451/latest/. License:
of NAD+, when NAD+ is unavailable, red blood cells will be unable CC BY: Attribution
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to produce a sufficient amount of ATP in order to survive. Located at: http://cnx.org/content/m44432/latest...e_07_02_01.jpg. License:
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Additionally, the last step in glycolysis will not occur if pyruvate OpenStax College, Glycolysis. October 16, 2013. Provided by: OpenStax CNX.
kinase, the enzyme that catalyzes the formation of pyruvate, is not Located at: http://cnx.org/content/m44432/latest...e_07_02_02.jpg. License:
CC BY: Attribution
available in sufficient quantities. In this situation, the entire
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glycolysis pathway will continue to proceed, but only two ATP Located at: http://cnx.org/content/m44432/latest/?collection=col11448/latest.
molecules will be made in the second half (instead of the usual four License: CC BY: Attribution
pyruvate. Provided by: Wiktionary. Located at:
ATP molecules). Thus, pyruvate kinase is a rate-limiting enzyme for en.wiktionary.org/wiki/pyruvate. License: CC BY-SA: Attribution-ShareAlike
glycolysis. OpenStax College, Carbohydrate Metabolism. November 10, 2013. Provided by:
OpenStax CNX. Located at: http://cnx.org/content/m46451/latest/. License:
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OpenStax College, Glycolysis. October 16, 2013. Provided by: OpenStax CNX. ShareAlike
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CC BY: Attribution This page titled 7.7: Glycolysis - Outcomes of Glycolysis is shared under a
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Located at: http://cnx.org/content/m44432/latest...e_07_02_02.jpg. License: CC BY-SA 4.0 license and was authored, remixed, and/or curated by
CC BY: Attribution Boundless.
Glycolysis. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/File:Glycolysis.svg. License: CC BY-SA: Attribution-

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7.8: OXIDATION OF PYRUVATE AND THE CITRIC ACID CYCLE - BREAKDOWN
OF PYRUVATE
pyruvate dehydrogenase; the lost carbon dioxide is the first of the six
 LEARNING OBJECTIVES carbons from the original glucose molecule to be removed. This step
proceeds twice for every molecule of glucose metabolized
Explain why cells break down pyruvate
(remember: there are two pyruvate molecules produced at the end of
glycolysis); thus, two of the six carbons will have been removed at
BREAKDOWN OF PYRUVATE the end of both of these steps.
In order for pyruvate, the product of glycolysis, to enter the next Step 2. The hydroxyethyl group is oxidized to an acetyl group, and
pathway, it must undergo several changes to become acetyl the electrons are picked up by NAD+, forming NADH (the reduced
Coenzyme A (acetyl CoA). Acetyl CoA is a molecule that is further form of NAD+). The high- energy electrons from NADH will be
converted to oxaloacetate, which enters the citric acid cycle (Krebs used later by the cell to generate ATP for energy.
cycle). The conversion of pyruvate to acetyl CoA is a three-step
Step 3. The enzyme-bound acetyl group is transferred to CoA,
process.
producing a molecule of acetyl CoA. This molecule of acetyl CoA is
then further converted to be used in the next pathway of metabolism,
the citric acid cycle.

KEY POINTS
In the conversion of pyruvate to acetyl CoA, each pyruvate
molecule loses one carbon atom with the release of carbon
dioxide.
During the breakdown of pyruvate, electrons are transferred to
NAD+ to produce NADH, which will be used by the cell to
produce ATP.
In the final step of the breakdown of pyruvate, an acetyl group is
Figure 7.8.1: Breakdown of Pyruvate: Each pyruvate molecule loses
transferred to Coenzyme A to produce acetyl CoA.
a carboxylic group in the form of carbon dioxide. The remaining two
carbons are then transferred to the enzyme CoA to produce Acetyl KEY TERMS
CoA. acetyl CoA: a molecule that conveys the carbon atoms from
Step 1. A carboxyl group is removed from pyruvate, releasing a glycolysis (pyruvate) to the citric acid cycle to be oxidized for
molecule of carbon dioxide into the surrounding medium. (Note: energy production
carbon dioxide is one carbon attached to two oxygen atoms and is
one of the major end products of cellular respiration. ) The result of This page titled 7.8: Oxidation of Pyruvate and the Citric Acid Cycle -
this step is a two-carbon hydroxyethyl group bound to the enzyme Breakdown of Pyruvate is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

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7.9: OXIDATION OF PYRUVATE AND THE CITRIC ACID CYCLE - ACETYL COA
TO CO₂
In addition to the citric acid cycle, named for the first intermediate
 LEARNING OBJECTIVES formed, citric acid, or citrate, when acetate joins to the oxaloacetate,
the cycle is also known by two other names. The TCA cycle is
Describe the fate of the acetyl CoA carbons in the citric acid
named for tricarboxylic acids (TCA) because citric acid (or citrate)
cycle
and isocitrate, the first two intermediates that are formed, are
tricarboxylic acids. Additionally, the cycle is known as the Krebs
ACETYL COA TO CO2 cycle, named after Hans Krebs, who first identified the steps in the
Acetyl CoA links glycolysis and pyruvate oxidation with the citric pathway in the 1930s in pigeon flight muscle.
acid cycle. In the presence of oxygen, acetyl CoA delivers its acetyl
group to a four-carbon molecule, oxaloacetate, to form citrate, a six- KEY POINTS
carbon molecule with three carboxyl groups. During this first step of The citric acid cycle is also known as the Krebs cycle or the
the citric acid cycle, the CoA enzyme, which contains a sulfhydryl TCA (tricarboxylic acid) cycle.
group (-SH), is recycled and becomes available to attach another Acetyl CoA transfers its acetyl group to oxaloacetate to form
acetyl group. The citrate will then harvest the remainder of the citrate and begin the citric acid cycle.
extractable energy from what began as a glucose molecule and The release of carbon dioxide is coupled with the reduction of
continue through the citric acid cycle. NAD+ to NADH in the citric acid cycle.
In the citric acid cycle, the two carbons that were originally the
KEY TERMS
acetyl group of acetyl CoA are released as carbon dioxide, one of the
major products of cellular respiration, through a series of enzymatic TCA cycle: an alternative name for the Krebs cycle or citric acid
reactions. For each acetyl CoA that enters the citric acid cycle, two cycle
carbon dioxide molecules are released in reactions that are coupled Krebs cycle: a series of enzymatic reactions that occurs in all
with the production of NADH molecules from the reduction of aerobic organisms; it involves the oxidative metabolism of acetyl
NAD+ molecules. units and serves as the main source of cellular energy
oxaloacetate: a four carbon molecule that receives an acetyl
group from acetyl CoA to form citrate, which enters the citric
acid cycle

This page titled 7.9: Oxidation of Pyruvate and the Citric Acid Cycle -
Acetyl CoA to CO₂ is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

Figure 7.9.1: Acetyl CoA and the Citric Acid Cycle: For each
molecule of acetyl CoA that enters the citric acid cycle, two carbon
dioxide molecules are released, removing the carbons from the
acetyl group.

7.9.1 https://bio.libretexts.org/@go/page/13149
7.10: OXIDATION OF PYRUVATE AND THE CITRIC ACID CYCLE - CITRIC ACID
CYCLE

 LEARNING OBJECTIVES

List the steps of the Krebs (or citric acid) cycle

CITRIC ACID CYCLE (KREBS CYCLE)

Like the conversion of pyruvate to acetyl CoA, the citric acid cycle
takes place in the matrix of the mitochondria. Almost all of the
enzymes of the citric acid cycle are soluble, with the single
exception of the enzyme succinate dehydrogenase, which is
embedded in the inner membrane of the mitochondrion. Unlike
glycolysis, the citric acid cycle is a closed loop: the last part of the
pathway regenerates the compound used in the first step. The eight
steps of the cycle are a series of redox, dehydration, hydration, and
decarboxylation reactions that produce two carbon dioxide
molecules, one GTP/ATP, and reduced forms of NADH and FADH2.
This is considered an aerobic pathway because the NADH and
FADH2 produced must transfer their electrons to the next pathway
in the system, which will use oxygen. If this transfer does not occur,
the oxidation steps of the citric acid cycle also do not occur. Note
that the citric acid cycle produces very little ATP directly and does
not directly consume oxygen.

Figure 7.10.1: The citric acid cycle: In the citric acid cycle, the
acetyl group from acetyl CoA is attached to a four-carbon
oxaloacetate molecule to form a six-carbon citrate molecule.
Through a series of steps, citrate is oxidized, releasing two carbon
dioxide molecules for each acetyl group fed into the cycle. In the
process, three NAD+ molecules are reduced to NADH, one FAD
molecule is reduced to FADH2, and one ATP or GTP (depending on
the cell type) is produced (by substrate-level phosphorylation).
Because the final product of the citric acid cycle is also the first
reactant, the cycle runs continuously in the presence of sufficient
reactants.

STEPS IN THE CITRIC ACID CYCLE

Step 1. The first step is a condensation step, combining the two-
carbon acetyl group (from acetyl CoA) with a four-carbon
oxaloacetate molecule to form a six-carbon molecule of citrate. CoA
is bound to a sulfhydryl group (-SH) and diffuses away to eventually
combine with another acetyl group. This step is irreversible because
it is highly exergonic. The rate of this reaction is controlled by
negative feedback and the amount of ATP available. If ATP levels
increase, the rate of this reaction decreases. If ATP is in short supply,
the rate increases.
Step 2. Citrate loses one water molecule and gains another as citrate
is converted into its isomer, isocitrate.
Steps 3 and 4. In step three, isocitrate is oxidized, producing a five-
carbon molecule, α-ketoglutarate, together with a molecule of
CO2and two electrons, which reduce NAD+ to NADH. This step is
also regulated by negative feedback from ATP and NADH and by a
positive effect of ADP. Steps three and four are both oxidation and
decarboxylation steps, which release electrons that reduce NAD+ to
NADH and release carboxyl groups that form CO2 molecules. α-

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Ketoglutarate is the product of step three, and a succinyl group is the in further steps of cellular respiration to produce ATP for the cell.
product of step four. CoA binds the succinyl group to form succinyl
CoA. The enzyme that catalyzes step four is regulated by feedback KEY TERMS
inhibition of ATP, succinyl CoA, and NADH. citric acid cycle: a series of chemical reactions used by all
Step 5. A phosphate group is substituted for coenzyme A, and a aerobic organisms to generate energy through the oxidization of
high- energy bond is formed. This energy is used in substrate-level acetate derived from carbohydrates, fats, and proteins into carbon
phosphorylation (during the conversion of the succinyl group to dioxide
succinate) to form either guanine triphosphate (GTP) or ATP. There Krebs cycle: a series of enzymatic reactions that occurs in all
are two forms of the enzyme, called isoenzymes, for this step, aerobic organisms; it involves the oxidative metabolism of acetyl
depending upon the type of animal tissue in which they are found. units and serves as the main source of cellular energy
One form is found in tissues that use large amounts of ATP, such as mitochondria: in cell biology, a mitochondrion (plural
heart and skeletal muscle. This form produces ATP. The second form mitochondria) is a membrane-enclosed organelle, often described
of the enzyme is found in tissues that have a high number of as “cellular power plants” because they generate most of the ATP
anabolic pathways, such as liver. This form produces GTP. GTP is
CONTRIBUTIONS AND ATTRIBUTIONS
energetically equivalent to ATP; however, its use is more restricted.
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
In particular, protein synthesis primarily uses GTP. Located at: http://cnx.org/content/m44433/latest...ol11448/latest. License: CC
BY: Attribution
Step 6. Step six is a dehydration process that converts succinate into acetyl CoA. Provided by: Wikipedia. Located at:
fumarate. Two hydrogen atoms are transferred to FAD, producing en.Wikipedia.org/wiki/acetyl%20CoA. License: CC BY-SA: Attribution-
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FADH2. The energy contained in the electrons of these atoms is
09 10PyruvateToAcetylCoA-L. Provided by: Wikimedia. Located at:
insufficient to reduce NAD+ but adequate to reduce FAD. Unlike commons.wikimedia.org/wiki/Fi...cetylCoA-L.jpg. License: CC BY:
NADH, this carrier remains attached to the enzyme and transfers the Attribution
Krebs cycle. Provided by: Wiktionary. Located at:
electrons to the electron transport chain directly. This process is en.wiktionary.org/wiki/Krebs_cycle. License: CC BY-SA: Attribution-
made possible by the localization of the enzyme catalyzing this step ShareAlike
OpenStax College, Biology. October 29, 2013. Provided by: OpenStax CNX.
inside the inner membrane of the mitochondrion. Located at: http://cnx.org/content/m44433/latest...ol11448/latest. License: CC
BY: Attribution
Step 7. Water is added to fumarate during step seven, and malate is Boundless. Provided by: Boundless Learning. Located at:
produced. The last step in the citric acid cycle regenerates www.boundless.com//biology/de...tion/tca-cycle. License: CC BY-SA:
Attribution-ShareAlike
oxaloacetate by oxidizing malate. Another molecule of NADH is Boundless. Provided by: Boundless Learning. Located at:
produced. www.boundless.com//biology/de...n/oxaloacetate. License: CC BY-SA:
Attribution-ShareAlike
09 10PyruvateToAcetylCoA-L. Provided by: Wikimedia. Located at:
PRODUCTS OF THE CITRIC ACID CYCLE commons.wikimedia.org/wiki/Fi...cetylCoA-L.jpg. License: CC BY:
Two carbon atoms come into the citric acid cycle from each acetyl Attribution
OpenStax College, Oxidation of Pyruvate and the Citric Acid Cycle. November
group, representing four out of the six carbons of one glucose 10, 2013. Provided by: OpenStax CNX. Located at:
molecule. Two carbon dioxide molecules are released on each turn http://cnx.org/content/m44433/latest/. License: CC BY: Attribution
Krebs cycle. Provided by: Wiktionary. Located at:
of the cycle; however, these do not necessarily contain the most en.wiktionary.org/wiki/Krebs_cycle. License: CC BY-SA: Attribution-
recently-added carbon atoms. The two acetyl carbon atoms will ShareAlike
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eventually be released on later turns of the cycle; thus, all six carbon Located at: http://cnx.org/content/m44433/latest...ol11448/latest. License: CC
atoms from the original glucose molecule are eventually BY: Attribution
incorporated into carbon dioxide. Each turn of the cycle forms three mitochondria. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/mitochondria. License: CC BY-SA: Attribution-
NADH molecules and one FADH2 molecule. These carriers will ShareAlike
connect with the last portion of aerobic respiration to produce ATP citric acid cycle. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/citric%20acid%20cycle. License: CC BY-SA:
molecules. One GTP or ATP is also made in each cycle. Several of Attribution-ShareAlike
the intermediate compounds in the citric acid cycle can be used in 09 10PyruvateToAcetylCoA-L. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/Fi...cetylCoA-L.jpg. License: CC BY:
synthesizing non-essential amino acids; therefore, the cycle is Attribution
amphibolic (both catabolic and anabolic). OpenStax College, Oxidation of Pyruvate and the Citric Acid Cycle. November
10, 2013. Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44433/latest/. License: CC BY: Attribution
KEY POINTS OpenStax College, Oxidation of Pyruvate and the Citric Acid Cycle. October 16,
The four-carbon molecule, oxaloacetate, that began the cycle is 2013. Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44433/latest...e_07_03_02.jpg. License: CC BY:
regenerated after the eight steps of the citric acid cycle. Attribution
The eight steps of the citric acid cycle are a series of redox,
dehydration, hydration, and decarboxylation reactions.
Each turn of the cycle forms one GTP or ATP as well as three This page titled 7.10: Oxidation of Pyruvate and the Citric Acid Cycle -
NADH molecules and one FADH2 molecule, which will be used Citric Acid Cycle is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

7.10.2 https://bio.libretexts.org/@go/page/13150
7.11: OXIDATIVE PHOSPHORYLATION - ELECTRON TRANSPORT CHAIN
The electron transport chain uses the electrons from electron carriers electrons reduce molecular oxygen, producing water. This
to create a chemical gradient that can be used to power oxidative requirement for oxygen in the final stages of the chain can be seen in
phosphorylation. the overall equation for cellular respiration, which requires both
glucose and oxygen.
 LEARNING OBJECTIVES A complex is a structure consisting of a central atom, molecule, or
protein weakly connected to surrounding atoms, molecules, or
Describe how electrons move through the electron transport
proteins. The electron transport chain is an aggregation of four of
chain
these complexes (labeled I through IV), together with associated
mobile electron carriers. The electron transport chain is present in
KEY POINTS multiple copies in the inner mitochondrial membrane of eukaryotes
Oxidative phosphorylation is the metabolic pathway in which and the plasma membrane of prokaryotes.
electrons are transferred from electron donors to electron
acceptors in redox reactions; this series of reactions releases
energy which is used to form ATP.
There are four protein complexes (labeled complex I-IV) in the
electron transport chain, which are involved in moving electrons
from NADH and FADH2 to molecular oxygen.
Complex I establishes the hydrogen ion gradient by pumping
four hydrogen ions across the membrane from the matrix into the
intermembrane space.
Complex II receives FADH2, which bypasses complex I, and
delivers electrons directly to the electron transport chain.
Ubiquinone (Q) accepts the electrons from both complex I and
complex II and delivers them to complex III.
Complex III pumps protons through the membrane and passes its
electrons to cytochrome c for transport to the fourth complex of
proteins and enzymes.
Figure 7.11.1: The electron transport chain: The electron transport
Complex IV reduces oxygen; the reduced oxygen then picks up chain is a series of electron transporters embedded in the inner
two hydrogen ions from the surrounding medium to make water. mitochondrial membrane that shuttles electrons from NADH and
FADH2 to molecular oxygen. In the process, protons are pumped
KEY TERMS from the mitochondrial matrix to the intermembrane space, and
oxygen is reduced to form water.
prosthetic group: The non-protein component of a conjugated
protein. COMPLEX I
complex: A structure consisting of a central atom, molecule, or To start, two electrons are carried to the first complex aboard
protein weakly connected to surrounding atoms, molecules, or NADH. Complex I is composed of flavin mononucleotide (FMN)
proteins. and an enzyme containing iron-sulfur (Fe-S). FMN, which is derived
ubiquinone: A lipid soluble substance that is a component of the from vitamin B2 (also called riboflavin), is one of several prosthetic
electron transport chain and accepts electrons from complexes I groups or co-factors in the electron transport chain. A prosthetic
and II. group is a non-protein molecule required for the activity of a protein.
Oxidative phosphorylation is a highly efficient method of producing Prosthetic groups can be organic or inorganic and are non-peptide
large amounts of ATP, the basic unit of energy for metabolic molecules bound to a protein that facilitate its function.
processes. During this process electrons are exchanged between Prosthetic groups include co-enzymes, which are the prosthetic
molecules, which creates a chemical gradient that allows for the groups of enzymes. The enzyme in complex I is NADH
production of ATP. The most vital part of this process is the electron dehydrogenase, a very large protein containing 45 amino acid
transport chain, which produces more ATP than any other part of chains. Complex I can pump four hydrogen ions across the
cellular respiration. membrane from the matrix into the intermembrane space; it is in this
way that the hydrogen ion gradient is established and maintained
ELECTRON TRANSPORT CHAIN between the two compartments separated by the inner mitochondrial
The electron transport chain is the final component of aerobic membrane.
respiration and is the only part of glucose metabolism that uses
atmospheric oxygen. Electron transport is a series of redox reactions
that resemble a relay race. Electrons are passed rapidly from one
component to the next to the endpoint of the chain, where the

7.11.1 https://bio.libretexts.org/@go/page/13152
Q AND COMPLEX II electrons, fluctuating between different oxidation states: Fe2+
Complex II directly receives FADH2, which does not pass through (reduced) and Fe3+ (oxidized). The heme molecules in the
complex I. The compound connecting the first and second cytochromes have slightly different characteristics due to the effects
complexes to the third is ubiquinone (Q). The Q molecule is lipid of the different proteins binding them, which makes each complex.
soluble and freely moves through the hydrophobic core of the Complex III pumps protons through the membrane and passes its
membrane. Once it is reduced to QH2, ubiquinone delivers its electrons to cytochrome c for transport to the fourth complex of
electrons to the next complex in the electron transport chain. Q proteins and enzymes. Cytochrome c is the acceptor of electrons
receives the electrons derived from NADH from complex I and the from Q; however, whereas Q carries pairs of electrons, cytochrome c
electrons derived from FADH2 from complex II, including succinate can accept only one at a time.
dehydrogenase. This enzyme and FADH2 form a small complex that
COMPLEX IV
delivers electrons directly to the electron transport chain, bypassing
the first complex. Since these electrons bypass, and thus do not The fourth complex is composed of cytochrome proteins c, a, and a3.
energize, the proton pump in the first complex, fewer ATP This complex contains two heme groups (one in each of the
molecules are made from the FADH2 electrons. The number of ATP cytochromes a and a3) and three copper ions (a pair of CuA and one
molecules ultimately obtained is directly proportional to the number CuB in cytochrome a3). The cytochromes hold an oxygen molecule
of protons pumped across the inner mitochondrial membrane. very tightly between the iron and copper ions until the oxygen is
completely reduced. The reduced oxygen then picks up two
COMPLEX III hydrogen ions from the surrounding medium to produce water
The third complex is composed of cytochrome b, another Fe-S (H2O). The removal of the hydrogen ions from the system also
protein, Rieske center (2Fe-2S center), and cytochrome c proteins; contributes to the ion gradient used in the process of chemiosmosis.
this complex is also called cytochrome oxidoreductase. Cytochrome
This page titled 7.11: Oxidative Phosphorylation - Electron Transport Chain
proteins have a prosthetic heme group. The heme molecule is similar
is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
to the heme in hemoglobin, but it carries electrons, not oxygen. As a curated by Boundless.
result, the iron ion at its core is reduced and oxidized as it passes the

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7.12: OXIDATIVE PHOSPHORYLATION - CHEMIOSMOSIS AND OXIDATIVE
PHOSPHORYLATION

 LEARNING OBJECTIVES

Describe how the energy obtained from the electron

transport chain powers chemiosmosis and discuss the role of
hydrogen ions in the synthesis of ATP

During chemiosmosis, electron carriers like NADH and FADH

donate electrons to the electron transport chain. The electrons cause
conformation changes in the shapes of the proteins to pump H+
across a selectively permeable cell membrane. The uneven
distribution of H+ ions across the membrane establishes both
concentration and electrical gradients (thus, an electrochemical
gradient) owing to the hydrogen ions’ positive charge and their
aggregation on one side of the membrane.

Figure 7.12.1: ATP Synthase: ATP synthase is a complex, molecular

machine that uses a proton (H+) gradient to form ATP from ADP
and inorganic phosphate (Pi).
Chemiosmosis is used to generate 90 percent of the ATP made
during aerobic glucose catabolism. The production of ATP using the
process of chemiosmosis in mitochondria is called oxidative
phosphorylation. It is also the method used in the light reactions of
photosynthesis to harness the energy of sunlight in the process of
photophosphorylation. The overall result of these reactions is the
production of ATP from the energy of the electrons removed from
Figure 7.12.1: Chemiosmosis: In oxidative phosphorylation, the hydrogen atoms. These atoms were originally part of a glucose
hydrogen ion gradient formed by the electron transport chain is used
by ATP synthase to form ATP. molecule. At the end of the pathway, the electrons are used to reduce
an oxygen molecule to oxygen ions. The extra electrons on the
If the membrane were open to diffusion by the hydrogen ions, the
oxygen attract hydrogen ions (protons) from the surrounding
ions would tend to spontaneously diffuse back across into the
medium and water is formed.
matrix, driven by their electrochemical gradient. However, many
ions cannot diffuse through the nonpolar regions of phospholipid KEY POINTS
membranes without the aid of ion channels. Similarly, hydrogen ions
During chemiosmosis, the free energy from the series of
in the matrix space can only pass through the inner mitochondrial
reactions that make up the electron transport chain is used to
membrane through a membrane protein called ATP synthase. This
pump hydrogen ions across the membrane, establishing an
protein acts as a tiny generator turned by the force of the hydrogen
electrochemical gradient.
ions diffusing through it, down their electrochemical gradient. The
Hydrogen ions in the matrix space can only pass through the
turning of this molecular machine harnesses the potential energy
inner mitochondrial membrane through a membrane protein
stored in the hydrogen ion gradient to add a phosphate to ADP,
called ATP synthase.
forming ATP.
As protons move through ATP synthase, ADP is turned into ATP.
The production of ATP using the process of chemiosmosis in
mitochondria is called oxidative phosphorylation.

KEY TERMS
ATP synthase: An important enzyme that provides energy for
the cell to use through the synthesis of adenosine triphosphate
(ATP).

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oxidative phosphorylation: A metabolic pathway that uses
This page titled 7.12: Oxidative Phosphorylation - Chemiosmosis and
energy released by the oxidation of nutrients to produce Oxidative Phosphorylation is shared under a CC BY-SA 4.0 license and was
adenosine triphosphate (ATP). authored, remixed, and/or curated by Boundless.
chemiosmosis: The movement of ions across a selectively
permeable membrane, down their electrochemical gradient.

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7.13: OXIDATIVE PHOSPHORYLATION - ATP YIELD
ese atoms were originally part of a glucose molecule. At the end of
the pathway, the electrons are used to reduce an oxygen molecule to
oxygen ions. The extra electrons on the oxygen attract hydrogen ions
(protons) from the surrounding medium and water is formed.

ATP YIELD
The amount of energy (as ATP) gained from glucose catabolism
varies across species and depends on other related cellular processes.

LEARNING OBJECTIVES
Describe the origins of variability in the amount of ATP that is
produced per molecule of glucose consumed

KEY TAKEAWAYS

KEY POINTS
Figure 7.13.1: Cellular respiration in a eukaryotic cell: Glycolysis
While glucose catabolism always produces energy, the amount of on the left portion of this illustration can be seen to yield 2 ATP
energy (in terms of ATP equivalents) produced can vary, molecules, while the Electron Transport Chain portion at the upper
especially across different species. right will yield the remaining 30-32 ATP molecules under the
presence of oxygen.
The number of hydrogen ions the electron transport chain
complexes can pump through the membrane varies between The number of ATP molecules generated via the catabolism of
species. glucose can vary substantially. For example, the number of hydrogen
NAD+ provides more ATP than FAD+ in the electron transport ions the electron transport chain complexes can pump through the
chain and can lead to variance in ATP production. membrane varies between species. Another source of variance
The use of intermediates from glucose catabolism in other occurs during the shuttle of electrons across the membranes of the
biosynthetic pathways, such as amino acid synthesis, can lower mitochondria. The NADH generated from glycolysis cannot easily
the yield of ATP. enter mitochondria. Thus, electrons are picked up on the inside of
mitochondria by either NAD+ or FAD+. These FAD+ molecules can
KEY TERMS transport fewer ions; consequently, fewer ATP molecules are
catabolism: Destructive metabolism, usually including the generated when FAD+ acts as a carrier. NAD+ is used as the electron
release of energy and breakdown of materials. transporter in the liver, and FAD+ acts in the brain.

ATP YIELD
In a eukaryotic cell, the process of cellular respiration can
metabolize one molecule of glucose into 30 to 32 ATP. The process
of glycolysis only produces two ATP, while all the rest are produced
during the electron transport chain. Clearly, the electron transport
chain is vastly more efficient, but it can only be carried out in the
presence of oxygen.

Figure 7.13.1: Adenosine triphosphate: ATP is the main source of

energy in many living organisms.
Another factor that affects the yield of ATP molecules generated
from glucose is the fact that intermediate compounds in these
pathways are used for other purposes. Glucose catabolism connects
with the pathways that build or break down all other biochemical
compounds in cells, but the result is not always ideal. For example,
sugars other than glucose are fed into the glycolytic pathway for
energy extraction. Moreover, the five-carbon sugars that form

7.13.1 https://bio.libretexts.org/@go/page/13155
nucleic acids are made from intermediates in glycolysis. Certain of glucose catabolism extract about 34 percent of the energy
nonessential amino acids can be made from intermediates of both contained in glucose.
glycolysis and the citric acid cycle. Lipids, such as cholesterol and
triglycerides, are also made from intermediates in these pathways, This page titled 7.13: Oxidative Phosphorylation - ATP Yield is shared
and both amino acids and triglycerides are broken down for energy under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.
through these pathways. Overall, in living systems, these pathways

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7.14: METABOLISM WITHOUT OXYGEN - ANAEROBIC CELLULAR
RESPIRATION
Some prokaryotes and eukaryotes use anaerobic respiration in which bacteria and archaea, most of which are anaerobic, reduce sulfate to
they can create energy for use in the absence of oxygen. hydrogen sulfide to regenerate NAD+ from NADH.

 LEARNING OBJECTIVES

Describe the process of anaerobic cellular respiration.

KEY POINTS
Anaerobic respiration is a type of respiration where oxygen is not
used; instead, organic or inorganic molecules are used as final
electron acceptors.
Fermentation includes processes that use an organic molecule to
regenerate NAD+ from NADH.
Types of fermentation include lactic acid fermentation and
alcohol fermentation, in which ethanol is produced.
All forms of fermentation except lactic acid fermentation
Figure 7.14.1: Anaerobic bacteria: The green color seen in these
produce gas, which plays a role in the laboratory identification of coastal waters is from an eruption of hydrogen sulfide-producing
bacteria. bacteria. These anaerobic, sulfate-reducing bacteria release
Some types of prokaryotes are facultatively anaerobic, which hydrogen sulfide gas as they decompose algae in the water.
means that they can switch between aerobic respiration and Eukaryotes can also undergo anaerobic respiration. Some examples
fermentation, depending on the availability of oxygen. include alcohol fermentation in yeast and lactic acid fermentation in
mammals.
KEY TERMS
archaea: A group of single-celled microorganisms. They have LACTIC ACID FERMENTATION
no cell nucleus or any other membrane-bound organelles within The fermentation method used by animals and certain bacteria (like
their cells. those in yogurt) is called lactic acid fermentation. This type of
anaerobic respiration: A form of respiration using electron fermentation is used routinely in mammalian red blood cells and in
acceptors other than oxygen. skeletal muscle that has an insufficient oxygen supply to allow
fermentation: An anaerobic biochemical reaction. When this aerobic respiration to continue (that is, in muscles used to the point
reaction occurs in yeast, enzymes catalyze the conversion of of fatigue). The excess amount of lactate in those muscles is what
sugars to alcohol or acetic acid with the evolution of carbon causes the burning sensation in your legs while running. This pain is
dioxide. a signal to rest the overworked muscles so they can recover. In these
muscles, lactic acid accumulation must be removed by the blood
ANAEROBIC CELLULAR RESPIRATION circulation and the lactate brought to the liver for further
The production of energy requires oxygen. The electron transport metabolism. The chemical reactions of lactic acid fermentation are
chain, where the majority of ATP is formed, requires a large input of the following:
oxygen. However, many organisms have developed strategies to Pyruvic acid + NADH ↔ lactic acid + NAD+
carry out metabolism without oxygen, or can switch from aerobic to
anaerobic cell respiration when oxygen is scarce.
During cellular respiration, some living systems use an organic
molecule as the final electron acceptor. Processes that use an organic
molecule to regenerate NAD+ from NADH are collectively referred
to as fermentation. In contrast, some living systems use an inorganic
molecule as a final electron acceptor. Both methods are called
anaerobic cellular respiration, where organisms convert energy for
their use in the absence of oxygen.
Certain prokaryotes, including some species of bacteria and archaea,
use anaerobic respiration. For example, the group of archaea called
methanogens reduces carbon dioxide to methane to oxidize NADH.
These microorganisms are found in soil and in the digestive tracts of
ruminants, such as cows and sheep. Similarly, sulfate-reducing

7.14.1 https://bio.libretexts.org/@go/page/13157
 Figure 7.14.1: Alcohol Fermentation: Fermentation of grape juice
into wine produces CO2 as a byproduct. Fermentation tanks have
valves so that the pressure inside the tanks created by the carbon
dioxide produced can be released.
The first reaction is catalyzed by pyruvate decarboxylase, a
cytoplasmic enzyme, with a coenzyme of thiamine pyrophosphate
(TPP, derived from vitamin B1 and also called thiamine). A carboxyl
group is removed from pyruvic acid, releasing carbon dioxide as a
gas. The loss of carbon dioxide reduces the size of the molecule by
one carbon, making acetaldehyde. The second reaction is catalyzed
Figure 7.14.1: Lactic acid fermentation: Lactic acid fermentation is by alcohol dehydrogenase to oxidize NADH to NAD+ and reduce
common in muscle cells that have run out of oxygen. acetaldehyde to ethanol.
The enzyme used in this reaction is lactate dehydrogenase (LDH). The fermentation of pyruvic acid by yeast produces the ethanol
The reaction can proceed in either direction, but the reaction from found in alcoholic beverages. Ethanol tolerance of yeast is variable,
left to right is inhibited by acidic conditions. Such lactic acid ranging from about 5 percent to 21 percent, depending on the yeast
accumulation was once believed to cause muscle stiffness, fatigue, strain and environmental conditions.
and soreness, although more recent research disputes this
hypothesis. Once the lactic acid has been removed from the muscle OTHER TYPES OF FERMENTATION
and circulated to the liver, it can be reconverted into pyruvic acid Various methods of fermentation are used by assorted organisms to
and further catabolized for energy. ensure an adequate supply of NAD+ for the sixth step in glycolysis.
Without these pathways, that step would not occur and no ATP
ALCOHOL FERMENTATION
would be harvested from the breakdown of glucose.Other
Another familiar fermentation process is alcohol fermentation, fermentation methods also occur in bacteria. Many prokaryotes are
which produces ethanol, an alcohol. The use of alcohol fermentation facultatively anaerobic. This means that they can switch between
can be traced back in history for thousands of years. The chemical aerobic respiration and fermentation, depending on the availability
reactions of alcoholic fermentation are the following (Note: CO2 of oxygen. Certain prokaryotes, like Clostridia, are obligate
does not participate in the second reaction): anaerobes. Obligate anaerobes live and grow in the absence of
Pyruvic acid → CO2 + acetaldehyde + NADH → ethanol + NAD+ molecular oxygen. Oxygen is a poison to these microorganisms,
killing them on exposure.
It should be noted that all forms of fermentation, except lactic acid
fermentation, produce gas. The production of particular types of gas
is used as an indicator of the fermentation of specific carbohydrates,
which plays a role in the laboratory identification of the bacteria.

CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44444/latest...ol11448/latest. License: CC
BY: Attribution
OpenStax College, Biology. October 28, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44444/latest...ol11448/latest. License: CC
BY: Attribution
archaea. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/archaea.
License: CC BY-SA: Attribution-ShareAlike
anaerobic respiration. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/anaerobic%20respiration. License: CC BY-SA:

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Attribution-ShareAlike http://cnx.org/content/m44444/latest...e_07_05_02.png. License: CC BY:
fermentation. Provided by: Wiktionary. Located at: Attribution
en.wiktionary.org/wiki/fermentation. License: CC BY-SA: Attribution- OpenStax College, Metabolism Without Oxygen. October 16, 2013. Provided
ShareAlike by: OpenStax CNX. Located at:
OpenStax College, Metabolism Without Oxygen. October 16, 2013. Provided http://cnx.org/content/m44444/latest...e_07_05_03.jpg. License: CC BY:
by: OpenStax CNX. Located at: Attribution
http://cnx.org/content/m44444/latest...e_07_05_01.jpg. License: CC BY:
Attribution This page titled 7.14: Metabolism without Oxygen - Anaerobic Cellular
OpenStax College, Metabolism Without Oxygen. October 16, 2013. Provided
by: OpenStax CNX. Located at: Respiration is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.

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7.15: CONNECTIONS OF CARBOHYDRATE, PROTEIN, AND LIPID METABOLIC
PATHWAYS - CONNECTING OTHER SUGARS TO GLUCOSE METABOLISM
Sugars, such as galactose, fructose, and glycogen, are catabolized shunted into glycogen for storage. Glycogen is made and stored in
into new products in order to enter the glycolytic pathway. both the liver and muscles. The glycogen is hydrolyzed into the
glucose monomer, glucose-1-phosphate (G-1-P), if blood sugar
 LEARNING OBJECTIVES levels drop. The presence of glycogen as a source of glucose allows
ATP to be produced for a longer period of time during exercise.
Identify the types of sugars involved in glucose metabolism Glycogen is broken down into G-1-P and converted into glucose-6-
phosphate (G-6-P) in both muscle and liver cells; this product enters
KEY POINTS the glycolytic pathway.
When blood sugar levels drop, glycogen is broken down into
glucose -1-phosphate, which is then converted to glucose-6-
phosphate and enters glycolysis for ATP production.
In the liver, galactose is converted to glucose-6-phosphate in
order to enter the glycolytic pathway.
Fructose is converted into glycogen in the liver and then follows
the same pathway as glycogen to enter glycolysis.
Sucrose is broken down into glucose and fructose; glucose enters
the pathway directly while fructose is converted to glycogen.

KEY TERMS
disaccharide: A sugar, such as sucrose, maltose, or lactose,
consisting of two monosaccharides combined together.
glycogen: A polysaccharide that is the main form of
carbohydrate storage in animals; converted to glucose as needed.
monosaccharide: A simple sugar such as glucose, fructose, or
deoxyribose that has a single ring.
You have learned about the catabolism of glucose, which provides
energy to living cells. But living things consume more than glucose
for food. How does a turkey sandwich end up as ATP in your cells?
This happens because all of the catabolic pathways for Figure 7.15.1: Glycogen Structure: Schematic two-dimensional
carbohydrates, proteins, and lipids eventually connect into glycolysis cross-sectional view of glycogen: A core protein of glycogenin is
and the citric acid cycle pathways. surrounded by branches of glucose units. The entire globular granule
may contain around 30,000 glucose units.
Metabolic pathways should be thought of as porous; that is,
Galactose is the sugar in milk. Infants have an enzyme in the small
substances enter from other pathways, and intermediates leave for
intestine that metabolizes lactose to galactose and glucose. In areas
other pathways. These pathways are not closed systems. Many of the
where milk products are regularly consumed, adults have also
substrates, intermediates, and products in a particular pathway are
evolved this enzyme. Galactose is converted in the liver to G-6-P
reactants in other pathways. Like sugars and amino acids, the
and can thus enter the glycolytic pathway.
catabolic pathways of lipids are also connected to the glucose
catabolism pathways. Fructose is one of the three dietary monosaccharides (along with
glucose and galactose) which are absorbed directly into the
bloodstream during digestion. Fructose is absorbed from the small
intestine and then passes to the liver to be metabolized, primarily to
glycogen. The catabolism of both fructose and galactose produces
the same number of ATP molecules as glucose.

Figure 7.15.1: Glycogen Pathway: Glycogen from the liver and

muscles, hydrolyzed into glucose-1-phosphate, together with fats
and proteins, can feed into the catabolic pathways for carbohydrates.
Glycogen, a polymer of glucose, is an energy-storage molecule in
animals. When there is adequate ATP present, excess glucose is

7.15.1 https://bio.libretexts.org/@go/page/13159
 Sucrose is a disaccharide with a molecule of glucose and a molecule
of fructose bonded together with a glycosidic linkage. The
catabolism of sucrose breaks it down to monomers of glucose and
fructose. The glucose can directly enter the glycolytic pathway while
fructose must first be converted to glycogen, which can be broken
down to G-1-P and enter the glycolytic pathway as described above.

This page titled 7.15: Connections of Carbohydrate, Protein, and Lipid

Metabolic Pathways - Connecting Other Sugars to Glucose Metabolism is
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

Figure 7.15.1: Fructose Metabolism: Although the metabolism of

fructose and glucose share many of the same intermediate structures,
they have very different metabolic fates in human metabolism.

7.15.2 https://bio.libretexts.org/@go/page/13159
7.16: CONNECTIONS OF CARBOHYDRATE, PROTEIN, AND LIPID METABOLIC
PATHWAYS - CONNECTING PROTEINS TO GLUCOSE METABOLISM
Excess amino acids are converted into molecules that can enter the
pathways of glucose catabolism.

 LEARNING OBJECTIVES

Describe the role played by proteins in glucose metabolism

KEY POINTS
Amino acids must be deaminated before entering any of the
pathways of glucose catabolism: the amino group is converted to
ammonia, which is used by the liver in the synthesis of urea.
Deaminated amino acids can be converted into pyruvate, acetyl
CoA, or some components of the citric acid cycle to enter the
pathways of glucose catabolism.
Figure 7.16.1: Connection of Amino Acids to Glucose Metabolism
Several amino acids can enter the glucose catabolism pathways Pathways: The carbon skeletons of certain amino acids (indicated in
at multiple locations. boxes) are derived from proteins and can feed into pyruvate, acetyl
CoA, and the citric acid cycle.
KEY TERMS Each amino acid must have its amino group removed (deamination)
catabolism: Destructive metabolism, usually including the prior to the carbon chain’s entry into these pathways. When the
release of energy and breakdown of materials. amino group is removed from an amino acid, it is converted into
keto acid: Any carboxylic acid that also contains a ketone group. ammonia through the urea cycle. The remaining atoms of the amino
deamination: The removal of an amino group from a compound. acid result in a keto acid: a carbon chain with one ketone and one
carboxylic acid group. In mammals, the liver synthesizes urea from
Metabolic pathways should be thought of as porous; that is,
two ammonia molecules and a carbon dioxide molecule. Thus, urea
substances enter from other pathways and intermediates leave for
is the principal waste product in mammals produced from the
other pathways. These pathways are not closed systems. Many of the
nitrogen originating in amino acids; it leaves the body in urine. The
substrates, intermediates, and products in a particular pathway are
keto acid can then enter the citric acid cycle.
reactants in other pathways. Proteins are a good example of this
phenomenon. They can be broken down into their constituent amino When deaminated, amino acids can enter the pathways of glucose
acids and used at various steps of the pathway of glucose metabolism as pyruvate, acetyl CoA, or several components of the
catabolism. citric acid cycle. For example, deaminated asparagine and aspartate
are converted into oxaloacetate and enter glucose catabolism in the
Proteins are hydrolyzed by a variety of enzymes in cells. Most of the
citric acid cycle. Deaminated amino acids can also be converted into
time, the amino acids are recycled into the synthesis of new proteins
another intermediate molecule before entering the pathways. Several
or are used as precursors in the synthesis of other important
amino acids can enter glucose catabolism at multiple locations.
biological molecules, such as hormones, nucleotides, or
neurotransmitters. However, if there are excess amino acids, or if the This page titled 7.16: Connections of Carbohydrate, Protein, and Lipid
body is in a state of starvation, some amino acids will be shunted Metabolic Pathways - Connecting Proteins to Glucose Metabolism is shared
into the pathways of glucose catabolism. under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

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7.17: CONNECTIONS OF CARBOHYDRATE, PROTEIN, AND LIPID METABOLIC
PATHWAYS - CONNECTING LIPIDS TO GLUCOSE METABOLISM
Lipids can be both made and broken down through parts of the combines with oxaloacetate. The NADH and FADH2 are then used
glucose catabolism pathways. by the electron transport chain.

 LEARNING OBJECTIVES CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44441/latest...ol11448/latest. License: CC
Explain the connection of lipids to glucose metabolism BY: Attribution
Medical Physiology/Basic Biochemistry/Sugars. Provided by: Wikibooks.
Located at: en.wikibooks.org/wiki/Medical...ose_Metabolism. License: CC
KEY POINTS BY-SA: Attribution-ShareAlike
glycogen. Provided by: Wiktionary. Located at:
Many types of lipids exist, but cholesterol and triglycerides are en.wiktionary.org/wiki/glycogen. License: CC BY-SA: Attribution-ShareAlike
the lipids that enter the pathways of glucose catabolism. monosaccharide. Provided by: Wiktionary. Located at:
Through the process of phosphorylation, glycerol can be en.wiktionary.org/wiki/monosaccharide. License: CC BY-SA: Attribution-
ShareAlike
converted to glycerol-3-phosphate during the glycolytic pathway. disaccharide. Provided by: Wiktionary. Located at:
When fatty acids are broken down into acetyl groups through en.wiktionary.org/wiki/disaccharide. License: CC BY-SA: Attribution-
ShareAlike
beta-oxidation, the acetyl groups are used by CoA to form Medical Physiology/Basic Biochemistry/Sugars. Provided by: Wikibooks.
acetyl-CoA, which enters the citric acid cycle to produce ATP. Located at: en.wikibooks.org/wiki/Medical...ose_Metabolism. License: CC
BY-SA: Attribution-ShareAlike
Beta-oxidation produces FADH2 and NADH, which are used by Glycogen Structure. Provided by: Wikipedia. Located at:
the electron transport chain for ATP production. en.Wikipedia.org/wiki/Glycog..._structure.svg. License: Public Domain: No
Known Copyright
Glycogen pathway.jpg. Provided by: OpenStax CNX. Located at:
KEY TERMS https://cnx.org/contents/ZP457F64@7/...arbohydrate-Pr. License: CC BY-
beta-oxidation: A process that takes place in the matrix of the SA: Attribution-ShareAlike
keto acid. Provided by: Wiktionary. Located at:
mitochondria and catabolizes fatty acids by converting them to en.wiktionary.org/wiki/keto_acid. License: CC BY-SA: Attribution-ShareAlike
acetyl groups while producing NADH and FADH2. OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44441/latest...ol11448/latest. License: CC
lipid: A group of organic compounds including fats, oils, waxes, BY: Attribution
sterols, and triglycerides; characterized by being insoluble in deamination. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/deamination. License: CC BY-SA: Attribution-
water; account for most of the fat present in the human body. ShareAlike
catabolism. Provided by: Wiktionary. Located at:
Like sugars and amino acids, the catabolic pathways of lipids are en.wiktionary.org/wiki/catabolism. License: CC BY-SA: Attribution-
also connected to the glucose catabolism pathways. The lipids that ShareAlike
Medical Physiology/Basic Biochemistry/Sugars. Provided by: Wikibooks.
are connected to the glucose pathways are cholesterol and Located at:
triglycerides. en.wikibooks.org/wiki/Medical_Physiology/Basic_Biochemistry/Sugars%23
Galactose_Metabolism. License: CC BY-SA: Attribution-ShareAlike
Glycogen Structure. Provided by: Wikipedia. Located at:
CHOLESTEROL en.Wikipedia.org/wiki/Glycogen%23mediaviewer/File:Glycogen_structure.s
Cholesterol contributes to cell membrane flexibility and is a vg. License: Public Domain: No Known Copyright
Glycogen pathway.jpg. Provided by: OpenStax CNX. Located at:
precursor to steroid hormones. The synthesis of cholesterol starts https://cnx.org/contents/ZP457F64@7/Connections-of-Carbohydrate-Pr.
with acetyl groups, which are transferred from acetyl CoA, and License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Connections of Carbohydrate, Protein, and Lipid Metabolic
proceeds in only one direction; the process cannot be reversed. Thus, Pathways. October 16, 2013. Provided by: OpenStax CNX. Located at:
synthesis of cholesterol requires an intermediate of glucose http://cnx.org/content/m44441/latest...e_07_06_01.jpg. License: CC BY:
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metabolism. OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44441/latest/?collection=col11448/latest.
TRIGLYCERIDES License: CC BY: Attribution
Metabolomics/Metabolites/Lipids/Energy Storage. Provided by: Wikibooks.
Triglycerides, a form of long-term energy storage in animals, are Located at: en.wikibooks.org/wiki/Metabol...Energy_Storage. License: CC
made of glycerol and three fatty acids. Animals can make most of BY-SA: Attribution-ShareAlike
Boundless. Provided by: Boundless Learning. Located at:
the fatty acids they need. Triglycerides can be both made and broken www.boundless.com//biology/de...beta-oxidation. License: CC BY-SA:
down through parts of the glucose catabolism pathways. Glycerol Attribution-ShareAlike
lipid. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/lipid.
can be phosphorylated to glycerol-3-phosphate, which continues License: CC BY-SA: Attribution-ShareAlike
through glycolysis. Medical Physiology/Basic Biochemistry/Sugars. Provided by: Wikibooks.
Located at:
Fatty acids are catabolized in a process called beta-oxidation that en.wikibooks.org/wiki/Medical_Physiology/Basic_Biochemistry/Sugars%23
takes place in the matrix of the mitochondria and converts their fatty Galactose_Metabolism. License: CC BY-SA: Attribution-ShareAlike
Glycogen Structure. Provided by: Wikipedia. Located at:
acid chains into two carbon units of acetyl groups, while producing en.Wikipedia.org/wiki/Glycogen%23mediaviewer/File:Glycogen_structure.s
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License: CC BY-SA: Attribution-ShareAlike

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OpenStax College, Connections of Carbohydrate, Protein, and Lipid Metabolic This page titled 7.17: Connections of Carbohydrate, Protein, and Lipid
Pathways. October 16, 2013. Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44441/latest...e_07_06_01.jpg. License: CC BY:
Metabolic Pathways - Connecting Lipids to Glucose Metabolism is shared
Attribution under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

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7.18: REGULATION OF CELLULAR RESPIRATION - REGULATORY
MECHANISMS FOR CELLULAR RESPIRATION
(non-active) site on the protein. This site has an effect on the
 LEARNING OBJECTIVES enzyme’s activity, often by changing the conformation of the
protein. The molecules most commonly used in this capacity are the
Explain the mechanisms that regulate cellular respiration.
nucleotides ATP, ADP, AMP, NAD+, and NADH. These regulators,
known as allosteric effectors, may increase or decrease enzyme
REGULATORY MECHANISMS activity, depending on the prevailing conditions, altering the steric
Various mechanisms are used to control cellular respiration. As such, structure of the enzyme, usually affecting the configuration of the
some type of control exists at each stage of glucose metabolism. active site. This alteration of the protein’s (the enzyme’s) structure
Access of glucose to the cell can be regulated using the GLUT either increases or decreases its affinity for its substrate, with the
proteins that transport glucose. In addition, different forms of the effect of increasing or decreasing the rate of the reaction. The
GLUT protein control passage of glucose into the cells of specific attachment of a molecule to the allosteric site serves to send a signal
tissues. to the enzyme, providing feedback. This feedback type of control is
effective as long as the chemical affecting it is bound to the enzyme.
Once the overall concentration of the chemical decreases, it will
diffuse away from the protein, and the control is relaxed.

KEY POINTS
Varying forms of the GLUT protein control the passage of
glucose into the cells of specific tissues, thereby regulating
cellular respiration.
Reactions that are catalyzed by only one enzyme can go to
equilibrium, which can cause the reaction to stall.
If two different enzymes are necessary for a reversible reaction,
there is greater opportunity to control the rate of the reaction and,
as a result, equilibrium is reached less often.
Enzymes are often controlled by binding of a molecule to an
Figure 7.18.1: Glucose Transport: GLUT4 is a glucose transporter
that is stored in vesicles. A cascade of events that occurs upon allosteric site on the protein.
insulin binding to a receptor in the plasma membrane causes
GLUT4-containing vesicles to fuse with the plasma membrane so KEY TERMS
that glucose may be transported into the cell.
enzyme: a globular protein that catalyses a biological chemical
Some reactions are controlled by having two different enzymes: one reaction
each for the two directions of a reversible reaction. Reactions that allosteric: a compound that binds to an inactive site, affecting
are catalyzed by only one enzyme can go to equilibrium, stalling the the activity of an enzyme by changing the conformation of the
reaction. In contrast, if two different enzymes (each specific for a protein (can activate or deactivate)
given direction) are necessary for a reversible reaction, the metabolism: the complete set of chemical reactions that occur in
opportunity to control the rate of the reaction increases and
living cells
equilibrium is not reached.
A number of enzymes involved in each of the pathways (in This page titled 7.18: Regulation of Cellular Respiration - Regulatory
particular, the enzyme catalyzing the first committed reaction of the Mechanisms for Cellular Respiration is shared under a CC BY-SA 4.0
pathway) are controlled by attachment of a molecule to an allosteric license and was authored, remixed, and/or curated by Boundless.

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7.19: REGULATION OF CELLULAR RESPIRATION - CONTROL OF CATABOLIC
PATHWAYS
increased when fructose-1,6-bisphosphate levels increase. (Recall
 LEARNING OBJECTIVES that fructose-1,6-bisphosphate is an intermediate in the first half of
glycolysis. ) The regulation of pyruvate kinase involves
Explain how catabolic pathways are controlled
phosphorylation, resulting in a less-active enzyme.
Dephosphorylation by a phosphatase reactivates it. Pyruvate kinase
CONTROL OF CATABOLIC PATHWAYS is also regulated by ATP (a negative allosteric effect).
Enzymes, proteins, electron carriers, and pumps that play roles in If more energy is needed, more pyruvate will be converted into
glycolysis, the citric acid cycle, and the electron transport chain tend acetyl CoA through the action of pyruvate dehydrogenase. If either
to catalyze non-reversible reactions. In other words, if the initial acetyl groups or NADH accumulate, there is less need for the
reaction takes place, the pathway is committed to proceeding with reaction and the rate decreases. Pyruvate dehydrogenase is also
the remaining reactions. Whether a particular enzyme activity is regulated by phosphorylation: a kinase phosphorylates it to form an
released depends upon the energy needs of the cell (as reflected by inactive enzyme, and a phosphatase reactivates it. The kinase and
the levels of ATP, ADP, and AMP). the phosphatase are also regulated.

GLYCOLYSIS CITRIC ACID CYCLE

The control of glycolysis begins with the first enzyme in the The citric acid cycle is controlled through the enzymes that catalyze
pathway, hexokinase. This enzyme catalyzes the phosphorylation of the reactions that make the first two molecules of NADH. These
glucose, which helps to prepare the compound for cleavage in a later enzymes are isocitrate dehydrogenase and α-ketoglutarate
step. The presence of the negatively-charged phosphate in the dehydrogenase. When adequate ATP and NADH levels are
molecule also prevents the sugar from leaving the cell. When available, the rates of these reactions decrease. When more ATP is
hexokinase is inhibited, glucose diffuses out of the cell and does not needed, as reflected in rising ADP levels, the rate increases. α-
become a substrate for the respiration pathways in that tissue. The Ketoglutarate dehydrogenase will also be affected by the levels of
product of the hexokinase reaction is glucose-6-phosphate, which succinyl CoA, a subsequent intermediate in the cycle, causing a
accumulates when a later enzyme, phosphofructokinase, is inhibited. decrease in activity. A decrease in the rate of operation of the
pathway at this point is not necessarily negative as the increased
levels of the α-ketoglutarate not used by the citric acid cycle can be
used by the cell for amino acid (glutamate) synthesis.

Figure 7.19.1: Citric Acid Cycle: Enzymes, isocitrate

dehydrogenase and α-ketoglutarate dehydrogenase, catalyze the
Figure 7.19.1: Glycolysis: The glycolysis pathway is primarily reactions that make the first two molecules of NADH in the citric
regulated at the three key enzymatic steps (1, 2, and 7) as indicated. acid cycle. Rates of the reaction decrease when sufficient ATP and
Note that the first two steps that are regulated occur early in the NADH levels are reached.
pathway and involve hydrolysis of ATP.
Phosphofructokinase is the main enzyme controlled in glycolysis. ELECTRON TRANSPORT CHAIN
High levels of ATP, citrate, or a lower, more acidic pH decrease the Specific enzymes of the electron transport chain are unaffected by
enzyme’s activity. An increase in citrate concentration can occur feedback inhibition, but the rate of electron transport through the
because of a blockage in the citric acid cycle. Fermentation, with its pathway is affected by the levels of ADP and ATP. Greater ATP
production of organic acids like lactic acid, frequently accounts for consumption by a cell is indicated by a buildup of ADP. As ATP
the increased acidity in a cell; however, the products of fermentation usage decreases, the concentration of ADP decreases: ATP begins to
do not typically accumulate in cells. build up in the cell. This change in the relative concentration of ADP
to ATP triggers the cell to slow down the electron transport chain.
The last step in glycolysis is catalyzed by pyruvate kinase. The
pyruvate produced can proceed to be catabolized or converted into
the amino acid alanine. If no more energy is needed and alanine is in
adequate supply, the enzyme is inhibited. The enzyme’s activity is

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 specific target molecules (substrates); the process is termed
phosphorylation

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License: CC BY: Attribution
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KEY TERMS Attribution-ShareAlike
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 CHAPTER OVERVIEW

8: PHOTOSYNTHESIS

Topic hierarchy
8.1: Overview of Photosynthesis - The Purpose and Process of Photosynthesis
8.2: Overview of Photosynthesis - Main Structures and Summary of Photosynthesis
8.3: Overview of Photosynthesis - The Two Parts of Photosynthesis
8.4: The Light-Dependent Reactions of Photosynthesis - Introduction to Light Energy
8.5: The Light-Dependent Reactions of Photosynthesis - Absorption of Light
8.6: The Light-Dependent Reactions of Photosynthesis - Processes of the Light-Dependent Reactions
8.7: The Light-Independent Reactions of Photosynthesis - AM and C4 Photosynthesis
8.8: The Light-Independent Reactions of Photosynthesis - The Calvin Cycle
8.9: The Light-Independent Reactions of Photosynthesis - The Carbon Cycle

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1
8.1: OVERVIEW OF PHOTOSYNTHESIS - THE PURPOSE AND PROCESS OF
PHOTOSYNTHESIS

 LEARNING OBJECTIVES

Describe the process of photosynthesis

THE IMPORTANCE OF PHOTOSYNTHESIS

The processes of all organisms—from bacteria to humans—require
energy. To get this energy, many organisms access stored energy by
eating food. Carnivores eat other animals and herbivores eat plants.
But where does the stored energy in food originate? All of this
energy can be traced back to the process of photosynthesis and light
energy from the sun.
Figure 8.1.1: Photosynthetic and Chemosynthetic Organisms:
Photosynthesis is essential to all life on earth. It is the only Photoautotrophs, including (a) plants, (b) algae, and (c)
biological process that captures energy from outer space (sunlight) cyanobacteria, synthesize their organic compounds via
photosynthesis using sunlight as an energy source. Cyanobacteria
and converts it into chemical energy in the form of G3P ( and planktonic algae can grow over enormous areas in water, at
Glyceraldehyde 3-phosphate) which in turn can be made into sugars times completely covering the surface. In a (d) deep sea vent,
and other molecular compounds. Plants use these compounds in all chemoautotrophs, such as these (e) thermophilic bacteria, capture
energy from inorganic compounds to produce organic compounds.
of their metabolic processes; plants do not need to consume other The ecosystem surrounding the vents has a diverse array of animals,
organisms for food because they build all the molecules they need. such as tubeworms, crustaceans, and octopuses that derive energy
Unlike plants, animals need to consume other organisms to consume from the bacteria.
the molecules they need for their metabolic processes. The importance of photosynthesis is not just that it can capture
sunlight’s energy. A lizard sunning itself on a cold day can use the
THE PROCESS OF PHOTOSYNTHESIS sun’s energy to warm up. Photosynthesis is vital because it evolved
During photosynthesis, molecules in leaves capture sunlight and as a way to store the energy in solar radiation (the “photo-” part) as
energize electrons, which are then stored in the covalent bonds of high-energy electrons in the carbon-carbon bonds of carbohydrate
carbohydrate molecules. That energy within those covalent bonds molecules (the “-synthesis” part). Those carbohydrates are the
will be released when they are broken during cell respiration. How energy source that heterotrophs use to power the synthesis of ATP
long lasting and stable are those covalent bonds? The energy via respiration. Therefore, photosynthesis powers 99 percent of
extracted today by the burning of coal and petroleum products Earth’s ecosystems. When a top predator, such as a wolf, preys on a
represents sunlight energy captured and stored by photosynthesis deer, the wolf is at the end of an energy path that went from nuclear
almost 200 million years ago. reactions on the surface of the sun, to light, to photosynthesis, to
Plants, algae, and a group of bacteria called cyanobacteria are the vegetation, to deer, and finally to wolf.
only organisms capable of performing photosynthesis. Because they
KEY POINTS
use light to manufacture their own food, they are called
Photosynthesis evolved as a way to store the energy in solar
photoautotrophs (“self-feeders using light”). Other organisms, such
as animals, fungi, and most other bacteria, are termed heterotrophs radiation as high-energy electrons in carbohydrate molecules.
Plants, algae, and cyanobacteria, known as photoautotrophs, are
(“other feeders”) because they must rely on the sugars produced by
the only organisms capable of performing photosynthesis.
photosynthetic organisms for their energy needs. A third very
Heterotrophs, unable to produce their own food, rely on the
interesting group of bacteria synthesize sugars, not by using
sunlight’s energy, but by extracting energy from inorganic chemical carbohydrates produced by photosynthetic organisms for their
energy needs.
compounds; hence, they are referred to as chemoautotrophs.
KEY TERMS
photosynthesis: the process by which plants and other
photoautotrophs generate carbohydrates and oxygen from carbon
dioxide, water, and light energy in chloroplasts
photoautotroph: an organism that can synthesize its own food
by using light as a source of energy
chemoautotroph: a simple organism, such as a protozoan, that
derives its energy from chemical processes rather than
photosynthesis

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8.2: OVERVIEW OF PHOTOSYNTHESIS - MAIN STRUCTURES AND SUMMARY
OF PHOTOSYNTHESIS

 LEARNING OBJECTIVES

Describe the main structures involved in photosynthesis and

recall the chemical equation that summarizes the process of
photosynthesis
Figure 8.2.1: Chemical equation for photosynthesis: The basic
equation for photosynthesis is deceptively simple. In reality, the
OVERVIEW OF PHOTOSYNTHESIS process includes many steps involving intermediate reactants and
Photosynthesis is a multi-step process that requires sunlight, carbon products. Glucose, the primary energy source in cells, is made from
two three-carbon GA3P molecules.
dioxide, and water as substrates. It produces oxygen and
glyceraldehyde-3-phosphate (G3P or GA3P), simple carbohydrate PHOTOSYNTHESIS AND THE LEAF
molecules that are high in energy and can subsequently be converted In plants, photosynthesis generally takes place in leaves, which
into glucose, sucrose, or other sugar molecules. These sugar consist of several layers of cells. The process of photosynthesis
molecules contain covalent bonds that store energy. Organisms occurs in a middle layer called the mesophyll. The gas exchange of
break down these molecules to release energy for use in cellular carbon dioxide and oxygen occurs through small, regulated openings
work. called stomata (singular: stoma ), which also play a role in the
plant’s regulation of water balance. The stomata are typically located
on the underside of the leaf, which minimizes water loss. Each
stoma is flanked by guard cells that regulate the opening and closing
of the stomata by swelling or shrinking in response to osmotic
changes.

Figure 8.2.1: Structure of a leaf (cross-section): Photosynthesis

takes place in the mesophyll. The palisade layer contains most of the
chloroplast and principal region in which photosynthesis is carried
out. The airy spongy layer is the region of storage and gas exchange.
The stomata regulate carbon dioxide and water balance.
Figure 8.2.1: Photosynthesis: Photosynthesis uses solar energy,
carbon dioxide, and water to produce energy-storing carbohydrates. PHOTOSYNTHESIS WITHIN THE CHLOROPLAST
Oxygen is generated as a waste product of photosynthesis.
The energy from sunlight drives the reaction of carbon dioxide and
In all autotrophic eukaryotes, photosynthesis takes place inside an
water molecules to produce sugar and oxygen, as seen in the
organelle called a chloroplast. For plants, chloroplast-containing
chemical equation for photosynthesis. Though the equation looks
simple, it is carried out through many complex steps. Before cells exist in the mesophyll. Chloroplasts have a double membrane
envelope composed of an outer membrane and an inner membrane.
learning the details of how photoautotrophs convert light energy into
chemical energy, it is important to become familiar with the Within the double membrane are stacked, disc-shaped structures
called thylakoids.
structures involved.
Embedded in the thylakoid membrane is chlorophyll, a pigment that
absorbs certain portions of the visible spectrum and captures energy
from sunlight. Chlorophyll gives plants their green color and is
responsible for the initial interaction between light and plant

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material, as well as numerous proteins that make up the electron KEY POINTS
transport chain. The thylakoid membrane encloses an internal space The chemical equation for photosynthesis is
called the thylakoid lumen. A stack of thylakoids is called a granum, 6CO2+6H2O→C6H12O6+6O2.6CO2+6H2O→C6H12O6+6O2.
and the liquid-filled space surrounding the granum is the stroma or In plants, the process of photosynthesis takes place in the
“bed.” mesophyll of the leaves, inside the chloroplasts.
Chloroplasts contain disc-shaped structures called thylakoids,
which contain the pigment chlorophyll.
Chlorophyll absorbs certain portions of the visible spectrum and
captures energy from sunlight.

KEY TERMS
chloroplast: An organelle found in the cells of green plants and
photosynthetic algae where photosynthesis takes place.
mesophyll: A layer of cells that comprises most of the interior of
the leaf between the upper and lower layers of epidermis.
stoma: A pore in the leaf and stem epidermis that is used for
gaseous exchange.

Figure 8.2.1: Structure of the Chloroplast: Photosynthesis takes This page titled 8.2: Overview of Photosynthesis - Main Structures and
place in chloroplasts, which have an outer membrane and an inner Summary of Photosynthesis is shared under a CC BY-SA 4.0 license and
membrane. Stacks of thylakoids called grana form a third membrane was authored, remixed, and/or curated by Boundless.
layer.

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8.3: OVERVIEW OF PHOTOSYNTHESIS - THE TWO PARTS OF
PHOTOSYNTHESIS

 LEARNING OBJECTIVES

Distinguish between the two parts of photosynthesis

LIGHT-DEPENDENT REACTIONS
Just as the name implies, light-dependent reactions require sunlight.
In the light-dependent reactions, energy from sunlight is absorbed by
chlorophyll and converted into stored chemical energy, in the form
of the electron carrier molecule NADPH (nicotinamide adenine
dinucleotide phosphate) and the energy currency molecule ATP
(adenosine triphosphate). The light-dependent reactions take place in
the thylakoid membranes in the granum (stack of thylakoids), within
the chloroplast.

Figure 8.3.1: Photosystems I & II: As explained above, the

photosystems manipulate electrons with energy harvested from light.
The process that converts light energy into chemical energy takes
place in a multi-protein complex called a photosystem. Two types of
photosystems are embedded in the thylakoid membrane:
Figure 8.3.1: The two stages of photosynthesis: Photosynthesis photosystem II ( PSII) and photosystem I (PSI). Each photosystem
takes place in two stages: light-dependent reactions and the Calvin plays a key role in capturing the energy from sunlight by exciting
cycle (light-independent reactions). Light-dependent reactions, electrons. These energized electrons are transported by “energy
which take place in the thylakoid membrane, use light energy to
make ATP and NADPH. The Calvin cycle, which takes place in the carrier” molecules, which power the light-independent reactions.
stroma, uses energy derived from these compounds to make GA3P Photosystems consist of a light-harvesting complex and a reaction
from CO2.
center. Pigments in the light-harvesting complex pass light energy to
PHOTOSYSTEMS two special chlorophyll a molecules in the reaction center. The light
excites an electron from the chlorophyll a pair, which passes to the
primary electron acceptor. The excited electron must then be
replaced. In photosystem II, the electron comes from the splitting of
water, which releases oxygen as a waste product. In photosystem I,
the electron comes from the chloroplast electron transport chain.
The two photosystems oxidize different sources of the low-energy
electron supply, deliver their energized electrons to different places,
and respond to different wavelengths of light.

LIGHT-INDEPENDENT REACTIONS
In the light-independent reactions or Calvin cycle, the energized
electrons from the light-dependent reactions provide the energy to
form carbohydrates from carbon dioxide molecules. The light-
independent reactions are sometimes called the Calvin cycle because
of the cyclical nature of the process.

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Light energy is harnessed in Photosystems I and II, both of OpenStax College, Overview of Photosynthesis. October 16, 2013. Provided by:
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8.4: THE LIGHT-DEPENDENT REACTIONS OF PHOTOSYNTHESIS -
INTRODUCTION TO LIGHT ENERGY

 LEARNING OBJECTIVES

Explain the difference between short and long wavelengths.

WHAT IS LIGHT ENERGY?

The sun emits an enormous amount of electromagnetic radiation
(solar or light energy). Humans can see only a fraction of this
energy, which is referred to as “visible light.” The manner in which
solar energy travels is described as waves. Scientists can determine
the amount of energy of a wave by measuring its wavelength, the Figure 8.4.1: The Electromagnetic Spectrum: The sun emits energy
distance between consecutive points of a wave, such as from crest to in the form of electromagnetic radiation. This radiation exists at
crest or from trough to trough. different wavelengths, each of which has its own characteristic
energy. All electromagnetic radiation, including visible light, is
characterized by its wavelength.
Each type of electromagnetic radiation travels at a particular
wavelength. The longer the wavelength, the less energy is carried.
Short, tight waves carry the most energy. This may seem illogical,
but think of it in terms of a person moving a heavy rope. It takes
little effort by a person to move a rope in long, wide waves. To make
a rope move in short, tight waves, a person would need to apply
significantly more energy.

KEY POINTS
The amount of energy of a wave can be determined by measuring
Figure 8.4.1: Wavelengths: The wavelength of a single wave is the its wavelength, the distance between consecutive points of a
distance between two consecutive points of similar position (two wave.
crests or two troughs) along the wave.
Visible light is a type of radiant energy within the
Visible light constitutes only one of many types of electromagnetic electromagnetic spectrum; other types of electromagnetic
radiation emitted from the sun and other stars. The electromagnetic radiation include UV, infrared, gamma, and radio rays as well as
spectrum is the range of all possible frequencies of radiation. The X-rays.
electromagnetic spectrum shows several types of electromagnetic The difference between wavelengths relates to the amount of
radiation originating from the sun, including X-rays and ultraviolet energy carried by them; short, tight waves carry more energy
(UV) rays. The higher-energy waves can penetrate tissues and than long, wide waves.
damage cells and DNA, which explains why both X-rays and UV
rays can be harmful to living organisms. Scientists differentiate the KEY TERMS
various types of radiant energy from the sun within the electromagnetic spectrum: the entire range of wavelengths of
electromagnetic spectrum.The difference between wavelengths all known radiations consisting of oscillating electric and
relates to the amount of energy carried by them. magnetic fields, including gamma rays, visible light, infrared,
radio waves, and X-rays
wavelength: the length of a single cycle of a wave, as measured
by the distance between one peak or trough of a wave and the
next; it corresponds to the velocity of the wave divided by its
frequency
visible light: the part of the electromagnetic spectrum, between
infrared and ultraviolet, that is visible to the human eye

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8.5: THE LIGHT-DEPENDENT REACTIONS OF PHOTOSYNTHESIS -
ABSORPTION OF LIGHT
photosynthetic pigments that are very efficient molecules for the
 LEARNING OBJECTIVES disposal of excess energy. When a leaf is exposed to full sun, the
light-dependent reactions are required to process an enormous
Differentiate between chlorophyll and carotenoids.
amount of energy; if that energy is not handled properly, it can do
significant damage. Therefore, many carotenoids are stored in the
ABSORPTION OF LIGHT thylakoid membrane to absorb excess energy and safely release that
Light energy initiates the process of photosynthesis when pigments energy as heat.
absorb the light. Organic pigments have a narrow range of energy Each type of pigment can be identified by the specific pattern of
levels that they can absorb. Energy levels lower than those wavelengths it absorbs from visible light, which is the absorption
represented by red light are insufficient to raise an orbital electron to spectrum. Chlorophyll a absorbs light in the blue-violet region,
an excited, or quantum, state. Energy levels higher than those in blue while chlorophyll b absorbs red-blue light. Neither a or b absorb
light will physically tear the molecules apart, a process called green light; because green is reflected or transmitted, chlorophyll
bleaching. For example, retinal pigments can only “see” (absorb) appears green. Carotenoids absorb light in the blue-green and violet
700 nm to 400 nm light; this is visible light. For the same reasons, region and reflect the longer yellow, red, and orange wavelengths.
plant pigment molecules absorb only light in the wavelength range
of 700 nm to 400 nm; plant physiologists refer to this range for
plants as photosynthetically-active radiation.
The visible light seen by humans as the color white light actually
exists in a rainbow of colors in the electromagnetic spectrum, with
violet and blue having shorter wavelengths and, thus, higher energy.
At the other end of the spectrum, toward red, the wavelengths are
longer and have lower energy.

Figure 8.5.1: Chlorophyll a and b: (a) Chlorophyll a, (b) chlorophyll

b, and (c) β-carotene are hydrophobic organic pigments found in the
thylakoid membrane. Chlorophyll a and b, which are identical
except for the part indicated in the red box, are responsible for the
green color of leaves. β-carotene is responsible for the orange color
in carrots. Each pigment has (d) a unique absorbance spectrum.
Many photosynthetic organisms have a mixture of pigments. In this
Figure 8.5.1: Visible Light: The colors of visible light do not carry
way organisms can absorb energy from a wider range of
the same amount of energy. Violet has the shortest wavelength and,
therefore, carries the most energy, whereas red has the longest wavelengths. Not all photosynthetic organisms have full access to
wavelength and carries the least amount of energy. sunlight. Some organisms grow underwater where light intensity and
quality decrease and change with depth. Other organisms grow in
UNDERSTANDING PIGMENTS
competition for light. Plants on the rainforest floor must be able to
Different kinds of pigments exist, each of which has evolved to absorb any light that comes through because the taller trees absorb
absorb only certain wavelengths or colors of visible light. Pigments most of the sunlight and scatter the remaining solar radiation
reflect or transmit the wavelengths they cannot absorb, making them
appear in the corresponding color.
Chlorophylls and carotenoids are the two major classes of
photosynthetic pigments found in plants and algae; each class has
multiple types of pigment molecules. There are five major
chlorophylls: a, b, c and d, along with a related molecule found in
prokaryotes called bacteriochlorophyll.
With dozens of different forms, carotenoids are a much larger group
of pigments. The carotenoids found in fruit, such as the red of
tomato (lycopene), the yellow of corn seeds (zeaxanthin), or the
orange of an orange peel (β-carotene), are used to attract seed-
dispersing organisms. In photosynthesis, carotenoids function as

8.5.1 https://bio.libretexts.org/@go/page/13207
 Violet and blue have the shortest wavelengths and the most
energy, whereas red has the longest wavelengths and carries the
least amount of energy.
Pigments reflect or transmit the wavelengths they cannot absorb,
making them appear in the corresponding color.
Chorophylls and carotenoids are the major pigments in plants;
while there are dozens of carotenoids, there are only five
important chorophylls: a, b, c, d, and bacteriochlorophyll.
Chlorophyll a absorbs light in the blue-violet region, chlorophyll
b absorbs red-blue light, and both a and b reflect green light
(which is why chlorophyll appears green).
Figure 8.5.1: Pigments in Plants: Plants that commonly grow in the Carotenoids absorb light in the blue-green and violet region and
shade have adapted to low levels of light by changing the relative
reflect the longer yellow, red, and orange wavelengths; these
concentrations of their chlorophyll pigments.
pigments also dispose excess energy out of the cell.
When studying a photosynthetic organism, scientists can determine
the types of pigments present by using a spectrophotometer. These KEY TERMS
instruments can differentiate which wavelengths of light a substance
chlorophyll: Any of a group of green pigments that are found in
can absorb. Spectrophotometers measure transmitted light and
the chloroplasts of plants and in other photosynthetic organisms
compute its absorption. By extracting pigments from leaves and
such as cyanobacteria.
placing these samples into a spectrophotometer, scientists can
carotenoid: Any of a class of yellow to red plant pigments
identify which wavelengths of light an organism can absorb.
including the carotenes and xanthophylls.
KEY POINTS spectrophotometer: An instrument used to measure the intensity
of electromagnetic radiation at different wavelengths.
Plant pigment molecules absorb only light in the wavelength
range of 700 nm to 400 nm; this range is referred to as This page titled 8.5: The Light-Dependent Reactions of Photosynthesis -
photosynthetically-active radiation. Absorption of Light is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

8.5.2 https://bio.libretexts.org/@go/page/13207
8.6: THE LIGHT-DEPENDENT REACTIONS OF PHOTOSYNTHESIS -
PROCESSES OF THE LIGHT-DEPENDENT REACTIONS

 LEARNING OBJECTIVES

Describe how light energy is converted into ATP and

NADPH.

HOW LIGHT-DEPENDENT REACTIONS WORK

The overall function of light-dependent reactions, the first stage of
photosynthesis, is to convert solar energy into chemical energy in
the form of NADPH and ATP, which are used in light-independent
reactions and fuel the assembly of sugar molecules. Protein
complexes and pigment molecules work together to produce
NADPH and ATP.

PRODUCING CHEMICAL ENERGY

Light energy is converted into chemical energy in a multiprotein
complex called a photosystem. Two types of photosystems,
photosystem I (PSI) and photosystem II (PSII), are found in the
thylakoid membrane inside the chloroplast. Each photosystem
consists of multiple antenna proteins that contain a mixture of 300–
400 chlorophyll a and b molecules, as well as other pigments like
carotenoids. Cytochrome b6f complex and ATP synthase are also
major protein complexes in the thylakoid membrane that work with
the photosystems to create ATP and NADPH.

Figure 8.6.1: Photosystems I & II: A photosystem consists of a

light-harvesting complex and a reaction center. Pigments in the
light-harvesting complex pass light energy to two special
chlorophyll a molecules in the reaction center. The light excites an
electron from the chlorophyll a pair, which passes to the primary
electron acceptor. The excited electron must then be replaced. In (a)
photosystem II, the electron comes from the splitting of water, which
releases oxygen as a waste product. In (b) photosystem I, the
electron comes from the chloroplast electron transport chain.
The two photosystems absorb light energy through proteins
containing pigments, such as chlorophyll. The light-dependent
reactions begin in photosystem II. In PSII, energy from sunlight is
used to split water, which releases two electrons, two hydrogen
atoms, and one oxygen atom. When a chlorophyll a molecule within
the reaction center of PSII absorbs a photon, the electron in this
molecule attains a higher energy level. Because this state of an
electron is very unstable, the electron is transferred to another
molecule creating a chain of redox reactions called an electron
transport chain (ETC). The electron flow goes from PSII to
cytochrome b6f to PSI; as electrons move between these two
photosystems, they lose energy. Because the electrons have lost
energy prior to their arrival at PSI, they must be re-energized by PSI.
Therefore, another photon is absorbed by the PSI antenna. That

8.6.1 https://bio.libretexts.org/@go/page/13208
energy is transmitted to the PSI reaction center. This reaction center, lumen to the stroma; this energy allows ATP synthase to attach a
known as P700, is oxidized and sends a high-energy electron to third phosphate group to ADP, which forms ATP.
reduce NADP+ to NADPH. This process illustrates oxygenic In cyclic photophosphorylation, cytochrome b6f uses the energy
photosynthesis, wherein the first electron donor is water and oxygen of electrons from both PSII and PSI to create more ATP and to
is created as a waste product. stop the production of NADPH, maintaining the right
proportions of NADPH and ATP.

KEY TERMS
photosystem: Either of two biochemical systems, active in
chloroplasts, that are part of photosynthesis.
photophosphorylation: The addition of a phosphate (PO43-)
group to a protein or other organic molecule by photosynthesis.
chemiosmosis: The movement of ions across a selectively
permeable membrane, down their electrochemical gradient.

CONTRIBUTIONS AND ATTRIBUTIONS

visible light. Provided by: Wiktionary. Located at:
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Located at: http://cnx.org/content/m44448/latest...ol11448/latest. License: CC
BY: Attribution
Boundless. Provided by: Boundless Learning. Located at:
www.boundless.com//biology/de...netic-spectrum. License: CC BY-SA:
Figure 8.6.1: Photosystem II: In the photosystem II (PSII) reaction Attribution-ShareAlike
center, energy from sunlight is used to extract electrons from water. wavelength. Provided by: Wiktionary. Located at:
The electrons travel through the chloroplast electron transport chain en.wiktionary.org/wiki/wavelength. License: CC BY-SA: Attribution-
to photosystem I (PSI), which reduces NADP+ to NADPH. The ShareAlike
electron transport chain moves protons across the thylakoid OpenStax College, The Light-Dependent Reactions of Photosynthesis. October
membrane into the lumen. At the same time, splitting of water adds 16, 2013. Provided by: OpenStax CNX. Located at:
protons to the lumen while reduction of NADPH removes protons http://cnx.org/content/m44448/latest...e_08_02_03.jpg. License: CC BY:
from the stroma. The net result is a low pH in the thylakoid lumen Attribution
and a high pH in the stroma. ATP synthase uses this electrochemical OpenStax College, The Light-Dependent Reactions of Photosynthesis. October
gradient to make ATP. 16, 2013. Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44448/latest...e_08_02_02.jpg. License: CC BY:
Cytochrome b6f and ATP synthase work together to create ATP. This Attribution
process, called photophosphorylation, occurs in two different ways. OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44448/latest...ol11448/latest. License: CC
In non-cyclic photophosphorylation, cytochrome b6f uses the energy BY: Attribution
of electrons from PSII to pump hydrogen ions from the lumen (an spectrophotometer. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/spectrophotometer. License: CC BY-SA: Attribution-
area of high concentration) to the stroma (an area of low ShareAlike
concentration). The energy released by the hydrogen ion stream carotenoid. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/carotenoid. License: CC BY-SA: Attribution-
allows ATP synthase to attach a third phosphate group to ADP, ShareAlike
which forms ATP. This flow of hydrogen ions through ATP synthase chlorophyll. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/chlorophyll. License: CC BY-SA: Attribution-
is called chemiosmosis because the ions move from an area of high ShareAlike
to an area of low concentration through a semi-permeable structure. OpenStax College, The Light-Dependent Reactions of Photosynthesis. October
16, 2013. Provided by: OpenStax CNX. Located at:
In cyclic photophosphorylation, cytochrome b6f uses the energy of
http://cnx.org/content/m44448/latest...e_08_02_03.jpg. License: CC BY:
electrons from both PSII and PSI to create more ATP and to stop the Attribution
production of NADPH. Cyclic phosphorylation is important to OpenStax College, The Light-Dependent Reactions of Photosynthesis. October
16, 2013. Provided by: OpenStax CNX. Located at:
maintain the right proportions of NADPH and ATP, which will carry http://cnx.org/content/m44448/latest...e_08_02_02.jpg. License: CC BY:
out light-independent reactions later on. Attribution
OpenStax College, The Light-Dependent Reactions of Photosynthesis. October
The net-reaction of all light-dependent reactions in oxygenic 16, 2013. Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44448/latest...e_08_02_04.jpg. License: CC BY:
photosynthesis is: 2H2O + 2NADP+ + 3ADP + 3Pi → O2 + Attribution
2NADPH + 3ATP OpenStax College, The Light-Dependent Reactions of Photosynthesis. October
16, 2013. Provided by: OpenStax CNX. Located at:
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KEY POINTS Attribution
Light energy splits water and extracts electrons in photosystem II OpenStax College, The Light-Dependent Reactions of Photosynthesis. October
16, 2013. Provided by: OpenStax CNX. Located at:
(PSII); then electrons are moved from PSII to cytochrome b6f to http://cnx.org/content/m44448/latest...e_08_02_06.jpg. License: CC BY:
photosystem I (PSI) and reduce in energy. Attribution
photosystem. Provided by: Wiktionary. Located at:
Electrons are re-energized in PSI and those high energy electrons en.wiktionary.org/wiki/photosystem. License: CC BY-SA: Attribution-
reduce NADP+ to NADPH. ShareAlike
Cell Biology/Energy supply/Light Dependent Reactions. Provided by:
In non-cyclic photophosphorylation, cytochrome b6f uses the Wikibooks. Located at: en.wikibooks.org/wiki/Cell_Bi...dent_Reactions.
energy of electrons from PSII to pump hydrogen ions from the License: CC BY-SA: Attribution-ShareAlike

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OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. http://cnx.org/content/m44448/latest..._02_05abcd.jpg. License: CC BY:
Located at: http://cnx.org/content/m44448/latest...ol11448/latest. License: CC Attribution
BY: Attribution OpenStax College, The Light-Dependent Reactions of Photosynthesis. October
photophosphorylation. Provided by: Wikipedia. Located at: 16, 2013. Provided by: OpenStax CNX. Located at:
en.Wikipedia.org/wiki/photophosphorylation. License: CC BY-SA: http://cnx.org/content/m44448/latest...e_08_02_06.jpg. License: CC BY:
Attribution-ShareAlike Attribution
chemiosmosis. Provided by: Wikipedia. Located at: OpenStax College, The Light-Dependent Reactions of Photosynthesis. October
en.Wikipedia.org/wiki/chemiosmosis. License: CC BY-SA: Attribution- 16, 2013. Provided by: OpenStax CNX. Located at:
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16, 2013. Provided by: OpenStax CNX. Located at:
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Attribution
OpenStax College, The Light-Dependent Reactions of Photosynthesis. October This page titled 8.6: The Light-Dependent Reactions of Photosynthesis -
16, 2013. Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44448/latest...e_08_02_04.jpg. License: CC BY:
Processes of the Light-Dependent Reactions is shared under a CC BY-SA
Attribution 4.0 license and was authored, remixed, and/or curated by Boundless.
OpenStax College, The Light-Dependent Reactions of Photosynthesis. October
16, 2013. Provided by: OpenStax CNX. Located at:

8.6.3 https://bio.libretexts.org/@go/page/13208
8.7: THE LIGHT-INDEPENDENT REACTIONS OF PHOTOSYNTHESIS - AM AND
C4 PHOTOSYNTHESIS
phosphoglycerate by RuBisCO. Sixteen thousand species of plants
 LEARNING OBJECTIVES use CAM.

Compare C4 and CAM photosynthesis

Photosynthesis in desert plants has evolved adaptations that

conserve water. In harsh, dry heat, every drop of water must be used
to survive. Because stomata must open to allow for the uptake of
CO2, water escapes from the leaf during active photosynthesis.
Desert plants have evolved processes to conserve water and deal
with harsh conditions. A more efficient use of CO2 allows plants to
adapt to living with less water.
Some plants such as cacti can prepare materials for photosynthesis
during the night by a temporary carbon fixation and storage process,
because opening the stomata at this time conserves water due to
cooler temperatures. In addition, cacti have evolved the ability to Figure 8.7.1: Cross section of agave, a CAM plant: Cross section of
a CAM (crassulacean acid metabolism) plant, specifically of an
carry out low levels of photosynthesis without opening stomata at agave leaf. Vascular bundles shown. Drawing based on microscopic
all, a mechanism for surviving extremely dry periods. images courtesy of Cambridge University Plant Sciences
Department.

C4
CARBON FIXATION
The C4 pathway bears resemblance to CAM; both act to concentrate
CO2 around RuBisCO, thereby increasing its efficiency. CAM
concentrates it temporally, providing CO2 during the day and not at
night, when respiration is the dominant reaction.
C4 plants, in contrast, concentrate CO2 spatially, with a RuBisCO
reaction centre in a “bundle sheath cell” that is inundated with CO2.
Due to the inactivity required by the CAM mechanism, C4 carbon
fixation has a greater efficiency in terms of PGA synthesis.

Figure 8.7.1: Cactus: The harsh conditions of the desert have led
plants like these cacti to evolve variations of the light-independent
reactions of photosynthesis. These variations increase the efficiency
of water usage, helping to conserve water and energy.

CAM PHOTOSYNTHESIS
Xerophytes, such as cacti and most succulents, also use
phosphoenolpyruvate (PEP) carboxylase to capture carbon dioxide
in a process called crassulacean acid metabolism (CAM). In contrast
to C4 metabolism, which physically separates the CO2 fixation to
PEP from the Calvin cycle, CAM temporally separates these two
processes.
CAM plants have a different leaf anatomy from C3 plants, and fix CROSS SECTION OF MAIZE, A C4 PLANT
the CO2 at night, when their stomata are open. CAM plants store the Cross section of a C4 plant, specifically of a maize leaf. Drawing
CO2 mostly in the form of malic acid via carboxylation of based on microscopic images courtesy of Cambridge University
phosphoenolpyruvate to oxaloacetate, which is then reduced to Plant Sciences Department.
malate. Decarboxylation of malate during the day releases CO2 C4 plants can produce more sugar than C3 plants in conditions of
inside the leaves, thus allowing carbon fixation to 3- high light and temperature. Many important crop plants are C4
plants, including maize, sorghum, sugarcane, and millet. Plants that

8.7.1 https://bio.libretexts.org/@go/page/13210
do not use PEP-carboxylase in carbon fixation are called C3 plants KEY TERMS
because the primary carboxylation reaction, catalyzed by RuBisCO, crassulacean acid metabolism: A carbon fixation pathway that
produces the three-carbon 3-phosphoglyceric acids directly in the evolved in some plants as an adaptation to arid conditions, in
Calvin-Benson cycle. Over 90% of plants use C3 carbon fixation, which the stomata in the leaves remain shut during the day to
compared to 3% that use C4 carbon fixation; however, the evolution reduce evapotranspiration, but open at night to collect carbon
of C4 in over 60 plant lineages makes it a striking example of dioxide (CO2).
convergent evolution. C4 carbon fixation: A form of photosynthesis in which plants
concentrate CO2 spatially, with a RuBisCO reaction centre in a
KEY POINTS
“bundle sheath cell” that is inundated with CO2
The process of photosynthesis in desert plants has evolved
mechanisms to conserve water. This page titled 8.7: The Light-Independent Reactions of Photosynthesis -
Plants that use crassulacean acid metabolism (CAM) AM and C4 Photosynthesis is shared under a CC BY-SA 4.0 license and
photosynthesis fix CO2 at night, when their stomata are open. was authored, remixed, and/or curated by Boundless.
Plants that use C4 carbon fixation concentrate carbon dioxide
spatially, using “bundle sheath cells” which are inundated with
CO2.

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8.8: THE LIGHT-INDEPENDENT REACTIONS OF PHOTOSYNTHESIS - THE
CALVIN CYCLE

 LEARNING OBJECTIVES

Describe the Calvin Cycle

THE CALVIN CYCLE

In plants, carbon dioxide (CO2) enters the leaves through stomata,
where it diffuses over short distances through intercellular spaces
until it reaches the mesophyll cells. Once in the mesophyll cells,
CO2 diffuses into the stroma of the chloroplast, the site of light-
independent reactions of photosynthesis. These reactions actually
have several names associated with them. Other names for light-
independent reactions include the Calvin cycle, the Calvin-Benson
cycle, and dark reactions. The most outdated name is dark reactions,
which can be misleading because it implies incorrectly that the
reaction only occurs at night or is independent of light, which is why
most scientists and instructors no longer use it.

Figure 8.8.1: The Calvin Cycle: The Calvin cycle has three stages.
In stage 1, the enzyme RuBisCO incorporates carbon dioxide into an
organic molecule, 3-PGA. In stage 2, the organic molecule is
reduced using electrons supplied by NADPH. In stage 3, RuBP, the
molecule that starts the cycle, is regenerated so that the cycle can
continue. Only one carbon dioxide molecule is incorporated at a
time, so the cycle must be completed three times to produce a single
three-carbon GA3P molecule, and six times to produce a six-carbon
glucose molecule.

STAGE 2: REDUCTION
Figure 8.8.1: Light Reactions: Light-dependent reactions harness ATP and NADPH are used to convert the six molecules of 3-PGA
energy from the sun to produce chemical bonds, ATP, and NADPH. into six molecules of a chemical called glyceraldehyde 3-phosphate
These energy-carrying molecules are made in the stroma where the (G3P). This is a reduction reaction because it involves the gain of
Calvin cycle takes place. The Calvin cycle is not totally independent
of light since it relies on ATP and NADH, which are products of the electrons by 3-PGA. Recall that a reduction is the gain of an electron
light-dependent reactions. by an atom or molecule. Six molecules of both ATP and NADPH are
The light-independent reactions of the Calvin cycle can be organized used. For ATP, energy is released with the loss of the terminal
into three basic stages: fixation, reduction, and regeneration. phosphate atom, converting it to ADP; for NADPH, both energy and
a hydrogen atom are lost, converting it into NADP+. Both of these
STAGE 1: FIXATION molecules return to the nearby light-dependent reactions to be reused
In the stroma, in addition to CO2,two other components are present and reenergized.
to initiate the light-independent reactions: an enzyme called ribulose
bisphosphate carboxylase (RuBisCO) and three molecules of STAGE 3: REGENERATION
ribulose bisphosphate (RuBP). RuBP has five atoms of carbon, At this point, only one of the G3P molecules leaves the Calvin cycle
flanked by two phosphates. RuBisCO catalyzes a reaction between and is sent to the cytoplasm to contribute to the formation of other
CO2 and RuBP. For each CO2 molecule that reacts with one RuBP, compounds needed by the plant. Because the G3P exported from the
two molecules of 3-phosphoglyceric acid (3-PGA) form. 3-PGA has chloroplast has three carbon atoms, it takes three “turns” of the
three carbons and one phosphate. Each turn of the cycle involves Calvin cycle to fix enough net carbon to export one G3P. But each
only one RuBP and one carbon dioxide and forms two molecules of turn makes two G3Ps, thus three turns make six G3Ps. One is
3-PGA. The number of carbon atoms remains the same, as the atoms exported while the remaining five G3P molecules remain in the
move to form new bonds during the reactions (3 atoms from 3CO2 + cycle and are used to regenerate RuBP, which enables the system to
15 atoms from 3RuBP = 18 atoms in 3 atoms of 3-PGA). This prepare for more CO2 to be fixed. Three more molecules of ATP are
process is called carbon fixation because CO2 is “fixed” from an used in these regeneration reactions.
inorganic form into organic molecules.

8.8.1 https://bio.libretexts.org/@go/page/13211
KEY POINTS KEY TERMS
The Calvin cycle refers to the light-independent reactions in light-independent reaction: chemical reactions during
photosynthesis that take place in three key steps. photosynthesis that convert carbon dioxide and other compounds
Although the Calvin Cycle is not directly dependent on light, it is into glucose, taking place in the stroma
indirectly dependent on light since the necessary energy carriers ( rubisco: (ribulose bisphosphate carboxylase) a plant enzyme
ATP and NADPH) are products of light-dependent reactions. which catalyzes the fixing of atmospheric carbon dioxide during
In fixation, the first stage of the Calvin cycle, light-independent photosynthesis by catalyzing the reaction between carbon
reactions are initiated; CO2 is fixed from an inorganic to an dioxide and RuBP
organic molecule. ribulose bisphosphate: an organic substance that is involved in
In the second stage, ATP and NADPH are used to reduce 3-PGA photosynthesis, reacts with carbon dioxide to form 3-PGA
into G3P; then ATP and NADPH are converted to ADP and
NADP+, respectively. This page titled 8.8: The Light-Independent Reactions of Photosynthesis -
In the last stage of the Calvin Cycle, RuBP is regenerated, which The Calvin Cycle is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.
enables the system to prepare for more CO2 to be fixed.

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8.9: THE LIGHT-INDEPENDENT REACTIONS OF PHOTOSYNTHESIS - THE
CARBON CYCLE
KEY POINTS
 LEARNING OBJECTIVES Every single atom of energy is conserved by changing form or
Describe the importance of the carbon cycle moving from one type of energy to another, so waste does not
exist in nature.
Photosynthesis absorbs light energy to build carbohydrates, and
THE CARBON CYCLE
aerobic cellular respiration releases energy by using oxygen to
Whether the organism is a bacterium, plant, or animal, all living metabolize carbohydrates.
things access energy by breaking down carbohydrate molecules. But Photosynthesis consumes carbon dioxide and produces oxygen,
if plants make carbohydrate molecules, why would they need to and aerobic respiration consumes oxygen and produces carbon
break them down, especially when it has been shown that the gas dioxide.
organisms release as a “waste product” (CO2) acts as a substrate for Both photosynthesis and cellular respiration use electron
the formation of more food in photosynthesis? Living things need transport chains to capture the energy necessary to drive other
energy to perform life functions. In addition, an organism can either reactions.
make its own food or eat another organism; either way, the food still
needs to be broken down. Finally, in the process of breaking down KEY TERMS
food, called cellular respiration, heterotrophs release needed energy heterotroph: an organism that requires an external supply of
and produce “waste” in the form of CO2 gas. energy in the form of food, as it cannot synthesize its own
cellular respiration: the set of the metabolic reactions and
processes that take place in the cells of organisms to convert
biochemical energy from nutrients into adenosine triphosphate
(ATP)
aerobic: living or occurring only in the presence of oxygen

CONTRIBUTIONS AND ATTRIBUTIONS

Using Light Energy. Provided by: OpenStax CNX. Located at:
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Energy-to-Make-Org. License: CC BY-SA: Attribution-ShareAlike
Using Light Energy. Provided by: Wikipedia. Located at:
https://en.Wikipedia.org/wiki/Photosynthesis. License: CC BY-SA:
Attribution-ShareAlike
C4 Carbon Fixation. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/C4_carbon_fixation. License: CC BY-SA: Attribution-
ShareAlike
CAM. Provided by: Wikipedia. Located at:
Figure 8.9.1: Photosynthesis and Aerobic Respiration: en.Wikipedia.org/wiki/Crassulacean_acid_metabolism. License: CC BY-SA:
Photosynthesis consumes carbon dioxide and produces oxygen. Attribution-ShareAlike
Aerobic respiration consumes oxygen and produces carbon dioxide. 400px-Cross_section_of_maize_a_C4_plant..jpg. Provided by: Wikipedia.
These two processes play an important role in the carbon cycle. Located at: en.Wikipedia.org/wiki/C4_car..._C4_plant..jpg. License: CC BY-
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matter and energy is conserved, recycling over and over, infinitely. commons.wikimedia.org/wiki/F...Cactus1web.jpg. License: Public Domain:
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another, but their constituent atoms never disappear. at: en.Wikipedia.org/wiki/Crassu...CAM_plant..jpg. License: CC BY-SA:
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Located at: http://cnx.org/content/m44449/latest...ol11448/latest. License: CC
photosynthesis. Both are byproducts of reactions that move on to
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other reactions. Photosynthesis absorbs light energy to build ribulose bisphosphate. Provided by: Wikipedia. Located at:
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releases energy by using oxygen to metabolize carbohydrates in the light-independent reaction. Provided by: Wikipedia. Located at:
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dioxide and produces oxygen. Aerobic respiration consumes oxygen rubisco. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/rubisco.
and produces carbon dioxide. Both processes use electron transport License: CC BY-SA: Attribution-ShareAlike
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 CHAPTER OVERVIEW

9: CELL COMMUNICATION
9.1: Signaling Molecules and Cellular Receptors - Signaling Molecules and Cellular Receptors
9.2: Signaling Molecules and Cellular Receptors - Forms of Signaling
9.3: Signaling Molecules and Cellular Receptors - Types of Receptors
9.4: Signaling Molecules and Cellular Receptors - Signaling Molecules
9.5: Propagation of the Cellular Signal - Binding Initiates a Signaling Pathway
9.6: Propagation of the Cellular Signal - Methods of Intracellular Signaling
9.7: Response to the Cellular Signal - Termination of the Signal Cascade
9.8: Response to the Cellular Signal - Cell Signaling and Gene Expression
9.9: Response to the Cellular Signal - Cell Signaling and Cellular Metabolism
9.10: Response to the Cellular Signal - Cell Signaling and Cell Growth
9.11: Response to the Cellular Signal - Cell Signaling and Cell Death
9.12: Signaling in Single-Celled Organisms - Signaling in Yeast
9.13: Signaling in Single-Celled Organisms - Signaling in Bacteria

This page titled 9: Cell Communication is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

1
9.1: SIGNALING MOLECULES AND CELLULAR RECEPTORS - SIGNALING
MOLECULES AND CELLULAR RECEPTORS
In multicellular organisms, cells send and receive chemical
 LEARNING OBJECTIVES messages constantly to coordinate the actions of distant organs,
tissues, and cells. The ability to send messages quickly and
Explain the importance of cell communication
efficiently enables cells to coordinate and fine-tune their functions.
While the necessity for cellular communication in larger organisms
INTRODUCTION: SIGNALING MOLECULES AND
seems obvious, even single-celled organisms communicate with
CELLULAR RECEPTORS
each other. Yeast cells signal each other to aid mating. Some forms
Imagine what life would be like if you and the people around you of bacteria coordinate their actions in order to form large complexes
could not communicate. You would not be able to express your called biofilms or to organize the production of toxins to remove
wishes to others, nor could you ask questions to find out more about competing organisms. The ability of cells to communicate through
your environment. Social organization is dependent on chemical signals originated in single cells and was essential for the
communication between the individuals that comprise that society; evolution of multicellular organisms. The efficient and error-free
without communication, society would fall apart. function of communication systems is vital for all forms of life.

KEY POINTS
The ability of cells to communicate through chemical signals
originated in single cells and was essential for the evolution of
multicellular organisms.
In multicellular organisms, cells send and receive chemical
messages constantly to coordinate the actions of distant organs,
tissues, and cells.
Cells can receive a message, transfer the information across the
plasma membrane, and then produce changes within the cell in
response to the message.
Single-celled organisms, like yeast and bacteria, communicate
with each other to aid in mating and coordination.
Figure 9.1.1: Communication is Key: Have you ever become
separated from a friend while in a crowd? If so, you know the Cellular communication has developed as a means to
challenge of searching for someone when surrounded by thousands communicate with the environment, produce biological changes,
of other people. If you and your friend have cell phones, your and, if necessary, ensure survival.
chances of finding each other are good. A cell phone’s ability to
send and receive messages makes it an ideal communication device.
KEY TERMS
As with people, it is vital for individual cells to be able to interact
with their environment. This is true whether a cell is growing by biofilm: a thin film of mucus created by and containing a colony
itself in a pond or is one of many cells that form a larger organism. of bacteria and other microorganisms
In order to properly respond to external stimuli, cells have developed
This page titled 9.1: Signaling Molecules and Cellular Receptors - Signaling
complex mechanisms of communication that can receive a message,
Molecules and Cellular Receptors is shared under a CC BY-SA 4.0 license
transfer the information across the plasma membrane, and then and was authored, remixed, and/or curated by Boundless.
produce changes within the cell in response to the message.

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9.2: SIGNALING MOLECULES AND CELLULAR RECEPTORS - FORMS OF
SIGNALING
where signal transmission occurs is called a synapse. A synaptic
 LEARNING OBJECTIVES signal is a chemical signal that travels between nerve cells. Signals
within the nerve cells are propagated by fast-moving electrical
Describe four types of signaling found in multicellular
impulses. When these impulses reach the end of the axon, the signal
organisms
continues on to a dendrite of the next cell by the release of chemical
ligands called neurotransmitters by the presynaptic cell (the cell
FORMS OF SIGNALING emitting the signal). The neurotransmitters are transported across the
There are four categories of chemical signaling found in very small distances between nerve cells, which are called chemical
multicellular organisms: paracrine signaling, endocrine signaling, synapses. The small distance between nerve cells allows the signal
autocrine signaling, and direct signaling across gap junctions. The to travel quickly; this enables an immediate response.
main difference between the different categories of signaling is the
distance that the signal travels through the organism to reach the
target cell. It is also important to note that not all cells are affected
by the same signals.

Figure 9.2.1: Forms of Chemical Signaling: In chemical signaling, a

cell may target itself (autocrine signaling), a cell connected by gap
junctions, a nearby cell (paracrine signaling), or a distant cell
(endocrine signaling). Paracrine signaling acts on nearby cells,
endocrine signaling uses the circulatory system to transport ligands, Figure 9.2.1: Synapsis: The distance between the presynaptic cell
and autocrine signaling acts on the signaling cell. Signaling via gap and the postsynaptic cell—called the synaptic gap—is very small
junctions involves signaling molecules moving directly between and allows for rapid diffusion of the neurotransmitter. Enzymes in
adjacent cells. the synapatic cleft degrade some types of neurotransmitters to
terminate the signal.
PARACRINE SIGNALING
ENDOCRINE SIGNALING
Signals that act locally between cells that are close together are
called paracrine signals. Paracrine signals move by diffusion through Signals from distant cells are called endocrine signals; they originate
from endocrine cells. In the body, many endocrine cells are located
the extracellular matrix. These types of signals usually elicit quick
responses that last only a short amount of time. In order to keep the in endocrine glands, such as the thyroid gland, the hypothalamus,
and the pituitary gland. These types of signals usually produce a
response localized, paracrine ligand molecules are normally quickly
degraded by enzymes or removed by neighboring cells. Removing slower response, but have a longer-lasting effect. The ligands
released in endocrine signaling are called hormones, signaling
the signals will reestablish the concentration gradient for the signal,
allowing them to quickly diffuse through the intracellular space if molecules that are produced in one part of the body, but affect other
body regions some distance away.
released again.
One example of paracrine signaling is the transfer of signals across Hormones travel the large distances between endocrine cells and
synapses between nerve cells. A nerve cell consists of a cell body, their target cells via the bloodstream, which is a relatively slow way
several short, branched extensions called dendrites that receive to move throughout the body. Because of their form of transport,
stimuli, and a long extension called an axon, which transmits signals hormones get diluted and are present in low concentrations when
to other nerve cells or muscle cells. The junction between nerve cells

9.2.1 https://bio.libretexts.org/@go/page/13215
they act on their target cells. This is different from paracrine to coordinate their response to a signal that only one of them may
signaling in which local concentrations of ligands can be very high. have received. In plants, plasmodesmata are ubiquitous, making the
entire plant into a giant communication network.
AUTOCRINE SIGNALING
Autocrine signals are produced by signaling cells that can also bind KEY POINTS
to the ligand that is released. This means the signaling cell and the Cells communicate via various types of signaling that allow
target cell can be the same or a similar cell (the prefix auto- means chemicals to travel to target sites in order to elicit a response.
self, a reminder that the signaling cell sends a signal to itself). This Paracrine signaling occurs between local cells where the signals
type of signaling often occurs during the early development of an elicit quick responses and last only a short amount of time due to
organism to ensure that cells develop into the correct tissues and the degradation of the paracrine ligands.
take on the proper function. Autocrine signaling also regulates pain Endocrine signaling occurs between distant cells and is mediated
sensation and inflammatory responses. Further, if a cell is infected by hormones released from specific endocrine cells that travel to
with a virus, the cell can signal itself to undergo programmed cell target cells, producing a slower, long-lasting response.
death, killing the virus in the process. In some cases, neighboring Autocrine signals are produced by signaling cells that can also
cells of the same type are also influenced by the released ligand. In bind to the ligand that is released, which means the signaling cell
embryological development, this process of stimulating a group of and the target cell can be the same or a similar cell.
neighboring cells may help to direct the differentiation of identical Direct signaling can occur by transferring signaling molecules
cells into the same cell type, thus ensuring the proper developmental across gap junctions between neighboring cells.
outcome.
KEY TERMS
DIRECT SIGNALING ACROSS GAP JUNCTIONS endocrine signaling: signals from distant cells that originate
from endocrine cells, usually producing a slow response, but
Gap junctions in animals and plasmodesmata in plants are having a long-lasting effect
connections between the plasma membranes of neighboring cells. autocrine signaling: produced by signaling cells that can also
These water-filled channels allow small signaling molecules, called bind to the ligand that is released: the signaling cell and the
intracellular mediators, to diffuse between the two cells. Small target cell can be the same or a similar cell (prefix auto- means
molecules, such as calcium ions (Ca2+), are able to move between self)
cells, but large molecules, like proteins and DNA, cannot fit through paracrine signaling: a form of cell signaling in which the target
the channels. The specificity of the channels ensures that the cells cell is near (para = near) the signal-releasing cell
remain independent, but can quickly and easily transmit signals. The
transfer of signaling molecules communicates the current state of the This page titled 9.2: Signaling Molecules and Cellular Receptors - Forms of
cell that is directly next to the target cell; this allows a group of cells Signaling is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.

9.2.2 https://bio.libretexts.org/@go/page/13215
9.3: SIGNALING MOLECULES AND CELLULAR RECEPTORS - TYPES OF
RECEPTORS
affect. Cell-surface receptors are also called cell-specific proteins or
 LEARNING OBJECTIVES markers because they are specific to individual cell types.
Each cell-surface receptor has three main components: an external
Compare internal receptors with cell-surface receptors
ligand-binding domain (extracellular domain), a hydrophobic
membrane-spanning region, and an intracellular domain inside the
TYPES OF RECEPTORS
cell. The size and extent of each of these domains vary widely,
Receptors are protein molecules in the target cell or on its surface depending on the type of receptor.
that bind ligands. There are two types of receptors: internal receptors
Cell-surface receptors are involved in most of the signaling in
and cell-surface receptors.
multicellular organisms. There are three general categories of cell-
INTERNAL RECEPTORS surface receptors: ion channel-linked receptors, G-protein-linked
receptors, and enzyme-linked receptors.
Internal receptors, also known as intracellular or cytoplasmic
receptors, are found in the cytoplasm of the cell and respond to ION CHANNEL-LINKED RECEPTORS
hydrophobic ligand molecules that are able to travel across the
Ion channel-linked receptors bind a ligand and open a channel
plasma membrane. Once inside the cell, many of these molecules
through the membrane that allows specific ions to pass through. To
bind to proteins that act as regulators of mRNA synthesis to mediate
form a channel, this type of cell-surface receptor has an extensive
gene expression. Gene expression is the cellular process of
membrane-spanning region. In order to interact with the
transforming the information in a cell’s DNA into a sequence of
phospholipid fatty acid tails that form the center of the plasma
amino acids that ultimately forms a protein. When the ligand binds
membrane, many of the amino acids in the membrane-spanning
to the internal receptor, a conformational change exposes a DNA-
region are hydrophobic in nature. Conversely, the amino acids that
binding site on the protein. The ligand-receptor complex moves into
line the inside of the channel are hydrophilic to allow for the passage
the nucleus, binds to specific regulatory regions of the chromosomal
of water or ions. When a ligand binds to the extracellular region of
DNA, and promotes the initiation of transcription. Internal receptors
the channel, there is a conformational change in the protein’s
can directly influence gene expression without having to pass the
structure that allows ions such as sodium, calcium, magnesium, and
signal on to other receptors or messengers.
hydrogen to pass through.

Figure 9.3.1: Gated-Ion Channels: Gated ion channels form a pore

through the plasma membrane that opens when the signaling
molecule binds. The open pore then allows ions to flow into or out
of the cell.
Figure 9.3.1: Intracellular Receptors: Hydrophobic signaling
molecules typically diffuse across the plasma membrane and interact
with intracellular receptors in the cytoplasm. Many intracellular
G-PROTEIN LINKED RECEPTORS
receptors are transcription factors that interact with DNA in the G-protein-linked receptors bind a ligand and activate a membrane
nucleus and regulate gene expression. protein called a G-protein. The activated G-protein then interacts
with either an ion channel or an enzyme in the membrane. All G-
CELL-SURFACE RECEPTORS
protein-linked receptors have seven transmembrane domains, but
Cell-surface receptors, also known as transmembrane receptors, are
each receptor has its own specific extracellular domain and G-
cell surface, membrane-anchored, or integral proteins that bind to
protein-binding site.
external ligand molecules. This type of receptor spans the plasma
membrane and performs signal transduction, converting an Cell signaling using G-protein-linked receptors occurs as a cyclic
extracellular signal into an intracellular signal. Ligands that interact series of events. Before the ligand binds, the inactive G-protein can
with cell-surface receptors do not have to enter the cell that they bind to a newly-revealed site on the receptor specific for its binding.
Once the G-protein binds to the receptor, the resultant shape change

9.3.1 https://bio.libretexts.org/@go/page/13216
activates the G-protein, which releases GDP and picks up GTP. The intracellular domain of the receptor itself is an enzyme or the
subunits of the G-protein then split into the α subunit and the β enzyme-linked receptor has an intracellular domain that interacts
subunit. One or both of these G-protein fragments may be able to directly with an enzyme. The enzyme-linked receptors normally
activate other proteins as a result. Later, the GTP on the active α have large extracellular and intracellular domains, but the
subunit of the G-protein is hydrolyzed to GDP and the β subunit is membrane-spanning region consists of a single alpha-helical region
deactivated. The subunits reassociate to form the inactive G-protein, of the peptide strand. When a ligand binds to the extracellular
and the cycle starts over. domain, a signal is transferred through the membrane and activates
the enzyme, which sets off a chain of events within the cell that
eventually leads to a response. An example of this type of enzyme-
linked receptor is the tyrosine kinase receptor. The tyrosine kinase
receptor transfers phosphate groups to tyrosine molecules. Signaling
molecules bind to the extracellular domain of two nearby tyrosine
kinase receptors, which then dimerize. Phosphates are then added to
tyrosine residues on the intracellular domain of the receptors and can
then transmit the signal to the next messenger within the cytoplasm.

KEY POINTS
Intracellular receptors are located in the cytoplasm of the cell and
are activated by hydrophobic ligand molecules that can pass
through the plasma membrane.
Cell-surface receptors bind to an external ligand molecule and
convert an extracellular signal into an intracellular signal.
Three general categories of cell-surface receptors include: ion -
channel, G- protein, and enzyme -linked protein receptors.
Ion channel -linked receptors bind a ligand and open a channel
through the membrane that allows specific ions to pass through.
G-protein-linked receptors bind a ligand and activate a
membrane protein called a G-protein, which then interacts with
either an ion channel or an enzyme in the membrane.
Enzyme-linked receptors are cell-surface receptors with
intracellular domains that are associated with an enzyme.
Figure 9.3.1: G-proteins: Heterotrimeric G proteins have three
subunits: α, β, and γ. When a signaling molecule binds to a G-
protein-coupled receptor in the plasma membrane, a GDP molecule
KEY TERMS
associated with the α subunit is exchanged for GTP. The β and γ integral protein: a protein molecule (or assembly of proteins)
subunits dissociate from the α subunit, and a cellular response is that is permanently attached to the biological membrane
triggered either by the α subunit or the dissociated β pair. Hydrolysis
of GTP to GDP terminates the signal. transcription: the synthesis of RNA under the direction of DNA

ENZYME-LINKED RECEPTORS This page titled 9.3: Signaling Molecules and Cellular Receptors - Types of
Receptors is shared under a CC BY-SA 4.0 license and was authored,
Enzyme-linked receptors are cell-surface receptors with intracellular
remixed, and/or curated by Boundless.
domains that are associated with an enzyme. In some cases, the

9.3.2 https://bio.libretexts.org/@go/page/13216
9.4: SIGNALING MOLECULES AND CELLULAR RECEPTORS - SIGNALING
MOLECULES
WATER-SOLUBLE LIGANDS
 LEARNING OBJECTIVES Water-soluble ligands are polar and, therefore, cannot pass through
Compare and contrast the different types of signaling the plasma membrane unaided; sometimes, they are too large to pass
molecules: hydrophobic, water-soluble, and gas ligands through the membrane at all. Instead, most water-soluble ligands
bind to the extracellular domain of cell-surface receptors. Cell-
surface receptors include: ion-channel, G-protein, and enzyme-
SIGNALING MOLECULES
linked protein receptors. The binding of these ligands to these
Produced by signaling cells and the subsequent binding to receptors receptors results in a series of cellular changes. These water soluble
in target cells, ligands act as chemical signals that travel to the target ligands are quite diverse and include small molecules, peptides, and
cells to coordinate responses. The types of molecules that serve as proteins.
ligands are incredibly varied and range from small proteins to small
ions like calcium (Ca2+). OTHER LIGANDS
Nitric oxide (NO) is a gas that also acts as a ligand. It is able to
SMALL HYDROPHOBIC LIGANDS
diffuse directly across the plasma membrane; one of its roles is to
Small hydrophobic ligands can directly diffuse through the plasma interact with receptors in smooth muscle and induce relaxation of
membrane and interact with internal receptors. Important members the tissue. NO has a very short half-life; therefore, it only functions
of this class of ligands are the steroid hormones. Steroids are lipids over short distances. Nitroglycerin, a treatment for heart disease,
that have a hydrocarbon skeleton with four fused rings; different acts by triggering the release of NO, which causes blood vessels to
steroids have different functional groups attached to the carbon dilate (expand), thus restoring blood flow to the heart.
skeleton. Steroid hormones include the female sex hormone,
estradiol, which is a type of estrogen; the male sex hormone, KEY POINTS
testosterone; and cholesterol, which is an important structural Signaling molecules can range from small proteins to small ions
component of biological membranes and a precursor of steriod and can be hydrophobic, water-soluble, or even a gas.
hormones. Other hydrophobic hormones include thyroid hormones Hydrophobic signaling molecules ( ligands ) can diffuse through
and vitamin D. In order to be soluble in blood, hydrophobic ligands the plasma membrane and bind to internal receptors.
must bind to carrier proteins while they are being transported Water-soluble ligands are unable to pass freely through the
through the bloodstream. plasma membrane due to their polarity and must bind to an
extracellular domain of a cell -surface receptor.
Other types of ligands can include gases, such as nitric oxide,
which can freely diffuse through the plasma membrane and bind
to internal receptors.

KEY TERMS
ligand: an ion, molecule, or functional group that binds to
another chemical entity to form a larger complex
hydrophobic: lacking an affinity for water; unable to absorb, or
be wetted by water

This page titled 9.4: Signaling Molecules and Cellular Receptors - Signaling
Molecules is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.

Figure 9.4.1: Steroid Hormones: Steroid hormones have similar

chemical structures to their precursor, cholesterol. Because these
molecules are small and hydrophobic, they can diffuse directly
across the plasma membrane into the cell, where they interact with
internal receptors.

9.4.1 https://bio.libretexts.org/@go/page/13217
9.5: PROPAGATION OF THE CELLULAR SIGNAL - BINDING INITIATES A
SIGNALING PATHWAY
The events in the cascade occur in a series, much like a current
 LEARNING OBJECTIVES flows in a river. Interactions that occur before a certain point are
defined as upstream events; events after that point are called
Recognize the relationship between a ligand’s structure and
downstream events.
its mechanism of action.

Cell-surface receptors, also known as transmembrane receptors, are

membrane-anchored (integral) proteins that bind to external ligand
molecules. This type of receptor spans the plasma membrane and
performs signal transduction in which an extracellular signal is
converted into an intracellular signal. Ligands that interact with cell-
surface receptors do not have to enter the cell that they affect. Cell-
surface receptors are also called cell-specific proteins or markers
because they are specific to individual cell types. Each cell-surface
receptor has three main components: an external ligand-binding
domain, a hydrophobic membrane-spanning region, and an
intracellular domain inside the cell. The ligand-binding domain is
also called the extracellular domain. The size and extent of each of
these domains vary widely, depending on the type of receptor. Cell-
surface receptors are involved in most of the signaling in
multicellular organisms.
There are three general categories of cell-surface receptors: ion
channel-linked receptors, G-protein-linked receptors, and enzyme-
linked receptors.
1. Ion channel-linked receptors bind a ligand and open a channel
through the membrane that allows specific ions to pass through.
To form a channel, this type of cell-surface receptor has an
extensive membrane-spanning region. When a ligand binds to
the extracellular region of the channel, there is a conformational
change in the protein’s structure that allows ions such as sodium,
calcium, magnesium, and hydrogen to pass through.
2. G-protein-linked receptors bind a ligand and activate a
membrane protein called a G-protein. The activated G-protein
then interacts with either an ion channel or an enzyme in the
membrane. All G-protein-linked receptors have seven
transmembrane domains, but each receptor has its own specific
extracellular domain and G-protein-binding site.
3. Enzyme-linked receptors are cell-surface receptors with
intracellular domains that are associated with an enzyme. In
Figure 9.5.1: Ligand Initiated Signaling Pathway: An example of
some cases, the intracellular domain of the receptor itself is an ligand initiated signaling pathways is when epidermal growth factor
enzyme. Other enzyme-linked receptors have a small (EGF) binds to its receptor. A complex cascade of downstream
intracellular domain that interacts directly with an enzyme. events causes the cell to grow and divide.
When a ligand binds to the extracellular domain, a signal is Signaling pathways can get very complicated very quickly because
transferred through the membrane, activating the enzyme. most cellular proteins can affect different downstream events,
Activation of the enzyme sets off a chain of events within the depending on the conditions within the cell. A single pathway can
cell that eventually leads to a response. branch off toward different endpoints based on the interplay between
two or more signaling pathways. The same ligands are often used to
After the ligand binds to the cell-surface receptor, the activation of
initiate different signals in different cell types. This variation in
the receptor’s intracellular components sets off a chain of events that
response is due to differences in protein expression in different cell
is called a signaling pathway or a signaling cascade. In a signaling
types. Another complicating element is signal integration of the
pathway, second messengers, enzymes, and activated proteins
pathways in which signals from two or more different cell-surface
interact with specific proteins, which are in turn activated in a chain
receptors merge to activate the same response in the cell. This
reaction that eventually leads to a change in the cell’s environment.

9.5.1 https://bio.libretexts.org/@go/page/13219
process can ensure that multiple external requirements are met G-protein-linked receptors activate a membrane protein called a
before a cell commits to a specific response. G-protein once a ligand binds.
The effects of extracellular signals can also be amplified by Enzyme -linked receptors are cell-surface receptors with
enzymatic cascades. At the initiation of the signal, a single ligand intracellular domains.
binds to a single receptor. However, activation of a receptor-linked
KEY TERMS
enzyme can activate many copies of a component of the signaling
cascade, which amplifies the signal. ligand: an ion, molecule, or functional group that binds to
another chemical entity to form a larger complex
KEY POINTS receptor: a protein on a cell wall that binds with specific
Signaling pathways can be complicated since most cellular molecules so that they can be absorbed into the cell in order to
proteins can affect different downstream events. control certain functions
Cell -surface receptors are integral in signaling pathways.
This page titled 9.5: Propagation of the Cellular Signal - Binding Initiates a
Ion channel -linked receptors open a channel once a ligand binds
Signaling Pathway is shared under a CC BY-SA 4.0 license and was
allowing specific ions to pass through the membrane.
authored, remixed, and/or curated by Boundless.

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9.6: PROPAGATION OF THE CELLULAR SIGNAL - METHODS OF
INTRACELLULAR SIGNALING
propagate a signal after it has been initiated by the binding of the
 LEARNING OBJECTIVES signaling molecule to the receptor. These molecules help to spread a
signal through the cytoplasm by altering the behavior of certain
Explain how the binding of a ligand initiates signal
cellular proteins.
transduction throughout a cell
Calcium ion is a widely-used second messenger. The free
concentration of calcium ions (Ca2+) within a cell is very low
The induction of a signaling pathway depends on the modification of
because ion pumps in the plasma membrane continuously use
a cellular component by an enzyme. There are numerous enzymatic
adenosine-5′-triphosphate ( ATP ) to remove it. For signaling
modifications that can occur which are recognized in turn by the
purposes, Ca2+ is stored in cytoplasmic vesicles, such as the
next component downstream.
endoplasmic reticulum, or accessed from outside the cell. When
One of the most common chemical modifications that occurs in
signaling occurs, ligand-gated calcium ion channels allow the higher
signaling pathways is the addition of a phosphate group (PO4–3) to a
levels of Ca2+ that are present outside the cell (or in intracellular
molecule such as a protein in a process called phosphorylation. The storage compartments) to flow into the cytoplasm, which raises the
phosphate can be added to a nucleotide such as GMP to form GDP concentration of cytoplasmic Ca2+. The response to the increase in
or GTP. Phosphates are also often added to serine, threonine, and Ca2+ varies, depending on the cell type involved. For example, in the
tyrosine residues of proteins where they replace the hydroxyl group β-cells of the pancreas, Ca2+ signaling leads to the release of insulin,
of the amino acid. The transfer of the phosphate is catalyzed by an whereas in muscle cells, an increase in Ca2+ leads to muscle
enzyme called a kinase. Various kinases are named for the substrate contractions.
they phosphorylate. Phosphorylation of serine and threonine
residues often activates enzymes. Phosphorylation of tyrosine Another second messenger utilized in many different cell types is
cyclic AMP (cAMP). Cyclic AMP is synthesized by the enzyme
residues can either affect the activity of an enzyme or create a
adenylyl cyclase from ATP. The main role of cAMP in cells is to
binding site that interacts with downstream components in the
bind to and activate an enzyme called cAMP-dependent kinase (A-
signaling cascade. Phosphorylation may activate or inactivate
kinase). A-kinase regulates many vital metabolic pathways. It
enzymes; the reversal of phosphorylation, dephosphorylation by a
phosphorylates serine and threonine residues of its target proteins,
phosphatase, will reverse the effect.
activating them in the process. A-kinase is found in many different
types of cells; the target proteins in each kind of cell are different.
Differences give rise to the variation of the responses to cAMP in
different cells.

Figure 9.6.1: Example of cAMP as a second messenger: This

diagram shows the mechanism for the formation of cyclic AMP
(cAMP). cAMP serves as a second messenger to activate or
inactivate proteins within the cell. Termination of the signal occurs
when an enzyme called phosphodiesterase converts cAMP into
AMP.
Present in small concentrations in the plasma membrane, inositol
phospholipids are lipids that can also be converted into second
messengers. Because these molecules are membrane components,
they are located near membrane-bound receptors and can easily
interact with them. Phosphatidylinositol (PI) is the main
Figure 9.6.1: Example of phosphorylation: In protein
phosphorylation, a phosphate group (PO4-3 ) is added to residues of phospholipid that plays a role in cellular signaling. Enzymes known
the amino acids serine, threonine, and tyrosine. as kinases phosphorylate PI to form PI-phosphate (PIP) and PI-
The activation of second messengers is also a common event after bisphosphate (PIP2).
the induction of a signaling pathway. They are small molecules that

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KEY POINTS ligand. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/ligand.
License: CC BY-SA: Attribution-ShareAlike
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Calcium ion, cyclic AMP, and inositol phospholipids are phosphorylation. Provided by: Wiktionary. Located at:
examples of widely-used second messengers. en.wiktionary.org/wiki/phosphorylation. License: CC BY-SA: Attribution-
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authored, remixed, and/or curated by Boundless.

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9.7: RESPONSE TO THE CELLULAR SIGNAL - TERMINATION OF THE SIGNAL
CASCADE
The aberrant signaling often seen in tumor cells is proof that the
 LEARNING OBJECTIVES termination of a signal at the appropriate time can be just as
important as the initiation of a signal. One method of terminating or
Describe the process by which the signal cascade in cell
stopping a specific signal is to degrade or remove the ligand so that
communication is terminated
it can no longer access its receptor. One reason that hydrophobic
hormones like estrogen and testosterone trigger long-lasting events
TERMINATION OF THE SIGNAL CASCADE is because they bind carrier proteins. These proteins allow the
Ligand binding to the receptor allows for signal transduction through insoluble molecules to be soluble in blood, but they also protect the
the cell. The chain of events that conveys the signal through the cell hormones from degradation by circulating enzymes.
is called a signaling pathway or cascade. Signaling pathways are Inside the cell, many different enzymes reverse the cellular
often very complex because of the interplay between different modifications that result from signaling cascades. For example,
proteins. A major component of cell signaling cascades is the phosphatases are enzymes that remove the phosphate group attached
phosphorylation of molecules by enzymes known as kinases. to proteins by kinases in a process called dephosphorylation. Cyclic
Phosphorylation adds a phosphate group to serine, threonine, and AMP (cAMP) is degraded into AMP by phosphodiesterase, and the
tyrosine residues in a protein, changing their shapes, and activating release of calcium stores is reversed by the Ca2+ pumps that are
or inactivating the protein. located in the external and internal membranes of the cell.

KEY POINTS
The chain of events that conveys the signal through the cell is
called a signaling pathway or cascade.
Phosphorylation, a major component of signal cascades, adds a
phosphate group to proteins, thereby changing their shapes and
activating or inactivating the protein.
Degrading or removing the ligand so it can no longer access its
receptor terminates the signal.
Enzymes like phosphotases can remove phosphate groups on
proteins during dephosphorylation and reverse the cellular
modifications produced by signaling cascades.

KEY TERMS
signaling cascade: the chain of events that conveys the signal
through the cell
phosphorylation: the addition of a phosphate group to a
compound; often catalyzed by enzymes
dephosphorylation: the removal of phosphate groups from a
compound; often catalyzed by enzymes

Figure 9.7.1: Phosphorylation: In protein phosphorylation, a This page titled 9.7: Response to the Cellular Signal - Termination of the
phosphate group is added to residues of the amino acids serine, Signal Cascade is shared under a CC BY-SA 4.0 license and was authored,
threonine, and tyrosine. remixed, and/or curated by Boundless.

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9.8: RESPONSE TO THE CELLULAR SIGNAL - CELL SIGNALING AND GENE
EXPRESSION

 LEARNING OBJECTIVES

Describe the regulation of gene expression

GENE EXPRESSION
For a cell to function properly, necessary proteins must be
synthesized at the proper time. All cells control or regulate the
synthesis of proteins from information encoded in their DNA. The
process of turning on a gene to produce RNA and protein is called Figure 9.8.1: Prokaryotic & Eukaryotic Gene Expression:
Prokaryotic transcription and translation occur simultaneously in the
gene expression. cytoplasm; regulation occurs at the transcriptional level. Eukaryotic
Whether in a simple unicellular organism or a complex multi- gene expression is regulated during transcription and RNA
processing, which take place in the nucleus, and during protein
cellular organism, each cell controls when and how its genes are
translation, which takes place in the cytoplasm. Further regulation
expressed. For this to occur, there must be a mechanism to control may occur through post-translational modifications of proteins.
when a gene is expressed to make RNA and protein; how much of Eukaryotic cells, in contrast, have intracellular organelles that add to
the protein is made; and when it is time to stop making that protein their complexity. In eukaryotic cells, the DNA is contained inside
because it is no longer needed. The regulation of gene expression the cell’s nucleus where it is transcribed into RNA. The newly-
conserves energy and space. It would require a significant amount of synthesized RNA is then transported out of the nucleus into the
energy for an organism to express every gene at all times, so it is cytoplasm where ribosomes translate the RNA into protein. The
more energy efficient to turn on the genes only when they are processes of transcription and translation are physically separated by
required. In addition, only expressing a subset of genes in each cell the nuclear membrane: transcription occurs only within the nucleus,
saves space because DNA must be unwound from its tightly-coiled and translation occurs only outside the nucleus in the cytoplasm. The
structure to transcribe and translate the DNA. Cells would have to be regulation of gene expression can occur at all stages of the process.
enormous if every protein were expressed in every cell all the time. Regulation may occur when the DNA is uncoiled and loosened from
The control of gene expression is extremely complex. Malfunctions nucleosomes to bind transcription factors (epigenetic level); when
in this process are detrimental to the cell and can lead to the the RNA is transcribed (transcriptional level); when the RNA is
development of many diseases, including cancer. processed and exported to the cytoplasm after it is transcribed (post-
transcriptional level); when the RNA is translated into protein
PROKARYOTIC VERSUS EUKARYOTIC GENE
(translational level); or after the protein has been made (post-
EXPRESSION
translational level).
To understand how gene expression is regulated, we must first
understand how a gene codes for a functional protein in a cell. The KEY POINTS
process occurs in both prokaryotic and eukaryotic cells, just in Each cell controls when and how its genes are expressed.
slightly different manners. Prokaryotic organisms are single-celled Malfunctions in the control of gene expression are detrimental to
organisms that lack a cell nucleus; their DNA floats freely in the cell the cell and can lead to the development of many diseases, such
cytoplasm. To synthesize a protein, the processes of transcription as cancer.
and translation occur almost simultaneously. When the resulting In prokaryotic cells, the control of gene expression is mostly at
protein is no longer needed, transcription stops. As a result, the the transcriptional level.
primary method to control what type of protein and how much of In eukaryotic cells, the control of gene expression is at the
each protein is expressed in a prokaryotic cell is the regulation of epigenetic, transcriptional, post-transcriptional, translational, and
DNA transcription. All of the subsequent steps occur automatically. post-translational levels.
When more protein is required, more transcription occurs.
Therefore, in prokaryotic cells, the control of gene expression is KEY TERMS
mostly at the transcriptional level. translation: a process occurring in the ribosome in which a
strand of messenger RNA (mRNA) guides assembly of a
sequence of amino acids to make a protein
gene expression: the transcription and translation of a gene into
messenger RNA and, thus, into a protein
transcription: the synthesis of RNA under the direction of DNA

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Gene Expression is shared under a CC BY-SA 4.0 license and was authored,

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9.9: RESPONSE TO THE CELLULAR SIGNAL - CELL SIGNALING AND
CELLULAR METABOLISM
then available for use by the muscle cell in response to a sudden
 LEARNING OBJECTIVES surge of adrenaline—the “fight or flight” reflex.

Explain how cellular metabolism can be altered

INCREASE IN CELLULAR METABOLISM

As the environments of most organisms are constantly changing, the
reactions of metabolism must be finely regulated to maintain a
constant set of conditions within cells. Metabolic regulation also
allows organisms to respond to signals and interact actively with
their environments. Two closely-linked concepts are important for
understanding how metabolic pathways are controlled. Firstly, the
Figure 9.9.1: Formation of Cyclic AMP: This diagram shows the
regulation of an enzyme in a pathway is how its activity is increased mechanism for the formation of cyclic AMP (cAMP). cAMP serves
and decreased in response to signals. Secondly, the controlexerted as a second messenger to activate or inactivate proteins within the
by this enzyme is the effect that these changes in its activity have on cell.
the overall rate of the pathway. For example, an enzyme may show
KEY POINTS
large changes in activity (i.e. it is highly regulated), but if these
changes have little effect on the rate of a metabolic pathway, then The activation of β-adrenergic receptors in muscle cells by
this enzyme is not involved in the control of the pathway. adrenaline leads to an increase in cyclic AMP.
Cyclic AMP activates PKA (protein kinase A), which
The result of one such signaling pathway affects muscle cells and is phosphorylates two enzymes.
a good example of an increase in cellular metabolism. The activation Phophorylation of the first enzyme promotes the degradation of
of β-adrenergic receptors in muscle cells by adrenaline leads to an glycogen by activating intermediate GPK that in turn activates
increase in cyclic adenosine monophosphate (also known as cyclic GP, which catabolizes glycogen into glucose.
AMP or cAMP) inside the cell. Also known as epinephrine, Phosphorylation of the second enzyme, glycogen synthase (GS),
adrenaline is a hormone (produced by the adrenal gland attached to inhibits its ability to form glycogen from glucose.
the kidney) that prepares the body for short-term emergencies. The inhibition of glucose to form glycogen prevents a futile
Cyclic AMP activates PKA (protein kinase A), which in turn cycle of glycogen degradation and synthesis, so glucose is then
phosphorylates two enzymes. The first enzyme promotes the available for use by the muscle cell.
degradation of glycogen by activating intermediate glycogen
phosphorylase kinase (GPK) that in turn activates glycogen KEY TERMS
phosphorylase (GP), which catabolizes glycogen into glucose. cyclic adenosine monophosphate: cAMP, a second messenger
(Recall that your body converts excess glucose to glycogen for derived from ATP that is involved in the activation of protein
short-term storage. When energy is needed, glycogen is quickly kinases and regulates the effects of adrenaline
reconverted to glucose. ) Phosphorylation of the second enzyme, epinephrine: (adrenaline) an amino acid-derived hormone
glycogen synthase (GS), inhibits its ability to form glycogen from secreted by the adrenal gland in response to stress
glucose. In this manner, a muscle cell obtains a ready pool of
protein kinase A: a family of enzymes whose activity is
glucose by activating its formation via glycogen degradation and by dependent on cellular levels of cyclic AMP (cAMP)
inhibiting the use of glucose to form glycogen, thus preventing a
futile cycle of glycogen degradation and synthesis. The glucose is This page titled 9.9: Response to the Cellular Signal - Cell Signaling and
Cellular Metabolism is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

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9.10: RESPONSE TO THE CELLULAR SIGNAL - CELL SIGNALING AND CELL
GROWTH
signaling proteins. This prevents the cell from regulating its cell
 LEARNING OBJECTIVES cycle, triggering unrestricted cell division and cancer. The genes that
regulate the signaling proteins are one type of oncogene: a gene that
Explain how cell growth is affected by growth factors.
has the potential to cause cancer. The gene encoding RAS is an
oncogene that was originally discovered when mutations in the RAS
CELL GROWTH protein were linked to cancer. Further studies have indicated that 30
Cell signaling pathways play a major role in cell division. Cells do percent of cancer cells have a mutation in the RAS gene that leads to
not normally divide unless they are stimulated by signals from other uncontrolled growth. If left unchecked, uncontrolled cell division
cells. The ligands that promote cell growth are called growth factors. can lead tumor formation and metastasis, the growth of cancer cells
Most growth factors bind to cell-surface receptors that are linked to in new locations in the body.
tyrosine kinases. These cell-surface receptors are called receptor Cancer biologists have been able to identify many other oncogenes
tyrosine kinases (RTKs). Activation of RTKs initiates a signaling that contribute to the development of cancer. For example, HER2 is
pathway that includes a G-protein called RAS, which activates the a cell-surface receptor that is present in excessive amounts in 20
MAP kinase pathway described earlier. The enzyme MAP kinase percent of human breast cancers. Cancer biologists realized that
then stimulates the expression of proteins that interact with other gene duplication led to HER2 overexpression in 25 percent of breast
cellular components to initiate cell division. In addition, cancer patients and developed a drug called Herceptin
uncontrolled cell growth leads to cancer; mutations in the genes (trastuzumab), a monoclonal antibody that targets HER2 for removal
encoding protein components of signaling pathways are often found by the immune system. Herceptin therapy helps to control signaling
in tumor cells. through HER2. Its use, in combination with chemotherapy, has
helped to increase the overall survival rate of patients with
metastatic breast cancer.

KEY POINTS
Normally, cells do not divide unless they are stimulated by
signals from other cells.
Most growth factors, which promote cell growth, bind to cell-
surface receptors that are linked to tyrosine kinases.
MAP kinase stimulates the expression of proteins that interact
with other cellular components to initiate cell division.
Uncontrolled cell growth leads to cancer.

KEY TERMS
receptor: a protein on a cell wall that binds with specific
molecules so that they can be absorbed into the cell in order to
control certain functions
Figure 9.10.1: Uncontrolled Cell Growth: Colorectal cancer occurs
after numerous mutations to a normal cell. growth factor: a naturally-occurring substance capable of
stimulating cellular growth, proliferation, and cellular
CANCER BIOLOGISTS & UNCONTROLLED CELL differentiation
GROWTH oncogene: any gene that contributes to the conversion of a
Cancer biologists study the molecular origins of cancer with the goal normal cell into a cancerous cell when mutated or expressed at
of developing new prevention methods and treatment strategies that high levels
will inhibit the growth of tumors without harming the normal cells
of the body. Signaling pathways control cell growth. These pathways This page titled 9.10: Response to the Cellular Signal - Cell Signaling and
are controlled by signaling proteins, which are, in turn, expressed by Cell Growth is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.
genes. Mutations in these genes can result in malfunctioning

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9.11: RESPONSE TO THE CELLULAR SIGNAL - CELL SIGNALING AND CELL
DEATH

 LEARNING OBJECTIVES

Describe how apoptosis is initiated

APOPTOSIS
When a cell is damaged, superfluous, or potentially dangerous to an
organism, a cell can initiate a mechanism to trigger programmed cell
death, or apoptosis. Apoptosis allows a cell to die in a controlled
manner that prevents the release of potentially damaging molecules
from inside the cell.

INTERNAL SIGNALING
There are many internal checkpoints that monitor a cell’s health; if
abnormalities are observed, a cell can spontaneously initiate the
process of apoptosis. However, in some cases such as a viral
infection or uncontrolled cell division due to cancer, the cell’s
normal checks and balances fail.

EXTERNAL SIGNALING
External signaling can also initiate apoptosis. For example, most
normal animal cells have receptors that interact with the
extracellular matrix, a network of glycoproteins that provides
structural support for cells in an organism. The binding of cellular
receptors to the extracellular matrix initiates a signaling cascade
within the cell. However, if the cell moves away from the
extracellular matrix, the signaling ceases, and the cell undergoes
apoptosis. This system keeps cells from traveling through the body
and proliferating out of control, as happens with tumor cells that
metastasize. Figure 9.11.1: Apoptosis: The histological section of a foot of a 15-
Another example of external signaling that leads to apoptosis occurs day-old mouse embryo, visualized using light microscopy, reveals
areas of tissue between the toes which apoptosis will eliminate
in T-cell development. T-cells are immune cells that bind to foreign before the mouse reaches its full gestational age at 27 days.
macromolecules and particles, targeting them for destruction by the
immune system. Normally, T-cells do not target “self” proteins KEY POINTS
(those of their own organism), a process that can lead to Apoptosis allows a cell to die in a controlled manner by
autoimmune diseases. In order to develop the ability to discriminate preventing the release of damaging molecules from inside the
between self and non-self, immature T-cells undergo screening to cell.
determine whether they bind to so-called self proteins. If the T-cell Internal checkpoints to monitor a cell’s health exist; if
receptor binds to self proteins, the cell initiates apoptosis to remove abnormalities are observed, a cell can also spontaneously initiate
the potentially dangerous cell. the process of apoptosis.
In some cases, such as a viral infection or cancer, the cell’s
APOPTOSIS AND EMBRYOS normal checks and balances fail.
Apoptosis is also essential for normal embryological development. External signaling can also initiate apoptosis.
In vertebrates, for example, early stages of development include the Apoptosis is also essential for normal embryological
formation of web-like tissue between individual fingers and toes. development; unnecessary cells that appear during the early
During the course of normal development, these unnecessary cells stages of development will eventually be eliminated through cell
must be eliminated, enabling fully separated fingers and toes to signaling.
form. A cell signaling mechanism triggers apoptosis, which destroys
the cells between the developing digits. KEY TERMS
apoptosis: a process of programmed cell death
glycoprotein: a protein with covalently-bonded carbohydrates

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Cellular metabolism. Provided by: Wikipedia. Located at: Molecular Cancer | Full text | Role of APCand DNA mismatch repair genes in the
en.Wikipedia.org/wiki/Cellula...on_and_control. License: CC BY-SA: development of colorectal cancers. Provided by: Molecular Cancer Journal.
Attribution-ShareAlike Located at: http://www.molecular-cancer.com/content/2/1/41. License: CC
epinephrine. Provided by: Wiktionary. Located at: BY: Attribution
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cyclic adenosine monophosphate. Provided by: Wikipedia. Located at: http://cnx.org/content/m44453/latest...e_09_03_02.jpg. License: CC BY:
en.Wikipedia.org/wiki/cyclic%...0monophosphate. License: CC BY-SA: Attribution
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protein kinase A. Provided by: Wikipedia. Located at:
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This page titled 9.11: Response to the Cellular Signal - Cell Signaling and
Attribution-ShareAlike Cell Death is shared under a CC BY-SA 4.0 license and was authored,
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9.12: SIGNALING IN SINGLE-CELLED ORGANISMS - SIGNALING IN YEAST
species. Yeasts have 130 types of kinases. More complex organisms
 LEARNING OBJECTIVES such as nematode worms and fruit flies have 454 and 239 kinases,
respectively. Of the 130 kinase types in yeast, 97 belong to the 55
Describe how cell signaling occurs in single-celled
subfamilies of kinases that are found in other eukaryotic organisms.
organisms such as yeast
The only obvious deficiency seen in yeasts is the complete absence
of tyrosine kinases. It is hypothesized that phosphorylation of
SIGNALING IN YEAST tyrosine residues is needed to control the more sophisticated
Yeasts are single-celled eukaryotes; therefore, they have a nucleus functions of development, differentiation, and cellular
and organelles characteristic of more complex life forms. communication used in multicellular organisms.
Comparisons of the genomes of yeasts, nematode worms, fruit flies, Because yeasts contain many of the same classes of signaling
and humans illustrate the evolution of increasingly-complex proteins as humans, these organisms are ideal for studying signaling
signaling systems that allow for the efficient inner workings that cascades. Yeasts multiply quickly and are much simpler organisms
keep humans and other complex life forms functioning correctly. than humans or other multicellular animals. Therefore, the signaling
The components and processes found in yeast signals are similar to cascades are also simpler and easier to study, although they contain
those of cell-surface receptor signals in multicellular organisms. similar counterparts to human signaling
Budding yeasts are able to participate in a process that is similar to
sexual reproduction that entails two haploid cells combining to form KEY POINTS
a diploid cell. In order to find another haploid yeast cell that is Budding yeasts participate in a process that is similar to sexual
prepared to mate, budding yeasts secrete a signaling molecule called reproduction that entails two haploid cells combining to form a
mating factor. When mating factor binds to cell-surface receptors in diploid cell.
other yeast cells that are nearby, they stop their normal growth Budding yeasts secrete a signaling molecule called mating factor
cycles and initiate a cell signaling cascade that includes protein when trying to find another haploid yeast cell that is ready to
kinases and GTP-binding proteins that are similar to G-proteins. mate.
In yeast, a cell signaling cascade is initiated when a mating factor
binds to cell-surface receptors in other yeast cells.
A cell signaling cascade includes protein kinases and GTP-
binding proteins that are similar to G-proteins.
Yeasts have 130 types of kinases, but they do not contain
tyrosine kinases, which are utilized by multicellular organisms to
control complex forms of development and communication.

KEY TERMS
kinase: any of a group of enzymes that transfers phosphate
groups from high-energy donor molecules, such as ATP, to
specific target molecules (substrates); the process is termed
phosphorylation
GTP-binding protein: a protein which binds GTP and catalyzes
Figure 9.12.1: Budding Yeasts: Budding Saccharomyces cerevisiae its conversion to GDP
yeast cells can communicate by releasing a signaling molecule
called mating factor. In this micrograph, they are visualized using G protein: any of a class of proteins, found in cell membranes,
differential interference contrast microscopy, a light microscopy that pass signals between hormone receptors and effector
technique that enhances the contrast of the sample. enzymes
CELLULAR COMMUNICATION IN YEASTS This page titled 9.12: Signaling in Single-Celled Organisms - Signaling in
Kinases are a major component of cellular communication. Studies Yeast is shared under a CC BY-SA 4.0 license and was authored, remixed,
of these enzymes illustrate the evolutionary connectivity of different and/or curated by Boundless.

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9.13: SIGNALING IN SINGLE-CELLED ORGANISMS - SIGNALING IN BACTERIA

 LEARNING OBJECTIVES

Describe how cell signaling occurs in single-celled

organisms such as bacteria

SIGNALING IN BACTERIA
Signaling in bacteria, known as quorum sensing, enables bacteria to
monitor extracellular conditions, ensure sufficient amounts of
nutrients are present, and avoid hazardous situations. There are
circumstances, however, when bacteria communicate with each
other.
The first evidence of bacterial communication was observed in a
bacterium that has a symbiotic relationship with Hawaiian bobtail
squid. When the population density of the bacteria reached a certain
level, specific gene expression was initiated: the bacteria produced
bioluminescent proteins that emitted light. Because the number of
cells present in the environment (the cell density) is the determining
factor for signaling, bacterial signaling was named quorum sensing.
Interestingly, in politics and business, a quorum is the minimum
number of members required to be present to vote on an issue.
Quorum sensing uses autoinducers as signaling molecules.
Autoinducers are signaling molecules secreted by bacteria to
communicate with other bacteria of the same kind. The secreted
autoinducers can be small, hydrophobic molecules, such as acyl-
homoserine lactone (AHL), or larger peptide-based molecules. Each Figure 9.13.1: Autoinducers: Autoinducers are small molecules or
type of molecule has a different mode of action. When AHL enters proteins produced by bacteria that regulate gene expression.
target bacteria, it binds to transcription factors, which then switch Some species of bacteria that use quorum sensing form biofilms,
gene expression on or off. The peptide autoinducers stimulate more which are complex colonies of bacteria (often containing several
complicated signaling pathways that include bacterial kinases. The species) that exchange chemical signals to coordinate the release of
changes in bacteria following exposure to autoinducers can be quite toxins that attack the host. Bacterial biofilms can sometimes be
extensive. The pathogenic bacterium Pseudomonas aeruginosa has found on medical equipment. When biofilms invade implants, such
616 different genes that respond to autoinducers. as hip or knee replacements or heart pacemakers, they can cause
life-threatening infections.

Figure 9.13.1: Bacterial Biofilms: Cell-cell communication enables

these (a) Staphylococcus aureus bacteria to work together to form a
biofilm inside a hospital patient’s catheter, seen here via scanning
electron microscopy. S. aureus is the main cause of hospital-acquired
infections. (b) Hawaiian bobtail squid have a symbiotic relationship
with the bioluminescent bacteria Vibrio fischeri. The luminescence
makes it difficult to see the squid from below because it effectively
eliminates its shadow. In return for camouflage, the squid provides
food for the bacteria. Free-living V. fischeri do not produce
luciferase, the enzyme responsible for luminescence, but V. fischeri
living in a symbiotic relationship with the squid do. Quorum sensing
determines whether the bacteria should produce the luciferase
enzyme.

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Gene expression in bacteria is initiated when the population Located at: http://cnx.org/content/m44454/latest/?collection=col11448/latest.
License: CC BY: Attribution
density of the bacteria reaches a certain level. kinase. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/kinase.
Bacterial signaling is called quorum sensing because cell density License: CC BY-SA: Attribution-ShareAlike
G protein. Provided by: Wiktionary. Located at:
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Quorum sensing uses autoinducers, which are secreted by ShareAlike
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Provided by: OpenStax CNX. Located at:
signaling molecules. http://cnx.org/content/m44454/latest...e_09_04_01.jpg. License: CC BY:
Autoinducers may be small, hydrophobic molecules, or they can Attribution
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molecule has a different mode of action. License: CC BY: Attribution
autoinducer. Provided by: Wiktionary. Located at:
Some bacteria form biofilms, which are complex colonies of en.wiktionary.org/wiki/autoinducer. License: CC BY-SA: Attribution-
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biofilm. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/biofilm.
of toxins that attack the host. License: CC BY-SA: Attribution-ShareAlike
quorum sensing. Provided by: Wiktionary. Located at:
KEY TERMS en.wiktionary.org/wiki/quorum_sensing. License: CC BY-SA: Attribution-
ShareAlike
quorum sensing: a method of communication between bacterial OpenStax College, Signaling in Single-Celled Organisms. October 16, 2013.
cells by the release and sensing of small diffusible signal Provided by: OpenStax CNX. Located at:
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molecules Attribution
autoinducer: any of several compounds, synthesized by OpenStax College, Signaling in Single-Celled Organisms. October 16, 2013.
Provided by: OpenStax CNX. Located at:
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biofilm: a thin film of mucus created by and containing a colony Attribution
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 CHAPTER OVERVIEW

10: CELL REPRODUCTION

10.1: Cell Division
10.1A: The Role of the Cell Cycle
10.1B: Genomic DNA and Chromosomes
10.1C: Eukaryotic Chromosomal Structure and Compaction
10.2: The Cell Cycle
10.2A: Interphase
10.2B: The Mitotic Phase and the G0 Phase
10.3: Control of the Cell Cycle
10.3A: Regulation of the Cell Cycle by External Events
10.3B: Regulation of the Cell Cycle at Internal Checkpoints
10.3C: Regulator Molecules of the Cell Cycle
10.4: Cancer and the Cell Cycle
10.4A: Proto-oncogenes
10.4B: Tumor Suppressor Genes
10.5: Prokaryotic Cell Division
10.5A: Binary Fission

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1
SECTION OVERVIEW

10.1: CELL DIVISION

10.1B: GENOMIC DNA AND CHROMOSOMES
Topic hierarchy
10.1C: EUKARYOTIC CHROMOSOMAL
STRUCTURE AND COMPACTION
10.1A: THE ROLE OF THE CELL CYCLE

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and was authored, remixed, and/or curated by Boundless.

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10.1A: THE ROLE OF THE CELL CYCLE
replicated DNA and cytoplasmic contents are separated and the cell
 LEARNING OBJECTIVES divides.

Explain the role of the cell cycle in carrying out the cell’s
essential functions

INTRODUCTION: CELL DIVISION AND

REPRODUCTION
A human, as well as every sexually-reproducing organism, begins
life as a fertilized egg or zygote. Trillions of cell divisions
subsequently occur in a controlled manner to produce a complex,
multicellular human. In other words, that original single cell is the
ancestor of every other cell in the body. Once a being is fully grown,
cell reproduction is still necessary to repair or regenerate tissues. For
example, new blood and skin cells are constantly being produced.
All multicellular organisms use cell division for growth and the
maintenance and repair of cells and tissues. Cell division is tightly
regulated because the occasional failure of regulation can have life- Figure 10.1A. 1 : The Cell Cycle: The cell cycle consists of
interphase and the mitotic phase. During interphase, the cell grows
threatening consequences. Single-celled organisms use cell division and the nuclear DNA is duplicated. Interphase is followed by the
as their method of reproduction. mitotic phase. During the mitotic phase, the duplicated
chromosomes are segregated and distributed into daughter nuclei.
The cytoplasm is usually divided as well, resulting in two daughter
cells

KEY POINTS
All multicellular organisms use cell division for growth and the
Figure 10.1A. 1 : Cell Division and Growth: A sea urchin begins life maintenance and repair of cells and tissues.
as a single cell that (a) divides to form two cells, visible by scanning Single-celled organisms use cell division as their method of
electron microscopy. After four rounds of cell division, (b) there are reproduction.
16 cells, as seen in this SEM image. After many rounds of cell
division, the individual develops into a complex, multicellular Somatic cells divide regularly; all human cells (except for the
organism, as seen in this (c) mature sea urchin. cells that produce eggs and sperm) are somatic cells.
While there are a few cells in the body that do not undergo cell Somatic cells contain two copies of each of their chromosomes
division, most somatic cells divide regularly. A somatic cell is a (one copy from each parent).
general term for a body cell: all human cells, except for the cells that The cell cycle has two major phases: interphase and the mitotic
produce eggs and sperm (which are referred to as germ cells), are phase.
somatic cells. Somatic cells contain two copies of each of their During interphase, the cell grows and DNA is replicated; during
chromosomes (one copy received from each parent). Cells in the the mitotic phase, the replicated DNA and cytoplasmic contents
body replace themselves over the lifetime of a person. For example, are separated and the cell divides.
the cells lining the gastrointestinal tract must be frequently replaced
KEY TERMS
when constantly “worn off” by the movement of food through the
gut. But what triggers a cell to divide and how does it prepare for somatic cell: any normal body cell of an organism that is not
and complete cell division? involved in reproduction; a cell that is not on the germline
interphase: the stage in the life cycle of a cell where the cell
The cell cycle is an ordered series of events involving cell growth
grows and DNA is replicated
and cell division that produces two new daughter cells. Cells on the
mitotic phase: replicated DNA and the cytoplasmic material are
path to cell division proceed through a series of precisely timed and
divided into two identical cells
carefully regulated stages of growth, DNA replication, and division
that produces two identical (clone) cells. The cell cycle has two This page titled 10.1A: The Role of the Cell Cycle is shared under a CC BY-
major phases: interphase and the mitotic phase. During interphase, SA 4.0 license and was authored, remixed, and/or curated by Boundless.
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10.1B: GENOMIC DNA AND CHROMOSOMES

 LEARNING OBJECTIVES

Explain the importance of a genome to an organism

GENOMIC DNA
Before discussing the steps a cell must undertake to replicate, a
deeper understanding of the structure and function of a cell’s genetic
information is necessary. A cell’s DNA, packaged as a double-
stranded DNA molecule, is called its genome. In prokaryotes, the
genome is composed of a single, double-stranded DNA molecule in
the form of a loop or circle. The region in the cell containing this
genetic material is called a nucleoid. Some prokaryotes also have
smaller loops of DNA called plasmids that are not essential for Figure 10.1B. 1: Eukaryotic Genome: There are 23 pairs of
normal growth. Bacteria can exchange these plasmids with other homologous chromosomes in a female human somatic cell. The
bacteria, sometimes receiving beneficial new genes that the recipient condensed chromosomes are viewed within the nucleus (top),
removed from a cell in mitosis and spread out on a slide (right), and
can add to their chromosomal DNA. Antibiotic resistance is one trait artificially arranged according to length (left); an arrangement like
that often spreads through a bacterial colony through plasmid this is called a karyotype. In this image, the chromosomes were
exchange. exposed to fluorescent stains for differentiation of the different
chromosomes. A method of staining called “chromosome painting”
employs fluorescent dyes that highlight chromosomes in different
colors.
Matched pairs of chromosomes in a diploid organism are called
homologous (“same knowledge”) chromosomes. Homologous
chromosomes are the same length and have specific nucleotide
segments called genes in exactly the same location, or locus. Genes,
the functional units of chromosomes, determine specific
characteristics, or traits, by coding for specific proteins. For
example, hair color is a trait that can be blonde, brown, or black.
Each copy of a homologous pair of chromosomes originates from a
different parent; therefore, the genes themselves are not identical.
The variation of individuals within a species is due to the specific
combination of the genes inherited from both parents. Even a
Figure 10.1B. 1: Prokaryotic Genome: Prokaryotes, including
bacteria and archaea, have a single, circular chromosome located in slightly altered sequence of nucleotides within a gene can result in
a central region called the nucleoid. an alternative trait. For example, there are three possible gene
In eukaryotes, the genome consists of several double-stranded linear sequences on the human chromosome that code for blood type:
DNA molecules packaged into chromosomes. Each species of sequence A, sequence B, and sequence O. Because all diploid
eukaryotes has a characteristic number of chromosomes in the nuclei human cells have two copies of the chromosome that determines
of its cells. Human body cells have 46 chromosomes, while human blood type, the blood type (the trait) is determined by which two
gametes (sperm or eggs) have 23 chromosomes each. A typical body versions of the marker gene are inherited. It is possible to have two
cell, or somatic cell, contains two matched sets of chromosomes, a copies of the same gene sequence on both homologous
configuration known as diploid. The letter n is used to represent a chromosomes, with one on each (for example, AA, BB, or OO), or
single set of chromosomes; therefore, a diploid organism is two different sequences, such as AB, AO, or BO.
designated 2n. Human cells that contain one set of chromosomes are Minor variations of traits, such as blood type, eye color, and
called gametes, or sex cells; these are eggs and sperm, and are handedness, contribute to the natural variation found within a
designated 1n, or haploid. species. However, if the entire DNA sequence from any pair of
human homologous chromosomes is compared, the difference is less
than one percent. The sex chromosomes, X and Y, are the single
exception to the rule of homologous chromosome uniformity. Other
than a small amount of homology that is necessary to accurately
produce gametes, the genes found on the X and Y chromosomes are
different.

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KEY POINTS chromosomes, X and Y, are the single exception to this rule since
A cell ‘s DNA, packaged as a double-stranded DNA molecule, is their genes are different.
called its genome.
KEY TERMS
In prokaryotes, the genome is composed of a single, double-
stranded DNA molecule in the form of a loop or circle; the genome: the cell’s complete genetic information packaged as a
region in the cell containing this genetic material is called a double-stranded DNA molecule
nucleoid. nucleoid: the irregularly-shaped region within a prokaryote cell
In eukaryotes, the genome consists of several double-stranded where the genetic material is localized
linear DNA molecules; each species of eukaryotes has a gene: a unit of heredity; the functional units of chromosomes that
characteristic number of chromosomes in the nuclei of its cells. determine specific characteristics by coding for specific proteins
Matched pairs of chromosomes in a diploid organism are called chromosome: a structure in the cell nucleus that contains DNA,
homologous chromosomes, which are the same length and have histone protein, and other structural proteins
specific nucleotide segments called genes in exactly the same locus: a fixed position on a chromosome that may be occupied
location, or locus. by one or more genes
Each copy of a homologous pair of chromosomes originates
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10.1C: EUKARYOTIC CHROMOSOMAL STRUCTURE AND COMPACTION

 LEARNING OBJECTIVES

Describe the levels of chromsomal structure and compaction

EUKARYOTIC CHROMOSOMAL STRUCTURE AND

COMPACTION
If the DNA from all 46 chromosomes in a human cell nucleus was
laid out end to end, it would measure approximately two meters.
However, the diameter would be only 2 nm. Considering that the
size of a typical human cell is about 10 µm (100,000 cells lined up
to equal one meter), DNA must be tightly packaged to fit in the
cell’s nucleus. At the same time, it must also be readily accessible
for the genes to be expressed. During some stages of the cell cycle,
the long strands of DNA are condensed into compact chromosomes.
There are a number of ways that chromosomes are compacted to fit
in the cell’s nucleus and be accessible for gene expression.
In the first level of compaction, short stretches of the DNA double
helix wrap around a core of eight histone proteins at regular intervals
along the entire length of the chromosome. The DNA-histone
complex is called chromatin. The beadlike, histone DNA complex is
called a nucleosome. DNA connecting the nucleosomes is called
linker DNA. A DNA molecule in this form is about seven times
shorter than the double helix without the histones. The beads are
about 10 nm in diameter, in contrast with the 2-nm diameter of a
DNA double helix. The next level of compaction occurs as the
nucleosomes and the linker DNA between them are coiled into a 30-
nm chromatin fiber. This coiling further shortens the chromosome so
that it is now about 50 times shorter than the extended form. In the Figure 10.1C. 1 : Levels of DNA Compaction: Double-stranded
third level of packing, a variety of fibrous proteins is used to pack DNA wraps around histone proteins to form nucleosomes that have
the chromatin. These fibrous proteins also ensure that each the appearance of “beads on a string.” The nucleosomes are coiled
into a 30-nm chromatin fiber. When a cell undergoes mitosis, the
chromosome in a non-dividing cell occupies a particular area of the chromosomes condense even further.
nucleus that does not overlap with that of any other chromosome.
DNA replicates in the S phase of interphase. After replication, the
chromosomes are composed of two linked sister chromatids. When
fully compact, the pairs of identically-packed chromosomes are
bound to each other by cohesin proteins. The connection between
the sister chromatids is closest in a region called the centromere. The
conjoined sister chromatids, with a diameter of about 1 µm, are
visible under a light microscope. The centromeric region is highly
condensed and will appear as a constricted area.

KEY POINTS
During some stages of the cell cycle, the long strands of DNA
are condensed into compact chromosomes to fit in the cell’s
nucleus.
In the first level of compaction, short stretches of the DNA
double helix wrap around a core of eight histone proteins at
regular intervals along the entire length of the chromosome.
The DNA surrouding the histone core is called a nucleosome; the
DNA-histone complex is called chromatin.
The second level of compaction occurs as the nucleosomes and
the linker DNA between them are coiled into a 30-nm chromatin

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gene. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/gene.
region called the centromere; this region is highly condensed. License: CC BY-SA: Attribution-ShareAlike
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chromatin: a complex of DNA, RNA, and proteins within the
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CONTRIBUTIONS AND ATTRIBUTIONS nucleosome. Provided by: Wiktionary. Located at:
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interphase. Provided by: Wiktionary. Located at: CNX. Located at: http://cnx.org/content/m44457/latest...0_00_02abc.jpg.
en.wiktionary.org/wiki/interphase. License: CC BY-SA: Attribution- License: CC BY: Attribution
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en.Wikipedia.org/wiki/mitotic%20phase. License: CC BY-SA: Attribution- License: CC BY: Attribution
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somatic cell. Provided by: Wiktionary. Located at: CNX. Located at: http://cnx.org/content/m44459/latest...e_10_01_02.jpg.
en.wiktionary.org/wiki/somatic_cell. License: CC BY-SA: Attribution- License: CC BY: Attribution
ShareAlike OpenStax College, Cell Division October 16, 2013. Provided by: OpenStax
OpenStax College, Biology. November 5, 2013. Provided by: OpenStax CNX. CNX. Located at: http://cnx.org/content/m44459/latest...e_10_01_03.jpg.
Located at: http://cnx.org/content/m44460/latest...ol11448/latest. License: CC License: CC BY: Attribution
BY: Attribution
OpenStax College, Introduction. October 16, 2013. Provided by: OpenStax This page titled 10.1C: Eukaryotic Chromosomal Structure and Compaction
CNX. Located at: http://cnx.org/content/m44457/latest...0_00_02abc.jpg. is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
License: CC BY: Attribution
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SECTION OVERVIEW

10.2: THE CELL CYCLE

10.2B: THE MITOTIC PHASE AND THE G0 PHASE
Topic hierarchy
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10.2A: INTERPHASE
interphase, nuclear DNA remains in a semi-condensed chromatin
 LEARNING OBJECTIVES configuration. In the S phase, DNA replication results in the
formation of identical pairs of DNA molecules, sister chromatids,
Describe the events that occur during Interphase
that are firmly attached to the centromeric region. The centrosome is
duplicated during the S phase. The two centrosomes will give rise to
INTERPHASE the mitotic spindle, the apparatus that orchestrates the movement of
During interphase, the cell undergoes normal growth processes chromosomes during mitosis. At the center of each animal cell, the
while also preparing for cell division. In order for a cell to move centrosomes of animal cells are associated with a pair of rod-like
from interphase into the mitotic phase, many internal and external objects, the centrioles, which are at right angles to each other.
conditions must be met. The three stages of interphase are called G1, Centrioles help organize cell division. Centrioles are not present in
S, and G2 . the centrosomes of other eukaryotic species, such as plants and most
fungi.

G2 PHASE (SECOND GAP)

In the G2 phase, the cell replenishes its energy stores and synthesizes
proteins necessary for chromosome manipulation. Some cell
organelles are duplicated, and the cytoskeleton is dismantled to
provide resources for the mitotic phase. There may be additional cell
growth during G2. The final preparations for the mitotic phase must
be completed before the cell is able to enter the first stage of mitosis.

KEY POINTS
There are three stages of interphase: G1 (first gap), S (synthesis
of new DNA ), and G2 (second gap).
Cells spend most of their lives in interphase, specifically in the S
phase where genetic material must be copied.
Figure 10.2A. 1 : The Stages of Interphase and the Cell Cycle: The The cell grows and carries out biochemical functions, such as
cell cycle consists of interphase and the mitotic phase. During
interphase, the cell grows and the nuclear DNA is duplicated.
protein synthesis, in the G1 phase.
Interphase is followed by the mitotic phase. During the mitotic During the S phase, DNA is duplicated into two sister
phase, the duplicated chromosomes are segregated and distributed chromatids, and centrosomes, which give rise to the mitotic
into daughter nuclei. The cytoplasm is usually divided as well,
resulting in two daughter cells.
spindle, are also replicated.
In the G2 phase, energy is replenished, new proteins are
G1 PHASE (FIRST GAP) synthesized, the cytoskeleton is dismantled, and additional
The first stage of interphase is called the G1 phase (first gap) growth occurs.
because, from a microscopic aspect, little change is visible.
KEY TERMS
However, during the G1 stage, the cell is quite active at the
biochemical level. The cell grows and accumulates the building interphase: the stage in the life cycle of a cell where the cell
blocks of chromosomal DNA and the associated proteins as well as grows and DNA is replicated
sufficient energy reserves to complete the task of replicating each sister chromatid: either of the two identical strands of a
chromosome in the nucleus. chromosome (DNA material) that separate during mitosis
mitotic spindle: the apparatus that orchestrates the movement of
S PHASE (SYNTHESIS OF DNA) chromosomes during mitosis
The synthesis phase of interphase takes the longest because of the
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complexity of the genetic material being duplicated. Throughout
and was authored, remixed, and/or curated by Boundless.

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10.2B: THE MITOTIC PHASE AND THE G0 PHASE
more microtubules assemble and stretch across the length of the
 LEARNING OBJECTIVES former nuclear area. Chromosomes become more condensed and
discrete. Each sister chromatid develops a protein structure called a
Describe the events that occur at the different stages of
kinetochore in the centromeric region. The proteins of the
mitosis
kinetochore attract and bind mitotic spindle microtubules.

THE MITOTIC PHASE

The mitotic phase is a multistep process during which the duplicated
chromosomes are aligned, separated, and move into two new,
identical daughter cells. The first portion of the mitotic phase is
called karyokinesis or nuclear division. The second portion of the
mitotic phase, called cytokinesis, is the physical separation of the
cytoplasmic components into the two daughter cells.

KARYOKINESIS (MITOSIS)
Karyokinesis, also known as mitosis, is divided into a series of
phases (prophase, prometaphase, metaphase, anaphase, and Figure 10.2B. 1: Kinetochore and Mitotic Spindle: During
prometaphase, mitotic spindle microtubules from opposite poles
telophase) that result in the division of the cell nucleus. attach to each sister chromatid at the kinetochore. In anaphase, the
connection between the sister chromatids breaks down and the
microtubules pull the chromosomes toward opposite poles.
During metaphase, the “change phase,” all the chromosomes are
aligned on a plane called the metaphase plate, or the equatorial
plane, midway between the two poles of the cell. The sister
chromatids are still tightly attached to each other by cohesin
proteins. At this time, the chromosomes are maximally condensed.
During anaphase, the “upward phase,” the cohesin proteins degrade,
and the sister chromatids separate at the centromere. Each
chromatid, now called a chromosome, is pulled rapidly toward the
centrosome to which its microtubule is attached. The cell becomes
visibly elongated (oval shaped) as the polar microtubules slide
against each other at the metaphase plate where they overlap.
During telophase, the “distance phase,” the chromosomes reach the
opposite poles and begin to decondense (unravel), relaxing into a
chromatin configuration. The mitotic spindles are depolymerized
Figure 10.2B. 1: Stages of the Cell Cycle: Karyokinesis (or mitosis) into tubulin monomers that will be used to assemble cytoskeletal
is divided into five stages: prophase, prometaphase, metaphase, components for each daughter cell. Nuclear envelopes form around
anaphase, and telophase. The images at the bottom were taken by the chromosomes and nucleosomes appear within the nuclear area.
fluorescence microscopy (hence, the black background) of cells
artificially stained by fluorescent dyes: blue fluorescence indicates
DNA (chromosomes) and green fluorescence indicates microtubules CYTOKINESIS
(spindle apparatus). Cytokinesis, or “cell motion,” is the second main stage of the mitotic
During prophase, the “first phase,” the nuclear envelope starts to phase during which cell division is completed via the physical
dissociate into small vesicles. The membranous organelles (such as separation of the cytoplasmic components into two daughter cells.
the Golgi apparatus and endoplasmic reticulum) fragment and Division is not complete until the cell components have been
disperse toward the periphery of the cell. The nucleolus disappears apportioned and completely separated into the two daughter cells.
and the centrosomes begin to move to opposite poles of the cell. Although the stages of mitosis are similar for most eukaryotes, the
Microtubules that will eventually form the mitotic spindle extend process of cytokinesis is quite different for eukaryotes that have cell
between the centrosomes, pushing them farther apart as the walls, such as plant cells.
microtubule fibers lengthen. The sister chromatids begin to coil In cells such as animal cells, which lack cell walls, cytokinesis
more tightly with the aid of condensin proteins and become visible
follows the onset of anaphase. A contractile ring composed of actin
under a light microscope. filaments forms just inside the plasma membrane at the former
During prometaphase, the “first change phase,” many processes that metaphase plate. The actin filaments pull the equator of the cell
began in prophase continue to advance. The remnants of the nuclear inward, forming a fissure. This fissure or “crack” is called the
envelope fragment. The mitotic spindle continues to develop as

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cleavage furrow. The furrow deepens as the actin ring contracts; During metaphase, the sister chromatids align along the equator
eventually the membrane is cleaved in two. of the cell by attaching their centromeres to the spindle fibers.
During anaphase, sister chromatids are separated at the
centromere and are pulled towards opposite poles of the cell by
the mitotic spindle.
During telophase, chromosomes arrive at opposite poles and
unwind into thin strands of DNA, the spindle fibers disappear,
and the nuclear membrane reappears.
Cytokinesis is the actual splitting of the cell membrane; animal
cells pinch apart, while plant cells form a cell plate that becomes
the new cell wall.
Cells enter the G0 (inactive) phase after they exit the cell cycle
when they are not actively preparing to divide; some cells remain
in G0 phase permanently.

KEY TERMS
karyokinesis: (mitosis) the first portion of mitotic phase in
which division of the cell nucleus takes place
centrosome: an organelle near the nucleus in the cytoplasm of
most organisms that controls the organization of its microtubules
and gives rise to the mitotic spindle
cytokinesis: the second portion of the mitotic phase in which the
Figure 10.2B. 1: Cytokinesis: During cytokinesis in animal cells, a cytoplasm of a cell divides following the division of the nucleus
ring of actin filaments forms at the metaphase plate. The ring
contracts, forming a cleavage furrow, which divides the cell in two. CONTRIBUTIONS AND ATTRIBUTIONS
In plant cells, Golgi vesicles coalesce at the former metaphase plate, OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
forming a phragmoplast. A cell plate formed by the fusion of the Located at: http://cnx.org/content/m44460/latest...ol11448/latest. License: CC
vesicles of the phragmoplast grows from the center toward the cell BY: Attribution
walls and the membranes of the vesicles fuse to form a plasma Boundless. Provided by: Boundless Learning. Located at:
membrane that divides the cell in two. www.boundless.com//biology/de...itotic-spindle. License: CC BY-SA:
Attribution-ShareAlike
In plant cells, a new cell wall must form between the daughter cells. interphase. Provided by: Wiktionary. Located at:
During interphase, the Golgi apparatus accumulates enzymes, en.wiktionary.org/wiki/interphase. License: CC BY-SA: Attribution-
ShareAlike
structural proteins, and glucose molecules prior to breaking into sister chromatid. Provided by: Wiktionary. Located at:
vesicles and dispersing throughout the dividing cell. During en.wiktionary.org/wiki/sister_chromatid. License: CC BY-SA: Attribution-
telophase, these Golgi vesicles are transported on microtubules to ShareAlike
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form a phragmoplast (a vesicular structure) at the metaphase plate. Located at: http://cnx.org/content/m44460/latest...ol11448/latest. License: CC
There, the vesicles fuse and coalesce from the center toward the cell BY: Attribution
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
walls; this structure is called a cell plate. As more vesicles fuse, the Located at: http://cnx.org/content/m44460/latest...ol11448/latest. License: CC
cell plate enlarges until it merges with the cell walls at the periphery BY: Attribution
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of the cell. Enzymes use the glucose that has accumulated between Located at: http://cnx.org/content/m44460/latest...ol11448/latest. License: CC
the membrane layers to build a new cell wall. The Golgi membranes BY: Attribution
karyokinesis. Provided by: Wiktionary. Located at:
become parts of the plasma membrane on either side of the new cell en.wiktionary.org/wiki/karyokinesis. License: CC BY-SA: Attribution-
wall. ShareAlike
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www.boundless.com//biology/de...on/cytokinesis. License: CC BY-SA:
G0 PHASE Attribution-ShareAlike
Not all cells adhere to the classic cell cycle pattern in which a centrosome. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/centrosome. License: CC BY-SA: Attribution-
newly-formed daughter cell immediately enters the preparatory ShareAlike
phases of interphase, closely followed by the mitotic phase. Cells in OpenStax College, Biology. November 4, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44460/latest...ol11448/latest. License: CC
G0 phase are not actively preparing to divide. The cell is in a BY: Attribution
quiescent (inactive) stage that occurs when cells exit the cell cycle. OpenStax College, The Cell Cycle. October 16, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44460/latest...e_10_02_02.png.
Some cells enter G0 temporarily until an external signal triggers the License: CC BY: Attribution
onset of G1. Other cells that never or rarely divide, such as mature OpenStax College, The Cell Cycle. October 16, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44460/latest...e_10_02_03.jpg.
cardiac muscle and nerve cells, remain in G0 permanently.
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KEY POINTS CNX. Located at: http://cnx.org/content/m44460/latest...e_10_02_04.jpg.
License: CC BY: Attribution
During prophase, the nucleus disappears, spindle fibers form,
and DNA condenses into chromosomes ( sister chromatids ).

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This page titled 10.2B: The Mitotic Phase and the G0 Phase is shared under Boundless.
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SECTION OVERVIEW

10.3: CONTROL OF THE CELL CYCLE

10.3B: REGULATION OF THE CELL CYCLE AT
Topic hierarchy INTERNAL CHECKPOINTS

10.3C: REGULATOR MOLECULES OF THE CELL

10.3A: REGULATION OF THE CELL CYCLE BY
CYCLE
EXTERNAL EVENTS

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10.3A: REGULATION OF THE CELL CYCLE BY EXTERNAL EVENTS
Whatever the source of the message, the cell receives the signal, and
 LEARNING OBJECTIVES a series of events within the cell allows it to proceed into interphase.
Moving forward from this initiation point, every parameter required
Describe external events that can affect cell cycle regulation
during each cell cycle phase must be met or the cycle cannot
progress.
REGULATION OF THE CELL CYCLE BY
EXTERNAL EVENTS KEY POINTS
Unlike the life of organisms, which is a straight progression from The death of nearby cells and the presence or absence of certain
birth to death, the life of a cell takes place in a cyclical pattern. Each hormones can impact the cell cycle.
cell is produced as part of its parent cell. When a daughter cell The release of growth-promoting hormones, such as HGH, can
divides, it turns into two new cells, which would lead to the initiate cell division, and a lack of these hormones can inhibit
assumption that each cell is capable of being immortal as long as its cell division.
descendants can continue to divide. However, all cells in the body Cell growth initiates cell division because cells must divide as
only live as long as the organism lives. Some cells do live longer the surface-to-volume ratio decreases; cell crowding inhibits cell
than others, but eventually all cells die when their vital functions division.
cease. Most cells in the body exist in the state of interphase, the non- Key conditions must be met before the cell can move into
dividing stage of the cell life cycle. When this stage ends, cells move interphase.
into the dividing part of their lives called mitosis.
KEY TERMS
Both the initiation and inhibition of cell division are triggered by
events external to the cell when it is about to begin the replication gigantism: a condition caused by an over-production of growth
process. An event may be as simple as the death of a nearby cell or hormone, resulting in excessive bone growth
as sweeping as the release of growth-promoting hormones, such as growth hormone: any polypeptide hormone secreted by the
human growth hormone (HGH). A lack of HGH can inhibit cell pituitary gland that promotes growth and regulates the
division, resulting in dwarfism, whereas too much HGH can result in metabolism of carbohydrates, proteins, and lipids
gigantism. Crowding of cells can also inhibit cell division. Another dwarfism: a condition caused by a lack of growth hormone,
factor that can initiate cell division is the size of the cell; as a cell resulting in short stature and limbs that are disproportionately
grows, it becomes inefficient due to its decreasing surface-to-volume small in relation to the body
ratio. The solution to this problem is to divide.
This page titled 10.3A: Regulation of the Cell Cycle by External Events is
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

Figure 10.3A. 1 : Dwarfism: Commodore Nut (right) was a famous

circus performer afflicted with dwarfism. This was a result of a lack
of Human Growth Hormone.

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10.3B: REGULATION OF THE CELL CYCLE AT INTERNAL CHECKPOINTS
THE G2 CHECKPOINT
 LEARNING OBJECTIVES The G2 checkpoint bars entry into the mitotic phase if certain
Explain the effects of internal checkpoints on the regulation conditions are not met. As with the G1 checkpoint, cell size and
of the cell cycle protein reserves are assessed. However, the most important role of
the G2 checkpoint is to ensure that all of the chromosomes have
been accurately replicated without mistakes or damage. If the
REGULATION AT INTERNAL CHECKPOINTS
checkpoint mechanisms detect problems with the DNA, the cell
It is essential that the daughter cells are exact duplicates of the cycle is halted and the cell attempts to either complete DNA
parent cell. Mistakes in the duplication or distribution of the replication or repair the damaged DNA. If the DNA has been
chromosomes lead to mutations that may be passed forward to every correctly replicated, cyclin dependent kinases (CDKs) signal the
new cell produced from an abnormal cell. To prevent a compromised beginning of mitotic cell division.
cell from continuing to divide, internal control mechanisms operate
at three main cell cycle checkpoints. A checkpoint is one of several THE M CHECKPOINT
points in the eukaryotic cell cycle at which the progression of a cell The M checkpoint occurs near the end of the metaphase stage of
to the next stage in the cycle can be halted until conditions are mitosis. The M checkpoint is also known as the spindle checkpoint
favorable (e.g. the DNA is repaired). These checkpoints occur near because it determines whether all the sister chromatids are correctly
the end of G1, at the G2/M transition, and during metaphase. attached to the spindle microtubules. Because the separation of the
sister chromatids during anaphase is an irreversible step, the cycle
will not proceed until the kinetochores of each pair of sister
chromatids are firmly anchored to at least two spindle fibers arising
from opposite poles of the cell.

KEY POINTS
A checkpoint is one of several points in the eukaryotic cell cycle
at which the progression of a cell to the next stage in the cycle
can be halted until conditions are favorable.
Damage to DNA and other external factors are evaluated at the
G1 checkpoint; if conditions are inadequate, the cell will not be
allowed to continue to the S phase of interphase.
The G2 checkpoint ensures all of the chromosomes have been
Figure 10.3B. 1: Internal Checkpoints During the Cell Cycle: The replicated and that the replicated DNA is not damaged before
cell cycle is controlled at three checkpoints. The integrity of the cell enters mitosis.
DNA is assessed at the G1 checkpoint. Proper chromosome
duplication is assessed at the G2 checkpoint. Attachment of each The M checkpoint determines whether all the sister chromatids
kinetochore to a spindle fiber is assessed at the M checkpoint. are correctly attached to the spindle microtubules before the cell
enters the irreversible anaphase stage.
THE G1 CHECKPOINT
The G1 checkpoint determines whether all conditions are favorable KEY TERMS
for cell division to proceed. The G1 checkpoint, also called the restriction point: (G1 checkpoint) a point in the animal cell
restriction point (in yeast), is a point at which the cell irreversibly cycle at which the cell becomes “committed” to the cell cycle,
commits to the cell division process. External influences, such as which is determined by external factors and signals
growth factors, play a large role in carrying the cell past the G1 spindle checkpoint: (M checkpoint) prevents separation of the
checkpoint. The cell will only pass the checkpoint if it is an duplicated chromosomes until each chromosome is properly
appropriate size and has adequate energy reserves. At this point, the attached to the spindle apparatus
cell also checks for DNA damage. A cell that does not meet all the cyclin: any of a group of proteins that regulates the cell cycle by
requirements will not progress to the S phase. The cell can halt the forming a complex with kinases
cycle and attempt to remedy the problematic condition, or the cell G2 checkpoint: ensures all of the chromosomes have been
can advance into G0 (inactive) phase and await further signals when replicated and that the replicated DNA is not damaged
conditions improve.
This page titled 10.3B: Regulation of the Cell Cycle at Internal Checkpoints
If a cell meets the requirements for the G1 checkpoint, the cell will
is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
enter S phase and begin DNA replication. This transition, as with all
curated by Boundless.
of the major checkpoint transitions in the cell cycle, is signaled by
cyclins and cyclin dependent kinases (CDKs). Cyclins are cell-
signaling molecules that regulate the cell cycle.

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10.3C: REGULATOR MOLECULES OF THE CELL CYCLE
different cyclins and Cdks bind at specific points in the cell cycle
 LEARNING OBJECTIVES and thus regulate different checkpoints.

Differentiate among the molecules that regulate the cell

cycle

REGULATOR MOLECULES OF THE CELL CYCLE

In addition to the internally controlled checkpoints, there are two

groups of intracellular molecules that regulate the cell cycle. These
regulatory molecules either promote progress of the cell to the next
phase (positive regulation) or halt the cycle (negative regulation).
Regulator molecules may act individually or they can influence the
activity or production of other regulatory proteins. Therefore, the
failure of a single regulator may have almost no effect on the cell
cycle, especially if more than one mechanism controls the same
event. Conversely, the effect of a deficient or non-functioning
regulator can be wide-ranging and possibly fatal to the cell if
multiple processes are affected.

POSITIVE REGULATION OF THE CELL CYCLE

Two groups of proteins, called cyclins and cyclin-dependent kinases
(Cdks), are responsible for the progress of the cell through the
various checkpoints. The levels of the four cyclin proteins fluctuate
throughout the cell cycle in a predictable pattern. Increases in the
concentration of cyclin proteins are triggered by both external and
internal signals. After the cell moves to the next stage of the cell
cycle, the cyclins that were active in the previous stage are degraded.

Figure 10.3C. 1 : Activation of Cdks: Cyclin-dependent kinases

(Cdks) are protein kinases that, when fully activated, can
Figure 10.3C. 1 : Cyclin Concentrations at Checkpoints: The
phosphorylate and activate other proteins that advance the cell cycle
concentrations of cyclin proteins change throughout the cell cycle.
past a checkpoint. To become fully activated, a Cdk must bind to a
There is a direct correlation between cyclin accumulation and the
cyclin protein and then be phosphorylated by another kinase.
three major cell cycle checkpoints. Also, note the sharp decline of
cyclin levels following each checkpoint (the transition between Although the cyclins are the main regulatory molecules that
phases of the cell cycle) as cyclin is degraded by cytoplasmic determine the forward momentum of the cell cycle, there are several
enzymes.
other mechanisms that fine tune the progress of the cycle with
Cyclins regulate the cell cycle only when they are tightly bound to negative, rather than positive, effects. These mechanisms essentially
Cdks. To be fully active, the Cdk/cyclin complex must also be block the progression of the cell cycle until problematic conditions
phosphorylated in specific locations. Like all kinases, Cdks are are resolved. Molecules that prevent the full activation of Cdks are
enzymes (kinases) that phosphorylate other proteins. called Cdk inhibitors. Many of these inhibitor molecules directly or
Phosphorylation activates the protein by changing its shape. The indirectly monitor a particular cell cycle event. The block placed on
proteins phosphorylated by Cdks are involved in advancing the cell Cdks by inhibitor molecules will not be removed until the specific
to the next phase.. The levels of Cdk proteins are relatively stable event being monitored is completed.
throughout the cell cycle; however, the concentrations of cyclin
fluctuate and determine when Cdk/cyclin complexes form. The

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NEGATIVE REGULATION OF THE CELL CYCLE KEY POINTS
Two groups of proteins, cyclins and cyclin-dependent kinases
The second group of cell cycle regulatory molecules are negative (Cdks), are responsible for promoting the cell cycle.
regulators. Negative regulators halt the cell cycle. Remember that in Cyclins regulate the cell cycle only when they are bound to
positive regulation, active molecules cause the cycle to progress. Cdks; to be fully active, the Cdk/cyclin complex must be
The best understood negative regulatory molecules are phosphorylated, which allows it to phosphorylate other proteins
retinoblastoma protein (Rb), p53, and p21. Retinoblastoma proteins that advance the cell cycle.
are a group of tumor-suppressor proteins common in many cells. Negative regulator molecules (Rb, p53, and p21) act primarily at
Much of what is known about cell cycle regulation comes from the G1 checkpoint and prevent the cell from moving forward to
research conducted with cells that have lost regulatory control. All division until damaged DNA is repaired.
three of these regulatory proteins were discovered to be damaged or p53 halts the cell cycle and recruits enzymes to repair damaged
non-functional in cells that had begun to replicate uncontrollably DNA; if DNA cannot be repaired, p53 triggers apoptosis to
(became cancerous). In each case, the main cause of the unchecked prevent duplication.
progress through the cell cycle was a faulty copy of the regulatory Production of p21 is triggered by p53; p21 halts the cycle by
protein. binding to and inhibiting the activity of the Cdk/cyclin complex.
Dephosphorylated Rb binds to E2F, which halts the cell cycle;
Rb, p53, and p21 act primarily at the G1 checkpoint. p53 is a multi-
when the cell grows, Rb is phosphorylated and releases E2F,
functional protein that has a major impact on the cell’s commitment
which advances the cell cycle.
to division; it acts when there is damaged DNA in cells that are
undergoing the preparatory processes during G1. If damaged DNA is KEY TERMS
detected, p53 halts the cell cycle and recruits enzymes to repair the
cyclin: any of a group of proteins that regulates the cell cycle by
DNA. If the DNA cannot be repaired, p53 can trigger apoptosis (cell
forming a complex with kinases
suicide) to prevent the duplication of damaged chromosomes. As
cyclin-dependent kinase: (CDK) a member of a family of
p53 levels rise, the production of p21 is triggered. p21 enforces the
protein kinases first discovered for its role in regulating the cell
halt in the cycle dictated by p53 by binding to and inhibiting the
cycle through phosphorylation
activity of the Cdk/cyclin complexes. As a cell is exposed to more
retinoblastoma protein: (Rb) a group of tumor-suppressor
stress, higher levels of p53 and p21 accumulate, making it less likely
proteins that regulates the cell cycle by monitoring cell size
that the cell will move into the S phase.
Rb exerts its regulatory influence on other positive regulator CONTRIBUTIONS AND ATTRIBUTIONS
proteins. Rb monitors cell size. In the active, dephosphorylated state, 03. Cell Physiology. Provided by: ptahumanphysiology Wikispace. Located at:
http://ptahumanphysiology.wikispaces...ell+Physiology. License: CC BY-SA:
Rb binds to proteins called transcription factors, most commonly to Attribution-ShareAlike
E2F. Transcription factors “turn on” specific genes, allowing the OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
production of proteins encoded by that gene. When Rb is bound to Located at: http://cnx.org/content/m44466/latest...ol11448/latest. License: CC
BY: Attribution
E2F, production of proteins necessary for the G1/S transition is gigantism. Provided by: Wiktionary. Located at:
blocked. As the cell increases in size, Rb is slowly phosphorylated en.wiktionary.org/wiki/gigantism. License: CC BY-SA: Attribution-ShareAlike
Boundless. Provided by: Boundless Learning. Located at:
until it becomes inactivated. Rb releases E2F, which can now turn on www.boundless.com//biology/definition/dwarfism. License: CC BY-SA:
the gene that produces the transition protein and this particular block Attribution-ShareAlike
growth hormone. Provided by: Wiktionary. Located at:
is removed. For the cell to move past each of the checkpoints, all en.wiktionary.org/wiki/growth_hormone. License: CC BY-SA: Attribution-
positive regulators must be “turned on” and all negative regulators ShareAlike
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must be “turned off.” http://upload.wikimedia.org/wikipedi...odore_Nutt.jpg. License: CC BY-SA:
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Located at: http://cnx.org/content/m44466/latest...ol11448/latest. License: CC
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S phase. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/S_phase.
License: CC BY-SA: Attribution-ShareAlike
spindle checkpoint. Provided by: Wikipedia. Located at:
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restriction point. Provided by: Wikipedia. Located at:
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Boundless. Provided by: Boundless Learning. Located at:
www.boundless.com//biology/de.../g2-checkpoint. License: CC BY-SA:
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cyclin. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/cyclin.
License: CC BY-SA: Attribution-ShareAlike
Provided by: Wikimedia. Located at:
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Figure 10.3C. 1 : Function of the Rb Regulator Molecule: Rb halts OpenStax College, Control of the Cell Cycle. October 16, 2013. Provided by:
the cell cycle by binding E2F. Rb releases its hold on E2F in OpenStax CNX. Located at:
response to cell growth to advance the cell cycle.

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http://cnx.org/content/m44466/latest...e_10_03_01.jpg. License: CC BY: OpenStax College, Control of the Cell Cycle. October 16, 2013. Provided by:
Attribution OpenStax CNX. Located at:
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Located at: http://cnx.org/content/m44466/latest...ol11448/latest. License: CC Attribution
BY: Attribution OpenStax College, Control of the Cell Cycle. October 16, 2013. Provided by:
cyclin-dependent kinase. Provided by: Wikipedia. Located at: OpenStax CNX. Located at:
en.Wikipedia.org/wiki/cyclin-...ndent%20kinase. License: CC BY-SA: http://cnx.org/content/m44466/latest...e_10_03_02.jpg. License: CC BY:
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Boundless. Provided by: Boundless Learning. Located at: OpenStax College, Control of the Cell Cycle. October 16, 2013. Provided by:
www.boundless.com//biology/de...astoma-protein. License: CC BY-SA: OpenStax CNX. Located at:
Attribution-ShareAlike http://cnx.org/content/m44466/latest...03_03_prev.jpg. License: CC BY:
cyclin. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/cyclin. Attribution
License: CC BY-SA: Attribution-ShareAlike
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SECTION OVERVIEW

10.4: CANCER AND THE CELL CYCLE

10.4B: TUMOR SUPPRESSOR GENES
Topic hierarchy
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10.4A: PROTO-ONCOGENES

 LEARNING OBJECTIVES

Explain regulation of the cell cycle by proto-oncogenes

PROTO-ONCOGENES
The genes that code for the positive cell cycle regulators are called
proto-oncogenes. Proto-oncogenes are normal genes that, when
mutated in certain ways, become oncogenes: genes that cause a cell
to become cancerous. There are several ways by which a proto-
oncogene can be converted into an oncogene. Consider what might
happen to the cell cycle in a cell with a recently-acquired oncogene.
In most instances, the alteration of the DNA sequence will result in a Figure 10.4A. 1 : Proto-oncogene Conversion to Oncogene:
less functional (or non-functional) protein. The result is detrimental Examples of ways to convert proto-oncogenes into cancer-causing
genes (oncogenes).
to the cell and will likely prevent the cell from completing the cell
cycle; however, the organism is not harmed because the mutation The Cdk gene in the above example is only one of many genes that
will not be carried forward. If a cell cannot reproduce, the mutation are considered proto-oncogenes. In addition to the cell cycle
is not propagated and the damage is minimal. regulatory proteins, any protein that influences the cycle can be
altered in such a way as to override cell cycle checkpoints. An
Occasionally, however, a gene mutation causes a change that
oncogene is any gene that, when altered, leads to an increase in the
increases the activity of a positive regulator. For example, a
rate of cell cycle progression.
mutation that allows the Cdk gene to be activated without being
partnered with cyclin could push the cell cycle past a checkpoint KEY POINTS
before all of the required conditions are met. If the resulting
Proto- oncogenes positively regulate the cell cycle.
daughter cells are too damaged to undergo further cell divisions, the
Mutations may cause proto-oncogenes to become oncogenes,
mutation would not be propagated and no harm would come to the
disrupting normal cell division and causing cancers to form.
organism. However, if the atypical daughter cells are able to undergo
Some mutations prevent the cell from reproducing, which keeps
further cell divisions, subsequent generations of cells will probably
the mutations from being passed on.
accumulate even more mutations, some possibly in additional genes
If a mutated cell is able to reproduce because the cell division
that regulate the cell cycle.
regulators are damaged, then the mutation will be passed on,
possibly accumulating more mutations with successive divisions.

KEY TERMS
proto-oncogene: a gene that promotes the specialization and
division of normal cells that becomes an oncogene following
mutation
mutation: any heritable change of the base-pair sequence of
genetic material
oncogene: any gene that contributes to the conversion of a
normal cell into a cancerous cell when mutated or expressed at
high levels

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10.4B: TUMOR SUPPRESSOR GENES
adequate levels of p21, there is no effective block on Cdk activation.
 LEARNING OBJECTIVES Essentially, without a fully functional p53, the G1 checkpoint is
severely compromised and the cell proceeds directly from G1 to S
Describe the role played by tumor suppressor genes in the
regardless of internal and external conditions. At the completion of
cell cycle
this shortened cell cycle, two daughter cells are produced that have
inherited the mutated p53 gene. Given the non-optimal conditions
Like proto- oncogenes, many of the negative cell cycle regulatory
under which the parent cell reproduced, it is likely that the daughter
proteins were discovered in cells that had become cancerous. Tumor
cells will have acquired other mutations in addition to the faulty
suppressor genes are segments of DNA that code for negative tumor suppressor gene. Cells such as these daughter cells quickly
regulator proteins: the type of regulators that, when activated, can
accumulate both oncogenes and non-functional tumor suppressor
prevent the cell from undergoing uncontrolled division. The
genes. Again, the result is tumor growth.
collective function of the best-understood tumor suppressor gene
proteins, Rb, p53, and p21, is to put up a roadblock to cell cycle KEY POINTS
progression until certain events are completed. A cell that carries a Tumor suppressor genes are segments of DNA that code for
mutated form of a negative regulator might not be able to halt the negative regulator proteins, which keep the cell from undergoing
cell cycle if there is a problem. Tumor suppressors are similar to uncontrolled division.
brakes in a vehicle: malfunctioning brakes can contribute to a car Mutated p53 genes are believed to be responsible for causing
crash. tumor growth because they turn off the regulatory mechanisms
Mutated p53 genes have been identified in more than one-half of all that keep cells from dividing out of control.
human tumor cells. This discovery is not surprising in light of the Sometimes cells with negative regulators can halt their
multiple roles that the p53 protein plays at the G1 checkpoint. A cell transmission by inducing pre-programmed cell death called
with a faulty p53 may fail to detect errors present in the genomic apoptosis.
DNA. Even if a partially-functional p53 does identify the mutations, Without a fully functional p53, the G1 checkpoint of interphase
it may no longer be able to signal the necessary DNA repair is severely compromised and the cell proceeds directly from G1
enzymes. Either way, damaged DNA will remain uncorrected. At to S; this creates two daughter cells that have inherited the
this point, a functional p53 will deem the cell unsalvageable and mutated p53 gene.
trigger programmed cell death (apoptosis). The damaged version of
p53 found in cancer cells, however, cannot trigger apoptosis. KEY TERMS
apoptosis: a process of programmed cell death

CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44463/latest...ol11448/latest. License: CC
BY: Attribution
oncogene. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/oncogene. License: CC BY-SA: Attribution-ShareAlike
proto-oncogene. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/proto-oncogene. License: CC BY-SA: Attribution-
ShareAlike
mutation. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/mutation. License: CC BY-SA: Attribution-ShareAlike
Possible Pathways that turn Proto-oncogenes into Oncogenes. Provided by:
Wikimedia. Located at: commons.wikimedia.org/wiki/Fi..._Oncogenes.jpg.
License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44463/latest...ol11448/latest. License: CC
BY: Attribution
apoptosis. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/apoptosis. License: CC BY-SA: Attribution-ShareAlike
Possible Pathways that turn Proto-oncogenes into Oncogenes. Provided by:
Wikimedia. Located at: commons.wikimedia.org/wiki/Fi..._Oncogenes.jpg.
License: CC BY-SA: Attribution-ShareAlike
Figure 10.4B. 1: Function of Normal and Mutated p53 genes: The OpenStax College, Cancer and the Cell Cycle. October 16, 2013. Provided by:
role of normal p53 is to monitor DNA and the supply of oxygen OpenStax CNX. Located at:
(hypoxia is a condition of reduced oxygen supply). If damage is http://cnx.org/content/m44463/latest...e_10_04_01.png. License: CC BY:
detected, p53 triggers repair mechanisms. If repairs are unsuccessful, Attribution
p53 signals apoptosis. A cell with an abnormal p53 protein cannot
repair damaged DNA and cannot signal apoptosis. Cells with
abnormal p53 can become cancerous.
This page titled 10.4B: Tumor Suppressor Genes is shared under a CC BY-
The loss of p53 function has other repercussions for the cell cycle. SA 4.0 license and was authored, remixed, and/or curated by Boundless.
Mutated p53 might lose its ability to trigger p21 production. Without

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SECTION OVERVIEW

10.5: PROKARYOTIC CELL DIVISION

10.5A: BINARY FISSION
Topic hierarchy

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10.5A: BINARY FISSION

 LEARNING OBJECTIVES

Describe the process of binary fission in prokaryotes

Prokaryotes, such as bacteria, propagate by binary fission. For

unicellular organisms, cell division is the only method used to
produce new individuals. In both prokaryotic and eukaryotic cells,
the outcome of cell reproduction is a pair of daughter cells that are
genetically identical to the parent cell. In unicellular organisms,
daughter cells are individuals.
Due to the relative simplicity of the prokaryotes, the cell division
process, or binary fission, is a less complicated and much more rapid
process than cell division in eukaryotes. The single, circular DNA
chromosome of bacteria is not enclosed in a nucleus, but instead
occupies a specific location, the nucleoid, within the cell. Although
the DNA of the nucleoid is associated with proteins that aid in
packaging the molecule into a compact size, there are no histone
proteins and thus, no nucleosomes in prokaryotes. The packing
proteins of bacteria are, however, related to the cohesin and
condensin proteins involved in the chromosome compaction of
eukaryotes.
The bacterial chromosome is attached to the plasma membrane at
about the midpoint of the cell. The starting point of replication, the
origin, is close to the binding site of the chromosome at the plasma
membrane. Replication of the DNA is bidirectional, moving away
from the origin on both strands of the loop simultaneously. As the
new double strands are formed, each origin point moves away from
the cell wall attachment toward the opposite ends of the cell. As the
cell elongates, the growing membrane aids in the transport of the
chromosomes. After the chromosomes have cleared the midpoint of
the elongated cell, cytoplasmic separation begins. The formation of a Figure 10.5A. 1 : Binary Fission: These images show the steps of
ring composed of repeating units of a protein, FtsZ, directs the binary fission in prokaryotes.
partition between the nucleoids. Formation of the FtsZ ring triggers
the accumulation of other proteins that work together to recruit new MITOTIC SPINDLE APPARATUS
membrane and cell wall materials to the site. A septum is formed The precise timing and formation of the mitotic spindle is critical to
between the nucleoids, extending gradually from the periphery the success of eukaryotic cell division. Prokaryotic cells, on the
toward the center of the cell. When the new cell walls are in place, other hand, do not undergo karyokinesis and, therefore, have no
the daughter cells separate. need for a mitotic spindle. However, the FtsZ protein that plays such
a vital role in prokaryotic cytokinesis is structurally and functionally
very similar to tubulin, the building block of the microtubules that
make up the mitotic spindle fibers that are necessary for eukaryotes.
FtsZ proteins can form filaments, rings, and other three-dimensional
structures that resemble the way tubulin forms microtubules,
centrioles, and various cytoskeletal components. In addition, both
FtsZ and tubulin employ the same energy source, GTP (guanosine
triphosphate), to rapidly assemble and disassemble complex
structures.
FtsZ and tubulin are homologous structures derived from common
evolutionary origins. In this example, FtsZ is the ancestor protein to
tubulin (a modern protein). While both proteins are found in extant
organisms, tubulin function has evolved and diversified
tremendously since evolving from its FtsZ prokaryotic origin. A

10.5A.1 https://bio.libretexts.org/@go/page/13246
survey of mitotic assembly components found in present-day karyokinesis: (mitosis) the first portion of mitotic phase where
unicellular eukaryotes reveals crucial intermediary steps to the division of the cell nucleus takes place
complex membrane-enclosed genomes of multicellular eukaryotes. binary fission: the process whereby a cell divides asexually to
produce two daughter cells
KEY POINTS
In bacterial replication, the DNA is attached to the plasma CONTRIBUTIONS AND ATTRIBUTIONS
membrane at about the midpoint of the cell. OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44467/latest...ol11448/latest. License: CC
The origin, or starting point of bacterial replication, is close to BY: Attribution
the binding site of the DNA to the plasma membrane. OpenStax College, Prokaryotic Cell Division. October 28, 2013. Provided by:
OpenStax CNX. Located at: http://cnx.org/content/m44467/latest/. License:
Replication of the bacterial DNA is bidirectional, which means it CC BY: Attribution
moves away from the origin on both strands simultaneously. karyokinesis. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/karyokinesis. License: CC BY-SA: Attribution-
The formation of the FtsZ ring, a ring composed of repeating ShareAlike
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www.boundless.com//biology/de...itotic-spindle. License: CC BY-SA:
work together to acquire and bring new membrane and cell wall Attribution-ShareAlike
materials to the site. binary fission. Provided by: Wiktionary. Located at:
When new cell walls are in place, due to the formation of a en.wiktionary.org/wiki/binary_fission. License: CC BY-SA: Attribution-
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septum, the daughter cells separate to form individual cells. OpenStax College, Prokaryotic Cell Division. October 16, 2013. Provided by:
OpenStax CNX. Located at:
http://cnx.org/content/m44467/latest...e_10_05_01.jpg. License: CC BY:
KEY TERMS Attribution
mitotic spindle: the apparatus that orchestrates the movement of
DNA during mitosis This page titled 10.5A: Binary Fission is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

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 CHAPTER OVERVIEW

11: MEIOSIS AND SEXUAL REPRODUCTION

11.1: The Process of Meiosis - Introduction to Meiosis
11.2: The Process of Meiosis - Meiosis I
11.3: The Process of Meiosis - Meiosis II
11.4: The Process of Meiosis - Comparing Meiosis and Mitosis
11.5: Sexual Reproduction - Advantages and Disadvantages of Sexual Reproduction
11.6: Sexual Reproduction - Life Cycles of Sexually Reproducing Organisms

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1
11.1: THE PROCESS OF MEIOSIS - INTRODUCTION TO MEIOSIS
Haploid cells that are part of the sexual reproductive cycle are
 LEARNING OBJECTIVES produced by a type of cell division called meiosis. Meiosis employs
many of the same mechanisms as mitosis. However, the starting
Describe the importance of meiosis in sexual reproduction
nucleus is always diploid and the nuclei that result at the end of a
meiotic cell division are haploid, so the resulting cells have half the
INTRODUCTION: MEIOSIS AND SEXUAL chromosomes as the original. To achieve this reduction in
REPRODUCTION chromosomes, meiosis consists of one round of chromosome
The ability to reproduce in kind is a basic characteristic of all living duplication and two rounds of nuclear division. Because the events
things. In kind means that the offspring of any organism closely that occur during each of the division stages are analogous to the
resemble their parent or parents. Sexual reproduction requires events of mitosis, the same stage names are assigned. However,
fertilization: the union of two cells from two individual organisms. because there are two rounds of division, the major process and the
Haploid cells contain one set of chromosomes. Cells containing two stages are designated with a “I” or a “II.” Thus, meiosis I is the first
sets of chromosomes are called diploid. The number of sets of round of meiotic division and consists of prophase I, prometaphase
chromosomes in a cell is called its ploidy level. If the reproductive I, and so on. Meiosis II, the second round of meiotic division,
cycle is to continue, then the diploid cell must somehow reduce its includes prophase II, prometaphase II, and so on.
number of chromosome sets before fertilization can occur again or
there will be a continual doubling in the number of chromosome sets KEY POINTS
in every generation. Therefore, sexual reproduction includes a Sexual reproduction is the production of haploid cells and the
nuclear division that reduces the number of chromosome sets. fusion of two of those cells to form a diploid cell.
Before sexual reproduction can occur, the number of
chromosomes in a diploid cell must decrease by half.
Meiosis produces cells with half the number of chromosomes as
the original cell.
Haploid cells used in sexual reproduction, gametes, are formed
during meiosis, which consists of one round of chromosome
replication and two rounds of nuclear division.
Meiosis I is the first round of meiotic division, while meiosis II
Figure 11.1.1: Offspring Closely Resemble Their Parents: In kind
means that the offspring of any organism closely resemble their is the second round.
parent or parents. The hippopotamus gives birth to hippopotamus
calves (a). Joshua trees produce seeds from which Joshua tree KEY TERMS
seedlings emerge (b). Adult flamingos lay eggs that hatch into
flamingo chicks (c).
haploid: of a cell having a single set of unpaired chromosomes
gamete: a reproductive cell, male (sperm) or female (egg), that
Sexual reproduction is the production of haploid cells (gametes) and
has only half the usual number of chromosomes
the fusion (fertilization) of two gametes to form a single, unique
diploid: of a cell, having a pair of each type of chromosome, one
diploid cell called a zygote. All animals and most plants produce
of the pair being derived from the ovum and the other from the
these gametes, or eggs and sperm. In most plants and animals,
spermatozoon
through tens of rounds of mitotic cell division, this diploid cell will
develop into an adult organism. This page titled 11.1: The Process of Meiosis - Introduction to Meiosis is
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

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11.2: THE PROCESS OF MEIOSIS - MEIOSIS I

 LEARNING OBJECTIVES

Describe the stages and results of meiosis I

MEIOSIS I
Meiosis is preceded by an interphase consisting of three stages. The
G1 phase (also called the first gap phase) initiates this stage and is
focused on cell growth. The S phase is next, during which the DNA
of the chromosomes is replicated. This replication produces two
identical copies, called sister chromatids, that are held together at the
centromere by cohesin proteins. The centrosomes, which are the
structures that organize the microtubules of the meiotic spindle, also
replicate. Finally, during the G2 phase (also called the second gap
phase), the cell undergoes the final preparations for meiosis.

PROPHASE I
During prophase I, chromosomes condense and become visible
inside the nucleus. As the nuclear envelope begins to break down,
homologous chromosomes move closer together. The synaptonemal
complex, a lattice of proteins between the homologous
chromosomes, forms at specific locations, spreading to cover the
entire length of the chromosomes. The tight pairing of the
homologous chromosomes is called synapsis. In synapsis, the genes
on the chromatids of the homologous chromosomes are aligned with
each other. The synaptonemal complex also supports the exchange
of chromosomal segments between non-sister homologous
chromatids in a process called crossing over. The crossover events
are the first source of genetic variation produced by meiosis. A
single crossover event between homologous non-sister chromatids Figure 11.2.1: Crossover between homologous chromosomes:
leads to an exchange of DNA between chromosomes. Following Crossover occurs between non-sister chromatids of homologous
crossover, the synaptonemal complex breaks down and the cohesin chromosomes. The result is an exchange of genetic material between
homologous chromosomes.
connection between homologous pairs is also removed. At the end of
prophase I, the pairs are held together only at the chiasmata; they are
called tetrads because the four sister chromatids of each pair of
homologous chromosomes are now visible.

Figure 11.2.1: Synapsis holds pairs of homologous chromosomes

together: Early in prophase I, homologous chromosomes come
together to form a synapse. The chromosomes are bound tightly
together and in perfect alignment by a protein lattice called a
synaptonemal complex and by cohesin proteins at the centromere.

PROMETAPHASE I
The key event in prometaphase I is the formation of the spindle fiber
apparatus where spindle fiber microtubules attach to the kinetochore

11.2.1 https://bio.libretexts.org/@go/page/13249
proteins at the centromeres. Microtubules grow from centrosomes centromere. The chiasmata are broken in anaphase I as the
placed at opposite poles of the cell. The microtubules move toward microtubules attached to the fused kinetochores pull the homologous
the middle of the cell and attach to one of the two fused homologous chromosomes apart.
chromosomes at the kinetochores. At the end of prometaphase I,
each tetrad is attached to microtubules from both poles, with one TELOPHASE I AND CYTOKINESIS
homologous chromosome facing each pole. In addition, the nuclear In telophase I, the separated chromosomes arrive at opposite poles.
membrane has broken down entirely. In some organisms, the chromosomes decondense and nuclear
envelopes form around the chromatids in telophase I. Then
METAPHASE I cytokinesis, the physical separation of the cytoplasmic components
During metaphase I, the tetrads move to the metaphase plate with into two daughter cells, occurs without reformation of the nuclei. In
kinetochores facing opposite poles. The homologous pairs orient nearly all species of animals and some fungi, cytokinesis separates
themselves randomly at the equator. This event is the second the cell contents via a cleavage furrow (constriction of the actin ring
mechanism that introduces variation into the gametes or spores. In that leads to cytoplasmic division). In plants, a cell plate is formed
each cell that undergoes meiosis, the arrangement of the tetrads is during cell cytokinesis by Golgi vesicles fusing at the metaphase
different. The number of variations is dependent on the number of plate. This cell plate will ultimately lead to the formation of cell
chromosomes making up a set. There are two possibilities for walls that separate the two daughter cells.
orientation at the metaphase plate. The possible number of Two haploid cells are the end result of the first meiotic division. The
alignments, therefore, equals 2n, where n is the number of cells are haploid because at each pole there is just one of each pair of
chromosomes per set. Given these two mechanisms, it is highly the homologous chromosomes. Therefore, only one full set of the
unlikely that any two haploid cells resulting from meiosis will have chromosomes is present. Although there is only one chromosome
the same genetic composition. set, each homolog still consists of two sister chromatids.

KEY POINTS
Meiosis is preceded by interphase which consists of the G1 phase
(growth), the S phase ( DNA replication), and the G2 phase.
During prophase I, the homologous chromosomes condense and
become visible as the x shape we know, pair up to form a tetrad,
and exchange genetic material by crossing over.
During prometaphase I, microtubules attach at the chromosomes’
kinetochores and the nuclear envelope breaks down.
In metaphase I, the tetrads line themselves up at the metaphase
plate and homologous pairs orient themselves randomly.
In anaphase I, centromeres break down and homologous
chromosomes separate.
In telophase I, chromosomes move to opposite poles; during
cytokinesis the cell separates into two haploid cells.

KEY TERMS
crossing over: the exchange of genetic material between
homologous chromosomes that results in recombinant
Figure 11.2.1: Meiosis I ensures unique gametes: Random,
independent assortment during metaphase I can be demonstrated by chromosomes
considering a cell with a set of two chromosomes (n = 2). In this tetrad: two pairs of sister chromatids (a dyad pair) aligned in a
case, there are two possible arrangements at the equatorial plane in certain way and often on the equatorial plane during the meiosis
metaphase I. The total possible number of different gametes is 2n,
where n equals the number of chromosomes in a set. In this process
example, there are four possible genetic combinations for the chromatid: either of the two strands of a chromosome that
gametes. With n = 23 in human cells, there are over 8 million separate during meiosis
possible combinations of paternal and maternal chromosomes.
This page titled 11.2: The Process of Meiosis - Meiosis I is shared under a
ANAPHASE I
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In anaphase I, the microtubules pull the attached chromosomes Boundless.
apart. The sister chromatids remain tightly bound together at the

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11.3: THE PROCESS OF MEIOSIS - MEIOSIS II

 LEARNING OBJECTIVES

Describe the stages and results of Meiosis II

MEIOSIS II
Meiosis II initiates immediately after cytokinesis, usually before the
chromosomes have fully decondensed. In contrast to meiosis I,
meiosis II resembles a normal mitosis. In some species, cells enter a
brief interphase, or interkinesis, before entering meiosis II.
Interkinesis lacks an S phase, so chromosomes are not duplicated.
The two cells produced in meiosis I go through the events of meiosis
II together. During meiosis II, the sister chromatids within the two
daughter cells separate, forming four new haploid gametes. The
mechanics of meiosis II is similar to mitosis, except that each
dividing cell has only one set of homologous chromosomes.

PROPHASE II Figure 11.3.1: Meiosis I vs. Meiosis II: The process of chromosome
If the chromosomes decondensed in telophase I, they condense alignment differs between meiosis I and meiosis II. In prometaphase
again. If nuclear envelopes were formed, they fragment into I, microtubules attach to the fused kinetochores of homologous
chromosomes, and the homologous chromosomes are arranged at the
vesicles. The centrosomes that were duplicated during interphase I midpoint of the cell in metaphase I. In anaphase I, the homologous
move away from each other toward opposite poles and new spindles chromosomes are separated. In prometaphase II, microtubules attach
are formed. to the kinetochores of sister chromatids, and the sister chromatids
are arranged at the midpoint of the cells in metaphase II. In anaphase
PROMETAPHASE II II, the sister chromatids are separated.

The nuclear envelopes are completely broken down and the spindle TELOPHASE II AND CYTOKINESIS
is fully formed. Each sister chromatid forms an individual The chromosomes arrive at opposite poles and begin to decondense.
kinetochore that attaches to microtubules from opposite poles. Nuclear envelopes form around the chromosomes. Cytokinesis
separates the two cells into four unique haploid cells. At this point,
METAPHASE II
the newly-formed nuclei are both haploid. The cells produced are
The sister chromatids are maximally condensed and aligned at the
genetically unique because of the random assortment of paternal and
equator of the cell.
maternal homologs and because of the recombining of maternal and
ANAPHASE II paternal segments of chromosomes (with their sets of genes) that
The sister chromatids are pulled apart by the kinetochore occurs during crossover.
microtubules and move toward opposite poles. Non-kinetochore
microtubules elongate the cell.

11.3.1 https://bio.libretexts.org/@go/page/13250
 KEY POINTS
During prophase II, chromsomes condense again, centrosomes
that were duplicated during interphase I move away from each
other toward opposite poles, and new spindles are formed.
During prometaphase II, the nuclear envelopes are completely
broken down, and each sister chromatid forms an individual
kinetochore that attaches to microtubules from opposite poles.
During metaphase II, sister chromatids are condensed and
aligned at the equator of the cell.
During anaphase II sister chromatids are pulled apart by the
kinetochore microtubules and move toward opposite poles.
During telophase II and cytokinesis, chromosomes arrive at
opposite poles and begin to decondense; the two cells divide into
four unique haploid cells.

KEY TERMS
meiosis II: the second part of the meiotic process; the end result
is production of four haploid cells from the two haploid cells
produced in meiosis I

This page titled 11.3: The Process of Meiosis - Meiosis II is shared under a
CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Boundless.

Figure 11.3.1: Complete Stages of Meiosis: An animal cell with a

diploid number of four (2n = 4) proceeds through the stages of
meiosis to form four haploid daughter cells.

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11.4: THE PROCESS OF MEIOSIS - COMPARING MEIOSIS AND MITOSIS
spindle poles attached to each kinetochore of a homolog in a tetrad.
 LEARNING OBJECTIVES All of these events occur only in meiosis I.
When the tetrad is broken up and the homologous chromosomes
Compare and contrast mitosis and meiosis
move to opposite poles, the ploidy level is reduced from two to one.
For this reason, meiosis I is referred to as a reduction division. There
Mitosis and meiosis are both forms of division of the nucleus in
is no such reduction in ploidy level during mitosis.
eukaryotic cells. They share some similarities, but also exhibit
distinct differences that lead to very different outcomes. The purpose Meiosis II is much more similar to a mitotic division. In this case,
of mitosis is cell regeneration, growth, and asexual the duplicated chromosomes (only one set, as the homologous pairs
reproduction,while the purpose of meiosis is the production of have now been separated into two different cells) line up on the
gametes for sexual reproduction. Mitosis is a single nuclear division metaphase plate with divided kinetochores attached to kinetochore
that results in two nuclei that are usually partitioned into two new fibers from opposite poles. During anaphase II and mitotic anaphase,
daughter cells. The nuclei resulting from a mitotic division are the kinetochores divide and sister chromatids, now referred to as
genetically identical to the original nucleus. They have the same chromosomes, are pulled to opposite poles. The two daughter cells
number of sets of chromosomes, one set in the case of haploid cells of mitosis, however, are identical, unlike the daughter cells produced
and two sets in the case of diploid cells. In most plants and all by meiosis. They are different because there has been at least one
animal species, it is typically diploid cells that undergo mitosis to crossover per chromosome. Meiosis II is not a reduction division
form new diploid cells. In contrast, meiosis consists of two nuclear because, although there are fewer copies of the genome in the
divisions resulting in four nuclei that are usually partitioned into resulting cells, there is still one set of chromosomes, as there was at
four new haploid daughter cells. The nuclei resulting from meiosis the end of meiosis I. Meiosis II is, therefore, referred to as equatorial
are not genetically identical and they contain one chromosome set division.
only. This is half the number of chromosome sets in the original cell,
which is diploid.
KEY POINTS
For the most part, in mitosis, diploid cells are partitioned into
two new diploid cells, while in meiosis, diploid cells are
partitioned into four new haploid cells.
In mitosis, the daughter cells have the same number of
chromosomes as the parent cell, while in meiosis, the daughter
cells have half the number of chromosomes as the parent.
The daughter cells produced by mitosis are identical, whereas the
daughter cells produced by meiosis are different because
crossing over has occurred.
The events that occur in meiosis but not mitosis include
homologous chromosomes pairing up, crossing over, and lining
up along the metaphase plate in tetrads.
Meiosis II and mitosis are not reduction division like meiosis I
because the number of chromosomes remains the same;
therefore, meiosis II is referred to as equatorial division.
When the homologous chromosomes separate and move to
opposite poles during meiosis I, the ploidy level is reduced from
two to one, which is referred to as a reduction division.

KEY TERMS
Figure 11.4.1: Comparing Meiosis and Mitosis: Meiosis and mitosis reduction division: the first of the two divisions of meiosis, a
are both preceded by one round of DNA replication; however,
meiosis includes two nuclear divisions. The four daughter cells type of cell division
resulting from meiosis are haploid and genetically distinct. The ploidy: the number of homologous sets of chromosomes in a cell
daughter cells resulting from mitosis are diploid and identical to the equatorial division: a process of nuclear division in which each
parent cell.
chromosome divides equally such that the number of
The main differences between mitosis and meiosis occur in meiosis
chromosomes remains the same from parent to daughter cells
I. In meiosis I, the homologous chromosome pairs become
associated with each other and are bound together with the CONTRIBUTIONS AND ATTRIBUTIONS
synaptonemal complex. Chiasmata develop and crossover occurs OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
between homologous chromosomes, which then line up along the Located at: http://cnx.org/content/m44468/latest...ol11448/latest. License: CC
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CC BY: Attribution http://cnx.org/content/m44469/latest...e_11_01_03.jpg. License: CC BY:
haploid. Provided by: Wiktionary. Located at: Attribution
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License: CC BY-SA: Attribution-ShareAlike Attribution
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License: CC BY-SA: Attribution-ShareAlike OpenStax CNX. Located at:
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License: CC BY: Attribution OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44469/latest...ol11448/latest. License: CC
Located at: http://cnx.org/content/m44469/latest...ol11448/latest. License: CC BY: Attribution
BY: Attribution Boundless. Provided by: Boundless Learning. Located at:
crossing over. Provided by: Wikipedia. Located at: www.boundless.com//biology/de...orial-division. License: CC BY-SA:
en.Wikipedia.org/wiki/crossing%20over. License: CC BY-SA: Attribution- Attribution-ShareAlike
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License: CC BY-SA: Attribution-ShareAlike reduction division. Provided by: Wiktionary. Located at:
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11.5: SEXUAL REPRODUCTION - ADVANTAGES AND DISADVANTAGES OF
SEXUAL REPRODUCTION
The process of meiosis produces unique reproductive cells called
 LEARNING OBJECTIVES gametes, which have half the number of chromosomes as the parent
cell. Fertilization, the fusion of haploid gametes from two
Describe the benefits of sexual reproduction
individuals, restores the diploid condition. Thus, sexually-
reproducing organisms alternate between haploid and diploid stages.
AN INTRODUCTION TO SEXUAL REPRODUCTION However, the ways in which reproductive cells are produced and the
timing between meiosis and fertilization vary greatly. There are
Sexual reproduction was an early evolutionary innovation after the three main categories of sexual life cycles: diploid-dominant,
appearance of eukaryotic cells. During sexual reproduction, the demonstrated by most animals; haploid-dominant, demonstrated by
genetic material of two individuals is combined to produce all fungi and some algae; and the alternation of generations,
genetically-diverse offspring that differ from their parents. The fact demonstrated by plants and some algae.
that most eukaryotes reproduce sexually is evidence of its
evolutionary success. In many animals, it is actually the only mode
of reproduction. The genetic diversity of sexually-produced
offspring is thought to give species a better chance of surviving in an
unpredictable or changing environment.
Scientists recognize some real disadvantages to sexual reproduction.
On the surface, creating offspring that are genetic clones of the
parent appears to be a better system. If the parent organism is
successfully occupying a habitat, offspring with the same traits
would be similarly successful. Species that reproduce sexually must
maintain two different types of individuals, males and females,
which can limit the ability to colonize new habitats as both sexes
must be present. Therefore, there is an obvious benefit to an Figure 11.5.1: The Sexual Life Cycle: In animals, sexually-
organism that can produce offspring whenever circumstances are reproducing adults form haploid gametes from diploid germ cells.
favorable by asexual budding, fragmentation, or asexual eggs. These Fusion of the gametes gives rise to a fertilized egg cell, or zygote.
The zygote will undergo multiple rounds of mitosis to produce a
methods of asexual reproduction do not require another organism of multicellular offspring.
the opposite sex. Indeed, some organisms that lead a solitary
lifestyle have retained the ability to reproduce asexually. In addition, THE RED QUEEN HYPOTHESIS
in asexual populations, every individual is capable of reproduction. It is not in dispute that sexual reproduction provides evolutionary
In sexual populations, the males are not producing the offspring advantages to organisms that employ this mechanism to produce
themselves. In theory, an asexual population could grow twice as offspring. But why, even in the face of fairly stable conditions, does
fast. sexual reproduction persist when it is more difficult and costly for
Nevertheless, multicellular organisms that exclusively depend on individual organisms? Variation is the outcome of sexual
asexual reproduction are exceedingly rare. Why is sexuality (and reproduction, but why are ongoing variations necessary? Possible
meiosis ) so common? This is one of the important unanswered answers to these questions are explained in the Red Queen
questions in biology and has been the focus of much research hypothesis, first proposed by Leigh Van Valen in 1973.
beginning in the latter half of the twentieth century. There are All species co-evolve with other organisms; for example, predators
several possible explanations, one of which is that the variation that evolve with their prey and parasites evolve with their hosts. Each
sexual reproduction creates among offspring is very important to the tiny advantage gained by favorable variation gives a species an edge
survival and reproduction of the population. Thus, on average, a over close competitors, predators, parasites, or even prey. The only
sexually-reproducing population will leave more descendants than method that will allow a co-evolving species to maintain its own
an otherwise similar asexually-reproducing population. The only share of the resources is to also continually improve its fitness. As
source of variation in asexual organisms is mutation. This is the one species gains an advantage, this increases selection on the other
ultimate source of variation in sexual organisms, but, in addition, species; they must also develop an advantage or they will be out-
those different mutations are continually reshuffled from one competed. No single species progresses too far ahead because
generation to the next when different parents combine their unique genetic variation among the progeny of sexual reproduction provides
genomes and the genes are mixed into different combinations by the all species with a mechanism to improve rapidly. Species that cannot
process of meiosis. Meiosis is the division of the contents of the keep up become extinct. The Red Queen’s catchphrase was, “It takes
nucleus, dividing the chromosomes among gametes. all the running you can do to stay in the same place.” This is an apt
description of co-evolution between competing species.

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KEY POINTS KEY TERMS
The variation that sexual reproduction creates among offspring is sexual reproduction: Sexual reproduction is the creation of a
very important to the survival and reproduction of the new organism by combining the genetic material of two
population. organisms. There are two main processes during sexual
In sexual reproduction, different mutations are continually reproduction: meiosis, involving the halving of the number of
reshuffled from one generation to the next when different parents chromosomes, and fertilization, involving the fusion of two
combine their unique genomes; this results in an increase of gametes and the restoration of the original number of
genetic diversity. chromosomes.
On average, a sexually-reproducing population will leave more asexual reproduction: any form of reproduction that involves
offspring than an otherwise similar asexually-reproducing neither meiosis nor fusion of gametes
population.
This page titled 11.5: Sexual Reproduction - Advantages and Disadvantages
of Sexual Reproduction is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

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11.6: SEXUAL REPRODUCTION - LIFE CYCLES OF SEXUALLY REPRODUCING
ORGANISMS
important part of the life cycle, is haploid. The haploid cells that
 LEARNING OBJECTIVES make up the tissues of the dominant multicellular stage are formed
by mitosis. During sexual reproduction, specialized haploid cells
Explain the life cycles in sexual reproduction
from two individuals, designated the (+) and (−) mating types, join
to form a diploid zygote. The zygote immediately undergoes meiosis
In sexual reproduction, the genetic material of two individuals is to form four haploid cells called spores. Although haploid like the
combined to produce genetically diverse offspring that differ from
“parents,” these spores contain a new genetic combination from two
their parents. Fertilization and meiosis alternate in sexual life cycles. parents. The spores can remain dormant for various time periods.
What happens between these two events depends upon the organism. Eventually, when conditions are conducive, the spores form
The process of meiosis, the division of the contents of the nucleus multicellular haploid structures by many rounds of mitosis.
that divides the chromosomes among gametes, reduces the
chromosome number by half, while fertilization, the joining of two
haploid gametes, restores the diploid condition. There are three main
categories of life cycles in eukaryotic organisms: diploid-dominant,
haploid-dominant, and alternation of generations.

DIPLOID-DOMINANT LIFE CYCLE

In the diploid-dominant life cycle, the multicellular diploid stage is
the most obvious life stage, as occurs with most animals, including
humans. Nearly all animals employ a diploid-dominant life cycle
strategy in which the only haploid cells produced by the organism
are the gametes. Early in the development of the embryo, specialized
diploid cells, called germ cells, are produced within the gonads (e.g.
testes and ovaries). Germ cells are capable of mitosis to perpetuate
the cell line and meiosis to produce gametes. Once the haploid Figure 11.6.1: Haploid-Dominant Life Cycle: Fungi, such as black
gametes are formed, they lose the ability to divide again. There is no bread mold (Rhizopus nigricans), have haploid-dominant life cycles.
The haploid multicellular stage produces specialized haploid cells by
multicellular haploid life stage. Fertilization occurs with the fusion mitosis that fuse to form a diploid zygote. The zygote undergoes
of two gametes, usually from different individuals, restoring the meiosis to produce haploid spores. Each spore gives rise to a
diploid state. multicellular haploid organism by mitosis.

ALTERNATION OF GENERATIONS
The third life-cycle type, employed by some algae and all plants, is a
blend of the haploid-dominant and diploid-dominant extremes.
Species with alternation of generations have both haploid and
diploid multicellular organisms as part of their life cycle. The
haploid multicellular plants are called gametophytes because they
produce gametes from specialized cells. Meiosis is not directly
involved in the production of gametes because the organism that
produces the gametes is already a haploid. Fertilization between the
gametes forms a diploid zygote. The zygote will undergo many
rounds of mitosis and give rise to a diploid multicellular plant called
a sporophyte. Specialized cells of the sporophyte will undergo
meiosis and produce haploid spores. The spores will subsequently
develop into the gametophytes.
Figure 11.6.1: Diploid-Dominant Life Cycle: In animals, sexually-
reproducing adults form haploid gametes from diploid germ cells.
Fusion of the gametes gives rise to a fertilized egg cell, or zygote.
The zygote will undergo multiple rounds of mitosis to produce a
multicellular offspring. The germ cells are generated early in the
development of the zygote.

HAPLOID-DOMINANT LIFE CYCLE

Within haploid-dominant life cycles, the multicellular haploid stage
is the most obvious life stage. Most fungi and algae employ a life
cycle type in which the “body” of the organism, the ecologically

11.6.1 https://bio.libretexts.org/@go/page/13254
 haploid cells from two individuals join to form a diploid zygote.
Observed in all plants and some algae, species with alternation of
generations have both haploid and diploid multicellular
organisms as part of their life cycle.

KEY TERMS
zygote: a diploid fertilized egg cell
gametophyte: a plant (or the haploid phase in its life cycle) that
produces gametes by mitosis in order to produce a zygote
sporophyte: a plant (or the diploid phase in its life cycle) that
produces spores by meiosis in order to produce gametophytes

Figure 11.6.1: Alternation of Generations: Plants have a life cycle CONTRIBUTIONS AND ATTRIBUTIONS
that alternates between a multicellular haploid organism and a asexual reproduction. Provided by: Wiktionary. Located at:
multicellular diploid organism. In some plants, such as ferns, both en.wiktionary.org/wiki/asexual_reproduction. License: CC BY-SA:
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The spores develop into multicellular, haploid plants called Located at: http://cnx.org/content/m44470/latest...ol11448/latest. License: CC
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and the relationship between them vary greatly. In plants such as sexual reproduction. Provided by: Wikipedia. Located at:
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KEY POINTS OpenStax College, Sexual Reproduction. October 16, 2013. Provided by:
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In the diploid – dominant cycle, the multicellular diploid stage is
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the most obvious life stage; the only haploid cells produced by Attribution
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 CHAPTER OVERVIEW

12: MENDEL'S EXPERIMENTS AND HEREDITY

Topic hierarchy
12.1: Mendels Experiments and the Laws of Probability
12.1A: Introduction to Mendelian Inheritance
12.1B: Mendel’s Model System
12.1C: Mendelian Crosses
12.1D: Garden Pea Characteristics Revealed the Basics of Heredity
12.1E: Rules of Probability for Mendelian Inheritance
12.2: Patterns of Inheritance
12.2A: Genes as the Unit of Heredity
12.2B: Phenotypes and Genotypes
12.2C: The Punnett Square Approach for a Monohybrid Cross
12.2D: Alternatives to Dominance and Recessiveness
12.2E: Sex-Linked Traits
12.2F: Lethal Inheritance Patterns
12.3: Laws of Inheritance
12.3A: Mendel’s Laws of Heredity
12.3B: Mendel’s Law of Dominance
12.3C: Mendel’s Law of Segregation
12.3D: Mendel’s Law of Independent Assortment
12.3E: Genetic Linkage and Violation of the Law of Independent Assortment
12.3F: Epistasis

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1
SECTION OVERVIEW

12.1: MENDELS EXPERIMENTS AND THE LAWS OF PROBABILITY

12.1D: GARDEN PEA CHARACTERISTICS
Topic hierarchy REVEALED THE BASICS OF HEREDITY

12.1E: RULES OF PROBABILITY FOR MENDELIAN

12.1A: INTRODUCTION TO MENDELIAN
INHERITANCE
INHERITANCE

12.1B: MENDEL’S MODEL SYSTEM This page titled 12.1: Mendels Experiments and the Laws of Probability is
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
12.1C: MENDELIAN CROSSES curated by Boundless.

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12.1A: INTRODUCTION TO MENDELIAN INHERITANCE

 LEARNING OBJECTIVES

Describe the traits of pea plants that were studied by Mendel

GREGOR MENDEL AND THE STUDY OF

GENETICS
Genetics is the study of heredity, or the passing of traits from parents
to offspring. Gregor Johann Mendel set the framework for genetics
long before chromosomes or genes had been identified, at a time
when meiosis was not well understood. For his work, Mendel is Figure 12.1A. 1 : Appearance and genetic makeup of garden pea
often referred to as the “father of modern genetics. ” Mendel plant flowers: Based on Mendel’s experiments, the genotype of the
pea flowers could be determined from the phenotypes of the flowers.
selected a simple biological system, garden peas, and conducted
Because of Mendel’s work, the fundamental principles of heredity
methodical, quantitative analyses using large sample sizes.
were revealed, which are often referred to as Mendel’s Laws of
Inheritance. We now know that genes, carried on chromosomes, are
the basic functional units of heredity with the capability to be
replicated, expressed, or mutated. Today, the postulates put forth by
Mendel form the basis of classical, or Mendelian, genetics. Not all
genes are transmitted from parents to offspring according to
Mendelian genetics, but Mendel’s experiments serve as an excellent
starting point for thinking about inheritance.
Mendel made all of his observations and findings crossing individual
plants. We can now view a human karyotype of all of the
chromosomes in an individual to visualize chromosomal
abnormalities in offspring, even before birth. Shortly after Mendel
Figure 12.1A. 1 : Gregor Mendel: Gregor Johann Mendel was a proposed that traits were determined by what are now known as
German-speaking Moravian scientist and Augustinian friar who
gained posthumous fame as the founder of the modern science of genes, other researchers observed that different traits were often
genetics. inherited together, and thereby deduced that the genes were
Mendel entered the Augustinian St. Thomas’s Abbey and began his physically linked by being located on the same chromosome.
training as a priest. He began studying heredity using mice, but his Mendel’s work was the beginning of many of the advances in
bishop did not like one of his friars studying animal sex, so he molecular biology over the years.
switched to plants. Mendel grew and studied around 29,000 garden
pea plants in a monastery’s garden, where he analyzed seven
KEY POINTS
characteristics of the garden pea plants: flower color (purple or Mendel studied seven characteristics of the garden pea plants:
white), seed texture (wrinkled or round), seed color (yellow or flower color, seed texture, seed color, stem length, pod color, pod
green), stem length (long or short), pod color (yellow or green), pod texture, and flower position to develop his Laws of Inheritance.
texture (inflated or constricted), and flower position (axial or Genetics is the study of genes passed from parents to offspring.
terminal). Based on the appearance, or phenotypes, of the seven Genes are the basic fundamental units of heredity.
traits, Mendel developed genotypes for those traits.
KEY TERMS
genetics: The branch of biology that deals with the transmission
and variation of inherited characteristics, in particular
chromosomes and DNA.

This page titled 12.1A: Introduction to Mendelian Inheritance is shared

under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

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12.1B: MENDEL’S MODEL SYSTEM
form in the second generation. By experimenting with true-breeding
 LEARNING OBJECTIVES pea plants, Mendel avoided the appearance of unexpected
(recombinant) traits in offspring that might occur if the plants were
Describe the scientific reasons for the success of Mendel’s
not true breeding.
experimental work

MENDEL’S MODEL SYSTEM

Mendel’s seminal work was accomplished using the garden pea,
Pisum sativum, to study inheritance. Pea plant reproduction is easily
manipulated; large quantities of garden peas could be cultivated
simultaneously, allowing Mendel to conclude that his results did not
occur simply by chance. The garden pea also grows to maturity
within one season; several generations could be evaluated over a
relatively short time.
Pea plants have both male and female parts and can easily be grown
in large numbers. For this reason, garden pea plants can either self- Figure 12.1B. 1: Mendel’s Experiments With Peas: Experimenting
pollinate or cross-pollinate with other pea plants. In the absence of with thousands of garden peas, Mendel uncovered the fundamentals
outside manipulation, this species naturally self-fertilizes: ova (the of genetics.
eggs) within individual flowers are fertilized by pollen (containing KEY POINTS
the sperm cell) from the same flower. The sperm and the eggs that
Mendel used true-breeding plants in his experiments. These
produce the next generation of plants both come from the same
plants, when self-fertilized, always produce offspring with the
parent. What’s more, the flower petals remain sealed tightly until
same phenotype.
after pollination, preventing pollination from other plants. The result
Pea plants are easily manipulated, grow in one season, and can
is highly inbred, or “true-breeding,” pea plants. These are plants that
be grown in large quantities; these qualities allowed Mendel to
always produce offspring that look like the parent. Today, we know
conduct methodical, quantitative analyses using large sample
that these “true-breeding” plants are homozygous for most traits.
sizes.
A gardener or researcher, such as Mendel, can cross-pollinate these Based on his experiments with the garden peas, Mendel found
same plants by manually applying sperm from one plant to the pistil that one phenotype was always dominant over another recessive
(containing the ova) of another plant. Now the sperm and eggs come phenotype for the same trait.
from different parent plants. When Mendel cross-pollinated a true-
breeding plant that only produced yellow peas with a true-breeding KEY TERMS
plant that only produced green peas, he found that the first phenotype: the observable characteristics of an organism, often
generation of offspring is always all yellow peas. The green pea trait resulting from its genetic information or a combination of
did not show up. However, if this first generation of yellow pea genetic information and environmental factors
plants were allowed to self-pollinate, the following or second genotype: the specific genetic information of a cell or organism,
generation had a ratio of 3:1 yellow to green peas. usually a description of the allele or alleles relating to a specific
In this and all the other pea plant traits Mendel followed, one form gene.
of the trait was “dominant” over another so it masked the presence true-breeding plant: a plant that always produces offspring of
of the other “recessive” form in the first generation after cross- the same phenotype when self-fertilized; one that is homozygous
breeding two homozygous plants.. Even if the phenotype (visible for the trait being followed.
form) is hidden, the genotype (allele controlling that form of the
trait) can be passed on to next generation and produce the recessive This page titled 12.1B: Mendel’s Model System is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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12.1C: MENDELIAN CROSSES

 LEARNING OBJECTIVES

Identify Mendelian crosses

MENDELIAN CROSSES
Mendel performed crosses, which involved mating two true-
breeding individuals that have different traits. In the pea, which is a
naturally self-pollinating plant, this is done by manually transferring
pollen from the anther of a mature pea plant of one variety to the
stigma of a separate mature pea plant of the second variety. In
plants, pollen carries the male gametes (sperm) to the stigma, a
sticky organ that traps pollen and allows the sperm to move down
the pistil to the female gametes (ova) below. To prevent the pea plant
that was receiving pollen from self-fertilizing and confounding his
results, Mendel painstakingly removed all of the anthers from the
plant’s flowers before they had a chance to mature.

Figure 12.1C. 1 : Mendelian Crosses: In one of his experiments on

inheritance patterns, Mendel crossed plants that were true-breeding
for violet flower color with plants true-breeding for white flower
color (the P generation). The resulting hybrids in the F1 generation
all had violet flowers. In the F2 generation, approximately three-
quarters of the plants had violet flowers, while one-quarter had white
flowers.
Plants used in first-generation crosses were called P0, or parental
generation one, plants. Mendel collected the seeds belonging to the
P0 plants that resulted from each cross and grew them the following
season. These offspring were called the F1, or the first filial (filial =
offspring, daughter or son), generation. Once Mendel examined the
characteristics in the F1generation of plants, he allowed them to self-
fertilize naturally. He then collected and grew the seeds from the F1
plants to produce the F2, or second filial, generation. Mendel’s
experiments extended beyond the F2 generation to the F3 and
F4generations, and so on, but it was the ratio of characteristics in the
P0−F1−F2 generations that were the most intriguing and became the
basis for Mendel’s postulates.

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KEY POINTS KEY TERMS
Mendel carefully controlled his experiments by removing the filial: of a generation or generations descending from a specific
anthers from the pea plants before they matured. previous one
First generation pea plants were called parental generation, P0, parental: of the generation of organisms that produce a hybrid
while the following generations were called filial, Fn, where n is
the number of generations from P0. This page titled 12.1C: Mendelian Crosses is shared under a CC BY-SA 4.0
The ratio of characteristics in the P0−F1−F2 generations became license and was authored, remixed, and/or curated by Boundless.
the basis for Mendel’s postulates.

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12.1D: GARDEN PEA CHARACTERISTICS REVEALED THE BASICS OF
HEREDITY
example of a dominant trait is the violet-flower trait. For this same
 LEARNING OBJECTIVES characteristic (flower color), white-colored flowers are a recessive
trait. The fact that the recessive trait reappeared in the F2 generation
Evaluate the results of F1 and F2 generations from
meant that the traits remained separate (not blended) in the plants of
Mendelian crosses of peas
the F1 generation. Mendel also proposed that plants possessed two
copies of the trait for the flower-color characteristic and that each
GARDEN PEA CHARACTERISTICS REVEALED parent transmitted one of its two copies to its offspring, where they
THE BASICS OF HEREDITY came together. Moreover, the physical observation of a dominant
To fully examine each of the seven traits in garden peas, Mendel trait could mean that the genetic composition of the organism
generated large numbers of F1 and F2 plants, reporting results from included two dominant versions of the characteristic or that it
19,959 F2 plants alone. His findings were consistent. included one dominant and one recessive version. Conversely, the
What results did Mendel find in his crosses for flower color? First, observation of a recessive trait meant that the organism lacked any
Mendel confirmed that he had plants that bred true for white or dominant versions of this characteristic.
violet flower color. Regardless of how many generations Mendel
examined, all self-crossed offspring of parents with white flowers
had white flowers, and all self-crossed offspring of parents with
violet flowers had violet flowers. In addition, Mendel confirmed
that, other than flower color, the pea plants were physically
identical.
Once these validations were complete, Mendel applied the pollen
from a plant with violet flowers to the stigma of a plant with white
flowers. After gathering and sowing the seeds that resulted from this
cross, Mendel found that 100 percent of the F1hybrid generation had
violet flowers. Conventional wisdom at that time would have
predicted the hybrid flowers to be pale violet or for hybrid plants to
have equal numbers of white and violet flowers. In other words, the
contrasting parental traits were expected to blend in the offspring.
Instead, Mendel’s results demonstrated that the white flower trait in
the F1 generation had completely disappeared.
Importantly, Mendel did not stop his experimentation there. He
allowed the F1 plants to self-fertilize and found that, of F2-
generation plants, 705 had violet flowers and 224 had white flowers.
This was a ratio of 3.15 violet flowers per one white flower, or
approximately 3:1. When Mendel transferred pollen from a plant
with violet flowers to the stigma of a plant with white flowers and
vice versa, he obtained about the same ratio regardless of which
parent, male or female, contributed which trait. This is called a
reciprocal cross: a paired cross in which the respective traits of the Figure 12.1D. 1 : Results of Mendel’s Garden Pea Hybridizations:
Mendel conducted thousands of experiments and found the same
male and female in one cross become the respective traits of the ratios of offspring every time, regardless of which trait he examined.
female and male in the other cross. For the other six characteristics
Mendel examined, the F1 and F2generations behaved in the same KEY POINTS
way as they had for flower color. One of the two traits would Dominant traits are inherited unchanged from one generation to
disappear completely from the F1 generation only to reappear in the the next.
F2 generation at a ratio of approximately 3:1. Recessive traits disappear in the first filial generation, but
Upon compiling his results for many thousands of plants, Mendel reappear in the second filial generation at a ratio of 3:1,
concluded that the characteristics could be divided into expressed dominant:recessive.
and latent traits. He called these, respectively, dominant and In the F1 generation, Mendel found that one of the two options
recessive traits. Dominant traits are those that are inherited for each trait had disappeared (all offspring were identical
unchanged in a hybridization. Recessive traits become latent, or phenotypes), while in the F2 generation, the trait reappeared in
disappear, in the offspring of a hybridization. The recessive trait 1/4 of the offspring (a 3:1 ratio).
does, however, reappear in the progeny of the hybrid offspring. An

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KEY TERMS same locus
hybrid: offspring resulting from cross-breeding different entities,
This page titled 12.1D: Garden Pea Characteristics Revealed the Basics of
e.g. two different species or two purebred parent strains
Heredity is shared under a CC BY-SA 4.0 license and was authored,
recessive: able to be covered up by a dominant trait
remixed, and/or curated by Boundless.
dominant: a relationship between alleles of a gene, in which one
allele masks the expression (phenotype) of another allele at the

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12.1E: RULES OF PROBABILITY FOR MENDELIAN INHERITANCE

 LEARNING OBJECTIVES

Calculate the probability of traits of pea plants using

Mendelian crosses

PROBABILITY BASICS
Probabilities are mathematical measures of likelihood. The empirical
probability of an event is calculated by dividing the number of times
the event occurs by the total number of opportunities for the event to
occur. Empirical probabilities come from observations such as those
of Mendel. An example of a genetic event is a round seed produced
by a pea plant. Mendel demonstrated that the probability of the event
“round seed” was guaranteed to occur in the F1 offspring of true-
breeding parents, one of which has round seeds and one of which
has wrinkled seeds. When the F1 plants were subsequently self-
crossed, the probability of any given F2 offspring having round
seeds was now three out of four. In other words, in a large
population of F2 offspring chosen at random, 75 percent were
expected to have round seeds, whereas 25 percent were expected to Figure 12.1E. 1: Role of probability in segregation of alleles and
have wrinkled seeds. Using large numbers of crosses, Mendel was fertilization: In a genetic cross, the probability of the dominant trait
being expressed is dependent upon its frequency. In this case, both
able to calculate probabilities and use these to predict the outcomes parents possessed a dominant and a recessive gene for the trait of
of other crosses. flower color. The dominant trait is expressed in 3/4 of the offspring
and the recessive trait is expressed in 1/4.
THE PRODUCT RULE
Mendel demonstrated that the pea-plant characteristics he studied THE SUM RULE
were transmitted as discrete units from parent to offspring. Mendel
The sum rule is applied when considering two mutually-exclusive
also determined that different characteristics were transmitted
outcomes that can result from more than one pathway. It states that
independently of one another and could be considered in separate
the probability of the occurrence of one event or the other, of two
probability analyses. For instance, performing a cross between a
mutually-exclusive events, is the sum of their individual
plant with green, wrinkled seeds and a plant with yellow, round
probabilities. The word “or” indicates that you should apply the sum
seeds produced offspring that had a 3:1 ratio of green:yellow seeds
rule. Let’s imagine you are flipping a penny (P) and a quarter (Q).
and a 3:1 ratio of round:wrinkled seeds. The characteristics of color
What is the probability of one coin coming up heads and one coming
and texture did not influence each other.
up tails? This can be achieved by two cases: the penny is heads (PH)
The product rule of probability can be applied to this phenomenon of and the quarter is tails (QT), or the quarter is heads (QH) and the
the independent transmission of characteristics. It states that the penny is tails (PT). Either case fulfills the outcome. We calculate the
probability of two independent events occurring together can be probability of obtaining one head and one tail as [(PH) × (QT)] +
calculated by multiplying the individual probabilities of each event [(QH) × (PT)] = [(1/2) × (1/2)] + [(1/2) × (1/2)] = 1/2. You should
occurring alone. Imagine that you are rolling a six-sided die (D) and also notice that we used the product rule to calculate the probability
flipping a penny (P) at the same time. The die may roll any number of PH and QT and also the probability of PT and QH, before we
from 1–6 (D#), whereas the penny may turn up heads (PH) or tails summed them. The sum rule can be applied to show the probability
(PT). The outcome of rolling the die has no effect on the outcome of of having just one dominant trait in the F2 generation of a dihybrid
flipping the penny and vice versa. There are 12 possible outcomes, cross.
and each is expected to occur with equal probability: D1PH, D1PT,
To use probability laws in practice, it is necessary to work with large
D2PH, D2PT, D3PH, D3PT, D4PH, D4PT, D5PH, D5PT, D6PH, D6PT.
sample sizes because small sample sizes are prone to deviations
Of the 12 possible outcomes, the die has a 2/12 (or 1/6) probability caused by chance. The large quantities of pea plants that Mendel
of rolling a two, and the penny has a 6/12 (or 1/2) probability of examined allowed him to calculate the probabilities of the traits
coming up heads. The probability that you will obtain the combined appearing in his F2 generation. This discovery meant that when
outcome 2 and heads is: (D2) x (PH) = (1/6) x (1/2) or 1/12. The parental traits were known, the offspring’s traits could be predicted
word “and” is a signal to apply the product rule. Consider how the accurately even before fertilization.
product rule is applied to a dihybrid: the probability of having both
dominant traits in the F2 progeny is the product of the probabilities
of having the dominant trait for each characteristic.

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KEY POINTS OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44476/latest...ol11448/latest. License: CC
The Product Rule is used to determine the outcome of an event BY: Attribution
filial. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/filial.
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two mutually exclusive events from multiple pathways; the commons.wikimedia.org/wiki/Fi...el_flowers.svg. License: CC BY-SA:
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that trait.
recessive. Provided by: Wiktionary. Located at:
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KEY TERMS hybrid. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/hybrid.
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This page titled 12.1E: Rules of Probability for Mendelian Inheritance is curated by Boundless.
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or

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SECTION OVERVIEW

12.2: PATTERNS OF INHERITANCE

12.2D: ALTERNATIVES TO DOMINANCE AND
Topic hierarchy RECESSIVENESS

12.2E: SEX-LINKED TRAITS

12.2A: GENES AS THE UNIT OF HEREDITY
12.2F: LETHAL INHERITANCE PATTERNS
12.2B: PHENOTYPES AND GENOTYPES

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12.2A: GENES AS THE UNIT OF HEREDITY

 LEARNING OBJECTIVES

Describe the structure of a gene and how offspring inherit

genes from each parent

PAIRS OF UNIT FACTORS, OR GENES

Mendel proposed that paired unit factors of heredity were
transmitted faithfully from generation to generation by the
dissociation and reassociation of paired factors during
gametogenesis and fertilization, respectively. After he crossed peas
with contrasting traits and found that the recessive trait resurfaced in
the F2 generation, Mendel deduced that hereditary factors must be
inherited as discrete units. This finding contradicted the belief at that
time that parental traits were blended in the offspring.
A gene is made up of short sections of DNA that are contained on a
chromosome within the nucleus of a cell. Genes control the
development and function of all organs and all working systems in
the body. A gene has a certain influence on how the cell works; the
same gene in many different cells determines a certain physical or
biochemical feature of the whole body (e.g., eye color or
reproductive functions). All human cells hold approximately 21,000
different genes.
Genetics is the science of the way traits are passed from parent to
offspring. For all forms of life, continuity of the species depends
upon the genetic code being passed from parent to offspring.
Evolution by natural selection is dependent on traits being heritable.
Genetics is very important in human physiology because all
attributes of the human body are affected by a person’s genetic code.
It can be as simple as eye color, height, or hair color. Or it can be as Figure 12.2A. 1 : Gene pairs enable genetic combinations: A child
complex as how well your liver processes toxins, whether you will will inherit half of its genes (one of each of its 23 pairs) from its
mother and the other half from its father.
be prone to heart disease or breast cancer, and whether you will be
color blind. KEY POINTS
Genetic inheritance begins at the time of conception. You inherited A gene is a stretch of DNA that helps to control the development
23 chromosomes from your mother and 23 from your father. and function of all organs and working systems in the body.
Together they form 22 pairs of autosomal chromosomes and a pair Genes are passed from parent to offspring; the combination of
of sex chromosomes (either XX if you are female, or XY if you are these genes affects all aspects of the human body, from eye and
male). Homologous chromosomes have the same genes in the same hair color to how well the liver can process toxins.
positions, but may have different alleles (varieties) of those genes. A human will inherit 23 chromosomes from its mother and 23
There can be many alleles of a gene within a population, but an from its father; together, these form 23 pairs of chromosomes
individual within that population only has two copies and can be that direct the inherited characteristics of the individual.
homozygous (both copies the same) or heterozygous (the two copies If the two copies of a gene inherited from each parent are the
are different) for any given gene. same, that individual is said to be homozygous for the gene; if
the two copies inherited from each parent are different, that
individual is said to be heterozygous for the gene.

KEY TERMS
gene: a unit of heredity; the functional units of chromosomes that
determine specific characteristics by coding for specific proteins
chromosome: a structure in the cell nucleus that contains DNA,
histone protein, and other structural proteins
genetics: the branch of biology that deals with the transmission
and variation of inherited characteristics, in particular

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12.2B: PHENOTYPES AND GENOTYPES

 LEARNING OBJECTIVES

Distinguish between the phenotype and the genotype of an

organism

PHENOTYPES AND GENOTYPES

The observable traits expressed by an organism are referred to as its
phenotype. An organism’s underlying genetic makeup, consisting of
both physically visible and non-expressed alleles, is called its
genotype. Johann Gregor Mendel’s (1822–1884) hybridization
experiments demonstrate the difference between phenotype and
genotype.
Mendel crossed or mated two true-breeding (self-pollinating) garden
peas, Pisum saivum, by manually transferring pollen from the anther
of a mature pea plant of one variety to the stigma of a separate
mature pea plant of the second variety. Plants used in first-
generation crosses were called P0, or parental generation one, plants.
Mendel collected the seeds belonging to the P0plants that resulted
from each cross and grew them the following season. These
offspring were called the F1, or the first filial (filial = offspring,
daughter or son), generation. Once Mendel examined the
characteristics in the F1 generation of plants, he allowed them to
self-fertilize naturally. He then collected and grew the seeds from the
F1 plants to produce the F2, or second filial, generation.

Figure 12.2B. 1: Mendelian crosses: In one of his experiments on

inheritance patterns, Mendel crossed plants that were true-breeding
for violet flower color with plants true-breeding for white flower
color (the P generation). The resulting hybrids in the F1 generation
all had violet flowers. In the F2 generation, approximately three-
quarters of the plants had violet flowers, and one-quarter had white
flowers.
When true-breeding plants in which one parent had white flowers
and one had violet flowers were cross-fertilized, all of the F1 hybrid
offspring had violet flowers. That is, the hybrid offspring were
phenotypically identical to the true-breeding parent with violet
flowers. However, we know that the allele donated by the parent
with white flowers was not simply lost because it reappeared in
some of the F2 offspring. Therefore, the F1 plants must have been
genotypically different from the parent with violet flowers.
In his 1865 publication, Mendel reported the results of his crosses
involving seven different phenotypes, each with two contrasting
traits. A trait is defined as a variation in the physical appearance of a

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heritable characteristic. The characteristics included plant height, Mendel found that crossing two purebred pea plants which
seed texture, seed color, flower color, pea pod size, pea pod color, expressed different traits resulted in an F1generation where all
and flower position. To fully examine each characteristic, Mendel the pea plants expressed the same trait or phenotype.
generated large numbers of F1 and F2 plants, reporting results from When Mendel allowed the F1 plants to self-fertilize, the F2
19,959 F2 plants alone. His findings were consistent. First, Mendel generation showed two different phenotypes, indicating that the
confirmed that he had plants that bred true for white or violet flower F1 plants had different genotypes.
color. Regardless of how many generations Mendel examined, all
self-crossed offspring of parents with white flowers had white KEY TERMS
flowers, and all self-crossed offspring of parents with violet flowers phenotype: the appearance of an organism based on a
had violet flowers. In addition, Mendel confirmed that, other than multifactorial combination of genetic traits and environmental
flower color, the pea plants were physically identical. factors, especially used in pedigrees
genotype: the combination of alleles, situated on corresponding
KEY POINTS chromosomes, that determines a specific trait of an individual,
Mendel used pea plants with seven distinct traits or phenotypes such as “Aa” or “aa”
to determine the pattern of inheritance and the underlying
genotypes. This page titled 12.2B: Phenotypes and Genotypes is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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12.2C: THE PUNNETT SQUARE APPROACH FOR A MONOHYBRID CROSS

 LEARNING OBJECTIVES

Describe the Punnett square approach to a monohybrid cross

PUNNETT SQUARE APPROACH TO A

MONOHYBRID CROSS
When fertilization occurs between two true-breeding parents that
differ in only one characteristic, the process is called a monohybrid
cross, and the resulting offspring are monohybrids. Mendel
performed seven monohybrid crosses involving contrasting traits for
each characteristic. On the basis of his results in F1 and F2
generations, Mendel postulated that each parent in the monohybrid
cross contributed one of two paired unit factors to each offspring and
that every possible combination of unit factors was equally likely.
To demonstrate a monohybrid cross, consider the case of true-
breeding pea plants with yellow versus green pea seeds. The
dominant seed color is yellow; therefore, the parental genotypes
were YY ( homozygous dominant) for the plants with yellow seeds
and yy (homozygous recessive ) for the plants with green seeds,
respectively. A Punnett square, devised by the British geneticist
Reginald Punnett, can be drawn that applies the rules of probability
to predict the possible outcomes of a genetic cross or mating and
their expected frequencies.To prepare a Punnett square, all possible
combinations of the parental alleles are listed along the top (for one Figure 12.2C. 1 : Punnett square analysis of a monohytbrid cross: In
parent) and side (for the other parent) of a grid, representing their the P generation, pea plants that are true-breeding for the dominant
yellow phenotype are crossed with plants with the recessive green
meiotic segregation into haploid gametes. Then the combinations of phenotype. This cross produces F1 heterozygotes with a yellow
egg and sperm are made in the boxes in the table to show which phenotype. Punnett square analysis can be used to predict the
alleles are combining. Each box then represents the diploid genotype genotypes of the F2 generation.
of a zygote, or fertilized egg, that could result from this mating. A self-cross of one of the Yy heterozygous offspring can be
Because each possibility is equally likely, genotypic ratios can be represented in a 2 × 2 Punnett square because each parent can donate
determined from a Punnett square. If the pattern of inheritance one of two different alleles. Therefore, the offspring can potentially
(dominant or recessive) is known, the phenotypic ratios can be have one of four allele combinations: YY, Yy, yY, or yy. Notice that
inferred as well. For a monohybrid cross of two true-breeding there are two ways to obtain the Yy genotype: a Y from the egg and
parents, each parent contributes one type of allele. In this case, only a y from the sperm, or a y from the egg and a Y from the sperm.
one genotype is possible. All offspring are Yy and have yellow Both of these possibilities must be counted. Recall that Mendel’s
seeds. pea-plant characteristics behaved in the same way in reciprocal
crosses. Therefore, the two possible heterozygous combinations
produce offspring that are genotypically and phenotypically identical
despite their dominant and recessive alleles deriving from different
parents. They are grouped together. Because fertilization is a random
event, we expect each combination to be equally likely and for the
offspring to exhibit a ratio of YY:Yy:yy genotypes of 1:2:1.
Furthermore, because the YY and Yy offspring have yellow seeds
and are phenotypically identical, applying the sum rule of
probability, we expect the offspring to exhibit a phenotypic ratio of 3
yellow:1 green. Indeed, working with large sample sizes, Mendel
observed approximately this ratio in every F2 generation resulting
from crosses for individual traits.
Beyond predicting the offspring of a cross between known
homozygous or heterozygous parents, Mendel also developed a way
to determine whether an organism that expressed a dominant trait
was a heterozygote or a homozygote. Called the test cross, this

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technique is still used by plant and animal breeders. In a test cross, KEY POINTS
the dominant-expressing organism is crossed with an organism that Fertilization between two true-breeding parents that differ in
is homozygous recessive for the same characteristic. If the only one characteristic is called a monohybrid cross.
dominant-expressing organism is a homozygote, then all F1 For a monohybrid cross of two true-breeding parents, each parent
offspring will be heterozygotes expressing the dominant trait. contributes one type of allele resulting in all of the offspring with
Alternatively, if the dominant expressing organism is a heterozygote, the same genotype.
the F1 offspring will exhibit a 1:1 ratio of heterozygotes and A test cross is a way to determine whether an organism that
recessive homozygotes. The test cross further validates Mendel’s expressed a dominant trait was a heterozygote or a homozygote.
postulate that pairs of unit factors segregate equally.
KEY TERMS
monohybrid: a hybrid between two species that only have a
difference of one gene
homozygous: of an organism in which both copies of a given
gene have the same allele
heterozygous: of an organism which has two different alleles of
a given gene
Punnett square: a graphical representation used to determine the
probability of an offspring expressing a particular genotype

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Figure 12.2C. 1 : Example of a test cross: A test cross can be

performed to determine whether an organism expressing a dominant
trait is a homozygote or a heterozygote.

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12.2D: ALTERNATIVES TO DOMINANCE AND RECESSIVENESS

 LEARNING OBJECTIVES

Discuss incomplete dominance, codominance, and multiple

alleles as alternatives to dominance and recessiveness

ALTERNATIVES TO DOMINANCE AND

RECESSIVENESS
Mendel’s experiments with pea plants suggested that: (1) two “units”
or alleles exist for every gene; (2) alleles maintain their integrity in
each generation (no blending); and (3) in the presence of the
dominant allele, the recessive allele is hidden and makes no
contribution to the phenotype. Therefore, recessive alleles can be
“carried” and not expressed by individuals. Such heterozygous
individuals are sometimes referred to as “carriers.” Further genetic
studies in other plants and animals have shown that much more
complexity exists, but that the fundamental principles of Mendelian
genetics still hold true.
Mendel’s results, that traits are inherited as dominant and recessive
pairs, contradicted the view at that time that offspring exhibited a Figure 12.2D. 1 : Example of incomplete dominance: These pink
blend of their parents’ traits. However, the heterozygote phenotype flowers of a heterozygote snapdragon result from incomplete
dominance.
occasionally does appear to be intermediate between the two
parents. For example, in the snapdragon, Antirrhinum majus, a cross A variation on incomplete dominance is codominance, in which both
between a homozygous parent with white flowers (CWCW) and a alleles for the same characteristic are simultaneously expressed in
homozygous parent with red flowers (CRCR) will produce offspring the heterozygote. An example of codominance is the MN blood
with pink flowers (CRCW). This pattern of inheritance is described groups of humans. The M and N alleles are expressed in the form of
as incomplete dominance, denoting the expression of two an M or N antigen present on the surface of red blood cells.
contrasting alleles such that the individual displays an intermediate Homozygotes (LMLMand LNLN) express either the M or the N allele,
phenotype. The allele for red flowers is incompletely dominant over and heterozygotes (LMLN) express both alleles equally. In a self-
the allele for white flowers. However, the results of a heterozygote cross between heterozygotes expressing a codominant trait, the three
self-cross can still be predicted, just as with Mendelian dominant possible offspring genotypes are phenotypically distinct. However,
and recessive crosses. In this case, the genotypic ratio would be 1 the 1:2:1 genotypic ratio characteristic of a Mendelian monohybrid
CRCR:2 CRCW:1 CWCW, and the phenotypic ratio would be 1:2:1 for cross still applies.
red:pink:white. Mendel implied that only two alleles, one dominant and one
recessive, could exist for a given gene. We now know that this is an
oversimplification. Although individual humans (and all diploid
organisms) can only have two alleles for a given gene, multiple
alleles may exist at the population level such that many
combinations of two alleles are observed. Note that when many
alleles exist for the same gene, the convention is to denote the most
common phenotype or genotype among wild animals as the wild
type (often abbreviated “+”); this is considered the standard or norm.
All other phenotypes or genotypes are considered variants of this
standard, meaning that they deviate from the wild type. The variant
may be recessive or dominant to the wild-type allele. An example of
multiple alleles is coat color in rabbits. Here, four alleles exist for
the c gene. The wild-type version, C+C+, is expressed as brown fur.
The chinchilla phenotype, cchcch, is expressed as black-tipped white
fur. The Himalayan phenotype, chch, has black fur on the extremities
and white fur elsewhere. Finally, the albino, or “colorless”
phenotype, cc, is expressed as white fur. In cases of multiple alleles,
dominance hierarchies can exist. In this case, the wild-type allele is
dominant over all the others, chinchilla is incompletely dominant

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over Himalayan and albino, and Himalayan is dominant over albino.
This hierarchy, or allelic series, was revealed by observing the
phenotypes of each possible heterozygote offspring.

Figure 12.2D. 1 : Example of a mutant allele interfering with the

function of a wild-type gene: As seen in comparing the wild-type
Drosophila(left) and the Antennapedia mutant (right), the
Antennapedia mutant has legs on its head in place of antennae.
Figure 12.2D. 1 : Example of multiple alleles for rabbit coat color:
Four different alleles exist for the rabbit coat color (C) gene. KEY POINTS
The complete dominance of a wild-type phenotype over all other Incomplete dominance is the expression of two contrasting
mutants often occurs as an effect of “dosage” of a specific gene alleles such that the individual displays an intermediate
product, such that the wild-type allele supplies the correct amount of phenotype.
gene product whereas the mutant alleles cannot. For the allelic series Codominance is a variation on incomplete dominance in which
in rabbits, the wild-type allele may supply a given dosage of fur both alleles for the same characteristic are simultaneously
pigment, whereas the mutants supply a lesser dosage or none at all. expressed in the heterozygote.
Alternatively, one mutant allele can be dominant over all other Diploid organisms can only have two alleles for a given gene;
phenotypes, including the wild type. This may occur when the however, multiple alleles may exist at the population level such
mutant allele somehow interferes with the genetic message so that that many combinations of two alleles are observed.
even a heterozygote with one wild-type allele copy expresses the The complete dominance of a wild-type phenotype over all other
mutant phenotype. One way in which the mutant allele can interfere mutants often occurs as an effect of “dosage” of a specific gene
is by enhancing the function of the wild-type gene product or product: the wild-type allele supplies the correct amount of gene
changing its distribution in the body. One example of this is the product whereas the mutant alleles cannot.
Antennapedia mutation in Drosophila. In this case, the mutant allele One mutant allele can also be dominant over all other
expands the distribution of the gene product; as a result, the phenotypes, including the wild type.
Antennapedia heterozygote develops legs on its head where its
antennae should be. KEY TERMS
allele: one of a number of alternative forms of the same gene
occupying a given position on a chromosome
incomplete dominance: a condition in which the phenotype of
the heterozygous genotype is distinct from and often
intermediate to the phenotypes of the homozygous genotypes
codominance: a condition in which both alleles of a gene pair in
a heterozygote are fully expressed, with neither one being
dominant or recessive to the other

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12.2E: SEX-LINKED TRAITS
In fruit flies, the wild-type eye color is red (XW) and is dominant to
 LEARNING OBJECTIVES white eye color (Xw). Because this eye-color gene is located on the
X chromosome only, reciprocal crosses do not produce the same
Distinguish between sex-linked traits and other forms of
offspring ratios. Males are said to be hemizygous, because they have
inheritance
only one allele for any X-linked characteristic. Hemizygosity makes
the descriptions of dominance and recessiveness irrelevant for XY
SEX DETERMINATION males because each male only has one copy of the gene. Drosophila
In humans, as well as in many other animals and some plants, the males lack a second allele copy on the Y chromosome; their
sex of the individual is determined by sex chromosomes. However, genotype can only be XWY or XwY. In contrast, females have two
there are other sex determination systems in nature. For example, allele copies of this gene and can be XWXW, XWXw, or XwXw.
temperature-dependent sex determination is relatively common,
and there are many other types of environmental sex determination.
Some species, such as some snails, practice sex change adults start
out male, then become female. In tropical clown fish, the dominant
individual in a group becomes female while the others are male.
The sex chromosomes are one pair of non-homologous
chromosomes. Until now, we have only considered inheritance
patterns among non-sex chromosomes, or autosomes. In addition to
22 homologous pairs of autosomes, human females have a
homologous pair of X chromosomes, whereas human males have an
XY chromosome pair. Although the Y chromosome contains a small
region of similarity to the X chromosome so that they can pair
during meiosis, the Y chromosome is much shorter and contains
many fewer genes. When a gene being examined is present on the X
chromosome, but not on the Y chromosome, it is said to be X-
linked.

Figure 12.2E. 1: Eye color in Drosophila is an example of a X-

linked trait: In Drosophila, the gene for eye color is located on the X
chromosome. Clockwise from top left are brown, cinnabar, sepia,
vermilion, white, and red. Red eye color is wild-type and is
dominant to white eye color.

X-LINKED CROSSES
In an X-linked cross, the genotypes of F1 and F2 offspring depend on
whether the recessive trait was expressed by the male or the female
in the P1 generation. With regard to Drosophila eye color, when the
P1 male expresses the white-eye phenotype and the female is
homozygous red-eyed, all members of the F1 generation exhibit red
Figure 12.2E. 1: Human male karyotype: A human males possesses eyes. The F1 females are heterozygous (XWXw), and the males are
XY chromosomes, as seen in the bottom left of this karyotype. The all XWY, having received their X chromosome from the homozygous
Y chromosome is much shorter than the X chromosome, unlike all
of the other homologous chromosome pairs. dominant P1 female and their Y chromosome from the P1 male.
A subsequent cross between the XWXw female and the XWY male
X-LINKED TRAITS would produce only red-eyed females (with XWXW or
Insects also follow an XY sex-determination pattern and like XWXwgenotypes) and both red- and white-eyed males (with XWY or
humans, Drosophila males have an XY chromosome pair and XwY genotypes). Now, consider a cross between a homozygous
females are XX. Eye color in Drosophila was one of the first X- white-eyed female and a male with red eyes. The F1 generation
linked traits to be identified, and Thomas Hunt Morgan mapped this would exhibit only heterozygous red-eyed females (XWXw) and only
trait to the X chromosome in 1910. white-eyed males (XwY). Half of the F2 females would be red-eyed

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(XWXw) and half would be white-eyed (XwXw). Similarly, half of of the X chromosomes. However, female carriers can contribute the
the F2 males would be red-eyed (XWY) and half would be white- trait to their sons, resulting in the son exhibiting the trait, or they can
eyed (XwY). contribute the recessive allele to their daughters, resulting in the
daughters being carriers of the trait. Although some Y-linked
recessive disorders exist, typically they are associated with infertility
in males and are, therefore, not transmitted to subsequent
generations.

Figure 12.2E. 1: Punnett square analysis of Drosophila eye color:

Punnett square analysis is used to determine the ratio of offspring
from a cross between a red-eyed male fruit fly (XWY) and a white-
eyed female fruit fly (XwXw).

X-LINKED RECESSIVE DISORDERS IN HUMANS

Sex-linkage studies provided the fundamentals for understanding X- Figure 12.2E. 1: Inheritance of a recessive X-linked disorder: The
son of a woman who is a carrier of a recessive X-linked disorder will
linked recessive disorders in humans, which include red-green color have a 50 percent chance of being affected. A daughter will not be
blindness and Types A and B hemophilia. Because human males affected, but she will have a 50 percent chance of being a carrier like
need to inherit only one recessive mutant X allele to be affected, X- her mother.
linked disorders are disproportionately observed in males. Females
KEY POINTS
must inherit recessive X-linked alleles from both of their parents in
order to express the trait. In mammals, females have a homologous pair of X
chromosomes, whereas males have an XY chromosome pair.
The Y chromosome contains a small region of similarity to the X
chromosome so that they can pair during meiosis, but the Y is
much shorter and contains fewer genes.
Males are said to be hemizygous because they have only one
allele for any X-linked characteristic; males will exhibit the trait
of any gene on the X-chromosome regardless of dominance and
recessiveness.
Most sex-linked traits are actually X-linked, such as eye color in
Drosophila or color blindness in humans.
Figure 12.2E. 1: Color perception in different types of color
blindness: In this chart you can see what people with different types KEY TERMS
of color blindness can see versus the normal color vision line at top.
hemizygous: Having some single copies of genes in an
RECESSIVE CARRIERS otherwise diploid cell or organism.
X-linked: Associated with the X chromosome.
When they inherit one recessive X-linked mutant allele and one
carrier: A person or animal that transmits a disease to others
dominant X-linked wild-type allele, they are carriers of the trait and
without itself contracting the disease.
are typically unaffected. Carrier females can manifest mild forms of
sex chromosomes: A chromosome involved with determining
the trait due to the inactivation of the dominant allele located on one
the sex of an organism, typically one of two kinds.

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12.2F: LETHAL INHERITANCE PATTERNS

 LEARNING OBJECTIVES

Describe recessive and dominant lethal inheritance patterns

LETHAL INHERITANCE PATTERNS

A large proportion of genes in an individual’s genome are essential
for survival. Occasionally, a nonfunctional allele for an essential
gene can arise by mutation and be transmitted in a population as
long as individuals with this allele also have a wild-type, functional
copy. The wild-type allele functions at a capacity sufficient to
sustain life and is, therefore, considered to be dominant over the
nonfunctional allele. However, consider two heterozygous parents
that have a genotype of wild-type/nonfunctional mutant for a
hypothetical essential gene. In one quarter of their offspring, we
would expect to observe individuals that are homozygous recessive
for the nonfunctional allele. Because the gene is essential, these
individuals might fail to develop past fertilization, die in utero, or
die later in life, depending on what life stage requires this gene. An
inheritance pattern in which an allele is only lethal in the
homozygous form and in which the heterozygote may be normal or
have some altered non-lethal phenotype is referred to as recessive
lethal. Figure 12.2F . 1 : Effects of Huntington’s disease on neurons: The
For crosses between heterozygous individuals with a recessive lethal neuron in the center of this micrograph (yellow) has nuclear
inclusions characteristic of Huntington’s disease (orange area in the
allele that causes death before birth when homozygous, only wild- center of the neuron). Huntington’s disease occurs when an
type homozygotes and heterozygotes would be observed. The abnormal dominant allele for the Huntington gene is present.
genotypic ratio would therefore be 2:1. In other instances, the
recessive lethal allele might also exhibit a dominant (but not lethal) KEY POINTS
phenotype in the heterozygote. For instance, the recessive lethal An inheritance pattern in which an allele is only lethal in the
Curly allele in Drosophila affects wing shape in the heterozygote homozygous form and in which the heterozygote may be normal
form, but is lethal in the homozygote. or have some altered non-lethal phenotype is referred to as
recessive lethal.
DOMINANT LETHAL ALLELES
The dominant lethal inheritance pattern is one in which an allele
A single copy of the wild-type allele is not always sufficient for is lethal both in the homozygote and the heterozygote; this allele
normal functioning or even survival. The dominant lethal inheritance can only be transmitted if the lethality phenotype occurs after
pattern is one in which an allele is lethal both in the homozygote and reproductive age.
the heterozygote; this allele can only be transmitted if the lethality Dominant lethal alleles are very rare because the allele only lasts
phenotype occurs after reproductive age. Individuals with mutations one generation and is, therefore, not usually transmitted.
that result in dominant lethal alleles fail to survive even in the In the case where dominant lethal alleles might not be expressed
heterozygote form. Dominant lethal alleles are very rare because, as until adulthood, the allele may be unknowingly passed on,
you might expect, the allele only lasts one generation and is not resulting in a delayed death in both generations.
transmitted. However, just as the recessive lethal allele might not
immediately manifest the phenotype of death, dominant lethal alleles KEY TERMS
also might not be expressed until adulthood. Once the individual mutation: any heritable change of the base-pair sequence of
reaches reproductive age, the allele may be unknowingly passed on, genetic material
resulting in a delayed death in both generations. An example of this recessive lethal: an inheritance pattern in which an allele is only
in humans is Huntington’s disease in which the nervous system lethal in the homozygous form and in which the heterozygote
gradually wastes away. People who are heterozygous for the may be normal or have some altered non-lethal phenotype
dominant Huntington allele (Hh) will inevitably develop the fatal dominant lethal: an inheritance pattern is one in which an allele
disease. However, the onset of Huntington’s disease may not occur is lethal both in the homozygote and the heterozygote; this allele
until age 40, at which point the afflicted persons may have already can only be transmitted if the lethality phenotype occurs after
passed the allele to 50 percent of their offspring. reproductive age

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SECTION OVERVIEW

12.3: LAWS OF INHERITANCE

12.3D: MENDEL’S LAW OF INDEPENDENT
Topic hierarchy ASSORTMENT

12.3E: GENETIC LINKAGE AND VIOLATION OF

12.3A: MENDEL’S LAWS OF HEREDITY
THE LAW OF INDEPENDENT ASSORTMENT
12.3B: MENDEL’S LAW OF DOMINANCE
12.3F: EPISTASIS
12.3C: MENDEL’S LAW OF SEGREGATION
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12.3A: MENDEL’S LAWS OF HEREDITY

 LEARNING OBJECTIVES

Discuss the methods Mendel utilized in his research that led

to his success in understanding the process of inheritance

INTRODUCTION
Mendelian inheritance (or Mendelian genetics or Mendelism) is a set
of primary tenets relating to the transmission of hereditary
characteristics from parent organisms to their children; it underlies
much of genetics. The tenets were initially derived from the work of
Gregor Mendel published in 1865 and 1866, which was “re-
discovered” in 1900; they were initially very controversial, but they
soon became the core of classical genetics.
The laws of inheritance were derived by Gregor Mendel, a 19th
century monk conducting hybridization experiments in garden peas
(Pisum sativum). Between 1856 and 1863, he cultivated and tested
some 28,000 pea plants. From these experiments, he deduced two
generalizations that later became known as Mendel’s Laws of
Heredity or Mendelian inheritance. He described these laws in a two
part paper, “Experiments on Plant Hybridization”, which was
published in 1866.

MENDEL’S LAWS
Mendel discovered that by crossing true-breeding white flower and
true-breeding purple flower plants, the result was a hybrid offspring.
Rather than being a mix of the two colors, the offspring was purple
flowered. He then conceived the idea of heredity units, which he
called “factors”, one of which is a recessive characteristic and the
other dominant. Mendel said that factors, later called genes,
normally occur in pairs in ordinary body cells, yet segregate during
the formation of sex cells. Each member of the pair becomes part of
the separate sex cell. The dominant gene, such as the purple flower
in Mendel’s plants, will hide the recessive gene, the white flower.
After Mendel self-fertilized the F1 generation and obtained an F2
generation with a 3:1 ratio, he correctly theorized that genes can be
paired in three different ways for each trait: AA, aa, and Aa. The
capital A represents the dominant factor while the lowercase a
represents the recessive.
Figure 12.3A. 1 : Mendel’s Pea Plants: In one of his experiments on
inheritance patterns, Mendel crossed plants that were true-breeding
for violet flower color with plants true-breeding for white flower
color (the P generation). The resulting hybrids in the F1 generation
all had violet flowers. In the F2 generation, approximately three-
quarters of the plants had violet flowers, and one-quarter had white
flowers.
Mendel stated that each individual has two alleles for each trait, one
from each parent. Thus, he formed the “first rule”, the Law of
Segregation, which states individuals possess two alleles and a
parent passes only one allele to his/her offspring. One allele is given
by the female parent and the other is given by the male parent. The
two factors may or may not contain the same information. If the two
alleles are identical, the individual is called homozygous for the
trait. If the two alleles are different, the individual is called
heterozygous. The presence of an allele does not promise that the

12.3A.1 https://bio.libretexts.org/@go/page/13268
trait will be expressed in the individual that possesses it. In KEY POINTS
heterozygous individuals, the only allele that is expressed is the By crossing purple and white pea plants, Mendel found the
dominant. The recessive allele is present, but its expression is offspring were purple rather than mixed, indicating one color
hidden. The genotype of an individual is made up of the many was dominant over the other.
alleles it possesses. An individual’s physical appearance, or Mendel’s Law of Segregation states individuals possess two
phenotype, is determined by its alleles as well as by its environment. alleles and a parent passes only one allele to his/her offspring.
Mendel also analyzed the pattern of inheritance of seven pairs of Mendel’s Law of Independent Assortment states the inheritance
contrasting traits in the domestic pea plant. He did this by cross- of one pair of factors ( genes ) is independent of the inheritance
breeding dihybrids; that is, plants that were heterozygous for the of the other pair.
alleles controlling two different traits. Mendel then crossed these If the two alleles are identical, the individual is called
dihybrids. If it is inevitable that round seeds must always be yellow homozygous for the trait; if the two alleles are different, the
and wrinkled seeds must be green, then he would have expected that individual is called heterozygous.
this would produce a typical monohybrid cross: 75 percent round- Mendel cross-bred dihybrids and found that traits were inherited
yellow; 25 percent wrinkled-green. But, in fact, his mating generated independently of each other.
seeds that showed all possible combinations of the color and texture
traits. He found 9/16 of the offspring were round-yellow, 3/16 were KEY TERMS
round-green, 3/16 were wrinkled-yellow, and 1/16 were wrinkled- homozygous: of an organism in which both copies of a given
green. Finding in every case that each of his seven traits was gene have the same allele
inherited independently of the others, he formed his “second rule”, heterozygous: of an organism which has two different alleles of
the Law of Independent Assortment, which states the inheritance of a given gene
one pair of factors (genes) is independent of the inheritance of the allele: one of a number of alternative forms of the same gene
other pair. Today we know that this rule holds only if the genes are occupying a given position on a chromosome
on separate chromosomes
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12.3B: MENDEL’S LAW OF DOMINANCE
which the dominant allele is transmitted. The recessive trait will
 LEARNING OBJECTIVES only be expressed by offspring that have two copies of this allele;
these offspring will breed true when self-crossed.
Explain the concept of dominance versus recessiveness
By definition, the terms dominant and recessive refer to the
genotypic interaction of alleles in producing the phenotype of the
ALLELES CAN BE DOMINANT OR RECESSIVE
heterozygote. The key concept is genetic: which of the two alleles
Most familiar animals and some plants have paired chromosomes present in the heterozygote is expressed, such that the organism is
and are described as diploid. They have two versions of each phenotypically identical to one of the two homozygotes. It is
chromosome: one contributed by the female parent in her ovum and sometimes convenient to talk about the trait corresponding to the
one by the male parent in his sperm. These are joined at fertilization. dominant allele as the dominant trait and the trait corresponding to
The ovum and sperm cells (the gametes) have only one copy of each the hidden allele as the recessive trait. However, this can easily lead
chromosome and are described as haploid. to confusion in understanding the concept as phenotypic. For
example, to say that “green peas” dominate “yellow peas” confuses
inherited genotypes and expressed phenotypes. This will
subsequently confuse discussion of the molecular basis of the
phenotypic difference. Dominance is not inherent. One allele can be
dominant to a second allele, recessive to a third allele, and
codominant to a fourth. If a genetic trait is recessive, a person needs
to inherit two copies of the gene for the trait to be expressed. Thus,
both parents have to be carriers of a recessive trait in order for a
child to express that trait.
Since Mendel’s experiments with pea plants, other researchers have
found that the law of dominance does not always hold true. Instead,
several different patterns of inheritance have been found to exist.

KEY POINTS
Dominant alleles are expressed exclusively in a heterozygote,
while recessive traits are expressed only if the organism is
homozygous for the recessive allele.
A single allele may be dominant over one allele, but recessive to
another.
Not all traits are controlled by simple dominance as a form of
inheritance; more complex forms of inheritance have been found
to exist.
Figure 12.3B. 1: Recessive traits are only visible if an individual
inherits two copies of the recessive allele: The child in the photo KEY TERMS
expresses albinism, a recessive trait. dominant: a relationship between alleles of a gene, in which one
Mendel’s law of dominance states that in a heterozygote, one trait allele masks the expression (phenotype) of another allele at the
will conceal the presence of another trait for the same characteristic. same locus
Rather than both alleles contributing to a phenotype, the dominant recessive: able to be covered up by a dominant trait
allele will be expressed exclusively. The recessive allele will remain
“latent,” but will be transmitted to offspring by the same manner in This page titled 12.3B: Mendel’s Law of Dominance is shared under a CC
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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12.3C: MENDEL’S LAW OF SEGREGATION
For the F2 generation of a monohybrid cross, the following three
 LEARNING OBJECTIVES possible combinations of genotypes could result: homozygous
dominant, heterozygous, or homozygous recessive. Because
Apply the law of segregation to determine the chances of a
heterozygotes could arise from two different pathways (receiving
particular genotype arising from a genetic cross
one dominant and one recessive allele from either parent), and
because heterozygotes and homozygous dominant individuals are
EQUAL SEGREGATION OF ALLELES phenotypically identical, the law supports Mendel’s observed 3:1
Observing that true-breeding pea plants with contrasting traits gave phenotypic ratio. The equal segregation of alleles is the reason we
rise to F1 generations that all expressed the dominant trait and F2 can apply the Punnett square to accurately predict the offspring of
generations that expressed the dominant and recessive traits in a 3:1 parents with known genotypes.
ratio, Mendel proposed the law of segregation. The law of The physical basis of Mendel’s law of segregation is the first
segregation states that each individual that is a diploid has a pair of division of meiosis in which the homologous chromosomes with
alleles (copy) for a particular trait. Each parent passes an allele at their different versions of each gene are segregated into daughter
random to their offspring resulting in a diploid organism. The allele nuclei. The behavior of homologous chromosomes during meiosis
that contains the dominant trait determines the phenotype of the can account for the segregation of the alleles at each genetic locus to
offspring. In essence, the law states that copies of genes separate or different gametes. As chromosomes separate into different gametes
segregate so that each gamete receives only one allele. during meiosis, the two different alleles for a particular gene also
segregate so that each gamete acquires one of the two alleles. In
Mendel’s experiments, the segregation and the independent
assortment during meiosis in the F1 generation give rise to the F2
Unaffected Unaffected phenotypic ratios observed by Mendel. The role of the meiotic
"Carrier" "Carrier" segregation of chromosomes in sexual reproduction was not
Father Mother understood by the scientific community during Mendel’s lifetime.

KEY POINTS
Each gamete acquires one of the two alleles as chromosomes
R r R r separate into different gametes during meiosis.
Heterozygotes, which posess one dominant and one recessive
allele, can receive each allele from either parent and will look
R R R r R r r r identical to homozygous dominant individuals; the Law of
Segregation supports Mendel’s observed 3:1 phenotypic ratio.
Mendel proposed the Law of Segregation after observing that
pea plants with two different traits produced offspring that all
expressed the dominant trait, but the following generation
expressed the dominant and recessive traits in a 3:1 ratio.

KEY TERMS
law of segregation: a diploid individual possesses a pair of
alleles for any particular trait and each parent passes one of these
Unaffected Unaffected "Carrier" Affected randomly to its offspring
1 in 4 chance 2 in 4 chance 1 in 4 chance
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segregate randomly into gametes: When gametes are formed, each
allele of one parent segregates randomly into the gametes, such that
half of the parent’s gametes carry each allele.

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12.3D: MENDEL’S LAW OF INDEPENDENT ASSORTMENT

 LEARNING OBJECTIVES

Use the probability or forked line method to calculate the

chance of any particular genotype arising from a genetic
cross

INDEPENDENT ASSORTMENT
Mendel’s law of independent assortment states that genes do not
influence each other with regard to the sorting of alleles into
gametes: every possible combination of alleles for every gene is
equally likely to occur. The independent assortment of genes can be
illustrated by the dihybrid cross: a cross between two true-breeding
parents that express different traits for two characteristics. Consider
the characteristics of seed color and seed texture for two pea plants:
one that has green, wrinkled seeds (yyrr) and another that has
yellow, round seeds (YYRR). Because each parent is homozygous,
the law of segregation indicates that the gametes for the
green/wrinkled plant all are yr, while the gametes for the
yellow/round plant are all YR. Therefore, the F1 generation of
offspring all are YyRr. Figure 12.3D. 1 : Independent assortment of 2 genes: This dihybrid
cross of pea plants involves the genes for seed color and texture.
For the F2 generation, the law of segregation requires that each
gamete receive either an R allele or an r allele along with either a Y Because of independent assortment and dominance, the 9:3:3:1
allele or a y allele. The law of independent assortment states that a dihybrid phenotypic ratio can be collapsed into two 3:1 ratios,
gamete into which an r allele sorted would be equally likely to characteristic of any monohybrid cross that follows a dominant and
contain either a Y allele or a y allele. Thus, there are four equally recessive pattern. Ignoring seed color and considering only seed
likely gametes that can be formed when the YyRr heterozygote is texture in the above dihybrid cross, we would expect that three-
self-crossed as follows: YR, Yr, yR, and yr. Arranging these gametes quarters of the F2 generation offspring would be round and one-
along the top and left of a 4 × 4 Punnett square gives us 16 equally quarter would be wrinkled. Similarly, isolating only seed color, we
likely genotypic combinations. From these genotypes, we infer a would assume that three-quarters of the F2offspring would be yellow
phenotypic ratio of 9 round/yellow:3 round/green:3 and one-quarter would be green. The sorting of alleles for texture
wrinkled/yellow:1 wrinkled/green. These are the offspring ratios we and color are independent events, so we can apply the product rule.
would expect, assuming we performed the crosses with a large Therefore, the proportion of round and yellow F2 offspring is
enough sample size. expected to be (3/4) × (3/4) = 9/16, and the proportion of wrinkled
and green offspring is expected to be (1/4) × (1/4) = 1/16. These
proportions are identical to those obtained using a Punnett square.
Round/green and wrinkled/yellow offspring can also be calculated
using the product rule as each of these genotypes includes one
dominant and one recessive phenotype. Therefore, the proportion of
each is calculated as (3/4) × (1/4) = 3/16.

FORKED-LINE METHOD
When more than two genes are being considered, the Punnett-square
method becomes unwieldy. For instance, examining a cross
involving four genes would require a 16 × 16 grid containing 256
boxes. It would be extremely cumbersome to manually enter each
genotype. For more complex crosses, the forked-line and probability
methods are preferred.
To prepare a forked-line diagram for a cross between F1
heterozygotes resulting from a cross between AABBCC and aabbcc
parents, we first create rows equal to the number of genes being
considered and then segregate the alleles in each row on forked lines
according to the probabilities for individual monohybrid crosses. We

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then multiply the values along each forked path to obtain the F2 a dominant and recessive pattern, what proportion of the offspring
offspring probabilities. Note that this process is a diagrammatic will be expected to be homozygous recessive for all four alleles?
version of the product rule. The values along each forked pathway Rather than writing out every possible genotype, we can use the
can be multiplied because each gene assorts independently. For a probability method. We know that for each gene the fraction of
trihybrid cross, the F2phenotypic ratio is 27:9:9:9:3:3:3:1. homozygous recessive offspring will be 1/4. Therefore, multiplying
this fraction for each of the four genes, (1/4) × (1/4) × (1/4) × (1/4),
we determine that 1/256 of the offspring will be quadruply
homozygous recessive.

KEY POINTS
Mendel’s law of independent assortment states that genes do not
influence each other with regard to the sorting of alleles into
gametes; every possible combination of alleles for every gene is
Figure 12.3D. 1 : Independent assortment of 3 genes: The forked- equally likely to occur.
line method can be used to analyze a trihybrid cross. Here, the
probability for color in the F2 generation occupies the top row (3 The calculation of any particular genotypic combination of more
yellow:1 green). The probability for shape occupies the second row than one gene is, therefore, the probability of the desired
(3 round:1 wrinked), and the probability for height occupies the third genotype at the first locus multiplied by the probability of the
row (3 tall:1 dwarf). The probability for each possible combination
of traits is calculated by multiplying the probability for each desired genotype at the other loci.
individual trait. Thus, the probability of F2 offspring having yellow, The forked line method can be used to calculate the chances of
round, and tall traits is 3 × 3 × 3, or 27. all possible genotypic combinations from a cross, while the
probability method can be used to calculate the chance of any
PROBABILITY METHOD
one particular genotype that might result from that cross.
While the forked-line method is a diagrammatic approach to keeping
track of probabilities in a cross, the probability method gives the KEY TERMS
proportions of offspring expected to exhibit each phenotype (or independent assortment: separate genes for separate traits are
genotype) without the added visual assistance.
passed independently of one another from parents to offspring
To fully demonstrate the power of the probability method, however,
we can consider specific genetic calculations. For instance, for a This page titled 12.3D: Mendel’s Law of Independent Assortment is shared
tetrahybrid cross between individuals that are heterozygotes for all under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
four genes, and in which all four genes are sorting independently in by Boundless.

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12.3E: GENETIC LINKAGE AND VIOLATION OF THE LAW OF INDEPENDENT
ASSORTMENT

 LEARNING OBJECTIVES

Describe how recombination can separate linked genes

LINKED GENES VIOLATE THE LAW OF

INDEPENDENT ASSORTMENT
Although all of Mendel’s pea characteristics behaved according to
the law of independent assortment, we now know that some allele
combinations are not inherited independently of each other. Genes
that are located on separate non-homologous chromosomes will
always sort independently. However, each chromosome contains
hundreds or thousands of genes organized linearly on chromosomes
like beads on a string. The segregation of alleles into gametes can be
influenced by linkage, in which genes that are located physically
close to each other on the same chromosome are more likely to be
inherited as a pair. However, because of the process of
recombination, or “crossover,” it is possible for two genes on the
same chromosome to behave independently, or as if they are not
linked. To understand this, let’s consider the biological basis of gene
linkage and recombination.

Figure 12.3E. 1: Linked genes can be separated by recombination:

The process of crossover, or recombination, occurs when two
homologous chromosomes align during meiosis and exchange a
segment of genetic material. Here, the alleles for gene C were
exchanged. The result is two recombinant and two non-recombinant
chromosomes.

When two genes are located in close proximity on the same

chromosome, they are considered linked, and their alleles tend to be
transmitted through meiosis together. To exemplify this, imagine a
dihybrid cross involving flower color and plant height in which the
genes are next to each other on the chromosome. If one homologous
Figure 12.3E. 1: Unlinked genes assort independently: This figure chromosome has alleles for tall plants and red flowers, and the other
shows all possible combinations of offspring resulting from a chromosome has genes for short plants and yellow flowers, then
dihybrid cross of pea plants that are heterozygous for the tall/dwarf when the gametes are formed, the tall and red alleles will go together
and inflated/constricted alleles.
into a gamete and the short and yellow alleles will go into other
Homologous chromosomes possess the same genes in the same gametes. These are called the parental genotypes because they have
linear order. The alleles may differ on homologous chromosome been inherited intact from the parents of the individual producing
pairs, but the genes to which they correspond do not. In preparation gametes. But unlike if the genes were on different chromosomes,
for the first division of meiosis, homologous chromosomes replicate there will be no gametes with tall and yellow alleles and no gametes
and synapse. Like genes on the homologs align with each other. At with short and red alleles. If you create the Punnett square with these
this stage, segments of homologous chromosomes exchange linear gametes, you will see that the classical Mendelian prediction of a
segments of genetic material. This process is called recombination, 9:3:3:1 outcome of a dihybrid cross would not apply. As the distance
or crossover, and it is a common genetic process. Because the genes between two genes increases, the probability of one or more
are aligned during recombination, the gene order is not altered. crossovers between them increases, and the genes behave more like
Instead, the result of recombination is that maternal and paternal they are on separate chromosomes. Geneticists have used the
alleles are combined onto the same chromosome. Across a given proportion of recombinant gametes (the ones not like the parents) as
chromosome, several recombination events may occur, causing a measure of how far apart genes are on a chromosome. Using this
extensive shuffling of alleles.

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information, they have constructed elaborate maps of genes on Linked genes can be separated by recombination in which
chromosomes for well-studied organisms, including humans. homologous chromosomes exchange genetic information during
Mendel’s seminal publication makes no mention of linkage, and meiosis; this results in parental, or nonrecombinant genotypes, as
many researchers have questioned whether he encountered linkage, well as a smaller proportion of recombinant genotypes.
but chose not to publish those crosses out of concern that they would Geneticists can use the amount of recombination between genes
invalidate his independent assortment postulate. The garden pea has to estimate the distance between them on a chromosome.
seven chromosomes and some have suggested that his choice of
KEY TERMS
seven characteristics was not a coincidence. However, even if the
genes he examined were not located on separate chromosomes, it is linkage: the property of genes of being inherited together
possible that he simply did not observe linkage because of the recombination: the formation of genetic combinations in
extensive shuffling effects of recombination. offspring that are not present in the parents

KEY POINTS This page titled 12.3E: Genetic Linkage and Violation of the Law of
Independent Assortment is shared under a CC BY-SA 4.0 license and was
Two genes close together on the same chromosome tend to be
authored, remixed, and/or curated by Boundless.
inherited together and are said to be linked.

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12.3F: EPISTASIS

 LEARNING OBJECTIVES

Explain the phenotypic outcomes of epistatic effects

between genes

EPISTASIS
Mendel’s studies in pea plants implied that the sum of an
individual’s phenotype was controlled by genes (or as he called
them, unit factors): every characteristic was distinctly and
completely controlled by a single gene. In fact, single observable
characteristics are almost always under the influence of multiple
genes (each with two or more alleles) acting in unison. For example,
at least eight genes contribute to eye color in humans.
In some cases, several genes can contribute to aspects of a common
phenotype without their gene products ever directly interacting. In
the case of organ development, for instance, genes may be expressed
sequentially, with each gene adding to the complexity and specificity
of the organ. Genes may function in complementary or synergistic
fashions: two or more genes need to be expressed simultaneously to
affect a phenotype. Genes may also oppose each other with one gene
modifying the expression of another.
In epistasis, the interaction between genes is antagonistic: one gene
masks or interferes with the expression of another. “Epistasis” is a
word composed of Greek roots that mean “standing upon.” The Figure 12.3F . 1 : Epistasis in mouse coat color: In mice, the mottled
alleles that are being masked or silenced are said to be hypostatic to agouti coat color (A) is dominant to a solid coloration, such as black
the epistatic alleles that are doing the masking. Often the or gray. A gene at a separate locus (C) is responsible for pigment
production. The recessive c allele does not produce pigmentnand a
biochemical basis of epistasis is a gene pathway in which the mouse with the homozygous recessive cc genotype is albino
expression of one gene is dependent on the function of a gene that regardless of the allele present at the A locus. Thus, the C gene is
precedes or follows it in the pathway. epistatic to the A gene.

An example of epistasis is pigmentation in mice. The wild-type coat Epistasis can also occur when a dominant allele masks expression at
color, agouti (AA), is dominant to solid-colored fur (aa). However, a a separate gene. Fruit color in summer squash is expressed in this
separate gene (C) is necessary for pigment production. A mouse way. Homozygous recessive expression of the W gene (ww) coupled
with a recessive c allele at this locus is unable to produce pigment with homozygous dominant or heterozygous expression of the Y
and is albino regardless of the allele present at locus A. Therefore, gene (YY or Yy) generates yellow fruit, while the wwyy genotype
produces green fruit. However, if a dominant copy of the W gene is
the genotypes AAcc, Aacc, and aacc all produce the same albino
phenotype. A cross between heterozygotes for both genes (AaCc x present in the homozygous or heterozygous form, the summer
squash will produce white fruit regardless of the Y alleles. A cross
AaCc) would generate offspring with a phenotypic ratio of 9
between white heterozygotes for both genes (WwYy × WwYy)
agouti:3 solid color:4 albino. In this case, the C gene is epistatic to
would produce offspring with a phenotypic ratio of 12 white:3
the A gene.
yellow:1 green.
Finally, epistasis can be reciprocal: either gene, when present in the
dominant (or recessive) form, expresses the same phenotype. In the
shepherd’s purse plant (Capsella bursa-pastoris), the characteristic
of seed shape is controlled by two genes in a dominant epistatic
relationship. When the genes A and B are both homozygous
recessive (aabb), the seeds are ovoid. If the dominant allele for either
of these genes is present, the result is triangular seeds. That is, every
possible genotype other than aabb results in triangular seeds; a cross
between heterozygotes for both genes (AaBb x AaBb) would yield
offspring with a phenotypic ratio of 15 triangular:1 ovoid.
Keep in mind that any single characteristic that results in a
phenotypic ratio that totals 16 is typical of a two-gene interaction.

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CONTRIBUTIONS AND ATTRIBUTIONS recombination. Provided by: Wiktionary. Located at:
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 CHAPTER OVERVIEW

13: MODERN UNDERSTANDINGS OF INHERITANCE

13.1: Chromosomal Theory and Genetic Linkage
13.1A: Chromosomal Theory of Inheritance
13.1B: Genetic Linkage and Distances
13.1C: Identification of Chromosomes and Karyotypes
13.2: Chromosomal Basis of Inherited Disorders
13.2A: Disorders in Chromosome Number
13.2B: Chromosomal Structural Rearrangements
13.2C: X-Inactivation

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1
SECTION OVERVIEW

13.1: CHROMOSOMAL THEORY AND GENETIC LINKAGE

13.1B: GENETIC LINKAGE AND DISTANCES
Topic hierarchy
13.1C: IDENTIFICATION OF CHROMOSOMES AND
KARYOTYPES
13.1A: CHROMOSOMAL THEORY OF
INHERITANCE
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under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
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13.1A: CHROMOSOMAL THEORY OF INHERITANCE
parents.
 LEARNING OBJECTIVES
Despite compelling correlations between the behavior of
List the reasons that fruit flies are excellent model organisms chromosomes during meiosis and Mendel’s abstract laws, the
for genetic research Chromosomal Theory of Inheritance was proposed long before there
was any direct evidence that traits were carried on chromosomes.
CHROMOSOMAL THEORY OF INHERITANCE Critics pointed out that individuals had far more independently
segregating traits than they had chromosomes. It was only after
The speculation that chromosomes might be the key to
several years of carrying out crosses with the fruit fly, Drosophila
understanding heredity led several scientists to examine Mendel’s
melanogaster, that Thomas Hunt Morgan provided experimental
publications and re-evaluate his model in terms of the behavior of
evidence to support the Chromosomal Theory of Inheritance.
chromosomes during mitosis and meiosis. In 1902, Theodor Boveri
observed that proper embryonic development of sea urchins does not In 1910, Thomas Hunt Morgan started his work with Drosophila
occur unless chromosomes are present. That same year, Walter melanogaster, a fruit fly. He chose fruit flies because they can be
Sutton observed the separation of chromosomes into daughter cells cultured easily, are present in large numbers, have a short generation
time, and have only four pair of chromosomes that can be easily
during meiosis. Together, these observations led to the development
of the Chromosomal Theory of Inheritance, which identified identified under the microscope. They have three pair of autosomes
chromosomes as the genetic material responsible for Mendelian and a pair of sex chromosomes. At that time, he already knew that X
inheritance. and Y have to do with gender. He used normal flies with red eyes
and mutated flies with white eyes and cross bred them. In flies, the
wild type eye color is red (XW) and is dominant to white eye color
(Xw). He was able to conclude that the gene for eye color was on the
X chromosome. This trait was thus determined to be X-linked and
was the first X-linked trait to be identified. Males are said to be
hemizygous, in that they have only one allele for any X-linked
characteristic.

Figure 13.1A. 1 : Sutton and Boveri: (a) Walter Sutton and (b)
Theodor Boveri are credited with developing the Chromosomal
Theory of Inheritance, which states that chromosomes carry the unit
of heredity (genes). Figure 13.1A. 1 : Eye Color in Fruit Flies: In Drosophila, the gene
The Chromosomal Theory of Inheritance was consistent with for eye color is located on the X chromosome. Red eye color is wild
type and is dominant to white eye color.
Mendel’s laws and was supported by the following observations:
During meiosis, homologous chromosome pairs migrate as KEY POINTS
discrete structures that are independent of other chromosome Homologous chromosome pairs are independent of other
pairs. chromosome pairs.
The sorting of chromosomes from each homologous pair into Chromosomes from each homologous pair are sorted randomly
pre-gametes appears to be random. into pre- gametes.
Each parent synthesizes gametes that contain only half of their Parents synthesize gametes that contain only half of their
chromosomal complement. chromosomes; eggs and sperm have the same number of
Even though male and female gametes (sperm and egg) differ in chromosomes.
size and morphology, they have the same number of Gametic chromosomes combine during fertilization to produce
chromosomes, suggesting equal genetic contributions from each offspring with the same chromosome number as their parents.
parent. Eye color in fruit flies was the first X-linked trait to be
The gametic chromosomes combine during fertilization to discovered; thus, Morgan’s experiments with fruit flies solidified
produce offspring with the same chromosome number as their the Chromosomal Theory of Inheritance.

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KEY TERMS wild type: the typical form of an organism, strain, gene or
autosome: any chromosome other than sex chromosomes characteristic as it occurs in nature
hemizygous: having some single copies of genes in an otherwise
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13.1B: GENETIC LINKAGE AND DISTANCES

 LEARNING OBJECTIVES

Discuss how linked genes can be inherited separately

GENETIC LINKAGE AND DISTANCES

Mendel’s work suggested that traits are inherited independently of
each other. Morgan identified a 1:1 ratio between a segregating trait
and the X chromosome, suggesting that the random segregation of
chromosomes was the physical basis of Mendel’s model. This also
demonstrated that linked genes disrupt Mendel’s predicted
outcomes. The fact that each chromosome can carry many linked
genes explains how individuals can have many more traits than they
have chromosomes. However, observations by researchers in
Morgan’s laboratory suggested that alleles positioned on the same
chromosome were not always inherited together. During meiosis,
linked genes somehow became unlinked.

HOMOLOGOUS RECOMBINATION
In 1909, Frans Janssen observed chiasmata (the point at which
chromatids are in contact with each other and may exchange
segments) prior to the first division of meiosis. He suggested that
alleles become unlinked when chromosomes physically exchange
segments. As chromosomes condensed and paired with their
homologs, they appeared to interact at distinct points. Janssen
suggested that these points corresponded to regions in which
chromosome segments were exchanged. It is now known that the
pairing and interaction between homologous chromosomes, known
Figure 13.1B. 1: Inheritance Patterns of Unlinked and Linked
as synapsis, does more than simply organize the homologs for Genes: In (a), two genes are located on different chromosomes so
migration to separate daughter cells. When synapsed, homologous independent assortment occurs during meiosis. The offspring have
chromosomes undergo reciprocal physical exchanges of DNA at an equal chance of being the parental type (inheriting the same
combination of traits as the parents) or a nonparental type (inheriting
their arms in a process called homologous recombination, or more a different combination of traits than the parents). In (b), two genes
simply, “crossing over.” are very close together on the same chromosome so that no crossing
over occurs between them. The genes are, therefore, always
inherited together and all of the offspring are the parental type. In
GENETIC MAPS (c), two genes are far apart on the chromosome such that crossing
In 1913, Alfred Sturtevant, a student in Morgan’s laboratory, created over occurs during every meiotic event. The recombination
the first “chromosome map,” a linear representation of gene order frequency will be the same as if the genes were on separate
chromosomes. (d) The actual recombination frequency of fruit fly
and relative distance on a chromosome.To construct a chromosome wing length and body color that Thomas Morgan observed in 1912
map, Sturtevant assumed that genes were ordered serially on was 17 percent. A crossover frequency between 0 percent and 50
threadlike chromosomes. He also assumed that the incidence of percent indicates that the genes are on the same chromosome and
crossover occurs some of the time.
recombination between two homologous chromosomes could occur
with equal likelihood anywhere along the length of the chromosome.
Operating under these assumptions, Sturtevant hypothesized alleles
that were far apart on a chromosome were more likely to dissociate
during meiosis simply because there was a larger region over which
recombination could occur. Conversely, alleles that were close to
each other on the chromosome were likely to be inherited together.
The average number of crossovers between two alleles, or their
recombination frequency, correlated with their genetic distance from
each other, relative to the locations of other genes on that
chromosome. Sturtevant divided his genetic map into map units, or
centimorgans (cM), in which a recombination frequency of 0.01
corresponds to 1 cM.

13.1B.1 https://bio.libretexts.org/@go/page/13277
 Sturtevant to calculate distances between several genes on the same
chromosome.

KEY POINTS
Alleles positioned on the same chromosome are not always
inherited together because during meiosis linked genes can
became unlinked.
Frans Janssen suggested chromosomes become unlinked during
homologous recombination, a process where homologous
chromosomes exchange segments of DNA.
Alfred Sturtevant hypothesized that alleles that were closer
together on a gene were more likely to be inherited together
rather than alleles that were farther apart and used measurements
of recombination between genes to create the first genetic map.
When genes are perfectly linked, they have a recombination
frequency of 0.
When genes are unlinked, they have a recombination frequency
Figure 13.1B. 1: Genetic Maps: This genetic map orders Drosophila of 0.5, which means 50 percent of offspring are recombinants
genes on the basis of recombination frequency.
and the other 50 percent are parental types.
By representing alleles in a linear map, Sturtevant suggested that
genes can range from being perfectly linked (recombination KEY TERMS
frequency = 0) to being perfectly unlinked (recombination frequency homologous recombination: a type of genetic recombination in
= 0.5) when genes are on different chromosomes or genes are which nucleotide sequences are exchanged between two similar
separated very far apart on the same chromosome. Perfectly or identical molecules of DNA
unlinked genes correspond to the frequencies predicted by Mendel to linkage: the property of genes of being inherited together
assort independently in a dihybrid cross. A recombination frequency synapsis: the association of homologous maternal and paternal
of 0.5 indicates that 50 percent of offspring are recombinants and the chromosomes during the initial part of meiosis
other 50 percent are parental types. That is, every type of allele
combination is represented with equal frequency. This allowed This page titled 13.1B: Genetic Linkage and Distances is shared under a CC
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

13.1B.2 https://bio.libretexts.org/@go/page/13277
13.1C: IDENTIFICATION OF CHROMOSOMES AND KARYOTYPES
along all of the 23 chromosome pairs. An experienced geneticist can
 LEARNING OBJECTIVES identify each chromosome based on its characteristic banding
pattern. In addition to the banding patterns, chromosomes are further
Describe a normal human karyotype and discuss the various
identified on the basis of size and centromere location. To obtain the
abnormalities that can be detected using this technique
classic depiction of the karyotype in which homologous pairs of
chromosomes are aligned in numerical order from longest to
IDENTIFICATION OF CHROMOSOMES shortest, the geneticist obtains a digital image, identifies each
The isolation and microscopic observation of chromosomes forms chromosome, and manually arranges the chromosomes into this
the basis of cytogenetics and is the primary method by which pattern.
clinicians detect chromosomal abnormalities in humans. A
karyotype is the number and appearance of chromosomes. To obtain
a view of an individual’s karyotype, cytologists photograph the
chromosomes and then cut and paste each chromosome into a chart,
or karyogram, also known as an ideogram.
In a given species, chromosomes can be identified by their number,
size, centromere position, and banding pattern. In a human
karyotype, autosomes or “body chromosomes” (all of the non–sex
chromosomes) are generally organized in approximate order of size
from largest (chromosome 1) to smallest (chromosome 22).
However, chromosome 21 is actually shorter than chromosome 22.
This was discovered after the naming of Down syndrome as trisomy
21, reflecting how this disease results from possessing one extra
chromosome 21 (three total). Not wanting to change the name of this
important disease, chromosome 21 retained its numbering, despite
describing the shortest set of chromosomes. The X and Y
chromosomes are not autosomes and are referred to as the sex Figure 13.1C. 1 : A human karyotype: This karyotype is of a male
chromosomes. human. Notice that homologous chromosomes are the same size, and
The chromosome “arms” projecting from either end of the have the same centromere positions and banding patterns. A human
female would have an XX chromosome pair instead of the XY pair
centromere may be designated as short or long, depending on their shown.
relative lengths. The short arm is abbreviated p (for “petite”), At its most basic, the karyotype may reveal genetic abnormalities in
whereas the long arm is abbreviated q (because it follows “p” which an individual has too many or too few chromosomes per cell.
alphabetically). Each arm is further subdivided and denoted by a Examples of this are Down Syndrome, which is identified by a third
number. Using this naming system, locations on chromosomes can copy of chromosome 21, and Turner Syndrome, which is
be described consistently in the scientific literature. characterized by the presence of only one X chromosome in women
Although Mendel is referred to as the “father of modern genetics,” instead of the normal two. Geneticists can also identify large
he performed his experiments with none of the tools that the deletions or insertions of DNA. For instance, Jacobsen Syndrome,
geneticists of today routinely employ. One such powerful cytological which involves distinctive facial features as well as heart and
technique is karyotyping, a method in which traits characterized by bleeding defects, is identified by a deletion on chromosome 11.
chromosomal abnormalities can be identified from a single cell. To Finally, the karyotype can pinpoint translocations, which occur when
observe an individual’s karyotype, a person’s cells (like white blood a segment of genetic material breaks from one chromosome and
cells) are first collected from a blood sample or other tissue. In thereattaches to another chromosome or to a different part of the same
laboratory, the isolated cells are stimulated to begin actively chromosome. Translocations are implicated in certain cancers,
dividing. A chemical called colchicine is then applied to cells to including chronic myelogenous leukemia.
arrest condensed chromosomes in metaphase. Cells are then made to During Mendel’s lifetime, inheritance was an abstract concept that
swell using a hypotonic solution so the chromosomes spread apart. could only be inferred by performing crosses and observing the traits
Finally, the sample is preserved in a fixative and applied to a slide. expressed by offspring. By observing a karyotype, today’s
The geneticist then stains chromosomes with one of several dyes to geneticists can actually visualize the chromosomal composition of
better visualize the distinct and reproducible banding patterns of an individual to confirm or predict genetic abnormalities in
each chromosome pair. Following staining, the chromosomes are offspring, even before birth.
viewed using bright-field microscopy. A common stain choice is the
Giemsa stain. Giemsa staining results in approximately 400–800
bands (of tightly coiled DNA and condensed proteins) arranged

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KEY POINTS BY: Attribution
homologous recombination. Provided by: Wikipedia. Located at:
A normal human karyotype contains 23 pairs of chromosomes: en.Wikipedia.org/wiki/homolog...0recombination. License: CC BY-SA:
Attribution-ShareAlike
22 pairs of autosomes and 1 pair of sex chromosomes, generally synapsis. Provided by: Wiktionary. Located at:
arranged in order from largest to smallest. en.wiktionary.org/wiki/synapsis. License: CC BY-SA: Attribution-ShareAlike
linkage. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/linkage.
The short arm of a chromosome is referred to as the p arm, while License: CC BY-SA: Attribution-ShareAlike
the long arm is designated the q arm. OpenStax College, Chromosomal Theory and Genetic Linkage October 16, 2013.
To observe a karyotype, cells are collected from a blood or tissue Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44481/latest...e_13_01_01.jpg. License: CC BY:
sample and stimulated to begin dividing; the chromosomes are Attribution
arrested in metaphase, preserved in a fixative and applied to a Robert Bear and David Rintoul, Extensions of the Laws of Inheritance. October
31, 2013. Provided by: OpenStax CNX. Located at:
slide where they are stained with a dye to visualize the distinct http://cnx.org/content/m47304/latest/. License: CC BY: Attribution
banding patterns of each chromosome pair. OpenStax College, Chromosomal Theory and Genetic Linkage October 16, 2013.
Provided by: OpenStax CNX. Located at:
A karyotype can be used to visualize abnormalities in the http://cnx.org/content/m44481/latest...e_13_01_02.jpg. License: CC BY:
chromosomes, such as an incorrect number of chromosomes, Attribution
OpenStax College, Chromosomal Theory and Genetic Linkage October 16, 2013.
deletions, insertions, or translocations of DNA. Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44481/latest...e_13_01_03.png. License: CC BY:
KEY TERMS Attribution
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
autosome: any chromosome other than sex chromosomes Located at: http://cnx.org/content/m44483/latest...ol11448/latest. License: CC
karyotype: the observed characteristics (number, type, shape BY: Attribution
karyotype. Provided by: Wiktionary. Located at:
etc) of the chromosomes of an individual or species en.wiktionary.org/wiki/karyotype. License: CC BY-SA: Attribution-
translocation: a transfer of a chromosomal segment to a new ShareAlike
translocation. Provided by: Wiktionary. Located at:
position, especially on a nonhomologous chromosome en.wiktionary.org/wiki/translocation. License: CC BY-SA: Attribution-
ShareAlike
CONTRIBUTIONS AND ATTRIBUTIONS autosome. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/autosome. License: CC BY-SA: Attribution-ShareAlike
hemizygous. Provided by: Wiktionary. Located at: OpenStax College, Chromosomal Theory and Genetic Linkage October 16, 2013.
http://en.wiktionary.org/wiki/hemizygous. License: CC BY-SA: Attribution- Provided by: OpenStax CNX. Located at:
ShareAlike http://cnx.org/content/m44481/latest...e_13_01_01.jpg. License: CC BY:
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. Attribution
Located at: http://cnx.org/content/m44481/latest...ol11448/latest. License: CC Robert Bear and David Rintoul, Extensions of the Laws of Inheritance. October
BY: Attribution 31, 2013. Provided by: OpenStax CNX. Located at:
Robert Bear and David Rintoul, Extensions of the Laws of Inheritance. October http://cnx.org/content/m47304/latest/. License: CC BY: Attribution
31, 2013. Provided by: OpenStax CNX. Located at: OpenStax College, Chromosomal Theory and Genetic Linkage October 16, 2013.
http://cnx.org/content/m47304/latest/. License: CC BY: Attribution Provided by: OpenStax CNX. Located at:
Structural Biochemistry/Chromosomes. Provided by: Wikibooks. Located at: http://cnx.org/content/m44481/latest...e_13_01_02.jpg. License: CC BY:
en.wikibooks.org/wiki/Structu...ry/Chromosomes. License: CC BY-SA: Attribution
Attribution-ShareAlike OpenStax College, Chromosomal Theory and Genetic Linkage October 16, 2013.
autosome. Provided by: Wiktionary. Located at: Provided by: OpenStax CNX. Located at:
en.wiktionary.org/wiki/autosome. License: CC BY-SA: Attribution-ShareAlike http://cnx.org/content/m44481/latest...e_13_01_03.png. License: CC BY:
wild type. Provided by: Wiktionary. Located at: Attribution
en.wiktionary.org/wiki/wild_type. License: CC BY-SA: Attribution-ShareAlike NHGRI human male karyotype. Provided by: Wikipedia. Located at:
OpenStax College, Chromosomal Theory and Genetic Linkage October 16, 2013. en.Wikipedia.org/wiki/File:NH..._karyotype.png. License: Public Domain:
Provided by: OpenStax CNX. Located at: No Known Copyright
http://cnx.org/content/m44481/latest...e_13_01_01.jpg. License: CC BY:
Attribution
Robert Bear and David Rintoul, Extensions of the Laws of Inheritance. October This page titled 13.1C: Identification of Chromosomes and Karyotypes is
31, 2013. Provided by: OpenStax CNX. Located at: shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
http://cnx.org/content/m47304/latest/. License: CC BY: Attribution
curated by Boundless.
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44481/latest...ol11448/latest. License: CC

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SECTION OVERVIEW

13.2: CHROMOSOMAL BASIS OF INHERITED DISORDERS

13.2C: X-INACTIVATION
Topic hierarchy
This page titled 13.2: Chromosomal Basis of Inherited Disorders is shared
13.2A: DISORDERS IN CHROMOSOME NUMBER under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.
13.2B: CHROMOSOMAL STRUCTURAL
REARRANGEMENTS

13.2.1 https://bio.libretexts.org/@go/page/12525
13.2A: DISORDERS IN CHROMOSOME NUMBER
Aneuploidy, an abnormal number of chromosomes in a cell, is two gametes with two copies of the chromosome. If sister
caused by nondisjunction, or the failure of chromosomes to separate chromatids fail to separate during meiosis II, the result is one gamete
at meiosis. that lacks that chromosome, two normal gametes with one copy of
the chromosome, and one gamete with two copies of the
 LEARNING OBJECTIVES chromosome. If a gamete with two copies of the chromosome
combines with a normal gamete during fertilization, the result is
Define aneuploidy and explain how this condition results trisomy; if a gamete with no copies of the chromosomes combines
from nondisjunction with a normal gamete during fertilization, the result is monosomy.

KEY POINTS
Aneuploidy is caused by nondisjunction, which occurs when
pairs of homologous chromosomes or sister chromatids fail to
separate during meiosis.
The loss of a single chromosome from a diploid genome is called
monosomy (2n-1), while the gain of one chromosome is called
trisomy (2n+1).
If homologous chromosomes fail to separate during meiosis I,
the result is no gametes with the normal number (one) of
chromosomes.
If sister chromatids fail to separate during meiosis II, the result is
two normal gametes each with one copy of the chromosome, and
two abnormal gametes in which one carries two copies and the
other carries none.
Aneuploidy can be lethal or result in serious developmental
disorders such as Turner Syndrome (X monosomy) or Downs
Syndrome (trisomy 21).

KEY TERMS
aneuploidy: the state of possessing a chromosome number that
is not an exact multiple of the haploid number
nondisjunction: the failure of chromosome pairs to separate
properly during meiosis

DISORDERS IN CHROMOSOME NUMBER Figure 13.2A. 1 : Nondisjunction in Meiosis: Nondisjunction occurs

when homologous chromosomes or sister chromatids fail to separate
Of all of the chromosomal disorders, abnormalities in chromosome during meiosis, resulting in an abnormal chromosome number.
number are the most obviously identifiable from a karyotype and are Nondisjunction may occur during meiosis I or meiosis II.
referred to as aneuploidy. Aneuploidy is a condition in which one or Aneuploidy often results in serious problems such as Turner
more chromosomes are present in extra copies or are deficient in syndrome, a monosomy in which females may contain all or part of
number, but not a complete set. To be more specific, the loss of a an X chromosome. Monosomy for autosomes is usually lethal in
single chromosome from a diploid genome is called monosomy (2n- humans and other animals. Klinefelter syndrome is a trisomy genetic
1). The gain of one chromosome is called trisomy (2n+1).They are disorder in males caused by the presence of one or more X
caused by nondisjunction, which occurs when pairs of homologous chromosomes. The effects of trisomy are similar to those of
chromosomes or sister chromatids fail to separate during meiosis. monosomy. Down syndrome is the only autosomal trisomy in
Misaligned or incomplete synapsis, or a dysfunction of the spindle humans that has a substantial number of survivors one year after
apparatus that facilitates chromosome migration, can cause birth. Trisomy in chromosome 21 is the cause of Down syndrome; it
nondisjunction. The risk of nondisjunction occurring increases with affects 1 infant in every 800 live births.
the age of the parents.
This page titled 13.2A: Disorders in Chromosome Number is shared under a
Nondisjunction can occur during either meiosis I or II, with differing CC BY-SA 4.0 license and was authored, remixed, and/or curated by
results. If homologous chromosomes fail to separate during meiosis Boundless.
I, the result is two gametes that lack that particular chromosome and

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13.2B: CHROMOSOMAL STRUCTURAL REARRANGEMENTS
Structural rearrangements of chromosomes include both inversions likely to have milder effects than aneuploid errors. However, altered
and translocations, which may have detrimental effects on an gene orientation can result in functional changes because regulators
organism. of gene expression could be moved out of position with respect to
their targets, causing aberrant levels of gene products.
 LEARNING OBJECTIVES An inversion can be pericentric and include the centromere, or
paracentric and occur outside of the centromere. A pericentric
Describe the various types of structural rearrangements of
inversion that is asymmetric about the centromere can change the
chromosomes and how they can impact an organism
relative lengths of the chromosome arms, making these inversions
easily identifiable.
KEY POINTS
A chromosome inversion is the detachment, 180° rotation, and
reinsertion of part of a chromosome; this may have no effect on
the organism, but if the inversion occurs within a gene or moves
a gene away from its regulatory elements it can have an adverse
effect.
Pericentric inversions include the centromere, while paracentric
inversions occur outside of the centromere; a pericentric
inversion can change the length of the chromosome arms above
and below the centromere.
A pericentric inversion on chromsome 18 appears to have been
involved in the evolution of humans.
Figure 13.2B. 1: Inversions can be pericentric or paracentric:
A translocation occurs when a segment of a chromosome Pericentric inversions include the centromere, and paracentric
dissociates and reattaches to a different, nonhomologous inversions do not. A pericentric inversion can change the relative
chromosome and can be benign or detrimental; in reciprocal lengths of the chromosome arms; a paracentric inversion cannot.
translocations, there is no gain or loss of genetic information, so When one homologous chromosome undergoes an inversion, but the
these are usually benign. other does not, the individual is described as an inversion
heterozygote. To maintain point-for-point synapsis during meiosis,
KEY TERMS one homolog must form a loop, and the other homolog must mold
inversion: a segment of DNA in the context of a chromosome around it. Although this topology can ensure that the genes are
that is reversed in orientation relative to a reference karyotype or correctly aligned, it also forces the homologs to stretch and can be
genome associated with regions of imprecise synapsis.
translocation: a transfer of a chromosomal segment to a new
position, especially on a nonhomologous chromosome

CHROMOSOMAL STRUCTURAL
REARRANGEMENTS
Cytologists have characterized numerous structural rearrangements
in chromosomes, but chromosome inversions and translocations are
the most common. Both are identified during meiosis by the
adaptive pairing of rearranged chromosomes with their former
homologs to maintain appropriate gene alignment. If the genes Figure 13.2B. 1: Inversion heterozygotes: When one chromosome
undergoes an inversion, but the other does not, one chromosome
carried on two homologs are not oriented correctly, a recombination must form an inverted loop to retain point-for-point interaction
event could result in the loss of genes from one chromosome and the during synapsis. This inversion pairing is essential to maintaining
gain of genes on the other. This would produce aneuploid gametes. gene alignment during meiosis and to allow for recombination.
Not all structural rearrangements of chromosomes produce
CHROMOSOME INVERSIONS nonviable, impaired, or infertile individuals. In rare instances, such a
A chromosome inversion is the detachment, 180° rotation, and change can result in the evolution of a new species. In fact, a
reinsertion of part of a chromosome. Inversions may occur in nature pericentric inversion in chromosome 18 appears to have contributed
as a result of mechanical shear, or from the action of transposable to the evolution of humans. This inversion is not present in our
elements (special DNA sequences capable of facilitating the closest genetic relatives, the chimpanzees. Humans and chimpanzees
rearrangement of chromosome segments with the help of enzymes differ cytogenetically by pericentric inversions on several
that cut and paste DNA sequences). Unless they disrupt a gene chromosomes and by the fusion of two separate chromosomes in
sequence, inversions only change the orientation of genes and are chimpanzees that correspond to chromosome two in humans.

13.2B.1 https://bio.libretexts.org/@go/page/13281
The pericentric chromosome 18 inversion is believed to have sequences. Notably, specific translocations have been associated
occurred in early humans following their divergence from a common with several cancers and with schizophrenia. Reciprocal
ancestor with chimpanzees approximately five million years ago. translocations result from the exchange of chromosome segments
Researchers characterizing this inversion have suggested that between two nonhomologous chromosomes such that there is no
approximately 19,000 nucleotide bases were duplicated on 18p, and gain or loss of genetic information.
the duplicated region inverted and reinserted on chromosome 18 of
an ancestral human.
A comparison of human and chimpanzee genes in the region of this
inversion indicates that two genes—ROCK1 and USP14—that are
adjacent on chimpanzee chromosome 17 (which corresponds to
human chromosome 18) are more distantly positioned on human
chromosome 18. This suggests that one of the inversion breakpoints
occurred between these two genes. Interestingly, humans and
chimpanzees express USP14 at distinct levels in specific cell types,
including cortical cells and fibroblasts. Perhaps the chromosome 18
inversion in an ancestral human repositioned specific genes and reset
their expression levels in a useful way. Because both ROCK1 and
USP14 encode cellular enzymes, a change in their expression could
alter cellular function. It is not known how this inversion contributed
to hominid evolution, but it appears to be a significant factor in the
divergence of humans from other primates. Figure 13.2B. 1: Reciprocal translocations do not involve loss of
genetic information: A reciprocal translocation occurs when a
TRANSLOCATIONS segment of DNA is transferred from one chromosome to another,
nonhomologous chromosome.
A translocation occurs when a segment of a chromosome dissociates
and reattaches to a different, nonhomologous chromosome. This page titled 13.2B: Chromosomal Structural Rearrangements is shared
Translocations can be benign or have devastating effects depending under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
on how the positions of genes are altered with respect to regulatory by Boundless.

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13.2C: X-INACTIVATION
The presence of extra X chromosomes in a cell is compensated for female, they would produce twice as much product from the genes
by X-inactivation in which all but one X chromosome are silenced. on the X chromosomes as males.

 LEARNING OBJECTIVES

Explain how and why X inactivation occurs in humans

KEY POINTS
Extra copies of the X chromosome are silenced by becoming
Barr bodies.
X chromosomal abnormalities are typically associated with mild
mental and physical defects, as well as sterility.
Conditions associated with aneuploidy of the sex chromosomes
include individuals with three X chromosomes, called triplo-X;
the XXY genotype, known as Klinefelter syndrome; and Turner
syndrome, characterized as X monosomy.
X-inactivation is a form of dosage compensation, in which an
organism attempts to equalize the amount of X chromosome Figure 13.2C. 1 : Sex Chromosome Nondisjunction: The symptoms
gene products in males and females. of Klinefelter’s syndrome (XXY) in a human male.
Since males only have one X chromosome, females inactivate So how does X-inactivation help alleviate the effects of extra X
one of theirs so that only one X chromosome is active in each chromosomes? An individual carrying an abnormal number of X
gender. chromosomes will inactivate all but one X chromosome in each of
her cells. If three X chromosomes are present, the cell will inactivate
KEY TERMS two of them. If four X chromosomes are present, three will be
dosage compensation: a genetic regulatory mechanism that inactivated, and so on. This results in an individual that is relatively
equalizes the phenotypic expression of characteristics determined phenotypically normal. However, even inactivated X chromosomes
by genes on the X chromosome so that they are equally continue to express a few genes, and X chromosomes must
expressed in males and females. reactivate for the proper maturation of female ovaries. As a result,
Barr body: a sex chromosome inactivated by packing in X-chromosomal abnormalities are typically associated with mild
heterochromatin mental and physical defects, as well as sterility. If the X
X inactivation: a process by which one of the two copies of the chromosome is absent altogether, the individual will not develop in
X chromosome present in female mammals is inactivated utero.

SEX CHROMOSOME NONDISJUNCTION IN Several errors in sex chromosome number have been characterized.
HUMANS Individuals with three X chromosomes, called triplo-X, are
phenotypically female, but express developmental delays and
Humans display dramatic deleterious effects with autosomal
reduced fertility. The XXY genotype, corresponding to one type of
trisomies and monosomies. Therefore, it may seem counterintuitive
Klinefelter syndrome, corresponds to phenotypically male
that human females and males can function normally, despite
individuals with small testes, enlarged breasts, and reduced body
carrying different numbers of the X chromosome. Rather than a gain
hair. More complex types of Klinefelter syndrome exist in which the
or loss of autosomes, variations in the number of X chromosomes
individual has as many as five X chromosomes. In all types, every X
are associated with relatively mild effects. In part, this occurs
chromosome except one undergoes inactivation to compensate for
because of a molecular process called X inactivation. Early in
the excess genetic dosage. This can be seen as several Barr bodies in
development, when female mammalian embryos consist of just a
each cell nucleus. Turner syndrome, characterized as an X0
few thousand cells (relative to trillions in the newborn), one X
genotype (i.e., only a single sex chromosome), corresponds to a
chromosome in each cell inactivates by tightly condensing into a
phenotypically female individual with short stature, webbed skin in
quiescent (dormant) structure called a Barr body. The chance that an
the neck region, hearing and cardiac impairments, and sterility.
X chromosome (maternally or paternally derived ) is inactivated in
each cell is random, but once the inactivation occurs, all cells DUPLICATIONS AND DELETIONS
derived from that single cell will have the same inactive X In addition to the loss or gain of an entire chromosome, a
chromosome or Barr body.
chromosomal segment may be duplicated or lost. Duplications and
By this process, a phenomenon called dosage compensation is deletions often produce offspring that survive but exhibit physical
achieved. Females possess two X chromosomes, while males have and mental abnormalities. Duplicated chromosomal segments may
only one; therefore, if both X chromosomes remained active in the fuse to existing chromosomes or may be free in the nucleus. Cri-du-

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chat (from the French for “cry of the cat”) is a syndrome associated http://cnx.org/content/m44483/latest...e_13_03_02.png. License: CC BY:
Attribution
with nervous system abnormalities and identifiable physical features translocation. Provided by: Wiktionary. Located at:
that result from a deletion of most of 5p (the small arm of en.wiktionary.org/wiki/translocation. License: CC BY-SA: Attribution-
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chromosome 5). Infants with this genotype emit a characteristic OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
high-pitched cry on which the disorder’s name is based. Located at: http://cnx.org/content/m44483/latest...ol11448/latest. License: CC
BY: Attribution
inversion. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/inversion. License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Chromosomal Basis of Inherited Disorders. October 16, 2013.
Provided by: OpenStax CNX. Located at:
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OpenStax College, Chromosomal Basis of Inherited Disorders. October 16, 2013.
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OpenStax College, Chromosomal Basis of Inherited Disorders. October 16, 2013.
Provided by: OpenStax CNX. Located at:
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Attribution
OpenStax College, Chromosomal Basis of Inherited Disorders. October 16, 2013.
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X inactivation. Provided by: Wiktionary. Located at:
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Structural Biochemistry/Chromosomes. Provided by: Wikibooks. Located at: Attribution
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 CHAPTER OVERVIEW

14: DNA STRUCTURE AND FUNCTION

Topic hierarchy
14.1: Historical Basis of Modern Understanding
14.1A: Discovery of DNA
14.1B: Modern Applications of DNA
14.2: DNA Structure and Sequencing
14.2A: The Structure and Sequence of DNA
14.2B: DNA Sequencing Techniques
14.3: DNA Replication
14.3A: Basics of DNA Replication
14.3B: DNA Replication in Prokaryotes
14.3C: DNA Replication in Eukaryotes
14.3D: Telomere Replication
14.4: DNA Repair
14.4A: DNA Repair

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1
SECTION OVERVIEW

14.1: HISTORICAL BASIS OF MODERN UNDERSTANDING

14.1B: MODERN APPLICATIONS OF DNA
Topic hierarchy
This page titled 14.1: Historical Basis of Modern Understanding is shared
14.1A: DISCOVERY OF DNA under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

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14.1A: DISCOVERY OF DNA
The discovery of DNA is generally credited to Watson and Crick, personally, he explained his findings to them. Chargaff’s Rule
but many other scientists contributed to the discovery. showed that in natural DNA, the number of guanine units equals the
number of cytosine units and the number of adenine units equals the
 LEARNING OBJECTIVES number of thymine units. This strongly hinted towards the base pair
makeup of the DNA. Chargaff’s research would help the Watson and
Explain the sequence of events leading up to the discovery Crick laboratory team to deduce the double helical structure of
of the structure of DNA DNA.

KEY POINTS FRANKLIN AND X-RAY DIFFRACTION

DNA is one of the most basic molecules of life; it carries genetic In Wilkins’ lab, researcher Rosalind Franklin used X-ray diffraction
instructions for the development, functioning and reproduction of methods to understand the structure of DNA. Watson and Crick were
all known living organisms. able to piece together the puzzle of the DNA molecule on the basis
The discovery of DNA’s double-helix structure is largely credited of Franklin’s data, because Crick had also studied X-ray diffraction.
to the scientists Watson and Crick, for which they won a Nobel In 1962, James Watson, Francis Crick, and Maurice Wilkins were
Prize. awarded the Nobel Prize in Medicine.
However, the X-ray crystallography work of Rosalind Franklin
and Erwin Chargaff’s work in discovering the composition of
DNA were instrumental to the discovery of DNA’s structure.

KEY TERMS
double helix: The structure formed by double-stranded
molecules of nucleic acids such as DNA.

WHAT IS DNA?
Deoxyribonucleic acid (DNA) is a molecule that carries most of the
genetic instructions used in the development, functioning and
Figure 14.1A. 1 : A team effort: The work of pioneering scientists
reproduction of all known living organisms and many viruses. James Watson, Francis Crick, and Maclyn McCarty (pictured at left)
DNA is a nucleic acid; alongside proteins and carbohydrates, nucleic led to our present day understanding of DNA. Scientist Rosalind
Franklin discovered the X-ray diffraction pattern of DNA (pictured
acids are one of the three major macromolecules essential for all at right), which helped to elucidate its double helix structure. (credit
known forms of life. DNA stores biological information and is a: modification of work by Marjorie McCarty, Public Library of
involved in the expression of traits in all living organisms. Science)
Unfortunately by then, Franklin had died. Nobel prizes are not
THE PATH TO DISCOVERY awarded posthumously, and though her work was crucial to the
In the 1950s, Francis Crick and James Watson worked together to discovery of DNA, Franklin was never nominated for a Nobel Prize.
determine the structure of DNA at the University of Cambridge, Francis Crick, James Watson, and Maurice Wilkins were awarded a
England. At the time, other scientists like Linus Pauling and Nobel Prize for the discovery of the structure of DNA in 1962.
Maurice Wilkins were also actively exploring this field. Pauling had There is still much controversy on how her image was given to
discovered the secondary structure of proteins using X-ray Watson and Crick and why she was not given due credit.
crystallography.
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CHARGAFF’S RULE license and was authored, remixed, and/or curated by Boundless.
Erwin Chargaff met Francis Crick and James D. Watson at
Cambridge in 1952, and, despite not getting along with them

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14.1B: MODERN APPLICATIONS OF DNA
DNA has many applications in a variety of fields including forensics
and medicine.

 LEARNING OBJECTIVES

Explain why DNA is a practical tool in various fields, such

as forensics and medicine

KEY POINTS
DNA is unique to each individual, and therefore can be used for
identification purposes.
The human genome consists of about 3 billion base pairs, Figure 14.1B. 1: Modern understanding of DNA structure and
function has led to cloning: Dolly the sheep was the first large
corresponding to about 20,000 to 25,000 functional genes. mammal to be cloned.
Each person’s DNA is inherited from their parents: 23
There have been attempts at producing cloned human embryos as
chromosomes from the mother, and 23 chromosomes from the
sources of embryonic stem cells, sometimes referred to as ‘cloning
father.
for therapeutic purposes’. Therapeutic cloning produces stem cells to
KEY TERMS attempt to remedy detrimental diseases or defects (unlike
reproductive cloning, which aims to reproduce an organism). Still,
genotype: The combination of alleles, situated on corresponding
therapeutic cloning efforts have met with resistance because of
chromosomes, that determines a specific trait of an individual.
bioethical considerations.
zygote: The single cell that arises from the union of two
gametes; in animals, the cell that arises from the union of sperm CRISPR
and ovum.
CRISPR (Clustered, Regularly-Interspaced Short Palindromic
gene: A unit of heredity; the functional units of chromosomes
Repeats) allows scientists to edit genomes, far better than older
that determine specific characteristics by coding for specific
techniques for gene splicing and editing. The CRISPR technique has
RNAs or proteins.
enormous potential application, including altering the germline of
phenotype: The appearance of an organism based on a
humans, animals and other organisms, and modifying the genes of
multifactorial combination of genetic traits and environmental
food crops.
factors.
Ethical concerns have surfaced about this biotechnology and the
The acronym “DNA” has become synonymous with solving crimes, prospect of editing the human germline and making so-called
testing for paternity, identifying human remains, and genetic testing. ‘designer babies’.
DNA can be retrieved from hair, blood, or saliva. Each person’s
DNA sequences are unique, and it is possible to detect differences
between individuals within a species on the basis of these unique Genome Editing with CRISPR-Cas9
features. DNA testing can also be used to identify pathogens,
identify biological remains in archaeological digs, trace disease
outbreaks, and study human migration patterns. In the medical field,
DNA is used in diagnostics, new vaccine development, and cancer
therapy. It is now also possible to determine predispositions to some
diseases by looking at genes.

CLONING
Reproductive cloning is a method used to make a clone or an
identical copy of an entire multicellular organism.
In cloning both the original organism and the clone have identical
DNA. Identical twins are, in one sense, clones of each other; they
have identical DNA, having developed from the same fertilized egg.
Cloning became an issue in scientific ethics when a sheep became
the first mammal cloned from an adult cell in 1996.
Since then several animals such as horses, bulls, and goats have been
successfully cloned, although these individuals often exhibit facial,
limb, and cardiac abnormalities.

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 Bacteria, plants, and animals have been genetically modified since
the early 1970s for academic, medical, agricultural, and industrial
purposes. In the US, GMOs such as Roundup-Ready soybeans and
borer-resistant corn are part of many common processed foods. As
in many of these biotechnology areas there is considerable
controversy in the use of GMOs.

CONTRIBUTIONS AND ATTRIBUTIONS

Erwin Chargaff. Provided by: Wikipedia. Located at:
https://en.Wikipedia.org/wiki/Erwin_Chargaff. License: CC BY-SA:
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DNA. Provided by: OpenStax CNX. Located at:
http://cnx.org/contents/GFy_h8cu@9.87:Q01G1mzh@2/Introduction.
License: CC BY-SA: Attribution-ShareAlike
DNA. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/DNA.
License: CC BY-SA: Attribution-ShareAlike
DNA Structure. Provided by: OpenStax CNX. Located at:
http://cnx.org/contents/GFy_h8cu@9.87:U7tPDRxK@7/DNA-Structure-
and-Sequencing. License: CC BY-SA: Attribution-ShareAlike
CRISPR Technique: This movie goes through the process of figure2.jpg. Provided by: OpenStax CNX. Located at:
CRISPR. http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.87.
License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
GENETICALLY MODIFIED ORGANISMS Located at: http://cnx.org/content/m44484/latest/?collection=col11448/latest.
A genetically modified organism (GMO) is any organism whose License: CC BY: Attribution
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genetic material has been altered using genetic engineering Located at: http://cnx.org/content/m44485/latest/?collection=col11448/latest.
techniques. GMOs are a source of medicines and genetically License: CC BY: Attribution
phenotype. Provided by: Wiktionary. Located at:
modified foods and are also widely used in scientific research, along en.wiktionary.org/wiki/phenotype. License: CC BY-SA: Attribution-
with the production of other goods. ShareAlike
genotype. Provided by: Wiktionary. Located at:
Genetic modification involves the mutation, insertion, or deletion of en.wiktionary.org/wiki/genotype. License: CC BY-SA: Attribution-ShareAlike
genes. Inserted genes usually come from a different species in a gene. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/gene.
License: CC BY-SA: Attribution-ShareAlike
form of horizontal gene-transfer. GMO. Provided by: Wikipedia. Located at:
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License: CC BY-SA: Attribution-ShareAlike
CRISPR Technique. Located at: http://www.youtube.com/watch?
v=2pp17E4E-O8. License: Public Domain: No Known Copyright. License
Terms: Standard YouTube license
GloFish.jpg. Provided by: Wikipedia. Located at:
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ish.jpg. License: CC BY-SA: Attribution-ShareAlike
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by: Connexions. License: CC BY: Attribution

Figure 14.1B. 1: Glo Fish: The GloFish is a patented and This page titled 14.1B: Modern Applications of DNA is shared under a CC
trademarked brand of genetically modified (GM) fluorescent fish. A BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
gene that encodes the green fluorescent protein, originally extracted
from a jellyfish, that naturally produced bright green fluorescence
was inserted into a zebrafish embryo.

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SECTION OVERVIEW

14.2: DNA STRUCTURE AND SEQUENCING

14.2B: DNA SEQUENCING TECHNIQUES
Topic hierarchy
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14.2A: THE STRUCTURE AND SEQUENCE OF BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
DNA

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14.2A: THE STRUCTURE AND SEQUENCE OF DNA
DNA is a double helix of two anti-parallel, complementary strands
having a phosphate-sugar backbone with nitrogenous bases stacked
inside.

 LEARNING OBJECTIVES

Describe the structure of DNA and summarize the

importance of DNA sequencing

KEY POINTS Figure 14.2A. 1 : Nucleotide Structure: Each nucleotide is made up

The two DNA strands are anti-parallel in nature; that is, the 3′ of a sugar, a phosphate group, and a nitrogenous base. The sugar is
end of one strand faces the 5′ end of the other strand. deoxyribose in DNA and ribose in RNA. In their mononucleotide
form, nucleotides can have one, two, or three phosphates attached to
The nucleotides that comprise DNA contain a nitrogenous base, them. When linked together in polynucleotide chains, the
a deoxyribose sugar, and a phosphate group which covalently nucleotides always have just one phosphate. A molecule with just a
link with other nucleotides to form phosphodiester bonds. nitrogenous base and a sugar is known as a nucleoside. Once at least
one phosphate is covalently attached, it is known as a nucleotide.
Nucleotide bases can be classified as purines (containing a
double-ring structure) or pyrimidines (containing a single-ring
James Watson and Francis Crick, with some help from Rosalind
structure).
Franklin and Maurice Wilkins, are credited with figuring out the
Adenine (purine) and thymine (pyrimidine) are complementary
structure of DNA. Watson and Crick proposed that DNA is made up
base pairs as are guanine (purine) and cytosine (pyrimidine).
of two polynucleotide strands that are twisted around each other to
DNA sequencing is the process of determining the precise order
form a right-handed helix.
of nucleotides within a DNA molecule.
The two polynucleotide strands are anti-parallel in nature. That is,
KEY TERMS they run in opposite directions.
deoxyribose: a derivative of the pentose sugar ribose in which The sugars and phosphates of the nucleotides form the backbone of
the 2′ hydroxyl (-OH) is reduced to a hydrogen (H); a constituent the structure, whereas the pairs of nitrogenous bases are pointed
of the nucleotides that comprise deoxyribonucleic acid, or DNA towards the interior of the molecule.
hydrogen bond: A weak bond in which a hydrogen atom already The twisting of the two strands around each other results in the
covalently bonded to a oxygen or nitrogen atom in one molecule formation of uniformly-spaced major and minor grooves bordered
is attracted to an electronegative atom (usually nitrogen or by the sugar-phosphate backbones of the two strands.
oxygen) in the same or different molecule. image
nucleotide: the monomer comprising DNA or RNA molecules;
consists of a nitrogenous heterocyclic base that can be a purine
or pyrimidine, a five-carbon pentose sugar, and a phosphate
group
The monomeric building blocks of DNA are
deoxyribomononucleotides (usually referred to as just nucleotides),
and DNA is formed from linear chains, or polymers, of these
nucleotides. The components of the nucleotide used in DNA
synthesis are a nitrogenous base, a deoxyribose, and a phosphate
group. The nucleotide is named depending on which nitrogenous
base is present. The nitrogenous base can be a purine such as
adenine (A) and guanine (G), characterized by double-ring
Figure 14.2A. 1 : Three representations of DNA’s double helical
structures, or a pyrimidine such as cytosine (C) and thymine (T), structure.: A is a spacefill model of DNA, where every atom is
characterized by single-ring structures. In polynucleotides (the linear represented as a sphere. The two anti-parallel polynucleotide strands
polymers of nucleotides) the nucleotides are connected to each other are colored differently to illustrate how they coil around each other.
B is a cartoon model of DNA, where the sugar-phosphate backbones
by covalent bonds known as phosphodiester bonds or are represented as violet strands and the nitrogenous bases are
phosphodiester linkages. represented as color-coded rings. C is another spacefill model, with
the sugar-phosphate atoms colored violet and all nitrogenous base
atoms colored green. The major and minor grooves, which wrap
around the entire molecule, are apparent as the spaces between the
sugar-phosphate backbones.
The diameter of the DNA double helix is 2 nm and is uniform
throughout. Only the pairing between a purine and pyrimidine can

14.2A.1 https://bio.libretexts.org/@go/page/13288
explain the uniform diameter. That is to say, at each point along the
DNA molecule, the two sugar phosphate backbones are always
separated by three rings, two from a purine and one from a
pyrimidine.
The two strands are held together by base pairing between
nitrogenous bases of one strand and nitrogenous bases from the
other strand. Base pairing takes place between a purine and
pyrimidine stabilized by hydrogen bonds: A pairs with T via two Figure 14.2A. 1 : DNA Structure: DNA has (a) a double helix
hydrogen bonds and G pairs with C via three hydrogen bonds. structure and (b) phosphodiester bonds. The (c) major and minor
grooves are binding sites for DNA binding proteins during processes
The interior basepairs rotate with respect to one another, but are also such as transcription (the copying of RNA from DNA) and
stacked on top of each other when the molecule is viewed looking replication.
up or down its long axis. DNA sequencing is the process of determining the precise order of
Each base pair is separated from the previous base pair by a height nucleotides within a DNA molecule. Rapid DNA sequencing
methods has greatly accelerated biological and medical research and
of 0.34 nm and each 360o turn of the helix travels 3.4 nm along the
long axis of the molecule. Therefore, ten base pairs are present per discovery. Knowledge of DNA sequences has become indispensable
for basic biological research, and in numerous applied fields such as
turn of the helix.
diagnostics, biotechnology, forensic biology, and biological
systematics. The rapid speed of sequencing attained with modern
technology has been instrumental in obtaining complete DNA
sequences, or genomes, of numerous types and species of life,
including the human genome and those of other animal, plant, and
microbial species.

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Boundless.

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14.2B: DNA SEQUENCING TECHNIQUES
DNA sequencing techniques are used to determine the order of
nucleotides (A,T,C,G) in a DNA molecule.

 LEARNING OBJECTIVES

Differentiate among the techniques used to sequence DNA

KEY POINTS
Genome sequencing will greatly advance our understanding of
genetic biology and has vast potential for medical diagnosis and
treatment. Figure 14.2B. 1: Sanger Method: In Frederick Sanger’s dideoxy
chain termination method, fluorescent-labeled dideoxynucleotides
DNA sequencing technologies have gone through at least three are used to generate DNA fragments that terminate at each
“generations”: Sanger sequencing and Gilbert sequencing were nucleotide along the template strand. The DNA is separated by
first-generation, pyrosequencing was second-generation, and capillary electrophoresis on the basis of size. From the order of
fragments formed, the DNA sequence can be read. The smallest
Illumina sequencing is next-generation. fragments were terminated earliest, and they come out of the column
Sanger sequencing is based on the use of chain terminators, first, so the order in which different fluorescent tags exit the column
ddNTPs, that are added to growing DNA strands and terminate is also the sequence of the strand. The DNA sequence readout is
shown on an electropherogram that is generated by a laser scanner.
synthesis at different points.
Illumina sequencing involves running up to 500,000,000 SANGER SEQUENCING
different sequencing reactions simultaneously on a single small The Sanger method is also known as the dideoxy chain termination
slide. It makes use of a modified replication reaction and uses method. This sequencing method is based on the use of chain
fluorescently-tagged nucleotides. terminators, the dideoxynucleotides (ddNTPs). The
Shotgun sequencing is a technique for determining the sequence dideoxynucleotides, or ddNTPSs, differ from deoxynucleotides by
of entire chromosomes and entire genomes based on producing the lack of a free 3′ OH group on the five-carbon sugar. If a ddNTP
random fragments of DNA that are then assembled by computers is added to a growing DNA strand, the chain is not extended any
which order fragments by finding overlapping ends. further because the free 3′ OH group needed to add another
nucleotide is not available. By using a predetermined ratio of
KEY TERMS
deoxyribonucleotides to dideoxynucleotides, it is possible to
DNA sequencing: a technique used in molecular biology that generate DNA fragments of different sizes when replicating DNA in
determines the sequence of nucleotides (A, C, G, and T) in a vitro.
particular region of DNA
A Sanger sequencing reaction is just a modified in vitro DNA
dideoxynucleotide: any nucleotide formed from a
deoxynucleotide by loss of an a second hydroxyl group from the replication reaction. As such the following components are needed:
deoxyribose group template DNA (which will the be DNA whose sequence will be
in vitro: any biochemical process done outside of its natural determined), DNA Polymerase to catalyze the replication reactions,
biological environment, such as in a test tube, petri dish, etc. a primer that basepairs prior to the portion of the DNA you want to
sequence, dNTPs, and ddNTPs. The ddNTPs are what distinguish a
(from the Latin for “in glass”)
Sanger sequencing reaction from just a replication reaction. Most of
DNA SEQUENCING TECHNIQUES the time in a Sanger sequencing reaction, DNA Polymerase will add
While techniques to sequence proteins have been around since the a proper dNTP to the growing strand it is synthesizing in vitro. But
1950s, techniques to sequence DNA were not developed until the at random locations, it will instead add a ddNTP. When it does, that
mid-1970s, when two distinct sequencing methods were developed strand will be terminated at the ddNTP just added. If enough
almost simultaneously, one by Walter Gilbert’s group at Harvard template DNAs are included in the reaction mix, each one will have
University, the other by Frederick Sanger’s group at Cambridge the ddNTP inserted at a different random location, and there will be
University. However, until the 1990s, the sequencing of DNA was a at least one DNA terminated at each different nucleotide along its
relatively expensive and long process. Using radiolabeled length for as long as the in vitro reaction can take place (about 900
nucleotides also compounded the problem through safety concerns. nucleotides under optimal conditions.)
With currently-available technology and automated machines, the The ddNTPs which terminate the strands have fluorescent labels
process is cheaper, safer, and can be completed in a matter of hours. covalently attached to them. Each of the four ddNTPs carries a
The Sanger sequencing method was used for the human genome different label, so each different ddNTP will fluoresce a different
sequencing project, which was finished its sequencing phase in color.
2003, but today both it and the Gilbert method have been largely After the reaction is over, the reaction is subject to capillary
replaced by better methods. electrophoresis. All the newly synthesized fragments, each

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terminated at a different nucleotide and so each a different length, chromosome is fragmented at different locations and the fragments
are separated by size. As each differently-sized fragment exits the from the same part of the chromosome will overlap each other. Each
capillary column, a laser excites the flourescent tag on its terminal fragment is sequenced and sophisticated computer algorithms
nucleotide. From the color of the resulting flouresence, a computer compare all the different fragments to find which overlaps with
can keep track of which nucleotide was present as the terminating which. By lining up the overlapped regions, a process called tiling,
nucleotide. The computer also keeps track of the order in which the the computer can find the largest possible continuous sequences that
terminating nucleotides appeared, which is the sequence of the DNA can be generated from the fragments. Ultimately, the sequence of
used in the original reaction. entire chromosomes are assembled.
image
SECOND GENERATION AND NEXT-GENERATION
SEQUENCING
The Sanger and Gilbert methods of sequencing DNA are often
called “first-generation” sequencing because they were the first to be
developed. In the late 1990s, new methods, called second-generation
sequencing methods, that were faster and cheaper, began to be
developed. The most popular, widely-used second-generation
sequencing method was one called Pyrosequencing.
Today a number of newer sequencing methods are available and
others are in the process of being developed. These are often called
next-generation sequencing methods. The most widely-used
sequencing method currently is one called Illumina sequencing
(after the name of the company which commercialized the
technique), but numerous competing methods are in the
developmental pipeline and may supplant Illumina sequencing.
In Illumina sequencing, up to 500,000,000 separate sequencing
reactions are run simultaneously on a single slide (the size of a
microscope slide) put into a single machine. Each reaction is
analyzed separately and the sequences generated from all 500 Figure 14.2B. 1: Whole genome shotgun sequencing.: In shotgun
million DNAs are stored in an attached computer. Each sequencing sequencing, multiple copies of the same chromosome are isolated
reaction is a modified replication reaction involving flourescently- and then fragmented in random locations. The different copies of the
chromosome end up generating different length fragments. When the
tagged nucleotides, but no chain-terminating dideoxy nucleotides are complete collection of fragments has been sequenced, comparing the
needed. sequences of all the fragments will reveal which fragments have
ends that overlap with other fragments. The complete sequence from
When the human genome was first sequenced using Sanger one end of the original DNA to the other can be assembled by
sequencing, it took several years, hundreds of labs working together, following the sequence from the first overlapping fragment to the
and a cost of around $100 million to sequence it to almost last.
completion. Next generation sequencing can sequence a Genome sequencing will greatly advance our understanding of
comparably-sized genome in a matter of days, using a single genetic biology. It has vast potential for medical diagnosis and
machine, at a cost of under $10,000. Many researchers have set a treatment.
goal of improving sequencing methods even more until a single
human genome can be sequenced for under $1000. CONTRIBUTIONS AND ATTRIBUTIONS
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SECTION OVERVIEW

14.3: DNA REPLICATION

14.3C: DNA REPLICATION IN EUKARYOTES
Topic hierarchy
14.3D: TELOMERE REPLICATION
14.3A: BASICS OF DNA REPLICATION
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14.3A: BASICS OF DNA REPLICATION
DNA replication uses a semi-conservative method that results in a each other; one of the two DNA molecules after replication would
double-stranded DNA with one parental strand and a new daughter be “all-old” and the other would be “all-new”. In semi-conservative
strand. replication, each of the two parental DNA strands would act as a
template for new DNA strands to be synthesized, but after
 LEARNING OBJECTIVES replication, each parental DNA strand would basepair with the
complementary newly-synthesized strand just synthesized, and both
Explain how the Meselson and Stahl experiment double-stranded DNAs would include one parental or “old” strand
conclusively established that DNA replication is semi- and one daughter or “new” strand. In dispersive replication, after
conservative. replication both copies of the new DNAs would somehow have
alternating segments of parental DNA and newly-synthesized DNA
KEY POINTS on each of their two strands.
There were three models suggested for DNA replication:
conservative, semi-conservative, and dispersive.
The conservative method of replication suggests that parental
DNA remains together and newly-formed daughter strands are
also together.
The semi-conservative method of replication suggests that the
two parental DNA strands serve as a template for new DNA and
after replication, each double-stranded DNA contains one strand
from the parental DNA and one new (daughter) strand.
The dispersive method of replication suggests that, after
replication, the two daughter DNAs have alternating segments of
both parental and newly-synthesized DNA interspersed on both
strands.
Meselson and Stahl, using E. coli DNA made with two nitrogen
istopes (14N and 15N) and density gradient centrifugation,
determined that DNA replicated via the semi-conservative
method of replication.

KEY TERMS
DNA replication: a biological process occuring in all living
organisms that is the basis for biological inheritance
isotope: any of two or more forms of an element where the
Figure 14.3A. 1 : Suggested Models of DNA Replication: The three
atoms have the same number of protons, but a different number suggested models of DNA replication. Grey indicates the original
of neutrons within their nuclei parental DNA strands or segments and blue indicates newly-
synthesized daughter DNA strands or segments.
BASICS OF DNA REPLICATION To determine which model of replication was accurate, a seminal
Watson and Crick’s discovery that DNA was a two-stranded double experiment was performed in 1958 by two researchers: Matthew
helix provided a hint as to how DNA is replicated. During cell Meselson and Franklin Stahl.
division, each DNA molecule has to be perfectly copied to ensure
identical DNA molecules to move to each of the two daughter cells. MESELSON AND STAHL
The double-stranded structure of DNA suggested that the two Meselson and Stahl were interested in understanding how DNA
strands might separate during replication with each strand serving as replicates. They grew E. coli for several generations in a medium
a template from which the new complementary strand for each is containing a “heavy” isotope of nitrogen (15N) that is incorporated
copied, generating two double-stranded molecules from one. into nitrogenous bases and, eventually, into the DNA. The E. coli
culture was then shifted into medium containing the common “light”
MODELS OF REPLICATION isotope of nitrogen (14N) and allowed to grow for one generation.
There were three models of replication possible from such a scheme: The cells were harvested and the DNA was isolated. The DNA was
conservative, semi-conservative, and dispersive. In conservative centrifuged at high speeds in an ultracentrifuge in a tube in which a
replication, the two original DNA strands, known as the parental cesium chloride density gradient had been established. Some cells
strands, would re-basepair with each other after being used as were allowed to grow for one more life cycle in 14N and spun again.
templates to synthesize new strands; and the two newly-synthesized
strands, known as the daughter strands, would also basepair with

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 determine the relative densities of different molecules. The
molecules that form the lowest bands have the highest densities.
DNA from cells grown exclusively in 15N produced a lower band
than DNA from cells grown exclusively in 14N. So DNA grown in
15
N had a higher density, as would be expected of a molecule with a
heavier isotope of nitrogen incorporated into its nitrogenous bases.
Meselson and Stahl noted that after one generation of growth in 14N
(after cells had been shifted from 15N), the DNA molecules
produced only single band intermediate in position in between DNA
of cells grown exclusively in 15N and DNA of cells grown
exclusively in 14N. This suggested either a semi-conservative or
dispersive mode of replication. Conservative replication would have
resulted in two bands; one representing the parental DNA still with
exclusively 15N in its nitrogenous bases and the other representing
the daughter DNA with exclusively 14N in its nitrogenous bases. The
single band actually seen indicated that all the DNA molecules
contained equal amounts of both 15N and 14N.
The DNA harvested from cells grown for two generations in 14N
Figure 14.3A. 1 : Meselson and Stahl: Meselson and Stahl formed two bands: one DNA band was at the intermediate position
experimented with E. coli grown first in heavy nitrogen (15N) then in
ligher nitrogen (14N.) DNA grown in 15N (red band) is heavier than between 15N and 14N and the other corresponded to the band of
DNA grown in 14N (orange band) and sediments to a lower level in exclusively 14N DNA. These results could only be explained if DNA
the cesium chloride density gradient in an ultracentrifuge. When replicates in a semi-conservative manner. Dispersive replication
DNA grown in 15N is switched to media containing 14N, after one
round of cell division the DNA sediments halfway between the 15N would have resulted in exclusively a single band in each new
and 14N levels, indicating that it now contains fifty percent 14N and generation, with the band slowly moving up closer to the height of
fifty percent 15N.. In subsequent cell divisions, an increasing amount the 14N DNA band. Therefore, dispersive replication could also be
of DNA contains 14N only. These data support the semi-conservative
replication model. ruled out.
During the density gradient ultracentrifugation, the DNA was loaded Meselson and Stahl’s results established that during DNA
into a gradient (Meselson and Stahl used a gradient of cesium replication, each of the two strands that make up the double helix
chloride salt, although other materials such as sucrose can also be serves as a template from which new strands are synthesized. The
used to create a gradient) and spun at high speeds of 50,000 to new strand will be complementary to the parental or “old” strand
60,000 rpm. In the ultracentrifuge tube, the cesium chloride salt and the new strand will remain basepaired to the old strand. So each
created a density gradient, with the cesium chloride solution being “daughter” DNA actually consists of one “old” DNA strand and one
more dense the farther down the tube you went. Under these newly-synthesized strand. When two daughter DNA copies are
circumstances, during the spin the DNA was pulled down the formed, they have the identical sequences to one another and
ultracentrifuge tube by centrifugal force until it arrived at the spot in identical sequences to the original parental DNA, and the two
the salt gradient where the DNA molecules’ density matched that of daughter DNAs are divided equally into the two daughter cells,
the surrounding salt solution. At the point, the molecules stopped producing daughter cells that are genetically identical to one another
sedimenting and formed a stable band. By looking at the relative and genetically identical to the parent cell.
positions of bands of molecules run in the same gradients, you can
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14.3B: DNA REPLICATION IN PROKARYOTES
Prokaryotic DNA is replicated by DNA polymerase III in the 5′ to 3′ single-stranded DNA from winding back into a double helix. DNA
direction at a rate of 1000 nucleotides per second. polymerase is able to add nucleotides only in the 5′ to 3′ direction (a
new DNA strand can be extended only in this direction). It also
 LEARNING OBJECTIVES requires a free 3′-OH group to which it can add nucleotides by
forming a phosphodiester bond between the 3′-OH end and the 5′
Explain the functions of the enzymes involved in phosphate of the next nucleotide. This means that it cannot add
prokaryotic DNA replication nucleotides if a free 3′-OH group is not available. Another enzyme,
RNA primase, synthesizes an RNA primer that is about five to ten
KEY POINTS nucleotides long and complementary to the DNA, priming DNA
Helicase separates the DNA to form a replication fork at the synthesis. A primer provides the free 3′-OH end to start replication.
origin of replication where DNA replication begins. DNA polymerase then extends this RNA primer, adding nucleotides
Replication forks extend bi-directionally as replication continues. one by one that are complementary to the template strand.
Okazaki fragments are formed on the lagging strand, while the DNA primase
RNA primer
leading strand is replicated continuously. DNA-ligase
DNA-Polymerase (Polα)

DNA ligase seals the gaps between the Okazaki fragments.

3’
Primase synthesizes an RNA primer with a free 3′-OH, which Lagging
strand 3’
DNA polymerase III uses to synthesize the daughter strands.
5’
Okazaki fragment
5’
KEY TERMS Leading
strand
5’

Topoisomerase
DNA replication: a biological process occuring in all living 3’
DNA Polymerase (Polδ)
organisms that is the basis for biological inheritance Helicase
Single strand,
helicase: an enzyme that unwinds the DNA helix ahead of the Binding proteins

replication machinery Figure 14.3B. 1: DNA Replication in Prokaryotes: A replication

origin of replication: a particular sequence in a genome at fork is formed when helicase separates the DNA strands at the origin
which replication is initiated of replication. The DNA tends to become more highly coiled ahead
of the replication fork. Topoisomerase breaks and reforms DNA’s
phosphate backbone ahead of the replication fork, thereby relieving
DNA REPLICATION IN PROKARYOTES the pressure that results from this supercoiling. Single-strand binding
DNA replication employs a large number of proteins and enzymes, proteins bind to the single-stranded DNA to prevent the helix from
re-forming. Primase synthesizes an RNA primer. DNA polymerase
each of which plays a critical role during the process. One of the key III uses this primer to synthesize the daughter DNA strand. On the
players is the enzyme DNA polymerase, which adds nucleotides one leading strand, DNA is synthesized continuously, whereas on the
by one to the growing DNA chain that are complementary to the lagging strand, DNA is synthesized in short stretches called Okazaki
fragments. DNA polymerase I replaces the RNA primer with DNA.
template strand. The addition of nucleotides requires energy; this
DNA ligase seals the gaps between the Okazaki fragments, joining
energy is obtained from the nucleotides that have three phosphates the fragments into a single DNA molecule.
attached to them, similar to ATP which has three phosphate groups The replication fork moves at the rate of 1000 nucleotides per
attached. When the bond between the phosphates is broken, the second. DNA polymerase can only extend in the 5′ to 3′ direction,
energy released is used to form the phosphodiester bond between the which poses a slight problem at the replication fork. As we know,
incoming nucleotide and the growing chain. In prokaryotes, three the DNA double helix is anti-parallel; that is, one strand is in the 5′
main types of polymerases are known: DNA pol I, DNA pol II, and to 3′ direction and the other is oriented in the 3′ to 5′ direction. One
DNA pol III. DNA pol III is the enzyme required for DNA strand (the leading strand), complementary to the 3′ to 5′ parental
synthesis; DNA pol I and DNA pol II are primarily required for DNA strand, is synthesized continuously towards the replication
repair. fork because the polymerase can add nucleotides in this direction.
There are specific nucleotide sequences called origins of replication The other strand (the lagging strand), complementary to the 5′ to 3′
where replication begins. In E. coli, which has a single origin of parental DNA, is extended away from the replication fork in small
replication on its one chromosome (as do most prokaryotes), it is fragments known as Okazaki fragments, each requiring a primer to
approximately 245 base pairs long and is rich in AT sequences. The start the synthesis. Okazaki fragments are named after the Japanese
origin of replication is recognized by certain proteins that bind to scientist who first discovered them.
this site. An enzyme called helicase unwinds the DNA by breaking The leading strand can be extended by one primer alone, whereas
the hydrogen bonds between the nitrogenous base pairs. ATP the lagging strand needs a new primer for each of the short Okazaki
hydrolysis is required for this process. As the DNA opens up, Y- fragments. The overall direction of the lagging strand will be 3′ to 5′,
shaped structures called replication forks are formed. Two while that of the leading strand will be 5′ to 3′. The sliding clamp (a
replication forks at the origin of replication are extended bi- ring-shaped protein that binds to the DNA) holds the DNA
directionally as replication proceeds. Single-strand binding proteins polymerase in place as it continues to add nucleotides.
coat the strands of DNA near the replication fork to prevent the Topoisomerase prevents the over-winding of the DNA double helix

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ahead of the replication fork as the DNA is opening up; it does so by
causing temporary nicks in the DNA helix and then resealing it. As
synthesis proceeds, the RNA primers are replaced by DNA. The
primers are removed by the exonuclease activity of DNA pol I,
while the gaps are filled in by deoxyribonucleotides. The nicks that
remain between the newly-synthesized DNA (that replaced the RNA
primer) and the previously-synthesized DNA are sealed by the
enzyme DNA ligase that catalyzes the formation of phosphodiester
linkage between the 3′-OH end of one nucleotide and the 5′
phosphate end of the other fragment. Figure 14.3B. 1: Prokaryotic DNA Replication: Enzymes and Their
Function: The enzymes involved in prokaryotic DNA replication
The table summarizes the enzymes involved in prokaryotic DNA and their functions are summarized on this table.
replication and the functions of each.
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Boundless.

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14.3C: DNA REPLICATION IN EUKARYOTES
DNA replication in eukaryotes occurs in three stages: initiation, formed at the origin of replication; these are extended in both
elongation, and termination, which are aided by several enzymes. directions as replication proceeds creating a replication bubble.
There are multiple origins of replication on the eukaryotic
 LEARNING OBJECTIVES chromosome which allow replication to occur simultaneously in
hundreds to thousands of locations along each chromosome.
Describe how DNA is replicated in eukaryotes

KEY POINTS
During initiation, proteins bind to the origin of replication while
helicase unwinds the DNA helix and two replication forks are
formed at the origin of replication.
During elongation, a primer sequence is added with
complementary RNA nucleotides, which are then replaced by
DNA nucleotides.
During elongation the leading strand is made continuously, while Figure 14.3C. 1 : Replication Fork Formation: A replication fork is
formed by the opening of the origin of replication; helicase separates
the lagging strand is made in pieces called Okazaki fragments. the DNA strands. An RNA primer is synthesized by primase and is
During termination, primers are removed and replaced with new elongated by the DNA polymerase. On the leading strand, only a
DNA nucleotides and the backbone is sealed by DNA ligase. single RNA primer is needed, and DNA is synthesized continuously,
whereas on the lagging strand, DNA is synthesized in short
stretches, each of which must start with its own RNA primer. The
KEY TERMS DNA fragments are joined by DNA ligase (not shown).
origin of replication: a particular sequence in a genome at
which replication is initiated ELONGATION
leading strand: the template strand of the DNA double helix that During elongation, an enzyme called DNA polymerase adds DNA
is oriented so that the replication fork moves along it in the 3′ to nucleotides to the 3′ end of the newly synthesized polynucleotide
5′ direction strand. The template strand specifies which of the four DNA
lagging strand: the strand of the template DNA double helix that nucleotides (A, T, C, or G) is added at each position along the new
is oriented so that the replication fork moves along it in a 5′ to 3′ chain. Only the nucleotide complementary to the template nucleotide
manner at that position is added to the new strand.

Because eukaryotic genomes are quite complex, DNA replication is DNA polymerase contains a groove that allows it to bind to a single-
a very complicated process that involves several enzymes and other stranded template DNA and travel one nucleotide at at time. For
proteins. It occurs in three main stages: initiation, elongation, and example, when DNA polymerase meets an adenosine nucleotide on
termination. the template strand, it adds a thymidine to the 3′ end of the newly
synthesized strand, and then moves to the next nucleotide on the
INITIATION template strand. This process will continue until the DNA
Eukaryotic DNA is bound to proteins known as histones to form polymerase reaches the end of the template strand.
structures called nucleosomes. During initiation, the DNA is made DNA polymerase cannot initiate new strand synthesis; it only adds
accessible to the proteins and enzymes involved in the replication new nucleotides at the 3′ end of an existing strand. All newly
process. There are specific chromosomal locations called origins of synthesized polynucleotide strands must be initiated by a specialized
replication where replication begins. In some eukaryotes, like yeast, RNA polymerase called primase. Primase initiates polynucleotide
these locations are defined by having a specific sequence of synthesis and by creating a short RNA polynucleotide strand
basepairs to which the replication initiation proteins bind. In other complementary to template DNA strand. This short stretch of RNA
eukaryotes, like humans, there does not appear to be a consensus nucleotides is called the primer. Once RNA primer has been
sequence for their origins of replication. Instead, the replication synthesized at the template DNA, primase exits, and DNA
initiation proteins might identify and bind to specific modifications polymerase extends the new strand with nucleotides complementary
to the nucleosomes in the origin region. to the template DNA.
Certain proteins recognize and bind to the origin of replication and Eventually, the RNA nucleotides in the primer are removed and
then allow the other proteins necessary for DNA replication to bind replaced with DNA nucleotides. Once DNA replication is finished,
the same region. The first proteins to bind the DNA are said to the daughter molecules are made entirely of continuous DNA
“recruit” the other proteins. Two copies of an enzyme called helicase nucleotides, with no RNA portions.
are among the proteins recruited to the origin. Each helicase
unwinds and separates the DNA helix into single-stranded DNA. As
the DNA opens up, Y-shaped structures called replication forks are
formed. Because two helicases bind, two replication forks are

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THE LEADING AND LAGGING STRANDS
DNA polymerase can only synthesize new strands in the 5′ to 3′
direction. Therefore, the two newly-synthesized strands grow in
DNA Replication
opposite directions because the template strands at each replication
fork are antiparallel. The “leading strand” is synthesized
continuously toward the replication fork as helicase unwinds the
template double-stranded DNA.
The “lagging strand” is synthesized in the direction away from the
replication fork and away from the DNA helicase unwinds. This
lagging strand is synthesized in pieces because the DNA polymerase
can only synthesize in the 5′ to 3′ direction, and so it constantly
encounters the previously-synthesized new strand. The pieces are
called Okazaki fragments, and each fragment begins with its own
RNA primer.

TERMINATION
Eukaryotic chromosomes have multiple origins of replication, which
initiate replication almost simultaneously. Each origin of replication
forms a bubble of duplicated DNA on either side of the origin of
replication. Eventually, the leading strand of one replication bubble
reaches the lagging strand of another bubble, and the lagging strand
will reach the 5′ end of the previous Okazaki fragment in the same
bubble.
DNA polymerase halts when it reaches a section of DNA template
that has already been replicated. However, DNA polymerase cannot
catalyze the formation of a phosphodiester bond between the two
segments of the new DNA strand, and it drops off. These unattached
sections of the sugar-phosphate backbone in an otherwise full-
replicated DNA strand are called nicks.
Once all the template nucleotides have been replicated, the
replication process is not yet over. RNA primers need to be replaced
with DNA, and nicks in the sugar-phosphate backbone need to be
connected.
The group of cellular enzymes that remove RNA primers include the
proteins FEN1 (flap endonulcease 1) and RNase H. The enzymes
FEN1 and RNase H remove RNA primers at the start of each
leading strand and at the start of each Okazaki fragment, leaving
gaps of unreplicated template DNA. Once the primers are removed,
a free-floating DNA polymerase lands at the 3′ end of the preceding DNA Replication: This is a clip from a PBS production called
DNA fragment and extends the DNA over the gap. However, this “DNA: The Secret of Life.” It details the latest research (as of 2005)
creates new nicks (unconnected sugar-phosphate backbone). concerning the process of DNA replication.
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sugar-phosphate backbones at each nick site. After ligase has CC BY-SA 4.0 license and was authored, remixed, and/or curated by
connected all nicks, the new strand is one long continuous DNA Boundless.
strand, and the daughter DNA molecule is complete.

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14.3D: TELOMERE REPLICATION
image
As DNA polymerase alone cannot replicate the ends of
chromosomes, telomerase aids in their replication and prevents
chromosome degradation.

 LEARNING OBJECTIVES

Describe the role played by telomerase in replication of

telomeres

KEY POINTS
DNA polymerase cannot replicate and repair DNA molecules at
the ends of linear chromosomes.
The ends of linear chromosomes, called telomeres, protect genes
from getting deleted as cells continue to divide.
The telomerase enzyme attaches to the end of the chromosome;
complementary bases to the RNA template are added on the 3′
end of the DNA strand. Figure 14.3D. 1 : The telomere end problem: A simplified schematic
Once the lagging strand is elongated by telomerase, DNA of DNA replication where the parental DNA (top) is replicated from
three origins of replication, yielding three replication bubbles
polymerase can add the complementary nucleotides to the ends (middle) before giving rise to two daughter DNAs (bottom). Parental
of the chromosomes and the telomeres can finally be replicated. DNA strands are black, newly synthesized DNA strands are blue,
Cells that undergo cell division continue to have their telomeres and RNA primers are red. All RNA primers will be removed by
Rnase H and FEN1, leaving gaps in the newly-synthesized DNA
shortened because most somatic cells do not make telomerase; strands (not shown.) DNA Polymerase and Ligase will replace all
telomere shortening is associated with aging. the RNA primers with DNA except the RNA primer at the 5′ ends of
Telomerase reactivation in telomerase-deficient mice causes each newly-synthesized (blue) strand. This means that each newly-
synthesized DNA strand is shorter at its 5′ end than the equivalent
extension of telomeres; this may have potential for treating age- strand in the parental DNA.
related diseases in humans.
Every RNA primer synthesized during replication can be removed
KEY TERMS and replaced with DNA strands except the RNA primer at the 5′ end
of the newly synthesized strand. This small section of RNA can only
telomere: either of the repetitive nucleotide sequences at each
be removed, not replaced with DNA. Enzymes RNase H and FEN1
end of a eukaryotic chromosome, which protect the chromosome
remove RNA primers, but DNA Polymerase will add new DNA only
from degradation
if the DNA Polymerase has an existing strand 5′ to it (“behind” it) to
telomerase: an enzyme in eukaryotic cells that adds a specific
extend. However, there is no more DNA in the 5′ direction after the
sequence of DNA to the telomeres of chromosomes after they
final RNA primer, so DNA polymerse cannot replace the RNA with
divide, giving the chromosomes stability over time
DNA. Therefore, both daughter DNA strands have an incomplete 5′
THE END PROBLEM OF LINEAR DNA strand with 3′ overhang.
REPLICATION In the absence of additional cellular processes, nucleases would
Linear chromosomes have an end problem. After DNA replication, digest these single-stranded 3′ overhangs. Each daughter DNA
each newly synthesized DNA strand is shorter at its 5′ end than at would become shorter than the parental DNA, and eventually entire
the parental DNA strand’s 5′ end. This produces a 3′ overhang at one DNA would be lost. To prevent this shortening, the ends of linear
end (and one end only) of each daughter DNA strand, such that the eukaryotic chromosomes have special structures called telomeres.
two daughter DNAs have their 3′ overhangs at opposite ends
TELOMERE REPLICATION
The ends of the linear chromosomes are known as telomeres:
repetitive sequences that code for no particular gene. These
telomeres protect the important genes from being deleted as cells
divide and as DNA strands shorten during replication.
In humans, a six base pair sequence, TTAGGG, is repeated 100 to
1000 times. After each round of DNA replication, some telomeric
sequences are lost at the 5′ end of the newly synthesized strand on
each daughter DNA, but because these are noncoding sequences,
their loss does not adversely affect the cell. However, even these
sequences are not unlimited. After sufficient rounds of replication,

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all the telomeric repeats are lost, and the DNA risks losing coding CONTRIBUTIONS AND ATTRIBUTIONS
sequences with subsequent rounds. OpenStax College, Biology. October 22, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44487/latest...ol11448/latest. License: CC
The discovery of the enzyme telomerase helped in the understanding BY: Attribution
of how chromosome ends are maintained. The telomerase enzyme OpenStax College, Biology. October 29, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44487/latest...ol11448/latest. License: CC
attaches to the end of a chromosome and contains a catalytic part BY: Attribution
and a built-in RNA template. Telomerase adds complementary RNA DNA replication. Provided by: Wikipedia. Located at:
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bases to the 3′ end of the DNA strand. Once the 3′ end of the lagging ShareAlike
strand template is sufficiently elongated, DNA polymerase adds the isotope. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/isotope.
License: CC BY-SA: Attribution-ShareAlike
complementary nucleotides to the ends of the chromosomes; thus, OpenStax College, Biology. October 29, 2013. Provided by: OpenStax CNX.
the ends of the chromosomes are replicated. Located at: http://cnx.org/content/m44487/latest...ol11448/latest. License: CC
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chromosome integrity: The ends of linear chromosomes are en.Wikipedia.org/wiki/leading%20strand. License: CC BY-SA: Attribution-
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SECTION OVERVIEW

14.4: DNA REPAIR

14.4A: DNA REPAIR
Topic hierarchy
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14.4A: DNA REPAIR
Most mistakes during replication are corrected by DNA polymerase
during replication or by post-replication repair mechanisms.

 LEARNING OBJECTIVES

Explain how errors during replication are repaired

KEY POINTS
Mismatch repair enzymes recognize mis-incorporated bases, Figure 14.4A. 1 : DNA polymerase proofreading: Proofreading by
remove them from DNA, and replace them with the correct DNA polymerase corrects errors during replication.
bases.
Some errors are not corrected during replication, but are instead
In nucleotide excision repair, enzymes remove incorrect bases
corrected after replication is completed; this type of repair is known
with a few surrounding bases, which are replaced with the
as mismatch repair. The enzymes recognize the incorrectly-added
correct bases with the help of a DNA polymerase and the
nucleotide and excise it; this is then replaced by the correct base. If
template DNA.
this remains uncorrected, it may lead to more permanent damage.
When replication mistakes are not corrected, they may result in
How do mismatch repair enzymes recognize which of the two bases
mutations, which sometimes can have serious consequences.
is the incorrect one? In E. coli, after replication, the nitrogenous base
Point mutations, one base substituted for another, can be silent
adenine acquires a methyl group; the parental DNA strand will have
(no effect) or may have effects ranging from mild to severe.
methyl groups, whereas the newly-synthesized strand lacks them.
Mutations may also involve insertions (addition of a base),
Thus, DNA polymerase is able to remove the incorrectly-
deletion (loss of a base), or translocation (movement of a DNA
incorporated bases from the newly-synthesized, non-methylated
section to a new location on the same or another chromosome ).
strand. In eukaryotes, the mechanism is not very well understood,
KEY TERMS but it is believed to involve recognition of unsealed nicks in the new
mismatch repair: a system for recognizing and repairing some strand, as well as a short-term continuing association of some of the
forms of DNA damage and erroneous insertion, deletion, or mis- replication proteins with the new daughter strand after replication
incorporation of bases that can arise during DNA replication and has been completed.
recombination
nucleotide excision repair: a DNA repair mechanism that
corrects damage done by UV radiation, including thymine
dimers and 6,4 photoproducts that cause bulky distortions in the
DNA

ERRORS DURING REPLICATION

DNA replication is a highly accurate process, but mistakes can
occasionally occur as when a DNA polymerase inserts a wrong base.
Uncorrected mistakes may sometimes lead to serious consequences,
such as cancer. Repair mechanisms can correct the mistakes, but in
rare cases mistakes are not corrected, leading to mutations; in other
cases, repair enzymes are themselves mutated or defective.
Mutations: In this interactive, you can “edit” a DNA strand and
cause a mutation. Take a look at the effects!
Most of the mistakes during DNA replication are promptly corrected
by DNA polymerase which proofreads the base that has just been
added. In proofreading, the DNA pol reads the newly-added base
before adding the next one so a correction can be made. The
polymerase checks whether the newly-added base has paired Figure 14.4A. 1 : Mismatch Repair: In mismatch repair, the
correctly with the base in the template strand. If it is the correct base, incorrectly-added base is detected after replication. The mismatch-
repair proteins detect this base and remove it from the newly-
the next nucleotide is added. If an incorrect base has been added, the synthesized strand by nuclease action. The gap is now filled with the
enzyme makes a cut at the phosphodiester bond and releases the correctly-paired base.
incorrect nucleotide. This is performed by the exonuclease action of In another type of repair mechanism, nucleotide excision repair,
DNA pol III. Once the incorrect nucleotide has been removed, a new enzymes replace incorrect bases by making a cut on both the 3′ and
one will be added again. 5′ ends of the incorrect base. The segment of DNA is removed and

14.4A.1 https://bio.libretexts.org/@go/page/13296
replaced with the correctly-paired nucleotides by the action of DNA DNA DAMAGE AND MUTATIONS
pol. Once the bases are filled in, the remaining gap is sealed with a Errors during DNA replication are not the only reason why
phosphodiester linkage catalyzed by DNA ligase. This repair mutations arise in DNA. Mutations, variations in the nucleotide
mechanism is often employed when UV exposure causes the sequence of a genome, can also occur because of damage to DNA.
formation of pyrimidine dimers. Such mutations may be of two types: induced or spontaneous.
Induced mutations are those that result from an exposure to
chemicals, UV rays, X-rays, or some other environmental agent.
Spontaneous mutations occur without any exposure to any
environmental agent; they are a result of natural reactions taking
place within the body.
Mutations may have a wide range of effects. Some mutations are not
expressed; these are known as silent mutations. Point mutations are
those mutations that affect a single base pair. The most common
nucleotide mutations are substitutions, in which one base is replaced
by another. These can be of two types: transitions or transversions.
Transition substitution refers to a purine or pyrimidine being
replaced by a base of the same kind; for example, a purine such as
adenine may be replaced by the purine guanine. Transversion
substitution refers to a purine being replaced by a pyrimidine or vice
versa; for example, cytosine, a pyrimidine, is replaced by adenine, a
purine. Mutations can also be the result of the addition of a base,
known as an insertion, or the removal of a base, known as a deletion.
Sometimes a piece of DNA from one chromosome may get
Figure 14.4A. 1 : DNA Ligase I Repairing Chromosomal Damage: translocated to another chromosome or to another region of the same
DNA damage, due to environmental factors and normal metabolic
chromosome.
processes inside the cell, occurs at a rate of 1,000 to 1,000,000
molecular lesions per cell per day. A special enzyme, DNA ligase
(shown here in color), encircles the double helix to repair a broken CONTRIBUTIONS AND ATTRIBUTIONS
strand of DNA. DNA ligase is responsible for repairing the millions Boundless. Provided by: Boundless Learning. Located at:
of DNA breaks generated during the normal course of a cell’s life. www.boundless.com//biology/de...xcision-repair. License: CC BY-SA:
Without molecules that can mend such breaks, cells can Attribution-ShareAlike
malfunction, die, or become cancerous. DNA ligases catalyse the OpenStax College, Biology. November 4, 2013. Provided by: OpenStax CNX.
crucial step of joining breaks in duplex DNA during DNA repair, Located at: http://cnx.org/content/m44513/latest...ol11448/latest. License: CC
replication and recombination, and require either Adenosine BY: Attribution
triphosphate (ATP) or Nicotinamide adenine dinucleotide (NAD+) mismatch repair. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/mismatch%20repair. License: CC BY-SA: Attribution-
as a cofactor.
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Figure 14.4A. 1 : Nucleotide Excision Repairs: Nucleotide excision

repairs thymine dimers. When exposed to UV, thymines lying
adjacent to each other can form thymine dimers. In normal cells,
they are excised and replaced.

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 CHAPTER OVERVIEW

15: GENES AND PROTEINS

15.1: The Genetic Code - The Relationship Between Genes and Proteins
15.2: The Genetic Code - The Central Dogma- DNA Encodes RNA and RNA Encodes Protein
15.3: Prokaryotic Transcription - Transcription in Prokaryotes
15.4: Prokaryotic Transcription - Initiation of Transcription in Prokaryotes
15.5: Prokaryotic Transcription - Elongation and Termination in Prokaryotes
15.6: Eukaryotic Transcription - Initiation of Transcription in Eukaryotes
15.7: Eukaryotic Transcription - Elongation and Termination in Eukaryotes
15.8: RNA Processing in Eukaryotes - mRNA Processing
15.9: RNA Processing in Eukaryotes - Processing of tRNAs and rRNAs
15.10: Ribosomes and Protein Synthesis - The Protein Synthesis Machinery
15.11: Ribosomes and Protein Synthesis - The Mechanism of Protein Synthesis
15.12: Ribosomes and Protein Synthesis - Protein Folding, Modification, and Targeting

This page titled 15: Genes and Proteins is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

1
15.1: THE GENETIC CODE - THE RELATIONSHIP BETWEEN GENES AND
PROTEINS
Proteins, encoded by individual genes, orchestrate nearly every protein-encoding genes typically found in plant cells, nonetheless
function of the cell. have huge impacts on cellular functioning.
Protein-encoding genes specify the sequences of amino acids, which
 LEARNING OBJECTIVES are the building blocks of proteins. In turn, proteins are responsible
for orchestrating nearly every function of the cell. Both protein-
Describe transcription and translation
encoding genes and the proteins that are their gene products are
absolutely essential to life as we know it.
KEY POINTS
Genes are composed of DNA arranged on chromosomes.
Some genes encode structural or regulatory RNAs. Other genes
encode proteins.
Replication copies DNA; transcription uses DNA to make
complementary RNAs; translation uses mRNAs to make
proteins.
In eukaryotic cells, replication and transcription take place
within the nucleus while translation takes place in the cytoplasm. Figure 15.1.1: Genes Encode Proteins: Genes, which are carried on
In prokaryotic cells, replication, transcription, and translation (a) chromosomes, are linearly-organized instructions for making the
RNA and protein molecules that are necessary for all of processes of
occur in the cytoplasm. life. The (b) interleukin-2 protein and (c) alpha-2u-globulin protein
are just two examples of the array of different molecular structures
KEY TERMS that are encoded by genes.
DNA: a biopolymer of deoxyribonucleic acids (a type of nucleic Replication, Transcription, and Translation are the three main
acid) that has four different chemical groups, called bases: processes used by all cells to maintain their genetic information and
adenine, guanine, cytosine, and thymine to convert the genetic information encoded in DNA into gene
messenger RNA: Messenger RNA (mRNA) is a molecule of products, which are either RNAs or proteins, depending on the gene.
RNA that encodes a chemical “blueprint” for a protein product. In eukaryotic cells, or those cells that have a nucleus, replication and
protein: any of numerous large, complex naturally-produced transcription take place within the nucleus while translation takes
molecules composed of one or more long chains of amino acids, place outside of the nucleus in cytoplasm. In prokaryotic cells, or
in which the amino acid groups are held together by peptide those cells that do not have a nucleus, all three processes occur in
bonds the cytoplasm.
GENES AND PROTEINS Replication is the basis for biological inheritance. It copies a cell’s
Since the rediscovery of Mendel’s work in 1900, the definition of DNA. The enzyme DNA polymerase copies a single parental
the gene has progressed from an abstract unit of heredity to a double-stranded DNA molecule into two daughter double-stranded
tangible molecular entity capable of replication, transcription, DNA molecules. Transcription makes RNA from DNA. The enzyme
RNA polymerase creates an RNA molecule that is complementary to
translation, and mutation. Genes are composed of DNA and are
a gene-encoding stretch of DNA. Translation makes protein from
linearly arranged on chromosomes. Some genes encode structural
mRNA. The ribosome generates a polypeptide chain of amino acids
and regulatory RNAs. There is increasing evidence from research
using mRNA as a template. The polypeptide chain folds up to
that profiles the transcriptome of cells (the complete set all RNA
become a protein.
transcripts present in a cell) that these may be the largest classes of
RNAs produced by eukaryotic cells, far outnumbering the protein- This page titled 15.1: The Genetic Code - The Relationship Between Genes
encoding messenger RNAs (mRNAs), but the 20,000 protein- and Proteins is shared under a CC BY-SA 4.0 license and was authored,
encoding genes typically found in animal cells, and the 30,o00 remixed, and/or curated by Boundless.

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15.2: THE GENETIC CODE - THE CENTRAL DOGMA- DNA ENCODES RNA AND
RNA ENCODES PROTEIN
THE CENTRAL DOGMA: DNA ENCODES RNA,
 LEARNING OBJECTIVES RNA ENCODES PROTEIN
The central dogma of molecular biology describes the flow of
Recall the central dogma of biology
genetic information in cells from DNA to messenger RNA (mRNA)
to protein. It states that genes specify the sequence of mRNA
THE GENETIC CODE IS DEGENERATE AND
molecules, which in turn specify the sequence of proteins. Because
UNIVERSAL
the information stored in DNA is so central to cellular function, the
The genetic code is degenerate as there are 64 possible nucleotide cell keeps the DNA protected and copies it in the form of RNA. An
triplets (43), which is far more than the number of amino acids. enzyme adds one nucleotide to the mRNA strand for every
These nucleotide triplets are called codons; they instruct the addition nucleotide it reads in the DNA strand. The translation of this
of a specific amino acid to a polypeptide chain. Sixty-one of the information to a protein is more complex because three mRNA
codons encode twenty different amino acids. Most of these amino nucleotides correspond to one amino acid in the polypeptide
acids can be encoded by more than one codon. Three of the 64 sequence.
codons terminate protein synthesis and release the polypeptide from
the translation machinery. These triplets are called stop codons. The
stop codon UGA is sometimes used to encode a 21st amino acid
called selenocysteine (Sec), but only if the mRNA additionally
contains a specific sequence of nucleotides called a selenocysteine
insertion sequence (SECIS). The stop codon UAG is sometimes
used by a few species of microorganisms to encode a 22nd amino
acid called pyrrolysine (Pyl). The codon AUG, also has a special
function. In addition to specifying the amino acid methionine, it also
serves as the start codon to initiate translation. The reading frame for
translation is set by the AUG start codon.
The genetic code is universal. With a few exceptions, virtually all
species use the same genetic code for protein synthesis. The
universal nature of the genetic code is powerful evidence that all of
life on Earth shares a common origin.

Figure 15.2.1: Codons and the universal genetic code.: The genetic
code for translating each nucleotide triplet (codon) in mRNA into an
amino acid or a translation termination signal.

15.2.1 https://bio.libretexts.org/@go/page/13299
 transcription is messenger RNA (mRNA), which will then be used to
create that protein in the process of translation.

TRANSLATION: RNA TO PROTEIN

Translation is the process by which mRNA is decoded and translated
to produce a polypeptide sequence, otherwise known as a protein.
This method of synthesizing proteins is directed by the mRNA and
accomplished with the help of a ribosome, a large complex of
ribosomal RNAs (rRNAs) and proteins. In translation, a cell decodes
the mRNA’s genetic message and assembles the brand-new
polypeptide chain. Transfer RNA, or tRNA, translates the sequence
of codons on the mRNA strand. The main function of tRNA is to
transfer a free amino acid from the cytoplasm to a ribosome, where
it is attached to the growing polypeptide chain. tRNAs continue to
add amino acids to the growing end of the polypeptide chain until
they reach a stop codon on the mRNA. The ribosome then releases
the completed protein into the cell.

KEY POINTS
The genetic code is degenerate because 64 codons encode only
22 amino acids.
The genetic code is universal because it is the same among all
organisms.
Replication is the process of copying a molecule of DNA.
Transcription is the process of converting a specific sequence of
DNA into RNA.
Translation is the process where a ribosome decodes mRNA into
a protein.

KEY TERMS
codon: a sequence of three adjacent nucleotides, which encode
for a specific amino acid during protein synthesis or translation
ribosome: protein/mRNA complexes found in all cells that are
involved in the production of proteins by translating messenger
RNA
degenerate: the redundancy of the genetic code (more than one
codon codes for each amino acid)

Figure 15.2.1: The central dogma: Instructions on DNA are

CONTRIBUTIONS AND ATTRIBUTIONS
transcribed onto messenger RNA. Ribosomes are able to read the OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44518/latest...ol11448/latest. License: CC
genetic information inscribed on a strand of messenger RNA and use
BY: Attribution
this information to string amino acids together into a protein. Principles of Biochemistry/Cell Metabolism I: DNA replication. Provided by:
Wikibooks. Located at: en.wikibooks.org/wiki/Princip...NA_replication.
TRANSCRIPTION: DNA TO RNA License: CC BY-SA: Attribution-ShareAlike
Cell Biology/Genes/Gene translation. Provided by: Wikibooks. Located at:
Transcription is the process of creating a complementary RNA copy en.wikibooks.org/wiki/Cell_Bi...ne_translation. License: CC BY-SA:
of a sequence of DNA. Both RNA and DNA are nucleic acids, Attribution-ShareAlike
translation. Provided by: Wiktionary. Located at:
which use base pairs of nucleotides as a complementary language en.wiktionary.org/wiki/translation. License: CC BY-SA: Attribution-
that enzymes can convert back and forth from DNA to RNA. During ShareAlike
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transcription, a DNA sequence is read by RNA polymerase, which en.wiktionary.org/wiki/gene%23English. License: CC BY-SA: Attribution-
produces a complementary, antiparallel RNA strand. Unlike DNA ShareAlike
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en.wiktionary.org/wiki/transcription. License: CC BY-SA: Attribution-
substitutes the RNA uracil (U) in all instances where the DNA ShareAlike
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License: CC BY-SA: Attribution-ShareAlike
gene expression. The stretch of DNA transcribed into an RNA OpenStax College, How Genes Are Regulated. October 30, 2013. Provided by:
molecule is called a transcript. Some transcripts are used as OpenStax CNX. Located at: http://cnx.org/content/m45480/latest/. License:
CC BY: Attribution
structural or regulatory RNAs, and others encode one or more Structural Biochemistry/Nucleic Acid/Translation. Provided by: Wikibooks.
proteins. If the transcribed gene encodes a protein, the result of Located at: en.wikibooks.org/wiki/Structu...id/Translation. License: CC BY-

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SA: Attribution-ShareAlike Structural Biochemistry/Nucleic Acid/Translation. Provided by: Wikibooks.
Principles of Biochemistry/Cell Metabolism II: RNA transcription. Provided by: Located at: en.wikibooks.org/wiki/Structu...id/Translation. License: CC BY-
Wikibooks. Located at: en.wikibooks.org/wiki/Princip..._transcription. SA: Attribution-ShareAlike
License: CC BY-SA: Attribution-ShareAlike codon. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/codon.
protein. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/protein. License: CC BY-SA: Attribution-ShareAlike
License: CC BY-SA: Attribution-ShareAlike Boundless. Provided by: Boundless Learning. Located at:
messenger RNA. Provided by: Wikipedia. Located at: www.boundless.com//biology/de...ion/degenerate. License: CC BY-SA:
en.Wikipedia.org/wiki/messenger%20RNA. License: CC BY-SA: Attribution- Attribution-ShareAlike
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DNA. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/DNA. en.wiktionary.org/wiki/ribosome. License: CC BY-SA: Attribution-ShareAlike
License: CC BY-SA: Attribution-ShareAlike OpenStax College, Introduction. October 16, 2013. Provided by: OpenStax
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CNX. Located at: http://cnx.org/content/m44518/latest...e_15_00_01.jpg. License: CC BY: Attribution
License: CC BY: Attribution OpenStax College, The Genetic Code. October 16, 2013. Provided by: OpenStax
Principles of Biochemistry/Cell Metabolism II: RNA transcription. Provided by: CNX. Located at: http://cnx.org/content/m44522/latest...e_15_01_02.jpg.
Wikibooks. Located at: en.wikibooks.org/wiki/Princip..._transcription. License: CC BY: Attribution
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BY: Attribution This page titled 15.2: The Genetic Code - The Central Dogma- DNA
Principles of Biochemistry/Cell Metabolism I: DNA replication. Provided by:
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15.3: PROKARYOTIC TRANSCRIPTION - TRANSCRIPTION IN PROKARYOTES
image
The genetic code is a degenerate, non-overlapping set of 64 codons
that encodes for 21 amino acids and 3 stop codons.

 LEARNING OBJECTIVES

Describe the genetic code and how the nucleotide sequence

prescribes the amino acid and the protein sequence

KEY POINTS
The relationship between DNA base sequences and the amino
acid sequence in proteins is called the genetic code.
There are 61 codons that encode amino acids and 3 codons that
code for chain termination for a total of 64 codons.
Unlike, eukayrotes, a bacterial chromosome is a covalently-
closed circle.
The DNA double helix must partially unwind for transcription to
occur; this unwound region is called a transcription bubble.
Figure 15.3.1: Genetic Code Table.: A codon is made of three
KEY TERMS nucleotides. Consequently there are 43 (=64) different codons. The
nucleotide: the monomer comprising DNA or RNA molecules; 64 codons encode 22 different amino acids and three termination
codons, also called stop codons.
consists of a nitrogenous heterocyclic base that can be a purine
or pyrimidine, a five-carbon pentose sugar, and a phosphate Degeneracy is the redundancy of the genetic code. The genetic code
group has redundancy, but no ambiguity. For example, although codons
amino acid: Any of 20 naturally occurring α-amino acids GAA and GAG both specify glutamic acid (redundancy), neither of
(having the amino, and carboxylic acid groups on the same them specifies any other amino acid (no ambiguity). The codons
carbon atom), and a variety of side chains, that combine, via encoding one amino acid may differ in any of their three positions.
peptide bonds, to form proteins. For example, the amino acid glutamic acid is specified by GAA and
redundancy: duplication of components, such as amino acid GAG codons (difference in the third position); the amino acid
codons, to provide survival of the total system in case of failure leucine is specified by UUA, UUG, CUU, CUC, CUA, CUG codons
of single components (difference in the first or third position); while the amino acid serine
is specified by UCA, UCG, UCC, UCU, AGU, AGC (difference in
THE GENETIC CODE: NUCLEOTIDE SEQUENCES the first, second or third position). These properties of the genetic
PRESCRIBE THE AMINO ACIDS code make it more fault-tolerant for point mutations.
The genetic code is the relationship between DNA base sequences
ORIGIN OF TRANSCRIPTION ON PROKARYOTIC
and the amino acid sequence in proteins. Features of the genetic
ORGANISMS
code include:
Prokaryotes are mostly single-celled organisms that, by definition,
Amino acids are encoded by three nucleotides.
lack membrane-bound nuclei and other organelles. The central
It is non-overlapping.
region of the cell in which prokaryotic DNA resides is called the
It is degenerate.
nucleoid region. Bacterial and Archaeal chromosomes are
There are 21 genetically-encoded amino acids universally found in covalently-closed circles that are not as extensively compacted as
the species from all three domains of life. ( There is a 22nd eukaryotic chromosomes, but are compacted nonetheless as the
genetically-encooded amino acid, Pyl, but so far it has only been diameter of a typical prokaryotic chromosome is larger than the
found in a handful of Archaea and Bacteria species.) Yet there are diameter of a typical prokaryotic cell. Additionally, prokaryotes
only four different nucleotides in DNA or RNA, so a minimum of often have abundant plasmids, which are shorter, circular DNA
three nucleotides are needed to code each of the 21 (or 22) amino molecules that may only contain one or a few genes and often carry
acids. The set of three nucleotides that codes for a single amino acid traits such as antibiotic resistance.
is known as a codon. There are 64 codons in total, 61 that encode Transcription in prokaryotes (as in eukaryotes) requires the DNA
amino acids and 3 that code for chain termination. Two of the double helix to partially unwind in the region of RNA synthesis. The
codons for chain termination can, under certain circumstances, region of unwinding is called a transcription bubble. Transcription
instead code for amino acids. always proceeds from the same DNA strand for each gene, which is
called the template strand. The RNA product is complementary to
the template strand and is almost identical to the other (non-

15.3.1 https://bio.libretexts.org/@go/page/13300
template) DNA strand, called the sense or coding strand. The only are designated upstream. Conversely, nucleotides following, or 3′ to,
difference is that in RNA all of the T nucleotides are replaced with the template strand initiation site are denoted with “+” numbering
U nucleotides. and are called downstream nucleotides.
The nucleotide on the DNA template strand that corresponds to the
This page titled 15.3: Prokaryotic Transcription - Transcription in
site from which the first 5′ RNA nucleotide is transcribed is called
Prokaryotes is shared under a CC BY-SA 4.0 license and was authored,
the +1 nucleotide, or the initiation site. Nucleotides preceding, or 5′
remixed, and/or curated by Boundless.
to, the template strand initiation site are given negative numbers and

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15.4: PROKARYOTIC TRANSCRIPTION - INITIATION OF TRANSCRIPTION IN
PROKARYOTES
RNA polymerase initiates transcription at specific DNA sequences PROKARYOTIC PROMOTERS AND INITIATION OF
called promoters. TRANSCRIPTION
The nucleotide pair in the DNA double helix that corresponds to the
 LEARNING OBJECTIVES site from which the first 5′ mRNA nucleotide is transcribed is called
the +1 site, or the initiation site. Nucleotides preceding the initiation
Summarize the initial steps of transcription in prokaryotes
site are given negative numbers and are designated upstream.
Conversely, nucleotides following the initiation site are denoted with
KEY POINTS “+” numbering and are called downstream nucleotides.
Transcription of mRNA begins at the initiation site. A promoter is a DNA sequence onto which the transcription
Two promoter consensus sequences are at the -10 and -35 machinery binds and initiates transcription. In most cases, promoters
regions upstream of the initiation site. exist upstream of the genes they regulate. The specific sequence of a
The σ subunit of RNA polymerase recognizes and binds the -35 promoter is very important because it determines whether the
region. corresponding gene is transcribed all the time, some of the time, or
Five subunits (α, α, β, β’, and σ) make up the complete RNA infrequently. Although promoters vary among prokaryotic genomes,
polymerase holoenzyme. a few elements are conserved. At the -10 and -35 regions upstream
of the initiation site, there are two promoter consensus sequences, or
KEY TERMS
regions that are similar across all promoters and across various
holoenzyme: a fully functioning enzyme, composed of all its bacterial species. The -10 consensus sequence, called the -10 region,
subunits is TATAAT. The -35 sequence, TTGACA, is recognized and bound
promoter: the section of DNA that controls the initiation of by σ. Once this interaction is made, the subunits of the core enzyme
RNA transcription bind to the site. The A–T-rich -10 region facilitates unwinding of the
DNA template; several phosphodiester bonds are made. The
PROKARYOTIC RNA POLYMERASE
transcription initiation phase ends with the production of abortive
Prokaryotes use the same RNA polymerase to transcribe all of their
transcripts, which are polymers of approximately 10 nucleotides that
genes. In E. coli, the polymerase is composed of five polypeptide
are made and released.
subunits, two of which are identical. Four of these subunits, denoted
α, α, β, and β’, comprise the polymerase core enzyme. These
subunits assemble each time a gene is transcribed; they disassemble
once transcription is complete. Each subunit has a unique role: the
two α-subunits are necessary to assemble the polymerase on the
DNA; the β-subunit binds to the ribonucleoside triphosphate that
will become part of the nascent “recently-born” mRNA molecule;
and the β’ binds the DNA template strand. The fifth subunit, σ, is
involved only in transcription initiation. It confers transcriptional
specificity such that the polymerase begins to synthesize mRNA
from an appropriate initiation site. Without σ, the core enzyme Figure 15.4.1: Promoter: The σ subunit of prokaryotic RNA
polymerase recognizes consensus sequences found in the promoter
would transcribe from random sites and would produce mRNA region upstream of the transcription start sight. The σ subunit
molecules that specified protein gibberish. The polymerase dissociates from the polymerase after transcription has been
comprised of all five subunits is called the holoenzyme. initiated.

This page titled 15.4: Prokaryotic Transcription - Initiation of Transcription

in Prokaryotes is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.

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15.5: PROKARYOTIC TRANSCRIPTION - ELONGATION AND TERMINATION IN
PROKARYOTES
Transcription elongation begins with the release of the polymerase σ are two kinds of termination signals: one is protein-based and the
subunit and terminates via the rho protein or via a stable hairpin. other is RNA-based.
Rho-dependent termination is controlled by the rho protein, which
 LEARNING OBJECTIVES tracks along behind the polymerase on the growing mRNA chain.
Near the end of the gene, the polymerase encounters a run of G
Explain the process of elongation and termination in
nucleotides on the DNA template and it stalls. As a result, the rho
prokaryotes
protein collides with the polymerase. The interaction with rho
releases the mRNA from the transcription bubble.
KEY POINTS
Rho-independent termination is controlled by specific sequences in
The transcription elongation phase begins with the dissociation the DNA template strand. As the polymerase nears the end of the
of the σ subunit, which allows the core RNA polymerase enzyme gene being transcribed, it encounters a region rich in C–G
to proceed along the DNA template. nucleotides. The mRNA folds back on itself, and the complementary
Rho-dependent termination is caused by the rho protein colliding C–G nucleotides bind together. The result is a stable hairpin that
with the stalled polymerase at a stretch of G nucleotides on the causes the polymerase to stall as soon as it begins to transcribe a
DNA template near the end of the gene. region rich in A–T nucleotides. The complementary U–A region of
Rho-independent termination is caused the polymerase stalling at the mRNA transcript forms only a weak interaction with the
a stable hairpin formed by a region of complementary C–G template DNA. This, coupled with the stalled polymerase, induces
nucleotides at the end of the mRNA. enough instability for the core enzyme to break away and liberate
the new mRNA transcript.
KEY TERMS
elongation: the addition of nucleotides to the 3′-end of a growing Upon termination, the process of transcription is complete. By the
RNA chain during transcription time termination occurs, the prokaryotic transcript would already
have been used to begin synthesis of numerous copies of the
ELONGATION IN PROKARYOTES encoded protein because these processes can occur concurrently in
The transcription elongation phase begins with the release of the σ the cytoplasm. The unification of transcription, translation, and even
subunit from the polymerase. The dissociation of σ allows the core mRNA degradation is possible because all of these processes occur
RNA polymerase enzyme to proceed along the DNA template, in the same 5′ to 3′ direction and because there is no membranous
synthesizing mRNA in the 5′ to 3′ direction at a rate of compartmentalization in the prokaryotic cell. In contrast, the
approximately 40 nucleotides per second. As elongation proceeds, presence of a nucleus in eukaryotic cells prevents simultaneous
the DNA is continuously unwound ahead of the core enzyme and transcription and translation.
rewound behind it. Since the base pairing between DNA and RNA is
CONTRIBUTIONS AND ATTRIBUTIONS
not stable enough to maintain the stability of the mRNA synthesis
amino acid. Provided by: Wiktionary. Located at:
components, RNA polymerase acts as a stable linker between the http://en.wiktionary.org/wiki/amino_acid. License: CC BY-SA: Attribution-
DNA template and the nascent RNA strands to ensure that ShareAlike
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BY: Attribution
An Introduction to Molecular Biology/Genetic Code. Provided by: Wikibooks.
Located at: en.wikibooks.org/wiki/An_Intr...y/Genetic_Code. License: CC
BY-SA: Attribution-ShareAlike
Structural Biochemistry/Genetic code. Provided by: Wikibooks. Located at:
en.wikibooks.org/wiki/Structu...y/Genetic_code. License: CC BY-SA:
Attribution-ShareAlike
redundancy. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/redundancy. License: CC BY-SA: Attribution-
ShareAlike
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prokaryotic RNA polymerase tracks along the DNA template, Located at: http://cnx.org/content/m44523/latest...ol11448/latest. License: CC
synthesizes mRNA in the 5′ to 3′ direction, and unwinds and BY: Attribution
rewinds the DNA as it is read. promoter. Provided by: Wiktionary. Located at:
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Boundless. Provided by: Boundless Learning. Located at:
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Once a gene is transcribed, the prokaryotic polymerase needs to be Attribution-ShareAlike
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newly-made mRNA. Depending on the gene being transcribed, there http://cnx.org/content/m44523/latest...e_15_02_01.jpg. License: CC BY:
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15.6: EUKARYOTIC TRANSCRIPTION - INITIATION OF TRANSCRIPTION IN
EUKARYOTES

 LEARNING OBJECTIVES

Describe how transcription is initiated and proceeds along

the DNA strand

STEPS IN EUKARYOTIC TRANSCRIPTION

Eukaryotic transcription is carried out in the nucleus of the cell by
one of three RNA polymerases, depending on the RNA being
transcribed, and proceeds in three sequential stages:
1. Initiation
2. Elongation
3. Termination.

INITIATION OF TRANSCRIPTION IN EUKARYOTES

Unlike the prokaryotic RNA polymerase that can bind to a DNA
template on its own, eukaryotes require several other proteins, called
transcription factors, to first bind to the promoter region and then
help recruit the appropriate polymerase. The completed assembly of
transcription factors and RNA polymerase bind to the promoter,
forming a transcription pre-initiation complex (PIC).
The most-extensively studied core promoter element in eukaryotes is
a short DNA sequence known as a TATA box, found 25-30 base Figure 15.6.1: Eukaryotic Transcription Initiation: A generalized
promoter of a gene transcribed by RNA polymerase II is shown.
pairs upstream from the start site of transcription. Only about 10- Transcription factors recognize the promoter, RNA polymerase II
15% of mammalian genes contain TATA boxes, while the rest then binds and forms the transcription initiation complex.
contain other core promoter elements, but the mechanisms by which
transcription is initiated at promoters with TATA boxes is well
THE THREE EUKARYOTIC RNA POLYMERASES
characterized.
(RNAPS)
The features of eukaryotic mRNA synthesis are markedly more
The TATA box, as a core promoter element, is the binding site for a
complex those of prokaryotes. Instead of a single polymerase
transcription factor known as TATA-binding protein (TBP), which is
comprising five subunits, the eukaryotes have three polymerases that
itself a subunit of another transcription factor: Transcription Factor
are each made up of 10 subunits or more. Each eukaryotic
II D (TFIID). After TFIID binds to the TATA box via the TBP, five
polymerase also requires a distinct set of transcription factors to
more transcription factors and RNA polymerase combine around the
bring it to the DNA template.
TATA box in a series of stages to form a pre-initiation complex. One
transcription factor, Transcription Factor II H (TFIIH), is involved in RNA polymerase I is located in the nucleolus, a specialized nuclear
separating opposing strands of double-stranded DNA to provide the substructure in which ribosomal RNA (rRNA) is transcribed,
RNA Polymerase access to a single-stranded DNA template. processed, and assembled into ribosomes. The rRNA molecules are
However, only a low, or basal, rate of transcription is driven by the considered structural RNAs because they have a cellular role but are
pre-initiation complex alone. Other proteins known as activators and not translated into protein. The rRNAs are components of the
repressors, along with any associated coactivators or corepressors, ribosome and are essential to the process of translation. RNA
are responsible for modulating transcription rate. Activator proteins polymerase I synthesizes all of the rRNAs except for the 5S rRNA
increase the transcription rate, and repressor proteins decrease the molecule.
transcription rate. RNA polymerase II is located in the nucleus and synthesizes all
protein-coding nuclear pre-mRNAs. Eukaryotic pre-mRNAs
undergo extensive processing after transcription, but before
translation. RNA polymerase II is responsible for transcribing the
overwhelming majority of eukaryotic genes, including all of the
protein-encoding genes which ultimately are translated into proteins
and genes for several types of regulatory RNAs, including
microRNAs (miRNAs) and long-coding RNAs (lncRNAs).

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RNA polymerase III is also located in the nucleus. This polymerase RNA Polymerase II is the polymerase responsible for
transcribes a variety of structural RNAs that includes the 5S pre- transcribing mRNA.
rRNA, transfer pre-RNAs (pre-tRNAs), and small nuclear pre-
RNAs. The tRNAs have a critical role in translation: they serve as KEY TERMS
the adaptor molecules between the mRNA template and the growing repressor: any protein that binds to DNA and thus regulates the
polypeptide chain. Small nuclear RNAs have a variety of functions, expression of genes by decreasing the rate of transcription
including “splicing” pre-mRNAs and regulating transcription activator: any chemical or agent which regulates one or more
factors. Not all miRNAs are transcribed by RNA Polymerase II, genes by increasing the rate of transcription
RNA Polymerase III transcribes some of them. polymerase: any of various enzymes that catalyze the formation
of polymers of DNA or RNA using an existing strand of DNA or
KEY POINTS RNA as a template
Eukaryotic transcription is carried out in the nucleus of the cell
and proceeds in three sequential stages: initiation, elongation, This page titled 15.6: Eukaryotic Transcription - Initiation of Transcription
and termination. in Eukaryotes is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.
Eukaryotes require transcription factors to first bind to the
promoter region and then help recruit the appropriate
polymerase.

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15.7: EUKARYOTIC TRANSCRIPTION - ELONGATION AND TERMINATION IN
EUKARYOTES
Elongation synthesizes pre-mRNA in a 5′ to 3′ direction, and spaced and include 146 nucleotides of DNA wound twice around the
termination occurs in response to termination sequences and signals. eight histones in a nucleosome like thread around a spool.
For polynucleotide synthesis to occur, the transcription machinery
 LEARNING OBJECTIVES needs to move histones out of the way every time it encounters a
nucleosome. This is accomplished by a special protein dimer called
Describe what is happening during transcription elongation
FACT, which stands for “facilitates chromatin transcription.” FACT
and termination
partially disassembles the nucleosome immediately ahead
(upstream) of a transcribing RNA Polymerase II by removing two of
KEY POINTS the eight histones (a single dimer of H2A and H2B histones is
RNA polymerase II (RNAPII) transcribes the major share of removed.) This presumably sufficiently loosens the DNA wrapped
eukaryotic genes. around that nucleosome so that RNA Polymerase II can transcribe
During elongation, the transcription machinery needs to move through it. FACT reassembles the nucleosome behind the RNA
histones out of the way every time it encounters a nucleosome. Polymerase II by returning the missing histones to it. RNA
Transcription elongation occurs in a bubble of unwound DNA, Polymerase II will continue to elongate the newly-synthesized RNA
where the RNA Polymerase uses one strand of DNA as a until transcription terminates.
template to catalyze the synthesis of a new RNA strand in the 5′ image

to 3′ direction.
RNA Polymerase I and RNA Polymerase III terminate
transcription in response to specific termination sequences in
either the DNA being transcribed (RNA Polymerase I) or in the
newly-synthesized RNA (RNA Polymerase III).
RNA Polymerase II terminates transcription at random locations
past the end of the gene being transcribed. The newly-
synthesized RNA is cleaved at a sequence-specified location and
released before transcription terminates. Figure 15.7.1: The FACT protein dimer allows RNA Polymerase II
to transcribe through packaged DNA: DNA in eukaryotes is
KEY TERMS packaged in nucleosomes, which consist of an octomer of 4 different
histone proteins. When DNA is tightly wound twice around a
nucleosome: any of the subunits that repeat in chromatin; a coil nucleosome, RNA Polymerase II cannot access it for transcription.
of DNA surrounding a histone core FACT removes two of the histones from the nucleosome
histone: any of various simple water-soluble proteins that are immediately ahead of RNA Polymerase, loosening the packaging so
that RNA Polymerase II can continue transcription. FACT also
rich in the basic amino acids lysine and arginine and are reassembles the nucleosome immediately behindd the RNA
complexed with DNA in the nucleosomes of eukaryotic Polymerase by returning the missing histones.
chromatin
chromatin: a complex of DNA, RNA, and proteins within the ELONGATION
cell nucleus out of which chromosomes condense during cell RNA Polymerase II is a complex of 12 protein subunits. Specific
division subunits within the protein allow RNA Polymerase II to act as its
own helicase, sliding clamp, single-stranded DNA binding protein,
TRANSCRIPTION THROUGH NUCLEOSOMES as well as carry out other functions. Consequently, RNA Polymerase
Following the formation of the pre-initiation complex, the II does not need as many accessory proteins to catalyze the synthesis
polymerase is released from the other transcription factors, and of new RNA strands during transcription elongation as DNA
elongation is allowed to proceed with the polymerase synthesizing Polymerase does to catalyze the synthesis of new DNA strands
RNA in the 5′ to 3′ direction. RNA Polymerase II (RNAPII) during replication elongation.
transcribes the major share of eukaryotic genes, so this section will However, RNA Polymerase II does need a large collection of
mainly focus on how this specific polymerase accomplishes accessory proteins to initiate transcription at gene promoters, but
elongation and termination. once the double-stranded DNA in the transcription start region has
Although the enzymatic process of elongation is essentially the same been unwound, the RNA Polymerase II has been positioned at the
in eukaryotes and prokaryotes, the eukaryotic DNA template is more +1 initiation nucleotide, and has started catalyzing new RNA strand
complex. When eukaryotic cells are not dividing, their genes exist as synthesis, RNA Polymerase II clears or “escapes” the promoter
a diffuse, but still extensively packaged and compacted mass of region and leaves most of the transcription initiation proteins behind.
DNA and proteins called chromatin. The DNA is tightly packaged All RNA Polymerases travel along the template DNA strand in the
around charged histone proteins at repeated intervals. These DNA– 3′ to 5′ direction and catalyze the synthesis of new RNA strands in
histone complexes, collectively called nucleosomes, are regularly

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the 5′ to 3′ direction, adding new nucleotides to the 3′ end of the nucleotides downstream from the AAUAAA site. The Poly(A)
growing RNA strand. Polymerase enzyme which catalyzes the addition of a 3′ poly-A tail
RNA Polymerases unwind the double stranded DNA ahead of them on the pre-mRNA is part of the complex that forms with CPSF and
and allow the unwound DNA behind them to rewind. As a result, CstF.
image
RNA strand synthesis occurs in a transcription bubble of about 25
unwound DNA basebairs. Only about 8 nucleotides of newly-
synthesized RNA remain basepaired to the template DNA. The rest
of the RNA molecules falls off the template to allow the DNA
behind it to rewind.
RNA Polymerases use the DNA strand below them as a template to
direct which nucleotide to add to the 3′ end of the growing RNA
strand at each point in the sequence. The RNA Polymerase travels
along the template DNA one nucleotide at at time. Whichever RNA
nucleotide is capable of basepairing to the template nucleotide below
the RNA Polymerase is the next nucleotide to be added. Once the
addition of a new nucleotide to the 3′ end of the growing strand has
been catalyzed, the RNA Polymerase moves to the next DNA
nucleotide on the template below it. This process continues until
transcription termination occurs.

TERMINATION
The termination of transcription is different for the three different
eukaryotic RNA polymerases.
The ribosomal rRNA genes transcribed by RNA Polymerase I
contain a specific sequence of basepairs (11 bp long in humans; 18
bp in mice) that is recognized by a termination protein called TTF-1
(Transcription Termination Factor for RNA Polymerase I.) This
protein binds the DNA at its recognition sequence and blocks further
transcription, causing the RNA Polymerase I to disengage from the
template DNA strand and to release its newly-synthesized RNA.
The protein-encoding, structural RNA, and regulatory RNA genes
transcribed by RNA Polymerse II lack any specific signals or Figure 15.7.1: Transcription termination by RNA Polymerase II on a
protein-encoding gene.: RNA Polymerase II has no specific signals
sequences that direct RNA Polymerase II to terminate at specific that terminate its transcription. In the case of protein-encoding
locations. RNA Polymerase II can continue to transcribe RNA genes, a protein complex will bind to two locations on the growing
anywhere from a few bp to thousands of bp past the actual end of the pre-mRNA once the RNA Polymerase has transcribed past the end
of the gene. CPSF in the complex will bind a AAUAAA sequence,
gene. However, the transcript is cleaved at an internal site before and CstF in the complex will bind a GU-rich sequence (top figure).
RNA Polymerase II finishes transcribing. This releases the upstream CPSF in the complex will cleave the pre-mRNA at a site between
portion of the transcript, which will serve as the initial RNA prior to the two bound sequences, releasing the pre-mRNA (middle figure).
Poly(A) Polymerase is a part of the same complex and will begin to
further processing (the pre-mRNA in the case of protein-encoding add a poly-A tail to the pre-mRNA. At the same time, Xrn2 protein,
genes.) This cleavage site is considered the “end” of the gene. The which is an exonuclease, attacks the 5′ end of the RNA strand still
remainder of the transcript is digested by a 5′-exonuclease (called associated with the RNA Polymerase. Xrn2 will start digesting the
non-released portion of the newly synthesized RNA until Xrn2
Xrn2 in humans) while it is still being transcribed by the RNA reaches the RNA Polymerase, where it aids in displacing the RNA
Polymerase II. When the 5′-exonulease “catches up” to RNA Polymerase from the template DNA strand. This terminates
Polymerase II by digesting away all the overhanging RNA, it helps transcription at some random location downstream from the true end
of the gene (bottom figure).
disengage the polymerase from its DNA template strand, finally
The tRNA, 5S rRNA, and structural RNAs genes transcribed by
terminating that round of transcription.
RNA Polymerase III have a not-entirely-understood termination
In the case of protein-encoding genes, the cleavage site which signal. The RNAs transcribed by RNA Polymerase III have a short
determines the “end” of the emerging pre-mRNA occurs between an stretch of four to seven U’s at their 3′ end. This somehow triggers
upstream AAUAAA sequence and a downstream GU-rich sequence RNA Polymerase III to both release the nascent RNA and disengage
separated by about 40-60 nucleotides in the emerging RNA. Once from the template DNA strand.
both of these sequences have been transcribed, a protein called
CPSF in humans binds the AAUAAA sequence and a protein called CONTRIBUTIONS AND ATTRIBUTIONS
CstF in humans binds the GU-rich sequence. These two proteins An Introduction to Molecular Biology/Transcription of RNA and its
modification. Provided by: Wikibooks. Located at:
form the base of a complicated protein complex that forms in this en.wikibooks.org/wiki/An_Intr...s_modification. License: CC BY-SA:
region before CPSF cleaves the nascent pre-mRNA at a site 10-30 Attribution-ShareAlike

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BY: Attribution BY: Attribution
Structural Biochemistry/Transcription. Provided by: Wikibooks. Located at: chromatin. Provided by: Wiktionary. Located at:
en.wikibooks.org/wiki/Structu...RNA_Polymerase. License: CC BY-SA: en.wiktionary.org/wiki/chromatin. License: CC BY-SA: Attribution-
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Eukaryotic transcription. Provided by: Wikipedia. Located at: histone. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/histone.
en.Wikipedia.org/wiki/Eukaryo...n%23Initiation. License: CC BY-SA: License: CC BY-SA: Attribution-ShareAlike
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polymerase. Provided by: Wiktionary. Located at: OpenStax College, Eukaryotic Transcription October 16, 2013. Provided by:
en.wiktionary.org/wiki/polymerase. License: CC BY-SA: Attribution- OpenStax CNX. Located at:
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activator. Provided by: Wiktionary. Located at: Attribution
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OpenStax College, Eukaryotic Transcription October 16, 2013. Provided by: This page titled 15.7: Eukaryotic Transcription - Elongation and
OpenStax CNX. Located at:
http://cnx.org/content/m44524/latest...e_15_03_01.jpg. License: CC BY: Termination in Eukaryotes is shared under a CC BY-SA 4.0 license and was
Attribution authored, remixed, and/or curated by Boundless.

15.7.3 https://bio.libretexts.org/@go/page/13306
15.8: RNA PROCESSING IN EUKARYOTES - MRNA PROCESSING
Eukaryotic pre-mRNA receives a 5′ cap and a 3′ poly (A) tail before
introns are removed and the mRNA is considered ready for
translation.

 LEARNING OBJECTIVES

Outline the steps of pre-mRNA processing

KEY POINTS
A 7-methylguanosine cap is added to the 5′ end of the pre-
mRNA while elongation is still in progress. The 5′ cap protects
the nascent mRNA from degradation and assists in ribosome
binding during translation.
A poly (A) tail is added to the 3′ end of the pre-mRNA once
elongation is complete. The poly (A) tail protects the mRNA
Figure 15.8.1: 5′ cap structure: Capping of the pre-mRNA involves
from degradation, aids in the export of the mature mRNA to the the addition of 7-methylguanosine (m7G) to the 5′ end. The cap
cytoplasm, and is involved in binding proteins involved in protects the 5′ end of the primary RNA transcript from attack by
initiating translation. ribonucleases and is recognized by eukaryotic initiation factors
involved in assembling the ribosome on the mature mRNA prior to
Introns are removed from the pre-mRNA before the mRNA is initiating translation.
exported to the cytoplasm.

KEY TERMS 3′ POLY-A TAIL

intron: a portion of a split gene that is included in pre-RNA While RNA Polymerase II is still transcribing downstream of the
transcripts but is removed during RNA processing and rapidly proper end of a gene, the pre-mRNA is cleaved by an endonuclease-
degraded containing protein complex between an AAUAAA consensus
moiety: a specific segment of a molecule sequence and a GU-rich sequence. This releases the functional pre-
spliceosome: a dynamic complex of RNA and protein subunits mRNA from the rest of the transcript, which is still attached to the
that removes introns from precursor mRNA RNA Polymerase. An enzyme called poly (A) polymerase (PAP) is
part of the same protein complex that cleaves the pre-mRNA and it
PRE-MRNA PROCESSING immediately adds a string of approximately 200 A nucleotides,
The eukaryotic pre-mRNA undergoes extensive processing before it called the poly (A) tail, to the 3′ end of the just-cleaved pre-mRNA.
is ready to be translated. The additional steps involved in eukaryotic The poly (A) tail protects the mRNA from degradation, aids in the
mRNA maturation create a molecule with a much longer half-life export of the mature mRNA to the cytoplasm, and is involved in
than a prokaryotic mRNA. Eukaryotic mRNAs last for several binding proteins involved in initiating translation.
hours, whereas the typical E. coli mRNA lasts no more than five
seconds.
Pre-mRNAs are first coated in RNA-stabilizing proteins; these
protect the pre-mRNA from degradation while it is processed and
exported out of the nucleus. The three most important steps of pre-
mRNA processing are the addition of stabilizing and signaling
factors at the 5′ and 3′ ends of the molecule, and the removal of
intervening sequences that do not specify the appropriate amino
acids. In rare cases, the mRNA transcript can be “edited” after it is
transcribed.

5′ CAPPING
While the pre-mRNA is still being synthesized, a 7-methylguanosine
cap is added to the 5′ end of the growing transcript by a 5′-to-5′
phosphate linkage. This moiety protects the nascent mRNA from
degradation. In addition, initiation factors involved in protein
synthesis recognize the cap to help initiate translation by ribosomes.

15.8.1 https://bio.libretexts.org/@go/page/13307
 image
that separate exons often encode separate protein subunits or
domains. For the most part, the sequences of introns can be mutated
without ultimately affecting the protein product.

INTRON PROCESSING
All introns in a pre-mRNA must be completely and precisely
removed before protein synthesis. If the process errs by even a
single nucleotide, the reading frame of the rejoined exons would
shift, and the resulting protein would be dysfunctional. The process
of removing introns and reconnecting exons is called splicing.
Introns are removed and degraded while the pre-mRNA is still in the
nucleus. Splicing occurs by a sequence-specific mechanism that
ensures introns will be removed and exons rejoined with the
accuracy and precision of a single nucleotide. The splicing of pre-
mRNAs is conducted by complexes of proteins and RNA molecules
called spliceosomes.

Figure 15.8.1: Poly (A) Polymerase adds a 3′ poly (A) tail to the
pre-mRNA.: The pre-mRNA is cleaved off the rest of the growing
transcript before RNA Polymerase II has stopped transcribing. This
cleavage is done by an endonuclease-containing protein complex
that binds to an AAUAAA sequence upstream of the cleavage site
and to a GU-rich sequence downstream of the cut site. Immediately
after the cleavage, Poly (A) Polymerase (PAP), which is also part of
the protein complex, catalyzes the addition of up to 200 A
nucleotides to the 3′ end of the just-cleaved pre-mRNA.
Figure 15.8.1: Pre-mRNA splicing: Pre-mRNA splicing involves the
PRE-MRNA SPLICING precise removal of introns from the primary RNA transcript. The
Eukaryotic genes are composed of exons, which correspond to splicing process is catalyzed by large complexes called
spliceosomes. Each spliceosome is composed of five subunits called
protein-coding sequences (ex-on signifies that they are expressed), snRNPs. The spliceseome’s actions result in the splicing together of
and intervening sequences called introns (int-ron denotes their the two exons and the release of the intron in a lariat form.
intervening role), which may be involved in gene regulation, but are Each spliceosome is composed of five subunits called snRNPs (for
removed from the pre-mRNA during processing. Intron sequences in small nuclear ribonucleoparticles, and pronounced “snurps”.) Each
mRNA do not encode functional proteins. snRNP is itself a complex of proteins and a special type of RNA
found only in the nucleus called snRNAs (small nuclear RNAs).
DISCOVERY OF INTRONS Spliceosomes recognize sequences at the 5′ end of the intron
The discovery of introns came as a surprise to researchers in the because introns always start with the nucleotides GU and they
1970s who expected that pre-mRNAs would specify protein recognize sequences at the 3′ end of the intron because they always
sequences without further processing, as they had observed in end with the nucleotides AG. The spliceosome cleaves the pre-
prokaryotes. The genes of higher eukaryotes very often contain one mRNA’s sugar phosphate backbone at the G that starts the intron and
or more introns. While these regions may correspond to regulatory then covalently attaches that G to an internal A nucleotide within the
sequences, the biological significance of having many introns or intron. Then the spliceosme connects the 3′ end of the first exon to
having very long introns in a gene is unclear. It is possible that the 5′ end of the following exon, cleaving the 3′ end of the intron in
introns slow down gene expression because it takes longer to the process. This results in the splicing together of the two exons and
transcribe pre-mRNAs with lots of introns. Alternatively, introns the release of the intron in a lariat form.
may be nonfunctional sequence remnants left over from the fusion
of ancient genes throughout evolution. This is supported by the fact

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 This page titled 15.8: RNA Processing in Eukaryotes - mRNA Processing is
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

Figure 15.8.1: Mechanism of pre-mRNA splicing.: The snRNPs of

the spliceosome were left out of this figure, but it shows the sites
within the intron whose interactions are catalyzed by the
spliceosome. Initially, the conserved G which starts an intron is
cleaved from the 3′ end of the exon upstream to it and the G is
covalently attached to an internal A within the intron. Then the 3′
end of the just-released exon is joined to the 5′ end of the next exon,
cleaving the bond that attaches the 3′ end of the intron to its adjacent
exon. This both joins the two exons and removes the intron in lariat
form.

15.8.3 https://bio.libretexts.org/@go/page/13307
15.9: RNA PROCESSING IN EUKARYOTES - PROCESSING OF TRNAS AND
RRNAS
rRNA and tRNA are structural molecules that aid in protein proteins, while the bacterial small subunit is called the 30S subunit
synthesis but are not themselves translated into protein. and is composed of the 16S rRNA and 21 proteins.
The two subunits join to constitute a functioning ribosome that is
 LEARNING OBJECTIVES capable of creating proteins.

Describe how pre-rRNAs and pre-tRNAs are processed into TRANSFER RNA (TRNA)
mature rRNAs and tRNAs. Each different tRNA binds to a specific amino acid and transfers it
to the ribosome. Mature tRNAs take on a three-dimensional
KEY POINTS structure through intramolecular basepairing to position the amino
Ribosomal RNA (rRNA) is a structural molecule that makes up acid binding site at one end and the anticodon in an unbasepaired
over half of the mass of a ribosome and aids in protein synthesis. loop of nucleotides at the other end. The anticodon is a three-
Transfer RNA (tRNA) recognizes a codon on mRNA and brings nucleotide sequence, unique to each different tRNA, that interacts
the appropriate amino acid to that site. with a messenger RNA (mRNA) codon through complementary
rRNAs are processed from larger pre-rRNAs by trimming the base pairing.
larger rRNAs down and methylating some of the nucleotides. There are different tRNAs for the 21 different amino acids. Most
tRNAs are processed from pre-tRNAs by trimming both ends of amino acids can be carried by more than one tRNA.
the pre-tRNA, adding a CCA trinucleotide to the 3′ end, if
needed, removing any introns present, and chemically modified
12 nucleotides on average per tRNA.

KEY TERMS
anticodon: a sequence of three nucleotides in transfer RNA that
binds to the complementary triplet (codon) in messenger RNA,
specifying an amino acid during protein synthesis

PROCESSING OF TRNAS AND RRNAS

The tRNAs and rRNAs are structural molecules that have roles in
protein synthesis; however, these RNAs are not themselves
translated. In eukaryotes, pre-rRNAs are transcribed, processed, and
assembled into ribosomes in the nucleolus, while pre-tRNAs are
transcribed and processed in the nucleus and then released into the
cytoplasm where they are linked to free amino acids for protein
synthesis.

RIBOSOMAL RNA (RRNA)

The four rRNAs in eukaryotes are first transcribed as two long
precursor molecules. One contains just the pre-rRNA that will be
processed into the 5S rRNA; the other spans the 28S, 5.8S, and 18S
rRNAs. Enzymes then cleave the precursors into subunits
corresponding to each rRNA. In bacteria, there are only three rRNAs Figure 15.9.1: Structure of tRNA: This is a space-filling model of a
and all are transcribed in one long precursor molecule that is cleaved tRNA molecule that adds the amino acid phenylalanine to a growing
into the individual rRNAs. Some of the bases of pre-rRNAs are polypeptide chain. The anticodon AAG binds the codon UUC on the
mRNA. The amino acid phenylalanine is attached to the other end of
methylated for added stability. Mature rRNAs make up 50-60% of the tRNA.
each ribosome. Some of a ribosome’s RNA molecules are purely In all organisms, tRNAs are transcribed in a pre-tRNA form that
structural, whereas others have catalytic or binding activities. requires multiple processing steps before the mature tRNA is ready
The eukaryotic ribosome is composed of two subunits: a large for use in translation. In bacteria, multiple tRNAs are often
subunit (60S) and a small subunit (40S). The 60S subunit is transcribed as a single RNA. The first step in their processing is the
composed of the 28S rRNA, 5.8S rRNA, 5S rRNA, and 50 proteins. digestion of the RNA to release individual pre-tRNAs. In archaea
The 40S subunit is composed of the 18S rRNA and 33 proteins. The and eukaryotes, each pre-tRNA is transcribed as a separate
bacterial ribosome is composed of two similar subunits, with slightly transcript.
different components. The bacterial large subunit is called the 50S
subunit and is composed of the 23S rRNA, 5S rRNA, and 31

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The processing to convert the pre-tRNA to a mature tRNA involves CONTRIBUTIONS AND ATTRIBUTIONS
five steps. OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44532/latest...ol11448/latest. License: CC
1. The 5′ end of the pre-tRNA, called the 5′ leader sequence, is BY: Attribution
cleaved off. spliceosome. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/spliceosome. License: CC BY-SA: Attribution-
2. The 3′ end of the pre-tRNA is cleaved off. ShareAlike
moiety. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/moiety.
3. In all eukaryote pre-tRNAs, but in only some bacterial and License: CC BY-SA: Attribution-ShareAlike
archaeal pre-tRNAs, a CCA sequence of nucleotides is added to the intron. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/intron.
License: CC BY-SA: Attribution-ShareAlike
3′ end of the pre-tRNA after the original 3′ end is trimmed off. Some RNA splicing. Provided by: Wikipedia. Located at:
bacteria and archaea pre-tRNAs already have the CCA encoded in en.Wikipedia.org/wiki/RNA_splicing. License: CC BY-SA: Attribution-
ShareAlike
their transcript immediately upstream of the 3′ cleavage site, so they RNA Splicing. Provided by: WikiPedia. Located at:
don’t need to add one. The CCA at the 3′ end of the mature tRNA en.Wikipedia.org/wiki/RNA_splicing. License: CC BY-SA: Attribution-
ShareAlike
will be the site at which the tRNA’s amino acid will be added. OpenStax College, RNA Processing in Eukaryotes. October 16, 2013. Provided
4. Multiple nucleotides in the pre-tRNA are chemically modified, by: OpenStax CNX. Located at:
http://cnx.org/content/m44532/latest...e_15_04_02.png. License: CC BY:
altering their nitorgen bases. On average about 12 nucleotides are Attribution
modified per tRNA. The most common modifications are the An Introduction to Molecular Biology/Transcription of RNA and its
modification. Provided by: Wikibooks. Located at:
conversion of adenine (A) to pseudouridine (ψ), the conversion of en.wikibooks.org/wiki/An_Intr...plicing_of_RNA. License: CC BY-SA:
adenine to inosine (I), and the conversion of uridine to Attribution-ShareAlike
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
dihydrouridine (D). But over 100 other modifications can occur.
Located at: http://cnx.org/content/m44532/latest...ol11448/latest. License: CC
5. A significant number of eukaryotic and archaeal pre-tRNAs have BY: Attribution
Structural Biochemistry/Cell Organelles/Ribosome. Provided by: Wikibooks.
introns that have to be spliced out. Introns are rarer in bacterial pre- Located at: en.wikibooks.org/wiki/Structu...elles/Ribosome. License: CC BY-
tRNAs, but do occur occasionally and are spliced out. SA: Attribution-ShareAlike
Structural Biochemistry/Nucleic Acid/RNA/Transfer RNA (tRNA). Provided by:
After processing, the mature pre-tRNA is ready to have its cognate Wikibooks. Located at: en.wikibooks.org/wiki/Structu...fer_RNA_(tRNA).
License: CC BY-SA: Attribution-ShareAlike
amino acid attached. The cognate amino acid for a tRNA is the one Structural Biochemistry/Nucleic Acid/RNA/Ribosomal RNA (rRNA). Provided
specified by its anticodon. Attaching this amino acid is called by: Wikibooks. Located at:
charging the tRNA. In eukaryotes, the mature tRNA is generated in en.wikibooks.org/wiki/Structu...mal_RNA_(rRNA). License: CC BY-SA:
Attribution-ShareAlike
the nucleus, and then exported to the cytoplasm for charging. anticodon. Provided by: Wiktionary. Located at:
image en.wiktionary.org/wiki/anticodon. License: CC BY-SA: Attribution-
ShareAlike
RNA Splicing. Provided by: WikiPedia. Located at:
en.Wikipedia.org/wiki/RNA_splicing. License: CC BY-SA: Attribution-
ShareAlike
OpenStax College, RNA Processing in Eukaryotes. October 16, 2013. Provided
by: OpenStax CNX. Located at:
http://cnx.org/content/m44532/latest...e_15_04_02.png. License: CC BY:
Attribution
An Introduction to Molecular Biology/Transcription of RNA and its
modification. Provided by: Wikibooks. Located at:
en.wikibooks.org/wiki/An_Intr...plicing_of_RNA. License: CC BY-SA:
Attribution-ShareAlike
OpenStax College, RNA Processing in Eukaryotes. October 16, 2013. Provided
by: OpenStax CNX. Located at:
Figure 15.9.1: Processing of a pre-tRNA.: A typical pre-tRNA http://cnx.org/content/m44532/latest...e_15_04_03.jpg. License: CC BY:
Attribution
undergoing processing steps to generate a mature tRNA ready to
have its cognate amino acid attached. Nucleotides that are cleaved
away are shown in green. Chemically-modified nucleotides are in This page titled 15.9: RNA Processing in Eukaryotes - Processing of tRNAs
yellow, as is the CAA trinucleotide that is added to the 3′ end of the and rRNAs is shared under a CC BY-SA 4.0 license and was authored,
pre-tRNA during processing. The anticodon nucleotides are shown remixed, and/or curated by Boundless.
in a lighter shade of red.

15.9.2 https://bio.libretexts.org/@go/page/13308
15.10: RIBOSOMES AND PROTEIN SYNTHESIS - THE PROTEIN SYNTHESIS
MACHINERY
Protein synthesis, or translation of mRNA into protein, occurs with
the help of ribosomes, tRNAs, and aminoacyl tRNA synthetases.

 LEARNING OBJECTIVES

Explain the role played by ribosomes, tRNA, and aminoacyl

tRNA synthetases in protein synthesis

KEY POINTS
Ribosomes, macromolecular structures composed of rRNA and
polypeptide chains, are formed of two subunits (in bacteria and
archaea, 30S and 50S; in eukaryotes, 40S and 60S), that bring
together mRNA and tRNAs to catalyze protein synthesis. Figure 15.10.1: The ribosome in action: Structure and role of
Fully assembled ribosomes have three tRNA binding sites: an A ribosomes during translation
site for incoming aminoacyl-tRNAs, a P site for peptidyl-tRNAs, Ribosomes exist in the cytoplasm in prokaryotes and in the
and an E site where empty tRNAs exit. cytoplasm and on rough endoplasmic reticulum membranes in
tRNAs (transfer ribonucleic acids), which serve to deliver the eukaryotes. Mitochondria and chloroplasts also have their own
appropriate amino acid to the growing peptide chain, consist of a ribosomes, and these look more similar to prokaryotic ribosomes
modified RNA chain with the appropriate amino acid covalently (and have similar drug sensitivities) than the cytoplasmic ribosomes.
attached. Ribosomes dissociate into large and small subunits when they are
tRNAs have a loop of unbasepaired nucleotides at one end of the not synthesizing proteins and reassociate during the initiation of
molecule that contains three nucleotides that act as the anticodon translation.E. coli have a 30S small subunit and a 50S large subunit,
that basepairs to the mRNA codon. for a total of 70S when assembled (recall that Svedberg units are not
Aminoacyl tRNA synthetases are enzymes that load the additive). Mammalian ribosomes have a small 40S subunit and a
individual amino acids onto the tRNAs. large 60S subunit, for a total of 80S. The small subunit is
responsible for binding the mRNA template, whereas the large
KEY TERMS subunit sequentially binds tRNAs.
ribosome: protein/mRNA complexes found in all cells that are In bacteria, archaea, and eukaryotes, the intact ribosome has three
involved in the production of proteins by translating messenger binding sites that accomodate tRNAs: The A site, the P site, and the
RNA E site. Incoming aminoacy-tRNAs (a tRNA with an amino acid
covalently attached is called an aminoacyl-tRNA) enter the
THE PROTEIN SYNTHESIS MACHINERY ribosome at the A site. The peptidyl-tRNA carrying the growing
In addition to the mRNA template, many molecules and polypeptide chain is held in the P site. The E site holds empty
macromolecules contribute to the process of translation. The tRNAs just before they exit the ribosome.
composition of each component may vary across species. For
instance, ribosomes may consist of different numbers of rRNAs and
polypeptides depending on the organism. However, the general
structures and functions of the protein synthesis machinery are
comparable from bacteria to archaea to human cells. Translation
requires the input of an mRNA template, ribosomes, tRNAs, and
various enzymatic factors.

RIBOSOMES
A ribosome is a complex macromolecule composed of structural and
catalytic rRNAs, and many distinct polypeptides. In eukaryotes, the
synthesis and assembly of rRNAs occurs in the nucleolus.

15.10.1 https://bio.libretexts.org/@go/page/13311
 the “cloverleaf” structure. All tRNAs fold into very similar
cloverleaf structures of four major stems and three major loops.

Figure 15.10.1: Ribosome structure: The large ribosomal subunit

sits atop the small ribosomal subunit and the mRNA is threaded
through a groove near the interface of the two subunits. The intact
ribosome has three tRNA binding sites: the A site for incoming
aminoacyl-tRNAs; the P site for the peptidyl-tRNA carrying the Figure 15.10.1: The two-dimensional cloverleaf structure of a
growing polypeptide chain; and the E site where empty tRNAs exit typical tRNA.: All tRNAs, regardless of the species they come from
(not shown in this figure but immediately adjacent to the P site.) or the amino acid they carry, self-basepair to produce a cloverleaf
Each mRNA molecule is simultaneously translated by many structure of four main stems and three main loops. The amino acid
carried by the tRNA is covalently attached to the nucleotide at the 3′
ribosomes, all reading the mRNA from 5′ to 3′ and synthesizing the end of the tRNA, known as the tRNA’s acceptor arm. The opposite
polypeptide from the N terminus to the C terminus. The complete end of the folded tRNA has the anticodon loop where the tRNA will
mRNA/poly-ribosome structure is called a polysome. basepair to the mRNA codon.
If viewed as a three-dimensional structure, all the basepaired regions
TRNAS IN EUKARYOTES of the tRNA are helical, and the tRNA folds into a L-shaped
The tRNA molecules are transcribed by RNA polymerase III. structure.
Depending on the species, 40 to 60 types of tRNAs exist in the
cytoplasm. Specific tRNAs bind to codons on the mRNA template
and add the corresponding amino acid to the polypeptide chain.
(More accurately, the growing polypeptide chain is added to each
new amino acid bound in by a tRNA.)
The transfer RNAs (tRNAs) are structural RNA molecules. In
eukaryotes, tRNA mole are transcribed from tRNA genes by RNA
polymerase III. Depending on the species, 40 to 60 types of tRNAs
exist in the cytoplasm. Serving as adaptors, specific tRNAs bind to
sequences on the mRNA template and add the corresponding amino
acid to the polypeptide chain. (More accurately, the growing
polypeptide chain is added to each new amino acid brought in by a
tRNA.) Therefore, tRNAs are the molecules that actually “translate”
the language of RNA into the language of proteins.
Of the 64 possible mRNA codons (triplet combinations of A, U, G,
and C) three specify the termination of protein synthesis and 61
specify the addition of amino acids to the polypeptide chain. Of the
three termination codons, one (UGA) can also be used to encode the
21st amino acid, selenocysteine, but only if the mRNA contains a
specific sequence of nucleotides known as a SECIS sequence. Of the
61 non-termination codons, one codon (AUG) also encodes the
Figure 15.10.1: The three dimensional shape taken by tRNAs.: If
initiation of translation. viewed as a three-dimensional structure, all tRNAs are partially
Each tRNA polynucleotide chain folds up so that some internal helical molecules that are vaguely L-shaped. The anticodon-
containing loop is at one end of the molecule (in grey here) and the
sections basepair with other internal sections. If just diagrammed in amino acid acceptor arm is at the other end of the molecule (in
two dimensions, the regions where basepairing occurs are called yellow here) past the bend of the “L”.
stems, and the regions where no basepairs form are called loops, and Each tRNA has a sequence of three nucleotides located in a loop at
the entire pattern of stems and loops that forms for a tRNA is called one end of the molecule that can basepair with an mRNA codon.
This is called the tRNA’s anticodon. Each different tRNA has a

15.10.2 https://bio.libretexts.org/@go/page/13311
different anticodon. When the tRNA anticodon basepairs with one of aminoacyl-tRNA. At least one type of aminoacyl tRNA synthetase
the mRNA codons, the tRNA will add an amino acid to a growing exists for each of the 21 amino acids; the exact number of aminoacyl
polypeptide chain or terminate translation, according to the genetic tRNA synthetases varies by species. These enzymes first bind and
code. For instance, if the sequence CUA occurred on a mRNA hydrolyze ATP to catalyze the formation of a covalent bond between
template in the proper reading frame, it would bind a tRNA with an an amino acid and adenosine monophosphate (AMP); a
anticodon expressing the complementary sequence, GAU. The pyrophosphate molecule is expelled in this reaction. This is called
tRNA with this anticodon would be linked to the amino acid leucine. “activating” the amino acid. The same enzyme then catalyzes the
attachment of the activated amino acid to the tRNA and the
AMINOACYL TRNA SYNTHETASES simultaneous release of AMP. After the correct amino acid
The process of pre-tRNA synthesis by RNA polymerase III only covalently attached to the tRNA, it is released by the enzyme. The
creates the RNA portion of the adaptor molecule. The corresponding tRNA is said to be charged with its cognate amino acid. (the amino
amino acid must be added later, once the tRNA is processed and acid specified by its anticodon is a tRNA’s cognate amino acid.)
exported to the cytoplasm. Through the process of tRNA “charging,”
each tRNA molecule is linked to its correct amino acid by a group of This page titled 15.10: Ribosomes and Protein Synthesis - The Protein
enzymes called aminoacyl tRNA synthetases. When an amino acid Synthesis Machinery is shared under a CC BY-SA 4.0 license and was
is covalently linked to a tRNA, the resulting complex is known as an authored, remixed, and/or curated by Boundless.

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15.11: RIBOSOMES AND PROTEIN SYNTHESIS - THE MECHANISM OF
PROTEIN SYNTHESIS
positioned in the ribosome P site. The remaining eIFs dissociate
 LEARNING OBJECTIVES from the ribosome and translation is ready to begins.
In archaea, translation initiation is similar to that seen in eukaryotes,
Describe the process of translation
except that the initiation factors involved are called aIFs (archaeal
inititiaion factors), not eIFs.
As with mRNA synthesis, protein synthesis can be divided into three
phases: initiation, elongation, and termination.

INITIATION OF TRANSLATION
Protein synthesis begins with the formation of a pre-initiation
complex. In E. coli, this complex involves the small 30S ribosome,
the mRNA template, three initiation factors (IFs; IF-1, IF-2, and IF-
3), and a special initiator tRNA, called fMet-tRNA. The initiator
tRNA basepairs to the start codon AUG (or rarely, GUG) and is
covalently linked to a formylated methionine called fMet.
Methionine is one of the 21 amino acids used in protein synthesis;
formylated methionine is a methione to which a formyl group (a
one-carbon aldehyde) has been covalently attached at the amino
nitrogen. Formylated methionine is inserted by fMet-tRNA at the
beginning of every polypeptide chain synthesized by E. coli, and is
usually clipped off after translation is complete. When an in-frame
AUG is encountered during translation elongation, a non-formylated
methionine is inserted by a regular Met-tRNA. In E. coli mRNA, a
sequence upstream of the first AUG codon, called the Shine-
Dalgarno sequence (AGGAGG), interacts with the rRNA molecules
that compose the ribosome. This interaction anchors the 30S
ribosomal subunit at the correct location on the mRNA template.
Figure 15.11.1: Translation initiation in eukaryotes.: In eukaryotes, a
In eukaryotes, a pre-initiation complex forms when an initiation preinitiation complex forms made of the small 40S subunit, the
factor called eIF2 ( eukaryotic initiation factor 2) binds GTP, and the initiator Met-tRNAi, and eIF2-GTP. This preinitiation complex
GTP-eIF2 recruits the eukaryotic initiator tRNA to the 40s small binds to the 5′-m7G cap of the mRNA with the help of other eIFS
and PAB, which binds the poly(A) tail of the mRNA, and loops the
ribosomal subunit. The initiator tRNA, called Met-tRNAi, carries tail to the cap. Once at the cap, the preinitiation complex slides
unmodified methionine in eukaryotes, not fMet, but it is distinct along the mRNA until it encounters the initiator AUG codon. There,
from other cellular Met-tRNAs in that it can bind eIFs and it can GTP is hydrolyzed by eIF2 and the Met-tRNAi is loaded onto the
AUG. Next, eIF5-GTP recruits the 60S large ribosomal subunit to
bind at the ribosome P site. The eukaryotic pre-initiation complex the 40S subunit at the AUG and hydrolyzes GTP. This allows the
then recognizes the 7-methylguanosine cap at the 5′ end of a mRNA. large ribosomal subunit to assemble on top of the small subunit,
Several other eIFs, specifically eIF1, eIF3, and eIF4, act as cap- generating the intact 80S ribosome, and places the Met-tRNAi in the
P site of the intact ribosome. The ribosome A site is positioned over
binding proteins and assist the recruitment of the pre-initiation the second codon in the mRNA reading frame, and translation
complex to the 5′ cap. Poly (A)-Binding Protein (PAB) binds both elongation can begin. (CC BY-SA 3.0; Zephyris via Wikipedia)
the poly (A) tail of the mRNA and the complex of proteins at the cap
and also assists in the process. Once at the cap, the pre-initiation TRANSLATION ELONGATION
complex tracks along the mRNA in the 5′ to 3′ direction, searching The basics of elongation are the same in prokaryotes and eukaryotes.
for the AUG start codon. Many, but not all, eukaryotic mRNAs are The intact ribosome has three compartments: the A site binds
translated from the first AUG sequence. The nucleotides around the incoming aminoacyl tRNAs; the P site binds tRNAs carrying the
AUG indicate whether it is the correct start codon. growing polypeptide chain; the E site releases dissociated tRNAs so
that they can be recharged with amino acids. The initiator tRNA,
Once the appropriate AUG is identified, eIF2 hydrolyzes GTP to
rMet-tRNA in E. coli and Met-tRNAi in eukaryotes and archaea,
GDP and powers the delivery of the tRNAi-Met to the start codon,
binds directly to the P site. This creates an initiation complex with a
where the tRNAi anticodon basepairs to the AUG codon. After this,
free A site ready to accept the aminoacyl-tRNA corresponding to the
eIF2-GDP is released from the complex, and eIF5-GTP binds. The
first codon after the AUG.
60S ribosomal subunit is recruited to the pre-initiation complex by
eIF5-GTP, which hydrolyzes its GTP to GDP to power the assembly The aminoacyl-tRNA with an anticodon complementary to the A
of the full ribosome at the translation start site with the Met-tRNAi site codon lands in the A site. A peptide bond is formed between the
amino group of the A site amino acid and the carboxyl group of the

15.11.1 https://bio.libretexts.org/@go/page/13312
most-recently attached amino acid in the growing polypeptide chain
attached to the P-site tRNA.The formation of the peptide bond is
catalyzed by peptidyl transferase, an RNA-based enzyme that is
integrated into the large ribosomal subunit. The energy for the
peptide bond formation is derived from GTP hydrolysis, which is
catalyzed by a separate elongation factor.
Catalyzing the formation of a peptide bond removes the bond
holding the growing polypeptide chain to the P-site tRNA. The
growing polypeptide chain is transferred to the amino end of the
incoming amino acid, and the A-site tRNA temporarily holds the
growing polypeptide chain, while the P-site tRNA is now empty or
uncharged.
The ribosome moves three nucleotides down the mRNA. The tRNAs
are basepaired to a codon on the mRNA, so as the ribosome moves
over the mRNA, the tRNAs stay in place while the ribosome moves
and each tRNA is moved into the next tRNA binding site. The E site Figure 15.11.2: Translation elongation in eukaryotes.: During
moves over the former P-site tRNA, now empty or uncharged, the P translation elongation, the incoming aminoacyl-tRNA enters the
site moves over the former A-site tRNA, now carrying the growing ribosome A site, where it binds if the tRNA anticodon is
complementary to the A site mRNA codon. The elongation factor
polypeptide chain, and the A site moves over a new codon. In the E eEF1 assists in loading the aminoacyl-tRNA, powering the process
site, the uncharged tRNA detaches from its anticodon and is through the hydrolysis of GTP. The growing polypeptide chain is
expelled. A new aminoacyl-tRNA with an anticodon complementary attached to the tRNA in the ribosome P site. The ribosome’s peptidyl
transferase catalyses the transfer of the growing polypeptide chain
to the new A-site codon enters the ribosome at the A site and the from the P site tRNA to the amino group of the A site amino acid.
elongation process repeats itself. The energy for each step of the This creates a peptide bond between the C terminus of the growing
ribosome is donated by an elongation factor that hydrolyzes GTP. polypeptide chain and the A site amino acid. After the peptide bond
is created, the growing polypeptide chain is attached to the A site
tRNA, and the tRNA in the P site is empty. The ribosome
translocates once codon on the mRNA. The elongation factor eEF2
assists in the translocation, powering the process through the
hydrolysis of GTP. During translocation, the two tRNAs remain
basepaired to their mRNA codons, so the ribosome moves over
them, putting the empty tRNA in the E site (where it will be
expelled from the ribosome) and the tRNA with the growing
polypeptide chain in the P site. The A site moves over an empty
codon, and the process repeats itself until a stop codon is reached.
(CC BY-SA 4.0; Jordan Nguyen via Wikipedia)

TRANSLATION TERMINATION
Termination of translation occurs when the ribosome moves over a
stop codon (UAA, UAG, or UGA). There are no tRNAs with
anticodons complementary to stop codons, so no tRNAs enter the A
site. Instead, in both prokaryotes and eukaryotes, a protein called a
release factor enters the A site. The release factors cause the
ribosome peptidyl transferase to add a water molecule to the
carboxyl end of the most recently added amino acid in the growing
polypeptide chain attached to the P-site tRNA. This causes the
polypeptide chain to detach from its tRNA, and the newly-made
polypeptide is released. The small and large ribosomal subunits
dissociate from the mRNA and from each other; they are recruited
almost immediately into another translation initiation complex. After
many ribosomes have completed translation, the mRNA is degraded
so the nucleotides can be reused in another transcription reaction.

KEY POINTS
Protein synthesis, or translation, begins with a process known as
pre-initiation, when the small ribosmal subunit, the mRNA
template, initiator factors, and a special initiator tRNA, come
together.
During translocation and elongation, the ribosome moves one
codon 3′ down the mRNA, brings in a charged tRNA to the A

15.11.2 https://bio.libretexts.org/@go/page/13312
site, transfers the growing polypeptide chain from the P-site KEY TERMS
tRNA to the carboxyl group of the A-site amino acid, and ejects translation: a process occurring in the ribosome in which a
the uncharged tRNA at the E site. strand of messenger RNA (mRNA) guides assembly of a
When a stop or nonsense codon (UAA, UAG, or UGA) is sequence of amino acids to make a protein
reached on the mRNA, the ribosome terminates translation.
This page titled 15.11: Ribosomes and Protein Synthesis - The Mechanism
of Protein Synthesis is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

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15.12: RIBOSOMES AND PROTEIN SYNTHESIS - PROTEIN FOLDING,
MODIFICATION, AND TARGETING
In order to function, proteins must fold into the correct three-
dimensional shape, and be targeted to the correct part of the cell.

 LEARNING OBJECTIVES

Discuss how post-translational events affect the proper

function of a protein

KEY POINTS
Protein folding is a process in which a linear chain of amino Figure 15.12.1: Protein folding: A protein starts as a linear sequence
acids attains a defined three-dimensional structure, but there is a of amino acids, then folds into a 3-dimensional shape imbued with
all the functional properties required inside the cell.
possibility of forming misfolded or denatured proteins, which are
often inactive. PROTEIN MODIFICATION AND TARGETING
Proteins must also be located in the correct part of the cell in
During and after translation, individual amino acids may be
order to function correctly; therefore, a signal sequence is often
chemically modified and signal sequences may be appended to the
attached to direct the protein to its proper location, which is
protein. A signal sequence is a short tail of amino acids that directs a
removed after it attains its location.
protein to a specific cellular compartment. These sequences at the
Protein misfolding is the cause of numerous diseases, such as
amino end or the carboxyl end of the protein can be thought of as the
mad cow disease, Creutzfeldt-Jakob disease, and cystic fibrosis.
protein’s “train ticket” to its ultimate destination. Other cellular
KEY TERMS factors recognize each signal sequence and help transport the protein
from the cytoplasm to its correct compartment. For instance, a
prion: a self-propagating misfolded conformer of a protein that
specific sequence at the amino terminus will direct a protein to the
is responsible for a number of diseases that affect the brain and
mitochondria or chloroplasts (in plants). Once the protein reaches its
other neural tissue
cellular destination, the signal sequence is usually clipped off.
chaperone: a protein that assists the non-covalent
folding/unfolding of other proteins MISFOLDING
PROTEIN FOLDING It is very important for proteins to achieve their native conformation
since failure to do so may lead to serious problems in the
After being translated from mRNA, all proteins start out on a
accomplishment of its biological function. Defects in protein folding
ribosome as a linear sequence of amino acids. This linear sequence
may be the molecular cause of a range of human genetic disorders.
must “fold” during and after the synthesis so that the protein can
For example, cystic fibrosis is caused by defects in a membrane-
acquire what is known as its native conformation. The native
bound protein called cystic fibrosis transmembrane conductance
conformation of a protein is a stable three-dimensional structure that
regulator (CFTR). This protein serves as a channel for chloride ions.
strongly determines a protein’s biological function. When a protein
The most common cystic fibrosis-causing mutation is the deletion of
loses its biological function as a result of a loss of three-dimensional
a Phe residue at position 508 in CFTR, which causes improper
structure, we say that the protein has undergone denaturation.
folding of the protein. Many of the disease-related mutations in
Proteins can be denatured not only by heat, but also by extremes of
collagen also cause defective folding.
pH; these two conditions affect the weak interactions and the
hydrogen bonds that are responsible for a protein’s three- A misfolded protein, known as prion, appears to be the agent of a
dimensional structure. Even if a protein is properly specified by its number of rare degenerative brain diseases in mammals, like the
corresponding mRNA, it could take on a completely dysfunctional mad cow disease. Related diseases include kuru and Creutzfeldt-
shape if abnormal temperature or pH conditions prevent it from Jakob. The diseases are sometimes referred to as spongiform
folding correctly. The denatured state of the protein does not equate encephalopathies, so named because the brain becomes riddled with
with the unfolding of the protein and randomization of holes. Prion, the misfolded protein, is a normal constituent of brain
conformation. Actually, denatured proteins exist in a set of partially- tissue in all mammals, but its function is not yet known. Prions
folded states that are currently poorly understood. Many proteins cannot reproduce independently and not considered living
fold spontaneously, but some proteins require helper molecules, microoganisms. A complete understanding of prion diseases awaits
called chaperones, to prevent them from aggregating during the new information about how prion protein affects brain function, as
complicated process of folding. well as more detailed structural information about the protein.
Therefore, improved understanding of protein folding may lead to
new therapies for cystic fibrosis, Creutzfeldt-Jakob, and many other
diseases.

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CONTRIBUTIONS AND ATTRIBUTIONS Translation (biology). Provided by: Wikepedia. Located at:
en.Wikipedia.org/wiki/Transla...slation_en.svg. License: Public Domain: No
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. Known Copyright
Located at: http://cnx.org/content/m44529/latest...ol11448/latest. License: CC OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
BY: Attribution Located at: http://cnx.org/content/m44529/latest...ol11448/latest. License: CC
ribosome. Provided by: Wiktionary. Located at: BY: Attribution
en.wiktionary.org/wiki/ribosome. License: CC BY-SA: Attribution-ShareAlike Lydia Kavraki, Protein Folding. November 4, 2013. Provided by: OpenStax
Ribosome. Provided by: Wikipedia. Located at: CNX. Located at: http://cnx.org/content/m11467/latest/. License: CC BY:
en.Wikipedia.org/wiki/Ribosome. License: CC BY-SA: Attribution-ShareAlike Attribution
Transfer RNA. Provided by: Wikipedia. Located at: prion. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/prion.
commons.wikimedia.org/wiki/Fi...e_yeast_en.svg. License: CC BY-SA: License: CC BY-SA: Attribution-ShareAlike
Attribution-ShareAlike chaperone. Provided by: Wiktionary. Located at:
Transfer RNA. Provided by: Wikepedia. Located at: en.wiktionary.org/wiki/chaperone. License: CC BY-SA: Attribution-
commons.wikimedia.org/wiki/Fi...e_yeast_en.svg. License: CC BY-SA: ShareAlike
Attribution-ShareAlike Ribosome. Provided by: Wikipedia. Located at:
Translation (biology). Provided by: Wikepedia. Located at: en.Wikipedia.org/wiki/Ribosome. License: CC BY-SA: Attribution-ShareAlike
en.Wikipedia.org/wiki/Transla...slation_en.svg. License: Public Domain: No Transfer RNA. Provided by: Wikipedia. Located at:
Known Copyright commons.wikimedia.org/wiki/Fi...e_yeast_en.svg. License: CC BY-SA:
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. Attribution-ShareAlike
Located at: http://cnx.org/content/m44529/latest...ol11448/latest. License: CC Transfer RNA. Provided by: Wikepedia. Located at:
BY: Attribution commons.wikimedia.org/wiki/Fi...e_yeast_en.svg. License: CC BY-SA:
translation. Provided by: Wiktionary. Located at: Attribution-ShareAlike
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ShareAlike en.Wikipedia.org/wiki/Transla...slation_en.svg. License: Public Domain: No
Ribosome. Provided by: Wikipedia. Located at: Known Copyright
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Transfer RNA. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/File:Protein_folding.png. License: Public Domain: No
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Attribution-ShareAlike Modification, and Targeting is shared under a CC BY-SA 4.0 license and
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 CHAPTER OVERVIEW

16: GENE EXPRESSION

16.1: Regulation of Gene Expression - The Process and Purpose of Gene Expression Regulation
16.2: Regulation of Gene Expression - Prokaryotic versus Eukaryotic Gene Expression
16.3: Prokaryotic Gene Regulation - The trp Operon- A Repressor Operon
16.4: Prokaryotic Gene Regulation - Catabolite Activator Protein (CAP)- An Activator Regulator
16.5: Prokaryotic Gene Regulation - The lac Operon- An Inducer Operon
16.6: Eukaryotic Gene Regulation - The Promoter and the Transcription Machinery
16.7: Eukaryotic Gene Regulation - Transcriptional Enhancers and Repressors
16.8: Eukaryotic Gene Regulation - Epigenetic Control- Regulating Access to Genes within the Chromosome
16.9: Eukaryotic Gene Regulation - RNA Splicing
16.10: Eukaryotic Gene Regulation - The Initiation Complex and Translation Rate
16.11: Eukaryotic Gene Regulation - Regulating Protein Activity and Longevity
16.12: Regulating Gene Expression in Cell Development - Gene Expression in Stem Cells
16.13: Regulating Gene Expression in Cell Development - Cellular Differentiation
16.14: Regulating Gene Expression in Cell Development - Mechanics of Cellular Differentation
16.15: Regulating Gene Expression in Cell Development - Establishing Body Axes during Development
16.16: Regulating Gene Expression in Cell Development - Gene Expression for Spatial Positioning
16.17: Regulating Gene Expression in Cell Development - Cell Migration in Multicellular Organisms
16.18: Regulating Gene Expression in Cell Development - Programmed Cell Death
16.19: Cancer and Gene Regulation - Altered Gene Expression in Cancer
16.20: Cancer and Gene Regulation - Epigenetic Alterations in Cancer
16.21: Cancer and Gene Regulation - Cancer and Transcriptional Control
16.22: Cancer and Gene Regulation - Cancer and Post-Transcriptional Control
16.23: Cancer and Gene Regulation - Cancer and Translational Control

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1
16.1: REGULATION OF GENE EXPRESSION - THE PROCESS AND PURPOSE
OF GENE EXPRESSION REGULATION
protein. In a given cell type, not all genes encoded in the DNA are
 LEARNING OBJECTIVES transcribed into RNA or translated into protein because specific cells
in our body have specific functions. Specialized proteins that make
Discuss how the genome and proteome contribute to the
up the eye (iris, lens, and cornea) are only expressed in the eye,
specialization of a cell
whereas the specialized proteins in the heart (pacemaker cells, heart
muscle, and valves) are only expressed in the heart. At any given
Each somatic cell in the body generally contains the same DNA. A
time, only a subset of all of the genes encoded by our DNA are
few exceptions include red blood cells, which contain no DNA in expressed and translated into proteins. The expression of specific
their mature state, and some immune system cells that rearrange genes is a highly-regulated process with many levels and stages of
their DNA while producing antibodies. In general, however, the control. This complexity ensures the proper expression in the proper
genes that determine whether you have green eyes, brown hair, and
cell at the proper time.
how fast you metabolize food are the same in the cells in your eyes
In this section, you will learn about the various methods of gene
and your liver, even though these organs function quite differently. If
regulation and the mechanisms used to control gene expression, such
each cell has the same DNA, how is it that cells or organs are
as: epigenetic, transcriptional, post-transcriptional, translational, and
different? Why do cells in the eye differ so dramatically from cells
post-translational controls in eukaryotic gene expression, and
in the liver ?
transcriptional control in prokaryotic gene expression.

KEY POINTS
Every cell within an organism shares the same genome (with
exceptions, i.e. mature red blood cells), but has variation
between its proteomes.
Gene expression involves the process of transcribing DNA into
RNA and then translating RNA into proteins.
Gene expression is a highly complex and tightly-regulated
process.
Figure 16.1.1: Gene Expression: The genetic content of each
somatic cell in an organism is the same, but not all genes are
expressed in every cell. The control of which genes are expressed
KEY TERMS
dictates whether a cell is (a) an eye cell or (b) a liver cell. It is the somatic: part of, or relating to the body of an organism
differential gene expression patterns that arise in different cells that genome: the cell’s complete genetic information packaged as a
give rise to (c) a complete organism.
double-stranded DNA molecule
Whereas each cell shares the same genome and DNA sequence, each
proteome: the complete set of proteins encoded by a particular
cell does not turn on, or express, the same set of genes. Each cell
genome
type needs a different set of proteins to perform its function.
Therefore, only a small subset of proteins is expressed in a cell that This page titled 16.1: Regulation of Gene Expression - The Process and
constitutes its proteome. For the proteins to be expressed, the DNA Purpose of Gene Expression Regulation is shared under a CC BY-SA 4.0
must be transcribed into RNA and the RNA must be translated into license and was authored, remixed, and/or curated by Boundless.

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16.2: REGULATION OF GENE EXPRESSION - PROKARYOTIC VERSUS
EUKARYOTIC GENE EXPRESSION

 LEARNING OBJECTIVES

Compare and contrast prokaryotic and eukaryotic gene

expression

PROKARYOTIC VERSUS EUKARYOTIC GENE

EXPRESSION
To understand how gene expression is regulated, we must first
understand how a gene codes for a functional protein in a cell. The Figure 16.2.1: Prokaryotic vs Eukaryotic Gene Expression:
Prokaryotic transcription and translation occur simultaneously in the
process occurs in both prokaryotic and eukaryotic cells, just in cytoplasm, and regulation occurs at the transcriptional level.
slightly different manners. Eukaryotic gene expression is regulated during transcription and
Prokaryotic organisms are single-celled organisms that lack a RNA processing, which take place in the nucleus, and during protein
translation, which takes place in the cytoplasm. Further regulation
defined nucleus; therefore, their DNA floats freely within the cell may occur through post-translational modifications of proteins.
cytoplasm. To synthesize a protein, the processes of transcription
(DNA to RNA) and translation (RNA to protein) occur almost KEY POINTS
simultaneously. When the resulting protein is no longer needed, Prokaryotic gene expression is primarily controlled at the level
transcription stops. Thus, the regulation of transcription is the of transcription.
primary method to control what type of protein and how much of Eukaryotic gene expression is controlled at the levels of
each protein is expressed in a prokaryotic cell. All of the subsequent epigenetics, transcription, post-transcription, translation, and
steps occur automatically. When more protein is required, more post-translation.
transcription occurs. Therefore, in prokaryotic cells, the control of Prokaryotic gene expression (both transcription and translation)
gene expression is mostly at the transcriptional level. occurs within the cytoplasm of a cell due to the lack of a defined
Eukaryotic cells, in contrast, have intracellular organelles that add to nucleus; thus, the DNA is freely located within the cytoplasm.
their complexity. In eukaryotic cells, the DNA is contained inside Eukaryotic gene expression occurs in both the nucleus
the cell’s nucleus where it is transcribed into RNA. The newly- (transcription) and cytoplasm (translation).
synthesized RNA is then transported out of the nucleus into the
KEY TERMS
cytoplasm where ribosomes translate the RNA into protein. The
processes of transcription and translation are physically separated by epigenetics: the study of heritable changes caused by the
the nuclear membrane; transcription occurs only within the nucleus, activation and deactivation of genes without any change in DNA
and translation occurs only outside the nucleus within the cytoplasm. sequence
The regulation of gene expression can occur at all stages of the nucleosome: any of the subunits that repeat in chromatin; a coil
process. Regulation may occur when the DNA is uncoiled and of DNA surrounding a histone core
loosened from nucleosomes to bind transcription factors
CONTRIBUTIONS AND ATTRIBUTIONS
(epigenetics), when the RNA is transcribed (transcriptional level),
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
when the RNA is processed and exported to the cytoplasm after it is Located at: http://cnx.org/content/m44533/latest...ol11448/latest. License: CC
transcribed (post-transcriptional level), when the RNA is translated BY: Attribution
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(post-translational level). proteome. Provided by: Wiktionary. Located at:
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genome. Provided by: Wiktionary. Located at:
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OpenStax College, Regulation of Gene Expression. October 16, 2013. Provided This page titled 16.2: Regulation of Gene Expression - Prokaryotic versus
by: OpenStax CNX. Located at:
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16.3: PROKARYOTIC GENE REGULATION - THE TRP OPERON- A REPRESSOR
OPERON
initiate transcription. The promoter sequence is upstream of the
 LEARNING OBJECTIVES transcriptional start site. Each operon has a sequence within or near
the promoter to which proteins (activators or repressors) can bind
Explain the relationship between structure and function of
and regulate transcription.
an operon and the ways in which repressors regulate gene
expression A DNA sequence called the operator sequence is encoded between
the promoter region and the first trp-coding gene. This operator
contains the DNA code to which the repressor protein can bind.
Bacteria such as E. coli need amino acids to survive. Tryptophan is
When tryptophan is present in the cell, two tryptophan molecules
one such amino acid that E. coli can ingest from the environment. E.
bind to the trp repressor, which changes shape to bind to the trp
coli can also synthesize tryptophan using enzymes that are encoded
operator. Binding of the tryptophan–repressor complex at the
by five genes. These five genes are next to each other in what is
operator physically prevents the RNA polymerase from binding and
called the tryptophan (trp) operon. If tryptophan is present in the
transcribing the downstream genes.
environment, then E. coli does not need to synthesize it; the switch
controlling the activation of the genes in the trp operon is turned off. When tryptophan is not present in the cell, the repressor by itself
However, when tryptophan availability is low, the switch controlling does not bind to the operator; therefore, the operon is active and
the operon is turned on, transcription is initiated, the genes are tryptophan is synthesized. Because the repressor protein actively
expressed, and tryptophan is synthesized. binds to the operator to keep the genes turned off, the trp operon is
negatively regulated and the proteins that bind to the operator to
silence trp expression are negative regulators.

KEY POINTS
The operator sequence is encoded between the promoter region
and the first trp-coding gene.
The trp operon is repressed when tryptophan levels are high by
binding the repressor protein to the operator sequence via a
corepressor which blocks RNA polymerase from transcribing the
trp-related genes.
The trp operon is activated when tryptophan levels are low by
dissociation of the repressor protein to the operator sequence
which allows RNA polymerase to transcribe the trp genes in the
operon.
Figure 16.3.1: The trp operon: The five genes that are needed to
synthesize tryptophan in E. coli are located next to each other in the KEY TERMS
trp operon. When tryptophan is plentiful, two tryptophan molecules
bind the repressor protein at the operator sequence. This physically repressor: any protein that binds to DNA and thus regulates the
blocks the RNA polymerase from transcribing the tryptophan genes. expression of genes by decreasing the rate of transcription
When tryptophan is absent, the repressor protein does not bind to the
operator and the genes are transcribed. operon: a unit of genetic material that functions in a coordinated
manner by means of an operator, a promoter, and structural genes
A DNA sequence that codes for proteins is referred to as the coding
that are transcribed together
region. The five coding regions for the tryptophan biosynthesis
enzymes are arranged sequentially on the chromosome in the This page titled 16.3: Prokaryotic Gene Regulation - The trp Operon- A
operon. Just before the coding region is the transcriptional start site. Repressor Operon is shared under a CC BY-SA 4.0 license and was
This is the region of DNA to which RNA polymerase binds to authored, remixed, and/or curated by Boundless.

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16.4: PROKARYOTIC GENE REGULATION - CATABOLITE ACTIVATOR PROTEIN
(CAP)- AN ACTIVATOR REGULATOR
greater simplicity with which glucose may be metabolized in
 LEARNING OBJECTIVES comparison to lactose.

Explain how an activator works to increase transcription of a

gene

Just as the trp operon is negatively regulated by tryptophan

molecules, there are proteins that bind to the operator sequences that
act as a positive regulator to turn genes on and activate them. For
example, when glucose is scarce, E. coli bacteria can turn to other
sugar sources for fuel. To do this, new genes to process these
alternate genes must be transcribed. This type of process can be seen
in the lac operon which is turned on in the presence of lactose and
absence of glucose.
When glucose levels drop, cyclic AMP (cAMP) begins to
accumulate in the cell. The cAMP molecule is a signaling molecule Figure 16.4.1: Catabolite Activator Protein (CAP) Regulation:
When glucose levels fall, E. coli may use other sugars for fuel, but
that is involved in glucose and energy metabolism in E. coli. When must transcribe new genes to do so. As glucose supplies become
glucose levels decline in the cell, accumulating cAMP binds to the limited, cAMP levels increase. This cAMP binds to the CAP protein,
positive regulator catabolite activator protein (CAP), a protein that a positive regulator that binds to an operator region upstream of the
genes required to use other sugar sources.
binds to the promoters of operons that control the processing of
alternative sugars, such as the lac operon. The CAP assists in KEY POINTS
production in the absence of glucose. CAP is a transcriptional Catabolite activator protein (CAP) must bind to cAMP to
activator that exists as a homodimer in solution, with each subunit activate transcription of the lac operon by RNA polymerase.
comprising a ligand-binding domain at the N-terminus, which is also CAP is a transcriptional activator with a ligand-binding domain
responsible for the dimerization of the protein and a DNA-binding at the N-terminus and a DNA -binding domain at the C-terminus.
domain at the C-terminus. Two cAMP molecules bind dimeric CAP cAMP molecules bind to CAP and function as allosteric effectors
with negative cooperativity and function as allosteric effectors by by increasing CAP’s affinity to DNA.
increasing the protein’s affinity for DNA. CAP has a characteristic
helix-turn-helix structure that allows it to bind to successive major KEY TERMS
grooves on DNA. This opens up the DNA molecule, allowing RNA RNA polymerase: a DNA-dependent RNA polymerase, an
polymerase to bind and transcribe the genes involved in lactose enzyme, that produces RNA
catabolism. When cAMP binds to CAP, the complex binds to the operon: a unit of genetic material that functions in a coordinated
promoter region of the genes that are needed to use the alternate manner by means of an operator, a promoter, and structural genes
sugar sources. In these operons, a CAP-binding site is located that are transcribed together
upstream of the RNA-polymerase-binding site in the promoter. This promoter: the section of DNA that controls the initiation of
increases the binding ability of RNA polymerase to the promoter RNA transcription
region and the transcription of the genes. As cAMP-CAP is required
for transcription of the lac operon, this requirement reflects the This page titled 16.4: Prokaryotic Gene Regulation - Catabolite Activator
Protein (CAP)- An Activator Regulator is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

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16.5: PROKARYOTIC GENE REGULATION - THE LAC OPERON- AN INDUCER
OPERON

 LEARNING OBJECTIVES

Describe the components of the lac operon and their role in

its function

A major type of gene regulation that occurs in prokaryotic cells

utilizes and occurs through inducible operons. Inducible operons
have proteins that can bind to either activate or repress transcription
depending on the local environment and the needs of the cell. The
lac operon is a typical inducible operon. As mentioned previously, E.
coli is able to use other sugars as energy sources when glucose
concentrations are low. To do so, the cAMP–CAP protein complex
serves as a positive regulator to induce transcription. One such sugar
source is lactose. The lac operon encodes the genes necessary to
acquire and process the lactose from the local environment, which
includes the structural genes lacZ, lacY, and lacA. lacZ encodes β-
galactosidase (LacZ), an intracellular enzyme that cleaves the
disaccharide lactose into glucose and galactose. lacY encodes β-
galactoside permease (LacY), a membrane-bound transport protein
that pumps lactose into the cell. lacA encodes β-galactoside
transacetylase (LacA), an enzyme that transfers an acetyl group from
acetyl-CoA to β-galactosides. Only lacZ and lacY appear to be
necessary for lactose catabolism.
CAP binds to the operator sequence upstream of the promoter that Figure 16.5.1: The lac Operon: Transcription of the lac operon is
initiates transcription of the lac operon. The lac operon uses a two- carefully regulated so that its expression only occurs when glucose
part control mechanism to ensure that the cell expends energy is limited and lactose is present to serve as an alternative fuel source.
producing β-galactosidase, β-galactoside permease, and KEY POINTS
thiogalactoside transacetylase (also known as galactoside O-
The lac operon contains an operator, promoter, and structural
acetyltransferase) only when necessary. However, for the lac operon
genes that are transcribed together and are under the control of
to be activated, two conditions must be met. First, the level of
the catabolite activator protein (CAP) or repressor.
glucose must be very low or non-existent. Second, lactose must be
The lac operon is not activated and transcription remains off
present. If glucose is absent, then CAP can bind to the operator
when the level of glucose is low or non-existent, but lactose is
sequence to activate transcription. If lactose is absent, then the
absent.
repressor binds to the operator to prevent transcription. If either of
The lac operon encodes for the genes needed to utilize lactose as
these requirements is met, then transcription remains off. The cell
an energy source.
can use lactose as an energy source by producing the enzyme b-
galactosidase to digest that lactose into glucose and galactose. Only KEY TERMS
when both conditions are satisfied is the lac operon transcribed, such
operator: a segment of DNA to which a transcription factor
as when glucose is absent and lactose is present. This process is protein binds
beneficial and makes most sense for the cell as it would be
repressor: any protein that binds to DNA and thus regulates the
energetically wasteful to create the proteins to process lactose if expression of genes by decreasing the rate of transcription
glucose were plentiful or if lactose were not available.
CONTRIBUTIONS AND ATTRIBUTIONS
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44535/latest...ol11448/latest. License: CC
BY: Attribution
operon. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/operon.
License: CC BY-SA: Attribution-ShareAlike
repressor. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/repressor. License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Prokaryotic Gene Regulation. October 16, 2013. Provided
by: OpenStax CNX. Located at:
http://cnx.org/content/m44535/latest...e_16_02_01.jpg. License: CC BY:
Attribution

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OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. BY: Attribution
Located at: http://cnx.org/content/m44535/latest...ol11448/latest. License: CC An Introduction to Molecular Biology/Gene Expression. Provided by:
BY: Attribution Wikibooks. Located at: en.wikibooks.org/wiki/An_Intr...ene_Expression.
Catabolite activator protein. Provided by: Wikipedia. Located at: License: CC BY-SA: Attribution-ShareAlike
en.Wikipedia.org/wiki/Catabol...ivator_protein. License: CC BY-SA: operator. Provided by: Wikipedia. Located at:
Attribution-ShareAlike en.Wikipedia.org/wiki/operator. License: CC BY-SA: Attribution-ShareAlike
RNA polymerase. Provided by: Wikipedia. Located at: OpenStax College, Prokaryotic Gene Regulation. October 16, 2013. Provided
en.Wikipedia.org/wiki/RNA%20polymerase. License: CC BY-SA: by: OpenStax CNX. Located at:
Attribution-ShareAlike http://cnx.org/content/m44535/latest...e_16_02_01.jpg. License: CC BY:
operon. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/operon. Attribution
License: CC BY-SA: Attribution-ShareAlike OpenStax College, Prokaryotic Gene Regulation. October 16, 2013. Provided
promoter. Provided by: Wiktionary. Located at: by: OpenStax CNX. Located at:
en.wiktionary.org/wiki/promoter. License: CC BY-SA: Attribution-ShareAlike http://cnx.org/content/m44535/latest...e_16_02_02.jpg. License: CC BY:
OpenStax College, Prokaryotic Gene Regulation. October 16, 2013. Provided Attribution
by: OpenStax CNX. Located at: OpenStax College, Prokaryotic Gene Regulation. October 16, 2013. Provided
http://cnx.org/content/m44535/latest...e_16_02_01.jpg. License: CC BY: by: OpenStax CNX. Located at:
Attribution http://cnx.org/content/m44535/latest...e_16_02_03.png. License: CC BY:
OpenStax College, Prokaryotic Gene Regulation. October 16, 2013. Provided Attribution
by: OpenStax CNX. Located at:
http://cnx.org/content/m44535/latest...e_16_02_02.jpg. License: CC BY: This page titled 16.5: Prokaryotic Gene Regulation - The lac Operon- An
Attribution
repressor. Provided by: Wiktionary. Located at: Inducer Operon is shared under a CC BY-SA 4.0 license and was authored,
en.wiktionary.org/wiki/repressor. License: CC BY-SA: Attribution-ShareAlike remixed, and/or curated by Boundless.
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44535/latest...ol11448/latest. License: CC

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16.6: EUKARYOTIC GENE REGULATION - THE PROMOTER AND THE
TRANSCRIPTION MACHINERY

 LEARNING OBJECTIVES

Describe the role of promoters in RNA transcription

Genes are organized to make the control of gene expression easier.

The promoter region is immediately upstream of the coding
sequence. This region can be short (only a few nucleotides in length)
or quite long (hundreds of nucleotides long). The longer the
promoter, the more available space for proteins to bind. This also
adds more control to the transcription process. The length of the
promoter is gene-specific and can differ dramatically between genes.
Consequently, the level of control of gene expression can also differ
quite dramatically between genes. The purpose of the promoter is to
bind transcription factors that control the initiation of transcription.
Within the promoter region, just upstream of the transcriptional start
site, resides the TATA box. This box is simply a repeat of thymine
and adenine dinucleotides (literally, TATA repeats). RNA
polymerase binds to the transcription initiation complex, allowing
transcription to occur. To initiate transcription, a transcription factor
(TFIID) is the first to bind to the TATA box. Binding of TFIID
recruits other transcription factors, including TFIIB, TFIIE, TFIIF,
and TFIIH to the TATA box. Once this transcription initiation
complex is assembled, RNA polymerase can bind to its upstream
sequence. When bound along with the transcription factors, RNA Figure 16.6.1: Promoters: A generalized promoter of a gene
polymerase is phosphorylated. This releases part of the protein from transcribed by RNA polymerase II is shown. Transcription factors
the DNA to activate the transcription initiation complex and places recognize the promoter. RNA polymerase II then binds and forms
the transcription initiation complex.
RNA polymerase in the correct orientation to begin transcription;
DNA-bending protein brings the enhancer, which can be quite a In addition to the general transcription factors, other transcription
distance from the gene, in contact with transcription factors and factors can bind to the promoter to regulate gene transcription. These
mediator proteins. transcription factors bind to the promoters of a specific set of genes.
They are not general transcription factors that bind to every
promoter complex, but are recruited to a specific sequence on the
promoter of a specific gene. There are hundreds of transcription
factors in a cell that each bind specifically to a particular DNA
sequence motif. When transcription factors bind to the promoter just
upstream of the encoded gene, they are referred to as cis-acting
elements because they are on the same chromosome, just next to the
gene. The region that a particular transcription factor binds to is
called the transcription factor binding site. Transcription factors
respond to environmental stimuli that cause the proteins to find their
binding sites and initiate transcription of the gene that is needed.

KEY POINTS
The purpose of the promoter is to bind transcription factors that
control the initiation of transcription.
The promoter region can be short or quite long; the longer the
promoter is, the more available space for proteins to bind.
To initiate transcription, a transcription factor (TFIID) binds to
the TATA box, which causes other transcription factors to
subsequently bind to the TATA box.
Once the transcription initiation complex is assembled, RNA
polymerase can bind to its upstream sequence and is then

16.6.1 https://bio.libretexts.org/@go/page/13359
phosphorylated. KEY TERMS
Phosphorylation of RNA polymerase releases part of the protein TATA box: a DNA sequence (cis-regulatory element) found in
from the DNA to activate the transcription initiation complex the promoter region of genes in archaea and eukaryotes
and places RNA polymerase in the correct orientation to begin transcription factor: a protein that binds to specific DNA
transcription. sequences, thereby controlling the flow (or transcription) of
Transcription factors respond to environmental stimuli that cause genetic information from DNA to mRNA
the proteins to find their binding sites and initiate transcription of promoter: the section of DNA that controls the initiation of
the gene that is needed. RNA transcription

This page titled 16.6: Eukaryotic Gene Regulation - The Promoter and the
Transcription Machinery is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

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16.7: EUKARYOTIC GENE REGULATION - TRANSCRIPTIONAL ENHANCERS
AND REPRESSORS
TURNING GENES OFF: TRANSCRIPTIONAL
 LEARNING OBJECTIVES REPRESSORS
Like prokaryotic cells, eukaryotic cells also have mechanisms to
Explain how enhancers and repressors regulate gene
prevent transcription. Transcriptional repressors can bind to
expression
promoter or enhancer regions and block transcription. Like the
transcriptional activators, repressors respond to external stimuli to
ENHANCERS AND TRANSCRIPTION
prevent the binding of activating transcription factors.
In some eukaryotic genes, there are regions that help increase or
A corepressor is a protein that decreases gene expression by binding
enhance transcription. These regions, called enhancers, are not
to a transcription factor that contains a DNA-binding domain. The
necessarily close to the genes they enhance. They can be located
corepressor is unable to bind DNA by itself. The corepressor can
upstream of a gene, within the coding region of the gene,
repress transcriptional initiation by recruiting histone deacetylase,
downstream of a gene, or may be thousands of nucleotides away.
which catalyzes the removal of acetyl groups from lysine residues.
Enhancer regions are binding sequences, or sites, for transcription This increases the positive charge on histones, which strengthens the
factors. When a DNA-bending protein binds to an enhancer, the interaction between the histones and DNA, making the DNA less
shape of the DNA changes. This shape change allows the interaction accessible to the process of transcription.
between the activators bound to the enhancers and the transcription
factors bound to the promoter region and the RNA polymerase to KEY POINTS
occur. Whereas DNA is generally depicted as a straight line in two Enhancers can be located upstream of a gene, within the coding
dimensions, it is actually a three-dimensional object. Therefore, a region of the gene, downstream of a gene, or thousands of
nucleotide sequence thousands of nucleotides away can fold over nucleotides away.
and interact with a specific promoter. When a DNA -bending protein binds to the enhancer, the shape
of the DNA changes, which allows interactions between the
activators and transcription factors to occur.
Repressors respond to external stimuli to prevent the binding of
activating transcription factors.
Corepressors can repress transcriptional initiation by recruiting
histone deacetylase.
Histone deactylation increases the positive charge on histones,
which strengthens the interaction between the histones and DNA,
making the DNA less accessible to transcription.

KEY TERMS
enhancer: a short region of DNA that can increase transcription
of genes
repressor: any protein that binds to DNA and thus regulates the
expression of genes by decreasing the rate of transcription
activator: any chemical or agent which regulates one or more
genes by increasing the rate of transcription

This page titled 16.7: Eukaryotic Gene Regulation - Transcriptional

Enhancers and Repressors is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.
Figure 16.7.1: Enhancers: An enhancer is a DNA sequence that
promotes transcription. Each enhancer is made up of short DNA
sequences called distal control elements. Activators bound to the
distal control elements interact with mediator proteins and
transcription factors.

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16.8: EUKARYOTIC GENE REGULATION - EPIGENETIC CONTROL-
REGULATING ACCESS TO GENES WITHIN THE CHROMOSOME

 LEARNING OBJECTIVES

Discuss how eukaryotic gene regulation occurs at the

epigenetic level and the various epigenetic changes that can
be made to DNA

EPIGENETIC CONTROL: REGULATING ACCESS

TO GENES WITHIN THE CHROMOSOME
The human genome encodes over 20,000 genes; each of the 23 pairs
of human chromosomes encodes thousands of genes. The DNA in
the nucleus is precisely wound, folded, and compacted into
chromosomes so that it will fit into the nucleus. It is also organized
so that specific segments can be accessed as needed by a specific
cell type.
The first level of organization, or packing, is the winding of DNA
strands around histone proteins. Histones package and order DNA Figure 16.8.1: Nucleosomes can change position to allow
into structural units called nucleosome complexes, which can control transcription of genes: Nucleosomes can slide along DNA. When
nucleosomes are spaced closely together (top), transcription factors
the access of proteins to the DNA regions. Under the electron cannot bind and gene expression is turned off. When the
microscope, this winding of DNA around histone proteins to form nucleosomes are spaced far apart (bottom), the DNA is exposed.
nucleosomes looks like small beads on a string. These beads (histone Transcription factors can bind, allowing gene expression to occur.
Modifications to the histones and DNA affect nucleosome spacing.
proteins) can move along the string (DNA) and change the structure
How the histone proteins move is dependent on signals found on
of the molecule.
both the histone proteins and on the DNA. These signals are tags, or
modifications, added to histone proteins and DNA that tell the
histones if a chromosomal region should be open or closed. These
tags are not permanent, but may be added or removed as needed.
They are chemical modifications (phosphate, methyl, or acetyl
groups) that are attached to specific amino acids in the protein or to
the nucleotides of the DNA. The tags do not alter the DNA base
sequence, but they do alter how tightly wound the DNA is around
the histone proteins. DNA is a negatively-charged molecule;
Figure 16.8.1: DNA Packaging: DNA is folded around histone therefore, changes in the charge of the histone will change how
proteins to create (a) nucleosome complexes. These nucleosomes tightly wound the DNA molecule will be. When unmodified, the
control the access of proteins to the underlying DNA. When viewed histone proteins have a large positive charge; by adding chemical
through an electron microscope (b), the nucleosomes look like beads
on a string. modifications, such as acetyl groups, the charge becomes less
positive.
If DNA encoding a specific gene is to be transcribed into RNA, the
nucleosomes surrounding that region of DNA can slide down the
DNA to open that specific chromosomal region and allow for the
transcriptional machinery ( RNA polymerase ) to initiate
transcription. Nucleosomes can move to open the chromosome
structure to expose a segment of DNA, but do so in a very controlled
manner.

16.8.1 https://bio.libretexts.org/@go/page/13362
 chromosomal region to allow access for RNA polymerase and other
proteins, called transcription factors, to bind to the promoter region,
located just upstream of the gene, and initiate transcription. If a gene
is to remain turned off, or silenced, the histone proteins and DNA
have different modifications that signal a closed chromosomal
configuration. In this closed configuration, the RNA polymerase and
transcription factors do not have access to the DNA and
transcription cannot occur.

KEY POINTS
DNA is packaged by wrapping around histone proteins into
structures called nucleosomes, which resemble beads on a string.
When DNA is to be transcribed, the nucleosomes can slide away
from that region of DNA, opening it up to the transcription
Figure 16.8.1: Modifications to histones and DNA can alter gene machinery of the cell.
expression: Histone proteins and DNA nucleotides can be modified Chemical modifications to either the histone proteins or the DNA
chemically. Modifications affect nucleosome spacing and gene
expression. itself signals whether or not a particular region of the genome
should be “open” or “closed” to the transcription machinery.
The DNA molecule itself can also be modified. This occurs within
Modifications such as acetylation or methylation of the histones
very specific regions called CpG islands. These are stretches with a
can alter how tightly DNA is wrapped around them, while
high frequency of cytosine and guanine dinucleotide DNA pairs
methylation of DNA changes how the DNA interacts with
(CG) found in the promoter regions of genes. When this
proteins, including the histone proteins that control access to the
configuration exists, the cytosine member of the pair can be
region.
methylated (a methyl group is added). This modification changes
This type of genetic regulation is called epigenetic regulation
how the DNA interacts with proteins, including the histone proteins
(“above genetics”) as it does not change the nucleotide sequence
that control access to the region. Highly-methylated
of the DNA.
(hypermethylated) DNA regions with deacetylated histones are
tightly coiled and transcriptionally inactive. These changes to DNA KEY TERMS
are inherited from parent to offspring, such that while the DNA
nucleosome: any of the subunits that repeat in chromatin; a coil
sequence is not altered, the pattern of gene expression is passed to
of DNA surrounding a histone core
the next generation.
epigenetics: the study of heritable changes caused by the
This type of gene regulation is called epigenetic regulation. activation and deactivation of genes without any change in DNA
Epigenetics means “above genetics.” The changes that occur to the sequence
histone proteins and DNA do not alter the nucleotide sequence and histone: any of various simple water-soluble proteins that are
are not permanent. Instead, these changes are temporary (although rich in the basic amino acids lysine and arginine and are
they often persist through multiple rounds of cell division) and alter complexed with DNA in the nucleosomes of eukaryotic
the chromosomal structure (open or closed) as needed. A gene can chromatin
be turned on or off depending upon the location and modifications to
the histone proteins and DNA. If a gene is to be transcribed, the This page titled 16.8: Eukaryotic Gene Regulation - Epigenetic Control-
histone proteins and DNA are modified surrounding the Regulating Access to Genes within the Chromosome is shared under a CC
chromosomal region encoding that gene. This opens the BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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16.9: EUKARYOTIC GENE REGULATION - RNA SPLICING

 LEARNING OBJECTIVES

Explain the role of RNA splicing in regulating gene

expression

RNA SPLICING, THE FIRST STAGE OF POST- Figure 16.9.1: Alternative Splicing: Pre-mRNA can be alternatively
TRANSCRIPTIONAL CONTROL spliced to create different proteins.
Gene expression is the process that transfers genetic information ALTERNATIVE SPLICING
from a gene made of DNA to a functional gene product made of
Alternative splicing is a process that occurs during gene expression
RNA or protein. Genetic Information flows from DNA to RNA by
and allows for the production of multiple proteins (protein isoforms)
the process of transcription and then from RNA to protein by the
from a single gene coding. Alternative splicing can occur due to the
process of translation. In order to ensure that the proper products are
different ways in which an exon can be excluded from or included in
produced, gene expression is regulated at many different stages
the messenger RNA. It can also occur if portions on an exon are
during and in between transcription and translation. In eukaryotes,
excluded/included or if there is an inclusion of introns. For example,
the gene contains extra sequences that do not code for protein. In
if a pre-mRNA has four exons (A, B, C, and D), these can be spliced
these organisms, transcription of DNA produces pre-mRNA. These
and translated in a number of different combinations. Exons A, B,
pre-mRNA transcripts often contain regions, called introns, that are
and C can be translated together or Exons A, C, and D can be
intervening sequences which must be removed prior to translation by
translated. This results in what is called alternative splicing. The
the process of splicing. The regions of RNA that code for protein are
pattern of splicing and production of alternatively-spliced messenger
called exons. Splicing can be regulated so that different mRNAs can
RNA is controlled by the binding of regulatory proteins (trans-acting
contain or lack exons, in a process called alternative splicing.
proteins that contain the genes) to cis-acting sites that are found on
Alternative splicing allows more than one protein to be produced
the pre-RNA. Some of these regulatory proteins include splicing
from a gene and is an important regulatory step in determining
activators (proteins that promote certain splicing sites) and splicing
which functional proteins are produced from gene expression. Thus,
repressors (proteins that reduce the use of certain sites). Some
splicing is the first stage of post-transcriptional control.
common splicing repressors include: heterogeneous nuclear
ribonucleoprotein (hnRNP) and polypyrimidine tract binding protein
(PTB). Proteins that are translated from alternatively-spliced
messenger RNAs differ in the sequence of their amino acids which
results in altered function of the protein. This is one reason why the
human genome can encode a wide diversity of proteins. Alternative
splicing is a common process that occurs in eukaryotes; most of the
multi-exonic genes in humans are spliced alternatively.
Unfortunately, abnormal variations in splicing are also the reason
why there are many genetic diseases and disorders.

Figure 16.9.1: Alternative Splicing: There are five basic modes of

alternative splicing.

Figure 16.9.1: Mechanism of Splicing: Alternative splicing can

result in protein isoforms.

16.9.1 https://bio.libretexts.org/@go/page/13363
SPLICEOSOME processing by a spliceosome.
The splicing of messenger RNA is accomplished and catalyzed by a Exons are expressing sequences within a pre-mRNA molecule
macro-molecule complex known as the spliceosome. The areas for that are spliced together once introns are removed to form mature
ligation and cleavage are determined by the many sub-units of the mRNA molecules that are translated into proteins.
spliceosome which include the branch site (A) and the 5′ and 3′ Alternative splicing allows for the production of various protein
splice sites. Interactions between these sub-units and the small isoforms from one single gene coding.
nuclear ribonucleoproteins (snRNP) found in the spliceosome create A spliceosome is a complex comprised of both RNA molecules
a spliceosome A complex which helps determine which introns to and proteins which determine which introns to leave out and
leave out and which exons to keep and bind together. Once the which exons to keep and bind together.
introns are cleaved and removed, the exons are joined together by a
KEY TERMS
phosphodiester bond.
intron: a portion of a split gene that is included in pre-RNA
REGULATORY PROTEINS transcripts but is removed during RNA processing and rapidly
As noted above, splicing is regulated by repressor proteins and degraded
activator proteins, which are are also known as trans-acting proteins. exon: a region of a transcribed gene present in the final
Equally as important are the silencers and enhancers that are found functional RNA molecule
on the messenger RNAs, also known as cis-acting sites. These spliceosome: a dynamic complex of RNA and protein subunits
regulatory functions work together in order to create splicing code that removes introns from precursor mRNA
that determines alternative splicing.
This page titled 16.9: Eukaryotic Gene Regulation - RNA Splicing is shared
KEY POINTS under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.
Introns are intervening sequences within a pre-mRNA molecule
that do not code for proteins and are removed during RNA

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16.10: EUKARYOTIC GENE REGULATION - THE INITIATION COMPLEX AND
TRANSLATION RATE
Once the 43S complex is at the initiation AUG, the tRNAi-Met is
 LEARNING OBJECTIVES positioned over the AUG. The anticodon on tRNAi-Met basepairs
with the AUG codon. At this point, the GTP bound to eIF2 in the
Discuss how eukaryotes assemble ribosomes on the mRNA
43S complexx is hydrolyzed to GDP + phosphate, and energy is
to begin translation
released. This energy is used to release the eIF2 (with GDP bound to
it) from the 43S complex, leaving the 40S ribosomal subunit and the
RIBOSOME ASSEMBLY AND TRANSLATION RATE tRNAi-Met at the translation start site of the mRNA.
Like transcription, translation is controlled by proteins that bind and Next, eIF5 with GTP bound binds to the 40S ribosomal subunit
initiate the process. In translation, before protein synthesis can complexed to the mRNA and the tRNAi-Met. The eIF5-GTP allows
begin, ribosome assembly has to be completed. This is a multi-step the 60S large ribosomal subunit to bind. Once the 60S ribosomal
process. subunit arrives, eIF5 hydrolyzes its bound GTP to GDP + phosphate,
In ribosome assembly, the large and small ribosomal subunits and an and energy is released. This energy powers assembly of the two
initiator tRNA (tRNAi) containing the first amino acid of the final ribosomal subunits into the intact 80S ribosome, with tRNAi-Met in
polypeptide chain all come together at the translation start codon on its P site while also basepaired to the initiation AUG codon on the
an mRNA to allow translation to begin. First, the small ribosomal mRNA. Translation is ready to begin.
subunit binds to the tRNAi which carries methionine in eukaryotes The binding of eIF-2 to the 40S ribosomal subunit is controlled by
and archaea and carries N-formyl-methionine in bacteria. (Because phosphorylation. If eIF-2 is phosphorylated, it undergoes a
the tRNAi is carrying an amino acid, it is said to be charged.) Next, conformational change and cannot bind to GTP. Therefore, the 43S
the small ribosomal subunit with the charged tRNAi still bound complex cannot form properly and translation is impeded. When
scans along the mRNA strand until it reaches the start codon AUG, eIF-2 remains unphosphorylated, it binds the 40S ribosomal subunit
which indicates where translation will begin. The start codon also and actively translates the protein.
establishes the reading frame for the mRNA strand, which is crucial
to synthesizing the correct sequence of amino acids. A shift in the
reading frame results in mistranslation of the mRNA. The anticodon
on the tRNAi then binds to the start codon via basepairing. The
complex consisting of mRNA, charged tRNAi, and the small
ribosomal subunit attaches to the large ribosomal subunit, which
completes ribosome assembly. These components are brought
together by the help of proteins called initiation factors which bind
to the small ribosomal subunit during initiation and are found in all
three domains of life. In addition, the cell spends GTP energy to
help form the initiation complex. Once ribosome assembly is
Figure 16.10.1: Translation Initiation Complex: Gene expression
complete, the charged tRNAi is positioned in the P site of the can be controlled by factors that bind the translation initiation
ribosome and the empty A site is ready for the next aminoacyl- complex.
tRNA. The polypeptide synthesis begins and always proceeds from The ability to fully assemble the ribosome directly affects the rate at
the N-terminus to the C-terminus, called the N-to-C direction. which translation occurs. But protein synthesis is regulated at
In eukaryotes, several eukaryotic initiation factor proteins (eIFs) various other levels as well, including mRNA synthesis, tRNA
assist in ribosome assembly. The eukaryotic initiation factor-2 (eIF- synthesis, rRNA synthesis, and eukaryotic initiation factor synthesis.
2) is active when it binds to guanosine triphosphate (GTP). With Alteration in any of these components affects the rate at which
GTP bound to it, eIF-2 protein binds to the small 40S ribosomal translation can occur.
subunit. Next, the initiatior tRNA charged with methionine (Met-
tRNAi) associates with the GTP-eIF-2/40S ribosome complex, and KEY POINTS
once all these components are bound to each other, they are The components involved in ribosome assembly are brought
collectively called the 43S complex. together by the help of proteins called initiation factors which
Eukaryotic initiation factors eIF1, eIF3, eIF4, and eIF5 help bring bind to the small ribosomal subunit.
Initiator tRNA is used to locate the start codon AUG (the amino
the 43S complex to the 5′-m7G cap of an mRNA be translated. Once
bound to the mRNA’s 5′ m7G cap, the 43S complex starts travelling acid methionine) which establishes the reading frame for the
mRNA strand.
down the mRNA until it reaches the initiation AUG codon at the
start of the mRNA’s reading frame. Sequences around the AUG may GTP carried by eIF2 is the energy source used for loading the
initiator tRNA carried by the small ribosomal subunit on the
help ensure the correct AUG is used as the initiation codon in the
mRNA. correct start codon in the mRNA.

16.10.1 https://bio.libretexts.org/@go/page/13364
 GTP carried by eIF5 is the energy source for assembling the phosphorylation: the addition of a phosphate group to a
large and small ribosomal subunits together. compound; often catalyzed by enzymes

KEY TERMS This page titled 16.10: Eukaryotic Gene Regulation - The Initiation
reading frame: either of three possible triplets of codons in Complex and Translation Rate is shared under a CC BY-SA 4.0 license and
which a DNA sequence could be transcribed was authored, remixed, and/or curated by Boundless.

16.10.2 https://bio.libretexts.org/@go/page/13364
16.11: EUKARYOTIC GENE REGULATION - REGULATING PROTEIN ACTIVITY
AND LONGEVITY
organelle that functions to remove proteins to be degraded. One way
 LEARNING OBJECTIVES to control gene expression is to alter the longevity of the protein:
ubiquitination shortens a protein’s lifespan.
Explain how chemical modifications affect protein activity
and longevity

CHEMICAL MODIFICATIONS, PROTEIN ACTIVITY,

AND LONGEVITY
Proteins can be chemically modified with the addition of methyl,
phosphate, acetyl, and ubiquitin groups. The addition or removal of
these groups from proteins regulates their activity or the length of
time they exist in the cell. Sometimes these modifications can
regulate where a protein is found in the cell; for example, in the
nucleus, the cytoplasm, or attached to the plasma membrane. Figure 16.11.1: Ubiquitin Tags: Proteins with ubiquitin tags are
marked for degradation within the proteasome.
Chemical modifications occur in response to external stimuli such as
stress, the lack of nutrients, heat, or ultraviolet light exposure. These KEY POINTS
changes can alter protein function, epigenetic accessibility, Proteins can be chemically modified by adding methyl,
transcription, mRNA stability, or translation; all resulting in changes phosphate, acetyl, and ubiquitin groups.
in expression of various genes. This is an efficient way for the cell to Protein longevity can be affected by altering stages of gene
rapidly change the abundance levels of specific proteins in response regulation, including but not limited to altering: accessibility to
to the environment. Because proteins are involved in every stage of chromosomal DNA for transcription, rate of translation, nuclear
gene regulation, the phosphorylation of a protein (depending on the shuttling, RNA stability, and post-translational modifications.
protein that is modified) can alter accessibility to the chromosome, Ubiquitin is added to a protein to mark it for degradation by the
can alter translation (by altering transcription factor binding or proteasome.
function), can change nuclear shuttling (by influencing
modifications to the nuclear pore complex), can alter RNA stability KEY TERMS
(by binding or not binding to the RNA to regulate its stability), can ubiquitin: a small polypeptide present in the cells of all
modify translation (increase or decrease), or can change post- eukaryotes; it plays a part in modifying and degrading proteins
translational modifications (add or remove phosphates or other proteasome: a complex protein, found in bacterial, archaeal and
chemical modifications). All of these protein activities are affected eukaryotic cells, that breaks down other proteins via proteolysis
by the phosphorylation process. The enzymes which are responsible
for phosphorylation are known as protein kinases. The addition of a CONTRIBUTIONS AND ATTRIBUTIONS
phosphate group to a protein can result in either activation or OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44538/latest...ol11448/latest. License: CC
deactivation; it is protein dependent. BY: Attribution
promoter. Provided by: Wiktionary. Located at:
Another example of chemical modifications affecting protein
en.wiktionary.org/wiki/promoter. License: CC BY-SA: Attribution-ShareAlike
activity include the addition or removal of methyl groups. Methyl TATA box. Provided by: Wiktionary. Located at:
groups are added to proteins via the process of methylation; this is en.wiktionary.org/wiki/TATA_box. License: CC BY-SA: Attribution-
ShareAlike
the most common form of post-translational modification. The transcription factor. Provided by: Wiktionary. Located at:
addition of methyl groups to a protein can result in protein-protein en.wiktionary.org/wiki/transcription_factor. License: CC BY-SA: Attribution-
ShareAlike
interactions that allows for transcriptional regulation, response to OpenStax College, Eukaryotic Transcription November 2, 2013. Provided by:
stress, protein repair, nuclear transport, and even differentiation OpenStax CNX. Located at: http://cnx.org/content/m44524/latest/. License:
CC BY: Attribution
processes. Methylation on side chain nitrogens is considered largely repressor. Provided by: Wiktionary. Located at:
irreversible while methylation of the carboxyl groups is potentially en.wiktionary.org/wiki/repressor. License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
reversible. Methylation in the proteins negates the negative charge Located at: http://cnx.org/content/m44538/latest...ol11448/latest. License: CC
on it and increases the hydrophobicity of the protein. Methylation on BY: Attribution
OpenStax College, Biology. October 22, 2013. Provided by: OpenStax CNX.
carboxylate side chains covers up a negative charge and adds
Located at: http://cnx.org/content/m44538/latest...ol11448/latest. License: CC
hydrophobicity. The addition of this chemical group changes the BY: Attribution
property of the protein and, thus, affects it activity. An Introduction to Molecular Biology/Gene Expression. Provided by:
Wikibooks. Located at: en.wikibooks.org/wiki/An_Intr...ion%23Enhancer.
The addition of an ubiquitin group to a protein marks that protein for License: CC BY-SA: Attribution-ShareAlike
enhancer. Provided by: Wiktionary. Located at:
degradation. Ubiquitin acts like a flag indicating that the protein en.wiktionary.org/wiki/enhancer. License: CC BY-SA: Attribution-ShareAlike
lifespan is complete. These proteins are moved to the proteasome, an activator. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/activator. License: CC BY-SA: Attribution-ShareAlike

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OpenStax College, Eukaryotic Transcription November 2, 2013. Provided by: OpenStax College, Eukaryotic Post-transcriptional Gene Regulation. October 16,
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2013. Provided by: OpenStax CNX. Located at: OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
http://cnx.org/content/m44538/latest/. License: CC BY: Attribution Located at: http://cnx.org/content/m44542/latest...ol11448/latest. License: CC
nucleosome. Provided by: Wiktionary. Located at: BY: Attribution
en.wiktionary.org/wiki/nucleosome. License: CC BY-SA: Attribution- Structural Biochemistry/Nucleic Acid/Translation. Provided by: Wikibooks.
ShareAlike Located at:
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Located at: http://cnx.org/content/m44536/latest...ol11448/latest. License: CC License: CC BY-SA: Attribution-ShareAlike
BY: Attribution phosphorylation. Provided by: Wiktionary. Located at:
histone. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/histone. en.wiktionary.org/wiki/phosphorylation. License: CC BY-SA: Attribution-
License: CC BY-SA: Attribution-ShareAlike ShareAlike
epigenetics. Provided by: Wiktionary. Located at: reading frame. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/epigenetics. License: CC BY-SA: Attribution- en.wiktionary.org/wiki/reading_frame. License: CC BY-SA: Attribution-
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OpenStax College, Eukaryotic Epigenetic Gene Regulation. October 16, 2013. OpenStax College, Eukaryotic Epigenetic Gene Regulation. October 16, 2013.
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OpenStax College, Eukaryotic Epigenetic Gene Regulation. October 16, 2013. OpenStax College, Eukaryotic Epigenetic Gene Regulation. October 16, 2013.
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OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. Splicing overview. Provided by: Wikimedia. Located at:
Located at: http://cnx.org/content/m44539/latest...ol11448/latest. License: CC commons.wikimedia.org/wiki/File:Splicing_overview.jpg. License: CC BY-
BY: Attribution SA: Attribution-ShareAlike
OpenStax College, Biology. October 22, 2013. Provided by: OpenStax CNX. OpenStax College, Eukaryotic Post-transcriptional Gene Regulation. October 16,
Located at: http://cnx.org/content/m44539/latest...ol11448/latest. License: CC 2013. Provided by: OpenStax CNX. Located at:
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Structural Biochemistry/Proteins/Signaling Control of Splicing Proteins. Attribution
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License: CC BY-SA: Attribution-ShareAlike Attribution
intron. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/intron. OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
License: CC BY-SA: Attribution-ShareAlike Located at: http://cnx.org/content/m44542/latest...ol11448/latest. License: CC
spliceosome. Provided by: Wiktionary. Located at: BY: Attribution
en.wiktionary.org/wiki/spliceosome. License: CC BY-SA: Attribution- Proteomics/Post-translational Modification/Amino Group Modification.
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2013. Provided by: OpenStax CNX. Located at: en.wikibooks.org/wiki/Structural_Biochemistry/Enzyme_Regulation/Phosph
http://cnx.org/content/m44538/latest/. License: CC BY: Attribution orylation. License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Eukaryotic Epigenetic Gene Regulation. October 16, 2013. proteasome. Provided by: Wiktionary. Located at:
Provided by: OpenStax CNX. Located at: en.wiktionary.org/wiki/proteasome. License: CC BY-SA: Attribution-
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2013. Provided by: OpenStax CNX. Located at: Provided by: OpenStax CNX. Located at:
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OpenStax College, Eukaryotic Epigenetic Gene Regulation. October 16, 2013. OpenStax College, Eukaryotic Translational and Post-translational Gene
Provided by: OpenStax CNX. Located at: Regulation. October 16, 2013. Provided by: OpenStax CNX. Located at:
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OpenStax College, Eukaryotic Post-transcriptional Gene Regulation. October 16, This page titled 16.11: Eukaryotic Gene Regulation - Regulating Protein
2013. Provided by: OpenStax CNX. Located at:
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Activity and Longevity is shared under a CC BY-SA 4.0 license and was
Attribution authored, remixed, and/or curated by Boundless.

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16.12: REGULATING GENE EXPRESSION IN CELL DEVELOPMENT - GENE
EXPRESSION IN STEM CELLS
extraction through apheresis (wherein blood is drawn from the
 LEARNING OBJECTIVES donor, passed through a machine that extracts the stem cells, and
returned to the donor). Stem cells can also be taken from umbilical
Discuss the types of cell division that can occur to add cells
cord blood just after birth. Of all the stem cell types, autologous
during development
harvesting involves the least risk. By definition, autologous cells are
obtained from one’s own body, just as one may bank his or her own
ADDING CELLS THROUGH CELLULAR DIVISION blood for elective surgical procedures. Highly plastic adult stem
Stem cells are undifferentiated biological cells found in multicellular cells are routinely used in medical therapies, for example in bone
organisms, that can differentiate into specialized cells (asymmetric marrow transplantation. Stem cells can now be artificially grown
division) or can divide to produce more stem cells (symmetric and differentiated into specialized cell types with characteristics
division). In mammals, there are two broad types of stem cells: consistent with muscle or nerve cells through cell culture.
embryonic stem cells, which are isolated from the inner cell mass of Embryonic cell lines and autologous embryonic stem cells generated
blastocysts, and adult stem cells, which are found in various tissues. through therapeutic cloning have also been proposed as promising
In adult organisms, stem cells and progenitor cells act as a repair candidates for future therapies.
system for the body by replenishing adult tissues. In a developing
embryo, stem cells can differentiate into all of the specialized cells SYMMETRIC AND ASYMMETRIC CELL DIVISION
(including ectoderm, endoderm and mesoderm cells) but also To ensure self-renewal, stem cells undergo two types of cell
maintain the normal turnover of regenerative organs, such as blood, division: symmetric and asymmetric. Symmetric division gives rise
skin, or intestinal tissues. The pathway that is taken to produced to two identical daughter cells both endowed with stem cell
specialized cells included: the embryonic cells develop from properties. Asymmetric division, on the other hand, produces only
totipotent cells, to pluripotent cells which undergo differentiation one stem cell and a progenitor cell with limited self-renewal
and become more specialized. The key component however, in the potential. Progenitors can go through several rounds of cell division
ability to maintain tissues is the ability to maintain a key of stem themselves before terminally differentiating into a mature cell.. It is
cells. possible that the molecular distinction between symmetric and
asymmetric division lies in differential segregation of cell membrane
proteins between the daughter cells. An alternative theory is that
stem cells remain undifferentiated due to environmental cues in their
particular niche. Stem cells differentiate when they leave that niche
or no longer receive those signals.

Figure 16.12.1: Stem Cells: Pluripotent, embryonic stem cells

originate as inner cell mass (ICM) cells within a blastocyst. These
stem cells can become any tissue in the body, excluding a placenta.
Only cells from an earlier stage of the embryo, known as the morula,
are totipotent, able to become all tissues in the body and the
extraembryonic placenta.
There are three accessible sources of autologous adult stem cells in
humans: (1) bone marrow, which requires extraction by harvesting
(i.e., drilling into bone); (2) adipose tissue (lipid cells), which
requires extraction by liposuction; and (3) blood, which requires

16.12.1 https://bio.libretexts.org/@go/page/13367
 Animals are made up of a vast number of distinct cell types. During
development, the zygote undergoes many cell divisions that give rise
1 to various cell types, including embryonic stem cells. Asymmetric
divisions of these embryonic cells gives rise to one cell of the same
potency (self-renewal), and another that may be of the same potency
or stimulated to further differentiate into specialized cell types such
2 as neurons. Asymmetric division of stem cells plays a key role in
development by allowing for the differentiation of a subset of
daughter cells while maintaining stem cell pluripotency. Since it can
be controlled by both intrinsic and extrinsic factors, upon delineating
3
these particular factors it may be possible to use this knowledge in
applications of tissue and whole organ generation.
4 4 KEY POINTS
Symmetric cell division of stem cells ensures that a constant pool
of stem cells is available by giving rise to two identical daughter
cells both endowed with stem cell properties.
A B C Asymmetric division of stem cells results in the production of
only one stem cell and a progenitor cell with limited self-renewal
Figure 16.12.1: Symmetric and Asymmetric Division: This diagram potential.
illustrates stem cell division and differentiation, through the
processes of (1) symmetric stem cell division, (2) asymmetric stem Progenitor cells that are produced via asymmetric cell division
cell division, (3) progenitor division, and (4) terminal differentiation. will go through additional rounds of cell division until they are
Stem cells are indicated by (A), progenitor cells by (B), and terminally differentiated into a mature, specialized cell.
differentiated cells by (C).
Asymmetric division can be controlled by both intrinsic and
An asymmetric cell division produces two daughter cells with extrinsic factors.
different cellular fates. This is in contrast to normal symmetric cell
Intrinsic factors involve differing amounts of cell-fate
divisions, which give rise to daughter cells of equivalent fates.
determinants being distributed into each daughter cell, while
Notably, stem cells divide asymmetrically to give rise to two distinct
extrinsic factors involve interactions with neighboring cells and
daughter cells: one copy of the original stem cell as well as a second
the micro and macro environment of the precursor cell.
daughter programmed to differentiate into a non-stem cell fate.
In principle, there are two mechanisms by which distinct properties KEY TERMS
may be conferred on the daughters of a dividing cell. In one, the totipotency: the ability of a cell to produce differentiated cells
daughter cells are initially equivalent but a difference is induced by upon division
signaling between the cells, from surrounding cells, or from the progenitor cell: a biological cell that, like a stem cell, has a
precursor cell. This mechanism is known as extrinsic asymmetric tendency to differentiate into a specific type of cell, but is
cell division. Extrinsic factors involve interactions with neighboring already more specific than a stem cell and is pushed to
cells and the micro and macro environment of the precursor cell. differentiate into its “target” cell.
In the second mechanism, the prospective daughter cells are autologous: derived from part of the same individual (i.e. from
inherently different at the time of division of the mother cell. the recipient rather than the donor)
Because this latter mechanism does not depend on interactions of morula: a spherical mass of blastomeres that forms following
cells with each other or with their environment, it must rely on the splitting of a zygote; it becomes the blastula
intrinsic asymmetry. The term asymmetric cell division usually pluripotent: able to develop into more than one mature cell or
refers to such intrinsic asymmetric divisions. Intrinsic factors tissue type, but not all
generally involve differing amounts of cell-fate determinants being
This page titled 16.12: Regulating Gene Expression in Cell Development -
distributed into each daughter cell.
Gene Expression in Stem Cells is shared under a CC BY-SA 4.0 license and
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16.13: REGULATING GENE EXPRESSION IN CELL DEVELOPMENT - CELLULAR
DIFFERENTIATION
outer layer of cells, and inside this hollow sphere, there is a cluster
 LEARNING OBJECTIVES of cells called the inner cell mass. The cells of the inner cell mass go
on to form virtually all of the tissues of the human body. Although
Discuss how differentiated cells can serve different
the cells of the inner cell mass can form virtually every type of cell
functions
found in the human body, they cannot form an organism. These cells
are referred to as pluripotent.
To develop a multicellular organisms, cells must differentiate to
Pluripotent stem cells undergo further specialization into multipotent
specialize for different functions. Three basic categories of cells
progenitor cells that then give rise to functional cells. Examples of
make up the mammalian body: germ cells, somatic cells, and stem
stem and progenitor cells include:
cells. Each of the approximately 100 trillion cells in an adult human
has its own copy or copies of the genome except certain cell types, 1. Hematopoietic stem cells (adult stem cells) from the bone
such as red blood cells, that lack nuclei in their fully differentiated marrow that give rise to red blood cells, white blood cells, and
state. Most cells are diploid; they have two copies of each platelets
chromosome. The process of cellular differentiation is regulated by 2. Mesenchymal stem cells (adult stem cells) from the bone marrow
transcription factors and growth factors, and results in expression or that give rise to stromal cells, fat cells, and types of bone cells;
inhibition of various genes between the cell types, thereby resulting 3. Epithelial stem cells (progenitor cells) that give rise to the
in varying proteomes between cell types. The variation in proteomes various types of skin cells
between cell types is what drives differentiation and thus, 4. Muscle satellite cells (progenitor cells) that contribute to
specialization of cells. The ability of transcription factors to control differentiated muscle tissue
whether a gene will be transcribed or not that contributes to A pathway that is guided by the cell adhesion molecules is created
specialization and growth factors to aid in the division process are as the cellular blastomere differentiates from the single-layered
key components of cell differentiation. blastula to the three primary layers of germ cells in mammals,
namely the ectoderm, mesoderm and endoderm (listed from most
distal, or exterior, to the most proximal, or interior). The ectoderm
ends up forming the skin and the nervous system, the mesoderm
forms the bones and muscular tissue, and the endoderm forms the
internal organ tissues.

KEY POINTS
The three major cell types in the mammalian body include germ
cells (which develop into gametes), somatic cells ( diploid cells
that develop into a majority of the human body) and stem cells
(cells that can divide indefinitely).
Figure 16.13.1: Cell Differentiation: Mechanics of cellular In human development, the inner cell mass exhibits the ability to
differentiation can be controlled by growth factors which can induce differentiate and form all tissues of the body; however, they
cell division. In asymetric cell division the cell will be induced to
differentiate into a specialized cell and the growth factors will work cannot form an organism.
in tandem. The various types of stem and progenitor cells included in the
Somatic cells are diploid cells that make up most of the human body, body that will differentiate to develop more specialized cells
such as the skin and muscle. Germ cells are any line of cells that includes: hematopoietic stem cells, mesenchymal stem cells,
give rise to gametes—eggs and sperm—and thus are continuous epithelial stem cells and muscle satellite cells.
through the generations. Stem cells, on the other hand, have the To develop a multicellular oragnisms, cells must differentiate to
ability to divide for indefinite periods and to give rise to specialized specialize for different functions.
cells. They are best described in the context of normal human
KEY TERMS
development.
blastocyst: the mammalian blastula formed during development
EMBRYONIC DEVELOPMENT where the inner cell mass can be found which forms the embryo
Development begins when a sperm fertilizes an egg and creates a inner cell mass: a mass of cells within a primordial embryo that
single cell that has the potential to form an entire organism. In the will eventually develop into the distinct form of a fetus in most
first hours after fertilization, this cell divides into identical cells. In eutherian mammals
humans, approximately four days after fertilization and after several proteome: the complete set of proteins encoded by a particular
cycles of cell division, these cells begin to specialize, forming a genome
hollow sphere of cells, called a blastocyst. The blastocyst has an

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pluripotent: able to develop into more than one mature cell or This page titled 16.13: Regulating Gene Expression in Cell Development -
tissue type, but not all Cellular Differentiation is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

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16.14: REGULATING GENE EXPRESSION IN CELL DEVELOPMENT -
MECHANICS OF CELLULAR DIFFERENTATION

 LEARNING OBJECTIVES

Summarize how a cell can differentiate into a specialized

cell

CELLULAR DIFFERENTIATION
How does a complex organism such as a human develop from a
single cell—a fertilized egg—into the vast array of cell types such as
nerve cells, muscle cells, and epithelial cells that characterize the
adult? Throughout development and adulthood, the process of
cellular differentiation leads cells to assume their final morphology
and physiology. Differentiation is the process by which
unspecialized cells become specialized to carry out distinct
functions.

STEM CELLS
A stem cell is an unspecialized cell that can divide without limit as Figure 16.14.1: Hematopoiesis: the differentiation of multipotent
needed and can, under specific conditions, differentiate into cells: The process of hematopoiesis involves the differentiation of
specialized cells. Stem cells are divided into several categories multipotent cells into blood and immune cells. The multipotent
hematopoietic stem cells give rise to many different cell types,
according to their potential to differentiate. The first embryonic cells including the cells of the immune system and red blood cells.
that arise from the division of the zygote are the ultimate stem cells; Finally, multipotent cells can become further specialized oligopotent
these stems cells are described as totipotent because they have the cells. An oligopotent stem cell is limited to becoming one of a few
potential to differentiate into any of the cells needed to enable an different cell types. In contrast, a unipotent cell is fully specialized
organism to grow and develop. The embryonic cells that develop and can only reproduce to generate more of its own specific cell
from totipotent stem cells and are precursors to the fundamental type. Stem cells are unique in that they can also continually divide
tissue layers of the embryo are classified as pluripotent. A and regenerate new stem cells instead of further specializing.
pluripotent stem cell is one that has the potential to differentiate into
There are different stem cells present at different stages of a human’s
any type of human tissue but cannot support the full development of
life, including the embryonic stem cells of the embryo, fetal stem
an organism. These cells then become slightly more specialized, and
cells of the fetus, and adult stem cells in the adult. One type of adult
are referred to as multipotent cells. A multipotent stem cell has the
stem cell is the epithelial stem cell, which gives rise to the
potential to differentiate into different types of cells within a given
keratinocytes (cells that produce keratin, the primary protein in nails
cell lineage or small number of lineages, such as a red blood cell or
and hair) in the multiple layers of epithelial cells in the epidermis of
white blood cell.
skin. Adult bone marrow has three distinct types of stem cells:
hematopoietic stem cells, which give rise to red blood cells, white
blood cells, and platelets; endothelial stem cells, which give rise to
the endothelial cell types that line blood and lymph vessels; and
mesenchymal stem cells, which give rise to the different types of
muscle cells.

DIFFERENTIATION
When a cell differentiates (i.e., becomes more specialized), it may
undertake major changes in its size, shape, metabolic activity, and
overall function. Because all cells in the body, beginning with the
fertilized egg, contain the same DNA, how do the different cell types
come to be so different? The answer is analogous to a movie script.
Different actors in a movie all read from the same script, but each
one only reads their own part of the script. Similarly, all cells
contain the same full complement of DNA, but each type of cell
only “reads” the portions of DNA that are relevant to its own
functioning. In other terms, each cell has the genome but will only
express specific genes, thereby having unique proteomes. In biology,

16.14.1 https://bio.libretexts.org/@go/page/13370
this is referred to as the unique genetic expression of each cell. In cell to the most restricted): totipotent, pluripotent, multipotent to
order for a cell to differentiate into its specialized form and function, oligopotent.
it need only manipulate those genes (and thus those proteins) that Totipotent cells have the potential to differentiate into any of the
will be expressed, and not those that will remain silent. cells needed to enable an organism to grow and develop;
pluripotent cells have the potential to differentiate into any type
MECHANISM of human tissue but cannot support the full development of an
The primary mechanism by which genes are turned “on” or “off” is organism.
through transcription factors. A transcription factor is one of a class A multipotent stem cell has the potential to differentiate into
of proteins that bind to specific genes on the DNA molecule and different types of cells within a given cell lineage or small
either promote or inhibit their transcription. The primary mechanism number of lineages, while an oligopotent stem cell is limited to
that determines which genes will be expressed and which ones will becoming one of a few different cell types.
not is through the use of different transcription factor proteins, The process of cellular differentiation is under strict regulation
which bind to DNA and promote or hinder the transcription of by transcription factors which can either activate or repress
different genes. Through the action of these transcription factors, expression of genes that will affect the proteome of the cell and
cells specialize into one of hundreds of different cell types in the thus, provide the necessary components it needs to become a
human body. specialized cell.
All cells contain the same complement of DNA, or genome, but
once differentiation occurs, it is the changes in the proteome that
will distinguish one cell type from another.

KEY TERMS
Figure 16.14.1: Transcription Factors Regulate Gene Expression: differentiate: to produce distinct cells, organs or to achieve
While each body cell contains the organism’s entire genome,
different cells regulate gene expression with the use of various specific functions by a process of development
transcription factors. Transcription factors are proteins that affect the proteome: the complete set of proteins encoded by a particular
binding of RNA polymerase to a particular gene on the DNA genome
molecule.
transcription: the synthesis of RNA under the direction of DNA
KEY POINTS
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16.15: REGULATING GENE EXPRESSION IN CELL DEVELOPMENT -
ESTABLISHING BODY AXES DURING DEVELOPMENT

 LEARNING OBJECTIVES

Summarize the mechanisms and cell types that establish the

body axes

VERTEBRATE AXIS FORMATION

During development, as the germ layers form, the ball of cells still Figure 16.15.1: Neural Tube: Transverse section of half of a chick
retains its spherical shape. However, animal bodies have lateral- embryo of forty-five hours’ incubation. The dorsal (back) surface of
medial (left-right), dorsal-ventral (back-belly), and anterior-posterior the embryo is toward the top of this page, while the ventral (front)
surface is toward the bottom. (Neural tube is in green. )
(head-feet) axes. How are these established? In one of the most
seminal experiments ever to be carried out in developmental PRIMARY AND SECONDARY NEURULATION
biology, Spemann and Mangold took dorsal cells from one embryo The neural tube develops in two ways: primary neurulation and
and transplanted them into the belly region of another embryo. They secondary neurulation. Primary neurulation divides the ectoderm
found that the transplanted embryo now had two notochords: one at into three cell types: the internally located neural tube, the externally
the dorsal site from the original cells and another at the transplanted located epidermis, and the neural crest cells, which develop in the
site. This suggested that the dorsal cells were genetically region between the neural tube and epidermis but then migrate to
programmed to form the notochord and define the axis. Since then, new locations. Primary neurulation begins after the neural plate
researchers have identified many genes that are responsible for axis forms. The edges of the neural plate start to thicken and lift upward,
formation. Mutations in these genes leads to the loss of symmetry forming the neural folds. The center of the neural plate remains
required for organism development. Animal bodies have externally grounded, allowing a U-shaped neural groove to form. This neural
visible symmetry. However, the internal organs are not symmetric. groove sets the boundary between the right and left sides of the
For example, the heart is on the left side and the liver on the right. embryo. The neural folds pinch in towards the midline of the
The formation of the central left-right axis is an important process embryo and fuse together to form the neural tube. In secondary
during development. This internal asymmetry is established very neurulation, the cells of the neural plate form a cord-like structure
early during development and involves many genes. Research is still that migrates inside the embryo and hollows to form the tube.
ongoing to fully understand the developmental implications of these
genes.

Figure 16.15.1: Vertebrate Axis Formation: Animal bodies have

three axes for symmetry:lateral-medial (left-right), dorsal-ventral
(back-belly), and anterior-posterior (head-feet).

NEURAL TUBE
In the developing chordate (including vertebrates), the neural tube is
the embryo’s precursor to the central nervous system, which
comprises the brain and spinal cord. The neural groove gradually
deepens as the neural folds become elevated, and ultimately the
folds meet and coalesce in the middle line and convert the groove
into a closed tube, the neural tube or neural canal, the ectodermal
wall of which forms the rudiment of the nervous system.

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 with sensation. The ventral part of the neural tube contains the basal
plate, which is primarily associated with motor (i.e., muscle) control.

SIGNALING MOLECULES AND OTHER FACTORS

The neural tube patterns along the dorsal-ventral axis establish
defined compartments of neural progenitor cells that lead to distinct
classes of neurons. This patterning occurs early in development and
results from the activity of several secreted signaling molecules.
Sonic hedgehog (Shh) is a key player in patterning the ventral axis,
while Bone morphogenic proteins (Bmp) and Wnt family members
play an important role in patterning the dorsal axis. Other factors
shown to provide positional information to the neural progenitor
cells include Fibroblast growth factors (FGF) and Retinoic Acid.
Retinoic acid is required ventrally along with Shh to induce Pax6
and Olig2 during differentiation of motor neurons. Three main
ventral cell types are established during early neural tube
development: the floor plate cells, which form at the ventral midline
during the neural fold stage; as well as the more dorsally located
motor neurons and interneurons. These cell types are specified by
the secretion of Shh from the notochord (located ventrally to the
neural tube), and later from the floor plate cells. Shh acts as a
morphogen, meaning that it acts in a concentration-dependent
manner to specify cell types as it moves further from its source. The
different combinations of expression of transcription factors along
the dorsal-ventral axis of the neural tube are responsible for creating
the identity of the neuronal progenitor cells.
Figure 16.15.1: Neural Tube Formation: The central region of the
ectoderm forms the neural tube, which gives rise to the brain and the KEY POINTS
spinal cord. The three axes of the animal body are established in development
Each organism uses primary and secondary neurulation to varying via the expression of specific sets of genes that regulate which
degrees. Neurulation in fish proceeds only via the secondary form. cells will develop into specific structures.
In avian species the posterior regions of the tube develop using During development, the dorsal cells are genetically programmed
secondary neurulation and the anterior regions develop by primary to develop into the notochord and define the axis.
neurulation. In mammals, secondary neurulation begins around the The neural tube can develop in two ways: primary or secondary
35th somite. Mammalian neural tubes close in the head in the neurulation, which are used by organisms in varying degrees to
opposite order that they close in the trunk. In the head, neural crest establish the neural tube that will develop into the central
cells migrate, the neural tube closes, and the overlying ectoderm nervous system (brain and spinal cord).
closes. In the trunk, overlying ectoderm closes, the neural tube Specific patterns along the neural tube that are established via
closes and neural crest cells migrate. secretion and production of specific signaling molecules (such as
Wnt, Shh, BMP and retinoic acid) play a key role in patterning
NEURAL TUBE SUBDIVISIONS the dorsal and ventral axes.
Four neural tube subdivisions eventually develop into distinct
regions of the central nervous system by the division of KEY TERMS
neuroepithelial cells: the prosencephalon, the mesencephalon, the neural tube: hollow longitudinal dorsal tube formed in the
rhombencephalon and the spinal cord. The prosencephalon further folding and subsequent fusion of the opposite ectodermal folds in
goes on to develop into the telencephalon (the forebrain or the embryo that gives rise to the brain and spinal cord
cerebrum) and the diencephalon (the optic vesicles and neurulation: the process by which the beginnings of the
hypothalamus). The mesencephalon develops into the midbrain. The vertebrate nervous system is formed in embryos
rhombencephalon develops into the metencephalon (the pons and anencephaly: a lethal birth defect in which most of the brain and
cerebellum) and the myelencephalon (the medulla oblongata). parts of the skull are missing; absence of the encephalon
For a short time, the neural tube is open both cranially and caudally. notochord: a flexible rodlike structure that forms the main
These openings, called neuropores, close during the fourth week in support of the body in the lowest chordates; a primitive spine
the human. Improper closure of the neuropores can result in neural
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tube defects such as anencephaly or spina bifida. The dorsal part of
Establishing Body Axes during Development is shared under a CC BY-SA
the neural tube contains the alar plate, which is primarily associated
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16.16: REGULATING GENE EXPRESSION IN CELL DEVELOPMENT - GENE
EXPRESSION FOR SPATIAL POSITIONING

 LEARNING OBJECTIVES

Describe the role genes play in development and ensuring

proper spatial positioning

GENES PROVIDE POSITIONAL INFORMATION

Gastrulation leads to the formation of the three germ layers that give
rise, during further development, to the different organs in the
animal body. This process, known as organogenesis, is characterized
by rapid and precise movements of the cells within the embryo.

ORGANOGENESIS
Organs form from the germ layers through the process of
differentiation. During differentiation, the embryonic stem cells
express specific sets of genes which will determine their ultimate
cell type. For example, some cells in the ectoderm (the outer tissue
layer of the embryo) will express the genes specific to skin cells. As
a result, these cells will differentiate into epidermal cells. The
process of differentiation is regulated by cellular signaling cascades. Figure 16.16.1: Somites: In this five-week old human embryo,
somites are segments along the length of the body.
Scientists study organogenesis extensively in the lab in fruit flies
(Drosophila) and the nematode Caenorhabditis elegans. Drosophila
have segments along their bodies, and the patterning associated with
the segment formation has allowed scientists to study which genes
play important roles in organogenesis along the length of the embryo
at different time points. The nematode C.elegans has roughly 1000
somatic cells and scientists have studied the fate of each of these
cells during their development in the nematode life cycle. There is
little variation in patterns of cell lineage between individuals, unlike
in mammals where cell development from the embryo is dependent
on cellular cues.
In vertebrates, one of the primary steps during organogenesis is the
formation of the neural system. The ectoderm forms epithelial cells
and tissues, as well as neuronal tissues. During the formation of the
neural system, special signaling molecules called growth factors
signal some cells at the edge of the ectoderm to become epidermis
cells. The remaining cells in the center form the neural plate. If the
signaling by growth factors were disrupted, then the entire ectoderm
would differentiate into neural tissue. The neural plate undergoes a
series of cell movements where it rolls up and forms a tube called
the neural tube. In further development, the neural tube will give rise
to the brain and the spinal cord. The mesoderm that lies on either
side of the vertebrate neural tube will develop into the various
connective tissues of the animal body. A spatial pattern of gene
expression reorganizes the mesoderm into groups of cells called
somites with spaces between them. The somites will further develop
into the ribs, lungs, and segmental (spine) muscle. The mesoderm
also forms a structure called the notochord, which is rod-shaped and
forms the central axis of the animal body.
Figure 16.16.1: Neural Tube Formation: The central region of the
ectoderm forms the neural tube, which gives rise to the brain and the
spinal cord.

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KEY POINTS KEY TERMS
Organogenesis results in the formation of the various organs in gastrulation: the stage of embryo development at which a
the body; however it will only occur if specific sets of genes are gastrula is formed from the blastula by the inward migration of
expressed to determine ultimate cell type. cells
The ability of specific cells to migrate to the the edge of the organogenesis: the formation and development of the organs of
ectoderm is highly regulated by specific gene expression and an organism from embryonic cells
allows for differentiation into epidermal cells; in contrast, the somite: one of the paired masses of mesoderm distributed along
cells which remain in the center will develop into the neural the sides of the neural tube that will eventually become dermis,
plate. skeletal muscle, or vertebrae
The expression of specific sets of genes will also regulate the
reorganization of the mesoderm into distinct groups of cells, This page titled 16.16: Regulating Gene Expression in Cell Development -
called somites, which develop into the ribs, lungs, spine muscle Gene Expression for Spatial Positioning is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.
and notochord.

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16.17: REGULATING GENE EXPRESSION IN CELL DEVELOPMENT - CELL
MIGRATION IN MULTICELLULAR ORGANISMS
Cell Migration: Phase images of BSC 1 cells migrating in a scratch
 LEARNING OBJECTIVES assay in the absence of serum over a period of 15 hours.

Describe how cells can migrate within an organism COMMON FEATURES OF CELL MIGRATION
The processes underlying mammalian cell migration are believed to
CELL MIGRATION be consistent with those of (non-spermatozoic) locomotion.
Cell migration is a central process in the development and Observations in common include cytoplasmic displacement at the
maintenance of multicellular organisms. Processes such as tissue leading front and laminar removal of dorsally-accumulated debris
formation during embryonic development, wound healing, and toward trailing end. The latter feature is most easily observed when
immune responses, all require the orchestrated movement of cells in aggregates of a surface molecule are cross-linked with a fluorescent
particular directions to specific locations. Errors during this process antibody or when small beads become artificially bound to the front
have serious consequences, including intellectual disability, vascular of the cell. Other eukaryotic cells are observed to migrate similarly.
disease, tumor formation and metastasis. An understanding of the The amoeba Dictyostelium discoideum is useful to researchers
mechanism by which cells migrate may lead to the development of because they consistently exhibit chemotaxis in response to cyclic
novel therapeutic strategies for controlling, for example, invasive AMP; they move more quickly than cultured mammalian cells; and
tumor cells. they have a haploid genome that simplifies the process of connecting
Cells often migrate in response to specific external signals, including a particular gene product with its effect on cellular behavior.
chemical signals and mechanical signals. Due to a highly viscous
environment, cells need to permanently produce forces in order to
KEY POINTS
move. Cells achieve active movement by very different mechanisms. The disruption or dysfunction of cell migration processes can
Many less complex prokaryotic organisms (and sperm cells) use lead to formation of various diseases such as metastasis, tumor
flagella or cilia to propel themselves. Eukaryotic cell migration formation and vascular disease.
typically is far more complex and can consist of combinations of In prokaryotic organisms, and some eukaryotic cells such as
different migration mechanisms. It generally involves drastic sperm cells, cell migration occurs via the use of a cilia or flagella
changes in cell shape which are driven by the cytoskeleton, for to propel forward.
instance a series of contractions and expansions due to cytoplasmic In eukaryotic organisms, cell migration is a much more complex
displacement. Two very distinct migration scenarios are crawling process and can include, but is not excluded to, changes in the
motion (most commonly studied) and blebbing motility. cytoskeleton, motor proteins, blebbing, and cytoplasmic
displacement; it involves both external and internal signals that
The migration of cultured cells attached to a surface is commonly
mediate these processes.
studied using microscopy. As cell movement is very slow (only a
few µm/minute), time-lapse microscopy videos are recorded of the KEY TERMS
migrating cells to speed up the movement. Such videos reveal that bleb: an irregular bulge in the plasma membrane of a cell
the leading cell front is very active with a characteristic behavior of chemotaxis: the movement of a cell or an organism in response
successive contractions and expansions. It is generally accepted that to a chemical stimulant
the leading front is the main motor that pulls the cell forward.
laminar: of fluid motion, smooth and regular, flowing as though
in different layers
metastasis: the transference of a bodily function or disease to
Cell migration another part of the body; specifically the development of a
secondary area of disease remote from the original site, as with
some cancers

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16.18: REGULATING GENE EXPRESSION IN CELL DEVELOPMENT -
PROGRAMMED CELL DEATH
and proliferating out of control, as happens with tumor cells that
 LEARNING OBJECTIVES metastasize.

Explain the importance of programmed cell death, including

apoptosis and autophagy

PROGRAMMED CELL DEATH

Programmed cell-death (or PCD) is death of a cell in any form,
mediated by an intracellular program. PCD is carried out in a
regulated process, which usually confers advantage during an
organism’s life-cycle. For example, the differentiation of fingers and
toes in a developing human embryo occurs because cells between
the fingers apoptose, or die; the result is that the digits are separate.
PCD serves fundamental functions during both plant and metazoa
(multicellular animal) tissue development. Apoptosis and autophagy
are both forms of PCD, but necrosis is a non-physiological process
that occurs as a result of infection or injury.

APOPTOSIS
Apoptosis is the process of PCD that may occur in multicellular
organisms. Biochemical events lead to characteristic cell changes (
morphology ) and death. These changes include blebbing, cell
shrinkage, nuclear fragmentation, chromatin condensation, and
chromosomal DNA fragmentation. There appears to be some
variation in the morphology and indeed the biochemistry of these
“suicide” pathways. Some tread the path of apoptosis, while others
follow a more generalized pathway to deletion; however both are
usually genetically and synthetically motivated. There is some
Figure 16.18.1: Programmed Cell Death: This histological section of
evidence that certain symptoms of apoptosis, such as endonuclease a foot of a 15-day-old mouse embryo, visualized using light
activation, can be spuriously induced without engaging a genetic microscopy, reveals areas of tissue between the toes, which
cascade; however, it is presumed that true apoptosis and PCD must apoptosis will eliminate before the mouse reaches its full gestational
age at 27 days.
be genetically mediated. It is also becoming clear that mitosis (the
division of the cell nucleus) and apoptosis are linked in some way, Another example of external signaling that leads to apoptosis occurs
and that the balance achieved depends on signals received from in T-cell development. T-cells are immune cells that bind to foreign
appropriate growth or survival factors. macromolecules and particles, and target them for destruction by the
immune system. Normally, T-cells do not target “self” proteins
When a cell is damaged, superfluous, or potentially dangerous to an
(those of their own organism), a process that can lead to
organism, a cell can initiate a mechanism to trigger apoptosis.
autoimmune diseases. In order to develop the ability to discriminate
Apoptosis allows a cell to die in a controlled manner that prevents
between self and non-self, immature T-cells undergo screening to
the release of potentially damaging molecules from inside the cell.
determine whether they bind to so-called self proteins. If the T-cell
There are many internal checkpoints that monitor a cell’s health; if
receptor binds to self proteins, the cell initiates apoptosis to remove
abnormalities are observed, a cell can spontaneously initiate the
the potentially dangerous cell.
process of apoptosis. However, in some cases, such as a viral
infection or uncontrolled cell division due to cancer, the cell’s Apoptosis is also essential for normal embryological development.
normal checks and balances fail. In vertebrates, for example, early stages of development include the
formation of web-like tissue between individual fingers and toes.
Apoptosis can also be initiated via external signaling. For example,
During the course of normal development, these unneeded cells
most normal animal cells have receptors that interact with the
must be eliminated, enabling fully separated fingers and toes to
extracellular matrix, a network of glycoproteins that provides
form. A cell signaling mechanism triggers apoptosis, which destroys
structural support for cells in an organism. The binding of cellular
the cells between the developing digits.
receptors to the extracellular matrix initiates a signaling cascade
within the cell. However, if the cell moves away from the
extracellular matrix, the signaling ceases, and the cell undergoes
apoptosis. This system keeps cells from traveling through the body

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AUTOPHAGY ShareAlike
Stem cell division and differentiation. Provided by: Wikipedia. Located at:
Macroautophagy, often referred to as autophagy, is a type of en.Wikipedia.org/wiki/File:St...rentiation.svg. License: Public Domain: No
Known Copyright
programmed cell death accomplished through self-digestion. It is a Stem cells diagram. Provided by: Wikipedia. Located at:
catabolic process that results in the autophagosomic-lysosomal en.Wikipedia.org/wiki/File:St...ls_diagram.png. License: CC BY-SA:
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NECROSIS proteome. Provided by: Wiktionary. Located at:
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KEY POINTS Attribution-ShareAlike
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16.19: CANCER AND GENE REGULATION - ALTERED GENE EXPRESSION IN
CANCER
activated in Burkett’s Lymphoma, a cancer of the lymph system.
 LEARNING OBJECTIVES Overexpression of myc transforms normal B cells into cancerous
cells that continue to grow uncontrollably. High B-cell numbers can
Describe how cancer is caused by uncontrolled cell growth
result in tumors that can interfere with normal bodily function.
Patients with Burkett’s lymphoma can develop tumors on their jaw
CANCER: DISEASE OF ALTERED GENE or in their mouth that interfere with the ability to eat.
EXPRESSION
Cancer can be described as a disease of altered gene expression.
There are many proteins that are turned on or off (gene activation or
gene silencing) that dramatically alter the overall activity of the cell.
A gene that is not normally expressed in that cell can be switched on
and expressed at high levels. This can be the result of gene mutation
or changes in gene regulation ( epigenetic, transcription, post-
transcription, translation, or post-translation).
Changes in epigenetic regulation, transcription, RNA stability,
protein translation, and post-translational control can be detected in
cancer. While these changes do not occur simultaneously in one
cancer, changes at each of these levels can be detected when
observing cancer at different sites in different individuals. Therefore,
changes in histone acetylation (epigenetic modification that leads to Figure 16.19.1: Proto-oncogenes Can Become Oncogenes: When
mutated, proto-oncogenes can become oncogenes and cause cancer
gene silencing), activation of transcription factors by due to uncontrolled cell growth.
phosphorylation, increased RNA stability, increased translational
control, and protein modification can all be detected at some point in KEY POINTS
various cancer cells. Scientists are working to understand the Cancer results from a gene that is not normally expressed in a
common changes that give rise to certain types of cancer or how a cell, but is switched on and expressed at high levels due to
modification might be exploited to destroy a tumor cell. mutations or alterations in gene regulation.
Alterations in histone acetylation, activation of transcription
TUMOR SUPPRESSOR GENES, ONCOGENES,
factors, increased RNA stability, increased translational control,
AND CANCER
and protein modification are all observed in cancer cells.
In normal cells, some genes function to prevent excess,
Tumor suppressor genes, active in normal cells, work to prevent
inappropriate cell growth. These are tumor suppressor genes, which
uncontrolled cell growth.
are active in normal cells to prevent uncontrolled cell growth. There
Proto- oncogenes, which are positive cell-cycle regulators, can
are many tumor suppressor genes in cells, but the one most studied
become oncogenes and cause cancer when mutated.
is p53, which is mutated in over 50 percent of all cancer types. The
p53 protein itself functions as a transcription factor. It can bind to KEY TERMS
sites in the promoters of genes to initiate transcription. Therefore,
oncogene: any gene that contributes to the conversion of a
the mutation of p53 in cancer will dramatically alter the normal cell into a cancerous cell when mutated or expressed at
transcriptional activity of its target genes.
high levels
Another type of gene often deregulated in cancers are proto- proto-oncogene: a gene that promotes the specialization and
oncogenes which are positive cell-cycle regulators. When mutated, division of normal cells that becomes an oncogene following
proto-oncogenes can become oncogenes and cause cancer. mutation
Overexpression of the oncogene can lead to uncontrolled cell growth cancer: a disease in which the cells of a tissue undergo
because oncogenes can alter transcriptional activity, stability, or uncontrolled (and often rapid) proliferation
protein translation of another gene that directly or indirectly controls
cell growth. An example of an oncogene involved in cancer is a This page titled 16.19: Cancer and Gene Regulation - Altered Gene
protein called myc. Myc is a transcription factor that is aberrantly Expression in Cancer is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

16.19.1 https://bio.libretexts.org/@go/page/13377
16.20: CANCER AND GENE REGULATION - EPIGENETIC ALTERATIONS IN
CANCER
that removes acetyl groups, or by DNA methyl transferase enzymes
 LEARNING OBJECTIVES that add methyl groups to cytosines in DNA) it is possible to design
new drugs and new therapies to take advantage of the reversible
Describe the role played by epigenetic alterations to gene
nature of these processes. Indeed, many researchers are testing how
expression in the development of cancer
a silenced gene can be switched back on in a cancer cell to help re-
establish normal growth patterns.
CANCER AND EPIGENETIC ALTERATIONS
Genes involved in the development of many other illnesses, ranging
Cancer epigenetics is the study of epigenetic modifications to the from allergies to inflammation to autism, are also thought to be
genome of cancer cells that do not involve a change in the regulated by epigenetic mechanisms. As our knowledge deepens of
nucleotide sequence. Epigenetic alterations are as important as how genes are controlled, new ways to treat these diseases and
genetic mutations in a cell’s transformation to cancer. Mechanisms cancer will emerge.
of epigenetic silencing of tumor suppressor genes and activation of
oncogenes include: alteration in CpG island methylation patterns, KEY POINTS
histone modifications, and dysregulation of DNA binding proteins. The DNA in the promoter region of silenced genes in cancer
cells is methylated on cytosine DNA residues in CpG islands.
normal tissue hyperplasia

basement membrane
neoplasia

invasion
Histone proteins that surround the promoter region of silenced
5mC
genes lack the acetylation modification that is present when the
CpG-island methylation
genes are expressed in normal cells.
When the combination of DNA methylation and histone
altered histone modification pattern

Figure 16.20.1: Epigenetic Alterations in Cancer Cells: In cancer

deacetylation occur within cancer cells, the gene present in that
cells, silencing genes through epigenetic mechanisms is a common
occurrence. Mechanisms can include modifications to histone chromosomal region is silenced.
proteins and DNA associated with these silencing genes. Epigenetic changes that are altered in cancer can be reversed and
Silencing genes through epigenetic mechanisms is very common in may, therefore, be helpful in new drug and therapy design.
cancer cells and include modifications to histone proteins and DNA
that are associated with silenced genes. In cancer cells, the DNA in KEY TERMS
the promoter region of silenced genes is methylated on cytosine epigenetic: the study of heritable changes in gene expression or
DNA residues in CpG islands, genomic regions that contain a high cellular phenotype caused by mechanisms other than changes in
frequency of CpG sites, where a cytosine nucleotide occurs next to a the underlying DNA sequence
guanine nucleotide. Histone proteins that surround that region lack methylation: the addition of a methyl group to cytosine and
the acetylation modification (the addition of an acetyl group) that is adenine residues in DNA that leads to the epigenetic
present when the genes are expressed in normal cells. This modification of DNA and the reduction of gene expression and
combination of DNA methylation and histone deacetylation protein production
(epigenetic modifications that lead to gene silencing) is commonly acetylation: the reaction of a substance with acetic acid or one of
found in cancer. When these modifications occur, the gene present in its derivatives; the introduction of one or more acetyl groups into
that chromosomal region is silenced. Increasingly, scientists are a substance
understanding how these epigenetic changes are altered in cancer.
Because these changes are temporary and can be reversed (for This page titled 16.20: Cancer and Gene Regulation - Epigenetic Alterations
example, by preventing the action of the histone deacetylase protein in Cancer is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.

16.20.1 https://bio.libretexts.org/@go/page/13378
16.21: CANCER AND GENE REGULATION - CANCER AND TRANSCRIPTIONAL
CONTROL
how a transcription factor binds, or a pathway that activates where a
 LEARNING OBJECTIVES gene can be turned off, has led to new drugs and new ways to treat
cancer. In breast cancer, for example, many proteins are
Explain the role of transcription factors in cancer
overexpressed. This can lead to increased phosphorylation of key
transcription factors that increase transcription. One such example is
CANCER AND TRANSCRIPTIONAL CONTROL the overexpression of the epidermal growth factor receptor (EGFR)
Many transcription factors, especially some that are proto-oncogenes in a subset of breast cancers. The EGFR pathway activates many
or tumor suppressors, help regulate the cell cycle and, as such, protein kinases that, in turn, activate many transcription factors that
determine how large a cell will get and when it can divide into two control genes involved in cell growth. New drugs that prevent the
daughter cells. Alterations in cells that give rise to cancer can affect activation of EGFR have been developed and are used to treat these
the transcriptional control of gene expression. Mutations that cancers.
activate transcription factors, such as increased phosphorylation, can
increase the binding of a transcription factor to its binding site in a KEY POINTS
promoter. This could lead to increased transcriptional activation of The mutations that activate transcription factors can increase the
that gene that results in modified cell growth. Alternatively, a binding of a transcription factor to its binding site in a promoter
mutation in the DNA of a promoter or enhancer region can increase leading to increased transcriptional activation of that gene and
the binding ability of a transcription factor. This could also lead to resulting in altered cell growth.
the increased transcription and aberrant gene expression that is seen A mutation in the DNA of a promoter or enhancer region may
in cancer cells. increase the binding ability of a transcription factor, which may
then lead to the increased transcription and anomalous gene
expression that is seen in cancer cells.
Studying how to control the transcriptional activation of gene
expression in cancer cells along with identifying how a
transcription factor binds or a pathway activates where a gene
can be turned off has led researchers to new drugs and novel
ways of treating cancer.

KEY TERMS
transcription factor: a protein that binds to specific DNA
sequences, thereby controlling the flow (or transcription) of
genetic information from DNA to mRNA
Figure 16.21.1: Transcription Factors: Transcription factors,
especially some that are proto-oncogenes or tumor suppressors, help This page titled 16.21: Cancer and Gene Regulation - Cancer and
regulate the cell cycle; however, when regulation gives rise to cancer
cells, then transcriptional control of gene expression is affected. Transcriptional Control is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.
Researchers have been investigating how to control the
transcriptional activation of gene expression in cancer. Identifying

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16.22: CANCER AND GENE REGULATION - CANCER AND POST-
TRANSCRIPTIONAL CONTROL

 LEARNING OBJECTIVES

Explain how post-transcriptional control can result in cancer

CANCER AND POST-TRANSCRIPTIONAL

CONTROL
Post-transcriptional regulation is the control of gene expression at
the RNA level; therefore, between the transcription and the
translation of the gene. After being produced, the stability and
distribution of the different transcripts is regulated (post-
transcriptional regulation) by means of RNA-binding proteins (RBP)
that control the various steps and rates of the transcripts: events such Figure 16.22.1: MicroRNA: Overexpression of miRNAs could be
detrimental to normal cellular activity because miRNAs bind to the
as alternative splicing, nuclear degradation (exosome), processing, 3′ UTR of RNA molecules to degrade them. Specific types of
nuclear export (three alternative pathways), sequestration in DCP2- miRNAs are only found in cancer cells.
bodies for storage or degradation, and, ultimately, translation.
KEY POINTS
Changes in the post-transcriptional control of a gene can result in
cancer. Recently, several groups of researchers have shown that Specific cancers have altered expression of miRNAs; changes in
specific cancers have altered expression of microRNAs (miRNAs). the miRNA population of particular cancers varies depending on
miRNAs bind to the 3′ UTR or 5′ UTR of RNA molecules to the type of cancer.
degrade them. Overexpression of these miRNAs could be Having too many miRNAs can dramatically decrease the RNA
detrimental to normal cellular activity. An increase in many population leading to a decrease in protein expression.
Studies have found that some miRNAs are specifically expressed
miRNAs could dramatically decrease the RNA population leading to
only in cancer cells.
a decrease in protein expression. Several studies have demonstrated
a change in the miRNA population in specific cancer types. It KEY TERMS
appears that the subset of miRNAs expressed in breast cancer cells is
microRNA: a single-stranded, non-coding form of RNA, having
quite different from the subset expressed in lung cancer cells or even
only about 20-30 nucleotides, that has a number of functions
from normal breast cells. This suggests that alterations in miRNA
including the regulation of gene expression
activity can contribute to the growth of breast cancer cells. These
exosome: a vesicle responsible for the selective removal of
types of studies also suggest that if some miRNAs are specifically
plasma membrane proteins
expressed only in cancer cells, they could be potential drug targets.
It would, therefore, be conceivable that new drugs that turn off This page titled 16.22: Cancer and Gene Regulation - Cancer and Post-
miRNA expression in cancer could be an effective method to treat Transcriptional Control is shared under a CC BY-SA 4.0 license and was
cancer. authored, remixed, and/or curated by Boundless.

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16.23: CANCER AND GENE REGULATION - CANCER AND TRANSLATIONAL
CONTROL

 LEARNING OBJECTIVES

Determine how translational or post-translational

modifications can lead to cancer

There are many examples of translational or post-translational

modifications of proteins that arise in cancer. Modifications are
found in cancer cells from the increased translation of a protein to
changes in protein phosphorylation to alternative splice variants of a
protein. An example of how the expression of an alternative form of
a protein can have dramatically different outcomes is seen in colon
cancer cells. The c-Flip protein, a protein involved in mediating the
cell death pathway, comes in two forms: long (c-FLIPL) and short
(c-FLIPS). Both forms appear to be involved in initiating controlled
cell death mechanisms in normal cells. However, in colon cancer
cells, expression of the long form results in increased cell growth Figure 16.23.1: Using Gene Expression in Targeted Therapy:
instead of cell death. Clearly, the expression of the wrong protein Scientists are using knowledge of the regulation of gene expression
in individual cancers to develop new ways to treat target diseased
dramatically alters cell function and contributes to the development cells and prevent the disease from occurring. Target therapies exploit
of cancer. the overexpression of a specific protein or gene mutation to develop
new medications against the specific cancer.
NEW DRUGS TO COMBAT CANCER: TARGETED
THERAPY KEY POINTS
Scientists are using what is known about the regulation of gene Protein modifications from the increased translation of a protein
expression in disease states, including cancer, to develop new ways to changes in protein phosphorylation to alternative splice
to treat and prevent disease development. Many scientists are variants of a protein are found in cancer cells.
designing drugs on the basis of the gene expression patterns within The expression of the wrong protein dramatically alters cell
individual tumors. This idea, that therapy and medicines can be function and contributes to the progression of cancer.
tailored to an individual, has given rise to the field of personalized Gene regulation and gene function provide scientists with the
medicine. With an increased understanding of gene regulation and opportunity to design medicines and therapies that specifically
gene function, medicines can be designed to specifically target target diseased cells or exploit the overexpression of specific
diseased cells without harming healthy cells. Some new medicines, proteins as cancer treatment.
called targeted therapies, have exploited the overexpression of a
KEY TERMS
specific protein or the mutation of a gene to develop a new
medication to treat disease. One such example is the use of anti-EGF targeted therapy: a type of medication that blocks the growth of
receptor medications to treat the subset of breast cancer tumors that cancer cells by interfering with specific targeted molecules rather
than by interfering with rapidly dividing cells
have very high levels of the EGF protein. Undoubtedly, more
targeted therapies will be developed as scientists learn more about cancer: a disease in which the cells of a tissue undergo
uncontrolled (and often rapid) proliferation
how gene expression changes can cause cancer.
post-translational modification: the chemical modification of a
protein after its translation; one of the later steps in protein
biosynthesis, and thus gene expression, for many proteins

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 CHAPTER OVERVIEW

17: BIOTECHNOLOGY AND GENOMICS

17.1: Biotechnology
17.1A: Biotechnology
17.1B: Basic Techniques to Manipulate Genetic Material (DNA and RNA)
17.1C: Molecular and Cellular Cloning
17.1D: Reproductive Cloning
17.1E: Genetic Engineering
17.1F: Genetically Modified Organisms (GMOs)
17.1G: Biotechnology in Medicine
17.1H: Production of Vaccines, Antibiotics, and Hormones
17.2: Mapping Genomes
17.2A: Genetic Maps
17.2B: Physical Maps and Integration with Genetic Maps
17.3: Whole-Genome Sequencing
17.3A: Strategies Used in Sequencing Projects
17.3B: Use of Whole-Genome Sequences of Model Organisms
17.3C: Uses of Genome Sequences
17.4: Applying Genomics
17.4A: Predicting Disease Risk at the Individual Level
17.4B: Pharmacogenomics, Toxicogenomics, and Metagenomics
17.4C: Genomics and Biofuels
17.5: Genomics and Proteomics
17.5A: Genomics and Proteomics
17.5B: Basic Techniques in Protein Analysis
17.5C: Cancer Proteomics

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1
SECTION OVERVIEW

17.1: BIOTECHNOLOGY
17.1E: GENETIC ENGINEERING
Topic hierarchy
17.1F: GENETICALLY MODIFIED ORGANISMS
(GMOS)
17.1A: BIOTECHNOLOGY
17.1G: BIOTECHNOLOGY IN MEDICINE
17.1B: BASIC TECHNIQUES TO MANIPULATE
GENETIC MATERIAL (DNA AND RNA) 17.1H: PRODUCTION OF VACCINES,
ANTIBIOTICS, AND HORMONES
17.1C: MOLECULAR AND CELLULAR CLONING

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17.1A: BIOTECHNOLOGY
Relying on the study of DNA, genomics analyzes entire genomes, technology has led to Google maps that enable people to get detailed
while biotechnology uses biological agents for technological information about locations around the globe, genomic information
advancements. is used to create similar maps of the DNA of different organisms.
These findings have helped anthropologists to better understand
 LEARNING OBJECTIVES human migration and have aided the field of medicine through the
mapping of human genetic diseases. The ways in which genomic
Justify an overview of the field of biotechnology information can contribute to scientific understanding are varied and
quickly growing.
KEY POINTS
Genomics includes the study of a complete set of genes, their
nucleotide sequence and organization, and their interactions
within a species and with other species.
Through DNA sequencing, genomic information is used to create
maps of the DNA of different organisms.
Biotechnology, or the use of biological agents for technological
progression, has applications in medicine, agriculture, and in
industry, which include processes such as fermentation and the
production of biofuels. Figure 17.1A. 1 : Genomics: In genomics, the DNA of different
organisms is compared, enabling scientists to create maps with
KEY TERMS which to navigate the DNA of different organisms.

genomics: the study of the complete genome of an organism Another rapidly-advancing field that utilizes DNA is biotechnology.
sequencing: the procedure of determining the order of amino This field involves the use of biological agents for technological
acids in the polypeptide chain of a protein (protein sequencing) advancement. Biotechnology was used for breeding livestock and
or of nucleotides in a DNA section comprising a gene (gene crops long before the scientific basis of these techniques was
sequencing) understood. Since the discovery of the structure of DNA in 1953, the
biotechnology: the use of living organisms (especially field of biotechnology has grown rapidly through both academic
microorganisms) in industrial, agricultural, medical, and other research and private companies. The primary applications of this
technological applications technology are in medicine (production of vaccines and antibiotics)
and agriculture (genetic modification of crops, such as to increase
The study of nucleic acids began with the discovery of DNA, yields). Biotechnology also has many industrial applications, such as
progressed to the study of genes and small fragments, and has now fermentation, the treatment of oil spills, and the production of
exploded to the field of genomics. Genomics is the study of entire biofuels.
genomes, including the complete set of genes, their nucleotide
sequence and organization, and their interactions within a species This page titled 17.1A: Biotechnology is shared under a CC BY-SA 4.0
and with other species. The advances in genomics have been made license and was authored, remixed, and/or curated by Boundless.
possible by DNA sequencing technology. Just as information

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17.1B: BASIC TECHNIQUES TO MANIPULATE GENETIC MATERIAL (DNA AND
RNA)
Basic techniques used in genetic material manipulation include various techniques. Most nucleic acid extraction techniques involve
extraction, gel electrophoresis, PCR, and blotting methods. steps to break open the cell and use enzymatic reactions to destroy
all macromolecules that are not desired (such as degradation of
 LEARNING OBJECTIVES unwanted molecules and separation from the DNA sample). Cells
are broken using a lysis buffer (a solution that is mostly a detergent);
Distinguish among the basic techniques used to manipulate lysis means “to split.” These enzymes break apart lipid molecules in
DNA and RNA the membranes of the cell and the nucleus. Macromolecules are
inactivated using enzymes such as proteases that break down
KEY POINTS proteins, and ribonucleases (RNAses) that break down RNA. The
The first step to study or work with nucleic acids includes the DNA is then precipitated using alcohol. Human genomic DNA is
isolation or extraction of DNA or RNA from cells. usually visible as a gelatinous, white mass. Samples can be stored at
Gel electrophoresis depends on the negatively-charged ions –80°C for years.
present on nucleic acids at neutral or basic pH to separate
molecules on the basis of size.
Specific regions of DNA can be amplified through the use of
polymerase chain reaction for further analysis.
Southern blotting involves the transfer of DNA to a nylon
membrane, while northern blotting is the transfer of RNA to a
nylon membrane; these techniques allow samples to be probed
for the presence of certain sequences.

KEY TERMS
denaturation: the change of folding structure of a protein (and
thus of physical properties) caused by heating, changes in pH, or
exposure to certain chemicals
electrophoresis: a method for the separation and analysis of
large molecules, such as proteins or nucleic acids, by migrating a Figure 17.1B. 1: DNA Extraction: This diagram shows the basic
colloidal solution of them through a gel under the influence of an method used for extraction of DNA.
electric field RNA analysis is performed to study gene expression patterns in
polymerase chain reaction: a technique in molecular biology cells. RNA is naturally very unstable because RNAses are
for creating multiple copies of DNA from a sample commonly present in nature and very difficult to inactivate. Similar
to DNA, RNA extraction involves the use of various buffers and
BASIC TECHNIQUES TO MANIPULATE GENETIC enzymes to inactivate macromolecules and preserve the RNA.
MATERIAL (DNA AND RNA)
To understand the basic techniques used to work with nucleic acids,
GEL ELECTROPHORESIS
remember that nucleic acids are macromolecules made of Because nucleic acids are negatively-charged ions at neutral or basic
nucleotides (a sugar, a phosphate, and a nitrogenous base) linked by pH in an aqueous environment, they can be mobilized by an electric
phosphodiester bonds. The phosphate groups on these molecules field. Gel electrophoresis is a technique used to separate molecules
each have a net negative charge. An entire set of DNA molecules in on the basis of size using this charge and may be separated as whole
the nucleus is called the genome. DNA has two complementary chromosomes or fragments. The nucleic acids are loaded into a slot
strands linked by hydrogen bonds between the paired bases. The two near the negative electrode of a porous gel matrix and pulled toward
strands can be separated by exposure to high temperatures (DNA the positive electrode at the opposite end of the gel. Smaller
denaturation) and can be reannealed by cooling. The DNA can be molecules move through the pores in the gel faster than larger
replicated by the DNA polymerase enzyme. Unlike DNA, which is molecules; this difference in the rate of migration separates the
located in the nucleus of eukaryotic cells, RNA molecules leave the fragments on the basis of size. There are molecular-weight standard
nucleus. The most common type of RNA that is analyzed is the samples that can be run alongside the molecules to provide a size
messenger RNA (mRNA) because it represents the protein -coding comparison. Nucleic acids in a gel matrix can be observed using
genes that are actively expressed. various fluorescent or colored dyes. Distinct nucleic acid fragments
appear as bands at specific distances from the top of the gel (the
DNA AND RNA EXTRACTION negative electrode end) on the basis of their size.
To study or manipulate nucleic acids, the DNA or RNA must first be
isolated or extracted from the cells. This can be done through

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 Figure 17.1B. 1: PCR Amplification: Polymerase chain reaction, or
PCR, is used to amplify a specific sequence of DNA. Primers—short
pieces of DNA complementary to each end of the target sequence—
are combined with genomic DNA, Taq polymerase, and
deoxynucleotides. Taq polymerase is a DNA polymerase isolated
from the thermostable bacterium Thermus aquaticus that is able to
Figure 17.1B. 1: Gel Electrophoresis: Shown are DNA fragments withstand the high temperatures used in PCR. Thermus aquaticus
from seven samples run on a gel, stained with a fluorescent dye, and grows in the Lower Geyser Basin of Yellowstone National Park.
viewed under UV light. Reverse transcriptase PCR (RT-PCR) is similar to PCR, but cDNA is
made from an RNA template before PCR begins.
AMPLIFICATION OF NUCLEIC ACID FRAGMENTS DNA fragments can also be amplified from an RNA template in a
BY POLYMERASE CHAIN REACTION process called reverse transcriptase PCR (RT-PCR). The first step is
Polymerase chain reaction (PCR) is a technique used to amplify to recreate the original DNA template strand (called cDNA) by
specific regions of DNA for further analysis. PCR is used for many applying DNA nucleotides to the mRNA. This process is called
purposes in laboratories, such as the cloning of gene fragments to reverse transcription. This requires the presence of an enzyme called
analyze genetic diseases, identification of contaminant foreign DNA reverse transcriptase. After the cDNA is made, regular PCR can be
in a sample, and the amplification of DNA for sequencing. More used to amplify it.
practical applications include the determination of paternity and
detection of genetic diseases. HYBRIDIZATION, SOUTHERN BLOTTING, AND
NORTHERN BLOTTING
Nucleic acid samples, such as fragmented genomic DNA and RNA
extracts, can be probed for the presence of certain sequences. Short
DNA fragments called probes are designed and labeled with
radioactive or fluorescent dyes to aid detection. Gel electrophoresis
separates the nucleic acid fragments according to their size. The
fragments in the gel are then transferred onto a nylon membrane in a
procedure called blotting. The nucleic acid fragments that are bound
to the surface of the membrane can then be probed with specific
radioactively- or fluorescently-labeled probe sequences. When DNA
is transferred to a nylon membrane, the technique is called Southern
blotting; when RNA is transferred to a nylon membrane, it is called
northern blotting. Southern blots are used to detect the presence of
certain DNA sequences in a given genome, and northern blots are
used to detect gene expression.

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 This page titled 17.1B: Basic Techniques to Manipulate Genetic Material
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Figure 17.1B. 1: Blotting Techniques: Southern blotting is used to

find a particular sequence in a sample of DNA. DNA fragments are
separated on a gel, transferred to a nylon membrane, and incubated
with a DNA probe complementary to the sequence of interest.
Northern blotting is similar to Southern blotting, but RNA is run on
the gel instead of DNA. In western blotting, proteins are run on a gel
and detected using antibodies.

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17.1C: MOLECULAR AND CELLULAR CLONING
Molecular cloning reproduces the desired regions or fragments of a unaffected by antibiotics). Plasmids have been repurposed and
genome, enabling the manipulation and study of genes. engineered as vectors for molecular cloning and the large-scale
production of important reagents such as insulin and human growth
 LEARNING OBJECTIVES hormone. An important feature of plasmid vectors is the ease with
which a foreign DNA fragment can be introduced via the multiple
Describe the process of molecular cloning cloning site (MCS). The MCS is a short DNA sequence containing
multiple sites that can be cut with different commonly-available
KEY POINTS restriction endonucleases. Restriction endonucleases recognize
Cloning small fragments of a genome allows specific genes, their specific DNA sequences and cut them in a predictable manner; they
protein products, and non-coding regions to be studied in are naturally produced by bacteria as a defense mechanism against
isolation. foreign DNA. Many restriction endonucleases make staggered cuts
A plasmid, also known as a vector, is a small circular DNA in the two strands of DNA, such that the cut ends have a 2- or 4-base
molecule that replicates independently of the chromosomal single-stranded overhang. Because these overhangs are capable of
DNA; it can be used to provide a “folder” in which to insert a annealing with complementary overhangs, these are called “sticky
desired DNA fragment. ends.” Addition of an enzyme called DNA ligase permanently joins
Recombinant DNA molecules are plasmids with foreign DNA the DNA fragments via phosphodiester bonds. In this way, any DNA
inserted into them; they are created artificially as they do not fragment generated by restriction endonuclease cleavage can be
occur in nature. spliced between the two ends of a plasmid DNA that has been cut
Bacteria and yeast naturally produce clones of themselves when with the same restriction endonuclease.
they replicate asexually through cellular cloning.

KEY TERMS
recombinant DNA: DNA that has been engineered by splicing
together fragments of DNA from multiple species and introduced
into the cells of a host
molecular cloning: a biological method that creates many
identical DNA molecules and directs their replication within a
host organism
plasmid: a circle of double-stranded DNA that is separate from
the chromosomes, which is found in bacteria and protozoa

MOLECULAR CLONING
In general, the word “cloning” means the creation of a perfect
replica; however, in biology, the re-creation of a whole organism is
referred to as “reproductive cloning.” Long before attempts were
made to clone an entire organism, researchers learned how to
reproduce desired regions or fragments of the genome, a process that
is referred to as molecular cloning.
Cloning small fragments of the genome allows for the manipulation Figure 17.1C. 1 : Molecular Cloning: This diagram shows the steps
and study of specific genes (and their protein products) or noncoding involved in molecular cloning, where regions or fragments of a
genome are reproduced to allow the study or manipulation of genes
regions in isolation. A plasmid (also called a vector) is a small and their protein products.
circular DNA molecule that replicates independently of the
chromosomal DNA. In cloning, the plasmid molecules can be used RECOMBINANT DNA MOLECULES
to provide a “folder” in which to insert a desired DNA fragment. Plasmids with foreign DNA inserted into them are called
Plasmids are usually introduced into a bacterial host for recombinant DNA molecules because they are created artificially
proliferation. In the bacterial context, the fragment of DNA from the and do not occur in nature. They are also called chimeric molecules
human genome (or the genome of another organism that is being because the origin of different parts of the molecules can be traced
studied) is referred to as foreign DNA (or a transgene) to back to different species of biological organisms or even to chemical
differentiate it from the DNA of the bacterium, which is called the synthesis. Proteins that are expressed from recombinant DNA
host DNA. molecules are called recombinant proteins. Not all recombinant
Plasmids occur naturally in bacterial populations (such as plasmids are capable of expressing genes. The recombinant DNA
Escherichia coli) and have genes that can contribute favorable traits may need to be moved into a different vector (or host) that is better
to the organism such as antibiotic resistance (the ability to be designed for gene expression. Plasmids may also be engineered to

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express proteins only when stimulated by certain environmental fission; this is known as cellular cloning. The nuclear DNA
factors so that scientists can control the expression of the duplicates by the process of mitosis, which creates an exact replica
recombinant proteins. of the genetic material.

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17.1D: REPRODUCTIVE CLONING
Reproductive cloning, possible through artificially-induced asexual contains the genetic information to produce a new individual.
reproduction, is a method used to make a clone of an entire However, early embryonic development requires the cytoplasmic
organism. material contained in the egg cell. This idea forms the basis for
reproductive cloning. If the haploid nucleus of an egg cell is
 LEARNING OBJECTIVES replaced with a diploid nucleus from the cell of any individual of the
same species (called a donor), it will become a zygote that is
Differentiate reproductive cloning from cellular and genetically identical to the donor. Somatic cell nuclear transfer is the
molecular cloning technique of transferring a diploid nucleus into an enucleated egg. It
can be used for either therapeutic cloning or reproductive cloning.
KEY POINTS The first cloned animal was Dolly, a sheep who was born in 1996.
A form of asexual reproduction, parthenogenesis, occurs when The success rate of reproductive cloning at the time was very low.
an embryo grows and develops without the fertilization of the Dolly lived for seven years and died of respiratory complications.
egg. There is speculation that because the cell DNA belongs to an older
In reproductive cloning, if the haploid nucleus of an egg cell is individual, the age of the DNA may affect the life expectancy of a
replaced with a diploid nucleus from the cell of an individual of cloned individual. Since Dolly, several animals (e.g. horses, bulls,
the same species, it will become a zygote that is genetically and goats) have been successfully cloned, although these individuals
identical to the donor. often exhibit facial, limb, and cardiac abnormalities. There have
Reproductive cloning has become successful, but still has been attempts at producing cloned human embryos as sources of
limitations as cloned individuals often exhibit facial, limb, and embryonic stem cells. Sometimes referred to as cloning for
cardiac abnormalities. therapeutic purposes, the technique produces stem cells that attempt
Therapeutic cloning, the cloning of human embryos as a source to remedy detrimental diseases or defects (unlike reproductive
of embryonic stem cells, has been attempted in order to produce cloning, which aims to reproduce an organism). Still, therapeutic
cells that can be used to treat detrimental diseases or defects. cloning efforts have met with resistance because of bioethical
considerations.
KEY TERMS
clone: a living organism produced asexually from a single
ancestor, to which it is genetically identical
stem cell: a primal undifferentiated cell from which a variety of
other cells can develop through the process of cellular
differentiation
parthenogenesis: a form of asexual reproduction where growth
and development of embryos occur without fertilization

REPRODUCTIVE CLONING
Reproductive cloning is a method used to make a clone or an
identical copy of an entire multicellular organism. Most
multicellular organisms undergo reproduction by sexual means,
which involves genetic hybridization of two individuals (parents),
making it impossible to generate an identical copy or clone of either
parent. Recent advances in biotechnology have made it possible to
artificially induce asexual reproduction of mammals in the
laboratory.
Parthenogenesis, or “virgin birth,” occurs when an embryo grows
and develops without the fertilization of the egg occurring; this is a
form of asexual reproduction. An example of parthenogenesis occurs
in species in which the female lays an egg. If the egg is fertilized, it
is a diploid egg and the individual develops into a female; if the egg
is not fertilized, it remains a haploid egg and develops into a male. Figure 17.1D. 1 : Reproductive Cloning of Dolly, the Sheep: Dolly
The unfertilized egg is called a parthenogenic, or virgin, egg. Some the sheep was the first mammal to be cloned. To create Dolly, the
nucleus was removed from a donor egg cell. The nucleus from a
insects and reptiles lay parthenogenic eggs that can develop into
second sheep was then introduced into the cell, which was allowed
adults. to divide to the blastocyst stage before being implanted in a
Sexual reproduction requires two cells; when the haploid egg and surrogate mother.
sperm cells fuse, a diploid zygote results. The zygote nucleus

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17.1E: GENETIC ENGINEERING
In genetic engineering, an organism’s genotype is altered using is called transgenic. Bacteria, plants, and animals have been
recombinant DNA, created by molecular cloning, to modify an genetically modified since the early 1970s for academic, medical,
organism’s DNA. agricultural, and industrial purposes. In the US, GMOs such as
Roundup-ready soybeans and borer-resistant corn are part of many
 LEARNING OBJECTIVES common processed foods.

Discuss how genetic engineering leads to DNA

modification.

KEY POINTS
A genetically modified organism receives recombinant DNA
generated through molecular cloning.
Transgenic host organisms receive their foreign DNA from a
different species.
The use of recombinant DNA vectors to alter the expression of a
particular gene is known as gene targeting, which is done
through the addition of mutations in a gene or the exclusion of
the expression of a certain gene.
Recombinant DNA technology involves transferring a DNA
fragment of interest from one organism to another by inserting it Figure 17.1E. 1: GMO Corn: Borer-resistant corn is an example of a
into a vector. genetically- modified organism made possible through genetic
engineering methods that allow scientists to alter an organism’s
DNA to achieve specific traits, such as herbicide resistance.
KEY TERMS
recombinant DNA: DNA that has been engineered by splicing GENE TARGETING
together fragments of DNA from multiple species and introduced Although classical methods of studying the function of genes began
into the cells of a host with a given phenotype and determined the genetic basis of that
genetic engineering: the deliberate modification of the genetic phenotype, modern techniques allow researchers to start at the DNA
structure of an organism sequence level and ask: “What does this gene or DNA element do? ”
genetically modified organism: an organism whose genetic This technique, called reverse genetics, has resulted in reversing the
material has been altered using genetic engineering techniques classic genetic methodology. This method would be similar to
damaging a body part to determine its function. An insect that loses
GENETIC ENGINEERING
a wing cannot fly, which means that the function of the wing is
Genetic engineering is the alteration of an organism’s genotype flight. The classical genetic method would compare insects that
using recombinant DNA technology to modify an organism’s DNA cannot fly with insects that can fly, and observe that the non-flying
to achieve desirable traits. Recombinant DNA technology, or DNA insects have lost wings. Similarly, mutating or deleting genes
cloning, is the process of transferring a DNA fragment of interest provides researchers with clues about gene function. The methods
from one organism to a self-replicating genetic element, such as a used to disable gene function are collectively called gene targeting.
bacteria plasmid, which is called a vector. The DNA of interest can Gene targeting is the use of recombinant DNA vectors to alter the
then be propagated in another organism. The addition of foreign expression of a particular gene, either by introducing mutations in a
DNA in the form of recombinant DNA vectors generated by gene, or by eliminating the expression of a certain gene by deleting a
molecular cloning is the most common method of genetic part or all of the gene sequence from the genome of an organism.
engineering. The organism that receives the recombinant DNA is
called a genetically-modified organism (GMO). If the foreign DNA This page titled 17.1E: Genetic Engineering is shared under a CC BY-SA
that is introduced comes from a different species, the host organism 4.0 license and was authored, remixed, and/or curated by Boundless.

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17.1F: GENETICALLY MODIFIED ORGANISMS (GMOS)
Transgenic modification, adding recombinant DNA to a species, has
led to the expression of desirable genes in plants and animals.

 LEARNING OBJECTIVES

Describe how research on transgenic plants and animals aids

humans.

KEY POINTS
Transgenic animals are those that have been modified to express
recombinant DNA from another species.
Manipulation of transgenic plants, those that have received
recombinant DNA from other species, has led to the creation of
species that display disease resistance, herbicide and pesticide
resistance, better nutritional value, and better shelf-life.
The thickness of a plant’s cell wall makes the artificial
introduction of DNA into plant cells much more challenging than
in animal cells. Figure 17.1F . 1 : Transgenic Plants: Corn, a major agricultural crop
used to create products for a variety of industries, is often modified
through plant biotechnology.
KEY TERMS
transgenic: of or pertaining to an organism whose genome has TRANSFORMATION OF PLANTS USING
been changed by the addition of a gene from another species; AGROBACTERIUM TUMEFACIENS
genetically modified Gene transfer occurs naturally between species in microbial
genetically modified organism: an organism whose genetic populations. Many viruses that cause human diseases, such as
material has been altered using genetic engineering techniques cancer, act by incorporating their DNA into the human genome. In
plants, tumors caused by the bacterium Agrobacterium tumefaciens
TRANSGENIC ANIMALS occur by transfer of DNA from the bacterium to the plant. Although
Although several recombinant proteins used in medicine are the tumors do not kill the plants, they stunt the plants, which become
successfully produced in bacteria, some proteins require a eukaryotic more susceptible to harsh environmental conditions. Many plants,
animal host for proper processing. For this reason, the desired genes such as walnuts, grapes, nut trees, and beets, are affected by A.
are cloned and expressed in animals, such as sheep, goats, chickens, tumefaciens. The artificial introduction of DNA into plant cells is
and mice. Animals that have been modified to express recombinant more challenging than in animal cells because of the thick plant cell
DNA are called transgenic animals. Several human proteins are wall.
expressed in the milk of transgenic sheep and goats, while others are
Researchers used the natural transfer of DNA from Agrobacterium
expressed in the eggs of chickens. Mice have been used extensively
to a plant host to introduce DNA fragments of their choice into plant
for expressing and studying the effects of recombinant genes and
hosts. In nature, the disease-causing A. tumefaciens have a set of
mutations.
plasmids, called the Ti plasmids (tumor-inducing plasmids), that
TRANSGENIC PLANTS contain genes for the production of tumors in plants. DNA from the
Ti plasmid integrates into the infected plant cell’s genome.
Manipulating the DNA of plants (or creating genetically modified
Researchers manipulate the Ti plasmids to remove the tumor-
organisms called GMOs) has helped to create desirable traits, such
causing genes and insert the desired DNA fragment for transfer into
as disease resistance, herbicide and pesticide resistance, better
the plant genome. The Ti plasmids carry antibiotic resistance genes
nutritional value, and better shelf-life. Plants are the most important
to aid selection and can be propagated in E. coli cells as well.
source of food for the human population. Farmers developed ways to
select for plant varieties with desirable traits long before modern-day THE ORGANIC INSECTICIDE BACILLUS
biotechnology practices were established. Plants that have received THURINGIENSIS
recombinant DNA from other species are called transgenic plants.
Bacillus thuringiensis (Bt) is a bacterium that produces protein
Because foreign genes can spread to other species in the
crystals during sporulation that are toxic to many insect species that
environment, extensive testing is required to ensure ecological
affect plants. Bt toxin has to be ingested by insects for the toxin to
stability. Staples like corn, potatoes, and tomatoes were the first crop
be activated. Insects that have eaten Bt toxin stop feeding on the
plants to be genetically engineered.
plants within a few hours. After the toxin is activated in the
intestines of the insects, death occurs within a couple of days.
Modern biotechnology has allowed plants to encode their own

17.1F.1 https://bio.libretexts.org/@go/page/13342
crystal Bt toxin that acts against insects. The crystal toxin genes
have been cloned from Bt and introduced into plants. Bt toxin has
been found to be safe for the environment, non-toxic to humans and
other mammals, and is approved for use by organic farmers as a
natural insecticide.

FLAVR SAVR TOMATO

The first GM crop to be introduced into the market was the Flavr
Savr Tomato, produced in 1994. Antisense RNA technology was
used to slow down the process of softening and rotting caused by
fungal infections, which led to increased shelf life of the GM
tomatoes. Additional genetic modification improved the flavor of
this tomato. The Flavr Savr tomato did not successfully stay in the
market because of problems maintaining and shipping the crop.

Figure 17.1F . 1 : The Flavr Savr Tomato: Plant physiologist

Athanasios Theologis with tomatoes that contain the bioengineered
ACC synthase gene (the Flavr Savr Tomato).

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under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

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17.1G: BIOTECHNOLOGY IN MEDICINE
From manipulation of mutant genes to enhanced resistance to will maximize the value of the medicine and decrease the likelihood
disease, biotechnology has allowed advances in medicine. of overdose.
3. Improvements in the drug discovery and approval process. The
 LEARNING OBJECTIVES discovery of potential therapies will be made easier using genome
targets. Genes have been associated with numerous diseases and
Give examples of how biotechnology is used in medicine.
disorders. With modern biotechnology, these genes can be used as
targets for the development of effective new therapies, which could
KEY POINTS significantly shorten the drug discovery process.
The study of pharmacogenomics can result in the development of 4. Better vaccines. Safer vaccines can be designed and produced by
tailor-made vaccines for people, more accurate means of organisms transformed by means of genetic engineering. These
determining drug dosages, improvements in drug discovery and vaccines will elicit the immune response without the attendant risks
approval, and the development of safer vaccines. of infection. They will be inexpensive, stable, easy to store, and
Modern biotechnology can be used to manufacture drugs more capable of being engineered to carry several strains of pathogen at
easily and cheaply, as they can be produced in larger quantities once.
from existing genetic sources.
Modern biotechnology can be used to manufacture existing drugs
Genetic diagnosis involves the process of testing for suspected
more easily and cheaply. The first genetically-engineered products
genetic defects before administering treatment through genetic
were medicines designed to combat human diseases. In 1978,
testing.
Genentech joined a gene for insulin with a plasmid vector and put
In gene therapy, a good gene is introduced at a random location
the resulting gene into a bacterium called Escherichia coli. Insulin,
in the genome to aid the cure of a disease that is caused by a
widely used for the treatment of diabetes, was previously extracted
mutated gene.
from sheep and pigs. It was very expensive and often elicited
KEY TERMS unwanted allergic responses. The resulting genetically-engineered
gene therapy: any of several therapies involving the insertion of bacterium enabled the production of vast quantities of human insulin
genes into a patient’s cells in order to replace defective ones at low cost. Since then, modern biotechnology has made it possible
pharmacogenomics: the study of genes that code for enzymes to produce more easily and cheaply the human growth hormone,
that metabolize drugs, and the design of tailor-made drugs clotting factors for hemophiliacs, fertility drugs, erythropoietin, and
other drugs. Genomic knowledge of the genes involved in diseases,
adapted to an individual’s genetic make-up
disease pathways, and drug-response sites are expected to lead to the
immunodeficiency: a depletion in the body’s natural immune
discovery of thousands more new targets.
system, or in some component of it

BIOTECHNOLOGY IN MEDICINE GENETIC DIAGNOSIS AND GENE THERAPY

The process of testing for suspected genetic defects before
It is easy to see how biotechnology can be used for medicinal
administering treatment is called genetic diagnosis by genetic
purposes. Knowledge of the genetic makeup of our species, the
testing. Depending on the inheritance patterns of a disease-causing
genetic basis of heritable diseases, and the invention of technology
gene, family members are advised to undergo genetic testing.
to manipulate and fix mutant genes provides methods to treat the
Treatment plans are based on the findings of genetic tests that
disease.
determine the type of cancer. If the cancer is caused by inherited
Pharmacogenomics is the study of how the genetic inheritance of an gene mutations, other female relatives are also advised to undergo
individual affects his/her body’s response to drugs. It is a coined genetic testing and periodic screening for breast cancer. Genetic
word derived from the words “pharmacology” and ” genomics “. It testing is also offered for fetuses to determine the presence or
is, therefore, the study of the relationship between pharmaceuticals absence of disease-causing genes in families with specific,
and genetics. The vision of pharmacogenomics is to be able to debilitating diseases.
design and produce drugs that are adapted to each person’s genetic
Genetic testing involves the direct examination of the DNA
makeup. Pharmacogenomics results in the following benefits:
molecule itself. A scientist scans a patient’s DNA sample for
1. Development of tailor-made medicines. Using
mutated sequences. There are two major types of gene tests. In the
pharmacogenomics, pharmaceutical companies can create drugs first type, a researcher may design short pieces of DNA whose
based on the proteins, enzymes, and RNA molecules that are
sequences are complementary to the mutated sequences. These
associated with specific genes and diseases. These tailor-made drugs probes will seek their complement among the base pairs of an
promise not only to maximize therapeutic effects, but also to
individual’s genome. If the mutated sequence is present in the
decrease damage to nearby healthy cells. patient’s genome, the probe will bind to it and flag the mutation. In
2. More accurate methods of determining appropriate drug dosages. the second type, a researcher may conduct the gene test by
Knowing a patient’s genetics will enable doctors to determine how
well the patient’s body can process and metabolize a medicine. This

17.1G.1 https://bio.libretexts.org/@go/page/13343
comparing the sequence of DNA bases in a patient’s gene to a
normal version of the gene.
Gene therapy is a genetic engineering technique used to cure
disease. In its simplest form, it involves the introduction of a good
gene at a random location in the genome to aid the cure of a disease
that is caused by a mutated gene. The good gene is usually
introduced into diseased cells as part of a vector transmitted by a
virus that can infect the host cell and deliver the foreign DNA. More
advanced forms of gene therapy try to correct the mutation at the
original site in the genome, such as is the case with treatment of
severe combined immunodeficiency (SCID).

Figure 17.1G. 1: Gene Therapy: Gene therapy using an adenovirus

vector can be used to cure certain genetic diseases in which a person
has a defective gene.

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17.1H: PRODUCTION OF VACCINES, ANTIBIOTICS, AND HORMONES
Biotechnological advances in gene manipulation techniques have
further resulted in the production of vaccines, antibiotics, and
hormones.

 LEARNING OBJECTIVES

Discuss the methods by which biotechnology is used to

produce vaccines, antibiotics, and hormones.

KEY POINTS
Vaccines use weakened or inactive forms of microorganisms to
mount the initial immune response through the use of antigens,
which are produced through use the genes of microbes that are
cloned into vectors.
Antibiotics, agents that inhibit bacterial growth or kill bacteria,
are produced by cultivating and manipulating fungal cells.
Figure 17.1H. 1 : Antibiotic Treatment: Assays such as the one
Hormones, such as the human growth hormone (HGH), can be shown help scientists understand the effects of antibiotics on
formulated through recombinant DNA technology; for example, bacterial species. Clear rings around the round inserts, which contain
HGH can be cloned from a cDNA library and inserted into E. antibiotic, mean that bacteria on the plate are inhibited or killed by
the compound.
coli cells by cloning it into a bacterial vector.

KEY TERMS HORMONES

bactericidal: that which kills bacteria
Recombinant DNA technology was used to produce large-scale
bacteriostatic: that which slows down or stalls bacterial growth
quantities of human insulin (a hormone) in E. coli as early as 1978.
antigen: a substance that binds to a specific antibody; may cause
Previously, it was only possible to treat diabetes with pig insulin,
an immune response
which caused allergic reactions in humans because of differences in
PRODUCTION OF VACCINES, ANTIBIOTICS, AND the gene product. In recent times, human growth hormone (HGH)
HORMONES has been used to treat growth disorders in children. The HGH gene
was cloned from a cDNA library and inserted into E. coli cells by
VACCINES cloning it into a bacterial vector. The bacteria was then grown and
Traditional vaccination strategies use weakened or inactive forms of the hormone isolated, enabling large scale commercial production.
microorganisms to mount the initial immune response. Modern
techniques use the genes of microorganisms cloned into vectors to
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GMO corn label RoundUp Liberty Link Herculex I Cruiser Mid Rate. Provided Tomatoes ARS. Provided by: Wikipedia. Located at:
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SECTION OVERVIEW

17.2: MAPPING GENOMES

17.2B: PHYSICAL MAPS AND INTEGRATION WITH
Topic hierarchy GENETIC MAPS

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license and was authored, remixed, and/or curated by Boundless.

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17.2A: GENETIC MAPS
Genetic maps provide information about which chromosomes Observations that certain traits were always linked and certain others
contain specific genes and precisely where the genes lie on that were not linked came from studying the offspring of crosses
chromosome. between parents with different traits. For example, in experiments
performed on the garden pea, it was discovered that the color of the
 LEARNING OBJECTIVES flower and shape of the plant’s pollen were linked traits; therefore,
the genes encoding these traits were in close proximity on the same
Describe the different types of genetic markers that are used chromosome. The exchange of DNA between homologous pairs of
in generating genetic maps of DNA chromosomes is called genetic recombination, which occurs by the
crossing over of DNA between homologous strands of DNA, such as
KEY POINTS nonsister chromatids. Linkage analysis involves studying the
Genetic mapping, often called linkage mapping, provides recombination frequency between any two genes. The greater the
information about the location of a specific gene along a distance between two genes, the higher the chance that a
chromosome. recombination event will occur between them, and the higher the
Gene linkage describes the phenomenon that certain genes are recombination frequency between them. If the recombination
physically linked by being located on the same chromosome and frequency between two genes is less than 50 percent, they are said to
have a tendency to be inherited together. be linked.
Genetic recombination involves the production of a novel set of
genetic information by breaking and rejoining DNA fragments
that have a great distance between them along the chromosome.
The construction of genetic maps is reliant on the natural process
of recombination which results in the ability to identify genetic
markers with variability within a population.
Genetic markers that can be used in generating genetic maps
include restriction length polymorphisms ( RFLP ); variable
number of tandem repeats (VNTRs); microsatellite
polymorphisms; and single nucleotide polymorphisms ( SNPs ).

KEY TERMS
polymorphism: the regular existence of two or more different
genotypes within a given species or population
SNP: single nucleotide polymorphism is single base pair of DNA
Figure 17.2A. 1 : Crossovers and Recombination: Crossover may
which is polymorphic with respect to a population occur at different locations on the chromosome. Recombination
microsatellite: any of a group of polymorphic loci in DNA that between genes A and B is more frequent than recombination
consist of repeat units of just a few base pairs between genes B and C because genes A and B are farther apart; a
crossover is, therefore, more likely to occur between them.
RFLP: restriction fragment length polymorphism is a section of
The generation of genetic maps requires markers, just as a road map
DNA whose length varies among individuals and which is
requires landmarks (such as rivers and mountains). Early genetic
delimited by a base which does not occur within it
maps were based on the use of known genes as markers. More
GENETIC MAPS sophisticated markers, including those based on non-coding DNA,
The study of genetic maps begins with linkage analysis, a procedure are now used to compare the genomes of individuals in a population.
that analyzes the recombination frequency between genes to Although individuals of a given species are genetically similar, they
are not identical; every individual has a unique set of traits. These
determine if they are linked or show independent assortment. The
term linkage was used before the discovery of DNA. Early minor differences in the genome between individuals in a population
are useful for the purposes of genetic mapping. In general, a good
geneticists relied on the observation of phenotypic changes to
understand the genotype of an organism. Shortly after Gregor genetic marker is a region on the chromosome that shows variability
or polymorphism (multiple forms) in the population.
Mendel (the father of modern genetics) proposed that traits were
determined by what are now known as genes, other researchers Some genetic markers used in generating genetic maps are
observed that different traits were often inherited together and, restriction fragment length polymorphisms (RFLP), variable number
thereby, deduced that the genes were physically linked by being of tandem repeats (VNTRs), microsatellite polymorphisms, and the
located on the same chromosome. The mapping of genes relative to single nucleotide polymorphisms (SNPs). RFLPs (sometimes
each other based on linkage analysis led to the development of the pronounced “rif-lips”) are detected when the DNA of an individual
first genetic maps. is cut with a restriction endonuclease that recognizes specific
sequences in the DNA to generate a series of DNA fragments, which

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are then analyzed by gel electrophoresis. The DNA of every are similar to VNTRs, but the repeat unit is very small; thus, it is
individual will give rise to a unique pattern of bands when cut with a often referred to as short tandem repeats(STRs). SNPs are variations
particular set of restriction endonucleases; this is sometimes referred in a single nucleotide.
to as an individual’s DNA “fingerprint.” Certain regions of the Because genetic maps rely completely on the natural process of
chromosome that are subject to polymorphism will lead to the recombination, mapping is affected by natural increases or decreases
generation of the unique banding pattern. VNTRs are repeated sets in the level of recombination in any given area of the genome. Some
of nucleotides present in the non-coding regions of DNA. Non- parts of the genome are recombination hotspots, whereas others do
coding DNA has no known biological function; however, research not show a propensity for recombination. For this reason, it is
shows that much of this DNA is actually transcribed. While its important to look at mapping information developed by multiple
function is uncertain, it is certainly active; it may be involved in the methods.
regulation of coding genes. The number of repeats may vary in
individual organisms of a population. Microsatellite polymorphisms This page titled 17.2A: Genetic Maps is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

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17.2B: PHYSICAL MAPS AND INTEGRATION WITH GENETIC MAPS
Physical maps display the physical distance between genes and can unique sequence in the genome with a known exact chromosomal
be constructed using cytogenetic, radiation hybrid, or sequence location. An expressed sequence tag (EST) and a single sequence
mapping. length polymorphism (SSLP) are common STSs. An EST is a short
STS that is identified with cDNA libraries, while SSLPs are
 LEARNING OBJECTIVES obtained from known genetic markers and provide a link between
genetic maps and physical maps.
Describe the methods used to physically map genes:
cytogenetic mapping, radiation hybrid mapping, and
sequence mapping

KEY POINTS
Physical maps provide specified detail about the number of bases
and physical distance that exists between genetic markers.
Cytogenetic mapping is a method used to construct physical
maps that uses stained sections of chromosomes to approximate
the distance between genetic markers.
Radiation hybrid mapping is a method used to construct physical
maps that uses radiation or x-rays to break DNA into fragments
to determine the distance between genetic markers and their
order on the chromosome.
Sequence mapping is a method used to construct physical maps
that uses already-known locations of genetic markers to
determine distances in number of base pairs.

KEY TERMS
cytogenetic: of or pertaining to the origin and development of
cells
physical map: a map showing how much DNA separates two
genes and is measured in base pairs
expressed sequence tag: a short sub-sequence of a cDNA
sequence that may be used to identify gene transcripts

PHYSICAL MAPS
A physical map provides detail of the actual physical distance Figure 17.2B. 1: Cytogenetic Map: A cytogenetic map shows the
appearance of a chromosome after it is stained and examined under a
between genetic markers, as well as the number of nucleotides. microscope.
There are three methods used to create a physical map: cytogenetic
mapping, radiation hybrid mapping, and sequence mapping. INTEGRATION OF GENETIC AND PHYSICAL
Cytogenetic mapping uses information obtained by microscopic MAPS
analysis of stained sections of the chromosome. It is possible to Genetic maps provide the outline and physical maps provide the
determine the approximate distance between genetic markers using details. It is easy to understand why both types of genome mapping
cytogenetic mapping, but not the exact distance (number of base techniques are important to show the big picture. Information
pairs). Radiation hybrid mapping uses radiation, such as x-rays, to obtained from each technique is used in combination to study the
break the DNA into fragments. The amount of radiation can be genome. Genomic mapping is being used with different model
adjusted to create smaller or larger fragments. This technique research organisms. Genome mapping is an-ongoing process; as
overcomes the limitation of genetic mapping and is not affected by better techniques are developed, more advances are expected.
increased or decreased recombination frequency. Sequence mapping Genome mapping is similar to completing a complicated puzzle
resulted from DNA sequencing technology that allowed for the using every piece of available data. Mapping information generated
creation of detailed physical maps with distances measured in terms in laboratories worldwide is entered into central databases, such as
of the number of base pairs. The creation of genomic libraries and GenBank at the National Center for Biotechnology Information
complementary DNA (cDNA) libraries (collections of cloned (NCBI). Efforts are being made to make the information more easily
sequences or all DNA from a genome ) has sped up the process of accessible to researchers and the general public. Just as we use
physical mapping. A genetic site used to generate a physical map global positioning systems instead of paper maps to navigate
with sequencing technology (a sequence-tagged site, or STS) is a

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curated by Boundless.

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SECTION OVERVIEW

17.3: WHOLE-GENOME SEQUENCING

17.3B: USE OF WHOLE-GENOME SEQUENCES
Topic hierarchy OF MODEL ORGANISMS

17.3C: USES OF GENOME SEQUENCES

17.3A: STRATEGIES USED IN SEQUENCING
PROJECTS
This page titled 17.3: Whole-Genome Sequencing is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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17.3A: STRATEGIES USED IN SEQUENCING PROJECTS
The strategies used for sequencing genomes include the Sanger different point during replication. When the reaction mixture is
method, shotgun sequencing, pairwise end, and next-generation processed by gel electrophoresis after being separated into single
sequencing. strands, the multiple, newly-replicated DNA strands form a ladder
due to their differing sizes. Because the ddNTPs are fluorescently
 LEARNING OBJECTIVES labeled, each band on the gel reflects the size of the DNA strand and
the ddNTP that terminated the reaction. The different colors of the
Compare the different strategies used for whole-genome fluorophore-labeled ddNTPs help identify the ddNTP incorporated
sequencing: Sanger method, shotgun sequencing, pairwise- at that position. Reading the gel on the basis of the color of each
end sequencing, and next-generation sequencing band on the ladder produces the sequence of the template strand.

KEY POINTS
The Sanger method is a basic sequencing technique that uses
fluorescently-labeled dideoxynucleotides (ddNTPs) during DNA
replication which results in multiple short strands of replicated
DNA that terminate at different points, based on where the
ddNTP was incorporated.
Shotgun sequencing is a method that randomly cuts DNA
fragments into smaller pieces and then, with the help of a
computer, takes the DNA fragments, analyzes them for
overlapping sequences, and reassembles the entire DNA Figure 17.3A. 1 : Sanger’s Method: Frederick Sanger’s dideoxy
chain termination method uses dideoxynucleotides, in which the
sequence.
DNA fragment can be terminated at different points. The DNA is
Pairwise-end sequencing is a type of shotgun sequencing which separated on the basis of size, and these bands, based on the size of
is used for larger genomes and analyzes both ends of the DNA the fragments, can be read.
fragments for overlap.
Next-generation sequencing is a type of sequencing which is
automated and relies on sophisticated software for rapid DNA
sequencing.

KEY TERMS
fluorophore: a molecule or functional group which is capable of
fluorescence
contig: a set of overlapping DNA segments, derived from a
single source of genetic material, from which the complete
sequence may be deduced
dideoxynucleotide: any nucleotide formed from a
deoxynucleotide by loss of an a second hydroxyl group from the
deoxyribose group

STRATEGIES USED IN SEQUENCING PROJECTS Figure 17.3A. 1 : Structure of a Dideoxynucleotide: A

dideoxynucleotide is similar in structure to a deoxynucleotide, but is
The basic sequencing technique used in all modern day sequencing missing the 3′ hydroxyl group (indicated by the box). When a
projects is the chain termination method (also known as the dideoxy dideoxynucleotide is incorporated into a DNA strand, DNA
method), which was developed by Fred Sanger in the 1970s. The synthesis stops.
chain termination method involves DNA replication of a single-
EARLY STRATEGIES: SHOTGUN SEQUENCING
stranded template with the use of a primer and a regular
AND PAIR-WISE END SEQUENCING
deoxynucleotide (dNTP), which is a monomer, or a single unit, of
In the shotgun sequencing method, several copies of a DNA
DNA. The primer and dNTP are mixed with a small proportion of
fluorescently-labeled dideoxynucleotides (ddNTPs). The ddNTPs fragment are cut randomly into many smaller pieces (somewhat like
what happens to a round shot cartridge when fired from a shotgun).
are monomers that are missing a hydroxyl group (–OH) at the site at
which another nucleotide usually attaches to form a chain. Each All of the segments are then sequenced using the chain-sequencing
method. Then, with the help of a computer, the fragments are
ddNTP is labeled with a different color of fluorophore. Every time a
analyzed to see where their sequences overlap. By matching
ddNTP is incorporated in the growing complementary strand, it
overlapping sequences at the end of each fragment, the entire DNA
terminates the process of DNA replication, which results in multiple
sequence can be reformed. A larger sequence that is assembled from
short strands of replicated DNA that are each terminated at a
overlapping shorter sequences is called a contig. As an analogy,

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consider that someone has four copies of a landscape photograph pairwise-end sequencing, both ends of each fragment are analyzed
that you have never seen before and know nothing about how it for overlap. Pairwise-end sequencing is, therefore, more
should appear. The person then rips up each photograph with their cumbersome than shotgun sequencing, but it is easier to reconstruct
hands, so that different size pieces are present from each copy. The the sequence because there is more available information.
person then mixes all of the pieces together and asks you to
reconstruct the photograph. In one of the smaller pieces you see a NEXT-GENERATION SEQUENCING
mountain. In a larger piece, you see that the same mountain is Since 2005, automated sequencing techniques used by laboratories
behind a lake. A third fragment shows only the lake, but it reveals are under the umbrella of next-generation sequencing, which is a
that there is a cabin on the shore of the lake. Therefore, from looking group of automated techniques used for rapid DNA sequencing.
at the overlapping information in these three fragments, you know These automated, low-cost sequencers can generate sequences of
that the picture contains a mountain behind a lake that has a cabin on hundreds of thousands or millions of short fragments (25 to 500 base
its shore. This is the principle behind reconstructing entire DNA pairs) in the span of one day. Sophisticated software is used to
sequences using shotgun sequencing. manage the cumbersome process of putting all the fragments in
Originally, shotgun sequencing only analyzed one end of each order.
fragment for overlaps. This was sufficient for sequencing small
This page titled 17.3A: Strategies Used in Sequencing Projects is shared
genomes. However, the desire to sequence larger genomes, such as
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
that of a human, led to the development of double-barrel shotgun by Boundless.
sequencing, more formally known as pairwise-end sequencing. In

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17.3B: USE OF WHOLE-GENOME SEQUENCES OF MODEL ORGANISMS
Sequencing genomes of model organisms allows scientists to study The process of attaching biological information to gene sequences is
homologous proteins in more complex eukaryotes, such as humans. called genome annotation. Annotation aids researchers doing basic
experiments in molecular biology, such as designing PCR primers
 LEARNING OBJECTIVES and RNA targets.
Sequencing genomes allows scientists to identify homologous
Describe the model organisms used in whole-genome
proteins and establish evolutionary relationships. Furthermore, if a
sequencing
newly-discovered protein is homologous to a known protein,
through homology, scientists can make an educated guess as to how
KEY POINTS the new protein functions.
The first genome to be completely sequenced was the bacterial Eukaryotes are organisms containing cells that enclose complex
virus, bacteriophage fx174, which is 5368 base pairs. organelles within a well-defined cell membrane. The defining
Scientists utilize genome sequencing from model organisms to characteristic that sets eukaryotes and prokaryotes apart is the
study homologous proteins and establish evolutionary eukaryotes’ nucleus, or nuclear envelope, in which an organism’s
relationships. genetic information is contained. The first eukaryotic genome to be
Genome annotation is the process of attaching biological sequenced was that of S. cerevisiae, which is the yeast used in
information to gene sequences identified using whole-genome baking and brewing. It is the most-studied eukaryotic model
sequencing. organism in molecular and cell biology, similar to E. coli‘s role in
Model organisms include the fruit fly (Drosophila the study of prokaryotic organisms. Research on many proteins that
melanogaster), brewers yeast (Saccharomyces cerevisiae), the are important to humans is done by examining their homologs in
nematode, Caenorhabditis elegans, and the mouse (Mus yeasts. For example, signaling proteins and protein-processing
musculus). enzymes were discovered through the help of yeast genome.

KEY TERMS
genome annotation: the process of attaching biological
information to gene sequences.
model organism: any organism (e.g. the fruit fly) that has been
extensively studied as an example of many others and from
which general principles may be established

USE OF WHOLE-GENOME SEQUENCES OF

MODEL ORGANISMS
The first genome to be completely sequenced was of a bacterial
virus, the bacteriophage fx174 (5368 base pairs). This was
accomplished by Fred Sanger using shotgun sequencing. Several
other organelle and viral genomes were later sequenced. The first
organism whose genome was sequenced was the bacterium
Haemophilus influenzae, which was accomplished by Craig Venter
in the 1980s. Approximately 74 different laboratories collaborated
on the sequencing of the genome of the yeast Saccharomyces
cerevisiae, which began in 1989 and was completed in 1996. It took
this long because it was 60 times bigger than any other genome that
had been sequenced at that point. By 1997, the genome sequences of
two important model organisms were available: the bacterium
Figure 17.3B. 1: S. cerevisiae: A Model Organism for Eukaryotes:
Escherichia coli K12 and the yeast Saccharomyces cerevisiae. Saccharomyces cerevisiae, a yeast, is used as a model organism for
Genomes of other model organisms, such as the mouse Mus studying signaling proteins and protein-processing enzymes which
musculus, the fruit fly Drosophila melanogaster, the nematode have homologs in humans.
Caenorhabditis elegans, and the human Homo sapiens are now This page titled 17.3B: Use of Whole-Genome Sequences of Model
known. Much basic research is performed using model organisms Organisms is shared under a CC BY-SA 4.0 license and was authored,
because the information can be applied to the biological processes of remixed, and/or curated by Boundless.
genetically-similar organisms. Having entire genomes sequenced
aids these research efforts.

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17.3C: USES OF GENOME SEQUENCES
Genome sequences and expression can be analyzed using DNA
microarrays, which can contribute to detection of disease and
genetic disorders.

 LEARNING OBJECTIVES

Describe the various uses of genome sequences

KEY POINTS
DNA microarrays can be used to detect gene expression within
specific samples by analyzing active genes and sequences using
an array of DNA fragments fixed to a slide.
Genome sequences can be used to discover the possibility of
disease and genetic disorders prior to onset.
Genome sequences can also be used to develop agrochemicals
and pharmaceuticals.

KEY TERMS
microarray: any of several devices containing a two-
dimensional array of small quantities of biological material used
for various types of assays
genomics: the study of the complete genome of an organism Figure 17.3C. 1 : DNA Microarray: DNA microarrays can be used to
analyze gene expression within the genome.
USES OF GENOME SEQUENCES In addition to disease and medicine, genomics can contribute to the
DNA microarrays are methods used to detect gene expression by development of novel enzymes that convert biomass to biofuel,
analyzing an array of DNA fragments that are fixed to a glass slide which results in higher crop and fuel production, and lower cost to
or a silicon chip to identify active genes and identify sequences. the consumer. This knowledge should allow better methods of
Almost one million genotypic abnormalities can be discovered using control over the microbes that are used in the production of biofuels.
microarrays, whereas whole- genome sequencing can provide Genomics could also improve the methods used to monitor the
information about all six billion base pairs in the human genome. impact of pollutants on ecosystems and help clean up environmental
Although the study of medical applications of genome sequencing is contaminants. In addition, genomics has allowed for the
interesting, this discipline tends to dwell on abnormal gene function. development of agrochemicals and pharmaceuticals that could
Knowledge of the entire genome will allow future onset diseases and benefit medical science and agriculture.
other genetic disorders to be discovered early, which will allow for It sounds great to have all the knowledge we can get from whole-
more informed decisions to be made about lifestyle, medication, and genome sequencing; however, humans have a responsibility to use
having children. Genomics is still in its infancy, although someday it this knowledge wisely. Otherwise, it could be easy to misuse the
may become routine to use whole-genome sequencing to screen power of such knowledge, leading to discrimination based on a
every newborn to detect genetic abnormalities. person’s genetics, human genetic engineering, and other ethical
concerns. This information could also lead to legal issues regarding
health and privacy.

CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44555/latest/?collection=col11448/latest.
License: CC BY: Attribution
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License: CC BY-SA: Attribution-ShareAlike
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SECTION OVERVIEW

17.4: APPLYING GENOMICS

17.4B: PHARMACOGENOMICS,
Topic hierarchy TOXICOGENOMICS, AND METAGENOMICS

17.4C: GENOMICS AND BIOFUELS

17.4A: PREDICTING DISEASE RISK AT THE
INDIVIDUAL LEVEL
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17.4A: PREDICTING DISEASE RISK AT THE INDIVIDUAL LEVEL
Genome analysis is used to predict the level of disease risk in of the common diseases, such as heart disease, are multi-factored or
healthy individuals. polygenic, which is a phenotypic characteristic that involves two or
more genes interacting with environmental factors such as diet. In
 LEARNING OBJECTIVES April 2010, scientists at Stanford University published the genome
analysis of a healthy individual (Stephen Quake, a scientist at
Explain how analysis of an individual’s genome can aid in Stanford University, who had his genome sequenced); the analysis
predicting disease risk predicted his propensity to acquire various diseases. A risk
assessment was performed to analyze Quake’s percentage of risk for
KEY POINTS 55 different medical conditions. A rare genetic mutation was found,
Genome sequencing can predict the risk of developing diseases which showed him to be at risk for sudden heart attack. He was also
brought on by a single gene defect, but these defects only predicted to have a 23 percent risk of developing prostate cancer and
account for five percent of common diseases. a 1.4 percent risk of developing Alzheimer’s. The scientists used
Most diseases are polygenic or are brought on by environmental databases and several publications to analyze the genomic data.
factors; genome sequencing cannot predict the risk of acquiring Even though genomic sequencing is becoming more affordable and
these diseases. analytical tools are becoming more reliable, ethical issues
Genome sequencing is becoming more reliable, but many surrounding genomic analysis at a population level remain to be
scientists still question if it reduces the risk of death from certain addressed.
diseases such as prostate cancer. In 2011, the United States Preventative Services Task Force
recommended against using the PSA test to screen healthy men for
KEY TERMS prostate cancer. Their recommendation was based on evidence that
Human Genome Project: an organized international scientific screening does not reduce the risk of death from prostate cancer.
endeavor to determine the complete structure of human genetic Prostate cancer often develops very slowly and does not cause
material (DNA) to identify all the genes and understand their problems, while the cancer treatment can have severe side effects.
function The PCA3 test is considered to be more accurate, but screening may
genome sequencing: a laboratory process that determines the still result in men suffering side effects from treatment who would
complete DNA sequence of an organism’s genome at a single not have been harmed by the cancer itself.
time
polygenic: a phenotypic characteristic controlled by the
interaction of two or more genes with the environment

PREDICTING DISEASE RISK AT THE INDIVIDUAL

LEVEL
The introduction of DNA sequencing and whole genome sequencing
projects, particularly the Human Genome project, has expanded the
applicability of DNA sequence information. Genomics is now being
used in a wide variety of fields, such as metagenomics,
pharmacogenomics, and mitochondrial genomics. The most
commonly-known application of genomics is to understand and find
cures for diseases.
Predicting the risk of disease involves screening currently-healthy Figure 17.4A. 1 : PCA3: PCA3 is a gene that is expressed in prostate
epithelial cells and overexpressed in cancerous cells. A high
individuals by genome analysis at the individual level. Intervention concentration of PCA3 in urine is indicative of prostate cancer.
with lifestyle changes and drugs can be recommended before disease
onset. However, this approach is most applicable when the problem This page titled 17.4A: Predicting Disease Risk at the Individual Level is
resides within a single gene defect. Such defects only account for shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
approximately five percent of diseases in developed countries. Most curated by Boundless.

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17.4B: PHARMACOGENOMICS, TOXICOGENOMICS, AND METAGENOMICS

 LEARNING OBJECTIVES

Explain how microbial metagenomics can give researchers a

more complete picture of a particular environment

Pharmacogenomics, also called toxicogenomics, involves evaluating

the effectiveness and safety of drugs on the basis of information
from an individual’s genomic sequence. Genomic responses to drugs
can be studied using experimental animals (such as laboratory rats or
mice) or live cells in the laboratory before embarking on studies
with humans. Studying changes in gene expression could provide
information about the transcription profile in the presence of the
drug, which can be used as an early indicator of the potential for
toxic effects. For example, genes involved in cellular growth and
controlled cell death, when disturbed, could lead to the growth of
cancerous cells. Genome-wide studies can also help to find new
genes involved in drug toxicity. Personal genome sequence
information can be used to prescribe medications that will be most
effective and least toxic on the basis of the individual patient’s Figure 17.4B. 1: Metagenomics: Metagenomics involves isolating
genotype. The gene signatures may not be completely accurate, but DNA from multiple species within an environmental niche.
can be tested further before pathologic symptoms arise.
KEY POINTS
MICROBIAL GENOMICS: METAGENOMICS Pharmacogenomics involves evaluating the effectiveness and
Traditionally, microbiology has been taught with the view that safety of drugs on the basis of information from an individual’s
microorganisms are best studied under pure culture conditions, genomic sequence.
which involves isolating a single type of cell and culturing it in the Genomic responses to drugs can be studied using experimental
laboratory. Because microorganisms can go through several animals or live cells in the laboratory, which help indicate the
generations in a matter of hours, their gene expression profiles adapt potentially toxic effects of a drug.
to the new laboratory environment very quickly. In addition, the vast Personal genome sequence information can be used to prescribe
majority of bacterial species resist being cultured in isolation. Most medications that will be most effective and least toxic on an
microorganisms do not live as isolated entities, but in microbial individual level.
communities known as biofilms. For all of these reasons, pure Metagenomics, the study of the collective genomes of multiple
culture is not always the best way to study microorganisms. species that grow and interact in an environmental niche, is often
Metagenomics is the study of the collective genomes of multiple a better way to study microrganisms rather than pure culture.
species that grow and interact in an environmental niche.
KEY TERMS
Metagenomics can be used to identify new species more rapidly and
to analyze the effect of pollutants on the environment. pharmacogenomics: the study of genes that code for enzymes
that metabolize drugs, and the design of tailor-made drugs
adapted to an individual’s genetic make-up
metagenomics: the study of the collective genomes of multiple
species that grow and interact in an environmental niche

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17.4C: GENOMICS AND BIOFUELS
Microbial genomics can be used to create new biofuels. cellulose and hemicellulose and tolerate higher ethanol
concentrations to optimize fuel yields. For instance, the hindgut
 LEARNING OBJECTIVES contents of nature’s own bioreactor, the termite, has yielded more
than 500 genes related to the enzymatic deconstruction of cellulose
Explain the process of creating new biofuels by using and hemicellulose.
microbial genomics

KEY POINTS
Microorganisms can encode new enzymes and produce new
organic compounds that can be used as biofuels.
Genomic analysis of the fungus Pichia will allow optimization of
its use in fermenting ethanol fuels.
Analysis of the microbes in the hindgut of termites have found
500 genes that may be useful in enzymatic destruction of
cellulose.
Genetic markers have been used in forensic analysis, like in 2001
when the FBI used microbial genomics to determine a specific
strain of anthrax that was found in several pieces of mail.
Genomics is used in agriculture to develop plants with more
desirable traits, such as drought and disease resistance.

KEY TERMS
renewable resource: a natural resource such that it is
replenished by natural processes at a rate comparable to its rate
of consumption by humans or other users
biofuel: any fuel that is obtained from a renewable biological
resource
Knowledge of the genomics of microorganisms is being used to find
better ways to harness biofuels from algae and cyanobacteria. The
primary sources of fuel today are coal, oil, wood, and other plant
products, such as ethanol. Although plants are renewable resources,
there is still a need to find more alternative renewable sources of
energy to meet our population ‘s energy demands. The microbial
world is one of the largest resources for genes that encode new
enzymes and produce new organic compounds, and it remains
largely untapped.
For microbial biomass breakdown, many candidates have already
been identified. These include Clostridia species for their ability to
degrade cellulose, and fungi that express genes associated with the
decomposition of the most recalcitrant features of the plant cell wall,
lignin, the phenolic “glue” that imbues the plant with structural
integrity and pest resistance. The white rot fungus Phanerochaete
Figure 17.4C. 1 : Termites: Nature’s Bioreactors: The hindgut of the
chrysosporium produces unique extracellular oxidative enzymes that termite has yielded more than 500 genes of microbes related to the
effectively degrade lignin by gaining access through the protective enzymatic deconstruction of cellulose.
matrix surrounding the cellulose microfibrils of plant cell walls.
CONTRIBUTIONS AND ATTRIBUTIONS
Another fungus, the yeast Pichia stipitis, ferments the five-carbon Human Genome Project. Provided by: Wiktionary. Located at:
“wood sugar” xylose abundant in hardwoods and agricultural en.wiktionary.org/wiki/Human_Genome_Project. License: CC BY-SA:
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harvest residue. Pichia‘s recently-sequenced genome has revealed
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insights into the metabolic pathways responsible for this process, Located at: http://cnx.org/content/m44560/latest/?collection=col11448/latest.
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genome sequencing. Provided by: Wikipedia. Located at: Genomics of Plant-based Biofuels in the Journal Nature. Provided by:
en.Wikipedia.org/wiki/genome%20sequencing. License: CC BY-SA: http://www.jgi.doe.gov/News/news_8_13_08.html. Located at:
Attribution-ShareAlike http://www.jgi.doe.gov/News/news_8_13_08.html. License: Public Domain:
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http://cnx.org/content/m44560/latest/Figure_B17_02_02.png. License: CC Ancient History/Human Evolution/Multiregional Origin. Provided by:
BY: Attribution Wikibooks. Located at:
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Located at: http://cnx.org/content/m44560/latest/?collection=col11448/latest. igin%23Mitochondrial_DNA. License: CC BY-SA: Attribution-ShareAlike
License: CC BY: Attribution Ancient History/Human Evolution/Recent African Origin. Provided by:
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License: CC BY: Attribution Origin. License: CC BY-SA: Attribution-ShareAlike
Boundless. Provided by: Boundless Learning. Located at: biofuel. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/biofuel.
www.boundless.com//biology/definition/metagenomics. License: CC BY-SA: License: CC BY-SA: Attribution-ShareAlike
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SECTION OVERVIEW

17.5: GENOMICS AND PROTEOMICS

17.5B: BASIC TECHNIQUES IN PROTEIN
Topic hierarchy ANALYSIS

17.5C: CANCER PROTEOMICS

17.5A: GENOMICS AND PROTEOMICS

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17.5A: GENOMICS AND PROTEOMICS
proteolytic cleavage, phosphorylation, glycosylation, and
 LEARNING OBJECTIVES ubiquitination. There are also protein-protein interactions, which
complicate the study of proteomes. Although the genome provides a
Explain how the field of genomics led to the development of
blueprint, the final architecture depends on several factors that can
proteomics
change the progression of events that generate the proteome.

Proteomics is a relatively-recent field; the term was coined in 1994

while the science itself had its origins in electrophoresis techniques
of the 1970’s and 1980’s. The study of proteins, however, has been a
scientific focus for a much longer time. Studying proteins generates
insight into how they affect cell processes. Conversely, this study
also investigates how proteins themselves are affected by cell
processes or the external environment. Proteins provide intricate
control of cellular machinery; they are, in many cases, components
of that same machinery. They serve a variety of functions within the
cell; there are thousands of distinct proteins and peptides in almost
every organism. The goal of proteomics is to analyze the varying
proteomes of an organism at different times in order to highlight
differences between them. Put more simply, proteomics analyzes the
structure and function of biological systems. For example, the
protein content of a cancerous cell is often different from that of a
healthy cell. Certain proteins in the cancerous cell may not be Figure 17.5A. 1 : Large-scale proteomics machinery: This machine
present in the healthy cell, making these unique proteins good is preparing to do a proteomic pattern analysis to identify specific
targets for anti-cancer drugs. The realization of this goal is difficult; cancers so that an accurate cancer prognosis can be made.
both purification and identification of proteins in any organism can KEY POINTS
be hindered by a multitude of biological and environmental factors.
Proteomics investigates how proteins affect and are affected by
The study of the function of proteomes is called proteomics. A cell processes or the external environment.
proteome is the entire set of proteins produced by a cell type. Within an individual organism, the genome is constant, but the
Genomics led to proteomics (via transcriptomics) as a logical step. proteome varies and is dynamic.
Proteomes can be studied using the knowledge of genomes because Every cell in an individual organism has the same set of genes,
genes code for mRNAs and the mRNAs encode proteins. Although but the set of proteins produced in different tissues differ from
mRNA analysis is a step in the right direction, not all mRNAs are one another and are dependent on gene expression.
translated into proteins. Proteomics complements genomics and is
useful when scientists want to test their hypotheses that were based KEY TERMS
on genes. Even though all cells of a multicellular organism have the proteomics: the branch of molecular biology that studies the set
same set of genes, the set of proteins produced in different tissues is of proteins expressed by the genome of an organism
different and dependent on gene expression. Thus, the genome is proteome: the complete set of proteins encoded by a particular
constant, but the proteome varies and is dynamic within an genome
organism. In addition, RNAs can be alternately spliced (cut and genomics: the study of the complete genome of an organism
pasted to create novel combinations and novel proteins) and many
proteins are modified after translation by processes such as This page titled 17.5A: Genomics and Proteomics is shared under a CC BY-
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17.5B: BASIC TECHNIQUES IN PROTEIN ANALYSIS
X-RAY CRYSTALLOGRAPHY AND NUCLEAR
 LEARNING OBJECTIVES MAGNETIC RESONANCE
X-ray crystallography enables scientists to determine the three-
Describe the techniques used in proteomics to analyze
proteins dimensional structure of a protein crystal at atomic resolution.
Crystallographers aim high-powered X-rays at a tiny crystal
containing trillions of identical molecules. The crystal scatters the
BASIC TECHNIQUES IN PROTEIN ANALYSIS
X-rays onto an electronic detector that is the same type used to
The ultimate goal of proteomics is to identify or compare the capture images in a digital camera. After each blast of X-rays,
proteins expressed in a given genome under specific conditions, lasting from a few seconds to several hours, the researchers precisely
study the interactions between the proteins, and use the information rotate the crystal by entering its desired orientation into the
to predict cell behavior or develop drug targets. Just as the genome computer that controls the X-ray apparatus. This enables the
is analyzed using the basic technique of DNA sequencing, scientists to capture in three dimensions how the crystal scatters, or
proteomics requires techniques for protein analysis. The basic diffracts, X-rays. The intensity of each diffracted ray is fed into a
technique for protein analysis, analogous to DNA sequencing, is computer, which uses a mathematical equation to calculate the
mass spectrometry. position of every atom in the crystallized molecule. The result is a
three-dimensional digital image of the molecule.

Figure 17.5B. 1: X-ray crystallography: X-rays that hit atomic

nuclei are diffracted onto a detector.
Another protein imaging technique, nuclear magnetic resonance
(NMR), uses the magnetic properties of atoms to determine the
three-dimensional structure of proteins. NMR spectroscopy is
unique in being able to reveal the atomic structure of
macromolecules in solution, provided that highly-concentrated
solution can be obtained. This technique depends on the fact that
Figure 17.5B. 1: Mass Spectrometer: Matrix-Assisted Laser
Desorbtion Ionisation – Time Of Flight (MALDI-TOF) Mass certain atomic nuclei are intrinsically magnetic. The chemical shift
Spectrometer. Mass spectrometry can be used in protein analysis. of nuclei depends on their local environment. The spins of
neighboring nuclei interact with each other in ways that provide
MASS SPECTROMETRY definitive structural information that can be used to determine
Mass spectrometry is used to identify and determine the complete three-dimensional structures of proteins.
characteristics of a molecule. It is a technique in which gas phase
molecules are ionized and their mass-to-charge ratio is measured by PROTEIN MICROARRAYS AND TWO- HYBRID
observing acceleration differences of ions when an electric field is SCREENING
applied. Lighter ions will accelerate faster and be detected first. If Protein microarrays have also been used to study interactions
the mass is measured with precision, then the composition of the between proteins. These are large-scale adaptations of the basic two-
molecule can be identified. In the case of proteins, the sequence can hybrid screen. The premise behind the two-hybrid screen is that
be identified. The challenge of techniques used for proteomic most eukaryotic transcription factors have modular activating and
analyses is the difficulty in detecting small quantities of proteins, but binding domains that can still activate transcription even when split
advances in spectrometry have allowed researchers to analyze very into two separate fragments, as long as the fragments are brought
small samples of protein. Variations in protein expression in within close proximity to each other. Generally, the transcription
diseased states, however, can be difficult to discern. Proteins are factor is split into a DNA-binding domain (BD) and an activation
naturally-unstable molecules, which makes proteomic analysis much domain (AD). One protein of interest is genetically fused to the BD
more difficult than genomic analysis. and another protein is fused to the AD. If the two proteins of interest
bind each other, then the BD and AD will also come together and
activate a reporter gene that signals interaction of the two hybrid
proteins.

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 electrophoresis, then transferred to a nitrocellulose or other type of
membrane, and finally stained with a primary antibody that
specifically binds the protein of interest. A fluorescent or
radioactive-labeled secondary antibody binds to the primary
antibody and provides a means of detection via either photography
or x-ray film, respectively.

KEY POINTS
Mass Spectrometry is a technique that is useful for determining
the size of a protein or protein complex.
X-ray crystallography and NMR are techniques useful for
determining the 3-D structure of a protein or protein complex.
Protein microarrays are useful for determining protein-protein
Figure 17.5B. 1: Two-hybrid screening: Two-hybrid screening is interactions.
used to determine whether two proteins interact. In this method, a
transcription factor is split into a DNA-binding domain (BD) and an KEY TERMS
activation domain (AD). The binding domain is able to bind the
promoter in the absence of the activator domain, but it does not turn microarray: any of several devices containing a two-
on transcription. A protein called the bait is attached to the BD, and dimensional array of small quantities of biological material used
a protein called the prey is attached to the AD. Transcription occurs
for various types of assays
only if the prey “catches” the bait.
reporter gene: a gene that researchers attach to a regulatory
WESTERN BLOT sequence of another gene of interest and whose product is easily
identifiable in assays
The western blot, or protein immunoblot, is a technique that
combines protein electrophoresis and antibodies to detect proteins in This page titled 17.5B: Basic Techniques in Protein Analysis is shared under
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above techniques and, thus, can serve as an assay to validate results Boundless.
from other experiments. The protein sample is first separated by gel

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17.5C: CANCER PROTEOMICS
possibility of disease recurrence. The National Cancer Institute has
 LEARNING OBJECTIVES developed programs to improve the detection and treatment of
cancer. The Clinical Proteomic Technologies for Cancer and the
Explain the ways in which cancer proteomics may lead to
Early Detection Research Network are efforts to identify protein
better treatments
signatures specific to different types of cancers. The Biomedical
Proteomics Program is designed to identify protein signatures and
Genomes and proteomes of patients suffering from specific diseases
design effective therapies for cancer patients.
are being studied to understand the genetic basis of diseases. The
most prominent set of diseases being studied with proteomic KEY POINTS
approaches is cancer. Proteomic approaches are being used to Identifying those proteins whose expression is affected by
improve screening and early detection of cancer, which is achieved disease processes can be used to improve screening and early
by identifying proteins whose expression is affected by the disease detection of cancer.
process. Different biomarkers and protein signatures are being used to
An individual protein that indicates disease is called a biomarker, analyze each type of cancer.
whereas a set of proteins with altered expression levels is called a A future goal of cancer proteomics is to have a personalized
protein signature. For a biomarker or protein signature to be useful treatment plan for each individual.
as a candidate for early screening and detection of a cancer, it must
be secreted in body fluids (e.g. sweat, blood, or urine) such that KEY TERMS
large-scale screenings can be performed in a non-invasive fashion. biomarker: a substance used as an indicator of a biological state,
The current problem with using biomarkers for the early detection of most commonly disease
cancer is the high rate of false-negative results. A false-negative is
an incorrect test result that should have been positive. In other CONTRIBUTIONS AND ATTRIBUTIONS
words, many cases of cancer go undetected, which makes OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44558/latest...ol11448/latest. License: CC
biomarkers unreliable. Some examples of protein biomarkers used in BY: Attribution
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 CHAPTER OVERVIEW

18: EVOLUTION AND THE ORIGIN OF SPECIES

Topic hierarchy
18.1: Understanding Evolution
18.1A: What is Evolution?
18.1B: Charles Darwin and Natural Selection
18.1C: The Galapagos Finches and Natural Selection
18.1D: Processes and Patterns of Evolution
18.1E: Evidence of Evolution
18.1F: Misconceptions of Evolution
18.2: Formation of New Species
18.2A: The Biological Species Concept
18.2B: Reproductive Isolation
18.2C: Speciation
18.2D: Allopatric Speciation
18.2E: Sympatric Speciation
18.3: Hybrid Zones and Rates of Speciation
18.3A: Hybrid Zones
18.3B: Varying Rates of Speciation
18.4: Evolution of Genomes
18.4A: Genomic Similiarities between Distant Species
18.4B: Genome Evolution
18.4C: Whole-Genome Duplication
18.4D: Gene Duplications and Divergence
18.4E: Noncoding DNA
18.4F: Variations in Size and Number of Genes
18.5: Evidence of Evolution
18.5A: The Fossil Record as Evidence for Evolution
18.5B: Fossil Formation
18.5C: Gaps in the Fossil Record
18.5D: Carbon Dating and Estimating Fossil Age
18.5E: The Fossil Record and the Evolution of the Modern Horse
18.5F: Homologous Structures
18.5G: Convergent Evolution
18.5H: Vestigial Structures
18.5I: Biogeography and the Distribution of Species

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1
SECTION OVERVIEW

18.1: UNDERSTANDING EVOLUTION

18.1D: PROCESSES AND PATTERNS OF
Topic hierarchy EVOLUTION

18.1E: EVIDENCE OF EVOLUTION

18.1A: WHAT IS EVOLUTION?
18.1F: MISCONCEPTIONS OF EVOLUTION
18.1B: CHARLES DARWIN AND NATURAL
SELECTION
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SELECTION

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18.1A: WHAT IS EVOLUTION?
Evolution, the unifying theory of biology, describes a mechanism for already been suggested and debated. The view that species are static
the change and diversification of species over time. and unchanging was grounded in the writings of Plato, yet there
were also ancient Greeks who expressed ideas about evolution.
 LEARNING OBJECTIVES During the eighteenth century, ideas about the evolution of animals
were reintroduced by the naturalist Georges-Louis Leclerc Comte de
Describe the historical influences on Darwin’s theory of Buffon who observed that various geographic regions have different
evolution plant and animal populations, even when the environments are
similar. It was also accepted that there are extinct species.
KEY POINTS
Ancient Greeks expressed ideas about evolution, which were
reintroduced in the eighteenth century by Georges-Louis Leclerc
Comte de Buffon who observed different environments had
different plant and animal populations.
James Hutton proposed that geological changes occur gradually
over time via the accumulation of small changes rather than
through large catastrophic events. Figure 18.1A. 1 : Evolution by Natural Selection: All organisms are
Charles Lyell popularized James Hutton’s theory; this theory of products of evolution adapted to their environment. (a) Saguaro
incremental change influenced Darwin’s theory of evolution. (Carnegiea gigantea) can soak up 750 liters of water in a single rain
storm, enabling these cacti to survive the dry conditions of the
Jean-Baptiste Lamarck proposed the theory of the inheritance of Sonoran desert in Mexico and the Southwestern United States. (b)
acquired characterstics; this theory has now been discredited, but The Andean semiaquatic lizard (Potamites montanicola), discovered
it served as an important influence on the theory of evolution. in Peru in 2010, lives between 1,570 to 2,100 meters in elevation
and, unlike most lizards, is nocturnal and swims. Scientists still do
not know how these cold-blood animals are able to move in the cold
KEY TERMS (10 to 15°C) temperatures of the Andean night.
evolution: the change in the genetic composition of a population During this time, a Scottish naturalist named James Hutton proposed
over successive generations that geological change occurs gradually by the accumulation of
inheritance of acquired characteristics: hypothesis that small changes over long periods of time. This theory contrasted with
physiological changes acquired over the life of an organism may the predominant view of the time: that the geology of the planet is a
be transmitted to its offspring consequence of catastrophic events that occurred during a relatively
brief past. During the nineteenth century, Hutton’s views were
INTRODUCTION: EVOLUTION popularized by the geologist Charles Lyell, who was a friend of
All species of living organisms, including bacteria and chimpanzees, Charles Darwin. Lyell’s ideas, in turn, influenced Darwin’s concept
evolved at some point from a different species. Although it may of evolution. The greater age of the earth proposed by Lyell
seem that living things today stay the same, this is not the case: supported the gradual evolution that Darwin proposed, and the slow
evolution is a gradual and ongoing process. process of geological change provided an analogy for the gradual
The theory of evolution is the unifying theory of biology, meaning it change in species.
is the framework within which biologists ask questions about the In the early nineteenth century, Jean-Baptiste Lamarck published a
living world. The Ukrainian-born American geneticist Theodosius book that detailed a different mechanism for evolutionary change.
Dobzhansky famously wrote that “nothing makes sense in biology This mechanism is now referred to as an inheritance of acquired
except in the light of evolution.” The tenet that all species have characteristics. This idea states that modifications in an individual
evolved and diversified from a common ancestor is the foundation are caused by its environment, or the use or disuse of a structure
from which we approach all questions in biology. It provides a during its lifetime, and that these changes can be inherited by its
direction for predictions about living things, which has been offspring, bringing about change in a species. While this mechanism
validated through extensive scientific experimentation. for evolutionary change was discredited, Lamarck’s ideas were an
Evolution by natural selection describes a mechanism for the change important influence on the concept of evolution.
of species over time. Well before Darwin began to explore the
concept of evolution, the idea that species change over time had This page titled 18.1A: What is Evolution? is shared under a CC BY-SA 4.0
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18.1B: CHARLES DARWIN AND NATURAL SELECTION
Charles Darwin and Alfred Wallace independently developed the finch species on the mainland of South America. Darwin imagined
theories of evolution and its main operating principle: natural that the island species might be modified from one of the original
selection. mainland species. Upon further study, he realized that the varied
beaks of each finch helped the birds acquire a specific type of food.
 LEARNING OBJECTIVES For example, seed-eating finches had stronger, thicker beaks for
breaking seeds, while insect-eating finches had spear-like beaks for
Explain how natural selection can lead to evolution stabbing their prey.

KEY POINTS
Wallace traveled to Brazil to collect and observe insects from the
Amazon rainforest.
Darwin observed that finches in the Galápagos Islands had
different beaks than finches in South America; these adaptations
equiped the birds to acquire specific food sources.
Wallace and Darwin observed similar patterns in the variation of
organisms and independently developed the same explanation for
how such variations could occur over time, a mechanism Darwin
called natural selection.
According to natural selection, also known as “survival of the
fittest,” individuals with traits that enable them to survive are
more reproductively successful; this leads to those traits
becoming predominant within a population.
Natural selection is an inevitable outcome of three principles: Figure 18.1B. 1: Beak Shape Among Finch Species: Darwin
observed that beak shape varies among finch species. He postulated
most characteristics are inherited, more offspring are produced that the beak of an ancestral species had adapted over time to equip
than are able to survive, and offspring with more favorable the finches to acquire different food sources.
characteristics will survive and have more offspring than those
individuals with less favorable traits. NATURAL SELECTION
Wallace and Darwin observed similar patterns in other organisms
KEY TERMS and independently developed the same explanation for how and why
natural selection: a process in which individual organisms or such changes could take place. Darwin called this mechanism
phenotypes that possess favorable traits are more likely to natural selection. Natural selection, also known as “survival of the
survive and reproduce fittest,” is the more prolific reproduction of individuals with
descent with modification: change in populations over favorable traits that survive environmental change because of those
generations traits. This leads to evolutionary change, the trait becoming
predominant within a population. For example, Darwin observed
CHARLES DARWIN AND NATURAL SELECTION that a population of giant tortoises found in the Galapagos
In the mid-nineteenth century, the mechanism for evolution was Archipelago have longer necks than those that lived on other islands
independently conceived of and described by two naturalists: with dry lowlands. These tortoises were “selected” because they
Charles Darwin and Alfred Russel Wallace. Importantly, each could reach more leaves and access more food than those with short
naturalist spent time exploring the natural world on expeditions to necks. In times of drought, when fewer leaves would be available,
the tropics. From 1831 to 1836, Darwin traveled around the world to those that could reach more leaves had a better chance to eat and
places like South America, Australia, and the southern tip of Africa. survive than those that could not reach the food source.
Wallace traveled to Brazil to collect insects in the Amazon rainforest Consequently, long-necked tortoises would more probably be
from 1848 to 1852 and to the Malay Archipelago from 1854 to 1862. reproductively successful and pass the long-necked trait to their
Darwin’s journey, as with Wallace’s later journeys to the Malay offspring. Over time, only long-necked tortoises would be present in
Archipelago, included stops at several island chains, the last being the population.
the Galápagos Islands west of Ecuador. On these islands, Darwin Natural selection, Darwin argued, was an inevitable outcome of
observed that species of organisms on different islands were clearly three principles that operated in nature. First, most characteristics of
similar, yet had distinct differences. For example, the ground finches organisms are inherited, or passed from parent to offspring, although
inhabiting the Galápagos Islands comprised several species with a how traits were inherited was unknown. Second, more offspring are
unique beak shape. The species on the islands had a graded series of produced than are able to survive. The capacity for reproduction in
beak sizes and shapes with very small differences between the most all organisms outstrips the availability of resources to support their
similar. He observed that these finches closely resembled another numbers. Thus, there is competition for those resources in each

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generation. Both Darwin and Wallace were influenced by an essay
written by economist Thomas Malthus who discussed this principle
in relation to human populations. Third, Darwin and Wallace
reasoned that offspring with the inherited characteristics that allow
them to best compete for limited resources will survive and have
more offspring than those individuals with variations that are less
able to compete. Because characteristics are inherited, these traits
will be better represented in the next generation. This will lead to
change in populations over successive generations in a process that
Darwin called descent with modification. Ultimately, natural
selection leads to greater adaptation of the population to its local
environment; it is the only mechanism known for adaptive
evolution.
Papers by Darwin and Wallace presenting the idea of natural
selection were read together in 1858 before the Linnean Society in Figure 18.1B. 1: Charles Darwin and Alfred Wallace: Both (a)
Charles Darwin and (b) Alfred Wallace wrote scientific papers on
London. The following year, Darwin’s book, On the Origin of
natural selection that were presented together before the Linnean
Species, was published. His book outlined his arguments for Society in 1858.
evolution by natural selection.
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18.1C: THE GALAPAGOS FINCHES AND NATURAL SELECTION
The differences in shape and size of beaks in Darwin’s finches
illustrate ongoing evolutionary change.

 LEARNING OBJECTIVES

Describe how finches provide visible evidence of evolution

KEY POINTS
Darwin observed the Galapagos finches had a graded series of
beak sizes and shapes and predicted these species were modified
from one original mainland species.
Darwin called differences among species natural selection, which
is caused by the inheritance of traits, competition between
individuals, and the variation of traits.
Offspring with inherited characteristics that allow them to best
compete will survive and have more offspring than those Figure 18.1C. 1 : Darwin’s Finches: Darwin observed that beak
shape varies among finch species. He postulated that the beak of an
individuals with variations that are less able to compete. ancestral species had adapted over time to equip the finches to
Large-billed finches feed more efficiently on large, hard seeds, acquire different food sources. This illustration shows the beak
whereas smaller billed finches feed more efficiently on small, shapes for four species of ground finch: 1. Geospiza magnirostris
(the large ground finch), 2. G. fortis (the medium ground finch), 3.
soft seeds.
G. parvula (the small tree finch), and 4. Certhidea olivacea (the
When small, soft seeds become rare, large-billed finches will green-warbler finch).
survive better, and there will be more larger-billed birds in the
following generation; when large, hard seeds become rare, the NATURAL SELECTION
opposite will occur. Darwin called this mechanism of change natural selection. Natural
selection, Darwin argued, was an inevitable outcome of three
KEY TERMS principles that operated in nature. First, the characteristics of
natural selection: a process in which individual organisms or organisms are inherited, or passed from parent to offspring. Second,
phenotypes that possess favorable traits are more likely to more offspring are produced than are able to survive; in other words,
survive and reproduce resources for survival and reproduction are limited. The capacity for
evolution: the change in the genetic composition of a population reproduction in all organisms exceeds the availability of resources to
over successive generations support their numbers. Thus, there is a competition for those
resources in each generation. Third, offspring vary among each other
VISIBLE EVIDENCE OF ONGOING EVOLUTION: in regard to their characteristics and those variations are inherited.
DARWIN’S FINCHES Out of these three principles, Darwin reasoned that offspring with
From 1831 to 1836, Darwin traveled around the world, observing inherited characteristics that allow them to best compete for limited
animals on different continents and islands. On the Galapagos resources will survive and have more offspring than those
Islands, Darwin observed several species of finches with unique individuals with variations that are less able to compete. Because
beak shapes. He observed these finches closely resembled another characteristics are inherited, these traits will be better represented in
finch species on the mainland of South America and that the group the next generation. This will lead to change in populations over
of species in the Galápagos formed a graded series of beak sizes and generations in a process that Darwin called “descent with
shapes, with very small differences between the most similar. modification,” or evolution.
Darwin imagined that the island species might be all species
modified from one original mainland species. In 1860, he wrote, STUDIES OF NATURAL SELECTION AFTER
“seeing this gradation and diversity of structure in one small, DARWIN
intimately related group of birds, one might really fancy that from an Demonstrations of evolution by natural selection can be time
original paucity of birds in this archipelago, one species had been consuming. Peter and Rosemary Grant and their colleagues have
taken and modified for different ends.” studied Galápagos finch populations every year since 1976 and have
provided important demonstrations of the operation of natural
selection. The Grants found changes from one generation to the next
in the beak shapes of the medium ground finches on the Galápagos
island of Daphne Major.
The medium ground finch feeds on seeds. The birds have inherited
variation in the bill shape with some individuals having wide, deep

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bills and others having thinner bills. Large-billed birds feed more in this species in response to other changing conditions on the
efficiently on large, hard seeds, whereas smaller billed birds feed island. The evolution has occurred both to larger bills, as in this
more efficiently on small, soft seeds. During 1977, a drought period case, and to smaller bills when large seeds became rare.
altered vegetation on the island. After this period, the number of
seeds declined dramatically; the decline in small, soft seeds was
greater than the decline in large, hard seeds. The large-billed birds
were able to survive better than the small-billed birds the following
year.
The year following the drought when the Grants measured beak
Figure 18.1C. 1 : Finches of Daphne Major: A drought on the
sizes in the much-reduced population, they found that the average
Galápagos island of Daphne Major in 1977 reduced the number of
bill size was larger. This was clear evidence for natural selection of small seeds available to finches, causing many of the small-beaked
bill size caused by the availability of seeds. The Grants had studied finches to die. This caused an increase in the finches’ average beak
size between 1976 and 1978.
the inheritance of bill sizes and knew that the surviving large-billed
birds would tend to produce offspring with larger bills, so the This page titled 18.1C: The Galapagos Finches and Natural Selection is
selection would lead to evolution of bill size. Subsequent studies by shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
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18.1D: PROCESSES AND PATTERNS OF EVOLUTION
Natural selection can only occur in the presence of genetic variation; A mutation may produce a phenotype with a beneficial effect on
environmental conditions determine which traits are selected. fitness. Different mutations will have a range of effects on the
fitness of an organism that expresses them in their phenotype,
 LEARNING OBJECTIVES from a small effect to a great effect.
Sexual reproduction also leads to genetic diversity: when two
Explain why only heritable variation can be acted upon by
parents reproduce, unique combinations of alleles assemble to
natural selection
produce the unique genotypes and thus phenotypes in each of the
offspring. However, sexual reproduction can not lead to new genes,
KEY POINTS but rather provides a new combination of genes in a given
Genetic variation within a population is a result of mutations and individual.
sexual reproduction.
A mutation may be neutral, reduce an organism’s fitness, or ADAPTATIONS
increase an organism’s fitness. A heritable trait that aids the survival and reproduction of an
An adaptation is a heritable trait that increases the survival and organism in its present environment is called an adaptation.
rate of reproduction of an organism in its present environment. Scientists describe groups of organisms becoming adapted to their
Divergent evolution describes the process in which two species environment when a change in the range of genetic variation occurs
evolve in diverse directions from a common point. over time that increases or maintains the “fitness” of the population
Convergent evolution is the process in which similar traits evolve to its environment. The webbed feet of platypuses are an adaptation
independently in species that do not share a recent common for swimming. The snow leopards’ thick fur is an adaptation for
ancestry. living in the cold. The cheetahs’ fast speed is an adaptation for
catching prey.
KEY TERMS
Whether or not a trait is favorable depends on the environmental
adaptation: modification of something or its parts that makes it
conditions at the time. The same traits are not always selected
more fit for existence under the conditions of its current
because environmental conditions can change. For example,
environment
consider a species of plant that grew in a moist climate and did not
divergent evolution: the process by which a species with similar
need to conserve water. Large leaves were selected because they
traits become groups that are tremendously different from each
allowed the plant to obtain more energy from the sun. Large leaves
other over many generations
require more water to maintain than small leaves, and the moist
convergent evolution: a trait of evolution in which species not
environment provided favorable conditions to support large leaves.
of similar recent origin acquire similar properties due to natural
After thousands of years, the climate changed and the area no longer
selection
had excess water. The direction of natural selection shifted so that
PROCESSES AND PATTERNS OF EVOLUTION plants with small leaves were selected because those populations
were able to conserve water to survive the new environmental
VARIATION conditions.
Natural selection can only take place if there is variation, or The evolution of species has resulted in enormous variation in form
differences, among individuals in a population. Importantly, these and function. Sometimes, evolution gives rise to groups of
differences must have some genetic basis; otherwise, the selection organisms that become tremendously different from each other.
will not lead to change in the next generation. This is critical When two species evolve in diverse directions from a common
because variation among individuals can be caused by non-genetic point, it is called divergent evolution. Such divergent evolution can
reasons, such as an individual being taller due to better nutrition be seen in the forms of the reproductive organs of flowering plants
rather than different genes. which share the same basic anatomies; however, they can look very
Genetic diversity within a population comes from two main different as a result of selection in different physical environments
mechanisms: mutation and sexual reproduction. Mutation, a change and adaptation to different kinds of pollinators.
in the DNA sequence, is the ultimate source of new alleles, or new
genetic variation in any population. The genetic changes caused by
mutation can have one of three outcomes:
Many mutations will have no effect on the fitness of the
phenotype; these are called neutral mutations.
A mutation may affect the phenotype of the organism in a way
that gives it reduced fitness (a lower likelihood of survival or
fewer offspring).

18.1D.1 https://bio.libretexts.org/@go/page/13416
 adaptations to flight. However, the wings of bats and insects have
evolved from very different original structures. This phenomenon is
called convergent evolution, where similar traits evolve
independently in species that do not share a recent common ancestry.
The two species came to the same function, flying, but did so
separately from each other.
These physical changes occur over enormous spans of time and help
explain how evolution occurs. Natural selection acts on individual
organisms, which in turn can shape an entire species. Although
natural selection may work in a single generation on an individual, it
Figure 18.1D. 1 : Flowering Plants: Flowering plants evolved from a can take thousands or even millions of years for the genotype of an
common ancestor. Notice that the (a) dense blazing star (Liatrus entire species to evolve. It is over these large time spans that life on
spicata) and the (b) purple coneflower (Echinacea purpurea) vary in
appearance, yet both share a similar basic morphology. earth has changed and continues to change.
In other cases, similar phenotypes evolve independently in distantly- This page titled 18.1D: Processes and Patterns of Evolution is shared under
related species. For example, flight has evolved in both bats and a CC BY-SA 4.0 license and was authored, remixed, and/or curated by
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18.1E: EVIDENCE OF EVOLUTION
Evidence for evolution has been obtained through fossil records, shapes and sizes of these bones in different species, but they have
embryology, geography, and molecular biology. maintained the same overall layout. Scientists call these synonymous
parts homologous structures.
 LEARNING OBJECTIVES

Explain the development of the theory of evolution

KEY POINTS
Fossils serve to highlight the differences and similarities between
current and extinct species, showing the evolution of form over
time.
Similar anatomy across different species highlights their
common origin and can be seen in homologous and vestigial
structures.
Embryology provides evidence for evolution since the
embryonic forms of divergent groups are extremely similar.
The natural distribution of species across different continents
supports evolution; species that evolved before the breakup of Figure 18.1E. 1: Common Ancestors: The similar construction of
the supercontinent are distributed worldwide, whereas species these appendages indicates that these organisms share a common
that evolved more recently are more localized. ancestor.
Molecular biology indicates that the molecular basis for life
evolved very early and has been maintained with little variation
across all life on the planet.

KEY TERMS
homologous structure: the traits of organisms that result from
sharing a common ancestor; such traits often have similar
embryological origins and development
biogeography: the study of the geographical distribution of
Figure 18.1E. 1: Evolution of Humans and Horses: (a) In this
living things display, fossil hominids are arranged from oldest (bottom) to newest
vestigial structure: genetically determined structures or (top). As hominids evolved, the shape of the skull changed. (b) An
attributes that have apparently lost most or all of their ancestral artist’s rendition of extinct species of the genus Equus reveals that
these ancient species resembled the modern horse (Equus ferus), but
function in a given species varied in size.

EVIDENCE OF EVOLUTION Some structures exist in organisms that have no apparent function at
all, appearing to be residual parts from a common ancestor. These
The evidence for evolution is compelling and extensive. Looking at
unused structures (such as wings on flightless birds, leaves on some
every level of organization in living systems, biologists see the
cacti, and hind leg bones in whales) are vestigial.
signature of past and present evolution. Darwin dedicated a large
Embryology, the study of the development of the anatomy of an
portion of his book, On the Origin of Species, to identifying patterns
in nature that were consistent with evolution. Since Darwin, our organism to its adult form, provides evidence for evolution as
understanding has become clearer and broader. embryo formation in widely-divergent groups of organisms tends to
be conserved. Structures that are absent in the adults of some groups
FOSSILS, ANATOMY, AND EMBRYOLOGY often appear in their embryonic forms, disappearing by the time the
Fossils provide solid evidence that organisms from the past are not adult or juvenile form is reached. For example, all vertebrate
the same as those found today; they show a progression of evolution. embryos, including humans, exhibit gill slits and tails at some point
Scientists calculate the age of fossils and categorize them to in their early development. These disappear in the adults of
determine when the organisms lived relative to each other. The terrestrial groups, but are maintained in adults of aquatic groups,
resulting fossil record tells the story of the past and shows the such as fish and some amphibians. Great ape embryos, including
evolution of form over millions of years. For example, scientists humans, have a tail structure during their development that is lost by
have recovered highly-detailed records showing the evolution of birth.
humans and horses. The whale flipper shares a similar morphology Another form of evidence of evolution is the convergence of form in
to appendages of birds and mammals, indicating that these species organisms that share similar environments. For example, species of
share a common ancestor. Over time, evolution led to changes in the unrelated animals, such as the arctic fox and ptarmigan living in the

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arctic region, have been selected for seasonal white phenotypes The great diversification of marsupials in Australia and the absence
during winter to blend with the snow and ice. These similarities of other mammals reflect Australia’s long isolation. Australia has an
occur not because of common ancestry, but because of similar abundance of endemic species (those found nowhere else) which is
selection pressures: the benefits of not being seen by predators. typical of islands whose isolation by expanses of water prevents
species from migrating. Over time, these species diverge
evolutionarily into new species that look very different from their
ancestors that may exist on the mainland. The marsupials of
Australia, the finches on the Galápagos, and many species on the
Hawaiian Islands are all unique to their one point of origin, yet they
display distant relationships to ancestral species on mainlands.

MOLECULAR BIOLOGY
Like anatomical structures, the structures of the molecules of life
reflect descent with modification. Evidence of a common ancestor
for all of life is reflected in the universality of DNA as the genetic
material, in the near universality of the genetic code, and in the
machinery of DNA replication and expression. In general, the
relatedness of groups of organisms is reflected in the similarity of
Figure 18.1E. 1: Adaptations: Winter Coats: The white winter coat their DNA sequences. This is exactly the pattern that would be
of the (a) arctic fox and the (b) ptarmigan’s plumage are adaptations
to their environments. expected from descent and diversification from a common ancestor.
DNA sequences have also shed light on some of the mechanisms of
BIOGEOGRAPHY evolution. For example, it is clear that the evolution of new
The geographic distribution of organisms on the planet follows functions for proteins commonly occurs after gene duplications that
patterns that are best explained by evolution in conjunction with the allow the free modification of one copy by mutation, selection, or
movement of tectonic plates over geological time. Broad groups that drift (changes in a population ‘s gene pool resulting from chance),
evolved before the breakup of the supercontinent Pangaea (about while the second copy continues to produce a functional protein.
200 million years ago) are distributed worldwide. Groups that
evolved since the breakup appear uniquely in regions of the planet, This page titled 18.1E: Evidence of Evolution is shared under a CC BY-SA
such as the unique flora and fauna of northern continents that formed 4.0 license and was authored, remixed, and/or curated by Boundless.
from the supercontinent Laurasia compared to that of the southern
continents that formed from the supercontinent Gondwana.

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18.1F: MISCONCEPTIONS OF EVOLUTION
There are many misconceptions about evolution, including the When critics of evolution say evolution is “just a theory,” they are
meaning of the word theory, the way populations change, and the implying that there is little evidence supporting it and that it is still
origin of life. in the process of being rigorously tested. This is a mis-
characterization.
 LEARNING OBJECTIVES
INDIVIDUALS EVOLVE
Discuss misconceptions about the theory of evolution Evolution is the change in genetic composition of a population over
time, specifically over generations, resulting from differential
KEY POINTS reproduction of individuals with certain alleles. Individuals do
Attacks on the theory of evolution sometimes take issue with the change over their lifetime, obviously, but this is called development
word “theory”, which in the vernacular means a guess or and involves changes programmed by the set of genes the individual
suggested explanation. In scientific language, “theory” indicates acquired at birth in coordination with the individual’s environment.
a body of thoroughly-tested and verified explanations for a set of When thinking about the evolution of a characteristic, it is probably
observations of the natural world. best to think about the change of the average value of the
Evolution does not take place on an individual level; evolution is characteristic in the population over time. For example, when natural
the average change of a characteristic within an entire selection leads to bill-size change in medium-ground finches in the
population. Galápagos, this does not mean that individual bills on the finches are
Evolution does not explain the origin of life; the theory of changing. If one measures the average bill size among all individuals
evolution instead explains how populations change over time and in the population at one time and then measures the average bill size
how traits are selected in order to increase the fitness of a in that population several years later, this average value of the
population. population will be different as a result of evolution.
Favorable traits do not arise as a result of the environment as
EVOLUTION EXPLAINS THE ORIGIN OF LIFE
these traits are already present; individuals with favorable traits
are more likely to survive and, thus, will have greater fitness than It is a common misunderstanding that evolution includes an
individuals with less desirable traits. explanation of life’s origins. The theory of evolution explains how
Evolution and natural selection are not synonymous. Natural populations change over time. It does not shed light on the
selection is just one mechanism by which evolution occurs. beginnings of life, including the origins of the first cells, which is
how life is defined. The mechanisms of the origin of life on earth are
KEY TERMS a particularly difficult problem because it occurred a very long time
theory: a well-substantiated explanation of some aspect of the ago and, presumably, it occurred just once. However, while
natural world based on knowledge that has been repeatedly evolution does not explain the origin of life, it may have something
confirmed through observation and experimentation to say about some of the processes operating once pre-living entities
acquired certain properties. Once a mechanism of inheritance was in
MISCONCEPTIONS OF EVOLUTION place in the form of a molecule like DNA, either within a cell or pre-
Although the theory of evolution generated controversy when it was cell, these entities would be subject to the principle of natural
first proposed, it was almost universally accepted by biologists selection. More effective reproducers would increase in frequency at
within 20 years of the publication of On the Origin of Species. the expense of inefficient reproducers.
Nevertheless, the theory of evolution is a difficult concept and
misconceptions about it abound.
ORGANISMS EVOLVE ON PURPOSE
Statements such as “organisms evolve in response to a change in an
EVOLUTION IS JUST A THEORY environment” may lead to the misunderstanding that evolution is
Critics of the theory of evolution dismiss its importance by somehow intentional. A changed environment results in some
purposefully confounding the everyday usage of the word “theory” individuals in the population, those with particular phenotypes,
with the way scientists use the word. In science, a “theory” is benefiting and, therefore, producing proportionately more offspring
understood to be a body of thoroughly-tested and verified than other phenotypes. This results in change in the population if the
explanations for a set of observations of the natural world. Scientists characteristics are genetically determined.
have a theory of the atom, a theory of gravity, and the theory of It is important to understand that the variation that natural selection
relativity, each of which describes understood facts about the world. works on is already present in a population and does not arise in
In the same way, the theory of evolution describes facts about the response to an environmental change. For example, applying
living world. A theory in science has also survived significant efforts antibiotics to a population of bacteria will, over time, select a
to discredit it by scientists. In contrast, a “theory” in common population of bacteria that are resistant to antibiotics. The resistance,
vernacular is a word meaning a guess or suggested explanation; this which is caused by a gene, did not arise by mutation because of the
meaning is more akin to the scientific concept of “hypothesis. ” application of the antibiotic. The gene for resistance was already

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present in the gene pool of the bacteria, probably at a low frequency. CONTRIBUTIONS AND ATTRIBUTIONS
The antibiotic, which kills the bacterial cells without the resistance OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
gene, strongly selects individuals that are resistant, since these Located at: http://cnx.org/content/m44561/latest...ol11448/latest. License: CC
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demonstrated that mutations for antibiotic resistance do not arise as Located at: http://cnx.org/content/m44568/latest...ol11448/latest. License: CC
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a result of antibiotics. evolution. Provided by: Wiktionary. Located at:
http://en.wiktionary.org/wiki/evolution. License: CC BY-SA: Attribution-
In a larger sense, evolution is not goal directed. Species do not ShareAlike
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http://en.Wikipedia.org/wiki/inherit...haracteristics. License: CC BY-SA:
with adaptations that maximize their reproduction. The Attribution-ShareAlike
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changing non-directionally. A trait that is fit in one environment at OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
one time may also be fatal at some point in the future. Located at: http://cnx.org/content/m44568/latest...ol11448/latest. License: CC
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Boundless. Provided by: Boundless Learning. Located at:
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Attribution-ShareAlike
The terms “evolution” and “natural selection” are often conflated, as
natural selection. Provided by: Wiktionary. Located at:
the two concepts are closely related. They are not, however, en.wiktionary.org/wiki/natural_selection. License: CC BY-SA: Attribution-
synonymous. Natural selection refers to the process by which ShareAlike
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genetics in a population. Evolution is defined more broadly as any OpenStax CNX. Located at: http://cnx.org/content/m44568/latest/. License:
change in the genetic makeup of a population over time. As CC BY: Attribution
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expounded by Darwin, natural selection is a major driving force of OpenStax CNX. Located at:
evolution, but it is not the only one. http://cnx.org/content/m44568/latest...18_01_02ab.jpg. License: CC BY:
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Genetic drift, for example, is another mechanism by which evolution natural selection. Provided by: Wiktionary. Located at:
may occurs. Genetic drift occurs when allelic frequency is altered en.wiktionary.org/wiki/natural_selection. License: CC BY-SA: Attribution-
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due to random sampling. It is evolution by chance, and the smaller Animal Behavior/Darwin's Finches. Provided by: Wikibooks. Located at:
the population, the more significant the effects on genetic en.wikibooks.org/wiki/Animal_Behavior/Darwin's_Finches. License: CC BY-
SA: Attribution-ShareAlike
distribution due to sampling error. For example, a population OpenStax College, Discovering How Populations Change. December 6, 2013.
bottleneck, which occurs when an event such as a natural disaster Provided by: OpenStax CNX. Located at:
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dramatically reduces the size of a population, can result in the evolution. Provided by: Wiktionary. Located at:
elimination or significant reduction of a trait within a population, en.wiktionary.org/wiki/evolution. License: CC BY-SA: Attribution-ShareAlike
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regardless of how beneficial that trait may be to survival or CNX. Located at: http://cnx.org/content/m44561/latest...18_00_01ab.jpg.
reproduction. Thus evolution can occur without natural selection. License: CC BY: Attribution
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Figure 18.1F . 1 : Misconceptions of Evolution: Lamarckian
adaptation. Provided by: Wiktionary. Located at:
evolution and giraffes: Lamarck’s hypothesis for how evolution en.wiktionary.org/wiki/adaptation. License: CC BY-SA: Attribution-
worked stated that organisms could pass on traits they had acquired ShareAlike
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SECTION OVERVIEW

18.2: FORMATION OF NEW SPECIES

18.2C: SPECIATION
Topic hierarchy
18.2D: ALLOPATRIC SPECIATION
18.2A: THE BIOLOGICAL SPECIES CONCEPT 18.2E: SYMPATRIC SPECIATION
18.2B: REPRODUCTIVE ISOLATION
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18.2A: THE BIOLOGICAL SPECIES CONCEPT
A species is defined as a group of individuals that, in nature, are able
to mate and produce viable, fertile offspring.

 LEARNING OBJECTIVES

Explain the biological species concept

KEY POINTS Figure 18.2A. 1 : Interbreeding in Dogs: Dogs of different breeds

Members of the same species are similar both in their external still have the ability to reproduce. The (a) poodle and (b) cocker
appearance and their internal physiology; the closer the spaniel can reproduce to produce a breed known as (c) the cockapoo.
relationship between two organisms, the more similar they will In other cases, individuals may appear similar although they are not
be in these features. members of the same species. For example, even though bald eagles
Some species can look very dissimilar, such as two very different (Haliaeetus leucocephalus) and African fish eagles (Haliaeetus
breeds of dogs, but can still mate and produce viable offspring, vocifer) are both birds and eagles, each belongs to a separate species
which signifies that they belong to the same species. group. If humans were to artificially intervene and fertilize the egg
Some species may look very similar externally, but can be of a bald eagle with the sperm of an African fish eagle and a chick
dissimilar enough in their genetic makeup that they cannot did hatch, that offspring, called a hybrid (a cross between two
produce viable offspring and are, therefore, different species. species), would probably be infertile: unable to successfully
Mutations can occur in any cell of the body, but if a change does reproduce after it reached maturity. Different species may have
not occur in a sperm or egg cell, it cannot be passed on to the different genes that are active in development; therefore, it may not
organism’s offspring. be possible to develop a viable offspring with two different sets of
directions. Thus, even though hybridization may take place, the two
KEY TERMS species still remain separate.
species: a group of organsms that, in nature, are capable of
mating and producing viable, fertile offspring
hybrid: offspring resulting from cross-breeding different entities,
e.g. two different species or two purebred parent strains
gene pool: the complete set of unique alleles that would be found
by inspecting the genetic material of every living member of a
species or population

SPECIES AND THE ABILITY TO REPRODUCE

Figure 18.2A. 1 : Species Similarity & Reproduction: Species that
A species is a group of individual organisms that interbreed and appear similar may not be able to reproduce. The (a) African fish
produce fertile, viable offspring. According to this definition, one eagle is similar in appearance to the (b) bald eagle, but the two birds
species is distinguished from another when, in nature, it is not are members of different species.
possible for matings between individuals from each species to Populations of species share a gene pool: a collection of all the
produce fertile offspring. variants of genes in the species. Again, the basis to any changes in a
group or population of organisms must be genetic for this is the only
Members of the same species share both external and internal
way to share and pass on traits. When variations occur within a
characteristics which develop from their DNA. The closer
species, they can only be passed to the next generation along two
relationship two organisms share, the more DNA they have in
main pathways: asexual reproduction or sexual reproduction. The
common, just like people and their families. People’s DNA is likely
change will be passed on asexually simply if the reproducing cell
to be more like their father or mother’s DNA than their cousin’s or
possesses the changed trait. For the changed trait to be passed on by
grandparent’s DNA. Organisms of the same species have the highest
sexual reproduction, a gamete, such as a sperm or egg cell, must
level of DNA alignment and, therefore, share characteristics and
possess the changed trait. In other words, sexually-reproducing
behaviors that lead to successful reproduction.
organisms can experience several genetic changes in their body
Species’ appearance can be misleading in suggesting an ability or cells, but if these changes do not occur in a sperm or egg cell, the
inability to mate. For example, even though domestic dogs (Canis
changed trait will never reach the next generation. Only heritable
lupus familiaris) display phenotypic differences, such as size, build, traits can evolve. Therefore, reproduction plays a paramount role for
and coat, most dogs can interbreed and produce viable puppies that
genetic change to take root in a population or species. In short,
can mature and sexually reproduce. organisms must be able to reproduce with each other to pass new
traits to offspring.

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18.2B: REPRODUCTIVE ISOLATION
Reproductive isolation, through mechanical, behavioral, and REPRODUCTIVE ISOLATION
physiological barriers, is an important component of speciation. Given enough time, the genetic and phenotypic divergence between
populations will affect characters that influence reproduction: if
 LEARNING OBJECTIVES individuals of the two populations were to be brought together,
mating would be improbable, but if mating did occur, offspring
Explain how reproductive isolation can result in speciation
would be non-viable or infertile. Many types of diverging characters
may affect reproductive isolation, the ability to interbreed, of the
KEY POINTS two populations. Reproductive isolation is a collection of
Reproductive isolation can be either prezygotic (barriers that mechanisms, behaviors, and physiological processes that prevent the
prevent fertilization ) or postzygotic (barriers that occur after members of two different species that cross or mate from producing
zygote formation such as organisms that die as embryos or those offspring, or which ensure that any offspring that may be produced
that are born sterile). is not fertile.
Some species may be prevented from mating with each other by Scientists classify reproductive isolation in two groups: prezygotic
the incompatibility of their anatomical mating structures, or a barriers and postzygotic barriers. Recall that a zygote is a fertilized
resulting offspring may be prevented by the incompatibility of egg: the first cell of the development of an organism that reproduces
their gametes. sexually. Therefore, a prezygotic barrier is a mechanism that blocks
Postzygotic barriers include the creation of hybrid individuals reproduction from taking place; this includes barriers that prevent
that do not survive past the embryonic stages ( hybrid inviability fertilization when organisms attempt reproduction. A postzygotic
) or the creation of a hybrid that is sterile and unable to produce barrier occurs after zygote formation; this includes organisms that
offspring ( hybrid sterility ). don’t survive the embryonic stage and those that are born sterile.
Temporal isolation can result in species that are physically
Some types of prezygotic barriers prevent reproduction entirely.
similar and may even live in the same habitat, but if their
Many organisms only reproduce at certain times of the year, often
breeding schedules do not overlap then interbreeding will never
just annually. Differences in breeding schedules, called temporal
occur.
isolation, can act as a form of reproductive isolation. For example,
Behavioral isolation, in which the behaviors involved in mating
two species of frogs inhabit the same area, but one reproduces from
are so unique as to prevent mating, is a prezygotic barrier that
January to March, whereas the other reproduces from March to May.
can cause two otherwise-compatible species to be uninterested in
mating with each other.
Behavioral isolation, in which the behaviors involved in mating
are so unique as to prevent mating, is a prezygotic barrier that
can cause two otherwise compatible species to be uninterested in
mating with each other.

KEY TERMS
reproductive isolation: a collection of mechanisms, behaviors,
and physiological processes that prevent two different species
Figure 18.2B. 1: Temporal isolation: These two related frog species
that mate from producing offspring, or which ensure that any exhibit temporal reproductive isolation. (a) Rana aurora breeds
offspring produced is not fertile earlier in the year than (b) Rana boylii.
temporal isolation: factors that prevent potentially fertile In some cases, populations of a species move to a new habitat and
individuals from meeting that reproductively isolate the members take up residence in a place that no longer overlaps with other
of distinct species populations of the same species; this is called habitat isolation.
behavioral isolation: the presence or absence of a specific Reproduction with the parent species ceases and a new group exists
behavior that prevents reproduction between two species from that is now reproductively and genetically independent. For
taking place example, a cricket population that was divided after a flood could no
prezygotic barrier: a mechanism that blocks reproduction from longer interact with each other. Over time, the forces of natural
taking place by preventing fertilization selection, mutation, and genetic drift will likely result in the
postzygotic barrier: a mechanism that blocks reproduction after divergence of the two groups.
fertilization and zygote formation
hybrid inviability: a situation in which a mating between two
individuals creates a hybrid that does not survive past the
embryonic stages
hybrid sterility: a situation in which a mating between two
individuals creates a hybrid that is sterile

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 In plants, certain structures aimed to attract one type of pollinator
simultaneously prevent a different pollinator from accessing the
pollen. The tunnel through which an animal must access nectar can
vary in length and diameter, which prevents the plant from being
cross-pollinated with a different species.

Figure 18.2B. 1: Habitat isolation: Speciation can occur when two

populations occupy different habitats. The habitats need not be far
apart. The cricket (a) Gryllus pennsylvanicus prefers sandy soil,
while the cricket (b) Gryllus firmus prefers loamy soil. The two
species can live in close proximity, but because of their different soil
preferences, they became genetically isolated.
Figure 18.2B. 1: Reproductive isolation in plants: Some flowers
Behavioral isolation occurs when the presence or absence of a have evolved to attract certain pollinators. The (a) wide foxglove
specific behavior prevents reproduction from taking place. For flower is adapted for pollination by bees, while the (b) long, tube-
example, male fireflies use specific light patterns to attract females. shaped trumpet creeper flower is adapted for pollination by
humming birds.
Various species display their lights differently; if a male of one
species tried to attract the female of another, she would not When fertilization takes place and a zygote forms, postzygotic
recognize the light pattern and would not mate with the male. barriers can prevent reproduction. Hybrid individuals in many cases
cannot form normally in the womb and simply do not survive past
Other prezygotic barriers work when differences in their gamete
the embryonic stages; this is called hybrid inviability. In another
cells prevent fertilization from taking place; this is called a gametic
postzygotic situation, reproduction leads to the birth and growth of a
barrier. Similarly, in some cases, closely-related organisms try to
hybrid that is sterile and unable to reproduce offspring of their own;
mate, but their reproductive structures simply do not fit together. For
this is called hybrid sterility.
example, damselfly males of different species have differently-
shaped reproductive organs. If one species tries to mate with the This page titled 18.2B: Reproductive Isolation is shared under a CC BY-SA
female of another, their body parts simply do not fit together.. 4.0 license and was authored, remixed, and/or curated by Boundless.

Figure 18.2B. 1: Differences in reproductive structures in male

damselflies: The shape of the male reproductive organ varies among
male damselfly species and is only compatible with the female of
that species. Reproductive organ incompatibility keeps the species
reproductively isolated.

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18.2C: SPECIATION
Speciation is an event in which a single species may branch to form Given the extraordinary diversity of life on the planet, there must be
two or more new species. mechanisms for speciation: the formation of two species from one
original species. Darwin envisioned this process as a branching
 LEARNING OBJECTIVES event and diagrammed the process in the only illustration found in
On the Origin of Species, which bears some resemblance to the more
Define speciation and discuss the ways in which it may modern phylogenetic diagram of elephant evolution. The diagram
occur shows that as one species changes over time, it branches repeatedly
to form more than one new species as long as the population
KEY POINTS survives or until the organism becomes extinct.
For the majority of species, the definition of a species is a group
of animals that can potentially interbreed, although some
different species are capable of producing hybrid offspring.
Darwin was the first to envision speciation as the branching of
two or more new species from one ancestral species; indicated by
a diagram he made that bears a striking resemblance to modern-
day phylogenetic diagrams.
For a new species to be formed from an old species, certain
events or changes must occur such that the new population is no
longer capable of interbreeding with the old one.
Figure 18.2C. 1 : The Evolution of Species: The only illustration in
Speciation can occur either through allopatric speciation, when a Darwin’s On the Origin of Species is (a) a diagram showing
population is geographically separated from one another, or speciation events leading to biological diversity. The diagram shows
through sympatric speciation, in which the two new species are similarities to phylogenetic charts that are drawn today to illustrate
the relationships of species. (b) Modern elephants evolved from the
not geographically separated. Palaeomastodon, a species that lived in Egypt 35–50 million years
Speciation, the formation of two species from one original ago.
species, occurs as one species changes over time and branches to For speciation to occur, two new populations must be formed from
form more than one new species. one original population; they must evolve in such a way that it
becomes impossible for individuals from the two new populations to
KEY TERMS interbreed. Biologists have proposed mechanisms by which this
sympatric: living in the same territory without interbreeding could occur that fall into two broad categories: allopatric speciation
allopatric: not living in the same territory; geographically and sympatric speciation. Allopatric speciation (allo- = “other”; -
isolated and thus unable to crossbreed patric = “homeland”) involves geographic separation of populations
speciation: the process by which new distinct species evolve from a parent species and subsequent evolution. Sympatric
speciation (sym- = “same”; -patric = “homeland”) involves
SPECIATION
speciation occurring within a parent species remaining in one
The biological definition of species, which works for sexually- location.
reproducing organisms, is a group of actually or potentially
Biologists think of speciation events as the splitting of one ancestral
interbreeding individuals. There are exceptions to this rule. Many
species into two descendant species. There is no reason why there
species are similar enough that hybrid offspring are possible and
might not be more than two species formed at one time except that it
may often occur in nature, but for the majority of species this rule
generally holds. In fact, the presence in nature of hybrids between is less likely; multiple events can be conceptualized as single splits
similar species suggests that they may have descended from a single occurring close in time.
interbreeding species: the speciation process may not yet be This page titled 18.2C: Speciation is shared under a CC BY-SA 4.0 license
completed. and was authored, remixed, and/or curated by Boundless.

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18.2D: ALLOPATRIC SPECIATION
Allopatric speciation occurs when a single species becomes Typically, environmental conditions, such as climate, resources,
geographically separated; each group evolves new and distinctive predators, and competitors for the two populations will differ
traits. causing natural selection to favor divergent adaptations in each
group.
 LEARNING OBJECTIVES Isolation of populations leading to allopatric speciation can occur in
a variety of ways: a river forming a new branch, erosion forming a
Give examples of allopatric speciation
new valley, a group of organisms traveling to a new location without
the ability to return, or seeds floating over the ocean to an island.
KEY POINTS The nature of the geographic separation necessary to isolate
When a population is geographically continuous, the allele populations depends entirely on the biology of the organism and its
frequencies among its members are similar; however, when a potential for dispersal. If two flying insect populations took up
population becomes separated, the allele frequencies between the residence in separate nearby valleys, chances are individuals from
two groups can begin to vary. each population would fly back and forth, continuing gene flow.
If the separation between groups continues for a long period of However, if two rodent populations became divided by the
time, the differences between their alleles can become more and formation of a new lake, continued gene flow would be improbable;
more pronounced due to differences in climate, predation, food therefore, speciation would be probably occur.
sources, and other factors, eventually leading to the formation of Biologists group allopatric processes into two categories: dispersal
a new species. and vicariance. Dispersal occurs when a few members of a species
Geographic separation between populations can occur in many move to a new geographical area, while vicariance occurs when a
ways; the severity of the separation depends on the travel natural situation arises to physically divide organisms.
capabilities of the species.
Scientists have documented numerous cases of allopatric speciation.
Allopatric speciation events can occur either by dispersal, when a
For example, along the west coast of the United States, two separate
few members of a species move to a new geographical area, or
sub-species of spotted owls exist. The northern spotted owl has
by vicariance, when a natural situation, such as the formation of
genetic and phenotypic differences from its close relative, the
a river or valley, physically divide organisms.
Mexican spotted owl, which lives in the south.
When a population disperses throughout an area, into new,
different and often isolated habitats, multiple speciation events
can occur in which the single original species gives rise to many
new species; this phenomenon is called adaptive radiation.

KEY TERMS
vicariance: the separation of a group of organisms by a
geographic barrier, resulting in differentiation of the original
group into new varieties or species
adaptive radiation: the diversification of species into separate
forms that each adapt to occupy a specific environmental niche
dispersal: the movement of a few members of a species to a new
geographical area, resulting in differentiation of the original
group into new varieties or species

ALLOPATRIC SPECIATION
A geographically-continuous population has a gene pool that is
relatively homogeneous. Gene flow, the movement of alleles across
the range of the species, is relatively free because individuals can Figure 18.2D. 1 : Allopatric speciation due to geographic separation:
The northern spotted owl and the Mexican spotted owl inhabit
move and then mate with individuals in their new location. Thus, the geographically separate locations with different climates and
frequency of an allele at one end of a distribution will be similar to ecosystems. The owl is an example of allopatric speciation.
the frequency of the allele at the other end. When populations Additionally, scientists have found that the further the distance
become geographically discontinuous, that free-flow of alleles is between two groups that once were the same species, the more
prevented. When that separation continues for a period of time, the probable it is that speciation will occur. This seems logical because
two populations are able to evolve along different trajectories. This as the distance increases, the various environmental factors would
is known as allopatric speciation. Thus, their allele frequencies at generally have less in common than locations in close proximity.
numerous genetic loci gradually become more and more different as Consider the two owls: in the north, the climate is cooler than in the
new alleles independently arise by mutation in each population. south causing the types of organisms in each ecosystem differ, as do

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their behaviors and habits. Also, the hunting habits and prey choices
of the southern owls vary from the northern owls. These variances
can lead to evolved differences in the owls, resulting in speciation.

ADAPTIVE RADIATION
In some cases, a population of one species disperses throughout an
area with each finding a distinct niche or isolated habitat. Over time,
the varied demands of their new lifestyles lead to multiple speciation
events originating from a single species. This is called adaptive
radiation because many adaptations evolve from a single point of
origin, causing the species to radiate into several new ones. Island
archipelagos like the Hawaiian Islands provide an ideal context for
adaptive radiation events because water surrounds each island which
leads to geographical isolation for many organisms. The Hawaiian
honeycreeper illustrates one example of adaptive radiation. From a
single species, called the founder species, numerous species have
evolved.

Figure 18.2D. 1 : Adaptive Radiation: The honeycreeper birds

illustrate adaptive radiation. From one original species of bird,
multiple others evolved, each with its own distinctive characteristics.
In Hawaiian honeycreepers, the response to natural selection based
on specific food sources in each new habitat led to the evolution of a
different beak suited to the specific food source. The seed-eating
birds have a thicker, stronger beak which is suited to break hard
nuts. The nectar-eating birds have long beaks to dip into flowers to
reach the nectar. The insect-eating birds have beaks like swords,
appropriate for stabbing and impaling insects.

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18.2E: SYMPATRIC SPECIATION
Sympatric speciation occurs when two individual populations
diverge from an ancestral species without being separated
geographically.

 LEARNING OBJECTIVES

Give examples of sympatric speciation

KEY POINTS
Sympatric speciation can occur when one individual develops an
abnormal number of chromosomes, either extra chromosomes (
polyploidy ) or fewer, such that viable interbreeding can no
longer occur.
When the extra sets of chromosomes in a polyploid originate
with the individual because their own gametes do not undergo Figure 18.2E. 1: Aneuploidy of chromosomes: Aneuploidy results
cytokinesis after meiosis, the result is autopolyploidy. when the gametes have too many or too few chromosomes due to
When individuals of two different species reproduce to form a nondisjunction during meiosis. In the example shown here, the
resulting offspring will have 2n+1 or 2n-1 chromosomes
viable offspring, such that the extra chromosomes come from
Polyploidy is a condition in which a cell or organism has an extra
two different species, the result is an allopolyploid.
set, or sets, of chromosomes. Scientists have identified two main
Once a species develops an abnormal number of chromosomes, it
types of polyploidy that can lead to reproductive isolation, or the
can then only interbreed with members of the population that
inability to interbreed with normal individuals, of an individual in
have the same abnormal number, which can lead to the
the polyploidy state. In some cases, a polyploid individual will have
development of a new species.
two or more complete sets of chromosomes from its own species in
KEY TERMS a condition called autopolyploidy. The prefix “auto-” means “self,”
so the term means multiple chromosomes from one’s own species.
sympatric speciation: the process through which new species
Polyploidy results from an error in meiosis in which all of the
evolve from a single ancestral species while inhabiting the same
chromosomes move into one cell instead of separating.
geographic region
autopolyploid: having more than two sets of chromosomes,
derived from the same species, as a result of redoubling
allopolyploid: having multiple complete sets of chromosomes
derived from different species

SYMPATRIC SPECIATION
Can divergence occur if no physical barriers are in place to separate
individuals who continue to live and reproduce in the same habitat?
The answer is yes. The process of speciation within the same space
is called sympatric speciation. The prefix “sym” means same, so
Figure 18.2E. 1: The generation of autopolyploidy: Autopolyploidy
“sympatric” means “same homeland” in contrast to “allopatric” results when meiosis is not followed by cytokinesis.
meaning “other homeland.” A number of mechanisms for sympatric
For example, if a plant species with 2n = 6 produces autopolyploid
speciation have been proposed and studied.
gametes that are also diploid (2n = 6, when they should be n = 3),
One form of sympatric speciation can begin with a serious the gametes now have twice as many chromosomes as they should
chromosomal error during cell division. In a normal cell division have. These new gametes will be incompatible with the normal
event, chromosomes replicate, pair up, and then separate so that each gametes produced by this plant species. However, they could either
new cell has the same number of chromosomes. However, self-pollinate or reproduce with other autopolyploid plants with
sometimes the pairs separate and the end cell product has too many gametes having the same diploid number. In this way, sympatric
or too few individual chromosomes in a condition called aneuploidy. speciation can occur quickly by forming offspring with 4n: a
tetraploid. These individuals would immediately be able to
reproduce only with those of this new kind and not those of the
ancestral species.
The other form of polyploidy occurs when individuals of two
different species reproduce to form a viable offspring called an
allopolyploid. The prefix “allo-” means “other” (recall from

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SECTION OVERVIEW

18.3: HYBRID ZONES AND RATES OF SPECIATION

18.3B: VARYING RATES OF SPECIATION
Topic hierarchy
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18.3A: HYBRID ZONES
Over time, two species may further diverge or reconnect, depending
on the fitness strength and the reproductive barriers of the hybrids.

 LEARNING OBJECTIVES

Discuss how the fitness of a hybrid will lead to changes in

the hybrid zone over time

KEY POINTS
After speciation, or sufficient evolutionary change for one
species to become two distinct species, the two species may
continue to co-habitate and interact.
Figure 18.3A. 1 : Speciation and the Hybrid Zone: After speciation
The area in which two closely-related species interact and has occurred, the two separate-but-closely-related species may
reproduce is known as the hybrid zone; their offspring are known continue to produce offspring in an area called the hybrid zone.
Reinforcement, fusion, or stability may result, depending on
as hybrids. reproductive barriers and the relative fitness of the hybrids.
Depending on the fitness of the hybrid offspring relative to the
Hybrids can have less fitness, more fitness, or about the same fitness
parents, the two species may either stay as two distinct species
level as the purebred parents. Usually, hybrids tend to be less fit;
(reinforcement), or become one species again ( reconnection ).
therefore, reproduction to produce hybrids will diminish over time,
KEY TERMS which nudges the two species to diverge further in a process called
reinforcement. This term is used because the low success of the
hybrid zone: an area where the ranges of two interbreeding
hybrids reinforces the original speciation. If the hybrids are less fit
species meet and interbreed
than the parents, reinforcement of speciation occurs, and the species
hybrid speciation: the formation of a new species as the direct
will continue to diverge until they can no longer mate and produce
result of mating between members of two existing species
viable offspring.
reconnection: a convergence of two species over time
If the hybrids are as fit or more fit than the parents, or the
RECONNECTION AFTER SPECIATION reproductive barriers weaken, the two species may fuse back into
Speciation occurs over a span of evolutionary time. When a new one species (reconnection). For a hybrid form to persist, it will
species arises, there is a transition period during which the closely- generally have to be able to exploit the available resources better
related species continue to interact. than either parent species, with which, in most cases, it will have to
After speciation, two species may recombine or even continue compete.
interacting indefinitely. Individual organisms will mate with any Over time, via a process called hybrid speciation, the hybrids
nearby individual with which they are capable of breeding. An area themselves can become a separate species. Reproductive isolation
where two closely-related species continue to interact and reproduce, between hybrids and their parents was once thought to be
forming hybrids, is called a hybrid zone. Over time, the hybrid zone particularly difficult to achieve; thus, hybrid species were thought to
may change depending on the fitness strength and the reproductive be extremely rare. With DNA analysis becoming more accessible in
barriers of the hybrids. the 1990s, hybrid speciation has been shown to be a fairly common
phenomenon, particularly in plants.
Scientists have also observed that sometimes two species will
remain separate, but continue to interact to produce some hybrid
individuals; this is classified as stability because no real net change
is taking place. For a hybrid zone to be stable, the offspring
produced by the hybrids have to be less fit than members of the
parent species.

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18.3B: VARYING RATES OF SPECIATION
Two patterns are currently observed in the rates of speciation:
gradual speciation and punctuated equilibrium.

 LEARNING OBJECTIVES

Explain how the interaction of an organism’s population size

in association with environmental changes can lead to
different rates of speciation

KEY POINTS
In the gradual speciation model, species diverge slowly over time
in small steps while in the punctuated equilibrium model, a new
species diverges rapidly from the parent species.
The two key influencing factors on the change in speciation rate
are the environmental conditions and the population size.
Gradual speciation is most likely to occur in large populations Figure 18.3B. 1: Graduated Speciation vs Punctuated Equilibrium:
In (a) gradual speciation, species diverge at a slow, steady pace as
that live in a stable environment, while the punctuation traits change incrementally. In (b) punctuated equilibrium, species
equilibrium model is more likely to occur in a small population diverge quickly and then remain unchanged for long periods of time.
with rapid environmental change. The primary influencing factor on changes in speciation rate is
environmental conditions. Under some conditions, selection occurs
KEY TERMS quickly or radically. Consider a species of snails that had been living
punctuated equilibrium: a theory of evolution holding that with the same basic form for many thousands of years. Layers of
evolutionary change tends to be characterized by long periods of their fossils would appear similar for a long time. When a change in
stability, with infrequent episodes of very fast development the environment takes place, such as a drop in the water level, a
gradualism: in evolutionary biology, belief that evolution small number of organisms are separated from the rest in a brief
proceeds at a steady pace, without the sudden development of period of time, essentially forming one large and one tiny
new species or biological features from one generation to the population. The tiny population faces new environmental conditions.
next Because its gene pool quickly became so small, any variation that
surfaces and that aids in surviving the new conditions becomes the
VARYING RATES OF SPECIATION
predominant form.
Scientists around the world study speciation, documenting
observations both of living organisms and those found in the fossil CONTRIBUTIONS AND ATTRIBUTIONS
record. As their ideas take shape and as research reveals new details
about how life evolves, they develop models to help explain rates of CONTRIBUTIONS AND ATTRIBUTIONS
speciation. In terms of how quickly speciation occurs, two patterns OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
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SECTION OVERVIEW

18.4: EVOLUTION OF GENOMES

18.4D: GENE DUPLICATIONS AND DIVERGENCE
Topic hierarchy
18.4E: NONCODING DNA
18.4A: GENOMIC SIMILIARITIES BETWEEN 18.4F: VARIATIONS IN SIZE AND NUMBER OF
DISTANT SPECIES GENES
18.4B: GENOME EVOLUTION
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18.4A: GENOMIC SIMILIARITIES BETWEEN DISTANT SPECIES
Genomic similarities between distant species can be established via establish phylogenetic trees, which map the relationship between
analysis of genomes using advanced technology. species at a genetic and molecular level. The ability to use these
technologies has established previously unknown relationships and
 LEARNING OBJECTIVES has contributed to a more complex evolutionary history. These
technologies have established genomic similarities between distant
Discuss the evolutionary implications of observed genome species by establishing genetic distances. In addition, the
similarities between distant species mechanisms by which genomic similarities between distant species
occur can include horizontal gene transfer.
KEY POINTS
Gram-positives
Genomic similarities between distant species can be explained by Animals Fungi
Chlamydiae
the theory that all organisms share a common ancestor. Slime molds
Green nonsulfur bacteria
Plants
Genomic similarities between distant species can be analysed Actinobacteria
Algae
using genomic analysis tools to create phylogenetic trees that Planctomycetes
explain these relationships. Spirochaetes
Protozoa
Genetic distance is used to explain the genetic divergence
between species or between populations within a species and can Fusobacteria
Crenarchaeota
indicate how closely related they are and whether they have a Nanoarchaeota
Cyanobacteria
(blue-green algae)
recent common ancestor or recent interbreeding has taken place.
Euryarchaeota Thermophilic
Horizontal gene transfer (HGT) occurs when two unrelated sulfate-reducers
species exchange genes, usually two prokaryotes, although HGT Acidobacteria
occurs in some eurokaryotes as well. Protoeobacteria

KEY TERMS Figure 18.4A. 1 : Tree of Life: Diagrammatic representation of the

divergence of modern taxonomic groups from their common
conjugation: the temporary fusion of organisms, especially as
ancestor. This shows the genomic similarities that can exist between
part of sexual reproduction distant species based on their relationship with this ancestor.
phylogeny: the evolutionary history of an organism
horizontal gene transfer: the transfer of genetic material from HORIZONTAL GENE TRANSFER
one organism to another one that is not its offspring; especially Horizontal gene transfer (HGT), also known as lateral gene transfer,
common among bacteria is the transfer of genes between unrelated species. Genes have been
transformation: the alteration of a bacterial cell caused by the shown to be passed between species which are only distantly related
transfer of DNA from another, especially if pathogenic using standard phylogeny, thus adding a layer of complexity to the
transduction: horizontal gene transfer mechanism in understanding of phylogenetic relationships. The various ways that
prokaryotes where genes are transferred using a virus HGT occurs in prokaryotes is important to understanding
phylogenies. Although at present, HGT is not viewed as important to
GENOMIC SIMILARITIES BETWEEN DISTANT eukaryotic evolution, HGT does occur in this domain as well.
SPECIES Finally, as an example of the ultimate gene transfer, theories of
Genetic distance refers to the genetic divergence between species or genome fusion between symbiotic or endosymbiotic organisms have
between populations within a species. Smaller genetic distances been proposed to explain an event of great importance—the
indicate that the populations have more similar genes, which evolution of the first eukaryotic cell, without which humans could
indicates they are closely related; they have a recent common not have come into existence.
ancestor, or recent interbreeding has taken place. Genetic distance is
The mechanism of HGT has been shown to be quite common in the
useful in reconstructing the history of populations. For example, prokaryotic domains of Bacteria and Archaea, significantly changing
evidence from genetic distance suggests that humans arrived in the way their evolution is viewed. The majority of evolutionary
America about 30,000 years ago. By examining the difference models, such as in the endosymbiont theory, propose that eukaryotes
between allele frequencies between the populations, genetic distance
descended from multiple prokaryotes, which makes HGT all the
can estimate how long ago the two populations were together. more important to understanding the phylogenetic relationships of
all extant and extinct species. These gene transfers between species
PHYLOGENETIC RELATIONSHIPS
are the major mechanism whereby bacteria acquire resistance to
Phylogeny describes the relationships of an organism, such as the antibiotics. Classically, this type of transfer has been thought to
relationship with its ancestors and the species it is most closely occur by three different mechanisms: transformation, transduction
related. Phylogenetic relationships provide information on shared and conjugation.
ancestry but not necessarily on how organisms are similar or
Although it is easy to see how prokaryotes exchange genetic
different. The use of advanced genomic analysis has allowed us to
material by HGT, it was initially thought that this process was absent

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in eukaryotes, followed by the idea that the gene transfers between
multicellular eukaryotes should be more difficult. In spite of this
fact, HGT between distantly related organisms has been
demonstrated in several eukaryotic species.
In animals, a particularly interesting example of HGT occurs within
the aphid species. Aphids are insects that vary in color based on
carotenoid content. Carotenoids are pigments made by a variety of
plants, fungi, and microbes, and they serve a variety of functions in
animals, who obtain these chemicals from their food. Humans Figure 18.4A. 1 : Horizontal Gene Transfer in Animals: (a) Red
aphids get their color from red carotenoid pigment. Genes necessary
require carotenoids to synthesize vitamin A, and we obtain them by to make this pigment are present in certain fungi, and scientists
eating orange fruits and vegetables: carrots, apricots, mangoes, and speculate that aphids acquired these genes through HGT after
sweet potatoes. On the other hand, aphids have acquired the ability consuming fungi for food. If genes for making carotenoids are
inactivated by mutation, the aphids revert back to (b) their green
to make the carotenoids on their own. According to DNA analysis, color. Red coloration makes the aphids a lot more conspicuous to
this ability is due to the transfer of fungal genes into the insect by predators, but evidence suggests that red aphids are more resistant to
HGT, presumably as the insect consumed fungi for food. insecticides than green ones. Thus, red aphids may be more fit to
survive in some environments than green ones.

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18.4B: GENOME EVOLUTION
Processes such as mutations, duplications, exon shuffling, mutation which causes the entire code to be read in the wrong order
transposable elements and pseudogenes have contributed to genomic and thus often results in a protein becoming non-functional. A
evolution. mutation in a promoter region, enhancer region or a region coding
for transcription factors can also result in either a loss of function or
 LEARNING OBJECTIVES and upregulation or downregulation in transcription of that gene.
Mutations are constantly occurring in an organism’s genome and can
Explain the importance of genomic changes in an cause either a negative effect, positive effect or no effect at all.
evolutionary context Single chromosome mutations

Deletion Duplication Inversion

KEY POINTS
Gene and whole genome duplications have contributed
accumulations that have contributed to genome evolution.
Mutations are constantly occurring in an organism’s genome and
can cause either a negative effect, positive effect or no effect at
all; however, it will still result in changes to the genome.
Transposable elements are regions of DNA that can be inserted
into the genetic code and will causes changes within the genome.
Pseudogenes are dysfunctional genes derived from previously
functional gene relatives and will become a pseudogene by
deletion or insertion of one or multiple nucleotides. Insertion
Exon shuffling occurs when two or more exons from different
genes are combined together or when exons are duplicated, and
will result in new genes.
Species can also exhibit genome reduction when subsets of their
genes are not needed anymore.
Chromosome 20

KEY TERMS Chromosome 4

Chromosome 20

intron: a portion of a split gene that is included in pre-RNA

transcripts but is removed during RNA processing and rapidly Chromosome 4

degraded Translocation
exon: a region of a transcribed gene present in the final Derivative
Chromosome 20 chromosome 20
functional RNA molecule
pseudogene: a segment of DNA that is part of the genome of an
organism, and which is similar to a gene but does not code for a
gene product

ACCUMULATING CHANGES OVER TIME

The evolution of the genome is characterized by the accumulation of Derivative chromosome 4
changes. The analaysis of genomes and their changes in sequence or
Chromosome 4
size over time involves various fields. There are various mechanisms
Figure 18.4B. 1: Chromosomal Mutations: Chromosomal mutations
that have contributed to genome evolution and these include gene over time can accumulate and promote diversity and evolution if a
and genome duplications, polyploidy, mutation rates, transposable produced trait is favorable.
elements, pseudogenes, exon shuffling and genomic reduction and
TRANSPOSABLE ELEMENTS
gene loss. The concepts of gene and whole-genome duplication are
discussed as their own independent concepts, thus, the focus will be Transposable elements are regions of DNA that can be inserted into
on other mechanisms. the genetic code through one of two mechanisms. These
mechanisms work similarly to “cut-and-paste” and “copy-and-paste”
MUTATION RATES functionalities in word processing programs. The “cut-and-paste”
Mutation rates differ between species and even between different mechanism works by excising DNA from one place in the genome
regions of the genome of a single species. Spontaneous mutations and inserting itself into another location in the code. The “copy-and-
often occur which can cause various changes in the genome. paste” mechanism works by making a genetic copy or copies of a
Mutations can result in the addition or deletion of one or more specific region of DNA and inserting these copies elsewhere in the
nucleotide bases. A change in the code can result in a frameshift code. The most common transposable element in the human genome

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is the Alu sequence, which is present in the genome over one million genome that can be either selected against and deleted or selectively
times. favored and conserved.

PSEUDOGENES GENOME REDUCTION AND GENE LOSS

Often a result of spontaneous mutation, pseudogenes are Many species exhibit genome reduction when subsets of their genes
dysfunctional genes derived from previously functional gene are not needed anymore. This typically happens when organisms
relatives. There are many mechanisms by which a functional gene adapt to a parasitic life style, e.g. when their nutrients are supplied
can become a pseudogene including the deletion or insertion of one by a host. As a consequence, they lose the genes need to produce
or multiple nucleotides. This can result in a shift of reading frame, these nutrients. In many cases, there are both free living and
causing the gene to longer code for the expected protein, a parasitic species that can be compared and their lost genes identified.
premature stop codon or a mutation in the promoter region. Often Good examples are the genomes of Mycobacterium tuberculosis and
cited examples of pseudogenes within the human genome include Mycobacterium leprae, the latter of which has a dramatically
the once functional olfactory gene families. Over time, many reduced genome. Another beautiful example are endosymbiont
olfactory genes in the human genome became pseudogenes and were species. For instance, Polynucleobacter necessarius was first
no longer able to produce functional proteins, explaining the poor described as a cytoplasmic endosymbiont of the ciliate Euplotes
sense of smell humans possess in comparison to their mammalian aediculatus. The latter species dies soon after being cured of the
relatives. endosymbiont. In the few cases in which P. necessarius is not
present, a different and rarer bacterium apparently supplies the same
EXON SHUFFLING function. No attempt to grow symbiotic P. necessarius outside their
Exon shuffling is a mechanism by which new genes are created. hosts has yet been successful, strongly suggesting that the
This can occur when two or more exons from different genes are relationship is obligate for both partners. Yet, closely related free-
combined together or when exons are duplicated. Exon shuffling living relatives of P. necessarius have been identified. The
results in new genes by altering the current intron-exon structure. endosymbionts have a significantly reduced genome when compared
This can occur by any of the following processes: transposon to their free-living relatives (1.56 Mbp vs. 2.16 Mbp).
mediated shuffling, sexual recombination or illegitimate
recombination. Exon shuffling may introduce new genes into the This page titled 18.4B: Genome Evolution is shared under a CC BY-SA 4.0
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18.4C: WHOLE-GENOME DUPLICATION
Whole-genome duplication is characterized by an organisms entire
genetic information being copied once or multiple times.

 LEARNING OBJECTIVES

State the evolutionary implications of whole-genome

duplication

KEY POINTS
Whole- genome duplication can provide an evolutionary
advantage by providing the organism with multiple copies of a
gene that is considered favorable.
Whole-genome duplication can result in divergence and
formation of new species over time.
Whole-genome duplication can result in mutation and cause
disease if the genes are rendered non-functional.

KEY TERMS
Figure 18.4C. 1 : Polyploidy: This image shows haploid (single),
polyploidy: having more than the usual two homologous sets of diploid (double), triploid (triple), and tetraploid (quadruple) sets of
chromosomes chromosomes. Triploid and tetraploid chromosomes are examples of
polyploidy.
palaeopolyploidization: the development of polyploid
organisms in the geologic past EVOLUTIONARY IMPORTANCE
sympatric speciation: the process through which new species
Paleopolyploidization events lead to massive cellular changes,
evolve from a single ancestral species while inhabiting the same
including doubling of the genetic material, changes in gene
geographic region
expression and increased cell size. Gene loss during diploidization is
WHOLE-GENOME DUPLICATION not completely random, but heavily selected. Genes from large gene
families are duplicated. On the other hand, individual genes are not
Gene duplication is the process by which a region of DNA coding
duplicated. Overall, paleopolyploidy can have both short-term and
for a gene creates additional copies of the gene. Similar to gene
long-term evolutionary effects on an organism’s fitness in the natural
duplication, whole-genome duplication is the process by which an
environment.
organism’s entire genetic information is copied, once or multiple
times, which is known as polyploidy. This may provide an GENOME DIVERSITY
evolutionary benefit to the organism by supplying it with multiple
Genome doubling provides organisms with redundant alleles that
copies of a gene, thus, creating a greater possibility of functional and
can evolve freely with little selection pressure. The duplicated genes
selectively favored genes.
can undergo neofunctionalization or subfunctionalization which
could help the organism adapt to the new environment or survive
different stress conditions.

SPECIATION
Sympatric speciation can begin with a chromosomal error during
meiosis or the formation of a hybrid individual with too many
chromosomes, such as polyploidy which can occur during whole-
genome duplication. Scientists have identified types of polyploidy
that can lead to reproductive isolation of an individual in the
polyploid state. In some cases a polyploid individual will have two
or more complete sets of chromosomes from its own species in a
condition called autopolyploidy. The other form of polyploidy
occurs when individuals of two different species reproduce to form a
viable offspring called an allopolyploid. The prefix “allo” means
“other” (recall from allopatric); therefore, an allopolyploid occurs
when gametes from two different species combine.
It has been suggested that many polyploidization events created new
species, via a gain of adaptive traits, or by sexual incompatibility

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with their diploid counterparts. An example would be the recent chromosomal regions, accounting for over half of the yeast’s
speciation of allopolyploid Spartina — S. anglica; the polyploid genome. They also noted that although homologs were present, they
plant is so successful that it is listed as an invasive species in many were often located on different chromosomes. Based on these
regions. observations, they determined that Saccharomyces cerevisiae
underwent a whole-genome duplication soon after its evolutionary
EVIDENCE OF WHOLE-GENOME DUPLICATION split from Kluyveromyces, a genus of ascomycetous yeasts. Over
In 1997, Wolfe & Shields gave evidence for an ancient duplication time, many of the duplicate genes were deleted and rendered non-
of the Saccharomyces cerevisiae (Yeast) genome. It was initially functional. A number of chromosomal rearrangements broke the
noted that this yeast genome contained many individual gene original duplicate chromosomes into the current manifestation of
duplications. Wolfe & Shields hypothesized that this was actually homologous chromosomal regions.
the result of an entire genome duplication in the yeast’s distant
evolutionary history. They found 32 pairs of homologous This page titled 18.4C: Whole-Genome Duplication is shared under a CC
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18.4D: GENE DUPLICATIONS AND DIVERGENCE
REPLICATION SLIPPAGE
 LEARNING OBJECTIVES Replication slippage is an error in DNA replication, which can
produce duplications of short genetic sequences. During replication,
Explain the mechanisms of gene duplication and divergence
DNA polymerase begins to copy the DNA, and at some point during
the replication process, the polymerase dissociates from the DNA
GENE DUPLICATION
and replication stalls. When the polymerase reattaches to the DNA
Gene duplication is the process by which a region of DNA coding strand, it aligns the replicating strand to an incorrect position and
for a gene is copied. Gene duplication can occur as the result of an incidentally copies the same section more than once. Replication
error in recombination or through a retrotransposition event. slippage is also often facilitated by repetitive sequence but requires
Duplicate genes are often immune to the selective pressure under only a few bases of similarity.
which genes normally exist. This can result in a large number of
mutations accumulating in the duplicate gene code. This may render RETROTRANSPOSITION
the gene non-functional or in some cases confer some benefit to the During cellular invasion by a replicating retroelement or retrovirus,
organism. There are multiple mechanisms by which gene duplication viral proteins copy their genome by reverse transcribing RNA to
can occur. DNA. If viral proteins attach irregularly to cellular mRNA, they can
reverse-transcribe copies of genes to create retrogenes. Retrogenes
ECTOPIC RECOMBINATION
usually lack intronic sequence and often contain poly A sequences
Duplications can arise from unequal crossing-over that occurs that are also integrated into the genome. Many retrogenes display
during meiosis between misaligned homologous chromosomes. The changes in gene regulation in comparison to their parental gene
product of this recombination is a duplication at the site of the sequences, which sometimes results in novel functions.
exchange and a reciprocal deletion. Ectopic recombination is
typically mediated by sequence similarity at the duplicate ANEUPLOIDY
breakpoints, which form direct repeats. Repetitive genetic elements, Aneuploidy occurs when nondisjunction at a single chromosome
such as transposable elements, offer one source of repetitive DNA results in an abnormal number of chromosomes. Aneuploidy is often
that can facilitate recombination, and they are often found at harmful and in mammals regularly leads to spontaneous abortions.
duplication breakpoints in plants and mammals. Some aneuploid individuals are viable. For example, trisomy 21 in
humans leads to Down syndrome, but it is not fatal. Aneuploidy
often alters gene dosage in ways that are detrimental to the organism
and therefore, will not likely spread through populations.

GENE DUPLICATION AS AN EVOLUTIONARY

EVENT
Gene duplications are an essential source of genetic novelty that can
lead to evolutionary innovation. Duplication creates genetic
redundancy and if one copy of a gene experiences a mutation that
affects its original function, the second copy can serve as a ‘spare
part’ and continue to function correctly. Thus, duplicate genes
accumulate mutations faster than a functional single-copy gene, over
generations of organisms, and it is possible for one of the two copies
to develop a new and different function. This is an examples of
neofunctionalization.
Gene duplication is believed to play a major role in evolution; this
stance has been held by members of the scientific community for
over 100 years. It has been argued that gene duplication is the most
important evolutionary force since the emergence of the universal
common ancestor.
Another possible fate for duplicate genes is that both copies are
Figure 18.4D. 1 : Gene Duplication: This figure indicates a equally free to accumulate degenerative mutations, so long as any
schematic of a region of a chromosome before and after a defects are complemented by the other copy. This leads to a neutral
duplication event. Ectopic recombination is typically mediated by
sequence similarity at the duplicate breakpoints, which form direct
“subfunctionalization” model, in which the functionality of the
repeats. original gene is distributed among the two copies. Neither gene can
be lost, as both now perform important non-redundant functions, but
ultimately neither is able to achieve novel functionality.

18.4D.1 https://bio.libretexts.org/@go/page/13432
Subfunctionalization can occur through neutral processes in which such as nucleotide sequences or protein sequences that are derived
mutations accumulate with no detrimental or beneficial effects. from two or more homologous genes. Both orthologous genes
However, in some cases subfunctionalization can occur with clear (resulting from a speciation event) and paralogous genes (resulting
adaptive benefits. If an ancestral gene is pleiotropic and performs from gene duplication within a population) can be said to display
two functions, often times neither one of these two functions can be divergent evolution.
changed without affecting the other function. In this way,
partitioning the ancestral functions into two separate genes can allow KEY POINTS
for adaptive specialization of subfunctions, thereby providing an Ectopic recombination occurs when there is an unequal crossing-
adaptive benefit. over and the product of this recombination are a duplication at
the site of the exchange and a reciprocal deletion.
DIVERGENCE Gene duplications do not always result in detrimental mutations;
Genetic divergence is the process in which two or more populations they can contribute to divergent evolution, which causes genetic
of an ancestral species accumulate independent genetic changes differences between groups to develop and eventually form new
through time, often after the populations have become species.
reproductively isolated for some period of time. In some cases, Replication slippage can occur when there is an error during
subpopulations living in ecologically distinct peripheral DNA replication and duplications of short genetic sequences are
environments can exhibit genetic divergence from the remainder of a produced.
population, especially where the range of a population is very large. Retrotranspositions occur when a retrovirus copies their genome
The genetic differences among divergent populations can involve by reverse transcribing RNA to DNA and aberrantly attach to
silent mutations (that have no effect on the phenotype) or give rise to cellular mRNA and reverse transcribe copies of genes to create
significant morphological and/or physiological changes. Genetic retrogenes.
divergence will always accompany reproductive isolation, either due Aneuploidy can occur when there is a nondisjunction even at a
to novel adaptations via selection and/or due to genetic drift, and is single chromosome thus, the result is an abnormal number of
the principal mechanism underlying speciation. chromosomes.
Genetic drift or allelic drift is the change in the frequency of a gene Genetic divergence can occur by mechanisms such as genetic
variant ( allele ) in a population due to random sampling. The alleles drift which contibute to the accumulation of independent genetic
in the offspring are a sample of those in the parents, and chance has changes of two or more populations derived from a common
a role in determining whether a given individual survives and ancestor.
reproduces. A population’s allele frequency is the fraction of the
copies of one gene that share a particular form. Genetic drift may
KEY TERMS
cause gene variants to disappear completely and thereby reduce paralogous: having a similar structure indicating divergence
genetic variation. When there are few copies of an allele, the effect from a common ancestral gene
of genetic drift is larger, and when there are many copies the effect is nondisjunction: the failure of chromosome pairs to separate
smaller. These changes in gene frequency can contribute to properly during meiosis
divergence. retrogene: a DNA gene copied back from RNA by reverse
transcription
Divergent evolution is usually a result of diffusion of the same
genetic drift: an overall shift of allele distribution in an isolated
species to different and isolated environments, which blocks the
population, due to random fluctuations in the frequencies of
gene flow among the distinct populations allowing differentiated
individual alleles of the genes
fixation of characteristics through genetic drift and natural
selection.Divergent evolution can also be applied to molecular This page titled 18.4D: Gene Duplications and Divergence is shared under a
biology characteristics. This could apply to a pathway in two or CC BY-SA 4.0 license and was authored, remixed, and/or curated by
more organisms or cell types. This can apply to genes and proteins, Boundless.

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18.4E: NONCODING DNA
Noncoding DNA are sequences of DNA that do not encode protein GENOMIC VARIATION BETWEEN ORGANISMS
sequences but can be transcribed to produce important regulatory The amount of total genomic DNA varies widely between
molecules. organisms, and the proportion of coding and noncoding DNA within
these genomes varies greatly as well. More than 98% of the human
 LEARNING OBJECTIVES genome does not encode protein sequences, including most
sequences within introns and most intergenic DNA. While overall
Summarize the importance of noncoding DNA
genome size, and by extension the amount of noncoding DNA, are
correlated to organism complexity, there are many exceptions. For
KEY POINTS example, the genome of the unicellular Polychaos dubium (formerly
In the human genome, over 98% of DNA is classified as known as Amoeba dubia) has been reported to contain more than
noncoding DNA and can be transcribed to regulatory noncoding 200 times the amount of DNA in humans. The pufferfish Takifugu
RNAs (i.e. tRNAs, rRNAs), origins of DNA replication, rubripes genome is only about one eighth the size of the human
centromeres, telomeres and scaffold attachment regions (SARs). genome, yet seems to have a comparable number of genes;
Noncoding regions are most commonly referred to as ‘junk approximately 90% of the Takifugu genome is noncoding DNA.
DNA’, however, this term is misleading as noncoding DNA does In 2013, a new “record” for most efficient genome was discovered.
have functional importance. Utricularia gibba, a bladderwort plant, has only 3% noncoding
The proportion of coding and noncoding DNA within organisms DNA. The extensive variation in nuclear genome size among
varies and the amount of noncoding DNA typically correlates eukaryotic species is known as the C-value enigma or C-value
with organism complexity, though there are many notable paradox. Most of the genome size difference appears to lie in the
exceptions. noncoding DNA. About 80 percent of the nucleotide bases in the
human genome may be transcribed, but transcription does not
KEY TERMS
necessarily imply function.
intergenic: describing the noncoding sections of nucleic acid
between genes
noncoding: DNA which does not code for protein
intron: a portion of a split gene that is included in pre-RNA
transcripts but is removed during RNA processing and rapidly
degraded

NONCODING DNA
In genomics and related disciplines, noncoding DNA sequences are
components of an organism’s DNA that do not encode protein
sequences. Some noncoding DNA is transcribed into functional
noncoding RNA molecules (e.g. transfer RNA, ribosomal RNA, and
regulatory RNAs), while others are not transcribed or give rise to
RNA transcripts of unknown function. The amount of noncoding
DNA varies greatly among species. For example, over 98% of the
human genome is noncoding DNA, while only about 2% of a typical
bacterial genome is noncoding DNA.
Initially, a large proportion of noncoding DNA had no known
biological function and was therefore sometimes referred to as “junk
DNA”, particularly in the lay press. However, many types of
noncoding DNA sequences do have important biological functions,
including the transcriptional and translational regulation of protein-
coding sequences, origins of DNA replication, centromeres,
telomeres, scaffold attachment regions (SARs), genes for functional
RNAs, and many others. Other noncoding sequences have likely, but
as-yet undetermined, functions. Some sequences may have no
biological function for the organism, such as endogenous
retroviruses.

18.4E.1 https://bio.libretexts.org/@go/page/13433
 This page titled 18.4E: Noncoding DNA is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

Figure 18.4E. 1: Utricularia gibba flower: Utricularia gibba has 3%

noncoding DNA, which is low for flowering plants. This 3% has
given this plant the title the ‘most efficient’ genome.

18.4E.2 https://bio.libretexts.org/@go/page/13433
18.4F: VARIATIONS IN SIZE AND NUMBER OF GENES
The genome size does not always correlate with the complexity of During sexual reproduction, offspring inherit alleles from both
the organism and, in fact, shows great variation in size and gene parents and these alleles might be slightly different, especially if
number. there has been migration or hybridization of organisms, so that the
parents may come from different populations and gene pools. Also,
 LEARNING OBJECTIVES when the offspring’s chromosomes are copied after fertilization,
genes can be exchanged in a process called sexual recombination.
Describe how variations in the size and number of genes can Harmless mutations and sexual recombination may allow the
arise through evolutionary mechanisms evolution of new characteristics.

KEY POINTS GENOME SIZE AND NUMBER

Harmless mutations and sexual recombination of chromosomes Genome size is usually measured in base pairs (or bases in single-
may allow the evolution of new characteristics. stranded DNA or RNA). The C-value is another measure of genome
Genome size can be affected by various events, including size. The C-value refers to the amount, in picograms, of DNA
duplication, insertion, recombination, deletion and contained within a haploid nucleus (e.g. a gamete) or one half the
polyploidization events. amount in a diploid somatic cell of a eukaryotic organism. In some
Genome size can be affected by evolution of an organism and cases (notably among diploid organisms), the terms C-value and
result is an increased or decreased need for specific genes for genome size are used interchangeably, however in polyploids the C-
survival based on behavior. value may represent two or more genomes contained within the
The human genome exemplifies the concept that complexity same nucleus.
does not always correlate with an increase in genome size; there Different species can have different numbers of genes within the
are fewer protein coding genes present than expected relative to entire DNA or genome of the organism. However, a greater total
the genome size. number of genes might not correspond with a greater observable
complexity in the anatomy and physiology of the organism (i.e.
KEY TERMS greater phenotypic complexity). For example, the predicted size of
polyploidization: hybridization that leads to polyploidy the human genome is not much larger than the genomes of some
pseudogene: a segment of DNA that is part of the genome of an invertebrates and plants, and may even be smaller than the Indian
organism, and which is similar to a gene but does not code for a rice genome. In humans, more proteins are encoded per gene than in
gene product other species. In prokaryotic genomes, research has shown that there
genome: the cell’s complete genetic information packaged as a is a significant positive correlation between the C-value of
double-stranded DNA molecule prokaryotes and the amount of genes that compose the genome. This
indicates that gene number is the main factor influencing the size of
VARIATIONS IN SIZE AND NUMBER OF GENES the prokaryotic genome.
Genetic diversity refers to any variation in the nucleotides, genes,
chromosomes, or whole genomes of organisms. Genetic diversity at GENES VS GENOME SIZE
its most elementary level is represented by differences in the In eukaryotic organisms, there is a paradox observed, namely that
sequences of nucleotides (adenine, cytosine, guanine, and thymine) the number of genes that make up the genome does not correlate
that form the DNA (deoxyribonucleic acid) within the cells of the with genome size. In other words, the genome size is much larger
organism. The DNA is contained in the chromosomes present within than would be expected given the total number of protein coding
the cell; some chromosomes are contained within specific organelles genes. Genome size can increase by duplication, insertion, or
in the cell (for example, the chromosomes of mitochondria and polyploidization and the process of recombination can lead to both
chloroplast). Nucleotide variation is measured for discrete sections DNA loss or gain. It is also possible that genomes can shrink due to
of the chromosomes, called genes. Thus, each gene compromises a deletions.
hereditary section of DNA that occupies a specific place of the
chromosome, and controls a particular characteristic of an organism.

CHROMOSOMES
Most organisms are diploid, having two sets of chromosomes, and
therefore two copies (called alleles ) of each gene. However, some
organisms can be haploid, triploid, or tetraploid (having one, three,
or four sets of chromosomes respectively). Within any single
organism, there may be variation between the two (or more) alleles
for each gene. This variation is introduced either through mutation
of one of the alleles, or as a result of sexual reproduction.

18.4F.1 https://bio.libretexts.org/@go/page/13434
 isomerases; 94; 0,5%
unclassified; 4061; 23,6%
extracellular matrix proteins; 72; 0,4%
License: CC BY: Attribution
receptors; 1076; 6,3%
proteases; 476; 2,8% CollapsedtreeLabels-simplified. Provided by: Wikipedia. Located at:
storage proteins; 15; 0,1%
cytoskeletal proteins; 441; 2,6% en.Wikipedia.org/wiki/File:Co...simplified.svg. License: Public Domain: No
structural proteins; 280; 1,6%
transporters; 1098; 6,4% Known Copyright
surfactants; 15; 0,1%
cell junction proteins; 67; 0,4%
transmembrane receptor regulatory/ Mutation rate. Provided by: Wikipedia. Located at:
/adaptor proteins; 84; 0,5%
chaperones; 130; 0,8%
transferases; 1512; 8,8%
en.Wikipedia.org/wiki/Mutation_rate. License: CC BY-SA: Attribution-
transcription factors; 2067; 12,0%
oxidoreductases; 550; 3,2%
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phosphatases; 230; 1,3%
lyases; 104; 0,6% Mutation rate. Provided by: Wikipedia. Located at:
membrane traffic proteins; 321; 1,9%
transfer/carrier proteins; 248; 1,4%
cell adhesion molecules; 93; 0,5% en.Wikipedia.org/wiki/Mutation_rate. License: CC BY-SA: Attribution-
hydrolases; 454; 2,6%
ligases; 260; 1,5% ShareAlike
defense/immunity proteins; 107; 0,6% nucleic acid binding; 1466; 8,5% Genome evolution. Provided by: Wikipedia. Located at:
signaling molecules; 961; 5,6%
calcium-binding proteins; 63; 0,4% en.Wikipedia.org/wiki/Genome_evolution. License: CC BY-SA: Attribution-
viral proteins; 7; 0,0% enzyme modulators; 857; 5,0%
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Figure 18.4F . 1 : Gene variation in the Genome: This figure intron. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/intron.
represents the human genome, categorized by function of each gene License: CC BY-SA: Attribution-ShareAlike
product, given both as number of genes and as percentage of all exon. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/exon.
License: CC BY-SA: Attribution-ShareAlike
genes. Importantly, genome size does not necessarily correlate with
pseudogene. Provided by: Wiktionary. Located at:
complexity. en.wiktionary.org/wiki/pseudogene. License: CC BY-SA: Attribution-
A famous example for such gene decay is the genome of ShareAlike
OpenStax College, Perspectives on the Phylogenetic Tree. December 15, 2013.
Mycobacterium leprae, the causative agent of leprosy. M.leprae has Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44593/1.5/.
lost many once-functional genes over time due to the formation of License: CC BY: Attribution
CollapsedtreeLabels-simplified. Provided by: Wikipedia. Located at:
pseudogenes. This is evident in looking at its closest ancestor en.Wikipedia.org/wiki/File:Co...simplified.svg. License: Public Domain: No
Mycobacterium tuberculosis. M. leprae lives inside and replicates Known Copyright
Chromosomes mutations-en. Provided by: Wikipedia. Located at:
inside of a host and due to this arrangement it does not have a need
en.Wikipedia.org/wiki/File:Ch...tations-en.svg. License: Public Domain: No
for many of the genes it once carried which allowed it to live and Known Copyright
prosper outside of the host. Thus over time these genes have lost Paleopolyploidy. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/Paleopolyploidy%23Evolutionary_importance.
their function through mechanisms such as mutation causing them to License: CC BY-SA: Attribution-ShareAlike
become pseudogenes. It is beneficial to an organism to rid itself of Genome evolution. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/Genome_evolution%23Gene_Duplication. License:
non-essential genes because it makes replicating its DNA much CC BY-SA: Attribution-ShareAlike
faster and more energy-efficient. OpenStax College, Speciation. December 15, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m45493/latest/. License: CC BY:
An example of increasing genome size over time is seen in Attribution
filamentous plant pathogens. These plant pathogen genomes have sympatric speciation. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/sympatric%20speciation. License: CC BY-SA:
been growing larger over the years due to repeat-driven expansion. Attribution-ShareAlike
The repeat-rich regions contain genes coding for host interaction polyploidy. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/polyploidy. License: CC BY-SA: Attribution-
proteins. With the addition of more and more repeats to these ShareAlike
regions the plants increase the possibility of developing new palaeopolyploidization. Provided by: Wiktionary. Located at:
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virulence factors through mutation and other forms of genetic Attribution-ShareAlike
recombination. In this way it is beneficial for these plant pathogens OpenStax College, Perspectives on the Phylogenetic Tree. December 15, 2013.
Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44593/1.5/.
to have larger genomes. License: CC BY: Attribution
CollapsedtreeLabels-simplified. Provided by: Wikipedia. Located at:
CONTRIBUTIONS AND ATTRIBUTIONS en.Wikipedia.org/wiki/File:Co...simplified.svg. License: Public Domain: No
Known Copyright
horizontal gene transfer. Provided by: Wiktionary. Located at:
Chromosomes mutations-en. Provided by: Wikipedia. Located at:
en.wiktionary.org/wiki/horizontal_gene_transfer. License: CC BY-SA:
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Known Copyright
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Haploid, diploid ,triploid and tetraploid. Provided by: Wikipedia. Located at:
Provided by: OpenStax CNX. Located at: http://cnx.org/content/m44593/1.5/.
en.Wikipedia.org/wiki/File:Ha...tetraploid.svg. License: CC BY-SA:
License: CC BY: Attribution
Attribution-ShareAlike
Genetic distance. Provided by: Wikipedia. Located at:
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Phylogenetics. Provided by: Wikipedia. Located at:
Gene duplication. Provided by: Wikipedia. Located at:
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en.Wikipedia.org/wiki/Gene_duplication. License: CC BY-SA: Attribution-
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OpenStax College, Biology. December 6, 2013. Provided by: OpenStax CNX.
Genome evolution. Provided by: Wikipedia. Located at:
Located at: http://cnx.org/content/m44588/latest/?collection=col11448/latest.
en.Wikipedia.org/wiki/Genome_evolution%23Gene_Duplication. License:
License: CC BY: Attribution
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transformation. Provided by: Wiktionary. Located at:
Divergent evolution. Provided by: Wikipedia. Located at:
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Genetic divergence. Provided by: Wikipedia. Located at:
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en.Wikipedia.org/wiki/Genetic_divergence. License: CC BY-SA: Attribution-
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Genetic drift. Provided by: Wikipedia. Located at:
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paralogous. Provided by: Wiktionary. Located at: License: CC BY-SA: Attribution-ShareAlike
en.wiktionary.org/wiki/paralogous. License: CC BY-SA: Attribution- Gene-duplication. Provided by: Wikipedia. Located at:
ShareAlike en.Wikipedia.org/wiki/File:Ge...uplication.png. License: Public Domain: No
retrogene. Provided by: Wiktionary. Located at: Known Copyright
en.wiktionary.org/wiki/retrogene. License: CC BY-SA: Attribution-ShareAlike Utricularia gibba flower 01. Provided by: Wikipedia. Located at:
genetic drift. Provided by: Wiktionary. Located at: en.Wikipedia.org/wiki/File:Ut..._flower_01.jpg. License: CC BY: Attribution
en.wiktionary.org/wiki/genetic_drift. License: CC BY-SA: Attribution- Genome evolution. Provided by: Wikipedia. Located at:
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OpenStax College, Perspectives on the Phylogenetic Tree. December 15, 2013. ShareAlike
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License: CC BY: Attribution License: CC BY-SA: Attribution-ShareAlike
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Public Domain: No Known Copyright http://cnx.org/content/m12158/latest/. License: CC BY: Attribution
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en.Wikipedia.org/wiki/File:Chromosomes_mutations-en.svg. License: Public en.wiktionary.org/wiki/genome. License: CC BY-SA: Attribution-ShareAlike
Domain: No Known Copyright polyploidization. Provided by: Wiktionary. Located at:
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en.Wikipedia.org/wiki/Noncoding_DNA. License: CC BY-SA: Attribution- License: CC BY: Attribution
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License: CC BY: Attribution Gene. Provided by: Wikipedia. Located at:
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18.4F.3 https://bio.libretexts.org/@go/page/13434
SECTION OVERVIEW

18.5: EVIDENCE OF EVOLUTION

18.5E: THE FOSSIL RECORD AND THE
Topic hierarchy EVOLUTION OF THE MODERN HORSE

18.5F: HOMOLOGOUS STRUCTURES

18.5A: THE FOSSIL RECORD AS EVIDENCE FOR
EVOLUTION 18.5G: CONVERGENT EVOLUTION
18.5B: FOSSIL FORMATION 18.5H: VESTIGIAL STRUCTURES
18.5C: GAPS IN THE FOSSIL RECORD 18.5I: BIOGEOGRAPHY AND THE DISTRIBUTION
OF SPECIES
18.5D: CARBON DATING AND ESTIMATING
FOSSIL AGE
This page titled 18.5: Evidence of Evolution is shared under a CC BY-SA
4.0 license and was authored, remixed, and/or curated by Boundless.

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18.5A: THE FOSSIL RECORD AS EVIDENCE FOR EVOLUTION
Fossils tell us when organisms lived, as well as provide evidence for
the progression and evolution of life on earth over millions of years.

 LEARNING OBJECTIVES

Synthesize the contributions of the fossil record to our

understanding of evolution

KEY POINTS
Fossils are the preserved remains or traces of animals, plants, and
other organisms from the past.
Fossils are important evidence for evolution because they show
that life on earth was once different from life found on earth
today. Figure 18.5A. 1 : “Sue” T-rex skeleton: The bones of this
Tyrannosaurus rex were preserved through the process of
Usually only a portion of an organism is preserved as a fossil, permineralization, which suggests that this organism was covered by
such as body fossils (bones and exoskeletons ), trace fossils sediment soon after death.
(feces and footprints), and chemofossils (biochemical signals).
Paleontologists can determine the age of fossils using methods
PERMINERALIZATION
like radiometric dating and categorize them to determine the Permineralization is a process of fossilization that occurs when an
evolutionary relationships between organisms. organism is buried. The empty spaces within an organism (spaces
filled with liquid or gas during life) become filled with mineral-rich
KEY TERMS groundwater. Minerals precipitate from the groundwater, occupying
biomarker: A substance used as an indicator of a biological the empty spaces. This process can occur in very small spaces, such
state, most commonly disease. as within the cell wall of a plant cell. Small-scale permineralization
trace fossil: A type of fossil reflecting the reworking of can produce very detailed fossils. For permineralization to occur, the
sediments and hard substrates by organisms including structures organism must be covered by sediment soon after death, or soon
like burrows, trails, and impressions. after the initial decay process.
fossil record: All discovered and undiscovered fossils and their The degree to which the remains are decayed when covered
placement in rock formations and sedimentary layers. determines the later details of the fossil. Fossils usually consist of
strata: Layers of sedimentary rock. the portion of the organisms that was partially mineralized during
fossiliferous: Containing fossils. life, such as the bones and teeth of vertebrates or the chitinous or
calcareous exoskeletons of invertebrates. However, other fossils
WHAT FOSSILS TELL US contain traces of skin, feathers or even soft tissues.
Fossils are the preserved remains or traces of animals, plants, and
other organisms from the past. Fossils range in age from 10,000 to TRACE FOSSILS
3.48 billion years old. The observation that certain fossils were Fossils may also consist of the marks left behind by the organism
associated with certain rock strata led 19th century geologists to while it was alive, such as footprints or feces. These types of fossils
recognize a geological timescale. Like extant organisms, fossils vary are called trace fossils, or ichnofossils, as opposed to body fossils.
in size from microscopic, like single-celled bacteria, to gigantic, like Past life may also leave some markers that cannot be seen but can be
dinosaurs and trees. detected in the form of biochemical signals; these are known as
chemofossils or biomarkers.

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 sedimentary layers (strata) is known as the fossil record. The fossil
record was one of the early sources of data underlying the study of
evolution and continues to be relevant to the history of life on Earth.
The development of radiometric dating techniques in the early 20th
century allowed geologists to determine the numerical or “absolute”
age of various strata and their included fossils.

EVIDENCE FOR EVOLUTION

Fossils provide solid evidence that organisms from the past are not
the same as those found today; fossils show a progression of
evolution. Fossils, along with the comparative anatomy of present-
day organisms, constitute the morphological, or anatomical, record.
By comparing the anatomies of both modern and extinct species,
paleontologists can infer the lineages of those species. This approach
is most successful for organisms that had hard body parts, such as
shells, bones or teeth. The resulting fossil record tells the story of the
Figure 18.5A. 1 : Dinosaur footprints: Footprints are examples of past and shows the evolution of form over millions of years.
trace fossils, which contribute to the fossil record.

THE FOSSIL RECORD This page titled 18.5A: The Fossil Record as Evidence for Evolution is
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
The totality of fossils, both discovered and undiscovered, and their
curated by Boundless.
placement in fossiliferous (fossil-containing) rock formations and

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18.5B: FOSSIL FORMATION
Fossils can form under ideal conditions by preservation, PRESERVED REMAINS
permineralization, molding (casting), replacement, or compression. The rarest form of fossilization is the preservation of original
skeletal material and even soft tissue. For example, some insects
 LEARNING OBJECTIVES have been preserved perfectly in amber, which is ancient tree sap. In
addition, several mammoths and even a Neanderthal hunter have
Predict the conditions suitable to fossil formation
been discovered frozen in glaciers. These preserved remains allow
scientists the rare opportunity to examine the skin, hair, and organs
KEY POINTS of ancient creatures. Scientists have collected DNA from these
Preservation of remains in amber or other substances is the rarest remains and compared the DNA sequences to those of modern
from of fossilization; this mechanism allows scientists to study creatures.
the skin, hair, and organs of ancient creatures.
Permineralization, where minerals like silica fill the empty
spaces of shells, is the most common form of fossilization.
Molds form when shells or bones dissolve, leaving behind an
empty depression; a cast is then formed when the depression is
filled by sediment.
Replacement occurs when the original shell or bone dissolves
away and is replaced by a different mineral; when this occurs
with permineralization, it is called petrification.
In compression, the most common form of fossilization of leaves
and ferns, a dark imprint of the fossil remains.
Decay, chemical weathering, erosion, and predators are factors
that deter fossilization.
Fossilization of soft body parts is rare, and hard parts are better
preserved when buried.

KEY TERMS
Figure 18.5B. 1: Amber: The image depicts a gnat preserved in
amber: a hard, generally yellow to brown translucent fossil resin amber. A lot of insects have been found to be perfectly maintained in
permineralization: form of fossilization in which minerals are this ancient tree sap.
deposited in the pores of bone and similar hard animal parts
petrification: process by which organic material is converted PERMINERALIZATION
into stone through the replacement of the original material and The most common method of fossilization is permineralization.
the filling of the original pore spaces with minerals After a bone, wood fragment, or shell is buried in sediment, it may
be exposed to mineral-rich water that moves through the sediment.
FOSSIL FORMATION This water will deposit minerals, typically silica, into empty spaces,
The process of a once living organism becoming a fossil is called producing a fossil. Fossilized dinosaur bones, petrified wood, and
fossilization. Fossilization is a very rare process, and of all the many marine fossils were formed by permineralization.
organisms that have lived on Earth, only a tiny percentage of them
ever become fossils. To see why, imagine an antelope that dies on
the African plain. Most of its body is quickly eaten by scavengers,
and the remaining flesh is soon eaten by insects and bacteria, leaving
behind only scattered bones. As the years go by, the bones are
scattered and fragmented into small pieces, eventually turning into
dust and returning their nutrients to the soil. As a result, it would be
rare for any of the antelope’s remains to actually be preserved as a
fossil.
Fossilization can occur in many ways. Most fossils are preserved in
one of five processes:
preserved remains
permineralization Figure 18.5B. 1: Permineralization: These fossils from the Road
Canyon Formation (Middle Permian of Texas) have been silicified
molds and casts
(replaced with silica), which is a form of permineralization.
replacement
compression

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MOLDS AND CASTS process of turning organic material into stone. However,
In some cases, the original bone or shell dissolves away, leaving replacement can occur without permineralization and vice versa.
behind an empty space in the shape of the shell or bone. This
COMPRESSION
depression is called a mold. Later, the space may be filled with other
sediments to form a matching cast in the shape of the original Some fossils form when their remains are compressed by high
organism. Many mollusks (bivalves, snails, and squid) are pressure. This can leave behind a dark imprint of the fossil.
commonly found as molds and casts because their shells dissolve Compression is most common for fossils of leaves and ferns but also
easily. can occur with other organisms.

CONDITIONS FOR FOSSILIZATION

Following the death of an organism, several forces contribute to the
dissolution of its remains. Decay, predators, or scavengers will
typically rapidly remove the flesh. The hard parts, if they are
separable at all, can be dispersed by predators, scavengers, or
currents. The individual hard parts are subject to chemical
weathering and erosion, as well as to splintering by predators or
scavengers, which will crunch up bones for marrow and shells to
extract the flesh inside. Also, an animal swallowed whole by a
predator, such as a mouse swallowed by a snake, will have not just
its flesh but some, and perhaps all, its bones destroyed by the gastric
juices of the predator.
It would not be an exaggeration to say that the typical vertebrate
Figure 18.5B. 1: Molds: The depression in the image is an external
mold of a bivalve from the Logan Formation, Lower Carboniferous,
fossil consists of a single bone, or tooth, or fish scale. The
Ohio preservation of an intact skeleton with the bones in the relative
positions they had in life requires a remarkable circumstances, such
REPLACEMENT as burial in volcanic ash, burial in aeolian sand due to the sudden
In some cases, the original shell or bone dissolves away and is slumping of a sand dune, burial in a mudslide, burial by a turbidity
replaced by a different mineral. For example, shells that were current, and so forth. The mineralization of soft parts is even less
originally calcite may be replaced by dolomite, quartz, or pyrite. If common and is seen only in exceptionally rare chemical and
quartz fossils are surrounded by a calcite matrix, the calcite can be biological conditions.
dissolved away by acid, leaving behind an exquisitely preserved
quartz fossil. When permineralization and replacement occur This page titled 18.5B: Fossil Formation is shared under a CC BY-SA 4.0
together, the organism is said to have undergone petrification, the license and was authored, remixed, and/or curated by Boundless.

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18.5C: GAPS IN THE FOSSIL RECORD
Because not all animals have bodies which fossilize easily, the fossil bodied organisms with little to no fossil record. Groups considered
record is considered incomplete. to have a good fossil record, including transitional fossils between
these groups, are the vertebrates, the echinoderms, the brachiopods,
 LEARNING OBJECTIVES and some groups of arthropods. Their hard bones and shells fossilize
easily, unlike the bodies of organisms like cephalopods or jellyfish.
Explain the gap in the fossil record
ROMER’S GAP
KEY POINTS Romer’s gap is an example of an apparent gap in the tetrapod fossil
The number of species known about through fossils is less than record used in the study of evolutionary biology. These gaps
1% of all species that have ever lived. represent periods from which no relevant fossils have been found.
Because hard body parts are more easily preserved than soft Romer’s gap is named after paleontologist Alfred Romer, who first
body parts, there are more fossils of animals with hard body recognized it. Romer’s gap spanned from approximately 360 to 345
parts, such as vertebrates, echinoderms, brachiopods, and some million years ago, corresponding to the first 15 million years of the
groups of arthropods. Carboniferous Period.
Very few fossils have been found in the period from 360 to 345
million years ago, known as Romer’s gap. Theories to explain
this include the period’s geochemistry, errors in excavation, and
limited vertebrate diversity.

KEY TERMS
transitional fossil: Fossilized remains of a life form that exhibits
traits common to both an ancestral group and its derived
descendant group.
Romer’s gap: A period in the tetrapod fossil record (360 to 345
million years ago) from which excavators have not yet found
relevant fossils.

INCOMPLETENESS OF THE FOSSIL RECORD

Each fossil discovery represents a snapshot of the process of
evolution. Because of the specialized and rare conditions required
Figure 18.5C. 1 : Romer’s Gap: The bank of the Whiteadder Water
for a biological structure to fossilize, many important species or in Scotland is one of the few known localities bearing fossils of
groups may never leave fossils at all. Even if they do leave fossils, tetrapods from Romer’s gap.
humans may never find them—for example, if they are buried under There has been much debate over why there are so few fossils from
hundreds of feet of ice in Antarctica. The number of species known this time period. Some scientists have suggested that the
about through the fossil record is less than 5% of the number of geochemistry of the time period caused bad conditions for fossil
species alive today. Fossilized species may represent less than 1% of formation, so few organisms were fossilized. Another theory
all the species that have ever lived. suggests that scientists have simply not yet discovered an excavation
site for these fossils, due to inaccessibility or random chance.
TYPES OF FOSSILS IN THE FOSSIL RECORD
The fossil record is very uneven and is mostly comprised of fossils This page titled 18.5C: Gaps in the Fossil Record is shared under a CC BY-
of organisms with hard body parts, leaving most groups of soft- SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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18.5D: CARBON DATING AND ESTIMATING FOSSIL AGE
The age of fossils can be determined using stratigraphy,
biostratigraphy, and radiocarbon dating.

 LEARNING OBJECTIVES

Summarize the available methods for dating fossils

KEY POINTS
Determining the ages of fossils is an important step in mapping
out how life evolved across geologic time.
The study of stratigraphy enables scientists to determine the age
of a fossil if they know the age of layers of rock that surround it.
Biostratigraphy enables scientists to match rocks with particular
fossils to other rocks with those fossils to determine age. Figure 18.5D. 1 : Sedimentary layers: The layers of sedimentary
Paleontology seeks to map out how life evolved across geologic rock, or strata, can be seen as horizontal bands of differently colored
time. A substantial hurdle is the difficulty of working out fossil or differently structured materials exposed in this cliff. The deeper
layers are older than the layers found at the top, which aids in
ages. determining the relative age of fossils found within the strata.
Scientists use carbon dating when determining the age of fossils
that are less than 60,000 years old, and that are composed of
organic materials such as wood or leather. BIOSTRATIGRAPHY
Fossils of species that survived for a relatively short time can be
KEY TERMS
used to match isolated rocks: this technique is called biostratigraphy.
half-life: The time required for half of the nuclei in a sample of a
For instance, the extinct chordate Eoplacognathus pseudoplanus is
specific isotope to undergo radioactive decay.
thought to have existed during a short range in the Middle
stratigraphy: The study of rock layers and the layering process.
Ordovician period. If rocks of unknown age have traces of E.
radiocarbon dating: A method of estimating the age of an
pseudoplanus, they have a mid-Ordovician age. Such index fossils
artifact or biological vestige based on the relative amounts of
must be distinctive, globally distributed, and occupy a short time
various isotopes of carbon present in a sample.
range to be useful. Misleading results can occur if the index fossils
DETERMINING FOSSIL AGES are incorrectly dated.
Paleontology seeks to map out how life evolved across geologic RELATIVE DATING
time. A substantial hurdle is the difficulty of working out fossil ages.
Stratigraphy and biostratigraphy can in general provide only relative
There are several different methods for estimating the ages of
dating (A was before B), which is often sufficient for studying
fossils, including:
evolution. This is difficult for some time periods, however, because
1. stratigraphy of the barriers involved in matching rocks of the same age across
2. biostratigraphy continents. Family-tree relationships can help to narrow down the
3. carbon dating date when lineages first appeared. For example, if fossils of B date
to X million years ago and the calculated “family tree” says A was
STRATIGRAPHY an ancestor of B, then A must have evolved earlier.
Paleontologists rely on stratigraphy to date fossils. Stratigraphy is
It is also possible to estimate how long ago two living branches of a
the science of understanding the strata, or layers, that form the
family tree diverged by assuming that DNA mutations accumulate at
sedimentary record. Strata are differentiated from each other by their
a constant rate. However, these “molecular clocks” are sometimes
different colors or compositions and are exposed in cliffs, quarries,
inaccurate and provide only approximate timing. For example, they
and river banks. These rocks normally form relatively horizontal,
are not sufficiently precise and reliable for estimating when the
parallel layers, with younger layers forming on top.
groups that feature in the Cambrian explosion first evolved, and
If a fossil is found between two layers of rock whose ages are estimates produced by different approaches to this method may vary
known, the fossil’s age is thought to be between those two known as well.
ages. Because rock sequences are not continuous, but may be broken
up by faults or periods of erosion, it is difficult to match up rock CARBON DATING
beds that are not directly adjacent. Together with stratigraphic principles, radiometric dating methods
are used in geochronology to establish the geological time scale.
Beds that preserve fossils typically lack the radioactive elements
needed for radiometric dating (” radiocarbon dating ” or simply

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“carbon dating”). The principle of radiocarbon dating is simple: the The half-life of carbon-14 is 5,730 years, so carbon dating is only
rates at which various radioactive elements decay are known, and relevant for dating fossils less than 60,000 years old. Radioactive
the ratio of the radioactive element to its decay products shows how elements are common only in rocks with a volcanic origin, so the
long the radioactive element has existed in the rock. This rate is only fossil-bearing rocks that can be dated radiometrically are
represented by the half-life, which is the time it takes for half of a volcanic ash layers. Carbon dating uses the decay of carbon-14 to
sample to decay. estimate the age of organic materials, such as wood and leather.

This page titled 18.5D: Carbon Dating and Estimating Fossil Age is shared
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

Figure 18.5D. 1 : Half-life of Carbon-14: Radiometric dating is a

technique used to date materials such as rocks or carbon, usually
based on a comparison between the observed abundance of a
naturally occurring radioactive isotope and its decay products, using
known decay rates.

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18.5E: THE FOSSIL RECORD AND THE EVOLUTION OF THE MODERN HORSE

 LEARNING OBJECTIVES

Analyze the fossil record to understand the evolution of

horses

Fossils provide evidence that organisms from the past are not the
Figure 18.5E. 1: Horse evolution: This illustration shows an artist’s
same as those found today, and demonstrate a progression of renderings of species derived from fossils of the evolutionary history
evolution. Scientists date and categorize fossils to determine when of the horse and its ancestors. The species depicted are only four
the organisms lived relative to each other. The resulting fossil record from a very diverse lineage that contains many branches, dead ends,
and adaptive radiations. One of the trends, depicted here, is the
tells the story of the past and shows the evolution of forms over evolutionary tracking of a drying climate and increase in prairie
millions of years. versus forest habitat reflected in forms that are more adapted to
grazing and predator escape through running.
CASE STUDY: EVOLUTION OF THE MODERN Since then, as the number of equid fossils has increased, the actual
HORSE evolutionary progression from Eohippus to Equus has been
Highly detailed fossil records have been recovered for sequences in discovered to be much more complex and multibranched than was
the evolution of modern horses. The fossil record of horses in North initially supposed. Detailed fossil information on the rate and
America is especially rich and contains transition fossils: fossils that distribution of new equid species has also revealed that the
show intermediate stages between earlier and later forms. The fossil progression between species was not as smooth and consistent as
record extends back to a dog-like ancestor some 55 million years was once believed.
ago, which gave rise to the first horse-like species 55 to 42 million Although some transitions were indeed gradual progressions, a
years ago in the genus Eohippus. number of others were relatively abrupt in geologic time, taking
The first equid fossil was found in the gypsum quarries in place over only a few million years. Both anagenesis, a gradual
Montmartre, Paris in the 1820s. The tooth was sent to the Paris change in an entire population ‘s gene frequency, and cladogenesis,
Conservatory, where Georges Cuvier identified it as a browsing a population “splitting” into two distinct evolutionary branches,
equine related to the tapir. His sketch of the entire animal matched occurred, and many species coexisted with “ancestor” species at
later skeletons found at the site. During the H.M.S. Beagle survey various times.
expedition, Charles Darwin had remarkable success with fossil
hunting in Patagonia. In 1833 in Santa Fe, Argentina, he was “filled
ADAPTATION FOR GRAZING
with astonishment” when he found a horse’s tooth in the same The series of fossils tracks the change in anatomy resulting from a
stratum as fossils of giant armadillos and wondered if it might have gradual drying trend that changed the landscape from a forested
been washed down from a later layer, but concluded this was “not habitat to a prairie habitat. Early horse ancestors were originally
very probable.” In 1836, the anatomist Richard Owen confirmed the specialized for tropical forests, while modern horses are now
tooth was from an extinct species, which he subsequently named adapted to life on drier land. Successive fossils show the evolution
Equus curvidens. of teeth shapes and foot and leg anatomy to a grazing habit with
adaptations for escaping predators.
The original sequence of species believed to have evolved into the
horse was based on fossils discovered in North America in the 1870s The horse belongs to the order Perissodactyla (odd-toed ungulates),
by paleontologist Othniel Charles Marsh. The sequence, from the members of which all share hoofed feet and an odd number of
Eohippus to the modern horse (Equus), was popularized by Thomas toes on each foot, as well as mobile upper lips and a similar tooth
Huxley and became one of the most widely known examples of a structure. This means that horses share a common ancestry with
clear evolutionary progression. The sequence of transitional fossils tapirs and rhinoceroses. Later species showed gains in size, such as
was assembled by the American Museum of Natural History into an those of Hipparion, which existed from about 23 to 2 million years
exhibit that emphasized the gradual, “straight-line” evolution of the ago. The fossil record shows several adaptive radiations in the horse
horse. lineage, which is now much reduced to only one genus, Equus, with
several species. Paleozoologists have been able to piece together a
more complete outline of the modern horse’s evolutionary lineage
than that of any other animal.

KEY POINTS
A dog-like organism gave rise to the first horse ancestors 55-42
million years ago.
The fossil record shows modern horses moved from tropical
forests to prairie habitats, developed teeth, and grew in size.

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The first equid fossil was a tooth from the extinct species Equus KEY TERMS
curvidens found in Paris in the 1820s. cladogenesis: An evolutionary splitting event in which each
Thomas Huxley popularized the evolutionary sequence of horses, branch and its smaller branches forms a clade.
which became one of the most common examples of clear equid: A member of the horse family.
evolutionary progression. anagenesis: Evolution of a new species through a large scale
Horse evolution was previously believed to be a linear progress, change in gene frequency so that the new species replaces the
but after more fossils were discovered, it was determined the old, rather than branching to produce an additional species.
evolution of horses was more complex and multi-branched.
Horses have evolved from gradual change ( anagenesis ) as well This page titled 18.5E: The Fossil Record and the Evolution of the Modern
as abrupt progression and division ( cladogenesis ). Horse is shared under a CC BY-SA 4.0 license and was authored, remixed,
and/or curated by Boundless.

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18.5F: HOMOLOGOUS STRUCTURES
Homologous structures are similar structures that evolved from a
common ancestor.

 LEARNING OBJECTIVES

Describe the connection between evolution and the

appearance of homologous structures

KEY POINTS
Homology is a relationship defined between structures or DNA
derived from a common ancestor and illustrates descent from a
common ancestor.
Analogous structures are physically (but not genetically) similar
structures that were not present the last common ancestor.
Homology can also be partial; new structures can evolve through
the combination or parts of developmental pathways. Figure 18.5F . 1 : Homology in the forelimbs of vertebrates: The
principle of homology illustrated by the adaptive radiation of the
Analogy may also be referred to as homoplasy, which is further forelimb of mammals. All conform to the basic pentadactyl pattern
divided into parallelism, reversal, and convergence. but are modified for different usages. The third metacarpal is shaded
throughout; the shoulder is crossed-hatched.
KEY TERMS In genetics, homology is measured by comparing protein or DNA
homology: A correspondence of structures in two life forms with sequences. Homologous gene sequences share a high similarity,
a common evolutionary origin, such as flippers and hands. supporting the hypothesis that they share a common ancestor.
analogy: The relationship between characteristics that are Homology can also be partial: new structures can evolve through the
apparently similar but did not develop from the same structure combination of developmental pathways or parts of them. As a
homoplasy: A correspondence between the parts or organs of result, hybrid or mosaic structures can evolve that exhibit partial
different species acquired as the result of parallel evolution or homologies. For example, certain compound leaves of flowering
convergence. plants are partially homologous both to leaves and shoots because
they combine some traits of leaves and some of shoots.
HOMOLOGOUS STRUCTURES
Homology is the relationship between structures or DNA derived PARALOGOUS STRUCTURES
from the most recent common ancestor. A common example of Homologous sequences are considered paralogous if they were
homologous structures in evolutionary biology are the wings of bats separated by a gene duplication event; if a gene in an organism is
and the arms of primates. Although these two structures do not look duplicated to occupy two different positions in the same genome,
similar or have the same function, genetically, they come from the then the two copies are paralogous.
same structure of the last common ancestor. Homologous traits of A set of sequences that are paralogous are called paralogs of each
organisms are therefore explained by descent from a common other. Paralogs typically have the same or similar function, but
ancestor. sometimes do not. It is considered that due to lack of the original
It’s important to note that defining two structures as homologous selective pressure upon one copy of the duplicated gene, this copy is
depends on what ancestor is being described as the common free to mutate and acquire new functions.
ancestor. If we go all the way back to the beginning of life, all
structures are homologous!

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 ANALOGOUS STRUCTURES
The opposite of homologous structures are analogous structures,
which are physically similar structures between two taxa that
evolved separately (rather than being present in the last common
ancestor). Bat wings and bird wings evolved independently and are
considered analogous structures. Genetically, a bat wing and a bird
wing have very little in common; the last common ancestor of bats
and birds did not have wings like either bats or birds. Wings evolved
independently in each lineage after diverging from ancestors with
forelimbs that were not used as wings (terrestrial mammals and
theropod dinosaurs, respectively).
It is important to distinguish between different hierarchical levels of
homology in order to make informative biological comparisons. In
the above example, the bird and bat wings are analogous as wings,
Figure 18.5F . 1 : Homology vs. analogy: The wings of pterosaurs but homologous as forelimbs because the organ served as a forearm
(1), bats (2), and birds (3) are analogous as wings, but homologous (not a wing) in the last common ancestor of tetrapods.
as forelimbs. This is because they are similar characteristically and
even functionally, but evolved from different ancestral roots. Analogy is different than homology. Although analogous
Paralogous genes often belong to the same species, but not always. characteristics are superficially similar, they are not homologous
For example, the hemoglobin gene of humans and the myoglobin because they are phylogenetically independent. The wings of a
gene of chimpanzees are considered paralogs. This is a common maple seed and the wings of an albatross are analogous but not
problem in bioinformatics; when genomes of different species have homologous (they both allow the organism to travel on the wind, but
been sequenced and homologous genes have been found, one can they didn’t both develop from the same structure). Analogy is
not immediately conclude that these genes have the same or similar commonly also referred to as homoplasy.
function, as they could be paralogs whose function has diverged.
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18.5G: CONVERGENT EVOLUTION
Convergent evolution occurs in different species that have evolved One of the most well-known examples of convergent evolution is
similar traits independently of each other. the camera eye of cephalopods (e.g., octopus), vertebrates (e.g.,
mammals), and cnidaria (e.g., box jellies). Their last common
 LEARNING OBJECTIVES ancestor had at most a very simple photoreceptive spot, but a range
of processes led to the progressive refinement of this structure to the
Predict the circumstances supporting convergent evolution advanced camera eye. There is, however, one subtle difference: the
of two species cephalopod eye is “wired” in the opposite direction, with blood and
nerve vessels entering from the back of the retina, rather than the
KEY POINTS front as in vertebrates.
Examples of convergent evolution include the relationship
between bat and insect wings, shark and dolphin bodies, and
vertebrate and cephalopod eyes.
Analogous structures arise from convergent evolution, but 1 1
homologous structures do not. 2 2
Convergent evolution is the opposite of divergent evolution, in
which related species evolve different traits. 4
Convergent evolution is similar to parallel evolution, in which
two similar but independent species evolve in the same direction 3 3

and independently acquire similar characteristics.

KEY TERMS
parallel evolution: the development of a similar trait in related, Figure 18.5G. 1: Eye evolution: Vertebrates and octopuses
but distinct, species descending from the same ancestor, but from developed the camera eye independently. In the vertebrate version
the nerve fibers pass in front of the retina, and there is a blind spot
different clades (4) where the nerves pass through the retina. In the octopus version,
convergent evolution: a trait of evolution in which species not the eye is constructed the “right way out,” with the nerves attached
of similar recent origin acquire similar properties due to natural to the rear of the retina. This means that octopuses do not have a
blind spot.
selection
Convergent evolution is similar to, but distinguishable from, the
divergent evolution: the process by which a species with similar
phenomenon of parallel evolution. Parallel evolution occurs when
traits become groups that are tremendously different from each
two independent but similar species evolve in the same direction and
other over many generations
thus independently acquire similar characteristics; for example,
morphology: the form and structure of an organism
gliding frogs have evolved in parallel from multiple types of tree
CONVERGENT EVOLUTION frog.
Sometimes, similar phenotypes evolve independently in distantly
ANALOGOUS STRUCTURES
related species. For example, flight has evolved in both bats and
Traits arising through convergent evolution are analogous structures,
insects, and they both have wings, which are adaptations to flight.
in contrast to homologous structures, which have a common origin,
However, the wings of bats and insects have evolved from very
but not necessarily similar function. The British anatomist Richard
different original structures. This phenomenon is called convergent
Owen was the first scientist to recognize the fundamental difference
evolution, where similar traits evolve independently in species that
between analogies and homologies. Bat and pterosaur wings are an
do not share a recent common ancestry.
example of analogous structures, while the bat wing is homologous
EXAMPLES OF CONVERGENT EVOLUTION to human and other mammal forearms, sharing an ancestral state
Convergent evolution describes the independent evolution of similar despite serving different functions.
features in species of different lineages. The two species came to the
DIVERGENT EVOLUTION
same function, flying, but did so separately from each other. They
The opposite of convergent evolution is divergent evolution,
have “converged” on this useful trait. Both sharks and dolphins have
whereby related species evolve different traits. On a molecular level,
similar body forms, yet are only distantly related: sharks are fish and
this can happen due to random mutation unrelated to adaptive
dolphins are mammals. Such similarities are a result of both
changes.
populations being exposed to the same selective pressures. Within
both groups, changes that aid swimming have been favored. Thus, This page titled 18.5G: Convergent Evolution is shared under a CC BY-SA
over time, they developed similar appearances (morphology), even 4.0 license and was authored, remixed, and/or curated by Boundless.
though they are not closely related.

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18.5H: VESTIGIAL STRUCTURES
Vestigial structures have no function but may still be inherited to
maintain fitness.

 LEARNING OBJECTIVES

Discuss the connection between evolution and the existence

of vestigial structures

KEY POINTS
Structures that have no apparent function and appear to be
residual parts from a past ancestor are called vestigial structures.
Examples of vestigial structures include the human appendix, the
pelvic bone of a snake, and the wings of flightless birds.
Vestigial structures can become detrimental, but in most cases
Figure 18.5H. 1 : Vestigial appendix: In humans the vermiform
these structures are harmless; however, these structures, like any appendix is a vestigial structure; it has lost much of its ancestral
other structure, require extra energy and are at risk for disease. function.
Vestigial structures, especially non-harmful ones, take a long There are also several reflexes and behaviors that are considered to
time to be phased out since eliminating them would require be vestigial. The formation of goose bumps in humans under stress
major alterations that could result in negative side effects. is a vestigial reflex its function in human ancestors was to raise the
body’s hair, making the ancestor appear larger and scaring off
KEY TERMS predators. The arrector pili muscle, which is a band of smooth
vestigial structure: Genetically determined structures or muscle that connects the hair follicle to connective tissue, contracts
attributes that have lost most or all of their ancestral function in a and creates the goose bumps on skin.
given species.
adaptation: A modification of something or its parts that makes VESTIGIAL STRUCTURES IN EVOLUTION
it more fit for existence under the conditions of its current Vestigial structures are often homologous to structures that function
environment. normally in other species. Therefore, vestigial structures can be
considered evidence for evolution, the process by which beneficial
WHAT ARE VESTIGIAL STRUCTURES?
heritable traits arise in populations over an extended period of time.
Some organisms possess structures with no apparent function which The existence of vestigial traits can be attributed to changes in the
appear to be residual parts from a past ancestor. For example, some environment and behavior patterns of the organism in question. As
snakes have pelvic bones despite having no legs because they the function of the trait is no longer beneficial for survival, the
descended from reptiles that did have legs. Another example of a likelihood that future offspring will inherit the “normal” form of it
structure with no function is the human vermiform appendix. These decreases. In some cases the structure becomes detrimental to the
unused structures without function are called vestigial structures. organism.
Other examples of vestigial structures are wings (which may have
other functions) on flightless birds like the ostrich, leaves on some
cacti, traces of pelvic bones in whales, and the sightless eyes of cave
animals.

Figure 18.5H. 1 : Whale Skeleton: The pelvic bones in whales are

also a good example of vestigial evolution (whales evolved from
four-legged land mammals and secondarily lost their hind legs).
Letter c in the picture indicates the undeveloped hind legs of a
baleen whale.
If there are no selection pressures actively lowering the fitness of the
individual, the trait will persist in future generations unless the trait
is eliminated through genetic drift or other random events.
Although in many cases the vestigial structure is of no direct harm,
all structures require extra energy in terms of development,
maintenance, and weight and are also a risk in terms of disease (e.g.,
infection, cancer). This provides some selective pressure for the

18.5H.1 https://bio.libretexts.org/@go/page/13473
removal of parts that do not contribute to an organism’s fitness, but a homology of the structure. Homologous structures indicate common
structure that is not directly harmful will take longer to be ‘phased ancestry with those organisms that have a functional version of the
out’ than one that is. Some vestigial structures persist due to structure. Vestigial traits can still be considered adaptations because
limitations in development, such that complete loss of the structure an adaptation is often defined as a trait that has been favored by
could not occur without major alterations of the organism’s natural selection. Adaptations, therefore, need not be adaptive, as
developmental pattern, and such alterations would likely produce long as they were at some point.
numerous negative side-effects.
This page titled 18.5H: Vestigial Structures is shared under a CC BY-SA 4.0
The vestigial versions of a structure can be compared to the original
license and was authored, remixed, and/or curated by Boundless.
version of the structure in other species in order to determine the

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18.5I: BIOGEOGRAPHY AND THE DISTRIBUTION OF SPECIES
The biological distribution of species is based on the movement of plant and mammal species are endemic species found solely in
tectonic plates over a period of time. Australia.

 LEARNING OBJECTIVES

Relate biogeography and the distribution of species

KEY POINTS
Biogeography is the study of geological species distribution,
which is influenced by both biotic and abiotic factors.
Some species are endemic and are only found in a particular
region, while others are generalists and are distributed
worldwide.
Species that evolved before the breakup of continents are
Figure 18.5I . 1: Australia: Australia is home to many endemic
distributed worldwide. species. The (a) wallaby (Wallabia bicolor), a medium-sized
Species that evolved after the breakup of continents are found in member of the kangaroo family, is a pouched mammal, or marsupial.
only certain regions of the planet. The (b) echidna (Tachyglossus aculeatus) is an egg-laying mammal.
The geographic distribution of organisms on the planet follows
KEY TERMS patterns that are best explained by evolution in conjunction with the
endemic: unique to a particular area or region; not found in other movement of tectonic plates over geological time. Broad groups that
places evolved before the breakup of the supercontinent Pangaea (about
generalist: species which can thrive in a wide variety of 200 million years ago) are distributed worldwide. Groups that
environmental conditions evolved since the breakup appear uniquely in regions of the planet,
Pangaea: supercontinent that included all the landmasses of the such as the unique flora and fauna of northern continents that formed
earth before the Triassic period and that broke up into Laurasia from the supercontinent Laurasia and of the southern continents that
and Gondwana formed from the supercontinent Gondwana. The presence of
Proteaceae in Australia, southern Africa, and South America is best
DISTRIBUTION OF SPECIES explained by the plant family’s presence there prior to the southern
Biogeography is the study of the geographic distribution of living supercontinent Gondwana breaking up.
things and the abiotic factors that affect their distribution. Abiotic
factors, such as temperature and rainfall, vary based on latitude and
elevation, primarily. As these abiotic factors change, the
composition of plant and animal communities also changes.

PATTERNS OF SPECIES DISTRIBUTION

Ecologists who study biogeography examine patterns of species
distribution. No species exists everywhere; for example, the Venus
flytrap is endemic to a small area in North and South Carolina. An
endemic species is one which is naturally found only in a specific
geographic area that is usually restricted in size. Other species are Figure 18.5I . 1: Biogeography: The Proteacea family of plants
generalists: species which live in a wide variety of geographic areas; evolved before the supercontinent Gondwana broke up. Today,
the raccoon, for example, is native to most of North and Central members of this plant family are found throughout the southern
hemisphere (shown in red).
America.
Since species distribution patterns are based on biotic and abiotic CONTRIBUTIONS AND ATTRIBUTIONS
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 CHAPTER OVERVIEW

19: THE EVOLUTION OF POPULATIONS

Topic hierarchy
19.1: Population Evolution
19.1A: Defining Population Evolution
19.1B: Population Genetics
19.1C: Hardy-Weinberg Principle of Equilibrium
19.2: Population Genetics
19.2A: Genetic Variation
19.2B: Genetic Drift
19.2C: Gene Flow and Mutation
19.2D: Nonrandom Mating and Environmental Variance
19.3: Adaptive Evolution
19.3A: Natural Selection and Adaptive Evolution
19.3B: Stabilizing, Directional, and Diversifying Selection
19.3C: Frequency-Dependent Selection
19.3D: Sexual Selection
19.3E: No Perfect Organism

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1
SECTION OVERVIEW

19.1: POPULATION EVOLUTION

19.1B: POPULATION GENETICS
Topic hierarchy
19.1C: HARDY-WEINBERG PRINCIPLE OF
EQUILIBRIUM
19.1A: DEFINING POPULATION EVOLUTION

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19.1A: DEFINING POPULATION EVOLUTION
Genetic variation in a population is determined by mutations, natural
selection, genetic drift, genetic hitchhiking, and gene flow.

 LEARNING OBJECTIVES

Describe how the forces of genetic drift, genetic hitchhiking,

gene flow, and mutation can lead to differences in
population variation

KEY POINTS
The theory of evolution gives us a unifying theory to explain the
similarities and differences within life’s organisms and processes. Figure 19.1A. 1 : Evolution on earth: Evolution has resulted in living
Populations (or gene pools ) evolve as gene frequencies change; things that may be single-celled or complex, multicellular
organisms. They may be plants, animals, fungi, bacteria, or archaea.
individual organisms cannot evolve. This diversity results from evolution.
Variation in populations is determined by the genes present in the
population’s gene pool, which may be directly altered by GENETIC VARIATION IN POPULATIONS
mutation. A population is a group of individuals that can all interbreed, often
Natural selection is the gradual process that increases the distinguished as a species. Because these individuals can share genes
frequency of advantageous inherited traits (allowing it to survive and pass on combinations of genes to the next generation, the
and reproduce) and decreases the frequency of detrimental collection of these genes is called a gene pool. The process of
inherited traits within a population. evolution occurs only in populations and not in individuals. A single
A population’s genetic makeup can also be affected by random individual cannot evolve alone; evolution is the process of changing
chance events like genetic drift, or when genes are inherited the gene frequencies within a gene pool. Five forces can cause
together in genetic hitchhiking. genetic variation and evolution in a population: mutations, natural
selection, genetic drift, genetic hitchhiking, and gene flow.
KEY TERMS
gene flow: the transfer of alleles or genes from one population to MUTATIONS
another Why do some organisms survive while others die? These surviving
genetic hitchhiking: a phenomenon in which a gene increases in organisms generally possess traits or characteristics that bestow
a population because it lies near genes on the same chromosome benefits that help them survive (e.g., better camouflage, faster
that are advantageous to an organism swimming, or more efficient digestion). Each of these characteristics
genetic drift: an overall shift of allele distribution in an isolated is the result of a mutation, or a change in the genetic code. Mutations
population, due to random fluctuations in the frequencies of occur spontaneously, but not all mutations are heritable; they are
individual alleles of the genes passed down to offspring only if the mutations occur in the gametes.
fitness: an individual’s ability to propagate its genes These heritable mutations are responsible for the rise of new traits in
natural selection: a process in which individual organisms or a population.
phenotypes that possess favorable traits are more likely to
survive and reproduce NATURAL SELECTION
mutation: any heritable change of the base-pair sequence of Just as mutations cause new traits in a population, natural selection
genetic material acts on the frequency of those traits. Because there are more
organisms than resources, all organisms are in a constant struggle for
THE EVOLUTION OF POPULATIONS existence. In natural selection, those individuals with superior traits
According to evolutionary theory, every organism from humans to will be able to produce more offspring. The more offspring an
beetles to plants to bacteria share a common ancestor. Millions of organism can produce, the higher its fitness. As novel traits and
years of evolutionary pressure caused some organisms to died while behaviors arise from mutation, natural selection perpetuates the traits
others survived, leaving earth with the diverse life forms we have that confer a benefit.
today. Within this diversity is unity; for example, all organisms are
composed of cells and use DNA. The theory of evolution gives us a
unifying theory to explain the similarities and differences within
life’s organisms and processes.

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 Mutation creates
variation

Unfavorable mutations
selected against

Reproduction and
mutation occur Figure 19.1A. 1 : Genetic drift and gene fixation: In this simulation,
there is fixation in the blue gene variation within five generations.
Favorable mutations Images these dots are beetles and some of them are destroyed by a
more likely to survive
wildfire. As the surviving population changes over time, some traits
(red) may be completely eliminated from the population, leaving
… and reproduce only the beetles with other traits (blue).

Figure 19.1A. 1 : Mutation and natural selection: As mutations GENETIC HITCHHIKING

create variation, natural selection affects the frequency of that trait in
a population. Mutations that confer a benefit (such as running faster When recombination occurs during sexual reproduction, genes are
or digesting food more efficiently) can help that organism survive usually shuffled so that each parent gives its offspring a random
and reproduce, carrying the mutation to the next generation. assortment of its genetic variation. However, genes that are close
together on the same chromosome are often assorted together.
GENETIC DRIFT
Therefore, the frequency of a gene may increase in a population
When selective forces are absent or relatively weak, gene
through genetic hitchhiking if its proximal genes confer a benefit.
frequencies tend to “drift” due to random events. This drift halts
when the variation of the gene becomes “fixed” by either GENE FLOW
disappearing from the population or replacing the other variations
Gene flow is the exchange of genes between populations or between
completely. Even in the absence of selective forces, genetic drift can species.If the gene pools between two populations are different, the
cause two separate populations that began with the same genetic
exchange of genes can introduce variation that is advantageous or
structure to drift apart into two divergent populations. disadvantageous to one of the populations. If advantageous, this
gene variation may replace all the other variations until the entire
population exhibits that trait.

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19.1B: POPULATION GENETICS
A A A B
Population genetics is the study of the distributions and changes of such as I I or I I . Although each organism can only carry two
allele frequency in a population. alleles, more than those two alleles may be present in the larger
population. For example, in a population of fifty people where all
 LEARNING OBJECTIVES the blood types are represented, there may be more IA alleles than i
alleles. Population genetics is the study of how selective forces
Define a population gene pool and explain how the size of change a population through changes in allele and genotypic
the gene pool can affect the evolutionary success of a frequencies.
population
ALLELE FREQUENCY
KEY POINTS The allele frequency (or gene frequency) is the rate at which a
A gene pool is the sum of all the alleles (variants of a gene) in a specific allele appears within a population. In population genetics,
population. the term evolution is defined as a change in the frequency of an
Allele frequencies range from 0 (present in no individuals) to 1 allele in a population. Frequencies range from 0, present in no
(present in all individuals); all allele frequencies for a given gene individuals, to 1, present in all individuals. The gene pool is the sum
add up to 100 percent in a population. of all the alleles at all genes in a population.
The smaller a population, the more susceptible it is to Using the ABO blood type system as an example, the frequency of
mechanisms like natural selection and genetic drift, as the effects one of the alleles, for example IA, is the number of copies of that
of such mechanisms are magnified when the gene pool is small. allele divided by all the copies of the ABO gene in the population,
The founder effect occurs when part of an original population i.e. all the alleles. Allele frequencies can be expressed as a decimal
establishes a new population with a separate gene pool, leading or as a percent and always add up to 1, or 100 percent, of the total
to less genetic variation in the new population. population. For example, in a sample population of humans, the
frequency of the IA allele might be 0.26, which would mean that
KEY TERMS 26% of the chromosomes in that population carry the IA allele. If we
allele: one of a number of alternative forms of the same gene also know that the frequency of the IB allele in this population is
occupying a given position on a chromosome 0.14, then the frequency of the i allele is 0.6, which we obtain by
gene pool: the complete set of unique alleles that would be found subtracting all the known allele frequencies from 1 (thus: 1 – 0.26 –
by inspecting the genetic material of every living member of a 0.14 = 0.6). A change in any of these allele frequencies over time
species or population would constitute evolution in the population.
founder effect: a decrease in genetic variation that occurs when
an entire population descends from a small number of founders POPULATION SIZE AND EVOLUTION
When allele frequencies within a population change randomly with
POPULATION GENETICS no advantage to the population over existing allele frequencies, the
A gene for a particular characteristic may have several variations phenomenon is called genetic drift. The smaller a population, the
called alleles. These variations code for different traits associated more susceptible it is to mechanisms such as genetic drift as alleles
with that characteristic. For example, in the ABO blood type system are more likely to become fixed at 0 (absent) or 1 (universally
in humans, three alleles (IA, IB, or i) determine the particular blood- present). Random events that alter allele frequencies will have a
type protein on the surface of red blood cells. A human with a type much larger effect when the gene pool is small. Genetic drift and
IA allele will display A-type proteins (antigens) on the surface of natural selection usually occur simultaneously in populations, but
their red blood cells. Individuals with the phenotype of type A blood the cause of the frequency change is often impossible to determine.
have the genotype IAIA or IAi, type B have IBIB or IBi, type AB have Natural selection also affects allele frequency. If an allele confers a
IAIB, and type O have ii. phenotype that enables an individual to better survive or have more
offspring, the frequency of that allele will increase. Because many of
those offspring will also carry the beneficial allele and, therefore, the
phenotype, they will have more offspring of their own that also carry
the allele. Over time, the allele will spread throughout the population
and may become fixed: every individual in the population carries the
allele. If an allele is dominant but detrimental, it may be swiftly
eliminated from the gene pool when the individual with the allele
Figure 19.1B. 1: ABO blood type in humans: In humans, each blood
type corresponds to a combination of two alleles, which represent a does not reproduce. However, a detrimental recessive allele can
the type of antigens displayed on the outside of a red blood cell. linger for generations in a population, hidden by the dominant allele
Human blood types are A, B, AB, and O. in heterozygotes. In such cases, the only individuals to be eliminated
A diploid organism can only carry two alleles for a particular gene. from the population are those unlucky enough to inherit two copies
In human blood type, the combinations are composed of two alleles of such an allele.

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THE FOUNDER EFFECT
The founder effect occurs when part of a population becomes
isolated and establishes a separate gene pool with its own allele
frequencies. When a small number of individuals become the basis
of a new population, this new population can be very different
genetically from the original population if the founders are not
representative of the original. Therefore, many different populations,
with very different and uniform gene pools, can all originate from
the same, larger population. Together, the forces of natural selection,
genetic drift, and founder effect can lead to significant changes in
the gene pool of a population.

Figure 19.1B. 1: The Founder Effect: Here are three possible

outcomes of the founder effect, each with gene pools separate from
the original populations.

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19.1C: HARDY-WEINBERG PRINCIPLE OF EQUILIBRIUM

 LEARNING OBJECTIVES

Use the Hardy Weinberg equation to calculate allelic and

genotypic frequencies in a population

The Hardy-Weinberg principle states that a population’s allele and

genotype frequencies will remain constant in the absence of
evolutionary mechanisms. Ultimately, the Hardy-Weinberg principle
models a population without evolution under the following
conditions:
1. no mutations
2. no immigration/emigration
3. no natural selection
4. no sexual selection
5. a large population
Although no real-world population can satisfy all of these
Figure 19.1C. 1 : Hardy-Weinberg proportions for two alleles: The
conditions, the principle still offers a useful model for population horizontal axis shows the two allele frequencies p and q and the
analysis. vertical axis shows the expected genotype frequencies.Each line
shows one of the three possible genotypes.
HARDY-WEINBERG EQUATIONS AND ANALYSIS In our example, the possible genotypes are homozygous dominant
According to the Hardy-Weinberg principle, the variable p often (YY), heterozygous (Yy), and homozygous recessive (yy). If we can
represents the frequency of a particular allele, usually a dominant only observe the phenotypes in the population, then we know only
one. For example, assume that p represents the frequency of the the recessive phenotype (yy). For example, in a garden of 100 pea
dominant allele, Y, for yellow pea pods. The variable q represents plants, 86 might have yellow peas and 16 have green peas. We do
the frequency of the recessive allele, y, for green pea pods. If p and q not know how many are homozygous dominant (Yy) or
are the only two possible alleles for this characteristic, then the sum heterozygous (Yy), but we do know that 16 of them are homozygous
of the frequencies must add up to 1, or 100 percent. We can also recessive (yy).
write this as p + q = 1.If the frequency of the Y allele in the Therefore, by knowing the recessive phenotype and, thereby, the
population is 0.6, then we know that the frequency of the y allele is frequency of that genotype (16 out of 100 individuals or 0.16), we
0.4. can calculate the number of other genotypes. If q2 represents the
From the Hardy-Weinberg principle and the known allele frequency of homozygous recessive plants, then q2 = 0.16.
frequencies, we can also infer the frequencies of the genotypes. Therefore, q = 0.4.Because p + q = 1, then 1 – 0.4 = p, and we know
Since each individual carries two alleles per gene (Y or y), we can that p = 0.6. The frequency of homozygous dominant plants (p2) is
predict the frequencies of these genotypes with a chi square. If two (0.6)2 = 0.36. Out of 100 individuals, there are 36 homozygous
alleles are drawn at random from the gene pool, we can determine dominant (YY) plants. The frequency of heterozygous plants (2pq)
the probability of each genotype. is 2(0.6)(0.4) = 0.48. Therefore, 48 out of 100 plants are
In the example, our three genotype possibilities are: pp (YY), heterozygous yellow (Yy).
producing yellow peas; pq (Yy), also yellow; or qq (yy), producing
green peas. The frequency of homozygous pp individuals is p2; the
frequency of hereozygous pq individuals is 2pq; and the frequency
of homozygous qq individuals is q2. If p and q are the only two
possible alleles for a given trait in the population, these genotypes
frequencies will sum to one: p2 + 2pq + q2 = 1.

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 The frequency of alleles can be estimated by calculating the
frequency of the recessive genotype, then calculating the square
root of that frequency in order to determine the frequency of the
recessive allele.

KEY TERMS
genotype: the combination of alleles, situated on corresponding
chromosomes, that determines a specific trait of an individual,
such as “Aa” or “aa”
phenotype: the appearance of an organism based on a
multifactorial combination of genetic traits and environmental
factors, especially used in pedigrees

CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44575/latest...ol11448/latest. License: CC
BY: Attribution
Evolution. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/Evolution%23Mechanisms. License: CC BY-SA:
Attribution-ShareAlike
gene flow. Provided by: Wikipedia. Located at:
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Boundless. Provided by: Boundless Learning. Located at:
www.boundless.com//biology/de...ic-hitchhiking. License: CC BY-SA:
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fitness. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/fitness.
License: CC BY-SA: Attribution-ShareAlike
mutation. Provided by: Wiktionary. Located at:
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Natural Selection. Provided by: Wikipedia. Located at:
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are in the Hardy-Weinberg equilibrium, the allelic frequency is en.wiktionary.org/wiki/genetic_drift. License: CC BY-SA: Attribution-
stable from generation to generation and the distribution of alleles ShareAlike
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differs from the predicted value, scientists can make inferences en.wiktionary.org/wiki/natural_selection. License: CC BY-SA: Attribution-
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about what evolutionary forces are at play.
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CNX. Located at: http://cnx.org/content/m44575/latest...e_19_00_01.jpg.
APPLICATIONS OF HARDY-WEINBERG License: CC BY: Attribution
Mutation and selection diagram. Provided by: Wikipedia. Located at:
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from genetic drift, mutation, migration, and natural and sexual Attribution-ShareAlike
Random sampling genetic drift. Provided by: Wikipedia. Located at:
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License: CC BY-SA: Attribution-ShareAlike
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KEY POINTS ShareAlike
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SECTION OVERVIEW

19.2: POPULATION GENETICS

19.2C: GENE FLOW AND MUTATION
Topic hierarchy
19.2D: NONRANDOM MATING AND
ENVIRONMENTAL VARIANCE
19.2A: GENETIC VARIATION

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license and was authored, remixed, and/or curated by Boundless.

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19.2A: GENETIC VARIATION
Genetic variation is a measure of the variation that exists in the from others.
genetic makeup of individuals within population.

 LEARNING OBJECTIVES

Assess the ways in which genetic variance affects the

evolution of populations

KEY POINTS
Genetic variation is an important force in evolution as it allows
natural selection to increase or decrease frequency of alleles
already in the population.
Genetic variation can be caused by mutation (which can create
entirely new alleles in a population), random mating, random
fertilization, and recombination between homologous
chromosomes during meiosis (which reshuffles alleles within an
organism’s offspring).
Genetic variation is advantageous to a population because it
enables some individuals to adapt to the environment while
maintaining the survival of the population.

KEY TERMS
genetic diversity: the level of biodiversity, refers to the total Figure 19.2A. 1 : Genetic variation in the shells of Donax variabilis:
number of genetic characteristics in the genetic makeup of a An enormous amount of phenotypic variation exists in the shells of
Donax varabilis, otherwise known as the coquina mollusc. This
species phenotypic variation is due at least partly to genetic variation within
crossing over: the exchange of genetic material between the coquina population.
homologous chromosomes that results in recombinant
chromosomes
EVOLUTION AND ADAPTATION TO THE
phenotypic variation: variation (due to underlying heritable
ENVIRONMENT
genetic variation); a fundamental prerequisite for evolution by Variation allows some individuals within a population to adapt to the
natural selection changing environment. Because natural selection acts directly only
genetic variation: variation in alleles of genes that occurs both on phenotypes, more genetic variation within a population usually
within and among populations enables more phenotypic variation. Some new alleles increase an
organism’s ability to survive and reproduce, which then ensures the
GENETIC VARIATION survival of the allele in the population. Other new alleles may be
Genetic variation is a measure of the genetic differences that exist immediately detrimental (such as a malformed oxygen-carrying
within a population. The genetic variation of an entire species is protein) and organisms carrying these new mutations will die out.
often called genetic diversity. Genetic variations are the differences Neutral alleles are neither selected for nor against and usually
in DNA segments or genes between individuals and each variation remain in the population. Genetic variation is advantageous because
of a gene is called an allele.For example, a population with many it enables some individuals and, therefore, a population, to survive
different alleles at a single chromosome locus has a high amount of despite a changing environment.
genetic variation. Genetic variation is essential for natural selection
because natural selection can only increase or decrease frequency of
alleles that already exist in the population.
Genetic variation is caused by:
mutation
random mating between organisms
random fertilization
crossing over (or recombination) between chromatids of
homologous chromosomes during meiosis
The last three of these factors reshuffle alleles within a population,
giving offspring combinations which differ from their parents and

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 GEOGRAPHIC VARIATION
Some species display geographic variation as well as variation
within a population. Geographic variation, or the distinctions in the
genetic makeup of different populations, often occurs when
populations are geographically separated by environmental barriers
or when they are under selection pressures from a different
environment. One example of geographic variation are clines:
graded changes in a character down a geographic axis.

SOURCES OF GENETIC VARIATION

Gene duplication, mutation, or other processes can produce new
genes and alleles and increase genetic variation. New genetic
variation can be created within generations in a population, so a
population with rapid reproduction rates will probably have high
genetic variation. However, existing genes can be arranged in new
ways from chromosomal crossing over and recombination in sexual
reproduction. Overall, the main sources of genetic variation are the
formation of new alleles, the altering of gene number or position,
rapid reproduction, and sexual reproduction.

This page titled 19.2A: Genetic Variation is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

Figure 19.2A. 1 : Low genetic diversity in the wild cheetah

population: Populations of wild cheetahs have very low genetic
variation. Because wild cheetahs are threatened, their species has a
very low genetic diversity. This low genetic diversity means they are
often susceptible to disease and often pass on lethal recessive
mutations; only about 5% of cheetahs survive to adulthood.

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19.2B: GENETIC DRIFT
Genetic drift is the change in allele frequencies of a population due of alleles within a population due to chance events that cause
to random chance events, such as natural disasters. random samples of the population to reproduce or not.

 LEARNING OBJECTIVES

Distinguish between selection and genetic drift

KEY POINTS
Genetic drift is the change in the frequency of an allele in a
population due to random sampling and the random events that
influence the survival and reproduction of those individuals.
The bottleneck effect occurs when a natural disaster or similar
event randomly kills a large portion (i.e. random sample) of the
population, leaving survivors that have allele frequencies that
were very different from the previous population.
The founder effect occurs when a portion of the population (i.e.
“founders”) separates from the old population to start a new
population with different allele frequencies.
Small populations are more susceptible genetic drift than large
populations, whose larger numbers can buffer the population
against chance events.

KEY TERMS
genetic drift: an overall shift of allele distribution in an isolated
population, due to random sampling
founder effect: a decrease in genetic variation that occurs when
an entire population descends from a small number of founders
random sampling: a subset of individuals (a sample) chosen
from a larger set (a population) by chance

GENETIC DRIFT VS. NATURAL SELECTION

Genetic drift is the converse of natural selection. The theory of
natural selection maintains that some individuals in a population
have traits that enable to survive and produce more offspring, while
other individuals have traits that are detrimental and may cause them
to die before reproducing. Over successive generation, these
selection pressures can change the gene pool and the traits within the
population. For example, a big, powerful male gorilla will mate with
more females than a small, weak male and therefore more of his
genes will be passed on to the next generation. His offspring may
continue to dominate the troop and pass on their genes as well. Over
time, the selection pressure will cause the allele frequencies in the
gorilla population to shift toward large, strong males. Figure 19.2B. 1: Effect of genetic drift: Genetic drift in a population
can lead to the elimination of an allele from that population by
Unlike natural selection, genetic drift describes the effect of chance chance. In this example, the brown coat color allele (B) is dominant
on populations in the absence of positive or negative selection over the white coat color allele (b). In the first generation, the two
pressure. Through random sampling, or the survival or and alleles occur with equal frequency in the population, resulting in p
and q values of.5. Only half of the individuals reproduce, resulting
reproduction of a random sample of individuals within a population, in a second generation with p and q values of.7 and.3, respectively.
allele frequencies within a population may change. Rather than a Only two individuals in the second generation reproduce and, by
male gorilla producing more offspring because he is stronger, he chance, these individuals are homozygous dominant for brown coat
color. As a result, in the third generation the recessive b allele is lost.
may be the only male available when a female is ready to mate. His
Small populations are more susceptible to the forces of genetic drift.
genes are passed on to future generation because of chance, not
Large populations, on the other hand, are buffered against the effects
because he was the biggest or the strongest. Genetic drift is the shift
of chance. If one individual of a population of 10 individuals

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happens to die at a young age before leaving any offspring to the
next generation, all of its genes (1/10 of the population’s gene pool)
will be suddenly lost. In a population of 100, that individual
represents only 1 percent of the overall gene pool; therefore, genetic
drift has much less impact on the larger population’s genetic
structure.

THE BOTTLENECK EFFECT

Genetic drift can also be magnified by natural events, such as a
natural disaster that kills a large portion of the population at random.
The bottleneck effect occurs when only a few individuals survive
and reduces variation in the gene pool of a population. The genetic
structure of the survivors becomes the genetic structure of the entire
population, which may be very different from the pre-disaster Figure 19.2B. 1: The Founder Effect: The founder effect occurs
when a portion of the population (i.e. “founders”) separates from the
population.
old population to start a new population with different allele
frequencies.
The founder effect is believed to have been a key factor in the
genetic history of the Afrikaner population of Dutch settlers in South
Africa, as evidenced by mutations that are common in Afrikaners,
but rare in most other populations. This was probably due to the fact
that a higher-than-normal proportion of the founding colonists
carried these mutations. As a result, the population expresses
unusually high incidences of Huntington’s disease (HD) and Fanconi
anemia (FA), a genetic disorder known to cause blood marrow and
congenital abnormalities, even cancer.

DRIFT AND FIXATION

The Hardy–Weinberg principle states that within sufficiently large
populations, the allele frequencies remain constant from one
generation to the next unless the equilibrium is disturbed by
Figure 19.2B. 1: Effect of a bottleneck on a population: A chance migration, genetic mutation, or selection.
event or catastrophe can reduce the genetic variability within a Because the random sampling can remove, but not replace, an allele,
population.
and because random declines or increases in allele frequency
THE FOUNDER EFFECT influence expected allele distributions for the next generation,
Another scenario in which populations might experience a strong genetic drift drives a population towards genetic uniformity over
influence of genetic drift is if some portion of the population leaves time. When an allele reaches a frequency of 1 (100%) it is said to be
to start a new population in a new location or if a population gets “fixed” in the population and when an allele reaches a frequency of
divided by a physical barrier of some kind. In this situation, it is 0 (0%) it is lost. Once an allele becomes fixed, genetic drift for that
improbable that those individuals are representative of the entire allele comes to a halt, and the allele frequency cannot change unless
population, which results in the founder effect. The founder effect a new allele is introduced in the population via mutation or gene
occurs when the genetic structure changes to match that of the new flow. Thus even while genetic drift is a random, directionless
population’s founding fathers and mothers. process, it acts to eliminate genetic variation over time.

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 This page titled 19.2B: Genetic Drift is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

Figure 19.2B. 1: Genetic drift over time: Ten simulations of random

genetic drift of a single given allele with an initial frequency
distribution 0.5 measured over the course of 50 generations, repeated
in three reproductively synchronous populations of different sizes. In
these simulations, alleles drift to loss or fixation (frequency of 0.0 or
1.0) only in the smallest population.Effect of population size on
genetic drift: Ten simulations each of random change in the
frequency distribution of a single hypothetical allele over 50
generations for different sized populations; first population size
n=20, second population n=200, and third population n=2000.

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19.2C: GENE FLOW AND MUTATION
A population’s genetic variation changes as individuals migrate into
or out of a population and when mutations introduce new alleles.

 LEARNING OBJECTIVES

Explain how gene flow and mutations can influence the

allele frequencies of a population

KEY POINTS
Plant populations experience gene flow by spreading their pollen
long distances. Figure 19.2C. 1 : Gene flow: Gene flow can occur when an
Animals experience gene flow when individuals leave a family individual travels from one geographic location to another.
group or herd to join other populations. Maintained gene flow between two populations can also lead to a
The flow of individuals in and out of a population introduces combination of the two gene pools, reducing the genetic variation
new alleles and increases genetic variation within that between the two groups. Gene flow strongly acts against speciation,
population. by recombining the gene pools of the groups, and thus, repairing the
Mutations are changes to an organism’s DNA that create developing differences in genetic variation that would have led to
diversity within a population by introducing new alleles. full speciation and creation of daughter species.
Some mutations are harmful and are quickly eliminated from the For example, if a species of grass grows on both sides of a highway,
population by natural selection; harmful mutations prevent pollen is likely to be transported from one side to the other and vice
organisms from reaching sexual maturity and reproducing. versa. If this pollen is able to fertilize the plant where it ends up and
Other mutations are beneficial and can increase in a population if produce viable offspring, then the alleles in the pollen have
they help organisms reach sexual maturity and reproduce. effectively linked the population on one side of the highway with the
other.
KEY TERMS
gene flow: the transfer of alleles or genes from one population to MUTATION
another Mutations are changes to an organism’s DNA and are an important
mutation: any heritable change of the base-pair sequence of driver of diversity in populations. Species evolve because of the
genetic material accumulation of mutations that occur over time. The appearance of
new mutations is the most common way to introduce novel
GENE FLOW genotypic and phenotypic variance. Some mutations are unfavorable
An important evolutionary force is gene flow: the flow of alleles in or harmful and are quickly eliminated from the population by natural
and out of a population due to the migration of individuals or selection. Others are beneficial and will spread through the
gametes. While some populations are fairly stable, others experience population. Whether or not a mutation is beneficial or harmful is
more movement and fluctuation. Many plants, for example, send determined by whether it helps an organism survive to sexual
their pollen by wind, insects, or birds to pollinate other populations maturity and reproduce. Some mutations have no effect on an
of the same species some distance away. Even a population that may organism and can linger, unaffected by natural selection, in the
initially appear to be stable, such as a pride of lions, can receive new genome while others can have a dramatic effect on a gene and the
genetic variation as developing males leave their mothers to form resulting phenotype.
new prides with genetically-unrelated females. This variable flow of
individuals in and out of the group not only changes the gene
structure of the population, but can also introduce new genetic
variation to populations in different geological locations and
habitats.

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 This page titled 19.2C: Gene Flow and Mutation is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

Figure 19.2C. 1 : Mutation in a garden rose: A mutation has caused

this garden moss rose to produce flowers of different colors. This
mutation has introduce a new allele into the population that
increases genetic variation and may be passed on to the next
generation.

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19.2D: NONRANDOM MATING AND ENVIRONMENTAL VARIANCE
Population structure can be altered by nonrandom mating (the
preference of certain individuals for mates) as well as the
environment.

 LEARNING OBJECTIVES

Explain how environmental variance and nonrandom mating

can change gene frequencies in a population

KEY POINTS
Nonrandom mating can occur when individuals prefer mates
with particular superior physical characteristics or by the
preference of individuals to mate with individuals similar to
themselves.
Nonrandom mating can also occur when mates are chosen based
on physical accessibility; that is, the availability of some mates
Figure 19.2D. 1 : Assortative mating in the American Robin: The
over others. American Robin may practice assortative mating on plumage color,
Phenotypes of individuals can also be influenced by the a melanin based trait, and mate with other robins who have the most
environment in which they live, such as temperature, terrain, or similar shade of color. However, there may also be some sexual
selection for more vibrant plumage which indicates health and
other factors. reproductive performance.
A cline occurs when populations of a given species vary Another cause of nonrandom mating is physical location. This is
gradually across an ecological gradient. especially true in large populations spread over large geographic
distances where not all individuals will have equal access to one
KEY TERMS another. Some might be miles apart through woods or over rough
cline: a gradation in a character or phenotype within a species or terrain, while others might live immediately nearby.
other group
sexual selection: a mode of natural selection in which some ENVIRONMENTAL VARIANCE
individuals out-reproduce others of a population because they are Genes are not the only players involved in determining population
better at securing mates variation. Phenotypes are also influenced by other factors, such as
assortative mating: between males and females of a species, the the environment. A beachgoer is likely to have darker skin than a
mutual attraction or selection, for reproductive purposes, of city dweller, for example, due to regular exposure to the sun, an
individuals with similar characteristics environmental factor. Some major characteristics, such as gender,
are determined by the environment for some species. For example,
NONRANDOM MATING
some turtles and other reptiles have temperature-dependent sex
If individuals nonrandomly mate with other individuals in the determination (TSD). TSD means that individuals develop into
population, i.e. they choose their mate, choices can drive evolution males if their eggs are incubated within a certain temperature range,
within a population. There are many reasons nonrandom mating or females at a different temperature range.
occurs. One reason is simple mate choice or sexual selection; for
example, female peahens may prefer peacocks with bigger, brighter
tails. Traits that lead to more matings for an individual lead to more
offspring and through natural selection, eventually lead to a higher
frequency of that trait in the population. One common form of mate
choice, called positive assortative mating, is an individual’s
preference to mate with partners that are phenotypically similar to
themselves.

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 genetic variation. Provided by: Wikipedia. Located at:
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phenotypic variation. Provided by: Wikipedia. Located at:
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Genetic Diversity. Provided by: Wikipedia. Located at:
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Figure 19.2D. 1 : Temperature-dependent sex determination: The
founder effect. Provided by: Wiktionary. Located at:
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determined by the temperature at which the eggs are incubated. Eggs ShareAlike
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degrees C produce males. en.wiktionary.org/wiki/genetic_drift. License: CC BY-SA: Attribution-
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Geographic separation between populations can lead to differences
Coquina variation3. Provided by: Wikipedia. Located at:
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geographical variation is seen between most populations and can be Attribution-ShareAlike
Cheetah genetic diversity. Provided by: Wikimedia. Located at:
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variation in moose; body mass increase positively with latitude. Attribution
Bergmann’s Rule is an ecologic principle which states that as Random genetic drift chart. Provided by: Wikimedia. Located at:
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between the two, supporting Bergmann’s Rule. Attribution
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Species of warm-blooded animals, for example, tend to have larger Located at: http://cnx.org/content/m44584/latest...ol11448/latest. License: CC
bodies in the cooler climates closer to the earth’s poles, allowing BY: Attribution
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them to better conserve heat. This is considered a latitudinal cline. Located at: http://cnx.org/content/m44584/latest...ol11448/latest. License: CC
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If there is gene flow between the populations, the individuals will en.wiktionary.org/wiki/mutation. License: CC BY-SA: Attribution-ShareAlike
likely show gradual differences in phenotype along the cline. Coquina variation3. Provided by: Wikipedia. Located at:
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CONTRIBUTIONS AND ATTRIBUTIONS Founder effect with drift. Provided by: Wikimedia. Located at:
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A-level Biology/Central Concepts/Classification, selection and evolution. Random genetic drift chart. Provided by: Wikimedia. Located at:
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crossing over. Provided by: Wikipedia. Located at: Attribution
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SECTION OVERVIEW

19.3: ADAPTIVE EVOLUTION

19.3C: FREQUENCY-DEPENDENT SELECTION
Topic hierarchy
19.3D: SEXUAL SELECTION
19.3A: NATURAL SELECTION AND ADAPTIVE 19.3E: NO PERFECT ORGANISM
EVOLUTION

19.3B: STABILIZING, DIRECTIONAL, AND This page titled 19.3: Adaptive Evolution is shared under a CC BY-SA 4.0
DIVERSIFYING SELECTION license and was authored, remixed, and/or curated by Boundless.

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19.3A: NATURAL SELECTION AND ADAPTIVE EVOLUTION
Natural selection drives adaptive evolution by selecting for and
increasing the occurrence of beneficial traits in a population.

 LEARNING OBJECTIVES

Explain how natural selection leads to adaptive evolution

KEY POINTS
Natural selection increases or decreases biological traits within a
population, thereby selecting for individuals with greater
evolutionary fitness.
An individual with a high evolutionary fitness will provide more Figure 19.3A. 1 : Adaptive evolution in finches: Through natural
selection, a population of finches evolved into three separate species
beneficial contributions to the gene pool of the next generation.
by adapting to several difference selection pressures. Each of the
Relative fitness, which compares an organism’s fitness to the three modern finches has a beak adapted to its life history and diet.
others in the population, allows researchers to establish how a Fitness is often quantifiable and is measured by scientists in the
population may evolve by determining which individuals are field. However, it is not the absolute fitness of an individual that
contributing additional offspring to the next generation. counts, but rather how it compares to the other organisms in the
Stabilizing selection, directional selection, diversifying selection, population. This concept, called relative fitness, allows researchers
frequency -dependent selection, and sexual selection all to determine which individuals are contributing additional offspring
contribute to the way natural selection can affect variation within to the next generation and, thus, how the population might evolve.
a population.
There are several ways selection can affect population variation:
KEY TERMS stabilizing selection
natural selection: a process in which individual organisms or directional selection
phenotypes that possess favorable traits are more likely to diversifying selection
survive and reproduce frequency-dependent selection
fecundity: number, rate, or capacity of offspring production sexual selection
Darwinian fitness: the average contribution to the gene pool of As natural selection influences the allele frequencies in a population,
the next generation that is made by an average individual of the individuals can either become more or less genetically similar and
specified genotype or phenotype the phenotypes displayed can become more similar or more
disparate. In the end, natural selection cannot produce perfect
AN INTRODUCTION TO ADAPTIVE EVOLUTION organisms from scratch, it can only generate populations that are
Natural selection only acts on the population’s heritable traits: better adapted to survive and successfully reproduce in their
selecting for beneficial alleles and, thus, increasing their frequency environments through the aforementioned selections.
in the population, while selecting against deleterious alleles and,
thereby, decreasing their frequency. This process is known as
adaptive evolution. Natural selection does not act on individual Galápagos with David Attenborough
alleles, however, but on entire organisms. An individual may carry a
very beneficial genotype with a resulting phenotype that, for
example, increases the ability to reproduce ( fecundity ), but if that
same individual also carries an allele that results in a fatal childhood
disease, that fecundity phenotype will not be passed on to the next
generation because the individual will not live to reach reproductive
age. Natural selection acts at the level of the individual; it selects for
individuals with greater contributions to the gene pool of the next
generation, known as an organism’s evolutionary fitness (or
Darwinian fitness).

19.3A.1 https://bio.libretexts.org/@go/page/13487
 Galápagos with David Attenborough: Two hundred years after
Charles Darwin set foot on the shores of the Galápagos Islands,
David Attenborough travels to this wild and mysterious archipelago.
Amongst the flora and fauna of these enchanted volcanic islands,
Darwin formulated his groundbreaking theories on evolution.
Journey with Attenborough to explore how life on the islands has
continued to evolve in biological isolation, and how the ever-
changing volcanic landscape has given birth to species and sub-
species that exist nowhere else in the world.

This page titled 19.3A: Natural Selection and Adaptive Evolution is shared
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

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19.3B: STABILIZING, DIRECTIONAL, AND DIVERSIFYING SELECTION
Stabilizing, directional, and diversifying selection either decrease, DIRECTIONAL SELECTION
shift, or increase the genetic variance of a population. When the environment changes, populations will often undergo
directional selection, which selects for phenotypes at one end of the
 LEARNING OBJECTIVES spectrum of existing variation.

Contrast stabilizing selection, directional selection, and A classic example of this type of selection is the evolution of the
diversifying selection. peppered moth in eighteenth- and nineteenth-century England. Prior
to the Industrial Revolution, the moths were predominately light in
color, which allowed them to blend in with the light-colored trees
KEY POINTS
and lichens in their environment. As soot began spewing from
Stabilizing selection results in a decrease of a population ‘s factories, the trees darkened and the light-colored moths became
genetic variance when natural selection favors an average easier for predatory birds to spot.
phenotype and selects against extreme variations.
image
In directional selection, a population’s genetic variance shifts
toward a new phenotype when exposed to environmental Directional selection: Directional selection occurs when a single
changes. phenotype is favored, causing the allele frequency to continuously
Diversifying or disruptive selection increases genetic variance shift in one direction.
when natural selection selects for two or more extreme Over time, the frequency of the melanic form of the moth increased
phenotypes that each have specific advantages. because their darker coloration provided camouflage against the
In diversifying or disruptive selection, average or intermediate sooty tree; they had a higher survival rate in habitats affected by air
phenotypes are often less fit than either extreme phenotype and pollution. Similarly, the hypothetical mouse population may evolve
are unlikely to feature prominently in a population. to take on a different coloration if their forest floor habitat changed.
The result of this type of selection is a shift in the population’s
KEY TERMS genetic variance toward the new, fit phenotype.
directional selection: a mode of natural selection in which a
single phenotype is favored, causing the allele frequency to
continuously shift in one direction
disruptive selection: (or diversifying selection) a mode of
natural selection in which extreme values for a trait are favored
over intermediate values
stabilizing selection: a type of natural selection in which genetic
diversity decreases as the population stabilizes on a particular
trait value

STABILIZING SELECTION
If natural selection favors an average phenotype by selecting against
extreme variation, the population will undergo stabilizing selection.
For example, in a population of mice that live in the woods, natural
selection will tend to favor individuals that best blend in with the
forest floor and are less likely to be spotted by predators. Assuming
the ground is a fairly consistent shade of brown, those mice whose Figure 19.3B. 1: The Evolution of the Peppered Moth: Typica and
fur is most-closely matched to that color will most probably survive carbonaria morphs resting on the same tree.The light-colored typica
(below the bark’s scar) is nearly invisible on this pollution-free tree,
and reproduce, passing on their genes for their brown coat. Mice that camouflaging it from predators.
carry alleles that make them slightly lighter or slightly darker will
stand out against the ground and will more probably die from DIVERSIFYING (OR DISRUPTIVE) SELECTION
predation. As a result of this stabilizing selection, the population’s Sometimes natural selection can select for two or more distinct
genetic variance will decrease. phenotypes that each have their advantages. In these cases, the
image
intermediate phenotypes are often less fit than their extreme
counterparts. Known as diversifying or disruptive selection, this is
Stabilizing selection: Stabilizing selection occurs when the
seen in many populations of animals that have multiple male mating
population stabilizes on a particular trait value and genetic diversity
strategies, such as lobsters. Large, dominant alpha males obtain
decreases.
mates by brute force, while small males can sneak in for furtive
copulations with the females in an alpha male’s territory. In this
case, both the alpha males and the “sneaking” males will be selected

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for, but medium-sized males, which cannot overtake the alpha males
and are too big to sneak copulations, are selected against.
image

Diversifying (or disruptive) selection: Diversifying selection

occurs when extreme values for a trait are favored over the
intermediate values.This type of selection often drives speciation.
Diversifying selection can also occur when environmental changes
favor individuals on either end of the phenotypic spectrum. Imagine
a population of mice living at the beach where there is light-colored
sand interspersed with patches of tall grass. In this scenario, light-
colored mice that blend in with the sand would be favored, as well
as dark-colored mice that can hide in the grass. Medium-colored
mice, on the other hand, would not blend in with either the grass or
the sand and, thus, would more probably be eaten by predators. The
result of this type of selection is increased genetic variance as the
population becomes more diverse.

COMPARING TYPES OF NATURAL SELECTION

Figure 19.3B. 1: Types of natural selection: Different types of

natural selection can impact the distribution of phenotypes within a
population.In (a) stabilizing selection, an average phenotype is
favored.In (b) directional selection, a change in the environment
shifts the spectrum of phenotypes observed.In (c) diversifying
selection, two or more extreme phenotypes are selected for, while
the average phenotype is selected against.

This page titled 19.3B: Stabilizing, Directional, and Diversifying Selection

is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

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19.3C: FREQUENCY-DEPENDENT SELECTION
In frequency-dependent selection, phenotypes that are either strong orange males can fight off the blue males to mate with the
common or rare are favored through natural selection. blue’s pair-bonded females; the blue males are successful at
guarding their mates against yellow sneaker males; and the yellow
 LEARNING OBJECTIVES males can sneak copulations from the potential mates of the large,
polygynous orange males.
Describe frequency-dependent selection

KEY POINTS
Negative frequency -dependent selection selects for rare
phenotypes in a population and increases a population’s genetic
variance.
Positive frequency-dependent selection selects for common
phenotypes in a population and decreases genetic variance.
In the example of male side-blotched lizards, populations of each
color pattern increase or decrease at various stages depending on
their frequency; this ensures that both common and rare
phenotypes continue to be cyclically present.
Infectious agents such as microbes can exhibit negative
frequency-dependent selection; as a host population becomes
immune to a common strain of the microbe, less common strains
of the microbe are automatically favored.
Variation in color pattern mimicry by the scarlet kingsnake is Figure 19.3C. 1 : Frequency-dependent selection in side-blotched
lizards: A yellow-throated side-blotched lizard is smaller than either
dependent on the prevalence of the eastern coral snake, the the blue-throated or orange-throated males and appears a bit like the
model for this mimicry, in a particular geographical region. The females of the species, allowing it to sneak copulations. Frequency-
more prevalent the coral snake is in a region, the more common dependent selection allows for both common and rare phenotypes of
the population to appear in a frequency-aided cycle.
and variable the scarlet kingsnake’s color pattern will be, making
this an example of positive frequency-dependent selection. In this scenario, orange males will be favored by natural selection
when the population is dominated by blue males, blue males will
KEY TERMS thrive when the population is mostly yellow males, and yellow
frequency-dependent selection: the term given to an males will be selected for when orange males are the most populous.
evolutionary process where the fitness of a phenotype is As a result, populations of side-blotched lizards cycle in the
dependent on its frequency relative to other phenotypes in a distribution of these phenotypes. In one generation, orange might be
given population predominant and then yellow males will begin to rise in frequency.
polygynous: having more than one female as mate Once yellow males make up a majority of the population, blue males
will be selected for.Finally, when blue males become common,
FREQUENCY-DEPENDENT SELECTION orange males will once again be favored.
Another type of selection, called frequency-dependent selection, An example of negative frequency-dependent selection can also be
favors phenotypes that are either common (positive frequency- seen in the interaction between the human immune system and
dependent selection) or rare (negative frequency-dependent various infectious microbes such as pathogenic bacteria or viruses.
selection). As a particular human population is infected by a common strain of
microbe, the majority of individuals in the population become
NEGATIVE FREQUENCY-DEPENDENT immune to it. This then selects for rarer strains of the microbe which
SELECTION can still infect the population because of genome mutations; these
An interesting example of this type of selection is seen in a unique strains have greater evolutionary fitness because they are less
group of lizards of the Pacific Northwest. Male common side- common.
blotched lizards come in three throat-color patterns: orange, blue,
and yellow. Each of these forms has a different reproductive POSITIVE FREQUENCY-DEPENDENT SELECTION
strategy: orange males are the strongest and can fight other males for An example of positive frequency-dependent selection is the
access to their females; blue males are medium-sized and form mimicry of the warning coloration of dangerous species of animals
strong pair bonds with their mates; and yellow males are the smallest by other species that are harmless. The scarlet kingsnake, a harmless
and look a bit like female, allowing them to sneak copulations. Like species, mimics the coloration of the eastern coral snake, a
a game of rock-paper-scissors, orange beats blue, blue beats yellow, venomous species typically found in the same geographical region.
and yellow beats orange in the competition for females. The big, Predators learn to avoid both species of snake due to the similar

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coloration, and as a result the scarlet kingsnake becomes more
common, and its coloration phenotype becomes more variable due to
relaxed selection. This phenotype is therefore more “fit” as the
population of species that possess it (both dangerous and harmless)
becomes more numerous. In geographic areas where the coral snake
is less common, the pattern becomes less advantageous to the
kingsnake, and much less variable in its expression, presumably
because predators in these regions are not “educated” to avoid the
pattern.

Figure 19.3C. 1 : Micrurus fulvius, the eastern coral snake: The

eastern coral snake is poisonous.
Negative frequency-dependent selection serves to increase the
population’s genetic variance by selecting for rare phenotypes,
whereas positive frequency-dependent selection usually decreases
genetic variance by selecting for common phenotypes.
Figure 19.3C. 1 : Lampropeltis elapsoides, the scarlet kingsnake: This page titled 19.3C: Frequency-Dependent Selection is shared under a
The scarlet kingsnake mimics the coloration of the poisonous eastern
coral snake. Positive frequency-dependent selection reinforces the CC BY-SA 4.0 license and was authored, remixed, and/or curated by
common phenotype because predators avoid the distinct coloration. Boundless.

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19.3D: SEXUAL SELECTION
Sexual selection, the selection pressure on males and females to
obtain matings, can result in traits designed to maximize sexual
success.

 LEARNING OBJECTIVES

Discuss the effects of sexual dimorphism on the

reproductive potential of an organism

KEY POINTS
Sexual selection often results in the development of secondary
sexual characteristics, which help to maximize a species ‘ Figure 19.3D. 1 : Sexual selection in elk: This male elk has large
antlers to compete with rival males for available females (intrasexual
reproductive success, but do not provide any survival benefits.
competition).Tn addition, the many points on his antlers represent
The handicap principle states that only the best males survive the health and longevity, and therefore he may be more desirable to
risks from traits that may actually be detrimental to a species; females (intersexual selection).
therefore, they are more fit as mating partners.
SEXUAL DIMORPHISM
In the good genes hypothesis, females will choose males that
show off impressive traits to ensure they pass on genetic Males and females of certain species are often quite different from
superiority to their offspring. one another in ways beyond the reproductive organs. Males are often
Sexual dimorphisms, obvious morphological differences between larger, for example, and display many elaborate colors and
the sexes of a species, arise when there is more variance in the adornments, such as the peacock’s tail, while females tend to be
reproductive success of either males or females. smaller and duller in decoration. These differences are called sexual
dimorphisms and arise from the variation in male reproductive
KEY TERMS success.
sexual dimorphism: a physical difference between male and Females almost always mate, while mating is not guaranteed for
female individuals of the same species males. The bigger, stronger, or more decorated males usually obtain
sexual selection: a type of natural selection, where members of the vast majority of the total matings, while other males receive
the sexes acquire distinct forms because members choose mates none. This can occur because the males are better at fighting off
with particular features or because competition for mates with other males, or because females will choose to mate with the bigger
certain traits succeed or more decorated males. In either case, this variation in
handicap principle: a theory that suggests that animals of reproductive success generates a strong selection pressure among
greater biological fitness signal this status through a behavior or males to obtain those matings, resulting in the evolution of bigger
morphology that effectively lowers their chances of survival body size and elaborate ornaments in order to increase their chances
of mating. Females, on the other hand, tend to get a handful of
SEXUAL SELECTION selected matings; therefore, they are more likely to select more
The selection pressures on males and females to obtain matings is desirable males.
known as sexual selection. Sexual selection takes two major forms:
intersexual selection (also known as ‘mate choice’ or ‘female
choice’) in which males compete with each other to be chosen by
females; and intrasexual selection (also known as ‘male–male
competition’) in which members of the less limited sex (typically
males) compete aggressively among themselves for access to the
limiting sex. The limiting sex is the sex which has the higher
parental investment, which therefore faces the most pressure to Figure 19.3D. 1 : Sexual dimorphism: Morphological differences
make a good mate decision. between males and females of the same species is known as sexual
dimorphism.These differences can be observed in (a) peacocks and
peahens, (b) Argiope appensa spiders (the female spider is the large
one), and (c) wood ducks.
Sexual dimorphism varies widely among species; some species are
even sex-role reversed. In such cases, females tend to have a greater
variation in their reproductive success than males and are,
correspondingly, selected for the bigger body size and elaborate
traits usually characteristic of males.

19.3D.1 https://bio.libretexts.org/@go/page/13490
THE HANDICAP PRINCIPLE ability to fight disease. Females then choose males with the most
Sexual selection can be so strong that it selects for traits that are impressive traits because it signals their genetic superiority, which
actually detrimental to the individual’s survival, even though they they will then pass on to their offspring. Though it might be argued
maximize its reproductive success. For example, while the male that females should not be so selective because it will likely reduce
peacock’s tail is beautiful and the male with the largest, most their number of offspring, if better males father more fit offspring, it
colorful tail will more probably win the female, it is not a practical may be beneficial. Fewer, healthier offspring may increase the
appendage. In addition to being more visible to predators, it makes chances of survival more than many, weaker offspring.
the males slower in their attempted escapes. There is some evidence
that this risk, in fact, is why females like the big tails in the first
place. Because large tails carry risk, only the best males survive that BBC Planet Earth - Birds of Paradise
risk and therefore the bigger the tail, the more fit the male. This idea
is known as the handicap principle.

Figure 19.3D. 1 : A male bird of paradise: This male bird of paradise

carries an extremely long tail as the result of sexual selection.The BBC Planet Earth – Birds of Paradise mating dance:
tail is flamboyant and detrimental to the bird’s own survival, but it Extraordinary Courtship displays from these weird and wonderful
increases his reproductive success.This may be an example of the creatures. From episode 1 “Pole to Pole”. This is an example of the
handicap principle.
extreme behaviors that arise from intense sexual selection pressure.
THE GOOD GENES HYPOTHESIS
This page titled 19.3D: Sexual Selection is shared under a CC BY-SA 4.0
The good genes hypothesis states that males develop these
license and was authored, remixed, and/or curated by Boundless.
impressive ornaments to show off their efficient metabolism or their

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19.3E: NO PERFECT ORGANISM
Natural selection cannot create novel, perfect species because it only alleles and corresponding fitness of the phenotype. As a result, good
selects on existing variations in a population. alleles can be lost if they are carried by individuals that also have
several overwhelmingly bad alleles; similarly, bad alleles can be
 LEARNING OBJECTIVES kept if they are carried by individuals that have enough good alleles
to result in an overall fitness benefit.
Explain the limitations encountered in natural selection
POLYMORPHISM
KEY POINTS Furthermore, natural selection can be constrained by the
Natural selection is limited by a population ‘s existing genetic relationships between different polymorphisms. One morph may
variation. confer a higher fitness than another, but may not increase in
Natural selection is limited through linkage disequilibrium, frequency because the intermediate morph is detrimental.
where alleles that are physically proximate on the chromosome
are passed on together at greater frequencies.
In a polymorphic population, two phenotypes may be maintained
in the population despite the higher fitness of one morph if the
intermediate phenotype is detrimental.
Evolution is not purposefully adaptive; it is the result of various Figure 19.3E. 1:
selection forces working together to influence genetic and Polymorphism in the grove snail: Color and pattern morphs of the
phenotypical variances within a population. grove snail, Cepaea nemoralis.The polymorphism, when two or
more different genotypes exist within a given species, in grove snails
KEY TERMS seems to have several causes, including predation by thrushes.
linkage disequilibrium: a non-random association of two or For example, consider a hypothetical population of mice that live in
more alleles at two or more loci; normally caused by an the desert. Some are light-colored and blend in with the sand, while
interaction between genes others are dark and blend in with the patches of black rock. The
genetic hitchhiking: changes in the frequency of an allele dark-colored mice may be more fit than the light-colored mice, and
because of linkage with a positively or negatively selected allele according to the principles of natural selection the frequency of
at another locus light-colored mice is expected to decrease over time. However, the
polymorphism: the regular existence of two or more different intermediate phenotype of a medium-colored coat is very bad for the
genotypes within a given species or population mice: these cannot blend in with either the sand or the rock and will
more vulnerable to predators. As a result, the frequency of a dark-
NO PERFECT ORGANISM
colored mice would not increase because the intermediate morphs
Natural selection is a driving force in evolution and can generate
are less fit than either light-colored or dark-colored mice. This a
populations that are adapted to survive and successfully reproduce in
common example of disruptive selection.
their environments. However, natural selection cannot produce the
perfect organism. Natural selection can only select on existing NOT ALL EVOLUTION IS ADAPTIVE
variation in the population; it cannot create anything from scratch. Finally, it is important to understand that not all evolution is
Therefore, the process of evolution is limited by a population’s adaptive. While natural selection selects the fittest individuals and
existing genetic variance, the physical proximity of alleles, non- often results in a more fit population overall, other forces of
beneficial intermediate morphs in a polymorphic population, and evolution, including genetic drift and gene flow, often do the
non-adaptive evolutionary forces. opposite by introducing deleterious alleles to the population’s gene
pool. Evolution has no purpose. It is not changing a population into
NATURAL SELECTION ACTS ON INDIVIDUALS,
a preconceived ideal. It is simply the sum of various forces and their
NOT ALLELES
influence on the genetic and phenotypic variance of a population.
Natural selection is also limited because it acts on the phenotypes of
individuals, not alleles. Some alleles may be more likely to be CONTRIBUTIONS AND ATTRIBUTIONS
passed on with alleles that confer a beneficial phenotype because of natural selection. Provided by: Wiktionary. Located at:
their physical proximity on the chromosomes. Alleles that are en.wiktionary.org/wiki/natural_selection. License: CC BY-SA: Attribution-
ShareAlike
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is linked to beneficial allele, consequently meaning that it has a Located at: http://cnx.org/content/m44586/latest...ol11448/latest. License: CC
BY: Attribution
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population through genetic hitchhiking (also called genetic draft). en.Wikipedia.org/wiki/Darwinian%20fitness. License: CC BY-SA:
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unfavorable alleles. Natural selection acts on the net effect of these en.wiktionary.org/wiki/fecundity. License: CC BY-SA: Attribution-ShareAlike

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sexual dimorphism. Provided by: Wiktionary. Located at: Lampropeltis elapsoides. Provided by: WikiPedia. Located at:
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www.boundless.com//biology/de...icap-principle. License: CC BY-SA: Bull elk bugling during the fall mating season. Provided by: Wikimedia.
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 CHAPTER OVERVIEW

20: PHYLOGENIES AND THE HISTORY OF LIFE

20.1: Organizing Life on Earth
20.1A: Phylogenetic Trees
20.1B: Limitations of Phylogenetic Trees
20.1C: The Levels of Classification
20.2: Determining Evolutionary Relationships
20.2A: Distinguishing between Similar Traits
20.2B: Building Phylogenetic Trees
20.3: Perspectives on the Phylogenetic Tree
20.3A: Limitations to the Classic Model of Phylogenetic Trees
20.3B: Horizontal Gene Transfer
20.3C: Endosymbiotic Theory and the Evolution of Eukaryotes
20.3D: Web, Network, and Ring of Life Models

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1
SECTION OVERVIEW

20.1: ORGANIZING LIFE ON EARTH

20.1C: THE LEVELS OF CLASSIFICATION
20.1A: PHYLOGENETIC TREES

20.1B: LIMITATIONS OF PHYLOGENETIC TREES This page titled 20.1: Organizing Life on Earth is shared under a CC BY-SA
4.0 license and was authored, remixed, and/or curated by Boundless.

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20.1A: PHYLOGENETIC TREES

 LEARNING OBJECTIVES

Describe the various types of phylogenetic trees and how

they organize life

Scientists use a tool called a phylogenetic tree, a type of diagram, to

show the evolutionary pathways and connections among organisms.
Scientists consider phylogenetic trees to be a hypothesis of the
evolutionary past since one cannot go back to confirm the proposed
relationships. In other words, a “tree of life”, as it is sometimes
called, can be constructed to illustrate when different organisms
evolved and to show the relationships among different organisms.
Unlike a taxonomic classification diagram, a phylogenetic tree can
be read like a map of evolutionary history. Many phylogenetic trees Figure 20.1A. 1 : Rooted phylogenetic trees: The root of a
have a single lineage at the base representing a common ancestor. phylogenetic tree indicates that an ancestral lineage gave rise to all
organisms on the tree. A branch point indicates where two lineages
Scientists call such trees ‘rooted,’ which means there is a single diverged. A lineage that evolved early and remains unbranched is a
ancestral lineage (typically drawn from the bottom or left) to which basal taxon. When two lineages stem from the same branch point,
all organisms represented in the diagram relate. Notice in the rooted they are sister taxa. A branch with more than two lineages is a
polytomy.
phylogenetic tree that the three domains (Bacteria, Archaea, and
Rooted phylogenetic trees can serve as a pathway to understanding
Eukarya) diverge from a single point and branch off. The small
evolutionary history. The pathway can be traced from the origin of
branch that plants and animals (including humans) occupy in this
life to any individual species by navigating through the evolutionary
diagram shows how recent and miniscule these groups are compared
branches between the two points. Also, by starting with a single
with other organisms. Unrooted trees don’t show a common ancestor
species and tracing back towards the “trunk” of the tree, one can
but do show relationships among species.
discover that species’ ancestors, as well as where lineages share a
common ancestry. In addition, the tree can be used to study entire
groups of organisms.
Another point to mention on phylogenetic tree structure is that
rotation at branch points does not change the information. For
example, if a branch point was rotated and the taxon order changed,
this would not alter the information because the evolution of each
Figure 20.1A. 1 : Phylogenetic trees: Both of these phylogenetic taxon from the branch point was independent of the other.
trees shows the relationship of the three domains of life (Bacteria,
Archaea, and Eukarya), but the (a) rooted tree attempts to identify Many disciplines within the study of biology contribute to
when various species diverged from a common ancestor, while the understanding how past and present life evolved over time; together,
(b) unrooted tree does not.
these disciplines contribute to building, updating, and maintaining
In a rooted tree, the branching indicates evolutionary relationships. the “tree of life.” Information is used to organize and classify
The point where a split occurs, called a branch point, represents organisms based on evolutionary relationships in a scientific field
where a single lineage evolved into a distinct new one. A lineage called systematics. Data may be collected from fossils, from
that evolved early from the root and remains unbranched is called studying the structure of body parts or molecules used by an
basal taxon. When two lineages stem from the same branch point, organism, and by DNA analysis. By combining data from many
they are called sister taxa. A branch with more than two lineages is sources, scientists can put together the phylogeny of an organism.
called a polytomy and serves to illustrate where scientists have not Since phylogenetic trees are hypotheses, they will continue to
definitively determined all of the relationships. It is important to change as new types of life are discovered and new information is
note that although sister taxa and polytomy do share an ancestor, it learned.
does not mean that the groups of organisms split or evolved from
each other. Organisms in two taxa may have split apart at a specific KEY POINTS
branch point, but neither taxa gave rise to the other. Rooted trees have a single lineage at the base representing a
common ancestor that connects all organisms presented in a
phylogenetic diagram.
Branch points in a phylogenetic tree represent a split where a
single lineage evolved into a distinct new one, while basal taxon
depict unbranched lineages that evolved early from the root.

20.1A.1 https://bio.libretexts.org/@go/page/13526
 Unrooted trees portray relationships among species, but do not basal taxon: a lineage, displayed using a phylogenetic tree, that
depict their common ancestor. evolved early from the root and from which no other branches
Phylogenetic trees are hypotheses and are, therefore, modified as have diverged
data becomes available. systematics: research into the relationships of organisms; the
Systematics uses data from fossils, the study of bodily structures, science of systematic classification
molecules used by a species, and DNA analysis to contribute to phylogeny: the visual representation of the evolutionary history
the building, updating, and maintaining of phylogenetic trees. of organisms; based on rigorous analyses

KEY TERMS This page titled 20.1A: Phylogenetic Trees is shared under a CC BY-SA 4.0
polytomy: a section of a phylogeny in which the evolutionary license and was authored, remixed, and/or curated by Boundless.
relationships cannot be fully resolved to dichotomies

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20.1B: LIMITATIONS OF PHYLOGENETIC TREES
evolutionary order. In other words, the length of a branch does not
 LEARNING OBJECTIVES typically mean more time passed; nor does a short branch mean less
time passed, unless specified on the diagram. A tree may not
Identify the limitations of phylogenetic trees as
indicate how much time passed between the evolution of amniotic
representations of the organization of life
eggs and hair. What the tree does show is the order in which things
took place. For example, the tree in the diagram shows that the
It may be easy to assume that more closely-related organisms look
oldest trait is the vertebral column, followed by hinged jaws, and so
more alike; while this is often the case, it is not always true. If two
forth. Remember, any phylogenetic tree is a part of the greater whole
closely-related lineages evolved under significantly varied and, as with a real tree, it does not grow in only one direction after a
surroundings or after the evolution of a major new adaptation, it is
new branch develops. So, simply because a vertebral column
possible for the two groups to appear more different than other
evolved does not mean that invertebrate evolution ceased. It only
groups that are not as closely related. For example, the phylogenetic
means that a new branch formed. Also, groups that are not closely
tree shows that lizards and rabbits both have amniotic eggs, whereas
related, but evolve under similar conditions, may appear more
frogs do not; yet lizards and frogs appear more similar than lizards
phenotypically similar to each other than to a close relative.
and rabbits.
KEY POINTS
Closely-related species may not always look more alike, while
groups that are not closely related yet evolved under similar
conditions, may appear more similar to each other.
In phylogenetic trees, branches do not usually account for length
of time and only depict evolutionary order.
Phylogenetic trees are like real trees in that they do not simply
grow in only one direction after a new branch forms; the
evolution of one organism does not necessarily signify the
evolutionary end of another.

KEY TERMS
phenotypical: of or pertaining to a phenotype: the appearance of
Figure 20.1B. 1: Limitations of phylogenetic trees: This ladder-like
phylogenetic tree of vertebrates is rooted by an organism that lacked an organism based on a multifactorial combination of genetic
a vertebral column. At each branch point, organisms with different traits and environmental factors
characters are placed in different groups based on the characteristics
they share. This page titled 20.1B: Limitations of Phylogenetic Trees is shared under a
Another aspect of phylogenetic trees is that, unless otherwise CC BY-SA 4.0 license and was authored, remixed, and/or curated by
indicated, the branches do not account for length of time, only the Boundless.

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20.1C: THE LEVELS OF CLASSIFICATION
The taxonomic classification system (also called the Linnaean
 LEARNING OBJECTIVES system after its inventor, Carl Linnaeus, a Swedish botanist,
zoologist, and physician) uses a hierarchical model. Moving from
Describe how taxonomic classification of organisms is
the point of origin, the groups become more specific, until one
accomplished and detail the levels of taxonomic
branch ends as a single species. For example, after the common
classification from domain to species
beginning of all life, scientists divide organisms into three large
categories called domains: Bacteria, Archaea, and Eukarya. Within
Taxonomy (which literally means “arrangement law”) is the science
each domain is a second category called a kingdom. After kingdoms,
of classifying organisms to construct internationally-shared the subsequent categories of increasing specificity are: phylum,
classification systems with each organism placed into more and
class, order, family, genus, and species.
more inclusive groupings. Think about how a grocery store is
organized. One large space is divided into departments, such as
produce, dairy, and meats. Then each department further divides into
aisles, then each aisle into categories and brands, and then, finally, a
single product. This organization from larger to smaller, more-
specific categories is called a hierarchical system.

Figure 20.1C. 1 : Levels in taxonomic classification: At each

sublevel in the taxonomic classification system, organisms become
more similar. Dogs and wolves are the same species because they
can breed and produce viable offspring, but they are different
enough to be classified as different subspecies.
The kingdom Animalia stems from the Eukarya domain. The full
name of an organism technically has eight terms. For dogs, it is:
Eukarya, Animalia, Chordata, Mammalia, Carnivora, Canidae,
Canis, and lupus. Notice that each name is capitalized except for
species and that genus and species names are italicized. Scientists
generally refer to an organism only by its genus and species, which
is its two-word scientific name, in what is called binomial
nomenclature. Therefore, the scientific name of the dog is Canis
lupus. The name at each level is also called a taxon. In other words,
dogs are in order Carnivora. Carnivora is the name of the taxon at
the order level; Canidae is the taxon at the family level, and so forth.
Organisms also have a common name that people typically use; in
this case, dog. Note that the dog is additionally a subspecies: the
Figure 20.1C. 1 : Hierarchical models: The taxonomic classification
system uses a hierarchical model to organize living organisms into “familiaris” in Canis lupus familiaris. Subspecies are members of
increasingly specific categories. The common dog, Canis lupus the same species that are capable of mating and reproducing viable
familiaris, is a subspecies of Canis lupus, which also includes the offspring, but they are considered separate subspecies due to
wolf and dingo.
geographic or behavioral isolation or other factors.

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phylogenies have been determined. ShareAlike
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Categories within taxonomic classification are arranged in OpenStax CNX. Located at:
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increasing specificity. Attribution
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binomial nomenclature. Provided by: Wiktionary. Located at:
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as the “father of modern taxonomy” Attribution
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OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. Attribution
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BY: Attribution This page titled 20.1C: The Levels of Classification is shared under a CC
phylogeny. Provided by: Wiktionary. Located at:
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SECTION OVERVIEW

20.2: DETERMINING EVOLUTIONARY RELATIONSHIPS

20.2B: BUILDING PHYLOGENETIC TREES
Topic hierarchy
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TRAITS by Boundless.

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20.2A: DISTINGUISHING BETWEEN SIMILAR TRAITS

 LEARNING OBJECTIVES

Explain the difference between homologous and analogous

structures

TWO OPTIONS FOR SIMILARITIES

In general, organisms that share similar physical features and
genomes tend to be more closely related than those that do not. Such
features that overlap both morphologically (in form) and genetically
are referred to as homologous structures; they stem from
developmental similarities that are based on evolution. For example,
the bones in the wings of bats and birds have homologous structures.

Figure 20.2A. 1 : Analogous structures: The (c) wing of a honeybee

is similar in shape to a (b) bird wing and (a) bat wing, and it serves
the same function. However, the honeybee wing is not composed of
bones and has a distinctly-different structure and embryonic origin.
These wing types (insect versus bat and bird) illustrate an analogy:
similar structures that do not share an evolutionary history.
Similar traits can be either homologous or analogous. Homologous
Figure 20.2A. 1 : Homologous structures: Bat and bird wings are structures share a similar embryonic origin; analogous organs have a
homologous structures, indicating that bats and birds share a similar function. For example, the bones in the front flipper of a
common evolutionary past. whale are homologous to the bones in the human arm. These
Notice it is not simply a single bone, but rather a grouping of several structures are not analogous. The wings of a butterfly and the wings
bones arranged in a similar way. The more complex the feature, the of a bird are analogous, but not homologous. Some structures are
more probable that any overlap is due to a common evolutionary both analogous and homologous: the wings of a bird and the wings
past. Imagine two people from different countries both inventing a of a bat are both homologous and analogous. Scientists must
car with all the same parts and in exactly the same arrangement determine which type of similarity a feature exhibits to decipher the
without any previous or shared knowledge. That outcome would be phylogeny of the organisms being studied.
highly improbable. However, if two people both invented a hammer,
it would be reasonable to conclude that both could have the original MOLECULAR COMPARISONS
idea without the help of the other. The same relationship between With the advancement of DNA technology, the area of molecular
complexity and shared evolutionary history is true for homologous systematics, which describes the use of information on the molecular
structures in organisms. level including DNA analysis, has blossomed. New computer
programs not only confirm many earlier classified organisms, but
MISLEADING APPEARANCES also uncover previously-made errors. As with physical
Some organisms may be very closely related, even though a minor characteristics, even the DNA sequence can be tricky to read in
genetic change caused a major morphological difference to make some cases. For some situations, two very closely-related organisms
them look quite different. Similarly, unrelated organisms may be can appear unrelated if a mutation occurred that caused a shift in the
distantly related, but appear very similar. This usually happens genetic code. An insertion or deletion mutation would move each
because both organisms developed common adaptations that evolved nucleotide base over one place, causing two similar codes to appear
within similar environmental conditions. When similar unrelated.
characteristics occur because of environmental constraints and not Sometimes two segments of DNA code in distantly-related
due to a close evolutionary relationship, it is called an analogy or organisms randomly share a high percentage of bases in the same
homoplasy. For example, insects use wings to fly like bats and birds, locations, causing these organisms to appear closely related when
but the wing structure and embryonic origin is completely different. they are not. For both of these situations, computer technologies
These are called analogous structures. have been developed to help identify the actual relationships.
Ultimately, the coupled use of both morphologic and molecular
information is more effective in determining phylogeny.

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KEY POINTS KEY TERMS
Organisms may be very closely related, even though they look analogous: when similar similar physical features occur in
quite different, due to a minor genetic change that caused a major organisms because of environmental constraints and not due to a
morphological difference. close evolutionary relationship
Unrelated organisms may appear very similar because both homologous: when similar physical features and genomes stem
organisms developed common adaptations that evolved within from developmental similarities that are based on evolution
similar environmental conditions. phylogeny: the evolutionary history of an organism
To determine the phylogeny of an organism, scientists must molecular systematics: molecular phylogenetics is the analysis
determine whether a similarity is homologous or analogous. of hereditary molecular differences, mainly in DNA sequences,
The advancement of DNA technology, the area of molecular to gain information on an organism’s evolutionary relationships
systematics, describes the use of information on the molecular
level, including DNA analysis. This page titled 20.2A: Distinguishing between Similar Traits is shared
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

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20.2B: BUILDING PHYLOGENETIC TREES

 LEARNING OBJECTIVES

Describe the cladistics as a method used to create

phylogenetic trees

After the homologous and analogous traits are sorted, scientists

often organize the homologous traits using a system called
cladistics. This system sorts organisms into clades: groups of
organisms that descended from a single ancestor. For example, all of
the organisms in the orange region evolved from a single ancestor
that had amniotic eggs. Consequently, all of these organisms also
have amniotic eggs and make a single clade, also called a
monophyletic group. Clades must include all of the descendants
from a branch point.

Figure 20.2B. 1: Examples of clades: All the organisms within a

clade stem from a single point on the tree. A clade may contain
multiple groups, as in the case of animals, fungi, and plants, or a
single group, as in the case of flagellates. Groups that diverge at a
different branch point, or that do not include all groups in a single
branch point, are not considered clades.

If a characteristic is found in the ancestor of a group, it is considered

Figure 20.2B. 1: Common ancestors: Lizards, rabbits, and humans a shared-ancestral character because all of the organisms in the taxon
all descend from a common ancestor that had an amniotic egg. Thus, or clade have that trait. Now, consider the amniotic egg
lizards, rabbits, and humans all belong to the clade Amniota.
Vertebrata is a larger clade that also includes fish and lamprey. characteristic in the same figure. Only some of the organisms have
Clades can vary in size depending on which branch point is being this trait; to those that do, it is called a shared-derived character
referenced. The important factor is that all of the organisms in the because this trait derived at some point, but does not include all of
clade or monophyletic group stem from a single point on the tree. the ancestors in the tree. The tricky aspect to shared-ancestral and
This can be remembered because monophyletic breaks down into shared-derived characters is the fact that these terms are relative.
“mono,” meaning one, and “phyletic,” meaning evolutionary The same trait can be considered one or the other depending on the
relationship. Notice in the various examples of clades how each particular diagram being used. These terms help scientists
clade comes from a single point, whereas the non-clade groups show distinguish between clades in the building of phylogenetic trees.
branches that do not share a single point.
CHOOSING THE RIGHT RELATIONSHIPS
SHARED CHARACTERISTICS Imagine being the person responsible for organizing all of the items
Organisms evolve from common ancestors and then diversify. in a department store properly; an overwhelming task. Organizing
Scientists use the phrase “descent with modification” because even the evolutionary relationships of all life on earth proves much more
though related organisms have many of the same characteristics and difficult: scientists must span enormous blocks of time and work
genetic codes, changes occur. This pattern repeats as one goes with information from long-extinct organisms. Trying to decipher
through the phylogenetic tree of life: the proper connections, especially given the presence of homologies
and analogies, makes the task of building an accurate tree of life
1. A change in the genetic makeup of an organism leads to a new
extraordinarily difficult. Add to that the advancement of DNA
trait which becomes prevalent in the group.
technology, which now provides large quantities of genetic
2. Many organisms descend from this point and have this trait.
sequences to be used and analyzed. Taxonomy is a subjective
3. New variations continue to arise: some are adaptive and persist,
discipline: many organisms have more than one connection to each
leading to new traits.
other, so each taxonomist will decide the order of connections.
4. With new traits, a new branch point is determined (go back to
step 1 and repeat). To aid in the tremendous task of describing phylogenies accurately,
scientists often use a concept called maximum parsimony, which
means that events occurred in the simplest, most obvious way. For
example, if a group of people entered a forest preserve to go hiking,
based on the principle of maximum parsimony, one could predict

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that most of the people would hike on established trails rather than analogous. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/analogous. License: CC BY-SA: Attribution-
forge new ones. For scientists deciphering evolutionary pathways, ShareAlike
the same idea is used: the pathway of evolution probably includes molecular systematics. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/molecular%20systematics. License: CC BY-SA:
the fewest major events that coincide with the evidence at hand. Attribution-ShareAlike
Starting with all of the homologous traits in a group of organisms, homologous. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/homologous. License: CC BY-SA: Attribution-
scientists look for the most obvious and simple order of evolutionary ShareAlike
events that led to the occurrence of those traits. phylogeny. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/phylogeny. License: CC BY-SA: Attribution-
ShareAlike
KEY POINTS OpenStax College, Determining Evolutionary Relationships. October 16, 2013.
Phylogenetic trees sort organisms into clades: groups of Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44591/latest...e_20_02_01.jpg. License: CC BY:
organisms that descended from a single ancestor. Attribution
Organisms of a single clade are called a monophyletic group. OpenStax College, Determining Evolutionary Relationships. October 16, 2013.
Provided by: OpenStax CNX. Located at:
Scientists use the phrase “descent with modification” because http://cnx.org/content/m44591/latest...e_20_02_02.jpg. License: CC BY:
genetic changes occur even though related organisms have many Attribution
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
of the same characteristics and genetic codes. Located at: http://cnx.org/content/m44591/latest...ol11448/latest. License: CC
A characteristic is considered a shared-ancestral character if it is BY: Attribution
derived. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/derived.
found in the ancestor of a group and all of the organisms in the
License: CC BY-SA: Attribution-ShareAlike
taxon or clade have that trait. monophyletic. Provided by: Wiktionary. Located at:
If only some of the organisms have a certain trait, it is called a en.wiktionary.org/wiki/monophyletic. License: CC BY-SA: Attribution-
ShareAlike
shared- derived character because this trait derived at some point, maximum parsimony. Provided by: Wikipedia. Located at:
but does not include all of the ancestors in the clade. en.Wikipedia.org/wiki/maximum%20parsimony. License: CC BY-SA:
Attribution-ShareAlike
Scientists often use a concept called maximum parsimony, which ancestral. Provided by: Wiktionary. Located at:
means that events occurred in the simplest, most obvious way, to en.wiktionary.org/wiki/ancestral. License: CC BY-SA: Attribution-ShareAlike
Boundless. Provided by: Boundless Learning. Located at:
aid in the tremendous task of describing phylogenies accurately. www.boundless.com//biology/definition/clades. License: CC BY-SA:
Attribution-ShareAlike
KEY TERMS OpenStax College, Determining Evolutionary Relationships. October 16, 2013.
Provided by: OpenStax CNX. Located at:
monophyletic: of, pertaining to, or affecting a single phylum (or http://cnx.org/content/m44591/latest...e_20_02_01.jpg. License: CC BY:
other taxon) of organisms Attribution
OpenStax College, Determining Evolutionary Relationships. October 16, 2013.
derived: of, or pertaining to, conditions unique to the descendant Provided by: OpenStax CNX. Located at:
species of a clade, and not found in earlier ancestral species http://cnx.org/content/m44591/latest...e_20_02_02.jpg. License: CC BY:
Attribution
clades: groups of organisms that descended from a single OpenStax College, Determining Evolutionary Relationships. October 16, 2013.
ancestor Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44591/latest...e_20_02_05.png. License: CC BY:
ancestral: of, pertaining to, derived from, or possessed by, an Attribution
ancestor or ancestors; as, an ancestral estate OpenStax College, Determining Evolutionary Relationships. October 16, 2013.
Provided by: OpenStax CNX. Located at:
maximum parsimony: the preferred phylogenetic tree is the tree http://cnx.org/content/m44591/latest...e_20_02_04.png. License: CC BY:
that requires the least evolutionary change to explain some Attribution
observed data

CONTRIBUTIONS AND ATTRIBUTIONS This page titled 20.2B: Building Phylogenetic Trees is shared under a CC
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
Located at: http://cnx.org/content/m44591/latest...ol11448/latest. License: CC
BY: Attribution

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SECTION OVERVIEW

20.3: PERSPECTIVES ON THE PHYLOGENETIC TREE

20.3C: ENDOSYMBIOTIC THEORY AND THE
Topic hierarchy EVOLUTION OF EUKARYOTES

20.3D: WEB, NETWORK, AND RING OF LIFE

20.3A: LIMITATIONS TO THE CLASSIC MODEL OF
MODELS
PHYLOGENETIC TREES

20.3B: HORIZONTAL GENE TRANSFER This page titled 20.3: Perspectives on the Phylogenetic Tree is shared under
a CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Boundless.

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20.3A: LIMITATIONS TO THE CLASSIC MODEL OF PHYLOGENETIC TREES
variation in offspring, again, to be a result of a mutation within the
 LEARNING OBJECTIVES species. The concept of genes being transferred between unrelated
species was not considered as a possibility until relatively recently.
Identify the limitations to the classic model of phylogenetic
Horizontal gene transfer (HGT), also known as lateral gene transfer,
trees
is the transfer of genes between unrelated species. HGT has been
shown to be an ever-present phenomenon, with many evolutionists
The concepts of phylogenetic modeling are constantly changing. It is
postulating a major role for this process in evolution, thus
one of the most dynamic fields of study in all of biology. Over the
complicating the simple tree model. Genes have been shown to be
last several decades, new research has challenged scientists’ ideas passed between species which are only distantly related using
about how organisms are related. New models of these relationships
standard phylogeny, thus adding a layer of complexity to the
have been proposed for consideration by the scientific community.
understanding of phylogenetic relationships. Finally, as an example
Many phylogenetic trees have been shown as models of the
of the ultimate gene transfer, theories of genome fusion between
evolutionary relationship among species. Phylogenetic trees
symbiotic or endosymbiotic organisms have been proposed to
originated with Charles Darwin, who sketched the first phylogenetic
explain an event of great importance: the evolution of the first
tree in 1837, which served as a pattern for subsequent studies for eukaryotic cell, without which humans could not have come into
more than a century. The concept of a phylogenetic tree with a single existence.
trunk representing a common ancestor, with the branches
representing the divergence of species from this ancestor, fits well KEY POINTS
with the structure of many common trees, such as the oak. However, Charles Darwin sketched the first phylogenetic tree in 1837.
evidence from modern DNA sequence analysis and newly- A single trunk on a phylogenetic tree represents a common
developed computer algorithms has caused skepticism about the ancestor and the branches represent the divergence of species
validity of the standard tree model in the scientific community. from this ancestor.
Prokaryotes are assumed to evolve clonally in the classic tree
model.
Horizontal gene transfer is the transfer of genes between
unrelated species and, as such, complicates the simple tree
model.
Ultimate gene transfer has provided theories of genome fusion
between symbiotic or endosymbiotic organisms.

KEY TERMS
phylogenetic: of, or relating to the evolutionary development of
organisms
Figure 20.3A. 1 : Tree of life: The (a) concept of the “tree of life” clonal: pertaining to asexual reproduction
goes back to an 1837 sketch by Charles Darwin. Like an (b) oak
tree, the “tree of life” has a single trunk and many branches. horizontal gene transfer: the transfer of genetic material from
Classical thinking about prokaryotic evolution, included in the one organism to another one that is not its offspring; especially
common among bacteria
classic tree model, is that species evolve clonally. That is, they
produce offspring themselves with only random mutations causing
This page titled 20.3A: Limitations to the Classic Model of Phylogenetic
the descent into the variety of modern and extinct species known to Trees is shared under a CC BY-SA 4.0 license and was authored, remixed,
science. This view is somewhat complicated in eukaryotes that and/or curated by Boundless.
reproduce sexually, but the laws of Mendelian genetics explain the

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20.3B: HORIZONTAL GENE TRANSFER
More recently, a fourth mechanism of gene transfer between
 LEARNING OBJECTIVES prokaryotes has been discovered. Small, virus-like particles called
gene transfer agents (GTAs) transfer random genomic segments
Explain how horizontal gene transfer can make resolution of
from one species of prokaryote to another. GTAs have been shown
phylogenies difficult
to be responsible for genetic changes, sometimes at a very high
frequency compared to other evolutionary processes. The first GTA
Horizontal gene transfer (HGT) is the introduction of genetic
was characterized in 1974 using purple, non-sulfur bacteria. These
material from one species to another species by mechanisms other
GTAs, which are thought to be bacteriophages that lost the ability to
than the vertical transmission from parent(s) to offspring. These reproduce on their own, carry random pieces of DNA from one
transfers allow even distantly-related species (using standard
organism to another. The ability of GTAs to act with high frequency
phylogeny) to share genes, influencing their phenotypes. It is
has been demonstrated in controlled studies using marine bacteria.
thought that HGT is more prevalent in prokaryotes, but that only
Gene transfer events in marine prokaryotes, either by GTAs or by
about 2% of the prokaryotic genome may be transferred by this
viruses, have been estimated to be as high as 1013 per year in the
process. Some researchers believe these estimates are premature; the
Mediterranean Sea alone. GTAs and viruses are thought to be
actual importance of HGT to evolutionary processes must be viewed efficient HGT vehicles with a major impact on prokaryotic
as a work in progress. As the phenomenon is investigated more evolution.
thoroughly, it may be revealed to be more common. Many
evolutionists postulate a major role for this process in evolution, thus HGT IN EUKARYOTES
complicating the simple tree model. A number of scientists believe Although it is easy to see how prokaryotes exchange genetic
that HGT and mutation appear to be (especially in prokaryotes) a material by HGT, it was initially thought that this process was absent
significant source of genetic variation, which is the raw material for in eukaryotes. After all, prokaryotes are only single cells exposed
the process of natural selection. These transfers may occur between directly to their environment, whereas the sex cells of multicellular
any two species that share an intimate relationship, thus adding a organisms are usually sequestered in protected parts of the body. It
layer of complexity to the understanding or resolution of follows from this idea that the gene transfers between multicellular
phylogenetic relationships. eukaryotes should be more difficult. Indeed, it is thought that this
process is rarer in eukaryotes and has a much smaller evolutionary
impact than in prokaryotes. In spite of this fact, HGT between
distantly-related organisms has been demonstrated in several
eukaryotic species. It is possible that more examples will be
discovered in the future.
In plants, gene transfer has been observed in species that cannot
cross-pollinate by normal means. Transposons or “jumping genes”
have been shown to transfer between rice and millet plant species.
Furthermore, fungal species feeding on yew trees, from which the
anti-cancer drug TAXOL® is derived from the bark, have acquired
Figure 20.3B. 1: Mechanisms of prokaryotic and eukaryotic the ability to make taxol themselves; a clear example of gene
horizontal gene transfer: Horizontal gene transfer is the introduction
of genetic material from one species to another species by transfer.
mechanisms other than the vertical transmission from parent(s) to In animals, a particularly interesting example of HGT occurs within
offspring. These transfers allow even distantly-related species (using
standard phylogeny) to share genes, influencing their phenotypes. the aphid species. Aphids are insects that vary in color based on
Examples of mechanisms of horizontal gene transfer are listed for carotenoid content. Carotenoids are pigments made by a variety of
both prokaryotic and eukaryotic organisms. plants, fungi, and microbes, which serve a variety of functions in
HGT IN PROKARYOTES animals who obtain these chemicals from their food. Humans
require carotenoids to synthesize vitamin A and we obtain them by
The mechanism of HGT has been shown to be quite common in the
eating orange fruits and vegetables: carrots, apricots, mangoes, and
prokaryotic domains of Bacteria and Archaea, significantly changing
sweet potatoes. On the other hand, aphids have acquired the ability
the way their evolution is viewed. These gene transfers between
to make the carotenoids on their own. According to DNA analysis,
species are the major mechanism whereby bacteria acquire
this ability is due to the transfer of fungal genes into the insect by
resistance to antibiotics. Classically, this type of transfer was thought
HGT, presumably as the insect consumed fungi for food. A
to occur by three different mechanisms:
carotenoid enzyme called a desaturase is responsible for the red
Transformation: naked DNA is taken up by a bacteria. coloration seen in certain aphids. Furthermore, it has been shown
Transduction: genes are transferred using a virus. that when this gene is inactivated by mutation, the aphids revert
Conjugation: the use a hollow tube called a pilus to transfer back to their more common green color.
genes between organisms.

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 Many scientists believe that HGT and mutation appear to be
(especially in prokaryotes) a significant source of genetic
variation, which is the raw material for the process of natural
selection.
HGT in prokaryotes occurs by four different mechanisms:
transformation, transduction, conjugation, and via gene transfer
agents.
HGT occurs in plants through transposons (jumping genes),
Figure 20.3B. 1: HGT within the aphid species: (a) Red aphids get which transfer between different species of plants.
their color from red carotenoid pigment. Genes necessary to make An example of HGT in animals is the transfer (through
this pigment are present in certain fungi. Scientists speculate that
aphids acquired these genes through HGT after consuming fungi for consumption) of fungal genes into insects called aphids, which
food. If genes for making carotenoids are inactivated by mutation, allows the aphids the ability to make carotenoids on their own.
the aphids revert back to (b) their green color. Red coloration makes
the aphids much more conspicuous to predators, but evidence KEY TERMS
suggests that red aphids are more resistant to insecticides than green
ones. Thus, red aphids may be more fit to survive in some transformation: the alteration of a bacterial cell caused by the
environments than green ones. transfer of DNA from another, especially if pathogenic
transduction: horizontal gene transfer mechanism in
KEY POINTS prokaryotes where genes are transferred using a virus
It is thought that HGT is more prevalent in prokaryotes than conjugation: the temporary fusion of organisms, especially as
eukaryotes, but that only about 2% of the prokaryotic genome part of sexual reproduction
may be transferred by this process.
This page titled 20.3B: Horizontal Gene Transfer is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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20.3C: ENDOSYMBIOTIC THEORY AND THE EVOLUTION OF EUKARYOTES

 LEARNING OBJECTIVES

Describe the genome fusion hypothesis and its relationship

to the evolution of eukaryotes

GENOME FUSION AND THE EVOLUTION OF

EUKARYOTES
Scientists believe the ultimate event in HGT (horizontal gene
transfer) occurs through genome fusion between different species
when two symbiotic organisms become endosymbiotic. This occurs
when one species is taken inside the cytoplasm of another species,
which ultimately results in a genome consisting of genes from both
the endosymbiont and the host. This mechanism is an aspect of the
Endosymbiont Theory, which is accepted by a majority of biologists
as the mechanism whereby eukaryotic cells obtained their
mitochondria and chloroplasts. However, the role of endosymbiosis
in the development of the nucleus is more controversial. Nuclear and
mitochondrial DNA are thought to be of different (separate)
evolutionary origin, with the mitochondrial DNA being derived from Figure 20.3C. 1 : Endosymbiosis in eukaryotes: The theory that
the circular genomes of bacteria that were engulfed by ancient mitochondria and chloroplasts are endosymbiotic in origin is now
widely accepted. More controversial is the proposal that (a) the
prokaryotic cells. Mitochondrial DNA can be regarded as the eukaryotic nucleus resulted from the fusion of archaeal and bacterial
smallest chromosome. Interestingly enough, mitochondrial DNA is genomes; and that (b) Gram-negative bacteria, which have two
inherited only from the mother. The mitochondrial DNA degrades in membranes, resulted from the fusion of Archaea and Gram-positive
bacteria, each of which has a single membrane.
sperm when the sperm degrades in the fertilized egg or, in other
instances, when the mitochondria located in the flagellum of the More recent work proposes that gram-negative bacteria, which are
sperm fails to enter the egg. unique within their domain in that they contain two lipid bilayer
membranes, did result from an endosymbiotic fusion of archaeal and
Within the past decade, the process of genome fusion by
bacterial species. The double membrane would be a direct result of
endosymbiosis has been proposed to be responsible for the evolution
endosymbiosis, with the endosymbiont picking up the second
of the first eukaryotic cells. Using DNA analysis and a new
membrane from the host as it was internalized. This mechanism has
mathematical algorithm called conditioned reconstruction (CR), it
also been used to explain the double membranes found in
has been proposed that eukaryotic cells developed from an
mitochondria and chloroplasts. A lot of skepticism still surrounds
endosymbiotic gene fusion between two species: one an Archaea
this hypothesis; the ideas are still debated within the biological
and the other a Bacteria. As mentioned, some eukaryotic genes
science community.
resemble those of Archaea, whereas others resemble those from
Bacteria. An endosymbiotic fusion event would clearly explain this There are several other competing hypotheses as to the origin of
observation. On the other hand, this work is new and the CR eukaryotes and the nucleus. One idea about how the eukaryotic
algorithm is relatively unsubstantiated, which causes many scientists nucleus evolved is that prokaryotic cells produced an additional
to resist this hypothesis. membrane which surrounded the bacterial chromosome. Some
bacteria have the DNA enclosed by two membranes; however, there
is no evidence of a nucleolus or nuclear pores. Other proteobacteria
also have membrane-bound chromosomes. If the eukaryotic nucleus
evolved this way, we would expect one of the two types of
prokaryotes to be more closely-related to eukaryotes. Another
hypothesis, the nucleus-first hypothesis, proposes the nucleus
evolved in prokaryotes first, followed by a later fusion of the new
eukaryote with bacteria that became mitochondria. The
mitochondria-first hypothesis, however, proposes mitochondria were
first established in a prokaryotic host, which subsequently acquired a
nucleus (by fusion or other mechanisms) to become the first
eukaryotic cell. Most interestingly, the eukaryote-first hypothesis
proposes prokaryotes actually evolved from eukaryotes by losing
genes and complexity. All of these hypotheses are testable. Only

20.3C.1 https://bio.libretexts.org/@go/page/13536
time and more experimentation will determine which hypothesis is in genome fusion.
best supported by data. Genome fusion, by endosymbiosis, between two species, one an
Archaea and the other a Bacteria, has been proposed as
responsible for the evolution of the first eukaryotic cells.
Gram-negative bacteria are proposed to result from an
endosymbiotic fusion of archaeal and bacterial species through a
mechanism that has also been used to explain the double
membranes found in mitochondria and chloroplasts.
The nucleus-first hypothesis proposes the nucleus evolved in
prokaryotes first, followed by a later fusion of the new eukaryote
with bacteria that became mitochondria.
The mitochondria-first hypothesis proposes mitochondria were
first established in a prokaryotic host, which subsequently
acquired a nucleus to become the first eukaryotic cell.
The eukaryote-first hypothesis proposes prokaryotes actually
evolved from eukaryotes by losing genes and complexity.

KEY TERMS
genome fusion: a result of endosymbiosis when a genome
consists of genes from both the endosymbiont and the host.
symbiotic: of a relationship with mutual benefit between two
Figure 20.3C. 1 : Three hypotheses of eukaryotic and prokaryotic individuals or organisms
evolution: Three alternate hypotheses of eukaryotic and prokaryotic
evolution are (a) the nucleus-first hypothesis, (b) the mitochondrion- endosymbiosis: when one symbiotic species is taken inside the
first hypothesis, and (c) the eukaryote-first hypothesis. cytoplasm of another symbiotic species and both become
endosymbiotic
KEY POINTS
Two symbiotic organisms become endosymbiotic when one This page titled 20.3C: Endosymbiotic Theory and the Evolution of
species is taken inside the cytoplasm of another species, resulting Eukaryotes is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.

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20.3D: WEB, NETWORK, AND RING OF LIFE MODELS

 LEARNING OBJECTIVES

Describe the web, network, and ring of life models of

phylogenetic trees

The recognition of the importance of Horizontal gene transfer

(HGT), especially in the evolution of prokaryotes, has caused some
to propose abandoning the classic “tree of life” model. In 1999, a
phylogenetic model that resembles a web or a network more than a
tree was proposed. The hypothesis is that eukaryotes evolved not
from a single prokaryotic ancestor, but from a pool of many species
that were sharing genes by HGT mechanisms. Some individual
prokaryotes were responsible for transferring the bacteria that
caused mitochondrial development in the new eukaryotes, whereas Figure 20.3D. 1 : Phylogenetic ring of life model: According to the
other species transferred the bacteria that gave rise to chloroplasts. “ring of life” phylogenetic model, the three domains of life evolved
from a pool of primitive prokaryotes.
This model is often called the “web of life.” In an effort to save the
tree analogy, some have proposed using the Ficus tree with its In summary, the “tree of life” model proposed by Darwin must be
multiple trunks as a phylogenetic tree to represent the evolutionary modified to include HGT. This does not mean a tree, web, or a ring
role for HGT. will correlate completely to an accurate description of phylogenetic
relationships of life. A consequence of the new thinking about
phylogenetic models is the idea that Darwin’s original conception of
the phylogenetic tree is too simple, but made sense based on what
was known at that time.

KEY POINTS
A phylogenetic model that resembles a web or a network was
proposed since eukaryotes evolved not from a single prokaryotic
ancestor, but from a pool of many species that were sharing
genes by HGT mechanisms.
A phylogenetic model that resembles a ring was proposed in
which species of all three domains, Archaea, Bacteria, and
Eukarya, evolved from a single pool of gene-swapping
prokaryotes.
Figure 20.3D. 1 : Phylogenetic web of life model: In the (a) Phylogenetic models will continue to evolve as phylogeneticists
phylogenetic model proposed by W. Ford Doolittle, the “tree of life” remain highly skeptical of the current tree, web, and ring models.
arose from a community of ancestral cells, has multiple trunks, and
has connections between branches where horizontal gene transfer
has occurred. Visually, this concept is better represented by (b) the KEY TERMS
multi-trunked Ficus than by the single trunk of the oak, similar to the web of life: a phylogenetic model that resembles a web or a
tree drawn by Darwin.
network more than a tree
Others have proposed abandoning any tree-like model of phylogeny
ring of life: a phylogenetic model where all three domains of life
in favor of a ring structure. The ” ring of life ” is a phylogenetic
(Archaea, Bacteria, and Eukarya) evolved from a pool of
model where all three domains of life evolved from a pool of
primitive prokaryotes
primitive prokaryotes. Using the conditioned reconstruction
algorithm, it proposes a ring-like model in which species of all three CONTRIBUTIONS AND ATTRIBUTIONS
domains (Archaea, Bacteria, and Eukarya) evolved from a single OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
pool of gene-swapping prokaryotes. This structure is proposed as the Located at: http://cnx.org/content/m44593/latest...ol11448/latest. License: CC
BY: Attribution
best fit for data from extensive DNA analyses; the ring model is the OpenStax College, Biology. November 5, 2013. Provided by: OpenStax CNX.
only one that adequately takes HGT and genomic fusion into Located at: http://cnx.org/content/m44593/latest...ol11448/latest. License: CC
BY: Attribution
account. However, phylogeneticists remain highly skeptical of this clonal. Provided by: Wiktionary. Located at:
model. http://en.wiktionary.org/wiki/clonal. License: CC BY-SA: Attribution-
ShareAlike
phylogenetic. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/phylogenetic. License: CC BY-SA: Attribution-
ShareAlike
horizontal gene transfer. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/horizontal_gene_transfer. License: CC BY-SA:

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Attribution-ShareAlike OpenStax College, Perspectives on the Phylogenetic Tree. October 16, 2013.
OpenStax College, Biology. November 5, 2013. Provided by: OpenStax CNX. Provided by: OpenStax CNX. Located at:
Located at: http://cnx.org/content/m44593/latest...ol11448/latest. License: CC http://cnx.org/content/m44593/latest...e_20_03_03.jpg. License: CC BY:
BY: Attribution Attribution
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX. OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44593/latest...ol11448/latest. License: CC Located at: http://cnx.org/content/m44593/latest...ol11448/latest. License: CC
BY: Attribution BY: Attribution
transduction. Provided by: Wiktionary. Located at: OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX.
en.wiktionary.org/wiki/transduction. License: CC BY-SA: Attribution- Located at: http://cnx.org/content/m44593/latest...ol11448/latest. License: CC
ShareAlike BY: Attribution
transformation. Provided by: Wiktionary. Located at: Boundless. Provided by: Boundless Learning. Located at:
en.wiktionary.org/wiki/transformation. License: CC BY-SA: Attribution- www.boundless.com//biology/de...on/web-of-life. License: CC BY-SA:
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 CHAPTER OVERVIEW

21: VIRUSES
21.1: Viral Evolution, Morphology, and Classification
21.1A: Discovery and Detection of Viruses
21.1B: Evolution of Viruses
21.1C: Viral Morphology
21.1D: Virus Classification
21.2: Virus Infections and Hosts
21.2A: Steps of Virus Infections
21.2B: The Lytic and Lysogenic Cycles of Bacteriophages
21.2C: Animal Viruses
21.2D: Plant Viruses
21.3: Prevention and Treatment of Viral Infections
21.3A: Vaccines and Immunity
21.3B: Vaccines and Anti-Viral Drugs for Treatment
21.4: Prions and Viroids
21.4.1: 21-4A- Prions and Viroids

Thumbnail: Ebola virus. (Public Domain; CDC).

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1
SECTION OVERVIEW

21.1: VIRAL EVOLUTION, MORPHOLOGY, AND CLASSIFICATION

21.1C: VIRAL MORPHOLOGY
Topic hierarchy
21.1D: VIRUS CLASSIFICATION
21.1A: DISCOVERY AND DETECTION OF
VIRUSES This page titled 21.1: Viral Evolution, Morphology, and Classification is
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21.1B: EVOLUTION OF VIRUSES curated by Boundless.

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21.1A: DISCOVERY AND DETECTION OF VIRUSES
viruses. The surface structure of virions can be observed by both
 LEARNING OBJECTIVES scanning and transmission electron microscopy, whereas the internal
structures of the virus can only be observed in images from a
Describe how viruses were first discovered and how they are
transmission electron microscope. The use of these technologies has
detected
enabled the discovery of many viruses of all types of living
organisms. They were initially grouped by shared morphology.
DISCOVERY AND DETECTION Later, groups of viruses were classified by the type of nucleic acid
Viruses were first discovered after the development of a porcelain they contained, DNA or RNA, and whether their nucleic acid was
filter, called the Chamberland-Pasteur filter, which could remove all single- or double-stranded. More recently, molecular analysis of
bacteria visible in the microscope from any liquid sample. In 1886, viral replicative cycles has further refined their classification.
Adolph Meyer demonstrated that a disease of tobacco plants,
tobacco mosaic disease, could be transferred from a diseased plant to
a healthy one via liquid plant extracts. In 1892, Dmitri Ivanowski
showed that this disease could be transmitted in this way even after
the Chamberland-Pasteur filter had removed all viable bacteria from
the extract. Still, it was many years before it was proven that these
“filterable” infectious agents were not simply very small bacteria,
but were a new type of tiny, disease-causing particle.

Figure 21.1A. 1 : Examples of transmission electron micrographs of

viruses: In these transmission electron micrographs, (a) a virus is
dwarfed by the bacterial cell it infects, while (b) these E. coli cells
are dwarfed by cultured colon cells.

KEY POINTS
Virions, single virus particles, are 20–250 nanometers in
diameter.
In the past, viruses were classified by the type of nucleic acid
they contained, DNA or RNA, and whether they had single- or
double-stranded nucleic acid.
Molecular analysis of viral replicative cycles is now more
Figure 21.1A. 1 : The structure of the icosahedral cowpea mosaic
virus: In the past, viruses were classified by the type of nucleic acid routinely used to classify viruses.
they contained, DNA or RNA, and whether they had single- or
double-stranded nucleic acid. KEY TERMS
virus: a submicroscopic infectious organism, now understood to
Virions, single virus particles, are very small, about 20–250 be a non-cellular structure consisting of a core of DNA or RNA
nanometers in diameter. These individual virus particles are the surrounded by a protein coat
infectious form of a virus outside the host cell. Unlike bacteria virion: a single individual particle of a virus (the viral equivalent
(which are about 100 times larger), we cannot see viruses with a of a cell)
light microscope, with the exception of some large virions of the
This page titled 21.1A: Discovery and Detection of Viruses is shared under
poxvirus family. It was not until the development of the electron
a CC BY-SA 4.0 license and was authored, remixed, and/or curated by
microscope in the late 1930s that scientists got their first good view
Boundless.
of the structure of the tobacco mosaic virus (TMV) and other

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21.1B: EVOLUTION OF VIRUSES

 LEARNING OBJECTIVES

Describe the difficulties in determining the origin of viruses

Although biologists have accumulated a significant amount of

knowledge about how present-day viruses evolve, much less is
known about how viruses originated in the first place. When
exploring the evolutionary history of most organisms, scientists can
look at fossil records and similar historic evidence. However, viruses
do not fossilize, so researchers must conjecture by investigating how
today’s viruses evolve and by using biochemical and genetic Figure 21.1B. 1: Common ancestor tree of life: This phylogenetic
information to create speculative virus histories. tree of the three domains of life (Bacteria, Archaea, and Eukarya)
attempts to identify when various species diverged from a common
While most findings agree that viruses don’t have a single common ancestor. Finding a common ancestor for viruses has proven to be far
ancestor, scholars have yet to find one hypothesis about virus origins more difficult, especially since they do not fossilize.
that is fully accepted in the field. One possible hypothesis, called As technology advances, scientists may develop and refine further
devolution or the regressive hypothesis, proposes to explain the hypotheses to explain the origin of viruses. The emerging field
origin of viruses by suggesting that viruses evolved from free-living called virus molecular systematics attempts to do just that through
cells. However, many components of how this process might have comparisons of sequenced genetic material. These researchers hope
occurred are a mystery. A second hypothesis (called escapist or the to one day better understand the origin of viruses, a discovery that
progressive hypothesis) accounts for viruses having either an RNA could lead to advances in the treatments for the ailments they
or a DNA genome and suggests that viruses originated from RNA produce.
and DNA molecules that escaped from a host cell. A third
hypothesis posits a system of self-replication similar to that of other KEY POINTS
self-replicating molecules, probably evolving alongside the cells Scientists agree that viruses don’t have a single common
they rely on as hosts; studies of some plant pathogens support this ancestor, but have yet to agree on a single hypothesis about virus
hypothesis. origins.
The devolution or the regressive hypothesis suggests that viruses
evolved from free-living cells.
The escapist or the progressive hypothesis suggests that viruses
originated from RNA and DNA molecules that escaped from a
host cell.
The self-replicating hypothesis posits a system of self-replication
that most probably involves evolution alongside the host cells.

KEY TERMS
self-replicating: able to generate a copy of itself
devolution: degeneration (as opposed to evolution)

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4.0 license and was authored, remixed, and/or curated by Boundless.

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21.1C: VIRAL MORPHOLOGY

 LEARNING OBJECTIVES

Describe the relationship between the viral genome, capsid,

and envelope

Viruses are acellular, meaning they are biological entities that do not
have a cellular structure. Therefore, they lack most of the
components of cells, such as organelles, ribosomes, and the plasma
membrane. A virion consists of a nucleic acid core, an outer protein
coating or capsid, and sometimes an outer envelope made of protein
and phospholipid membranes derived from the host cell. The capsid
is made up of protein subunits called capsomeres. Viruses may also
contain additional proteins, such as enzymes. The most obvious
difference between members of viral families is their morphology,
which is quite diverse. An interesting feature of viral complexity is
that host and virion complexity are uncorrelated. Some of the most
intricate virion structures are observed in bacteriophages, viruses Figure 21.1C. 1 : Example of a virus attaching to its host cell: The
that infect the simplest living organisms: bacteria. KSHV virus binds the xCT receptor on the surface of human cells.
This attachment allows for later penetration of the cell membrane
and replication inside the cell.
MORPHOLOGY
Overall, the shape of the virion and the presence or absence of an
Viruses come in many shapes and sizes, but these are consistent and
envelope tell us little about what disease the virus may cause or what
distinct for each viral family. In general, the shapes of viruses are
species it might infect, but they are still useful means to begin viral
classified into four groups: filamentous, isometric (or icosahedral),
classification. Among the most complex virions known, the T4
enveloped, and head and tail. Filamentous viruses are long and
bacteriophage, which infects the Escherichia coli bacterium, has a
cylindrical. Many plant viruses are filamentous, including TMV
tail structure that the virus uses to attach to host cells and a head
(tobacco mosaic virus). Isometric viruses have shapes that are
structure that houses its DNA. Adenovirus, a non-enveloped animal
roughly spherical, such as poliovirus or herpesviruses. Enveloped
virus that causes respiratory illnesses in humans, uses glycoprotein
viruses have membranes surrounding capsids. Animal viruses, such
spikes protruding from its capsomeres to attach to host cells. Non-
as HIV, are frequently enveloped. Head and tail viruses infect
enveloped viruses also include those that cause polio (poliovirus),
bacteria. They have a head that is similar to icosahedral viruses and
plantar warts (papillomavirus), and hepatitis A (hepatitis A virus).
a tail shape like filamentous viruses.
Many viruses use some sort of glycoprotein to attach to their host
cells via molecules on the cell called viral receptors. For these
viruses, attachment is a requirement for later penetration of the cell
membrane, allowing them to complete their replication inside the
cell. The receptors that viruses use are molecules that are normally
found on cell surfaces and have their own physiological functions.
Viruses have simply evolved to make use of these molecules for
their own replication.

Figure 21.1C. 1 : Examples of virus shapes: Viruses can be either

complex in shape or relatively simple. This figure shows three
relatively-complex virions: the bacteriophage T4, with its DNA-
containing head group and tail fibers that attach to host cells;
adenovirus, which uses spikes from its capsid to bind to host cells;
and HIV, which uses glycoproteins embedded in its envelope to bind
to host cells.

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Enveloped virions like HIV consist of nucleic acid and capsid make copying errors than DNA polymerases and, therefore, often
proteins surrounded by a phospholipid bilayer envelope and its make mistakes during transcription. For this reason, mutations in
associated proteins. Glycoproteins embedded in the viral envelope RNA viruses occur more frequently than in DNA viruses. This
are used to attach to host cells. Other envelope proteins include the causes them to change and adapt more rapidly to their host. Human
matrix proteins that stabilize the envelope and often play a role in diseases caused by RNA viruses include hepatitis C, measles, and
the assembly of progeny virions. Chicken pox, influenza, and rabies.
mumps are examples of diseases caused by viruses with envelopes.
Because of the fragility of the envelope, non-enveloped viruses are KEY POINTS
more resistant to changes in temperature, pH, and some disinfectants Viruses are classified into four groups based on shape:
than are enveloped viruses. filamentous, isometric (or icosahedral), enveloped, and head and
tail.
TYPES OF NUCLEIC ACID Many viruses attach to their host cells to facilitate penetration of
Unlike nearly all living organisms that use DNA as their genetic the cell membrane, allowing their replication inside the cell.
material, viruses may use either DNA or RNA. The virus core Non-enveloped viruses can be more resistant to changes in
contains the genome or total genetic content of the virus. Viral temperature, pH, and some disinfectants than are enveloped
genomes tend to be small, containing only those genes that encode viruses.
proteins that the virus cannot obtain from the host cell. This genetic The virus core contains the small single- or double-stranded
material may be single- or double-stranded. It may also be linear or genome that encodes the proteins that the virus cannot get from
circular. While most viruses contain a single nucleic acid, others the host cell.
have genomes that have several, called segments.
KEY TERMS
In DNA viruses, the viral DNA directs the host cell’s replication
proteins to synthesize new copies of the viral genome and to capsid: the outer protein shell of a virus
transcribe and translate that genome into viral proteins. DNA viruses envelope: an enclosing structure or cover, such as a membrane
cause human diseases, such as chickenpox, hepatitis B, and some filamentous: Having the form of threads or filaments
venereal diseases, like herpes and genital warts. isometric: of, or being a geometric system of three equal axes
lying at right angles to each other (especially in crystallography)
RNA viruses contain only RNA as their genetic material. To
replicate their genomes in the host cell, the RNA viruses encode This page titled 21.1C: Viral Morphology is shared under a CC BY-SA 4.0
enzymes that can replicate RNA into DNA, which cannot be done license and was authored, remixed, and/or curated by Boundless.
by the host cell. These RNA polymerase enzymes are more likely to

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21.1D: VIRUS CLASSIFICATION
enveloped. Head and tail viruses infect bacteria and have a head that
 LEARNING OBJECTIVES is similar to icosahedral viruses and a tail shape like filamentous
viruses. Capsids are classified as naked icosahedral, enveloped
Describe how viruses are classified
icosahedral, enveloped helical, naked helical, and complex. For
example, the tobacco mosaic virus has a naked helical capsid. The
To understand the features shared among different groups of viruses,
adenovirus has an icosahedral capsid.
a classification scheme is necessary. However, most viruses are not
thought to have evolved from a common ancestor, so the methods
that scientists use to classify living things are not very useful.
Biologists have used several classification systems in the past, based
on the morphology and genetics of the different viruses. However,
these earlier classification methods grouped viruses based on which
features of the virus they were using to classify them. The most
commonly-used classification method today is called the Baltimore
classification scheme which is based on how messenger RNA
(mRNA) is generated in each particular type of virus. The surface Figure 21.1D. 1 : Adenovirus classification: Adenovirus (left) is
structure of virions can be observed by both scanning and depicted with a double-stranded DNA genome enclosed in an
transmission electron microscopy, whereas the internal structures of icosahedral capsid that is 90–100 nm across. The virus, shown
clustered in the micrograph (right), is transmitted orally and causes a
the virus can only be observed in images from a transmission variety of illnesses in vertebrates, including human eye and
electron microscope. respiratory infections.

PAST SYSTEMS OF CLASSIFICATION

Viruses are classified in several ways: by factors such as their core
content, the structure of their capsids, and whether they have an
outer envelope. Viruses may use either DNA or RNA as their
genetic material. The virus core contains the genome or total genetic
content of the virus. Viral genomes tend to be small, containing only
those genes that encode proteins that the virus cannot obtain from
the host cell. This genetic material may be single- or double-
stranded. It may also be linear or circular. While most viruses
contain a single nucleic acid, others have genomes that have several,
which are called segments. The type of genetic material (DNA or
RNA) and its structure (single- or double-stranded, linear or circular,
and segmented or non-segmented) are used to classify the virus core
structures. Figure 21.1D. 1 : Transmission electron micrograph of viruses:
Transmission electron micrographs of various viruses show their
structures. The capsid of the (a) polio virus is naked icosahedral; (b)
the Epstein-Barr virus capsid is enveloped icosahedral; (c) the
mumps virus capsid is an enveloped helix; (d) the tobacco mosaic
virus capsid is naked helical; and (e) the herpesvirus capsid is
complex.

Figure 21.1D. 1 : Virus classification by capsid structure: Viruses

can also be classified by the design of their capsids which are
classified as naked icosahedral, enveloped icosahedral, enveloped
Figure 21.1D. 1 : Virus classification by genome structure and core: helical, naked helical, and complex.
The type of genetic material (DNA or RNA) and its structure
(single- or double-stranded, linear or circular, and segmented or non-
segmented) are used to classify the virus core structures.
Viruses can also be classified by the design of their capsids.
Isometric viruses have shapes that are roughly spherical, such as
poliovirus or herpesviruses. Enveloped viruses have membranes
surrounding capsids. Animal viruses, such as HIV, are frequently

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 KEY TERMS
Baltimore classification: a classification scheme that groups
viruses into seven classes according to how the mRNA is
produced during the replicative cycle of the virus
messenger RNA: Messenger RNA (mRNA) is a molecule of
RNA that encodes a chemical “blueprint” for a protein product.

CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44595/latest...ol11448/latest. License: CC
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virus. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/virus.
License: CC BY-SA: Attribution-ShareAlike
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License: CC BY-SA: Attribution-ShareAlike
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2013. Provided by: OpenStax CNX. Located at:
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Viruses are classified based on their core genetic material and capsid Attribution
design. (a) Rabies virus has a single-stranded RNA (ssRNA) core CowpeaMosaicVirus3D. Provided by: Wikimedia Commons. Located at:
and an enveloped helical capsid, whereas (b) variola virus, the en.Wikipedia.org/wiki/File:Co...aicVirus3D.png. License: CC BY-SA:
causative agent of smallpox, has a double-stranded DNA (dsDNA) Attribution-ShareAlike
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The most commonly-used system of virus classification was
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differences in morphology and genetics.
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segmented or non-segmented are factors for classification. Attribution
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icosahedral, enveloped helical, naked helical, and complex. 2013. Provided by: OpenStax CNX. Located at:
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Virus can either have an envelope or not. Attribution
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Located at: http://cnx.org/content/m44595/latest...ol11448/latest. License: CC
groups viruses into seven classes according to how the mRNA is BY: Attribution
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SECTION OVERVIEW

21.2: VIRUS INFECTIONS AND HOSTS

21.2C: ANIMAL VIRUSES
Topic hierarchy
21.2D: PLANT VIRUSES
21.2A: STEPS OF VIRUS INFECTIONS
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21.2B: THE LYTIC AND LYSOGENIC CYCLES OF SA 4.0 license and was authored, remixed, and/or curated by Boundless.
BACTERIOPHAGES

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21.2A: STEPS OF VIRUS INFECTIONS
Viral infection involves the incorporation of viral DNA into a host
cell, replication of that material, and the release of the new viruses.

 LEARNING OBJECTIVES

List the steps of viral replication and explain what occurs at

each step

KEY POINTS
Viral replication involves six steps: attachment, penetration,
uncoating, replication, assembly, and release.
During attachment and penetration, the virus attaches itself to a
host cell and injects its genetic material into it.
During uncoating, replication, and assembly, the viral DNA or
RNA incorporates itself into the host cell’s genetic material and
Figure 21.2A. 1 : Pathway to viral infection: In influenza virus
induces it to replicate the viral genome. infection, glycoproteins attach to a host epithelial cell. As a result,
During release, the newly-created viruses are released from the the virus is engulfed. RNA and proteins are made and assembled
host cell, either by causing the cell to break apart, waiting for the into new virions.
cell to die, or by budding off through the cell membrane.
ATTACHMENT
KEY TERMS A virus attaches to a specific receptor site on the host cell membrane
virion: a single individual particle of a virus (the viral equivalent through attachment proteins in the capsid or via glycoproteins
of a cell) embedded in the viral envelope. The specificity of this interaction
glycoprotein: a protein with covalently-bonded carbohydrates determines the host (and the cells within the host) that can be
retrovirus: a virus that has a genome consisting of RNA infected by a particular virus. This can be illustrated by thinking of
several keys and several locks where each key will fit only one
STEPS OF VIRUS INFECTIONS specific lock.
A virus must use cell processes to replicate. The viral replication
ENTRY
cycle can produce dramatic biochemical and structural changes in
the host cell, which may cause cell damage. These changes, called The nucleic acid of bacteriophages enters the host cell naked,
cytopathic (causing cell damage) effects, can change cell functions leaving the capsid outside the cell. Plant and animal viruses can
or even destroy the cell. Some infected cells, such as those infected enter through endocytosis, in which the cell membrane surrounds
by the common cold virus known as rhinovirus, die through lysis and engulfs the entire virus. Some enveloped viruses enter the cell
(bursting) or apoptosis (programmed cell death or “cell suicide”), when the viral envelope fuses directly with the cell membrane. Once
releasing all progeny virions at once. The symptoms of viral diseases inside the cell, the viral capsid is degraded and the viral nucleic acid
result from the immune response to the virus, which attempts to is released, which then becomes available for replication and
control and eliminate the virus from the body and from cell damage transcription.
caused by the virus. Many animal viruses, such as HIV (Human
REPLICATION AND ASSEMBLY
Immunodeficiency Virus), leave the infected cells of the immune
The replication mechanism depends on the viral genome. DNA
system by a process known as budding, where virions leave the cell
viruses usually use host cell proteins and enzymes to make
individually. During the budding process, the cell does not undergo
additional DNA that is transcribed to messenger RNA (mRNA),
lysis and is not immediately killed. However, the damage to the cells
which is then used to direct protein synthesis. RNA viruses usually
that the virus infects may make it impossible for the cells to function
use the RNA core as a template for synthesis of viral genomic RNA
normally, even though the cells remain alive for a period of time.
and mRNA. The viral mRNA directs the host cell to synthesize viral
Most productive viral infections follow similar steps in the virus
enzymes and capsid proteins, and to assemble new virions. Of
replication cycle: attachment, penetration, uncoating, replication,
course, there are exceptions to this pattern. If a host cell does not
assembly, and release.
provide the enzymes necessary for viral replication, viral genes
supply the information to direct synthesis of the missing proteins.
Retroviruses, such as HIV, have an RNA genome that must be
reverse transcribed into DNA, which then is incorporated into the
host cell genome.

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To convert RNA into DNA, retroviruses must contain genes that number of infectious virions (copies of viral RNA) in the blood to
encode the virus-specific enzyme reverse transcriptase, which non-detectable levels in many HIV-infected individuals.
transcribes an RNA template to DNA. Reverse transcription never
occurs in uninfected host cells; the needed enzyme, reverse EGRESS
transcriptase, is only derived from the expression of viral genes The last stage of viral replication is the release of the new virions
within the infected host cells. The fact that HIV produces some of its produced in the host organism. They are then able to infect adjacent
own enzymes not found in the host has allowed researchers to cells and repeat the replication cycle. As you have learned, some
develop drugs that inhibit these enzymes. These drugs, including the viruses are released when the host cell dies, while other viruses can
reverse transcriptase inhibitor AZT, inhibit HIV replication by leave infected cells by budding through the membrane without
reducing the activity of the enzyme without affecting the host’s directly killing the cell.
metabolism. This approach has led to the development of a variety
of drugs used to treat HIV and has been effective at reducing the This page titled 21.2A: Steps of Virus Infections is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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21.2B: THE LYTIC AND LYSOGENIC CYCLES OF BACTERIOPHAGES
Bacteriophages, viruses that infect bacteria, may undergo a lytic or
lysogenic cycle.

 LEARNING OBJECTIVES

Describe the lytic and lysogenic cycles of bacteriophages

KEY POINTS
Viruses are species specific, but almost every species on Earth
can be affected by some form of virus.
The lytic cycle involves the reproduction of viruses using a host
cell to manufacture more viruses; the viruses then burst out of
the cell.
The lysogenic cycle involves the incorporation of the viral
genome into the host cell genome, infecting it from within.

KEY TERMS
latency: The ability of a pathogenic virus to lie dormant within a
cell.
bacteriophage: A virus that specifically infects bacteria.
lytic cycle: The normal process of viral reproduction involving
penetration of the cell membrane, nucleic acid synthesis, and Figure 21.2B. 1: Bacteriophage: This transmission electron
micrograph shows bacteriophages attached to a bacterial cell.
lysis of the host cell.
lysogenic cycle: A form of viral reproduction involving the BACTERIOPHAGES
fusion of the nucleic acid of a bacteriophage with that of a host, Bacteriophages are viruses that infect bacteria. Bacteriophages may
followed by proliferation of the resulting prophage. have a lytic cycle or a lysogenic cycle, and a few viruses are capable
of carrying out both. When infection of a cell by a bacteriophage
DIFFERENT HOSTS AND THEIR VIRUSES
results in the production of new virions, the infection is said to be
Viruses are often very specific as to which hosts and which cells productive.
within the host they will infect. This feature of a virus makes it
specific to one or a few species of life on earth. So many different
types of viruses exist that nearly every living organism has its own
set of viruses that try to infect its cells. Even the smallest and
simplest of cells, prokaryotic bacteria, may be attacked by specific
types of viruses.

Figure 21.2B. 1: Lytic versus lysogenic cycle: A temperate

bacteriophage has both lytic and lysogenic cycles. In the lytic cycle,
the phage replicates and lyses the host cell. In the lysogenic cycle,
phage DNA is incorporated into the host genome, where it is passed
on to subsequent generations. Environmental stressors such as
starvation or exposure to toxic chemicals may cause the prophage to
excise and enter the lytic cycle.

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LYTIC CYCLE At this point they initiate the reproductive cycle, resulting in lysis of
With lytic phages, bacterial cells are broken open (lysed) and the host cell. As the lysogenic cycle allows the host cell to continue
destroyed after immediate replication of the virion. As soon as the to survive and reproduce, the virus is reproduced in all of the cell’s
cell is destroyed, the phage progeny can find new hosts to infect. An offspring. An example of a bacteriophage known to follow the
example of a lytic bacteriophage is T4, which infects E. coli found lysogenic cycle and the lytic cycle is the phage lambda of E. coli.
in the human intestinal tract. Lytic phages are more suitable for
LATENCY PERIOD
phage therapy.
Viruses that infect plant or animal cells may also undergo infections
Some lytic phages undergo a phenomenon known as lysis inhibition,
where they are not producing virions for long periods. An example
where completed phage progeny will not immediately lyse out of the
is the animal herpes viruses, including herpes simplex viruses, which
cell if extracellular phage concentrations are high.
cause oral and genital herpes in humans. In a process called latency,
LYSOGENIC CYCLE these viruses can exist in nervous tissue for long periods of time
without producing new virions, only to leave latency periodically
In contrast, the lysogenic cycle does not result in immediate lysing
and cause lesions in the skin where the virus replicates. Even though
of the host cell. Those phages able to undergo lysogeny are known
there are similarities between lysogeny and latency, the term
as temperate phages. Their viral genome will integrate with host
lysogenic cycle is usually reserved to describe bacteriophages.
DNA and replicate along with it fairly harmlessly, or may even
become established as a plasmid. The virus remains dormant until This page titled 21.2B: The Lytic and Lysogenic Cycles of Bacteriophages
host conditions deteriorate, perhaps due to depletion of nutrients; is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
then, the endogenous phages (known as prophages) become active. curated by Boundless.

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21.2C: ANIMAL VIRUSES
Animal viruses have their genetic material copied by a host cell after symptoms worsen for a short period followed by the elimination of
which they are released into the environment to cause disease. the virus from the body by the immune system with eventual
recovery from the infection. Examples of acute viral diseases are the
 LEARNING OBJECTIVES common cold and influenza. Other viruses cause long-term chronic
infections, such as the virus causing hepatitis C, whereas others, like
Describe various animal viruses and the diseases they cause herpes simplex virus, cause only intermittent symptoms. Still other
viruses, such as human herpes viruses 6 and 7, which in some cases
KEY POINTS can cause the minor childhood disease roseola, often successfully
Animal viruses may enter a host cell by either receptor -mediated cause productive infections without causing any symptoms at all in
endocytosis or by changing shape and entering the cell through the host; these patients have an asymptomatic infection.
the cell membrane. In hepatitis C infections, the virus grows and reproduces in liver
Viruses cause diseases in humans and other animals; they often cells, causing low levels of liver damage. The damage is so low that
have to run their course before symptoms disappear. infected individuals are often unaware that they are infected, with
Examples of viral animal diseases include hepatitis C, chicken many infections only detected by routine blood work on patients
pox, and shingles. with risk factors such as intravenous drug use. Since many of the
symptoms of viral diseases are caused by immune responses, a lack
KEY TERMS of symptoms is an indication of a weak immune response to the
receptor-mediated endocytosis: a process by which cells virus. This allows the virus to escape elimination by the immune
internalize molecules (endocytosis) by the inward budding of system and persist in individuals for years, while continuing to
plasma membrane vesicles containing proteins with receptor produce low levels of progeny virions in what is known as a chronic
sites specific to the molecules being internalized viral disease. Chronic infection of the liver by this virus leads to a
much greater chance of developing liver cancer, sometimes as much
ANIMAL VIRUSES as 30 years after the initial infection.
Animal viruses, unlike the viruses of plants and bacteria, do not have
As mentioned, herpes simplex virus can remain in a state of latency
to penetrate a cell wall to gain access to the host cell. Non-enveloped
in nervous tissue for months, even years. As the virus “hides” in the
or “naked” animal viruses may enter cells in two different ways.
tissue and makes few if any viral proteins, there is nothing for the
When a protein in the viral capsid binds to its receptor on the host
immune response to act against; immunity to the virus slowly
cell, the virus may be taken inside the cell via a vesicle during the
declines. Under certain conditions, including various types of
normal cell process of receptor-mediated endocytosis. An alternative
physical and psychological stress, the latent herpes simplex virus
method of cell penetration used by non-enveloped viruses is for
may be reactivated and undergo a lytic replication cycle in the skin,
capsid proteins to undergo shape changes after binding to the
causing the lesions associated with the disease. Once virions are
receptor, creating channels in the host cell membrane. The viral
produced in the skin and viral proteins are synthesized, the immune
genome is then “injected” into the host cell through these channels
response is again stimulated and resolves the skin lesions in a few
in a manner analogous to that used by many bacteriophages.
days by destroying viruses in the skin. As a result of this type of
Enveloped viruses also have two ways of entering cells after binding
replicative cycle, appearances of cold sores and genital herpes
to their receptors: receptor-mediated endocytosis and fusion. Many
outbreaks only occur intermittently, even though the viruses remain
enveloped viruses enter the cell by receptor-mediated endocytosis in
in the nervous tissue for life. Latent infections are common with
a fashion similar to some non-enveloped viruses. On the other hand,
other herpes viruses as well, including the varicella-zoster virus that
fusion only occurs with enveloped virions. These viruses, which
causes chickenpox. After having a chickenpox infection in
include HIV among others, use special fusion proteins in their
childhood, the varicella-zoster virus can remain latent for many
envelopes to cause the envelope to fuse with the plasma membrane
years and reactivate in adults to cause the painful condition known
of the cell, thus releasing the genome and capsid of the virus into the
as “shingles”.
cell cytoplasm.
After making their proteins and copying their genomes, animal
viruses complete the assembly of new virions and exit the cell.
Using the example of HIV, enveloped animal viruses may bud from
the cell membrane as they assemble themselves, taking a piece of
the cell’s plasma membrane in the process. On the other hand, non-
enveloped viral progeny, such as rhinoviruses, accumulate in
infected cells until there is a signal for lysis or apoptosis, and all
virions are released together.
Animal viruses are associated with a variety of human diseases.
Some of them follow the classic pattern of acute disease, where

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 This page titled 21.2C: Animal Viruses is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

Figure 21.2C. 1 : Chicken pox virus: (a) Varicella-zoster, the virus

that causes chickenpox, has an enveloped icosahedral capsid visible
in this transmission electron micrograph. Its double-stranded DNA
genome incorporates into the host DNA and reactivates after latency
in the form of (b) shingles, often exhibiting a rash.

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21.2D: PLANT VIRUSES
Plant viruses can cause damage to stems, leaves, and fruits and can malformed leaves, black streaks on the stems of the plants, altered
have a major impact on the economy because of food supply growth of stems, leaves, or fruits, and ring spots, which are circular
disruptions. or linear areas of discoloration found in a leaf.

 LEARNING OBJECTIVES

Give examples of plant viruses and explain why they can be

costly to the economy

KEY POINTS
Plants have cell walls which protect them from viruses entering
their cells, so some type of damage must occur in order for them
to become infected.
When viruses are passed between plants, it is called horizontal
transmission; when they are passed from the parent plant to the
offspring, it is called vertical transmission.
Symptoms of plant virus infection include malformed leaves, Figure 21.2D. 1 : Oak tree galls: Galls are abnormal plant growth or
black streaks on the stems, discoloration of the leaves and fruits, swellings comprised of plant tissue. Galls are usually found on
and ring spots. foliage or twigs. These unusual deformities are caused by plant
growth-regulating chemicals or stimuli produced by an insect or
Plant viruses can cause major disruptions to crop growth, which other arthropod pest species. The chemicals produced by these
in turn can have a major impact on the economy. causal organisms interfere with normal plant cell growth.
Plant viruses can seriously disrupt crop growth and development,
KEY TERMS significantly affecting our food supply. They are responsible for poor
horizontal transmission: the transmission of an infectious crop quality and quantity globally, and can bring about huge
agent, such as bacterial, fungal, or viral infection, between economic losses annually. Other viruses may damage plants used in
members of the same species that are not in a parent-child landscaping. Some viruses that infect agricultural food plants
relationship include the name of the plant they infect, such as tomato spotted wilt
vertical transmission: the transmission of an infection or other virus, bean common mosaic virus, and cucumber mosaic virus. In
disease from the female of the species to the offspring plants used for landscaping, two of the most common viruses are
peony ring spot and rose mosaic virus. There are far too many plant
PLANT VIRUSES
viruses to discuss each in detail, but symptoms of bean common
Plant viruses, like other viruses, contain a core of either DNA or mosaic virus result in lowered bean production and stunted,
RNA. As plant viruses have a cell wall to protect their cells, their unproductive plants. In the ornamental rose, the rose mosaic disease
viruses do not use receptor-mediated endocytosis to enter host cells causes wavy yellow lines and colored splotches on the leaves of the
as is seen with animal viruses. For many plant viruses to be plant.
transferred from plant to plant, damage to some of the plants’ cells
must occur to allow the virus to enter a new host. This damage is CONTRIBUTIONS AND ATTRIBUTIONS
often caused by weather, insects, animals, fire, or human activities OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
such as farming or landscaping. Additionally, plant offspring may Located at: http://cnx.org/content/m44597/latest...ol11448/latest. License: CC
BY: Attribution
inherit viral diseases from parent plants. virion. Provided by: Wiktionary. Located at:
http://en.wiktionary.org/wiki/virion. License: CC BY-SA: Attribution-
Plant viruses can be transmitted by a variety of vectors: through ShareAlike
contact with an infected plant’s sap, by living organisms such as retrovirus. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/retrovirus. License: CC BY-SA: Attribution-
insects and nematodes, and through pollen. When plant viruses are ShareAlike
transferred between different plants, this is known as horizontal glycoprotein. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/glycoprotein. License: CC BY-SA: Attribution-
transmission; when they are inherited from a parent, this is called ShareAlike
vertical transmission. OpenStax College, Virus Infections and Hosts. October 16, 2013. Provided by:
OpenStax CNX. Located at:
Symptoms of viral diseases vary according to the virus and its host. http://cnx.org/content/m44597/latest...e_21_02_01.png. License: CC BY:
One common symptom is hyperplasia: the abnormal proliferation of Attribution
lysogenic cycle. Provided by: Wiktionary. Located at:
cells that causes the appearance of plant tumors known as galls. en.wiktionary.org/wiki/lysogenic_cycle. License: CC BY-SA: Attribution-
Other viruses induce hypoplasia, or decreased cell growth, in the ShareAlike
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
leaves of plants, causing thin, yellow areas to appear. Still other
Located at: http://cnx.org/content/m44597/latest...ol11448/latest. License: CC
viruses affect the plant by directly killing plant cells; a process BY: Attribution
known as cell necrosis. Other symptoms of plant viruses include OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44597/latest...ol11448/latest. License: CC

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BY: Attribution OpenStax College, Virus Infections and Hosts. October 16, 2013. Provided by:
bacteriophage. Provided by: Wiktionary. Located at: OpenStax CNX. Located at:
en.wiktionary.org/wiki/bacteriophage. License: CC BY-SA: Attribution- http://cnx.org/content/m44597/latest...21_02_04ab.jpg. License: CC BY:
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lytic cycle. Provided by: Wiktionary. Located at: OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
en.wiktionary.org/wiki/lytic_cycle. License: CC BY-SA: Attribution- Located at: http://cnx.org/content/m44597/latest...ol11448/latest. License: CC
ShareAlike BY: Attribution
OpenStax College, Virus Infections and Hosts. October 16, 2013. Provided by: horizontal transmission. Provided by: Wikipedia. Located at:
OpenStax CNX. Located at: en.Wikipedia.org/wiki/horizon...20transmission. License: CC BY-SA:
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Located at: http://cnx.org/content/m44597/latest...ol11448/latest. License: CC Attribution
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en.Wikipedia.org/wiki/recepto...%20endocytosis. License: CC BY-SA: http://cnx.org/content/m44597/latest...e_21_02_02.jpg. License: CC BY:
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SECTION OVERVIEW

21.3: PREVENTION AND TREATMENT OF VIRAL INFECTIONS

21.3B: VACCINES AND ANTI-VIRAL DRUGS FOR
Topic hierarchy TREATMENT

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under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

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21.3A: VACCINES AND IMMUNITY
Vaccinations prevent viruses from spreading by building immunity Live viral vaccines are designed in the laboratory to cause few
to the virus. symptoms in recipients while giving them protective immunity
against future infections. Polio was one disease that represented a
 LEARNING OBJECTIVES milestone in the use of vaccines. Mass immunization campaigns in
the 1950s and 1960s significantly reduced the incidence of the
Explain how vaccination protects vaccinated individuals and disease, which caused muscle paralysis in children and generated
the community great fear in the general population when regional epidemics
occurred. The success of the polio vaccine paved the way for the
KEY POINTS routine dispensation of childhood vaccines against measles, mumps,
Vaccinations are prepared with live viruses, killed viruses, or rubella, chickenpox, and other diseases.
molecular subunits of the virus. The danger of using live vaccines, which are usually more effective
A live vaccine consists of a small dose of the active virus. than killed vaccines, is low, but significant since the possibility that
A killed vaccine contains the inactivated virus. these viruses will revert to their disease-causing form by back
It is possible, though rare, for live vaccines to cause the disease mutations is still present. Live vaccines are usually made by
they’re used to prevent. attenuating (weakening) the “wild-type” (disease-causing) virus by
Live vaccines are made by growing the virus in a lab, which growing it in the laboratory in tissues or at temperatures different
causes mutations that allow them to grow better in the lab than in from what the virus is accustomed to in the host. Adaptations to
the host, thereby inhibiting their ability to cause disease. these new cells or temperatures induce mutations in the genomes of
Even though live vaccines are designed to cause few symptoms, the virus, allowing it to grow better in the laboratory while inhibiting
back mutations can occur and cause the virus to readapt to the its ability to cause disease when reintroduced into conditions found
host and the disease to spread. in the host. These attenuated viruses still cause infection, but since
they do not grow very well, they allow the immune response to
KEY TERMS develop in time to prevent major disease. Back mutations occur
vaccination: inoculation in order to protect against a particular when the vaccine undergoes mutations in the host such that it
disease or strain of disease; causes a primary immune response readapts to the host and can again cause disease, which can then be
without illness, allowing the secondary response to destroy spread to other humans in an epidemic. This type of scenario
subsequent infection happened as recently as 2007 in Nigeria where mutations in a polio
live vaccine: consists of an active microbe (virus or bacteria) vaccine led to an epidemic of polio in that country.
killed vaccine: (inactivated vaccine) consists of virus particles Some vaccines are in continuous development because certain
which are grown in culture and then killed using a method such viruses, such as influenza and HIV, have a high mutation rate
as with heat or formaldehyde compared to other viruses and normal host cells. With influenza,
VACCINES FOR PREVENTION mutations in the surface molecules of the virus help the organism
evade the protective immunity that may have been obtained in a
While we do have limited numbers of effective antiviral drugs, such
previous influenza season, making it necessary for individuals to get
as those used to treat HIV and influenza, the primary method of
vaccinated every year. Other viruses, such as those that cause the
controlling viral disease is by vaccination, which is intended to
childhood diseases measles, mumps, and rubella, mutate so
prevent outbreaks by building immunity to a virus or virus family.
infrequently that the same vaccine is used year after year.
Vaccines may be prepared using live viruses, killed viruses, or
molecular subunits of the virus. The killed viral vaccines and subunit This page titled 21.3A: Vaccines and Immunity is shared under a CC BY-SA
viruses are both incapable of causing disease. 4.0 license and was authored, remixed, and/or curated by Boundless.

Figure 21.3A. 1 : Vaccinations: Vaccinations are designed to boost

immunity to a virus to prevent infection.

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21.3B: VACCINES AND ANTI-VIRAL DRUGS FOR TREATMENT
Vaccines and anti-viral drugs can be used to inhibit the virus and immune response in this way, there is hope that affected individuals
reduce symptoms in individuals suffering from viral infections. will be better able to control the virus, potentially saving a greater
percentage of infected persons from a rapid and very painful death.
 LEARNING OBJECTIVES Another way of treating viral infections is the use of antiviral drugs.
These drugs often have limited success in curing viral disease, but in
Give examples of treatments with anti-viral drugs
many cases, they have been used to control and reduce symptoms
for a wide variety of viral diseases. For most viruses, these drugs can
KEY POINTS inhibit the virus by blocking the actions of one or more of its
Vaccines can boost an individual’s immune response and control proteins. It is important that the targeted proteins be encoded by
viruses, such as Ebola and rabies, before they become deadly. viral genes and that these molecules are not present in a healthy host
Anti-viral drugs inhibit the virus by blocking the actions of its cell. In this way, viral growth is inhibited without damaging the host.
proteins; they are used to control and reduce symptoms for viral There are large numbers of antiviral drugs available to treat
diseases. infections, some specific for a particular virus and others that can
Tamiflu can reduce flu symptoms by inhibiting the enzyme affect multiple viruses.
neuraminidase, which blocks the virus from spreading to Antivirals have been developed to treat genital herpes (herpes
uninfected cells. simplex II) and influenza. For genital herpes, drugs such as
Anti-HIV drugs inhibit and control viral replication at many acyclovir can reduce the number and duration of episodes of active
different phases of the HIV replication cycle, so patients taking viral disease during which patients develop viral lesions in their skin
these drugs have a higher survival rate. cells. As the virus remains latent in nervous tissue of the body for
Viruses can develop resistance to individual anti-viral drugs. life, this drug is not curative, but can make the symptoms of the
The treatment of HIV involves a mixture of different drugs disease more manageable. For influenza, drugs like Tamiflu
(fusion inhibitors, reverse transcriptase inhibitors, integrase (oseltamivir) can reduce the duration of “flu” symptoms by one or
inhibitors, and protease inhibitors) in a cocktail; viruses have two days, but the drug does not prevent symptoms entirely. Tamiflu
greater difficulty gaining resistance to multiple drugs. works by inhibiting an enzyme (viral neuraminidase) that allows
new virions to leave their infected cells. Thus, Tamiflu inhibits the
KEY TERMS
spread of virus from infected to uninfected cells. Other antiviral
virion: a single individual particle of a virus (the viral equivalent
drugs, such as Ribavirin, have been used to treat a variety of viral
of a cell) infections, although its mechanism of action against certain viruses
anti-viral drug: a class of medication, such as antibiotics, that remains unclear.
inhibits the virus by blocking the actions of one or more of its
proteins
Ebola virus: an extremely contagious virus of African origin
that causes Ebola fever, spread through contact with bodily fluids
or secretions of infected persons and by airborne particles

VACCINES AND ANTI-VIRAL DRUGS FOR

TREATMENT
In some cases, vaccines can be used to treat an active viral infection.
The concept behind this is that by giving the vaccine, immunity is
boosted without adding more disease-causing virus. In the case of Figure 21.3B. 1: Tamiflu: (a) Tamiflu inhibits a viral enzyme called
rabies, a fatal neurological disease transmitted via the saliva of neuraminidase (NA) found in the influenza viral envelope. (b)
Neuraminidase cleaves the connection between viral hemagglutinin
rabies virus-infected animals, the progression of the disease from the (HA), also found in the viral envelope, and glycoproteins on the host
time of the animal bite to the time it enters the central nervous cell surface. Inhibition of neuraminidase prevents the virus from
system may be two weeks or longer. This is enough time to detaching from the host cell, thereby blocking further infection.
vaccinate an individual who suspects that they have been bitten by a
ANTI-HIV DRUGS
rabid animaL; their boosted immune response is sufficient to prevent
the virus from entering nervous tissue. Thus, the potentially-fatal By far, the most successful use of antivirals has been in the
treatment of the retrovirus HIV, which causes a disease that, if
neurological consequences of the disease are averted; the individual
only has to recover from the infected bite. This approach is also untreated, is usually fatal within 10–12 years after infection. Anti-
HIV drugs have been able to control viral replication to the point
being used for the treatment of Ebola virus, one of the fastest and
that individuals receiving these drugs survive for a significantly
most deadly viruses on earth. Transmitted by bats and great apes,
longer time than the untreated.
this disease can cause death in 70–90 percent of infected humans
within two weeks. Using newly-developed vaccines that boost the

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Anti-HIV drugs inhibit viral replication at many different phases of anti-retroviral therapy, which involves a mixture of different drugs,
the HIV replicative cycle. Drugs have been developed that inhibit sometimes called a drug “cocktail.” By attacking the virus at
the fusion of the HIV viral envelope with the plasma membrane of different stages of its replicative cycle, it is much more difficult for
the host cell (fusion inhibitors), the conversion of its RNA genome the virus to develop resistance to multiple drugs at the same time.
into double-stranded DNA (reverse transcriptase inhibitors), the Still, even with the use of combination HAART therapy, there is
integration of the viral DNA into the host genome (integrase concern that, over time, the virus will develop resistance to this
inhibitors), and the processing of viral proteins (protease inhibitors). therapy. Thus, new anti-HIV drugs are constantly being developed
with the hope of continuing the battle against this highly fatal virus.

CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
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License: CC BY: Attribution
live vaccine. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/live%20vaccine. License: CC BY-SA: Attribution-
ShareAlike
vaccination. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/vaccination. License: CC BY-SA: Attribution-
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killed vaccine. Provided by: Wikipedia. Located at:
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virion. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/virion.
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Boundless. Provided by: Boundless Learning. Located at:
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Ebola virus. Provided by: Wiktionary. Located at:
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Attribution
OpenStax College, Prevention and Treatment of Viral Infections. October 16,
Figure 21.3B. 1: HIV: HIV, an enveloped, icosahedral virus, 2013. Provided by: OpenStax CNX. Located at:
attaches to the CD4 receptor of an immune cell and fuses with the http://cnx.org/content/m44599/latest/Figure_21_03_03ab.jpg. License: CC
cell membrane. Viral contents are released into the cell where viral BY: Attribution
enzymes convert the single-stranded RNA genome into DNA and OpenStax College, Prevention and Treatment of Viral Infections. October 16,
2013. Provided by: OpenStax CNX. Located at:
incorporate it into the host genome.
http://cnx.org/content/m44599/latest/Figure_21_03_04.jpg. License: CC BY:
When any of these drugs are used individually, the high mutation Attribution
rate of the virus allows it to easily and rapidly develop resistance to
This page titled 21.3B: Vaccines and Anti-Viral Drugs for Treatment is
the drug, limiting the drug’s effectiveness. The breakthrough in the
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
treatment of HIV was the development of HAART, highly-active
curated by Boundless.

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SECTION OVERVIEW

21.4: PRIONS AND VIROIDS

21.4.1: 21-4A- PRIONS AND VIROIDS
Topic hierarchy
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21.4.1: 21-4A- PRIONS AND VIROIDS
Prions are infectious particles that contain no nucleic acids, and On the other hand, BSE was initially thought to affect only cattle.
viroids are small plant pathogens that do not encode proteins. Cattle that died of BSE had developed lesions or “holes” in the
brain, causing the brain tissue to resemble a sponge. Later on in the
 LEARNING OBJECTIVES outbreak, however, it was shown that a similar encephalopathy in
humans known as variant Creutzfeldt-Jakob disease (CJD) could be
Describe prions and viroids and their basic properties acquired from eating beef from animals with BSE, sparking bans by
various countries on the importation of British beef and causing
KEY POINTS considerable economic damage to the British beef industry. BSE still
The prion appears to be the first infectious agent found whose exists in various areas. Although a rare disease, individuals that
transmission is not reliant upon genes made of DNA or RNA. acquire CJD are difficult to treat. The disease spreads from human to
An infectious structural variant of a normal cellular protein human by blood, so many countries have banned blood donation
called PrP (prion protein) is known to cause spongiform from regions associated with BSE.
encephalopathies. The cause of spongiform encephalopathies, such as kuru and BSE, is
Prions have been implicated in fatal neurodegenerative diseases, an infectious structural variant of a normal cellular protein called
such as kuru in humans and bovine spongiform encephalopathy PrP (prion protein). It is this variant that constitutes the prion
(BSE) in cattle. particle. PrP exists in two forms: PrPc, the normal form of the
Loss of motor control and unusual behaviors are common protein, and PrPsc, the infectious form. Once introduced into the
symptoms of individuals with kuru and BSE; symptoms are body, the PrPsc contained within the prion binds to PrPc and converts
usually followed by death. it to PrPsc. This leads to an exponential increase of the PrPsc protein,
Viroids do not have a capsid or outer envelope and can reproduce which aggregates. PrPsc is folded abnormally; the resulting
only within a host cell. conformation (shape) is directly responsible for the lesions seen in
Viroids are not known to cause any human diseases, but they are the brains of infected cattle. Thus, although not without some
responsible for crop failures and the loss of millions of dollars in detractors among scientists, the prion appears to be an entirely new
agricultural revenue each year. form of infectious agent; the first one found whose transmission is
not reliant upon genes made of DNA or RNA.
KEY TERMS
prion: a self-propagating misfolded conformer of a protein that
is responsible for a number of diseases that affect the brain and
other neural tissue
proteinaceous: of, pertaining to, or consisting of protein
viroid: plant pathogens that consist of just a short section of
RNA, but without the protein coat typical of viruses

PRIONS
Prions, so-called because they are proteinaceous, are infectious Figure 21.4.1.1: Example of the formation of a prion: (a)
particles, smaller than viruses, that contain no nucleic acids (neither Endogenous normal prion protein (PrPc) is converted into the
DNA nor RNA). Historically, the idea of an infectious agent that did disease-causing form (PrPsc) when it encounters this variant form of
the protein. PrPsc may arise spontaneously in brain tissue, especially
not use nucleic acids was considered impossible, but pioneering if a mutant form of the protein is present, or it may occur via the
work by Nobel Prize-winning biologist Stanley Prusiner has spread of misfolded prions consumed in food into brain tissue. (b)
convinced the majority of biologists that such agents do indeed exist. This prion-infected brain tissue, visualized using light microscopy,
shows the vacuoles that give it a spongy texture, typical of
Fatal neurodegenerative diseases, such as kuru in humans and transmissible spongiform encephalopathies.
bovine spongiform encephalopathy (BSE) in cattle (commonly
known as “mad cow disease”), were shown to be transmitted by VIROIDS
prions. The disease was spread by the consumption of meat, nervous Viroids are plant pathogens: small, single-stranded, circular RNA
tissue, or internal organs between members of the same species. particles that are much simpler than a virus. They do not have a
Kuru, native to humans in Papua New Guinea, was spread from capsid or outer envelope, but, as with viruses, can reproduce only
human to human via ritualistic cannibalism. BSE, originally detected within a host cell. Viroids do not, however, manufacture any
in the United Kingdom, spread between cattle by the practice of proteins. They produce only a single, specific RNA molecule.
including cattle nervous tissue in feed for other cattle. Individuals Human diseases caused by viroids have yet to be identified.
with kuru and BSE show symptoms of loss of motor control and Viroid-infected plants are responsible for crop failures and the loss
unusual behaviors, such as uncontrolled bursts of laughter with kuru, of millions of dollars in agricultural revenue each year. Some of the
followed by death. Kuru was controlled by inducing the population plants they infect include potatoes, cucumbers, tomatoes,
to abandon its ritualistic cannibalism. chrysanthemums, avocados, and coconut palms.

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 CONTRIBUTIONS AND ATTRIBUTIONS
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44601/latest...ol11448/latest. License: CC
BY: Attribution
OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44601/latest...ol11448/latest. License: CC
BY: Attribution
proteinaceous. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/proteinaceous. License: CC BY-SA: Attribution-
ShareAlike
viroid. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/viroid.
License: CC BY-SA: Attribution-ShareAlike
prion. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/prion.
License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Other Acellular Entities: Prions and Viroids. October 16,
Figure 21.4.1.1: Potatoes infected by a viroid: These potatoes have 2013. Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44601/latest...21_04_01ab.jpg. License: CC BY:
been infected by the potato spindle tuber viroid (PSTV). It is
Attribution
typically spread when infected knives are used to cut healthy OpenStax College, Other Acellular Entities: Prions and Viroids. October 16,
potatoes, which are then planted. 2013. Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44601/latest...e_21_04_02.jpg. License: CC BY:
Attribution

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 CHAPTER OVERVIEW

22: PROKARYOTES- BACTERIA AND ARCHAEA

Topic hierarchy
22.1: Prokaryotic Diversity
22.1A: Classification of Prokaryotes
22.1B: The Origins of Archaea and Bacteria
22.1C: Extremophiles and Biofilms
22.2: Structure of Prokaryotes
22.2A: Basic Structures of Prokaryotic Cells
22.2B: Prokaryotic Reproduction
22.3: Prokaryotic Metabolism
22.3A: Energy and Nutrient Requirements for Prokaryotes
22.3B: The Role of Prokaryotes in Ecosystems
22.4: Bacterial Diseases in Humans
22.4A: History of Bacterial Diseases
22.4B: Biofilms and Disease
22.4C: Antibiotics- Are We Facing a Crisis?
22.4D: Bacterial Foodborne Diseases
22.5: Beneficial Prokaryotes
22.5A: Symbiosis between Bacteria and Eukaryotes
22.5B: Early Biotechnology- Cheese, Bread, Wine, Beer, and Yogurt
22.5C: Prokaryotes and Environmental Bioremediation

Thumbnail: Scanning electron micrograph of neutrophil ingesting methicillin-resistant Staphylococcus aureus bacteria. (Public Domain;
NIAID/NIH).

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1
SECTION OVERVIEW

22.1: PROKARYOTIC DIVERSITY

22.1B: THE ORIGINS OF ARCHAEA AND
Topic hierarchy BACTERIA

22.1C: EXTREMOPHILES AND BIOFILMS

22.1A: CLASSIFICATION OF PROKARYOTES

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22.1A: CLASSIFICATION OF PROKARYOTES
Prokaryotic organisms were the first living things on earth and still kingdom Bacteria, the domain Archaea comprises the rest of the
inhabit every environment, no matter how extreme. prokaryotes, and the domain Eukarya comprises all eukaryotes,
including organisms in the kingdoms Animalia, Plantae, Fungi, and
 LEARNING OBJECTIVES Protista.
The current model of the evolution of the first, living organisms is
Discuss the origins of prokaryotic organisms in terms of the
that these were some form of prokaryotes, which may have evolved
geologic timeline
out of protobionts. In general, the eukaryotes are thought to have
evolved later in the history of life. However, some authors have
KEY POINTS questioned this conclusion, arguing that the current set of
All living things can be classified into three main groups called prokaryotic species may have evolved from more complex
domains; these include the Archaea, the Bacteria, and the eukaryotic ancestors through a process of simplification. Others
Eukarya. have argued that the three domains of life arose simultaneously,
Prokaryotes arose during the Precambrian Period 3.5 to 3.8 from a set of varied cells that formed a single gene pool.
billion years ago. Two of the three domains, Bacteria and Archaea, are prokaryotic.
Prokaryotic organisms can live in every type of environment on Based on fossil evidence, prokaryotes were the first inhabitants on
Earth, from very hot, to very cold, to super haline, to very acidic. Earth, appearing 3.5 to 3.8 billion years ago during the Precambrian
The domains Bacteria and Archaea are the ones containing Period. These organisms are abundant and ubiquitous; that is, they
prokaryotic organisms. are present everywhere. In addition to inhabiting moderate
The Archaea are prokaryotes that inhabit extreme environments, environments, they are found in extreme conditions: from boiling
such as inside of volcanoes, while Bacteria are more common springs to permanently frozen environments in Antarctica; from
organisms, such as E. coli. salty environments like the Dead Sea to environments under
tremendous pressure, such as the depths of the ocean; and from areas
KEY TERMS
without oxygen, such as a waste management plant, to radioactively-
prokaryote: an organism whose cell (or cells) are characterized
contaminated regions, such as Chernobyl. Prokaryotes reside in the
by the absence of a nucleus or any other membrane-bound
human digestive system and on the skin, are responsible for certain
organelles
illnesses, and serve an important role in the preparation of many
domain: in the three-domain system, the highest rank in the
foods.
classification of organisms, above kingdom: Bacteria, Archaea,
and Eukarya
archaea: a taxonomic domain of single-celled organisms lacking
nuclei, formerly called archaebacteria, but now known to differ
fundamentally from bacteria

EVOLUTION OF PROKARYOTES
In the recent past, scientists grouped living things into five kingdoms
(animals, plants, fungi, protists, and prokaryotes) based on several
criteria such as: the absence or presence of a nucleus and other
membrane-bound organelles, the absence or presence of cell walls,
multicellularity, etc. In the late 20th century, the pioneering work of
Carl Woese and others compared sequences of small-subunit Figure 22.1A. 1 : Prokaryotes in extreme environments: Certain
ribosomal RNA (SSU rRNA) which resulted in a more fundamental prokaryotes can live in extreme environments such as the Morning
way to group organisms on earth. Based on differences in the Glory pool, a hot spring in Yellowstone National Park. The spring’s
vivid blue color is from the prokaryotes that thrive in its very hot
structure of cell membranes and in rRNA, Woese and his colleagues waters.
proposed that all life on earth evolved along three lineages, called
domains. The domain Bacteria comprises all organisms in the This page titled 22.1A: Classification of Prokaryotes is shared under a CC
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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22.1B: THE ORIGINS OF ARCHAEA AND BACTERIA
Archaea are believed to have evolved from gram-positive bacteria Within prokaryotes, archaeal cell structure is most similar to that of
and can occupy more extreme environments. gram-positive bacteria, largely because both have a single lipid
bilayer and usually contain a thick sacculus of varying chemical
 LEARNING OBJECTIVES composition. In phylogenetic trees based upon different gene /
protein sequences of prokaryotic homologs, the archaeal homologs
Distinguish bacteria from archaea in terms of their origins are more closely related to those of Gram-positive bacteria. Archaea
and gram-positive bacteria also share conserved indels in a number
KEY POINTS of important proteins, such as Hsp70 and glutamine synthetase.
The first prokaryotes were adapted to the extreme conditions of It has been proposed that the archaea evolved from gram-positive
early earth. bacteria in response to antibiotic selection pressure. This is
It has been proposed that archaea evolved from gram-positive suggested by the observation that archaea are resistant to a wide
bacteria as a response to antibiotic selection pressures. variety of antibiotics that are primarily produced by gram-positive
Microbial mats and stromatolites represent some of the earliest bacteria and that these antibiotics primarily act on the genes that
prokaryotic formations that have been found. distinguish archaea from bacteria. The evolution of Archaea in
response to antibiotic selection, or any other competitive selective
KEY TERMS pressure, could also explain their adaptation to extreme
stromatolite: a laminated, columnar, rock-like structure built environments (such as high temperature or acidity) as the result of a
over geologic time by microorganisms such as cyanobacteria search for unoccupied niches to escape from antibiotic-producing
gram-positive: that is stained violet by Gram’s method due to organisms.
the presence of a peptidoglycan cell wall
sacculus: a small sac MICROBIAL MATS
indel: either an insertion or deletion mutation in the genetic code Microbial mats or large biofilms may represent the earliest forms of
life on earth; there is fossil evidence of their presence starting about
PROKARYOTES, THE FIRST INHABITANTS OF 3.5 billion years ago. A microbial mat is a multi-layered sheet of
EARTH prokaryotes that includes mostly bacteria, but also archaea.
When and where did life begin? What were the conditions on earth Microbial mats are a few centimeters thick, typically growing where
when life began? Prokaryotes were the first forms of life on earth, different types of materials interface, mostly on moist surfaces. The
existing for billions of years before plants and animals appeared. various types of prokaryotes that comprise the mats use different
The earth and its moon are thought to be about 4.54 billion years metabolic pathways, which is the reason for their various colors.
old. This estimate is based on evidence from radiometric dating of Prokaryotes in a microbial mat are held together by a glue-like
meteorite material together with other substrate material from earth sticky substance that they secrete called extracellular matrix.
and the moon. Early earth had a very different atmosphere
(contained less molecular oxygen) than it does today and was
subjected to strong radiation; thus, the first organisms would have
flourished where they were more protected, such as in ocean depths
or beneath the surface of the earth. Also at this time, strong volcanic
activity was common on Earth. It is probable that these first
organisms, the first prokaryotes, were adapted to very high
temperatures. Early earth was prone to geological upheaval and
volcanic eruption, and was subject to bombardment by mutagenic
radiation from the sun. The first organisms were prokaryotes that Figure 22.1B. 1: Microbial mat: This (a) microbial mat, about one
meter in diameter, grows over a hydrothermal vent in the Pacific
could withstand these harsh conditions. Ocean in a region known as the “Pacific Ring of Fire.” The mat
Although probable prokaryotic cell fossils date to almost 3.5 billion helps retain microbial nutrients. Chimneys, such as the one indicated
by the arrow, allow gases to escape. (b) In this micrograph, bacteria
years ago, most prokaryotes do not have distinctive morphologies; are visualized using fluorescence microscopy.
fossil shapes cannot be used to identify them as Archaea. Instead, The first microbial mats likely obtained their energy from chemicals
chemical fossils of unique lipids are more informative because such
found near hydrothermal vents. A hydrothermal vent is a breakage
compounds do not occur in other organisms. Some publications or fissure in the earth’s surface that releases geothermally-heated
suggest that archaean or eukaryotic lipid remains are present in
water. With the evolution of photosynthesis about 3 billion years
shales dating from 2.7 billion years ago. Such lipids have also been ago, some prokaryotes in microbial mats came to use a more widely-
detected in Precambrian formations. The oldest such traces come
available energy source, sunlight, whereas others were still
from the Isua district of west Greenland, which include earth’s oldest dependent on chemicals from hydrothermal vents for energy and
sediments, formed 3.8 billion years ago. The archaeal lineage may
food.
be the most ancient that exists on earth.

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STROMATOLITES
Fossilized microbial mats represent the earliest record of life on
earth. A stromatolite is a sedimentary structure formed when
minerals are precipitated out of water by prokaryotes in a microbial
mat. Stromatolites form layered rocks made of carbonate or silicate.
Although most stromatolites are artifacts from the past, there are
places on earth where stromatolites are still forming. For example,
growing stromatolites have been found in the Anza-Borrego Desert
Figure 22.1B. 1: Stromatolites: (a) These living stromatolites are
State Park in San Diego County, California. located in Shark Bay, Australia. (b) These fossilized stromatolites,
found in Glacier National Park, Montana, are nearly 1.5 billion years
old.

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22.1C: EXTREMOPHILES AND BIOFILMS
Prokaryotes are well adapted to living in all types of conditions,
including extreme ones, and prefer to live in colonies called
biofilms.

 LEARNING OBJECTIVES

Discuss the distinguishing features of extremophiles and the

environments that produce biofilms

KEY POINTS
Prokaryotes live in all environments, no matter how extreme they
may be. Figure 22.1C. 1 : Bacteria and radiation tolerance: Deinococcus
Bacteria that prefer very salty environments are called radiodurans, visualized in this false color transmission electron
halophiles, while those that live in very acidic environments are micrograph, is a prokaryote that can tolerate very high doses of
ionizing radiation. It has developed DNA repair mechanisms that
called acidophiles. allow it to reconstruct its chromosome even if it has been broken
An example of a habitat that halophiles can colonize is the Dead into hundreds of pieces by radiation or heat.
Sea, a body of water that is 10 times saltier than regular ocean Other bacteria and archaea are adapted to grow under extreme
water. conditions and are called extremophiles, meaning “lovers of
A biofilm is a microbial community held together in a gummy- extremes.” Extremophiles have been found in all kinds of
textured matrix that consists primarily of polysaccharides environments: the depth of the oceans, hot springs, the Arctic and
secreted by the organisms. the Antarctic, in very dry places, deep inside earth, in harsh
Biofilms can be found clogging pipes, on kitchen counters, or chemical environments, and in high radiation environments, just to
even on the surface of one’s teeth. mention a few. These organisms give us a better understanding of
prokaryotic diversity and raise the possibility of finding new
KEY TERMS prokaryotic species that may lead to the discovery of new
extremophile: an organism that lives under extreme conditions therapeutic drugs or have industrial applications. Because they have
of temperature, salinity, etc; commercially important as a source specialized adaptations that allow them to live in extreme
of enzymes that operate under similar conditions conditions, many extremophiles cannot survive in moderate
halophile: an organism that lives and thrives in an environment environments. There are many different groups of extremophiles.
of high salinity, often requiring such an environment; a form of They are identified based on the conditions in which they grow best.
extremophile Several habitats are extreme in multiple ways. For example, a soda
alkaliphile: any organism that lives and thrives in an alkaline lake is both salty and alkaline, so organisms that live in a soda lake
environment, such as a soda lake; a form of extremophile must be both alkaliphiles and halophiles. Other extremophiles, like
radioresistant organisms, do not prefer an extreme environment (in
MICROBES ARE ADAPTABLE: LIFE IN MODERATE this case, one with high levels of radiation), but have adapted to
AND EXTREME ENVIRONMENTS survive in it.
Some organisms have developed strategies that allow them to
survive harsh conditions. Prokaryotes thrive in a vast array of PROKARYOTES IN THE DEAD SEA
environments; some grow in conditions that would seem very One example of a very harsh environment is the Dead Sea, a
normal to us, whereas others are able to thrive and grow under hypersaline basin that is located between Jordan and Israel.
conditions that would kill a plant or animal. Almost all prokaryotes Hypersaline environments are essentially concentrated seawater. In
have a cell wall: a protective structure that allows them to survive in the Dead Sea, the sodium concentration is 10 times higher than that
both hyper- and hypo-osmotic conditions. Some soil bacteria are of seawater. The water also contains high levels of magnesium
able to form endospores that resist heat and drought, thereby (about 40 times higher than in seawater) that would be toxic to most
allowing the organism to survive until favorable conditions recur. living things. Iron, calcium, and magnesium, elements that form
These adaptations, along with others, allow bacteria to be the most divalent ions (Fe2+, Ca2+, and Mg2+), produce what is commonly
abundant life form in all terrestrial and aquatic ecosystems. referred to as “hard” water. Taken together, the high concentration of
divalent cations, the acidic pH (6.0), and the intense solar radiation
flux make the Dead Sea a unique, and uniquely hostile, ecosystem.

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 these communities more robust than free-living, or planktonic,
prokaryotes. The sticky substance that holds bacteria together also
excludes most antibiotics and disinfectants, making biofilm bacteria
hardier than their planktonic counterparts. Overall, biofilms are very
difficult to destroy because they are resistant to many common
forms of sterilization.

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Figure 22.1C. 1 : Halophile habitats: (a) The Dead Sea is Located at: http://cnx.org/content/m44602/latest...ol11448/latest. License: CC
hypersaline. Nevertheless, salt-tolerant bacteria thrive in this sea. (b) BY: Attribution
These halobacteria cells can form salt-tolerant bacterial mats. Prokaryote. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/Prokaryote. License: CC BY-SA: Attribution-
THE ECOLOGY OF BIOFILMS ShareAlike
prokaryote. Provided by: Wiktionary. Located at:
Until a couple of decades ago, microbiologists used to think of en.wiktionary.org/wiki/prokaryote. License: CC BY-SA: Attribution-
prokaryotes as isolated entities living apart. This model, however, ShareAlike
archaea. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/archaea.
does not reflect the true ecology of prokaryotes, most of which License: CC BY-SA: Attribution-ShareAlike
prefer to live in communities where they can interact. A biofilm is a domain. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/domain.
License: CC BY-SA: Attribution-ShareAlike
microbial community held together in a gummy-textured matrix that OpenStax College, Introduction. October 16, 2013. Provided by: OpenStax
consists primarily of polysaccharides secreted by the organisms, CNX. Located at: http://cnx.org/content/m44602/latest...e_22_00_01.jpg.
License: CC BY: Attribution
together with some proteins and nucleic acids. Biofilms grow Archaea. Provided by: Wikipedia. Located at:
attached to surfaces. Some of the best-studied biofilms are en.Wikipedia.org/wiki/Archaea. License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
composed of prokaryotes, although fungal biofilms have also been Located at: http://cnx.org/content/m44603/latest...ol11448/latest. License: CC
described, as well as some composed of a mixture of fungi and BY: Attribution
stromatolite. Provided by: Wiktionary. Located at:
bacteria.
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License: CC BY-SA: Attribution-ShareAlike
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Figure 22.1C. 1 : Biofilm Development: Five stages of biofilm OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
development are shown. During stage 1, initial attachment, bacteria Located at: http://cnx.org/content/m44603/latest...ol11448/latest. License: CC
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pili permanently anchor the bacteria to the surface. During stage 3,
BY: Attribution
maturation I, the biofilm grows through cell division and recruitment extremophile. Provided by: Wiktionary. Located at:
of other bacteria. An extracellular matrix composed primarily of en.wiktionary.org/wiki/extremophile. License: CC BY-SA: Attribution-
polysaccharides holds the biofilm together. During stage 4, ShareAlike
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complex shape. During stage 5, dispersal, the biofilm matrix is en.wiktionary.org/wiki/halophile. License: CC BY-SA: Attribution-ShareAlike
partly broken down, allowing some bacteria to escape and colonize alkaliphile. Provided by: Wiktionary. Located at:
another surface. Micrographs of a Pseudomonas aeruginosa biofilm en.wiktionary.org/wiki/alkaliphile. License: CC BY-SA: Attribution-
in each of the stages of development are shown. ShareAlike
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Biofilms are present almost everywhere: they can cause the clogging CNX. Located at: http://cnx.org/content/m44602/latest...e_22_00_01.jpg.
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recent, large-scale outbreaks of bacterial contamination of food, OpenStax CNX. Located at:
biofilms have played a major role. They also colonize household http://cnx.org/content/m44603/latest...e_22_01_01.jpg. License: CC BY:
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as well as places on the human body, such as the surfaces of our OpenStax CNX. Located at:
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with their protective exopolysaccharidic (EPS) environment, make

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SECTION OVERVIEW

22.2: STRUCTURE OF PROKARYOTES

22.2B: PROKARYOTIC REPRODUCTION
Topic hierarchy
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CELLS

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22.2A: BASIC STRUCTURES OF PROKARYOTIC CELLS
Prokaryotes, found in both Domain Archaea and Bacteria, are The composition of the cell wall differs significantly between the
unicellular organisms that lack membrane-bound organelles and a domains Bacteria and Archaea, the two domains of life into which
defined nucleus. prokaryotes are divided. The composition of their cell walls also
differs from the eukaryotic cell walls found in plants (cellulose) or
 LEARNING OBJECTIVES fungi and insects (chitin). The cell wall functions as a protective
layer and is responsible for the organism’s shape. Some bacteria
Describe the basic structure of a typical prokaryote have a capsule outside the cell wall. Other structures are present in
some prokaryotic species, but not in others. For example, the capsule
KEY POINTS found in some species enables the organism to attach to surfaces,
Prokaryotic cells lack a defined nucleus, but have a region in the protects it from dehydration and attack by phagocytic cells, and
cell, termed the nucleoid, in which a single chromosomal, increases its resistance to our immune responses. Some species also
circular, double-stranded DNA molecule is located. have flagella used for locomotion and pili used for attachment to
Archaeal membranes have replaced the fatty acids of bacterial surfaces. Plasmids, which consist of extra-chromosomal DNA, are
membranes with isoprene; some archaeal membranes are also present in many species of bacteria and archaea.
monolayer rather than bilayer.
Prokaryotes can be further classified based on the composition of
the cell wall in terms of the amount of peptidoglycan present.
Gram-positive organisms typically lack the outer membrane
found in gram-negative organisms and contain a large amount of
peptidoglycan in the cell wall, roughly 90%.
Gram-negative bacteria have a relatively thin cell wall composed
of a few layers of peptidoglycan.
Gram-negative bacteria have a relatively thin cell wall composed
of a few layers of peptidoglycan.

KEY TERMS
nucleoid: the irregularly-shaped region within a prokaryote cell
where the genetic material is localized
Figure 22.2A. 1 : Domains of life: Bacteria and Archaea are both
plasmid: a circle of double-stranded DNA that is separate from prokaryotes, but differ enough to be placed in separate domains. An
the chromosomes, which is found in bacteria and protozoa ancestor of modern Archaea is believed to have given rise to
osmotic pressure: the hydrostatic pressure exerted by a solution Eukarya, the third domain of life. Archaeal and bacterial phyla are
shown; the evolutionary relationship between these phyla is still
across a semipermeable membrane from a pure solvent open to debate.

THE PROKARYOTIC CELL THE PLASMA MEMBRANE

Prokaryotes are unicellular organisms that lack organelles or other The plasma membrane is a thin lipid bilayer (6 to 8 nanometers) that
internal membrane-bound structures. Therefore, they do not have a completely surrounds the cell and separates the inside from the
nucleus, but, instead, generally have a single chromosome: a piece outside. Its selectively-permeable nature keeps ions, proteins, and
of circular, double-stranded DNA located in an area of the cell called other molecules within the cell, preventing them from diffusing into
the nucleoid. Most prokaryotes have a cell wall outside the plasma the extracellular environment, while other molecules may move
membrane. through the membrane. The general structure of a cell membrane is a
phospholipid bilayer composed of two layers of lipid molecules. In
archaeal cell membranes, isoprene (phytanyl) chains linked to
glycerol replace the fatty acids linked to glycerol in bacterial
membranes. Some archaeal membranes are lipid monolayers instead
of bilayers.

Figure 22.2A. 1 : Prokaryotic cell structure: The features of a typical

prokaryotic cell are shown.

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 Bacteria are divided into two major groups: gram-positive and gram-
negative, based on their reaction to gram staining. Note that all
gram-positive bacteria belong to one phylum; bacteria in the other
phyla (Proteobacteria, Chlamydias, Spirochetes, Cyanobacteria, and
others) are gram-negative. The gram-staining method is named after
its inventor, Danish scientist Hans Christian Gram (1853–1938). The
different bacterial responses to the staining procedure are ultimately
due to cell wall structure. Gram-positive organisms typically lack
the outer membrane found in gram-negative organisms. Up to 90
percent of the cell wall in gram-positive bacteria is composed of
peptidoglycan, with most of the rest composed of acidic substances
called teichoic acids. Teichoic acids may be covalently linked to
lipids in the plasma membrane to form lipoteichoic acids.
Lipoteichoic acids anchor the cell wall to the cell membrane. Gram-
negative bacteria have a relatively thin cell wall composed of a few
layers of peptidoglycan (only 10 percent of the total cell wall),
surrounded by an outer envelope containing lipopolysaccharides
(LPS) and lipoproteins. This outer envelope is sometimes referred to
as a second lipid bilayer. The chemistry of this outer envelope is
Figure 22.2A. 1 : Plasma membrane structure: Archaeal very different, however, from that of the typical lipid bilayer that
phospholipids differ from those found in Bacteria and Eukarya in forms plasma membranes.
two ways. First, they have branched phytanyl sidechains instead of
linear ones. Second, an ether bond instead of an ester bond connects
the lipid to the glycerol.

THE CELL WALL

The cytoplasm of prokaryotic cells has a high concentration of
dissolved solutes. Therefore, the osmotic pressure within the cell is
relatively high. The cell wall is a protective layer that surrounds
some cells and gives them shape and rigidity. It is located outside the
cell membrane and prevents osmotic lysis (bursting due to
increasing volume). The chemical composition of the cell walls
Figure 22.2A. 1 : Gram-positive and gram-negative bacteria:
varies between archaea and bacteria. It also varies between bacterial Bacteria are divided into two major groups: gram-positive and gram-
species. negative. Both groups have a cell wall composed of peptidoglycan:
in gram-positive bacteria, the wall is thick, whereas in gram-
Bacterial cell walls contain peptidoglycan composed of negative bacteria, the wall is thin. In gram-negative bacteria, the cell
polysaccharide chains that are cross-linked by unusual peptides wall is surrounded by an outer membrane that contains
containing both L- and D-amino acids, including D-glutamic acid lipopolysaccharides and lipoproteins. Porins, proteins in this cell
membrane, allow substances to pass through the outer membrane of
and D-alanine. Proteins normally have only L-amino acids; as a gram-negative bacteria. In gram-positive bacteria, lipoteichoic acid
consequence, many of our antibiotics work by mimicking D-amino anchors the cell wall to the cell membrane.
acids and, therefore, have specific effects on bacterial cell wall
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Boundless.
the outside of cell walls of both archaea and bacteria.

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22.2B: PROKARYOTIC REPRODUCTION
Prokaryotes reproduce asexually by binary fission; they can also
exchange genetic material by transformation, transduction, and
conjugation.

 LEARNING OBJECTIVES

Distinguish among the types of reproduction in prokaryotes

KEY POINTS
Binary fission is a type of reproduction in which the
Figure 22.2B. 1: Modes of prokaryote reproduction: Besides binary
chromosome is replicated and the resultant prokaryote is an exact fission, there are three other mechanisms by which prokaryotes can
copy of the parental prokaryate, thus leaving no opportunity for exchange DNA. In (a) transformation, the cell takes up prokaryotic
genetic diversity. DNA directly from the environment. The DNA may remain separate
as plasmid DNA or be incorporated into the host genome. In (b)
Transformation is a type of prokaryotic reproduction in which a transduction, a bacteriophage injects DNA into the cell that contains
prokaryote can take up DNA found within the environment that a small fragment of DNA from a different prokaryote. In (c)
has originated from other prokaryotes. conjugation, DNA is transferred from one cell to another via a
mating bridge that connects the two cells after the pilus draws the
Transduction is a type of prokaryotic reproduction in which a two bacteria close enough to form the bridge.
prokaryote is infected by a virus which injects short pieces of In transformation, the prokaryote takes in DNA found in its
chromosomal DNA from one bacterium to another. environment that is shed by other prokaryotes. If a nonpathogenic
Conjugation is a type of prokaryotic reproduction in which DNA
bacterium takes up DNA for a toxin gene from a pathogen and
is transferred between prokaryotes by means of a pilus.
incorporates the new DNA into its own chromosome, it, too, may
become pathogenic. In transduction, bacteriophages, the viruses that
KEY TERMS
infect bacteria, sometimes also move short pieces of chromosomal
transformation: the alteration of a bacterial cell caused by the
DNA from one bacterium to another. Transduction results in a
transfer of DNA from another, especially if pathogenic
recombinant organism. Archaea are not affected by bacteriophages,
transduction: horizontal gene transfer mechanism in
but instead have their own viruses that translocate genetic material
prokaryotes where genes are transferred using a virus
from one individual to another. In conjugation, DNA is transferred
binary fission: the process whereby a cell divides asexually to
from one prokaryote to another by means of a pilus, which brings
produce two daughter cells
the organisms into contact with one another. The DNA transferred
conjugation: the temporary fusion of organisms, especially as
can be in the form of a plasmid or as a hybrid, containing both
part of sexual reproduction
plasmid and chromosomal DNA.
pilus: a hairlike appendage found on the cell surface of many
bacteria Reproduction can be very rapid: a few minutes for some species.
This short generation time, coupled with mechanisms of genetic
REPRODUCTION recombination and high rates of mutation, result in the rapid
Reproduction in prokaryotes is asexual and usually takes place by evolution of prokaryotes, allowing them to respond to environmental
binary fission. The DNA of a prokaryote exists as as a single, changes (such as the introduction of an antibiotic) very rapidly.
circular chromosome. Prokaryotes do not undergo mitosis; rather the
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chromosome is replicated and the two resulting copies separate from
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one another, due to the growth of the cell. The prokaryote, now Located at: http://cnx.org/content/m44605/latest...ol11448/latest. License: CC
enlarged, is pinched inward at its equator and the two resulting cells, BY: Attribution
nucleoid. Provided by: Wiktionary. Located at:
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SECTION OVERVIEW

22.3: PROKARYOTIC METABOLISM

22.3B: THE ROLE OF PROKARYOTES IN
Topic hierarchy ECOSYSTEMS

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22.3A: ENERGY AND NUTRIENT REQUIREMENTS FOR PROKARYOTES
Prokaryotes need a source of energy, a source of carbon, acids, lipids, and many other compounds. Carbon accounts for about
macronutrients, and micronutrients to survive. 50 percent of the composition of the cell. Nitrogen represents 12
percent of the total dry weight of a typical cell and is a component of
 LEARNING OBJECTIVES proteins, nucleic acids, and other cell constituents. Most of the
nitrogen available in nature is either atmospheric nitrogen (N2) or
Summarize what prokaryotes need to remain alive and another inorganic form. Diatomic (N2) nitrogen, however, can be
functioning converted into an organic form only by certain organisms, called
nitrogen-fixing organisms. Both hydrogen and oxygen are part of
KEY POINTS many organic compounds and of water. Phosphorus is required by
The main components of the organic compounds in a prokaryotic all organisms for the synthesis of nucleotides and phospholipids.
cell are macronutrients (such as carbon, hydrogen, oxygen, Sulfur is part of the structure of some amino acids such as cysteine
nitrogen, phosphorus, and sulfur) that make up important and methionine. It is also present in several vitamins and
biomolecules, including proteins and nucleic acids. coenzymes. Other important macronutrients are potassium (K),
Nutrients that are needed by prokaryotes in small quantities to magnesium (Mg), calcium (Ca), and sodium (Na). Although these
perform cellular functions, including certain electron transport elements are required in smaller amounts, they are very important
chain reactions, are known as micronutrients (e.g., iron, boron, for the structure and function of the prokaryotic cell.
chromium, and manganese).
Autotrophic prokaryotes are able to fix inorganic compounds,
MICRONUTRIENTS
such as carbon dioxide, to obtain carbon, while heterotrophic
prokaryotes use organic compounds as their source of carbon.

KEY TERMS
macronutrient: any of the elements required in large amounts
by all living things
chemotroph: an organism that obtains energy by the oxidation
of electron-donating molecules in the environment
micronutrient: a mineral, vitamin, or other substance that is
essential, even in very small quantities, for growth or metabolism Figure 22.3A. 1 : Filaments of photosynthetic cyanobacteria:
Cyanobacteria are an example of phototrophic prokaryotes.
NEEDS OF PROKARYOTES In addition to these macronutrients, prokaryotes require various
The diverse environments and ecosystems on Earth have a wide metallic elements in small amounts. These are referred to as
range of conditions in terms of temperature, available nutrients, micronutrients or trace elements. For example, iron is necessary for
acidity, salinity, and energy sources. Prokaryotes are very well the function of the cytochromes involved in electron-transport
equipped to make their living out of a vast array of nutrients and reactions. Some prokaryotes require other elements (such as boron
conditions. To live, prokaryotes need a source of energy, a source of (B), chromium (Cr), and manganese (Mn)) primarily as enzyme
carbon, and some additional nutrients. cofactors.

MACRONUTRIENTS THE WAYS IN WHICH PROKARYOTES OBTAIN

Cells are essentially a well-organized assemblage of ENERGY
macromolecules and water. Recall that macromolecules are Prokaryotes can use different sources of energy to assemble
produced by the polymerization of smaller units called monomers. macromolecules from smaller molecules. Phototrophs (or
For cells to build all of the molecules required to sustain life, they phototrophic organisms) obtain their energy from sunlight.
need certain substances, collectively called nutrients. When Chemotrophs (or chemosynthetic organisms) obtain their energy
prokaryotes grow in nature, they obtain their nutrients from the from chemical compounds. Chemotrophs that can use organic
environment. Nutrients that are required in large amounts are called compounds as energy sources are called chemoorganotrophs. Those
macronutrients, whereas those required in smaller or trace amounts that can also use inorganic compounds as energy sources are called
are called micronutrients. Just a handful of elements are considered chemolithotrophs.
macronutrients: carbon, hydrogen, oxygen, nitrogen, phosphorus,
and sulfur. (A mnemonic for remembering these elements is the THE WAYS IN WHICH PROKARYOTES OBTAIN
acronym CHONPS. ) CARBON
Why are these macronutrients needed in large amounts? They are Just as prokaryotes can use different sources of energy, they can also
the components of organic compounds in cells. Carbon is the major utilize different sources of carbon compounds. Recall that organisms
element in all macromolecules: carbohydrates, proteins, nucleic that are able to fix inorganic carbon are called autotrophs.

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Autotrophic prokaryotes synthesize organic molecules from carbon
dioxide. In contrast, heterotrophic prokaryotes obtain carbon from
organic compounds. To make the picture more complex, the terms
that describe how prokaryotes obtain energy and carbon can be
combined. Thus, photoautotrophs use energy from sunlight and
carbon from carbon dioxide and water, whereas chemoheterotrophs
obtain energy and carbon from an organic chemical source.
Chemolithoautotrophs obtain their energy from inorganic Figure 22.3A. 1 : Table 1. Carbon and energy sources in prokaryotes:
compounds, while building their complex molecules from carbon This table summarizes the types of energy and carbon sources for
dioxide. Table 1 summarizes carbon and energy sources in different types of prokaryotes.
prokaryotes. This page titled 22.3A: Energy and Nutrient Requirements for Prokaryotes
is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

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22.3B: THE ROLE OF PROKARYOTES IN ECOSYSTEMS
Prokaryotes play vital roles in the movement of carbon dioxide and
nitrogen in the carbon and nitrogen cycles.

 LEARNING OBJECTIVES

Give examples of the beneficial roles played by prokaryotes

in different ecosystems

KEY POINTS
Carbon and nitrogen are both macronutrients that are necessary
for life on earth; prokaryotes play vital roles in their cycles.
The carbon cycle is maintained by prokaryotes that remove
carbon dioxide and return it to the atmosphere.
Prokaryotes play a major role in the nitrogen cycle by fixing
atomspheric nitrogen into ammonia that plants can use and by Figure 22.3B. 1: Carbon cycle: Prokaryotes play a significant role in
converting ammonia into other forms of nitrogen sources. continuously moving carbon through the biosphere.
A large amount of available carbon is found in land plants, which
KEY TERMS are producers that use carbon dioxide from the air to synthesize
carbon cycle: the physical cycle of carbon through the earth’s carbon compounds. Related to this, one very significant source of
biosphere, geosphere, hydrosphere, and atmosphere that includes carbon compounds is humus, which is a mixture of organic materials
such processes as photosynthesis, decomposition, respiration and from dead plants and prokaryotes that have resisted decomposition.
carbonification Consumers such as animals use organic compounds generated by
nitrogen cycle: the natural circulation of nitrogen, in which producers, releasing carbon dioxide to the atmosphere. Then,
atmospheric nitrogen is converted to nitrogen oxides and bacteria and fungi, collectively called decomposers, carry out the
deposited in the soil, where it is used by organisms or breakdown (decomposition) of plants and animals and their organic
decomposed back to elemental nitrogen compounds. The most important contributor of carbon dioxide to the
nitrogen fixation: the conversion of atmospheric nitrogen into atmosphere is microbial decomposition of dead material (dead
ammonia and organic derivatives, by natural means, especially animals, plants, and humus).
by microorganisms in the soil, into a form that can be assimilated
In aqueous environments and their anoxic sediments, there is
by plants
another carbon cycle taking place. In this case, the cycle is based on
ROLE OF PROKARYOTES IN ECOSYSTEMS one-carbon compounds. In anoxic sediments, prokaryotes, mostly
archaea, produce methane (CH4). This methane moves into the zone
Prokaryotes are ubiquitous: There is no niche or ecosystem in which
above the sediment, which is richer in oxygen and supports bacteria
they are not present. Prokaryotes play many roles in the
called methane oxidizers that oxidize methane to carbon dioxide,
environments they occupy, but the roles they play in the carbon and
which then returns to the atmosphere.
nitrogen cycles are vital to life on earth.

PROKARYOTES AND THE CARBON CYCLE PROKARYOTES AND THE NITROGEN CYCLE
Nitrogen is a very important element for life because it is part of
Carbon is one of the most important macronutrients. Prokaryotes
proteins and nucleic acids. As a macronutrient in nature, it is
play an important role in the carbon cycle. Carbon is cycled through
recycled from organic compounds to ammonia, ammonium ions,
earth’s major reservoirs: land, the atmosphere, aquatic environments,
nitrate, nitrite, and nitrogen gas by myriad processes, many of which
sediments and rocks, and biomass. The movement of carbon is via
are carried out solely by prokaryotes; they are key to the nitrogen
carbon dioxide, which is removed from the atmosphere by land
cycle. The largest pool of nitrogen available in the terrestrial
plants and marine prokaryotes and is returned to the atmosphere via
ecosystem is gaseous nitrogen from the air, but this nitrogen is not
the respiration of chemoorganotrophic organisms, including
usable by plants, which are primary producers. Gaseous nitrogen is
prokaryotes, fungi, and animals. Although the largest carbon
transformed, or “fixed,” into more-readily available forms such as
reservoir in terrestrial ecosystems is in rocks and sediments, that
ammonia through the process of nitrogen fixation by natural means,
carbon is not readily available.
especially by microorganisms (prokayotes) in the soil. Ammonia can
then be used by plants or converted to other forms.

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 CONTRIBUTIONS AND ATTRIBUTIONS
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44606/latest...ol11448/latest. License: CC
BY: Attribution
chemotroph. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/chemotroph. License: CC BY-SA: Attribution-
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micronutrient. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/micronutrient. License: CC BY-SA: Attribution-
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macronutrient. Provided by: Wiktionary. Located at:
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Located at: http://cnx.org/content/m44606/latest...ol11448/latest. License: CC
BY: Attribution
Bacteria. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/Bacteria. License: CC BY: Attribution
OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44606/latest...ol11448/latest. License: CC
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carbon cycle. Provided by: Wiktionary. Located at:
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nitrogen cycle. en.wiktionary.org/wiki/nitrogen_fixation. License: CC BY-SA: Attribution-
Another source of ammonia is ammonification, the process by which ShareAlike
nitrogen cycle. Provided by: Wiktionary. Located at:
ammonia is released during the decomposition of nitrogen- en.wiktionary.org/wiki/nitrogen_cycle. License: CC BY-SA: Attribution-
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atmosphere, however, represents only 15 percent of the total Located at: http://cnx.org/content/m44606/latest...ol11448/latest. License: CC
nitrogen released; the rest is as N2 and N2O. Ammonia is catabolized BY: Attribution
Bacteria. Provided by: Wikipedia. Located at:
anaerobically by some prokaryotes, yielding N2 as the final product. en.Wikipedia.org/wiki/Bacteria. License: CC BY: Attribution
Nitrification is the conversion of ammonium to nitrite and nitrate. OpenStax College, Prokaryotic Metabolism. October 16, 2013. Provided by:
OpenStax CNX. Located at:
Nitrification in soils is carried out by bacteria belonging to the http://cnx.org/content/m44606/latest...e_22_03_01.jpg. License: CC BY:
genera Nitrosomas, Nitrobacter, and Nitrospira. The bacteria Attribution
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perform the reverse process, the reduction of nitrate from the soils to OpenStax CNX. Located at:
gaseous compounds such as N2O, NO, and N2, a process called http://cnx.org/content/m44606/latest...e_22_03_02.png. License: CC BY:
denitrification. Attribution

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under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
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SECTION OVERVIEW

22.4: BACTERIAL DISEASES IN HUMANS

22.4C: ANTIBIOTICS- ARE WE FACING A CRISIS?
22.4A: HISTORY OF BACTERIAL DISEASES
22.4D: BACTERIAL FOODBORNE DISEASES
22.4B: BIOFILMS AND DISEASE
This page titled 22.4: Bacterial Diseases in Humans is shared under a CC
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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22.4A: HISTORY OF BACTERIAL DISEASES
BUBONIC PLAGUES
 LEARNING OBJECTIVES From 541 to 750 C.E.., an outbreak of what was likely a bubonic
plague (the Plague of Justinian), eliminated one-quarter to one-half
Give examples of historical, new, and re-emerging bacterial
diseases in humans of the human population in the eastern Mediterranean region. The
population in Europe dropped by 50 percent during this outbreak.
The bubonic plague would strike Europe more than once.
There are records about infectious diseases as far back as 3000
B.C.E. A number of significant pandemics caused by bacteria have One of the most devastating pandemics was the Black Death (1346
been documented over several hundred years. Some of the most to 1361) that is believed to have been another outbreak of bubonic
memorable pandemics led to the decline of cities and nations. In the plague caused by the bacterium Yersinia pestis. It is thought to have
21st century, infectious diseases remain among the leading causes of originated initially in China and spread along the Silk Road, a
death worldwide, despite advances made in medical research and network of land and sea trade routes, to the Mediterranean region
treatments in recent decades. A disease spreads when the pathogen and Europe, carried by rat fleas living on black rats that were always
that causes it is passed from one person to another. For a pathogen to present on ships. The Black Death reduced the world’s population
cause disease, it must be able to reproduce in the host’s body and from an estimated 450 million to about 350 to 375 million. Bubonic
damage the host in some way. plague struck London hard again in the mid-1600s. In modern times,
approximately 1,000 to 3,000 cases of plague arise globally each
THE PLAGUE OF ATHENS year. Although contracting bubonic plague before antibiotics meant
In 430 B.C.E., the Plague of Athens killed one-quarter of the almost certain death, the bacterium responds to several types of
Athenian troops that were fighting in the great Peloponnesian War modern antibiotics; mortality rates from plague are now very low.
and weakened Athens’ dominance and power. The plague impacted
people living in overcrowded Athens as well as troops aboard ships
that had to return to Athens. The source of the plague may have been
identified recently when researchers from the University of Athens
were able to use DNA from teeth recovered from a mass grave. The
scientists identified nucleotide sequences from a pathogenic
bacterium, Salmonella enterica serovar typhi, which causes typhoid
fever. This disease is commonly seen in overcrowded areas and has
caused epidemics throughout recorded history. Figure 22.4A. 1 : Bubonic plague: The (a) Great Plague of London
killed an estimated 200,000 people, or about twenty percent of the
city’s population. The causative agent, the (b) bacterium Yersinia
pestis, is a gram-negative, rod-shaped bacterium from the class
Gamma Proteobacteria. The disease is transmitted through the bite
of an infected flea, which is infected by a rodent. Symptoms include
swollen lymph nodes, fever, seizure, vomiting of blood, and (c)
gangrene.

MIGRATION OF DISEASES TO NEW

POPULATIONS
Over the centuries, Europeans tended to develop genetic immunity
to endemic infectious diseases, but when European conquerors
reached the western hemisphere, they brought with them disease-
causing bacteria and viruses, which triggered epidemics that
completely devastated populations of Native Americans who had no
natural resistance to many European diseases. It has been estimated
Figure 22.4A. 1 : Salmonella enterica serovar typhi: Salmonella that up to 90 percent of Native Americans died from infectious
enterica serovar typhi, the causative agent of typhoid fever, is a diseases after the arrival of Europeans, making conquest of the New
gram-negative, rod-shaped gamma protobacterium. Typhoid fever,
which is spread through feces, causes intestinal hemorrhage, high World a foregone conclusion.
fever, delirium and dehydration. Today, between 16 and 33 million
cases of this re-emerging disease occur annually, resulting in over EMERGING AND RE-EMERGING DISEASES
200,000 deaths. Carriers of the disease can be asymptomatic. In a
The distribution of a particular disease is dynamic. Therefore,
famous case in the early 1900s, a cook named Mary Mallon
unknowingly spread the disease to over fifty people, three of whom changes in the environment, the pathogen, or the host population can
died. Other Salmonella serotypes cause food poisoning. dramatically impact the spread of a disease. According to the World
Health Organization (WHO), an emerging disease is one that has
appeared in a population for the first time, or that may have existed

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previously, but is rapidly increasing in incidence or geographic re-emerging, mostly in urban centers with high concentrations of
range. This definition also includes re-emerging diseases that were immunocompromised people. The WHO has identified certain
previously under control. Approximately 75 percent of recently- diseases whose worldwide re-emergence should be monitored.
emerging infectious diseases affecting humans are zoonotic diseases. Among these are two viral diseases (dengue fever and yellow fever)
Zoonoses, diseases that primarily infect animals and are transmitted and three bacterial diseases (diphtheria, cholera, and bubonic
to humans, are of both viral and bacterial origins. Brucellosis is an plague). The war against infectious diseases has no foreseeable end.
example of a prokaryotic zoonosis that is re-emerging in some
regions. Necrotizing fasciitis (commonly known as flesh-eating KEY POINTS
bacteria) has been increasing in virulence for the last 80 years, for A pathogen must be able to reproduce in the host’s body and
unknown reasons. damage the host in some way to cause disease.
Before antibiotics, contracting plagues usually meant death;
however, most bacterium associated with these plagues respond
to modern antibiotics; mortality rates from these diseases are
now very low.
Emerging diseases include those that have appeared in a
population for the first time or that may have existed previously,
but are rapidly spreading; this also includes re-emerging diseases
that were previously under control.
The spread of disease can be impacted dramatically by changes
in the environment, the pathogen, or the host population.

KEY TERMS
zoonosis: an animal disease that can be transmitted to humans
Figure 22.4A. 1 : Regions of bacterial disease emergence: The map
shows regions where bacterial diseases are emerging or reemerging. plague: an epidemic or pandemic caused by any pestilence
pathogen: any organism or substance, especially a
Some of the currently-emerging diseases are not actually new, but
are diseases that were catastrophic in the past. They devastated microorganism, capable of causing disease, such as bacteria,
viruses, protozoa, or fungi
populations, became dormant for a while, but have re-emerged,
sometimes more virulent than before. Such was the case with This page titled 22.4A: History of Bacterial Diseases is shared under a CC
bubonic plague. Other diseases, like tuberculosis, were never BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
eradicated, but were under control in some regions of the world until

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22.4B: BIOFILMS AND DISEASE
Biofilms, complex colonies of bacteria acting as a unit in their burned tissue. In healthcare environments, biofilms grow on
release of toxins, are highly resistant to antibiotics and host defense. hemodialysis machines, mechanical ventilators, shunts, and other
medical equipment. In fact, 65 percent of all infections acquired in
 LEARNING OBJECTIVES the hospital (nosocomial infections) are attributed to biofilms.
Biofilms are also related to diseases contracted from food because
Give examples of the roles played by biofilms in human they colonize the surfaces of vegetable leaves and meat, as well as
diseases food-processing equipment that is not adequately cleaned.

KEY POINTS
Once a biofilm infection is established, it is very difficult to
eradicate because biofilms exhibit great resistance to most
methods used to control microbial growth, including antibiotics.
Biofilms are able to grow anywhere there is an optimal
combination of moisture, nutrients, and a surface.
Biofilms are responsible for diseases such as infections in
patients and readily settle within wounds and burns; they can
also easily colonize medical devices and other surfaces where
sterility is vital for health.

KEY TERMS
biofilm: a thin film of mucus created by and containing a colony Figure 22.4B. 1: The Five Stages of Biofilm Development: Stage 1:
of bacteria and other microorganisms initial attachment; stage 2: irreversible attachment; stage 3:
nosocomial: contracted in a hospital, or arising from hospital maturation I; stage 4: maturation II; stage 5: dispersion. Each stage
of development in the diagram is paired with a photomicrograph of a
treatment developing Pseudomonas aeruginosa biofilm. All photomicrographs
are shown at the same scale.
BIOFILMS AND DISEASE Biofilm infections develop gradually and often do not cause
Biofilms are complex colonies of bacteria (often containing several immediate symptoms. They are rarely resolved by host defense
species) that exchange chemical signals to coordinate the release of mechanisms. Once an infection by a biofilm is established, it is very
toxins that will attack the host. Once established, they are very difficult to eradicate because biofilms tend to be resistant to most of
difficult to destroy as they are highly resistant to antimicrobial the methods used to control microbial growth, including antibiotics.
treatments and host defense. Biofilms form when microorganisms Biofilms respond poorly or only temporarily to antibiotics. It has
adhere to the surface of some object in a moist environment and been said that they can resist up to 1,000 times the antibiotic
begin to reproduce. They grow virtually everywhere in almost any concentrations used to kill the same bacteria when they are free-
environment where there is a combination of moisture, nutrients, and living or planktonic. An antibiotic dose that large would harm the
a surface. Biofilms are responsible for diseases such as infections in patient; therefore, scientists are working on new ways to eradicate
patients with cystic fibrosis, Legionnaires’ disease, and otitis media. biofilms.
They produce dental plaque and colonize catheters, prostheses,
transcutaneous and orthopedic devices, contact lenses, and internal This page titled 22.4B: Biofilms and Disease is shared under a CC BY-SA
devices such as pacemakers. They also form in open wounds and 4.0 license and was authored, remixed, and/or curated by Boundless.

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22.4C: ANTIBIOTICS- ARE WE FACING A CRISIS?

 LEARNING OBJECTIVES

Discuss antibiotic resistance.

The word antibiotic comes from the Greek word “anti” meaning
“against” and “bios” meaning “life.” An antibiotic is a chemical,
produced either by microbes or synthetically, that is hostile to the
growth of other organisms. Today’s news and other media often
address concerns about an antibiotic crisis. Are the antibiotics that
easily treated bacterial infections in the past becoming obsolete? Are
there new “superbugs”: bacteria that have evolved to become more
resistant to our arsenal of antibiotics? Is this the beginning of the end Figure 22.4C. 1 : MRSA, a superbug: This scanning electron
of antibiotics? All these questions challenge the healthcare micrograph shows methicillin-resistant Staphylococcus aureus
bacteria, commonly known as MRSA. S. aureus is not always
community. pathogenic, but can cause diseases such as food poisoning and skin
One of the main causes of resistant bacteria is the abuse of and respiratory infections.
antibiotics. The imprudent and excessive use of antibiotics has In summary, the medical community is facing an antibiotic crisis.
resulted in the natural selection of resistant forms of bacteria. The Some scientists believe that after years of being protected from
antibiotic kills most of the infecting bacteria; therefore, only the bacterial infections by antibiotics, we may be returning to a time in
resistant forms remain. These resistant forms reproduce, resulting in which a simple bacterial infection could again devastate the human
an increase in the proportion of resistant forms over non-resistant population. Researchers are developing new antibiotics, but it takes
ones. Another major misuse of antibiotics is in patients with colds or many years of research and clinical trials, plus financial investments
the flu, for which antibiotics are useless. There is also the excessive in the millions of dollars, to generate an effective and approved
use of antibiotics in livestock along with the routine use of drug.
antibiotics in animal feed, both of which promote bacterial
resistance. In the United States, 70 percent of the antibiotics KEY POINTS
produced are fed to animals. Because they are given to livestock in In antibiotic resistance, antibiotics will kill most of the infecting
low doses, the probability of resistance developing is maximized. bacteria leaving behind only the resistant forms, which
These resistant bacteria are readily transferred to humans. reproduce, resulting in an increase in the proportion of resistant
forms over non-resistant ones.
ONE OF THE SUPERBUGS: MRSA Cold and flu treatments and the medication of livestock are
The imprudent use of antibiotics has paved the way for bacteria to examples of antibiotic misuse responsible for bacterial
expand populations of resistant forms. For example, Staphylococcus resistance.
aureus, often called “staph,” is a common bacterium that can live in Methicillin-resistant Staphylococcus aureus (MRSA) is an
the human body and is usually easily treated with antibiotics. A very example of a dangerous antibiotic-resistant strain of bacteria that
dangerous strain, however, methicillin-resistant Staphylococcus can infect sick, as well as healthy people.
aureus (MRSA) has made the news over the past few years. This Due to the growing resistance to antibiotics, scientists believe
strain is resistant to many commonly-used antibiotics, including that we may be returning to a time in which a simple bacterial
methicillin, amoxicillin, penicillin, and oxacillin. MRSA can cause infection could again detrimentally impact human populations.
infections of the skin, but it can also infect the bloodstream, lungs,
urinary tract, or sites of injury. While MRSA infections are common
KEY TERMS
among people in healthcare facilities, they have also appeared in antibiotic: any substance that can destroy or inhibit the growth
healthy people who have not been hospitalized, but who live or work of bacteria and similar microorganisms
in tight populations (like military personnel and prisoners).
REFERENCE
Researchers have expressed concern about the way this latter source
of MRSA targets a much younger population than those residing in Naimi, TS, LeDell, KH, Como-Sabetti, K, et al. Comparison of
care facilities. The Journal of the American Medical Association community- and health care-associated methicillin-resistant
(JAMA) reported that, among MRSA-afflicted persons in healthcare Staphylococcus aureus infection. JAMA 290 (2003): 2976–84, doi:
facilities, the average age is 68, whereas people with “community- 10.1001/jama.290.22.2976.
associated MRSA” (CA-MRSA) have an average age of 23.
This page titled 22.4C: Antibiotics- Are We Facing a Crisis? is shared under
a CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Boundless.

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22.4D: BACTERIAL FOODBORNE DISEASES

 LEARNING OBJECTIVES

Give examples of bacterial foodborne diseases in humans

Prokaryotes are everywhere. They readily colonize the surface of

any type of material. Food is not an exception. Most of the time, Figure 22.4D. 1 : Bacterial illnesses from food: (a) Vegetable sprouts
prokaryotes colonize food and food-processing equipment in the grown at an organic farm were the cause of an (b) E. coli outbreak
that killed 32 people and sickened 3,800 in Germany in 2011. The
form of a biofilm. Outbreaks of bacterial infection related to food strain responsible, E. coli O104:H4, produces Shiga toxin, a
consumption are common. A foodborne disease (colloquially called substance that inhibits protein synthesis in the host cell. The toxin
“food poisoning”) is an illness resulting from the consumption of (c) destroys red blood cells, resulting in bloody diarrhea. Deformed
red blood cells clog the capillaries of the kidney, which can lead to
pathogenic bacteria, viruses, or other parasites that contaminate kidney failure, as happened to 845 patients in the 2011 outbreak.
food. Although the United States has one of the safest food supplies Kidney failure is usually reversible, but some patients experience
in the world, the U.S. Centers for Disease Control and Prevention kidney problems years later.
(CDC) has reported that “76 million people get sick, more than
KEY POINTS
300,000 are hospitalized, and 5,000 Americans die each year from
foodborne illness.” Food and food-processing equipment are usually colonized by
biofilms.
The characteristics of foodborne illnesses have changed over time.
A foodborne disease is an illness resulting from the consumption
In the past, sporadic cases of botulism, the potentially fatal disease
of pathogenic bacteria, viruses, or other parasites that
produced by a toxin from the anaerobic bacterium Clostridium
contaminate animal or plant-based food.
botulinum, were relatively common. Some of the sources for this
Proper sterilization techniques and canning procedures have
bacterium were non-acidic canned foods, homemade pickles, and
reduced the incidence of botulism.
processed meat and sausages. The can, jar, or package created a
E. coli outbreaks have become more common as new strains
suitable anaerobic environment where Clostridium could grow.
continue to evolve.
However, proper sterilization and canning procedures have reduced
the incidence of this disease. KEY TERMS
While people may tend to think of foodborne illnesses as associated serotype: a group of microorganisms characterized by a specific
with animal-based foods, most cases are now linked to produce. set of antigens
There have been serious, produce-related outbreaks associated with botulism: poisoning caused by the toxin from Clostridium
raw spinach in the United States and with vegetable sprouts in botulinum, a type of anaerobic bacteria that grows in improperly-
Germany. These types of outbreaks have become more common. prepared food
The raw spinach outbreak in 2006 was produced by the bacterium E.
coli serotype O157:H7. A serotype is a strain of bacteria that carries CONTRIBUTIONS AND ATTRIBUTIONS
a set of similar antigens on its cell surface. There are often many OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44607/latest...ol11448/latest. License: CC
different serotypes of a bacterial species. Most E. coli are not BY: Attribution
particularly dangerous to humans, but serotype O157:H7 can cause OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44607/latest...ol11448/latest. License: CC
bloody diarrhea and is potentially fatal. BY: Attribution
All types of food can potentially be contaminated with bacteria. pathogen. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/pathogen. License: CC BY-SA: Attribution-ShareAlike
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foods as diverse as peanut butter, alfalfa sprouts, and eggs. A deadly en.wiktionary.org/wiki/zoonosis. License: CC BY-SA: Attribution-ShareAlike
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to be a new serotype not previously involved in other outbreaks, http://cnx.org/content/m44607/latest...e_22_04_02.jpg. License: CC BY:
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Attribution
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by: OpenStax CNX. Located at:
http://cnx.org/content/m44607/latest...e_22_04_03.jpg. License: CC BY:
Attribution
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OpenStax College, Signaling in Single-Celled Organisms. November 7, 2013. Biofilm. Provided by: Wikipedia. Located at:
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nosocomial. Provided by: Wiktionary. Located at: OpenStax College, Bacterial Diseases in Humans. October 16, 2013. Provided
en.wiktionary.org/wiki/nosocomial. License: CC BY-SA: Attribution- by: OpenStax CNX. Located at:
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biofilm. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/biofilm. Attribution
License: CC BY-SA: Attribution-ShareAlike OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
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SECTION OVERVIEW

22.5: BENEFICIAL PROKARYOTES

22.5B: EARLY BIOTECHNOLOGY- CHEESE,
Topic hierarchy BREAD, WINE, BEER, AND YOGURT

22.5C: PROKARYOTES AND ENVIRONMENTAL

22.5A: SYMBIOSIS BETWEEN BACTERIA AND
BIOREMEDIATION
EUKARYOTES

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4.0 license and was authored, remixed, and/or curated by Boundless.

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22.5A: SYMBIOSIS BETWEEN BACTERIA AND EUKARYOTES
Prokaryotes fix nitrogen into a form that can be used by eukaryotes. The total fixed nitrogen through BNF is about 100 to 180 million
metric tons per year. Biological processes contribute 65 percent of
 LEARNING OBJECTIVES the nitrogen used in agriculture.

Explain the need for nitrogen fixation and how it is TYPES OF BACTERIA
accomplished Cyanobacteria are the most important nitrogen fixers in aquatic
environments. In soil, members of the genus Clostridium are
KEY POINTS examples of free-living, nitrogen-fixing bacteria. Other bacteria live
Prokaryotes perform biological nitrogen fixation (BNF) to symbiotically with legume plants, providing the most important
convert nitrogen gas from the atmosphere into ammonia, which source of BNF. Symbionts may fix more nitrogen in soils than free-
can be used by eukaryotes to form important biomolecules such living organisms by a factor of 10. Soil bacteria, collectively called
as amino acids and nucleic acids. rhizobia, are able to symbiotically interact with legumes to form
Although most nitrogen fixation is performed by prokaryotes, nodules: specialized structures where nitrogen fixation occurs.
abiotic processes, such as industrial processes and lightning, can Nitrogenase, the enzyme that fixes nitrogen, is inactivated by
also fix nitrogen. oxygen, so the nodule provides an oxygen-free area for nitrogen
Some bacteria form symbiotic relationships with legumes, which fixation to take place. This process provides a natural and
provide oxygen-free nodules on their roots for nitrogen fixation inexpensive plant fertilizer as it converts (reduces) atmospheric
to occur; this process allows ammonia to form naturally from nitrogen to ammonia, which is easily usable by plants. The use of
atomspheric nitrogen to act as fertilizer for soils. legumes is an excellent alternative to chemical fertilization and is of
special interest to sustainable agriculture, which seeks to minimize
KEY TERMS the use of chemicals and conserve natural resources. Through
abiotic: nonliving, inanimate, characterized by the absence of symbiotic nitrogen fixation, the plant benefits from using an endless
life; of inorganic matter source of nitrogen: the atmosphere. Bacteria benefit from using
nitrogen fixation: the conversion of atmospheric nitrogen into photosynthates (carbohydrates produced during photosynthesis)
ammonia and organic derivatives, by natural means, especially from the plant and having a protected niche. Additionally, the soil
by microorganisms in the soil, into a form that can be assimilated benefits from being naturally fertilized. Therefore, the use of
by plants rhizobia as biofertilizers is a sustainable practice.
legume: a large family of herbs, shrubs, and trees that bear
nodules on the roots that contain nitrogen-fixing bacteria

COOPERATION BETWEEN BACTERIA AND

EUKARYOTES: NITROGEN FIXATION
Nitrogen is a very important element for living things because it is
part of the nucleotides and amino acids that are the building blocks
of nucleic acids and proteins, respectively. Nitrogen is usually the
most limiting element in terrestrial ecosystems. Atmospheric
nitrogen, N2, provides the largest pool of available nitrogen.
However, eukaryotes cannot use atmospheric, gaseous nitrogen to
synthesize macromolecules. Fortunately, nitrogen can be “fixed,”
meaning it is converted into ammonia (NH3) either biologically or
abiotically. Abiotic nitrogen fixation occurs as a result of lightning Figure 22.5A. 1 : Location of Nitrogen Fixation: Soybean (Glycine
or by industrial processes. max) is a legume that interacts symbiotically with the soil bacterium
Bradyrhizobium japonicum to form specialized structures on the
roots called nodules where nitrogen fixation occurs.
BACTERIAL NITROGEN FIXATION
Some legumes, like soybeans, are also key sources of agricultural
Biological nitrogen fixation (BNF) is exclusively carried out by
protein. Some of the most important legumes are soybean, peanuts,
prokaryotes: soil bacteria, cyanobacteria, and Frankia spp.
peas, chickpeas, and beans. Other legumes, such as alfalfa, are used
(filamentous bacteria interacting with actinorhizal plants such as
to feed cattle.
alder, bayberry, and sweet fern). After photosynthesis and cellular
respiration, BNF is the second most important biological process on This page titled 22.5A: Symbiosis between Bacteria and Eukaryotes is
Earth. The equation representing the process is as follows, where Pi shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
stands for inorganic phosphate: N2 + 16ATP + 8− + 8H+ 2NH3 + curated by Boundless.
16ADP + 16Pi + H2

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22.5B: EARLY BIOTECHNOLOGY- CHEESE, BREAD, WINE, BEER, AND
YOGURT
Some of the earliest biotechnology used prokaryotes for the
production of food products such as cheese, bread, wine, beer, and
yogurt.

 LEARNING OBJECTIVES

Discuss the origins of food biotechnology as indicated by

the production of cheese, bread, wine, beer, and yogurt

KEY POINTS
Prokaryotes and other microbes are beneficial to some food
production by transforming textures, providing flavors,
producing ethanol, and providing protection from unwanted
microbes.
Bacteria breakdown proteins and fats into a complex mix of
amino acids, amines, and fatty acids; this processing alters the
food product.
Many food production processes rely on the fermentation of
prokaryotes and other microbes to produce the desired flavors; in
the case of beer and wine, they also affect the desired amount of Figure 22.5B. 1: Products made using prokaryotes: Some of the
ethanol. products derived from the use of prokaryotes in early biotechnology
include (a) cheese, (b) wine, (c) beer and bread, and (d) yogurt.

KEY TERMS Cheese production began around 4,000–7,000 years ago when
humans began to breed animals and process their milk.
fermentation: an anaerobic biochemical reaction, in yeast, for
Fermentation, in this case, preserves nutrients because milk will
example, in which enzymes catalyze the conversion of sugars to
spoil relatively quickly, but when processed as cheese, it is more
alcohol or acetic acid with the evolution of carbon dioxide
stable. A required step in cheese-making is separating the milk into
biotechnology: the use of living organisms (especially
solid curds and liquid whey. This usually is done by acidifying the
microorganisms) in industrial, agricultural, medical, and other
milk and adding rennet. The acidification can be accomplished
technological applications
directly by the addition of an acid like vinegar, but usually starter
EARLY BIOTECHNOLOGY: CHEESE, BREAD, bacteria are employed instead. These starter bacteria convert milk
WINE, BEER, AND YOGURT sugars into lactic acid. The same bacteria (and the enzymes they
According to the United Nations Convention on Biological produce) also play a large role in the eventual flavor of aged
Diversity, biotechnology is “any technological application that uses cheeses. Most cheeses are made with starter bacteria from the
Lactococci, Lactobacilli, or Streptococci families. As a cheese ages,
biological systems, living organisms, or derivatives thereof, to make
or modify products or processes for specific use. ” The concept of microbes and enzymes transform texture and intensify flavor. This
“specific use” involves some sort of commercial application. transformation is largely a result of the breakdown of casein proteins
Genetic engineering, artificial selection, antibiotic production, and and milkfat into a complex mix of amino acids, amines, and fatty
cell culture are current topics of study in biotechnology. However, acids. Some cheeses have additional bacteria or molds intentionally
humans have used prokaryotes before the term biotechnology was introduced before or during aging. In traditional cheesemaking,
even coined. Some of the products are as simple as cheese, bread, these microbes might already be present in the aging room; they are
wine, beer, and yogurt,which employ both bacteria and other simply allowed to settle and grow on the stored cheeses. More often
microbes, such as yeast. today, prepared cultures are used, giving more consistent results and
putting fewer constraints on the environment where the cheese ages.
Records of brewing beer date back about 6,000 years to the
Sumerians. Evidence indicates that the Sumerians discovered
fermentation by chance. Wine has been produced for about 4,500
years. The production of beer and wine use microbes, including both
yeast and bacteria, to produce ethanol during fermentation as well as
provide flavor to the beverage. Similarly, bread is one of the oldest
prepared foods. Bread-making also uses the fermentation of yeast
and some bacteria for leavening and flavor. Additionally, evidence

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22.5C: PROKARYOTES AND ENVIRONMENTAL BIOREMEDIATION
Bioremediation occurs when prokaryotes clean up a polluted (2010). To clean up these spills, bioremediation is promoted by the
environment through the natural breakdown of pollutants. addition of inorganic nutrients that help bacteria to grow.
Hydrocarbon-degrading bacteria feed on hydrocarbons in the oil
 LEARNING OBJECTIVES droplet, breaking down the hydrocarbons. Some species, such as
Alcanivorax borkumensis, produce surfactants that solubilize the oil,
Give examples of the use of prokaryotes in enviromental whereas other bacteria degrade the oil into carbon dioxide. In the
bioremediation case of oil spills in the ocean, ongoing, natural bioremediation tends
to occur if there are oil-consuming bacteria in the ocean prior to the
KEY POINTS spill. In addition to naturally occurring oil-degrading bacteria,
To clean up oil spills, bacteria are introduced to the area of the humans select and engineer bacteria that possess the same capability
spill where they break down the hydrocarbons of the oil into with increased efficacy and the spectrum of hydrocarbon compounds
carbon dioxide; this is an example of bioremediation. that can be processed. Under ideal conditions, it has been reported
Toxic metals, such as mercury (II), can be converted into that up to 80 percent of the non-volatile components in oil can be
nontoxic forms, such as mercury (0), by bacteria. degraded within one year of the spill. Other oil fractions containing
Using natural organisms as examples, scientists can engineer aromatic and highly-branched hydrocarbon chains are more difficult
bacteria for improved bioremediation of desired pollutants. to remove and remain in the environment for longer periods of time.
Bioremediation can remove oil, some pesticides, fertilizers, and
toxic chemicals, such as arsenic, from the environment.

KEY TERMS
bioremediation: the use of biological organisms, usually
microorganisms, to remove contaminants, especially from soil or
polluted water
biotransformation: the changes (both chemical and physical)
Figure 22.5C. 1 : Bioremediation in the Exxon Valdez oil spill: (a)
that occur to a substance (especially a drug) by the actions of Cleaning up oil after the Valdez spill in Alaska, workers hosed oil
enzymes within an organism from beaches and then used a floating boom to corral the oil, which
was finally skimmed from the water surface. Some species of
USING PROKARYOTES TO CLEAN UP OUR bacteria are able to solubilize and degrade the oil. (b) One of the
most catastrophic consequences of oil spills is the damage to fauna.
PLANET: BIOREMEDIATION
Microbial bioremediation is the use of prokaryotes (or microbial CONTRIBUTIONS AND ATTRIBUTIONS
metabolism) to remove pollutants. Bioremediation has been used to OpenStax College, Biology. October 16, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44609/latest...ol11448/latest. License: CC
remove agricultural chemicals (pesticides, fertilizers) that leach from BY: Attribution
soil into groundwater and the subsurface. Certain toxic metals and abiotic. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/abiotic.
License: CC BY-SA: Attribution-ShareAlike
oxides, such as selenium and arsenic compounds, can also be legume. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/legume.
removed from water by bioremediation. The reduction of SeO4-2 to License: CC BY-SA: Attribution-ShareAlike
nitrogen fixation. Provided by: Wiktionary. Located at:
SeO3-2 and to Se0 (metallic selenium) is a method used to remove
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selenium ions from water. Mercury is an example of a toxic metal ShareAlike
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Cheese. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Cheese.
living tissues. Several species of bacteria can carry out the License: CC BY-SA: Attribution-ShareAlike
biotransformation of toxic mercury into nontoxic forms. These Bread. Provided by: Wikipedia. Located at:
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 CHAPTER OVERVIEW

23: PROTISTS
23.1: Eukaryotic Origins
23.1A: Early Eukaryotes
23.1B: Characteristics of Eukaryotic DNA
23.1C: Endosymbiosis and the Evolution of Eukaryotes
23.1D: The Evolution of Mitochondria
23.1E: The Evolution of Plastids
23.2: Characteristics of Protists
23.2A: Cell Structure, Metabolism, and Motility
23.2B: Protist Life Cycles and Habitats
23.3: Groups of Protists
23.3A: Excavata
23.3B: Chromalveolata- Alveolates
23.3C: Chromalveolata- Stramenopiles
23.3D: Rhizaria
23.3E: Archaeplastida
23.3F: Amoebozoa and Opisthokonta
23.4: Ecology of Protists
23.4A: Protists as Primary Producers, Food Sources, and Symbionts
23.4B: Protists as Human Pathogens
23.4C: Protists as Plant Pathogens

Thumbnail: This scanning electron micrograph (SEM) revealed some of the external ultrastructural details displayed by a flagellated
Giardia lamblia protozoan parasite. G. lamblia is the organism responsible for causing the diarrheal disease "giardiasis". (Public Domain;
CDC / Janice Haney Carr).

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1
SECTION OVERVIEW

23.1: EUKARYOTIC ORIGINS

23.1C: ENDOSYMBIOSIS AND THE EVOLUTION
Topic hierarchy OF EUKARYOTES

23.1D: THE EVOLUTION OF MITOCHONDRIA

23.1A: EARLY EUKARYOTES
23.1E: THE EVOLUTION OF PLASTIDS
23.1B: CHARACTERISTICS OF EUKARYOTIC DNA

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23.1A: EARLY EUKARYOTES
While today’s atmosphere is about one-fifth molecular oxygen (O2),
 LEARNING OBJECTIVES geological evidence shows that it originally lacked O2. Without
oxygen, aerobic respiration would not be expected; living things
Discuss the origins of eukaryotes in terms of the geologic
would have relied on fermentation instead. At some point before
time line
about 3.5 billion years ago, some prokaryotes evolved the ability to
photosynthesize. Cyanobacteria used water as a hydrogen source and
ORIGINS OF EUKARYOTES released O2 as a waste product. Originally, oxygen-rich
Humans have been familiar with macroscopic organisms (organisms environments were probably localized around places where
big enough to see with the unaided eye) since before there was a cyanobacteria were active, but by about 2 billion years ago,
written history. It is likely that most cultures distinguished between geological evidence shows that oxygen was building up to higher
animals and land plants, but most probably included the concentrations in the atmosphere. Oxygen levels similar to today’s
macroscopic fungi as plants. Therefore, it became an interesting levels only arose within the last 700 million years. Recall that the
challenge to deal with the world of microorganisms once first fossils that we believe to be eukaryotes date to about 2 billion
microscopes were developed a few centuries ago. Many different years old, so they appeared as oxygen levels were increasing.
naming schemes were used over the last couple of centuries, but it
has become the most common practice to refer to eukaryotes that are
not land plants, animals, or fungi as protists.
Most protists are microscopic, unicellular organisms that are
abundant in soil, freshwater, brackish, and marine environments.
They are also common in the digestive tracts of animals and in the
vascular tissues of plants. Others invade the cells of other protists,
animals, and plants. Not all protists are microscopic. Some have
huge, macroscopic cells, such as the plasmodia (giant amoebae) of
myxomycete slime molds or the marine green alga Caulerpa, which
can have single cells that can be several meters in size. Some protists Figure 23.1A. 1 : Protist varieties: Protists range from the
microscopic, single-celled (a) Acanthocystis turfacea and the (b)
are multicellular, such as the red, green, and brown seaweeds. It is ciliate Tetrahymena thermophila, both visualized here using light
among the protists that one finds the wealth of ways that organisms microscopy, to the enormous, multicellular (c) kelps
can grow. They are among the first organisms to evolve with the rise (Chromalveolata) that extend for hundreds of feet in underwater
“forests. “
of eukaryotes.
KEY POINTS
EUKARYOTES IN A GEOLOGICAL TIME FRAME
On a geological time line, protists are among the first organisms
The oldest fossil evidence of eukaryotes, cells measuring 10 µm or that evolved after prokaryotes.
greater, is about 2 billion years old. All fossils older than this appear Today’s eukaryotes evolved from a common ancestor with the
to be prokaryotes. It is probable that today’s eukaryotes are following features: a nucleus that divided via mitosis, DNA
descended from an ancestor that had a prokaryotic cellular associated with histones, a cytoskeleton and endomembrane
organization. The last common ancestor (LCA) of today’s Eukarya
system, the ability to make cilia/flagella.
had several characteristics that included: cells with nuclei that Protists vary widely in size, from single cells approximately 10
divided mitotically and contained linear chromosomes where the
µm in size to multicellular seaweeds that are visible with the
DNA was associated with histones; a cytoskeleton and naked eye.
endomembrane system; and the ability to make cilia/flagella during
at least part of its life cycle. The LCA was aerobic because it had KEY TERMS
mitochondria that were the result of an aerobic alpha- cyanobacteria: photosynthetic prokaryotic microorganisms, of
proteobacterium that lived inside a host cell. Whether this host had a phylum Cyanobacteria, once known as blue-green algae
nucleus at the time of the initial symbiosis remains unknown. The aerobic: living or occurring only in the presence of oxygen
LCA may have had a cell wall for at least part of its life cycle, but endomembrane: all the membraneous components inside a
more data are needed to confirm this hypothesis. Today’s eukaryotes eukaryotic cell, including the nuclear envelope, endoplastic
are very diverse in their shapes, organization, life cycles, and reticulum, and Golgi apparatus
number of cells per individual.
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23.1B: CHARACTERISTICS OF EUKARYOTIC DNA
Eukaryotes, having probably evolved from prokaryotes, have more contained within a nucleus, but rather is attached to the plasma
complex traits in both cell and DNA organization. membrane and contained in the form of a nucleoid, an irregularly-
shaped region that is not surrounded by a nuclear membrane.
 LEARNING OBJECTIVES

Compare and contrast prokaryotic DNA to eukaryotic DNA

KEY POINTS
Prokaryotic genomic DNA is attached to the plasma membrane
in the form of a nucleoid, in contrast to eukaryotic DNA, which
is located in a nucleus.
Eukaryotic DNA is linear, compacted into chromosomes by Figure 23.1B. 1: Cellular location of eukaryotic and prokaryotic
DNA: Eukaryotic DNA is stored in a nucleus, whereas prokaryotic
histones, and has telomeres at each end to protect from DNA is in the cytoplasm in the form of a nucleoid.
deterioration. Eukaryotic DNA is packed into bundles of chromosomes, each
Prokaryotes contain circular DNA in addition to smaller, consisting of a linear DNA molecule coiled around basic (alkaline)
transferable DNA plasmids. proteins called histones, which wind the DNA into a more compact
Eukaryotic cells contain mitochondrial DNA in addition to form. Prokaryotic DNA is found in circular, non-chromosomal form.
nuclear DNA. In addition, prokaryotes have plasmids, which are smaller pieces of
Eukaryotes separate replicated chromosomes by mitosis, using circular DNA that can replicate separately from prokaryotic genomic
cytoskeletal proteins, whereas prokaryotes divide more simply DNA. Because of the linear nature of eukaryotic DNA, repeating
via binary fission. non-coding DNA sequences called telomeres are present on either
end of the chromosomes as protection from deterioration.
KEY TERMS
Mitosis, a process of nuclear division wherein replicated
telomere: either of the repetitive nucleotide sequences at each
chromosomes are divided and separated using elements of the
end of a eukaryotic chromosome, which protect the chromosome
cytoskeleton, is universally present in eukaryotes. The cytoskeleton
from degradation
contains structural and motility components called actin
plasmid: a circle of double-stranded DNA that is separate from
microfilaments and microtubules. All extant eukaryotes have these
the chromosomes, which is found in bacteria and protozoa
cytoskeletal elements. Prokaryotes on the other hand undergo binary
CHARACTERISTICS OF EUKARYOTIC DNA fission in a process where the DNA is replicated, then separates to
COMPARED TO PROKARYOTIC DNA two poles of the cell, and, finally, the cell fully divides.
Prokaryotic cells are known to be much less complex than A major DNA difference between eukaryotes and prokaryotes is the
eukaryotic cells since eukaryotic cells are considered to be present at presence of mitochondrial DNA (mtDNA) in eukaryotes. Because
a later point of evolution. It is probable that eukaryotic cells evolved eukaryotes have mitochondria and prokaryotes do not, eukaryotic
from prokaryotic cells. Differences in complexity can be seen at the cells contain mitochondrial DNA in addition to DNA contained in
cellular level. the nucleus and ribosomes. The mtDNA is composed of
significantly fewer base pairs than nuclear DNA and encodes only a
The single characteristic that is both necessary and sufficient to
define an organism as a eukaryote is a nucleus surrounded by a few dozen genes, depending on the organism.
nuclear envelope with nuclear pores. All extant eukaryotes have
This page titled 23.1B: Characteristics of Eukaryotic DNA is shared under a
cells with nuclei; most of a eukaryotic cell’s genetic material is CC BY-SA 4.0 license and was authored, remixed, and/or curated by
contained within the nucleus. In contrast, prokaryotic DNA is not Boundless.

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23.1C: ENDOSYMBIOSIS AND THE EVOLUTION OF EUKARYOTES
Eukaryotes may have been a product of one cell engulfing another Endosymbiosis

and evolving over time until the separate cells became a single Ancestral host cell Modern cell

organism.

 LEARNING OBJECTIVES

Describe the general concept of endosymbiosis and the after many

evolution of eukaryotes generations

of evolution

KEY POINTS
Endosymbiosis is the concept of one cell engulfing another and Aerobic bacterium Cyanobacterium Mitochondrion Chloroplast

after Lynn Margulis

both cells benefiting from the relationship.

Figure 23.1C. 1 : Endosymbiosis: Modern eukaryotic cells evolved
Endosymbiosis was originally considered after the observation of
from more primitive cells that engulfed bacteria with useful
the similarity between plant chloroplasts and free-living properties, such as energy production. Combined, the once-
cyanobacteria. independent organisms flourished and evolved into a single
organism.
Peroxisomes may have been the first endosymbionts, caused by
the increasing amount of atmospheric oxygen at that point in ENDOSYMBIOTIC THEORY
geological time. The endosymbiotic theory was first articulated by the Russian
Over time, endosymbionts may have transferred some of their botanist Konstantin Mereschkowski in 1905. Mereschkowski was
DNA to the host nucleus, thus becoming dependent on the host
familiar with work by botanist Andreas Schimper, who had observed
for survival and completing full integration into a single
in 1883 that the division of chloroplasts in green plants closely
organism. resembled that of free-living cyanobacteria. Schimper had
tentatively proposed that green plants arose from a symbiotic union
KEY TERMS
of two organisms. Ivan Wallin extended the idea of an
cyanobacteria: photosynthetic prokaryotic microorganisms, of
endosymbiotic origin to mitochondria in the 1920s. These theories
phylum Cyanobacteria, once known as blue-green algae
were initially dismissed or ignored. More detailed electron
peroxisome: a eukaryotic organelle that is the source of the
microscopic comparisons between cyanobacteria and chloroplasts
enzymes that catalyze the production and breakdown of
combined with the discovery that plastids ( organelles associated
hydrogen peroxide and are responsible for the oxidation of long-
with photosynthesis) and mitochondria contain their own DNA led
chain fatty acids
to a resurrection of the idea in the 1960s. The endosymbiotic theory
endosymbiont: an organism that lives within the body or cells of
was advanced and substantiated with microbiological evidence by
another organism
Lynn Margulis in 1967.
ENDOSYMBIOSIS AND THE EVOLUTION OF
EUKARYOTES
To fully understand eukaryotic organisms, it is necessary to
understand that all extant eukaryotes are descendants of a chimeric
organism that was a composite of a host cell and the cell(s) of an
alpha-proteobacterium that “took up residence” inside the host. This
major theme in the origin of eukaryotes is known as endosymbiosis,
where one cell engulfs another such that the engulfed cell survives
and both cells benefit. Over many generations, a symbiotic
relationship can result in two organisms that depend on each other so
Figure 23.1C. 1 : Chloroplasts in plants: A eukaryote with
completely that neither could survive on its own. Endosymbiotic mitochondria engulfed a cyanobacterium in an event of serial
events probably contributed to the origin of the last common primary endosymbiosis, creating a lineage of cells with both
ancestor (LCA) of today’s eukaryotes. organelles. These cyanobacteria have become chloroplasts in
modern plant cells. The cyanobacterial endosymbiont already had a
double membrane.
In 1981 she argued that eukaryotic cells originated as communities
of interacting entities, including endosymbiotic spirochetes that
developed into eukaryotic flagella and cilia. This last idea has not
received much acceptance because flagella lack DNA and do not
show ultrastructural similarities to bacteria or archaea. According to
Margulis and Dorion Sagan, “Life did not take over the globe by

23.1C.1 https://bio.libretexts.org/@go/page/13579
combat, but by networking” (i.e., by cooperation). The possibility evolutionary transition from a symbiotic community to an instituted
that the peroxisome organelles may have an endosymbiotic origin eukaryotic cell (called “serial endosymbiosis”). This hypothesis is
has also been considered, although they lack DNA. Christian de thought to be possible because it is known today from scientific
Duve proposed that they may have been the first endosymbionts, observation that transfer of DNA occurs between bacteria species,
allowing cells to withstand growing amounts of free molecular even if they are not closely related. Bacteria can take up DNA from
oxygen in the earth’s atmosphere. However, it now appears that they their surroundings and have a limited ability to incorporate it into
may be formed de novo, contradicting the idea that they have a their own genome.
symbiotic origin.
This page titled 23.1C: Endosymbiosis and the Evolution of Eukaryotes is
It is believed that over millennia these endosymbionts transferred
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
some of their own DNA to the host cell’s nucleus during the
curated by Boundless.

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23.1D: THE EVOLUTION OF MITOCHONDRIA
Mitochondria are energy-producing organelles that are thought to shaped like alpha-proteobacteria and are surrounded by two
have once been a type of free-living alpha-proteobacterium. membranes, which would result when one membrane-bound
organism engulfs another into a vacuole. The mitochondrial inner
 LEARNING OBJECTIVES membrane involves substantial infoldings called cristae that
resemble the textured, outer surface of alpha-proteobacteria. The
Explain the relationship between endosymbiosis and matrix and inner membrane are rich with enzymes necessary for
mitochondria to the evolution of eukaryotes aerobic respiration.

KEY POINTS
Eukaryotic cells contain varying amounts of mitochondria,
depending on the cells’ energy needs.
Mitochondria have many features that suggest they were
formerly independent organisms, including their own DNA, cell-
independent division, and physical characteristics similar to
alpha-proteobacteria.
Some mitochondrial genes transferred to the nuclear genome
over time, yet mitochondria retained some genetic material for
reasons not completely understood.
The hypothesized transfer of genes from mitochondria to the host
cell’s nucleus likely explains why mitochondria are not able to
survive outside the host cell.
Figure 23.1D. 1 : Micrograph of mammaliam mitochondria: In this
KEY TERMS transmission electron micrograph of mitochondria in a mammalian
lung cell, the cristae, infoldings of the mitochondrial inner
crista: cristae (singular crista) are the internal compartments membrane, can be seen in cross-section.
formed by the inner membrane of a mitochondrion Mitochondria divide independently by a process that resembles
vacuole: a large, membrane-bound, fluid-filled compartment in a binary fission in prokaryotes. Specifically, mitochondria are not
cell’s cytoplasm formed de novo by the eukaryotic cell; they reproduce within the cell
endosymbiosis: when one symbiotic species is taken inside the and are distributed between two cells when cells divide. Therefore,
cytoplasm of another symbiotic species and both become although these organelles are highly integrated into the eukaryotic
endosymbiotic cell, they still reproduce as if they are independent organisms within
the cell. However, their reproduction is synchronized with the
RELATIONSHIP BETWEEN ENDOSYMBIOSIS AND activity and division of the cell. Mitochondria have their own
MITOCHONDRIA circular DNA chromosome that is stabilized by attachments to the
One of the major features distinguishing prokaryotes from inner membrane and carries genes similar to genes expressed by
eukaryotes is the presence of mitochondria. Eukaryotic cells contain alpha-proteobacteria. Mitochondria also have special ribosomes and
anywhere from one to several thousand mitochondria, depending on transfer RNAs that resemble these components in prokaryotes.
the cell’s level of energy consumption. Each mitochondrion These features all support that mitochondria were once free-living
measures between 1 to 10 µm in length and exists in the cell as an prokaryotes.
organelle that can be ovoid to worm-shaped to intricately branched.
Mitochondria arise from the division of existing mitochondria. They MITOCHONDRIAL GENES
may fuse together. They move around inside the cell by interactions Mitochondria that carry out aerobic respiration have their own
with the cytoskeleton. However, mitochondria cannot survive genomes, with genes similar to those in alpha-proteobacteria.
outside the cell. As the amount of oxygen increased in the However, many of the genes for respiratory proteins are located in
atmosphere billions of years ago and as successful aerobic the nucleus. When these genes are compared to those of other
prokaryotes evolved, evidence suggests that an ancestral cell with organisms, they appear to be of alpha-proteobacterial origin.
some membrane compartmentalization engulfed a free-living Additionally, in some eukaryotic groups, such genes are found in the
aerobic prokaryote, specifically an alpha-proteobacterium, thereby mitochondria, whereas in other groups, they are found in the
giving the host cell the ability to use oxygen to release energy stored nucleus. This has been interpreted as evidence that genes have been
in nutrients. Alpha-proteobacteria are a large group of bacteria that transferred from the endosymbiont chromosome to the host genome.
includes species symbiotic with plants, disease organisms that can This loss of genes by the endosymbiont is probably one explanation
infect humans via ticks, and many free-living species that use light why mitochondria cannot live without a host.
for energy. Several lines of evidence support the derivation of
Despite the transfer of genes between mitochondria and the nucleus,
mitochondria from this endosymbiotic event. Most mitochondria are mitochondria retain much of their own independent genetic material.

23.1D.1 https://bio.libretexts.org/@go/page/13580
One possible explanation for mitochondria retaining control over to fully transfer the genes. A third possible explanation is that
some genes is that it may be difficult to transport hydrophobic mitochondria need to produce their own genetic material so as to
proteins across the mitochondrial membrane as well as ensure that ensure metabolic control in eukaryotic cells, which indicates that
they are shipped to the correct location, which suggests that these mtDNA directly influences the respiratory chain and the
proteins must be produced within the mitochondria. Another reduction/oxidation processes of the mitochondria.
possible explanation is that there are differences in codon usage
between the nucleus and mitochondria, making it difficult to be able This page titled 23.1D: The Evolution of Mitochondria is shared under a CC
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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23.1E: THE EVOLUTION OF PLASTIDS
Plastids may derive from cyanobacteria engulfed via endosymbiosis
by early eukaryotes, giving cells the ability to conduct
photosynthesis.

 LEARNING OBJECTIVES

Explain the relationship between endosymbiosis and plastids

to the evolution of eukaryotes
Figure 23.1E. 1: Chloroplast: (a) This chloroplast cross-section
KEY POINTS illustrates its elaborate inner membrane organization. Stacks of
thylakoid membranes compartmentalize photosynthetic enzymes and
Chloroplasts, chromoplasts, and leucoplasts are each a type of
provide scaffolding for chloroplast DNA. (b) The chloroplasts can
plastid. be seen as small green spheres.
Plastids in eukaryotes derive from primary endosymbiosis with
ancient cyanobacteria. There is also, as with the case of mitochondria, strong evidence that
Chlorarachniophytes are a type of algae that resulted from many of the genes of the endosymbiont transferred to the nucleus.
secondary endosymbiosis, when a eukaryote engulfed a green Plastids, like mitochondria, cannot live independently outside the
alga (which itself was a product of primary endosymbiosis with a host. In addition, like mitochondria, plastids derive from the binary
cyanobacterium). fission of other plastids. Researchers have suggested that the
Plastids share several features with mitochondria, including endosymbiotic event that led to Archaeplastida (land plants, red and
having their own DNA and the ability to replicate by binary green algae) occurred 1 to 1.5 billion years ago, at least 500 million
fission. years after the fossil record suggests the presence of eukaryotes.

KEY TERMS SECONDARY ENDOSYMBIOSIS IN

chloroplast: an organelle found in the cells of green plants and CHLORARACHNIOPHYTES
photosynthetic algae where photosynthesis takes place Endosymbiosis involves one cell engulfing another to produce, over
thylakoid: a folded membrane within plant chloroplasts from time, a co-evolved relationship in which neither cell could survive
which grana are made, used in photosynthesis alone. The chloroplasts of red and green algae, for instance, are
plastid: any of various organelles found in the cells of plants and derived from the engulfment of a photosynthetic cyanobacterium by
algae, often concerned with photosynthesis an early prokaryote. This leads to the question of the possibility of a
cell containing an endosymbiont to become engulfed itself, resulting
PLASTIDS in a secondary endosymbiosis. Not all plastids in eukaryotes derive
Some groups of eukaryotes are photosynthetic: their cells contain, in directly from primary endosymbiosis. Some of the major groups of
addition to the standard eukaryotic organelles, another kind of algae became photosynthetic by secondary endosymbiosis; that is,
organelle called a plastid. There are three type of plastids: by taking in either green algae or red algae as endosymbionts.
chloroplasts, chromoplasts, and leucoplasts. Chloroplasts are plastids Numerous microscopic and genetic studies support this conclusion;
that conduct photosynthesis. Chromoplasts are plastids that secondary plastids are surrounded by three or more membranes;
synthesize and store pigments. Leucoplasts are plastids located in some secondary plastids even have clear remnants of the nucleus of
the non-synthetic tissues of a plant (e.g., roots) and generally store endosymbiotic algae.
non-pigment molecules.
Like mitochondria, plastids appear to have a primary endosymbiotic
origin, but differ in that they derive from cyanobacteria rather than
alpha-proteobacteria. Cyanobacteria are a group of photosynthetic
bacteria with all the conventional structures of prokaryotes. Unlike
most prokaryotes, however, they have extensive, internal membrane-
bound compartments called thylakoids, which contain chlorophyll
and are the site of the light-dependent reactions of photosynthesis. In
Figure 23.1E. 1: Primary and secondary endosymbiosis: The
addition to thylakoids, chloroplasts found in eukaryotes have a hypothesized process of endosymbiotic events leading to the
circular DNA chromosome and ribosomes similar to those of evolution of chlorarachniophytes is shown. In a primary
endosymbiotic event, a heterotrophic eukaryote consumed a
cyanobacteria. Each chloroplast is surrounded by two membranes, cyanobacterium. In a secondary endosymbiotic event, the cell
suggestive of primary endosymbiosis. The outer membrane resulting from primary endosymbiosis was consumed by a second
surrounding the plastid is thought have derived from the vacuole in cell. The resulting organelle became a plastid in modern
chlorarachniophytes.
the host, while the inner membrane is thought to have derived from
the plasma membrane of the endosymbiont.

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 Structural Biochemistry/Prokaryotes and Eukaryotes. Provided by: Wikibooks.
Located at: en.wikibooks.org/wiki/Structu...and_Eukaryotes. License: CC
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plasmid. Provided by: Wiktionary. Located at:
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telomere. Provided by: Wiktionary. Located at:
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Figure 23.1E. 1: Red and green algae: (a) Red algae and (b) green OpenStax College, Introduction. October 16, 2013. Provided by: OpenStax
algae (visualized by light microscopy) share similar DNA sequences CNX. Located at: http://cnx.org/content/m44612/latest...3_00_01abc.jpg.
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endosymbiotic event. Chlorarachniophytes are rare algae indigenous Located at: http://cnx.org/content/m44614/latest...ol11448/latest. License: CC
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photosynthetic cyanobacterium. Several lines of evidence support Wikibooks. Located at: en.wikibooks.org/wiki/An_Intr...mbiotic_theory.
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third corresponds to the green alga, and the fourth corresponds to the Plagiomnium affine laminazellen. Provided by: Wikipedia. Located at:
vacuole that surrounded the green alga when it was engulfed by the en.Wikipedia.org/wiki/File:Pl...inazellen.jpeg. License: CC BY: Attribution
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CONTRIBUTIONS AND ATTRIBUTIONS crista. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/crista.
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Located at: http://cnx.org/content/m44612/latest...ol11448/latest. License: CC endosymbiosis. Provided by: Wiktionary. Located at:
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SECTION OVERVIEW

23.2: CHARACTERISTICS OF PROTISTS

23.2B: PROTIST LIFE CYCLES AND HABITATS
Topic hierarchy
This page titled 23.2: Characteristics of Protists is shared under a CC BY-
23.2A: CELL STRUCTURE, METABOLISM, AND SA 4.0 license and was authored, remixed, and/or curated by Boundless.
MOTILITY

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23.2A: CELL STRUCTURE, METABOLISM, AND MOTILITY
Protists are an incredibly diverse set of eukaryotes of various sizes, group of photoautotrophs and are characterized by the presence of
cell structures, metabolisms, and methods of motility. chloroplasts. Other protists are heterotrophic and consume organic
materials (such as other organisms) to obtain nutrition. Amoebas and
 LEARNING OBJECTIVES some other heterotrophic protist species ingest particles by a process
called phagocytosis in which the cell membrane engulfs a food
Describe the metabolism and structure of protists, explaining particle and brings it inward, pinching off an intracellular
the structures that provide their motility membranous sac, or vesicle, called a food vacuole. The vesicle
containing the ingested particle, the phagosome, then fuses with a
KEY POINTS lysosome containing hydrolytic enzymes to produce a
Protist cells may contain a single nucleus or many nuclei; they phagolysosome, which breaks down the food particle into small
range in size from microscopic to thousands of meters in area. molecules that diffuse into the cytoplasm for use in cellular
Protists may have animal-like cell membranes, plant-like cell metabolism. Undigested remains ultimately exit the cell via
walls, or may be covered by a pellicle. exocytosis.
Some protists are heterotrophs and ingest food by phagocytosis,
while other types of protists are photoautotrophs and store energy
via photosynthesis.
Most protists are motile and generate movement with cilia,
flagella, or pseudopodia.

KEY TERMS
amorphous: lacking a definite form or clear shape
multinucleate: having more than one nucleus
pellicle: cuticle, the hard protective outer layer of certain life
forms
taxis: the movement of an organism in response to a stimulus;
similar to kinesis, but more direct
phagocytosis: the process where a cell incorporates a particle by
extending pseudopodia and drawing the particle into a vacuole of
its cytoplasm Figure 23.2A. 1 : Protist metabolism: The stages of phagocytosis
phagosome: a membrane-bound vacuole within a cell containing include the engulfment of a food particle, the digestion of the
particle using enzymes contained within a lysosome, and the
foreign material captured by phagocytosis expulsion of undigested materials from the cell.

CELL STRUCTURE Subtypes of heterotrophs, called saprobes, absorb nutrients from

dead organisms or their organic wastes. Some protists function as
The cells of protists are among the most elaborate and diverse of all
mixotrophs, obtaining nutrition by photoautotrophic or heterotrophic
cells. Most protists are microscopic and unicellular, but some true
routes, depending on whether sunlight or organic nutrients are
multicellular forms exist. A few protists live as colonies that behave
available.
in some ways as a group of free-living cells and in other ways as a
multicellular organism. Still other protists are composed of MOTILITY
enormous, multinucleate, single cells that look like amorphous blobs The majority of protists are motile, but different types of protists
of slime, or in other cases, similar to ferns. Many protist cells are
have evolved varied modes of movement. Protists such as euglena
multinucleated; in some species, the nuclei are different sizes and
have one or more flagella, which they rotate or whip to generate
have distinct roles in protist cell function. movement. Paramecia are covered in rows of tiny cilia that they beat
Single protist cells range in size from less than a micrometer to to swim through liquids. Other protists, such at amoebae, form
thousands of square meters (giant kelp). Animal-like cell membranes cytoplasmic extensions called pseudopodia anywhere on the cell,
or plant-like cell walls envelope protist cells. In other protists, glassy anchor the pseudopodia to a surface, and pull themselves forward.
silica-based shells or pellicles of interlocking protein strips encase Some protists can move toward or away from a stimulus; a
the cells. The pellicle functions like a flexible coat of armor, movement referred to as taxis. Protists accomplish phototaxis,
preventing the protist from external damage without compromising movement toward light, by coupling their locomotion strategy with a
its range of motion. light-sensing organ.

METABOLISM
Protists exhibit many forms of nutrition and may be aerobic or
anaerobic. Protists that store energy by photosynthesis belong to a

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 This page titled 23.2A: Cell Structure, Metabolism, and Motility is shared
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by Boundless.

Figure 23.2A. 1 : Different types of motility in protists: Protists use

various methods for transportation. (a) A paramecium waves hair-
like appendages called cilia. (b) An amoeba uses lobe-like
pseudopodia to anchor itself to a solid surface and pull itself
forward. (c) Euglena uses a whip-like tail called a flagellum.

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23.2B: PROTIST LIFE CYCLES AND HABITATS
Protists live in a wide variety of habitats, including most bodies of
water, as parasites in both plants and animals, and on dead
organisms.

 LEARNING OBJECTIVES

Describe the habitats and life cycles of various protists

KEY POINTS
Slime molds are categorized on the basis of their life cycles into
plasmodial or cellular types, both of which end their life cycle in
the form of dispersed spores.
Plasmodial slime molds form a single-celled, multinucleate
mass, whereas cellular slime molds form an aggregated mass of
separate amoebas that are able to migrate as a unified whole.
Slimes molds feed primarily on bacteria and fungi and contribute
to the decomposition of dead plants.
Figure 23.2B. 1: Plasmodial slime mold life cycle: Haploid spores
KEY TERMS develop into amoeboid or flagellated forms, which are then fertilized
haploid: of a cell having a single set of unpaired chromosomes to form a diploid, multinucleate mass called a plasmodium. This
plasmodium is net-like and, upon maturation, forms a sporangium
sporangia: an enclosure in which spores are formed (also called on top of a stalk. The sporangium forms haploid spores through
a fruiting body) meiosis, after which the spores disseminate, germinate, and begin
plasmodium: a mass of cytoplasm, containing many nuclei, the life cycle anew. The brightly-colored plasmodium in the inset
photo is a single-celled, multinucleate mass.
created by the aggregation of amoeboid cells of slime molds
during their vegetative phase
diploid: of a cell, having a pair of each type of chromosome, one
CELLULAR SLIME MOLDS
of the pair being derived from the ovum and the other from the The cellular slime molds function as independent amoeboid cells
spermatozoon when nutrients are abundant. When food is depleted, cellular slime
molds aggregate into a mass of cells that behaves as a single unit
LIFE CYCLE OF SLIME MOLDS called a slug. Some cells in the slug contribute to a 2–3-millimeter
Protist life cycles range from simple to extremely elaborate. Certain stalk, which dries up and dies in the process. Cells atop the stalk
parasitic protists have complicated life cycles and must infect form an asexual fruiting body that contains haploid spores. As with
different host species at different developmental stages to complete plasmodial slime molds, the spores are disseminated and can
their life cycle. Some protists are unicellular in the haploid form and germinate if they land in a moist environment. One representative
multicellular in the diploid form, which is a strategy also employed genus of the cellular slime molds is Dictyostelium, which commonly
by animals. Other protists have multicellular stages in both haploid exists in the damp soil of forests.
and diploid forms, a strategy called alternation of generations that is
also used by plants.

PLASMODIAL SLIME MOLDS

The slime molds are categorized on the basis of their life cycles into
plasmodial or cellular types. Plasmodial slime molds are composed
of large, multinucleate cells and move along surfaces like an
amorphous blob of slime during their feeding stage. The slime mold
glides along, lifting and engulfing food particles, especially bacteria.
Upon maturation, the plasmodium takes on a net-like appearance
with the ability to form fruiting bodies, or sporangia, during times of
stress. Meiosis produces haploid spores within the sporangia. Spores
disseminate through the air or water to potentially land in more
favorable environments. If this occurs, the spores germinate to form
amoeboid or flagellate haploid cells that can combine with each
other and produce a diploid zygotic slime mold to complete the life
cycle.

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HABITATS OF VARIOUS PROTISTS plasmodium. Provided by: Wiktionary. Located at:
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SECTION OVERVIEW

23.3: GROUPS OF PROTISTS

23.3D: RHIZARIA
Topic hierarchy
23.3E: ARCHAEPLASTIDA
23.3A: EXCAVATA 23.3F: AMOEBOZOA AND OPISTHOKONTA
23.3B: CHROMALVEOLATA- ALVEOLATES
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23.3A: EXCAVATA
Excavata, defined by a feeding groove that is “excavated” from one
side, includes Diplomonads, Parabasalids and Euglenozoans.

 LEARNING OBJECTIVES

Describe characteristics of Excavates, including

Diplomonads, Parabasalids and Euglenozoans

KEY POINTS
Excavata are a supergroup of protists that are defined by an
asymmetrical appearance with a feeding groove that is
“excavated” from one side; it includes various types of
organisms which are parasitic, photosynthetic and heterotrophic
predators.
Excavata includes the protists: Diplomonads, Parabasalids and
Euglenozoans.
GIARDIA LAMBLIA
Diplomonads are defined by the presence of a nonfunctional,
mitochrondrial-remnant organelle called a mitosome. The mammalian intestinal parasite Giardia lamblia,visualized here
Parabasalids are characterized by a semi-functional mitochondria using scanning electron microscopy, is a waterborne protist that
referred to as a hydrogenosome; they are comprised of parasitic causes severe diarrhea when ingested.
protists, such as Trichomonas vaginalis.
PARABASALIDS
Euglenozoans can be classified as mixotrophs, heterotrophs,
autotrophs, and parasites; they are defined by their use of flagella A second Excavata subgroup, the parabasalids, also exhibits semi-
for movement. functional mitochondria. In parabasalids, these structures function
anaerobically and are called hydrogenosomes because they produce
KEY TERMS hydrogen gas as a byproduct. Parabasalids move with flagella and
mitosome: an organelle found within certain unicellular membrane rippling. Trichomonas vaginalis, a parabasalid that causes
eukaryotes which lack mitochondria a sexually-transmitted disease in humans, employs these
hydrogenosome: a membrane-bound organelle found in ciliates, mechanisms to transit through the male and female urogenital tracts.
trichomonads, and fungi which produces molecular hydrogen T. vaginalis causes trichomoniasis, which appears in an estimated
and ATP 180 million cases worldwide each year. Whereas men rarely exhibit
kinetoplast: a disk-shaped mass of circular DNA inside a large symptoms during an infection with this protist, infected women may
mitochondrion, found specifically in protozoa of the class become more susceptible to secondary infection with human
Kinetoplastea immunodeficiency virus (HIV) or genital wart virus infection, which
causes over 90% of cervical cancer. Pregnant women infected with
EXCAVATA T. vaginalis are at an increased risk of serious complications, such as
Many of the protist species classified into the supergroup Excavata pre-term delivery.
are asymmetrical, single-celled organisms with a feeding groove
EUGLENOZOANS
“excavated” from one side. This supergroup includes heterotrophic
predators, photosynthetic species, and parasites. Its subgroups are Euglenozoans includes parasites, heterotrophs, autotrophs, and
the diplomonads, parabasalids, and euglenozoans. mixotrophs, ranging in size from 10 to 500 µm. Euglenoids move
through their aquatic habitats using two long flagella that guide them
DIPLOMONADS toward light sources sensed by a primitive ocular organ called an
Among the Excavata are the diplomonads, which include the eyespot. The familiar genus, Euglena, encompasses some
intestinal parasite, Giardia lamblia. Until recently, these protists mixotrophic species that display a photosynthetic capability only
were believed to lack mitochondria. Mitochondrial remnant when light is present. In the dark, the chloroplasts of Euglena shrink
organelles, called mitosomes, have since been identified in up and temporarily cease functioning; the cells, instead, take up
diplomonads, but these mitosomes are essentially nonfunctional. organic nutrients from their environment.
Diplomonads exist in anaerobic environments and use alternative The human parasite, Trypanosoma brucei, belongs to a different
pathways, such as glycolysis, to generate energy. Each diplomonad subgroup of Euglenozoa, the kinetoplastids. The kinetoplastid
cell has two identical nuclei and uses several flagella for locomotion. subgroup is named after the kinetoplast, a DNA mass carried within
the single, oversized mitochondrion possessed by each of these cells.
This subgroup includes several parasites, collectively called

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trypanosomes, which cause devastating human diseases by infecting
an insect species during a portion of their life cycle. T. brucei
develops in the gut of the tsetse fly after the fly bites an infected
human or other mammalian host. The parasite then travels to the
insect salivary glands to be transmitted to another human or other
mammal when the infected tsetse fly consumes another blood meal.
T. brucei is common in central Africa and is the causative agent of
African sleeping sickness, a disease associated with severe chronic
fatigue and coma; it can be fatal if left untreated.

Figure 23.3A. 1 : Life cycle of Trypanosoma brucei: Trypanosoma

brucei, the causative agent of sleeping sickness, spends part of its
life cycle in the tsetse fly and part in humans.

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23.3B: CHROMALVEOLATA- ALVEOLATES
Alveolates are defined by the presence of an alveolus beneath the categorized into the dinoflagellates, the apicomplexans, and the
cell membrane and include dinoflagellates, apicomplexans and ciliates.
ciliates.
DINOFLAGELLATES
 LEARNING OBJECTIVES Dinoflagellates exhibit extensive morphological diversity and can be
photosynthetic, heterotrophic, or mixotrophic. Many dinoflagellates
Evaluate traits associated with protists classified as are encased in interlocking plates of cellulose with two
alveolates which include dinoflagellates, apicomplexans, perpendicular flagella that fit into the grooves between the cellulose
and ciliates plates. One flagellum extends longitudinally and a second encircles
the dinoflagellate. Together, the flagella contribute to the
KEY POINTS characteristic spinning motion of dinoflagellates. These protists exist
Alveolates are classified under the group Chromalveolata which in freshwater and marine habitats; they are a component of plankton.
developed as a result of a secondary endosymbiotic event.
Dinoflagellates are defined by their flagella structure which lays
perpendicular and fits into the cellulose plates of the
dinoflagellate, promoting a spinning motion.
Apicomplexans are defined by the asymmetrical distribution of
their microtubules, fibrin, and vacuoles; they include the
parasitic protist Plasmodium which causes malaria.
Ciliates are defined by the presence of cilia (such as the oral
groove in the Paramecium), which beat synchronously to aid the
organism in locomotion and obtaining nutrients.
Ciliates are defined by the presence of cilia, which beat
synchronously, to aid the organism in locomotion and obtaining
nutrients, such as the oral groove in the Paramecium.

KEY TERMS Figure 23.3B. 1: Dinoflagellates: The dinoflagellates exhibit great

osmoregulation: the homeostatic regulation of osmotic pressure diversity in shape. Many are encased in cellulose armor and have
in the body in order to maintain a constant water content two flagella that fit in grooves between the plates. Movement of
these two perpendicular flagella causes a spinning motion.
plastid: any of various organelles found in the cells of plants and
Some dinoflagellates generate light, called bioluminescence, when
algae, often concerned with photosynthesis
they are jarred or stressed. Large numbers of marine dinoflagellates
conjugation: the temporary fusion of organisms, especially as
(billions or trillions of cells per wave) can emit light and cause an
part of sexual reproduction
entire breaking wave to twinkle or take on a brilliant blue color. For
CHROMALVEOLATA approximately 20 species of marine dinoflagellates, population
explosions (called blooms) during the summer months can tint the
Current evidence suggests that species classified as chromalveolates
ocean with a muddy red color. This phenomenon is called a red tide
are derived from a common ancestor that engulfed a photosynthetic
and results from the abundant red pigments present in dinoflagellate
red algal cell, which itself had already evolved chloroplasts from an
plastids. In large quantities, these dinoflagellate species secrete an
endosymbiotic relationship with a photosynthetic prokaryote.
Therefore, the ancestor of chromalveolates is believed to have asphyxiating toxin that can kill fish, birds, and marine mammals.
resulted from a secondary endosymbiotic event. However, some Red tides can be massively detrimental to commercial fisheries;
humans who consume these protists may become poisoned.
chromalveolates appear to have lost red alga-derived plastid
organelles or lack plastid genes altogether. Therefore, this
supergroup should be considered a hypothesis-based working group
that is subject to change and can be subdivided into alveolates and
stramenopiles.

ALVEOLATES
A large body of data supports that the alveolates are derived from a
shared common ancestor. The alveolates are named for the presence
of an alveolus, or membrane-enclosed sac, beneath the cell
membrane. The exact function of the alveolus is unknown, but it
may be involved in osmoregulation. The alveolates are further

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 osmoregulatory vesicles that fill with water as it enters the cell by
osmosis and then contract to squeeze water from the cell.

Figure 23.3B. 1: Paramecium: Paramecium has a primitive mouth

(called an oral groove) to ingest food and an anal pore to excrete it.
Contractile vacuoles allow the organism to excrete excess water.
Figure 23.3B. 1: Bioluminescence: Bioluminescence is emitted Cilia enable the organism to move.
from dinoflagellates in a breaking wave, as seen from the New Paramecium has two nuclei, a macronucleus and a micronucleus, in
Jersey coast.
each cell. The micronucleus is essential for sexual reproduction,
APICOMPLEXANS whereas the macronucleus directs asexual binary fission and all
The apicomplexan protists are so named because their microtubules, other biological functions. The process of sexual reproduction in
fibrin, and vacuoles are asymmetrically distributed at one end of the Paramecium underscores the importance of the micronucleus to
cell in a structure called an apical complex. The apical complex is these protists. Paramecium and most other ciliates reproduce
specialized for entry and infection of host cells. Indeed, all sexually by conjugation. This process begins when two different
apicomplexans are parasitic. This group includes the genus mating types of Paramecium make physical contact and join with a
Plasmodium, which causes malaria in humans. Apicomplexan life cytoplasmic bridge. The diploid micronucleus in each cell then
cycles are complex, involving multiple hosts and stages of sexual undergoes meiosis to produce four haploid micronuclei. Three of
and asexual reproduction. these degenerate in each cell, leaving one micronucleus that then
undergoes mitosis, generating two haploid micronuclei. The cells
each exchange one of these haploid nuclei and move away from
each other. A similar process occurs in bacteria that have plasmids.
Fusion of the haploid micronuclei generates a completely novel
diploid pre-micronucleus in each conjugative cell. This pre-
micronucleus undergoes three rounds of mitosis to produce eight
copies, while the original macronucleus disintegrates. Four of the
eight pre-micronuclei become full-fledged micronuclei, whereas the
other four perform multiple rounds of DNA replication and then
become new macronuclei. Two cell divisions then yield four new
paramecia from each original conjugative cell.
Figure 23.3B. 1: Parasitic apicomplexans: (a) Apicomplexans are
parasitic protists. They have a characteristic apical complex that
enables them to infect host cells. (b) Plasmodium, the causative
agent of malaria, has a complex life cycle typical of apicomplexans.

CILIATES
The ciliates, which include Paramecium and Tetrahymena, are a
group of protists 10 to 3,000 micrometers in length that are covered
in rows, tufts, or spirals of tiny cilia. By beating their cilia
synchronously or in waves, ciliates can coordinate directed
movements and ingest food particles. Certain ciliates have fused
cilia-based structures that function like paddles, funnels, or fins.
Ciliates also are surrounded by a pellicle, providing protection
without compromising agility. The genus Paramecium includes
protists that have organized their cilia into a plate-like primitive
mouth called an oral groove, which is used to capture and digest
bacteria. Food captured in the oral groove enters a food vacuole
where it combines with digestive enzymes. Waste particles are
expelled by an exocytic vesicle that fuses at a specific region on the
cell membrane: the anal pore. In addition to a vacuole-based
digestive system, Paramecium also uses contractile vacuoles:

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 This page titled 23.3B: Chromalveolata- Alveolates is shared under a CC
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Figure 23.3B. 1: Paramecium: sexual reproduction: The complex

process of sexual reproduction in Parameciumcreates eight daughter
cells from two original cells. Each cell has a macronucleus and a
micronucleus. During sexual reproduction, the macronucleus
dissolves and is replaced by a micronucleus.

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23.3C: CHROMALVEOLATA- STRAMENOPILES
Stramenophiles include photosynthetic marine algae and Members of this subgroup range in size from single-celled diatoms
heterotrophic protists such as diatoms, brown and golden algae, and to the massive and multicellular kelp.
oomycetes.

 LEARNING OBJECTIVES

Describe characteristics of the following Stramenophiles:

diatoms, brown algae, golden algae, and oomycetes

KEY POINTS
Stramenophiles, also referred to as heterokonts, are a subclass of
chromalveolata, and are identified by the presence of a “hairy”
flagellum.
Diatoms, present in both freshwater and marine plankton, are
unicellular photosynthetic protists that are characterized by the
presence of a cell wall composed of silicon dioxide that displays
intricate patterns.
Golden algae, present in both freshwater and marine plankton
communities, are unicellular photosynthetic protists
characterized by the presence of carotenoids (yellow-orange
photosynthetic pigments). Figure 23.3C. 1 : Stramenophile structure: This stramenopile cell has
Oomycetes, commonly referred to as water molds, are a single hairy flagellum and a secondary smooth flagellum.
characterized by their fungus-like morphology, a cellulose-based
DIATOMS
cell wall, and a filamentous network used for nutrient uptake.
Oomycetes, commonly referred to as water molds, are The diatoms are unicellular photosynthetic protists that encase
characterized by their fungus-like morphology, a cellulose-based themselves in intricately patterned, glassy cell walls composed of
cell wall and a filamentous network used for nutrient uptake. silicon dioxide in a matrix of organic particles. These protists are a
component of freshwater and marine plankton. Most species of
KEY TERMS diatoms reproduce asexually, although some instances of sexual
stipe: the stem of a kelp reproduction and sporulation also exist. Some diatoms exhibit a slit
raphe: a ridge or seam on an organ, bodily tissue, or other in their silica shell called a raphe. By expelling a stream of
structure, especially at the join between two halves or sections mucopolysaccharides from the raphe, the diatom can attach to
saprobe: an organism that lives off of dead or decaying organic surfaces or propel itself in one direction.
material

CHROMALVEOLATES
Current evidence suggests that chromalveolates have an ancestor
which resulted from a secondary endosymbiotic event. The species
which fall under the classification of chromalveolates have evolved
from a common ancestor that engulfed a photosynthetic red algal
cell. This red algal cell had previously evolved chloroplasts from an
endosymbiotic relationship with a photosynthetic prokaryote.
Chromalveolates include very important photosynthetic organisms,
such as diatoms, brown algae, and significant disease agents in
animals and plants. The chromalveolates can be subdivided into
alveolates and stramenopiles. Figure 23.3C. 1 : Diatoms: Assorted diatoms, visualized here using
light microscopy, live among annual sea ice in McMurdo Sound,
STRAMENOPILES Antarctica. Diatoms range in size from 2 to 200 µm.

A subgroup of chromalveolates, the stramenopiles, also referred to During periods of nutrient availability, diatom populations bloom to
numbers greater than can be consumed by aquatic organisms. The
as heterokonts, includes photosynthetic marine algae and
heterotrophic protists. The unifying feature of this group is the excess diatoms die and sink to the sea floor where they are not easily
reached by saprobes that feed on dead organisms. As a result, the
presence of a textured, or “hairy,” flagellum. Many stramenopiles
also have an additional flagellum that lacks hair-like projections. carbon dioxide that the diatoms had consumed and incorporated into
their cells during photosynthesis is not returned to the atmosphere.

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In general, this process by which carbon is transported deep into the OOMYCETES
ocean is described as the biological carbon pump because carbon is The water molds, oomycetes (“egg fungus”), were so-named based
“pumped” to the ocean depths where it is inaccessible to the on their fungus-like morphology, but molecular data have shown
atmosphere as carbon dioxide. The biological carbon pump is a that the water molds are not closely related to fungi. The oomycetes
crucial component of the carbon cycle that helps to maintain lower are characterized by a cellulose-based cell wall and an extensive
atmospheric carbon dioxide levels. network of filaments that allow for nutrient uptake. As diploid
spores, many oomycetes have two oppositely-directed flagella (one
GOLDEN ALGAE
hairy and one smooth) for locomotion. The oomycetes are non-
Like diatoms, golden algae are largely unicellular, although some photosynthetic and include many saprobes and parasites. The
species can form large colonies. Their characteristic gold color saprobes appear as white fluffy growths on dead organisms. Most
results from their extensive use of carotenoids, a group of oomycetes are aquatic, but some parasitize terrestrial plants. One
photosynthetic pigments that are generally yellow or orange in color. plant pathogen is Phytophthora infestans, the causative agent of late
Golden algae are found in both freshwater and marine environments, blight of potatoes, such as occurred in the nineteenth century Irish
where they form a major part of the plankton community. potato famine.
BROWN ALGAE
The brown algae are primarily marine, multicellular organisms that
are known colloquially as seaweeds. Giant kelps are a type of brown
algae. Some brown algae have evolved specialized tissues that
resemble terrestrial plants, with root-like holdfasts, stem-like stipes,
and leaf-like blades that are capable of photosynthesis. The stipes of
giant kelps are enormous, extending in some cases for 60 meters. A
variety of algal life cycles exists, but the most complex is alternation
of generations in which both haploid and diploid stages involve
multicellularity. For instance, compare this life cycle to that of
humans. In humans, haploid gametes produced by meiosis (sperm
and egg) combine in fertilization to generate a diploid zygote that
undergoes many rounds of mitosis to produce a multicellular embryo
and then a fetus. However, the individual sperm and egg themselves
never become multicellular beings. In the brown algae genus
Laminaria, haploid spores develop into multicellular gametophytes, Figure 23.3C. 1 : Oomycete: A saprobic oomycete engulfs a dead
which produce haploid gametes that combine to produce diploid insect.
organisms that then become multicellular organisms with a different
This page titled 23.3C: Chromalveolata- Stramenopiles is shared under a CC
structure from the haploid form. Terrestrial plants also have evolved
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
alternation of generations.

Figure 23.3C. 1 : Brown algae life cycle: Several species of brown

algae, such as the Laminaria shown here, have evolved life cycles in
which both the haploid (gametophyte) and diploid (sporophyte)
forms are multicellular. The gametophyte is different in structure
from the sporophyte.

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23.3D: RHIZARIA

 LEARNING OBJECTIVES

Describe characteristics associated with Rhizaria

The Rhizaria supergroup includes many of the amoebas, most of

which have threadlike or needle-like pseudopodia. Pseudopodia
function to trap and engulf food particles and to direct movement in
rhizarian protists. These pseudopods project outward from anywhere
on the cell surface and can anchor to a substrate. The protist then
transports its cytoplasm into the pseudopod, thereby moving the
entire cell. This type of motion, called cytoplasmic streaming, is
used by several diverse groups of protists as a means of locomotion
or as a method to distribute nutrients and oxygen.

Figure 23.3D. 1 : Forams: These shells from foraminifera sank to the

sea floor.

RADIOLARIANS
A second subtype of Rhizaria, the radiolarians, exhibit intricate
exteriors of glassy silica with radial or bilateral symmetry.
Radiolarians display needle-like pseudopods that are supported by
microtubules which radiate outward from the cell bodies of these
protists and function to catch food particles. The shells of dead
radiolarians sink to the ocean floor, where they may accumulate in
100 meter-thick depths. Preserved, sedimented radiolarians are very
common in the fossil record.

Figure 23.3D. 1 : Ammonia tepida: Ammonia tepida, a Rhizaria

species viewed here using phase contrast light microscopy, exhibits
many threadlike pseudopodia.

FORAMS
Foraminiferans, or forams, are unicellular heterotrophic protists,
ranging from approximately 20 micrometers to several centimeters
in length; they occasionally resemble tiny snails. As a group, the
forams exhibit porous shells, called tests, that are built from various
organic materials and typically hardened with calcium carbonate.
The tests may house photosynthetic algae, which the forams can
harvest for nutrition. Foram pseudopodia extend through the pores
and allow the forams to move, feed, and gather additional building
materials. Foraminiferans are also useful as indicators of pollution Figure 23.3D. 1 : Radiolarian shell: This fossilized radiolarian shell
and changes in global weather patterns. was imaged using a scanning electron microscope.
The life-cycle involves an alternation between haploid and diploid KEY POINTS
phases. The haploid phase initially has a single nucleus, and divides
The needle-like pseudopodia are used to carry out a process
to produce gametes with two flagella. The diploid phase is
called cytoplasmic streaming which is a means of locomotion or
multinucleate, and after meiosis fragments to produce new
distributing nutrients and oxygen.
organisms. The benthic forms has multiple rounds of asexual
Two major subclassifications of Rhizaria include Forams and
reproduction between sexual generations.
Radiolarians.
Forams are characterized as unicellular heterotrophic protists that
have porous shells, referred to as tests, which can contain
photosynthetic algae that the foram can use as a nutrient source.
Radiolarians are characterized by a glassy silica exterior that
displays either bilateral or radial symmetry.

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KEY TERMS This page titled 23.3D: Rhizaria is shared under a CC BY-SA 4.0 license
pseudopodia: temporary projections of eukaryotic cells and was authored, remixed, and/or curated by Boundless.
test: the external calciferous shell of a foram

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23.3E: ARCHAEPLASTIDA
daughter colonies, an example of basic cell specialization in this
 LEARNING OBJECTIVES organism.

Describe the relationship between red algae, green algae,

and land plants

Red algae and green algae are included in the supergroup

Archaeplastida. It is well documented that land plants evolved from
a common ancestor of these protists; their closest relatives are found
within this group. Molecular evidence supports that all
Archaeplastida are descendants of an endosymbiotic relationship
VOLVOX AUREUS
between a heterotrophic protist and a cyanobacterium. The red and
green algae include unicellular, multicellular, and colonial forms Volvox aureus is a green alga in the supergroup Archaeplastida. This
species exists as a colony, consisting of cells immersed in a gel-like
RED ALGAE matrix and intertwined with each other via hair-like cytoplasmic
Red algae, or rhodophytes, are primarily multicellular, lack flagella, extensions.
and range in size from microscopic, unicellular protists to large, True multicellular organisms, such as the sea lettuce, Ulva, are
multicellular forms grouped into the informal seaweed category. The represented among the chlorophytes. In addition, some chlorophytes
red algae life cycle is an alternation of generations. Some species of exist as large, multinucleate, single cells. Species in the genus
red algae contain phycoerythrins, photosynthetic accessory pigments Caulerpa exhibit flattened, fern-like foliage and can reach lengths of
that are red in color and outcompete the green tint of chlorophyll, 3 meters. Caulerpa species undergo nuclear division, but their cells
making these species appear as varying shades of red. Other protists do not complete cytokinesis, remaining instead as massive and
classified as red algae lack phycoerythrins and are parasites. Red elaborate single cells.
algae are common in tropical waters where they have been detected
at depths of 260 meters. Other red algae exist in terrestrial or
freshwater environments.

GREEN ALGAE: CHLOROPHYTES AND

CHAROPHYTES
The most abundant group of algae is the green algae. The green
algae exhibit similar features to the land plants, particularly in terms
of chloroplast structure. It is well supported that this group of
protists share a relatively-recent common ancestors with land plants.
The green algae are subdivided into the chlorophytes and the
charophytes. The charophytes are the closest-living relatives of land
plants, resembling them in morphology and reproductive strategies. CAULERPA TAXIFOLIA
Charophytes are common in wet habitats where their presence often Caulerpa taxifolia is a chlorophyte consisting of a single cell
signals a healthy ecosystem. containing potentially thousands of nuclei.
The chlorophytes exhibit great diversity of form and function.
KEY POINTS
Chlorophytes primarily inhabit freshwater and damp soil; they are a
common component of plankton. Chlamydomonas is a simple, Archaeplastida are typically associated with their relationship to
unicellular chlorophyte with a pear-shaped morphology and two land plants; in addition, molecular evidence shows that
opposing, anterior flagella that guide this protist toward light sensed Archaeplastida evolved from an endosymbiotic relationship
by its eyespot. More complex chlorophyte species exhibit haploid between a heterotrophic protist and a cyanobacterium.
gametes and spores that resemble Chlamydomonas. Red algae (rhodophytes), are classified as Archaeplastida and are
most often characterized by the presence of the red pigment
The chlorophyte Volvox is one of only a few examples of a colonial
phycoerythrin; however, there are red algae that lack
organism, which behaves in some ways like a collection of
phycoerythrins and can be classified as parasites.
individual cells, but in other ways like the specialized cells of a
Red algae typically exist as multicellular protists that lack
multicellular organism. Volvox colonies contain 500 to 60,000 cells,
flagella; however, they can also exist as unicellular organisms.
each with two flagella, contained within a hollow, spherical matrix
Green algae are the most abundant group of algae and can be
composed of a gelatinous glycoprotein secretion. Individual Volvox
further classified as chlorophytes and charophytes.
cells move in a coordinated fashion and are interconnected by
Charophytes are the green algae which resemble land plants and
cytoplasmic bridges. Only a few of the cells reproduce to create
are their closest living relative.

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 Chlorophytes are the green algae which exhibit a wide range of plankton: a generic term for all the organisms that float in the
forms; they can be unicellular, multicellular, or colonial. sea

KEY TERMS This page titled 23.3E: Archaeplastida is shared under a CC BY-SA 4.0
endosymbiotic: that lives within a body or cells of another license and was authored, remixed, and/or curated by Boundless.
organism

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23.3F: AMOEBOZOA AND OPISTHOKONTA
Amoebozoa are a type of protist that is characterized by the presence The Amoebozoa include several groups of unicellular amoeba-like
of pseudopodia which they use for locomotion and feeding. organisms that are free-living or parasites that are classified as
unikonts. The best known and most well-studied member of this
 LEARNING OBJECTIVES group is the slime mold. Additional members include the
Archamoebae, Tubulinea, and Flabellinea.
Describe characteristics of Amoebozoa

KEY POINTS
Amoebozoa (amoebas) can live in either marine and fresh water
or in soil.
Amoebozoa are characterized by the presence of pseudopodia,
which are extensions that can be either tube-like or flat lobes and
are used for locomotion and feeding.
Amooebozoa can be further divided into subclassifications that
include slime molds; these can be found as both plasmodial and
cellular types.
Plasmodial slime molds are characterized by the presence of
large, multinucleate cells that have the ability to glide along the Figure 23.3F . 1 : Pseudopodia structures: Amoebae with tubular and
surface and engulf food particles as they move. lobe-shaped pseudopodia, such as the ones seen under this
Cellular molds are characterized by the presence of independent microscope, would be morphologically classified as amoebozoans.
amoeboid cells during times of nutrient abundancy and the
SLIME MOLDS
development of a cellular mass, called a slug, during times of
nutrient depletion. A subset of the amoebozoans, the slime molds, has several
Archamoebae, Flabellinea, and Tubulinea are also groups of morphological similarities to fungi that are thought to be the result
Amoebozoa; their defining characteristics include: Archamoebae of convergent evolution. For instance, during times of stress, some
lack mitochondria; Flabellinea flatten during locomotion and slime molds develop into spore -generating fruiting bodies, similar
lack a shell and flagella; Tubulinea have a rough cylindrical form to fungi.
during locomotion with cylindrical pseudopodia. The slime molds are categorized on the basis of their life cycles into
plasmodial or cellular types. Plasmodial slime molds are composed
KEY TERMS of large, multinucleate cells that move along surfaces like an
rhizaria: a species-rich supergroup of mostly unicellular amorphous blob of slime during their feeding stage. Food particles
eukaryotes that for the most part are amoeboids with filose, are lifted and engulfed into the slime mold as it glides along. Upon
reticulose, or microtubule-supported pseudopods maturation, the plasmodium takes on a net-like appearance with the
plasmodium: a mass of cytoplasm, containing many nuclei, ability to form fruiting bodies, or sporangia, during times of stress.
created by the aggregation of amoeboid cells of slime molds Haploid spores are produced by meiosis within the sporangia. These
during their vegetative phase spores can be disseminated through the air or water to potentially
sporangia: an enclosure in which spores are formed (also called land in more favorable environments. If this occurs, the spores
a fruiting body) germinate to form ameboid or flagellate haploid cells that can
combine with each other and produce a diploid zygotic slime mold
AMOEBOZOA to complete the life cycle.
Protists are eukaryotic organisms that are classified as unicellular,
colonial, or multicellular organisms that do not have specialized
tissues. This identifying property sets protists apart from other
organisms within the Eukarya domain. The amoebozoans are
classified as protists with pseudopodia which are used in locomotion
and feeding. Amoebozoans live in marine environments, fresh water,
or in soil. In addition to the defining pseudopodia, they also lack a
shell and do not have a fixed body. The pseudopodia which are
characteristically exhibited include extensions which can be tube-
like or flat lobes, rather than the hair-like pseudopodia of rhizarian
amoeba. Rhizarian amoeba are amoeboids with filose, reticulose, or
microtubule-supported pseudopods and include the groups:
Cercozoa, Foraminifera, and Radiolaria and are classified as bikonts.

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 ARCHAMOEBAE, FLABELLINEA, AND TUBULINEA
The Archamoebae are a group of Amoebozoa distinguished by the
absence of mitochondria. They include genera that are internal
parasites or commensals of animals (Entamoeba and Endolimax). A
few species are human pathogens, causing diseases such as amoebic
dysentery. The other genera of archamoebae live in freshwater
habitats and are unusual among amoebae in possessing flagella.
Most have a single nucleus and flagellum, but the giant amoeba,
Pelomyxa, has many of each.
The Tubulinea are a major grouping of Amoebozoa, including most
of the larger and more familiar amoebae like Amoeba, Arcella, and
Difflugia. During locomotion, most Tubulinea have a roughly
cylindrical form or produce numerous cylindrical pseudopods. Each
cylinder advances by a single central stream of cytoplasm, granular
in appearance, and has no subpseudopodia. This distinguishes them
from other amoeboid groups, although in some members this is not
the normal type of locomotion.

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SECTION OVERVIEW

23.4: ECOLOGY OF PROTISTS

23.4B: PROTISTS AS HUMAN PATHOGENS
Topic hierarchy
23.4C: PROTISTS AS PLANT PATHOGENS
23.4A: PROTISTS AS PRIMARY PRODUCERS,
FOOD SOURCES, AND SYMBIONTS This page titled 23.4: Ecology of Protists is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

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23.4A: PROTISTS AS PRIMARY PRODUCERS, FOOD SOURCES, AND
SYMBIONTS
Protists function as sources of food for organisms on land and sea.

 LEARNING OBJECTIVES

Give examples of how protists act as primary producers

KEY POINTS
Photosynthetic protists serve as producers of nutrition for other
organisms.
Protists like zooxanthellae have a symbiotic relationship with
coral reefs; the protists act as a food source for coral and the
coral provides shelter and compounds for photosynthesis for the
protists.
Protists feed a large portion of the world’s aquatic species and Figure 23.4A. 1 : Corals and dinoflagellates: Coral polyps obtain
nutrition through a symbiotic relationship with dinoflagellates.
conduct a quarter of the world’s photosynthesis.
Protists help land-dwelling animals such as cockroaches and The protists themselves and their products of photosynthesis are
termites digest cellulose. essential, directly or indirectly, to the survival of organisms ranging
from bacteria to mammals. As primary producers, protists feed a
KEY TERMS large proportion of the world’s aquatic species. (On land, terrestrial
zooxanthella: an animal of the genus Symbiodinium, a yellow plants serve as primary producers. ) In fact, approximately one-
dinoflagellate, notably found in coral reefs quarter of the world’s photosynthesis is conducted by protists,
primary producer: an autotroph organism that produces particularly dinoflagellates, diatoms, and multicellular algae.
complex organic matter using photosynthesis or chemosynthesis

PRIMARY PRODUCERS/FOOD SOURCES

Protists function in various ecological niches. Some protist species
are essential components of the food chain and are generators of
biomass.
Protists are essential sources of nutrition for many other organisms.
In some cases, as in plankton, protists are consumed directly.
Alternatively, photosynthetic protists serve as producers of nutrition
for other organisms. For instance, photosynthetic dinoflagellates
called zooxanthellae use sunlight to fix inorganic carbon. In this
symbiotic relationship, these protists provide nutrients for the coral
polyps that house them, giving corals a boost of energy to secrete a
calcium carbonate skeleton. In turn, the corals provide the protists
with a protected environment and the compounds needed for
photosynthesis. This type of symbiotic relationship is important in
nutrient-poor environments. Without dinoflagellate symbionts,
corals lose algal pigments in a process called coral bleaching and
they eventually die. This explains why reef-building corals do not
reside in waters deeper than 20 meters: insufficient light reaches
those depths for dinoflagellates to photosynthesize.

Figure 23.4A. 1 : Protists and aquatic organisms: Virtually all aquatic

organisms depend directly or indirectly on protists for food.

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Protists do not only create food sources for sea-dwelling organisms. This page titled 23.4A: Protists as Primary Producers, Food Sources, and
Certain anaerobic parabasalid species exist in the digestive tracts of Symbionts is shared under a CC BY-SA 4.0 license and was authored,
termites and wood-eating cockroaches where they contribute an remixed, and/or curated by Boundless.
essential step in the digestion of cellulose ingested by these insects
as they bore through wood.

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23.4B: PROTISTS AS HUMAN PATHOGENS
Many protists exist as parasites that infect and cause diseases in their mounts a massive inflammatory response with episodes of delirium-
hosts. inducing fever as parasites lyse red blood cells, spilling parasitic
waste into the bloodstream. P. falciparum is transmitted to humans
 LEARNING OBJECTIVES by the African malaria mosquito, Anopheles gambiae. Techniques to
kill, sterilize, or avoid exposure to this highly-aggressive mosquito
Identify the effects on humans of protist pathogens species are crucial to malaria control.

KEY POINTS
The protist parasite Plasmodium must colonize both a mosquito
and a vertebrate; P. falciparum, which is responsible for 50
percent of malaria cases, is transmitted to humans by the African
malaria mosquito, Anopheles gambiae.
When P. falciparum infects and destroys red blood cells, they
burst, and parasitic waste leaks into the blood stream, causing
deliruim, fever, and anemia in patients.
Trypanosoma brucei is responsible for African sleeping sickness
which the human immune system is unable to guard against since
it has thousands of possible antigens and changes surface
glycoproteins with each infectious cycle.
Another Trypanosoma species, T. cruzi, inhabits the heart and
digestive system tissues, causing malnutrition and heart failure.
PLASMODIUM
KEY TERMS Red blood cells are shown to be infected with P. falciparum, the
Trypanosoma: infects a variety of hosts and cause various causative agent of malaria. In this light microscopic image taken
diseases, including the fatal African sleeping sickness in humans using a 100× oil immersion lens, the ring-shaped P. falciparumstains
plasmodium: parasitic protozoa that must colonize a mosquito purple.
and a vertebrate to complete its life cycle
pathogen: any organism or substance, especially a TRYPANOSOMES
microorganism, capable of causing disease, such as bacteria, Trypanosoma brucei, the parasite that is responsible for African
viruses, protozoa, or fungi sleeping sickness, confounds the human immune system by
changing its thick layer of surface glycoproteins with each infectious
HUMAN PATHOGENS cycle. The glycoproteins are identified by the immune system as
A pathogen is anything that causes disease. Parasites live in or on an foreign antigens and a specific antibody defense is mounted against
organism and harm that organism. A significant number of protists the parasite. However, T. brucei has thousands of possible antigens;
are pathogenic parasites that must infect other organisms to survive with each subsequent generation, the protist switches to a
and propagate. Protist parasites include the causative agents of glycoprotein coating of a different molecular structure. In this way,
malaria, African sleeping sickness, and waterborne gastroenteritis in T. brucei is capable of replicating continuously without the immune
humans. system ever succeeding in clearing the parasite. Without treatment,
T. brucei attacks red blood cells, causing the patient to lapse into a
PLASMODIUM SPECIES coma and eventually die. During epidemic periods, mortality from
Members of the genus Plasmodium must colonize both a mosquito the disease can be high. Greater surveillance and control measures
and a vertebrate to complete their life cycle. In vertebrates, the lead to a reduction in reported cases; some of the lowest numbers
parasite develops in liver cells and goes on to infect red blood cells, reported in 50 years (fewer than 10,000 cases in all of sub-Saharan
bursting from and destroying the blood cells with each asexual Africa) have happened since 2009.
replication cycle. Of the four Plasmodium species known to infect
humans, P. falciparum accounts for 50 percent of all malaria cases
and is the primary cause of disease-related fatalities in tropical
regions of the world. In 2010, it was estimated that malaria caused
between one and one-half million deaths, mostly in African children.
During the course of malaria, P. falciparum can infect and destroy
more than one-half of a human’s circulating blood cells, leading to
severe anemia. In response to waste products released as the
parasites burst from infected blood cells, the host immune system

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 In Latin America, another species, T. cruzi, is responsible for Chagas
disease. T. cruzi infections are mainly caused by a blood-sucking
bug. The parasite inhabits heart and digestive system tissues in the
chronic phase of infection, leading to malnutrition and heart failure
due to abnormal heart rhythms. An estimated 10 million people are
infected with Chagas disease; it caused 10,000 deaths in 2008.

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Figure 23.4B. 1: Trypanosomes: Trypanosomes are shown among

red blood cells.

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23.4C: PROTISTS AS PLANT PATHOGENS
Many protists act as parasites that prey on plants or as decomposers Because the downy mildew pathogen does not overwinter in
that feed on dead organisms. midwestern fields, crop rotations and tillage practices do not affect
disease development. The pathogen tends to become established in
 LEARNING OBJECTIVES late summer. Therefore, planting early season varieties may further
reduce the threat posed by downy mildew. Fungicides can also be
Describe the ways in which protists act as decomposers and applied to control downy mildew. Broad spectrum protectant
the actions of parasitic protists on plants fungicides such as chlorothalonil, mancozeb, and fixed copper are
somewhat effective in protecting against downy mildew infection.
KEY POINTS Phytophthora infestans is an oomycete responsible for potato late
Plasmopara viticola causes downy mildew in grape plants, blight. This disease causes potato stalks and stems to decay into
resulting in stunted growth and withered, discolored leaves. black slime. Widespread potato blight caused by P. infestans led to
Since downy mildew has a higher incidence in the late summer, the well-known Irish potato famine in the nineteenth century that
planting early in the season can reduce the threat of downy claimed the lives of approximately one million people and resulted
mildew; fungicides are also somewhat effective at preventing in the emigration of at least one million more from Ireland. Late
downy mildew. blight continues to plague potato crops in certain parts of the United
Phytophthora infestans causes potato late blight (potato stalks States and Russia, wiping out as much as 70 percent of crops when
and stems decay into black slime) and was responsible for the no pesticides are applied.
Irish potato famine in the nineteenth century.
Protist saprobes feed on dead organisms, which returns inorganic
nutrients to soil and water.

KEY TERMS
saprobe: an organism that lives off of dead or decaying organic
material
oomycete: fungus-like filamentous unicellular protists; the water
molds
downy mildew: plant disease caused by oomycetes; causes
stunted growth in plants as well as discolored, withered leaves

PLANT PARASITES
Protist parasites prey on terrestrial plants and include agents that
Figure 23.4C. 1 : Potato Late Blight: These unappetizing remnants
cause massive destruction to food crops. The oomycete Plasmopara result from an infection with P. infestans, the causative agent of
viticola parasitizes grape plants, which causes a disease called potato late blight.
downy mildew. Grape plants infected with P. viticola appear stunted
AGENTS OF DECOMPOSITION
and have discolored, withered leaves. The spread of downy mildew
The fungus-like protist saprobes are specialized to absorb nutrients
nearly collapsed the French wine industry in the nineteenth century.
from non-living organic matter, such as dead organisms or their
They are easily controlled once discovered, so careful monitoring of
wastes. For instance, many types of oomycetes grow on dead
susceptible hosts is key because if left unaddressed, the organism
animals or algae. Saprobic protists have the essential function of
can quickly spread and completely overwhelm the host species
returning inorganic nutrients to the soil and water. This process
allows for new plant growth, which in turn generates sustenance for
other organisms along the food chain. Indeed, without saprobe
species, such as protists, fungi, and bacteria, life would cease to exist
as all organic carbon became “tied up” in dead organisms.

CONTRIBUTIONS AND ATTRIBUTIONS

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mildews on this grape leaf are caused by an infection of P. viticola. ShareAlike
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 CHAPTER OVERVIEW

24: FUNGI
24.1: Characteristics of Fungi
24.1A: Characteristics of Fungi
24.1B: Fungi Cell Structure and Function
24.1C: Fungi Reproduction
24.2: Ecology of Fungi
24.2A: Fungi Habitat, Decomposition, and Recycling
24.2B: Mutualistic Relationships with Fungi and Fungivores
24.3: Classifications of Fungi
24.3A: Chytridiomycota- The Chytrids
24.3B: Zygomycota - The Conjugated Fungi
24.3C: Ascomycota - The Sac Fungi
24.3D: Basidiomycota- The Club Fungi
24.3E: Deuteromycota - The Imperfect Fungi
24.3F: Glomeromycota
24.4: Fungal Parasites and Pathogens
24.4A: Fungi as Plant, Animal, and Human Pathogens
24.5: Importance of Fungi in Human Life
24.5A: Importance of Fungi in Human Life

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1
SECTION OVERVIEW

24.1: CHARACTERISTICS OF FUNGI

24.1B: FUNGI CELL STRUCTURE AND FUNCTION
Topic hierarchy
24.1C: FUNGI REPRODUCTION
24.1A: CHARACTERISTICS OF FUNGI
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24.1A: CHARACTERISTICS OF FUNGI
Fungi, latin for mushroom, are eukaryotes which are responsible for
decomposition and nutrient cycling through the environment.

 LEARNING OBJECTIVES

Describe the role of fungi in the ecosystem

KEY POINTS
Fungi are more closely related to animals than plants.
Fungi are heterotrophic: they use complex organic compounds as Figure 24.1A. 1 : Examples of fungi: Many species of fungus
sources of energy and carbon, not photosynthesis. produce the familiar mushroom (a) which is a reproductive structure.
This (b) coral fungus displays brightly-colored fruiting bodies. This
Fungi multiply either asexually, sexually, or both.
electron micrograph shows (c) the spore-bearing structures of
The majority of fungi produce spores, which are defined as Aspergillus, a type of toxic fungi found mostly in soil and plants.
haploid cells that can undergo mitosis to form multicellular, Fungi, once considered plant-like organisms, are more closely
haploid individuals. related to animals than plants. Fungi are not capable of
Fungi interact with other organisms by either forming beneficial photosynthesis: they are heterotrophic because they use complex
or mutualistic associations (mycorrhizae and lichens ) or by organic compounds as sources of energy and carbon. Some fungal
causing serious infections. organisms multiply only asexually, whereas others undergo both
asexual reproduction and sexual reproduction with alternation of
KEY TERMS generations. Most fungi produce a large number of spores, which are
mycorrhiza: a symbiotic association between a fungus and the haploid cells that can undergo mitosis to form multicellular, haploid
roots of a vascular plant individuals. Like bacteria, fungi play an essential role in ecosystems
spore: a reproductive particle, usually a single cell, released by a because they are decomposers and participate in the cycling of
fungus, alga, or plant that may germinate into another nutrients by breaking down organic and inorganic materials to
lichen: any of many symbiotic organisms, being associations of simple molecules.
fungi and algae; often found as white or yellow patches on old
Fungi often interact with other organisms, forming beneficial or
walls, etc.
mutualistic associations. For example most terrestrial plants form
Ascomycota: a taxonomic division within the kingdom Fungi;
symbiotic relationships with fungi. The roots of the plant connect
those fungi that produce spores in a microscopic sporangium
with the underground parts of the fungus forming mycorrhizae.
called an ascus
Through mycorrhizae, the fungus and plant exchange nutrients and
heterotrophic: organisms that use complex organic compounds
water, greatly aiding the survival of both species Alternatively,
as sources of energy and carbon
lichens are an association between a fungus and its photosynthetic
INTRODUCTION TO FUNGI partner (usually an alga). Fungi also cause serious infections in
plants and animals. For example, Dutch elm disease, which is caused
The word fungus comes from the Latin word for mushrooms.
by the fungus Ophiostoma ulmi, is a particularly devastating type of
Indeed, the familiar mushroom is a reproductive structure used by
fungal infestation that destroys many native species of elm (Ulmus
many types of fungi. However, there are also many fungi species
sp.) by infecting the tree’s vascular system. The elm bark beetle acts
that don’t produce mushrooms at all. Being eukaryotes, a typical
as a vector, transmitting the disease from tree to tree. Accidentally
fungal cell contains a true nucleus and many membrane-bound
introduced in the 1900s, the fungus decimated elm trees across the
organelles. The kingdom Fungi includes an enormous variety of
continent. Many European and Asiatic elms are less susceptible to
living organisms collectively referred to as Ascomycota, or true
Dutch elm disease than American elms.
Fungi. While scientists have identified about 100,000 species of
fungi, this is only a fraction of the 1.5 million species of fungus In humans, fungal infections are generally considered challenging to
probably present on earth. Edible mushrooms, yeasts, black mold, treat. Unlike bacteria, fungi do not respond to traditional antibiotic
and the producer of the antibiotic penicillin, Penicillium notatum, therapy because they are eukaryotes. Fungal infections may prove
are all members of the kingdom Fungi, which belongs to the domain deadly for individuals with compromised immune systems.
Eukarya. Fungi have many commercial applications. The food industry uses
yeasts in baking, brewing, and cheese and wine making. Many
industrial compounds are byproducts of fungal fermentation. Fungi
are the source of many commercial enzymes and antibiotics.

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24.1B: FUNGI CELL STRUCTURE AND FUNCTION
Fungi are unicellular or multicellular thick-cell-walled heterotroph with white patches. Pigments in fungi are associated with the cell
decomposers that eat decaying matter and make tangles of filaments. wall. They play a protective role against ultraviolet radiation and can
be toxic.
 LEARNING OBJECTIVES

Describe the physical structures associated with fungi

KEY POINTS
Fungal cell walls are rigid and contain complex polysaccharides
called chitin (adds structural strength) and glucans.
Ergosterol is the steroid molecule in the cell membranes that
replaces the cholesterol found in animal cell membranes.
Fungi can be unicellular, multicellular, or dimorphic, which is
when the fungi is unicellular or multicellular depending on
environmental conditions.
Fungi in the morphological vegetative stage consist of a tangle of
slender, thread-like hyphae, whereas the reproductive stage is Figure 24.1B. 1: The poisonous Amanita muscaria: The poisonous
Amanita muscaria is native to temperate and boreal regions of North
usually more obvious. America.
Fungi like to be in a moist and slightly acidic environment; they
The rigid layers of fungal cell walls contain complex
can grow with or without light or oxygen.
polysaccharides called chitin and glucans. Chitin, also found in the
Fungi are saprophyte heterotrophs in that they use dead or
exoskeleton of insects, gives structural strength to the cell walls of
decomposing organic matter as a source of carbon.
fungi. The wall protects the cell from desiccation and predators.
KEY TERMS Fungi have plasma membranes similar to other eukaryotes, except
that the structure is stabilized by ergosterol: a steroid molecule that
glucan: any polysaccharide that is a polymer of glucose
replaces the cholesterol found in animal cell membranes. Most
ergosterol: the functional equivalent of cholesterol found in cell
members of the kingdom Fungi are nonmotile.
membranes of fungi and some protists, as well as, the steroid
precursor of vitamin D2 GROWTH
mycelium: the vegetative part of any fungus, consisting of a
The vegetative body of a fungus is a unicellular or multicellular
mass of branching, threadlike hyphae, often underground
thallus. Dimorphic fungi can change from the unicellular to
hypha: a long, branching, filamentous structure of a fungus that
multicellular state depending on environmental conditions.
is the main mode of vegetative growth
Unicellular fungi are generally referred to as yeasts. Saccharomyces
septum: cell wall division between hyphae of a fungus
cerevisiae (baker’s yeast) and Candida species (the agents of thrush,
thallus: vegetative body of a fungus
a common fungal infection) are examples of unicellular fungi.
saprophyte: any organism that lives on dead organic matter, as
certain fungi and bacteria
chitin: a complex polysaccharide, a polymer of N-
acetylglucosamine, found in the exoskeletons of arthropods and
in the cell walls of fungi; thought to be responsible for some
forms of asthma in humans

CELL STRUCTURE AND FUNCTION

Fungi are eukaryotes and have a complex cellular organization. As
eukaryotes, fungal cells contain a membrane-bound nucleus where
the DNA is wrapped around histone proteins. A few types of fungi
have structures comparable to bacterial plasmids (loops of DNA). Figure 24.1B. 1: Example of a unicellular fungus: Candida albicans
is a yeast cell and the agent of candidiasis and thrush. This organism
Fungal cells also contain mitochondria and a complex system of has a similar morphology to coccus bacteria; however, yeast is a
internal membranes, including the endoplasmic reticulum and Golgi eukaryotic organism (note the nucleus).
apparatus. Most fungi are multicellular organisms. They display two distinct
Unlike plant cells, fungal cells do not have chloroplasts or morphological stages: the vegetative and reproductive. The
chlorophyll. Many fungi display bright colors arising from other vegetative stage consists of a tangle of slender thread-like structures
cellular pigments, ranging from red to green to black. The poisonous called hyphae (singular, hypha ), whereas the reproductive stage can
Amanita muscaria (fly agaric) is recognizable by its bright red cap be more conspicuous. The mass of hyphae is a mycelium. It can

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grow on a surface, in soil or decaying material, in a liquid, or even NUTRITION
on living tissue. Although individual hyphae must be observed under Like animals, fungi are heterotrophs: they use complex organic
a microscope, the mycelium of a fungus can be very large, with compounds as a source of carbon, rather than fix carbon dioxide
some species truly being “the fungus humongous.” The giant from the atmosphere as do some bacteria and most plants. In
Armillaria solidipes (honey mushroom) is considered the largest addition, fungi do not fix nitrogen from the atmosphere. Like
organism on Earth, spreading across more than 2,000 acres of animals, they must obtain it from their diet. However, unlike most
underground soil in eastern Oregon; it is estimated to be at least animals, which ingest food and then digest it internally in
2,400 years old. specialized organs, fungi perform these steps in the reverse order:
digestion precedes ingestion. First, exoenzymes are transported out
of the hyphae, where they process nutrients in the environment.
Then, the smaller molecules produced by this external digestion are
absorbed through the large surface area of the mycelium. As with
animal cells, the polysaccharide of storage is glycogen rather than
the starch found in plants.
Fungi are mostly saprobes (saprophyte is an equivalent term):
organisms that derive nutrients from decaying organic matter. They
obtain their nutrients from dead or decomposing organic matter,
mainly plant material. Fungal exoenzymes are able to break down
insoluble polysaccharides, such as the cellulose and lignin of dead
Figure 24.1B. 1: Example of a mycelium of a fungus: The
mycelium of the fungus Neotestudina rosati can be pathogenic to wood, into readily-absorbable glucose molecules. The carbon,
humans. The fungus enters through a cut or scrape and develops a nitrogen, and other elements are thus released into the environment.
mycetoma, a chronic subcutaneous infection.
Because of their varied metabolic pathways, fungi fulfill an
Most fungal hyphae are divided into separate cells by endwalls important ecological role and are being investigated as potential
called septa (singular, septum) ( a, c). In most phyla of fungi, tiny tools in bioremediation.
holes in the septa allow for the rapid flow of nutrients and small
Some fungi are parasitic, infecting either plants or animals. Smut
molecules from cell to cell along the hypha. They are described as
and Dutch elm disease affect plants, whereas athlete’s foot and
perforated septa. The hyphae in bread molds (which belong to the
candidiasis (thrush) are medically important fungal infections in
Phylum Zygomycota) are not separated by septa. Instead, they are
humans.
formed by large cells containing many nuclei, an arrangement
described as coenocytic hyphae ( b). Fungi thrive in environments This page titled 24.1B: Fungi Cell Structure and Function is shared under a
that are moist and slightly acidic; they can grow with or without CC BY-SA 4.0 license and was authored, remixed, and/or curated by
light. Boundless.

Figure 24.1B. 1: Division of hyphae into separate cells: Fungal

hyphae may be (a) septated or (b) coenocytic (coeno- = “common”; -
cytic = “cell”) with many nuclei present in a single hypha. A bright
field light micrograph of (c) Phialophora richardsiae shows septa
that divide the hyphae.

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24.1C: FUNGI REPRODUCTION
Fungi can reproduce asexually by fragmentation, budding, or
producing spores, or sexually with homothallic or heterothallic
mycelia.

 LEARNING OBJECTIVES

Describe the mechanisms of sexual and asexual

reproduction in fungi

KEY POINTS
New colonies of fungi can grow from the fragmentation of
hyphae. Figure 24.1C. 1 : The release of fungal spores: The (a) giant puff
ball mushroom releases (b) a cloud of spores when it reaches
During budding, a bulge forms on the side of the cell; the bud
maturity.
ultimately detaches after the nucleus divides mitotically.
Asexual spores are genetically identical to the parent and may be ASEXUAL REPRODUCTION
released either outside or within a special reproductive sac called Fungi reproduce asexually by fragmentation, budding, or producing
a sporangium. spores. Fragments of hyphae can grow new colonies. Mycelial
Adverse environmental conditions often cause sexual fragmentation occurs when a fungal mycelium separates into pieces
reproduction in fungi. with each component growing into a separate mycelium. Somatic
Mycelium can either be homothallic or heterothallic when cells in yeast form buds. During budding (a type of cytokinesis), a
reproducing sexually. bulge forms on the side of the cell, the nucleus divides mitotically,
Fungal sexual reproduction includes the following three stages: and the bud ultimately detaches itself from the mother cell.
plasmogamy, karyogamy, and gametangia. The most common mode of asexual reproduction is through the
formation of asexual spores, which are produced by one parent only
KEY TERMS
(through mitosis) and are genetically identical to that parent. Spores
homothallic: male and female reproductive structures are
allow fungi to expand their distribution and colonize new
present in the same plant or fungal mycelium
environments. They may be released from the parent thallus, either
gametangium: an organ or cell in which gametes are produced
outside or within a special reproductive sac called a sporangium.
that is found in many multicellular protists, algae, fungi, and the
gametophytes of plants
spore: a reproductive particle, usually a single cell, released by a
fungus, alga, or plant that may germinate into another
sporangium: a case, capsule, or container in which spores are
produced by an organism
karyogamy: the fusion of two nuclei within a cell
plasmogamy: stage of sexual reproduction joining the cytoplasm
of two parent mycelia without the fusion of nuclei

REPRODUCTION
Fungi reproduce sexually and/or asexually. Perfect fungi reproduce
both sexually and asexually, while imperfect fungi reproduce only
asexually (by mitosis).
In both sexual and asexual reproduction, fungi produce spores that
disperse from the parent organism by either floating on the wind or
hitching a ride on an animal. Fungal spores are smaller and lighter
than plant seeds. The giant puffball mushroom bursts open and
releases trillions of spores. The huge number of spores released
increases the likelihood of landing in an environment that will Figure 24.1C. 1 : Types of fungal reproduction: Fungi may utilize
support growth. both asexual and sexual stages of reproduction; sexual reproduction
often occurs in response to adverse environmental conditions.
There are many types of asexual spores. Conidiospores are
unicellular or multicellular spores that are released directly from the
tip or side of the hypha. Other asexual spores originate in the

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SECTION OVERVIEW

24.2: ECOLOGY OF FUNGI

24.2B: MUTUALISTIC RELATIONSHIPS WITH
Topic hierarchy FUNGI AND FUNGIVORES

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24.2A: FUNGI HABITAT, DECOMPOSITION, AND RECYCLING
Fungi are the major decomposers of nature; they break down organic
matter which would otherwise not be recycled.

 LEARNING OBJECTIVES

Explain the roles played by fungi in decomposition and

recycling

KEY POINTS
Aiding the survival of species from other kingdoms through the
supply of nutrients, fungi play a major role as decomposers and
recyclers in the wide variety of habitats in which they exist.
Fungi provide a vital role in releasing scarce, yet biologically-
Figure 24.2A. 1 : Fungi as decomposers: Fungi are an important part
essential elements, such as nitrogen and phosphorus, from of ecosystem nutrient cycles. These bracket fungi growing on the
decaying matter. side of a tree are the fruiting structures of a basidiomycete. They
Their mode of nutrition, which involves digestion before receive their nutrients through their hyphae, which invade and decay
the tree trunk.
ingestion, allows fungi to degrade many large and insoluble
The ability of fungi to degrade many large and insoluble molecules
molecules that would otherwise remain trapped in a habitat.
is due to their mode of nutrition. As seen earlier, digestion precedes
KEY TERMS ingestion. Fungi produce a variety of exoenzymes to digest nutrients.
decomposer: any organism that feeds off decomposing organic These enzymes are either released into the substrate or remain bound
material, especially bacterium or fungi to the outside of the fungal cell wall. Large molecules are broken
exoenzyme: any enzyme, generated by a cell, that functions down into small molecules, which are transported into the cell by a
outside of that cell system of protein carriers embedded in the cell membrane. Because
saprobe: an organism that lives off of dead or decaying organic the movement of small molecules and enzymes is dependent on the
material presence of water, active growth depends on a relatively-high
percentage of moisture in the environment.
FUNGI & THEIR ROLES AS DECOMPOSERS AND As saprobes, fungi help maintain a sustainable ecosystem for the
RECYCLERS animals and plants that share the same habitat. In addition to
Fungi play a crucial role in the balance of ecosystems. They replenishing the environment with nutrients, fungi interact directly
colonize most habitats on earth, preferring dark, moist conditions. with other organisms in beneficial, but sometimes damaging, ways.
They can thrive in seemingly-hostile environments, such as the
tundra. However, most members of the Kingdom Fungi grow on the
forest floor where the dark and damp environment is rich in
decaying debris from plants and animals. In these environments,
fungi play a major role as decomposers and recyclers, making it
possible for members of the other kingdoms to be supplied with
nutrients and to live.
The food web would be incomplete without organisms that
decompose organic matter. Some elements, such as nitrogen and
phosphorus, are required in large quantities by biological systems;
yet, they are not abundant in the environment. The action of fungi
releases these elements from decaying matter, making them
available to other living organisms. Trace elements present in low
Figure 24.2A. 1 : Fungi: beneficial & pathogenic: Shelf fungi, so
amounts in many habitats are essential for growth, but would remain called because they grow on trees in a stack, attack and digest the
tied up in rotting organic matter if fungi and bacteria did not return trunk or branches of a tree. While some shelf fungi are found only
on dead trees, others can parasitize living trees, causing eventual
them to the environment via their metabolic activity. death. They are considered serious tree pathogens.

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24.2B: MUTUALISTIC RELATIONSHIPS WITH FUNGI AND FUNGIVORES
Members of Kingdom Fungi form ecologically beneficial Ectomycorrhizae (“outside” mycorrhiza) depend on fungi
mutualistic relationships with cyanobateria, plants, and animals. enveloping the roots in a sheath (called a mantle) and a Hartig net of
hyphae that extends into the roots between cells. The fungal partner
 LEARNING OBJECTIVES can belong to the Ascomycota, Basidiomycota, or Zygomycota. In a
second type, the Glomeromycete fungi form vesicular–arbuscular
Describe mutualistic relationships with fungi interactions with arbuscular mycorrhiza (sometimes called
endomycorrhizae). In these mycorrhiza, the fungi form arbuscules
KEY POINTS that penetrate root cells and are the site of the metabolic exchanges
Mutualistic relationships are those where both members of an between the fungus and the host plant. The arbuscules (from the
association benefit; Fungi form these types of relationships with Latin for “little trees”) have a shrub-like appearance. Orchids rely on
various other Kingdoms of life. a third type of mycorrhiza. Orchids are epiphytes that form small
Mycorrhiza, formed from an association between plant roots and seeds without much storage to sustain germination and growth.
primitive fungi, help increase a plant’s nutrient uptake; in return, Their seeds will not germinate without a mycorrhizal partner
the plant supplies the fungi with photosynthesis products for (usually a Basidiomycete). After nutrients in the seed are depleted,
their metabolic use. fungal symbionts support the growth of the orchid by providing
In lichen, fungi live in close proximity with photosynthetic necessary carbohydrates and minerals. Some orchids continue to be
cyanobateria; the algae provide fungi with carbon and energy mycorrhizal throughout their lifecycle.
while the fungi supplies minerals and protection to the algae.
Mutualistic relationships between fungi and animals involves
numerous insects; Arthropods depend on fungi for protection,
while fungi receive nutrients in return and ensure a way to
disseminate the spores into new environments.

KEY TERMS
mycorrhiza: a symbiotic association between a fungus and the
roots of a vascular plant
lichen: any of many symbiotic organisms, being associations of
fungi and algae; often found as white or yellow patches on old
walls, etc.
thallus: vegetative body of a fungus

MUTUALISTIC RELATIONSHIPS Figure 24.2B. 1: Mycorrhizal fungi: (a) Ectomycorrhiza and (b)
Symbiosis is the ecological interaction between two organisms that arbuscular mycorrhiza have different mechanisms for interacting
with the roots of plants.
live together. However, the definition does not describe the quality
of the interaction. When both members of the association benefit, LICHENS
the symbiotic relationship is called mutualistic. Fungi form Lichens display a range of colors and textures. They can survive in
mutualistic associations with many types of organisms, including the most unusual and hostile habitats. They cover rocks,
cyanobacteria, plants, and animals. gravestones, tree bark, and the ground in the tundra where plant
roots cannot penetrate. Lichens can survive extended periods of
FUNGI & PLANT MUTUALISM
drought: they become completely desiccated and then rapidly
Mycorrhiza, which comes from the Greek words “myco” meaning
become active once water is available again. Lichens fulfill many
fungus and “rhizo” meaning root, refers to the association between ecological roles, including acting as indicator species, which allow
vascular plant roots and their symbiotic fungi. About 90 percent of
scientists to track the health of a habitat because of their sensitivity
all plant species have mycorrhizal partners. In a mycorrhizal to air pollution.
association, the fungal mycelia use their extensive network of
hyphae and large surface area in contact with the soil to channel
water and minerals from the soil into the plant, thereby increasing a
plant’s nutrient uptake. In exchange, the plant supplies the products
of photosynthesis to fuel the metabolism of the fungus.
Mycorrhizae display many characteristics of primitive fungi: they
produce simple spores, show little diversification, do not have a Figure 24.2B. 1: Lichen: fungi and cyanobateria: Lichens have
sexual reproductive cycle, and cannot live outside of a mycorrhizal many forms. They may be (a) crust-like, (b) hair-like, or (c) leaf-
like.
association. There are a number of types of mycorrhizae.

24.2B.1 https://bio.libretexts.org/@go/page/13602
Lichens are not a single organism, but, rather, an example of a protects the insect colonies. The scale insects foster a flow of
mutualism in which a fungus (usually a member of the Ascomycota nutrients from the parasitized plant to the fungus. In a second
or Basidiomycota phyla) lives in close contact with a photosynthetic example, leaf-cutting ants of Central and South America literally
organism (a eukaryotic alga or a prokaryotic cyanobacterium). farm fungi. They cut disks of leaves from plants and pile them up in
Generally, neither the fungus nor the photosynthetic organism can gardens. Fungi are cultivated in these disk gardens, digesting the
survive alone outside of the symbiotic relationship. The body of a cellulose in the leaves that the ants cannot break down. Once smaller
lichen, referred to as a thallus, is formed of hyphae wrapped around sugar molecules are produced and consumed by the fungi, the fungi
the photosynthetic partner. The photosynthetic organism provides in turn become a meal for the ants. The insects also patrol their
carbon and energy in the form of carbohydrates. Some cyanobacteria garden, preying on competing fungi. Both ants and fungi benefit
fix nitrogen from the atmosphere, contributing nitrogenous from the association. The fungus receives a steady supply of leaves
compounds to the association. In return, the fungus supplies and freedom from competition, while the ants feed on the fungi they
minerals and protection from dryness and excessive light by cultivate.
encasing the algae in its mycelium. The fungus also attaches the
symbiotic organism to the substrate. CONTRIBUTIONS AND ATTRIBUTIONS
OpenStax College, Biology. October 22, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44632/latest...ol11448/latest. License: CC
BY: Attribution
OpenStax College, Biology. November 12, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44632/latest...ol11448/latest. License: CC
BY: Attribution
exoenzyme. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/exoenzyme. License: CC BY-SA: Attribution-
ShareAlike
decomposer. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/decomposer. License: CC BY-SA: Attribution-
ShareAlike
saprobe. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/saprobe. License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Biology. November 12, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44632/latest...ol11448/latest. License: CC
BY: Attribution
OpenStax College, Biology. November 12, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44632/latest...ol11448/latest. License: CC
BY: Attribution
Boundless. Provided by: Boundless Learning. Located at:
www.boundless.com//biology/definition/thallus. License: CC BY-SA:
Attribution-ShareAlike
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44632/latest...ol11448/latest. License: CC
BY: Attribution
OpenStax College, Biology. October 22, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44632/latest...ol11448/latest. License: CC
BY: Attribution
mycorrhiza. Provided by: Wikipedia. Located at:
Figure 24.2B. 1: Thallus of lichen: This cross-section of a lichen en.Wikipedia.org/wiki/mycorrhiza. License: CC BY-SA: Attribution-
thallus shows the (a) upper cortex of fungal hyphae, which provides ShareAlike
protection; the (b) algal zone where photosynthesis occurs, the (c) lichen. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/lichen.
medulla of fungal hyphae, and the (d) lower cortex, which also License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Biology. November 12, 2013. Provided by: OpenStax CNX.
provides protection and may have (e) rhizines to anchor the thallus
Located at: http://cnx.org/content/m44632/latest...ol11448/latest. License: CC
to the substrate. BY: Attribution
The thallus of lichens grows very slowly, expanding its diameter a OpenStax College, Biology. November 12, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44632/latest...ol11448/latest. License: CC
few millimeters per year. Both the fungus and the alga participate in BY: Attribution
the formation of dispersal units for reproduction. Lichens produce OpenStax College, Ecology of Fungi. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44632/latest...e_24_03_06.jpg.
soredia, clusters of algal cells surrounded by mycelia. Soredia are License: CC BY: Attribution
dispersed by wind and water and form new lichens. OpenStax College, Ecology of Fungi. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44632/latest...e_24_03_03.png.
License: CC BY: Attribution
FUNGI & ANIMAL MUTUALISM OpenStax College, Ecology of Fungi. October 17, 2013. Provided by: OpenStax
Fungi have evolved mutualisms with numerous insects. Arthropods CNX. Located at: http://cnx.org/content/m44632/latest..._03_05abcf.jpg.
License: CC BY: Attribution
(jointed, legged invertebrates, such as insects) depend on the fungus
for protection from predators and pathogens, while the fungus This page titled 24.2B: Mutualistic Relationships with Fungi and
obtains nutrients and a way to disseminate spores into new Fungivores is shared under a CC BY-SA 4.0 license and was authored,
environments. The association between species of Basidiomycota remixed, and/or curated by Boundless.
and scale insects is one example. The fungal mycelium covers and

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SECTION OVERVIEW

24.3: CLASSIFICATIONS OF FUNGI

24.3D: BASIDIOMYCOTA- THE CLUB FUNGI
Topic hierarchy
24.3E: DEUTEROMYCOTA - THE IMPERFECT
FUNGI
24.3A: CHYTRIDIOMYCOTA- THE CHYTRIDS
24.3F: GLOMEROMYCOTA
24.3B: ZYGOMYCOTA - THE CONJUGATED FUNGI

24.3C: ASCOMYCOTA - THE SAC FUNGI This page titled 24.3: Classifications of Fungi is shared under a CC BY-SA
4.0 license and was authored, remixed, and/or curated by Boundless.

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24.3A: CHYTRIDIOMYCOTA- THE CHYTRIDS
Chytrids are the most primitive group of fungi and the only group The ecological habitat and cell structure of chytrids have much in
that possess gametes with flagella. common with protists. Chytrids usually live in aquatic
environments, although some species live on land. Some species
 LEARNING OBJECTIVES thrive as parasites on plants, insects, or amphibians, while others are
saprobes. Some chytrids cause diseases in many species of
Describe the ecology and reproduction of chytrids amphibians, resulting in species decline and extinction. An example
of a harmful parasitic chytrid is Batrachochytrium dendrobatidis,
KEY POINTS which is known to cause skin disease. Another chytrid species,
The first recognizable chytrids appeared more than 500 million Allomyces, is well characterized as an experimental organism. Its
years ago during the late pre-Cambrian period. reproductive cycle includes both asexual and sexual phases.
Like protists, chytrids usually live in aquatic environments, but Allomyces produces diploid or haploid flagellated zoospores in a
some species live on land. sporangium.
Some chytrids are saprobes while others are parasites that may
be harmful to amphibians and other animals.
Chytrids reproduce both sexually and asexually, which leads to
the production of zoospores.
Chytrids have chitin in their cell walls; one unique group also
has cellulose along with chitin.
Chytrids are mostly unicellular, but multicellular organisms do
exist.

KEY TERMS
chytridiomycete: an organism of the phylum Chytridiomycota
zoospore: an asexual spore of some algae and fungi
flagellum: a flagellum is a lash-like appendage that protrudes
from the cell body of certain prokaryotic and eukaryotic cells
coenocytic: a multinucleate cell that can result from multiple
nuclear divisions without their accompanying cytokinesis

CHYTRIDIOMYCOTA: THE CHYTRIDS

The kingdom Fungi contains five major phyla, which were
established according to their mode of sexual reproduction or use of
molecular data. The Phylum Chytridiomycota (chytrids) is one of
the five true phyla of fungi. There is only one class in the Phylum
Chytridiomycota, the Chytridiomycetes. The chytrids are the
simplest and most primitive Eumycota, or true fungi. The
evolutionary record shows that the first, recognizable chytrids
appeared during the late pre-Cambrian period, more than 500 Figure 24.3A. 1 : Parasitic chytrids: The chytrid Batrachochytrium
million years ago. Like all fungi, chytrids have chitin in their cell dendrobatidisis seen in these light micrographs as transparent
spheres growing on (a) a freshwater arthropod and (b) algae. This
walls, but one group of chytrids has both cellulose and chitin in the chytrid causes skin diseases in many species of amphibians,
cell wall. Most chytrids are unicellular; a few form multicellular resulting in species decline and extinction.
organisms and hyphae, which have no septa between cells
This page titled 24.3A: Chytridiomycota- The Chytrids is shared under a CC
(coenocytic). They reproduce both sexually and asexually; the
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
asexual spores are called diploid zoospores. Their gametes are the
only fungal cells known to have a flagellum.

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24.3B: ZYGOMYCOTA - THE CONJUGATED FUNGI

 LEARNING OBJECTIVES

Describe the ecology and reproduction of Zygomycetes

The zygomycetes are a relatively small group in the fungi kingdom

and belong to the Phylum Zygomycota. They include the familiar
bread mold, Rhizopus stolonifer, which rapidly propagates on the
surfaces of breads, fruits, and vegetables. They are mostly terrestrial
in habitat, living in soil or on plants and animals. Most species are
saprobes meaning they live off decaying organic material. Some are
parasites of plants, insects, and small animals, while others form
symbiotic relationships with plants. Zygomycetes play a
considerable commercial role. The metabolic products of other
species of Rhizopus are intermediates in the synthesis of semi-
synthetic steroid hormones.
Zygomycetes have a thallus of coenocytic hyphae in which the
nuclei are haploid when the organism is in the vegetative stage. The
fungi usually reproduce asexually by producing sporangiospores.
The black tips of bread mold, Rhizopus stolonifer, are the swollen
sporangia packed with black spores. When spores land on a suitable
substrate, they germinate and produce a new mycelium.

Figure 24.3B. 1: Zygomycete life cycle: Zygomycetes have asexual

and sexual life cycles. In the sexual life cycle, plus and minus
mating types conjugate to form a zygosporangium.
Sexual reproduction starts when conditions become unfavorable.
Two opposing mating strains (type + and type –) must be in close
proximity for gametangia (singular: gametangium) from the hyphae
to be produced and fuse, leading to karyogamy. The developing
Figure 24.3B. 1: Sporangia of bread mold: Sporangia grow at the
end of stalks, which appear as (a) white fuzz seen on this bread diploid zygospores have thick coats that protect them from
mold, Rhizopus stolonifer. The (b) tips of bread mold are the spore- desiccation and other hazards. They may remain dormant until
containing sporangia. environmental conditions become favorable. When the zygospore
germinates, it undergoes meiosis and produces haploid spores,
which will, in turn, grow into a new organism. This form of sexual
reproduction in fungi is called conjugation (although it differs
markedly from conjugation in bacteria and protists), giving rise to
the name “conjugated fungi”.

KEY POINTS
Most zygomycota are saprobes, while a few species are parasites.
Zygomycota usually reproduce asexually by producing
sporangiospores.
Zygomycota reproduce sexually when environmental conditions
become unfavorable.
To reproduce sexually, two opposing mating strains must fuse or
conjugate, thereby, sharing genetic content and creating
zygospores.
The resulting diploid zygospores remain dormant and protected
by thick coats until environmental conditions have improved.
When conditions become favorable, zygospores undergo meiosis
to produce haploid spores, which will eventually grow into a new
organism.

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KEY TERMS conjugation: the temporary fusion of organisms, especially as
zygomycete: an organism of the phylum Zygomycota part of sexual reproduction
karyogamy: the fusion of two nuclei within a cell
This page titled 24.3B: Zygomycota - The Conjugated Fungi is shared under
zygospore: a spore formed by the union of several zoospores
a CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Boundless.

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24.3C: ASCOMYCOTA - THE SAC FUNGI

 LEARNING OBJECTIVES

Describe the ecology and the reproduction of Ascomycetes

The majority of known fungi belong to the Phylum Ascomycota,

which is characterized by the formation of an ascus (plural, asci), a
sac-like structure that contains haploid ascospores. Many
ascomycetes are of commercial importance. Some play a beneficial
role, such as the yeasts used in baking, brewing, and wine
fermentation, plus truffles and morels, which are held as gourmet
delicacies. Aspergillus oryzae is used in the fermentation of rice to
produce sake. Other ascomycetes parasitize plants and animals,
including humans. For example, fungal pneumonia poses a
significant threat to AIDS patients who have a compromised
immune system. Ascomycetes not only infest and destroy crops
directly, they also produce poisonous secondary metabolites that
make crops unfit for consumption. Filamentous ascomycetes
produce hyphae divided by perforated septa, allowing streaming of
cytoplasm from one cell to the other. Conidia and asci, which are
used respectively for asexual and sexual reproductions, are usually Figure 24.3C. 1 : Release of ascospores: The bright field light
separated from the vegetative hyphae by blocked (non-perforated) micrograph shows ascospores being released from asci in the fungus
septa. Talaromyces flavus var. flavus.
Asexual reproduction is frequent and involves the production of
conidiophores that release haploid conidiospores. Sexual
reproduction starts with the development of special hyphae from
either one of two types of mating strains. The “male” strain produces
an antheridium (plural: antheridia) and the “female” strain develops
an ascogonium (plural: ascogonia). At fertilization, the antheridium
and the ascogonium combine in plasmogamy without nuclear fusion.
Special ascogenous hyphae arise, in which pairs of nuclei migrate:
one from the “male” strain and one from the “female” strain. In each
ascus, two or more haploid ascospores fuse their nuclei in
karyogamy. During sexual reproduction, thousands of asci fill a
fruiting body called the ascocarp. The diploid nucleus gives rise to
haploid nuclei by meiosis. The ascospores are then released,
germinate, and form hyphae that are disseminated in the
environment and start new mycelia.

Figure 24.3C. 1 : Lifecycle of an ascomycete: The lifecycle of an

ascomycete is characterized by the production of asci during the
sexual phase. The haploid phase is the predominant phase of the life
cycle.

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KEY POINTS ascus: a sac-shaped cell present in ascomycete fungi; it is a
Ascomycota fungi are the yeasts used in baking, brewing, and reproductive cell in which meiosis and an additional cell division
wine fermentation, plus delicacies such as truffles and morels. produce eight spores
Ascomycetes are filamentous and produce hyphae divided by ascospore: a sexually-produced spore from the ascus of an
perforated septa. Ascomycetes fungus
Ascomycetes frequently reproduce asexually which leads to the conidia: asexual, non-motile spores of a fungus, named after the
production of conidiophores that release haploid conidiospores. Greek word for dust, conia; also known as conidiospores and
Two types of mating strains, a “male” strain which produces an mitospores
antheridium and a “female” strain which develops an antheridia: a haploid structure or organ producing and
ascogonium, are required for sexual reproduction. containing male gametes (called antherozoids or sperm) present
The antheridium and the ascogonium combine in plasmogamy at in lower plants like mosses and ferns, primitive vascular
the time of fertilization, followed by nuclei fusion in the asci. psilotophytes, and fungi
In the ascocarp, a fruiting body, thousands of asci undergo ascogonium: a haploid structure or organ producing and
meiosis to generate haploid ascospores ready to be released to containing female gametes in certain Ascomycota fungi
the world. ascocarp: the sporocarp of an ascomycete, typically bowl-
shaped
KEY TERMS ascomycete: any fungus of the phylum Ascomycota,
plasmogamy: stage of sexual reproduction joining the cytoplasm characterized by the production of a sac, or ascus, which contains
of two parent mycelia without the fusion of nuclei non-motile spores
Ascomycota: a taxonomic division within the kingdom Fungi;
This page titled 24.3C: Ascomycota - The Sac Fungi is shared under a CC
those fungi that produce spores in a microscopic sporangium
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
called an ascus

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24.3D: BASIDIOMYCOTA- THE CLUB FUNGI
The basidiomycota are mushroom-producing fungi with developing, basidiomycetes produce deadly toxins. For example, Cryptococcus
club-shaped fruiting bodies called basidia on the gills under its cap. neoformans causes severe respiratory illness.

 LEARNING OBJECTIVES

Describe the ecology and reproduction of the Basidiomycota

KEY POINTS
The majority of edible fungi belong to the Phylum
Basidiomycota.
The basidiomycota includes shelf fungus, toadstools, and smuts
and rusts.
Unlike most fungi, basidiomycota reproduce sexually as opposed
to asexually.
Two different mating strains are required for the fusion of genetic Figure 24.3D. 1 : Fruiting bodies of a basidiomycete: The fruiting
bodies of a basidiomycete form a ring in a meadow, commonly
material in the basidium which is followed by meiosis producing called “fairy ring.” The best-known fairy ring fungus has the
haploid basidiospores. scientific name Marasmius oreades. The body of this fungus, its
Mycelia of different mating strains combine to produce a mycelium, is underground and grows outward in a circle. As it
grows, the mycelium depletes the soil of nitrogen, causing the
secondary mycelium that contains haploid basidiospores in what mycelia to grow away from the center, leading to the “fairy ring” of
is called the dikaryotic stage, where the fungi remains until a fruiting bodies where there is adequate soil nitrogen.
basidiocarp (mushroom) is generated with the developing basidia The lifecycle of basidiomycetes includes alternation of generations.
on the gills under its cap. Spores are generally produced through sexual reproduction, rather
than asexual reproduction. The club-shaped basidium carries spores
KEY TERMS called basidiospores. In the basidium, nuclei of two different mating
basidiocarp: a fruiting body that protrudes from the ground, strains fuse (karyogamy), giving rise to a diploid zygote that then
known as a mushroom, which has a developing basidia on the undergoes meiosis. The haploid nuclei migrate into basidiospores,
gills under its cap which germinate and generate monokaryotic hyphae. The mycelium
basidiomycete: a fungus of the phylum Basidiomycota, which that results is called a primary mycelium. Mycelia of different
produces sexual spores on a basidium mating strains can combine and produce a secondary mycelium that
Basidiomycota: a taxonomic division within the kingdom Fungi: contains haploid nuclei of two different mating strains. This is the
30,000 species of fungi that produce spores from a basidium dikaryotic stage of the basidiomyces lifecyle and it is the dominant
basidium: a small structure, shaped like a club, found in the stage. Eventually, the secondary mycelium generates a basidiocarp,
Basidiomycota phylum of fungi, that bears four spores at the tips which is a fruiting body that protrudes from the ground; this is what
of small projections we think of as a mushroom. The basidiocarp bears the developing
basidiospore: a sexually-reproductive spore produced by fungi basidia on the gills under its cap.
of the phylum Basidiomycota

BASIDIOMYCOTA: THE CLUB FUNGI

The fungi in the Phylum Basidiomycota are easily recognizable
under a light microscope by their club-shaped fruiting bodies called
basidia (singular, basidium), which are the swollen terminal cell of a
hypha. The basidia, which are the reproductive organs of these
fungi, are often contained within the familiar mushroom, commonly
seen in fields after rain, on the supermarket shelves, and growing on
your lawn. These mushroom-producing basidiomyces are sometimes
referred to as “gill fungi” because of the presence of gill-like
structures on the underside of the cap. The “gills” are actually
compacted hyphae on which the basidia are borne. This group also
includes shelf fungus, which cling to the bark of trees like small
shelves. In addition, the basidiomycota includes smuts and rusts,
which are important plant pathogens, and toadstools. Most edible
fungi belong to the Phylum Basidiomycota; however, some

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 This page titled 24.3D: Basidiomycota- The Club Fungi is shared under a
CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Boundless.

Figure 24.3D. 1 : Lifecycle of a basidiomycete: The lifecycle of a

basidiomycete alternates generation with a prolonged stage in which
two nuclei (dikaryon) are present in the hyphae.

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24.3E: DEUTEROMYCOTA - THE IMPERFECT FUNGI
recombine and form heterokaryotic hyphae. Genetic recombination
 LEARNING OBJECTIVES is known to take place between the different nuclei.
Imperfect fungi have a large impact on everyday human life. The
Describe the ecology and reproduction of the
Deuteromycota food industry relies on them for ripening some cheeses. The blue
veins in Roquefort cheese and the white crust on Camembert are the
result of fungal growth. The antibiotic penicillin was originally
Imperfect fungi are those that do not display a sexual phase. They
discovered on an overgrown Petri plate on which a colony of
are classified as belonging to the form Phylum Deuteromycota.
Penicillium fungi killed the bacterial growth surrounding it. Many
Deuteromycota is a polyphyletic group where many species are
imperfect fungi cause serious diseases, either directly as parasites
more closely related to organisms in other phyla than to each other;
(which infect both plants and humans), or as producers of potent
hence it cannot be called a true phylum and must, instead, be given
toxic compounds, as seen in the aflatoxins released by fungi of the
the name form phylum. Since they do not possess the sexual
genus Aspergillus.
structures that are used to classify other fungi, they are less well
described in comparison to other divisions. Most members live on
KEY POINTS
land, with a few aquatic exceptions. They form visible mycelia with
Deuteromycota do not possess the sexual structures that are used
a fuzzy appearance and are commonly known as mold. Molecular
to classify other fungi.
analysis shows that the closest group to the deuteromycetes is the
Most deuteromycota live on land; they form visible mycelia with
ascomycetes. In fact, some species, such as Aspergillus, which were
a fuzzy appearance called mold.
once classified as imperfect fungi, are now classified as
Recombination of genetic material is known to take place
ascomycetes.
between the different nuclei after some hyphae recombine.

KEY TERMS
deuteromycete: an organism of the phylum Deuteromycota
Deuteromycota: a taxonomic morphological group within the
kingdom Fungi; the fungi have no sexual reproduction
polyphyletic: having multiple ancestral sources; referring to a
taxon that does not contain the most recent common ancestor of
its members
conidiospore: a unicellular spore produced asexually by a
fungus
Figure 24.3E. 1: Example of an imperfect fungus: Aspergillus niger
is an imperfect fungus commonly found as a food contaminant. The This page titled 24.3E: Deuteromycota - The Imperfect Fungi is shared
spherical structure in this light micrograph is a conidiophore. under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
Reproduction of Deuteromycota is strictly asexual, occuring mainly by Boundless.
by production of asexual conidiospores. Some hyphae may

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24.3F: GLOMEROMYCOTA
KEY POINTS
 LEARNING OBJECTIVES Most glomeromycetes form arbuscular mycorrhizae, a type of
symbiotic relationship between a fungus and plant roots; the
Describe the ecology and reproduction of Glomeromycetes
plants supply a source of energy to the fungus while the fungus
supplies essential minerals to the plant.
In the kingdom Fungi, the Glomeromycota is a newly-established
Glomeromycota that have arbuscular mycorrhizal are mostly
phylum comprised of about 230 species that live in close association
terrestrial, but can also be found in wetlands.
with the roots of trees and plants. Fossil records indicate that trees
The glomeromycetes reproduce asexually by producing
and their root symbionts share a long evolutionary history. It appears
glomerospores and cannot survive without the presence of plant
that most members of this family form arbuscular mycorrhizae: the
roots.
hyphae interact with the root cells forming a mutually-beneficial
DNA analysis shows that all glomeromycetes probably
association where the plants supply the carbon source and energy in
descended from a common ancestor 462 and 353 million years
the form of carbohydrates to the fungus while the fungus supplies
ago.
essential minerals from the soil to the plant. This association is
The classification of fungi as Glomeromycota has been redefined
termed biotrophic. The Glomeromycota species that have arbuscular
with adoption of molecular techniques.
mycorrhizal are terrestrial and widely distributed in soils worldwide
where they form symbioses with the roots of the majority of plant KEY TERMS
species. They can also be found in wetlands, including salt-marshes,
biotrophic: describing a parasite that needs its host to stay alive
and are associated with epiphytic plants.
arbuscular mycorrhizae: a type of symbiotic relationship
between a fungus and the roots of a plant where the plants supply
a source of energy to the fungus while the fungus supplies
essential minerals to the plant
glomeromycete: an organism of the phylum Glomeromycota

LICENSES AND ATTRIBUTIONS

CC LICENSED CONTENT, SHARED PREVIOUSLY
OpenStax College, Biology. October 17, 2013. Provided by:
OpenStax CNX. Located at:
http://cnx.org/content/m44625/latest...ol11448/latest. License:
Figure 24.3F . 1 : Glyomeromycetes and tree roots: This image
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illustrates the bitrophic relationship between a glomeromycota OpenStax College, Biology. November 15, 2013. Provided by:
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and reproduce asexually, producing glomerospores. The biochemical coenocytic. Provided by: Wikipedia. Located at:
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scientists were able to estimate the time of divergence of the fungi. Attribution-ShareAlike
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SECTION OVERVIEW

24.4: FUNGAL PARASITES AND PATHOGENS

24.4A: FUNGI AS PLANT, ANIMAL, AND HUMAN
Topic hierarchy PATHOGENS

This page titled 24.4: Fungal Parasites and Pathogens is shared under a CC
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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24.4A: FUNGI AS PLANT, ANIMAL, AND HUMAN PATHOGENS
From crop and food spoilage to severe infections in animal species, downy mildew are other examples of common fungal pathogens that
fungal parasites and pathogens are wide spread and difficult to treat. affect crops.

 LEARNING OBJECTIVES

Give examples of fungi that are plant and animal parasites

and pathogens

KEY POINTS
In plants, fungi can destroy plant tissue directly or through the
production of potent toxins, which usually ends in host death and
can even lead to ergotism in animals like humans.
During mycosis, fungi, like dermatophytes, successfully attack
hosts directly by colonizing and destroying their tissues.
Examples of fungal parasites and pathogens in animals that cause
mycoses include Batrachochytrium dendrobatidis, Geomyces
destructans, and Histoplasma capsulatum.
Systemic mycoses, such as valley fever, Histoplasmosis, or
pulmonary disease, are fungal diseases that spread to internal
organs and commonly enter the body through the respiratory Figure 24.4A. 1 : Fungal pathogens: Some fungal pathogens include
(a) green mold on grapefruit, (b) powdery mildew on a zinnia, (c)
system.
stem rust on a sheaf of barley, and (d) grey rot on grapes.
Opportunistic mycoses, fungal infections that are common in all
Aflatoxins are toxic, carcinogenic compounds released by fungi of
environments, mainly take advantage of individuals who have a
the genus Aspergillus. Periodically, harvests of nuts and grains are
compromised immune system, such as AIDS patients.
tainted by aflatoxins, leading to massive recall of produce. This
Fungi can also cause mycetismus, a disease caused by the
sometimes ruins producers and causes food shortages in developing
ingestion of toxic mushrooms that leads to poisoning.
countries.
KEY TERMS ANIMAL AND HUMAN PARASITES AND
mycosis: a fungal disease caused by infection and direct damage PATHOGENS
dermatophyte: a parasitic fungus that secretes extracellular
Fungi can affect animals, including humans, in several ways. A
enzymes that break down keratin, causing infections the skin,
mycosis is a fungal disease that results from infection and direct
such as jock itch and athlete’s foot
damage. Fungi attack animals directly by colonizing and destroying
aflatoxin: toxic, carcinogenic compounds released by fungi of
tissues. Mycotoxicosis is the poisoning of humans (and other
the genus Aspergillus; contaminate nut and grain harvests
animals) by foods contaminated by fungal toxins (mycotoxins).
ergot: any fungus in the genus Claviceps which are parasitic on
Mycetismus describes the ingestion of preformed toxins in
grasses
poisonous mushrooms. In addition, individuals who display
FUNGAL PARASITES AND PATHOGENS hypersensitivity to molds and spores develop strong and dangerous
allergic reactions. Fungal infections are generally very difficult to
The production of sufficient good-quality crops is essential to human
treat because, unlike bacteria, fungi are eukaryotes. Antibiotics only
existence. Plant diseases have ruined crops, bringing widespread
target prokaryotic cells, whereas compounds that kill fungi also
famine. Many plant pathogens are fungi that cause tissue decay and
harm the eukaryotic animal host.
eventual death of the host. In addition to destroying plant tissue
directly, some plant pathogens spoil crops by producing potent Many fungal infections are superficial; that is, they occur on the
toxins. Fungi are also responsible for food spoilage and the rotting animal’s skin. Termed cutaneous (“skin”) mycoses, they can have
of stored crops. For example, the fungus Claviceps purpurea causes devastating effects. For example, the decline of the world’s frog
ergot, a disease of cereal crops (especially of rye). Although the population in recent years may be caused by the fungus
fungus reduces the yield of cereals, the effects of the ergot’s alkaloid Batrachochytrium dendrobatidis, which infects the skin of frogs and
toxins on humans and animals are of much greater significance. In presumably interferes with gaseous exchange. Similarly, more than a
animals, the disease is referred to as ergotism. The most common million bats in the United States have been killed by white-nose
signs and symptoms are convulsions, hallucinations, gangrene, and syndrome, which appears as a white ring around the mouth of the
loss of milk in cattle. The active ingredient of ergot is lysergic acid, bat. It is caused by the cold-loving fungus Geomyces destructans,
which is a precursor of the drug LSD. Smuts, rusts, and powdery or which disseminates its deadly spores in caves where bats hibernate.

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Mycologists are researching the transmission, mechanism, and Opportunistic mycoses are fungal infections that are either common
control of G. destructans to stop its spread. in all environments or are part of the normal biota. They mainly
Fungi that cause the superficial mycoses of the epidermis, hair, and affect individuals who have a compromised immune system.
nails rarely spread to the underlying tissue. These fungi are often Patients in the late stages of AIDS suffer from opportunistic
misnamed “dermatophytes”, from the Greek words dermis meaning mycoses that can be life threatening. The yeast Candida sp., a
skin and phyte meaning plant, although they are not plants. common member of the natural biota, can grow unchecked and
Dermatophytes are also called “ringworms” because of the red ring infect the vagina or mouth (oral thrush) if the pH of the surrounding
they cause on skin. They secrete extracellular enzymes that break environment, the person’s immune defenses, or the normal
down keratin (a protein found in hair, skin, and nails), causing population of bacteria are altered.
conditions such as athlete’s foot and jock itch. These conditions are Mycetismus can occur when poisonous mushrooms are eaten. It
usually treated with over-the-counter topical creams and powders; causes a number of human fatalities during mushroom-picking
they are easily cleared. More persistent superficial mycoses may season. Many edible fruiting bodies of fungi resemble highly-
require prescription oral medications. poisonous relatives. Amateur mushroom hunters are cautioned to
carefully inspect their harvest and avoid eating mushrooms of
doubtful origin.

CONTRIBUTIONS AND ATTRIBUTIONS

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Figure 24.4A. 1 : Mycosis infection: (a) Ringworm presents as a red Located at: http://cnx.org/content/m44628/latest...ol11448/latest. License: CC
ring on skin; (b) Trichophyton violaceum, shown in this bright field BY: Attribution
light micrograph, causes superficial mycoses on the scalp; (c) dermatophyte. Provided by: Wiktionary. Located at:
Histoplasma capsulatum is an ascomycete that infects airways and en.wiktionary.org/wiki/dermatophyte. License: CC BY-SA: Attribution-
causes symptoms similar to influenza. ShareAlike
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Systemic mycoses spread to internal organs, most commonly en.wiktionary.org/wiki/mycosis. License: CC BY-SA: Attribution-ShareAlike
entering the body through the respiratory system. For example, aflatoxin. Provided by: Wiktionary. Located at:
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SECTION OVERVIEW

24.5: IMPORTANCE OF FUNGI IN HUMAN LIFE

24.5A: IMPORTANCE OF FUNGI IN HUMAN LIFE
Topic hierarchy
This page titled 24.5: Importance of Fungi in Human Life is shared under a
CC BY-SA 4.0 license and was authored, remixed, and/or curated by
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24.5A: IMPORTANCE OF FUNGI IN HUMAN LIFE
Fungi play important roles in many aspects of human life, including
medicine, food, and farming.

 LEARNING OBJECTIVES

Explain the important roles fungi play in human life

KEY POINTS
The majority of grasses and trees require a mycorrhizal
relationship with fungi to survive.
Yeasts have been used for thousands of years in the production of
beer, wine, and bread.
Fungi not only directly produce substances that humans use as Figure 24.5A. 1 : Fungal Insecticides: The emerald ash borer is an
insect that attacks ash trees. It is in turn parasitized by a pathogenic
medicine, but they are also versatile tools in the vast field of fungus that holds promise as a biological insecticide. The parasitic
medical research. fungus appears as white fuzz on the body of the insect.
Some fungi attack insects and, therefore, can be used as natural
pesticides. FARMING
The mycorrhizal relationship between fungi and plant roots is
KEY TERMS essential for the productivity of farm land. Without the fungal
inoculant: the active material used in an inoculation partner in root systems, 80–90 percent of trees and grasses would
ergot: any fungus in the genus Claviceps which are parasitic on not survive. Mycorrhizal fungal inoculants are available as soil
grasses additives from gardening supply stores and are promoted by
immunosuppressant: capable of immunosuppression, or the supporters of organic agriculture.
reduction of immune system efficacy
FOOD
IMPORTANCE OF FUNGI IN HUMAN LIFE Fungi figure prominently in the human diet. Morels, shiitake
Although we often think of fungi as organisms that cause disease mushrooms, chanterelles, and truffles are considered delicacies. The
and rot food, fungi are important to human life on many levels. They meadow mushroom, Agaricus campestris, appears in many dishes.
influence the well-being of human populations on a large scale Molds of the genus Penicilliumripen many cheeses. They originate
because they are part of the nutrient cycle in ecosystems. They also in the natural environment such as the caves of Roquefort, France,
have other ecosystem uses, such as pesticides. where wheels of sheep milk cheese are stacked to capture the molds
responsible for the blue veins and pungent taste of the cheese.
BIOLOGICAL INSECTICIDES
As animal pathogens, fungi help to control the population of
damaging pests. These fungi are very specific to the insects they
attack; they do not infect animals or plants. Fungi are currently
under investigation as potential microbial insecticides, with several
already on the market. For example, the fungus Beauveria bassiana
is a pesticide being tested as a possible biological control agent for
the recent spread of emerald ash borer.

Figure 24.5A. 1 : Morel mushroom: The morel mushroom is an

ascomycete much appreciated for its delicate taste.

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Fermentation of grains to produce beer and of fruits to produce wine bleeding. Psilocybin is a compound found in fungi such as Psilocybe
is an ancient art that humans in most cultures have practiced for semilanceata and Gymnopilus junonius, which have been used for
millennia. Ancient humans acquired wild yeasts from the their hallucinogenic properties by various cultures for thousands of
environment and used them to ferment sugars into CO2 and ethanol years.
under anaerobic conditions. It is now possible to purchase isolated As simple eukaryotic organisms, fungi are important model research
strains of wild yeasts from different wine-making regions. Louis organisms. Many advances in modern genetics were achieved by the
Pasteur was instrumental in developing a reliable strain of brewer’s use of the red bread mold Neurospora crassa. Additionally, many
yeast, Saccharomyces cerevisiae, for the French brewing industry in important genes originally discovered in S. cerevisiae served as a
the late 1850s. starting point in discovering analogous human genes. As a
Saccharomyces cerevisiae, also know as baker’s yeast, is an eukaryotic organism, the yeast cell produces and modifies proteins
important ingredient in bread, a food that has been considered a in a manner similar to human cells, as opposed to the bacterium
staple of human life for thousands of years. Before isolated yeast Escherichia coli, which lacks the internal membrane structures and
became available in modern times, humans simply let the dough enzymes to tag proteins for export. This makes yeast a much better
collect yeast from the air and rise over a period of hours or days. A organism for use in recombinant DNA technology experiments. Like
small piece of this leavened dough was saved and used as a starter bacteria, yeasts grow easily in culture, have a short generation time,
(source of the same yeast) for the next batch, much in the same way and are amenable to genetic modification.
sourdough bread is made today.
CONTRIBUTIONS AND ATTRIBUTIONS
OpenStax College, Biology. October 22, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44629/latest...ol11448/latest. License: CC
BY: Attribution
Bread. Provided by: Wikibooks. Located at: en.wikibooks.org/wiki/Bread.
License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Biology. November 14, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44629/latest...ol11448/latest. License: CC
BY: Attribution
immunosuppressant. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/immunosuppressant. License: CC BY-SA: Attribution-
ShareAlike
ergot. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/ergot.
Figure 24.5A. 1 : Saccharomyces cerevisiae: The yeast License: CC BY-SA: Attribution-ShareAlike
Saccharomyces cerevisiae is approximately 5 µm in diameter and is inoculant. Provided by: Wiktionary. Located at:
important for the production of wine, beer, and bread. The yeast also en.wiktionary.org/wiki/inoculant. License: CC BY-SA: Attribution-ShareAlike
has many applications in medical research. OpenStax College, Biology. November 14, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44629/latest...ol11448/latest. License: CC
BY: Attribution
MEDICINE Cell Biology/Models/Yeasts. Provided by: Wikibooks. Located at:
Many secondary metabolites of fungi are of great commercial en.wikibooks.org/wiki/Cell_Bi.../Models/Yeasts. License: CC BY-SA:
Attribution-ShareAlike
importance. Fungi naturally produce antibiotics to kill or inhibit the OpenStax College, Biology. November 14, 2013. Provided by: OpenStax CNX.
growth of bacteria, limiting their competition in the natural Located at: http://cnx.org/content/m44629/latest...ol11448/latest. License: CC
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environment. Important antibiotics, such as penicillin and the
cephalosporins, can be isolated from fungi. Valuable drugs isolated This page titled 24.5A: Importance of Fungi in Human Life is shared under
from fungi include the immunosuppressant drug cyclosporine a CC BY-SA 4.0 license and was authored, remixed, and/or curated by
(which reduces the risk of rejection after organ transplant), the Boundless.
precursors of steroid hormones, and ergot alkaloids used to stop

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 CHAPTER OVERVIEW

25: SEEDLESS PLANTS

Topic hierarchy
25.1: Early Plant Life
25.1A: Early Plant Life
25.1B: Evolution of Land Plants
25.1C: Plant Adaptations to Life on Land
25.1D: Sporophytes and Gametophytes in Seedless Plants
25.1E: Structural Adaptations for Land in Seedless Plants
25.1F: The Major Divisions of Land Plants
25.2: Green Algae- Precursors of Land Plants
25.2A: Streptophytes and Reproduction of Green Algae
25.2B: Charales
25.3: Bryophytes
25.3A: Bryophytes
25.3B: Liverworts and Hornworts
25.3C: Mosses
25.4: Seedless Vascular Plants
25.4A: Seedless Vascular Plants
25.4B: Vascular Tissue- Xylem and Phloem
25.4C: The Evolution of Roots in Seedless Plants
25.4D: Ferns and Other Seedless Vascular Plants
25.4E: The Importance of Seedless Vascular Plants

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1
SECTION OVERVIEW

25.1: EARLY PLANT LIFE

25.1D: SPOROPHYTES AND GAMETOPHYTES IN
Topic hierarchy SEEDLESS PLANTS

25.1E: STRUCTURAL ADAPTATIONS FOR LAND

25.1A: EARLY PLANT LIFE
IN SEEDLESS PLANTS
25.1B: EVOLUTION OF LAND PLANTS
25.1F: THE MAJOR DIVISIONS OF LAND PLANTS
25.1C: PLANT ADAPTATIONS TO LIFE ON LAND
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license and was authored, remixed, and/or curated by Boundless.

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25.1A: EARLY PLANT LIFE
A diverse array of seedless plants still populate and thrive in the
world today, particularly in moist environments.

 LEARNING OBJECTIVES

Describe the pervasiveness of seedless plants during the

history of the kingdom Plantae

KEY POINTS
Non-vascular seedless plants, or bryophytes, are the group of
plants that are the closest extant relative of early terrestrial
plants.
The vast majority of terrestrial plants today are seed plants,
which tend to be better adapted to the arid land environment. Figure 25.1A. 1 : Horsetails are seedless plants: Seedless plants, like
these horsetails (Equisetumsp.), thrive in damp, shaded
Seedless plants are classified into three main catagories: green environments under a tree canopy where dryness is rare.
algae, seedless non- vascular plants, and seedless vascular plants.
Current evolutionary thought holds that all plants, green algae as
KEY TERMS well as land dwellers, are monophyletic; that is, they are descendants
of a single common ancestor. The evolutionary transition from water
vascular plant: any plant possessing vascular tissue (xylem and
to land imposed severe constraints on plants. They had to develop
phloem), including ferns, conifers, and flowering plants
strategies: to avoid drying out, to disperse reproductive cells in air,
bryophyte: seedless, nonvascular plants that are the closest
for structural support, and for capturing and filtering sunlight. While
extant relative of early terrestrial plants
seed plants developed adaptations that allowed them to populate
INTRODUCTION TO EARLY PLANT LIFE even the most arid habitats on Earth, full independence from water
did not happen in all plants. Most seedless plants still require a moist
An incredible variety of seedless plants populates the terrestrial
environment.
landscape. Mosses may grow on a tree trunk and horsetails may
display their jointed stems and spindly leaves across the forest floor. Seedless plants are classified into three main categories: green algae,
Today, however, seedless plants represent only a small fraction of seedless non-vascular plants, and seedless vascular plants. Seedless
the plants in our environment. The kingdom Plantae constitutes a non-vascular plants (bryophytes), such as mosses, are the group of
large and varied group of organisms with more than 300,000 species plants that are the closest extant relative of early terrestrial plants.
of cataloged plants. Of these, more than 260,000 are seed plants. Seedless vascular plants include horsetails and ferns.
However, three hundred million years ago, seedless plants
This page titled 25.1A: Early Plant Life is shared under a CC BY-SA 4.0
dominated the landscape and grew in the enormous swampy forests
license and was authored, remixed, and/or curated by Boundless.
of the Carboniferous period. Their decomposition created large
deposits of coal that we mine today.

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25.1B: EVOLUTION OF LAND PLANTS
The geologic periods of the Paleozoic are marked by changes in the
plant life that inhabited the earth.

 LEARNING OBJECTIVES

Summarize the development of adaptations in land plants

KEY POINTS
Land plants first appeared during the Ordovician period, more
than 500 million years ago.
The evolution of plants occurred by a stepwise development of
physical structures and reproductive mechanisms such as
vascular tissue, seed production, and flowering.
Paleobotonists trace the evolution of plant morphology through a
study of the fossil record in the context of the surrounding Figure 25.1B. 1: The Rhynie chert sedimentary rock deposit: This
geological sediments. Rhynie chert contains fossilized material from vascular plants. The
area inside the circle contains bulbous underground stems called
corms and root-like structures called rhizoids.
KEY TERMS
image
Paleobotany: the branch of paleontology or paleobiology
dealing with the recovery and identification of plant remains Gradual evolution of land plants: The adaptation of plants to life
from geological contexts on land occurred gradually through the stepwise development of
mycorrhiza: a symbiotic association between a fungus and the physical structures and reproduction mechanisms
roots of a vascular plant How organisms acquired traits that allow them to colonize new
environments, and how the contemporary ecosystem is shaped, are
EVOLUTION OF LAND PLANTS fundamental questions of evolution. Paleobotany (the study of
No discussion of the evolution of plants on land can be undertaken extinct plants) addresses these questions through the analysis of
without a brief review of the timeline of the geological eras. The fossilized specimens retrieved from field studies, reconstituting the
early era, known as the Paleozoic, is divided into six periods. It morphology of organisms that disappeared long ago. Paleobotanists
starts with the Cambrian period, followed by the Ordovician, trace the evolution of plants by following the modifications in plant
Silurian, Devonian, Carboniferous, and Permian. The major event to morphology, which sheds light on the connection between existing
mark the Ordovician, more than 500 million years ago, was the plants by identifying common ancestors that display the same traits.
colonization of land by the ancestors of modern land plants. This field seeks to find transitional species that bridge gaps in the
Fossilized cells, cuticles, and spores of early land plants have been path to the development of modern organisms. Paleobotanists collect
dated as far back as the Ordovician period in the early Paleozoic era. fossil specimens in the field and place them in the context of the
The evolution of plants occurred by a gradual development of novel geological sediments and other fossilized organisms surrounding
structures and reproduction mechanisms. Embryo protection them.
developed prior to the development of vascular plants which, in turn, Paleobotanists distinguish between extinct species, as fossils, and
evolved before seed plants and flowering plants. The oldest-known extant species, which are still living. The extinct vascular plants,
vascular plants have been identified in deposits from the Devonian. classified as zosterophylls and trimerophytes, most probably lacked
One of the richest sources of information is the Rhynie chert, a true leaves and roots, forming low vegetation mats similar in size to
sedimentary rock deposit found in Rhynie, Scotland, where modern-day mosses, although some trimetophytes could reach one
embedded fossils of some of the earliest vascular plants have been meter in height. The later genus Cooksonia, which flourished during
identified. the Silurian, has been extensively studied from well-preserved
examples. Imprints of Cooksonia show slender, branching stems
ending in what appear to be sporangia. From the recovered
specimens, it is not possible to establish for certain whether
Cooksoniapossessed vascular tissues. Fossils indicate that by the end
of the Devonian period, ferns, horsetails, and seed plants populated
the landscape, giving rising to trees and forests. This luxuriant
vegetation helped enrich the atmosphere in oxygen, making it easier
for air-breathing animals to colonize dry land. Plants also
established early symbiotic relationships with fungi, creating
mycorrhizae: a relationship in which the fungal network of filaments

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increases the efficiency of the plant root system. The plants provide This page titled 25.1B: Evolution of Land Plants is shared under a CC BY-
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25.1C: PLANT ADAPTATIONS TO LIFE ON LAND
Plants adapted to the dehydrating land environment through the before land animals; therefore, until dry land was also colonized by
development of new physical structures and reproductive animals, no predators threatened plant life. This situation changed as
mechanisms. animals emerged from the water and fed on the abundant sources of
nutrients in the established flora. In turn, plants developed strategies
 LEARNING OBJECTIVES to deter predation: from spines and thorns to toxic chemicals.
Early land plants, like the early land animals, did not live far from an
Discuss how lack of water in the terrestrial environment led
abundant source of water and developed survival strategies to
to significant adaptations in plants
combat dryness. One of these strategies is called desiccation
tolerance. Many mosses can dry out to a brown and brittle mat, but
KEY POINTS as soon as rain or a flood makes water available, mosses will absorb
While some plants remain dependent on a moist and humid it and are restored to their healthy green appearance. Another
environment, many have adapted to a more arid climate by strategy is to colonize environments where droughts are uncommon.
developing tolerance or resistance to drought conditions. Ferns, which are considered an early lineage of plants, thrive in
Alternation of generations describes a life cycle in which an damp and cool places such as the understory of temperate forests.
organism has both haploid (1n) and diploid (2n) multicellular Later, plants moved away from moist or aquatic environments and
stages, although in different species the haploid or diploid stage developed resistance to desiccation, rather than tolerance. These
can be dominant. plants, like cacti, minimize the loss of water to such an extent they
The life on land presents significant challenges for plants, can survive in extremely dry environments.
including the potential for desiccation, mutagenic radiation from The most successful adaptation solution was the development of
the sun, and a lack of buoyancy from the water. new structures that gave plants the advantage when colonizing new
and dry environments. Four major adaptations are found in all
KEY TERMS
terrestrial plants: the alternation of generations, a sporangium in
desiccation tolerance: the ability of an organism to withstand or which the spores are formed, a gametangium that produces haploid
endure extreme dryness, or drought-like condition
cells, and apical meristem tissue in roots and shoots. The evolution
alternation of generation: the life cycle of plants with a
of a waxy cuticle and a cell wall with lignin also contributed to the
multicellular sporophyte, which is diploid, that alternates with a
success of land plants. These adaptations are noticeably lacking in
multicellular gametophyte, which is haploid
the closely-related green algae, which gives reason for the debate
PLANT ADAPTATIONS TO LIFE ON LAND over their placement in the plant kingdom.

As organisms adapted to life on land, they had to contend with ALTERNATION OF GENERATIONS
several challenges in the terrestrial environment. The cell ‘s interior
Alternation of generations describes a life cycle in which an
is mostly water: in this medium, small molecules dissolve and
organism has both haploid and diploid multicellular stages (n
diffuse and the majority of the chemical reactions of metabolism
represents the number of copies of chromosomes). Haplontic refers
take place. Desiccation, or drying out, is a constant danger for to a lifecycle in which there is a dominant haploid stage (1n), while
organisms exposed to air. Even when parts of a plant are close to a diplontic refers to a lifecycle in which the diploid (2n) is the
source of water, the aerial structures are prone to desiccation. Water
dominant life stage. Humans are diplontic. Most plants exhibit
also provides buoyancy to organisms. On land, plants need to alternation of generations, which is described as haplodiplodontic.
develop structural support in a medium that does not give the same
The haploid multicellular form, known as a gametophyte, is
lift as water. The organism is also subject to bombardment by followed in the development sequence by a multicellular diploid
mutagenic radiation because air does not filter out the ultraviolet
organism: the sporophyte. The gametophyte gives rise to the
rays of sunlight. Additionally, the male gametes must reach the
gametes (reproductive cells) by mitosis. This can be the most
female gametes using new strategies because swimming is no longer obvious phase of the life cycle of the plant, as in the mosses. In fact,
possible. As such, both gametes and zygotes must be protected from
the sporophyte stage is barely noticeable in lower plants (the
desiccation. Successful land plants have developed strategies to face collective term for the plant groups of mosses, liverworts, and
all of these challenges. Not all adaptations appeared at once; some
lichens). Alternatively, the gametophyte stage can occur in a
species never moved very far from the aquatic environment,
microscopic structure, such as a pollen grain, in the higher plants (a
although others went on to conquer the driest environments on
common collective term for the vascular plants). Towering trees are
Earth.
the diplontic phase in the life cycles of plants such as sequoias and
Despite these survival challenges, life on land does offer several pines.
advantages. First, sunlight is abundant. Water acts as a filter, altering
the spectral quality of light absorbed by the photosynthetic pigment
chlorophyll. Second, carbon dioxide is more readily available in air
than water since it diffuses faster in air. Third, land plants evolved

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 Protection of the embryo is a major requirement for land plants. The
vulnerable embryo must be sheltered from desiccation and other
environmental hazards. In both seedless and seed plants, the female
gametophyte provides protection and nutrients to the embryo as it
develops into the new generation of sporophyte. This distinguishing
feature of land plants gave the group its alternate name of
embryophytes.

This page titled 25.1C: Plant Adaptations to Life on Land is shared under a
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Boundless.

Figure 25.1C. 1 : Alternation of generations of plants: Plants exhibit

an alternation of generations between a 1n gametophyte and 2n
sporophyte.

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25.1D: SPOROPHYTES AND GAMETOPHYTES IN SEEDLESS PLANTS
Sporophytes (2n) undergo meiosis to produce spores that develop
into gametophytes (1n) which undergo mitosis.

 LEARNING OBJECTIVES

Describe the role of the sporophyte and gametophyte in

plant reproduction

KEY POINTS
The diploid stage of a plant (2n), the sporophyte, bears a
sporangium, an organ that produces spores during meiosis.
Homosporous plants produce one type of spore which develops
into a gametophyte (1n) with both male and female organs.
Heterosporous plants produce separate male and female
gametophytes, which produce sperm and eggs, respectively.
In seedless plants, male gametangia (antheridium) release sperm,
Figure 25.1D. 1 : Sporangia: Spore-producing sacs called sporangia
which can then swim to and fertilize an egg at the female grow at the ends of long, thin stalks in this photo of the moss
gametangia (archegonia); this mode of reproduction is replaced Esporangios bryum.
by pollen production in seed plants. Two different spore-forming methods are used in land plants,
resulting in the separation of sexes at different points in the
KEY TERMS lifecycle. Seedless, non- vascular plants produce only one kind of
gametophyte: a plant (or the haploid phase in its life cycle) that spore and are called homosporous. The gametophyte phase (1n) is
produces gametes by mitosis in order to produce a zygote dominant in these plants. After germinating from a spore, the
gametangium: an organ or cell in which gametes are produced resulting gametophyte produces both male and female gametangia,
that is found in many multicellular protists, algae, fungi, and the usually on the same individual. In contrast, heterosporous plants
gametophytes of plants produce two morphologically different types of spores. The male
sporopollenin: a combination of biopolymers observed in the spores are called microspores, because of their smaller size, and
tough outer layer of the spore and pollen wall develop into the male gametophyte; the comparatively larger
syngamy: the fusion of two gametes to form a zygote megaspores develop into the female gametophyte. Heterospory is
sporophyte: a plant (or the diploid phase in its life cycle) that observed in a few seedless vascular plants and in all seed plants.
produces spores by meiosis in order to produce gametophytes

SPORANGIA IN SEEDLESS PLANTS

The sporophyte of seedless plants is diploid and results from
syngamy (fusion) of two gametes. The sporophyte bears the
sporangia (singular, sporangium): organs that first appeared in the
land plants. The term “sporangia” literally means “spore in a
vessel”: it is a reproductive sac that contains spores. Inside the
multicellular sporangia, the diploid sporocytes, or mother cells, Figure 25.1D. 1 : Lifecycle of heterosporous plants: Heterosporous
produce haploid spores by meiosis, where the 2n chromosome plants produce two morphologically different types of spores:
number is reduced to 1n (note that many plant sporophytes are microspores, which develop into the male gametophyte, and
megaspores, which develop into the female gametophyte.
polyploid: for example, durum wheat is tetraploid, bread wheat is
hexaploid, and some ferns are 1000-ploid). The spores are later When the haploid spore germinates in a hospitable environment, it
generates a multicellular gametophyte by mitosis. The gametophyte
released by the sporangia and disperse in the environment.
supports the zygote formed from the fusion of gametes and the
resulting young sporophyte (vegetative form). The cycle then begins
anew.
The spores of seedless plants are surrounded by thick cell walls
containing a tough polymer known as sporopollenin. This complex
substance is characterized by long chains of organic molecules
related to fatty acids and carotenoids: hence the yellow color of most
pollen. Sporopollenin is unusually resistant to chemical and
biological degradation. In seed plants, which use pollen to transfer

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the male sperm to the female egg, the toughness of sporopollenin (antheridium) releases sperm. Many seedless plants produce sperm
explains the existence of well-preserved pollen fossils. equipped with flagella that enable them to swim in a moist
Sporopollenin was once thought to be an innovation of land plants; environment to the archegonia: the female gametangium. The
however, the green algae, Coleochaetes, also forms spores that embryo develops inside the archegonium as the sporophyte.
contain sporopollenin. Gametangia are prominent in seedless plants, but are replaced by
pollen grains in seed-producing plants.
GAMETANGIA IN SEEDLESS PLANTS
Gametangia (singular, gametangium) are organs observed on This page titled 25.1D: Sporophytes and Gametophytes in Seedless Plants is
multicellular haploid gametophytes. In the gametangia, precursor shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
cells give rise to gametes by mitosis. The male gametangium curated by Boundless.

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25.1E: STRUCTURAL ADAPTATIONS FOR LAND IN SEEDLESS PLANTS
Plants developed a series of organs and structures to facilitate life on
dry land independent from a constant source of water.

 LEARNING OBJECTIVES

Discuss the primary structural adaptations made by plants to

living on land

KEY POINTS
Many plants developed a vascular system: to distribute water
from the roots (via the xylem ) and sugars from the shoots (via
the phloem ) throughout the entire plant.
An apical meristem enables elongation of the shoots and roots, Figure 25.1E. 1: Apical meristem: Addition of new cells in a root
allowing a plant to access additional space and resources. occurs at the apical meristem. Subsequent enlargement of these cells
causes the organ to grow and elongate. The root cap protects the
Because of the waxy cuticle covering leaves to prevent water fragile apical meristem as the root tip is pushed through the soil by
loss, plants evolved stomata, or pores on the leaves, which open cell elongation.
and close to regulate traffic of gases and water vapor.
Plants evolved pathways for the synthesis of complex organic VASCULAR STRUCTURES
molecules, called secondary metabolites, for protection from In small plants such as single-celled algae, simple diffusion suffices
both UV lights and predators. to distribute water and nutrients throughout the organism. However,
for plants to develop larger forms, the evolution of vascular tissue
KEY TERMS for the distribution of water and solutes was a prerequisite. The
phloem: a vascular tissue in land plants primarily responsible for vascular system contains xylem and phloem tissues. Xylem conducts
the distribution of sugars and nutrients manufactured in the shoot water and minerals absorbed from the soil up to the shoot, while
stoma: a pore found in the leaf and stem epidermis used for phloem transports food derived from photosynthesis throughout the
gaseous exchange entire plant. A root system evolved to take up water and minerals
xylem: a vascular tissue in land plants primarily responsible for from the soil, while anchoring the increasingly taller shoot in the
the distribution of water and minerals taken up by the roots; also soil.
the primary component of wood
meristem: the plant tissue composed of totipotent cells that ADDITIONAL LAND PLANT ADAPTATIONS
allows plant growth In land plants, a waxy, waterproof cover called a cuticle protects the
leaves and stems from desiccation. However, the cuticle also
LAND PLANT ADAPTATIONS prevents intake of carbon dioxide needed for the synthesis of
As plants adapted to dry land and became independent from the carbohydrates through photosynthesis. To overcome this, stomata, or
constant presence of water in damp habitats, new organs and pores, that open and close to regulate traffic of gases and water
structures made their appearance. Early land plants did not grow vapor, appeared in plants as they moved away from moist
more than a few inches off the ground, competing for light on these environments into drier habitats.
low mats. By developing a shoot and growing taller, individual
Water filters ultraviolet-B (UVB) light, which is harmful to all
plants captured more light. Because air offers substantially less
organisms, especially those that must absorb light to survive. This
support than water, land plants incorporated more rigid molecules in
filtering does not occur for land plants. This presented an additional
their stems (and later, tree trunks).
challenge to land colonization, which was met by the evolution of
APICAL MERISTEMS biosynthetic pathways for the synthesis of protective flavonoids and
other compounds: pigments that absorb UV wavelengths of light and
Shoots and roots of plants increase in length through rapid cell
protect the aerial parts of plants from photodynamic damage.
division in a tissue called the apical meristem, which is a small zone
of cells found at the shoot tip or root tip. The apical meristem is Plants cannot avoid being eaten by animals. Instead, they synthesize
made of undifferentiated cells that continue to proliferate throughout a large range of poisonous secondary metabolites: complex organic
the life of the plant. Meristematic cells give rise to all the specialized molecules such as alkaloids, whose noxious smells and unpleasant
tissues of the organism. Elongation of the shoots and roots allows a taste deter animals. These toxic compounds can also cause severe
plant to access additional space and resources: light, in the case of diseases and even death, thus discouraging predation. Humans have
the shoot, and water and minerals, in the case of roots. A separate used many of these compounds for centuries as drugs, medications,
meristem, called the lateral meristem, produces cells that increase or spices. In contrast, as plants co-evolved with animals, the
the diameter of tree trunks. development of sweet and nutritious metabolites lured animals into
providing valuable assistance in dispersing pollen grains, fruit, or

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seeds. Plants have been enlisting animals to be their helpers in this This page titled 25.1E: Structural Adaptations for Land in Seedless Plants is
way for hundreds of millions of years. shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

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25.1F: THE MAJOR DIVISIONS OF LAND PLANTS
Land plants, or embryophytes, are classified by the presence or period of the Paleozoic Era (approximately 440-485 million years
absence of vascular tissue and how they reproduce (with or without ago). These early plants were probably most similar to modern day
seeds). lycophytes, which include club mosses (not to be confused with the
mosses), and pterophytes, which include ferns, horsetails, and whisk
 LEARNING OBJECTIVES ferns. Lycophytes and pterophytes are both referred to as seedless
vascular plants because they do not produce any seeds.
Identify the major divisions of land plants
The seed producing plants, or spermatophytes, form the largest
group of all existing plants, dominating the landscape. Seed-
KEY POINTS producing plants include gymnosperms, most notably conifers,
Non- vascular plants, or bryophytes, appeared early in plant which produce “naked seeds,” and the most successful of all
evolution and reproduce without seeds; they include mosses, modern-day plants, angiosperms, which are the flowering plants.
liverworts, and hornworts. Angiosperms protect their seeds inside chambers at the center of a
Vascular plants are subdivided into two classes: seedless plants, flower; the walls of the chamber later develop into a fruit.
which probably evolved first (including lycophytes and
pterophytes), and seed plants. CONTRIBUTIONS AND ATTRIBUTIONS
Seed-producing plants include gymnosperms, which produce vascular plant. Provided by: Wiktionary. Located at:
http://en.wiktionary.org/wiki/vascular_plant. License: CC BY-SA:
“naked” seeds, and angiosperms, which reproduce by flowering. Attribution-ShareAlike
OpenStax College, Biology. November 11, 2013. Provided by: OpenStax CNX.
KEY TERMS Located at: http://cnx.org/content/m44638/latest...ol11448/latest. License: CC
BY: Attribution
spermatophyte: any plant that bears seeds rather than spores OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44633/latest...ol11448/latest. License: CC
embryophyte: any member of the subkingdom Embryophyta; BY: Attribution
most land plants OpenStax College, Biology. November 11, 2013. Provided by: OpenStax CNX.
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THE MAJOR DIVISIONS OF LAND PLANTS BY: Attribution
bryophyte. Provided by: Wiktionary. Located at:
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embryophytes, are classified into two major groups according to the
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Paleobotany. Provided by: Wikipedia. Located at:
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CNX. Located at: http://cnx.org/content/m44635/latest...25_01_04ab.jpg.
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Figure 25.1F . 1 : Major divisions of land plants: Land plants are License: CC BY: Attribution
categorized by presence or absence of vascular tissue and their OpenStax College, Early Plant Life. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44635/latest...25_01_04ab.jpg.
reproduction with or without the use of seeds.
License: CC BY: Attribution
In contrast, vascular plants developed a network of cells, called OpenStax College, Early Plant Life. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44635/latest...e_25_01_01.jpg.
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BY: Attribution OpenStax College, Early Plant Life. October 17, 2013. Provided by: OpenStax
Gametangium. Provided by: Wikipedia. Located at: CNX. Located at: http://cnx.org/content/m44635/latest...e_25_01_01.jpg.
en.Wikipedia.org/wiki/Gametangium. License: CC BY-SA: Attribution- License: CC BY: Attribution
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en.wiktionary.org/wiki/gametophyte. License: CC BY-SA: Attribution- License: CC BY: Attribution
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en.wiktionary.org/wiki/sporopollenin. License: CC BY-SA: Attribution- CNX. Located at: http://cnx.org/content/m44635/latest...e_25_01_03.jpg.
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CNX. Located at: http://cnx.org/content/m44635/latest...e_25_01_01.jpg. CNX. Located at: http://cnx.org/content/m44633/latest/Figure_25_00_01.jpg.
License: CC BY: Attribution License: CC BY: Attribution
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License: CC BY: Attribution http://cnx.org/content/m44635/latest/Figure_25_01_04ab.jpg. License: CC
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License: CC BY: Attribution CNX. Located at: http://cnx.org/content/m44635/latest...e_25_01_02.jpg.
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License: CC BY-SA: Attribution-ShareAlike CNX. Located at: http://cnx.org/content/m44635/latest...e_25_01_03.jpg.
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SECTION OVERVIEW

25.2: GREEN ALGAE- PRECURSORS OF LAND PLANTS

25.2B: CHARALES
Topic hierarchy
This page titled 25.2: Green Algae- Precursors of Land Plants is shared
25.2A: STREPTOPHYTES AND REPRODUCTION under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
OF GREEN ALGAE by Boundless.

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25.2A: STREPTOPHYTES AND REPRODUCTION OF GREEN ALGAE
Land plants and closely-related green algae (charophytes) are two flagella per cell, as well as various colonial, coccoid, and
classified as Streptophytes; the remaining green algae are filamentous forms, along with macroscopic seaweeds, all of which
chlorophytes. add to the ambiguity of green algae classification since plants are
multicellular.
 LEARNING OBJECTIVES

Discuss the general similarities of green algae and land

plants

KEY POINTS
There is a diverse array of green algae including single-celled or
multicellular species, which can reproduce both sexually or
asexually.
The classification of green algae is challenging because they bear
many of the structural and biochemical traits of plants.
Species of green algae that are closely related to embryophytes Figure 25.2A. 1 : Chara vulgaris: A representative charophyte alga is
Chara vulgris, or common stonewort, which is a multicellular
are classified as charophytes while the remaining green algae are branching species that can grow up to 120m long.
classified as chlorophytes.
Green algae contain the same carotenoids and chlorophyll a and b as
Like plants, charophytes have chlorophyll a and b, store
land plants, whereas other algae have different accessory pigments
carbohydrates as starch, have cell walls consisting of cellulose,
and types of chlorophyll molecules in addition to chlorophyll a. Both
and undergo similar cell-division processes.
green algae and land plants also store carbohydrates as starch. Cells
Charophytes have unique reproductive organs that differ
in green algae divide along cell plates called phragmoplasts and their
considerably from that of other algae.
cell walls are layered with cellulose in the same manner as the cell
KEY TERMS walls of embryophytes. Consequently, land plants (embryophytes)
and closely-related green algae ( Charophyta ) are now part of a new
streptophytes: a subphylum consisting of several orders of green
monophyletic group called Streptophyta. The remaining green algae,
algae and embryophytes
which are more distantly related to plants, belong to a group called
Charophyta: a division of green algae that includes the closest
Chlorophyta that includes more than 7000 different species that live
relatives of the embryophyte plants
in fresh or brackish water, in seawater, or in snow patches.
Chlorophyta: a division of green algae that are considered more
distantly related to plants The Charophyta are a division of green algae that includes the
closest relatives of the embryophyte plants. Charophyta are a small
STREPTOPHYTES but important group of plants which show marked differences from
Until recently, all photosynthetic eukaryotes were considered both the Thallophyta and the Bryophyta. They are all specialized
members of the kingdom Plantae. The brown, red, and gold algae, water plants. The reproductive organs consist of antheridia and
however, have been reassigned to the Protista kingdom. This is oogonia, although the structure of these organs differs considerably
because, apart from their ability to capture light energy and fix CO2, from the corresponding organs in the Algae.
they lack many structural and biochemical traits that distinguish
This page titled 25.2A: Streptophytes and Reproduction of Green Algae is
plants from protists. The position of green algae is more ambiguous.
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
Green algae include unicellular and colonial flagellates, most with
curated by Boundless.

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25.2B: CHARALES
Algae in the order Charales live in fresh water and are often In Charales, large cells form the thallus: the main stem of the alga.
considered the closest-living relatives of embryophytes. Branches arising from the nodes are made of smaller cells. Male and
female reproductive structures are found on the nodes; the sperm
 LEARNING OBJECTIVES have flagella. Unlike land plants, Charales do not undergo
alternation of generations in their lifecycle. Like embryophytes,
Identify the principle features of charophyte algae Charales exhibit a number of traits that are significant in their
adaptation to land life. They produce the compounds lignin and
KEY POINTS sporopollenin. They form plasmodesmata, which are microscopic
The structure of charophyte algae consists of a thallus, which is channels that connect the cytoplasm of adjacent cells. The egg and,
the main stem, and branches that arise from nodes which bear later, the zygote, form in a protected chamber on the parent plant.
both male and female reproductive structures. New information from recent, extensive DNA sequence analysis of
Although charophyte algae do not exhibit alteration of green algae indicates that the Zygnematales are more closely-related
generations, they share a number of adaptations to life on land to the embryophytes than the Charales. The Zygnematales include
with embryophytes, including the encasement of eggs in the familiar genus Spirogyra. As techniques in DNA analysis
protective enclosures. improve and new information on comparative genomics arises, the
As new DNA sequence analysis techniques develop, revisions phylogenetic connections between species will probably continue to
may need to be made in our understanding of plant evolution, change. Clearly, plant biologists have yet to solve the mystery of the
such as indications that green algae in the order of Zygnematales origin of land plants.
may be more-closely related to embryophytes than is Charales.
CONTRIBUTIONS AND ATTRIBUTIONS
KEY TERMS OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44637/latest...ol11448/latest. License: CC
Charales: green algae in the division Charophyta which are BY: Attribution
green plants believed to be the closest relatives of the green land OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44637/latest/?collection=col11448/latest.
plants License: CC BY: Attribution
sporopollenin: a combination of biopolymers observed in the Charophyta. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/Charophyta. License: CC BY-SA: Attribution-
tough outer layer of the spore and pollen wall
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streptophytes. Provided by: Wikipedia. Located at:
CHARALES en.Wikipedia.org/wiki/streptophytes. License: CC BY-SA: Attribution-
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Green algae in the order Charales, and the coleochaetes, microscopic Charophyta. Provided by: Wikipedia. Located at:
green algae that enclose their spores in sporopollenin, are considered en.Wikipedia.org/wiki/Charophyta. License: CC BY-SA: Attribution-
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the closest-living relatives of embryophytes. The Charales can be Chlorophyta. Provided by: Wikipedia. Located at:
traced as far back as 420 million years. They live in a range of fresh en.Wikipedia.org/wiki/Chlorophyta. License: CC BY-SA: Attribution-
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water habitats and vary in size from as small as a few millimeters to Charophyta. Provided by: Wikipedia. Located at:
as large as a meter in length. A representative species of Charales is es.Wikipedia.org/wiki/Charophyta. License: CC BY: Attribution
sporopollenin. Provided by: Wiktionary. Located at:
Chara, which is often called muskgrass or skunkweed because of its en.wiktionary.org/wiki/sporopollenin. License: CC BY-SA: Attribution-
unpleasant smell. ShareAlike
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License: CC BY: Attribution
Charales. Provided by: Wikipedia. Located at:
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Charophyta. Provided by: Wikipedia. Located at:
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OpenStax College, Green Algae: Precursors of Land Plants. October 17, 2013.
Provided by: OpenStax CNX. Located at:
http://cnx.org/content/m44637/latest...e_25_02_02.jpg. License: CC BY:
Attribution

This page titled 25.2B: Charales is shared under a CC BY-SA 4.0 license
and was authored, remixed, and/or curated by Boundless.

Figure 25.2B. 1: Charophyte algae: A representative charophyte

alga, Chara, is a noxious weed in Florida, where it clogs waterways.

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SECTION OVERVIEW

25.3: BRYOPHYTES
25.3B: LIVERWORTS AND HORNWORTS
Topic hierarchy
25.3C: MOSSES
25.3A: BRYOPHYTES
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and was authored, remixed, and/or curated by Boundless.

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25.3A: BRYOPHYTES
Bryophytes (liverworts, mosses, and hornworts) are non-vascular This compelling fact is used as evidence that non-vascular plants
plants that appeared on earth over 450 million years ago. must have preceded the Silurian period.
More than 25,000 species of bryophytes thrive in mostly-damp
 LEARNING OBJECTIVES habitats, although some live in deserts. They constitute the major
flora of inhospitable environments like the tundra where their small
Describe the characteristics of bryophytes
size and tolerance to desiccation offer distinct advantages. They
generally lack lignin and do not have actual tracheids (xylem cells
KEY POINTS specialized for water conduction). Rather, water and nutrients
Bryophytes are the closest-living relative of early terrestrial circulate inside specialized conducting cells. Although the term non-
plants; liverworts were the first Bryophytes, probably appearing tracheophyte is more accurate, bryophytes are commonly called non-
during the Ordovician period. vascular plants.
Bryophytes fossil formation is improbable since they do not In a bryophyte, all the conspicuous vegetative organs, including the
possess lignin. photosynthetic leaf-like structures, the thallus, stem, and the rhizoid
Bryophytes thrive in mostly-damp habitats; however, some that anchors the plant to its substrate, belong to the haploid
species can live in deserts while others can inhabit hostile organism, or gametophyte. The sporophyte is barely noticeable.
environments such as the tundra. Thus, the gametophyte is the dominant and most familiar form; the
Bryophytes are nonvascular because they do not have tracheids; sporophyte appears for only a short period. The gametes formed by
instead, water and nutrients circulate inside specialized bryophytes swim with a flagellum. The sporangium, the
conducting cells. multicellular sexual reproductive structure, is present in bryophytes
In a bryophyte, all the vegetative organs belong to the and absent in the majority of algae. The sporophyte embryo also
gametophyte, which is the dominant and most familiar form; the remains attached to the parent plant, which protects and nourishes it.
sporophyte appears for only a short period. This is a characteristic of land plants. The bryophytes are divided
The sporophyte is dependent on the gametophyte and remains into three phyla: the liverworts (Hepaticophyta), the hornworts
permanently attached to it in order to gain nutrition and (Anthocerotophyta), and the mosses (true Bryophyta).
protection.

KEY TERMS
bryophyte: seedless, nonvascular plants that are the closest
extant relative of early terrestrial plants
tracheid: elongated cells in the xylem of vascular plants that
serve in the transport of water and mineral salts
sporangium: a case, capsule, or container in which spores are
produced by an organism

BRYOPHYTES
Bryophytes are the group of seedles plants that are the closest-extant
relative of early terrestrial plants. The first bryophytes (liverworts)
probably appeared in the Ordovician period, about 450 million years
ago. However, because they lack of lignin and other resistant
structures, bryophyte fossil formation is improbable and the fossil
Figure 25.3A. 1 : Moss: Mosses (true bryophyta) are one of the three
record is poor. Some spores protected by sporopollenin have kinds of bryophytes (along with liverworts and hornworts). This
survived and are attributed to early bryophytes. By the Silurian image shows a moss growing on a dry stone wall.
period, however, vascular plants had spread through the continents.
This page titled 25.3A: Bryophytes is shared under a CC BY-SA 4.0 license
and was authored, remixed, and/or curated by Boundless.

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25.3B: LIVERWORTS AND HORNWORTS
Liverworts and hornworts are both bryophytes, but aspects of their
structures and development are different.

 LEARNING OBJECTIVES

Describe the distinguishing traits of hornworts and

liverworts

KEY POINTS
The leaves of liverworts are lobate green structures similar to the
lobes of the liver, while hornworts have narrow, pipe-like
structures. Figure 25.3B. 1: Liverworts: A liverwort, Lunularia cruciata,
The gametophyte stage is the dominant stage in both liverworts displays its lobate, flat thallus. The organism in the photograph is in
the dominant gametophyte stage.
and hornworts; however, liverwort sporophytes do not contain
The liverwort’s life cycle begins with the release of haploid spores
stomata, while hornwort sporophytes do.
from the sporangium that developed on the sporophyte. Spores
The life cycle of liverworts and hornworts follows alternation of
disseminated by wind or water germinate into flattened thalli
generations: spores germinate into gametophytes, the zygote
gametophytes attached to the substrate by thin, single-celled
develops into a sporophyte that releases spores, and then spores
produce new gametophytes. filaments. Male and female gametangia develop on separate,
Liverworts develop short, small sporophytes, whereas hornworts individual plants. Once released, male gametes swim with the aid of
develop long, slender sporophytes. their flagella to the female gametangium (the archegonium), and
fertilization ensues. The zygote grows into a small sporophyte still
To aid in spore dispersal, liverworts utilize elaters, whereas
attached to the parent gametophyte and develops spore-producing
hornworts utilize pseudoelaters.
cells and elaters. The spore-producing cells undergo meiosis to form
Liverworts and hornworts can reproduce asexually through the
spores, which disperse (with the help of elaters), giving rise to new
fragmentation of leaves into gemmae that disperse and develop
gametophytes. Thus, the life cycle of liverworts follows the pattern
into gametophytes.
of alternation of generations.
KEY TERMS
alternation of generation: the life cycle of plants with a
multicellular sporophyte, which is diploid, that alternates with a
multicellular gametophyte, which is haploid
pseudoelater: single-celled structure that aids in spore dispersal
gemmae: small, intact, complete pieces of plant that are
produced in a cup on the surface of the thallus and develop into
gametophytes through asexual reproduction

LIVERWORTS AND HORNWORTS

LIVERWORTS
Liverworts (Hepaticophyta) are viewed as the plants most closely
related to the ancestor that moved to land. Liverworts have
colonized every terrestrial habitat on earth and diversified to more
than 7000 existing species. Liverwort gametophytes (the dominant
stage of the life cycle) form lobate green structures. The shape of
these leaves are similar to the lobes of the liver; hence, providing the
origin of the name given to the phylum. Openings that allow the
movement of gases may be observed in liverworts. However, these
are not stomata because they do not actively open and close. The
Figure 25.3B. 1: Liverwort Life Cycle: The life cycle of a typical
plant takes up water over its entire surface and has no cuticle to liverwort follows the pattern of alternation of generations. Spores
prevent desiccation. are released from sporophytes and form the gametophyte. Male
gametes fertilize female gametes to form a zygote, which grows into
a sporophyte. This sporophyte disperses spores with the help of
elaters; the process begins again.

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Liverwort plants can also reproduce asexually by the breaking of eggs. However, unlike liverworts, the zygote develops into a long
branches or the spreading of leaf fragments called gemmae. In this and slender sporophyte that eventually splits open, releasing spores.
latter type of reproduction, the gemmae (small, intact, complete Additionally, thin cells called pseudoelaters surround the spores and
pieces of plant that are produced in a cup on the surface of the help propel them further in the environment. Unlike the elaters
thallus ) are splashed out of the cup by raindrops. The gemmae then observed in liverworts, the hornwort pseudoelaters are single-celled
land nearby and develop into gametophytes. structures. The haploid spores germinate and produce the next
generation of gametophytes. Like liverworts, some hornworts may
HORNWORTS also produce asexually through fragmentation.
The hornworts (Anthocerotophyta) belong to the broad bryophyte
group that have colonized a variety of habitats on land, although
they are never far from a source of moisture. The short, blue-green
gametophyte is the dominant phase of the lifecycle of a hornwort.
The narrow, pipe-like sporophyte is the defining characteristic of the
group. The sporophytes emerge from the parent gametophyte and
continue to grow throughout the life of the plant. Stomata appear in
the hornworts and are abundant on the sporophyte. Photosynthetic
cells in the thallus contain a single chloroplast. Meristem cells at the
base of the plant keep dividing and adding to its height. Many
hornworts establish symbiotic relationships with cyanobacteria that
fix nitrogen from the environment.

Figure 25.3B. 1: Life Cycle of Hornworts: The life cycle of

hornworts is similar to that of liverworts. Both follow the pattern of
alternation of generations. However, liverworts develop a small
Figure 25.3B. 1: Hornworts: Unlike liverworts, hornworts grow a sporophyte, whereas hornworts develop a long, slender sporophyte.
tall and slender sporophyte. Liverworts also disperse their spores with the help of elaters, while
hornworts utilize pseudoelaters to aid in spore dispersal.
The life cycle of hornworts also follows the general pattern of
alternation of generations and has a similar life cycle to liverworts. This page titled 25.3B: Liverworts and Hornworts is shared under a CC BY-
The gametophytes grow as flat thalli on the soil with embedded SA 4.0 license and was authored, remixed, and/or curated by Boundless.
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25.3C: MOSSES
Mosses are bryophytes that live in many environments and are sporangium. Some mosses have small branches. Some primitive
characterized by their short flat leaves, root-like rhizoids, and traits of green algae, such as flagellated sperm, are still present in
peristomes. mosses that are dependent on water for reproduction. Other features
of mosses are adaptations to dry land. For example, stomata are
 LEARNING OBJECTIVES present on the stems of the sporophyte and a primitive vascular
system runs up the sporophyte’s stalk. Additionally, mosses are
Describe the distinguishing traits of mosses anchored to the substrate, whether it is soil, rock, or roof tiles, by
multicellular rhizoids. These structures are precursors of roots. They
KEY POINTS originate from the base of the gametophyte, but are not the major
Mosses slow down erosion, store moisture and soil nutrients, and route for the absorption of water and minerals. The lack of a true
provide shelter for small animals and food for larger herbivores. root system explains why it is so easy to rip moss mats from a tree
Mosses have green, flat structures that resemble true leaves, trunk.
which absorb water and nutrients; some mosses have small
branches.
Mosses have traits that are adaptations to dry land, such as
stomata present on the stems of the sporophyte.
Mosses are anchored to the substrate by rhizoids, which originate
from the base of the gametophyte.
The moss life cycle follows the pattern of alternation of
generations where gametophytes form male and female
gametophores, which fertilize to form the sporophyte; spores are
released from the sporophyte to produce new gametophytes.
The concentric tissue around the mouth of the capsule is made of
triangular, close-fitting units that open and close to release
spores, and the peristome increases the spread of spores after the
tip of the capsule falls off at dispersal.
Figure 25.3C. 1 : Setae: This photograph shows the long slender
KEY TERMS stems, called setae, connected to capsules of the moss
Thamnobryum alopecurum.
peristome: one or two rings of tooth-like appendages
The moss life cycle follows the pattern of alternation of generations.
surrounding the opening of the capsule of many mosses that aid
The most familiar structure is the haploid gametophyte, which
in spreading spores
germinates from a haploid spore and forms first a protonema:
rhizoid: a rootlike structure that acts as support and anchors the
plant to its substrate usually, a tangle of single-celled filaments that hug the ground. Cells
seta: the stalk of a moss sporangium, or occasionally in a akin to an apical meristem actively divide and give rise to a
liverwort gametophore, consisting of a photosynthetic stem and foliage-like
structures. Rhizoids form at the base of the gametophore.
MOSSES Gametangia of both sexes develop on separate gametophores. The
More than 10,000 species of mosses have been cataloged. Their male organ (the antheridium) produces many sperm, whereas the
habitats vary from the tundra, where they are the main vegetation, to archegonium (the female organ) forms a single egg. At fertilization,
the understory of tropical forests. In the tundra, the mosses’ shallow the sperm swims down the neck to the venter and unites with the egg
rhizoids allow them to fasten to a substrate without penetrating the inside the archegonium. The zygote, protected by the archegonium,
frozen soil. Mosses slow down erosion, store moisture and soil divides and grows into a sporophyte, still attached by its foot to the
nutrients, and provide shelter for small animals as well as food for gametophyte.
larger herbivores, such as the musk ox. Mosses are very sensitive to
air pollution and are used to monitor air quality. They are also
sensitive to copper salts. Such salts are a common ingredient of
compounds marketed to eliminate mosses from lawns.
Mosses form diminutive gametophytes, which are the dominant
phase of the life cycle. Green, flat structures resembling true leaves,
but lacking vascular tissue are attached in a spiral to a central stalk
or seta. The plants absorb water and nutrients directly through these
leaf-like structures. The seta (plural, setae) contains tubular cells that
transfer nutrients from the base of the sporophyte (the foot) to the

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Marchantiophyta. Provided by: Wikipedia. Located at:
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a sporophyte, which stays attached to the gametophyte. Spores rhizoid. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/rhizoid.
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SECTION OVERVIEW

25.4: SEEDLESS VASCULAR PLANTS

25.4D: FERNS AND OTHER SEEDLESS
Topic hierarchy VASCULAR PLANTS

25.4E: THE IMPORTANCE OF SEEDLESS

25.4A: SEEDLESS VASCULAR PLANTS
VASCULAR PLANTS
25.4B: VASCULAR TISSUE- XYLEM AND PHLOEM
This page titled 25.4: Seedless Vascular Plants is shared under a CC BY-SA
25.4C: THE EVOLUTION OF ROOTS IN SEEDLESS
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PLANTS

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25.4A: SEEDLESS VASCULAR PLANTS
Seedless vascular plants, which reproduce and spread through Seedless vascular plants are plants that contain vascular tissue, but
spores, are plants that contain vascular tissue, but do not flower or do not produce flowers or seeds. In seedless vascular plants, such as
seed. ferns and horsetails, the plants reproduce using haploid, unicellular
spores instead of seeds. The spores are very lightweight (unlike
 LEARNING OBJECTIVES many seeds), which allows for their easy dispersion in the wind and
for the plants to spread to new habitats. Although seedless vascular
Evaluate the evolution of seedless vascular plants plants have evolved to spread to all types of habitats, they still
depend on water during fertilization, as the sperm must swim on a
KEY POINTS layer of moisture to reach the egg. This step in reproduction explains
The life cycle of seedless vascular plants alternates between a why ferns and their relatives are more abundant in damp
diploid sporophyte and a haploid gametophyte phase. environments, including marshes and rainforests. The life cycle of
Seedless vascular plants reproduce through unicellular, haploid seedless vascular plants is an alternation of generations, where the
spores instead of seeds; the lightweight spores allow for easy diploid sporophyte alternates with the haploid gametophyte phase.
dispersion in the wind. The diploid sporophyte is the dominant phase of the life cycle, while
Seedless vascular plants require water for sperm motility during the gametophyte is an inconspicuous, but still-independent,
reproduction and, thus, are often found in moist environments. organism. Throughout plant evolution, there is a clear reversal of
roles in the dominant phase of the life cycle.
KEY TERMS
gametophyte: a plant (or the haploid phase in its life cycle) that
produces gametes by mitosis in order to produce a zygote
sporophyte: a plant (or the diploid phase in its life cycle) that
produces spores by meiosis in order to produce gametophytes
tracheophyte: any plant possessing vascular tissue (xylem and
phloem), including ferns, conifers, and flowering plants

SEEDLESS VASCULAR PLANTS

The vascular plants, or tracheophytes, are the dominant and most
conspicuous group of land plants. They contain tissue that transports
water and other substances throughout the plant. More than 260,000
species of tracheophytes represent more than 90 percent of the Figure 25.4A. 1 : Life cycle of a fern: This life cycle of a fern shows
earth’s vegetation. By the late Devonian period, plants had evolved alternation of generations with a dominant sporophyte stage.
vascular tissue, well-defined leaves, and root systems. With these
This page titled 25.4A: Seedless Vascular Plants is shared under a CC BY-
advantages, plants increased in height and size and were able to
SA 4.0 license and was authored, remixed, and/or curated by Boundless.
spread to all habitats.

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25.4B: VASCULAR TISSUE- XYLEM AND PHLOEM
Xylem and phloem form the vascular system of plants to transport long tubes. Vessels and tracheids are dead at maturity. Tracheids
water and other substances throughout the plant. have thick secondary cell walls and are tapered at the ends. It is the
thick walls of the tracheids that provide support for the plant and
 LEARNING OBJECTIVES allow it to achieve impressive heights. Tall plants have a selective
advantage by being able to reach unfiltered sunlight and disperse
Describe the functions of plant vascular tissue their spores or seeds further away, thus expanding their range. By
growing higher than other plants, tall trees cast their shadow on
KEY POINTS shorter plants and limit competition for water and precious nutrients
Xylem transports and stores water and water-soluble nutrients in in the soil. The tracheids do not have end openings like the vessels
vascular plants. do, but their ends overlap with each other, with pairs of pits present.
Phloem is responsible for transporting sugars, proteins, and other The pit pairs allow water to pass horizontally from cell to cell.
organic molecules in plants.
Vascular plants are able to grow higher than other plants due to
the rigidity of xylem cells, which support the plant.

KEY TERMS
xylem: a vascular tissue in land plants primarily responsible for
the distribution of water and minerals taken up by the roots; also
the primary component of wood
phloem: a vascular tissue in land plants primarily responsible for
the distribution of sugars and nutrients manufactured in the shoot
tracheid: elongated cells in the xylem of vascular plants that
serve in the transport of water and mineral salts

VASCULAR TISSUE: XYLEM AND PHLOEM

The first fossils that show the presence of vascular tissue date to the
Silurian period, about 430 million years ago. The simplest
arrangement of conductive cells shows a pattern of xylem at the
center surrounded by phloem. Together, xylem and phloem tissues
form the vascular system of plants.

Figure 25.4B. 1: Tracheids and vessel elements: Tracheids (top) and

vessel elements (bottom) are the water conducting cells of xylem
tissue.
Phloem tissue is responsible for translocation, which is the transport
of soluble organic substances, for example, sugar. The substances
travel along sieve elements, but other types of cells are also present:
the companion cells, parenchyma cells, and fibers. The end walls,
unlike vessel members in xylem, do not have large openings. The
end walls, however, are full of small pores where cytoplasm extends
from cell to cell. These porous connections are called sieve plates.
Figure 25.4B. 1: Xylem and phloem: Xylem and phloem tissue
make up the transport cells of stems. The direction of water and Despite the fact that their cytoplasm is actively involved in the
sugar transportation through each tissue is shown by the arrows. conduction of food materials, sieve-tube members do not have
Xylem is the tissue responsible for supporting the plant as well as nuclei at maturity. The activity of the sieve tubes is controlled by
for the storage and long-distance transport of water and nutrients, companion cells through plasmadesmata.
including the transfer of water-soluble growth factors from the
organs of synthesis to the target organs. The tissue consists of vessel This page titled 25.4B: Vascular Tissue- Xylem and Phloem is shared under
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25.4C: THE EVOLUTION OF ROOTS IN SEEDLESS PLANTS
Roots support plants by anchoring them to soil, absorbing water and
minerals, and storing products of photosynthesis.

 LEARNING OBJECTIVES

Explain how roots provide support for plants

KEY POINTS
There are two main types of root systems: tap root systems
consist of one main root that grows down vertically with smaller
lateral roots growing off of the main root, while fibrous root
systems form a dense network of roots near the soil surface.
Roots can be modified to store food or starches and to provide
additional support for plants; many vegetables, such as carrots,
are modified roots.
A zone of cell division, a zone of elongation, and a zone of
maturation and differentiation make up a root tip, where the root Figure 25.4C. 1 : Root types: (a) Tap root systems have a main root
cells divide, grow, and differentiate into specialized cells. that grows down, while (b) fibrous root systems consist of many
The vascular system of roots is surrounded by an epidermis, small roots.
which regulates materials that enter the root’s vascular system. A tap root system penetrates deep into the soil. In contrast, a fibrous
root system is located closer to the soil surface, forming a dense
KEY TERMS network of roots that also helps prevent soil erosion (lawn grasses
endodermis: in a plant stem or root, a cylinder of cells that are a good example, as are wheat, rice, and corn). In addition, some
separates the outer cortex from the central core and controls the plants actually have a combination of tap root and fibrous roots.
flow of water and minerals within the plant Plants that grow in dry areas often have deep root systems, whereas
suberin: a waxy material found in bark that can repel water plants growing in areas with abundant water tend to have shallower
pericycle: in a plant root, the cylinder of plant tissue between the root systems.
endodermis and phloem
ROOT GROWTH AND ANATOMY
ROOTS: SUPPORT FOR THE PLANT
Roots are not well preserved in the fossil record. Nevertheless, it
seems that roots appeared later in evolution than vascular tissue. The
development of an extensive network of roots represented a
significant new feature of vascular plants. Roots provided seed
plants with three major functions: anchoring the plant to the soil,
absorbing water and minerals and transporting them upwards, and
storing the products of photosynthesis. Importantly, roots are
modified to absorb moisture and exchange gases. In addition, while
most roots are underground, some plants have adventitious roots,
which emerge above the ground from the shoot.

TYPES OF ROOT SYSTEMS

There are mainly two types of root systems. Dicots (flowering plants
with two embryonic seed leaves) have a tap root system while
monocots (flowering plants with one embryonic seed leaf) have a
fibrous root system. A tap root system has a main root that grows
Figure 25.4C. 1 : Zones on a root tip: A longitudinal view of the root
down vertically from which many smaller lateral roots arise. reveals the zones of cell division, elongation, and maturation. Cell
Dandelions are a good example; their tap roots usually break off division occurs in the apical meristem.
when trying to pull these weeds; they can regrow another shoot from Root growth begins with seed germination. When the plant embryo
the remaining root. emerges from the seed, the radicle of the embryo forms the root
system. The tip of the root is protected by the root cap, a structure
exclusive to roots and unlike any other plant structure. The root cap
is continuously replaced because it gets damaged easily as the root

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pushes through soil. The root tip can be divided into three zones: a the endodermis, separates the vascular tissue from the ground tissue
zone of cell division, a zone of elongation, and a zone of maturation in the outer portion of the root. The endodermis is exclusive to roots,
and differentiation. The zone of cell division is closest to the root serving as a checkpoint for materials entering the root’s vascular
tip; it is made up of the actively-dividing cells of the root meristem. system. A waxy substance called suberin is present on the walls of
The zone of elongation is where the newly-formed cells increase in the endodermal cells. This waxy region, known as the Casparian
length, thereby lengthening the root. Beginning at the first root hair strip, forces water and solutes to cross the plasma membranes of
is the zone of cell maturation where the root cells begin to endodermal cells instead of slipping between the cells. This ensures
differentiate into special cell types. All three zones are in the first that only materials required by the root pass through the endodermis,
centimeter or so of the root tip. while toxic substances and pathogens are generally excluded. The
outermost cell layer of the root’s vascular tissue is the pericycle, an
area that can give rise to lateral roots. In dicot roots, the xylem and
phloem of the stele are arranged alternately in an X shape, whereas
in monocot roots, the vascular tissue is arranged in a ring around the
pith.

ROOT MODIFICATIONS
Root structures may be modified for specific purposes. For example,
some roots are bulbous and store starch. Aerial roots and prop roots
are two forms of aboveground roots that provide additional support
to anchor the plant. Tap roots, such as carrots, turnips, and beets, are
Figure 25.4C. 1 : Modified roots: Many vegetables are modified examples of roots that are modified for food storage.
roots, such as radishes and carrots, which store energy in the form of
starches and sugars. This page titled 25.4C: The Evolution of Roots in Seedless Plants is shared
The vascular tissue in the root is arranged in the inner portion of the under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
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25.4D: FERNS AND OTHER SEEDLESS VASCULAR PLANTS
Ferns, club mosses, horsetails, and whisk ferns are seedless vascular
plants that reproduce with spores and are found in moist
environments.

 LEARNING OBJECTIVES

Identify types of seedless vascular plants

KEY POINTS
Club mosses, which are the earliest form of seedless vascular
plants, are lycophytes that contain a stem and microphylls.
Horsetails are often found in marshes and are characterized by
jointed hollow stems with whorled leaves.
Photosynthesis occurs in the stems of whisk ferns, which lack
roots and leaves.
Figure 25.4D. 1 : Strobili of club mosses: In some club mosses such
Most ferns have branching roots and form large compound as Lycopodium clavatum, sporangia are arranged in clusters called
leaves, or fronds, that perform photosynthesis and carry the strobili.
reproductive organs of the plant.
PHYLUM MONILOPHYTA: CLASS
KEY TERMS EQUISETOPSIDA (HORSETAILS)
sorus: a cluster of sporangia associated with a fern leaf Horsetails, whisk ferns, and ferns belong to the phylum
lycophyte: a tracheophyte subdivision of the Kingdom Plantae; Monilophyta, with horsetails placed in the Class Equisetopsida. The
the oldest extant (living) vascular plant division at around 410 single extant genus Equisetum is the survivor of a large group of
million years old plants, which produced large trees, shrubs, and vines in the swamp
sporangia: enclosures in which spores are formed forests in the Carboniferous. The plants are usually found in damp
environments and marshes.
FERNS AND OTHER SEEDLESS VASCULAR The stem of a horsetail is characterized by the presence of joints or
PLANTS nodes, hence the old name Arthrophyta (arthro- = “joint”; -phyta =
Water is required for fertilization of seedless vascular plants; most “plant”). Leaves and branches come out as whorls from the evenly-
favor a moist environment. Modern-day seedless tracheophytes spaced joints. The needle-shaped leaves do not contribute greatly to
include lycophytes and monilophytes. photosynthesis, the majority of which takes place in the green stem.

PHYLUM LYCOPODIOPHYTA: CLUB MOSSES

The club mosses, or phylum Lycopodiophyta, are the earliest group
of seedless vascular plants. They dominated the landscape of the
Carboniferous, growing into tall trees and forming large swamp
forests. Today’s club mosses are diminutive, evergreen plants
consisting of a stem (which may be branched) and microphylls
(leaves with a single unbranched vein). The phylum Lycopodiophyta
consists of close to 1,200 species, including the quillworts
(Isoetales), the club mosses (Lycopodiales), and spike mosses
(Selaginellales), none of which are true mosses or bryophytes.
Lycophytes follow the pattern of alternation of generations seen in
the bryophytes, except that the sporophyte is the major stage of the
life cycle. The gametophytes do not depend on the sporophyte for
nutrients. Some gametophytes develop underground and form
mycorrhizal associations with fungi. In club mosses, the sporophyte
gives rise to sporophylls arranged in strobili, cone-like structures
that give the class its name. Lycophytes can be homosporous or
heterosporous.

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 environments ranging from tropics to temperate forests. Although
some species survive in dry environments, most ferns are restricted
to moist, shaded places. Ferns made their appearance in the fossil
record during the Devonian period and expanded during the
Carboniferous.
The dominant stage of the life cycle of a fern is the sporophyte,
which typically consists of large compound leaves called fronds.
Fronds fulfill a double role; they are photosynthetic organs that also
carry reproductive structure. The stem may be buried underground
as a rhizome from which adventitious roots grow to absorb water
and nutrients from the soil, or they may grow above ground as a
trunk in tree ferns. Adventitious organs are those that grow in
unusual places, such as roots growing from the side of a stem. Most
ferns produce the same type of spores and are, therefore,
homosporous. The diploid sporophyte is the most conspicuous stage
of the life cycle. On the underside of its mature fronds, sori
(singular, sorus) form as small clusters where sporangia develop.
Sporangia in a sorus produce spores by meiosis and release them
into the air. Those that land on a suitable substrate germinate and
form a heart-shaped gametophyte, which is attached to the ground
by thin filamentous rhizoids. The inconspicuous gametophyte
harbors both sex gametangia. Flagellated sperm are released and
swim on a wet surface to where the egg is fertilized. The newly-
Figure 25.4D. 1 : Leaves of a horsetail: The whorls of green formed zygote grows into a sporophyte that emerges from the
structures at the joints are actually stems. The leaves are barely
noticeable as brown rings just above each joint. Horsetails were once gametophyte, growing by mitosis into the next generation
used as scrubbing brushes and so were called scouring rushes. sporophyte.
Silica collects in the epidermal cells, contributing to the stiffness of
horsetail plants. Underground stems known as rhizomes anchor the
plants to the ground. Modern-day horsetails are homosporous and
produce bisexual gametophytes.

PHYLUM MONILOPHYTA: CLASS PSILOTOPSIDA

(WHISK FERNS)
While most ferns form large leaves and branching roots, the whisk
ferns, Class Psilotopsida, lack both roots and leaves, which were
probably lost by reduction. Photosynthesis takes place in their green
stems; small yellow knobs form at the tip of the branch stem and
contain the sporangia. Whisk ferns were considered an early
pterophytes. However, recent comparative DNA analysis suggests Figure 25.4D. 1 : Sori on a fern frond: Sori appear as small bumps
on the underside of a fern frond.
that this group may have lost both leaves and roots through
evolution and is more closely related to ferns. This page titled 25.4D: Ferns and Other Seedless Vascular Plants is shared
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
PHYLUM MONILOPHYTA: CLASS by Boundless.
POLYPODIOPSIDA (FERNS)
With their large fronds, ferns are the most-readily recognizable
seedless vascular plants. More than 20,000 species of ferns live in

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25.4E: THE IMPORTANCE OF SEEDLESS VASCULAR PLANTS
Seedless vascular plants provide many benefits to life in ecosystems, areas. Since bryophytes have neither a root system for absorption of
including food and shelter and, to humans, fuel and medicine. water and nutrients, nor a cuticle layer that protects them from
desiccation, pollutants in rainwater readily penetrate their tissues;
 LEARNING OBJECTIVES they absorb moisture and nutrients through their entire exposed
surfaces. Therefore, pollutants dissolved in rainwater penetrate plant
Explain the beneficial roles of seedless vascular plants tissues readily and have a larger impact on mosses than on other
plants. The disappearance of mosses can be considered a
KEY POINTS bioindicator for the level of pollution in the environment.
Mosses and liverworts provide food and shelter for other Ferns contribute to the environment by promoting the weathering of
organisms in otherwise barren or hostile environments. rock, accelerating the formation of topsoil, and slowing down
The level of pollution in an environment can be determined by erosion by spreading rhizomes in the soil. The water ferns of the
the disappearance of mosses, which absorb the pollutants with genus Azolla harbor nitrogen-fixing cyanobacteria and restore this
moisture through their entire surfaces. important nutrient to aquatic habitats.
Dried peat moss is used as a renewable resource for fuel. Seedless plants have historically played a role in human life through
Ferns prevent soil erosion, promote topsoil formation, restore uses as tools, fuel, and medicine. Dried peat moss, Sphagnum, is
nitrogen to aquatic habitats by harboring cyanobacteria, make commonly used as fuel in some parts of Europe and is considered a
good house plants, and have been used as food and for medicinal renewable resource. Sphagnum bogs are cultivated with cranberry
remedies. and blueberry bushes. The ability of Sphagnum to hold moisture
Coal, a major fuel source and contributor to global warming, was makes the moss a common soil conditioner. Florists use blocks of
deposited by the seedless vascular plants of the Carboniferous Sphagnum to maintain moisture for floral arrangements.
period.

KEY TERMS
bioindicator: any species that acts as a biological indicator of
the health of an environment
pharmacopoeia: an official book describing medicines or other
pharmacological substances, especially their use, preparation,
and regulation
sphagnum: any of various widely-distributed mosses, of the
genus Sphagnum, which slowly decompose to form peat; often
used for fuel

THE IMPORTANCE OF SEEDLESS VASCULAR

PLANTS
Mosses and liverworts are often the first macroscopic organisms to
Figure 25.4E. 1: Plants as a renewable resource for fuel: Sphagnum
colonize an area, both in a primary succession (where bare land is
acutifolium is dried peat moss and can be used as fuel.
settled for the first time by living organisms) or in a secondary
The attractive fronds of ferns make them a favorite ornamental plant.
succession (where soil remains intact after a catastrophic event
Because they thrive in low light, they are well suited as house plants.
wipes out many existing species ). Their spores are carried by the
More importantly, fiddleheads are a traditional spring food of Native
wind, birds, or insects. Once mosses and liverworts are established,
Americans in the Pacific Northwest and are popular as a side dish in
they provide food and shelter for other species. In a hostile
French cuisine. The licorice fern, Polypodium glycyrrhiza, is part of
environment, such as the tundra where the soil is frozen, bryophytes
the diet of the Pacific Northwest coastal tribes, owing in part to the
grow well because they do not have roots and can dry and rehydrate
sweetness of its rhizomes. It has a faint licorice taste and serves as a
rapidly once water is again available. Mosses are at the base of the
sweetener. The rhizome also figures in the pharmacopoeia of Native
food chain in the tundra biome. Many species, from small insects to
Americans for its medicinal properties and is used as a remedy for
musk oxen and reindeer, depend on mosses for food. In turn,
sore throat.
predators feed on the herbivores, which are the primary consumers.
Some reports indicate that bryophytes make the soil more amenable
to colonization by other plants. Because they establish symbiotic
relationships with nitrogen-fixing cyanobacteria, mosses replenish
the soil with nitrogen.
At the end of the nineteenth century, scientists observed that lichens
and mosses were becoming increasingly rare in urban and suburban

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Figure 25.4E. 1: Fiddlehead ferns as food: A chicken dish with License: CC BY-SA: Attribution-ShareAlike
fiddlehead ferns as a side is shown. Native Americans traditionally phloem. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/phloem.
cook fiddleheads with meals during the spring. License: CC BY-SA: Attribution-ShareAlike
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OpenStax CNX. Located at:
however, comes from their extinct progenitors. The tall club mosses, http://cnx.org/content/m44640/latest...e_25_04_07.png. License: CC BY:
horsetails, and tree-like ferns that flourished in the swampy forests Attribution
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throughout the world. Coal provided an abundant source of energy http://cnx.org/content/m43140/latest/. License: CC BY: Attribution
Vessel element. Provided by: Wikipedia. Located at:
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deposited the large amounts of coal throughout the world.
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 CHAPTER OVERVIEW

26: SEED PLANTS

Topic hierarchy
26.1: Evolution of Seed Plants
26.1A: The Evolution of Seed Plants and Adaptations for Land
26.1B: Evolution of Gymnosperms
26.1C: Evolution of Angiosperms
26.2: Gymnosperms
26.2A: Characteristics of Gymnosperms
26.2B: Life Cycle of a Conifer
26.2C: Diversity of Gymnosperms
26.3: Angiosperms
26.3A: Angiosperm Flowers
26.3B: Angsiosperm Fruit
26.3C: The Life Cycle of an Angiosperm
26.3D: Diversity of Angiosperms
26.4: The Role of Seed Plants
26.4A: Herbivory and Pollination
26.4B: The Importance of Seed Plants in Human Life
26.4C: Biodiversity of Plants

This page titled 26: Seed Plants is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

1
SECTION OVERVIEW

26.1: EVOLUTION OF SEED PLANTS

26.1B: EVOLUTION OF GYMNOSPERMS
Topic hierarchy
26.1C: EVOLUTION OF ANGIOSPERMS
26.1A: THE EVOLUTION OF SEED PLANTS AND
ADAPTATIONS FOR LAND This page titled 26.1: Evolution of Seed Plants is shared under a CC BY-SA
4.0 license and was authored, remixed, and/or curated by Boundless.

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26.1A: THE EVOLUTION OF SEED PLANTS AND ADAPTATIONS FOR LAND
The evolution of seeds allowed plants to reproduce independently of SEEDS AND POLLEN AS AN EVOLUTIONARY
water; pollen allows them to disperse their gametes great distances. ADAPTATION TO DRY LAND
Unlike bryophyte and fern spores (which are haploid cells dependent
 LEARNING OBJECTIVES on moisture for rapid development of gametophytes ), seeds contain
a diploid embryo that will germinate into a sporophyte. Storage
Recognize the significance of seed plant evolution
tissue to sustain growth and a protective coat give seeds their
superior evolutionary advantage. Several layers of hardened tissue
KEY POINTS prevent desiccation, freeing reproduction from the need for a
Plants are used for food, textiles, medicines, building materials, constant supply of water. Furthermore, seeds remain in a state of
and many other products that are important to humans. dormancy induced by desiccation and the hormone abscisic acid
The evolution of seeds allowed plants to decrease their until conditions for growth become favorable. Whether blown by the
dependency upon water for reproduction. wind, floating on water, or carried away by animals, seeds are
Seeds contain an embryo that can remain dormant until scattered in an expanding geographic range, thus avoiding
conditions are favorable when it grows into a diploid sporophyte. competition with the parent plant.
Seeds are transported by the wind, water, or by animals to Pollen grains are male gametophytes carried by wind, water, or a
encourage reproduction and reduce competition with the parent pollinator. The whole structure is protected from desiccation and can
plant. reach the female organs without dependence on water. Male gametes
reach female gametophyte and the egg cell gamete though a pollen
KEY TERMS
tube: an extension of a cell within the pollen grain. The sperm of
seed: a fertilized ovule, containing an embryonic plant modern gymnosperms lack flagella, but in cycads and the Gingko,
sporophyte: a plant (or the diploid phase in its life cycle) that the sperm still possess flagella that allow them to swim down the
produces spores by meiosis in order to produce gametophytes pollen tube to the female gamete; however, they are enclosed in a
pollen: microspores produced in the anthers of flowering plants pollen grain.
EVOLUTION OF SEED PLANTS
The lush palms on tropical shorelines do not depend upon water for
the dispersal of their pollen, fertilization, or the survival of the
zygote, unlike mosses, liverworts, and ferns of the terrain. Seed
plants, such as palms, have broken free from the need to rely on
water for their reproductive needs. They play an integral role in all
aspects of life on the planet, shaping the physical terrain, influencing
the climate, and maintaining life as we know it. For millennia,
human societies have depended upon seed plants for nutrition and
medicinal compounds; and more recently, for industrial by-products,
such as timber and paper, dyes, and textiles. Palms provide materials
including rattans, oils, and dates. Wheat is grown to feed both
human and animal populations. The fruit of the cotton boll flower is
harvested as a boll, with its fibers transformed into clothing or pulp
for paper. The showy opium poppy is valued both as an ornamental
flower and as a source of potent opiate compounds.

Figure 26.1A. 1 : Seed plants dominate the landscape: Seed plants

dominate the landscape and play an integral role in human societies. Figure 26.1A. 1 : Fossilized pollen grains: This fossilized pollen is
(a) Palm trees grow along the shoreline; (b) wheat is a crop grown in from a Buckbean fen core found in Yellowstone National Park,
most of the world; (c) the flower of the cotton plant produces fibers Wyoming. The pollen is magnified 1,054 times.
that are woven into fabric; (d) the potent alkaloids of the beautiful
opium poppy have influenced human life both as a medicinal This page titled 26.1A: The Evolution of Seed Plants and Adaptations for
remedy and as a dangerously-addictive drug. Land is shared under a CC BY-SA 4.0 license and was authored, remixed,

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and/or curated by Boundless.

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26.1B: EVOLUTION OF GYMNOSPERMS
Seed ferns gave rise to the gymnosperms during the Devonian and associated tissues) which develops into a seed upon fertilization.
Period, allowing them to adapt to dry conditions. Seed plants resembling modern tree ferns became more numerous
and diverse in the coal swamps of the Carboniferous period. This
 LEARNING OBJECTIVES appears to have been the result of a whole genome duplication event
around 319 million years ago.
Explain how and why gymnosperms became the dominant
plant group during the Permian period

KEY POINTS
Seed ferns were the first seed plants, protecting their
reproductive parts in structures called cupules.
Seed ferns gave rise to the gymnosperms during the Paleozoic
Era, about 390 million years ago.
Gymnosperms include the gingkoes and conifers and inhabit
many ecosystems, such as the taiga and the alpine forests, Figure 26.1B. 1: Gymnosperms of the taiga: This boreal forest
because they are well adapted for cold weather. (taiga) has low-lying plants and conifer trees, as these plants are
True seed plants became more numerous and diverse during the better suited to the colder, dryer conditions.
Carboniferous period around 319 million years ago; an explosion Fossil records indicate the first gymnosperms (progymnosperms)
that appears to be due to a whole genome duplication event. most likely originated in the Paleozoic era, during the middle
Devonian period about 390 million years ago. Following the wet
KEY TERMS Mississippian and Pennsylvanian periods, which were dominated by
cupule: any small structure shaped like a cup giant fern trees, the Permian period was dry. This gave a
gymnosperm: any plant, such as a conifer, whose seeds are not reproductive edge to seed plants, which are better adapted to survive
enclosed in an ovary dry spells. The Ginkgoales, a group of gymnosperms with only one
mutualism: any interaction between two species that benefits surviving species, the Gingko biloba, were the first gymnosperms to
both appear during the lower Jurassic. Gymnosperms expanded in the
Mesozoic era (about 240 million years ago), supplanting ferns in the
EVOLUTION OF GYMNOSPERMS landscape, and reaching their greatest diversity during this time. It
has been suggested that during the mid-Mesozoic era, pollination of
some extinct groups of gymnosperms was performed by extinct
species of scorpionflies that had a specialized proboscis for feeding
on pollination drops. The scorpionflies probably engaged in
pollination mutualisms with gymnosperms, long before the similar
and independent coevolution of nectar-feeding insects on
angiosperms.
The Jurassic period was as much the age of the cycads (palm-tree-
like gymnosperms) as the age of the dinosaurs. Gingkoales and the
more familiar conifers also dotted the landscape. Although
Figure 26.1B. 1: Seed ferns: This fossilized leaf is from angiosperms (flowering plants) are the major form of plant life in
Glossopteris, a seed fern that thrived during the Permian age (290–
240 million years ago). most biomes, gymnosperms still dominate some ecosystems, such as
The fossil plant Elkinsia polymorpha, a “seed fern” from the the taiga (boreal forests) and the alpine forests at higher mountain
Devonian period (about 400 million years ago) is considered the elevations because of their adaptation to cold and dry growth
earliest seed plant known to date. Seed ferns produced their seeds conditions.
along their branches without specialized structures. What makes
This page titled 26.1B: Evolution of Gymnosperms is shared under a CC
them the first true seed plants is that they developed structures called
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
cupules to enclose and protect the ovule (the female gametophyte

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26.1C: EVOLUTION OF ANGIOSPERMS
Angiosperms, which evolved in the Cretaceous period, are a diverse
group of plants which protect their seeds within an ovary called a
fruit.

 LEARNING OBJECTIVES

Discuss the evolution and adaptations of angiosperms

KEY POINTS
Angiosperms evolved during the late Cretaceous Period, about
125-100 million years ago.
Angiosperms have developed flowers and fruit as ways to attract
pollinators and protect their seeds, respectively.
Flowers have a wide array of colors, shapes, and smells, all of
which are for the purpose of attracting pollinators.
Once the egg is fertilized, it grows into a seed that is protected by
a fleshy fruit.
As angiosperms evolved in the Cretaceous period, many modern
groups of insects also appeared, including pollinating insects that
drove the evolution of angiosperms; in many instances, flowers
and their pollinators have coevolved.
Angiosperms did not evolve from gymnosperms, but instead Figure 26.1C. 1 : Fossil evidence of angiosperms: This leaf imprint
evolved in parallel with the gymnosperms; however, it is unclear shows a Ficus speciosissima, an angiosperm that flourished during
the Cretaceous period. A large number of pollinating insects also
as to what type of plant actually gave rise to angiosperms. appeared during this same time.
Although several hypotheses have been offered to explain this
KEY TERMS
sudden profusion and variety of flowering plants, none have
clade: a group of animals or other organisms derived from a
garnered the consensus of paleobotanists (scientists who study
common ancestor species ancient plants). New data in comparative genomics and paleobotany
angiosperm: a plant whose ovules are enclosed in an ovary have, however, shed some light on the evolution of angiosperms.
basal angiosperm: the first flowering plants to diverge from the Rather than being derived from gymnosperms, angiosperms form a
ancestral angiosperm, including a single species of shrub from sister clade (a species and its descendents) that developed in parallel
New Caledonia, water lilies and some other aquatic plants, and with the gymnosperms. The two innovative structures of flowers and
woody aromatic plants fruit represent an improved reproductive strategy that served to
EVOLUTION OF ANGIOSPERMS protect the embryo, while increasing genetic variability and range.
Paleobotanists debate whether angiosperms evolved from small
Undisputed fossil records place the massive appearance and
woody bushes, or were basal angiosperms related to tropical grasses.
diversification of angiosperms in the middle to late Mesozoic era.
Both views draw support from cladistic studies. The so-called
Angiosperms (“seed in a vessel”) produce a flower containing male
woody magnoliid hypothesis (which proposes that the early
and/or female reproductive structures. Fossil evidence indicates that
ancestors of angiosperms were shrubs) also offers molecular
flowering plants first appeared in the Lower Cretaceous, about 125
biological evidence.
million years ago, and were rapidly diversifying by the Middle
Cretaceous, about 100 million years ago. Earlier traces of The most primitive living angiosperm is considered to be
angiosperms are scarce. Fossilized pollen recovered from Jurassic Amborellatrichopoda, a small plant native to the rainforest of New
geological material has been attributed to angiosperms. A few early Caledonia, an island in the South Pacific. Analysis of the genome of
Cretaceous rocks show clear imprints of leaves resembling A. trichopoda has shown that it is related to all existing flowering
angiosperm leaves. By the mid-Cretaceous, a staggering number of plants and belongs to the oldest confirmed branch of the angiosperm
diverse, flowering plants crowd the fossil record. The same family tree. A few other angiosperm groups, known as basal
geological period is also marked by the appearance of many modern angiosperms, are viewed as primitive because they branched off
groups of insects, including pollinating insects that played a key role early from the phylogenetic tree. Most modern angiosperms are
in ecology and the evolution of flowering plants. classified as either monocots or eudicots based on the structure of
their leaves and embryos. Basal angiosperms, such as water lilies,
are considered more primitive because they share morphological
traits with both monocots and eudicots.

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FLOWERS AND FRUITS AS AN EVOLUTIONARY BY: Attribution
pollen. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/pollen.
ADAPTATION License: CC BY-SA: Attribution-ShareAlike
seed. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/seed.
Angiosperms produce their gametes in separate organs, which are License: CC BY-SA: Attribution-ShareAlike
usually housed in a flower. Both fertilization and embryo OpenStax College, Introduction. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44643/latest..._00_01abcd.jpg.
development take place inside an anatomical structure that provides License: CC BY: Attribution
a stable system of sexual reproduction largely sheltered from OpenStax College, Evolution of Seed Plants. November 12, 2013. Provided by:
environmental fluctuations. Flowering plants are the most diverse OpenStax CNX. Located at:
http://cnx.org/content/m44653/latest...e_26_01_04.jpg. License: CC BY:
phylum on Earth after insects; flowers come in a bewildering array Attribution
of sizes, shapes, colors, smells, and arrangements. Most flowers OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44653/latest...ol11448/latest. License: CC
have a mutualistic pollinator, with the distinctive features of flowers BY: Attribution
reflecting the nature of the pollination agent. The relationship Gymnosperm. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/Gymnosperm. License: CC BY-SA: Attribution-
between pollinator and flower characteristics is one of the great ShareAlike
examples of coevolution. mutualism. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/mutualism. License: CC BY-SA: Attribution-
ShareAlike
gymnosperm. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/gymnosperm. License: CC BY-SA: Attribution-
ShareAlike
cupule. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/cupule.
License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Introduction. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44643/latest..._00_01abcd.jpg.
License: CC BY: Attribution
OpenStax College, Evolution of Seed Plants. November 12, 2013. Provided by:
OpenStax CNX. Located at:
http://cnx.org/content/m44653/latest...e_26_01_04.jpg. License: CC BY:
Attribution
OpenStax College, Evolution of Seed Plants. October 17, 2013. Provided by:
OpenStax CNX. Located at:
http://cnx.org/content/m44653/latest...e_26_01_02.jpg. License: CC BY:
Attribution
OpenStax College, Evolution of Seed Plants. October 17, 2013. Provided by:
OpenStax CNX. Located at:
http://cnx.org/content/m44653/latest...e_26_01_03.jpg. License: CC BY:
Attribution
angiosperm. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/angiosperm. License: CC BY-SA: Attribution-
ShareAlike
Figure 26.1C. 1 : Coevolution of flowers and pollinators: Many OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
flowers have coevolved with particular pollinators, such that the Located at: http://cnx.org/content/m44653/latest...ol11448/latest. License: CC
flower is uniquely structured for the mouthparts of the pollinator. It BY: Attribution
often has features considered attractive to its particular pollinator. basal angiosperm. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/basal%20angiosperm. License: CC BY-SA:
Following fertilization of the egg, the ovule grows into a seed. The Attribution-ShareAlike
surrounding tissues of the ovary thicken, developing into a fruit that clade. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/clade.
License: CC BY-SA: Attribution-ShareAlike
will protect the seed and often ensure its dispersal over a wide OpenStax College, Introduction. October 17, 2013. Provided by: OpenStax
geographic range. Not all fruits develop from an ovary; such CNX. Located at:
http://cnx.org/content/m44643/latest/Figure_26_00_01abcd.jpg. License: CC
structures are “false fruits.” Like flowers, fruit can vary
BY: Attribution
tremendously in appearance, size, smell, and taste. Tomatoes, walnut OpenStax College, Evolution of Seed Plants. November 12, 2013. Provided by:
shells and avocados are all examples of fruit. As with pollen and OpenStax CNX. Located at:
http://cnx.org/content/m44653/latest/Figure_26_01_04.jpg. License: CC BY:
seeds, fruits also act as agents of dispersal. Some may be carried Attribution
away by the wind. Many attract animals that will eat the fruit and OpenStax College, Evolution of Seed Plants. October 17, 2013. Provided by:
OpenStax CNX. Located at:
pass the seeds through their digestive systems, then deposit the seeds http://cnx.org/content/m44653/latest/Figure_26_01_02.jpg. License: CC BY:
in another location. Cockleburs are covered with stiff, hooked spines Attribution
OpenStax College, Evolution of Seed Plants. October 17, 2013. Provided by:
that can hook into fur (or clothing) and hitch a ride on an animal for OpenStax CNX. Located at:
long distances. The cockleburs that clung to the velvet trousers of an http://cnx.org/content/m44653/latest/Figure_26_01_03.jpg. License: CC BY:
Attribution
enterprising Swiss hiker, George de Mestral, inspired his invention OpenStax College, Evolution of Seed Plants. October 17, 2013. Provided by:
of the loop and hook fastener he named Velcro. OpenStax CNX. Located at:
http://cnx.org/content/m44653/latest/Figure_26_01_05.jpg. License: CC BY:
Attribution
CONTRIBUTIONS AND ATTRIBUTIONS European honey bee extracts nectar. Provided by: Wikipedia. Located at:
sporophyte. Provided by: Wiktionary. Located at: en.Wikipedia.org/wiki/File:European_honey_bee_extracts_nectar.jpg.
en.wiktionary.org/wiki/sporophyte. License: CC BY-SA: Attribution- License: Public Domain: No Known Copyright
ShareAlike
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. This page titled 26.1C: Evolution of Angiosperms is shared under a CC BY-
Located at: http://cnx.org/content/m44643/latest...ol11448/latest. License: CC
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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SECTION OVERVIEW

26.2: GYMNOSPERMS
26.2B: LIFE CYCLE OF A CONIFER
Topic hierarchy
26.2C: DIVERSITY OF GYMNOSPERMS
26.2A: CHARACTERISTICS OF GYMNOSPERMS
This page titled 26.2: Gymnosperms is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

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26.2A: CHARACTERISTICS OF GYMNOSPERMS
Gymnosperms are seed plants that have evolved cones to carry their cone, which produces microspores that subsequently develop into
reproductive structures. pollen grains. The other type of cones, the larger “ovulate” cones,
make megaspores that develop into female gametophytes called
 LEARNING OBJECTIVES ovules. Incredibly, this whole sexual process can take three years:
from the production of the two sexes of gametophytes, to bringing
Discuss the type of seeds produced by gymnosperms the gametophytes together in the process of pollination, and finally
to forming mature seeds from fertilized ovules. After this process is
KEY POINTS completed, the individual sporophylls separate (the cone breaks
Gymnosperms produce both male and female cones, each apart) and float in the wind to a habitable place. This is concluded
making the gametes needed for fertilization; this makes them with germination and the formation of a seedling. Conifers have
heterosporous. sperm that do not have flagella, but instead are conveyed to the egg
Megaspores made in cones develop into the female via a pollen tube. It is important to note that the seeds of
gametophytes inside the ovules of gymnosperms, while pollen gymnosperms are not enclosed in their final state upon the cone.
grains develop from cones that produce microspores.
Conifer sperm do not have flagella but rather move by way of a
pollen tube once in contact with the ovule.

KEY TERMS
ovule: the structure in a plant that develops into a seed after
fertilization; the megasporangium of a seed plant with its
enclosing integuments
sporophyll: the equivalent to a leaf in ferns and mosses that
bears the sporangia
heterosporous: producing both male and female gametophytes

CHARACTERISTICS OF GYMNOSPERMS
Gymnosperms are seed plants adapted to life on land; thus, they are
autotrophic, photosynthetic organisms that tend to conserve water.
They have a vascular system (used for the transportation of water
and nutrients) that includes roots, xylem, and phloem. The name Figure 26.2A. 1 : Female cone of Tamarack pine: The female cone of
Pinus tontorta, the Tamarack Pine, showing the rough scales. This is
gymnosperm means “naked seed,” which is the major distinguishing the cone that produces ovules.
factor between gymnosperms and angiosperms, the two distinct
subgroups of seed plants. This term comes from the fact that the
ovules and seeds of gymnosperms develop on the scales of cones
rather than in enclosed chambers called ovaries.
Gymnosperms are older than angiosperms on the evolutionary scale.
They are found far earlier in the fossil record than angiosperms. As
will be discussed in subsequent sections, the various environmental
adaptations gymnosperms have represent a step on the path to the
most successful (diversity-wise) clade (monophyletic branch).

GYMNOSPERM REPRODUCTION AND SEEDS

Gymnosperms are sporophytes (a plant with two copies of its
genetic material, capable of producing spores ). Their sporangia
(receptacle in which sexual spores are formed) are found on Figure 26.2A. 1 : Male cone of Tamarack pine: The male cone of
sporophylls, plated scale-like structures that together make up cones. Pinus tontorta, the Tamarack pine, showing the close proximity of
The female gametophyte develops from the haploid (meaning one the scales. This is the cone that produces pollen.
set of genetic material) spores that are contained within the This page titled 26.2A: Characteristics of Gymnosperms is shared under a
sporangia. Like all seed plants, gymnosperms are heterosporous: CC BY-SA 4.0 license and was authored, remixed, and/or curated by
both sexes of gametophytes develop from different types of spores Boundless.
produced by separate cones. One type of cone is the small pollen

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26.2B: LIFE CYCLE OF A CONIFER
Conifers are monoecious plants that produce both male and female Female cones (ovulate cones) contain two ovules per scale. One
cones, each making the necessary gametes used for fertilization. megaspore mother cell (megasporocyte) undergoes meiosis in each
ovule. Three of the four cells break down leaving only a single
 LEARNING OBJECTIVES surviving cell which will develop into a female multicellular
gametophyte. It encloses archegonia (an archegonium is a
Describe the life cycle of a gymnosperm reproductive organ that contains a single large egg). Upon
fertilization, the diploid egg will give rise to the embryo, which is
KEY POINTS enclosed in a seed coat of tissue from the parent plant. Fertilization
Male cones give rise to microspores, which produce pollen and seed development is a long process in pine trees: it may take up
grains, while female cones give rise to megaspores, which to two years after pollination. The seed that is formed contains three
produce ovules. generations of tissues: the seed coat that originates from the
The pollen tube develops from the pollen grain to initiate sporophyte tissue, the gametophyte that will provide nutrients, and
fertilization; the pollen grain divides into two sperm cells by the embryo itself.
mitosis; one of the sperm cells unites with the egg cell during In the life cycle of a conifer, the sporophyte (2n) phase is the longest
fertilization. phase. The gametophytes (1n), microspores and megaspores, are
Once the ovule is fertilized, a diploid sporophyte is produced, reduced in size. This phase may take more than one year between
which gives rise to the embryo enclosed in a seed coat of tissue pollination and fertilization while the pollen tube grows towards the
from the parent plant. megasporocyte (2n), which undergoes meiosis into megaspores. The
Fetilization and seed development can take years; the seed that is megaspores will mature into eggs (1n).
formed is made up of three tissues: the seed coat, the
gametophyte, and the embryo.

KEY TERMS
megaspore: the larger spore of a heterosporous plant, typically
producing a female gametophyte
microspore: a small spore, as contrasted to the larger megaspore,
which develops into male gametophytes
monoecious: having the male (stamen) and female (carpel)
reproductive organs on the same plant rather than on separate
plants

LIFE CYCLE OF A CONIFER

Pine trees are conifers (cone bearing) and carry both male and
female sporophylls on the same mature sporophyte. Therefore, they
are monoecious plants. Like all gymnosperms, pines are
heterosporous, generating two different types of spores: male
microspores and female megaspores. In the male cones (staminate
cones), the microsporocytes give rise to pollen grains by meiosis. In
the spring, large amounts of yellow pollen are released and carried
by the wind. Some gametophytes will land on a female cone.
Pollination is defined as the initiation of pollen tube growth. The Figure 26.2B. 1: Life cycle of a conifer: This image shows the life
pollen tube develops slowly as the generative cell in the pollen grain cycle of a conifer. Pollen from male
divides into two haploid sperm cells by mitosis. At fertilization, one cones moves up into upper branches where it fertilizes female cones.
of the sperm cells will finally unite its haploid nucleus with the
haploid nucleus of an egg cell. This page titled 26.2B: Life Cycle of a Conifer is shared under a CC BY-SA
4.0 license and was authored, remixed, and/or curated by Boundless.

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26.2C: DIVERSITY OF GYMNOSPERMS
Gymnosperms are a diverse group of plants the protect their seeds than the wood of angiosperms; it contains tracheids, but no vessel
with cones and do not produce flowers or fruits. elements, and is, therefore, referred to as “soft wood.”

 LEARNING OBJECTIVES

Give examples showing the diversity of gymnosperms

KEY POINTS
Gymnosperms consist of four main phyla: the Coniferophyta,
Cycadophyta, Gingkophyta and Gnetophyta.
Conifers are the dominant plant of the gymnosperms, having
needle-like leaves and living in areas where the weather is cold
and dry.
Cycads live in warm climates, have large, compound leaves, and
are unusual in that they are pollinated by beetles rather than
wind.
Gingko biloba is the only remaining species of the Gingkophyta
and is usually resistant to pollution.
Gnetophytes are the gymnosperms believed to be most closely
related to the angiosperms because of the presence of vessel
elements within their stems.

KEY TERMS Figure 26.2C. 1 : Diversity of conifers: Conifers are the dominant
form of vegetation in cold or arid environments and at high altitudes.
tracheid: elongated cells in the xylem of vascular plants that Shown here are the (a) evergreen spruce Picea sp., (b) juniper
serve in the transport of water and mineral salts Juniperus sp., (c) sequoia Sequoia Semervirens, which is a
angiosperm: a plant whose ovules are enclosed in an ovary deciduous gymnosperm, and (d) the tamarack Larix larcinia. Notice
the yellow leaves of the tamarack.
conifer: a plant belonging to the conifers; a cone-bearing seed
plant with vascular tissue, usually a tree CYCADS
Cycads thrive in mild climates. They are often mistaken for palms
DIVERSITY OF GYMNOSPERMS
because of the shape of their large, compound leaves. Cycads bear
Modern gymnosperms are classified into four phyla. The first three
large cones and may be pollinated by beetles rather than wind,
(the Coniferophyta, Cycadophyta, and Gingkophyta) are similar in
which is unusual for a gymnosperm (). They dominated the
their production of secondary cambium (cells that generate the
landscape during the age of dinosaurs in the Mesozoic, but only a
vascular system of the trunk or stem and are partially specialized for hundred or so species persisted to modern times. Cycads face
water transportation) and their pattern of seed development. possible extinction; several species are protected through
However, these three phyla are not closely related phylogenetically
international conventions. Because of their attractive shape, they are
to each other. The fourth phylum (the Gnetophyta) are considered often used as ornamental plants in gardens in the tropics and
the closest group to angiosperms because they produce true xylem
subtropics.
tissue.

CONIFEROPHYTES
Conifers are the dominant phylum of gymnosperms, with the most
variety of species. They are typically tall trees that usually bear
scale-like or needle-like leaves. Water evaporation from leaves is
reduced by their thin shape and the thick cuticle. Snow slides easily
off needle-shaped leaves, keeping the load light and decreasing
breaking of branches. Adaptations to cold and dry weather explain
the predominance of conifers at high altitudes and in cold climates.
Conifers include familiar evergreen trees such as pines, spruces, firs,
cedars, sequoias, and yews. A few species are deciduous, losing
their leaves in fall. The European larch and the tamarack are
Figure 26.2C. 1 : Cycad leaves: This Encephalartos ferox cycad has
examples of deciduous conifers. Many coniferous trees are harvested large cones and broad, fern-like leaves.
for paper pulp and timber. The wood of conifers is more primitive

26.2C.1 https://bio.libretexts.org/@go/page/13678
GINGKOPHYTES
The single surviving species of the gingkophytes group is the
Gingko biloba. Its fan-shaped leaves, unique among seed plants
because they feature a dichotomous venation pattern, turn yellow in
autumn and fall from the tree. For centuries, G. biloba was
cultivated by Chinese Buddhist monks in monasteries, which Figure 26.2C. 1 : Gnetophytes: (a) Ephedra viridis, known by the
ensured its preservation. It is planted in public spaces because it is common name Mormon tea, grows on the West Coast of the United
unusually resistant to pollution. Male and female organs are States and Mexico. (b) Gnetum gnemon grows in Malaysia. (c) The
large Welwitschia mirabilis can be found in the Namibian desert.
produced on separate plants. Typically, gardeners plant only male
trees because the seeds produced by the female plant have an off- CONTRIBUTIONS AND ATTRIBUTIONS
putting smell of rancid butter. Plants-Gymnosperms. Provided by: sharonapbio-taxonomy Wikispace. Located
at: http://sharonapbio-taxonomy.wikispac...ts-Gymnosperms. License: CC
BY-SA: Attribution-ShareAlike
sporophyll. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/sporophyll. License: CC BY-SA: Attribution-
ShareAlike
heterosporous. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/heterosporous. License: CC BY-SA: Attribution-
ShareAlike
ovule. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/ovule.
License: CC BY-SA: Attribution-ShareAlike
Pinus contorta 8021. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/Fi...torta_8021.jpg. License: CC BY-SA:
Attribution-ShareAlike
Male Cones (3618723565). Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/File:Male_Cones_(3618723565).jpg. License:
CC BY-SA: Attribution-ShareAlike
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44648/latest/?collection=col11448/latest.
License: CC BY: Attribution
megaspore. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/megaspore. License: CC BY-SA: Attribution-
ShareAlike
monoecious. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/monoecious. License: CC BY-SA: Attribution-
ShareAlike
microspore. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/microspore. License: CC BY-SA: Attribution-
ShareAlike
Pinus contorta 8021. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/File:Pinus_contorta_8021.jpg. License: CC BY-
SA: Attribution-ShareAlike
Male Cones (3618723565). Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/File:Male_Cones_(3618723565).jpg. License:
CC BY-SA: Attribution-ShareAlike
GINGKO BILOBA OpenStax College, Gymnosperms. October 17, 2013. Provided by: OpenStax
Gingko biloba is the only surviving species of the phylum CNX. Located at:
http://cnx.org/content/m44648/latest/Figure_26_02_01.png. License: CC BY:
Gingkophyta. This plate from the 1870 book Flora Japonica, Sectio Attribution
Prima (Tafelband) depicts the leaves and fruit of Gingko biloba, as OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44648/latest/?collection=col11448/latest.
drawn by Philipp Franz von Siebold and Joseph Gerhard Zuccarini. License: CC BY: Attribution
tracheid. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/tracheid.
GNETOPHYTES License: CC BY-SA: Attribution-ShareAlike
conifer. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/conifer.
Gnetophytes are the closest relative to modern angiosperms and License: CC BY-SA: Attribution-ShareAlike
include three dissimilar genera of plants: Ephedra, Gnetum, and angiosperm. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/angiosperm. License: CC BY-SA: Attribution-
Welwitschia. Like angiosperms, they have broad leaves. In tropical ShareAlike
and subtropical zones, gnetophytes are vines or small shrubs. Pinus contorta 8021. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/File:Pinus_contorta_8021.jpg. License: CC BY-
Ephedra occurs in dry areas of the West Coast of the United States SA: Attribution-ShareAlike
and Mexico. Ephedra’s small, scale-like leaves are the source of the Male Cones (3618723565). Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/File:Male_Cones_(3618723565).jpg. License:
compound ephedrine, which is used in medicine as a potent CC BY-SA: Attribution-ShareAlike
decongestant. Because ephedrine is similar to amphetamines, both in OpenStax College, Gymnosperms. October 17, 2013. Provided by: OpenStax
chemical structure and neurological effects, its use is restricted to CNX. Located at:
http://cnx.org/content/m44648/latest/Figure_26_02_01.png. License: CC BY:
prescription drugs. Like angiosperms, but unlike other Attribution
gymnosperms, all gnetophytes possess vessel elements in their OpenStax College, Gymnosperms. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44648/latest/Figure_26_02_05.jpg.
xylem. License: CC BY: Attribution
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CNX. Located at: http://cnx.org/content/m44648/latest/Figure_26_02_04.jpg.
License: CC BY: Attribution

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OpenStax College, Gymnosperms. October 17, 2013. Provided by: OpenStax License: CC BY: Attribution
CNX. Located at:
http://cnx.org/content/m44648/latest/Figure_26_02_02abcd.jpg. License: CC This page titled 26.2C: Diversity of Gymnosperms is shared under a CC
BY: Attribution
OpenStax College, Gymnosperms. October 17, 2013. Provided by: OpenStax BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
CNX. Located at: http://cnx.org/content/m44648/latest/Figure_26_02_03.jpg.

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SECTION OVERVIEW

26.3: ANGIOSPERMS
26.3C: THE LIFE CYCLE OF AN ANGIOSPERM
Topic hierarchy
26.3D: DIVERSITY OF ANGIOSPERMS
26.3A: ANGIOSPERM FLOWERS
This page titled 26.3: Angiosperms is shared under a CC BY-SA 4.0 license
26.3B: ANGSIOSPERM FRUIT and was authored, remixed, and/or curated by Boundless.

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26.3A: ANGIOSPERM FLOWERS
Flowers are modified leaves containing the reproductive organs of corolla, are located inside the whorl of sepals and often display vivid
angiospems; their pollination is usually accomplished by animals or colors to attract pollinators. Flowers pollinated by wind are usually
wind. small, feathery, and visually inconspicuous. Sepals and petals
together form the perianth. The sexual organs (carpels and stamens)
 LEARNING OBJECTIVES are located at the center of the flower.
Styles, stigmas, and ovules constitute the female organ: the
Describe the main parts of a flower and their purposes
gynoecium or carpel. Flower structure is very diverse. Carpels may
be singular, multiple, or fused. Multiple fused carpels comprise a
KEY POINTS pistil. The megaspores and the female gametophytes are produced
Sepals, petals, carpels, and stamens are structures found in all and protected by the thick tissues of the carpel. A long, thin structure
flowers. called a style leads from the sticky stigma, where pollen is
To attract pollinators, petals usually exhibit vibrant colors; deposited, to the ovary, enclosed in the carpel. The ovary houses one
however, plants that depend on wind pollination contain flowers or more ovules, each of which will develop into a seed upon
that are small and light. fertilization. The male reproductive organs, the stamens (collectively
Carpels protect the female gametophytes and megaspores. called the androecium), surround the central carpel. Stamens are
The stigma is the structure where pollen is deposited and is composed of a thin stalk called a filament and a sac-like structure
connected to the ovary through the style. called the anther. The filament supports the anther, where the
The anther, which comprises the stamen, is the site of microspore microspores are produced by meiosis and develop into pollen grains.
production and their development into pollen.

KEY TERMS
sepal: a part of an angiosperm, and one of the component parts
of the calyx; collectively the sepals are called the calyx (plural
calyces), the outermost whorl of parts that form a flower
corolla: an outermost-but-one whorl of a flower, composed of
petals, when it is not the same in appearance as the outermost
whorl (the calyx); it usually comprises the petal, which may be
fused
stamen: in flowering plants, the structure in a flower that
produces pollen, typically consisting of an anther and a filament
carpel: one of the individual female reproductive organs in a
flower composed of an ovary, a style, and a stigma; also known
as the gynoecium

FLOWERS
Flowers are modified leaves, or sporophylls, organized around a
central stalk. Although they vary greatly in appearance, all flowers
contain the same structures: sepals, petals, carpels, and stamens. The
Figure 26.3A. 1 : Structure of flowers: This image depicts the
peduncle attaches the flower to the plant. A whorl of sepals structure of a flower. Perfect flowers produce both male and female
(collectively called the calyx) is located at the base of the peduncle floral organs. The flower shown has only one carpel, but some
and encloses the unopened floral bud. Sepals are usually flowers have a cluster of carpels. Together, all the carpels make up
the gynoecium.
photosynthetic organs, although there are some exceptions. For
example, the corolla in lilies and tulips consists of three sepals and This page titled 26.3A: Angiosperm Flowers is shared under a CC BY-SA
three petals that look virtually identical. Petals, collectively the 4.0 license and was authored, remixed, and/or curated by Boundless.

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26.3B: ANGSIOSPERM FRUIT
A fertilized, fully grown, and ripened ovary containing a seed forms herbivore’s feces. Other fruits have burs and hooks to cling to fur
what we know as fruit, important seed dispersal agents for plants. and hitch rides on animals.

 LEARNING OBJECTIVES

Recall the evolutionary advantage of fruits

KEY POINTS
Scientists classify fruit in many different categories that include
descriptions, such as mature, fleshy, and dry; only a few are
actually classified as being fleshy and sweet.
Some fruit are developed from ovaries, while others develop
from the pericarp, from clusters of flowers, or from separate
ovaries in a single flower.
Fruit are vital dispersal agents for plants; their unique shapes and
features evolved to take advantage of specific dispersal modes.
Dispersal methods of seeds within fruit include wind, water, Figure 26.3B. 1: Wind dispersal: The winged shape of Alsomitra
herbivores, and animal fur. macrocarpa’s seeds allow them to use wind for dispersal. They can,
therefore, glide for great distances.

KEY TERMS
fruit: the seed-bearing part of a plant, often edible, colorful, and
fragrant, produced from a floral ovary after fertilization
pericarp: the outermost layer, or skin, of a ripe fruit or ovary
hypanthium: the bowl-shaped part of a flower on which the
sepals, petals, and stamens are borne

FRUIT
In botany, a fertilized, fully-grown, and ripened ovary is a fruit. As
the seed develops, the walls of the ovary in which it forms thicken
and form the fruit, enlarging as the seeds grow. Many foods
commonly-called vegetables are actually fruit. Eggplants, zucchini,
string beans, and bell peppers are all technically fruit because they
contain seeds and are derived from the thick ovary tissue. Acorns are
nuts and winged, maple whirligigs (whose botanical name is samara)
are also fruit. Botanists classify fruit into more than two dozen
different categories, only a few of which are actually fleshy and
sweet.
Mature fruit can be fleshy or dry. Fleshy fruit include the familiar
berries, peaches, apples, grapes, and tomatoes. Rice, wheat, and nuts
are examples of dry fruit. Another distinction is that not all fruits are
derived from the ovary. For instance, strawberries are derived from
the receptacle, while apples are derived from the pericarp, or
hypanthium. Some fruits are derived from separate ovaries in a
single flower, such as the raspberry. Other fruits, such as the
pineapple, form from clusters of flowers. Additionally, some fruits,
like watermelon and oranges, have rinds.
Regardless of how they are formed, fruits are an agent of seed
dispersal. The variety of shapes and characteristics reflect the mode
of dispersal, whether it be wind, water, or animals. Wind carries the
light dry fruit of trees and dandelions. Water transports floating Figure 26.3B. 1: Fruit dispersal: A fruit’s distinctive shape and
specialized characteristics will determine its dispersal mechanism.
coconuts. Some fruits attract herbivores with color or perfume, or as
food. Once eaten, tough, undigested seeds are dispersed through the This page titled 26.3B: Angsiosperm Fruit is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

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26.3C: THE LIFE CYCLE OF AN ANGIOSPERM
Angiosperms are seed-producing plants that generate male and
female gametophytes, which allow them to carry out double
fertilization.

 LEARNING OBJECTIVES

Explain the life cycle of an angiosperm, including cross-

pollination and the ways in which it takes place

KEY POINTS
Microspores develop into pollen grains, which are the male
gametophytes, while megaspores form an ovule that contains the
female gametophytes.
In the ovule, the megasporocyte undergoes meiosis, generating
four megaspores; three small and one large; only the large
megaspore survives and produces the female gametophyte
(embryo sac).
When the pollen grain reaches the stigma, it extends its pollen
tube to enter the ovule and deposits two sperm cells in the
embryo sac.
The two available sperm cells allow for double fertilization to
occur, which results in a diploid zygote (the future embryo) and a
triploid cell (the future endosperm), which acts as a food store.
Some species are hermaphroditic (stamens and pistils are
contained on a single flower), some species are monoecious Figure 26.3C. 1 : Life cycle of angiosperms: The life cycle of an
(stamens and pistils occur on separate flowers, but the same angiosperm is shown. Anthers and carpels are structures that shelter
the actual gametophytes: the pollen grain and embryo sac. Double
plant), and some are dioecious (staminate and pistillate flowers fertilization is a process unique to angiosperms.
occur on separate plants). The ovule, sheltered within the ovary of the carpel, contains the
megasporangium protected by two layers of integuments and the
KEY TERMS
ovary wall. Within each megasporangium, a megasporocyte
cotyledon: the leaf of the embryo of a seed-bearing plant; after undergoes meiosis, generating four megaspores: three small and one
germination it becomes the first leaves of the seedling large. Only the large megaspore survives; it produces the female
heterosporous: producing both male and female gametophytes gametophyte referred to as the embryo sac. The megaspore divides
synergid: either of two nucleated cells at the top of the embryo three times to form an eight-cell stage. Four of these cells migrate to
sac that aid in the production of the embryo; helper cells each pole of the embryo sac; two come to the equator and will
THE LIFE CYCLE OF AN ANGIOSPERM eventually fuse to form a 2n polar nucleus. The three cells away
from the egg form antipodals while the two cells closest to the egg
The adult, or sporophyte, phase is the main phase of an angiosperm’s
become the synergids.
life cycle. As with gymnosperms, angiosperms are heterosporous.
Therefore, they generate microspores, which will produce pollen The mature embryo sac contains one egg cell, two synergids
grains as the male gametophytes, and megaspores, which will form (“helper” cells), three antipodal cells, and two polar nuclei in a
central cell. When a pollen grain reaches the stigma, a pollen tube
an ovule that contains female gametophytes. Inside the anthers’
microsporangia, male gametophytes divide by meiosis to generate extends from the grain, grows down the style, and enters through the
micropyle, an opening in the integuments of the ovule. The two
haploid microspores, which, in turn, undergo mitosis and give rise to
pollen grains. Each pollen grain contains two cells: one generative sperm cells are deposited in the embryo sac.
cell that will divide into two sperm and a second cell that will A double fertilization event then occurs. One sperm and the egg
become the pollen tube cell. combine, forming a diploid zygote, the future embryo. The other
sperm fuses with the 2n polar nuclei, forming a triploid cell that will
develop into the endosperm, which is tissue that serves as a food
reserve. The zygote develops into an embryo with a radicle, or small
root, and one ( monocot ) or two (dicot) leaf-like organs called
cotyledons. This difference in the number of embryonic leaves is the
basis for the two major groups of angiosperms: the monocots and the

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eudicots. Seed food reserves are stored outside the embryo in the Some species of angiosperms are hermaphroditic (stamens and
form of complex carbohydrates, lipids, or proteins. The cotyledons pistils are contained on a single flower), some species are
serve as conduits to transmit the broken-down food reserves from monoecious (stamens and pistils occur on separate flowers, but the
their storage site inside the seed to the developing embryo. The seed same plant), and some are dioecious (staminate and pistillate flowers
consists of a toughened layer of integuments forming the coat, the occur on separate plants). Both anatomical and environmental
endosperm with food reserves, and the well-protected embryo at the barriers promote cross-pollination mediated by a physical agent
center. (wind or water) or an animal, such as an insect or bird. Cross-
pollination increases genetic diversity in a species.

This page titled 26.3C: The Life Cycle of an Angiosperm is shared under a
CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Boundless.

Figure 26.3C. 1 : The fruit of the Aesculus or Horse Chestnut tree:

These seeds are enclosed a protective outer covering called the seed
coat, usually with some stored food. After fertilization and some
growth in the angiosperm, the ripened ovule is produced. The
formation of the seed completes the process of reproduction in seed
plants (started with the development of flowers and pollination),
with the embryo developed from the zygote and the seed coat from
the integuments of the ovule.

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26.3D: DIVERSITY OF ANGIOSPERMS
Angiosperm diversity is divided into two main groups, monocot and
dicots, based primarily on the number of cotyledons they possess.

 LEARNING OBJECTIVES

Explain how angiosperm diversity is classified

KEY POINTS
Angiosperm are flowering plants that are classified based on
characteristics that include (but are not limited to) cotyledon
structure, pollen grains, as well as flower and vascular tissue
arrangement.
Basal angiosperms, classified separately, contain features found
in both monocots and dicots, as they are believed to have
originated before the separation of these two main groups.
Monocots contain a single cotyledon and have veins that run
parallel to the length of their leaves; their flowers are arranged in
three to six-fold symmetry.
Dicots have flowers arranged in whorls, two cotyledons, and a
vein arrangement that forms networks within their leaves.
Monocots do not contain any true woody tissue while dicots can
be herbacious or woody and have vascular tissue that forms a
ring in the stem.

KEY TERMS Figure 26.3D. 1 : Examples of basal angiosperms: The (a) common
dicot: a plant whose seedlings have two cotyledons; a spicebush belongs to the Laurales, the same family as cinnamon and
bay laurel. The fruit of (b) the Piper nigrum plant is black pepper,
dicotyledon the main product that was traded along spice routes. Notice the
angiosperm: a plant whose ovules are enclosed in an ovary small, unobtrusive, clustered flowers. (c) Lotus flowers, Nelumbo
monocot: one of two major groups of flowering plants (or nucifera, have been cultivated since ancient times for their
ornamental value; the root of the lotus flower is eaten as a vegetable.
angiosperms) that are traditionally recognized; seedlings The red seeds of (d) a magnolia tree, characteristic of the final stage,
typically have one cotyledon (seed-leaf) are just starting to appear.
cotyledon: the leaf of the embryo of a seed-bearing plant; after
germination it becomes the first leaves of the seedling BASAL ANGIOSPERMS
basal angiosperm: the first flowering plants to diverge from the Examples of basal angiosperms include the Magnoliidae, Laurales,
ancestral angiosperm, including a single species of shrub from Nymphaeales, and the Piperales. Members in these groups all share
New Caledonia, water lilies and some other aquatic plants, and traits from both monocot and dicot groups. The Magnoliidae are
woody aromatic plants represented by the magnolias: tall trees bearing large, fragrant
flowers that have many parts and are considered archaic. Laurel
DIVERSITY OF ANGIOSPERMS trees produce fragrant leaves and small, inconspicuous flowers. The
Angiosperms are classified in a single phylum: the Anthophyta. Laurales grow mostly in warmer climates and are small trees and
Modern angiosperms appear to be a monophyletic group, which shrubs. Familiar plants in this group include the bay laurel,
means that they originated from a single ancestor. Flowering plants cinnamon, spice bush, and avocado tree. The Nymphaeales are
are divided into two major groups according to the structure of the comprised of the water lilies, lotus, and similar plants; all species
cotyledons and pollen grains, among others. Monocots include thrive in freshwater biomes and have leaves that float on the water
grasses and lilies while eudicots or dicots form a polyphyletic group. surface or grow underwater. Water lilies are particularly prized by
However, many species exhibit characteristics that belong to either gardeners and have graced ponds and pools for thousands of years.
group; as such, the classification of a plant as a monocot or a eudicot The Piperales are a group of herbs, shrubs, and small trees that grow
is not always clearly evident. Basal angiosperms are a group of in the tropical climates. They have small flowers without petals that
plants that are believed to have branched off before the separation are tightly arranged in long spikes. Many species are the source of
into monocots and eudicots because they exhibit traits from both prized fragrance or spices; for example, the berries of Piper nigrum
groups. They are categorized separately in many classification are the familiar black peppercorns that are used to flavor many
schemes. The Magnoliidae (magnolia trees, laurels, and water lilies) dishes.
and the Piperaceae (peppers) belong to the basal angiosperm group.

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MONOCOTS sepal. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/sepal.
License: CC BY-SA: Attribution-ShareAlike
Plants in the monocot group are primarily identified as such by the OpenStax College, Angiosperms. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44650/latest...e_26_03_02.jpg.
presence of a single cotyledon in the seedling. Other anatomical License: CC BY: Attribution
features shared by monocots include veins that run parallel to the OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44650/latest...ol11448/latest. License: CC
length of the leaves and flower parts that are arranged in a three- or BY: Attribution
six-fold symmetry. True woody tissue is rarely found in monocots. fruit. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/fruit.
In palm trees, vascular and parenchyma tissues produced by the License: CC BY-SA: Attribution-ShareAlike
hypanthium. Provided by: Wiktionary. Located at:
primary and secondary thickening of meristems form the trunk. The en.wiktionary.org/wiki/hypanthium. License: CC BY-SA: Attribution-
pollen from the first angiosperms was monosulcate, containing a ShareAlike
pericarp. Provided by: Wiktionary. Located at:
single furrow or pore through the outer layer. This feature is still en.wiktionary.org/wiki/pericarp. License: CC BY-SA: Attribution-ShareAlike
seen in the modern monocots. Vascular tissue of the stem is not OpenStax College, Angiosperms. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44650/latest...e_26_03_02.jpg.
arranged in any particular pattern. The root system is mostly License: CC BY: Attribution
adventitious and unusually positioned, with no major tap root. The Forest fruits from Barro Colorado. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/Fi...o_Colorado.png. License: CC BY:
monocots include familiar plants such as the true lilies (which are Attribution
the origin of their alternate name: Liliopsida), orchids, grasses, and Alsomitra macrocarpa seed (syn.nZanonia macrocarpa). Provided by:
Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...acrocarpa).jpg.
palms. Many important crops are monocots, such as rice and other
License: CC BY: Attribution
cereals, corn, sugar cane, and tropical fruits like bananas and OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
pineapples. Located at: http://cnx.org/content/m44650/latest/?collection=col11448/latest.
License: CC BY: Attribution
heterosporous. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/heterosporous. License: CC BY-SA: Attribution-
ShareAlike
synergid. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/synergid. License: CC BY-SA: Attribution-ShareAlike
cotyledon. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/cotyledon. License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Angiosperms. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44650/latest/Figure_26_03_02.jpg.
License: CC BY: Attribution
Forest fruits from Barro Colorado. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/File:Forest_fruits_from_Barro_Colorado.png.
License: CC BY: Attribution
Alsomitra macrocarpa seed (syn.nZanonia macrocarpa). Provided by:
Wikimedia. Located at: commons.wikimedia.org/wiki/Fi...acrocarpa).jpg.
License: CC BY: Attribution
Aesculus hippocastanum fruit. Provided by: Wikimedia commons. Located at:
en.Wikipedia.org/wiki/File:Ae...anum_fruit.jpg. License: CC BY-SA:
Attribution-ShareAlike
data-attribution-url=cnx.org/content/m44650/latest...e_26_03_03.png. Provided
by: Connexions. License: CC BY: Attribution
Figure 26.3D. 1 : Monocots and Dicots: major crops of the world: angiosperm. Provided by: Wiktionary. Located at:
The world’s major crops are flowering plants. (a) Rice, (b) wheat, en.wiktionary.org/wiki/angiosperm. License: CC BY-SA: Attribution-
and (c) bananas are monocots, while (d) cabbage, (e) beans, and (f) ShareAlike
peaches are dicots. OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44650/latest/?collection=col11448/latest.
License: CC BY: Attribution
EUDICOTS cotyledon. Provided by: Wiktionary. Located at:
Eudicots, or true dicots, are characterized by the presence of two en.wiktionary.org/wiki/cotyledon. License: CC BY-SA: Attribution-ShareAlike
basal angiosperm. Provided by: Wikipedia. Located at:
cotyledons in the developing shoot. Veins form a network in leaves, en.Wikipedia.org/wiki/basal%20angiosperm. License: CC BY-SA:
while flower parts come in four, five, or many whorls. Vascular Attribution-ShareAlike
dicot. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/dicot.
tissue forms a ring in the stem whereas in monocots, vascular tissue License: CC BY-SA: Attribution-ShareAlike
is scattered in the stem. Eudicots can be herbaceous (like grasses), or monocot. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/monocot. License: CC BY-SA: Attribution-ShareAlike
produce woody tissues. Most eudicots produce pollen that is
OpenStax College, Angiosperms. October 17, 2013. Provided by: OpenStax
trisulcate or triporate, with three furrows or pores. The root system is CNX. Located at: http://cnx.org/content/m44650/latest/Figure_26_03_02.jpg.
usually anchored by one main root developed from the embryonic License: CC BY: Attribution
Forest fruits from Barro Colorado. Provided by: Wikimedia. Located at:
radicle. Eudicots comprise two-thirds of all flowering plants. http://commons.wikimedia.org/wiki/File:Forest_fruits_from_Barro_Colorad
o.png. License: CC BY: Attribution
Alsomitra macrocarpa seed (syn.nZanonia macrocarpa). Provided by:
CONTRIBUTIONS AND ATTRIBUTIONS Wikimedia. Located at:
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. commons.wikimedia.org/wiki/File:Alsomitra_macrocarpa_seed_(syn._Zano
Located at: http://cnx.org/content/m44650/latest...ol11448/latest. License: CC nia_macrocarpa).jpg. License: CC BY: Attribution
BY: Attribution Aesculus hippocastanum fruit. Provided by: Wikimedia commons. Located at:
carpel. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/carpel. en.Wikipedia.org/wiki/File:Ae...anum_fruit.jpg. License: CC BY-SA:
License: CC BY-SA: Attribution-ShareAlike Attribution-ShareAlike
corolla. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/corolla. data-attribution-url=cnx.org/content/m44650/latest...e_26_03_03.png. Provided
License: CC BY-SA: Attribution-ShareAlike by: Connexions. License: CC BY: Attribution
stamen. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/stamen. OpenStax College, Angiosperms. October 17, 2013. Provided by: OpenStax
License: CC BY-SA: Attribution-ShareAlike CNX. Located at: http://cnx.org/content/m44650/latest...e_26_03_05.jpg.
License: CC BY: Attribution

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CNX. Located at: http://cnx.org/content/m44650/latest..._03_04abcd.jpg. This page titled 26.3D: Diversity of Angiosperms is shared under a CC BY-
License: CC BY: Attribution SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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SECTION OVERVIEW

26.4: THE ROLE OF SEED PLANTS

26.4B: THE IMPORTANCE OF SEED PLANTS IN
Topic hierarchy HUMAN LIFE

26.4C: BIODIVERSITY OF PLANTS

26.4A: HERBIVORY AND POLLINATION

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26.4A: HERBIVORY AND POLLINATION
The diversity of plants can be attributed to pollination and herbivory, therefore, unsavory to some animals. Other plants are protected by
both examples of coevolution between animals and plants. bark, although some animals have developed specialized mouth
pieces to tear and chew vegetal material. Spines and thorns deter
 LEARNING OBJECTIVES most animals, except for mammals with thick fur; some birds have
specialized beaks to get past such defenses.
Describe the interaction of plants and animals in achieving
pollination

KEY POINTS
Herbivory is believed to have been as much a driving force in the
evolution of plant diversity as pollination.
Coevolution between herbivores and plants is commonly seen in
nature; for example, plants have developed unique ways to fight
off herbivores while, in turn, herbivores have developed
specialized features to get around these defenses. Figure 26.4A. 1 : Plant defenses from herbivory: (a) Spines and (b)
Plants have developed unique pollination adaptations, such as the thorns are examples of plant defenses.
ability to capture the wind or attract specific classes of animals. Herbivory has been used by seed plants for their own benefit in a
Birds, insects, bats, lemurs, and lizards can act as pollinators; display of mutualistic relationships. The dispersal of fruit by animals
each is attracted to a specific plant adaptation, which has been is the most striking example. The plant offers to the herbivore a
developed to attract a suitable pollinator. nutritious source of food in return for spreading the plant’s genetic
Any disruption between pollinator and plant interactions, such as material to a wider area.
the extinction of a species, can lead to the collapse of an
An extreme example of collaboration between an animal and a plant
ecosystem and/or the demise of an agricultural industry.
is the case of acacia trees and ants. The trees support the insects with
KEY TERMS shelter and food. In return, ants discourage herbivores, both
invertebrates and vertebrates, by stinging and attacking leaf-eating
coevolution: the evolution of organisms of two or more species
organisms.
in which each adapts to changes in the other
pollination: the transfer of pollen from an anther to a stigma that POLLINATION
is carried out by insects, birds, bats, and the wind
Grasses are a successful group of flowering plants that are wind
herbivory: the consumption of living plant tissue by animals
pollinated. They produce large amounts of powdery pollen carried
ANIMAL & PLANT INTERACTIONS over large distances by the wind. The flowers are small and wisp-
like. Large trees such as oaks, maples, and birches are also wind
Angiosperm diversity is due in part to multiple interactions with
pollinated.
animals. Herbivory has favored the development of defense
mechanisms in plants and avoidance of those defense mechanisms in More than 80 percent of angiosperms depend on animals for
animals. Pollination (the transfer of pollen to a carpel) is mainly pollination: the transfer of pollen from the anther to the stigma.
carried out by wind and animals; therefore, angiosperms have Consequently, plants have developed many adaptations to attract
evolved numerous adaptations to capture the wind or attract specific pollinators. The specificity of specialized plant structures that target
classes of animals. animals can be very surprising. It is possible, for example, to
determine the type of pollinator favored by a plant just from the
Coevolution of flowering plants and insects is a hypothesis that has
flower’s characteristics. Many bird or insect-pollinated flowers
received much attention and support, especially because both
secrete nectar, a sugary liquid. They also produce both fertile pollen
angiosperms and insects diversified at about the same time in the
for reproduction and sterile pollen rich in nutrients for birds and
middle Mesozoic. Many authors have attributed the diversity of
insects. Butterflies and bees can detect ultraviolet light. Flowers that
plants and insects to pollination and herbivory, which is the
attract these pollinators usually display a pattern of low ultraviolet
consumption of plants by insects and other animals. This is believed
reflectance that helps them quickly locate the flower’s center to
to have been as much a driving force as pollination.
collect nectar while being dusted with pollen. Large, red flowers
HERBIVORY with little smell and a long funnel shape are preferred by
hummingbirds who have good color perception, a poor sense of
Coevolution of herbivores and plant defenses is observed in nature.
Unlike animals, most plants cannot outrun predators or use mimicry smell, and need a strong perch. White flowers that open at night
attract moths. Other animals (such as bats, lemurs, and lizards) can
to hide from hungry animals. A sort of arms race exists between
plants and herbivores. To “combat” herbivores, some plant seeds also act as pollinating agents. Any disruption to these interactions,
such as the disappearance of bees as a consequence of colony
(such as acorn and unripened persimmon) are high in alkaloids and,

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collapse disorders, can lead to disaster for agricultural industries that
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depend heavily on pollinated crops. SA 4.0 license and was authored, remixed, and/or curated by Boundless.

Figure 26.4A. 1 : Animal-aided pollination: As a bee collects nectar

from a flower, it is dusted by pollen, which it then disperses to other
flowers.

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26.4B: THE IMPORTANCE OF SEED PLANTS IN HUMAN LIFE
Human life has become dependent on plants for the qualities and
developments that they provide, which include medicine and food
production.

 LEARNING OBJECTIVES

Explain the importance of seed plants to humans

KEY POINTS
Providing much of the nutritional values that humans need, seed
plants are the foundation of human diets across the world.
Wood, paper, textiles, and dyes are just a few examples of plant
uses in everyday human life.
Traditionally, humans have also used plants as ornamental
species through their use as decorations and as inspiration in the
arts.
As medicinal sources, plants are vital to humans, as many
modern drugs have been derived from secondary plant
metabolites; ancient societies also depended on them for their
curative properties.
The important relationship that human cultures have developed
with plants can be studied through the field of ethnobotany.

KEY TERMS
ethnobotany: the scientific study of the relationships between
people and plants
Figure 26.4B. 1: Importance of plants to humans: Humans rely on
pharmacognosy: the branch of pharmacology that studies plants for a variety of reasons. (a) Cacao beans were introduced in
medical substances that are extracted from natural sources, Europe from the New World, where they were used by
including drugs derived from plants and herbs used for medicinal Mesoamerican civilizations. Combined with sugar, another plant
product, chocolate is a popular food. (b) Flowers like the tulip are
purposes cultivated for their beauty. (c) Quinine, extracted from cinchona
husbandry: the raising of livestock and the cultivation of crops; trees, is used to treat malaria, to reduce fever, and to alleviate pain.
agriculture (d) This violin is made of wood.

THE IMPORTANCE OF SEED PLANTS IN HUMAN Staple crops are not the only food derived from seed plants. Fruits
LIFE and vegetables provide nutrients, vitamins, and fiber. Sugar, to
Seed plants are cultivated for their beauty and smells, as well as their sweeten dishes, is produced from the monocot sugarcane and the
importance in the development of medicines. Plants are also the eudicot sugar beet. Drinks are made from infusions of tea leaves,
foundation of human diets across the world. Many societies eat, chamomile flowers, crushed coffee beans, or powdered cocoa beans.
almost exclusively, vegetarian fare and depend solely on seed plants Spices come from many different plant parts: saffron and cloves are
for their nutritional needs. A few crops (rice, wheat, and potatoes) stamens and buds, black pepper and vanilla are seeds, the bark of a
dominate the agricultural landscape. Many crops were developed bush in the Laurales family supplies cinnamon, and the herbs that
during the agricultural revolution when human societies made the flavor many dishes come from dried leaves and fruit, such as the
transition from nomadic hunter–gatherers to horticulture and pungent red chili pepper. The volatile oils of flowers and bark
agriculture. Cereals, rich in carbohydrates, provide the staple of provide the scent of perfumes. Additionally, no discussion of seed
many human diets. In addition, beans and nuts supply proteins. Fats plant contribution to human diet would be complete without the
are derived from crushed seeds, as is the case for peanut and mention of alcohol. Fermentation of plant-derived sugars and
rapeseed (canola) oils, or fruits such as olives. Animal husbandry starches is used to produce alcoholic beverages in all societies. In
also requires large amounts of crops. some cases, the beverages are derived from the fermentation of
sugars from fruit, as with wines, and, in other cases, from the
fermentation of carbohydrates derived from seeds, as with beers.
Seed plants have many other uses, including providing wood as a
source of timber for construction, fuel, and material to build
furniture. Most paper is derived from the pulp of coniferous trees.

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Fibers of seed plants, such as cotton, flax, and hemp, are woven into Pharmacognosy is the branch of pharmacology that focuses on
cloth. Textile dyes, such as indigo, were mostly of plant origin until medicines derived from natural sources. With massive globalization
the advent of synthetic chemical dyes. Lastly, it is more difficult to and industrialization, there is a concern that much human knowledge
quantify the benefits of ornamental seed plants. These grace private of plants and their medicinal purposes will disappear with the
and public spaces, adding beauty and serenity to human lives and cultures that fostered them. This is where ethnobotanists come in. To
inspiring painters and poets alike. learn about and understand the use of plants in a particular culture,
The medicinal properties of plants have been known to human an ethnobotanist must bring in knowledge of plant life and an
societies since ancient times. There are references to the use of understanding and appreciation of diverse cultures and traditions.
plants’ curative properties in Egyptian, Babylonian, and Chinese The Amazon forest is home to an incredible diversity of vegetation
writings from 5,000 years ago. Many modern synthetic therapeutic and is considered an untapped resource of medicinal plants; yet, both
drugs are derived or synthesized de novo from plant secondary the ecosystem and its indigenous cultures are threatened with
metabolites. It is important to note that the same plant extract can be extinction.
a therapeutic remedy at low concentrations, become an addictive To become an ethnobotanist, a person must acquire a broad
drug at higher doses, and can potentially kill at high concentrations. knowledge of plant biology, ecology, and sociology. Not only are the
plant specimens studied and collected, but also the stories, recipes,
ETHNOBOTANY and traditions that are linked to them. For ethnobotanists, plants are
The relatively new field of ethnobotany studies the interaction not viewed solely as biological organisms to be studied in a
between a particular culture and the plants native to the region. Seed laboratory; they are seen as an integral part of human culture. The
plants have a large influence on day-to-day human life. Not only are convergence of molecular biology, anthropology, and ecology make
plants the major source of food and medicine, they also influence the field of ethnobotany a truly multidisciplinary science.
many other aspects of society, from clothing to industry. The
medicinal properties of plants were recognized early on in human This page titled 26.4B: The Importance of Seed Plants in Human Life is
cultures. From the mid-1900s, synthetic chemicals began to supplant shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
plant-based remedies. curated by Boundless.

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26.4C: BIODIVERSITY OF PLANTS
Plant biodiversity, vital to ecosystems, food crops, and medicine
production, is threatened by habitat destruction and species
extinction.

 LEARNING OBJECTIVES

Discuss the threats to plant biodiversity

KEY POINTS
Plant biodiversity is invaluable because it balances ecosystems,
protects watersheds, mitigates erosion, moderates climate, and
provides shelter for animals.
Threats to plant biodiversity include the increasing human
population, pollution, deforestation, and species extinction.
Plant extinction is progressing at an alarming rate; this, in turn,
affects other species, which also become extinct because they
Figure 26.4C. 1 : Indiscriminate logging: Indiscriminate logging,
depend on the delicate ecological balance.Efforts to preserve which leads to the clearing of whole habitats, has become a severe
plant biodiversity currently include heirloom seed collections threat to plant biodiversity and has led to species extinction.
and barcoding DNA analysis. The number of plant species becoming extinct is increasing at an
alarming rate. Because ecosystems are in a delicate balance and
KEY TERMS because seed plants maintain close symbiotic relationships with
biodiversity: the diversity (number and variety of species) of animals, whether predators or pollinators, the disappearance of a
plant and animal life within a region single plant can lead to the extinction of connected animal species.
barcoding: a taxonomic method that uses a short genetic marker A real and pressing issue is that many plant species have not yet
in an organism’s DNA to identify it as belonging to a particular been cataloged; their place in the ecosystem is unknown. These
species unknown species are threatened by logging, habitat destruction, and
heirloom seed: seeds which are not of agricultural importance loss of pollinators. They may become extinct before we have the
yet hold traditional importance; these seeds are kept in seed chance to begin to understand the possible impacts resulting from
banks and are still maintained by some gardeners and farmers their disappearance. Efforts to preserve biodiversity take several
lines of action, from preserving heirloom seeds to barcoding species.
THREATS TO PLANT BIODIVERSITY Heirloom seeds come from plants that were traditionally grown in
Plants play a key role in ecosystems. They are a source of food and human populations, as opposed to the seeds used for large-scale
medicinal compounds while also providing raw materials for many agricultural production. Barcoding is a technique in which one or
industries. Rapid deforestation and industrialization, however, more short gene sequences, taken from a well-characterized portion
threaten plant biodiversity. In turn, this threatens the ecosystem. of the genome, are used to identify a species through DNA analysis.
Biodiversity of plants ensures a resource for new food crops and
medicines. Plant life balances ecosystems, protects watersheds, CONTRIBUTIONS AND ATTRIBUTIONS
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mitigates erosion, moderates climate, and provides shelter for many
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angles. The explosion of the human population, especially in tropical OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44652/latest...ol11448/latest. License: CC
countries where birth rates are highest and economic development is BY: Attribution
in full swing, is leading to human encroachment into forested areas. OpenStax College, Biology. November 13, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44652/latest...ol11448/latest. License: CC
To feed the larger population, humans need to obtain arable land BY: Attribution
which leads to massive clearing of trees. The need for more energy coevolution. Provided by: Wiktionary. Located at:
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come from poachers who log trees for their precious wood. Ebony pollination. Provided by: Wiktionary. Located at:
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and Brazilian rosewood, both on the endangered list, are examples ShareAlike
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 CHAPTER OVERVIEW

27: INTRODUCTION TO ANIMAL DIVERSITY

Topic hierarchy
27.1: Features of the Animal Kingdom
27.1A: Characteristics of the Animal Kingdom
27.1B: Complex Tissue Structure
27.1C: Animal Reproduction and Development
27.2: Features Used to Classify Animals
27.2A: Animal Characterization Based on Body Symmetry
27.2B: Animal Characterization Based on Features of Embryological Development
27.3: Animal Phylogeny
27.3A: Constructing an Animal Phylogenetic Tree
27.3B: Molecular Analyses and Modern Phylogenetic Trees
27.4: The Evolutionary History of the Animal Kingdom
27.4A: Pre-Cambrian Animal Life
27.4B: The Cambrian Explosion of Animal Life
27.4C: Post-Cambrian Evolution and Mass Extinctions

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1
SECTION OVERVIEW

27.1: FEATURES OF THE ANIMAL KINGDOM

27.1B: COMPLEX TISSUE STRUCTURE
Topic hierarchy
27.1C: ANIMAL REPRODUCTION AND
DEVELOPMENT
27.1A: CHARACTERISTICS OF THE ANIMAL
KINGDOM
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27.1A: CHARACTERISTICS OF THE ANIMAL KINGDOM
The animal kingdom is very diverse, but animals share many They must identify traits that are common to all animals as well as
common characteristics, such as methods of development and traits that can be used to distinguish among related groups of
reproduction. animals. The animal classification system characterizes animals
based on their anatomy, morphology, evolutionary history, features
 LEARNING OBJECTIVES of embryological development, and genetic makeup. This
classification scheme is constantly developing as new information
Describe the methods used to classify animals about species arises. Understanding and classifying the great variety
of living species help us better understand how to conserve the
KEY POINTS diversity of life on earth.
Animals vary in complexity and are classified based on anatomy, Even though members of the animal kingdom are incredibly diverse,
morphology, genetic makeup, and evolutionary history. most animals share certain features that distinguish them from
All animals are eukaryotic, multicellular organisms, and most organisms in other kingdoms. All animals are eukaryotic,
animals have complex tissue structure with differentiated and multicellular organisms, and almost all animals have a complex
specialized tissue. tissue structure with differentiated and specialized tissues. Most
Animals are heterotrophs; they must consume living or dead animals are motile, at least during certain life stages. All animals
organisms since they cannot synthesize their own food and can require a source of food and are, therefore, heterotrophic: ingesting
be carnivores, herbivores, omnivores, or parasites. other living or dead organisms. This feature distinguishes them from
Most animals are motile for at least some stages of their lives, autotrophic organisms, such as most plants, which synthesize their
and most animals reproduce sexually. own nutrients through photosynthesis. As heterotrophs, animals may
be carnivores, herbivores, omnivores, or parasites. Most animals
KEY TERMS reproduce sexually with the offspring passing through a series of
body plan: an assemblage of morphological features shared developmental stages that establish a fixed body plan. The body plan
among many members of a phylum-level group refers to the morphology of an animal, determined by developmental
heterotroph: an organism that requires an external supply of cues.
energy in the form of food, as it cannot synthesize its own
extant: still in existence; not extinct

INTRODUCTION: FEATURES OF THE ANIMAL

KINGDOM
Animal evolution began in the ocean over 600 million years ago
with tiny creatures that probably do not resemble any living
organism today. Since then, animals have evolved into a highly-
diverse kingdom. Although over one million extant (currently living)
species of animals have been identified, scientists are continually
discovering more species as they explore ecosystems around the
Figure 27.1A. 1 : Heterotrophs: All animals are heterotrophs that
world. The number of extant species is estimated to be between 3 derive energy from food. The (a) black bear is an omnivore, eating
and 30 million. both plants and animals. The (b) heartworm Dirofilaria immitis is a
parasite that derives energy from its hosts. It spends its larval stage
But what is an animal? While we can easily identify dogs, birds, in mosquitoes and its adult stage infesting the heart of dogs and
fish, spiders, and worms as animals, other organisms, such as corals other mammals.
and sponges, are not as easy to classify. Animals vary in complexity,
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with the difficult task of classifying them within a unified system.
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27.1B: COMPLEX TISSUE STRUCTURE
Animals, besides Parazoa (sponges), are characterized by specialized type of connective tissue that supports the entire body structure. The
tissues such as muscle, nerve, connective, and epithelial tissues. complex bodies and activities of vertebrates demand such supportive
tissues. Epithelial tissues cover, line, protect, and secrete; these
 LEARNING OBJECTIVES tissues include the epidermis of the integument: the lining of the
digestive tract and trachea. They also make up the ducts of the liver
List the various specialized tissue types found in animals and glands of advanced animals.
and describe their functions
The animal kingdom is divided into Parazoa (sponges) and
Eumetazoa (all other animals). As very simple animals, the
KEY POINTS organisms in group Parazoa (“beside animal”) do not contain true
Animal cells don’t have cell walls; their cells may be embedded specialized tissues. Although they do possess specialized cells that
in an extracellular matrix and have unique structures for perform different functions, those cells are not organized into
intercellular communication. tissues. These organisms are considered animals since they lack the
Animals have nerve and muscle tissues, which provide ability to make their own food. Animals with true tissues are in the
coordination and movement; these are not present in plants and group Eumetazoa (“true animals”). When we think of animals, we
fungi. usually think of Eumetazoans, since most animals fall into this
Complex animal bodies demand connective tissues made up of category.
organic and inorganic materials that provide support and
structure.
Animals are also characterized by epithelial tissues, like the
epidermis, which function in secretion and protection.
The animal kingdom is divided into Parazoa (sponges), which do
not contain true specialized tissues, and Eumetazoa (all other
animals), which do contain true specialized tissues.

KEY TERMS
Parazoa: a taxonomic subkingdom within the kingdom
Animalia; the sponges
Eumetazoa: a taxonomic subkingdom, within kingdom
Animalia; all animals except the sponges
epithelial tissue: one of the four basic types of animal tissue,
which line the cavities and surfaces of structures throughout the
body, and also form many glands
Figure 27.1B. 1: Sponges: Sponges, such as those in the Caribbean
COMPLEX TISSUE STRUCTURE Sea, are classified as Parazoans because they are very simple
animals that do not contain true specialized tissues.
As multicellular organisms, animals differ from plants and fungi
because their cells don’t have cell walls; their cells may be The different types of tissues in true animals are responsible for
embedded in an extracellular matrix (such as bone, skin, or carrying out specific functions for the organism. This differentiation
connective tissue); and their cells have unique structures for and specialization of tissues is part of what allows for such
intercellular communication (such as gap junctions). In addition, incredible animal diversity. For example, the evolution of nerve
animals possess unique tissues, absent in fungi and plants, which tissues and muscle tissues has resulted in animals’ unique ability to
allow coordination (nerve tissue) and motility (muscle tissue). rapidly sense and respond to changes in their environment. This
Animals are also characterized by specialized connective tissues that allows animals to survive in environments where they must compete
provide structural support for cells and organs. This connective with other species to meet their nutritional demands.
tissue constitutes the extracellular surroundings of cells and is made
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up of organic and inorganic materials. In vertebrates, bone tissue is a
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27.1C: ANIMAL REPRODUCTION AND DEVELOPMENT
Most animals undergo sexual reproduction and have similar forms of fertilization. Typically, the small, motile male sperm fertilizes the
development dictated by Hox genes. much larger, sessile female egg. This process produces a diploid
fertilized egg called a zygote.
 LEARNING OBJECTIVES Some animal species (including sea stars and sea anemones, as well
as some insects, reptiles, and fish) are capable of asexual
Explain the processes of animal reproduction and embryonic
reproduction. The most common forms of asexual reproduction for
development
stationary aquatic animals include budding and fragmentation where
part of a parent individual can separate and grow into a new
KEY POINTS individual. In contrast, a form of asexual reproduction found in
Most animals reproduce through sexual reproduction, but some certain insects and vertebrates is called parthenogenesis where
animals are capable of asexual reproduction through unfertilized eggs can develop into new offspring. This type of
parthenogenesis, budding, or fragmentation. parthenogenesis in insects is called haplodiploidy and results in male
Following fertilization, an embryo is formed, and animal tissues offspring. These types of asexual reproduction produce genetically
organize into organ systems; some animals may also undergo identical offspring, which is disadvantageous from the perspective of
incomplete or complete metamorphosis. evolutionary adaptability because of the potential buildup of
Cleavage of the zygote leads to the formation of a blastula, deleterious mutations. However, for animals that are limited in their
which undergoes further cell division and cellular rearrangement capacity to attract mates, asexual reproduction can ensure genetic
during a process called gastrulation, which leads to the formation propagation.
of the gastrula. After fertilization, a series of developmental stages occur during
During gastrulation, the digestive cavity and germ layers are which primary germ layers are established and reorganize to form an
formed; these will later develop into certain tissue types, organs, embryo. During this process, animal tissues begin to specialize and
and organ systems during a process called organogenesis. organize into organs and organ systems, determining their future
Hox genes are responsible for determining the general body plan, morphology and physiology. Some animals, such as grasshoppers,
such as the number of body segments of an animal, the number undergo incomplete metamorphosis, in which the young resemble
and placement of appendages, and animal head-tail directionality. the adult. Other animals, such as some insects, undergo complete
Hox genes, similar across most animals, can turn on or off other metamorphosis where individuals enter one or more larval stages
genes by coding transcription factors that control the expression that may differ in structure and function from the adult. In complete
of numerous other genes. metamorphosis, the young and the adult may have different diets,
limiting competition for food between them. Regardless of whether
KEY TERMS
a species undergoes complete or incomplete metamorphosis, the
metamorphosis: a change in the form and often habits of an series of developmental stages of the embryo remains largely the
animal after the embryonic stage during normal development same for most members of the animal kingdom.
Hox gene: genes responsible for determining the general body
plan, such as the number of body segments of an animal, the
number and placement of appendages, and animal head-tail
directionality
blastula: a 6-32-celled hollow structure that is formed after a
zygote undergoes cell division

ANIMAL REPRODUCTION AND DEVELOPMENT

Most animals are diploid organisms (their body, or somatic, cells are
diploid) with haploid reproductive ( gamete ) cells produced through
meiosis. The majority of animals undergo sexual reproduction. This
fact distinguishes animals from fungi, protists, and bacteria where
asexual reproduction is common or exclusive. However, a few
groups, such as cnidarians, flatworms, and roundworms, undergo
asexual reproduction, although nearly all of those animals also have
a sexual phase to their life cycle.

PROCESSES OF ANIMAL REPRODUCTION AND

EMBRYONIC DEVELOPMENT
Figure 27.1C. 1 : Incomplete and complete metamorphosis: (a) The
During sexual reproduction, the haploid gametes of the male and grasshopper undergoes incomplete metamorphosis. (b) The butterfly
female individuals of a species combine in a process called undergoes complete metamorphosis.

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The process of animal development begins with the cleavage, or
series of mitotic cell divisions, of the zygote. Three cell divisions
transform the single-celled zygote into an eight-celled structure.
After further cell division and rearrangement of existing cells, a 6–
32-celled hollow structure called a blastula is formed. Next, the
blastula undergoes further cell division and cellular rearrangement
during a process called gastrulation. This leads to the formation of
the next developmental stage, the gastrula, in which the future
digestive cavity is formed. Different cell layers (called germ layers)
are formed during gastrulation. These germ layers are programed to
develop into certain tissue types, organs, and organ systems during a
process called organogenesis.

Figure 27.1C. 1 : Hox genes: Hox genes are highly-conserved genes

encoding transcription factors that determine the course of
embryonic development in animals. In vertebrates, the genes have
been duplicated into four clusters: Hox-A, Hox-B, Hox-C, and Hox-
D. Genes within these clusters are expressed in certain body
segments at certain stages of development. Shown here is the
homology between Hox genes in mice and humans. Note how Hox
gene expression, as indicated with orange, pink, blue, and green
shading, occurs in the same body segments in both the mouse and
Figure 27.1C. 1 : Embryonic development: During embryonic the human.
development, the zygote undergoes a series of mitotic cell divisions,
or cleavages, to form an eight-cell stage, then a hollow blastula.
During a process called gastrulation, the blastula folds inward to CONTRIBUTIONS AND ATTRIBUTIONS
form a cavity in the gastrula. OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
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Near the end of the 20th century, a particular class of genes that Provided by: OpenStax CNX. Located at:
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dictate developmental direction was discovered. These genes that Parazoa. Provided by: Wiktionary. Located at:
determine animal structure are called “homeotic genes.” They en.wiktionary.org/wiki/Parazoa. License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
contain DNA sequences called homeoboxes, with specific sequences Located at: http://cnx.org/content/m44655/latest...ol11448/latest. License: CC
referred to as Hox genes. This family of genes is responsible for BY: Attribution
Eumetazoa. Provided by: Wiktionary. Located at:
determining the general body plan: the number of body segments of en.wiktionary.org/wiki/Eumetazoa. License: CC BY-SA: Attribution-
an animal, the number and placement of appendages, and animal ShareAlike
epithelial tissue. Provided by: Wikipedia. Located at:
head-tail directionality. The first Hox genes to be sequenced were en.Wikipedia.org/wiki/epithelial%20tissue. License: CC BY-SA: Attribution-
those from the fruit fly (Drosophila melanogaster). A single Hox ShareAlike
OpenStax College, Features of the Animal Kingdom. November 13, 2013.
mutation in the fruit fly can result in an extra pair of wings or even Provided by: OpenStax CNX. Located at:
appendages growing from the “wrong” body part. http://cnx.org/content/m44655/latest/. License: CC BY: Attribution
Sponges in Caribbean Sea, Cayman Islands. Provided by: Wikimedia. Located
There are many genes that play roles in the morphological at: commons.wikimedia.org/wiki/Fi...an_Islands.jpg. License: CC BY-SA:
development of an animal, but Hox genes are so powerful because Attribution-ShareAlike
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they can turn on or off large numbers of other genes. Hox genes do Located at: http://cnx.org/content/m44655/latest...ol11448/latest. License: CC
this by coding transcription factors that control the expression of BY: Attribution
Boundless. Provided by: Boundless Learning. Located at:
numerous other genes. Hox genes are homologous in the animal www.boundless.com//biology/de...5-42bc0b4412de. License: CC BY-SA:
kingdom: the genetic sequences and their positions on chromosomes Attribution-ShareAlike
are remarkably similar across most animals (e.g., worms, flies, mice, blastula. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/blastula. License: CC BY-SA: Attribution-ShareAlike
humans) because of their presence in a common ancestor. Hox genes metamorphosis. Provided by: Wiktionary. Located at:
have undergone at least two duplication events during animal en.wiktionary.org/wiki/metamorphosis. License: CC BY-SA: Attribution-
ShareAlike
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evolve. Provided by: OpenStax CNX. Located at:
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Sponges in Caribbean Sea, Cayman Islands. Provided by: Wikimedia. Located http://cnx.org/content/m44655/latest...e_27_01_04.png. License: CC BY:
at: Attribution
commons.wikimedia.org/wiki/File:Sponges_in_Caribbean_Sea,_Cayman_Isl OpenStax College, Features of the Animal Kingdom. October 17, 2013.
ands.jpg. License: CC BY-SA: Attribution-ShareAlike Provided by: OpenStax CNX. Located at:
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Attribution This page titled 27.1C: Animal Reproduction and Development is shared
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Provided by: OpenStax CNX. Located at: under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

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SECTION OVERVIEW

27.2: FEATURES USED TO CLASSIFY ANIMALS

27.2B: ANIMAL CHARACTERIZATION BASED ON
Topic hierarchy FEATURES OF EMBRYOLOGICAL
DEVELOPMENT
27.2A: ANIMAL CHARACTERIZATION BASED ON
BODY SYMMETRY This page titled 27.2: Features Used to Classify Animals is shared under a
CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Boundless.

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27.2A: ANIMAL CHARACTERIZATION BASED ON BODY SYMMETRY
Animals can be classified by three types of body plan symmetry:
radial symmetry, bilateral symmetry, and asymmetry.

 LEARNING OBJECTIVES

Differentiate among the ways in which animals can be

characterized by body symmetry

KEY POINTS
Animals with radial symmetry have no right or left sides, only a
top or bottom; these species are usually marine organisms like
jellyfish and corals.
Most animals are bilaterally symmetrical with a line of symmetry
dividing their body into left and right sides along with a “head”
and “tail” in addition to a top and bottom.
Only sponges (phylum Porifera) have asymmetrical body plans.
Some animals start life with one type of body symmetry, but
develop a different type as adults; for example, sea stars are
classified as bilaterally symmetrical even though their adult
forms are radially symmetrical.

KEY TERMS
sagittal plane: divides the body into right and left halves
Figure 27.2A. 1 : Radial symmetry: Some organisms, like sea
radial symmetry: a form of symmetry wherein identical parts anemones (phylum Cnidaria), have radial symmetry.
are arranged in a circular fashion around a central axis
bilateral symmetry: having equal arrangement of parts BILATERAL SYMMETRY
(symmetry) about a vertical plane running from head to tail Bilateral symmetry involves the division of the animal through a
sagittal plane, resulting in two mirror-image, right and left halves,
ANIMAL CHARACTERIZATION BASED ON BODY such as those of a butterfly, crab, or human body. Animals with
SYMMETRY bilateral symmetry have a “head” and “tail” (anterior vs. posterior),
At a very basic level of classification, true animals can be largely front and back (dorsal vs. ventral), and right and left sides. All true
divided into three groups based on the type of symmetry of their animals, except those with radial symmetry, are bilaterally
body plan: radially symmetrical, bilaterally symmetrical, and symmetrical. The evolution of bilateral symmetry and, therefore, the
asymmetrical. Only a few animal groups display radial symmetry, formation of anterior and posterior (head and tail) ends promoted a
while asymmetry is a unique feature of phyla Porifera (sponges). All phenomenon called cephalization, which refers to the collection of
types of symmetry are well suited to meet the unique demands of a an organized nervous system at the animal’s anterior end. In contrast
particular animal’s lifestyle. to radial symmetry, which is best suited for stationary or limited-
motion lifestyles, bilateral symmetry allows for streamlined and
RADIAL SYMMETRY directional motion. In evolutionary terms, this simple form of
Radial symmetry is the arrangement of body parts around a central symmetry promoted active mobility and increased sophistication of
axis, like rays on a sun or pieces in a pie. Radially symmetrical resource-seeking and predator-prey relationships.
animals have top and bottom surfaces, but no left and right sides, or
front and back. The two halves of a radially symmetrical animal may
be described as the side with a mouth (“oral side”) and the side
without a mouth (“aboral side”). This form of symmetry marks the
body plans of animals in the phyla Ctenophora (comb jellies) and
Cnidaria (corals, sea anemones, and other jellies). Radial symmetry
enables these sea creatures, which may be sedentary or only capable
of slow movement or floating, to experience the environment
equally from all directions.

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 Figure 27.2A. 1 : Bilateral symmetry: This monarch butterfly
demonstrates bilateral symmetry down the sagittal plane, with the
line of symmetry running from ventral to dorsal and dividing the Figure 27.2A. 1 : Secondary radial symmetry in echinoderms: The
body into two left and right halves. larvae of echinoderms (sea stars, sand dollars, and sea urchins) have
Animals in the phylum Echinodermata (such as sea stars, sand bilateral symmetry as larvae, but develop radial symmetry as full
dollars, and sea urchins) display radial symmetry as adults, but their adults.
larval stages exhibit bilateral symmetry. This is termed secondary ASYMMETRY
radial symmetry. They are believed to have evolved from bilaterally
Only members of the phylum Porifera (sponges) have no body plan
symmetrical animals; thus, they are classified as bilaterally
symmetry. There are some fish species, such as flounder, that lack
symmetrical.
symmetry as adults. However, the larval fish are bilaterally
symmetrical.

This page titled 27.2A: Animal Characterization Based on Body Symmetry

is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

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27.2B: ANIMAL CHARACTERIZATION BASED ON FEATURES OF
EMBRYOLOGICAL DEVELOPMENT
Animals may be characterized by the presence of a coelom, digestive tract
formation of the mouth, and type of cell cleavage during embryonic
development.
ANIMAL CHARACTERIZATION BASED ON
FEATURES OF EMBRYOLOGICAL
 LEARNING OBJECTIVES DEVELOPMENT
Most animal species undergo a separation of tissues into germ layers
Explain the ways in which animals can be characterized by during embryonic development. These germ layers are formed
features of embryological development during gastrulation, developing into the animal’s specialized tissues
and organs. Animals develop either two or three embryonic germs
KEY POINTS layers. Radially-symmetrical animals are diploblasts, developing
Diploblasts contain two germ layers (inner endoderm and outer two germ layers: an inner layer (endoderm) and an outer layer
ectoderm ), while triploblasts contain three germ layers (ectoderm). Diploblasts have a non-living layer between the
(endoderm, mesoderm, and ectoderm). endoderm and ectoderm. Bilaterally-symmetrical animals are called
The endoderm becomes the digestive and respiratory tracts; the triploblasts, developing three tissue layers: an inner layer
ectoderm becomes the outer epithelial covering of the body (endoderm), an outer layer (ectoderm), and a middle layer
surface and the central nervous system; and the mesoderm (mesoderm).
becomes all muscle tissues, connective tissues, and most other
organs.
Triploblasts can be further categorized into those without a
coelom ( acoelomates ), those with a true coelom
(eucoelomates), and those with “false” coeloms (
pseudocoelomates ).
Bilaterally symmetrical, tribloblastic eucoelomates can be
divided into protostomes, those animals that develop a mouth
first, and deuterstomes, those animals that develop an anus first
and a mouth second.
In protostomes, the coelom forms when the mesoderm splits Figure 27.2B. 1: Germ development in embryogenesis: During
embryogenesis, diploblasts develop two embryonic germ layers: an
through the process of schizocoely, while in deuterostomes, the ectoderm and an endoderm. Triploblasts develop a third layer, the
coelom forms when the mesoderm pinches off through the mesoderm, between the endoderm and ectoderm
process of enterocoely.
Protostomes undergo spiral cleavage, while deuterostomes GERM LAYERS
undergo radial cleavage. Each of the three germ layers in a blastula, or developing ball of
cells, becomes particular body tissues and organs. The endoderm
KEY TERMS gives rise to the stomach, intestines, liver, pancreas, and the lining of
protostome: any animal in which the mouth is derived first from the digestive tract, as well as to the lining of the trachea, bronchi,
the embryonic blastopore (“mouth first”) and lungs of the respiratory tract. The ectoderm develops into the
deuterostome: Any animal in which the initial pore formed outer epithelial covering of the body surface and the central nervous
during gastrulation becomes the anus, and the second pore system. The mesoderm, the third germ layer forming between the
becomes the mouth endoderm and ectoderm in triploblasts, gives rise to all muscle
diploblast: a blastula in which there are two primary germ tissues (including the cardiac tissues and muscles of the intestines),
layers: the ectoderm and endoderm connective tissues such as the skeleton and blood cells, and most
triploblast: a blastula in which there are three primary germ other visceral organs such as the kidneys and the spleen.
layers: the ectoderm, mesoderm, and endoderm; formed during
gastrulation of the blastula PRESENCE OR ABSENCE OF A COELOM
acoelomate: any animal without a coelom, or body cavity Triploblasts can be differentiated into three categories: those that do
coelomate: any animal possessing a fluid-filled cavity within not develop an internal body cavity called a coelom (acoelomates),
which the digestive system is suspended. those with a true coelom (eucoelomates), and those with “false”
schizocoely: the process by which protostome animal embryos coeloms (pseudocoelomates).
develop; it occurs when a coelom (body cavity) is formed by
splitting the mesodermal embryonic tissue
enterocoely: the process by which deuterostome animal embryos
develop; the coelom forms from pouches “pinched” off of the

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 from the word meaning “mouth second. ” Deuterostomes include
more complex animals such as chordates, but also some simple
animals such as echinoderms.

Figure 27.2B. 1: Differentiation in triploblasts: Triploblasts may be

(a) acoelomates, (b) eucoelomates, or (c) pseudocoelomates.
Acoelomates have no body cavity. Eucoelomates have a body cavity
within the mesoderm, called a coelom, which is lined with
mesoderm. Pseudocoelomates also have a body cavity, but it is
sandwiched between the endoderm and mesoderm.

ACOELOMATES
Triploblasts that do not develop a coelom are called acoelomates:
their mesoderm region is completely filled with tissue. Flatworms in
the phylum Platyhelminthes are acoelomates. Figure 27.2B. 1: Early embryonic development in eucoelomates:
Eucoelomates can be divided into two groups based on their early
embryonic development. In protostomes, part of the mesoderm
EUCOELOMATES separates to form the coelom in a process called schizocoely. In
Eucoelomates (or coelomates) have a true coelom that arises entirely deuterostomes, the mesoderm pinches off to form the coelom in a
process called enterocoely.
within the mesoderm germ layer and is lined by an epithelial
membrane. This coelomic cavity represents a fluid-filled space that DEVELOPMENT OF THE COELOM
lies between the visceral organs and the body wall. It houses the
The coelom of most protostomes is formed through a process called
digestive system, kidneys, reproductive organs, and heart, and it
schizocoely, when a solid mass of the mesoderm splits apart and
contains the circulatory system. The epithelial membrane also lines
forms the hollow opening of the coelom. Deuterostomes differ in
the organs within the coelom, connecting and holding them in
that their coelom forms through a process called enterocoely, when
position while allowing them some free motion. Annelids, mollusks,
the mesoderm develops as pouches that are pinched off from the
arthropods, echinoderms, and chordates are all eucoelomates. The
endoderm tissue. These pouches eventually fuse to form the
coelom also provides space for the diffusion of gases and nutrients,
mesoderm, which then gives rise to the coelom.
as well as body flexibility and improved animal motility. The
coelom also provides cushioning and shock absorption for the major EMBRYONIC CLEAVAGE
organ systems, while allowing organs to move freely for optimal Protostomes undergo spiral cleavage: the cells of one pole of the
development and placement. embryo are rotated and, thus, misaligned with respect to the cells of
the opposite pole. This spiral cleavage is due to the oblique angle of
PSEUDOCOELOMATES
the cleavage. Protostomes also undergo determinate cleavage: the
The pseudocoelomates have a coelom derived partly from mesoderm
developmental fate of each embryonic cell is pre-determined.
and partly from endoderm. Although still functional, these are
Deuterostomes undergo radial cleavage where the cleavage axes are
considered false coeloms. The phylum Nematoda (roundworms) is
either parallel or perpendicular to the polar axis, resulting in the
an example of a pseudocoelomate.
alignment of the cells between the two poles. Unlike protostomes,
EMBRYONIC DEVELOPMENT OF THE MOUTH deuterostomes undergo indeterminate cleavage: cells remain
undifferentiated until a later developmental stage. This characteristic
Bilaterally symmetrical, tribloblastic eucoelomates can be further
of deuterostomes is reflected in the existence of familiar embryonic
divided into two groups based on differences in their early
stem cells, which have the ability to develop into any cell type.
embryonic development. These two groups are separated based on
which opening of the digestive cavity develops first: mouth This page titled 27.2B: Animal Characterization Based on Features of
(protostomes) or anus (deuterostomes). The word protostome comes Embryological Development is shared under a CC BY-SA 4.0 license and
from the Greek word meaning “mouth first. ” The protostomes was authored, remixed, and/or curated by Boundless.
include arthropods, mollusks, and annelids. Deuterostome originates

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SECTION OVERVIEW

27.3: ANIMAL PHYLOGENY

27.3B: MOLECULAR ANALYSES AND MODERN
Topic hierarchy PHYLOGENETIC TREES

27.3A: CONSTRUCTING AN ANIMAL This page titled 27.3: Animal Phylogeny is shared under a CC BY-SA 4.0
PHYLOGENETIC TREE license and was authored, remixed, and/or curated by Boundless.

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27.3A: CONSTRUCTING AN ANIMAL PHYLOGENETIC TREE
Phylogenetic trees are constructed according to the evolutionary aligning the sequences; the length of the branch is proportional to
relationships that exist between organisms based on homologous the amount of amino acid differences between the sequences.
traits. Phylogenetic systematics informs the construction of phylogenetic
trees based on shared characters. Comparing nucleic acids or other
 LEARNING OBJECTIVES molecules to infer relationships is a valuable tool for tracing an
organism’s evolutionary history. The ability of molecular trees to
Describe the information needed to construct a phylogenetic
encompass both short and long periods of time is hinged on the
tree of animals
ability of genes to evolve at different rates, even in the same
evolutionary lineage. For example, the DNA that codes for rRNA
KEY POINTS changes relatively slowly, so comparisons of DNA sequences in
Phylogenetic trees are constructed using various data derived these genes are useful for investigating relationships between taxa
from studies on homologous traits, analagous traits, and that diverged a long time ago. Interestingly, 99% of the genes in
molecular evidence that can be used to establish relationships humans and mice are detectably orthologous, and 50% of our genes
using polymeric molecules ( DNA, RNA, and proteins ). are orthologous with those of yeast. The hemoglobin B genes in
Evolutionary relationships between animal phyla, or Metazoa, humans and in mice are orthologous. These genes serve similar
are based on the the presence or absence of differentiated tissues, functions, but their sequences have diverged since the time that
referred to as Eumetazoa or Parazoa, respectively. humans and mice had a common ancestor.
Eumetazoa can be further classified into categories that are based Evolutionary pathways relating the members of a family of proteins
on whether they have radial or bilateral symmetry, referred to as may be deduced by examination of sequence similarity. This
Radiata or Bilateria, respectively. approach is based on the notion that sequences that are more similar
to one another have had less evolutionary time to diverge than have
KEY TERMS
sequences that are less similar. Evolutionary trees are used today for
orthologous: having been separated by a speciation event DNA hybridization, which determines the percentage difference of
homoplasy: a correspondence between the parts or organs of
genetic material between two similar species. If there is a high
different species acquired as the result of parallel evolution or
resemblance of DNA between the two species, then the species are
convergence
closely related. If only a small percentage is identical, then they are
CONSTRUCTING AN ANIMAL PHYLOGENETIC distantly related.
TREE
Evolutionary trees, or phylogeny, is the formal study of organisms
and their evolutionary history with respect to each other.
Phylogenetic trees are most-commonly used to depict the
relationships that exist between species. In particular, they clarify
whether certain traits are homologous (found in the common
ancestor as a result of divergent evolution) or homoplasy (sometimes
referred to as analogous: a character that is not found in a common
ancestor, but whose function developed independently in two or
more organisms through convergent evolution). Evolutionary trees
are diagrams that show various biological species and their
evolutionary relationships. They consist of branches that flow from
lower forms of life to the higher forms of life.
Evolutionary trees differ from taxonomy which is an ordered
division of organisms into categories based on a set of
characteristics used to assess similarities and differences.
Evolutionary trees involve biological classification and use
morphology to show relationships. Phylogeny is evolutionary
history shown by the relationships found when comparing polymeric
molecules such as RNA, DNA, or proteins of various organisms.
The evolutionary pathway is analyzed by the sequence similarity of Figure 27.3A. 1 : Phylogenetic tree of life: A phylogenetic tree of
these polymeric molecules. This is based on the assumption that the life, showing the relationship between species whose genomes had
similarities of sequence result from having fewer evolutionary been sequenced as of 2006. The very center represents the last
universal ancestor of all life on earth. The different colors represent
divergences than others. The evolutionary tree is constructed by the three domains of life: pink represents eukaryota (animals, plants,
and fungi); blue represents bacteria; and green represents archaea.

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ANIMAL PHYLA further divided into deuterostomes (including chordates and
The current understanding of evolutionary relationships between echinoderms) and two distinct clades of protostomes (including
animal, or Metazoa, phyla begins with the distinction between “true” ecdysozoans and lophotrochozoans). Ecdysozoa includes nematodes
animals with true differentiated tissues, called Eumetazoa, and and arthropods; named for a commonly-found characteristic among
animal phyla that do not have true differentiated tissues (such as the the group: exoskeletal molting (termed ecdysis). Lophotrochozoa is
sponges), called Parazoa. Both Parazoa and Eumetazoa evolved named for two structural features, each common to certain phyla
from a common ancestral organism that resembles the modern-day within the clade. Some lophotrochozoan phyla are characterized by a
protists called choanoflagellates. These protist cells strongly larval stage called trochophore larvae, and other phyla are
resemble sponge choanocyte cells. characterized by the presence of a feeding structure called a
lophophore.
Eumetazoa are subdivided into radially-symmetrical animals and
bilaterally-symmetrical animals and are classified into clade Radiata This page titled 27.3A: Constructing an Animal Phylogenetic Tree is shared
or Bilateria, respectively. The cnidarians and ctenophores are animal under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
phyla with true radial symmetry. All other Eumetazoa are members by Boundless.
of the Bilateria clade. The bilaterally-symmetrical animals are

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27.3B: MOLECULAR ANALYSES AND MODERN PHYLOGENETIC TREES
The process of establishing relationships between organisms is calculate the approximate time when the sequence of interest
increasingly becoming more accurate due to advances in molecular diverged into monophyletic groups.
analysis.

 LEARNING OBJECTIVES

Distinguish between morphological and molecular data in

creating phylogenetic trees of animals

KEY POINTS
The construction of phylogenetic trees is now based on
similarities and differences within the molecular sources used for
analysis which include DNA, RNA, and proteins.
The ability to use molecular sources as a basis of phylogenetic
tree construction has allowed for determination of previously-
unknown evolutionary relationships between organisms.
In addition to the establishment of new relationships within Figure 27.3B. 1: Phlyogenetic tree of life: Advances in molecular
phylogenetic trees, the ability to use molecular sources for biology and analysis of polymeric molecules such as DNA, RNA,
analysis has also created an emergence of new phlyums that were and proteins have contributed to the development of phylogenetic
trees.
previously classified in larger groups.
Besides identifying molecular similarities and differences Sequence alignments can be performed on a variety of sequences.
For constructing an evolutionary tree from proteins, for example, the
between organisms, by assigning a constant mutation rate to a
sequences are aligned and then compared. rRNA (ribosomal RNA)
sequence and performing a sequence alignment, it is possible to
is typically used to compare organisms since rRNA has a slower
determine when two organisms diverged from one another.
mutation rate and is a better source for evolutionary tree
KEY TERMS construction. This is best supported by research of Dr. Carl Woese
monophyletic: of, pertaining to, or affecting a single phylum (or that was conducted in the late 1970s. Since the ribosomes are critical
other taxon) of organisms to the function of living organisms, they are not easily changed
through the process of evolution. Taking advantage of this fact, Dr.
MODERN ADVANCES IN PHYLOGENETIC Woese compared the minuscule differences in the sequences of
UNDERSTANDING COME FROM MOLECULAR ribosomes among a great array of bacteria and showed that they
ANALYSES were not all related.
The phylogenetic groupings are continually being debated and For example, a previously-classified group of animals called
refined by evolutionary biologists. Each year, new evidence emerges lophophorates, which included brachiopods and bryozoans, were
that further alters the relationships described by a phylogenetic tree long-thought to be primitive deuterostomes. Extensive molecular
diagram. Previously, phylogenetic trees were constructed based on analysis using rRNA data found these animals to be protostomes,
homologous and analogous morphology; however, with the more closely related to annelids and mollusks. This discovery
advances in molecular biology, construction of phylogenetic trees is allowed for the distinction of the protostome clade: the
increasingly performed using data derived from molecular analyses. lophotrochozoans. Molecular data have also shed light on some
Many evolutionary relationships in the modern tree have only differences within the lophotrochozoan group. Some scientists
recently been determined due to molecular evidence. Nucleic acid believe that the phyla Platyhelminthes and Rotifera within this group
and protein analyses have informed the construction of the modern should actually belong to their own group of protostomes termed
phylogenetic animal tree. These data come from a variety of Platyzoa.
molecular sources, such as mitochondrial DNA, nuclear DNA, Molecular research similar to the discoveries that brought about the
ribosomal RNA (rRNA), and certain cellular proteins. Evolutionary distinction of the lophotrochozoan clade has also revealed a dramatic
trees can be made by the determination of sequence information of rearrangement of the relationships between mollusks, annelids,
similar genes in different organisms. Sequences that are similar to arthropods, and nematodes; a new ecdysozoan clade was formed.
each other frequently are considered to have less time to diverge, Due to morphological similarities in their segmented body types,
while less similar sequences have more evolutionary time to diverge. annelids and arthropods were once thought to be closely related.
The evolutionary tree is created by aligning sequences and having However, molecular evidence has revealed that arthropods are
each branch length proportional to the amino acid differences of the actually more closely related to nematodes, now comprising the
sequences. Furthermore, by assigning a constant mutation rate to a ecdysozoan clade, and annelids are more closely related to mollusks,
sequence and performing a sequence alignment, it is possible to

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brachiopods, and other phyla in the lophotrochozoan clade. These orthologous. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/orthologous. License: CC BY-SA: Attribution-
two clades now make up the protostomes. ShareAlike
homoplasy. Provided by: Wiktionary. Located at:
Another change to former phylogenetic groupings because of en.wiktionary.org/wiki/homoplasy. License: CC BY-SA: Attribution-
molecular analyses includes the emergence of an entirely new ShareAlike
Tree of life SVG. Provided by: Wikimedia. Located at:
phylum of worm called Acoelomorpha. These acoel flatworms were commons.wikimedia.org/wiki/Fi...f_life_SVG.svg. License: Public Domain:
long thought to belong to the phylum Platyhelminthes because of No Known Copyright
their similar “flatworm” morphology. However, molecular analyses OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44658/latest...ol11448/latest. License: CC
revealed this to be a false relationship and originally suggested that BY: Attribution
acoels represented living species of some of the earliest divergent Structural Biochemistry/Bioinformatics/Evolution Trees. Provided by:
Wikibooks. Located at: en.wikibooks.org/wiki/Structu...volution_Trees.
bilaterians. More recent research into the acoelomorphs has called License: CC BY-SA: Attribution-ShareAlike
this hypothesis into question and suggested a closer relationship with monophyletic. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/monophyletic. License: CC BY-SA: Attribution-
deuterostomes. The placement of this new phylum remains disputed, ShareAlike
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SECTION OVERVIEW

27.4: THE EVOLUTIONARY HISTORY OF THE ANIMAL KINGDOM

27.4C: POST-CAMBRIAN EVOLUTION AND MASS
Topic hierarchy EXTINCTIONS

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LIFE

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27.4A: PRE-CAMBRIAN ANIMAL LIFE
Early animal life (Ediacaran biota) evolved from protists during the
pre-Cambrian period, which is also known as the Ediacaran period.

 LEARNING OBJECTIVES

Describe the types of animals found in the Ediacaran period

KEY POINTS
The pre-Cambrian period ( Ediacaran period ) took place
between 635-543 million years ago.
Figure 27.4A. 1 : Earth’s history: (a) Earth’s history is divided into
Early animal life, called Ediacaran biota, evolved from protists; eons, eras, and periods. The Ediacaran period was the final period of
it was previously believed early animal life included only tiny, the Proterozoic Era which ended in the Cambrian period of the
Phanerozoic Era. (b) Stages on the geological time scale are
sessile, soft-bodied sea creatures, but scientific evidence suggests
represented as a spiral.
more complex animals lived during this time.
The earliest life comprising Ediacaran biota was long believed to
Sponge-like fossils believed to represent the oldest animals with
include only tiny, sessile, soft-bodied sea creatures. However,
hard body parts, named Coronacollina acula, date back as far as
recently there has been increasing scientific evidence suggesting that
560 million years.
more varied and complex animal species lived during this time, and
The fossils of the earliest animal species ever found were small,
possibly even before the Ediacaran period.
one-centimeter long, sponge-like creatures, dating before 650
million years, which predates the Ediacaran period. Fossils believed to represent the oldest animals with hard body parts
The discovery of the fossils of the earliest animal species were recently discovered in South Australia. These sponge-like
provided evidence that animals may have evolved before the fossils, named Coronacollina acula, date back as far as 560 million
Ediacaran period during the Cryogenian period. years. They are believed to show the existence of hard body parts
and spicules that extended 20–40 cm from the main body (estimated
KEY TERMS about 5 cm long). Other organisms, such as Cyclomedusa and
Ediacaran period: period from about 635-543 million years Dickinsonia, also evolved during the Ediacaran period.
ago; the final period of the late Proterozoic Neoproterozoic Era
choanoflagellate: any of a group of flagellate protozoa thought
to be the closest unicellular ancestors of animals
Coronacollina acula: sponge-like fossils believed to represent
the oldest animals with hard body parts that date back as far as
560 million years

PRE-CAMBRIAN ANIMAL LIFE

The time before the Cambrian period is known as the Ediacaran
period (between 635-543 million years ago), the final period of the
late Proterozoic Neoproterozoic Era. It is believed that early animal Figure 27.4A. 1 : Fossils from Ediacaran period: Fossils of (a)
Cyclomedusa and (b) Dickinsonia that evolved during the Ediacaran
life, termed Ediacaran biota, evolved from protists at this time. Some period.
protist species called choanoflagellates closely resemble the Another recent fossil discovery may represent the earliest animal
choanocyte cells in the simplest animals, sponges. In addition to species ever found. While the validity of this claim is still under
their morphological similarity, molecular analyses have revealed investigation, these primitive fossils appear to be small, one-
similar sequence homologies in their DNA. centimeter long, sponge-like creatures. These fossils from South
Australia date back 650 million years, actually placing the putative
animal before the great ice age extinction event that marked the
transition between the Cryogenian period and the Ediacaran period.
Until this discovery, most scientists believed that there was no
animal life prior to the Ediacaran period. Many scientists now
believe that animals may, in fact, have evolved during the
Cryogenian period.

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27.4B: THE CAMBRIAN EXPLOSION OF ANIMAL LIFE
During the Cambrian period, the most rapid evolution of new animal
species occurred, but the cause of this explosion is still unknown.

 LEARNING OBJECTIVES

Compare the theories that attempt to explain the Cambrian

Explosion

KEY POINTS
Echinoderms, mollusks, worms, chordates, and arthropods
(including arthropods called trilobites which were the one of the
first species to exhibit a sense of vision) developed during the
Cambrian period.
Environmental changes such as rising levels of atmospheric
oxygen and an increase in oceanic calcium concentrations may
have caused The Cambrian Explosion.
A continental shelf with numerous shallow pools that provided
the necessary living space for larger numbers of different types
of animals to co-exist may have caused the Cambrian Explosion.
The Cambrian Explosion may have been a result of ecological Figure 27.4B. 1: Trilobites: These fossils (a–d) belong to trilobites,
extinct arthropods that appeared in the early Cambrian period 525
relationships between species, such as changes in the food web, million years ago and disappeared from the fossil record during a
competition for food and space, and predator-prey relationships. mass extinction at the end of the Permian period about 250 million
The evolution of Hox control genes resulting in animal years ago.
complexity and flexibility may have provided the necessary
opportunities for increases in possible animal morphologies.

KEY TERMS
Ordovician period: covers the time between 485-443 million
years ago; followed the Cambrian period
Cambrian explosion: the relatively rapid appearance (over a
period of many millions of years), around 530 million years ago,
of most major animal phyla as demonstrated in the fossil record

THE CAMBRIAN EXPLOSION OF ANIMAL LIFE

The Cambrian period, occurring between approximately 542–488
million years ago, marks the most rapid evolution of new animal
phyla and animal diversity in earth’s history. It is believed that most
of the animal phyla in existence today had their origins during this
time, often referred to as the Cambrian explosion. Echinoderms,
mollusks, worms, arthropods, and chordates arose during this period.
One of the most dominant species during the Cambrian period was
the trilobite, an arthropod that was among the first animals to exhibit
a sense of vision. Figure 27.4B. 1: Cambrian period: An artist’s rendition depicts
some organisms from the Cambrian period.
The causes of the Cambrian explosion are still debated. There are
many theories that attempt to answer this question. Environmental
changes may have created a more suitable environment for animal
life. Examples of these changes include rising atmospheric oxygen
levels and large increases in oceanic calcium concentrations that
preceded the Cambrian period. Some scientists believe that an
expansive, continental shelf with numerous shallow lagoons or pools
provided the necessary living space for larger numbers of different
types of animals to co-exist. There is also support for theories that
argue that ecological relationships between species, such as changes

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in the food web, competition for food and space, and predator-prey Unresolved questions about the animal diversification that took
relationships, were primed to promote a sudden, massive place during the Cambrian period remain. For example, we do not
coevolution of species. Yet other theories claim genetic and understand how the evolution of so many species occurred in such a
developmental reasons for the Cambrian explosion. The short period of time. Was there really an “explosion” of life at this
morphological flexibility and complexity of animal development particular time? Some scientists question the validity of this idea
afforded by the evolution of Hox control genes may have provided because there is increasing evidence to suggest that more animal life
the necessary opportunities for increases in possible animal existed prior to the Cambrian period and that other similar species’
morphologies at the time of the Cambrian period. Theories that so-called explosions (or radiations) occurred later in history as well.
attempt to explain why the Cambrian explosion happened must be Furthermore, the vast diversification of animal species that appears
able to provide valid reasons for the massive animal diversification, to have begun during the Cambrian period continued well into the
as well as explain why it happened when it did. There is evidence following Ordovician period. Despite some of these arguments, most
that both supports and refutes each of the theories described above. scientists agree that the Cambrian period marked a time of
The answer may very well be a combination of these and other impressively-rapid animal evolution and diversification that is
theories. unmatched elsewhere during history.

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Figure 27.4B. 1: Earth’s oxygen concentration: The oxygen

concentration in earth’s atmosphere rose sharply around 300 million
years ago.

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27.4C: POST-CAMBRIAN EVOLUTION AND MASS EXTINCTIONS
The post-Cambrian era was characterized by animal evolution and terrestrial existence in animals, such as limbs in amphibians and
diversity where mass extinctions were followed by adaptive epidermal scales in reptiles.
radiations. Changes in the environment often create new niches (living spaces)
that contribute to rapid speciation and increased diversity. On the
 LEARNING OBJECTIVES other hand, cataclysmic events, such as volcanic eruptions and
meteor strikes that obliterate life, can result in devastating losses of
Differentiate among the causes of mass extinctions and their
diversity. Such periods of mass extinction have occurred repeatedly
effects on animal life
in the evolutionary record of life, erasing some genetic lines while
creating room for others to evolve into the empty niches left behind.
KEY POINTS The end of the Permian period (and the Paleozoic Era) was marked
During the Ordovician period, plant life first appeared on land, by the largest mass extinction event in Earth’s history, a loss of
which allowed aquatic animals to move on to land. roughly 95 percent of the extant species at that time. Some of the
Periods of mass extinction caused by cataclysmic events like dominant phyla in the world’s oceans, such as the trilobites,
volcanic eruptions and meteor strikes have erased many genetic disappeared completely. On land, the disappearance of some
lines and created room for new species. dominant species of Permian reptiles made it possible for a new line
The largest mass extinction event in earth’s history, which of reptiles to emerge: the dinosaurs. The warm and stable climatic
occurred at the end of the Permian period, resulted in a loss of conditions of the ensuing Mesozoic Era promoted an explosive
roughly 95 percent of the existing species at that time. diversification of dinosaurs into every conceivable niche in land, air,
The disappearance of some dominant species of Permian reptiles and water. Plants, too, radiated into new landscapes and empty
and the warm and stable climate that followed made it possible niches, creating complex communities of producers and consumers,
for the dinosaurs to emerge and diversify. some of which became extremely large on the abundant food
Another mass extinction event caused by a meteor strike and available.
volcanic ash eruption occurred at the end of the Cretaceous
period, bringing the Mesozoic Era to an end and pushing
dinosaurs into extinction.
The disappearance of dinosaurs led to the dominance of plants,
which created new niches for birds, insects, and mammals;
animal diversity was also brought on by the creation of
continents, islands, and mountains.

KEY TERMS
Cenozoic: a geologic era about between 65 million years ago to
the present when the continents moved to their current position
and modern plants and animals evolved
mass extinction: a sharp decrease in the total number of species
in a relatively short period of time Figure 27.4C. 1 : Mass extinctions: Mass extinctions have occurred
Cretaceous: the last geologic period within the Mesozoic era repeatedly over geological time.
from about 146 to 65 million years ago; ended with a large mass Another mass extinction event occurred at the end of the Cretaceous
extinction period, bringing the Mesozoic Era to an end. Skies darkened and
temperatures fell as a large meteor impact expelled tons of volcanic
POST-CAMBRIAN EVOLUTION AND MASS ash, blocking incoming sunlight. Plants died, herbivores and
EXTINCTIONS carnivores starved, and the mostly cold-blooded dinosaurs ceded
The periods that followed the Cambrian during the Paleozoic Era their dominance of the landscape to more warm-blooded mammals.
were marked by further animal evolution and the emergence of In the following Cenozoic Era, mammals radiated into terrestrial and
many new orders, families, and species. As animal phyla continued aquatic niches once occupied by dinosaurs. Birds, the warm-blooded
to diversify, new species adapted to new ecological niches. During offshoots of one line of the ruling reptiles, became aerial specialists.
the Ordovician period, which followed the Cambrian period, plant The appearance and dominance of flowering plants in the Cenozoic
life first appeared on land. This change allowed formerly-aquatic Era created new niches for insects, as well as for birds and
animal species to invade land, feeding directly on plants or decaying mammals. Changes in animal species diversity during the late
vegetation. Continual changes in temperature and moisture Cretaceous and early Cenozoic were also promoted by a dramatic
throughout the remainder of the Paleozoic Era due to continental shift in earth’s geography, as continental plates slid over the crust
plate movements encouraged the development of new adaptations to into their current positions, leaving some animal groups isolated on
islands and continents or separated by mountain ranges or inland

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 CHAPTER OVERVIEW

28: INVERTEBRATES

Topic hierarchy
28.1: Phylum Porifera
28.1A: Phylum Porifera
28.1B: Morphology of Sponges
28.1C: Physiological Processes in Sponges
28.2: Phylum Cnidaria
28.2A: Phylum Cnidaria
28.2B: Class Anthozoa
28.2C: Class Scyphozoa
28.2D: Class Cubozoa and Class Hydrozoa
28.3: Superphylum Lophotrochozoa
28.3A: Superphylum Lophotrochozoa
28.3B: Phylum Platyhelminthes
28.3C: Phylum Rotifera
28.3D: Phylum Nemertea
28.3E: Phylum Mollusca
28.3F: Classification of Phylum Mollusca
28.3G: Phylum Annelida
28.4: Superphylum Ecdysozoa
28.4A: Superphylum Ecdysozoa
28.4B: Phylum Nematoda
28.4C: Phylum Arthropoda
28.4D: Subphyla of Arthropoda
28.5: Superphylum Deuterostomia
28.5A: Phylum Echinodermata
28.5B: Classes of Echinoderms
28.5C: Phylum Chordata

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1
SECTION OVERVIEW

28.1: PHYLUM PORIFERA

28.1B: MORPHOLOGY OF SPONGES
Topic hierarchy
28.1C: PHYSIOLOGICAL PROCESSES IN
SPONGES
28.1A: PHYLUM PORIFERA

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28.1A: PHYLUM PORIFERA
Sponges lack true tissues, have no body symmetry, and are sessile; including all freshwater ones, and have the widest range of habitats.
types are classified based on presence and composition of spicules. Calcareous sponges, which have calcium carbonate spicules and, in
some species, calcium carbonate exoskeletons, are restricted to
 LEARNING OBJECTIVES relatively shallow marine waters where production of calcium
carbonate is easiest. They contain no spongin. Hemoscleromorpha
Explain the position of the phylum Porifera in the sponges tend to be massive or encrusting in form and have a very
phylogenetic tree of invertebrates simple structure with very little variation in spicule form (all
spicules tend to be very small). Hexactinellid sponges have sturdy
KEY POINTS lattice-like internal skeletons made up of fused spicules of silica;
As larvae, sponges are able to swim, but as adults, they are they tend to be more-or-less cup-shaped.
sessile, spending their life attached to a substrate.
Although the majority of sponges live in marine habitats, one
family, the Spongillidae, is found in fresh water.
Calcarea, Hexactinellida, Demospongiae, and
Homoscleromorpha make up the four classes of sponges; each
type is classified based on the presence or composition of its
spicules or spongin.
Most sponges reproduce sexually; however, some can reproduce
through budding and the regeneration of fragments.
The majority of sponges are filter-feeders, but a few species are
carnivorous due to the nutrient -poor environment in which they
are found.

KEY TERMS
parazoan: include only one phylum known as the sponges Figure 28.1A. 1 : Sponge Spicule: Sponges are classified based on
the presence and types of spicules they contain.
endosymbiont: an organism that lives within the body or cells of
another organism
spongin: a horny, sulfur-containing protein related to keratin that
forms the skeletal structure of certain classes of sponges
spicule: a sharp, needle-like piece
holdfast: a root-like structure that anchors aquatic sessile
organisms, such as seaweed, other sessile algae, stalked crinoids,
benthic cnidarians, and sponges, to the substrate Figure 28.1A. 1 : Types of sponges: (a) Clathrina clathrus belongs to
class Calcarea, (b) Staurocalyptus spp. (common name: yellow
Picasso sponge) belongs to class Hexactinellida, and (c) Acarnus
INTRODUCTION erithacus belongs to class Demospongia.
The invertebrates, or Invertebrata, are animals that do not contain Unlike Protozoans, the Poriferans are multicellular. However, unlike
bony structures such as the cranium and vertebrae. The simplest of higher metazoans, the cells that make up a sponge are not organized
all the invertebrates are the Parazoans, which include only the into tissues. Therefore, sponges lack true tissues and organs; in
phylum Porifera. Phylum Porifera (“pori” = pores, “fera” = bearers) addition, they have no body symmetry. Sponges do, however, have
are popularly known as sponges. Sponge larvae are able to swim; specialized cells that perform specific functions. The shapes of their
however, adults are non-motile and spend their life attached to a bodies are adapted for maximal efficiency of water flow through the
substratum through a holdfast. The majority of sponges are marine, central cavity, where nutrients are deposited, and leaves through a
living in seas and oceans. There is, however, one family of fresh hole called the osculum. Many sponges have internal skeletons of
water sponges (Family Spongillidae). The great majority of the spongin and/or spicules of calcium carbonate or silica. Primarily,
marine species can be found in ocean habitats ranging from tidal their body consists of a thin sheet of cells over a frame (skeleton).
zones to depths exceeding 8,800 m (5.5 mi). As their name suggests, Poriferans are characterized by the presence
Sponges are classified within four classes: calcareous sponges of minute pores called ostia on their body.
(Calcarea), glass sponges (Hexactinellida), demosponges Since water is vital to sponges for excretion, feeding, and gas
(Demospongiae), and the recently-recognized, encrusting sponges exchange, their body structure facilitates the movement of water
(Homoscleromorpha). The presence and composition of spicules and through the sponge. Structures such as canals, chambers, and
spongin are the differentiating characteristics between the classes of cavities enable water to move through the sponge to nearly all body
sponges. Demosponges, which contain spongin and may or may not cells.
have spicules, constitute about 90% of all known sponge species,

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Most species use sexual reproduction, releasing sperm cells into the Most of the approximately 5,000–10,000 known species of sponges
water to fertilize ova that in some species are released and in others are filter-feeders, feeding on bacteria and other food particles in the
are retained by the “mother. ” The fertilized eggs form larvae which water. However, a few species of sponge that live in food-poor
swim off in search of places to settle. Sponges are also known for environments have become carnivores that prey mainly on small
regenerating from fragments that are broken off, although this only crustaceans. Other species host photosynthesizing micro-organisms
works if the fragments include the right types of cells. A few species as endosymbionts; these alliances often produce more food and
reproduce by budding. When conditions deteriorate, such as when oxygen than they consume.
temperatures drop, many freshwater species and a few marine ones
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28.1B: MORPHOLOGY OF SPONGES
Instead of true tissues or organs, sponges have specialized cells that
are in charge of important bodily functions and processes.

 LEARNING OBJECTIVES

Explain the various cell forms and bodily functions of

sponges

KEY POINTS
Although sponges do not have organized tissue, they depend on
specialized cells, such as choanocytes, porocytes, amoebocytes,
and pinacocytes, for specialized functions within their bodies.
The mesohyl acts as a type of endoskeleton, helping to maintain
the tubular shape of sponges.
Porocytes control the amount of water that enters pores into the
spongocoel, while choanocytes, which are flagellated cells, aid Figure 28.1B. 1: Sponge morphology: The sponge’s (a) basic body
the movement of water through the sponge, thereby helping the plan is a cylinder shape with a large central cavity. The specialized
sponge to trap and ingest food particles. cell types in sponges (b) each perform a distinct function.
Amoebocytes carry out several special functions: they deliver While sponges (excluding the Hexactinellids) do not exhibit tissue-
nutrients from choanocytes to other cells, give rise to eggs for layer organization, they do have different cell types that perform
sexual reproduction, deliver phagocytized sperm from distinct functions. Pinacocytes, which are epithelial-like cells, form
choanocytes to eggs, and can transform into other cell types. the outermost layer of sponges, enclosing a jelly-like substance
Collencytes, lophocytes, sclerocytes, and spongocytes are called mesohyl. Mesohyl is an extracellular matrix consisting of a
examples of cells that are derived from amoebocytes; these cells collagen -like gel with suspended cells that perform various
manage other vital functions in the body of sponges. functions. The gel-like consistency of mesohyl acts as an
endoskeleton, maintaining the tubular morphology of sponges. In
KEY TERMS addition to the osculum, sponges have multiple pores called ostia on
choanocyte: any of the cells in sponges that contain a flagellum their bodies that allow water to enter the sponge. In some sponges,
and are used to control the movement of water ostia are formed by porocytes: single, tube-shaped cells that act as
spongocoel: the large, central cavity of sponges valves to regulate the flow of water into the spongocoel. In other
osculum: an opening in a sponge from which water is expelled sponges, ostia are formed by folds in the body wall of the sponge.
mesohyl: the gelatinous matrix within a sponge Choanocytes (“collar cells”) are present at various locations,
depending on the type of sponge; however, they always line the
MORPHOLOGY OF SPONGES inner portions of some space through which water flows: the
The morphology of the simplest sponges takes the shape of a spongocoel in simple sponges; canals within the body wall in more
cylinder with a large central cavity, the spongocoel, occupying the complex sponges; and chambers scattered throughout the body in the
inside of the cylinder. Water can enter into the spongocoel from most complex sponges. Whereas pinacocytes line the outside of the
numerous pores in the body wall. Water entering the spongocoel is sponge, choanocytes tend to line certain inner portions of the sponge
extruded via a large, common opening called the osculum. However, body that surround the mesohyl. The structure of a choanocyte is
sponges exhibit a range of diversity in body forms, including critical to its function, which is to generate a water current through
variations in the size of the spongocoel, the number of osculi, and the sponge and to trap and ingest food particles by phagocytosis.
where the cells that filter food from the water are located. Note that there is a similarity in appearance between the sponge
choanocyte and choanoflagellates (Protista). This similarity suggests
that sponges and choanoflagellates are closely related and probably
share a recent, common ancestry.
The cell body is embedded in mesohyl. It contains all organelles
required for normal cell function, but protruding into the “open
space” inside of the sponge is a mesh-like collar composed of
microvilli with a single flagellum in the center of the column. The
cumulative effect of the flagella from all choanocytes aids the
movement of water through the sponge: drawing water into the
sponge through the numerous ostia, into the spaces lined by
choanocytes, and eventually out through the osculum (or osculi).

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Meanwhile, food particles, including waterborne bacteria and algae, within the sponge; giving rise to eggs for sexual reproduction (which
are trapped by the sieve-like collar of the choanocytes, slide down remain in the mesohyl); delivering phagocytized sperm from
into the body of the cell, are ingested by phagocytosis, and become choanocytes to eggs; and differentiating into more-specific cell
encased in a food vacuole. Finally, choanocytes will differentiate types. Some of these more-specific cell types include collencytes
into sperm for sexual reproduction; they will become dislodged from and lophocytes, which produce the collagen-like protein to maintain
the mesohyl, leaving the sponge with expelled water through the the mesohyl; sclerocytes, which produce spicules in some sponges;
osculum. and spongocytes, which produce the protein spongin in the majority
The second crucial cells in sponges are called amoebocytes (or of sponges. These cells produce collagen to maintain the consistency
archaeocytes), named for the fact that they move throughout the of the mesohyl.
mesohyl in an amoeba-like fashion. Amoebocytes have a variety of
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28.1C: PHYSIOLOGICAL PROCESSES IN SPONGES
Sponges are sessile, feed by phagocytosis, and reproduce sexually All other major body functions in the sponge (gas exchange,
and asexually; all major functions are regulated by water flow circulation, excretion) are performed by diffusion between the cells
diffusion. that line the openings within the sponge and the water that is passing
through those openings. All cell types within the sponge obtain
 LEARNING OBJECTIVES oxygen from water through diffusion. Likewise, carbon dioxide is
released into seawater by diffusion. In addition, nitrogenous waste
Summarize the physiological processes of sponges produced as a by-product of protein metabolism is excreted via
diffusion by individual cells into the water as it passes through the
KEY POINTS sponge.
Choanocytes trap bacteria and other food particles from water
flowing within the sponge: in through the ostia and out through
the osculum; particles are ingested by phagocytosis.
Sponges reproduce by sexual and asexual methods, which
include fragmentation or budding; the production of gemmules is
another asexual reproduction method, but is found only in
freshwater sponges.
Sponges are monoecious; depending on the species, production
of gametes may be continuous through the year or dependent on
water temperature.
In nature, sponges are sessile as adults; however, under
laboratory conditions, sponge cells are capable of localized Figure 28.1C. 1 : Water flow in a sponge: In a sponge, water enters
creeping movements through organizational plasticity. through the body pores and exits in the direction of the osculum
Gas exchange, circulation, and excretion are other major body (direction of blue arrow). This diffusion of water through the body
supports major functions in the sponge.
functions in the sponge; these are achieved through the diffusion
of water through the sponge body. REPRODUCTION
Sponges reproduce by sexual, as well as, asexual methods. The
KEY TERMS typical means of asexual reproduction is either fragmentation (where
oocyte: a cell that develops into an egg or ovum; a female a piece of the sponge breaks off, settles on a new substrate, and
gametocyte develops into a new individual) or budding (a genetically-identical
gemmule: a small gemma or bud of dormant embryonic cells outgrowth from the parent eventually detaches or remains attached
produced by some freshwater sponges to form a colony). An atypical type of asexual reproduction is found
phagocytosis: the process where a cell incorporates a particle by only in freshwater sponges, occurring through the formation of
extending pseudopodia and drawing the particle into a vacuole of gemmules. Gemmules are environmentally-resistant structures
its cytoplasm produced by adult sponges wherein the typical sponge morphology
is inverted. In gemmules, an inner layer of amoebocytes is
PHYSIOLOGICAL PROCESSES IN SPONGES
surrounded by a layer of collagen (spongin) that may be reinforced
Sponges, despite being simple organisms, regulate their different by spicules. The collagen that is normally found in the mesohyl
physiological processes through a variety of mechanisms. These
becomes the outer protective layer. In freshwater sponges, gemmules
mechanisms regulate metabolism, reproduction, and locomotion. may survive hostile environmental conditions such as changes in
METABOLISM temperature. They serve to recolonize the habitat once
environmental conditions stabilize. Gemmules are capable of
Sponges lack complex digestive, respiratory, circulatory,
attaching to a substratum and generating a new sponge. Since
reproductive, and nervous systems. Their food is trapped when water
gemmules can withstand harsh environments, are resistant to
passes through the ostia and out through the osculum. Bacteria
desiccation, and remain dormant for long periods, they are an
smaller than 0.5 microns in size are trapped by choanocytes, which
excellent means of colonization for a sessile organism.
are the principal cells engaged in nutrition, and are ingested by
phagocytosis. Particles that are larger than the ostia may be Sexual reproduction in sponges occurs when gametes are generated.
phagocytized by pinacocytes. In some sponges, amoebocytes Sponges are monoecious (hermaphroditic), which means that one
transport food from cells that have ingested food particles to those individual can produce both gametes (eggs and sperm)
that do not. For this type of digestion, in which food particles are simultaneously. In some sponges, production of gametes may occur
digested within individual cells, the sponge draws water through throughout the year, whereas other sponges may show sexual cycles
diffusion. The limit of this type of digestion is that food particles depending upon water temperature. Sponges may also become
must be smaller than individual cells. sequentially hermaphroditic, producing oocytes first and
spermatozoa later. Oocytes arise by the differentiation of

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amoebocytes and are retained within the spongocoel, whereas holdfast. Provided by: Wiktionary. Located at:
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spermatozoa result from the differentiation of choanocytes and are spicule. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/spicule.
ejected via the osculum. Ejection of spermatozoa may be a timed License: CC BY-SA: Attribution-ShareAlike
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and coordinated event, as seen in certain species. Spermatozoa Located at: http://cnx.org/content/m44663/latest...ol11448/latest. License: CC
carried along by water currents can fertilize the oocytes borne in the BY: Attribution
Sponge-spicule hg. Provided by: Wikimedia. Located at:
mesohyl of other sponges. Early larval development occurs within commons.wikimedia.org/wiki/Fi...spicule_hg.jpg. License: CC BY: Attribution
the sponge; free-swimming larvae are then released via the osculum. OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44663/latest...ol11448/latest. License: CC
BY: Attribution
LOCOMOTION choanocyte. Provided by: Wiktionary. Located at:
Sponges are generally sessile as adults and spend their lives attached en.wiktionary.org/wiki/choanocyte. License: CC BY-SA: Attribution-
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to a fixed substratum. They do not show movement over large osculum. Provided by: Wiktionary. Located at:
distances as do free-swimming marine invertebrates. However, en.wiktionary.org/wiki/osculum. License: CC BY-SA: Attribution-ShareAlike
spongocoel. Provided by: Wiktionary. Located at:
sponge cells are capable of creeping along substrata via en.wiktionary.org/wiki/spongocoel. License: CC BY-SA: Attribution-
organizational plasticity. Under experimental conditions, researchers ShareAlike
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demonstrate a leading edge for directed movement. It has been OpenStax College, Biology. November 16, 2013. Provided by: OpenStax CNX.
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adjust to microenvironments near the point of attachment. It must be Sponge-spicule hg. Provided by: Wikimedia. Located at:
noted, however, that this pattern of movement has been documented commons.wikimedia.org/wiki/File:Sponge-spicule_hg.jpg. License: CC BY:
Attribution
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habitats. Located at: http://cnx.org/content/m44663/latest...ol11448/latest. License: CC
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gemmule. Provided by: Wiktionary. Located at:
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spongin. Provided by: Wiktionary. Located at: OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
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Invertebrate Zoology/Sponges. Provided by: Wikibooks. Located at: Sponge-spicule hg. Provided by: Wikimedia. Located at:
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SECTION OVERVIEW

28.2: PHYLUM CNIDARIA

28.2C: CLASS SCYPHOZOA
Topic hierarchy
28.2D: CLASS CUBOZOA AND CLASS HYDROZOA
28.2A: PHYLUM CNIDARIA
This page titled 28.2: Phylum Cnidaria is shared under a CC BY-SA 4.0
28.2B: CLASS ANTHOZOA license and was authored, remixed, and/or curated by Boundless.

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28.2A: PHYLUM CNIDARIA
Cnidarians are diploblastic, have organized tissue, undergo
extracellular digestion, and use cnidocytes for protection and to
capture prey.

 LEARNING OBJECTIVES

Describe the fundamental anatomy of a Cnidarian

KEY POINTS
Cnidarians have two distinct morphological body plans known as
polyp, which are sessile as adults, and medusa, which are
mobile; some species exhibit both body plans in their lifecycle.
All cnidarians have two membrane layers in the body: the
Figure 28.2A. 1 : Cnidocytes: Animals from the phylum Cnidaria
epidermis and the gastrodermis; between both layers they have have stinging cells called cnidocytes. Cnidocytes contain large
the mesoglea, which is a connective layer. organelles called (a) nematocysts that store a coiled thread and barb.
When hairlike projections on the cell surface are touched, (b) the
Cnidarians carry out extracellular digestion, where enzymes thread, barb, and a toxin are fired from the organelle.
break down the food particles and cells lining the gastrovascular
Animals in this phylum display two distinct morphological body
cavity absorb the nutrients.
plans: polyp or “stalk” and medusa or “bell”. An example of the
Cnidarians have an incomplete digestive system with only one
polyp form is Hydra spp.; perhaps the most well-known medusoid
opening; the gastrovascular cavity serves as both a mouth and an
animals are the jellies (jellyfish). Polyp forms are sessile as adults,
anus.
with a single opening to the digestive system (the mouth) facing up
The nervous system of cnidarians, responsible for tentacle
with tentacles surrounding it. Medusa forms are motile, with the
movement, drawing of captured prey to the mouth, digestion of
mouth and tentacles hanging down from an umbrella-shaped bell.
food, and expulsion of waste, is composed of nerve cells
scattered across the body.
Anthozoa, Scyphozoa, Cubozoa, and Hydrozoa make up the four
different classes of Cnidarians.

KEY TERMS
diploblastic: having two embryonic germ layers (the ectoderm
and the endoderm)
cnidocyte: a capsule, in certain cnidarians, containing a barbed,
threadlike tube that delivers a paralyzing sting

INTRODUCTION TO PHYLUM CNIDARIA Figure 28.2A. 1 : Cnidarian morphology: Cnidarians have two
Phylum Cnidaria includes animals that show radial or biradial distinct body plans, the medusa (a) and the polyp (b). All cnidarians
have two membrane layers, with a jelly-like mesoglea between
symmetry and are diploblastic: they develop from two embryonic them.
layers. Nearly all (about 99 percent) cnidarians are marine species.
Some cnidarians are polymorphic, having two body plans during
Cnidarians contain specialized cells known as cnidocytes (“stinging their life cycle. An example is the colonial hydroid called an Obelia.
cells”), which contain organelles called nematocysts (stingers). The sessile polyp form has, in fact, two types of polyps. The first is
These cells are present around the mouth and tentacles, serving to the gastrozooid, which is adapted for capturing prey and feeding; the
immobilize prey with toxins contained within the cells. Nematocysts other type of polyp is the gonozooid, adapted for the asexual
contain coiled threads that may bear barbs. The outer wall of the cell budding of medusa. When the reproductive buds mature, they break
has hairlike projections called cnidocils, which are sensitive to off and become free-swimming medusa, which are either male or
touch. When touched, the cells are known to fire coiled threads that female (dioecious). The male medusa makes sperm, whereas the
can either penetrate the flesh of the prey or predators of cnidarians, female medusa makes eggs. After fertilization, the zygote develops
or ensnare it. These coiled threads release toxins into the target that into a blastula and then into a planula larva. The larva is free
can often immobilize prey or scare away predators (). swimming for a while, but eventually attaches and a new colonial
reproductive polyp is formed.

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 The nervous system is primitive, with nerve cells scattered across
the body. This nerve net may show the presence of groups of cells in
the form of nerve plexi (singular: plexus) or nerve cords. The nerve
cells show mixed characteristics of motor as well as sensory
neurons. The predominant signaling molecules in these primitive
nervous systems are chemical peptides, which perform both
excitatory and inhibitory functions. Despite the simplicity of the
nervous system, it coordinates the movement of tentacles, the
drawing of captured prey to the mouth, the digestion of food, and the
expulsion of waste.
The cnidarians perform extracellular digestion in which the food is
taken into the gastrovascular cavity, enzymes are secreted into the
cavity, and the cells lining the cavity absorb nutrients. The
gastrovascular cavity has only one opening that serves as both a
mouth and an anus; this is termed an incomplete digestive system.
Cnidarian cells exchange oxygen and carbon dioxide by diffusion
between cells in the epidermis with water in the environment, and
between cells in the gastrodermis with water in the gastrovascular
cavity. The lack of a circulatory system to move dissolved gases
limits the thickness of the body wall, necessitating a non-living
mesoglea between the layers. There is no excretory system or
Figure 28.2A. 1 : Types of polyps in Obelia: The sessile form of organs; nitrogenous wastes simply diffuse from the cells into the
Obelia geniculate has two types of polyps: gastrozooids, which are
adapted for capturing prey, and gonozooids, which bud to produce water outside the animal or in the gastrovascular cavity. There is
medusae asexually. also no circulatory system, so nutrients must move from the cells
All cnidarians show the presence of two membrane layers in the that absorb them in the lining of the gastrovascular cavity through
body that are derived from the endoderm and ectoderm of the the mesoglea to other cells.
embryo. The outer layer (from ectoderm) is called the epidermis and The phylum Cnidaria contains about 10,000 described species
lines the outside of the animal, whereas the inner layer (from divided into four classes: Anthozoa, Scyphozoa, Cubozoa, and
endoderm) is called the gastrodermis and lines the digestive cavity. Hydrozoa. The anthozoans, the sea anemones and corals, are all
Between these two membrane layers is a non-living, jelly-like sessile species, whereas the scyphozoans (jellyfish) and cubozoans
mesoglea connective layer. In terms of cellular complexity, (box jellies) are swimming forms. The hydrozoans contain sessile
cnidarians show the presence of differentiated cell types in each forms and swimming colonial forms like the Portuguese Man O’
tissue layer: nerve cells, contractile epithelial cells, enzyme- War.
secreting cells, and nutrient-absorbing cells, as well as the presence
of intercellular connections. However, the development of organs or This page titled 28.2A: Phylum Cnidaria is shared under a CC BY-SA 4.0
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28.2B: CLASS ANTHOZOA
Members of the class Anthozoa display only polyp morphology and
have cnidocyte-covered tentacles around their mouth opening.

 LEARNING OBJECTIVES

Identify the adaptive features of anthozoa

KEY POINTS
Anthozoans include sea anemones, sea pens, and corals.
The pharynx of anthozoans (ingesting as well as egesting food)
leads to the gastrovascular cavity, which is divided by
mesenteries.
In Anthozoans, gametes are produced by the polyp; if they fuse,
they will give rise to a free-swimming planula larva, which will
become sessile once it finds an optimal substrate.
Sea anemonies and coral are examples of anthozoans that form
unique mutualistic relationships with other animal species; both Figure 28.2B. 1: Anthozoans: The sea anemone (a), like all
anthozoans, has only a polyp body plan (b).
sea anemonies and coral benefit from food availability provided
The mouth of a sea anemone is surrounded by tentacles that bear
by their partners.
cnidocytes. They have slit-like mouth openings and a pharynx,
KEY TERMS which is the muscular part of the digestive system that serves to
mesentery: in invertebrates, it describes any tissue that divides ingest as well as egest food. It may extend for up to two-thirds the
the body cavity into partitions length of the body before opening into the gastrovascular cavity.
cnidocyte: a capsule, in certain cnidarians, containing a barbed, This cavity is divided into several chambers by longitudinal septa
threadlike tube that delivers a paralyzing sting called mesenteries. Each mesentery consists of one ectodermal and
hermatypic: of a coral that is a species that builds coral reefs one endodermal cell layer with the mesoglea sandwiched in
between. Mesenteries do not divide the gastrovascular cavity
CLASS ANTHOZOA completely; the smaller cavities coalesce at the pharyngeal opening.
The class Anthozoa includes all cnidarians that exhibit a polyp body The adaptive benefit of the mesenteries appears to be an increase in
plan only; in other words, there is no medusa stage within their life surface area for absorption of nutrients and gas exchange.
cycle. Examples include sea anemones, sea pens, and corals, with an Sea anemones feed on small fish and shrimp, usually by
estimated number of 6,100 described species. Sea anemones are immobilizing their prey using the cnidocytes. Some sea anemones
usually brightly colored and can attain a size of 1.8 to 10 cm in establish a mutualistic relationship with hermit crabs by attaching to
diameter. These animals are usually cylindrical in shape and are the crab’s shell. In this relationship, the anemone gets food particles
attached to a substrate. from prey caught by the crab, while the crab is protected from the
predators by the stinging cells of the anemone. Anemone fish, or
clownfish, are able to live in the anemone since they are immune to
the toxins contained within the nematocysts. Another type of
anthozoan that forms an important mutualistic relationship is reef
building coral. These hermatypic corals rely on a symbiotic
relationship with zooxanthellae. The coral gains photosynthetic
capability, while the zooxanthellae benefit by using nitrogenous
waste and carbon dioxide produced by the cnidarian host.
Anthozoans remain polypoid throughout their lives. They can
reproduce asexually by budding or fragmentation, or sexually by
producing gametes. Both gametes are produced by the polyp, which
can fuse to give rise to a free-swimming planula larva. The larva
settles on a suitable substratum and develops into a sessile polyp.

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28.2C: CLASS SCYPHOZOA
Scyphozoans are free-swimming, polymorphic, dioecious, and live most of their life cycle as free-swimming, solitary carnivores.
carnivorous cnidarians with a prominent medusa morphology. The mouth leads to the gastrovascular cavity, which may be
sectioned into four interconnected sacs, called diverticuli. In some
 LEARNING OBJECTIVES species, the digestive system may be further branched into radial
canals. Like the septa in anthozoans, the branched gastrovascular
Explain the key features of scyphozoa cells serves to increase the surface area for nutrient absorption and
diffusion; thus, more cells are in direct contact with the nutrients in
KEY POINTS the gastrovascular cavity.
Scyphozoans have a ring of muscles that lines the dome of their In scyphozoans, nerve cells are scattered over the entire body.
bodies; these structures provide them with the contractile force Neurons may even be present in clusters called rhopalia. These
they need to swim through water. animals possess a ring of muscles lining the dome of the body,
Scyphozoans have separate sexes and form planula larvae which provides the contractile force required to swim through water.
through external fertilization. Scyphozoans are dioecious animals, having separate sexes. The
Jellies exhibit the polyp form, known as a scyphistoma, after gonads are formed from the gastrodermis with gametes expelled
their larvae settle on a substrate; these forms will later bud-off through the mouth. Planula larvae are formed by external
and transform into their more prominenent medusa forms. fertilization; they settle on a substratum in a polypoid form known as
scyphistoma. These forms may produce additional polyps by
KEY TERMS budding or may transform into the medusoid form. The life cycle of
dioecious: having the male and female reproductive organs on these animals can be described as polymorphic because they exhibit
separate parts (of the same species) both a medusal and polypoid body plan at some point.
rhopalia: small sensory structures found within Scyphozoa that
are characterized by clusters of neurons that can be used to sense
light
scyphistoma: the polypoid form of scyphozoans
nematocyst: a capsule, in certain cnidarians, containing a
barbed, threadlike tube that delivers a paralyzing sting

CLASS SCYPHOZOA
Class Scyphozoa, an exclusively marine class of animals with about
200 known species, includes all the jellies. The defining
characteristic of this class is that the medusa is the prominent stage
in the life cycle, although there is a polyp stage present. Members of
this species range from 2 to 40 cm in length, but the largest
scyphozoan species, Cyanea capillata, can reach a size of 2 m
across. Scyphozoans display a characteristic bell-like morphology.

Figure 28.2C. 1 : Lifecycle of a jellyfish: The lifecycle of a jellyfish

includes two stages: the medusa stage and the polyp stage. The
polyp reproduces asexually by budding,while the medusa reproduces
sexually.
Figure 28.2C. 1 : Scyphozoans: For jellyfish (a), and all other
scyphozoans, the medusa (b) is the most prominent of the two life This page titled 28.2C: Class Scyphozoa is shared under a CC BY-SA 4.0
stages.
license and was authored, remixed, and/or curated by Boundless.
In the jellyfish, a mouth opening, surrounded by tentacles bearing
nematocysts, is present on the underside of the animal. Scyphozoans

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28.2D: CLASS CUBOZOA AND CLASS HYDROZOA
Cubozoans live as box-shaped medusae while Hydrozoans are true polyps to colonize a habitat. Polyp forms then transform into the
polymorphs and can be found as colonial or solitary organisms. medusoid forms.

 LEARNING OBJECTIVES CLASS HYDROZOA

Hydrozoa includes nearly 3,200 species; most are marine, although
Distinguish between cubozoa and hydrozoa cnidarians some freshwater species are known. Animals in this class are
polymorphs: most exhibit both polypoid and medusoid forms in
KEY POINTS their lifecycle, although this is variable.
Cubozoans differ from Scyphozoans in their arrangement of
tentacles; they are also known for their box-shaped medusa.
Out of all cnidarians, cubozoans are the most venomous.
Hydrozoans are polymorphs, existing as solitary polyps, solitary
medusae, or as colonies.
Hydrozoans are unique from all other cnidarians in that their
gonads are derived from epidermal tissue.

KEY TERMS
hydroid: any of many colonial coelenterates that exist mainly as
a polyp; a hydrozoan

CLASS CUBOZOA
Class Cubozoa includes jellies that have a box-shaped medusa: a
bell that is square in cross-section; hence, they are colloquially
known as “box jellyfish.” These species may achieve sizes of 15–25
cm. Cubozoans display overall morphological and anatomical
characteristics that are similar to those of the scyphozoans. A
prominent difference between the two classes is the arrangement of
tentacles. This is the most venomous group of all the cnidarians.

Figure 28.2D. 1 : Hydrozoans: (a) Obelia, (b) Physalia physalis,

known as the Portuguese Man O‘ War, (c) Velella bae, and (d)
Hydra have different body shapes, but all belong to the family
Hydrozoa.
The polyp form in these animals often shows a cylindrical
morphology with a central gastrovascular cavity lined by the
gastrodermis. The gastrodermis and epidermis have a simple layer of
Figure 28.2D. 1 : Cubozoans: The (a) tiny cubazoan jelly Malo kingi mesoglea sandwiched between them. A mouth opening, surrounded
is thimble shaped and, like all cubozoan jellies, (b) has four by tentacles, is present at the oral end of the animal. Many
muscular pedalia to which the tentacles attach. M. kingi is one of hydrozoans form colonies that are composed of a branched colony
two species of jellies known to cause Irukandji syndrome, a
condition characterized by excruciating muscle pain, vomiting, of specialized polyps that share a gastrovascular cavity, such as in
increased heart rate, and psychological symptoms. Two people in the colonial hydroid Obelia. Colonies may also be free-floating and
Australia, where Irukandji jellies are most-commonly found, are contain medusoid and polypoid individuals in the colony as in
believed to have died from Irukandji stings. (c) A sign on a beach in
northern Australia warns swimmers of the danger. Physalia (the Portuguese Man O’ War) or Velella (By-the-wind
The cubozoans contain muscular pads called pedalia at the corners sailor). Other species are solitary polyps (Hydra) or solitary
of the square bell canopy, with one or more tentacles attached to medusae (Gonionemus). The true characteristic shared by all these
each pedalium. These animals are further classified into orders based diverse species is that their gonads for sexual reproduction are
on the presence of single or multiple tentacles per pedalium. In some derived from epidermal tissue, whereas in all other cnidarians they
cases, the digestive system may extend into the pedalia. are derived from gastrodermal tissue.
Nematocysts may be arranged in a spiral configuration along the
CONTRIBUTIONS AND ATTRIBUTIONS
tentacles; this arrangement helps to effectively subdue and capture
OpenStax College, Biology. October 22, 2013. Provided by: OpenStax CNX.
prey. Cubozoans exist in a polypoid form that develops from a Located at: http://cnx.org/content/m44664/latest...ol11448/latest. License: CC
planula larva. These polyps show limited mobility along the BY: Attribution
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SECTION OVERVIEW

28.3: SUPERPHYLUM LOPHOTROCHOZOA

28.3D: PHYLUM NEMERTEA
Topic hierarchy
28.3E: PHYLUM MOLLUSCA
28.3A: SUPERPHYLUM LOPHOTROCHOZOA 28.3F: CLASSIFICATION OF PHYLUM MOLLUSCA
28.3B: PHYLUM PLATYHELMINTHES 28.3G: PHYLUM ANNELIDA
28.3C: PHYLUM ROTIFERA
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28.3A: SUPERPHYLUM LOPHOTROCHOZOA
The Lophotrochozoa are protostomes possessing a blastopore, an As lophotrochozoans, the organisms in this superphylum possess
early form of a mouth; they include the trochozoans and the either lophophore or trochophore larvae. The exact relationships
lophophorata. between the different phyla are not entirely certain. The lophophores
include groups that are united by the presence of the lophophore, a
 LEARNING OBJECTIVES set of ciliated tentacles surrounding the mouth. Lophophorata
include the flatworms and several other phyla, including the
Describe the phylogenetic position and basic features of Bryozoa, Entoprocta, Phoronida, and Brachiopoda. These clades are
lophotrochozoa upheld when RNA sequences are compared. Trochophore larvae are
characterized by two bands of cilia around the body. Previously,
KEY POINTS these were treated together as the Trochozoa, together with the
Lophotrochozoa have a blastopore, which is an involution of the arthropods, which do not produce trochophore larvae, but were
ectoderm that forms a rudimentary mouth opening to the considered close relatives of the annelids because they are both
alimentary canal, a condition called protostomy or “first mouth”. segmented. However, they show a number of important differences.
The Lophotrochozoa are comprised of the trochozoans and the Arthropods are now placed separately among the Ecdysozoa. The
lophophorata, although the exact relationships between the Trochozoa include the Nemertea, Mollusca, Sipuncula, and
different phyla are not clearly determined. Annelida.
Lophophores are characterized by the presence of the The lophotrochozoans are triploblastic, possessing an embryonic
lophophore, a set of ciliated tentacles surrounding the mouth; mesoderm sandwiched between the ectoderm and endoderm found
they include the flatworms and several other phyla whose in the diploblastic cnidarians. These phyla are also bilaterally
relationships are upheld by genetic evidence. symmetrical: a longitudinal section will divide them into right and
Trochophore larvae are distinguished from the lophophores by left sides that are symmetrical. They also show the beginning of
two bands of cilia around the body; they include the Nemertea, cephalization: the evolution of a concentration of nervous tissues
Mollusca, Sipuncula, and Annelida. and sensory organs in the head of the organism, which is where it
The lophotrochozoans have a mesoderm layer positioned first encounters its environment.
between the ectoderm and endoderm and are bilaterally
symmetrical, which signals the beginning of cephalization, the
concentration of nervous tissues and sensory organs in the head
of the organism.

KEY TERMS
blastopore: the opening into the archenteron
lophophore: a feeding organ of brachiopods, bryozoans, and
phoronids
cephalization: an evolutionary trend in which the neural and
sense organs become centralized at one end (the head) of an
animal

LOPHOTROCHOZOANS
Animals belonging to superphylum Lophotrochozoa are
protostomes: the blastopore (or the point of involution of the
ectoderm or outer germ layer) becomes the mouth opening to the Figure 28.3A. 1 : Lophotrochozoans: The Caribbean Reef Squid or
alimentary canal. This is called protostomy or “first mouth.” In Sepioteuthis sepioidea is a complex lophotrochozoan. Species in this
protostomy, solid groups of cells split from the endoderm or inner group have bilateral symmetry.
germ layer to form a central mesodermal layer of cells. This layer This page titled 28.3A: Superphylum Lophotrochozoa is shared under a CC
multiplies into a band which then splits internally to form the BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.
coelom; this protostomic coelom is termed schizocoelom.

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28.3B: PHYLUM PLATYHELMINTHES
The Platyhelminthes are flatworms that lack a coelom; many are longitudinal muscle. The mesodermal tissues include mesenchymal
parasitic; all lack either a circulatory or respiratory system. cells that contain collagen and support secretory cells that secrete
mucus and other materials at the surface. The flatworms are
 LEARNING OBJECTIVES acoelomates: their bodies are solid between the outer surface and the
cavity of the digestive system.
Differentiate among the classes of platyhelminthes
PHYSIOLOGICAL PROCESSES OF FLATWORMS
KEY POINTS The free-living species of flatworms are predators or scavengers.
The Platyhelminthes are acoelomate flatworms: their bodies are Parasitic forms feed on the tissues of their hosts. Most flatworms
solid between the outer surface and the cavity of the digestive have a gastrovascular cavity rather than a complete digestive system;
system. in such animals, the “mouth” is also used to expel waste materials
Most flatworms have a gastrovascular cavity rather than a from the digestive system. Some species also have an anal opening.
complete digestive system; the same cavity used to bring in food The gut may be a simple sac or highly branched. Digestion is
is used to expel waste materials. extracellular, with digested materials taken in to the cells of the gut
Platyhelminthes are either predators or scavengers; many are lining by phagocytosis. One group, the cestodes, lacks a digestive
parasites that feed on the tissues of their hosts. system. Flatworms have an excretory system with a network of
Flatworms posses a simple nervous system, no circulatory or tubules throughout the body with openings to the environment and
respiratory system, and most produce both eggs and sperm, with nearby flame cells, whose cilia beat to direct waste fluids
internal fertilization. concentrated in the tubules out of the body. The system is
Platyhelminthes are divided into four classes: Turbellaria, free- responsible for the regulation of dissolved salts and the excretion of
living marine species; Monogenea, ectoparasites of fish; nitrogenous wastes. The nervous system consists of a pair of nerve
Trematoda, internal parasites of humans and other species; and cords running the length of the body with connections between them
Cestoda (tapeworms), which are internal parasites of many and a large ganglion or concentration of nerves at the anterior end of
vertebrates. the worm, where there may also be a concentration of photosensory
In flatworms, digested materials are taken into the cells of the gut and chemosensory cells.
lining by phagocytosis, rather than being processed internally. There is neither a circulatory nor respiratory system, with gas and
nutrient exchange dependent on diffusion and cell-cell junctions.
KEY TERMS This necessarily limits the thickness of the body in these organisms,
acoelomate: any animal without a coelom, or body cavity constraining them to be “flat” worms. In addition, most flatworm
ectoparasite: a parasite that lives on the surface of a host species are monoecious; typically, fertilization is internal. Asexual
organism reproduction is common in some groups.
scolex: the structure at the rear end of a tapeworm which, in the
adult, has suckers and hooks by which it attaches itself to a host DIVERSITY OF FLATWORMS
proglottid: any of the segments of a tapeworm; they contain Platyhelminthes are traditionally divided into four classes:
both male and female reproductive organs Turbellaria, Monogenea, Trematoda, and Cestoda. The class
Turbellaria includes mainly free-living, marine species, although
PHYLUM PLATYHELMINTHES some species live in freshwater or moist terrestrial environments.
Phylum Platyhelminthes is composed of the flatworms: acoelomate The ventral epidermis of turbellarians is ciliated which facilitates
organisms that include many free-living and parasitic forms. Most of their locomotion. Some turbellarians are capable of remarkable feats
the flatworms are classified in the superphylum Lophotrochozoa, of regeneration: they may regrow the entire body from a small
which also includes the mollusks and annelids. The Platyhelminthes fragment.
consist of two lineages: the Catenulida and the Rhabditophora. The
Catenulida, or “chain worms” is a small clade of just over 100
species. These worms typically reproduce asexually by budding.
However, the offspring do not fully detach from the parents;
therefore, they resemble a chain. The remaining flatworms discussed
here are part of the Rhabditophora.
Many flatworms are parasitic, including important parasites of
humans. Flatworms have three embryonic tissue layers that give rise
to surfaces that cover tissues (from ectoderm), internal tissues (from
mesoderm), and line the digestive system (from endoderm). The
epidermal tissue is a single layer cells or a layer of fused cells
(syncytium) that covers a layer of circular muscle above a layer of

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 The cestodes, or tapeworms, are also internal parasites, mainly of
vertebrates. Tapeworms live in the intestinal tract of the primary
host, remaining fixed by using a sucker on the anterior end, or
scolex, of the tapeworm body. The remainder of the tapeworm is
composed of a long series of units called proglottids. Each may
contain an excretory system with flame cells and both female and
male reproductive structures. Tapeworms do not possess a digestive
system; instead, they absorb nutrients from the food matter passing
through them in the host’s intestine.

Figure 28.3B. 1: Turbellaria: Pseudobiceros bedfordi (Bedford’s

Flatworm), a member of the Turbellaria, is a marine species which
uses the epidermis of its belly for locomotion.
The monogeneans are ectoparasites, mostly of fish, with simple life
cycles that consist of a free-swimming larva that attaches to a fish to
begin transformation to the parasitic adult form. The worms may
produce enzymes that digest the host tissues or simply graze on
surface mucus and skin particles.
The trematodes, or flukes, are internal parasites of mollusks and
many other groups, including humans. Trematodes have complex
Figure 28.3B. 1: Cestodes: Taenia saginata, also known as
life cycles that involve a primary host in which sexual reproduction Taeniarhynchus saginata or the beef tapeworm, is a parasite of both
occurs and one or more secondary hosts in which asexual cattle and humans. It displays the long series of proglottid subunits
reproduction occurs. The primary host is almost always a mollusk. characteristic of the species.
Trematodes are responsible for serious human diseases including
This page titled 28.3B: Phylum Platyhelminthes is shared under a CC BY-
schistosomiasis, a blood fluke.
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

Figure 28.3B. 1: Trematodes: Botulus microporus is a trematode

that lives only in the intestinal tract of Lancetfish, Alepisaurus ferox.

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28.3C: PHYLUM ROTIFERA
Rotifers are microscopic organisms named for a rotating structure
(called the corona) at their anterior end that is covered with cilia.

 LEARNING OBJECTIVES

Identify the features of rotifers involved in movement and

feeding

KEY POINTS
The rotifer body form consists of a head (containing the sensory
organs in the form of a bi-lobed brain and small eyespots near
the corona), the trunk (containing organs), and the foot (which
can hold fast).
The foot of the rotifer secretes a sticky material to help it adhere
to surfaces.
Rotifers are filter feeders that generate a current using the corona
Figure 28.3C. 1 : Rotifers: A bdelloid rotifer is a member of a class
to pass food into the mouth, which then passes by digestive and of rotifers found in fresh water and moist soil. The rotifer body
salivary glands into the stomach and intestines. consists of a head, a truck, and a foot. They eat by filtering food into
Rotifers exhibit sexual dimorphism; the gender of many species the mouth by creating currents with the corona.
is determined by whether the egg is fertilized (and develops into The rotifer body form consists of a head (which contains the
a female) or unfertilized (and develops into a male). corona), a trunk (which contains the organs), and the foot. Rotifers
are typically free-swimming and truly planktonic organisms, but the
KEY TERMS toes or extensions of the foot can secrete a sticky material forming a
pseudocoelomate: any invertebrate animal with a three-layered holdfast to help them adhere to surfaces. The head contains sensory
body and a pseudocoel organs in the form of a bi-lobed brain and small eyespots near the
mastax: the pharynx of a rotifer which usually contains four corona.
horny pieces that work to crush the food The rotifers are filter feeders that will eat dead material, algae, and
other microscopic living organisms. Therefore, they are very
PHYLUM ROTIFERA important components of aquatic food webs. Rotifers obtain food
The rotifers are a microscopic (about 100 µm to 30 mm) group of that is directed toward the mouth by the current created from the
mostly-aquatic organisms that get their name from the corona: a movement of the corona. The food particles enter the mouth and
rotating, wheel-like structure that is covered with cilia at their travel to the mastax (pharynx with jaw-like structures). Food passes
anterior end. Although their taxonomy is currently in flux, one by digestive and salivary glands into the stomach and then into the
treatment places the rotifers in three classes: Bdelloidea, intestines. Digestive and excretory wastes are collected in a cloacal
Monogononta, and Seisonidea. The classification of the group is bladder before being released out the anus.
currently under revision, however, as more phylogenetic evidence
Rotifers are pseudocoelomates commonly found in fresh water and
becomes available. It is possible that the “spiny headed worms”
some salt water environments throughout the world. About 2,200
currently in phylum Acanthocephala will be incorporated into this
species of rotifers have been identified. Rotifers are dioecious
group in the future.
organisms (having either male or female genitalia) and exhibit
sexual dimorphism (males and females have different forms). Many
species are parthenogenic and exhibit haplodiploidy, a method of
gender determination in which a fertilized egg develops into a
female and an unfertilized egg develops into a male. In many
dioecious species, males are short-lived and smaller, with no
digestive system and a single testis. Females can produce eggs that
are capable of dormancy, which protects eggs during harsh
environmental conditions.

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28.3D: PHYLUM NEMERTEA
Nemertea, or ribbon worms, are distinguished by their proboscis, also show a flattened morphology: they are flat from front to back,
used for capturing prey and enclosed in a cavity called a like a flattened tube. In addition, nemertea are soft, unsegmented
rhynchocoel. animals.

 LEARNING OBJECTIVES

Identify the key features of the Phylum Nemertea

KEY POINTS
The Nemertini are mostly bottom-dwelling marine organisms,
although some are found in freshwater and terrestrial habitats.
Most nemerteans are carnivores, some are scavengers, and others
have evolved relationships with some mollusks that are benefit
the Nemertean but do not harm the mollusk.
Nemerteans vary greatly in size and are bilaterally symmetrical; Figure 28.3D. 1 : Morphology of Nemertea: A terrestrial
they are unsegmented and resemble a flat tube which can change Geonemertes, a Nemertean, displaying the flat, ribbon-like body of
morphological presentation in response to environmental cues. the organism that is unsegmented.
Nemertini have a simple nervous system comprised of a ring of A unique characteristic of this phylum is the presence of a proboscis
four nerve masses called “ganglia” at the anterior end between enclosed in a rhynchocoel. The proboscis serves to capture food and
the mouth and the foregut from which paired longitudinal nerve may be ornamented with barbs in some species. The rhynchocoel is
cords emerge and extend to the posterior end. a fluid-filled cavity that extends from the head to nearly two-thirds
Nemertini are mostly sexually dimorphic, fertilizing eggs of the length of the gut in these animals. The proboscis may be
externally by releasing both eggs and sperm into the water; a extended or retracted by the retractor muscle attached to the wall of
larva may develop inside the resulting young worm and devour the rhynchocoel.
its tissues before metamorphosing into the adult.

KEY TERMS
protonephridia: an invertebrate organ which occurs in pairs and
removes metabolic wastes from an animal’s body
rhynchocoel: a cavity which mostly runs above the midline and
ends a little short of the rear of the body of a nemertean and
extends or retracts the proboscis
proboscis: an elongated tube from the head or connected to the
mouth, of an animal

PHYLUM NEMERTEA
The Nemertea are colloquially known as ribbon worms. Most
species of phylum Nemertea are marine (predominantly benthic or
bottom dwellers) with an estimated 900 species known. However,
nemertini have been recorded in freshwater and terrestrial habitats as
well. Most nemerteans are carnivores, feeding on worms, clams, and
crustaceans. Some species are scavengers, while other nemertini
species, such as Malacobdella grossa, have also evolved
commensalistic relationships with some mollusks. Interestingly,
nemerteans have almost no predators, two species are sold as fish
Figure 28.3D. 1 : Internal structures of the Nemertini: This image
bait, and some species have devastated commercial fishing of clams shows the internal structures of a basic nemertean, including the
and crabs. proboscis and the rhynchocoel: 1: Proboscis 2: Rhynchocoel 3:
Dorsal commissure of brain 4: Rhynchodeum 5: Proboscis pore 6:
MORPHOLOGY Ventral commissure of brain 7: Mouth 8: Foregut 9: Stomach

Ribbon worms vary in size from 1 cm to several meters. They show METABOLISM
bilateral symmetry and remarkable contractile properties. Because of
The nemertini show a very well-developed digestive system. A
their contractility, they can change their morphological presentation
mouth opening that is ventral to the rhynchocoel leads into the
in response to environmental cues. Animals in phylum Nemertea
foregut, followed by the intestine. The intestine is present in the

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form of diverticular pouches which ends in a rectum that opens via “ganglia” comprises the brain in these animals. Paired longitudinal
an anus. Gonads are interspersed with the intestinal diverticular nerve cords emerge from the brain ganglia, extending to the
pouches, opening outwards via genital pores. A circulatory system posterior end. Ocelli or eyespots are present in pairs, in multiples of
consists of a closed loop of a pair of lateral blood vessels. The two in the anterior portion of the body. It is speculated that the
circulatory system is derived from the coelomic cavity of the eyespots originate from neural tissue and not from the epidermis.
embryo. Some animals may also have cross-connecting vessels in
addition to lateral ones. Although these are called blood vessels, REPRODUCTION
since they are of coelomic origin, the circulatory fluid is colorless. Animals in phylum Nemertea show sexual dimorphism, although
Some species bear hemoglobin as well as yellow or green pigments. freshwater species may be hermaphroditic. Eggs and sperm are
The blood vessels are connected to the rhynchocoel. The flow of released into the water; fertilization occurs externally. The zygote
fluid in these vessels is facilitated by the contraction of muscles in develops into a special kind of nemertean larvae called a planuliform
the body wall. A pair of protonephridia, or primitive kidneys, is larva. In some nemertine species, another larva specific to the
present in these animals to facilitate osmoregulation. Gaseous nemertinis, a pilidium, may develop inside the young worm from a
exchange occurs through the skin in the nemertini. series of imaginal discs. This larval form, characteristically shaped
like a deerstalker cap, devours tissues from the young worm for
NERVOUS SYSTEM survival before metamorphosing into the adult-like morphology.
Nemertini have a ganglion or “brain” situated at the anterior end
between the mouth and the foregut, surrounding the digestive system This page titled 28.3D: Phylum Nemertea is shared under a CC BY-SA 4.0
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28.3E: PHYLUM MOLLUSCA
Mollusks have a soft body and share several characteristics,
including a muscular foot, a visceral mass of internal organs, and a
mantle.

 LEARNING OBJECTIVES

Describe the unique anatomical and morphological features

of molluscs

KEY POINTS
A mollusk’s muscular foot is used for locomotion and anchorage,
varies in shape and function, and can both extend and retract.
The visceral mass inside the mollusk includes digestive, nervous,
excretory, reproductive, and respiratory systems.
Most mollusks possess a radula, which is similar to a tongue with
teeth-like projections, serving to shred or scrape food.
The mantle is the dorsal epidermis in mollusks; in some mollusks
it secretes a chitinous and hard calcareous shell.
Figure 28.3E. 1: Mollusk shells: Helix aspersa, a common land
KEY TERMS snail, has a calcium carbonate shell.
visceral mass: the soft, non-muscular metabolic region of the Mollusks have a muscular foot used for locomotion and anchorage
mollusc that contains the body organs that varies in shape and function, depending on the type of mollusk
mantle: the body wall of a mollusc, from which the shell is under study. In shelled mollusks, this foot is usually the same size as
secreted the opening of the shell. The foot is a retractable as well as an
radula: the rasping tongue of snails and most other mollusks extendable organ. It is the ventral-most organ, whereas the mantle is
the limiting dorsal organ. Mollusks are eucoelomate, but the cavity
PHYLUM MOLLUSCA
is restricted to a region around the heart in adult animals. The mantle
Phylum Mollusca is the predominant phylum in marine cavity develops independently of the coelomic cavity.
environments. It is estimated that 23 percent of all known marine
species are mollusks; there are around 85,000 described species, The visceral mass is present above the foot in the visceral hump.
making them the second most diverse phylum of animals. The name This includes digestive, nervous, excretory, reproductive, and
“mollusca” signifies a soft body; the earliest descriptions of respiratory systems. Mollusk species that are exclusively aquatic
mollusks came from observations of unshelled cuttlefish. Mollusks have gills for respiration, whereas some terrestrial species have
lungs for respiration. Additionally, a tongue-like organ called a
are predominantly a marine group of animals; however, they are
radula, which bears chitinous tooth-like ornamentation, is present in
known to inhabit freshwater as well as terrestrial habitats. Mollusks
many species, serving to shred or scrape food. The mantle (also
display a wide range of morphologies in each class and subclass.
known as the pallium) is the dorsal epidermis in mollusks; shelled
They range from large predatory squids and octopus, some of which
mollusks are specialized to secrete a chitinous and hard calcareous
show a high degree of intelligence, to grazing forms with
shell.
elaborately-sculpted and colored shells. In spite of their tremendous
diversity, however, they also share a few key characteristics,
including a muscular foot, a visceral mass containing internal
organs, and a mantle that may or may not secrete a shell of calcium
carbonate.

28.3E.1 https://bio.libretexts.org/@go/page/13721
 Most mollusks are dioecious animals where fertilization occurs
externally, although this is not the case in terrestrial mollusks, such
as snails and slugs, or in cephalopods. In some mollusks, the zygote
hatches and undergoes two larval stages, trochophore and veliger,
before becoming a young adult; bivalves may exhibit a third larval
stage, glochidia.

This page titled 28.3E: Phylum Mollusca is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

Figure 28.3E. 1: A “generalized mollusk”: An anatomical diagram

of a hypothetical ancestral mollusk, showing features common to
many mollusk types.

28.3E.2 https://bio.libretexts.org/@go/page/13721
28.3F: CLASSIFICATION OF PHYLUM MOLLUSCA
The phylum Mollusca includes a wide variety of animals including CLASS MONOPLACOPHORA
the gastropods (“stomach foot”), the cephalopods (“head foot”), and Members of class Monoplacophora (“bearing one plate”) posses a
the scaphopods (“boat foot”). single, cap-like shell that encloses the body. The morphology of the
shell and the underlying animal can vary from circular to ovate. A
 LEARNING OBJECTIVES looped digestive system, multiple pairs of excretory organs, many
gills, and a pair of gonads are present in these animals. The
Differentiate among the classes in the phylum mollusca
monoplacophorans were believed extinct and only known via fossil
records until the discovery of Neopilina galathaea in 1952. Today,
KEY POINTS scientists have identified nearly two dozen extant species.
Mollusks can be segregated into seven classes: Aplacophora,
Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, CLASS POLYPLACOPHORA
Cephalopoda, and Scaphopoda. These classes are distinguished Animals in the class Polyplacophora (“bearing many plates”) are
by, among other criteria, the presence and types of shells they commonly known as “chitons” and bear an armor-like, eight-plated
possess. dorsal shell. These animals have a broad, ventral foot that is adapted
Class Aplacophora includes worm-like animals with no shell and for suction to rocks and other substrates, and a mantle that extends
a rudimentary body structure. beyond the shell in the form of a girdle. Calcareous spines may be
Members of class Monoplacophora have a single shell that present on the girdle to offer protection from predators. Chitons live
encloses the body. worldwide, in cold water, warm water, and the tropics. Most chiton
Members of class Polyplacophora are better known as “chitons;” species inhabit intertidal or subtidal zones, and do not extend beyond
these molluscs have a large foot on the ventral side and a shell the photic zone. Some species live quite high in the intertidal zone
composed of eight hard plates on the dorsal side. and are exposed to the air and light for long periods.
Class Bivalvia consists of mollusks with two shells held together
by a muscle; these include oysters, clams, and mussels.
Members of class Gastropoda have an asymmetrical body plan
and usually have a shell, which can be planospiral or conispiral.
Their key characteristic is the torsion around the perpendicular
axis on the center of the foot that is modified for crawling.
Class Scaphopoda consists of mollusks with a single conical
shell through which the head protrudes, and a foot modified into
tentacles known as captaculae that are used to catch and
manipulate prey.

KEY TERMS
ctenidium: a respiratory system, in the form of a comb, in some
molluscs
captacula: the foot of a Scaphalopod, modified into tentacles for
Figure 28.3F . 1 : Chiton morphology: The underside of the gumboot
capturing prey chiton, Cryptochiton stellari, showing the foot in the center,
nephridium: a tubular excretory organ in some invertebrates surrounded by the gills and mantle. The mouth is visible to the left
in this image.
CLASSES IN PHYLUM MOLLUSCA
Phylum Mollusca is a very diverse (85,000 species ) group of mostly
CLASS BIVALVIA
marine species, with a dramatic variety of form. This phylum can be Bivalvia is a class of marine and freshwater molluscs with laterally
segregated into seven classes: Aplacophora, Monoplacophora, compressed bodies enclosed by a shell in two hinged parts. Bivalves
Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and include clams, oysters, mussels, scallops, and numerous other
Scaphopoda. families of shells. The majority are filter feeders and have no head or
radula. The gills have evolved into ctenidia, specialised organs for
CLASS APLACOPHORA feeding and breathing. Most bivalves bury themselves in sediment
Class Aplacophora (“bearing no plates”) includes worm-like animals on the seabed, while others lie on the sea floor or attach themselves
primarily found in benthic marine habitats. These animals lack a to rocks or other hard surfaces.
calcareous shell, but possess aragonite spicules on their epidermis. The shell of a bivalve is composed of calcium carbonate, and
They have a rudimentary mantle cavity and lack eyes, tentacles, and consists of two, usually similar, parts called valves. These are joined
nephridia (excretory organs). together along one edge by a flexible ligament that, in conjunction
with interlocking “teeth” on each of the valves, forms the hinge.

28.3F.1 https://bio.libretexts.org/@go/page/13722
 which is used for camouflage. All animals in this class are
carnivorous predators and have beak-like jaws at the anterior end.
All cephalopods show the presence of a very well-developed
nervous system along with eyes, as well as a closed circulatory
system. The foot is lobed and developed into tentacles and a funnel,
which is used as the mode of locomotion. Locomotion in
cephalopods is facilitated by ejecting a stream of water for
propulsion (“jet” propulsion). Cephalopods, such as squids and
octopuses, also produce sepia or a dark ink, which is squirted upon a
predator to assist in a quick getaway. Suckers are present on the
tentacles in octopuses and squid. Ctenidia are enclosed in a large
mantle cavity serviced by blood vessels, each with its own
associated heart. The mantle has siphonophores that facilitate
exchange of water.

Figure 28.3F . 1 : Empty shell of a bivalve: The empty shell of the

giant clam, Tridacna gigas. Note the pair of shells that are hinged
together, a characteristic of members of the class Bivalvia.

CLASS GASTROPODA
Animals in class Gastropoda (“stomach foot”) include well-known
mollusks like snails, slugs, conchs, sea hares, and sea butterflies.
Gastropoda includes shell-bearing species as well as species with a
reduced shell. These animals are asymmetrical and usually present a
coiled shell. Shells may be planospiral (like a garden hose wound
up), commonly seen in garden snails, or conispiral (like a spiral
staircase), commonly seen in marine conches.
The visceral mass in the shelled species displays torsion around the
perpendicular axis on the center of the foot, which is the key
characteristic of this group, along with a foot that is modified for
Figure 28.3F . 1 : Cephalopods: Cephalopods (“head foot”) include
crawling. Most gastropods bear a head with tentacles, eyes, and a this octopus, which ejects a stream of water from a funnel in its body
style. A complex radula is used by the digestive system and aids in to propel itself through the water.
the ingestion of food. Eyes may be absent in some gastropods A pair of nephridia is present within the mantle cavity. Sexual
species. The mantle cavity encloses the ctenidia (singluar: dimorphism is seen in this class of animals. Members of a species
ctenidium) as well as a pair of nephridia (singular: nephridium). mate, then the female lays the eggs in a secluded and protected
niche. Females of some species care for the eggs for an extended
period of time and may end up dying during that time period.
Reproduction in cephalopods is different from other mollusks in that
the egg hatches to produce a juvenile adult without undergoing the
trochophore and veliger larval stages.

CLASS SCAPHOPODA
Members of class Scaphopoda (“boat feet”) are known colloquially
as “tusk shells” or “tooth shells,” as evident when examining
Dentalium, one of the few remaining scaphopod genera. Scaphopods
are usually buried in sand with the anterior opening exposed to
water. These animals bear a single conical shell, which has both ends
Figure 28.3F . 1 : Gastropod foot: Gastropods, such as this Roman open. The head is rudimentary and protrudes out of the posterior end
snail, have a large foot that is modified for crawling. of the shell. These animals do not possess eyes, but they have a
radula, as well as a foot modified into tentacles with a bulbous end,
CLASS CEPHALOPODA
known as captaculae. Captaculae serve to catch and manipulate prey.
Class Cephalopoda (“head foot” animals) includes octopuses, Ctenidia are absent in these animals.
squids, cuttlefish, and nautilus. Cephalopods are a class of shell-
bearing animals as well as mollusks with a reduced shell. They
display vivid coloration, typically seen in squids and octopuses

28.3F.2 https://bio.libretexts.org/@go/page/13722
 This page titled 28.3F: Classification of Phylum Mollusca is shared under a
CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Boundless.

Figure 28.3F . 1 : The Scaphopods: The Scaphopods (“boat feet”)

include the Antalis vulgaris, the shell of which is depicted here.

28.3F.3 https://bio.libretexts.org/@go/page/13722
28.3G: PHYLUM ANNELIDA
Annelids include segmented worms, such as leeches and and external morphological features are repeated in each body
earthworms; they are the most advanced worms as they possess a segment. Metamerism allows animals to become bigger by adding
true coelom. “compartments,” while making their movement more efficient. This
metamerism is thought to arise from identical teloblast cells in the
 LEARNING OBJECTIVES embryonic stage, which develop into identical mesodermal
structures. The overall body can be divided into head, body, and
Describe the morphological and anatomical features of pygidium (or tail). The clitellum is a reproductive structure that
annelids generates mucus that aids in sperm transfer and gives rise to a
cocoon within which fertilization occurs; it appears as a fused band
KEY POINTS in the anterior third of the animal.
Annelids are often called “segmented worms” because they
possess true segmentation of their bodies, with both internal and
external morphological features repeated in each body segment.
The clitellum is a structure on the anterior portion of the worm
that generates mucus to aid in sperm transfer from one worm to
another; it also forms a cocoon within which fertilization occurs.
Most annelids have chitinous hairlike extensions in every
segment called chaetae that are anchored in the epidermis,
although the number and size of chaetae can vary in the different
classes.
Annelids possess a closed circulatory system, lack a well-
developed respiratory system, but have well-developed nervous
systems.
Annelids can either have distinct male and female forms or be
hermaphrodites (having both male and female reproductive
organs). Earthworms are hermaphrodites and can self-fertilize,
but prefer to cross-fertilize if possible.

KEY TERMS Figure 28.3G. 1: Clitellum: The clitellum is the reproductive

clitellum: a glandular swelling in the epidermis of some annelid structure of an annelid. It creates mucus that aids in sperm transfer
and gives rise to a cocoon within which fertilization occurs. It can be
worms; it secretes a viscous fluid in which the eggs are deposited seen in this image as the enlarged band around the animal.
chaeta: a chitinous bristle of an annelid worm
metamerism: the segmentation of the body into similar discrete ANATOMY
units The epidermis is protected by an acellular, external cuticle, but this
is much thinner than the cuticle found in the ecdysozoans and does
PHYLUM ANNELIDA not require periodic shedding for growth. Circular as well as
Phylum Annelida contains the class Polychaeta (the polychaetes) longitudinal muscles are located interior to the epidermis. Chitinous
and the class Oligochaeta (the earthworms, leeches, and their hairlike extensions, anchored in the epidermis and projecting from
relatives). These animals are found in marine, terrestrial, and the cuticle, called setae/chaetae are present in every segment.
freshwater habitats, but a presence of water or humidity is a critical Annelids show the presence of a true coelom, derived from
factor for their survival, especially in terrestrial habitats. The name embryonic mesoderm and protostomy. Hence, they are the most
of the phylum is derived from the Latin word annellus, which means advanced worms. A well-developed and complete digestive system
a small ring. Animals in this phylum show parasitic and commensal is present in earthworms (oligochaetes) with a mouth, muscular
symbioses with other species in their habitat. Approximately 16,500 pharynx, esophagus, crop, and gizzard being present. The gizzard
species have been described in phylum Annelida. The phylum leads to the intestine and ends in an anal opening. Each segment is
includes earthworms, polychaete worms, and leeches. Annelids limited by a membranous septum that divides the coelomic cavity
show protostomic development in embryonic stages and are often into a series of compartments.
called “segmented worms” due to their key characteristic of Annelids possess a closed circulatory system of dorsal and ventral
metamerism, or true segmentation. blood vessels that run parallel to the alimentary canal as well as
capillaries that service individual tissues. In addition, these vessels
MORPHOLOGY
are connected by transverse loops in every segment. These animals
Annelids display bilateral symmetry and are worm-like in overall
lack a well-developed respiratory system; gas exchange occurs
morphology. They have a segmented body plan where the internal
across the moist body surface. Excretion is facilitated by a pair of

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metanephridia (a type of primitive “kidney” that consists of a lophophore. Provided by: Wiktionary. Located at:
convoluted tubule and an open, ciliated funnel) that is present in en.wiktionary.org/wiki/lophophore. License: CC BY-SA:
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present around the pharynx. The nerve cord is ventral in position, en.wiktionary.org/wiki/blastopore. License: CC BY-SA:
bearing enlarged nodes or ganglia in each segment. Attribution-ShareAlike
Annelids may be either monoecious, with permanent gonads (as in Caribbean reef squid. Provided by: Wikipedia. Located at:
earthworms and leeches), or dioecious, with temporary or seasonal en.Wikipedia.org/wiki/File:Ca...reef_squid.jpg. License:
gonads that develop (as in polychaetes). However, cross-fertilization Public Domain: No Known Copyright
is preferred in hermaphroditic animals. These animals may also OpenStax College, Biology. October 17, 2013. Provided by:
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Earthworms are the most abundant members of the class
scolex. Provided by: Wiktionary. Located at:
Oligochaeta, distinguished by the presence of the clitellum as well as
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few, reduced chaetae (“oligo- = “few”; -chaetae = “hairs”). The
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number and size of chaetae are greatly diminished in Oligochaeta
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compared to the polychaetes (poly=many, chaetae = hairs). The
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many chetae of polychaetes are also arranged within fleshy, flat,
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paired appendages that protrude from each segment. These
proglottid. Provided by: Wiktionary. Located at:
parapodia may be specialized for different functions in the
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polychates. A significant difference between leeches and other
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annelids is the development of suckers at the anterior and posterior
acoelomate. Provided by: Wiktionary. Located at:
ends and an absence of chaetae. Additionally, the segmentation of
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the body wall may not correspond to the internal segmentation of the
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coelomic cavity. This adaptation possibly helps the leeches to
Caribbean reef squid. Provided by: Wikipedia. Located at:
elongate when they ingest copious quantities of blood from host
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Figure 28.3G. 1: Leeches: Unlike earthworms, leeches lack chaetae mastax. Provided by: Wiktionary. Located at:
and have suckers at both ends of the body. en.wiktionary.org/wiki/mastax. License: CC BY-SA:
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CC LICENSED CONTENT, SHARED PREVIOUSLY pseudocoelomate. Provided by: Wikipedia. Located at:
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Bedford's Flatworm. Provided by: Wikipedia. Located at: visceral mass. Provided by: Wikipedia. Located at:
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Attribution-ShareAlike Nemertea Anopla n Enopla Head Sagittal. Provided by:
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en.Wikipedia.org/wiki/File:Ne...d_Sagittal.png. License: CC Bivalvia. Provided by: WIKIPEDIA. Located at:
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SECTION OVERVIEW

28.4: SUPERPHYLUM ECDYSOZOA

28.4C: PHYLUM ARTHROPODA
Topic hierarchy
28.4D: SUBPHYLA OF ARTHROPODA
28.4A: SUPERPHYLUM ECDYSOZOA
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28.4A: SUPERPHYLUM ECDYSOZOA
The superphylum Ecdysozoa includes the nematode worms and the
arthropods, both of which have a tough external covering called a
cuticle.

 LEARNING OBJECTIVES

Discuss the phylogenetic position of Ecdysozoa

KEY POINTS
The Ecdysozoans are the most diverse group of animals,
containing the nematode worms and the arthropods.
These organisms have an external covering called a cuticle that
protects their soft internal organs from water loss and the outside
environment.
After they molt, or shed their cuticle, they grow in size and
secrete a new shell; this is called ecdysis.
Figure 28.4A. 1 : Molting in arthropods: This cicada is in the middle
The phylogeny of the Ecdysozoans has been the cause of much of the molting process. The old cuticle splits and the insect climbs
scientific debate with no definitive consensus in the scientific out. At this time, the insect’s body is very soft. The cicada will then
community. eat the old shell to replace nutrients that would otherwise be lost.
This encourages the new shell to harden.
KEY TERMS PHYLOGENETIC HYPOTHESES
cuticle: a noncellular protective covering outside the epidermis There are two main hypotheses about the phylogeny of the
of many invertebrates and plants Ecdysozoans. The first is called the Articulata hypothesis. This
coelomate: any animal possessing a fluid-filled cavity within
grouping scheme is widely accepted, although some zoologists still
which the digestive system is suspended.
hold to the original view that Panarthropoda should be classified
ecdysis: the shedding of an outer layer of skin in snakes,
with Annelida in a group called the Articulata, and that Ecdysozoa
crustaceans and insects; moulting
are polyphyletic. Others have suggested that a possible solution is to
SUPERPHYLUM ECDYSOZOA regard Ecdysozoa as a sister-group of Annelida, though many
scientists consider them unrelated. Inclusion of the roundworms
The superphylum Ecdysozoa contains an incredibly large number of
within the Ecdysozoa was initially contested, but since 2003, a broad
species. This is because it includes two of the most diverse animal
consensus has formed supporting the Ecdysozoa, placing them in a
groups: Phylum Nematoda (the roundworms) and Phylum
new set of groupings that include the Ecdysozoa, the
Arthropoda (the arthropods). The most distinguishing and prominent
Lophotrochozoa, and the Deuterostomia.
feature of Ecdysozoans is their cuticle: a tough, but flexible
The other idea about the phylogeny of the Ecdysozoa is called the
exoskeleton that protects these animals from water loss, predators,
coelomate hypothesis. Before Ecdysozoa, one of the prevailing
and other aspects of the external environment. All members of this
theories for the evolution of the bilateral animals was based on the
superphylum periodically molt or shed their cuticle as they grow.
morphology of their body cavities. There were three types, or
After molting, they secrete a new cuticle that will last until their next
grades, of organization: the Acoelomata (no coelom), the
growth phase. The process of molting and replacing the cuticle is
called ecdysis, which is the derivation of the superphylum’s name. Pseudocoelomata (partial coelom), and the Eucoelomata (true
coelom). With the introduction of molecular phylogenetics, the
coelomate hypothesis was abandoned, although some molecular,
phylogenetic support for the Coelomata continued until 2005.

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28.4B: PHYLUM NEMATODA
Nematodes are parasitic and free-living worms that are able to shed including the pharynx and rectum. The epidermis can be either a
their external cuticle in order to grow. single layer of cells or a syncytium, which is a multinucleated cell
formed from the fusion of uninucleated cells.
 LEARNING OBJECTIVES The overall morphology of these worms is cylindrical, while the
head is radially symmetrical. A mouth opening is present at the
Describe the features of animals classified in phylum
anterior end with three or six lips. Teeth occur in some species in the
Nematoda
form of cuticle extensions. Some nematodes may present other
external modifications such as rings, head shields, or warts. Rings,
KEY POINTS however, do not reflect true internal body segmentation. The mouth
Nematodes are in the same phylogenetic grouping as the leads to a muscular pharynx and intestine, which leads to a rectum
arthropods because of the presence of an external cuticle that and anal opening at the posterior end. In addition, the muscles of
protects the animal and keeps it from drying out. nematodes differ from those of most animals; they have a
There are an estimated 28,000 species of nematodes, with longitudinal layer only, which accounts for the whip-like motion of
approximately 16,000 of them being parasitic. their movement.
Nematodes are tubular in shape and are considered
pseudocoelomates because of they do not possess a true coelom.
Nematodes do not have a well-developed excretory system, but
do have a complete digestive system.
Nematodes possess the ability to shed their exoskeleton in order
to grow, a process called ecdysis.

KEY TERMS
exoskeleton: a hard outer structure that provides both structure
and protection to creatures such as insects, Crustacea, and
Nematoda

PHYLUM NEMATODA
The Nematoda, similar to most other animal phyla, are triploblastic,
possessing an embryonic mesoderm that is sandwiched between the Figure 28.4B. 1: Nematode shape: Scanning electron micrograph of
ectoderm and endoderm. They are also bilaterally symmetrical: a soybean cyst nematode and its egg. Nematodes are cylindrical in
longitudinal section will divide them into right and left sides that are shape, often looking like thin hairs. They possess an exoskeleton
that prevents them from drying out. It must be shed (a process called
symmetrical. Furthermore, the nematodes, or roundworms, possess a
ecdysis) in order for them to grow.
pseudocoelom and have both free-living and parasitic forms.
Both the nematodes and arthropods belong to the superphylum EXCRETORY SYSTEM
Ecdysozoa that is believed to be a clade consisting of all In nematodes, specialized excretory systems are not well developed.
evolutionary descendants from one common ancestor. The name Nitrogenous wastes may be lost by diffusion through the entire body
derives from the word ecdysis, which refers to the shedding, or or into the pseudocoelom (body cavity), where they are removed by
molting, of the exoskeleton. The phyla in this group have a hard specialized cells. Regulation of water and salt content of the body is
cuticle covering their bodies, which must be periodically shed and achieved by renette glands, present under the pharynx in marine
replaced for them to increase in size. nematodes.
Phylum Nematoda includes more than 28,000 species with an
NERVOUS SYSTEM
estimated 16,000 being parasitic in nature. Nematodes are present in
Most nematodes possess four longitudinal nerve cords that run along
all habitats.
the length of the body in dorsal, ventral, and lateral positions. The
MORPHOLOGY ventral nerve cord is better developed than the dorsal or lateral
In contrast with cnidarians, nematodes show a tubular morphology cords. All nerve cords fuse at the anterior end, around the pharynx,
and circular cross-section. These animals are pseudocoelomates; to form head ganglia, or the “brain” of the worm (taking the form of
they have a complete digestive system with a distinct mouth and a ring around the pharynx), as well as at the posterior end to form
anus. This is in contrast with the cnidarians where only one opening the tail ganglia. In C. elegans, the nervous system accounts for
is present (an incomplete digestive system). nearly one-third of the total number of cells in the animal.
The cuticle of Nematodes is rich in collagen and a carbohydrate-
protein polymer called chitin. It forms an external “skeleton” outside
the epidermis. The cuticle also lines many of the organs internally,

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REPRODUCTION development beginning very soon after fertilization. The embryo is
Nematodes employ a variety of reproductive strategies that range released from the vulva during the gastrulation stage. The embryonic
from monoecious to dioecious to parthenogenic, depending upon the development stage lasts for 14 hours; development then continues
species under consideration. C. elegans is a monoecious species, through four successive larval stages with ecdysis between each
having development of ova contained in a uterus as well as sperm stage (L1, L2, L3, and L4) ultimately leading to the development of
contained in the spermatheca. The uterus has an external opening a young male or female adult worm. Adverse environmental
known as the vulva. The female genital pore is near the middle of conditions such as overcrowding and lack of food can result in the
the body, whereas the male’s is at the tip. Specialized structures at formation of an intermediate larval stage known as the dauer larva.
the tail of the male keep him in place while he deposits sperm with
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28.4C: PHYLUM ARTHROPODA
Arthropods are the largest grouping of animals all of which have
jointed legs and an exoskeleton made of chitin.

 LEARNING OBJECTIVES

Describe the morphology of arthropoda

KEY POINTS
Arthropods include the Hexapoda (insects), the Crustacea
(lobsters, crabs, and shrimp), the Chelicerata (the spiders and
scorpions), and the Myriapoda (the centipedes and millipedes).
Arthropods have a segmented body plan that contains fused Figure 28.4C. 1 : Trilobite fossil: Acadoparadoxides, possibly A.
briareus, a large trilobite from about 500 million years ago from
segments divided into regions called tagma. Morocco, North Africa (Middle Cambrian)
Arthropods have an open circulatory system and can use book
gills, book lungs, or tracheal tubes for respiration. MORPHOLOGY
A unique feature of animals in the arthropod phylum is the presence
KEY TERMS of a segmented body and fusion of sets of segments that give rise to
tagma: a specialized grouping of arthropodan segments, such as functional body regions called tagma. Tagma may be in the form of
the head, the thorax, and the abdomen with a common function a head, thorax, and abdomen, or a cephalothorax and abdomen, or a
malpighian tubule: a tubule that extends from the alimentary head and trunk. A central cavity, called the hemocoel (or blood
canal to the exterior of the organism, excreting water and wastes cavity), is present; the open circulatory system is regulated by a
in the form of solid nitrogenous compounds tubular, or single-chambered, heart. Respiratory systems vary
spiracle: a pore or opening used (especially by spiders and some depending on the group of arthropod. Insects and myriapods use a
fish) for breathing series of tubes (tracheae) that branch through the body, open to the
outside through openings called spiracles, and perform gas exchange
PHYLUM ARTHROPODA directly between the cells and air in the tracheae. Other organisms
The name “arthropoda” means “jointed legs” (in the Greek, use variants of gills and lungs. Aquatic crustaceans utilize gills,
“arthros” means “joint” and “podos” means “leg”); it aptly describes terrestrial chelicerates employ book lungs, and aquatic chelicerates
the enormous number of invertebrates included in this phylum. use book gills. The book lungs of arachnids (scorpions, spiders,
Arthropods dominate the animal kingdom with an estimated 85 ticks, and mites) contain a vertical stack of hemocoel wall tissue that
percent of known species included in this phylum; many arthropods somewhat resembles the pages of a book. Between each of the
are as yet undocumented. The principal characteristics of all the “pages” of tissue is an air space. This allows both sides of the tissue
animals in this phylum are functional segmentation of the body and to be in contact with the air at all times, greatly increasing the
presence of jointed appendages. Arthropods also show the presence efficiency of gas exchange. The gills of crustaceans are filamentous
of an exoskeleton made principally of chitin, which is a waterproof, structures that exchange gases with the surrounding water.
tough polysaccharide. Phylum Arthropoda is the largest phylum in
the animal world; insects form the single largest class within this
phylum. Arthropods are eucoelomate, protostomic organisms.
Phylum Arthropoda includes animals that have been successful in
colonizing terrestrial, aquatic, and aerial habitats. This phylum is
further classified into five subphyla: Trilobitomorpha (trilobites, all
extinct), Hexapoda (insects and relatives), Myriapoda (millipedes,
centipedes, and relatives), Crustaceans (crabs, lobsters, crayfish,
isopods, barnacles, and some zooplankton), and Chelicerata
(horseshoe crabs, arachnids, scorpions, and daddy longlegs).
Trilobites are an extinct group of arthropods found chiefly in the Figure 28.4C. 1 : Book gills: The ventral side of a horseshoe crab
showing the book gills located near the telson (tail). These gills flap
pre-Cambrian Era that are probably most closely related to the back and forth bringing oxygen to the blood.
Chelicerata. These are identified based on fossil records.
Groups of arthropods also differ in the organs used for excretion.
Crustaceans possess green glands while insects use Malpighian
tubules, which work in conjunction with the hindgut to reabsorb
water while ridding the body of nitrogenous waste. The cuticle is the
covering of an arthropod. It is made up of two layers: the epicuticle,
which is a thin, waxy, water-resistant outer layer containing no

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chitin; and the chitinous procuticle, which is beneath the epicuticle. (“to strip off”); this is a cumbersome method of growth. During this
Chitin is a tough, flexible polysaccharide. In order to grow, the time, the animal is vulnerable to predation.
arthropod must shed the exoskeleton during a process called ecdysis
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28.4D: SUBPHYLA OF ARTHROPODA
The Phylum Arthropoda includes a wide range of species divided
into the subphyla: Hexapoda, Crustacea, Myriapoda, and
Chelicerata.

 LEARNING OBJECTIVES

Differentiate among the subphylums hexapoda, myriapoda,

crustacea, and chelicerata

KEY POINTS
The Hexapoda include insects; the Crustacea include lobster,
crabs, and shrimp; the Myriapoda include centipedes and
Figure 28.4D. 1 : Insect showing wings and body segments:
millipedes; and the Chelicerata include spiders, scorpions. Protaetia fieberi in flight posture. Hexapods are characterized by
The Hexapoda are the largest grouping of Arthropods, containing having three distinct tagma, or body segments. This beetle is just one
the more than one million species of insects, having of over one million different species of insects that inhabit the Earth.
representatives with six legs and one pair of antennae. SUBPHYLUM MYRIAPODA
The Myriapoda are terrestrial, prefering humid environments;
Subphylum Myriapoda includes arthropods with numerous legs.
they have between 10 and 750 legs.
Although the name is hyperbolic in suggesting that myriad legs are
The Crustacea are primarily aquatic arthropods, but also include
present in these invertebrates, the number of legs may vary from 10
terrestrial forms, which have a cephalothorax covered by a
to 750. This subphylum includes 13,000 species; the most
carapace.
commonly-found examples are millipedes and centipedes. All
The Chelicerata, which includes the spiders, horseshoe crabs,
myriapods are terrestrial animals, prefering a humid environment.
and scorpions, have mouth parts that are fang-like and used for
capturing prey. Myriapods are typically found in moist soils, decaying biological
material, and leaf litter. Centipedes, such as Scutigera
KEY TERMS coleoptrata,are classified as chilopods. These animals bear one pair
cephalothorax: the fused head and thorax of spiders and of legs per segment, mandibles as mouthparts, and are somewhat
crustaceans dorsoventrally flattened. The legs in the first segment are modified
forcipule: a modified pincer-like foreleg in centipedes, capable to form forcipules (poison claws) that deliver venom to prey such as
of injecting venom spiders and cockroaches, as centipedes are predatory. Millipedes
bear two pairs of legs per diplosegment, a feature that results from
REPRESENTATIVES OF PHYLUM ARTHROPODA embryonic fusion of adjacent pairs of body segments, are usually
rounder in cross-section, and are herbivores or detritivores.
SUBPHYLUM HEXAPODA Millipedes have visibly more numbers of legs as compared to
The name Hexapoda denotes the presence of six legs (three pairs) in centipedes, although they do not bear a thousand legs.
these animals, which differentiates them from the number of pairs
present in other arthropods. Hexapods are characterized by the
presence of a head, thorax, and abdomen, constituting three tagma.
The thorax bears the wings as well as six legs in three pairs. Many of
the common insects we encounter on a daily basis, including ants,
cockroaches, butterflies, and flies, are examples of Hexapoda.
Among the hexapods, the insects are the largest class in terms of
species diversity as well as biomass in terrestrial habitats ).
Typically, the head bears one pair of sensory antennae, mandibles as
mouthparts, a pair of compound eyes, and some ocelli (simple eyes),
along with numerous sensory hairs. The thorax bears three pairs of Figure 28.4D. 1 : House centipede: The house centipede (Scutigera
legs (one pair per segment) and two pairs of wings, with one pair coleoptrata) is one of the 13,000 species of Myriapoda. They bear
each on the second and third thoracic segments. The abdomen one pair of legs per segment and can inject venom. The Myriapods
contain the millipedes and centipedes.
usually has eleven segments and bears reproductive apertures.
Hexapoda includes insects that are winged (like fruit flies) and SUBPHYLUM CRUSTACEA
wingless (like fleas).
Crustaceans are the most dominant aquatic arthropods since the total
number of marine crustacean species stands at 67,000. However,
there are also freshwater and terrestrial crustacean species. Krill,

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shrimp, lobsters, crabs, and crayfish are all examples of crustaceans. species have gills, whereas terrestrial species have either trachea or
Terrestrial species like the wood lice (Armadillidium spp.) (also book lungs for gaseous exchange.
called pill bugs, rolly pollies, potato bugs, or isopods) are also
crustaceans, although the number of non-aquatic species in this
subphylum is relatively low.

Figure 28.4D. 1 : Mediterranean green Crab: This crab (Carcinus Figure 28.4D. 1 : Chelicera of spiders: This photo shows the
aestuarii) is one of the 67,000 species of crustaceans inhabiting the chelicera of a spider being held open with a stick. Some chelicerae,
world’s oceans. Most crustaceans are decapods, having ten legs. such as those found in spiders, are hollow and contain (or are
connected to) venom glands which are used to inject venom into
Crustaceans possess two pairs of antennae, mandibles as mouthparts, prey or a (perceived) threat.
and biramous (“two branched”) appendages: their legs are formed in The nervous system in chelicerates consists of a brain and two
two parts, as distinct from the uniramous (“one branched”) ventral nerve cords. These animals use external as well as internal
myriapods and hexapods. fertilization strategies for reproduction, depending upon the species
Unlike that of the Hexapoda, the head and thorax of most and its habitat. Parental care for the young ranges from absolutely
crustaceans is fused to form a cephalothorax, which is covered by a none to relatively-prolonged care.
plate called the carapace, thus producing a body structure of two
tagma. Crustaceans have a chitinous exoskeleton that is shed by CONTRIBUTIONS AND ATTRIBUTIONS
molting whenever the animal increases in size. The exoskeletons of OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44667/latest...ol11448/latest. License: CC
many species are also infused with calcium carbonate, which makes BY: Attribution
them even stronger than in other arthropods. Crustaceans have an Ecdysozoa. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/Ecdysozoa. License: CC BY-SA: Attribution-
open circulatory system where blood is pumped into the hemocoel ShareAlike
by the dorsally-located heart. Hemocyanin and hemoglobin are the ecdysis. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/ecdysis.
License: CC BY-SA: Attribution-ShareAlike
respiratory pigments present in these animals. cuticle. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/cuticle.
License: CC BY-SA: Attribution-ShareAlike
SUBPHYLUM CHELICERATA coelomate. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/coelomate. License: CC BY-SA: Attribution-
This subphylum includes animals such as spiders, scorpions, ShareAlike
horseshoe crabs, and sea spiders and is predominantly terrestrial, Cicada Molting. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/Fi...da_Molting.jpg. License: CC BY-SA:
although some marine species also exist. An estimated 77,000 Attribution-ShareAlike
species, found in almost all habitats, are included in subphylum OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44667/latest...ol11448/latest. License: CC
Chelicerata. BY: Attribution
exoskeleton. Provided by: Wiktionary. Located at:
The body of chelicerates may be divided into two parts: prosoma en.wiktionary.org/wiki/exoskeleton. License: CC BY-SA: Attribution-
and opisthosoma, which are basically the equivalents of ShareAlike
cephalothorax (usually smaller) and abdomen (usually larger). A Cicada Molting. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/Fi...da_Molting.jpg. License: CC BY-SA:
“head” tagmum is not usually discernible. The phylum derives its Attribution-ShareAlike
name from the first pair of appendages, the chelicerae, which are Soybean cyst nematode and egg SEM. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/Fi...nd_egg_SEM.jpg. License: CC BY:
specialized claw-like or fang-like mouthparts. These animals do not Attribution
possess antennae. The second pair of appendages is known as malpighian tubule. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/malpighian+tubule. License: CC BY-SA: Attribution-
pedipalps. In some species, such as sea spiders, an additional pair of ShareAlike
appendages, called ovigers, is present between the chelicerae and OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44667/latest...ol11448/latest. License: CC
pedipalps. BY: Attribution
Chelicerae are used primarily for feeding, but in spiders, these are spiracle. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/spiracle. License: CC BY-SA: Attribution-ShareAlike
often modified into fangs that inject venom into their prey before tagma. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/tagma.
feeding. Members of this subphylum have an open circulatory License: CC BY-SA: Attribution-ShareAlike
Cicada Molting. Provided by: Wikimedia. Located at:
system with a heart that pumps blood into the hemocoel. Aquatic commons.wikimedia.org/wiki/Fi...da_Molting.jpg. License: CC BY-SA:
Attribution-ShareAlike

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Soybean cyst nematode and egg SEM. Provided by: Wikimedia. Located at: Attribution
commons.wikimedia.org/wiki/Fi...nd_egg_SEM.jpg. License: CC BY: BLW Trilobite (Paradoxides sp.). Provided by: Wikimedia. Located at:
Attribution commons.wikimedia.org/wiki/Fi...xides_sp.).jpg. License: CC BY-SA:
BLW Trilobite (Paradoxides sp.). Provided by: Wikimedia. Located at: Attribution-ShareAlike
commons.wikimedia.org/wiki/Fi...xides_sp.).jpg. License: CC BY-SA: Horseshoecrab2. Provided by: Wikimedia. Located at:
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Attribution commons.wikimedia.org/wiki/Fi..._centipede.jpg. License: CC BY:
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Attribution
Located at: http://cnx.org/content/m44667/latest...ol11448/latest. License: CC Proteatia vol. Provided by: Wikimedia. Located at:
BY: Attribution commons.wikimedia.org/wiki/Fi...teatia_vol.jpg. License: CC BY-SA:
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en.wiktionary.org/wiki/forcipule. License: CC BY-SA: Attribution-ShareAlike Solifugae Chelicera lateral aspect 2012 01 24 0999s. Provided by: Wikimedia.
Cicada Molting. Provided by: Wikimedia. Located at: Located at: commons.wikimedia.org/wiki/Fi...1_24_0999s.JPG. License: CC
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SECTION OVERVIEW

28.5: SUPERPHYLUM DEUTEROSTOMIA

28.5B: CLASSES OF ECHINODERMS
Topic hierarchy
28.5C: PHYLUM CHORDATA
28.5A: PHYLUM ECHINODERMATA
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28.5A: PHYLUM ECHINODERMATA
Echinoderms are invertebrates that have pentaradial symmetry, a
spiny skin, a water vascular system, and a simple nervous system.

 LEARNING OBJECTIVES

Describe the characteristics of echinodermata

KEY POINTS
Echinoderms live exclusively in marine systems; they are widely
divergent, with over 7,000 known species in the phylum.
Echinoderms have pentaradial symmetry and a calcareous
endoskeleton that may possess pigment cells that give them a
wide range of colors, as well as cells that possess toxins.
Echinoderms have a water vascular system composed of a
central ring of canals that extend along each arm, through which
water circulates for gaseous exchange and nutrition.
Figure 28.5A. 1 : Sea stars: Star stars are among the most familiar of
Echinoderms have a very simple nervous system, comprised of a marine invertebrates. They are members of the phylum
nerve ring at the center and five radial nerves extending outward Echinodermata.
along the arms; there is no structure resembling a brain.
There are two sexes in echinoderms, which each release their MORPHOLOGY AND ANATOMY
eggs and sperm into the water; here, the sperm will fertilize the Adult echinoderms exhibit pentaradial symmetry and have a
eggs. calcareous endoskeleton made of ossicles, although the early larval
Echinoderms can reproduce asexually by regeneration. stages of all echinoderms have bilateral symmetry. The endoskeleton
is developed by epidermal cells and may possess pigment cells that
KEY TERMS give vivid colors to these animals, as well as cells laden with toxins.
madreporite: a lightcolored calcerous opening used to filter Echinoderms possess a simple digestive system which varies
water into the water vascular system of echinoderms according to the animal’s diet. Starfish are mostly carnivorous and
podocyte: cells that filter the bodily fluids in echinoderms have a mouth, oesophagus, two-part pyloric stomach with a pyloric
pentaradial symmetry: a variant of radial symmetry that duct leading to the intestine and rectum, with the anus located in the
arranges roughly equal parts around a central axis at orientations center of the aboral body surface. In many species, the large cardiac
of 72° apart stomach can be everted and digest food outside the body. Gonads are
water vascular system: a hydraulic system used by present in each arm. In echinoderms such as sea stars, every arm
echinoderms, such as sea stars and sea urchins, for locomotion, bears two rows of tube feet on the oral side which help in attachment
food and waste transportation, and respiration to the substratum. These animals possess a true coelom that is
ampulla: the dilated end of a duct modified into a unique circulatory system called a water vascular
system. The more notably distinct trait, which most echinoderms
PHYLUM ECHINODERMATA have, is their remarkable powers of regeneration of tissue, organs,
Echinodermata are so named owing to their spiny skin (from the limbs, and, in some cases, complete regeneration from a single limb.
Greek “echinos” meaning “spiny” and “dermos” meaning “skin”).
This phylum is a collection of about 7,000 described living species.
Echinodermata are exclusively marine organisms. Sea stars, sea
cucumbers, sea urchins, sand dollars, and brittle stars are all
examples of echinoderms. To date, no freshwater or terrestrial
echinoderms are known. Figure 28.5A. 1 : Sea cucumbers: Sea cucumbers are a member of
the phylum Echinodermata which are found on the sea floor
worldwide. As with all echinoderms, sea cucumbers have an
endoskeleton just below the skin, calcified structures that are usually
reduced to isolated microscopic ossicles joined by connective tissue.

WATER VASCULAR SYSTEM

Echinoderms possess a unique ambulacral or water vascular system,
consisting of a central ring canal and radial canals that extend along
each arm. Water circulates through these structures and facilitates
gaseous exchange as well as nutrition, predation, and locomotion.

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The water vascular system also projects from holes in the skeleton in pressure-equalizing valve, it is visible as a small red or yellow
the form of tube feet. These tube feet can expand or contract based button-like structure (similar to a small wart) on the aboral surface
on the volume of water (hydrostatic pressure) present in the system of the central disk of a sea star. Close up, it is visibly structured,
of that arm. resembling a “madrepore” colony. From this, it derives its name.
Water enters the madreporite on the aboral side of the echinoderm.
From there, it passes into the stone canal, which moves water into
1
11 the ring canal. The ring canal connects the radial canals (there are
10 five in a pentaradial animal), and the radial canals move water into
9 the ampullae, which have tube feet through which the water moves.
By moving water through the unique water vascular system, the
8
echinoderm can move and force open mollusk shells during feeding.
7
2
3 OTHER BODY SYSTEMS
4
The nervous system in these animals is a relatively simple structure
with a nerve ring at the center and five radial nerves extending
5 outward along the arms. Structures analogous to a brain or derived
from fusion of ganglia are not present in these animals.
6
Podocytes, cells specialized for ultrafiltration of bodily fluids, are
present near the center of echinoderms. These podocytes are
connected by an internal system of canals to the madreporite.
Echinoderms are sexually dimorphic and release their eggs and
sperm cells into water; fertilization is external. In some species, the
larvae divide asexually and multiply before they reach sexual
Figure 28.5A. 1 : The water vascular system of the sea star: 1. maturity. Echinoderms may also reproduce asexually, as well as
Pyloric stomach 2. Intestine 3. Rectal gland 4. Stone canal 5. regenerate body parts lost in trauma.
Madreporite 6. Pyloric duct 7. Pyloric cecum 8. Cardiac stomach 9.
Gonad 10. Ambulacral plates 11. Ampullae
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The madreporite is a light-colored, calcerous opening used to filter 4.0 license and was authored, remixed, and/or curated by Boundless.
water into the water vascular system of echinoderms. Acting as a

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28.5B: CLASSES OF ECHINODERMS

 LEARNING OBJECTIVES

Differentiate among the classes of echinoderms

The phylum echinoderms is divided into five extant classes:

Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea
urchins and sand dollars), Crinoidea (sea lilies or feather stars), and
Holothuroidea (sea cucumbers).
The most well-known echinoderms are members of class Asteroidea,
or sea stars. They come in a large variety of shapes, colors, and
sizes, with more than 1,800 species known so far. The key
characteristic of sea stars that distinguishes them from other
echinoderm classes includes thick arms (ambulacra; singular:
ambulacrum) that extend from a central disk where organs penetrate
into the arms. Sea stars use their tube feet not only for gripping
surfaces, but also for grasping prey. Sea stars have two stomachs, Figure 28.5B. 1: Sea urchins: Sea urchins do not have arms, but
have rows of tube feet that can be extended out of pores of the
one of which can protrude through their mouths and secrete internal shell.
digestive juices into or onto prey, even before ingestion. This Sea lilies and feather stars are examples of Crinoidea. Both of these
process can essentially liquefy the prey, making digestion easier.
species are suspension feeders. They live both in shallow water and
Brittle stars belong to the class Ophiuroidea. Unlike sea stars, which in depths as great as 6,000 meters. Sea lilies refer to the crinoids
have plump arms, brittle stars have long, thin arms that are sharply which, in their adult form, are attached to the sea bottom by a stalk.
demarcated from the central disk. Brittle stars move by lashing out Feather stars or comatulids refer to the unstalked forms. Crinoids are
their arms or wrapping them around objects and pulling themselves characterized by a mouth on the top surface that is surrounded by
forward. Of all echinoderms, the Ophiuroidea may have the feeding arms. They have a U-shaped gut; their anus is located next
strongest tendency toward 5-segment radial (pentaradial) symmetry. to the mouth. Although the basic echinoderm pattern of fivefold
Ophiuroids are generally scavengers or detritivores. Small organic symmetry can be recognized, most crinoids have many more than
particles are moved into the mouth by the tube feet. Ophiuroids may five arms. Crinoids usually have a stem used to attach themselves to
also prey on small crustaceans or worms. Some brittle stars, such as a substrate, but many live attached only as juveniles and become
the six-armed members of the family Ophiactidae, are fissiparous free-swimming as adults.
(divide though fission), with the disk splitting in half. Regrowth of
both the lost part of the disk and the arms occur, yielding an animal
with three large arms and three small arms during the period of
growth.
Sea urchins and sand dollars are examples of Echinoidea. These
echinoderms do not have arms, but are hemispherical or flattened
with five rows of tube feet that help them in slow movement; tube
feet are extruded through pores of a continuous internal shell called
a test. Like other echinoderms, sea urchins are bilaterans. Their early
larvae have bilateral symmetry, but they develop fivefold symmetry
as they mature. This is most apparent in the “regular” sea urchins,
which have roughly spherical bodies, with five equally-sized parts
radiating out from their central axes. Several sea urchins, however,
including the sand dollars, are oval in shape, with distinct front and
rear ends, giving them a degree of bilateral symmetry. In these
urchins, the upper surface of the body is slightly domed, but the
underside is flat, while the sides are devoid of tube feet. This
“irregular” body form has evolved to allow the animals to burrow
through sand or other soft materials.

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 Figure 28.5B. 1: Sea cucumbers: Sea cucumbers are the only
echinoderms that demonstrate “functional” bilateral symmetry as
adults, as they lie horizontally as opposed to the vertical axis of
other echinoderms.

KEY POINTS
Sea stars have thick arms called ambulacra that are used for
gripping surfaces and grabbing hold of prey.
Brittle stars have thin arms that wrap around prey or objects to
pull themselves forward.
Sea urchins and sand dollars embody flattened discs that do not
have arms, but do have rows of tube feet they use for movement.
Figure 28.5B. 1: Sea lilies: Sea lilies, like feather stars, have a Sea cucumbers demonstrate “functional” bilateral symmetry as
mouth on their upper surface that is surrounded by arms used for adults because they actually lie horizontally rather than stand
feeding.
vertically.
Sea cucumbers of class Holothuroidea are extended in the oral-
Sea lilies and feather stars are suspension feeders.
aboral axis and have five rows of tube feet. These are the only
echinoderms that demonstrate “functional” bilateral symmetry as KEY TERMS
adults because the uniquely-extended oral-aboral axis compels the ossicle: a small bone (or bony structure), especially one of the
animal to lie horizontally rather than stand vertically. Like all three of the middle ear
echinoderms, sea cucumbers have an endoskeleton just below the fissiparous: of cells that reproduce through fission, splitting into
skin: calcified structures that are usually reduced to isolated two
microscopic ossicles joined by connective tissue. In some species ambulacrum: a row of pores for the protrusion of appendages
these can sometimes be enlarged to flattened plates, forming armor. such as tube feet.
In pelagic species, such as Pelagothuria natatrix, the skeleton and a
calcareous ring are absent. This page titled 28.5B: Classes of Echinoderms is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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28.5C: PHYLUM CHORDATA
The phylum Chordata contains all animals that have a dorsal suspension feeding devices; in vertebrates, they have been
notochord at some stage of development; in most cases, this is the modified for gas exchange, jaw support, hearing, and other
backbone. functions.
A muscular, postanal tail which extends posterior to the anus.
 LEARNING OBJECTIVES The digestive tract of most nonchordates extends the length of
the body. In chordates, the tail has skeletal elements and
Name the features that distinguish the members of the musculature, and can provide most of the propulsion in aquatic
phylum chordata species.
In some groups, some of these traits are present only during
KEY POINTS embryonic development. In addition to containing vertebrate classes,
The phylum chordata is named for the notochord, a longitudinal, the phylum Chordata contains two clades of invertebrates:
flexible rod between the digestive tube and the nerve cord; in Urochordata (tunicates) and Cephalochordata (lancelets). However,
vertebrates, this is the spinal column. even though they are invertebrates, they share characteristics with
The chordates are also characterized by a dorsal nerve cord, other chordates that places them in this phylum. For example,
which splits into the brain and spinal cord. tunicate larvae have both a notochord and a nerve cord which are
Chordata contains two clades of invertebrates: Urochordata lost in adulthood. Most tunicates live on the ocean floor and are
(tunicates) and Cephalochordata (lancelets), both of which are suspension feeders. Cephalochordates, or lancelets, have a
suspension feeders. notochord and a nerve cord (but no brain or specialist sensory
The phylum chordata includes all animals that share four organs) and a very simple circulatory system. Lancelets are
characteristics, although they might each possess some of them suspension feeders that feed on phytoplankton and other
at different stages of their development: a notochord, a dorsal microorganisms.
nerve cord, pharyngeal slits, and a postanal tail.
Chordata contains five classes of animals: fish, amphibians,
reptiles, birds, and mammals; these classes are separated by
whether or not they can regulate their body temperature, the
manner by which they consume oxygen, and their method of
reproduction.

KEY TERMS Figure 28.5C. 1 : Structures present in a tunicate larva: While

dorsal nerve cord: a hollow cord dorsal to the notochord, tunicates are invertebrates and may seem very different from the
formed from a part of the ectoderm that rolls, forming a hollow more familiar members of Chordata, the tunicate larva possesses
both a notochord and a dorsal nerve cord, although both are lost in
tube. adulthood.
notochord: a flexible rodlike structure that forms the main The phylum Chordata contains all of the animals that have a rod-like
support of the body in the lowest chordates; a primitive spine structure used to give them support. In most cases this is the spine or
pharyngeal slit: filter-feeding organs found in non-vertebrate backbone. Within Chordata there are five classes of animals: fish,
chordates (lancelets and tunicates) and hemichordates living in amphibians, reptiles, birds, and mammals. Three dividing factors
aquatic environments separate these classes:
PHYLUM CHORDATA Regulation of body temperature: animals are either
Animals in the phylum Chordata share four key features that appear homeothermic (can regulate their internal temperature so that it
at some stage of their development: is kept at an optimum level) or poikilothermic (cannot regulate
their internal temperature, the environment affects how hot or
A notochord, or a longitudinal, flexible rod between the digestive
cold they are)
tube and the nerve cord. In most vertebrates, it is replaced Oxygen Absorption: the way in which oxygen is taken in from
developmentally by the vertebral column. This is the structure
the air, which can be through gills, the skin (amphibians), or
for which the phylum is named. lungs
A dorsal nerve cord which develops from a plate of ectoderm
Reproduction: this factor is particularly varied. Animals can be
that rolls into a tube located dorsal to the notochord. Other oviparous (lay eggs) or viviparous (birth live young).
animal phyla have solid nerve cords ventrally located. A
Fertilization can occur externally or internally. In mammals, the
chordate nerve cord splits into the central nervous system: the mother produces milk for the young.
brain and spinal cord.
Pharyngeal slits, which allow water that enters through the
mouth to exit without continuing through the entire digestive
tract. In many of the invertebrate chordates, these function as

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 ossicle. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/ossicle.
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ambulacrum. Provided by: Wiktionary. Located at:
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SeaDSC01286. Provided by: Wikipedia. Located at:
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Known Copyright
Figure 28.5C. 1 : Notochord: All chordates possess a notochord, or a Crinoid on the reef of Batu Moncho Island. Provided by: Wikipedia. Located at:
type of flexible support rod, at some point in their development. In en.Wikipedia.org/wiki/File:Cr...cho_Island.JPG. License: CC BY-SA:
this dissected lungfish, which is a member of the chordates, tissues Attribution-ShareAlike
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Located at: en.Wikipedia.org/wiki/File:Co...nd,_Hawaii.JPG. License: Public
CONTRIBUTIONS AND ATTRIBUTIONS Domain: No Known Copyright
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en.Wikipedia.org/wiki/Echinoderm. License: CC BY-SA: Attribution- en.Wikipedia.org/wiki/Chordate. License: CC BY-SA: Attribution-ShareAlike
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OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX. Crinoid on the reef of Batu Moncho Island. Provided by: Wikipedia. Located at:
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 CHAPTER OVERVIEW

29: VERTEBRATES

Topic hierarchy
29.1: Chordates
29.1A: Characteristics of Chordata
29.1B: Chordates and the Evolution of Vertebrates
29.1C: The Evolution of Craniata and Vertebrata
29.1D: Characteristics of Vertebrates
29.2: Fishes
29.2A: Agnathans- Jawless Fishes
29.2B: Gnathostomes - Jawed Fishes
29.3: Amphibians
29.3A: Characteristics and Evolution of Amphibians
29.3B: Modern Amphibians
29.3C: Evolution of Amniotes
29.4: Reptiles
29.4A: Characteristics of Amniotes
29.4B: Characteristics of Reptiles
29.4C: Evolution of Reptiles
29.4D: Modern Reptiles
29.5: Birds
29.5A: Characteristics of Birds
29.5B: Evolution of Birds
29.6: Mammals
29.6A: Characteristics of Mammals
29.6B: Evolution of Mammals
29.6C: Living Mammals
29.7: The Evolution of Primates
29.7A: Characteristics and Evolution of Primates
29.7B: Early Human Evolution
29.7C: Early Hominins
29.7D: Genus Homo

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1
SECTION OVERVIEW

29.1: CHORDATES
29.1C: THE EVOLUTION OF CRANIATA AND
Topic hierarchy VERTEBRATA

29.1D: CHARACTERISTICS OF VERTEBRATES

29.1A: CHARACTERISTICS OF CHORDATA

29.1B: CHORDATES AND THE EVOLUTION OF This page titled 29.1: Chordates is shared under a CC BY-SA 4.0 license
VERTEBRATES and was authored, remixed, and/or curated by Boundless.

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29.1A: CHARACTERISTICS OF CHORDATA
Animals in the phylum Chordata share four key features: a 3. pharyngeal slits
notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post- 4. post-anal tail
anal tail.
NOTOCHORD
 LEARNING OBJECTIVES The chordates are named for the notochord: a flexible, rod-shaped
structure that is found in the embryonic stage of all chordates and
Identify the key features of the chordates also in the adult stage of some chordate species. It is located
between the digestive tube and the nerve cord, providing skeletal
KEY POINTS support through the length of the body. In some chordates, the
These characteristics are only present during embryonic notochord acts as the primary axial support of the body throughout
development in some chordates. the animal’s lifetime.
The notochord provides skeletal support, gives the phylum its In vertebrates, the notochord is present during embryonic
name, and develops into the vertebral column in vertebrates. development, at which time it induces the development of the neural
The dorsal hollow nerve cord develops into the central nervous tube which serves as a support for the developing embryonic body.
system: the brain and spine. The notochord, however, is replaced by the vertebral column (spine)
Pharyngeal slits are openings in the pharynx that develop into in most adult vertebrates.
gill arches in bony fish and into the jaw and inner ear in
terrestrial animals. DORSAL HOLLOW NERVE CORD
The post-anal tail is a skeletal extension of the posterior end of The dorsal hollow nerve cord derives from ectoderm that rolls into a
the body, being absent in humans and apes, although present hollow tube during development. In chordates, it is located dorsally
during embryonic development. (at the top of the animal) to the notochord. In contrast to the
chordates, other animal phyla are characterized by solid nerve cords
KEY TERMS that are located either ventrally or laterally. The nerve cord found in
notochord: a flexible rodlike structure that forms the main most chordate embryos develops into the brain and spinal cord,
support of the body in the lowest chordates; a primitive spine which comprise the central nervous system.
nerve cord: a dorsal tubular cord of nervous tissue above the
notochord of a chordate PHARYNGEAL SLITS
pharyngeal slit: filter-feeding organs found in non-vertebrate Pharyngeal slits are openings in the pharynx (the region just
chordates (lancelets and tunicates) and hemichordates living in posterior to the mouth) that extend to the outside environment. In
aquatic environments organisms that live in aquatic environments, pharyngeal slits allow
for the exit of water that enters the mouth during feeding. Some
CHARACTERISTICS OF CHORDATA invertebrate chordates use the pharyngeal slits to filter food out of
Animals in the phylum Chordata share four key features that appear the water that enters the mouth. In vertebrate fishes, the pharyngeal
at some stage during their development (often, only during slits develop into gill arches, the bony or cartilaginous gill supports.
embryogenesis) (: In most terrestrial animals, including mammals and birds,
pharyngeal slits are present only during embryonic development. In
these animals, the pharyngeal slits develop into the jaw and inner ear
bones.

POST-ANAL TAIL
The post-anal tail is a posterior elongation of the body, extending
beyond the anus. The tail contains skeletal elements and muscles,
which provide a source of locomotion in aquatic species. In some
terrestrial vertebrates, the tail also helps with balance, courting, and
signaling when danger is near. In humans and other apes, the post-
Figure 29.1A. 1 : Defining characteristics of chordates: In chordates,
four common features appear at some point during development: a anal tail is present during embryonic development, but is vestigial as
notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post- an adult.
anal tail.
1. a notochord This page titled 29.1A: Characteristics of Chordata is shared under a CC
2. a dorsal hollow nerve cord BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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29.1B: CHORDATES AND THE EVOLUTION OF VERTEBRATES
Chordata contains two subphylums of invertebrates: Urochordata
(tunicates) and Cephalochordata (lancelets).

 LEARNING OBJECTIVES

Describe the features and phylogenetic history of lancelets

and urochordata Figure 29.1B. 1: Urochordates: (a) This photograph shows a colony
of the tunicate Botrylloides violaceus. (b) The larval stage of the
tunicate possesses all of the features characteristic of chordates: a
KEY POINTS notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-
Urochordata (tunicates) and Cephalochordata (lancelets) are anal tail. (c) In the adult stage, the notochord, nerve cord, and tail
invertebrates because they lack a backone. disappear.
Larval tunicates (Urochordata) posses all four structures that Most tunicates are hermaphrodites. Tunicate larvae hatch from eggs
classify chordates, but adult tunicates retain only pharyngeal inside the adult tunicate’s body. After hatching, a tunicate larva
slits. swims for a few days until it finds a suitable surface on which it can
Larval tunicates swim for a few days after hatching, then attach attach, usually in a dark or shaded location. It then attaches via the
to a marine surface and undergo metamorphosis into the sessile head to the surface and undergoes metamorphosis into the adult
adult form. form, at which point the notochord, nerve cord, and tail disappear.
Lancelets (Cephalochordata) are marine organisms that possess Most tunicates live a sessile existence on the ocean floor and are
all features of chordates; they are named Cephalochordata suspension feeders. The primary foods of tunicates are plankton and
because the notochord extends into the head. detritus. Seawater enters the tunicate’s body through its incurrent
Lancelets may be the closest-living relatives to vertebrates. siphon. Suspended material is filtered out of this water by a mucous
net (pharyngeal slits) and is passed into the intestine via the action of
KEY TERMS cilia. The anus empties into the excurrent siphon, which expels
Urochordata: a taxonomic subphylum within the phylum wastes and water. Tunicates are found in shallow ocean waters
Chordata: the tunicates or sea squirts around the world.
Cephalochordata: a taxonomic subphylum within the phylum
Chordata: the lancelets CEPHALOCHORDATA
sessile: permanently attached to a substrate; not free to move Members of Cephalochordata possess a notochord, dorsal hollow
about; “an attached oyster” nerve cord, pharyngeal slits, and a post-anal tail in the adult stage.
They do not have a true brain, but the notochord extends into the
CHORDATES AND THE EVOLUTION OF head, which gives the subphylum its name ( “cephalo” is Greek for
VERTEBRATES head). Extinct members of this subphylum include Pikaia, which is
The most familiar group of chordates is the vertebrates. However, in the oldest known cephalochordate. Pikaia fossils were recovered
addition to the subphylum Vertebrata, the phylum Chordata also from the Burgess shales of Canada and dated to the middle of the
contains two subphylums of invertebrates: Urochordata and Cambrian age, making them more than 500 million years old.
Cephalochordata. Members of these groups also possess the four Extant members of Cephalochordata are the lancelets, named for
distinctive features of chordates at some point during their their blade-like shape. Lancelets are only a few centimeters long and
development: a notochord, a dorsal hollow nerve cord, pharyngeal are usually found buried in sand at the bottom of warm temperate
slits, and a post-anal tail. Unlike vertebrates, urochordates and and tropical seas. Like tunicates, they are suspension feeders. With
cephalochordates never develop a bony backbone. notochord and paired muscle blocks, the lancelet and Pikaia may
belong to the chordate group of animals from which the vertebrates
UROCHORDATA
have descended.
Members of Urochordata are also known as tunicates. The name
tunicate derives from the cellulose-like carbohydrate material, called
the tunic, which covers the outer body of tunicates. Although
tunicates are classified as chordates, only the larval form possesses
all four common structures. Adults only maintain pharyngeal slits
and lack a notochord, a dorsal hollow nerve cord, and a post-anal
tail.

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 This page titled 29.1B: Chordates and the Evolution of Vertebrates is shared
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by Boundless.

Figure 29.1B. 1: Cephalochrodates: The lancelet, like all

cephalochordates, has a head. Adult lancelets retain the four key
features of chordates: a notochord, a dorsal hollow nerve cord,
pharyngeal slits, and a post-anal tail. Water from the mouth enters
the pharyngeal slits, which filter out food particles. The filtered
water then collects in the atrium and exits through the atriopore.

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29.1C: THE EVOLUTION OF CRANIATA AND VERTEBRATA
Both genomic and fossil evidence suggests that vertebrates evolved
from craniates, which evolved from invertebrate chordates.

 LEARNING OBJECTIVES

Explain how genomics informs scientists about chordate

evolution

KEY POINTS
The clade Craniata includes animals that have a cranium: a bony,
cartilaginous, or fibrous structure that surrounds the brain, jaw,
and facial bones.
Members of Craniata include the vertebrates and hagfish.
Genomic evidence suggests that vertebrates diverged from Figure 29.1C. 1 : Clade Craniata: Craniata, including this fish
cephalochordates (lancelets), which had previously diverged (Dunkleosteus), are characterized by the presence of a cranium,
mandible, and other facial bones.
from urochordates (tunicates).
Fossil evidence suggests that most vertebrate diversity originated Vertebrates are members of the subphylum Vertebrata, the clade
in the Cambrian explosion 540 million years ago. Craniata, and the phylum Chordata. Vertebrates display the four
Two whole- genome duplications occurred in early vertebrate characteristic features of chordates, but they are named for the
history.
vertebral column composed of a series of bony vertebrae joined
together as a backbone. In adult vertebrates, the vertebral column
KEY TERMS
replaces the embryonic notochord.
cranium: the part of the skull enclosing the brain, the braincase
genomics: the study of the complete genome of an organism
Cambrian explosion: the relatively rapid appearance (over a
period of many millions of years), around 530 million years ago,
of most major animal phyla as demonstrated in the fossil record

CRANIATA AND VERTEBRATA

The clade Craniata is a subdivision of Chordata. Members of
Craniata posses a cranium, which is a bony, cartilaginous, or fibrous
structure surrounding the brain, jaw, and facial bones. The clade
Craniata includes all vertebrates and the hagfishes (Myxini), which
Figure 29.1C. 1 : Vertebrates: Vertebrata are characterized by the
have a cranium but lack a backbone. Hagfish are the only known presence of a backbone, such as the one that runs through the middle
living animals that have a skull, but not a vertebral column. of this fish. All vertebrates are in the Craniata clade and have a
cranium.

VERTEBRATE EVOLUTION
In the phylum Chordata, the closest relatives of the vertebrates are
the invertebrate chordates. Based on the molecular analysis of
vertebrate and invertebrate genomes (genomics), scientists can
determine the evolutionary history of different phylogenetic groups.
According to these genomic analyses, vertebrates appear to be more
closely related to the lancelets (cephalochordates) than to the
tunicates (urochordates). This suggests that the cephalochordates
first diverged from urochordates, and that vertebrates subsequently
diverged from the cephalochordates. This hypothesis is further
supported by the fossil of a 530 million-year-old organism with a
brain and eyes like a vertebrate, but without the skull found in a
craniate. A comparison of the genomes of a lancelet, tunicate,
lamprey, fish, chicken, and human confirmed that two whole-
Figure 29.1C. 1 : Hagfish: Although it lacks a backbone, the hagfish genome duplications occurred in the early history of the Vertebrata
is a member of the Craniata clade because it possesses a bony skull. subphylum.

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Both fossil and genomic evidence suggests that vertebrates arose This page titled 29.1C: The Evolution of Craniata and Vertebrata is shared
during the Cambrian explosion.The Cambrian explosion was the under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
relatively brief span of time during the Cambrian period during by Boundless.
which many animal groups appeared and rapidly diversified. Most
modern animal phyla originated during the Cambrian explosion.

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29.1D: CHARACTERISTICS OF VERTEBRATES
Vertebrata is a subphlyum of Chordata that is further defined by their
bony backbone.

 LEARNING OBJECTIVES

Identify the defining characteristics of vertebrates

KEY POINTS
As chordates, vertebrates have the same common features: a
notochord, a dorsal hollow nerve cord, pharyngeal slits, and a
post-anal tail.
Vertebrates are further differentiated from chordates by their
vertebral column, which forms when their notochord develops
into the column of bony vertebrae separated by discs. Figure 29.1D. 1 : Phylum chordata: All chordates are deuterostomes,
Vertebrates are the only chordates that have a brain as part of possessing a notochord. Vertebrates are differentiated by having a
vertebral column.
their central nervous system.
As chordates, all vertebrates have a similar anatomy and
KEY TERMS morphology with the same qualifying characteristics: a notochord, a
vertebral column: the series of vertebrae that protect the spinal dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail.
cord; the spinal column However, the subphylum Vertebrata is distinguished from the
chordate: a member of the phylum Chordata; numerous animals phylum Chordata by the development of the notochord into a bony
having a notochord at some stage of their development; in backbone. Vertebrates include the amphibians, reptiles, mammals,
vertebrates this develops into the spine and birds, as well as the jawless fishes, bony fishes, sharks, and rays.
notochord: a flexible rodlike structure that forms the main
support of the body in the lowest chordates; a primitive spine

CHARACTERISTICS OF VERTEBRATES
Vertebrates are members of the subphylum Vertebrata, under the
phylum Chordata and under the kingdom Animalia. Animals that
possess bilateral symmetry can be divided into two groups,
protostomes and deuterostomes, based on their patterns of
embryonic development. The deuterostomes, whose name translates
as “second mouth,” consist of two phyla: Chordata and
Echinodermata. Echinoderms are invertebrate marine animals that
have pentaradial symmetry and a spiny body covering; the phylum
includes sea stars, sea urchins, and sea cucumbers. The phylum
Chordata contains two groups of invertebrate chordates, but the most
conspicuous and familiar members of Chordata are the vertebrates.

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 with a hollow tube of nervous tissue (the spinal cord) above it and
the gastrointestinal tract below. In all vertebrates, there is a mouth at
anterior end of the animal and an anus before the posterior end of the
body. There is a tail posterior to the anus during at least one phase of
the animal’s development.

THE VERTEBRAL COLUMN

Vertebrates are defined by the presence of the vertebral column. In
vertebrates, the notochord develops into the vertebral column or
spine: a series of bony vertebrae each separated by mobile discs.
These vertebrae are always found on the dorsal side of the animal.
However, a few vertebrates have secondarily lost their vertebrae
and, instead, retain the notochord into adulthood (e.g., the sturgeon
fish).

Figure 29.1D. 1 : Vertebral column: A fossilized skeleton of the

dinosaur Diplodocus carnegii shows an extreme example of the
backbone that characterizes vertebrates.

CENTRAL NERVOUS SYSTEM

Vertebrates are also the only members of Chordata to possess a
brain. In chordates, the central nervous system is based on a hollow
nerve tube that runs dorsal to the notochord along the length of the
animal. In vertebrates, the anterior end of the nerve tube expands
and differentiates into three brain vesicles.

VERTEBRATE CLASSIFICATION
Vertebrates are the largest group of chordates, with more than
Figure 29.1D. 1 : Diversity of vertebrates: animals with backbones: 62,000 living species. Vertebrates are grouped based on anatomical
The subphylum Vertebrata contains all animals that possess and physiological traits. The traditional groups include Agnatha,
backbones, gills, and a central nervous system in at least one phase
of development. Vertebrates include amphibians, reptiles, mammals, Chondrichthyes, Osteichthyes, Amphibia, Reptilia, Aves, and
and birds, as well as the jawless fishes, bony fishes, sharks, and rays. Mammalia.
More than 64,000 species of vertebrates have been described, but the Animals that possess jaws are known as gnathostomes, meaning
extant vertebrate species represent only a small portion of all the “jawed mouth.” Gnathostomes include fishes and tetrapods
vertebrates that have existed. Vertebrates range in size from the frog (amphibians, reptiles, birds, and mammals). Tetrapods can be further
species Paedophryne amauensis (as small as 7.7 mm (0.3 inch)) to divided into two groups: amphibians and amniotes. Amniotes are
the blue whale (as large as 33 m (110 ft)). Vertebrates comprise animals whose eggs are adapted for terrestrial living; this group
about 4 percent of all described animal species; the remainder are includes mammals, reptiles, and birds. Amniotic embryos,
invertebrates, which lack backbones. developing in either an externally-shelled egg or an egg carried by
the female, are provided with a water-retaining environment and are
ANATOMY AND MORPHOLOGY protected by amniotic membranes.
All vertebrates are built along the basic chordate body plan: a stiff
rod running through the length of the animal (vertebral column), This page titled 29.1D: Characteristics of Vertebrates is shared under a CC
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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SECTION OVERVIEW

29.2: FISHES
29.2B: GNATHOSTOMES - JAWED FISHES
29.2A: AGNATHANS- JAWLESS FISHES
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was authored, remixed, and/or curated by Boundless.

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29.2A: AGNATHANS- JAWLESS FISHES
The superclass Agnatha describes fish that lack jaws and includes
the extant species of hagfish and lampreys.

 LEARNING OBJECTIVES

Differentiate between the taxa of jawless fishes

KEY POINTS
Most agnathans are now extinct, but two branches exist today:
hagfishes (not true vertebrates) and lampreys (true vertebrates). Figure 29.2A. 1 : Ostracoderm: Ostracoderms were some of the
The earliest jawless fishes were the ostracoderms, which had earliest jawless fishes and were covered in bony armor. Present-day
jawless fishes lack bone in their scales.
bony scales as body armor.
Hagfish are eel-like marine scavengers in the clade Myxini that MYXINI: HAGFISHES
produce slime and can tie themselves into knots.
The clade Myxini includes at least 20 species of hagfishes.
Lampreys are in the clade Petromyzontidae and appear
Hagfishes are eel-like scavengers that live on the ocean floor and
morphologically similar to hagfish, but contain cartilaginous
feed on dead invertebrates, other fishes, and marine mammals.
vertebral elements as an adult; thus, they are considered true
Hagfishes are entirely marine and are found in oceans around the
vertebrates.
world, except for the polar regions. Hagfish have slime glands
KEY TERMS beneath the skin that constantly release mucus, allowing them to
escape from the grip of predators. Hagfish can also twist their bodies
hagfish: any of several primitive eellike creatures, of the family
into a knot to gain a mechanical advantage while feeding and are
Myxinidae, having a sucking mouth with rasping teeth;
notorious for eating carcasses from the inside out.
considered edible in Japan, their skin is used to make a form of
leather
lamprey: any long slender primitive eel-like freshwater and
saltwater fish of the Petromyzontidae family, having a sucking
mouth with rasping teeth, but no jaw
agnathan: a member of the superclass Agnatha of jawless
vertebrates

AGNATHANS: JAWLESS FISHES

Jawless fishes or agnathans are craniates that represent an ancient
vertebrate lineage that arose over one half-billion years ago.
“Gnathos” is Greek for “jaw” and the prefix “a” means “without,” so
agnathans are “without jaws. ” Most agnathans are now extinct, but Figure 29.2A. 1 : Hagfishes: Pacific hagfish are scavengers that live
two branches still exist today: hagfishes and lampreys. Hagfishes on the ocean floor. These agnathans are classified as Myxini and do
not have a vertebral column.
and lampreys are recognized as separate clades, primarily because
lampreys are true vertebrates, whereas hagfishes are not. A defining The skeleton of a hagfish is composed of cartilage, which includes a
feature of agnathans is the lack of paired lateral appendages or fins. cartilaginous notochord that runs the length of the body. This
notochord provides support to the hagfish’s body. Unlike true
Some of the earliest jawless fishes were the ostracoderms (Greek for
vertebrates, hagfishes do not replace the notochord with a vertebral
“bone-skin”). Ostracoderms were vertebrate fishes encased in bony
column during development. Since they have a cartilaginous skull,
armor, unlike present-day jawless fishes, which lack bone in their
they are classified in the clade Craniata.
scales.
PETROMYZONTIDAE: LAMPREYS
The clade Petromyzontidae includes approximately 35–40 or more
species of lampreys. Lampreys are morphologically similar to
hagfishes and also lack paired appendages. However, lampreys
develop some vertebral elements as an adult. Their notochord is
surrounded by a cartilaginous structure called an arcualia, which
may resemble an evolutionarily-early form of the vertebral column.
As adults, lampreys are characterized by a toothed, funnel-like
sucking mouth. Many species have a parasitic stage of their life

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cycle during which they are ectoparasites of fishes. Lampreys live
primarily in coastal and fresh waters. They are distributed
worldwide, except for the tropics and polar regions. Some species
are marine, but all species spawn in fresh water; eggs are fertilized
externally. The larvae differ distinctly from the adult form, spending
3 to 15 years as suspension feeders. Once they reach sexual maturity,
the adults die within days of reproduction.

Figure 29.2A. 1 : Parasitic lampreys: These parasitic sea lampreys

attach to their lake trout host by suction and use their rough tongues
to rasp away flesh in order to feed on the trout’s blood.

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29.2B: GNATHOSTOMES - JAWED FISHES

 LEARNING OBJECTIVES

Differentiate among the types of jawed fishes

Gnathostomes or “jaw-mouths” are vertebrates that possess jaws.

One of the most significant developments in early vertebrate
evolution was the development of the jaw, which is a hinged
structure attached to the cranium that allows an animal to grasp and
tear its food. The evolution of jaws allowed early gnathostomes to
exploit food resources that were unavailable to the jawless animals.
In early evolutionary history, there were gnathostomes (jawed
fishes) and agnathans (jawless fishes). Gnathostomes later evolved Figure 29.2B. 1: Hammerhead shark: Hammerhead sharks tend to
school during the day and hunt prey at night. As members of
into all tetrapods (animals with four limbs) including amphibians,
Chondrichthyes, their skeletons are composed of cartilage.
birds, and mammals.
Early gnathostomes were jawed fishes that possessed two sets of Most cartilaginous fishes live in marine habitats, although a few
paired fins, which increased their ability to maneuver accurately. species live in fresh water for part or all of their lives. Most sharks
These paired fins were pectoral fins, located on the anterior body, are carnivores that feed on live prey, either swallowing it whole or
and pelvic fins, on the posterior. The evolution of the jaw combined using their jaws and teeth to tear it into smaller pieces. Shark teeth
with paired fins permitted gnathostomes to expand from the probably evolved from the jagged scales that cover their skin called
sedentary suspension feeding of jawless fishes and become mobile placoid scales. Some species of sharks and rays are suspension
predators. The gnathostomes’ ability to exploit new nutrient sources feeders that feed on plankton.
led to their evolutionary success during the Devonian period. Two Sharks have well-developed sense organs that aid them in locating
early groups of gnathostomes were the acanthodians and prey, including a keen sense of smell and electroreception. Organs
placoderms, which arose in the late Silurian period and are now called ampullae of Lorenzini enable sharks to detect the
extinct. Most modern gnathostomes belong to the clades electromagnetic fields that are produced by all living things,
Chondrichthyes and Osteichthyes. including their prey. Only aquatic or amphibious animals possess
electroreception. Sharks, together with most fishes and aquatic and
larval amphibians, also have a sense organ called the lateral line,
which is used to detect movement and vibration in the surrounding
water. It is often considered homologous to “hearing” in terrestrial
vertebrates. The lateral line is visible as a darker stripe that runs
along the length of a fish’s body.
Rays and skates comprise more than 500 species and are closely
related to sharks. They can be distinguished from sharks by their
flattened bodies, pectoral fins that are enlarged and fused to the
head, and gill slits on their ventral surface. Like sharks, rays and
skates have a cartilaginous skeleton. Most species are marine and
live on the sea floor, with nearly a worldwide distribution.

Figure 29.2B. 1: Placoderms: Dunkleosteous was an enormous OSTEICHTHYES: BONY FISHES

placoderm from the Devonian period, 380–360 million years ago. It
measured up to 10 meters in length and weighed up to 3.6 tons. As Members of the clade Osteichthyes, also called bony fish, are
gnathostomes, they were more mobile and could exploit more food characterized by a bony skeleton. The vast majority of present-day
resources than the agnathostomes. fish belong to this group, which consists of approximately 30,000
CHONDRICHTHYES: CARTILAGINOUS FISHES species, making it the largest class of vertebrates in existence today.
The clade Chondrichthyes consists of sharks, rays, and skates, Nearly all bony fish have an ossified skeleton with specialized bone
together with sawfishes and a few dozen species of fishes called cells (osteocytes) that produce and maintain a calcium phosphate
chimaeras, or “ghost,” sharks. Chondrichthyes are jawed fishes that matrix. A few groups of Osteichthyes, such as sturgeons and
possess paired fins and a skeleton made of cartilage. This clade paddlefish, have primarily cartilaginous skeletons, but retain some
arose approximately 370 million years ago in the early or middle bony elements. The skin of bony fish is often covered by
Devonian. overlapping scales. Skin glands secrete mucus that reduces drag
when swimming and aids the fish in osmoregulation. Like sharks,
bony fish have a lateral line system that detects vibrations in water.

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All bony fish use gills for gas exchange. Water is drawn over gills operculum: a covering flap or lidlike structure in plants and
that are located in chambers covered and ventilated by a protective, animals, such as a gill cover
muscular flap called the operculum. Many bony fish also have a Chondrichthyes: a taxonomic class within the subphylum
swim bladder, a gas-filled organ that helps to control the buoyancy Vertebrata: the cartilaginous fish
of the fish. Osteichthyes: a taxonomic class within the subphylum
Bony fish are further divided into two extant clades: Actinopterygii vertebrata: the bony fish
(ray-finned fish) and Sarcopterygii (lobe-finned fish).
CONTRIBUTIONS AND ATTRIBUTIONS
Actinopterygii, the ray-finned fish include many familiar fish, such
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
as tuna, bass, trout, and salmon, among others. Ray-finned fish are Located at: http://cnx.org/content/m44687/latest/?collection=col11448/latest.
named for their fins that are webs of skin supported by bony spines License: CC BY: Attribution
hagfish. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/hagfish.
called rays. In contrast, the fins of Sarcopterygii are fleshy and License: CC BY-SA: Attribution-ShareAlike
lobed, supported by bone. Although most members of this clade are agnathan. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/agnathan. License: CC BY-SA: Attribution-ShareAlike
extinct, living members include the less-familiar lungfishes and lamprey. Provided by: Wiktionary. Located at:
coelacanths. Early Sarcopterygii evolved into modern tetrapods, en.wiktionary.org/wiki/lamprey. License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Fishes. October 17, 2013. Provided by: OpenStax CNX.
including reptiles, amphibians, birds, and mammals. Located at: http://cnx.org/content/m44687/latest...e_29_02_01.jpg. License:
CC BY: Attribution
Ostracoderm digital recreation.. Provided by: Wikimedia. Located at:
commons.wikimedia.org/wiki/File:Ostracoderm_digital_recreation..jpg.
License: CC BY-SA: Attribution-ShareAlike
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Located at: http://cnx.org/content/m44687/latest...e_29_02_02.jpg. License:
CC BY: Attribution
Figure 29.2B. 1: Actinopterygii and Sarcopterygii: The (a) sockeye Osteichthyes. Provided by: Wiktionary. Located at:
salmon (Actinopterygii) and (b) coelacanth (Sarcopterygii) are both en.wiktionary.org/wiki/Osteichthyes. License: CC BY-SA: Attribution-
bony fishes of the Osteichthyes clade. The coelacanth, sometimes ShareAlike
called a lobe-finned fish, was thought to have gone extinct in the OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Late Cretaceous period, 100 million years ago, until one was Located at: http://cnx.org/content/m44687/latest/?collection=col11448/latest.
discovered in 1938 near the Comoros Islands between Africa and License: CC BY: Attribution
Madagascar. ossified. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/ossified.
License: CC BY-SA: Attribution-ShareAlike
Chondrichthyes. Provided by: Wiktionary. Located at:
KEY POINTS en.wiktionary.org/wiki/Chondrichthyes. License: CC BY-SA: Attribution-
Early jawed fish (gnathostomes) were able to exploit new ShareAlike
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nutrient sources because of their jaws and paired fins. en.wiktionary.org/wiki/operculum. License: CC BY-SA: Attribution-
Chondrichthyes includes all jawed fish with cartilagenous ShareAlike
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skeletons, such as sharks, rays, skates, and chimaeras. Located at: http://cnx.org/content/m44687/latest/Figure_29_02_01.jpg.
Osteichthyes includes all jawed fish with ossified (bony) License: CC BY: Attribution
Ostracoderm digital recreation.. Provided by: Wikimedia. Located at:
skeletons; this includes the majority of modern fish. commons.wikimedia.org/wiki/File:Ostracoderm_digital_recreation..jpg.
Osteichthyes can be further separated into Actinopterygii (the License: CC BY-SA: Attribution-ShareAlike
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ray-finned fishes) and Sarcopterygii (lobe-finned fishes). Located at: http://cnx.org/content/m44687/latest/Figure_29_02_02.jpg.
The majority of modern fish species are actinopterygii, from License: CC BY: Attribution
trout to clownfish. OpenStax College, Fishes. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44687/latest...29_02_07ab.jpg. License:
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KEY TERMS OpenStax College, Fishes. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44687/latest...e_29_02_04.jpg. License:
ossified: composed of bone, which is a calcium phosphate matrix CC BY: Attribution
created by special cells called osteoblasts
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SECTION OVERVIEW

29.3: AMPHIBIANS
29.3B: MODERN AMPHIBIANS
Topic hierarchy
29.3C: EVOLUTION OF AMNIOTES
29.3A: CHARACTERISTICS AND EVOLUTION OF
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and was authored, remixed, and/or curated by Boundless.

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29.3A: CHARACTERISTICS AND EVOLUTION OF AMPHIBIANS
Amphibians evolved from fish 400 million years ago and are is an extra bone in the ear that transmits sounds to the inner ear. All
characterized by four limbs, moist skin, and sensitive inner ear extant adult amphibians are carnivorous. Some terrestrial
structures. amphibians have a sticky tongue that is used to capture prey.

LEARNING OBJECTIVES EVOLUTION OF AMPHIBIANS

Describe the evolutionary characteristics that distinguish the The fossil record provides evidence of the first tetrapods: now-
amphibian extinct amphibian species dating to nearly 400 million years ago.
Evolution of tetrapods from fishes represented a significant change
KEY TAKEAWAYS in body plan from one suited to organisms that respired and swam in
KEY POINTS water, to organisms that breathed air and moved onto land. These
Most amphibians are characterized by four well-developed changes occurred over a span of 50 million years during the
limbs, but some species of salamanders and all caecilians are Devonian period. One of the earliest known tetrapods is from the
functionally limbless. genus Acanthostega. Acanthostega was aquatic; fossils show that it
Amphibians have a moist, permeable skin that is achieved via had gills similar to fishes. However, it also had four limbs, with the
mucus glands that keep the skin lubricated in order to perform skeletal structure of limbs found in present-day tetrapods, including
cutaneous respiration. amphibians. Therefore, it is thought that Acanthostega lived in
All extant adult amphibians are carnivorous; some terrestrial shallow waters and was an intermediate form between lobe-finned
amphibians have a sticky tongue that is used to capture prey. fishes and early, fully terrestrial tetrapods. What preceded
Additional characteristics of amphibians include pedicellate Acanthostega?
teeth, papilla amphibiorum, papilla basilaris, and auricular In 2006, researchers published news of their discovery of a fossil of
operculum. a “tetrapod-like fish,” Tiktaalik roseae, which seems to be an
The tetrapod-like fish, Tiktaalik roseae, preceded Acanthostega intermediate form between fishes having fins and tetrapods having
and lived in a shallow water environment about 375 million limbs. Tiktaalik probably lived in a shallow water environment about
years ago. 375 million years ago.
All extant adult amphibians are carnivorous, and some terrestrial
amphibians have a sticky tongue that is used to capture prey.

KEY TERMS
cutaneous respiration: exchange of oxygen and carbon dioxide
with the environment that takes place through the permeable skin
pedicellate teeth: teeth in which the root and crown are
calcified, separated by a zone of noncalcified tissue
auricular operculum: an extra bone in the ear that transmits
sounds to the inner ear
CHARACTERISTICS OF AMPHIBIANS
As tetrapods, most amphibians are characterized by four well- Figure 29.3A. 1 : Tiktaalik roseae: The recent fossil discovery of
Tiktaalik roseae suggests evidence for an animal intermediate to
developed limbs. Some species of salamanders and all caecilians are
finned fish and legged tetrapods.
functionally limbless; their limbs are vestigial. An important
The early tetrapods that moved onto land had access to new nutrient
characteristic of extant amphibians is a moist, permeable skin that is
sources and relatively few predators. This led to the widespread
achieved via mucus glands that keep the skin moist; thus, exchange
distribution of tetrapods during the early Carboniferous period:
of oxygen and carbon dioxide with the environment can take place
sometimes called the “Age of the Amphibians.”
through it (cutaneous respiration). Additional characteristics of
amphibians include pedicellate teeth (teeth in which the root and MODERN
crown are calcified, separated by a zone of noncalcified tissue) and a
papilla amphibiorum and papilla basilaris (structures of the inner ear This page titled 29.3A: Characteristics and Evolution of Amphibians is
that are sensitive to frequencies below and above 10,00 hertz, shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
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29.3B: MODERN AMPHIBIANS
Amphibians can be divided into three groups: Urodela
(salamanders), Anura (frogs), and Apoda (caecilians).

 LEARNING OBJECTIVES

Differentiate among the orders of amphibians

KEY POINTS
Adult salamanders usually have four limbs and a tail, moving by
lateral undulation in a fish-like manner while “walking” their
arms and legs forward and back.
The majority of salamanders are lungless; respiration occurs
Figure 29.3B. 1: Salamanders: Most salamanders have legs and a
through the skin or through external gills; some terrestrial
tail, but respiration varies among species.
salamanders have primitive lungs; a few species have both lungs
and gills. URODELA: SALAMANDERS
Salamanders utilize internal fertilization after males transfer Salamanders are amphibians that belong to the order Urodela.
sperm to the eggs via the spermatophore; there is a prolonged Living salamanders include approximately 620 species, some of
egg phase; metamorphosis occurs before hatching. which are aquatic, other terrestrial, and some that live on land only
Caecilians are blind, limbless vertebrates that resemble as adults. Adult salamanders usually have a generalized tetrapod
earthworms and are adapted for a soil-burrowing or an aquatic body plan with four limbs and a tail. They move by bending their
lifestyle. bodies from side to side, called lateral undulation, in a fish-like
Adult frogs use their hind legs to jump; they fertilize externally, manner while “walking” their arms and legs back and forth. It is
laying their shell-less eggs in moist environments. thought that their gait is similar to that used by early tetrapods.
Tadpoles (the larval stage of a frog) have gills, a lateral line Respiration differs among the species. The majority of salamanders
system, long-finned tails, and lack limbs; when tadpoles become are lungless, with respiration occurring through the skin or through
adults, gills, tails, and the lateral line disappear, while an eardrum external gills. Some terrestrial salamanders have primitive lungs; a
and lungs develop. few species have both gills and lungs.
Unlike frogs, virtually all salamanders rely on internal fertilization
KEY TERMS
of the eggs. The only male amphibians that possess copulatory
lateral undulation: movement by bending the body from side to
structures are the caecilians, so fertilization among salamanders
side
typically involves an elaborate and often prolonged courtship. Such
spermatophore: a capsule or mass created by males, containing
a courtship allows the successful transfer of sperm from male to
sperm and transferred in entirety to the female during
female via a spermatophore. Development in many of the most
fertilization
highly-evolved salamanders, which are fully terrestrial, occurs
metamorphosis: a change in the form and often habits of an
during a prolonged egg stage, with the eggs guarded by the mother.
animal after the embryonic stage during normal development
During this time, the gilled larval stage is found only within the egg
MODERN AMPHIBIANS capsule, with the gills being resorbed, and metamorphosis being
completed, before hatching. Hatchlings resemble tiny adults.
Amphibia comprises an estimated 6,770 extant species that inhabit
tropical and temperate regions around the world. Amphibians can be
divided into three clades: Urodela (“tailed-ones”), the salamanders;
Anura (“tail-less ones”), the frogs; and Apoda (“legless ones”), the
caecilians.

Figure 29.3B. 1: Frogs: The Australian green tree frog is a nocturnal

predator that lives in the canopies of trees near a water source.

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ANURA: FROGS APODA: CAECILIANS
Frogs are amphibians that belong to the order Anura. Anurans are An estimated 185 species comprise caecilians, a group of
among the most diverse groups of vertebrates, with approximately amphibians that belong to the order Apoda. Although they are
5,965 species occurring on all of the continents except Antarctica. vertebrates, a complete lack of limbs leads to their resemblance to
Anurans have a body plan that is more specialized for movement. earthworms in appearance. They are adapted for a soil-burrowing or
Adult frogs use their hind limbs to jump on land. Frogs have a aquatic lifestyle; they are nearly blind. These animals are found in
number of modifications that allow them to avoid predators, the tropics of South America, Africa, and Southern Asia. They have
including skin that acts as camouflage. Many species of frogs and vestigial limbs which is evidence that they evolved from a legged
salamanders also release defensive chemicals that are poisonous to ancestor.
predators from glands in the skin.
CONTRIBUTIONS AND ATTRIBUTIONS
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44688/latest/?collection=col11448/latest.
License: CC BY: Attribution
OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44688/latest/?collection=col11448/latest.
License: CC BY: Attribution
Boundless. Provided by: Boundless Learning. Located at:
www.boundless.com//biology/definition/cutaneous-respiration. License: CC
BY-SA: Attribution-ShareAlike
Boundless. Provided by: Boundless Learning. Located at:
www.boundless.com//biology/definition/pedicellate-teeth. License: CC BY-
SA: Attribution-ShareAlike
Boundless. Provided by: Boundless Learning. Located at:
www.boundless.com//biology/definition/auricular-operculum. License: CC
Figure 29.3B. 1: Frog metamorphosis: A juvenile frog BY-SA: Attribution-ShareAlike
metamorphoses into a frog. Here, the frog has started to develop OpenStax College, Amphibians. October 17, 2013. Provided by: OpenStax
limbs, but its tadpole tail is still evident. CNX. Located at: http://cnx.org/content/m44688/latest...e_29_03_01.jpg.
Frog eggs are fertilized externally. As with other amphibians, frogs License: CC BY: Attribution
Boundless. Provided by: Boundless Learning. Located at:
generally lay their eggs in moist environments. This is required as www.boundless.com//biology/definition/lateral-undulation. License: CC BY-
eggs lack a shell and will dehydrate quickly in dry environments. SA: Attribution-ShareAlike
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Frogs demonstrate a great diversity of parental behaviors: some Located at: http://cnx.org/content/m44688/latest/?collection=col11448/latest.
species lay many eggs and exhibit little parental care; other species License: CC BY: Attribution
spermatophore. Provided by: Wikipedia. Located at:
carry eggs and tadpoles on their hind legs or backs. The life cycle of en.Wikipedia.org/wiki/spermatophore. License: CC BY-SA: Attribution-
frogs, as with other amphibians, consists of two distinct stages: 1) ShareAlike
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the larval stage followed by 2) metamorphosis to an adult stage. The en.wiktionary.org/wiki/metamorphosis. License: CC BY-SA: Attribution-
larval stage of a frog, the tadpole, is often a filter-feeding herbivore. ShareAlike
Tadpoles usually have gills, a lateral line system, long-finned tails, OpenStax College, Amphibians. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44688/latest/Figure_29_03_01.jpg.
and lack limbs. At the end of the tadpole stage, frogs undergo License: CC BY: Attribution
metamorphosis into the adult form. During this stage, the gills, tail, OpenStax College, Amphibians. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44688/latest...e_29_03_04.jpg.
and lateral line system disappear, and four limbs develop. The jaws License: CC BY: Attribution
become larger and are suited for carnivorous feeding, while the OpenStax College, Amphibians. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44688/latest...e_29_03_02.jpg.
digestive system transforms into the typical short gut of a predator. License: CC BY: Attribution
An eardrum and air-breathing lungs also develop. These changes OpenStax College, Amphibians. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44688/latest...e_29_03_03.jpg.
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adult stage.
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29.3C: EVOLUTION OF AMNIOTES
Modern amniotes, which includes mammals, reptiles, and birds, temporal fenestrae behind each eye. Temporal fenestrae are post-
evolved from an amphibian ancestor approximately 340 million orbital openings in the skull that allow muscles to expand and
years ago. lengthen. Anapsids have no temporal fenestrae, synapsids have one,
and diapsids have two. Anapsids include extinct organisms and may,
 LEARNING OBJECTIVES based on anatomy, include turtles (Testudines), which have an
anapsid-like skull with one opening. However, this is still
Outline the evolution of amniotes controversial, and turtles are sometimes classified as diapsids based
on molecular evidence. The diapsids include birds and all other
KEY POINTS living and extinct reptiles.
Synapsids include all mammals and therapsids, mammal-like
reptiles, from which mammals evolved.
Sauropsids, which are divided into the anapsids and diapsids,
include reptiles and birds.
The diapsids are divided into lepidosaurs (modern lizards,
Figure 29.3C. 1 : Tempral fenestrae: The image illustrates the
snakes, and tuataras) and archosaurs (modern crocodiles and differences in the skulls and temporal fenestrae of anapsids,
alligators, pterosaurs, and dinosaurs). synapsids, and diapsids. Anapsids have no openings, synapsids have
Skull structure and number of temporal fenestrae are the key one opening, and diapsids have two openings.
differences between the synapsids, anapsids, and diapsids; The diapsids diverged into two groups, the Archosauromorpha
anapsids have no temporal fenestrae, synapsids have one, and (“ancient lizard form”) and the Lepidosauromorpha (“scaly lizard
diapsids have two. form”) during the Mesozoic period. The lepidosaurs include modern
Turtle classification is still unclear, but based on molecular lizards, snakes, and tuataras. The archosaurs include modern
evidence, they are sometimes classified under diapsids. crocodiles and alligators, and the extinct pterosaurs (“winged
Although birds are considered distinct from reptiles, they lizard”) and dinosaurs (“terrible lizard”). Clade Dinosauria includes
evolved from a group of dinosaurs, so considering them birds, which evolved from a branch of dinosaurs.
separately from reptiles is not phylogenetically accurate.

KEY TERMS
synapsid: animals that have one opening low in the skull roof
behind each eye; includes all living and extinct mammals and
therapsids
anapsid: amniote whose skull does not have openings near the
temples; includes extinct organisms
diapsid: any of very many reptiles and birds that have a pair of
openings in the skull behind each eye
temporal fenestrae: post-orbital openings in the skull of some
amniotes that allow muscles to expand and lengthen

EVOLUTION OF AMNIOTES Figure 29.3C. 1 : Evolution of amniotes: This chart shows the
The first amniotes evolved from their amphibian ancestors evolution of amniotes. The placement of Testudines (turtles) is
approximately 340 million years ago during the Carboniferous currently still debated.
period. The early amniotes diverged into two main lines soon after In the past, the most common division of amniotes has been into the
the first amniotes arose. The initial split was into synapsids and classes Mammalia, Reptilia, and Aves. Birds are descended,
sauropsids. Synapsids include all mammals, including extinct however, from dinosaurs, so this classical scheme results in groups
mammalian species. Synapsids also include therapsids, which were that are not true clades. Birds are considered as a group distinct from
mammal-like reptiles from which mammals evolved. Sauropsids reptiles with the understanding that this does not completely reflect
include reptiles and birds and can be further divided into anapsids phylogenetic history and relationships.
and diapsids. The key differences between the synapsids, anapsids,
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SECTION OVERVIEW

29.4: REPTILES
29.4C: EVOLUTION OF REPTILES
Topic hierarchy
29.4D: MODERN REPTILES
29.4A: CHARACTERISTICS OF AMNIOTES
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29.4A: CHARACTERISTICS OF AMNIOTES
The distinguishing characteristic of amniotes, a shelled egg with an them from amphibians, which were restricted to moist environments
amniotic membrane, allowed them to venture onto land. due their shell-less eggs. Although the shells of various amniotic
species vary significantly, they all allow retention of water. The
 LEARNING OBJECTIVES shells of bird eggs are composed of calcium carbonate and are hard,
but fragile. The shells of reptile eggs are leathery and require a moist
Discuss the evolution of amniotes environment. Most mammals do not lay eggs (except for
monotremes). Instead, the embryo grows within the mother’s body;
KEY POINTS however, even with this internal gestation, amniotic membranes are
The amniotes include reptiles, birds, and mammals; shared still present.
characteristics between this group include a shelled egg protected The amniotic egg is the key characteristic of amniotes. In amniotes
by amniotic membranes, waterproof skin, and rib ventilation of that lay eggs, the shell of the egg provides protection for the
the lungs. developing embryo while being permeable enough to allow for the
In amniotes, the shell of the egg provides protection for the exchange of carbon dioxide and oxygen. The albumin, or egg white,
developing embryo and allows water retention while still being provides the embryo with water and protein, whereas the fattier egg
permeable to gas exchange. yolk is the energy supply for the embryo, as is the case with the eggs
Amniotic eggs contain albumin, which provides the embryo with of many other animals, such as amphibians. However, the eggs of
water and protein, and an egg yolk that supplies the embryo with amniotes contain three additional extra-embryonic membranes: the
energy. chorion, amnion, and allantois. Extra-embryonic membranes are
The chorion, amnion, and allantois are key membranes found those present in amniotic eggs that are not a part of the body of the
only in amniotic eggs. developing embryo. While the inner amniotic membrane surrounds
The chorion facilitates gas exchange between the embryo and the the embryo itself, the chorion surrounds the embryo and yolk sac.
egg’s external environment. The chorion facilitates exchange of oxygen and carbon dioxide
The amnion protects the embryo from mechanical shock and between the embryo and the egg’s external environment. The
supports hydration, while the allantois stores nitrogenous wastes amnion protects the embryo from mechanical shock and supports
and facilitates respiration. hydration. The allantois stores nitrogenous wastes produced by the
embryo and also facilitates respiration. In mammals, membranes that
KEY TERMS are homologous to the extra-embryonic membranes in eggs are
amnion: the innermost membrane of the fetal membranes of present in the placenta. Additional derived characteristics of
amniotes; the sac in which the embryo is suspended; protects the amniotes include waterproof skin, due to the presence of lipids, and
embryo from shock and carries out hydration costal (rib) ventilation of the lungs.
chorion: allows exchange of oxygen and carbon dioxide between
the embryo and the egg’s external environment
allantois: membrane in an egg that stores nitrogenous wastes
produced by the embryo and facilitates respiration
monotreme: a mammal that lays eggs and has a single urogenital
and digestive orifice; only the echidnas and platypuses

CHARACTERISTICS OF AMNIOTES
The amniotes, reptiles, birds, and mammals, are distinguished from
amphibians by their terrestrially-adapted egg, which is protected by
amniotic membranes. The evolution of amniotic membranes meant
that the embryos of amniotes were now provided with their own Figure 29.4A. 1 : Amniotic eggs: The key features of an amniotic
aquatic environment, which led to less dependence on water for egg are the chorion, amnion, and allantois.
development, allowing the amniotes to branch out into drier
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29.4B: CHARACTERISTICS OF REPTILES
Reptiles are ectothermic tetrapods that lay shelled eggs on land and One of the key adaptations that permitted reptiles to live on land was
possess scaly skin and lungs. the development of their scaly skin which contains the protein
keratin and waxy lipids, reducing water loss from the skin. Due to
 LEARNING OBJECTIVES this occlusive skin, reptiles cannot use their skin for respiration, as
do amphibians; all breathe with lungs.
Summarize the key adaptations of reptiles
Reptiles are ectotherms: animals whose main source of body heat
comes from the environment. This is in contrast to endotherms,
KEY POINTS which use heat produced by metabolism to regulate body
All reptiles, including aquatic ones, lay their eggs on land. temperature. In addition to being ectothermic, reptiles are
Reptiles reproduce sexually through internal fertilization; some categorized as poikilotherms: animals whose body temperatures vary
species are ovoviviparous (lay eggs) and others are viviparous rather than remain stable. Reptiles have behavioral adaptations to
(live birth). help regulate body temperature, such as basking in sunny places to
Because of the development of impermeable, scaly skin, reptiles warm up and finding shady spots or going underground to cool
were able to move onto land since their skin could not be used down. The advantage of ectothermy is that metabolic energy from
for respiration in water. food is not required to heat the body; therefore, reptiles can survive
Reptiles are ectotherms: they depend on their surrounding on about 10 percent of the calories required by a similarly-sized
environment to control their body temperature; this leads to endotherm. In cold weather, some reptiles, such as the garter snake,
advantages, such as not being dependent on metabolic energy brumate. Brumation is similar to hibernation in that the animal
from food for body heat. becomes less active and can go for long periods without eating, but
Reptiles are also poikilotherms: animals whose body differs from hibernation in that brumating reptiles are not asleep or
temperatures vary rather than remain stable. living off fat reserves. Rather, their metabolism is slowed in
Some reptiles go into brumation: a long period during cold response to cold temperatures; the animal becomes very sluggish.
weather that consists of no eating and a decreased metabolism.

KEY TERMS
viviparous: being born alive, as are most mammals, some
reptiles, and a few fish (as opposed to being laid as an egg)
ovoviviparous: a mode of reproduction in animals in which
embryos develop inside eggs that are retained within the
mother’s body until they are ready to hatch
ectotherm: a cold-blooded animal that regulates its body
temperature by exchanging heat with its surroundings

CHARACTERISTICS OF REPTILES
Reptiles are tetrapods. Limbless reptiles (snakes and other
squamates) have vestigial limbs and, as with caecilians, are
classified as tetrapods because they are descended from four-limbed
ancestors. Reptiles lay on land eggs enclosed in shells. Even aquatic
reptiles return to the land to lay eggs. They usually reproduce
sexually with internal fertilization. Some species are ovoviviparous, Figure 29.4B. 1: Ectotherms: Reptiles, such as these sunbathing
with the eggs remaining in the mother’s body until they are ready to Florida redbelly turtles, are ectotherms: they rely on their
environment for body heat.
hatch. Other species are viviparous, with the offspring born alive.
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29.4C: EVOLUTION OF REPTILES
Dinosaurs and pterosaurs diverged from early amniotes and attached to the long, fourth finger of each arm and extended along
dominated the Mesozoic Era. the body to the legs.

 LEARNING OBJECTIVES

Outline the evolution of reptiles

KEY POINTS
Diapsids diverged into archosaurs and lepidosaurs, but these
groups did not dominate the ecosystem until the Triassic
following the Permian extinction.
Archosaurs diverged into the dinosaurs and the pterosaurs about
250 million years ago.
Pterosaurs had the ability to fly because of their wings and
hollow bones, a trait convergent to modern birds, but were not
ancestral to birds.
Dinosaurs were quadrupeds or bipeds, carnivorous or
herbivorous, and laid eggs.
It is unknown whether dinosaurs were endothermic or Figure 29.4C. 1 : Pterosaurs: Pterosaurs, which existed from the late
Triassic to the Cretaceous period (210 to 65.5 million years ago),
ectothermic, but since birds are endothermic, the dinosaur possessed wings, but are not believed to have been capable of
ancestors of birds were probably endothermic. powered flight. Instead, they may have been able to soar after
Dinosaurs dominated the Mesozoic Era until the Cretaceous - launching from cliffs.
Tertiary extinction wiped out most of these large-bodied animals. The dinosaurs were a diverse group of terrestrial reptiles with more
than 1,000 species identified to date. Paleontologists continue to
KEY TERMS discover new species of dinosaurs. Some dinosaurs were
pterosaur: any of several extinct flying reptiles, of the order quadrupeds; others were bipeds. Some were carnivorous, whereas
Pterosauria, including the pterodactyls others were herbivorous. Dinosaurs laid eggs; a number of nests
Cretaceous-Tertiary extinction: mass extinction of three- containing fossilized eggs have been found. It is not known whether
quarters of plant and animal species on earth, including all non- dinosaurs were endotherms or ectotherms. However, given that
avian dinosaurs, that occurred over a geologically-short period of modern birds are endothermic, the dinosaurs that served as ancestors
time 66 million years ago to birds were probably endothermic as well. Some fossil evidence
exists for dinosaurian parental care. Comparative biology supports
EVOLUTION OF REPTILES this hypothesis since the archosaur birds and crocodilians display
Reptiles originated approximately 300 million years ago during the parental care.
Carboniferous period. One of the oldest-known amniotes is
Casineria, which had both amphibian and reptilian characteristics.
One of the earliest undisputed reptiles was Hylonomus. Soon after
the first amniotes appeared, they diverged into three groups
(synapsids, anapsids, and diapsids) during the Permian period. The
Permian period also saw a second major divergence of diapsid
reptiles into archosaurs (predecessors of crocodilians and dinosaurs)
and lepidosaurs (predecessors of snakes and lizards). These groups
remained inconspicuous until the Triassic period when the
archosaurs became the dominant terrestrial group due to the
extinction of large-bodied anapsids and synapsids during the
Permian-Triassic extinction. About 250 million years ago,
Figure 29.4C. 1 : Quadruped dinosaurs: Edmontonia, an example of
archosaurs radiated into the dinosaurs and the pterosaurs.
an extinct quadruped reptile, was an armored dinosaur that lived in
Although they are sometimes mistakenly called dinosaurs, the the late Cretaceous period, 145.5 to 65.6 million years ago.
pterosaurs were distinct from true dinosaurs. Pterosaurs had a Dinosaurs dominated the Mesozoic Era, which was known as the
number of adaptations that allowed for flight, including hollow “Age of Reptiles.” The dominance of dinosaurs lasted until the end
bones (birds also exhibit hollow bones, a case of convergent of the Cretaceous period, the end of the Mesozoic Era. The
evolution). Their wings were formed by membranes of skin that Cretaceous-Tertiary extinction resulted in the loss of most of the

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29.4D: MODERN REPTILES
Class Reptilia, amniotes that are neither mammals nor birds, has unidirectional system of air flow around the lungs; however, in
four living clades: Crocodilia, Sphenodontia, Squamata, and contrast to birds, they are ectotherms, as are all other reptiles.
Testudine. Crocodilians live throughout the tropics and subtropics of Africa,
South America, Southern Florida, Asia, and Australia. They are
 LEARNING OBJECTIVES found in freshwater, saltwater, and brackish habitats, such as rivers
and lakes; they spend most of their time in water. Some species are
Differentiate among the types of modern reptiles able to move on land due to their semi-erect posture.

KEY POINTS
Reptile are amniotes that lay their eggs on land; they have scales
or scutes and are ectothermic.
Crocodilia includes the alligators, crocodiles, and caimans; they
are mostly aquatic species, but some are able to move on land
because of their semi-erect posture.
Tuataras are classified as the only group under Sphenodontia;
they may be lizard-like, but skull and jaw differences set them
apart from true lizards.
Squamata, the largest group of reptiles, includes the lizards and
Figure 29.4D. 1 : Crocodilians: Crocodilians, such as this Siamese
snakes. crocodile (Crocodylus siamensis), have large flattened snouts and
Snakes, which lack the eyelids and external ears present in thick skin.
lizards, are believed to have evolved from burrowing or aquatic
lizards. SPHENODONTIA
Turtles are grouped under the Testudines; species in this group Sphenodontia (“wedge tooth”) arose in the Mesozoic era and
all have bony or cartilaginous shells. includes only one living genus, Tuatara, which comprises two
species that are found in New Zealand. Tuataras measure up to 80
KEY TERMS centimeters and weigh about 1 kilogram. Although quite lizard-like
scute: a horny, chitinous, or bony external plate or scale, as on in gross appearance, several unique features of the skull and jaws
the shell of a turtle or the skin of crocodiles clearly define them and distinguish the group from the squamates.
plastron: the nearly flat part of the shell structure of a tortoise or Their dentition, in which two rows of teeth in the upper jaw overlap
other animal, similar in composition to the carapace one row on the lower jaw, is unique among living species. They are
amniote: a group of vertebrates having an amnion during the also unusual in having a pronounced photoreceptive eye, dubbed the
development of the embryo; mammals, birds, and reptiles “third eye”, whose current function is a subject of ongoing research,
but is thought to be involved in setting circadian and seasonal
MODERN REPTILES cycles.
Class Reptilia includes many diverse species that are classified into
four living clades. These are the 25 species of Crocodilia, 2 species
of Sphenodontia, approximately 9,200 Squamata species, and the
Testudines, with about 325 species. A reptile is any amniote (a
tetrapod whose egg has an additional membrane, originally to allow
them to lay eggs on land) that is neither a mammal nor a bird. Unlike
mammals, birds, and certain extinct reptiles, living reptiles have
scales or scutes (rather than fur or feathers) and are cold-blooded.
Modern reptiles inhabit every continent with the exception of
Antarctica.
Figure 29.4D. 1 : Tuatara: Tuataras may resemble a lizard but
CROCODILIA belongs to a distinct lineage, the Sphenodontidae family.
Crocodilia (“small lizard”) arose with a distinct lineage by the
middle Triassic; extant species include alligators, crocodiles, and SQUAMATA
caimans. Crocodilians are large, solidly built lizard-like reptiles with Squamata (“scaly”) arose in the late Permian; extant species include
long flattened snouts and laterally-compressed tails, and eyes, ears, lizards and snakes. They are most closely-related to tuataras; both
and nostrils at the top of the head. Their skin is thick and covered in groups evolved from a lepidosaurian ancestor. Squamata is the
non-overlapping scales. They have conical, peg-like teeth and a largest extant clade of reptiles. Most lizards differ from snakes by
powerful bite. As with birds, they have a four-chambered heart and a having four limbs, although these have been variously lost or

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significantly reduced in at least 60 lineages. Snakes lack eyelids and
external ears, which are present in lizards. Lizard species range in
size from chameleons and geckos, which are a few centimeters in
length, to the Komodo dragon, which is about 3 meters in length.
Most lizards are carnivorous, but some large species, such as
iguanas, are herbivores.

Figure 29.4D. 1 : Testudines: Testudines include all shelled reptiles,

such as the African spurred tortoise (Geochelone sulcata) that lives
at the southern edge of the Sahara Desert. It is the third-largest
tortoise in the world.
Turtles arose approximately 200 million years ago, predating
Figure 29.4D. 1 : Squamata: The Squamata, which includes lizards
and snakes, are the largest group of reptiles. The garter snake crocodiles, lizards, and snakes. Similar to other reptiles, turtles are
belongs to the genus Thamnophis, the most widely-distributed ectotherms. They lay eggs on land, although many species live in or
reptile genus in North America. near water. None exhibit parental care. Turtles range in size from the
Snakes are thought to have descended from either burrowing lizards speckled padloper tortoise at 8 centimeters (3.1 inches) to the
or aquatic lizards over 100 million years ago. Snakes comprise about leatherback sea turtle at 200 centimeters (over 6 feet). The term
3,000 species and range in size from 10 centimeter-long thread “turtle” is sometimes used to describe only those species of
snakes to 10 meter-long pythons and anacondas. All snakes are Testudines that live in the sea, with the terms “tortoise” and
carnivorous, eating small animals, birds, eggs, fish, and insects. “terrapin” used to refer to species that live on land and in fresh
Although variations exist, most snakes have a skull that is very water, respectively.
flexible, involving eight rotational joints. They also differ from other
squamates by having mandibles (lower jaws) without either bony or CONTRIBUTIONS AND ATTRIBUTIONS
ligamentous attachment anteriorly. Having this connection via skin OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
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TESTUDINES Boundless. Provided by: Boundless Learning. Located at:
Turtles are members of the clade Testudines (“having a shell”). www.boundless.com//biology/definition/chorion. License: CC BY-SA:
Attribution-ShareAlike
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differentiate species of turtles. The two clades of turtles are most License: CC BY-SA: Attribution-ShareAlike
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group, which includes all North American species, retracts its neck Located at: http://cnx.org/content/m44689/latest...e_29_04_01.png. License:
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viviparous. Provided by: Wiktionary. Located at: Reptiles. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Reptiles.
en.wiktionary.org/wiki/viviparous. License: CC BY-SA: Attribution- License: CC BY-SA: Attribution-ShareAlike
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ovoviviparous. Provided by: Wikipedia. Located at: License: CC BY-SA: Attribution-ShareAlike
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SECTION OVERVIEW

29.5: BIRDS
29.5B: EVOLUTION OF BIRDS
Topic hierarchy
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29.5A: CHARACTERISTICS OF BIRDS authored, remixed, and/or curated by Boundless.

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29.5A: CHARACTERISTICS OF BIRDS
Birds are warm-blooded animals with wings having several wing and provide thrust. Secondary feathers are located closer to the
adaptations to flight, although not all species can fly. body, attach to the forearm portion of the wing, and provide lift.
Contour feathers are those found on the body. They help reduce drag
 LEARNING OBJECTIVES produced by wind resistance during flight, creating a smooth,
aerodynamic surface allowing air to flow smoothly over the bird’s
Summarize the derived characteristics of birds body for efficient flight.

KEY POINTS
Birds have down feathers that provide insulation and two types
of flight feathers found on the wings: thrust-producing primary
feathers at the tip of the wing and lift-providing secondary
feathers closer to the body.
Contour feathers found on the body create a smooth,
aerodynamic surface.
The chest muscles of birds are highly developed as they are
responsible for the flapping of the entire wing.
The two clavicles of birds are fused, forming the furcula or
wishbone, which is both flexible and strong enough to support to
the shoulder girdle during flapping.
In order to keep body weight low, birds have pneumatic bones,
no urinary bladders, and usually only one ovary.
Birds have developed an efficient respiratory system using air
sacs and unidirectional airflow and a cross-current exchange
system with the blood.

KEY TERMS
pneumatic: having cavities filled with air
Figure 29.5A. 1 : Bird feathers: Primary feathers are located at the
endothermic: an animal whose body temperature is regulated by wing tip and provide thrust; secondary feathers are located close to
internal factors the body and provide lift.
furcula: the forked bone formed by the fusion of the clavicles in Flapping of the entire wing occurs primarily through the actions of
birds; the wishbone the chest muscles: the pectoralis and the supracoracoideus. These
cloaca: the common duct in fish, reptiles, birds, and some muscles, highly developed in birds and accounting for a higher
primitive mammals that serves as the anus as well as the genital percentage of body mass than in most mammals, attach to a blade-
opening shaped keel, similar to that of a boat, located on the sternum. The
sternum of birds is larger than that of other vertebrates, which
CHARACTERISTICS OF BIRDS accommodates the large muscles required to generate enough
Birds are endothermic and, because they fly, they require large upward force to generate lift with the flapping of the wings. Another
amounts of energy, necessitating a high metabolic rate. As with skeletal modification found in most birds is the fusion of the two
mammals, which are also endothermic, birds have an insulating clavicles (collarbones), forming the furcula or wishbone. The furcula
covering that keeps heat in the body: feathers. Specialized feathers is flexible enough to bend and provide support to the shoulder girdle
called down feathers are especially insulating, trapping air in spaces during flapping.
between each feather to decrease the rate of heat loss. Certain parts
An important requirement of flight is a low body weight. As body
of a bird’s body are covered in down feathers; the base of other
weight increases, the muscle output required for flying increases.
feathers have a downy portion, while newly-hatched birds are
The largest living bird is the ostrich. While it is much smaller than
covered in down.
the largest mammals, it is flightless. For birds that do fly, reduction
Feathers not only act as insulation, but also allow for flight, enabling in body weight makes flight easier. Several modifications are found
the lift and thrust necessary to become airborne. The feathers on a in birds to reduce body weight, including pneumatization of bones.
wing are flexible, so the collective feathers move and separate as air Pneumatic bones are hollow rather than filled with tissue. They
moves through them, reducing the drag on the wing. Flight feathers contain air spaces that are sometimes connected to air sacs and they
are asymmetrical, which affects airflow over them and provides have struts of bone to provide structural reinforcement. Pneumatic
some of the lifting and thrusting force required for flight. Two types bones are not found in all birds; they are more extensive in large
of flight feathers are found on the wings: primary feathers and birds than in small birds. Not all bones of the skeleton are
secondary feathers. Primary feathers are located at the tip of the pneumatic, although the skulls of almost all birds are.

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 direction. Air sacs allow for this unidirectional airflow, which also
creates a cross-current exchange system with the blood. In a cross-
current or counter-current system, the air flows in one direction and
the blood flows in the opposite direction, creating a very efficient
means of gas exchange.

Figure 29.5A. 1 : Pneumatic bones of birds: Many birds have hollow,

pneumatic bones, which make flight easier.
Other modifications that reduce weight include the lack of a urinary
bladder. Birds possess a cloaca: a structure that allows water to be
reabsorbed from waste back into the bloodstream. Uric acid is not
expelled as a liquid, but is concentrated into urate salts, which are
expelled along with fecal matter. In this way, water is not held in the
Figure 29.5A. 1 : Avian respiration: Avian respiration is an efficient
urinary bladder, which would increase body weight. Most bird system of gas exchange with air flowing unidirectionally. During
species possess only one ovary rather than two, further reducing inhalation, air passes from the trachea into posterior air sacs, then
body mass. through the lungs to anterior air sacs. The air sacs are connected to
the hollow interior of bones. During exhalation, air from air sacs
The air sacs that extend into bones, making them pneumatic, also passes into the lungs and out the trachea.
join with the lungs and function in respiration. In contrast to
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29.5B: EVOLUTION OF BIRDS
Modern birds evolved from Saurichia, one of two subgroups of characteristics of both dinosaurs and birds. Some scientists propose
dinosaurs, although it is unclear how flight and/or endothermy arose classifying it as a bird, but others prefer to classify it as a dinosaur.
in birds. The fossilized skeleton of Archaeopteryx looks like that of a
dinosaur. It had teeth and birds do not, but it also had feathers
 LEARNING OBJECTIVES modified for flight, a trait associated only with birds among modern
animals. Fossils of older, feathered dinosaurs exist, but the feathers
Explain the evolution of birds do not have the characteristics of flight feathers.

KEY POINTS
Birds have two fenestrations, or openings, in their skulls making
them diapsids like crocodiles and dinosaurs.
Birds did not descend from bird-like dinosaurs (Ornithischia),
but rather from a divergent group of lizard-like dinosaurs
(Saurischia) called theropods, which were bipedal predators.
A Jurassic period fossil intermediate to dinosaurs and birds is
Archaeopteryx, which had teeth like dinosaurs, and feathers
modified for flight.
The arboreal (“tree”) hypothesis and the terrestrial (“land”) Figure 29.5B. 1: Bird fossils: (a) Archaeopteryx lived in the late
Jurassic Period around 150 million years ago. It had teeth like a
hypothesis are two theories on how flight evolved; these theories dinosaur, but had (b) flight feathers like modern birds, which can be
propose that wings developed to aid in jumping from branch to seen in this fossil.
branch or to aid in running, respectively. It is still unclear exactly how flight evolved in birds. Two main
It was not until after the extinction of Enantiornithes (a separate theories exist: the arboreal (“tree”) hypothesis and the terrestrial
evolutionary line of bird-like animals) during the Cretaceous (“land”) hypothesis. The arboreal hypothesis posits that tree-
period that the Ornithurae (the evolutionary line of modern birds) dwelling precursors to modern birds jumped from branch to branch
became dominant. and prospered. using their feathers for gliding before becoming fully capable of
flapping flight. In contrast to this, the terrestrial hypothesis holds
KEY TERMS that running was the stimulus for flight, as wings could be used to
diapsid: any of very many reptiles and birds that have a pair of improve running and then became used for flapping flight. As with
openings in the skull behind each eye the question of how flight evolved, the question of how endothermy
Archaeopteryx: a taxonomic genus within the family evolved in birds still is unanswered. Feathers provide insulation, but
Archaeopterygidae, known from fossils and widely accepted as this is only beneficial if body heat is being produced internally.
the earliest and most primitive known bird Similarly, internal heat production is only viable if insulation is
fenestration: an opening in the surface of an organ, etc. present to retain that heat. It has been suggested that one or the other
(feathers or endothermy) evolved in response to some other selective
EVOLUTION OF BIRDS pressure.
The evolutionary history of birds is still somewhat unclear. Due to
During the Cretaceous period, a group known as the Enantiornithes
the fragility of bird bones, they do not fossilize as well as other
was the dominant bird type. Enantiornithes means “opposite birds,”
vertebrates. Birds are diapsids, meaning they have two fenestrations,
which refers to the fact that certain bones of the feet are joined
or openings, in their skulls. Birds belong to a group of diapsids
differently than the way the bones are joined in modern birds. These
called the archosaurs, which also includes crocodiles and dinosaurs.
birds formed an evolutionary line separate from modern birds; they
It is commonly accepted that birds evolved from dinosaurs.
did not survive past the Cretaceous. Along with the Enantiornithes,
Dinosaurs were subdivided into two groups, the Saurischia (“lizard Ornithurae birds (the evolutionary line that includes modern birds)
like”) and the Ornithischia (“bird like”). Despite the names of these were also present in the Cretaceous. After the extinction of
groups, it was not the bird-like dinosaurs that gave rise to modern Enantiornithes, modern birds became the dominant bird, with a large
birds. Rather, Saurischia diverged into two groups. One included the radiation occurring during the Cenozoic Era. Referred to as
long-necked herbivorous dinosaurs, such as Apatosaurus. The Neornithes (“new birds”), modern birds are now classified into two
second group, bipedal predators called theropods, includes the groups, the Paleognathae (“old jaw”) or ratites (a group of flightless
ancestors of modern birds. This course of evolution is suggested by birds including ostriches, emus, rheas, and kiwis) and the
similarities between theropod fossils and birds, specifically in the Neognathae (“new jaw”), all other birds.
structure of the hip and wrist bones, as well as the presence of the
wishbone, formed by the fusing of the clavicles.
One important fossil of an animal intermediate to dinosaurs and
birds is Archaeopteryx, which is from the Jurassic period and has

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SECTION OVERVIEW

29.6: MAMMALS
29.6B: EVOLUTION OF MAMMALS
Topic hierarchy
29.6C: LIVING MAMMALS
29.6A: CHARACTERISTICS OF MAMMALS
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29.6A: CHARACTERISTICS OF MAMMALS
Mammalian traits include, among others: specialized glands,
modified jaw and inner ear bones, urinary bladder, and hair.

 LEARNING OBJECTIVES

Summarize the distinguishing characteristics of mammals

KEY POINTS
The various traits which are used to define mammals include: the
presence of hair; the integument system which contains
specialized secretory glands; the skeletal and muscular systems;
the heart and brain structure.
Mammals contain specialized glands which have various
functions: secretion of chemical compounds used for
Figure 29.6A. 1 : Mammalian fur as insulation: Polar bears use their
communication; glands that produce milk; glands that produce fur for warmth. While their skin is black, their transparent fur
perspiration used for thermoregulation; and glands that produce appears white, providing camouflage while hunting and serving as
sebum, which is used for lubrication. protection by hiding cubs in the snow.
Mammals have four-chambered hearts that are defined by the Mammalian integument, or skin, includes secretory glands with
ability to regulate the heart beat with the presence of specialized various functions. Sebaceous glands produce a lipid mixture called
pacemaker cells. sebum that is secreted onto the hair and skin for water resistance and
A mammal’s hair has many purposes, including insulation, lubrication. Sebaceous glands are located over most of the body.
sensory perception, protective coloration, and social signaling. Eccrine glands produce sweat, or perspiration, which is mainly
Mammals possess many unique skeletal structures including a composed of water. In most mammals, eccrine glands are limited to
single lower jaw bone that joins the skull at the squamosal bone certain areas of the body; some mammals do not possess them at all.
and three bones in the inner ear. However, in primates, especially humans, sweat figures prominently
in thermoregulation, regulating the body through evaporative
KEY TERMS cooling. Sweat glands are located over most of the body surface in
vibrissa: any of the tactile whiskers on the nose of an animal primates. Apocrine glands, or scent glands, secrete substances that
sebum: a thick oily substance, secreted by the sebaceous glands are used for chemical communication, such as in skunks. Mammary
of the skin, that consists of fat, keratin and cellular debris glands produce milk that is used to feed newborns. While male
diphyodont: having two successive sets of teeth (deciduous and monotremes and eutherians possess mammary glands, male
permanent), one succeeding the other marsupials do not. Mammary glands are probably modified
sinoatrial node: the impulse-generating (pacemaker) tissue sebaceous or eccrine glands, but their evolutionary origin is not
located in the right atrium of the heart, and thus the generator of entirely clear.
normal sinus rhythm The skeletal system of mammals possesses many unique features.
integument: an outer protective covering such as the feathers or The lower jaw of mammals consists of only one bone, the dentary.
skin of an animal, a rind or shell The jaws of other vertebrates are composed of more than one bone.
In mammals, the dentary bone joins the skull at the squamosal bone,
CHARACTERISTICS OF MAMMALS
while in other vertebrates, the quadrate bone of the jaw joins with
The presence of hair is one of the most obvious traits of a mammal. the articular bone of the skull. These bones are present in mammals,
Although it is not very extensive on certain species, such as whales, but they have been modified to function in hearing and form bones
hair has many important functions for mammals. Mammals are in the middle ear. Other vertebrates possess only one middle ear
endothermic so hair provides insulation to retain heat generated by bone, the stapes. Mammals have three: the malleus, incus, and
metabolic work by trapping a layer of air close to the body. Along stapes. The malleus originated from the articular bone, whereas the
with insulation, hair can serve as a sensory mechanism via incus originated from the quadrate bone. This arrangement of jaw
specialized hairs called vibrissae, better known as whiskers. These and ear bones aids in distinguishing fossil mammals from fossils of
attach to nerves that transmit information about sensation, which is other synapsids.
particularly useful to nocturnal or burrowing mammals. Hair can
also provide protective coloration or be part of social signaling, such
as when an animal’s hair stands “on end. ”

29.6A.1 https://bio.libretexts.org/@go/page/13936
 Upper body

Left lung
Right lung

Lower body

Figure 29.6A. 1 : Bones of the mammalian inner ear: Bones of the Figure 29.6A. 1 : Mammalian heart: Mammals possess a four-
mammalian inner ear are modified from bones of the jaw and skull. chambered heart, with two atria and two ventricles, that circulates
The adductor muscle that closes the jaw is composed of two muscles blood through the body.
in mammals: the temporalis and the masseter. These allow side-to- The kidneys of mammals have a portion of the nephron called the
side movement of the jaw, making chewing possible, which is loop of Henle or nephritic loop, which allows mammals to produce
unique to mammals. Most mammals have heterodont teeth, meaning urine with a high concentration of solutes; higher than that of the
that they have different types and shapes of teeth rather than just one blood. Mammals lack a renal portal system: a system of veins that
type and shape of tooth. Most mammals are diphyodonts, meaning moves blood from the hind or lower limbs and region of the tail to
that they have two sets of teeth in their lifetime: deciduous, or the kidneys. Renal portal systems are present in all other vertebrates
“baby” teeth, and permanent teeth. Other vertebrates are except jawless fishes. A urinary bladder is present in all mammals.
polyphyodonts: their teeth are replaced throughout their entire life. Mammalian brains have certain characteristics that differ from other
Mammals, like birds, possess a four-chambered heart. Mammals vertebrates. In some, but not all mammals, the cerebral cortex, the
also have a specialized group of cardiac fibers located in the walls of outermost part of the cerebrum, is highly folded, allowing for a
their right atrium called the sinoatrial node, or pacemaker, which greater surface area than is possible with a smooth cortex. The optic
determines the rate at which the heart beats. As for blood, lobes, located in the midbrain, are divided into two parts in
mammalian erythrocytes (red blood cells) do not have nuclei, mammals, whereas other vertebrates possess a single, undivided
whereas the erythrocytes of other vertebrates are nucleated. lobe. Eutherian mammals also possess a specialized structure that
links the two cerebral hemispheres, called the corpus callosum.

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29.6B: EVOLUTION OF MAMMALS
The modern mammals of today are synapsids: descendants of a
group called cynodonts which appeared in the Late Permian period.

 LEARNING OBJECTIVES

Outline the evolution of mammals

KEY POINTS
Synapsids are defined by a single opening in the skull and the
fact that they are endothermic.
Mammals are the only living synapsids, derived from a lineage
Figure 29.6B. 1: Cynodonts: Cynodonts, which first appeared in the
in the Jurassic period. Late Permian period 260 million years ago, are thought to be the
Two groups of mammals include the eutherians, which are ancestors of modern mammals.
closely related to placentals and the metatherians, which are Since Juramaia, the earliest-known eutherian, lived 160 million
more closely related to the marsupials. years ago in the Jurassic, this divergence must have occurred in the
Mammalian lineages from the Jurassic include Dryolestes, same period. After the Cretaceous–Paleogene extinction event wiped
related to placentals and marsupials, and Ambondro, related to out the non-avian dinosaurs (birds are generally regarded as the
monotremes. surviving dinosaurs) and several other mammalian groups, placental
Later synapsids had specialized structures for chewing, including and marsupial mammals diversified into many new forms and
teeth, cheeks that can hold food, and a secondary palate, which ecological niches throughout the Paleogene and Neogene, by the end
gave them the ability to chew and breathe at the same time. of which all modern orders had appeared.
The synapsid lineage became distinct from the sauropsid lineage in
KEY TERMS
the late Carboniferous period, between 320 and 315 million years
eutherian: the mammals more closely related to animals like
ago. The sauropsids are today’s reptiles and birds, along with all the
humans and rodents than to marsupials
extinct animals more closely related to them than to mammals. This
metatherian: belonging or pertaining to the infraclass
does not include the mammal-like reptiles, a group more closely
Metatheria of marsupials
related to the mammals. Throughout the Permian period, the
EVOLUTION OF MAMMALS synapsids included the dominant carnivores and several important
herbivores. In the subsequent Triassic period, however, a previously-
The evolution of mammals passed through many stages since the
obscure group of sauropsids, the archosaurs, became the dominant
first appearance of their synapsid ancestors in the late Carboniferous
vertebrates. The mammaliaforms appeared during this period; their
period. Mammals are synapsids: they have a single opening in the
superior sense of smell, backed up by a large brain, facilitated entry
skull. They are the only living synapsids as earlier forms became
into nocturnal niches with less exposure to archosaur predation. The
extinct by the Jurassic period. The early, non-mammalian synapsids
nocturnal lifestyle may have contributed greatly to the development
can be divided into two groups: the pelycosaurs and the therapsids.
of mammalian traits such as endothermy and hair. Later in the
Within the therapsids, a group called the cynodonts are thought to be
Mesozoic, after theropod dinosaurs replaced rauisuchians as the
the ancestors of mammals. By the mid-Triassic, there were many
dominant carnivores, mammals spread into other ecological niches.
synapsid species that looked like mammals. The lineage leading to
For example, some became aquatic, some were gliders, and some
today’s mammals split in the Jurassic. Synapsids from this period
even fed on juvenile dinosaurs. Most of the evidence consists of
include Dryolestes (more closely related to extant placentals and
fossils. For many years, fossils of Mesozoic mammals and their
marsupials than to monotremes) as well as Ambondro (more closely
immediate ancestors were very rare and fragmentary; however, since
related to monotremes). Later, the eutherian and metatherian
the mid-1990s, there have been many important new finds,
lineages separated. Metatherians are the animals more closely
especially in China. The relatively new techniques of molecular
related to the marsupials, while eutherians are those more closely
phylogenetics have also shed light on some aspects of mammalian
related to the placentals. Eutherians are distinguished from
evolution by estimating the timing of important divergence points
noneutherians by various features of the feet, ankles, jaws, and teeth.
for modern species. When used carefully, these techniques often, but
One of the major differences between placental and nonplacental
not always, agree with the fossil record. Although mammary glands
eutherians is that placentals lack epipubic bones, which are present
are a signature feature of modern mammals, little is known about the
in all other fossil and living mammals (marsupials and monotremes).
evolution of lactation. This is because these soft tissues are not often
preserved in the fossil record. Most study of the evolution of
mammals centers, rather, around the shapes of the teeth, the hardest
parts of the tetrapod body. Other much-studied aspects include the

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evolution of the middle ear bones, erect limb posture, a bony of the mouth where chewing occurs from the area above where
secondary palate, fur and hair, and warm-bloodedness. respiration occurs, allowing breathing to proceed uninterrupted
A key characteristic of synapsids is endothermy, rather than the during chewing. A secondary palate is not found in pelycosaurs, but
ectothermy seen in most other vertebrates. The increased metabolic is present in cynodonts and mammals. The jawbone also shows
rate required to internally-modify body temperature went hand-in- changes from early synapsids to later ones. The zygomatic arch, or
hand with changes to certain skeletal structures. The later synapsids, cheekbone, is present in mammals and advanced therapsids such as
which had more-evolved characteristics unique to mammals, possess cynodonts, but is not present in pelycosaurs. The presence of the
cheeks for holding food and heterodont teeth (specialized for zygomatic arch suggests the presence of the masseter muscle, which
chewing by mechanically breaking down food to speed digestion closes the jaw and functions in chewing.
and releasing the energy needed to produce heat). Chewing also
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29.6C: LIVING MAMMALS
includes humans. Eutherian mammals are sometimes called
 LEARNING OBJECTIVES placental mammals because all species possess a complex placenta
that connects a fetus to the mother, allowing for gas, fluid, and
Name and describe the distinguishing features of the three
nutrient exchange. While other mammals possess a less complex
main groups of mammals
placenta or briefly have a placenta, all eutherians possess a complex
placenta during gestation.
LIVING MAMMALS
Living mammals can be classified into three major classes:
eutherians, monotremes, and metatherians. The eutherians, or
placental mammals, and the metatherians, or marsupials, together
comprise the clade of therian mammals. Monotremes form their
sister clade.
There are three living species of monotremes: the platypus and two
species of echidnas, or spiny anteaters. The leathery-beaked platypus
belongs to the family Ornithorhynchidae (“bird beak”), whereas
echidnas belong to the family Tachyglossidae (“sticky tongue”). The
platypus and one species of echidna are found in Australia; the other
species of echidna is found in New Guinea. Monotremes are unique
among mammals as they lay eggs rather than giving birth to live
young. The shells of their eggs are not like the hard shells of birds,
but are leathery, similar to the shells of reptile eggs. All monotremes Figure 29.6C. 1 : A red fox: Red foxes are eutherian (placental)
possess a cloaca which serves as the opening for the intestinal, mammals because the mothers nourish their young via a placenta
during fetal development. The placenta enables a mother to
reproductive, and urinary tracts. Additionally, monotremes have no exchange gases, fluids, and nutrients with the growing embryos.
teeth.
Marsupials are found primarily in Australia,although the opossum is
KEY POINTS
found in North America. Australian marsupials include the Monotremes include the platypus which are defined by their
kangaroo, koala, bandicoot,Tasmanian devil, and several other ability to lay eggs instead of giving birth to live young.
species. Most species of marsupials possess a pouch in which the Metatherians are classified as the marsupials which possess a
very premature young reside after birth, receiving milk and pouch where the premature young reside and nurse while
continuing to develop. Marsupials differ from eutherians in that continuing to develop.
there is a less complex placental connection. The young are born at Eutherians are the most common type of mammal and are
an extremely early age and latch onto the nipple within the pouch. defined by the presence of a complex placenta which connects
the developing fetus to the mother during gestation.

KEY TERMS
marsupial: a mammal of which the female has a pouch in which
it rears its young, which are born immature, through early
infancy
placental: a mammal having a placenta; most members of
Mammalia

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SECTION OVERVIEW

29.7: THE EVOLUTION OF PRIMATES

29.7B: EARLY HUMAN EVOLUTION
Topic hierarchy
29.7C: EARLY HOMININS
29.7A: CHARACTERISTICS AND EVOLUTION OF 29.7D: GENUS HOMO
PRIMATES

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29.7A: CHARACTERISTICS AND EVOLUTION OF PRIMATES
All primates exhibit adaptations for climbing trees and have evolved some anthropoid-like features. Anthropoids include monkeys, apes,
into two main groups: Prosimians and Anthropoids. and humans. In general, prosimians tend to be nocturnal (in contrast
to diurnal anthropoids, excluding the nocturnal Aotus, owl monkey)
 LEARNING OBJECTIVES and have a smaller brain/body ratio than anthropoids.

Identify key characteristics of primates EVOLUTION OF PRIMATES

The first primate-like mammals are referred to as proto-primates.
KEY POINTS They were roughly similar to squirrels and tree shrews in size and
All primates are descended from tree-dwellers, exhibiting appearance. The existing fossil evidence (mostly from North Africa)
adaptations which allow for tree climbing that include: a rotating is very fragmentary. These proto-primates will remain largely
shoulder joint, separated big toes and thumb for grasping, and mysterious creatures until more fossil evidence becomes available.
stereoscopic vision. The oldest known primate-like mammal with a relatively robust
Other primate characteristics include: having one offspring per fossil record is Plesiadapis (although some researchers do not agree
pregnancy, claws evolved into flattened nails; and larger that Plesiadapis was a proto-primate). Fossils of this primate have
brain/body ratio than other mammals, and tendency to hold body been dated to approximately 55 million years ago. Plesiadapiforms
upright. had some features of the teeth and skeleton in common with true
True primates, ancestral to prosimians, first appear in the fossil primates. They were found in North America and Europe in the
record in the Eocene epoch around 55 million years ago; they Cenozoic, going extinct by the end of the Eocene.
were similar in form to lemurs. The first true primates were found in North America, Europe, Asia,
Anthropoids ancestral to both Old World and New World and Africa in the Eocene Epoch. These early primates resembled
monkeys appear in the fossil record in the Oligocene epoch present-day prosimians such as lemurs. Evolutionary changes
around 35 million years ago. continued in these early primates, with larger brains and eyes, and
Anthropoids ancestral to apes appear in the Miocene epoch smaller muzzles being the trend. By the end of the Eocene Epoch,
around 25 million years ago. many of the early prosimian species went extinct due either to cooler
Apes are divided into two main groups of hominoids: lesser apes temperatures or competition from the first monkeys.
or hylobatids (gibbons and siamangs) and great apes (Pongo: Evidence shows that the anthropoid monkeys evolved from
orangutans, Gorilla: gorillas, Pan:chimpanzees, and Homo: prosimians during the Oligocene Epoch. By 35 million years ago,
humans). evidence indicates that monkeys were present the Old World (Africa
and Asia) and in the New World (South America) by 30 million
KEY TERMS
years ago. New World monkeys are also called Platyrrhini: a
dimorphism: the occurrence in an animal species of two distinct reference to their broad noses. Old World monkeys (and apes) are
types of individual called Catarrhini: a reference to their narrow noses. There is still
adaptive radiation: the diversification of species into separate quite a bit of uncertainty about the origins of the New World
forms that each adapt to occupy a specific environmental niche monkeys. At the time the platyrrhines arose, the continents of South
American and Africa had drifted apart. Therefore, it is thought that
CHARACTERISTICS OF PRIMATES
monkeys arose in the Old World and reached the New World by
All primate species possess adaptations for climbing trees, as they drifting on log rafts or mangrove floating ‘islands’. Due to this
all descended from tree-dwellers. This arboreal heritage of primates
reproductive isolation, New World monkeys and Old World
has resulted in adaptations that include, but are not limited to: 1) a monkeys underwent separate adaptive radiations over millions of
rotating shoulder joint; 2) a big toe that is widely separated from the
years. The New World monkeys are all arboreal, whereas Old World
other toes and thumbs, that are widely separated from fingers
monkeys include arboreal and ground-dwelling species.
(except humans), which allow for gripping branches; and 3)
stereoscopic vision, two overlapping fields of vision from the eyes,
which allows for the perception of depth and gauging distance.
Other characteristics of primates are brains that are larger than those
of most other mammals (larger brain/body ratio than similar-sized
non-primates), claws that have been modified into flattened nails,
typically only one offspring per pregnancy, and a trend toward
holding the body upright.
The Order Primates is divided into two groups: prosimians and
anthropoids. Prosimians include the bush babies and pottos of
Africa, the lemurs of Madagascar, and the lorises of Southeast Asia.
Tarsier, also from Southeast Asia, show some prosimian-like and

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 The apes are divided into two groups. The lesser apes comprise the
family Hylobatidae, including gibbons and siamangs. The great apes
include the genera Pan (chimpanzees and bonobos), Gorilla
(gorillas), Pongo (orangutans), and Homo (humans). The very
arboreal gibbons are smaller than the great apes; they have low
sexual dimorphism (that is, the genders are not markedly different in
size); and they have relatively longer arms used for
swinging/brachiating through trees.

Figure 29.7A. 1 : Howler monkey: The howler monkey, a member of

the Platyrrhini, is native to Central and South America. It makes a
call that sounds like a lion roaring. Figure 29.7A. 1 : Chimpanzee: The (a) chimpanzee is one of the
great apes. It possesses a relatively large brain and has no tail. (b)
Apes evolved from the catarrhines in Africa midway through the All great apes have a similar skeletal structure.
Cenozoic during the Miocene epoch, approximately 25 million years
ago. Apes are generally larger than monkeys and do not possess a This page titled 29.7A: Characteristics and Evolution of Primates is shared
tail. All apes are capable of moving through trees, although many under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
species spend most their time on the ground. Apes are more by Boundless.
intelligent than monkeys as they have relatively larger brains
proportionate to body size.

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29.7B: EARLY HUMAN EVOLUTION
Modern humans and chimpanzees evolved from a common
hominoid ancestor that diverged approximately 6 million years ago.

 LEARNING OBJECTIVES

List the evolved physical traits used to differentiate

hominins from other hominoids

KEY POINTS
Modern humans are classified as hominins, which also includes
extinct bipedal human relatives, such as Australopithecus
africanus, Homo habilis , and Homo erectus.
Few very early (prior to 4 million years ago) hominin fossils
have been found so determining the lines of hominin descent is
extremely difficult.
Within the last 20 years, three new genera of hominoids were
discovered: Sahelanthropus tchadensis, Orrorin tugenensis, and
Ardipithecus ramidus and kadabba, but their status in regards to
human ancestry is somewhat uncertain. Figure 29.7B. 1: Evolution of modern humans: This chart shows the
evolution of modern humans and includes the point of divergence
that occurred between modern humans and the other great apes.
KEY TERMS
hominin: the evolutionary group that includes modern humans VERY EARLY HOMININS
and now-extinct bipedal relatives There have been three species of very early hominoids which have
hominoid: any great ape (such as humans) belonging to the made news in the past few years. The oldest of these,
superfamily Hominoidea Sahelanthropus tchadensis, has been dated to nearly seven million
years ago. There is a single specimen of this genus, a skull that was
HUMAN EVOLUTION
a surface find in Chad. The fossil, informally called “Toumai,” is a
The family Hominidae of order Primates includes chimpanzees and mosaic of primitive and evolved characteristics. To date, it is unclear
humans. Evidence from the fossil record and from a comparison of how this fossil fits with the picture given by molecular data. The line
human and chimpanzee DNA suggests that humans and
leading to modern humans and modern chimpanzees apparently
chimpanzees diverged from a common hominoid ancestor
bifurcated (divided into branches) about six million years ago. It is
approximately 6 million years ago. Several species evolved from the
not thought at this time that this species was an ancestor of modern
evolutionary branch that includes humans, although our species is
humans. It may not have been a hominin.
the only surviving member. The term hominin (or hominid) is used
A second, younger species (around 5.7 million years ago), Orrorin
to refer to those species that evolved after this split of the primate
tugenensis, is also a relatively-recent discovery, found in 2000.
line, thereby designating species that are more closely related to
There are several specimens of Orrorin. It is not known whether
humans than to chimpanzees. Hominins, who were bipedal in
Orrorin was a human ancestor, but this possibility has not been ruled
comparison to the other hominoids who were primarily quadrupedal,
out. Some features of Orrorin are more similar to those of modern
includes those groups that probably gave rise to our species:
humans than are the australopiths, although Orrorin is much older.
Australopithecus africanus, Homo habilis, and Homo erectus, along
with non- ancestral groups such as Australopithecus boisei. A third genus, Ardipithecus ramidus (4.4 million years ago), was
Determining the true lines of descent in hominins is difficult. In discovered in the 1990s. The scientists who discovered the first
years past, when relatively few hominin fossils had been recovered, fossil found that some other scientists did not believe the organism
some scientists believed that considering them in order, from oldest to be a biped (thus, it would not be considered a hominid). In the
to youngest, would demonstrate the course of evolution from early intervening years, several more specimens of Ardipithecus,
hominins to modern humans. In the past several years, however, including a new species, Ardipithecus kadabba (5.6 million years
many new fossils have been found. It is possible that there were ago), demonstrated that they were bipedal. Again, the status of this
often more than one species alive at any one time and that many of genus as a human ancestor is uncertain, but, given that it was
the fossils found (and species named) represent hominin species that bipedal, it was a hominin.
died out and are not ancestral to modern humans. However, it is also
This page titled 29.7B: Early Human Evolution is shared under a CC BY-SA
possible that too many new species have been named.
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29.7C: EARLY HOMININS
The hominin Australopithecus evolved 4 million years ago and is years ago. This species demonstrates a trend in human evolution: the
believed to be in the ancestral line of the genus Homo. reduction of the dentition and jaw in size. A. afarensis had smaller
canines and molars compared to apes, but these were larger than
 LEARNING OBJECTIVES those of modern humans. Its brain size was 380–450 cubic
centimeters, approximately the size of a modern chimpanzee brain.
Describe the physical characteristics of the Australopiths It also had prognathic jaws, which is a relatively longer jaw than that
and compare them to those of modern humans of modern humans. In the mid-1970s, the fossil of an adult female A.
afarensis was found in the Afar region of Ethiopia, dated to 3.24
KEY POINTS million years ago. The fossil, which is informally called “Lucy,” is
The early hominin Australopithecus displayed various significant because it was the most complete australopith fossil
characteristics which show more similarity to the great apes than found, with 40 percent of the skeleton recovered.
to modern humans: great sexual dimorphism, small brain size in
comparison to body mass, larger canines and molars, and a
prognathic jaw.
Australopithecus africanus lived between 2 and 3 million years
ago and had a larger brain than A. afarensis, but was still less
than one-third the size of the modern human brain.
The gracile australopiths had a relatively slender build and teeth
that were suited for soft food and may have had a partially
carnivorous diet, while the robust australopiths probably ate
tough vegetation.

KEY TERMS
dentition: the type, number and arrangement of the normal teeth
of an organism or of the actual teeth of an individual
sexual dimorphism: a physical difference between male and
female individuals of the same species
bipedalism: the habit of standing and walking on two feet

EARLY HOMININS: GENUS AUSTRALOPITHECUS

Australopithecus (“southern ape”) is a genus of hominin that
evolved in eastern Africa approximately 4 million years ago and
became extinct about 2 million years ago. This genus is of particular
interest to us as it is thought that our genus, genus Homo, evolved
from Australopithecus about 2 million years ago. Australopithecus
had a number of characteristics that were more similar to the great
apes than to modern humans. For example, sexual dimorphism was
more exaggerated than in modern humans. Males were up to 50
percent larger than females, a ratio that is similar to that seen in
modern gorillas and orangutans. In contrast, modern human males
are approximately 15 to 20 percent larger than females. The brain
size of Australopithecus relative to its body mass was also smaller
than modern humans and more similar (although larger) to that seen
in the great apes. A key feature that Australopithecus had in
common with modern humans was bipedalism, although it is likely
that Australopithecus also spent time in trees. Hominin footprints,
similar to those of modern humans, found in Laetoli, Tanzania, are
dated to 3.6 million years ago. They show that hominins at the time
of Australopithecus were walking upright.
There were a number of Australopithecus species, often referred to
as australopiths. Australopithecus anamensis lived about 4.2 million
years ago. More is known about another early species,
Australopithecus afarensis, which lived between 3.9 and 2.9 million

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 Figure 29.7C. 1 : Skull comparison: Australopithecus afarensis vs
modern humans: The skull of (a) Australopithecus afarensis, an
early hominid that lived between three and four million years ago,
resembled that of (b) modern humans, but was smaller with a sloped
forehead and prominent jaw.
Australopithecus africanus lived between 2 and 3 million years ago.
It had a slender build and was bipedal, but had robust arm bones
and, as with other early hominids, may have spent significant time in
trees. Its brain was larger than that of A. afarensis at 500 cubic
centimeters, which is slightly less than one-third the size of modern
human brains. Two other species, Australopithecus bahrelghazaliand
Australopithecus garhi, have been added to the roster of
australopiths in recent years.

A DEAD END
While most australopiths had a relatively slender, gracile build and
teeth suited for soft food, there were also australopiths of a more
robust build, dating to approximately 2.5 million years ago. These
hominids were larger and had large grinding teeth. Their molars
show heavy wear, suggesting that they had a coarse and fibrous
vegetarian diet as opposed to the partially carnivorous diet of the
more gracile australopiths. They include Australopithecus robustus
of South Africa, and Australopithecus aethiopicus and
Australopithecus boisei of East Africa. These hominids became
extinct more than 1 million years ago and are not thought to be
ancestral to modern humans, but rather members of an evolutionary
branch on the hominin tree that left no descendants.

This page titled 29.7C: Early Hominins is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

Figure 29.7C. 1 : Adult Female Australopithecus afarensis: This

adult female Australopithecus afarensis skeleton, nicknamed Lucy,
was discovered in the mid 1970s.

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29.7D: GENUS HOMO
The human genus Homo, which includes modern humans as well as H. erectus appeared approximately 1.8 million years ago. It is
extinct human relatives, appeared around 2.3 million years ago. believed to have originated in East Africa and was the first hominin
species to migrate out of Africa. Fossils of H. erectus have been
 LEARNING OBJECTIVES found in India, China, Java, and Europe, and were known in the past
as “Java Man” or “Peking Man.” H. erectus had a number of
Compare and contrast the evolution and characteristics features that were more similar to modern humans than those of H.
associated with the various Homo species: Homo habilis, habilis. H. erectus was larger in size than earlier hominins, reaching
erectus, and sapiens heights up to 1.85 meters and weighing up to 65 kilograms, sizes
similar to those of modern humans. Its degree of sexual dimorphism
KEY POINTS was less than earlier species, with males being 20 to 30 percent
Homo erectus, appearing 1.8 million years ago, was the first larger than females, which is close to the size difference seen in our
hominin species to migrate out of East Africa, use fire, and hunt. species. H. erectus had a larger brain than earlier species at 775–
Compared to Homo habilis, Homo erectus was more similar to 1,100 cubic centimeters, which compares to the 1,130–1,260 cubic
modern humans due to its height and weight, brain size, limited centimeters seen in modern human brains. H. erectus also had a nose
sexual dimorphism, and downward-facing nostrils. with downward-facing nostrils similar to modern humans, rather
Archaic Homo sapiens had a similar brain size to modern than the forward facing nostrils found in other primates. Longer,
humans (Homo sapiens sapiens), but, unlike modern humans, downward-facing nostrils allow for the warming of cold air before it
they had a thick skull, prominent brow ridge, and a receding enters the lungs and may have been an adaptation to colder climates.
chin. Artifacts found with fossils of H. erectus suggest that it was the first
The multiregional hypothesis of modern human origins states hominin to use fire, hunt, and have a home base. H. erectus is
that there is an unbroken line of evolution involving regional generally thought to have lived until about 50,000 years ago.
adaptations and gene flow from H. erectus to H. sapiens sapiens.
The recent out of Africa hypothesis of modern human origins
states that H. sapiens sapiens arose in Africa between 100,000 –
200,000 years, left Africa around 60,000 years ago, and replaced
all archaic humans, with very little inter-breeding.
All men today inherited a Y chromosome from a male that lived
in Africa about 140,000 years ago.

KEY TERMS
Homo habilis: (“handy man”) an extinct taxonomic species
within the genus Homo that had long arms and may have used
stone tools
Homo erectus: (“upright man) extinct species of hominin that
appeared 1.8 million years ago; the first hominin to use fire,
hunt, and have a home base
Homo sapiens: evolved from H. erectus starting about 500,000
years ago; humans

EARLY HOMININS: GENUS HOMO

The human genus, Homo, first appeared around 2.3 million years HOMO ERECTUS
ago. For many years, fossils of a species called Homo habilis were Homo erectus had a prominent brow and a nose that pointed
the oldest examples in the genus Homo, but in 2010, a new species downward rather than forward.
called Homo gautengensis was proposed that may be older, although
it is not well accepted. In comparison to Australopithecus africanus, HUMANS: HOMO SAPIENS
H. habilis had a number of features more similar to modern humans. A number of species, sometimes called archaic Homo sapiens,
H. habilis had a jaw that was less prognathic (forward projection of apparently evolved from H. erectus starting about 500,000 years
the jaw) than the australopiths and a larger brain, at 600–750 cubic ago. These archaic H. sapiens had a brain size similar to that of
centimeters. However, H. habilis retained some features of older modern humans, averaging 1,200–1,400 cubic centimeters. They
hominin species, such as long arms. The name H. habilis means differed from modern humans by having a thick skull, a prominent
“handy man,” which is a reference to the stone tools that have been brow ridge, and a receding chin. Some of these populations survived
found with its remains. until 30,000–10,000 years ago, overlapping with anatomically-
modern humans.

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Ardipithecus. Provided by: Wikipedia. Located at:
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modern humans or Homo sapiens sapiens. As discussed earlier, H. en.wiktionary.org/wiki/bipedalism. License: CC BY-SA: Attribution-
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major wave of migration about 1.5 million years ago. The en.wiktionary.org/wiki/dentition. License: CC BY-SA: Attribution-ShareAlike
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 CHAPTER OVERVIEW

30: PLANT FORM AND PHYSIOLOGY

30.1: The Plant Body - Plant Tissues and Organ Systems
30.2: Stems - Functions of Stems
30.3: Stems - Stem Anatomy
30.4: Stems - Primary and Secondary Growth in Stems
30.5: Stems - Stem Modifications
30.6: Roots - Types of Root Systems and Zones of Growth
30.7: Roots - Root Modifications
30.8: Leaves - Leaf Structure and Arrangment
30.9: Leaves - Types of Leaf Forms
30.10: Leaves - Leaf Structure, Function, and Adaptation
30.11: Plant Development - Meristems
30.12: Plant Development - Genetic Control of Flowers
30.13: Transport of Water and Solutes in Plants - Water and Solute Potential
30.14: Transport of Water and Solutes in Plants - Pressure, Gravity, and Matric Potential
30.15: Transport of Water and Solutes in Plants - Movement of Water and Minerals in the Xylem
30.16: Transport of Water and Solutes in Plants - Transportation of Photosynthates in the Phloem
30.17: Plant Sensory Systems and Responses - Plant Responses to Light
30.18: Plant Sensory Systems and Responses - The Phytochrome System and Red Light Response
30.19: Plant Sensory Systems and Responses - Blue Light Response
30.20: Plant Sensory Systems and Responses - Plant Responses to Gravity
30.21: Plant Sensory Systems and Responses - Auxins, Cytokinins, and Gibberellins
30.22: Plant Sensory Systems and Responses - Abscisic Acid, Ethylene, and Nontraditional Hormones
30.23: Plant Sensory Systems and Responses - Plant Responses to Wind and Touch
30.24: Plant Defense Mechanisms - Against Herbivores
30.25: Plant Defense Mechanisms - Against Pathogens

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1
30.1: THE PLANT BODY - PLANT TISSUES AND ORGAN SYSTEMS

 LEARNING OBJECTIVES

Differentiate among the types of plant tissues and organs

PLANT TISSUES
Plants are multicellular eukaryotes with tissue systems made of
various cell types that carry out specific functions. Plant tissue
systems fall into one of two general types: meristematic tissue and
permanent (or non-meristematic) tissue. Cells of the meristematic
tissue are found in meristems, which are plant regions of continuous
cell division and growth. Meristematic tissue cells are either
undifferentiated or incompletely differentiated; they continue to Figure 30.1.1: Cross section of a squash stem showing a vascular
bundle: This light micrograph shows a cross section of a squash
divide and contribute to the growth of the plant. In contrast, (Curcurbita maxima) stem. Each teardrop-shaped vascular bundle
permanent tissue consists of plant cells that are no longer actively consists of large xylem vessels toward the inside and smaller phloem
dividing. cells toward the outside. Xylem cells, which transport water and
nutrients from the roots to the rest of the plant, are dead at functional
Meristematic tissues consist of three types, based on their location in maturity. Phloem cells, which transport sugars and other organic
the plant. Apical meristems contain meristematic tissue located at compounds from photosynthetic tissue to the rest of the plant, are
living. The vascular bundles are encased in ground tissue and
the tips of stems and roots, which enable a plant to extend in length. surrounded by dermal tissue.
Lateral meristems facilitate growth in thickness or girth in a
maturing plant. Intercalary meristems occur only in monocots at the PLANT ORGAN SYSTEMS
bases of leaf blades and at nodes (the areas where leaves attach to a In plants, just as in animals, similar cells working together form a
stem). This tissue enables the monocot leaf blade to increase in tissue. When different types of tissues work together to perform a
length from the leaf base; for example, it allows lawn grass leaves to unique function, they form an organ; organs working together form
elongate even after repeated mowing. organ systems. Vascular plants have two distinct organ systems: a
Meristems produce cells that quickly differentiate, or specialize, and shoot system and a root system. The shoot system consists of two
become permanent tissue. Such cells take on specific roles and lose portions: the vegetative (non-reproductive) parts of the plant, such as
their ability to divide further. They differentiate into three main the leaves and the stems; and the reproductive parts of the plant,
types: dermal, vascular, and ground tissue. Dermal tissue covers and which include flowers and fruits. The shoot system generally grows
protects the plant. Vascular tissue transports water, minerals, and above ground, where it absorbs the light needed for photosynthesis.
sugars to different parts of the plant. Ground tissue serves as a site The root system, which supports the plants and absorbs water and
for photosynthesis, provides a supporting matrix for the vascular minerals, is usually underground.
tissue, and helps to store water and sugars.
Plant tissues are either simple (composed of similar cell types) or
complex (composed of different cell types). Dermal tissue, for
example, is a simple tissue that covers the outer surface of the plant
and controls gas exchange. Vascular tissue is an example of a
complex tissue. It is made of two specialized conducting tissues:
xylem and phloem. Xylem tissue transports water and nutrients from
the roots to different parts of the plant. It includes three different cell
types: vessel elements and tracheids (both of which conduct water)
and xylem parenchyma. Phloem tissue, which transports organic
compounds from the site of photosynthesis to other parts of the
plant, consists of four different cell types: sieve cells (which conduct
photosynthates), companion cells, phloem parenchyma, and phloem
fibers. Unlike xylem-conducting cells, phloem-conducting cells are
alive at maturity. The xylem and phloem always lie adjacent to each
other. In stems, the xylem and the phloem form a structure called a
vascular bundle; in roots, this is termed the vascular stele or vascular
cylinder. Figure 30.1.1: Example plant organ systems: The shoot system of a
plant consists of leaves, stems, flowers, and fruits. The root system
anchors the plant while absorbing water and minerals from the soil.

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KEY POINTS the primary component of wood
There are two types of plant tissues: meristematic tissue found in phloem: a vascular tissue in land plants primarily responsible for
plant regions of continuous cell division and growth, and the distribution of sugars and nutrients manufactured in the shoot
permanent (or non-meristematic) tissue consisting of cells that tracheid: elongated cells in the xylem of vascular plants that
are no longer actively dividing. serve in the transport of water and mineral salts
Meristems produce cells that differentiate into three secondary
CONTRIBUTIONS AND ATTRIBUTIONS
tissue types: dermal tissue which covers and protects the plant,
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
vascular tissue which transports water, minerals, and sugars and Located at: http://cnx.org/content/m44700/latest...ol11448/latest. License: CC
ground tissue which serves as a site for photosynthesis, supports BY: Attribution
Secondary growth. Provided by: Wikipedia. Located at:
vascular tissue, and stores nutrients. en.Wikipedia.org/wiki/Secondary_growth. License: CC BY-SA: Attribution-
Vascular tissue is made of xylem tissue which transports water ShareAlike
xylem. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/xylem.
and nutrients from the roots to different parts of the plant and License: CC BY-SA: Attribution-ShareAlike
phloem tissue which transports organic compounds from the site parenchyma. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/parenchyma. License: CC BY-SA: Attribution-
of photosynthesis to other parts of the plant.
ShareAlike
The xylem and phloem always lie next to each other forming a phloem. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/phloem.
structure called a vascular bundle in stems and a vascular stele or License: CC BY-SA: Attribution-ShareAlike
tracheid. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/tracheid.
vascular cylinder in roots. License: CC BY-SA: Attribution-ShareAlike
Parts of the shoot system include the vegetative parts, such as the meristem. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/meristem. License: CC BY-SA: Attribution-ShareAlike
leaves and the stems, and the reproductive parts, such as the OpenStax College, The Plant Body. October 17, 2013. Provided by: OpenStax
flowers and fruits. CNX. Located at: http://cnx.org/content/m44700/latest..._30_01_02f.jpg.
License: CC BY: Attribution
OpenStax College, The Plant Body. October 17, 2013. Provided by: OpenStax
KEY TERMS CNX. Located at: http://cnx.org/content/m44700/latest...e_30_01_01.jpg.
meristem: the plant tissue composed of totipotent cells that License: CC BY: Attribution

allows plant growth

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parenchyma: the ground tissue making up most of the non- shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
woody parts of a plant curated by Boundless.
xylem: a vascular tissue in land plants primarily responsible for
the distribution of water and minerals taken up by the roots; also

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30.2: STEMS - FUNCTIONS OF STEMS

 LEARNING OBJECTIVES

Summarize the main function and basic structure of stems

STEMS
Stems are a part of the shoot system of a plant. They may range in
length from a few millimeters to hundreds of meters. They also vary
in diameter, depending on the plant type. Stems are usually above
ground, although the stems of some plants, such as the potato, also
grow underground. Stems may be herbaceous (soft) or woody in
nature. Their main function is to provide support to the plant,
holding leaves, flowers, and buds; in some cases, stems also store
food for the plant. A stem may be unbranched, like that of a palm
tree, or it may be highly branched, like that of a magnolia tree. The
stem of the plant connects the roots to the leaves, helping to
transport absorbed water and minerals to different parts of the plant.
The stem also helps to transport the products of photosynthesis (i.e.,
sugars) from the leaves to the rest of the plant.
Plant stems, whether above or below ground, are characterized by
the presence of nodes and internodes. Nodes are points of
Figure 30.2.1: Parts of a stem: Leaves are attached to the plant stem
attachment for leaves, aerial roots, and flowers. The stem region at areas called nodes. An internode is the stem region between two
between two nodes is called an internode. The stalk that extends nodes. The petiole is the stalk connecting the leaf to the stem. The
from the stem to the base of the leaf is the petiole. An axillary bud is leaves just above the nodes arise from axillary buds.
usually found in the axil (the area between the base of a leaf and the KEY POINTS
stem) where it can give rise to a branch or a flower. The apex (tip) of
Most stems are found above ground, but some of them grow
the shoot contains the apical meristem within the apical bud.
underground.
Stems can be either unbranched or highly branched; they may be
herbaceous or woody.
Stems connect the roots to the leaves, helping to transport water,
minerals, and sugars to different parts of the plant.
Plant stems always have nodes (points of attachments for leaves,
roots, and flowers) and internodes (regions between nodes).
The petiole is the stalk that extends from the stem to the base of
the leaf.
An axillary bud gives rise to a branch or a flower; it is usually
found in the axil: the junction of the stem and petiole.

KEY TERMS
node: points of attachment for leaves, aerial roots, and flowers
internode: a section of stem between two stem nodes
petiole: stalk that extends from the stem to the base of the leaf
axillary bud: embryonic shoot that lies at the junction of the
stem and petiole that gives rise to a branch or flower

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30.3: STEMS - STEM ANATOMY

 LEARNING OBJECTIVES

Summarize the roles of dermal tissue, vascular tissue, and

ground tissue

STEM ANATOMY
The stem and other plant organs are primarily made from three
simple cell types: parenchyma, collenchyma, and sclerenchyma
cells. Parenchyma cells are the most common plant cells. They are
found in the stem, the root, the inside of the leaf, and the pulp of the
fruit. Parenchyma cells are responsible for metabolic functions, such
as photosynthesis. They also help repair and heal wounds. In Figure 30.3.1: Collenchyma cells in plants: Collenchyma cell walls
addition, some parenchyma cells store starch. are uneven in thickness, as seen in this light micrograph. They
provide support to plant structures.
Sclerenchyma cells also provide support to the plant, but unlike
collenchyma cells, many of them are dead at maturity. There are two
types of sclerenchyma cells: fibers and sclereids. Both types have
secondary cell walls that are thickened with deposits of lignin, an
organic compound that is a key component of wood. Fibers are long,
slender cells; sclereids are smaller-sized. Sclereids give pears their
gritty texture. Humans use sclerenchyma fibers to make linen and
rope.

Figure 30.3.1: Parenchyma cells in plants: The stem of common St

John’s Wort (Hypericum perforatum) is shown in cross section in
this light micrograph. The central pith (greenish-blue, in the center)
and peripheral cortex (narrow zone 3–5 cells thick, just inside the
epidermis) are composed of parenchyma cells. Vascular tissue
composed of xylem (red) and phloem tissue (green, between the
xylem and cortex) surrounds the pith.
Collenchyma cells are elongated cells with unevenly-thickened
walls. They provide structural support, mainly to the stem and
leaves. These cells are alive at maturity and are usually found below
the epidermis. The “strings” of a celery stalk are an example of
collenchyma cells.

Figure 30.3.1: Sclerenchyma cells in plants: The central pith and

outer cortex of the (a) flax stem are made up of parenchyma cells.
Inside the cortex is a layer of sclerenchyma cells, which make up the
fibers in flax rope and clothing. Humans have grown and harvested
flax for thousands of years. In (b) this drawing, fourteenth-century
women prepare linen. The (c) flax plant is grown and harvested for
its fibers, which are used to weave linen, and for its seeds, which are
the source of linseed oil.
As with the rest of the plant, the stem has three tissue systems:
dermal, vascular, and ground tissue. Each is distinguished by
characteristic cell types that perform specific tasks necessary for the
plant’s growth and survival.

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DERMAL TISSUE
The dermal tissue of the stem consists primarily of epidermis: a
single layer of cells covering and protecting the underlying tissue.
Woody plants have a tough, waterproof outer layer of cork cells
commonly known as bark, which further protects the plant from
damage. Epidermal cells are the most-numerous and least-
differentiated of the cells in the epidermis. The epidermis of a leaf
also contains openings, known as stomata, through which the
exchange of gases takes place. Two cells, known as guard cells, Figure 30.3.1: Vascular bundles: In (a) dicot stems, vascular bundles
surround each leaf stoma, controlling its opening and closing and, are arranged around the periphery of the ground tissue. The xylem
thus, regulating the uptake of carbon dioxide and the release of tissue is located toward the interior of the vascular bundle; phloem is
located toward the exterior. Sclerenchyma fibers cap the vascular
oxygen and water vapor. Trichomes are hair-like structures on the
bundles. In (b) monocot stems, vascular bundles composed of xylem
epidermal surface. They help to reduce transpiration (the loss of and phloem tissues are scattered throughout the ground tissue.
water by aboveground plant parts), increase solar reflectance, and Xylem tissue has three types of cells: xylem parenchyma, tracheids,
store compounds that defend the leaves against predation by and vessel elements. The latter two types conduct water and are dead
herbivores. at maturity. Tracheids are xylem cells with thick secondary cell
walls that are lignified. Water moves from one tracheid to another
through regions on the side walls known as pits where secondary
walls are absent. Vessel elements are xylem cells with thinner walls;
they are shorter than tracheids. Each vessel element is connected to
the next by means of a perforation plate at the end walls of the
element. Water moves through the perforation plates to travel up the
plant.
Phloem tissue is composed of sieve-tube cells, companion cells,
phloem parenchyma, and phloem fibers. A series of sieve-tube cells
(also called sieve-tube elements) are arranged end-to-end to create a
long sieve tube, which transports organic substances such as sugars
Figure 30.3.1: Stomata: Openings called stomata (singular: stoma) and amino acids. The sugars flow from one sieve-tube cell to the
allow a plant to take up carbon dioxide and release oxygen and
water vapor. The (a) colorized scanning-electron micrograph shows next through perforated sieve plates, which are found at the end
a closed stoma of a dicot. Each stoma is flanked by two guard cells junctions between two cells. Although still alive at maturity, the
that regulate its (b) opening and closing. The (c) guard cells sit nucleus and other cell components of the sieve-tube cells have
within the layer of epidermal cells.
disintegrated. Companion cells are found alongside the sieve-tube
VASCULAR TISSUE cells, providing them with metabolic support. The companion cells
The xylem and phloem that make up the vascular tissue of the stem contain more ribosomes and mitochondria than do the sieve-tube
cells, which lack some cellular organelles.
are arranged in distinct strands called vascular bundles, which run up
and down the length of the stem. Both are considered complex plant
GROUND TISSUE
tissue because they are composed of more than one simple cell type
Ground tissue is mostly made up of parenchyma cells, but may also
that work in concert with each other. When the stem is viewed in
contain collenchyma and sclerenchyma cells that help support the
cross section, the vascular bundles of dicot stems are arranged in a
stem. The ground tissue towards the interior of the vascular tissue in
ring. In plants with stems that live for more than one year, the
a stem or root is known as pith, while the layer of tissue between the
individual bundles grow together and produce the characteristic
vascular tissue and the epidermis is known as the cortex.
growth rings. In monocot stems, the vascular bundles are randomly
scattered throughout the ground tissue. KEY POINTS
The stem has three simple cell types: the parenchyma,
collenchyma, and sclerenchyma cells that are responsible for
metabolic functions, repairing and healing wounds, and storing
starch.
The stem is composed of three tissue systems that include the
epidermis, vascular, and ground tissues, all of which are made
from the simple cell types..
The xylem and phloem carry water and nutrients up and down
the length of the stem and are arranged in distinct strands called
vascular bundles.

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 The epidermis is a single layer of cells that makes up the dermal lignin: a complex, non-carbohydrate, aromatic polymer present
tissue covering the stem and protecting the underlying tissue. in all wood
Woody plants have an extra layer of protection on top of the stoma: a pore found in the leaf and stem epidermis used for
epidermis made of cork cells known as bark. gaseous exchange
The vascular tissue of the stem consists of the complex tissues trichome: a hair- or scale-like extension of the epidermis of a
xylem and phloem which carry water and nutrients up and down plant
the length of the stem and are arranged in distinct strands called xylem: a vascular tissue in land plants primarily responsible for
vascular bundles. the distribution of water and minerals taken up by the roots; also
Ground tissue helps support the stem and is called pith when it is the primary component of wood
located towards the middle of the stem and called the cortex phloem: a vascular tissue in land plants primarily responsible for
when it is between the vascular tissue and the epidermis. the distribution of sugars and nutrients manufactured in the shoot
tracheid: elongated cells in the xylem of vascular plants that
KEY TERMS serve in the transport of water and mineral salts
collenchyma: a supporting ground tissue just under the surface pith: the soft spongy substance in the center of the stems of
of various leaf structures formed before vascular differentiation many plants and trees
sclerenchyma: a mechanical, supportive ground tissue in plants cortex: the tissue of a stem or root that lies inward from the
consisting of aggregates of cells having thick, often mineralized epidermis, but exterior to the vascular tissue
walls parenchyma: the ground tissue making up most of the non-
sclereid: a reduced form of sclerenchyma cells with highly- woody parts of a plant
thickened, lignified walls
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30.4: STEMS - PRIMARY AND SECONDARY GROWTH IN STEMS
encouraging the axillary buds to grow out, giving the plant a bushy
 LEARNING OBJECTIVES shape.

Distinguish between primary and secondary growth in stems SECONDARY GROWTH

The increase in stem thickness that results from secondary growth is
Growth in plants occurs as the stems and roots lengthen. Some due to the activity of the lateral meristems, which are lacking in
plants, especially those that are woody, also increase in thickness herbaceous plants. Lateral meristems include the vascular cambium
during their life span. The increase in length of the shoot and the and, in woody plants, the cork cambium. The vascular cambium is
root is referred to as primary growth. It is the result of cell division located just outside the primary xylem and to the interior of the
in the shoot apical meristem. Secondary growth is characterized by primary phloem. The cells of the vascular cambium divide and form
an increase in thickness or girth of the plant. It is caused by cell secondary xylem ( tracheids and vessel elements) to the inside and
division in the lateral meristem. Herbaceous plants mostly undergo secondary phloem (sieve elements and companion cells) to the
primary growth, with little secondary growth or increase in outside. The thickening of the stem that occurs in secondary growth
thickness. Secondary growth, or “wood”, is noticeable in woody is due to the formation of secondary phloem and secondary xylem
plants; it occurs in some dicots, but occurs very rarely in monocots. by the vascular cambium, plus the action of cork cambium, which
forms the tough outermost layer of the stem. The cells of the
secondary xylem contain lignin, which provides hardiness and
strength.
In woody plants, cork cambium is the outermost lateral meristem. It
produces cork cells (bark) containing a waxy substance known as
suberin that can repel water. The bark protects the plant against
physical damage and helps reduce water loss. The cork cambium
also produces a layer of cells known as phelloderm, which grows
inward from the cambium. The cork cambium, cork cells, and
phelloderm are collectively termed the periderm. The periderm
substitutes for the epidermis in mature plants. In some plants, the
periderm has many openings, known as lenticels, which allow the
interior cells to exchange gases with the outside atmosphere. This
Figure 30.4.1: Primary and secondary growth: In woody plants,
primary growth is followed by secondary growth, which allows the supplies oxygen to the living- and metabolically-active cells of the
plant stem to increase in thickness or girth. Secondary vascular cortex, xylem, and phloem.
tissue is added as the plant grows, as well as a cork layer. The bark
of a tree extends from the vascular cambium to the epidermis.
Some plant parts, such as stems and roots, continue to grow
throughout a plant’s life: a phenomenon called indeterminate
growth. Other plant parts, such as leaves and flowers, exhibit
determinate growth, which ceases when a plant part reaches a
particular size.

PRIMARY GROWTH
Most primary growth occurs at the apices, or tips, of stems and
roots. Primary growth is a result of rapidly-dividing cells in the
apical meristems at the shoot tip and root tip. Subsequent cell
elongation also contributes to primary growth. The growth of shoots
and roots during primary growth enables plants to continuously seek
water (roots) or sunlight (shoots).
The influence of the apical bud on overall plant growth is known as
apical dominance, which diminishes the growth of axillary buds that
form along the sides of branches and stems. Most coniferous trees
exhibit strong apical dominance, thus producing the typical conical Figure 30.4.1: Example of lenticels: Lenticels on the bark of this
Christmas tree shape. If the apical bud is removed, then the axillary cherry tree enable the woody stem to exchange gases with the
buds will start forming lateral branches. Gardeners make use of this surrounding atmosphere.
fact when they prune plants by cutting off the tops of branches, thus

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ANNUAL RINGS KEY POINTS
The activity of the vascular cambium gives rise to annual growth Indeterminate growth continues throughout a plant’s life, while
rings. During the spring growing season, cells of the secondary determinate growth stops when a plant element (such as a leaf)
xylem have a large internal diameter; their primary cell walls are not reaches a particular size.
extensively thickened. This is known as early wood, or spring wood. Primary growth of stems is a result of rapidly-dividing cells in
During the fall season, the secondary xylem develops thickened cell the apical meristems at the shoot tips.
walls, forming late wood, or autumn wood, which is denser than Apical dominance reduces the growth along the sides of
early wood. This alternation of early and late wood is due largely to branches and stems, giving the tree a conical shape.
a seasonal decrease in the number of vessel elements and a seasonal The growth of the lateral meristems, which includes the vascular
increase in the number of tracheids. It results in the formation of an cambium and the cork cambium (in woody plants), increases the
annual ring, which can be seen as a circular ring in the cross section thickness of the stem during secondary growth.
of the stem. An examination of the number of annual rings and their Cork cells (bark) protect the plant against physical damage and
nature (such as their size and cell wall thickness) can reveal the age water loss; they contain a waxy substance known as suberin that
of the tree and the prevailing climatic conditions during each season. prevents water from penetrating the tissue.
The secondary xylem develops dense wood during the fall and
thin wood during the spring, which produces a characteristic ring
for each year of growth.

KEY TERMS
lenticel: small, oval, rounded spots upon the stem or branch of a
plant that allow the exchange of gases with the surrounding
atmosphere
periderm: the outer layer of plant tissue; the outer bark
suberin: a waxy material found in bark that can repel water

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curated by Boundless.
Figure 30.4.1: Annual growth rings: The rate of wood growth
increases in summer and decreases in winter, producing a
characteristic ring for each year of growth. Seasonal changes in
weather patterns can also affect the growth rate. Note how the rings
vary in thickness.

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30.5: STEMS - STEM MODIFICATIONS

 LEARNING OBJECTIVES

Explain the reasons for stem modifications

Some plant species have modified stems that are especially suited to
a particular habitat and environment. A rhizome is a modified stem
that grows horizontally underground; it has nodes and internodes.
Vertical shoots may arise from the buds on the rhizome of some
plants, such as ginger and ferns. Corms are similar to rhizomes,
except they are more rounded and fleshy (such as in gladiolus).
Corms contain stored food that enables some plants to survive the
winter. Stolons are stems that run almost parallel to the ground, or Figure 30.5.1: Aerial modifications of stems: Found in southeastern
United States, (a) buckwheat vine (Brunnichia ovata) is a weedy
just below the surface, and can give rise to new plants at the nodes.
plant that climbs with the aid of tendrils. This one is shown climbing
Runners are a type of stolon that runs above the ground and up a wooden stake. (b) Thorns are modified branches.
produces new clone plants at nodes at varying intervals: strawberries Tendrils are slender, twining strands that enable a plant (like the
are an example. Tubers are modified stems that may store starch, as buckwheat vine) to seek support by climbing on other surfaces.
seen in the potato. Tubers arise as swollen ends of stolons, and These may develop from either the axillary bud or the terminal
contain many adventitious or unusual buds (familiar to us as the bud of the stem.
“eyes” on potatoes). A bulb, which functions as an underground Thorns are modified branches appearing as hard, woody, sharp
storage unit, is a modification of a stem that has the appearance of outgrowths that protect the plant; common examples include
enlarged fleshy leaves emerging from the stem or surrounding the roses, osage orange, and devil’s walking stick.
base of the stem, as seen in the iris. Bulbils are axillary buds that have become fleshy and rounded
due to storage of food. They become detached from the plant,
fall on ground and develop into a new plant.
Cladodes are green branches of limited growth (usually one
internode long) which have taken up the functions of
photosynthesis.

KEY POINTS
Modified stems that grow horizontally underground are either
rhizomes, from which vertical shoots grow, or fleshier, food-
storing corms.
Figure 30.5.1: Stem modifications: Stem modifications enable
plants to thrive in a variety of environments. Shown are (a) ginger New plants can arise from the nodes of stolons and runners (an
(Zingiber officinale) rhizomes, (b) a carrion flower aboveground stolon): stems that run parallel to the ground, or
(Amorphophallus titanum) corm (c) Rhodes grass (Chloris gayana) just below the surface.
stolons, (d) strawberry (Fragaria ananassa) runners, (e) potato
(Solanum tuberosum) tubers, and (f) red onion (Allium) bulbs. Potatoes are examples of tubers: the swollen ends of stolons that
may store starch.
Modifications to the aerial stems, vegetative buds, and floral buds of
The stem modification that has enlarged fleshy leaves emerging
plants perform functions such as climbing, protection, and synthesis
from the stem or surrounding the base of the stem is called a
of food vegetative propagation. Aerial modifications of stems
bulb; it is also used to store food.
include the following:
Aerial modifications of stems include tendrils, thorns, bulbils,
and cladodes..

KEY TERMS
stolon: a shoot that grows along the ground and produces roots at
its nodes; a runner
tuber: a fleshy, thickened, underground stem of a plant, usually
containing stored starch, as for example a potato or arrowroot
cladode: green branches of limited growth which have taken up
the functions of photosynthesis
rhizome: a horizontal underground stem of some plants that
sends out roots and shoots from its nodes

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 corm: a short, vertical, swollen underground stem of a plant that OpenStax College, Stems. October 17, 2013. Provided by: OpenStax CNX.
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bulb: the bulb-shaped root portion of a plant such as a tulip, License: CC BY: Attribution
from which the rest of the plant may be regrown suberin. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/suberin.
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30.6: ROOTS - TYPES OF ROOT SYSTEMS AND ZONES OF GROWTH
elongation is where the newly-formed cells increase in length,
 LEARNING OBJECTIVES thereby lengthening the root. Beginning at the first root hair is the
zone of cell maturation where the root cells differentiate into
Describe the three zones of the root tip and summarize the
specialized cell types. All three zones are in approximately the first
role of each zone in root growth
centimeter of the root tip.

TYPES OF ROOT SYSTEMS

There are two main types of root systems. Dicots have a tap root
system, while monocots have a fibrous root system, which is also
known as an adventitious root system. A tap root system has a main
root that grows down vertically, from which many smaller lateral
roots arise. Dandelions are a common example; their tap roots
usually break off when these weeds are pulled from the ground; they
can regrow another shoot from the remaining root. A tap root system
penetrates deep into the soil. In contrast, a fibrous root system is
located closer to the soil surface where it forms a dense network of
roots that also helps prevent soil erosion (lawn grasses are a good
example, as are wheat, rice, and corn). Some plants have a
combination of tap roots and fibrous roots. Plants that grow in dry
areas often have deep root systems, whereas plants that grow in
areas with abundant water are likely to have shallower root systems.

Figure 30.6.1: Zones of the root tip: A longitudinal view of the root
reveals the zones of cell division, elongation, and maturation. Cell
division occurs in the apical meristem.

KEY POINTS
Root tips ultimately develop into two main types of root systems:
tap roots and fibrous roots.
The growing root tip is protected by a root cap.
Within the root tip, cells differentiate, actively divide, and
increase in length, depending on in which zone the cells are
located.
Dividing cells make up the zone of cell division in a germinating
plant.
The newly-forming root increases in size in the zone of
elongation.
Figure 30.6.1: Main types of root systems: (a) Tap root systems
have a main root that grows down, while (b) fibrous root systems Differentiating cells make up the zone of cell maturation.
consist of many small roots.
KEY TERMS
ZONES OF THE ROOT TIP
radicle: the rudimentary shoot of a plant that supports the
Root growth begins with seed germination. When the plant embryo cotyledons in the seed and from which the root is developed
emerges from the seed, the radicle of the embryo forms the root downward; the root of the embryo
system. The tip of the root is protected by the root cap, a structure meristem: the plant tissue composed of totipotent cells that
exclusive to roots and unlike any other plant structure. The root cap allows plant growth
is continuously replaced because it is easily damaged as the root germination: the beginning of vegetation or growth from a seed
pushes through soil. The root tip can be divided into three zones: a or spore
zone of cell division, a zone of elongation, and a zone of maturation.
The zone of cell division is closest to the root tip and is made up of This page titled 30.6: Roots - Types of Root Systems and Zones of Growth
the actively-dividing cells of the root meristem, which contains the is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
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30.7: ROOTS - ROOT MODIFICATIONS

 LEARNING OBJECTIVES

Explain the reasons for root modifications

Plants have different root structures for specific purposes. There are
many different types of specialized roots, but two of the more
familiar types of roots include aerial roots and storage roots. Aerial
roots grow above the ground, typically providing structural support.
Storage roots (for example, taproots and tuberous roots) are
modified for food storage.
Aerial roots are found in many different kinds of plants, offering
varying functions depending on the location of the plant. Epiphytic
Figure 30.7.1: Storage roots: Many vegetables, such as carrots and
roots are a type of aerial root that enable a plant to grow on another beets, are modified roots that store food and water.
plant in a non-parasitic manner. The banyan tree begins as an Other examples of modified roots are aerating roots and haustorial
epiphyte, germinating in the branches of a host tree. Aerial prop roots. Aerating roots, which rise above the ground, especially above
roots develop from the branches and eventually reach the ground, water, are commonly seen in mangrove forests that grow along salt
providing additional support. Over time, many roots will come water coastlines. Haustorial roots are often seen in parasitic plants
together to form what appears to be a trunk. The epiphytic roots of such as mistletoe. Their roots allow the plants to absorb water and
orchids develop a spongy tissue to absorb moisture and nutrients
nutrients from other plants.
from any organic material on their roots. In screwpine, a palm-like
tree that grows in sandy tropical soils, aerial roots develop to KEY POINTS
provide additional support that help the tree remain upright in Storage roots, which include a large number of edible vegetables
shifting sand and water conditions. such as potatoes and carrots, are some of the most commonly-
known types of modified roots.
Aerial roots encompass a variety of shapes, yet function similarly
as structural support for the plant.
Parasitic plants have special haustorial roots that allow the plant
to absorb nutrients from a host plant.

KEY TERMS
succulent: having fleshy leaves or other tissues that store water
Figure 30.7.1: Aerial roots: The (a) banyan tree, also known as the
epiphyte: a plant that grows on another, using it as a physical
strangler fig, begins life as an epiphyte in a host tree. Aerial roots support but neither obtaining nutrients from it nor causing it any
extend to the ground, supporting the growing plant, which damage if also offering no benefit
eventually strangles the host tree. The (b) screwpine develops aerial
roots that help support the plant in sandy soils.
CONTRIBUTIONS AND ATTRIBUTIONS
Storage roots, such as carrots, beets, and sweet potatoes, are meristem. Provided by: Wiktionary. Located at:
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eating animals. Some plants, however, such as leaf succulents and BY: Attribution
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30.8: LEAVES - LEAF STRUCTURE AND ARRANGMENT
are arranged in a spiral along the stem. In an opposite leaf
 LEARNING OBJECTIVES arrangement, two leaves arise at the same point, with the leaves
connecting opposite each other along the branch. If there are three or
Sketch the basic structure of a typical leaf
more leaves connected at a node, the leaf arrangement is classified
as whorled.
STRUCTURE OF A TYPICAL LEAF
Each leaf typically has a leaf blade called the lamina, which is also
the widest part of the leaf. Some leaves are attached to the plant
stem by a petiole. Leaves that do not have a petiole and are directly
attached to the plant stem are called sessile leaves. Leaves also have
stipules, small green appendages usually found at the base of the
petiole. Most leaves have a midrib, which travels the length of the
leaf and branches to each side to produce veins of vascular tissue.
The edge of the leaf is called the margin.

Figure 30.8.1: Venation patterns: (a) Tulip (Tulipa), a monocot, has

leaves with parallel venation. (b) The netlike venation in this linden
(Tilia cordata) leaf distinguishes it as a dicot. (c) The Ginkgo biloba
tree has dichotomous venation.

KEY POINTS
Each leaf typically has a leaf blade ( lamina ), stipules, a midrib,
and a margin.
Some leaves have a petiole, which attaches the leaf to the stem;
leaves that do not have petioles are directly attached to the plant
stem and are called sessile leaves.
The arrangement of veins in a leaf is called the venation pattern;
monocots have parallel venation, while dicots have reticulate
Figure 30.8.1: Parts of a leaf: A leaf may seem simple in
appearance, but it is a highly-efficient structure. Petioles, stipules, venation.
veins, and a midrib are all essential structures of a leaf. The arrangement of leaves on a stem is known as phyllotaxy;
Within each leaf, the vascular tissue forms veins. The arrangement leaves can be classified as either alternate, spiral, opposite, or
of veins in a leaf is called the venation pattern. Monocots and whorled.
dicots differ in their patterns of venation. Monocots have parallel Plants with alternate and spiral leaf arrangements have only one
venation in which the veins run in straight lines across the length of leaf per node.
the leaf without converging. In dicots, however, the veins of the leaf In an opposite leaf arrangement, two leaves connect at a node. In
have a net-like appearance, forming a pattern known as reticulate a whorled arrangement, three or more leaves connect at a node.
venation. Ginkgo biloba is an example of a plant with dichotomous
venation. KEY TERMS
petiole: stalk that extends from the stem to the base of the leaf
LEAF ARRANGEMENT lamina: the flat part of a leaf; the blade, which is the widest part
The arrangement of leaves on a stem is known as phyllotaxy. The of the leaf
number and placement of a plant’s leaves will vary depending on the stipule: small green appendage usually found at the base of the
species, with each species exhibiting a characteristic leaf petiole
arrangement. Leaves are classified as either alternate, spiral,
This page titled 30.8: Leaves - Leaf Structure and Arrangment is shared
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by Boundless.
alternate on each side of the stem in a flat plane, and spiral leaves

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30.9: LEAVES - TYPES OF LEAF FORMS
some families of higher plants. Each leaflet is attached to the rachis
 LEARNING OBJECTIVES (middle vein), but may have its own stalk. A palmately compound
leaf has its leaflets radiating outwards from the end of the petiole,
Differentiate among the types of leaf forms
like fingers off the palm of a hand. Examples of plants with
palmately compound leaves include poison ivy, the buckeye tree, or
LEAF FORM the familiar house plant Schefflera sp. (commonly called “umbrella
There are two basic forms of leaves that can be described plant”). Pinnately compound leaves take their name from their
considering the way the blade (or lamina) is divided. Leaves may be feather-like appearance; the leaflets are arranged along the middle
simple or compound. vein, as in rose leaves or the leaves of hickory, pecan, ash, or walnut
trees. In a pinnately compound leaf, the middle vein is called the
midrib. Bipinnately compound (or double compound) leaves are
twice divided; the leaflets are arranged along a secondary vein,
which is one of several veins branching off the middle vein. Each
leaflet is called a “pinnule”. The pinnules on one secondary vein are
called “pinna”. The silk tree (Albizia) is an example of a plant with
bipinnate leaves.

KEY POINTS
In a simple leaf, the blade is completely undivided; leaves may
also be formed of lobes where the gaps between lobes do not
reach to the main vein.
In a compound leaf, the leaf blade is divided, forming leaflets
that are attached to the middle vein, but have their own stalks.
The leaflets of palmately-compound leaves radiate outwards
from the end of the petiole.
Pinnately-compound leaves have their leaflets arranged along the
middle vein.
Bipinnately-compound (double-compound) leaves have their
Figure 30.9.1: Simple and compound leaves: Leaves may be simple leaflets arranged along a secondary vein, which is one of several
or compound. In simple leaves, the lamina is continuous. (a) The veins branching off the middle vein.
banana plant (Musa sp.) has simple leaves. In compound leaves, the
lamina is separated into leaflets. Compound leaves may be palmate
or pinnate. (b) In palmately compound leaves, such as those of the KEY TERMS
horse chestnut (Aesculus hippocastanum), the leaflets branch from simple leaf: a leaf with an undivided blade
the petiole. (c) In pinnately compound leaves, the leaflets branch
from the midrib, as on a scrub hickory (Carya floridana). (d) The
compound leaf: a leaf where the blade is divided, forming
honey locust has double compound leaves, in which leaflets branch leaflets
from the veins. palmately compound leaf: leaf that has its leaflets radiating
In a simple leaf, such as the banana leaf, the blade is completely outwards from the end of the petiole
undivided. The leaf shape may also be formed of lobes where the pinnately compound leaf: a leaf where the leaflets are arranged
gaps between lobes do not reach to the main vein. An example of along the middle vein
this type is the maple leaf.
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leaflets, as in the locust tree. Compound leaves are a characteristic of

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30.10: LEAVES - LEAF STRUCTURE, FUNCTION, AND ADAPTATION

 LEARNING OBJECTIVES

Describe the internal structure and function of a leaf

LEAF STRUCTURE AND FUNCTION

The outermost layer of the leaf is the epidermis. It consists of the
upper and lower epidermis, which are present on either side of the
leaf. Botanists call the upper side the adaxial surface (or adaxis) and
the lower side the abaxial surface (or abaxis). The epidermis aids in
the regulation of gas exchange. It contains stomata, which are
openings through which the exchange of gases takes place. Two
guard cells surround each stoma, regulating its opening and closing.
Guard cells are the only epidermal cells to contain chloroplasts.
The epidermis is usually one cell layer thick. However, in plants that
grow in very hot or very cold conditions, the epidermis may be
several layers thick to protect against excessive water loss from
transpiration. A waxy layer known as the cuticle covers the leaves of
all plant species. The cuticle reduces the rate of water loss from the
leaf surface. Other leaves may have small hairs (trichomes) on the
leaf surface. Trichomes help to avert herbivory by restricting insect
movements or by storing toxic or bad-tasting compounds. They can
also reduce the rate of transpiration by blocking air flow across the
leaf surface.

Figure 30.10.1: Mesophyll: (a) (top) The central mesophyll is

sandwiched between an upper and lower epidermis. The mesophyll
has two layers: an upper palisade layer and a lower spongy layer.
Figure 30.10.1: Trichomes: Trichomes give leaves a fuzzy Stomata on the leaf underside allow gas exchange. A waxy cuticle
appearance as in this (a) sundew (Drosera sp.). Leaf trichomes covers all aerial surfaces of land plants to minimize water loss. (b)
include (b) branched trichomes on the leaf of Arabidopsis lyrata and (bottom) These leaf layers are clearly visible in the scanning
(c) multibranched trichomes on a mature Quercus marilandica leaf. electron micrograph. The numerous small bumps in the palisade
Below the epidermis of dicot leaves are layers of cells known as the parenchyma cells are chloroplasts. The bumps protruding from the
lower surface of the leaf are glandular trichomes.
mesophyll, or “middle leaf.” The mesophyll of most leaves typically
contains two arrangements of parenchyma cells: the palisade Similar to the stem, the leaf contains vascular bundles composed of
parenchyma and spongy parenchyma. The palisade parenchyma xylem and phloem. The xylem consists of tracheids and vessels,
(also called the palisade mesophyll) aids in photosynthesis and has which transport water and minerals to the leaves. The phloem
column-shaped, tightly-packed cells. It may be present in one, two, transports the photosynthetic products from the leaf to the other
or three layers. Below the palisade parenchyma are loosely-arranged parts of the plant. A single vascular bundle, no matter how large or
cells of an irregular shape. These are the cells of the spongy small, always contains both xylem and phloem tissues.
parenchyma (or spongy mesophyll). The air space found between
the spongy parenchyma cells allows gaseous exchange between the
leaf and the outside atmosphere through the stomata. In aquatic
plants, the intercellular spaces in the spongy parenchyma help the
leaf float. Both layers of the mesophyll contain many chloroplasts.

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 KEY POINTS
The epidermis consists of the upper and lower epidermis; it aids
in the regulation of gas exchange via stomata.
The epidermis is one layer thick, but may have more layers to
prevent transpiration.
The cuticle is located outside the epidermis and protects against
water loss; trichomes discourage predation.
The mesophyll is found between the upper and lower epidermis;
it aids in gas exchange and photosynthesis via chloroplasts.
The xylem transports water and minerals to the leaves; the
phloem transports the photosynthetic products to the other parts
of the plant.
Figure 30.10.1: Xylem and phloem: This scanning electron Plants in cold climates have needle-like leaves that are reduced
micrograph shows xylem and phloem in the leaf vascular bundle. in size; plants in hot climates have succulent leaves that help to
LEAF ADAPTATIONS conserve water.
Coniferous plant species that thrive in cold environments, such as KEY TERMS
spruce, fir, and pine, have leaves that are reduced in size and needle- trichome: a hair- or scale-like extension of the epidermis of a
like in appearance. These needle-like leaves have sunken stomata
plant
and a smaller surface area, two attributes that aid in reducing water cuticle: a noncellular protective covering outside the epidermis
loss. In hot climates, plants such as cacti have succulent leaves that
of many invertebrates and plants
help to conserve water. Many aquatic plants have leaves with wide mesophyll: the inner tissue (parenchyma) of a leaf, containing
lamina that can float on the surface of the water; a thick waxy cuticle
many chloroplasts.
on the leaf surface that repels water.
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30.11: PLANT DEVELOPMENT - MERISTEMS

 LEARNING OBJECTIVES

Discuss the attributes of meristem tissue and its role in plant

development and growth

The adult body of vascular plants is the result of meristematic

activity. Plant meristems are centers of mitotic cell division, and are
composed of a group of undifferentiated self-renewing stem cells
from which most plant structures arise. Meristematic cells are also
responsible for keeping the plant growing. The Shoot Apical
Meristem (SAM) gives rise to organs like the leaves and flowers,
while the Root Apical Meristem (RAM) provides the meristematic
cells for the future root growth. The cells of the shoot and root apical
meristems divide rapidly and are considered to be indeterminate, Figure 30.11.1: Meristematic zones: Each zone of the apical
which means that they do not possess any defined end fate. In that meristem has a particular function. Pictured here are the (1) central
zone, (2) peripheral zone, (3) medullary meristem and (3) medullary
sense, the meristematic cells are frequently compared to the stem tissue.
cells in animals, which have an analogous behavior and function.

MERISTEM TISSUE AND PLANT DEVELOPMENT

Meristematic tissues are cells or group of cells that have the ability
to divide. These tissues in a plant consist of small, densely packed
cells that can keep dividing to form new cells. Meristematic tissue is
characterized by small cells, thin cell walls, large cell nuclei, absent
or small vacuoles, and no intercellular spaces.
Meristematic tissues are found in many locations, including near the
tips of roots and stems (apical meristems), in the buds and nodes of
stems, in the cambium between the xylem and phloem in
dicotyledonous trees and shrubs, under the epidermis of
dicotyledonous trees and shrubs (cork cambium), and in the
pericycle of roots, producing branch roots. The two types of
meristems are primary meristems and secondary meristems. Figure 30.11.1: Apical meristem: The apical meristem, pictured in
the center of the leaves of this image, is also termed the “growing
MERISTEM ZONES tip”. Its main function is to begin growth of new cells in young
The apical meristem, also known as the “growing tip,” is an seedlings at the tips of roots and shoots (forming buds, among other
things).
undifferentiated meristematic tissue found in the buds and growing
The central zone is located at the meristem summit, where a small
tips of roots in plants. Its main function is to trigger the growth of
group of slowly dividing cells can be found. Cells of this zone have
new cells in young seedlings at the tips of roots and shoots and
a stem cell function and are essential for meristem maintenance. The
forming buds. Apical meristems are organized into four zones: (1)
proliferation and growth rates at the meristem summit usually differ
the central zone, (2) the peripheral zone, (3) the medullary meristem
considerably from those at the periphery. Surrounding the central
and (3) the medullary tissue.
zone is the peripheral zone. The rate of cell division in the peripheral
zone is higher than that of the central zone. Peripheral zone cells
give rise to cells which contribute to the organs of the plant,
including leaves, inflorescence meristems, and floral meristems.
An active apical meristem lays down a growing root or shoot behind
itself, pushing itself forward. They are very small compared to the
cylinder-shaped lateral meristems, and are composed of several
layers, which varies according to plant type. The outermost layer is
called the tunica, while the innermost layers are cumulatively called
the corpus.

30.11.1 https://bio.libretexts.org/@go/page/13757
KEY POINTS The apical meristem is organized into four meristematic zones:
Mitotic cell division happens in plant meristems, which are (1) central zone, (2) peripheral zone, (3) medullary meristem and
composed of a group of self-renewing stem cells from which (3) medullary tissue.
most plant structures arise.
KEY TERMS
The cells of the shoot and root apical meristems divide rapidly
and are “indeterminate”, which means that they are not designed meristem: the plant tissue composed of totipotent cells that
for any specific end goal. allows plant growth
The Shoot Apical Meristem (SAM) gives rise to organs like the undifferentiated: describes tissues where the individual cells
leaves and flowers, while the Root Apical Meristem (RAM) have not yet developed mature or distinguishing features, or
provides cells for future root growth. describes embryonic organisms where the organs cannot be
Meristematic tissue has a number of defining features, including identified
small cells, thin cell walls, large cell nuclei, absent or small apical: situated at the growing tip of the plant or its roots, in
vacuoles, and no intercellular spaces. comparison with intercalary growth situated between zones of
The apical meristem (the growing tip) functions to trigger the permanent tissue
growth of new cells in young seedlings at the tips of roots and
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shoots and forming buds.
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30.12: PLANT DEVELOPMENT - GENETIC CONTROL OF FLOWERS
From a genetic perspective, two phenotypic changes that control
 LEARNING OBJECTIVES vegetative and floral growth are programmed in the plant. The first
genetic change involves the switch from the vegetative to the floral
Diagram the ABC model of flower development and
state. If this genetic change is not functioning properly, then
identify the genes that control that development
flowering will not occur. The second genetic event follows the
commitment of the plant to form flowers. The sequential
Flower development is the process by which angiosperms produce a
development of plant organs suggests that a genetic mechanism
pattern of gene expression in meristems that leads to the appearance
exists in which a series of genes are sequentially turned on and off.
of a flower. A flower (also referred to as a bloom or blossom) is the This switching is necessary for each whorl to obtain its final unique
reproductive structure found in flowering plants. There are three
identity.
physiological developments that must occur in order for
reproduction to take place: ABC MODEL OF FLOWER DEVELOPMENT
In the simple ABC model of floral development, three gene
activities (termed A, B, and C-functions) interact to determine the
developmental identities of the organ primordia (singular:
primordium) within the floral meristem. The ABC model of flower
development was first developed to describe the collection of
genetic mechanisms that establish floral organ identity in the Rosids
and the Asterids; both species have four verticils (sepals, petals,
stamens and carpels), which are defined by the differential
expression of a number of homeotic genes present in each verticil.
In the first floral whorl only A-genes are expressed, leading to the
Figure 30.12.1: Anatomy of a flower: Mature flowers aid in formation of sepals. In the second whorl both A- and B-genes are
reproduction for the plant. In order to achieve reproduction, the plant
must become sexually mature, the apical meristem must become a expressed, leading to the formation of petals. In the third whorl, B
floral meristem, and the flower must develop its individual and C genes interact to form stamens and in the center of the flower
reproductive organs. C-genes alone give rise to carpels. For example, when there is a loss
1. the plant must pass from sexual immaturity into a sexually of B-gene function, mutant flowers are produced with sepals in the
mature state first whorl as usual, but also in the second whorl instead of the
2. the apical meristem must transform from a vegetative meristem normal petal formation. In the third whorl the lack of B function but
into a floral meristem or inflorescence presence of C-function mimics the fourth whorl, leading to the
3. the flowers individual organs must grow (modeled using the formation of carpels also in the third whorl.
ABC model)

FLOWER DEVELOPMENT
A flower develops on a modified shoot or axis from a determinate
apical meristem (determinate meaning the axis grows to a set size).
The transition to flowering is one of the major phase changes that a
plant makes during its life cycle. The transition must take place at a
time that is favorable for fertilization and the formation of seeds,
hence ensuring maximal reproductive success. In order to flower at
an appropriate time, a plant can interpret important endogenous and
environmental cues such as changes in levels of plant hormones and
seasonable temperature and photoperiod changes. Many perennial
and most biennial plants require vernalization to flower.

GENETIC CONTROL OF FLOWER DEVELOPMENT

When plants recognize an opportunity to flower, signals are
transmitted through florigen, which involves a variety of genes,
including CONSTANS, FLOWERING LOCUS C and
Figure 30.12.1: ABC model of flower development: Class A genes
FLOWERING LOCUS T. Florigen is produced in the leaves in (blue) affect sepals and petals, class B genes (yellow) affect petals
reproductively favorable conditions and acts in buds and growing and stamens, class C genes (red) affect stamens and carpels.
tips to induce a number of different physiological and morphological Most genes central in this model belong to the MADS-box genes
changes. and are transcription factors that regulate the expression of the genes

30.12.1 https://bio.libretexts.org/@go/page/13758
specific for each floral organ. CONTRIBUTIONS AND ATTRIBUTIONS
Meristem. Provided by: Wikipedia. Located at:
KEY POINTS http://en.Wikipedia.org/wiki/Meristem. License: CC BY-SA: Attribution-
ShareAlike
Flower development describes the process by which angiosperms meristem. Provided by: Wiktionary. Located at:
(flowering plants) produce a pattern of gene expression in en.wiktionary.org/wiki/meristem. License: CC BY-SA: Attribution-ShareAlike
undifferentiated. Provided by: Wiktionary. Located at:
meristems that leads to the appearance of a flower; the biological en.wiktionary.org/wiki/undifferentiated. License: CC BY-SA: Attribution-
function of a flower is to aid in reproduction. ShareAlike
apical. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/apical.
In order for flowering to occur, three developments must take License: CC BY-SA: Attribution-ShareAlike
place: (1) the plant must reach sexual maturity, (2) the apical Provided by: Wikimedia. Located at:
http://upload.wikimedia.org/Wikipedia/commons/d/d7/M%C3%A9rist%C3
meristem must transform from a vegetative meristem to a floral %A8me_coupe_zones_chiffres.png. License: CC BY-SA: Attribution-
meristem, and (3) the plant must grow individual flower organs. ShareAlike
These developments are initiated using the transmission of a Provided by: Static Flckr. Located at:
http://farm3.staticflickr.com/2441/5717178292_fd834167b1_o.jpg. License:
complex signal known as florigen, which involves a variety of CC BY: Attribution
genes, including CONSTANS, FLOWERING LOCUS C and ABC model of flower development. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/ABC_model_of_flower_development. License: CC
FLOWERING LOCUS T. BY-SA: Attribution-ShareAlike
The last development (the growth of the flower’s individual Flower. Provided by: Wikipedia. Located at: en.Wikipedia.org/wiki/Flower.
License: CC BY-SA: Attribution-ShareAlike
organs) has been modeled using the ABC model of flower verticil. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/verticil.
development. License: CC BY-SA: Attribution-ShareAlike
primordium. Provided by: Wiktionary. Located at:
Class A genes affect sepals and petals, class B genes affect petals en.wiktionary.org/wiki/primordium. License: CC BY-SA: Attribution-
and stamens, class C genes affect stamens and carpels. ShareAlike
perennial. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/perennial. License: CC BY-SA: Attribution-ShareAlike
KEY TERMS apical meristem. Provided by: Wikipedia. Located at:
sepal: a part of an angiosperm, and one of the component parts en.Wikipedia.org/wiki/apical%20meristem. License: CC BY-SA: Attribution-
ShareAlike
of the calyx; collectively the sepals are called the calyx (plural sepal. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/sepal.
calyces), the outermost whorl of parts that form a flower License: CC BY-SA: Attribution-ShareAlike
biennial. Provided by: Wiktionary. Located at:
stamen: in flowering plants, the structure in a flower that en.wiktionary.org/wiki/biennial. License: CC BY-SA: Attribution-ShareAlike
produces pollen, typically consisting of an anther and a filament stamen. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/stamen.
License: CC BY-SA: Attribution-ShareAlike
verticil: a whorl; a group of similar parts such as leaves radiating angiosperm. Provided by: Wiktionary. Located at:
from a shared axis en.wiktionary.org/wiki/angiosperm. License: CC BY-SA: Attribution-
ShareAlike
biennial: a plant that requires two years to complete its life cycle whorl. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/whorl.
whorl: a circle of three or more leaves, flowers, or other organs, License: CC BY-SA: Attribution-ShareAlike
about the same part or joint of a stem Provided by: Wikimedia. Located at:
http://upload.wikimedia.org/Wikipedia/commons/d/d7/M%C3%A9rist%C3
apical meristem: the tissue in most plants containing %A8me_coupe_zones_chiffres.png. License: CC BY-SA: Attribution-
undifferentiated cells (meristematic cells), found in zones of the ShareAlike
Provided by: Static Flckr. Located at:
plant where growth can take place at the tip of a root or shoot. http://farm3.staticflickr.com/2441/5717178292_fd834167b1_o.jpg. License:
angiosperm: a plant whose ovules are enclosed in an ovary CC BY: Attribution
Provided by: Wikimedia. Located at:
perennial: a plant that is active throughout the year or survives http://upload.wikimedia.org/Wikipedia/commons/e/ee/ABC_flower_develop
for more than two growing seasons ment.svg. License: CC BY: Attribution
Mature flower diagram. Provided by: Wikipedia. Located at:
primordium: an aggregation of cells that is the first stage in the en.Wikipedia.org/wiki/File:Mature_flower_diagram.svg. License: CC BY-SA:
development of an organ Attribution-ShareAlike

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30.13: TRANSPORT OF WATER AND SOLUTES IN PLANTS - WATER AND
SOLUTE POTENTIAL
moves to equilibrate, moving from the system or compartment with
 LEARNING OBJECTIVES a higher water potential to the system or compartment with a lower
water potential. This brings the difference in water potential between
Describe the water and solute potential in plants
the two systems (Δ) back to zero (Δ = 0). Therefore, for water to
move through the plant from the soil to the air (a process called
WATER POTENTIAL transpiration), the conditions must exist as such:
Plants are phenomenal hydraulic engineers. Using only the basic Ψsoil > Ψroot > Ψstem > Ψleaf > Ψatmosphere.
laws of physics and the simple manipulation of potential energy,
Water only moves in response to Δ, not in response to the individual
plants can move water to the top of a 116-meter-tall tree. Plants can
components. However, because the individual components influence
also use hydraulics to generate enough force to split rocks and
the total Ψsystem, a plant can control water movement by
buckle sidewalks. Water potential is critical for moving water to
manipulating the individual components (especially Ψs).
leaves so that photosynthesis can take place.
SOLUTE POTENTIAL
Solute potential (Ψs), also called osmotic potential, is negative in a
plant cell and zero in distilled water. Typical values for cell
cytoplasm are –0.5 to –1.0 MPa. Solutes reduce water potential
(resulting in a negative Ψw) by consuming some of the potential
energy available in the water. Solute molecules can dissolve in water
because water molecules can bind to them via hydrogen bonds; a
hydrophobic molecule like oil, which cannot bind to water, cannot
go into solution. The energy in the hydrogen bonds between solute
molecules and water is no longer available to do work in the system
Figure 30.13.1: Water potential in plants: With heights nearing 116 because it is tied up in the bond. In other words, the amount of
meters, (a) coastal redwoods (Sequoia sempervirens) are the tallest available potential energy is reduced when solutes are added to an
trees in the world. Plant roots can easily generate enough force to (b)
buckle and break concrete sidewalks. aqueous system. Thus, Ψs decreases with increasing solute
concentration. Because Ψs is one of the four components of Ψsystem
Water potential is a measure of the potential energy in water, or the
or Ψtotal, a decrease in Ψs will cause a decrease in Ψtotal. The internal
difference in potential energy between a given water sample and
water potential of a plant cell is more negative than pure water
pure water (at atmospheric pressure and ambient temperature).
because of the cytoplasm’s high solute content. Because of this
Water potential is denoted by the Greek letter ψ (psi) and is
difference in water potential, water will move from the soil into a
expressed in units of pressure (pressure is a form of energy) called
megapascals (MPa). The potential of pure water (Ψwpure H2O) is plant’s root cells via the process of osmosis. This is why solute
designated a value of zero (even though pure water contains plenty potential is sometimes called osmotic potential.
of potential energy, that energy is ignored). Water potential values
for the water in a plant root, stem, or leaf are, therefore, expressed in
relation to Ψwpure H2O.
The water potential in plant solutions is influenced by solute
concentration, pressure, gravity, and factors called matrix effects.
Water potential can be broken down into its individual components
using the following equation:
Ψsystem = Ψtotal = Ψs + Ψp + Ψg + Ψm
where
Ψs = solute potential
Ψp, = pressure potential
Ψg, = gravity potential
Ψm = matric potential
“System” can refer to the water potential of the soil water (Ψsoil),
root water (Ψroot), stem water (Ψstem), leaf water (Ψleaf), or the water
in the atmosphere (Ψatmosphere), whichever aqueous system is under
consideration. As the individual components change, they raise or
lower the total water potential of a system. When this happens, water

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 KEY POINTS
Plants use water potential to transport water to the leaves so that
photosynthesis can take place.
Water potential is a measure of the potential energy in water as
well as the difference between the potential in a given water
sample and pure water.
Water potential is represented by the equation Ψsystem = Ψtotal =
Ψs + Ψp + Ψg + Ψm.
Water always moves from the system with a higher water
potential to the system with a lower water potential.
Solute potential (Ψs) decreases with increasing solute
concentration; a decrease in Ψs causes a decrease in the total
water potential.
The internal water potential of a plant cell is more negative than
pure water; this causes water to move from the soil into plant
roots via osmosis..

KEY TERMS
Figure 30.13.1: Solute potential: In this example with a solute potential: (osmotic potential) pressure which needs to be
semipermeable membrane between two aqueous systems, water will
applied to a solution to prevent the inward flow of water across a
move from a region of higher to lower water potential until
equilibrium is reached. Solutes (Ψs), pressure (Ψp), and gravity (Ψg) semipermeable membrane
influence total water potential for each side of the tube (Ψtotal right or transpiration: the loss of water by evaporation in terrestrial
left) and, therefore, the difference between Ψtotal on each side (Δ). plants, especially through the stomata; accompanied by a
(Ψm, the potential due to interaction of water with solid substrates, is
ignored in this example because glass is not especially hydrophilic). corresponding uptake from the roots
Water moves in response to the difference in water potential between water potential: the potential energy of water per unit volume;
two systems (the left and right sides of the tube).
designated by ψ
Plant cells can metabolically manipulate Ψs (and by extension,
Ψtotal) by adding or removing solute molecules. Therefore, plants This page titled 30.13: Transport of Water and Solutes in Plants - Water and
have control over Ψtotal via their ability to exert metabolic control Solute Potential is shared under a CC BY-SA 4.0 license and was authored,
over Ψs. remixed, and/or curated by Boundless.

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30.14: TRANSPORT OF WATER AND SOLUTES IN PLANTS - PRESSURE,
GRAVITY, AND MATRIC POTENTIAL
GRAVITY POTENTIAL
 LEARNING OBJECTIVES Gravity potential (Ψg) is always negative or zero in a plant with no
height. Without height, there is no potential energy in the system.
Differentiate among pressure, gravity, and matric potentials
The force of gravity pulls water downwards to the soil, which
in plants
reduces the total amount of potential energy in the water in the plant
(Ψtotal). The taller the plant, the taller the water column, and the
PRESSURE POTENTIAL
more influential Ψg becomes. On a cellular scale and in short plants,
Pressure potential is also called turgor potential or turgor pressure this effect is negligible and easily ignored. However, over the height
and is represented by Ψp. Pressure potential may be positive or of a tall tree like a giant coastal redwood, the plant must overcome
negative; the higher the pressure, the greater potential energy in a an extra 1MPa of resistance because of the gravitational pull of –0.1
system, and vice versa. Therefore, a positive Ψp (compression) MPa m-1.
increases Ψtotal, while a negative Ψp (tension) decreases Ψtotal.
Positive pressure inside cells is contained by the cell wall, producing MATRIC POTENTIAL
turgor pressure in a plant. Turgor pressure ensures that a plant can Matric potential (Ψm) is the amount of water bound to the matrix of
maintain its shape. A plant’s leaves wilt when the turgor pressure a plant via hydrogen bonds and is always negative to zero. In a dry
decreases and revive when the plant has been watered. Pressure system, it can be as low as –2 MPa in a dry seed or as high as zero in
potentials are typically around 0.6–0.8 MPa, but can reach as high as a water-saturated system. Every plant cell has a cellulosic cell wall,
1.5 MPa in a well-watered plant. As a comparison, most automobile which is hydrophilic and provides a matrix for water adhesion,
tires are kept at a pressure of 30–34 psi or about 0.207-0.234 MPa. hence the name matric potential. The binding of water to a matrix
Water is lost from the leaves via transpiration (approaching Ψp = 0 always removes or consumes potential energy from the system. Ψm
MPa at the wilting point) and restored by uptake via the roots. is similar to solute potential because the hydrogen bonds remove
energy from the total system. However, in solute potential, the other
components are soluble, hydrophilic solute molecules, whereas in
Ψm, the other components are insoluble, hydrophilic molecules of
the plant cell wall. m cannot be manipulated by the plant and is
typically ignored in well-watered roots, stems, and leaves.

KEY POINTS
The higher the pressure potential (Ψp), the more potential energy
in a system: a positive Ψp increases Ψtotal, while a negative Ψp
decreases Ψtotal.
Figure 30.14.1: Turgor pressure: When (a) total water potential Positive pressure inside cells is contained by the cell wall,
(Ψtotal) is lower outside the cells than inside, water moves out of the producing turgor pressure, which is responsible for maintaining
cells and the plant wilts. When (b) the total water potential is higher
outside the plant cells than inside, water moves into the cells,
the structure of leaves; absence of turgor pressure causes wilting.
resulting in turgor pressure (Ψp), keeping the plant erect. Plants lose water (and turgor pressure) via transpiration through
A plant can manipulate Ψp via its ability to manipulate Ψs (solute the stomata in the leaves and replenish it via positive pressure in
potential) and by the process of osmosis. Plants must overcome the the roots.
negative forces of gravity potential (Ψg) and matric potential (Ψm) Pressure potential is controlled by solute potential (when solute
to maintain a positive pressure potential. If a plant cell increases the potential decreases, pressure potential increases) and the opening
cytoplasmic solute concentration: and closing of stomata.
Gravity potential (Ψg) removes potential energy from the system
1. Ψs will decline
because gravity pulls water downwards to the soil, reducing
2. Ψtotal will decline
Ψtotal.
3. the Δ between the cell and the surrounding tissue will decline
Matric potential (Ψm) removes energy from the system because
4. water will move into the cell by osmosis
water molecules bind to the cellulose matrix of the plant’s cell
5. Ψp will increase.
walls.
Plants can also regulate Ψp by opening and closing the stomata.
Stomatal openings allow water to evaporate from the leaf, reducing KEY TERMS
Ψp and Ψtotal. This increases water potential between the water in the turgor pressure: pushes the plasma membrane against the cell
the petiole (base of the leaf) and in the leaf, thereby encouraging wall of plant; caused by the osmotic flow of water from outside
water to flow from the petiole into the leaf. of the cell into the cell’s vacuole

30.14.1 https://bio.libretexts.org/@go/page/13761
This page titled 30.14: Transport of Water and Solutes in Plants - Pressure, was authored, remixed, and/or curated by Boundless.
Gravity, and Matric Potential is shared under a CC BY-SA 4.0 license and

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30.15: TRANSPORT OF WATER AND SOLUTES IN PLANTS - MOVEMENT OF
WATER AND MINERALS IN THE XYLEM

 LEARNING OBJECTIVES

Outline the movement of water and minerals in the xylem

MOVEMENT OF WATER AND MINERALS IN THE

XYLEM
Most plants obtain the water and minerals they need through their
roots. The path taken is: soil -> roots -> stems -> leaves. The
minerals (e.g., K+, Ca2+) travel dissolved in the water (often
accompanied by various organic molecules supplied by root cells).
Water and minerals enter the root by separate paths which eventually
converge in the stele, or central vascular bundle in roots.
Transpiration is the loss of water from the plant through evaporation
at the leaf surface. It is the main driver of water movement in the
xylem. Transpiration is caused by the evaporation of water at the
leaf, or atmosphere interface; it creates negative pressure (tension)
equivalent to –2 MPa at the leaf surface. However, this value varies
greatly depending on the vapor pressure deficit, which can be
insignificant at high relative humidity (RH) and substantial at low
RH. Water from the roots is pulled up by this tension. At night,
when stomata close and transpiration stops, the water is held in the
stem and leaf by the cohesion of water molecules to each other as
Figure 30.15.1: Cohesion–Tension Theory of Sap Ascent: The
well as the adhesion of water to the cell walls of the xylem vessels cohesion–tension theory of sap ascent is shown. Evaporation from
and tracheids. This is called the cohesion–tension theory of sap the mesophyll cells produces a negative water potential gradient that
ascent. causes water to move upwards from the roots through the xylem.

The cohesion-tension theory explains how water moves up through CONTROL OF TRANSPIRATION
the xylem. Inside the leaf at the cellular level, water on the surface
Transpiration is a passive process: metabolic energy in the form of
of mesophyll cells saturates the cellulose microfibrils of the primary
ATP is not required for water movement. The energy driving
cell wall. The leaf contains many large intercellular air spaces for the
transpiration is the difference in energy between the water in the soil
exchange of oxygen for carbon dioxide, which is required for and the water in the atmosphere. However, transpiration is tightly
photosynthesis. The wet cell wall is exposed to the internal air space controlled. The atmosphere to which the leaf is exposed drives
and the water on the surface of the cells evaporates into the air transpiration, but it also causes massive water loss from the plant.
spaces. This decreases the thin film on the surface of the mesophyll Up to 90 percent of the water taken up by roots may be lost through
cells. The decrease creates a greater tension on the water in the
transpiration.
mesophyll cells, thereby increasing the pull on the water in the
Leaves are covered by a waxy cuticle on the outer surface that
xylem vessels. The xylem vessels and tracheids are structurally
prevents the loss of water. Regulation of transpiration, therefore, is
adapted to cope with large changes in pressure. Small perforations
achieved primarily through the opening and closing of stomata on
between vessel elements reduce the number and size of gas bubbles
the leaf surface. Stomata are surrounded by two specialized cells
that form via a process called cavitation. The formation of gas
called guard cells, which open and close in response to
bubbles in the xylem is detrimental since it interrupts the continuous
environmental cues such as light intensity and quality, leaf water
stream of water from the base to the top of the plant, causing a break
status, and carbon dioxide concentrations. Stomata must open to
(embolism) in the flow of xylem sap. The taller the tree, the greater
allow air containing carbon dioxide and oxygen to diffuse into the
the tension forces needed to pull water in a continuous column,
leaf for photosynthesis and respiration. When stomata are open,
increasing the number of cavitation events. In larger trees, the
however, water vapor is lost to the external environment, increasing
resulting embolisms can plug xylem vessels, making them non-
the rate of transpiration. Therefore, plants must maintain a balance
functional.
between efficient photosynthesis and water loss.
Plants have evolved over time to adapt to their local environment
and reduce transpiration. Desert plant (xerophytes) and plants that
grow on other plants ( epiphytes ) have limited access to water. Such
plants usually have a much thicker waxy cuticle than those growing

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in more moderate, well-watered environments (mesophytes). Xerophytes and epiphytes often have a thick covering of trichomes
Aquatic plants (hydrophytes) also have their own set of anatomical or stomata that are sunken below the leaf’s surface. Trichomes are
and morphological leaf adaptations. specialized hair-like epidermal cells that secrete oils and other
substances. These adaptations impede air flow across the stomatal
pore and reduce transpiration. Multiple epidermal layers are also
commonly found in these types of plants.

KEY POINTS
The cohesion – tension theory of sap ascent explains how how
water is pulled up from the roots to the top of the plant.
Evaporation from mesophyll cells in the leaves produces a
negative water potential gradient that causes water and minerals
to move upwards from the roots through the xylem.
Gas bubbles in the xylem can interrupt the flow of water in the
plant, so they must be reduced through small perforations
between vessel elements.
Transpiration is controlled by the opening and closing of stomata
in response to environmental cues.
Stomata must open for photosynthesis and respiration, but when
stomata are open, water vapor is lost to the external environment,
increasing the rate of transpiration.
Desert plants and plants with limited water access prevent
transpiration and excess water loss by utilizing a thicker cuticle,
trichomes, or multiple epidermal layers.

KEY TERMS
cohesion–tension theory of sap ascent: explains the process of
water flow upwards (against the force of gravity) through the
xylem of plants
cavitation: the formation, in a fluid, of vapor bubbles that can
Figure 30.15.1: Reducing Transpiration: Plants are suited to their interrupt water flow through the plant
local environment. (a) Xerophytes, like this prickly pear cactus trichome: a hair- or scale-like extension of the epidermis of a
(Opuntia sp.) and (b) epiphytes such as this tropical Aeschynanthus plant
perrottetii have adapted to very limited water resources. The leaves
of a prickly pear are modified into spines, which lowers the surface-
to-volume ratio and reduces water loss. Photosynthesis takes place This page titled 30.15: Transport of Water and Solutes in Plants - Movement
in the stem, which also stores water. (b) A. perrottetii leaves have a of Water and Minerals in the Xylem is shared under a CC BY-SA 4.0 license
waxy cuticle that prevents water loss. (c) Goldenrod (Solidago sp.) and was authored, remixed, and/or curated by Boundless.
is a mesophyte, well suited for moderate environments. (d)
Hydrophytes, like this fragrant water lily (Nymphaea odorata), are
adapted to thrive in aquatic environments.

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30.16: TRANSPORT OF WATER AND SOLUTES IN PLANTS - TRANSPORTATION
OF PHOTOSYNTHATES IN THE PHLOEM
Phloem STEs have reduced cytoplasmic contents and are connected
 LEARNING OBJECTIVES by sieve plates with pores that allow for pressure-driven bulk flow,
or translocation, of phloem sap. Companion cells are associated with
Explain the transport of photosynthates in the phloem
STEs. They assist with metabolic activities and produce energy for
the STEs.
TRANSPORTATION OF PHOTOSYNTHATES IN
THE PHLOEM
Plants need an energy source to grow. In seeds and bulbs, food is
stored in polymers (such as starch) that are converted by metabolic
processes into sucrose for newly-developing plants. Once green
shoots and leaves begin to grow, plants can produce their own food
by photosynthesis. The products of photosynthesis are called
photosynthates, which are usually in the form of simple sugars such
as sucrose.

SOURCES AND SINKS

Sources are the structures that produce photosynthates for the
growing plant. The sugars produced in the sources, such as leaves,
must be delivered to growing parts of the plant. These sugars are
transported through the plant via the phloem in a process called
translocation. The points of sugar delivery, such as roots, young
shoots, and developing seeds, are called sinks. Seeds, tubers, and
bulbs can be either a source or a sink, depending on the plant’s stage
of development and the season. Figure 30.16.1: Translocation to the phloem: Phloem is comprised
of cells called sieve-tube elements. Phloem sap travels through
The products from the source are usually translocated to the nearest perforations called sieve tube plates. Neighboring companion cells
sink through the phloem. For example, photosynthates produced in carry out metabolic functions for the sieve-tube elements and
provide them with energy. Lateral sieve areas connect the sieve-tube
the upper leaves will travel upward to the growing shoot tip, while
elements to the companion cells.
photosynthates in the lower leaves will travel downward to the roots.
Once in the phloem, the photosynthates are translocated to the
Intermediate leaves will send products in both directions. The
closest sink. Phloem sap is an aqueous solution that contains up to
multidirectional flow of phloem contrasts the flow of xylem, which
30 percent sugar, minerals, amino acids, and plant growth regulators.
is always unidirectional (soil to leaf to atmosphere). However, the
The high percentage of sugar decreases Ψs, which decreases the total
pattern of photosynthate flow changes as the plant grows and
water potential, causing water to move by osmosis from the adjacent
develops. Photosynthates are directed primarily to the roots during
xylem into the phloem tubes. This flow of water increases water
early development, to shoots and leaves during vegetative growth,
pressure inside the phloem, causing the bulk flow of phloem sap
and to seeds and fruits during reproductive development. They are
from source to sink. Sucrose concentration in the sink cells is lower
also directed to tubers for storage.
than in the phloem STEs because the sink sucrose has been
TRANSLOCATION: TRANSPORT FROM SOURCE metabolized for growth or converted to starch (for storage) or other
TO SINK polymers (for structural integrity). Unloading at the sink end of the
phloem tube occurs by either diffusion or active transport of sucrose
Photosynthates are produced in the mesophyll cells of
molecules from an area of high concentration to one of low
photosynthesizing leaves. From there, they are translocated through
concentration. Water diffuses from the phloem by osmosis and is
the phloem where they are used or stored. Mesophyll cells are
then transpired or recycled via the xylem back into the phloem sap.
connected by cytoplasmic channels called plasmodesmata.
Photosynthates move through plasmodesmata to reach phloem sieve-
tube elements (STEs) in the vascular bundles. From the mesophyll
cells, the photosynthates are loaded into the phloem STEs. The
sucrose is actively transported against its concentration gradient (a
process requiring ATP) into the phloem cells using the
electrochemical potential of the proton gradient. This is coupled to
the uptake of sucrose with a carrier protein called the sucrose-H+
symporter.

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 sink: where sugars are delivered in a plant, such as the roots,
young shoots, and developing seeds

CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44708/latest...ol11448/latest. License: CC
BY: Attribution
water potential. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/water_potential. License: CC BY-SA: Attribution-
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solute potential. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/solute%20potential. License: CC BY-SA: Attribution-
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BY: Attribution
turgor pressure. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/turgor%20pressure. License: CC BY-SA: Attribution-
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Figure 30.16.1: Translocation to the sink: Sucrose is actively OpenStax College, Transport of Water and Solutes in Plants. October 17, 2013.
transported from source cells into companion cells and then into the Provided by: OpenStax CNX. Located at:
sieve-tube elements. This reduces the water potential, which causes http://cnx.org/content/m44708/latest...e_30_05_01.jpg. License: CC BY:
water to enter the phloem from the xylem. The resulting positive Attribution
pressure forces the sucrose-water mixture down toward the roots, OpenStax College, Transport of Water and Solutes in Plants. October 17, 2013.
where sucrose is unloaded. Transpiration causes water to return to Provided by: OpenStax CNX. Located at:
the leaves through the xylem vessels. http://cnx.org/content/m44708/latest...e_30_05_02.png. License: CC BY:
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KEY POINTS Provided by: OpenStax CNX. Located at:
The products of photosynthesis are called photosynthates; they http://cnx.org/content/m44708/latest...e_30_05_03.jpg. License: CC BY:
Attribution
are usually in the form of simple sugars, such as sucrose. OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Photosynthates are produced by sources and are translocated to Located at: http://cnx.org/content/m44708/latest...ol11448/latest. License: CC
BY: Attribution
sinks. trichome. Provided by: Wiktionary. Located at:
Photosynthates are directed primarily to the roots during early en.wiktionary.org/wiki/trichome. License: CC BY-SA: Attribution-ShareAlike
Transport of Water and Minerals in Plants. Provided by: Kimball's Biology
development, to shoots and leaves during vegetative growth, and Pages. Located at: http://biology-pages.info/X/Xylem.html. License: CC BY:
to seeds and fruits during reproductive development. Attribution
Photosynthates are produced in the mesophyll cells of leaves and cavitation. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/cavitation. License: CC BY-SA: Attribution-
are translocated through the phloem; they are then transported to ShareAlike
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The high percentage of sugar in phloem sap causes water to BY-SA: Attribution-ShareAlike
move from the xylem into the phloem, which increases water OpenStax College, Transport of Water and Solutes in Plants. October 17, 2013.
Provided by: OpenStax CNX. Located at:
pressure inside the phloem, causing the sap to move from source http://cnx.org/content/m44708/latest...e_30_05_01.jpg. License: CC BY:
to sink. Attribution
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Sucrose concentration in the sink cells is lower than in the Provided by: OpenStax CNX. Located at:
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occurs by either diffusion or active transport of sucrose OpenStax College, Transport of Water and Solutes in Plants. October 17, 2013.
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concentration. http://cnx.org/content/m44708/latest...e_30_05_03.jpg. License: CC BY:
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source: structure that produces photosynthates Attribution
photosynthate: any compound that is a product of OpenStax College, Transport of Water and Solutes in Plants. October 17, 2013.
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source. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/source. OpenStax College, Transport of Water and Solutes in Plants. October 17, 2013.
License: CC BY-SA: Attribution-ShareAlike Provided by: OpenStax CNX. Located at:
Boundless. Provided by: Boundless Learning. Located at: http://cnx.org/content/m44708/latest/Figure_30_05_04.png. License: CC BY:
www.boundless.com//biology/definition/sink. License: CC BY-SA: Attribution- Attribution
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Provided by: OpenStax CNX. Located at: SA 4.0 license and was authored, remixed, and/or curated by Boundless.
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30.17: PLANT SENSORY SYSTEMS AND RESPONSES - PLANT RESPONSES
TO LIGHT
because of the quality of light available in the daylight spectrum. In
 LEARNING OBJECTIVES terrestrial habitats, light absorption by chlorophylls peaks in the blue
and red regions of the spectrum. As light filters through the canopy
Compare the ways plants respond to light
and the blue and red wavelengths are absorbed, the spectrum shifts
to the far-red end, shifting the plant community to those plants better
PLANT RESPONSES TO LIGHT adapted to respond to far-red light. Blue-light receptors allow plants
Plants have a number of sophisticated uses for light that go far to gauge the direction and abundance of sunlight, which is rich in
beyond their ability to perform photosynthesis. Plants can blue–green emissions. Water absorbs red light, which makes the
differentiate and develop in response to light (known as detection of blue light essential for algae and aquatic plants.
photomorphogenesis), which allows plants to optimize their use of
light and space. Plants use light to track time, which is known as KEY POINTS
photoperiodism. They can tell the time of day and time of year by Plants grow and differentiate to optimize their space, using light
sensing and using various wavelengths of sunlight. Light can also in a process known as photomorphogenesis.
elicit a directional response in plants that allows them to grow Plants grow and move toward or away from light depending on
toward, or even away from, light; this is known as phototropism. their needs; this process is known as phototropism.
Photoperiodism is illustrated by how plants flower and grow at
certain times of the day or year through the use of photoreceptors
that sense the wavelengths of sunlight available during the day
(versus night) and throughout the seasons.
The various wavelengths of light, red/far-red or blue regions of
the visible light spectrum, trigger structural responses in plants
suited for responding to those wavelengths.

Figure 30.17.1: Phototropism of an orchid plant: This orchid plant KEY TERMS
placed next to a window grows toward the sunlight through the photoreceptor: a specialized protein that is able to detect and
window. This is an example of positive phototropism.
react to light
The sensing of light in the environment is important to plants; it can
photoperiodism: the growth, development and other responses
be crucial for competition and survival. The response of plants to
of plants and animals according to the length of day and/or night
light is mediated by different photoreceptors: a protein covalently-
photomorphogenesis: the regulatory effect of light on the
bonded to a light-absorbing pigment called a chromophore; together,
growth, development and differentiation of plant cells, tissues
called a chromoprotein. The chromophore of the photoreceptor
and organs
absorbs light of specific wavelengths, causing structural changes in
phototropism: the movement of a plant toward or away from
the photoreceptor protein. The structural changes then elicit a
light
cascade of signaling throughout the plant.
The red, far-red, and violet-blue regions of the visible light spectrum This page titled 30.17: Plant Sensory Systems and Responses - Plant
trigger structural development in plants. Sensory photoreceptors Responses to Light is shared under a CC BY-SA 4.0 license and was
absorb light in these particular regions of the visible light spectrum authored, remixed, and/or curated by Boundless.

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30.18: PLANT SENSORY SYSTEMS AND RESPONSES - THE PHYTOCHROME
SYSTEM AND RED LIGHT RESPONSE
red light and have activated Pfr, which induces growth toward sunlit
 LEARNING OBJECTIVES areas. Because competition for light is so fierce in a dense plant
community, those plants who could grow toward light the fastest and
Explain the response of the phytochrome system to red/far-
most efficiently became the most successful.
red light
THE PHYTOCHROME SYSTEM IN SEEDS
The phytochromes are a family of chromoproteins with a linear In seeds, the phytochrome system is used to determine the presence
tetrapyrrole chromophore, similar to the ringed tetrapyrrole light- or absence of light, rather than the quality. This is especially
absorbing head group of chlorophyll. Phytochromes have two photo- important in species with very small seeds and, therefore, food
interconvertible forms: Pr and Pfr. Pr absorbs red light (~667 nm) reserves. For example, if lettuce seedlings germinated a centimeter
and is immediately converted to Pfr. Pfr absorbs far-red light (~730 under the soil surface, the seedling would exhaust its food resources
nm) and is quickly converted back to Pr. Absorption of red or far-red and die before reaching the surface. A seed will only germinate if
light causes a massive change to the shape of the chromophore, exposed to light at the surface of the soil, causing Pr to be converted
altering the conformation and activity of the phytochrome protein to to Pfr, signaling the start of germination. In the dark, phytochrome is
which it is bound. Pfr is the physiologically-active form of the in the inactive Pr form so the seed will not germinate.
protein; exposure to red light yields physiological activity in the
plant. Exposure to far-red light converts the Pfr to the inactive Pr PHOTOPERIODISM
form, inhibiting phytochrome activity. Together, the two forms Plants also use the phytochrome system to adjust growth according
represent the phytochrome system. to the seasons. Photoperiodism is a biological response to the timing
and duration of dark and light periods. Since unfiltered sunlight is
rich in red light, but deficient in far-red light, at dawn, all the
phytochrome molecules in a leaf convert to the active Pfr form and
remain in that form until sunset. Since Pfr reverts to Pr during
darkness, there will be no Pfr remaining at sunrise if the night is
long (winter) and some Pfr remaining if the night is short (summer).
The amount of Pfr present stimulates flowering, setting of winter
buds, and vegetative growth according to the seasons.
In addition, the phytochrome system enables plants to compare the
length of dark periods over several days. Shortening nights indicate
springtime to the plant; lengthening nights indicate autumn. This
Figure 30.18.1: Phytochrome system: The biologically-inactive information, along with sensing temperature and water availability,
form of phytochrome (Pr) is converted to the biologically-active allows plants to determine the time of the year and adjust their
form Pfr under illumination with red light. Far-red light and
darkness convert the molecule back to the inactive form. physiology accordingly. Short-day (long-night) plants use this
The phytochrome system acts as a biological light switch. It information to flower in the late summer and early fall when nights
monitors the level, intensity, duration, and color of environmental exceed a critical length (often eight or fewer hours). Long-day
(short-night) plants flower during the spring when darkness is less
light. The effect of red light is reversible by immediately shining far-
red light on the sample, which converts the chromoprotein to the than a critical length (often 8 to 15 hours). However, day-neutral
plants do not regulate flowering by day length. Not all plants use the
inactive Pr form. Additionally, Pfr can slowly revert to Pr in the dark
or break down over time. In all instances, the physiological response phyotochrome system to adjust their physiological responses to the
induced by red light is reversed. The active form of phytochrome seasons.
(Pfr) can directly activate other molecules in the cytoplasm, or it can
KEY POINTS
be trafficked to the nucleus, where it directly activates or represses
Exposure to red light converts the chromoprotein to the
specific gene expression.
functional, active form (Pfr), while darkness or exposure to far-
THE PHYTOCHROME SYSTEM AND GROWTH red light converts the chromophore to the inactive form (Pr).
Plants use the phytochrome system to grow away from shade and Plants grow toward sunlight because the red light from the sun
converts the chromoprotein into the active form (Pfr), which
toward light. Unfiltered, full sunlight contains much more red light
triggers plant growth; plants in shade slow growth because the
than far-red light. Any plant in the shade of another plant will be
inactive form (Pr) is produced.
exposed to red-depleted, far-red-enriched light because the other
If seeds sense light using the phytochrome system, they will
plant has absorbed most of the other red light. The exposure to red
light converts phytochrome in the shaded leaves to the Pr (inactive) germinate.
form, which slows growth. The leaves in full sunlight are exposed to

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 Plants regulate photoperiodism by measuring the Pfr/Pr ratio at chromophore: the group of atoms in a molecule in which the
dawn, which then stimulates physiological processes such as electronic transition responsible for a given spectral band is
flowering, setting winter buds, and vegetative growth. located
photoperiodism: the growth, development and other responses
KEY TERMS of plants and animals according to the length of day and/or night
phytochrome: any of a class of pigments that control most
photomorphogenic responses in higher plants This page titled 30.18: Plant Sensory Systems and Responses - The
Phytochrome System and Red Light Response is shared under a CC BY-SA
4.0 license and was authored, remixed, and/or curated by Boundless.

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30.19: PLANT SENSORY SYSTEMS AND RESPONSES - BLUE LIGHT
RESPONSE
light when illuminated even though the layer of gelatin was present.
 LEARNING OBJECTIVES However, when impermeable mica flakes were inserted between the
tip and the cut base, the seedling did not bend.
Differentiate among blue light responses of plants
A refinement of Boysen-Jensen’s experiment showed that the signal
traveled on the shaded side of the seedling. When the mica plate was
Phototropism is the directional bending of a plant toward or away
inserted on the illuminated side, the plant still bent toward the light.
from a light source of blue wavelengths of light. Positive
Therefore, the chemical signal from the sunlight, which is blue
phototropism is growth toward a light source, while negative
wavelengths of light, was a growth stimulant; the phototropic
phototropism (also called skototropism) is growth away from light.
response involved faster cell elongation on the shaded side than on
Several proteins use blue light to control various physiological
the illuminated side, causing the plant to bend. We now know that as
processes in the plant.
light passes through a plant stem, it is diffracted and generates
phototropin activation across the stem. Most activation occurs on the
lit side, causing the plant hormones indole acetic acid (IAA) or
auxin to accumulate on the shaded side. Stem cells elongate under
the influence of IAA.

Figure 30.19.1: Blue light response of azure bluets: Azure bluets

(Houstonia caerulea) display a phototropic response by bending
toward the light.

PHOTOTROPINS AND PHYSIOLOGICAL

RESPONSES E
Figure 30.19.1: Phototropism and the distribution of auxin:
Phototropins are protein-based receptors responsible for mediating Phototropism is the growth of plants in response to light. When the
the phototropic response in plants. Like all plant photoreceptors, sun is positioned almost directly over the plant, the hormone auxin
phototropins consist of a protein portion and a light-absorbing (pink dots) in the plant stem is evenly distributed. As the sun moves,
the auxin is repositioned on the other side of the plant. This overload
portion, called the chromophore, which senses blue wavelengths of of auxin next to these cells causes them to start to grow or elongate,
light. Phototropins belong to a class of proteins called flavoproteins tipping the growth of the stem toward the light.
because the chromophore is a covalently-bound molecule of flavin.
CRYPTOCHROMES
Phototropins control other physiological responses including leaf
Cryptochromes are another class of blue-light absorbing
opening and closing, chloroplast movement, and the opening of
stomata. However, of all responses controlled by phototropins, photoreceptors. Their chromophores also contain a flavin-based
chromophore. Cryptochromes set the plant’s circadian rhythm (the
phototropism has been studied the longest and is the best
understood. 24-hour activity cycle) using blue light receptors. There is some
evidence that cryptochromes work by sensing light-dependent redox
PHOTOTROPISM AND AUXIN reactions and that, together with phototropins, they mediate the
In 1880, Charles Darwin and his son Francis first described phototropic response.
phototropism as the bending of seedlings toward light. Darwin
KEY POINTS
observed that light was perceived by the the apical meristem (tip of
the plant), but that the plant bent in response in a different part of the In addition to phototropism, phototropins sense blue light to
plant. The Darwins concluded that the signal had to travel from the control leaf opening and closing, chloroplast movement, and the
apical meristem to the base of the plant, where it bent. opening of stomata.
When phototropins are activated by blue light, the hormone
In 1913, Peter Boysen-Jensen conducted an experiment that
auxin accumulates on the shaded side of the plant, triggering
demonstrated that a chemical signal produced in the plant tip was
elongation of stem cells and phototropism.
responsible for the plant’s bending response at the base. He cut off
Cryptochromes sense blue light-dependent redox reactions to
the tip of a seedling, covered the cut section with a permeable layer
control the circadian rhythm of plants.
of gelatin, and then replaced the tip. The seedling bent toward the

30.19.1 https://bio.libretexts.org/@go/page/13767
KEY TERMS cryptochrome: any of several light-sensitive flavoproteins, in
skototropism: growth or movement away from light the protoreceptors of plants, that regulate germination,
phototropin: any of a class of photoreceptor flavoproteins that elongation, and photoperiodism
mediate phototropism in higher plants
This page titled 30.19: Plant Sensory Systems and Responses - Blue Light
auxin: a class of plant growth hormones that is responsible for
Response is shared under a CC BY-SA 4.0 license and was authored,
elongation in phototropism and gravitropism and for other
remixed, and/or curated by Boundless.
growth processes in the plant life cycle

30.19.2 https://bio.libretexts.org/@go/page/13767
30.20: PLANT SENSORY SYSTEMS AND RESPONSES - PLANT RESPONSES
TO GRAVITY
Time-lapse of pea shoot and root growth: Time-lapse of a pea
 LEARNING OBJECTIVES plant growing from seed, showing both the shoot and root system.
The roots grown downward in the direction of gravity, which is
Describe the role of amyloplasts in gravitropism
positive gravitropism, and the shoot grows upward away from
gravity, which is negative gravitropism.
Whether or not they germinate in the light or in total darkness,
The reason plants know which way to grow in response to gravity is
shoots usually sprout up from the ground, while roots grow
due to amyloplasts in the plants. Amyloplasts (also known as
downward into the ground. A plant laid on its side in the dark will
statoliths ) are specialized plastids that contain starch granules and
send shoots upward when given enough time. Gravitropism ensures
settle downward in response to gravity. Amyloplasts are found in
that roots grow into the soil and that shoots grow toward sunlight.
shoots and in specialized cells of the root cap. When a plant is tilted,
Growth of the shoot apical tip upward is called negative
the statoliths drop to the new bottom cell wall. A few hours later, the
gravitropism, whereas growth of the roots downward is called
shoot or root will show growth in the new vertical direction.
positive gravitropism.

Time Lapse of Pea Shoot / Root Growth

Figure 30.20.1: Gravitropism: This is an image of an upright tree

with high curvature at the base as a result of negative gravitropism.
Despite being tilted, amyloplasts will cause the shoot to grow in a
vertical direction.
The mechanism that mediates gravitropism is reasonably well
understood. When amyloplasts settle to the bottom of the gravity-
sensing cells in the root or shoot, they physically contact the
endoplasmic reticulum (ER). This causes the release of calcium ions
from inside the ER. This calcium signaling in the cells causes polar
transport of the plant hormone indole acetic acid (IAA) to the
bottom of the cell. In roots, a high concentration of IAA inhibits cell
elongation. The effect slows growth on the lower side of the root

30.20.1 https://bio.libretexts.org/@go/page/13768
while cells develop normally on the upper side. IAA has the release of indole acetic acid.
opposite effect in shoots, where a higher concentration at the lower Indole acetic acid inhibits cell elongation in the lower side of
side of the shoot stimulates cell expansion and causes the shoot to roots, but stimulates cell expansion in shoots, which causes
grow up. After the shoot or root begin to grow vertically, the shoots to grow upward.
amyloplasts return to their normal position. Other hypotheses, which
involve the entire cell in the gravitropism effect, have been proposed KEY TERMS
to explain why some mutants that lack amyloplasts may still exhibit amyloplast: a non-pigmented organelle found in some plant cells
a weak gravitropic response. that is responsible for the synthesis and storage of starch granules
through the polymerization of glucose
KEY POINTS statolith: a specialized form of amyloplast involved in
Positive gravitropism occurs when roots grow into soil because graviperception by plant roots and most invertebrates
they grow in the direction of gravity while negative gravitropism gravitropism: a plant’s ability to change its growth in response
occurs when shoots grow up toward sunlight in the opposite to gravity
direction of gravity.
Amyloplasts settle at the bottom of the cells of the shoots and This page titled 30.20: Plant Sensory Systems and Responses - Plant
roots in response to gravity, causing calcium signaling and the Responses to Gravity is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

30.20.2 https://bio.libretexts.org/@go/page/13768
30.21: PLANT SENSORY SYSTEMS AND RESPONSES - AUXINS, CYTOKININS,
AND GIBBERELLINS
cytokinesis (cell division). Almost 200 naturally-occurring or
 LEARNING OBJECTIVES synthetic cytokinins are known, to date. Cytokinins are most
abundant in growing tissues, such as roots, embryos, and fruits,
Differentiate among the types of plant hormones and their
where cell division is occurring. Cytokinins are known to delay
effects on plant growth
senescence in leaf tissues, promote mitosis, and stimulate
differentiation of the meristem in shoots and roots. Many effects on
GROWTH RESPONSES plant development are under the influence of cytokinins, either in
A plant’s sensory response to external stimuli relies on hormones, conjunction with auxin or another hormone. For example, apical
which are simply chemical messengers. Plant hormones affect all dominance seems to result from a balance between auxins that
aspects of plant life, from flowering to fruit setting and maturation, inhibit lateral buds and cytokinins that promote bushier growth.
and from phototropism to leaf fall. Potentially, every cell in a plant
can produce plant hormones. The hormones can act in their cell of GIBBERELLINS
origin or be transported to other portions of the plant body, with Gibberellins (GAs) are a group of about 125 closely-related plant
many plant responses involving the synergistic or antagonistic hormones that stimulate shoot elongation, seed germination, and
interaction of two or more hormones. In contrast, animal hormones fruit and flower maturation. GAs are synthesized in the root and
are produced in specific glands and transported to a distant site for stem apical meristems, young leaves, and seed embryos. In urban
action, acting alone. areas, GA antagonists are sometimes applied to trees under power
Plant hormones are a group of unrelated chemical substances that lines to control growth and reduce the frequency of pruning.
affect plant morphogenesis. Five major plant hormones are GAs break dormancy (a state of inhibited growth and development)
traditionally described: auxins, cytokinins, gibberellins, ethylene, in the seeds of plants that require exposure to cold or light to
and abscisic acid. In addition, other nutrients and environmental germinate. Abscisic acid is a strong antagonist of GA action. Other
conditions can be characterized as growth factors. The first three effects of GAs include gender expression, seedless fruit
plant hormones largely affect plant growth, as described below. development, and the delay of senescence in leaves and fruit.
Seedless grapes are obtained through standard breeding methods;
AUXINS they contain inconspicuous seeds that fail to develop. Because GAs
The term auxin is derived from the Greek word auxein, which means are produced by the seeds and because fruit development and stem
“to grow. ” Auxins are the main hormones responsible for cell elongation are under GA control, these varieties of grapes would
elongation in phototropism and gravitropism. They also control the normally produce small fruit in compact clusters. Maturing grapes
differentiation of meristem into vascular tissue and promote leaf are routinely treated with GA to promote larger fruit size, as well as
development and arrangement. While many synthetic auxins are looser bunches (longer stems), which reduces the incidence of
used as herbicides, indole acetic acid (IAA) is the only naturally- mildew infection.
occurring auxin that shows physiological activity. Apical dominance
(the inhibition of lateral bud formation) is triggered by auxins
produced in the apical meristem. Flowering, fruit setting and
ripening, and inhibition of abscission (leaf falling) are other plant
responses under the direct or indirect control of auxins. Auxins also
act as a relay for the effects of the blue light and red/far-red
responses.
Commercial use of auxins is widespread in plant nurseries and for
crop production. IAA is used as a rooting hormone to promote
growth of adventitious roots on cuttings and detached leaves.
Applying synthetic auxins to tomato plants in greenhouses promotes
normal fruit development. Outdoor application of auxin promotes
synchronization of fruit setting and dropping, which coordinates the
harvesting season. Fruits such as seedless cucumbers can be induced
to set fruit by treating unfertilized plant flowers with auxins.

CYTOKININS
The effect of cytokinins was first reported when it was found that
adding the liquid endosperm of coconuts to developing plant
embryos in culture stimulated their growth. The stimulating growth
factor was found to be cytokinin, a hormone that promotes

30.21.1 https://bio.libretexts.org/@go/page/13769
 KEY POINTS
During phototropism and gravitropism, the plant hormone auxin
controls cell elongation.
The plant hormone cytokinin promotes cell division, controling
many developmental processes in plants.
Gibberellins control many aspects of plant physiology including
shoot elongation, seed germination, fruit and flower maturation,
seed dormancy, gender expression, seedless fruit development,
and the delay of senescence in leaves and fruit.

KEY TERMS
gibberellin: any of a class of diterpene plant growth hormones
that stimulate shoot elongation, seed germination, and fruit and
flower maturation
auxin: a class of plant growth hormones that is responsible for
elongation in phototropism and gravitropism and for other
growth processes in the plant life cycle
cytokinin: any of a class of plant hormones involved in cell
growth and division

This page titled 30.21: Plant Sensory Systems and Responses - Auxins,
Cytokinins, and Gibberellins is shared under a CC BY-SA 4.0 license and
was authored, remixed, and/or curated by Boundless.

Figure 30.21.1: Effect of gibberellins on grapes: In grapes,

application of gibberellic acid increases the size of fruit and loosens
clustering.

30.21.2 https://bio.libretexts.org/@go/page/13769
30.22: PLANT SENSORY SYSTEMS AND RESPONSES - ABSCISIC ACID,
ETHYLENE, AND NONTRADITIONAL HORMONES

 LEARNING OBJECTIVES

Describe the roles played by ethylene and nontraditional

hormones in plant development

GROWTH RESPONSES
In addition to the growth hormones auxins, cytokinins, gibberellins,
Figure 30.22.1: Date ripening: The plant hormone ethylene
there are two more major types of plant hormones, abscisic acid and promotes ripening, as seen in the ripening of dates.
ethylene, as well as several other less-studied compounds that Ethylene is widely used in agriculture. Commercial fruit growers
control plant physiology. control the timing of fruit ripening with application of the gas.
Horticulturalists inhibit leaf dropping in ornamental plants by
ABSCISIC ACID
removing ethylene from greenhouses using fans and ventilation.
The plant hormone abscisic acid (ABA) was first discovered as the
agent that causes the abscission or dropping of cotton bolls. NONTRADITIONAL HORMONES
However, more-recent studies indicate that ABA plays only a minor Recent research has discovered a number of compounds that also
role in the abscission process. ABA accumulates as a response to influence plant development. Their roles are less understood than the
stressful environmental conditions, such as dehydration, cold effects of the major hormones described so far.
temperatures, or shortened day lengths. Its activity counters many of
Jasmonates play a major role in defense responses to herbivory.
the growth-promoting effects of GAs and auxins. ABA inhibits stem
Their levels increase when a plant is wounded by a predator,
elongation and induces dormancy in lateral buds.
resulting in an increase in toxic secondary metabolites. They
ABA induces dormancy in seeds by blocking germination and contribute to the production of volatile compounds that attract
promoting the synthesis of storage proteins. Plants adapted to natural enemies of predators. For example, chewing of tomato plants
temperate climates require a long period of cold temperature before by caterpillars leads to an increase in jasmonic acid levels, which in
seeds germinate. This mechanism protects young plants from
turn triggers the release of volatile compounds that attract predators
sprouting too early during unseasonably warm weather in winter. As
of the pest.
the hormone gradually breaks down over winter, the seed is released
from dormancy and germinates when conditions are favorable in Oligosaccharins also play a role in plant defense against bacterial
and fungal infections. They act locally at the site of injury; they can
spring. Another effect of ABA is to promote the development of
also be transported to other tissues. Strigolactones promote seed
winter buds; it mediates the conversion of the apical meristem into a
germination in some species and inhibit lateral apical development
dormant bud. Low soil moisture causes an increase in ABA, which
in the absence of auxins. Strigolactones also play a role in the
causes stomata to close, reducing water loss in winter buds.
establishment of mycorrhizae, a mutualistic association of plant
ETHYLENE roots and fungi. Brassinosteroids are important to many
Ethylene is associated with fruit ripening, flower wilting, and leaf developmental and physiological processes. Signals between these
fall. Ethylene is unusual because it is a volatile gas (C2H4). compounds and other hormones, notably auxin and GAs, amplify
Hundreds of years ago, when gas street lamps were installed in city their physiological effect. Apical dominance, seed germination,
streets, trees that grew close to lamp posts developed twisted, gravitropism, and resistance to freezing are all positively influenced
thickened trunks, shedding their leaves earlier than expected. These by hormones. Root growth and fruit dropping are inhibited by
effects were caused by ethylene volatilizing from the lamps. steroids.

Aging tissues (especially senescing leaves) and nodes of stems KEY POINTS
produce ethylene. The best-known effect of the hormone, however,
Under stress, abscisic acid accumulates in plants, inhibiting stem
is the promotion of fruit ripening. Ethylene stimulates the elongation and inducing bud dormancy.
conversion of starch and acids to sugars. Some people store unripe
The plant hormone ethylene controls fruit ripening, flower
fruit, such as avocados, in a sealed paper bag to accelerate ripening; wilting, and leaf fall by stimulating the conversion of starch and
the gas released by the first fruit to mature will speed up the
acids to sugars.
maturation of the remaining fruit. Ethylene also triggers leaf and Other nontraditional hormones such as jasmonates and
fruit abscission, flower fading and dropping, and promotes
oligosaccharins control defense responses from herbivores and
germination in some cereals and sprouting of bulbs and potatoes. bacterial/fungal infections, respectively.

30.22.1 https://bio.libretexts.org/@go/page/13770
KEY TERMS ethylene: a plant hormone that is involved in fruit ripening,
abscisic acid: a plant hormone that functions in many plant flower wilting, and leaf fall
developmental processes, including bud dormancy, inhibition of
This page titled 30.22: Plant Sensory Systems and Responses - Abscisic
seed germination, and plant stress tolerance.
Acid, Ethylene, and Nontraditional Hormones is shared under a CC BY-SA
jasmonate: any of several esters of jasmonic acid that act as 4.0 license and was authored, remixed, and/or curated by Boundless.
plant hormones

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30.23: PLANT SENSORY SYSTEMS AND RESPONSES - PLANT RESPONSES
TO WIND AND TOUCH
KEY POINTS
 LEARNING OBJECTIVES When subjected to constant directional pressure, such as a trellis,
plants move to grow around the object providing the pressure;
Compare the ways plants respond to directional and non-
this process is known as thigmotropism.
directional stimuli
Thigmonastic responses include opening and closing leaves,
petals, or other parts of the plant as a reaction to touch.
The shoot of a pea plant wraps around a trellis while a tree grows on
Through thigmomorphogenesis, plants change their growth in
an angle in response to strong prevailing winds. These are examples
response to repeated mechanical stress from wind, rain, or other
of how plants respond to touch or wind.
living things.
The movement of a plant subjected to constant directional pressure
is called thigmotropism, from the Greek words thigma meaning KEY TERMS
“touch,” and tropism, implying “direction.” Tendrils are one thigmotropism: plant growth or motion in response to touch
example of this. A tendril is a specialized stem, leaf, or petiole with thigmomorphogenesis: the response by plants to mechanical
a threadlike shape that is used by climbing plants for support.The sensation (touch) by altering their growth patterns
meristematic region of tendrils is very touch sensitive; light touch thigmonastic response: a touch response independent of the
will evoke a quick coiling response. Cells in contact with a support direction of stimulus
surface contract, whereas cells on the opposite side of the support
expand. Application of jasmonic acid is sufficient to trigger tendril CONTRIBUTIONS AND ATTRIBUTIONS
coiling without a mechanical stimulus. OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44711/latest...ol11448/latest. License: CC
BY: Attribution
phototropism. Provided by: Wiktionary. Located at:
http://en.wiktionary.org/wiki/phototropism. License: CC BY-SA: Attribution-
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photoperiodism. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/photoperiodism. License: CC BY-SA: Attribution-
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photomorphogenesis. Provided by: Wiktionary. Located at:
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photoreceptor. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/photoreceptor. License: CC BY-SA: Attribution-
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Phototropism. Provided by: Wikipedia. Located at:
Figure 30.23.1: Thigmotropism in a redvine: Tendrils of a redvine en.Wikipedia.org/wiki/File:Phototropism.jpg. License: CC BY: Attribution
photoperiodism. Provided by: Wiktionary. Located at:
produce auxin in response to touching a support stick and then
en.wiktionary.org/wiki/photoperiodism. License: CC BY-SA: Attribution-
transfer the auxin to non-touching cells. The non-touching cells ShareAlike
elongate faster to curl around the support stick. OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
A thigmonastic response is a touch response independent of the Located at: http://cnx.org/content/m44711/latest...ol11448/latest. License: CC
BY: Attribution
direction of stimulus. In the Venus flytrap, two modified leaves are phytochrome. Provided by: Wiktionary. Located at:
joined at a hinge and lined with thin, fork-like tines along the outer en.wiktionary.org/wiki/phytochrome. License: CC BY-SA: Attribution-
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edges. Tiny hairs are located inside the trap. When an insect brushes chromophore. Provided by: Wiktionary. Located at:
against these trigger hairs, touching two or more of them in en.wiktionary.org/wiki/chromophore. License: CC BY-SA: Attribution-
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succession, the leaves close quickly, trapping the prey. Glands on the Phototropism. Provided by: Wikipedia. Located at:
leaf surface secrete enzymes that slowly digest the insect. The en.Wikipedia.org/wiki/File:Phototropism.jpg. License: CC BY: Attribution
OpenStax College, Plant Sensory Systems and Responses. October 17, 2013.
released nutrients are absorbed by the leaves, which reopen for the Provided by: OpenStax CNX. Located at:
next meal. http://cnx.org/content/m44711/latest...e_30_06_01.jpg. License: CC BY:
Attribution
Thigmomorphogenesis is a slow developmental change in the shape OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
of a plant subjected to continuous mechanical stress. When trees Located at: http://cnx.org/content/m44711/latest...ol11448/latest. License: CC
BY: Attribution
bend in the wind, for example, growth is usually stunted and the cryptochrome. Provided by: Wiktionary. Located at:
trunk thickens. Strengthening tissue, especially xylem, is produced en.wiktionary.org/wiki/cryptochrome. License: CC BY-SA: Attribution-
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to add stiffness to resist the wind’s force. Researchers hypothesize skototropism. Provided by: Wiktionary. Located at:
that mechanical strain from wind, rain, or movement by other living en.wiktionary.org/wiki/skototropism. License: CC BY-SA: Attribution-
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things induces growth and differentiation to strengthen the tissues.
phototropin. Provided by: Wiktionary. Located at:
Ethylene and jasmonate are likely involved in en.wiktionary.org/wiki/phototropin. License: CC BY-SA: Attribution-
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Boundless. Provided by: Boundless Learning. Located at:
www.boundless.com//biology/definition/auxin. License: CC BY-SA:
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Phototropism. Provided by: Wikipedia. Located at: jasmonate. Provided by: Wiktionary. Located at:
en.Wikipedia.org/wiki/File:Phototropism.jpg. License: CC BY: Attribution en.wiktionary.org/wiki/jasmonate. License: CC BY-SA: Attribution-
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http://www.youtube.com/watch?v=eDA8rmUP5ZM. License: Public Attribution-ShareAlike
Domain: No Known Copyright. License Terms: Standard YouTube license thigmotropism. Provided by: Wiktionary. Located at:
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License: CC BY: Attribution Boundless. Provided by: Boundless Learning. Located at:
Boundless. Provided by: Boundless Learning. Located at: www.boundless.com//biology/de...astic-response. License: CC BY-SA:
www.boundless.com//biology/definition/auxin. License: CC BY-SA: Attribution-ShareAlike
Attribution-ShareAlike Phototropism. Provided by: Wikipedia. Located at:
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Phototropism. Provided by: Wikipedia. Located at: OpenStax College, Plant Sensory Systems and Responses. October 17, 2013.
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Attribution Attribution
Phototropism Diagram. Provided by: Wikipedia. Located at: Time-lapse of pea shoot and root growth. Located at:
en.Wikipedia.org/wiki/File:Ph...sm_Diagram.svg. License: CC BY-SA: http://www.youtube.com/watch?v=eDA8rmUP5ZM. License: Public
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Time-lapse of pea shoot and root growth. Located at: Attribution
http://www.youtube.com/watch?v=eDA8rmUP5ZM. License: Public Ripening. Provided by: Wikipedia. Located at:
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OpenStax College, Plant Sensory Systems and Responses. October 17, 2013. Thigmonastic. Provided by: Wikipedia. Located at:
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30.24: PLANT DEFENSE MECHANISMS - AGAINST HERBIVORES

 LEARNING OBJECTIVES

Identify plant defense responses to herbivores

DEFENSE RESPONSES AGAINST HERBIVORES

Herbivores, both large and small, use plants as food and actively
chew them. Plants have developed a variety of strategies to
discourage or kill attackers.

MECHANICAL DEFENSES
The first line of defense in plants is an intact and impenetrable
barrier composed of bark and a waxy cuticle. Both protect plants
against herbivores. Other adaptations against herbivores include
hard shells, thorns (modified branches), and spines (modified
leaves). They discourage animals by causing physical damage or by
inducing rashes and allergic reactions. Some Acacia tree species
have developed mutualistic relationships with ant colonies: they
offer the ants shelter in their hollow thorns in exchange for the ants’
defense of the tree’s leaves.

Figure 30.24.1: Modified leaves on a cactus: The spines on cactus

plants are modified leaves that act as a mechanical defense against
predators.

CHEMICAL DEFENSES
A plant’s exterior protection can be compromised by mechanical
damage, which may provide an entry point for pathogens. If the first
line of defense is breached, the plant must resort to a different set of
defense mechanisms, such as toxins and enzymes. Secondary
metabolites are compounds that are not directly derived from
photosynthesis and are not necessary for respiration or plant growth
and development.
Many metabolites are toxic and can even be lethal to animals that
ingest them. Some metabolites are alkaloids, which discourage
predators with noxious odors (such as the volatile oils of mint and
sage) or repellent tastes (like the bitterness of quinine). Other
alkaloids affect herbivores by causing either excessive stimulation
(caffeine is one example) or the lethargy associated with opioids.
Some compounds become toxic after ingestion; for instance, glycol
Figure 30.24.1: Acacia collinsii: The large thorn-like stipules of cyanide in the cassava root releases cyanide only upon ingestion by
Acacia collinsii are hollow and offer shelter for ants, which in return
protect the plant against herbivores. the herbivore. Foxgloves produce several deadly chemicals, namely
cardiac and steroidal glycosides. Ingestion can cause nausea,
vomiting, hallucinations, convulsions, or death.

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 In addition, long-distance signaling elicits a systemic response
aimed at deterring predators. As tissue is damaged, jasmonates may
promote the synthesis of compounds that are toxic to predators.
Jasmonates also elicit the synthesis of volatile compounds that
attract parasitoids: insects that spend their developing stages in or on
another insect, eventually killing their host. The plant may activate
abscission of injured tissue if it is damaged beyond repair.

KEY POINTS
Many plants have impenetrable barriers, such as bark and waxy
cuticles, or adaptations, such as thorns and spines, to protect
them from herbivores.
If herbivores breach a plant’s barriers, the plant can respond with
secondary metabolites, which are often toxic compounds, such as
Figure 30.24.1: Foxgloves: Foxgloves produce several deadly glycol cyanide, that may harm the herbivore.
chemicals, namely cardiac and steroidal glycosides. Ingestion can
cause nausea, vomiting, hallucinations, convulsions, or death.
When attacked by a predator, damaged plant tissue releases
jasmonate hormones that promote the release of volatile
TIMING compounds, attracting parasitoids, which use, and eventually kill,
Mechanical wounding and predator attacks activate defense and the predators as host insects.
protective mechanisms in the damaged tissue and elicit long-
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30.25: PLANT DEFENSE MECHANISMS - AGAINST PATHOGENS
Plants defend against pathogens with barriers, secondary Mechanical wounding and predator attacks activate defense and
metabolites, and antimicrobial compounds. protective mechanisms in the damaged tissue and elicit long-
distancing signaling or activation of defense and protective
 LEARNING OBJECTIVES mechanisms at sites farther from the injury location. Some defense
reactions occur within minutes, while others may take several hours.
Identify plant defense responses to pathogens
KEY POINTS
DEFENSE RESPONSES AGAINST PATHOGENS Many plants have impenetrable barriers, such as bark and waxy
Pathogens are agents of disease. These infectious microorganisms, cuticles, or adaptations, such as thorns and spines, to protect
such as fungi, bacteria, and nematodes, live off of the plant and them from pathogens.
damage its tissues. Plants have developed a variety of strategies to If pathogens breach a plant’s barriers, the plant can respond with
discourage or kill attackers. secondary metabolites, which are often toxic compounds, such as
glycol cyanide, that may harm the pathogen.
The first line of defense in plants is an intact and impenetrable
Plants produce antimicrobial chemicals, antimicrobial proteins,
barrier composed of bark and a waxy cuticle. Both protect plants
and antimicrobial enzymes that are able to fight the pathogens.
against pathogens.
A plant’s exterior protection can be compromised by mechanical CONTRIBUTIONS AND ATTRIBUTIONS
damage, which may provide an entry point for pathogens. If the first Plant defense against herbivory. Provided by: Wikimedia. Located at:
en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No
line of defense is breached, the plant must resort to a different set of Known Copyright
defense mechanisms, such as toxins and enzymes. Secondary Plant defense against herbivory. Provided by: Wikimedia. Located at:
metabolites are compounds that are not directly derived from en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No
Known Copyright
photosynthesis and are not necessary for respiration or plant growth Plant defense against herbivory. Provided by: Wikipedia. Located at:
and development. Many metabolites are toxic and can even be lethal en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No
Known Copyright
to animals that ingest them. Plant defense against herbivory. Provided by: Wikimedia. Located at:
en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No
Additionally, plants have a variety of inducible defenses in the Known Copyright
presence of pathogens. In addition to secondary metabolites, plants Plant defense against herbivory. Provided by: Wikimedia. Located at:
produce antimicrobial chemicals, antimicrobial proteins, and en.Wikipedia.org/wiki/Plant_d...inst_herbivory. License: Public Domain: No
Known Copyright
antimicrobial enzymes that are able to fight the pathogens. Plants Plant defense against herbivory. Provided by: Wikipedia. Located at:
can close stomata to prevent the pathogen from entering the plant. A en.Wikipedia.org/wiki/Plant_defense_against_herbivory. License: Public
Domain: No Known Copyright
hypersensitive response, in which the plant experiences rapid cell
death to fight off the infection, can be initiated by the plant; or it This page titled 30.25: Plant Defense Mechanisms - Against Pathogens is
may use endophyte assistance: the roots release chemicals that shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
attract other beneficial bacteria to fight the infection. curated by Boundless.

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 CHAPTER OVERVIEW

31: SOIL AND PLANT NUTRITION

31.1: Nutritional Requirements of Plants
31.1A: Plant Nutrition
31.1B: The Chemical Composition of Plants
31.1C: Essential Nutrients for Plants
31.2: The Soil
31.2A: Soil Composition
31.2B: Soil Formation
31.2C: Physical Properties of Soil
31.3: Nutritional Adaptations of Plants
31.3A: Nitrogen Fixation- Root and Bacteria Interactions
31.3B: Mycorrhizae- The Symbiotic Relationship between Fungi and Roots
31.3C: Nutrients from Other Sources

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1
SECTION OVERVIEW

31.1: NUTRITIONAL REQUIREMENTS OF PLANTS

31.1C: ESSENTIAL NUTRIENTS FOR PLANTS
Topic hierarchy
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31.1B: THE CHEMICAL COMPOSITION OF
PLANTS

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31.1A: PLANT NUTRITION

 LEARNING OBJECTIVES

Describe how the nutritional requirements of plants are met

INTRODUCTION
Plants are unique organisms that can absorb nutrients and water
through their root system, as well as carbon dioxide from the
atmosphere. Soil quality and climate are the major determinants of Figure 31.1A. 1 : Examples of fruit bearing plants: For this (a)
squash seedling (Cucurbita maxima) to develop into a mature plant
plant distribution and growth. The combination of soil nutrients, bearing its (b) fruit, numerous nutritional requirements must be met.
water, and carbon dioxide, along with sunlight, allows plants to
There is a complex dynamic between plants and soils that ultimately
grow. In order to develop into mature, fruit -bearing plants, many
determines the outcome and viability of plant life. The following
requirements must be met and events must be coordinated.
sections of this chapter will discuss the many aspects of the
Take for example the Cucurbitaceae family of plants that were the nutritional requirements of plants in greater detail.
first cultivated in Mesoamerica, although several species are native
to North America. The family includes many edible species, such as KEY POINTS
squash and pumpkin, as well as inedible gourds. First, seeds must Nutrients and water are absorbed through the plants root system.
germinate under the right conditions in the soil; therefore, Carbon dioxide is absorbed through the leaves.
temperature, moisture, and soil quality are important factors that From seedling to mature plant, there is a complex dynamic
play a role in germination and seedling development. Soil quality between plants and their environment (soil and atmosphere).
and climate are significant to plant distribution and growth. Second,
the young seedling will eventually grow into a mature plant with the KEY TERMS
roots absorbing nutrients and water from the soil. At the same time, germinate: to sprout or produce buds
the aboveground parts of the plant will absorb carbon dioxide from photosynthesis: the process by which plants and other
the atmosphere and use energy from sunlight to produce organic photoautotrophs generate carbohydrates and oxygen from carbon
compounds through photosynthesis. Finally, the fruit are grown and dioxide, water, and light energy in chloroplasts
matured and the cycle begins all over again with the new seeds. nutrient: a source of nourishment, such as food, that can be
metabolized by an organism to give energy and build tissue

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31.1B: THE CHEMICAL COMPOSITION OF PLANTS
Plants are composed of water, carbon-containing organics, and non-
carbon-containing inorganic substances such as potassium and
nitrogen.

 LEARNING OBJECTIVES

Describe the chemical composition of plants

KEY POINTS
Water comprises a large percentage of a plant’s total weight and
is used to support cell structure, for metabolic functions, to carry
nutrients, and for photosynthesis.
Water is absorbed from the soil through root hairs and is carried
to the rest of the plant through the xylem.
Many essential organic and inorganic nutrients are required to
sustain plant life.
Figure 31.1B. 1: Water absorption by the roots: Water is absorbed
KEY TERMS through the root hairs and moves up the xylem to the leaves.
organic: relating to the compounds of carbon, relating to natural Since plants require nutrients in the form of elements such as carbon
products and potassium, it is important to understand the chemical
inorganic: relating to a compound that does not contain carbon composition of plants. The majority of volume in a plant cell is
xylem: a vascular tissue in land plants primarily responsible for water; it typically comprises 80 to 90 percent of the plant’s total
the distribution of water and minerals taken up by the roots; also weight. Soil is the water source for land plants. It can be an abundant
the primary component of wood source of water even if it appears dry. Plant roots absorb water from
transpiration: the loss of water by evaporation in terrestrial the soil through root hairs and transport it up to the leaves through
plants, especially through the stomata; accompanied by a the xylem. As water vapor is lost from the leaves, the process of
corresponding uptake from the roots transpiration and the polarity of water molecules (which enables
them to form hydrogen bonds) draws more water from the roots up
THE CHEMICAL COMPOSITION OF PLANTS through the plant to the leaves. Plants need water to support cell
structure, for metabolic functions, to carry nutrients, and for
WATER photosynthesis.

NUTRIENTS
Plant cells need essential substances, collectively called nutrients, to
sustain life. Plant nutrients may be composed of either organic or
inorganic compounds. An organic compound is a chemical
compound that contains carbon, such as carbon dioxide obtained
from the atmosphere. Carbon that was obtained from atmospheric
CO2 composes the majority of the dry mass within most plants. An
inorganic compound does not contain carbon and is not part of, or
produced by, a living organism. Inorganic substances (which form
the majority of the soil substance) are commonly called minerals:
those required by plants include nitrogen (N) and potassium (K), for
structure and regulation.

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31.1C: ESSENTIAL NUTRIENTS FOR PLANTS
Approximately 20 macronutrients and micronutrients are deemed
essential nutrients to support all the biochemical needs of plants.

 LEARNING OBJECTIVES

Distinguish among the essential nutrients for plants

KEY POINTS
An element is essential if a plant cannot complete its life cycle
without it, if no other element can perform the same function,
and if it is directly involved in nutrition.
An essential nutrient required by the plant in large amounts is
called a macronutrient, while one required in very small amounts
is termed a micronutrient.
Missing or inadequate supplies of nutrients adversely affect plant
growth, leading to stunted growth, slow growth, chlorosis, or cell
death.
About half the essential nutrients are micronutrients such as
boron, chlorine, manganese, iron, zinc, copper, molybdenum, Figure 31.1C. 1 : Essential elements required by plants: For an
element to be regarded as essential a plant cannot complete its life
nickel, silicon, and sodium. cycle without the element, no other element can perform the
function of the element, and the element is directly involved in plant
KEY TERMS nutrition.
micronutrient: a mineral, vitamin, or other substance that is The next-most-abundant element in plant cells is nitrogen (N); it is
essential, even in very small quantities, for growth or metabolism part of proteins and nucleic acids. Nitrogen is also used in the
chlorosis: a yellowing of plant tissue due to loss or absence of synthesis of some vitamins. Hydrogen and oxygen are
chlorophyll macronutrients that are part of many organic compounds and also
macronutrient: any of the elements required in large amounts form water. Oxygen is necessary for cellular respiration; plants use
by all living things oxygen to store energy in the form of ATP. Phosphorus (P), another
macromolecule, is necessary to synthesize nucleic acids and
ESSENTIAL NUTRIENTS phospholipids. As part of ATP, phosphorus enables food energy to
Plants require only light, water, and about 20 elements to support all be converted into chemical energy through oxidative
their biochemical needs. These 20 elements are called essential phosphorylation. Light energy is converted into chemical energy
nutrients. For an element to be regarded as essential, three criteria during photophosphorylation in photosynthesis; and into chemical
are required: energy to be extracted during respiration. Sulfur is part of certain
1. a plant cannot complete its life cycle without the element amino acids, such as cysteine and methionine, and is present in
2. no other element can perform the function of the element several coenzymes. Sulfur also plays a role in photosynthesis as part
3. the element is directly involved in plant nutrition of the electron transport chain where hydrogen gradients are key in
the conversion of light energy into ATP. Potassium (K) is important
MACRONUTRIENTS AND MICRONUTRIENTS because of its role in regulating stomatal opening and closing. As the
The essential elements can be divided into macronutrients and openings for gas exchange, stomata help maintain a healthy water
micronutrients. Nutrients that plants require in larger amounts are balance; a potassium ion pump supports this process.
called macronutrients. About half of the essential elements are Magnesium (Mg) and calcium (Ca) are also important
considered macronutrients: carbon, hydrogen, oxygen, nitrogen, macronutrients. The role of calcium is twofold: to regulate nutrient
phosphorus, potassium, calcium, magnesium, and sulfur. The first of transport and to support many enzyme functions. Magnesium is
these macronutrients, carbon (C), is required to form carbohydrates, important to the photosynthetic process. These minerals, along with
proteins, nucleic acids, and many other compounds; it is, therefore, the micronutrients, also contribute to the plant’s ionic balance.
present in all macromolecules. On average, the dry weight In addition to macronutrients, organisms require various elements in
(excluding water) of a cell is 50 percent carbon, making it a key part small amounts. These micronutrients, or trace elements, are present
of plant biomolecules. in very small quantities. The seven main micronutrients include
boron, chlorine, manganese, iron, zinc, copper, and molybdenum.
Boron (B) is believed to be involved in carbohydrate transport in
plants; it also assists in metabolic regulation. Boron deficiency will
often result in bud dieback. Chlorine (Cl) is necessary for osmosis

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and ionic balance; it also plays a role in photosynthesis. Copper (Cu) CONTRIBUTIONS AND ATTRIBUTIONS
is a component of some enzymes. Symptoms of copper deficiency OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
include browning of leaf tips and chlorosis (yellowing of the leaves). Located at: http://cnx.org/content/m44712/latest...ol11448/latest. License: CC
BY: Attribution
Iron (Fe) is essential for chlorophyll synthesis, which is why an iron OpenStax College, Biology. November 22, 2013. Provided by: OpenStax CNX.
deficiency results in chlorosis. Manganese (Mn) activates some Located at: http://cnx.org/content/m44714/latest...ol11448/latest. License: CC
BY: Attribution
important enzymes involved in chlorophyll formation. Manganese- photosynthesis. Provided by: Wiktionary. Located at:
deficient plants will develop chlorosis between the veins of its en.wiktionary.org/wiki/photosynthesis. License: CC BY-SA: Attribution-
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also activates many enzymes. Symptoms of zinc deficiency include License: CC BY: Attribution
chlorosis and stunted growth. transpiration. Provided by: Wiktionary. Located at:
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Deficiencies in any of these nutrients, particularly the ShareAlike
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macronutrients, can adversely affect plant growth. Depending on the Located at: http://cnx.org/content/m44714/latest...ol11448/latest. License: CC
specific nutrient, a lack can cause stunted growth, slow growth, or BY: Attribution
xylem. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/xylem.
chlorosis. Extreme deficiencies may result in leaves showing signs License: CC BY-SA: Attribution-ShareAlike
of cell death. organic. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/organic.
License: CC BY-SA: Attribution-ShareAlike
inorganic. Provided by: Wiktionary. Located at:
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Located at: http://cnx.org/content/m44714/latest...ol11448/latest. License: CC
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Micronutrient. Provided by: Wikipedia. Located at:
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chlorosis. Provided by: Wiktionary. Located at:
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micronutrient. Provided by: Wiktionary. Located at:
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Figure 31.1C. 1 : Nutrient deficiency in plants: Nutrient deficiency http://cnx.org/content/m44714/latest...e_31_01_01.jpg. License: CC BY:
is evident in the symptoms these plants show. This (a) grape tomato Attribution
suffers from blossom end rot caused by calcium deficiency. The OpenStax College, Biology. November 22, 2013. Provided by: OpenStax CNX.
yellowing in this (b) Frangula alnus results from magnesium Located at: http://cnx.org/content/m44714/latest...ol11448/latest. License: CC
BY: Attribution
deficiency. Inadequate magnesium also leads to (c) intervenal
OpenStax College, Nutritional Requirements of Plants. October 17, 2013.
chlorosis, seen here in a sweetgum leaf. This (d) palm is affected by Provided by: OpenStax CNX. Located at:
potassium deficiency. http://cnx.org/content/m44714/latest/Figure_31_01_03abcd.jpg. License: CC
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SECTION OVERVIEW

31.2: THE SOIL

31.2B: SOIL FORMATION
Topic hierarchy
31.2C: PHYSICAL PROPERTIES OF SOIL
31.2A: SOIL COMPOSITION
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31.2A: SOIL COMPOSITION
the soil. A good, healthy soil has sufficient air, water, minerals, and
 LEARNING OBJECTIVES organic material to promote and sustain plant life.
The organic material of soil, called humus, is made up of
Explain soil composition
microorganisms (dead and alive), and dead animals and plants in
varying stages of decay. Humus improves soil structure, providing
SOIL COMPOSITION
plants with water and minerals. The inorganic material of soil is
Plants obtain inorganic elements from the soil, which serves as a composed of rock, slowly broken down into smaller particles that
natural medium for land plants. Soil is the outer, loose layer that vary in size. Soil particles that are 0.1 to 2 mm in diameter are sand.
covers the surface of Earth. Soil quality, a major determinant, along Soil particles between 0.002 and 0.1 mm are called silt, and even
with climate, of plant distribution and growth, depends not only on smaller particles, less than 0.002 mm in diameter, are called clay.
the chemical composition of the soil, but also the topography Some soils have no dominant particle size, containing a mixture of
(regional surface features) and the presence of living organisms. sand, silt, and humus; these soils are called loams.
Soil consists of these major components:
KEY POINTS
The chemical composition of the soil, the topography, and the
presence of living organisms determines the quality of soil.
In general, soil contains 40-45% inorganic matter, 5% organic
matter, 25% water, and 25% air.
In order to sustain plant life, the proper mix of air, water,
minerals, and organic material is required.
Humus, the organic material in soil, is composed of
microorganisms (dead and alive) and decaying plants.
The inorganic material of soil is composed of rock, which is
broken down into small particles of sand (0.1 to 2 mm), silt
(0.002 to 0.1 mm), and clay (less than 0.002 mm).
Loam is a soil that is a mix sand, silt, and humus.
Figure 31.2A. 1 : Components of soil: The four major components of
soil are shown: inorganic minerals, organic matter, water, and air. KEY TERMS
inorganic mineral matter, about 40 to 45 percent of the soil
loam: soil with no dominant particle size that contains a mixture
volume
of sand, silt, and humus
organic matter, about 5 percent of the soil volume
humus: a large group of natural organic compounds found in the
water, about 25 percent of the soil volume
soil composed of decaying plants and dead and living
air, about 25 percent of the soil volume microorganisms
The amount of each of the four major components of soil depends
on the quantity of vegetation, soil compaction, and water present in This page titled 31.2A: Soil Composition is shared under a CC BY-SA 4.0
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31.2B: SOIL FORMATION

 LEARNING OBJECTIVES

Describe the five factors that account for soil formation

SOIL FORMATION
Soil formation is the consequence of a combination of biological,
physical, and chemical processes. Soil should ideally contain 50
percent solid material and 50 percent pore space. About one-half of
the pore space should contain water, while the other half should
contain air. The organic component of soil serves as a cementing
agent, returns nutrients to the plant, allows soil to store moisture,
makes soil tillable for farming, and provides energy for soil
microorganisms. Most soil microorganisms, bacteria, algae, or fungi,
are dormant in dry soil, but become active once moisture is
available.
Soil distribution is not homogenous because its formation results in
the production of layers; the vertical section of the layers of soil is
called the soil profile. Within the soil profile, soil scientists define
zones called horizons: a soil layer with distinct physical and
chemical properties that differ from those of other layers. Five
factors account for soil formation: parent material, climate,
topography, biological factors, and time.

PARENT MATERIAL Figure 31.2B. 1: Soft sediment deformation: Soil distribution is not
The organic and inorganic material in which soils form is the parent the same at all depths. The vertical section of soil layers is called the
soil profile. The soil profile contains defined zones called horizons
material. Mineral soils form directly from the weathering of whicht have distinct physical and chemical properties that differ
bedrock, the solid rock that lies beneath the soil; therefore, they have from those of other layers. An example is shown here in the soft
a similar composition to the original rock. Other soils form in sediment deformation in the Navajo Sandstone.
materials that came from elsewhere, such as sand and glacial drift.
CLIMATE
Materials located in the depth of the soil are relatively unchanged
Temperature, moisture, and wind cause different patterns of
compared with the deposited material. Sediments in rivers may have
different characteristics, depending on whether the stream moves weathering, which affect soil characteristics. The presence of
quickly or slowly. A fast-moving river could have sediments of moisture and nutrients from weathering will also promote biological
rocks and sand, whereas a slow-moving river could have fine- activity: a key component of a quality soil.
textured material, such as clay.
TOPOGRAPHY
Regional surface features (familiarly called “the lay of the land”)
can have a major influence on the characteristics and fertility of a
soil. Topography affects water runoff, which strips away parent
material and affects plant growth. Steep soils are more prone to
erosion and may be thinner than soils that are relatively flat or level.

BIOLOGICAL FACTORS
The presence of living organisms greatly affects soil formation and
structure. Animals and microorganisms can produce pores and
crevices. Plant roots can penetrate into crevices to produce more
fragmentation. Plant secretions promote the development of
microorganisms around the root in an area known as the rhizosphere.
Additionally, leaves and other material that fall from plants
decompose and contribute to soil composition.

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TIME steeper the soil, the more erosion takes place).
Time is an important factor in soil formation because soils develop The presence of microorganisms in soil creates pores and
over long periods. Soil formation is a dynamic process. Materials are crevices; plants promote the presence of microorganisms and
deposited over time, decompose, and transform into other materials contribute to soil formation.
that can be used by living organisms or deposited onto the surface of Soil formation takes place over long periods of time.
the soil.
KEY TERMS
KEY POINTS rhizosphere: the soil region subject to the influence of plant
Parent material is the organic and inorganic material from which roots and their associated microorganisms
soil is formed. bedrock: the solid rock that exists at some depth below the
Climate factors, such as temperature and wind, affect soil ground surface
formation and its characteristics; the presence of moisture and horizon: a soil layer with distinct physical and chemical
nutrients is also needed to form a quality soil. properties that differ from those of other layers
Topography, or regional surface features, affects water runoff,
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31.2C: PHYSICAL PROPERTIES OF SOIL
A, and C horizons, whereas mature soils may display all of these,
 LEARNING OBJECTIVES plus additional layers.

Describe the physical properties or profile of soil

PHYSICAL PROPERTIES OF THE SOIL

Soils are named and classified based on their horizons. The soil
profile has four distinct layers:

Figure 31.2C. 1 : Mature soil: The San Joaquin soil is a mature soil
that has an O horizon, A horizon, B horizon, and C horizon.

KEY POINTS
The O horizon, or topsoil, is made of decaying organisms and
plant life; it is responsible for plant production.
Figure 31.2C. 1 : Soil profile: This soil profile shows the different The A horizon is of a mixture of organic material and inorganic
soil layers (O horizon, A horizon, B horizon, and C horizon) found
in typical soils. products of weathering; it is the beginning of true mineral soil.
1. The O horizon has freshly-decomposing organic matter, humus, The B horizon, or subsoil, is a dense layer of mostly fine material
at its surface, with decomposed vegetation at its base. Humus that has been pushed down from the topsoil.
enriches the soil with nutrients, enhancing soil moisture The C horizon, or soil base, is located just above bedrock and is
retention. Topsoil, the top layer of soil, is usually two to three made of parent, organic, and inorganic material.
inches deep, but this depth can vary considerably. For instance,
KEY TERMS
river deltas, such as the Mississippi River delta, have deep layers
topsoil: top layer of soil containing humus at its surface and
of topsoil. Topsoil is rich in organic material. Microbial
decomposing vegetation at its base; the most fertile soil
processes occur there; it is responsible for plant production.
subsoil: dense layer of soil containing fine material that has
2. The A horizon consists of a mixture of organic material with
moved downward; the layer of earth that is below the topsoil
inorganic products of weathering; it is the beginning of true
mineral soil. This horizon is typically darkly colored because of CONTRIBUTIONS AND ATTRIBUTIONS
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BY: Attribution
3. The B horizon, or subsoil, is an accumulation of mostly fine
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humus. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/humus.
of calcium carbonate. License: CC BY-SA: Attribution-ShareAlike
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License: CC BY-SA: Attribution-ShareAlike
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The parent material may be either created in its natural place or Located at: http://cnx.org/content/m44715/latest...e_31_02_01.png. License:
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SECTION OVERVIEW

31.3: NUTRITIONAL ADAPTATIONS OF PLANTS

31.3B: MYCORRHIZAE- THE SYMBIOTIC
Topic hierarchy RELATIONSHIP BETWEEN FUNGI AND ROOTS

31.3C: NUTRIENTS FROM OTHER SOURCES

31.3A: NITROGEN FIXATION- ROOT AND
BACTERIA INTERACTIONS
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31.3A: NITROGEN FIXATION- ROOT AND BACTERIA INTERACTIONS
nitrogen fixation takes place. This process entails the reduction of
 LEARNING OBJECTIVES atmospheric nitrogen to ammonia by means of the enzyme
nitrogenase. Therefore, using rhizobia is a natural and
Explain the process and importance of nitrogen fixation
environmentally-friendly way to fertilize plants as opposed to
chemical fertilization that uses a non-renewable resource, such as
Nitrogen is an important macronutrient because it is part of nucleic
natural gas. Through symbiotic nitrogen fixation, the plant benefits
acids and proteins. Atmospheric nitrogen, which is the diatomic
from using an endless source of nitrogen from the atmosphere. The
molecule N2, or dinitrogen, is the largest pool of nitrogen in
process simultaneously contributes to soil fertility because the plant
terrestrial ecosystems. However, plants cannot take advantage of this root system leaves behind some of the biologically-available
nitrogen because they do not have the necessary enzymes to convert
nitrogen. As in any symbiosis, both organisms benefit from the
it into biologically useful forms. However, nitrogen can be “fixed.”
interaction: the plant obtains ammonia and bacteria obtain carbon
It can be converted to ammonia (NH3) through biological, physical,
compounds generated through photosynthesis, as well as a protected
or chemical processes. Biological nitrogen fixation (BNF), the
niche in which to grow.
conversion of atmospheric nitrogen (N2) into ammonia (NH3), is
exclusively carried out by prokaryotes, such as soil bacteria or
cyanobacteria. Biological processes contribute 65 percent of the
nitrogen used in agriculture.

Figure 31.3A. 1 : Rhizobia: Soybean roots contain (a) nitrogen-

fixing nodules. Cells within the nodules are infected with
Bradyrhyzobium japonicum, a rhizobia or “root-loving” bacterium.
The bacteria are encased in (b) vesicles inside the cell, as can be
seen in this transmission electron micrograph.

KEY POINTS
Diatomic nitrogen is abundant in the atmosphere and soil, but
plants are unable to use it because they do not have the necessary
enzyme, nitrogenase, to convert it into a form that they can use to
make proteins.
Soil bacteria, or rhizobia, are able to perform biological nitrogen
Figure 31.3A. 1 : Diagram of the Nitrogen Cycle: Schematic
representation of the nitrogen cycle. Abiotic nitrogen fixation has fixation in which atmospheric nitrogen gas (N2) is converted into
been omitted. the ammonia (NH3) that plants are able to use to synthesize
The most important source of BNF is the symbiotic interaction proteins.
between soil bacteria and legume plants, including many crops Both the plants and the bacteria benefit from the process of
important to humans. The NH3 resulting from fixation can be nitrogen fixation; the plant obtains the nitrogen it needs to
transported into plant tissue and incorporated into amino acids, synthesize proteins, while the bacteria obtain carbon from the
which are then made into plant proteins. Some legume seeds, such as plant and a secure environment to inhabit within the plant roots.
soybeans and peanuts, contain high levels of protein and are among
the most important agricultural sources of protein in the world. KEY TERMS
rhizobia: any of various bacteria, of the genus Rhizobium, that
form nodules on the roots of legumes and fix nitrogen
nitrogen fixation: the conversion of atmospheric nitrogen into
ammonia and organic derivatives, by natural means, especially
by microorganisms in the soil, into a form that can be assimilated
by plants
nodule: structures that occur on the roots of plants that associate
Figure 31.3A. 1 : Nitrogen fixation in crops: Some common edible
legumes, such as (a) peanuts, (b) beans, and (c) chickpeas, are able with symbiotic nitrogen-fixing bacteria
to interact symbiotically with soil bacteria that fix nitrogen.
Soil bacteria, collectively called rhizobia, symbiotically interact with This page titled 31.3A: Nitrogen Fixation- Root and Bacteria Interactions is
legume roots to form specialized structures called nodules in which shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
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31.3B: MYCORRHIZAE- THE SYMBIOTIC RELATIONSHIP BETWEEN FUNGI AND
ROOTS
around the roots, called a mantle. Hyphae from the fungi extend
 LEARNING OBJECTIVES from the mantle into the soil, which increases the surface area for
water and mineral absorption. This type of mycorrhizae is found in
Describe the symbiotic relationship of mycorrhizae and
forest trees, especially conifers, birches, and oaks.
plant roots
Endomycorrhizae, also called arbuscular mycorrhizae, do not form a
dense sheath over the root. Instead, the fungal mycelium is
MYCORRHIZAE: THE SYMBIOTIC RELATIONSHIP embedded within the root tissue. Endomycorrhizae are found in the
BETWEEN FUNGI AND ROOTS roots of more than 80 percent of terrestrial plants.
A nutrient depletion zone can develop when there is rapid soil
solution uptake, low nutrient concentration, low diffusion rate, or
low soil moisture. These conditions are very common; therefore,
most plants rely on fungi to facilitate the uptake of minerals from the
soil. Mycorrhizae, known as root fungi, form symbiotic associations
with plant roots. In these associations, the fungi are actually
integrated into the physical structure of the root. The fungi colonize
the living root tissue during active plant growth.
Through mycorrhization, the plant obtains phosphate and other
minerals, such as zinc and copper, from the soil. The fungus obtains
nutrients, such as sugars, from the plant root. Mycorrhizae help
increase the surface area of the plant root system because hyphae,
Figure 31.3B. 1: Ectomycorrhizae: Ectomycorrhizae form sheaths,
which are narrow, can spread beyond the nutrient depletion zone. called a mantle, around the roots of plants, as shown in this image.
Hyphae are long extensions of the fungus, which can grow into
small soil pores that allow access to phosphorus otherwise KEY POINTS
unavailable to the plant. The beneficial effect on the plant is best Because nutrients are often depleted in the soil, most plants form
observed in poor soils. The benefit to fungi is that they can obtain up symbiotic relationships called mycorrhizae with fungi that
to 20 percent of the total carbon accessed by plants. Mycorrhizae integrate into the plant’s root.
function as a physical barrier to pathogens. They also provides an The relationship between plants and fungi is symbiotic because
induction of generalized host defense mechanisms, which sometimes the plant obtains phosphate and other minerals through the
involves the production of antibiotic compounds by the fungi. Fungi fungus, while the fungus obtains sugars from the plant root.
have also been found to have a protective role for plants rooted in The long extensions of the fungus, called hyphae, help increase
soils with high metal concentrations, such as acidic and the surface area of the plant root system so that it can extend
contaminated soils. beyond the area of nutrient depletion.
Ectomycorrhizae are a type of mycorrhizae that form a dense
sheath around the plant roots, called a mantle, from which the
hyphae grow; in endomycorrhizae, mycelium is embedded
within the root tissue, as opposed to forming a sheath around it.
In endomycorrhizae, mycelium is embedded within the root
tissue, as opposed to forming a sheath around it; these are found
in the roots of most terrestrial plants.

KEY TERMS
mycorrhiza: a symbiotic association between a fungus and the
roots of a vascular plant
hypha: a long, branching, filamentous structure of a fungus that
is the main mode of vegetative growth
Figure 31.3B. 1: Mycorrhizae: Hyphae proliferate within the mycelium: the vegetative part of any fungus, consisting of a
mycorrhizae, which appears as off-white fuzz in this image. These mass of branching, threadlike hyphae, often underground
hyphae greatly increase the surface area of the plant root, allowing it
to reach areas that are not depleted of nutrients.
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31.3C: NUTRIENTS FROM OTHER SOURCES

 LEARNING OBJECTIVES

Differentiate among the sources of plant nutrition

NUTRIENTS FROM OTHER SOURCES

Some plants cannot produce their own food and must obtain their
nutrition from outside sources. This may occur with plants that are
parasitic or saprophytic: ingesting and utilizing dead matter as a
food source. In other cases, plants may be mutualistic symbionts,
epiphytes, or insectivorous.

PLANT PARASITES Figure 31.3C. 1 : Saprophytes: Saprophytes, like this Dutchmen’s

pipe (Monotropa hypopitys), obtain their food from dead matter and
A parasitic plant depends on its host for survival. Some parasitic do not have chlorophyll.
plants have no leaves. An example of this is the dodder, which has a
weak, cylindrical stem that coils around the host and forms suckers. SYMBIONTS
From these suckers, cells invade the host stem and grow to connect A symbiont is a plant in a symbiotic relationship with other
with the vascular bundles of the host. The parasitic plant obtains organisms, such as mycorrhizae (with fungi) or nodule formation.
water and nutrients through these connections. The plant is a total Root nodules occur on plant roots (primarily Fabaceae) that
parasite (a holoparasite) because it is completely dependent on its associate with symbiotic, nitrogen-fixing bacteria. Under nitrogen-
host. Other parasitic plants, called hemiparasites, are fully limiting conditions, capable plants form a symbiotic relationship
photosynthetic and only use the host for water and minerals. There with a host-specific strain of bacteria known as rhizobia. Within
are about 4,100 species of parasitic plants. legume nodules, nitrogen gas from the atmosphere is converted into
ammonia, which is then assimilated into amino acids (the building
blocks of proteins), nucleotides (the building blocks of DNA and
RNA, as well as the important energy molecule ATP), and other
cellular constituents such as vitamins, flavones, and hormones.
Fungi also form symbiotic associations with cyanobacteria and
green algae; the resulting symbiotic organism is called a lichen.
Lichens can sometimes be seen as colorful growths on the surface of
rocks and trees. The algal partner (phyco- or photobiont) makes food
autotrophically, some of which it shares with the fungus; the fungal
partner (mycobiont) absorbs water and minerals from the
environment, which are made available to the green alga. If one
Figure 31.3C. 1 : Parasitic plants: The dodder is a holoparasite that partner was separated from the other, they would both die.
penetrates the host’s vascular tissue and diverts nutrients for its own
growth. Note that the vines of the dodder, which has white flowers,
are beige. The dodder has no chlorophyll and cannot produce its
own food.

SAPROPHYTES
A saprophyte is a plant that does not have chlorophyll, obtaining its
food from dead matter, similar to bacteria and fungi. (Note that fungi
are often called saprophytes, which is incorrect, because fungi are
not plants). Plants such as these use enzymes to convert organic food
materials into simpler forms from which they can absorb nutrients.
Most saprophytes do not directly digest dead matter. Instead, they
parasitize mycorrhizae or other fungi that digest dead matter,
ultimately obtaining photosynthate from a fungus that derived
photosynthate from its host. Saprophytic plants are uncommon with
only a few, described species.

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 Figure 31.3C. 1 : Insectivorous plants: A Venus flytrap has
specialized leaves to trap insects, which it uses to supplement the
low level of nutrients in the soil in which it lives.

KEY POINTS
Some plants are parasites, which acquire all of some of their
nutrients from another host plant and are, therefore, entirely
dependent upon it for their survival.
Saprophytes acquire nutrients from dead matter, using enzymes
to convert complex organic compounds into simpler forms from
which the plant can absorb nutrients.
Figure 31.3C. 1 : Symbionts: Lichens, which result from the A symbiont experiences a mutually-beneficial arrangement with
symbiotic relationship between fungi and green algae, are often seen a plant; both partners contribute necessary nutrients to the other.
growing on trees. An epiphyte is a plant that grows on other plants, but is not
dependent upon the other plant for nutrition; instead, it uses the
EPIPHYTES
other plant for physical support.
An epiphyte is a plant that grows on other plants, but is not
Insectivorous plants have special adapatations for attracting and
dependent upon the other plant for nutrition; it is non-parasitic. The
trapping insects, which they use to supplement their own
epiphyte derives its moisture and nutrients from the air, rain, and
nutrients, depleted in the surrounding soil.
sometimes from debris accumulating around it instead of from the
structure to which it is fastened. Epiphytes have two types of roots: KEY TERMS
clinging aerial roots (which absorb nutrients from humus that photosynthate: any compound that is a product of
accumulates in the crevices of trees) and aerial roots (which absorb
photosynthesis
moisture from the atmosphere). photobiont: a photosynthetic symbiont
INSECTIVOROUS PLANTS mycobiont: the fungus that is a component of a lichen
saprophyte: any organism that lives on dead organic matter, as
An insectivorous plant has specialized leaves to attract and digest
certain fungi and bacteria
insects. The Venus flytrap is popularly known for its insectivorous
epiphyte: a plant that grows on another, using it as a physical
mode of nutrition and has leaves that work as traps. The minerals it
support but neither obtaining nutrients from it nor causing it any
obtains from prey compensate for those lacking in the boggy (low
damage if also offering no benefit
pH) soil of its native North Carolina coastal plains. There are three
insectivorous: capable of trapping and absorbing insects; such as
sensitive hairs in the center of each half of each leaf. The edges of
the sundew, pitcher plant and Venus flytrap
each leaf are covered with long spines. Nectar secreted by the plant
attracts flies to the leaf. When a fly touches the sensory hairs, the CONTRIBUTIONS AND ATTRIBUTIONS
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habitat. License: CC BY-SA: Attribution-ShareAlike
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BY: Attribution en.wiktionary.org/wiki/insectivorous. License: CC BY-SA: Attribution-
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http://cnx.org/content/m44718/latest/Figure_31_03_02ab.jpg. License: CC en.wiktionary.org/wiki/photobiont. License: CC BY-SA: Attribution-
BY: Attribution ShareAlike
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License: CC BY: Attribution photosynthate. Provided by: Wiktionary. Located at:
Mycorrhiza. Provided by: Wikipedia. Located at: en.wiktionary.org/wiki/photosynthate. License: CC BY-SA: Attribution-
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 CHAPTER OVERVIEW

32: PLANT REPRODUCTIVE DEVELOPMENT AND STRUCTURE

32.1: Plant Reproductive Development and Structure - Plant Reproductive Development and Structure
32.2: Plant Reproductive Development and Structure - Sexual Reproduction in Gymnosperms
32.3: Plant Reproductive Development and Structure - Sexual Reproduction in Angiosperms
32.4: Pollination and Fertilization - Introduction
32.5: Pollination and Fertilization - Pollination by Insects
32.6: Pollination and Fertilization - Pollination by Bats, Birds, Wind, and Water
32.7: Pollination and Fertilization - Double Fertilization in Plants
32.8: Pollination and Fertilization - Development of the Seed
32.9: Pollination and Fertilization - Development of Fruit and Fruit Types
32.10: Pollination and Fertilization - Fruit and Seed Dispersal
32.11: Asexual Reproduction - Asexual Reproduction in Plants
32.12: Asexual Reproduction - Natural and Artificial Methods of Asexual Reproduction in Plants
32.13: Asexual Reproduction - Plant Life Spans

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Boundless.

1
32.1: PLANT REPRODUCTIVE DEVELOPMENT AND STRUCTURE - PLANT
REPRODUCTIVE DEVELOPMENT AND STRUCTURE
to produce seeds through sexual reproduction. Seeds are the next
 LEARNING OBJECTIVES generation, serving as the primary method in most plants by which
individuals of the species are dispersed across the landscape. Actual
Differentiate among the ways in which plants reproduce
dispersal is, in most species, a function of the fruit (a structural part
that typically surrounds the seed).
INTRODUCTION
Plants have evolved different reproductive strategies for the
continuation of their species. Some plants reproduce sexually while
others reproduce asexually, in contrast to animal species, which rely
almost exclusively on sexual reproduction. Plant sexual reproduction
usually depends on pollinating agents, while asexual reproduction is
independent of these agents. Flowers are often the showiest or most Figure 32.1.1: Plants and sexual reproduction: Plants that reproduce
strongly-scented part of plants. With their bright colors, fragrances, sexually often achieve fertilization with the help of pollinators such
and interesting shapes and sizes, flowers attract insects, birds, and as (a) bees, (b) birds, and (c) butterflies.
animals to serve their pollination needs. Other plants pollinate via
KEY POINTS
wind or water; still others self-pollinate.
Vegetative reproduction is a type of asexual reproduction that
ASEXUAL REPRODUCTION results in new plant individuals without seed or spore production.
Vegetative reproduction is also utilized by horticulturists to
Vegetative reproduction is a type of asexual reproduction. Other
terms that apply are vegetative propagation, clonal growth, or ensure production of large quantities of valuable plants.
vegetative multiplication. Vegetative growth is enlargement of the Plants have flowers that produce seeds through sexual
individual plant, while vegetative reproduction is any process that reproduction; seeds are dispersed to increase propagation of the
results in new plant “individuals” without production of seeds or next generation.
spores. It is both a natural process in many, many species as well as Seeds are often dispersed by animals via ingestion of the fruits,
a process utilized or encouraged by horticulturists and farmers to which surround the seeds, promoting seed dispersal.
obtain quantities of economically-valuable plants. In this respect, it
KEY TERMS
is a form of cloning that has been carried out by humanity for
vegetative reproduction: a form of asexual reproduction in
thousands of years and by plants for hundreds of millions of years.
plants
SEXUAL REPRODUCTION AND THE FLOWER
This page titled 32.1: Plant Reproductive Development and Structure - Plant
The flower is the reproductive organ of plants classified as Reproductive Development and Structure is shared under a CC BY-SA 4.0
angiosperms. All plants have the means and corresponding license and was authored, remixed, and/or curated by Boundless.
structures for reproducing sexually. The basic function of a flower is

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32.2: PLANT REPRODUCTIVE DEVELOPMENT AND STRUCTURE - SEXUAL
REPRODUCTION IN GYMNOSPERMS

 LEARNING OBJECTIVES

Describe the process of sexual reproduction in

gymnosperms

SEXUAL REPRODUCTION IN GYMNOSPERMS

As with angiosperms, the life cycle of gymnosperms is also
characterized by alternation of generations. In conifers such as pines,
the green leafy part of the plant is the sporophyte; the cones contain
the male and female gametophytes. The female cones are larger than
the male cones and are positioned towards the top of the tree; the
small, male cones are located in the lower region of the tree.
Because the pollen is shed and blown by the wind, this arrangement
makes it difficult for a gymnosperm to self-pollinate.

Figure 32.2.1: Male and female gametophytes: These series of

micrographs shows male and female gymnosperm gametophytes. (a)
This male cone, shown in cross section, has approximately 20
microsporophylls, each of which produces hundreds of male
gametophytes (pollen grains). (b) Pollen grains are visible in this
single microsporophyll. (c) This micrograph shows an individual
pollen grain. (d) This cross section of a female cone shows portions
of about 15 megasporophylls. (e) The ovule can be seen in this
single megasporophyll. (f) Within this single ovule are the
megaspore mother cell (MMC), micropyle, and a pollen grain.

FEMALE GAMETOPHYTE
The female cone also has a central axis on which bracts known as
megasporophylls are present. In the female cone, megaspore mother
cells are present in the megasporangium. The megaspore mother cell
divides by meiosis to produce four haploid megaspores. One of the
Figure 32.2.1: Conifer life cycle: This image shows the life cycle of
a conifer. Pollen from male cones blows up into upper branches, megaspores divides to form the multicellular female gametophyte,
where it fertilizes female cones. Examples are shown for female and while the others divide to form the rest of the structure. The female
male cones. gametophyte is contained within a structure called the archegonium.
MALE GAMETOPHYTE REPRODUCTIVE PROCESS
A male cone has a central axis on which bracts, a type of modified Upon landing on the female cone, the tube cell of the pollen forms
leaf, are attached. The bracts, known as microsporophylls, are the the pollen tube, through which the generative cell migrates towards
sites where microspores will develop. The microspores develop the female gametophyte through the micropyle. It takes
inside the microsporangium. Within the microsporangium, cells approximately one year for the pollen tube to grow and migrate
known as microsporocytes divide by meiosis to produce four haploid towards the female gametophyte. The male gametophyte containing
microspores. Further mitosis of the microspore produces two nuclei: the generative cell splits into two sperm nuclei, one of which fuses
the generative nucleus and the tube nucleus. Upon maturity, the male with the egg, while the other degenerates. After fertilization of the
gametophyte (pollen) is released from the male cones and is carried egg, the diploid zygote is formed, which divides by mitosis to form
by the wind to land on female cones. the embryo. The scales of the cones are closed during development
of the seed. The seed is covered by a seed coat, which is derived
from the female sporophyte. Seed development takes another one to
two years. Once the seed is ready to be dispersed, the bracts of the
female cones open to allow the dispersal of seed; no fruit formation
takes place because gymnosperm seeds have no covering.

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ANGIOSPERMS VERSUS GYMNOSPERMS The megaspore mother cell in the female cone divides by meiosis
Gymnosperm reproduction differs from that of angiosperms in to produce four haploid megaspores; one of the megaspores
several ways. In angiosperms, the female gametophyte in the ovule divides to form the female gametophyte.
exists in an enclosed structure, the ovary; in gymnosperms, the The male gametophyte lands on the female cone, forming a
female gametophyte is present on exposed bracts of the female cone pollen tube through which the generative cell travels to meet the
and is not enclosed in an ovary. Double fertilization is a key event in female gametophyte.
the life cycle of angiosperms, but is completely absent in One of the two sperm cells released by the generative cell fuses
gymnosperms. The male and female gametophyte structures are with the egg, forming a diploid zygote that divides to form the
present on separate male and female cones in gymnosperms, embryo.
whereas in angiosperms, they are a part of the flower. Finally, wind Unlike angiosperms, ovaries are absent in gymnosperms, double
plays an important role in pollination in gymnosperms because fertilization does not take place, male and female gametophytes
pollen is blown by the wind to land on the female cones. Although are present on cones rather than flowers, and wind (not animals)
many angiosperms are also wind-pollinated, animal pollination is drives pollination.
more common.
KEY TERMS
KEY POINTS megasporophyll: bears megasporangium, which produces
In gymnosperms, a leafy green sporophyte generates cones megaspores that divide into the female gametophyte
containing male and female gametophytes; female cones are microsporophyll: a leaflike organ that bears microsporangium,
bigger than male cones and are located higher up in the tree. which produces microspores that divide into the male
A male cone contains microsporophylls where male gametophyte (pollen)
gametophytes ( pollen ) are produced and are later carried by
This page titled 32.2: Plant Reproductive Development and Structure -
wind to female gametophytes.
Sexual Reproduction in Gymnosperms is shared under a CC BY-SA 4.0
license and was authored, remixed, and/or curated by Boundless.

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32.3: PLANT REPRODUCTIVE DEVELOPMENT AND STRUCTURE - SEXUAL
REPRODUCTION IN ANGIOSPERMS
Flowers that contain both an androecium and a gynoecium are called
 LEARNING OBJECTIVES perfect, androgynous, or hermaphrodites. There are two types of
incomplete flowers: staminate flowers contain only an androecium;
Outline the components of a flower and their function
and carpellate flowers have only a gynoecium.

The lifecycle of angiosperms follows the alternation of generations.

In the angiosperm, the haploid gametophyte alternates with the
diploid sporophyte during the sexual reproduction process of
angiosperms. Flowers contain the plant’s reproductive structures.

FLOWER STRUCTURE
A typical flower has four main parts, or whorls: the calyx, corolla,
androecium, and gynoecium. The outermost whorl of the flower has
green, leafy structures known as sepals, which are collectively called
the calyx, and help to protect the unopened bud. The second whorl is
comprised of petals, usually brightly colored, collectively called the
corolla. The number of sepals and petals varies depending on
whether the plant is a monocot or dicot. Together, the calyx and
corolla are known as the perianth. The third whorl contains the male
reproductive structures and is known as the androecium. The
androecium has stamens with anthers that contain the
microsporangia. The innermost group of structures in the flower is
the gynoecium, or the female reproductive component(s). The carpel
is the individual unit of the gynoecium and has a stigma, style, and
ovary. A flower may have one or multiple carpels.

Figure 32.3.1: Staminate and carpellate flowers: The corn plant has
both staminate (male) and carpellate (female) flowers. Staminate
flowers, which are clustered in the tassel at the tip of the stem,
produce pollen grains. Carpellate flower are clustered in the
immature ears. Each strand of silk is a stigma. The corn kernels are
seeds that develop on the ear after fertilization. Also shown is the
lower stem and root.
If both male and female flowers are borne on the same plant (e.g.,
corn or peas), the species is called monoecious (meaning “one
home”). Species with male and female flowers borne on separate
plants (e.g., C. papaya or Cannabis)are termed dioecious, or “two
homes.” The ovary, which may contain one or multiple ovules, may
be placed above other flower parts (referred to as superior); or it
may be placed below the other flower parts (referred to as inferior).
Figure 32.3.1: Structures of the flower: The four main parts of the
flower are the calyx, corolla, androecium, and gynoecium. The
androecium is the sum of all the male reproductive organs, and the
gynoecium is the sum of the female reproductive organs.
If all four whorls are present, the flower is described as complete. If
any of the four parts is missing, the flower is known as incomplete.

32.3.1 https://bio.libretexts.org/@go/page/13802
 Within the microsporangium, the microspore mother cell divides by
meiosis to give rise to four microspores, each of which will
ultimately form a pollen grain. An inner layer of cells, known as the
tapetum, provides nutrition to the developing microspores,
contributing key components to the pollen wall. Mature pollen
grains contain two cells: a generative cell and a pollen tube cell. The
generative cell is contained within the larger pollen tube cell. Upon
germination, the tube cell forms the pollen tube through which the
generative cell migrates to enter the ovary. During its transit inside
the pollen tube, the generative cell divides to form two male
gametes. Upon maturity, the microsporangia burst, releasing the
pollen grains from the anther.
Each pollen grain has two coverings: the exine (thicker, outer layer)
and the intine. The exine contains sporopollenin, a complex
waterproofing substance supplied by the tapetal cells. Sporopollenin
allows the pollen to survive under unfavorable conditions and to be
carried by wind, water, or biological agents without undergoing
damage.

Figure 32.3.1: Superior and inferior flowers: The (a) lily is a

superior flower, which has the ovary above the other flower parts.
(b) Fuchsia is an inferior flower, which has the ovary beneath other
flower parts.

MALE GAMETOPHYTE
The male gametophyte develops and reaches maturity in an
immature anther. In a plant’s male reproductive organs, development
of pollen takes place in a structure known as the microsporangium.
The microsporangia, usually bi-lobed, are pollen sacs in which the
microspores develop into pollen grains.

Figure 32.3.1: Pollen grain structure: Pollen develops from the

microspore mother cells. The mature pollen grain is composed of
two cells: the pollen tube cell and the generative cell, which is inside
the tube cell. The pollen grain has two coverings: an inner layer
(intine) and an outer layer (exine). The inset scanning electron
micrograph shows Arabidopsis lyrata pollen grains.
Figure 32.3.1: Microsporangium: Shown is (a) a cross section of an
anther at two developmental stages. The immature anther (top) FEMALE GAMETOPHYTE (EMBRYO SAC)
contains four microsporangia, or pollen sacs. Each microsporangium
contains hundreds of microspore mother cells that will each give rise The overall development of the female gametophyte has two distinct
to four pollen grains. The tapetum supports the development and phases. First, in the process of megasporogenesis, a single cell in the
maturation of the pollen grains. Upon maturation of the pollen
(bottom), the pollen sac walls split open and the pollen grains (male diploid megasporangium undergoes meiosis to produce four
gametophytes) are released. (b) In these scanning electron megaspores, only one of which survives. During the second phase,
micrographs, pollen sacs are ready to burst, releasing their grains. megagametogenesis, the surviving haploid megaspore undergoes

32.3.2 https://bio.libretexts.org/@go/page/13802
mitosis to produce an eight-nucleate, seven-cell female megaspore undergoes mitosis to form an embryo sac (female
gametophyte, also known as the megagametophyte, or embryo sac. gametophyte).
The polar nuclei move to the equator and fuse, forming a single, The sperm, guided by the synergid cells, migrates to the ovary to
diploid central cell. This central cell later fuses with a sperm to form complete fertilization; the diploid zygote develops into the
the triploid endosperm. Three nuclei position themselves on the end embryo, while the fertilized ovule forms the other tissues of the
of the embryo sac opposite the micropyle and develop into the seed.
antipodal cells, which later degenerate. The nucleus closest to the
micropyle becomes the female gamete, or egg cell, and the two KEY TERMS
adjacent nuclei develop into synergid cells. The synergids help guide perianth: the calyx (sepals) and the corolla (petals)
the pollen tube for successful fertilization, after which they androecium: the set of a flower’s stamens (male reproductive
disintegrate. Once fertilization is complete, the resulting diploid organs)
zygote develops into the embryo; the fertilized ovule forms the other gynoecium: the set of a flower’s pistils (female reproductive
tissues of the seed. organs)

CONTRIBUTIONS AND ATTRIBUTIONS

vegetative reproduction. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/vegetat...20reproduction. License: CC BY-SA:
Attribution-ShareAlike
Botany/Plant reproduction. Provided by: Wikibooks. Located at:
en.wikibooks.org/wiki/Botany/...t_reproduction. License: CC BY-SA:
Attribution-ShareAlike
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44720/latest...ol11448/latest. License: CC
BY: Attribution
OpenStax College, Introduction. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44720/latest...2_00_01abc.jpg.
License: CC BY: Attribution
megasporophyll. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/megasporophyll. License: CC BY-SA: Attribution-
ShareAlike
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BY: Attribution
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Figure 32.3.1: Embryo sac: As shown in this diagram of the embryo Located at: http://cnx.org/content/m44722/latest...ol11448/latest. License: CC
BY: Attribution
sac in angiosperms, the ovule is covered by integuments and has an
microsporophyll. Provided by: Wiktionary. Located at:
opening called a micropyle. Inside the embryo sac are three en.wiktionary.org/wiki/microsporophyll. License: CC BY-SA: Attribution-
antipodal cells, two synergids, a central cell, and the egg cell. ShareAlike
A double-layered integument protects the megasporangium and, OpenStax College, Introduction. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44720/latest...2_00_01abc.jpg.
later, the embryo sac. The integument will develop into the seed coat License: CC BY: Attribution
after fertilization, protecting the entire seed. The ovule wall will OpenStax College, Reproductive Development and Structure. October 17, 2013.
Provided by: OpenStax CNX. Located at:
become part of the fruit. The integuments, while protecting the http://cnx.org/content/m44722/latest...e_32_01_08.png. License: CC BY:
megasporangium, do not enclose it completely, but leave an opening Attribution
OpenStax College, Reproductive Development and Structure. October 17, 2013.
called the micropyle. The micropyle allows the pollen tube to enter Provided by: OpenStax CNX. Located at:
the female gametophyte for fertilization. http://cnx.org/content/m44722/latest...e_32_01_09.jpg. License: CC BY:
Attribution
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KEY POINTS Located at: http://cnx.org/content/m44722/latest...ol11448/latest. License: CC
A typical flower has four main parts, or whorls: the calyx ( BY: Attribution
androecium. Provided by: Wiktionary. Located at:
sepals ), corolla (petals), androecium (male reproductive en.wiktionary.org/wiki/androecium. License: CC BY-SA: Attribution-
structure), and gynoecium (female reproductive structure). ShareAlike
gynoecium. Provided by: Wiktionary. Located at:
Angiosperms that contain both male and female gametophytes en.wiktionary.org/wiki/gynoecium. License: CC BY-SA: Attribution-
within the same flower are called complete and are considered to ShareAlike
Boundless. Provided by: Boundless Learning. Located at:
be androgynous or hermaphroditic. www.boundless.com//biology/definition/perianth. License: CC BY-SA:
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gametophytes are considered to be incomplete and are either OpenStax College, Introduction. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44720/latest...2_00_01abc.jpg.
staminate (contain only male structures) or carpellate (contain License: CC BY: Attribution
only female structures) flowers. OpenStax College, Reproductive Development and Structure. October 17, 2013.
Provided by: OpenStax CNX. Located at:
Microspores develop in the microsporangium and form mature http://cnx.org/content/m44722/latest...e_32_01_08.png. License: CC BY:
pollen grains (male gametophytes), which are then used to Attribution
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fertilize female gametophytes. Provided by: OpenStax CNX. Located at:
During megasporogenesis, four megaspores are produced with http://cnx.org/content/m44722/latest...e_32_01_09.jpg. License: CC BY:
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32.4: POLLINATION AND FERTILIZATION - INTRODUCTION

 LEARNING OBJECTIVES

Determine the differences between self-pollination and

cross-pollination, and describe how plants have developed
ways to avoid self-pollination

POLLINATION: AN INTRODUCTION
In angiosperms, pollination is defined as the placement or transfer of
pollen from the anther to the stigma of the same or a different
flower. In gymnosperms, pollination involves pollen transfer from
the male cone to the female cone. Upon transfer, the pollen
germinates to form the pollen tube and the sperm that fertilize the
egg.

SELF-POLLINATION AND CROSS-POLLINATION

Pollination takes two forms: self-pollination and cross-pollination.
Self-pollination occurs when the pollen from the anther is deposited Figure 32.4.1: Teosinte: Teosinte (left) is the ancestor of modern
on the stigma of the same flower or another flower on the same corn (far-right). Although they are morphologically dissimilar,
genetically they are not so different.
plant. Cross-pollination is the transfer of pollen from the anther of
one flower to the stigma of another flower on a different individual GENETIC DIVERSITY
of the same species. Self-pollination occurs in flowers where the Living species are designed to ensure survival of their progeny;
stamen and carpel mature at the same time and are positioned so that those that fail become extinct. Genetic diversity is, therefore,
the pollen can land on the flower’s stigma. This method of required so that in changing environmental or stress conditions,
pollination does not require an investment from the plant to provide some of the progeny can survive. Self-pollination leads to the
nectar and pollen as food for pollinators. These types of pollination production of plants with less genetic diversity since genetic
have been studied since the time of Gregor Mendel. Mendel material from the same plant is used to form gametes and,
successfully carried out self-pollination and cross-pollination in eventually, the zygote. In contrast, cross-pollination leads to greater
garden peas while studying how characteristics were passed on from genetic diversity because the male and female gametophytes are
one generation to the next. Today’s crops are a result of plant derived from different plants. Because cross-pollination allows for
breeding, which employs artificial selection to produce the present- more genetic diversity, plants have developed many ways to avoid
day cultivars. An example is modern corn, which is a result of self-pollination. In some species, the pollen and the ovary mature at
thousands of years of breeding that began with its ancestor, teosinte. different times. These flowers make self-pollination nearly
The teosinte that the ancient Mesoamericans originally began impossible. By the time pollen matures and has been shed, the
cultivating had tiny seeds, vastly different from today’s relatively stigma of this flower is mature and can only be pollinated by pollen
giant ears of corn. Interestingly, though these two plants appear to be from another flower. Some flowers have developed physical features
entirely different, the genetic difference between them is minuscule.
that prevent self-pollination. The primrose employs this technique.
Primroses have evolved two flower types with differences in anther
and stigma length: the pin-eyed flower and the thrum-eyed flower. In
the pin-eyed flower, anthers are positioned at the pollen tube’s
halfway point, and in the thrum-eyed flower, the stigma is found at
this same location. This allows insects to easily cross-pollinate while
seeking nectar at the pollen tube. This phenomenon is also known as
heterostyly. Many plants, such as cucumbers, have male and female
flowers located on different parts of the plant, thus making self-
pollination difficult. In other species, the male and female flowers
are borne on different plants, making them dioecious. All of these
are barriers to self-pollination; therefore, the plants depend on
pollinators to transfer pollen. The majority of pollinators are biotic
agents such as insects (bees, flies, and butterflies), bats, birds, and
other animals. Other plant species are pollinated by abiotic agents,
such as wind and water.

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 Cross-pollination is the most advantageous of the two types of
pollination since it provides species with greater genetic
diversity.
Maturation of pollen and ovaries at different times and
heterostyly are methods plants have developed to avoid self-
pollination.
The placement of male and female flowers on separate plants or
different parts of the plant are also barriers to self-pollination.

KEY TERMS
pollination: the transfer of pollen from an anther to a stigma that
is carried out by insects, birds, bats, and the wind
heterostyly: the condition of having unequal male (anther) and
Figure 32.4.1: Pollinators: To maximize their avoidance of self- female (stigma) reproductive organs
pollination, plants have evolved relationships with animals, such as cross-pollination: fertilization by the transfer of pollen from an
bees, to ensure cross-pollination between members of the same
species.
anther of one plant to a stigma of another
self-pollination: pollination of a flower by its own pollen in a
KEY POINTS flower that has both stamens and a pistil
Pollination, the transfer of pollen from flower-to-flower in This page titled 32.4: Pollination and Fertilization - Introduction is shared
angiosperms or cone -to-cone in gymnosperms, takes place under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
through self-pollination or cross-pollination. by Boundless.

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32.5: POLLINATION AND FERTILIZATION - POLLINATION BY INSECTS
brightly colored, have a strong fragrance, are open during the day,
 LEARNING OBJECTIVES and have nectar guides. The pollen is picked up and carried on the
butterfly’s limbs. Moths, on the other hand, pollinate flowers during
Explain how pollination by insects aids plant reproduction
the late afternoon and night. The flowers pollinated by moths are
pale or white and are flat, enabling the moths to land. One well-
BEES studied example of a moth-pollinated plant is the yucca plant, which
Bees are perhaps the most important pollinator of many garden is pollinated by the yucca moth. The shape of the flower and moth
plants and most commercial fruit trees. The most common species of have adapted in a way to allow successful pollination. The moth
bees are bumblebees and honeybees. Since bees cannot see the color deposits pollen on the sticky stigma for fertilization to occur later.
red, bee-pollinated flowers usually have shades of blue, yellow, or The female moth also deposits eggs into the ovary. As the eggs
other colors. Bees collect energy -rich pollen or nectar for their develop into larvae, they obtain food from the flower and developing
survival and energy needs. They visit flowers that are open during seeds. Thus, both the insect and flower benefit from each other in
the day, are brightly colored, have a strong aroma or scent, and have this symbiotic relationship. The corn earworm moth and Gaura plant
a tubular shape, typically with the presence of a nectar guide. A have a similar relationship.
nectar guide includes regions on the flower petals that are visible
only to bees, which help guide bees to the center of the flower, thus
making the pollination process more efficient. The pollen sticks to
the bees’ fuzzy hair; when the bee visits another flower, some of the
pollen is transferred to the second flower. Recently, there have been
many reports about the declining population of honeybees. Many
flowers will remain unpollinated, failing to bear seeds if honeybees
disappear. The impact on commercial fruit growers could be
devastating.
Figure 32.5.1: Moths as pollinators: A corn earworm (a moth) sips
nectar from a night-blooming Gaura plant. Both the moth and plant
benefit from each other as they have formed a symbiotic
relationship; the plant is pollinated while the moth is able to obtain
food.

KEY POINTS
Adaptations such as bright colors, strong fragrances, special
shapes, and nectar guides are used to attract suitable pollinators.
Important insect pollinators include bees, flies, wasps,
butterflies, and moths.
Bees and butterflies are attracted to brightly-colored flowers that
have a strong scent and are open during the day, whereas moths
are attracted to white flowers that are open at night.
Flies are attracted to dull brown and purple flowers that have an
Figure 32.5.1: Pollination by insects: Insects, such as bees, are
important agents of pollination. Bees are probably the most odor of decaying meat.
important species of pollinators for commercial and garden plant Nectar guides, which are only visible to certain insects, facilitate
species. pollination by guiding bees to the pollen at the center of flowers.
Insects and flowers both benefit from their specialized symbiotic
FLIES
relationships; plants are pollinated while insects obtain valuable
Many flies are attracted to flowers that have a decaying smell or an
sources of food.
odor of rotting flesh. These flowers, which produce nectar, usually
have dull colors, such as brown or purple. They are found on the KEY TERMS
corpse flower or voodoo lily (Amorphophallus), dragon arum nectar guide: markings or patterns seen in flowers of some
(Dracunculus), and carrion flower (Stapleia, Rafflesia). The nectar angiosperm species that guide pollinators to nectar or pollen
provides energy while the pollen provides protein. Wasps are also
important insect pollinators, pollinating many species of figs. This page titled 32.5: Pollination and Fertilization - Pollination by Insects is
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
BUTTERFLIES AND MOTHS curated by Boundless.
Butterflies, such as the monarch, pollinate many garden flowers and
wildflowers, which are usually found in clusters. These flowers are

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32.6: POLLINATION AND FERTILIZATION - POLLINATION BY BATS, BIRDS,
WIND, AND WATER
POLLINATION BY WIND
 LEARNING OBJECTIVES Most species of conifers and many angiosperms, such as grasses,
maples, and oaks, are pollinated by wind. Pine cones are brown and
Differentiate among the non-insect methods of pollination
unscented, while the flowers of wind-pollinated angiosperm species
are usually green, small, may have small or no petals, and produce
NON-INSECT METHODS OF POLLINATION
large amounts of pollen. Unlike the typical insect-pollinated flowers,
Plants have developed specialized adaptations to take advantage of flowers adapted to pollination by wind do not produce nectar or
non-insect forms of pollination. These methods include pollination scent. In wind-pollinated species, the microsporangia hang out of the
by bats, birds, wind, and water. flower, and, as the wind blows, the lightweight pollen is carried with
it. The flowers usually emerge early in the spring before the leaves
POLLINATION BY BATS
so that the leaves do not block the movement of the wind. The
In the tropics and deserts, bats are often the pollinators of nocturnal pollen is deposited on the exposed feathery stigma of the flower.
flowers such as agave, guava, and morning glory. The flowers are
usually large and white or pale-colored so that they can be
distinguished from their dark surroundings at night. The flowers
have a strong, fruity, or musky fragrance and produce large amounts
of nectar. They are naturally-large and wide-mouthed to
accommodate the head of the bat. As the bats seek the nectar, their
faces and heads become covered with pollen, which is then
transferred to the next flower.
Figure 32.6.1: Wind pollination: These male (a) and female (b)
POLLINATION BY BIRDS catkins from the goat willow tree (Salix caprea) have structures that
are light and feathery to better disperse and catch the wind-blown
Many species of small birds, such as hummingbirds and sun birds, pollen.
are pollinators for plants such as orchids and other wildflowers.
Flowers visited by birds are usually sturdy and are oriented in a way POLLINATION BY WATER
to allow the birds to stay near the flower without getting their wings Some weeds, such as Australian sea grass and pond weeds, are
entangled in the nearby flowers. The flower typically has a curved, pollinated by water. The pollen floats on water. When it comes into
tubular shape, which allows access for the bird’s beak. Brightly- contact with the flower, it is deposited inside the flower.
colored, odorless flowers that are open during the day are pollinated
by birds. As a bird seeks energy-rich nectar, pollen is deposited on POLLINATION BY DECEPTION
the bird’s head and neck and is then transferred to the next flower it Orchids are highly-valued flowers, with many rare varieties. They
visits. Botanists determine the range of extinct plants by collecting grow in a range of specific habitats, mainly in the tropics of Asia,
and identifying pollen from 200-year-old bird specimens from the South America, and Central America. At least 25,000 species of
same site. orchids have been identified.
Flowers often attract pollinators with food rewards, in the form of
nectar. However, some species of orchid are an exception to this
standard; they have evolved different ways to attract the desired
pollinators. They use a method known as food deception, in which
bright colors and perfumes are offered, but no food. Anacamptis
morio, commonly known as the green-winged orchid, bears bright
purple flowers and emits a strong scent. The bumblebee, its main
pollinator, is attracted to the flower because of the strong scent,
which usually indicates food for a bee. In the process, the bee picks
up the pollen to be transported to another flower.
Other orchids use sexual deception. Chiloglottis trapeziformis emits
a compound that smells the same as the pheromone emitted by a
female wasp to attract male wasps. The male wasp is attracted to the
Figure 32.6.1: Pollination by birds: Hummingbirds have adaptations scent, lands on the orchid flower, and, in the process, transfers
that allow them to reach the nectar of certain tubular flowers, pollen. Some orchids, like the Australian hammer orchid, use scent
thereby, aiding them in the process of pollination.
as well as visual trickery in yet another sexual deception strategy to
attract wasps. The flower of this orchid mimics the appearance of a
female wasp and emits a pheromone. The male wasp tries to mate

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with what appears to be a female wasp, but instead picks up pollen, KEY POINTS
which it then transfers to the next counterfeit mate. Flowers that are pollinated by bats bloom at night, tending to be
large, wide-mouthed, and pale-colored; they may also give off
strong scents.
Flowers that are pollinated by small birds usually have curved,
tubular shapes; birds carry the pollen off on their heads and neck
to the next flower they visit.
Wind-pollinated flowers do not produce scents or nectar; instead,
they tend to have small or no petals and to produce large
amounts of lightweight pollen.
Some species of flowers release pollen that can float on water;
pollination occurs when the pollen reaches another plant of the
same species.
Some flowers deceive pollinators through food or sexual
deception; the pollinators become attracted to the flowers with
false promises of food and mating opportunities.

KEY TERMS
food deception: a trickery method employed by some species of
orchids in which only bright colors and perfume are offered to
their pollinators with no food reward
Figure 32.6.1: Pollination by deception in orchids: Certain orchids
use food deception or sexual deception to attract pollinators. Shown
here is a bee orchid (Ophrys apifera). This page titled 32.6: Pollination and Fertilization - Pollination by Bats,
Birds, Wind, and Water is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

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32.7: POLLINATION AND FERTILIZATION - DOUBLE FERTILIZATION IN PLANTS
the seed is ready for dispersal. Embryonic development is suspended
 LEARNING OBJECTIVES after some time; growth resumes only when the seed germinates.
The developing seedling will rely on the food reserves stored in the
Describe the process of double fertilization in plants
cotyledons until the first set of leaves begin photosynthesis.

DOUBLE FERTILIZATION
After pollen is deposited on the stigma, it must germinate and grow
through the style to reach the ovule. The microspores, or the pollen,
contain two cells: the pollen tube cell and the generative cell. The
pollen tube cell grows into a pollen tube through which the
generative cell travels. The germination of the pollen tube requires
water, oxygen, and certain chemical signals. As it travels through the
style to reach the embryo sac, the pollen tube’s growth is supported
by the tissues of the style. During this process, if the generative cell
has not already split into two cells, it now divides to form two sperm
cells. The pollen tube is guided by the chemicals secreted by the
synergids present in the embryo sac; it enters the ovule sac through
the micropyle. Of the two sperm cells, one sperm fertilizes the egg
cell, forming a diploid zygote; the other sperm fuses with the two
polar nuclei, forming a triploid cell that develops into the
endosperm. Together, these two fertilization events in angiosperms
are known as double fertilization. After fertilization is complete, no
other sperm can enter. The fertilized ovule forms the seed, whereas
the tissues of the ovary become the fruit, usually enveloping the
seed.

Figure 32.7.1: Embryo development: Shown are the stages of

embryo development in the ovule of a shepherd’s purse (Capsella
bursa). After fertilization, the zygote divides to form an upper
terminal cell and a lower basal cell. (a) In the first stage of
development, the terminal cell divides, forming a globular pro-
embryo. The basal cell also divides, giving rise to the suspensor. (b)
In the second stage, the developing embryo has a heart shape due to
the presence of cotyledons. (c) In the third stage, the growing
embryo is crowded and begins to bend. (d) Eventually, it completely
Figure 32.7.1: Double fertilization: In angiosperms, one sperm fills the seed.
fertilizes the egg to form the 2n zygote, while the other sperm fuses
with two polar nuclei to form the 3n endosperm. This is called a KEY POINTS
double fertilization.
Double fertilization involves two sperm cells; one fertilizes the
After fertilization, embryonic development begins. The zygote
egg cell to form the zygote, while the other fuses with the two
divides to form two cells: the upper cell (terminal cell) and the lower
polar nuclei that form the endosperm.
cell (basal cell). The division of the basal cell gives rise to the
After fertilization, the fertilized ovule forms the seed while the
suspensor, which eventually makes connection with the maternal
tissues of the ovary become the fruit.
tissue. The suspensor provides a route for nutrition to be transported
In the first stage of embryonic development, the zygote divides
from the mother plant to the growing embryo. The terminal cell also
to form two cells; one will develop into a suspensor, while the
divides, giving rise to a globular-shaped proembryo. In dicots
other gives rise to a proembryo.
(eudicots), the developing embryo has a heart shape due to the
In the second stage of embryonic development (in eudicots), the
presence of the two rudimentary cotyledons. In non-endospermic
developing embryo has a heart shape due to the presence of
dicots, such as Capsella bursa, the endosperm develops initially, but
cotyledons.
is then digested. In this case, the food reserves are moved into the
As the embryo grows, it begins to bend as it fills the seed; at this
two cotyledons. As the embryo and cotyledons enlarge, they become
point, the seed is ready for dispersal.
crowded inside the developing seed and are forced to bend.
Ultimately, the embryo and cotyledons fill the seed, at which point,

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KEY TERMS the mother plant to the growing embryo
double fertilization: a complex fertilization mechanism that has proembryo: a cluster of cells in the ovule of a fertilized
evolved in flowering plants; involves the joining of a female flowering plant that has not yet formed into an embryo
gametophyte with two male gametes (sperm)
This page titled 32.7: Pollination and Fertilization - Double Fertilization in
suspensor: found in plant zygotes in angiosperms; connects the
Plants is shared under a CC BY-SA 4.0 license and was authored, remixed,
endosperm to the embryo and provides a route for nutrition from
and/or curated by Boundless.

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32.8: POLLINATION AND FERTILIZATION - DEVELOPMENT OF THE SEED

 LEARNING OBJECTIVES

Name the three parts of a seed and describe their functions

and development

PARTS OF A SEED
The seed, along with the ovule, is protected by a seed coat that is
formed from the integuments of the ovule sac. In dicots, the seed
coat is further divided into an outer coat, known as the testa, and Figure 32.8.1: Monocots and dicots: The structures of dicot and
inner coat, known as the tegmen. The embryonic axis consists of monocot seeds are shown. Dicots (left) have two cotyledons.
three parts: the plumule, the radicle, and the hypocotyl. The portion Monocots, such as corn (right), have one cotyledon, called the
scutellum, which channels nutrition to the growing embryo. Both
of the embryo between the cotyledon attachment point and the monocot and dicot embryos have a plumule that forms the leaves, a
radicle is known as the hypocotyl. The embryonic axis terminates in hypocotyl that forms the stem, and a radicle that forms the root. The
a radicle, which is the region from which the root will develop. embryonic axis comprises everything between the plumule and the
radicle, not including the cotyledon(s).
SEED GROWTH In endospermic dicots, the food reserves are stored in the
In angiosperms, the process of seed development begins with double endosperm. During germination, the two cotyledons act as
fertilization and involves the fusion of the egg and sperm nuclei into absorptive organs to take up the enzymatically-released food
a zygote. The second part of this process is the fusion of the polar reserves, similar to the process in monocots. In non-endospermic
nuclei with a second sperm cell nucleus, thus forming a primary dicots, the triploid endosperm develops normally following double
endosperm. Right after fertilization, the zygote is mostly inactive, fertilization, but the endosperm food reserves are quickly
but the primary endosperm divides rapidly to form the endosperm remobilized, moving into the developing cotyledon for storage.
tissue. This tissue becomes the food the young plant will consume
until the roots have developed after germination. The seed coat SEED GERMINATION
forms from the two integuments or outer layers of cells of the ovule, Upon germination in dicot seeds, the epicotyl is shaped like a hook
which derive from tissue from the mother plant: the inner with the plumule pointing downwards; this plumule hook persists as
integument forms the tegmen and the outer forms the testa. When long as germination proceeds in the dark. Therefore, as the epicotyl
the seed coat forms from only one layer, it is also called the testa, pushes through the tough and abrasive soil, the plumule is protected
though not all such testae are homologous from one species to the from damage. Upon exposure to light, the hypocotyl hook
next. straightens out, the young foliage leaves face the sun and expand,
In gymnosperms, the two sperm cells transferred from the pollen do and the epicotyl continues to elongate. During this time, the radicle
not develop seed by double fertilization, but one sperm nucleus is also growing and producing the primary root. As it grows
unites with the egg nucleus and the other sperm is not used. downward to form the tap root, lateral roots branch off to all sides,
Sometimes each sperm fertilizes an egg cell and one zygote is then producing the typical dicot tap root system.
aborted or absorbed during early development. The seed is
composed of the embryo and tissue from the mother plant, which
also form a cone around the seed in coniferous plants such as pine
and spruce. The ovules after fertilization develop into the seeds.

FOOD STORAGE IN THE SEED

The storage of food reserves in angiosperm seeds differs between
monocots and dicots. In monocots, the single cotyledon is called a
scutellum; it is connected directly to the embryo via vascular tissue.
Food reserves are stored in the large endosperm. Upon germination,
enzymes are secreted by the aleurone, a single layer of cells just
inside the seed coat that surrounds the endosperm and embryo. The Figure 32.8.1: Monocot seeds: As this monocot grass seed
enzymes degrade the stored carbohydrates, proteins, and lipids. germinates, the primary root, or radicle, emerges first, followed by
These products are absorbed by the scutellum and transported via a the primary shoot, or coleoptile, and the adventitious roots.
vasculature strand to the developing embryo. In monocot seeds, the testa and tegmen of the seed coat are fused.
As the seed germinates, the primary root emerges, protected by the
root-tip covering: the coleorhiza. Next, the primary shoot emerges,
protected by the coleoptile: the covering of the shoot tip. Upon

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exposure to light, elongation of the coleoptile ceases and the leaves In dicots, the hypocotyls extend above ground, giving rise to the
expand and unfold. At the other end of the embryonic axis, the stem of the plant, while in monocots, they remain below ground.
primary root soon dies, while other, adventitious roots emerge from In dicot seeds, the radicle grows downwards to form the tap root
the base of the stem. This produces the fibrous root system of the while lateral roots branch off to all sides, producing a dicot tap
monocot. root system; in contrast, the end of germination in monocot seeds
Depending on seed size, the time it takes a seedling to emerge may is marked by the production of a fibrous root system where
vary. However, many mature seeds enter a period of dormancy adventitious roots emerge from the stem.
marked by inactivity or extremely-low metabolic activity. This Seed germination is dependent on seed size and whether or not
period may last for months, years, or even centuries. Dormancy favorable conditions are present.
helps keep seeds viable during unfavorable conditions. Upon a
KEY TERMS
return to optimal conditions, seed germination takes place. These
conditions may be as diverse as moisture, light, cold, fire, or testa: the seed coat
chemical treatments. Scarification, the softening of the seed coat, radicle: the rudimentary shoot of a plant that supports the
presoaking in hot water, or passing through an acid environment, cotyledons in the seed and from which the root is developed
such as an animal’s digestive tract, may also be needed. downward; the root of the embryo
hypocotyl: in plants with seeds, the portion of the embryo or
KEY POINTS seedling between the root and cotyledons
In angiosperms, the process of seed production begins with plumule: consisting of the apical meristem and the first true
double fertilization while in gymnosperms it does not. leaves of the young plant
In both monocots and dicots, food reserves are stored in the coleoptile: a pointed sheath that protects the emerging shoot in
endosperm; however, in non-endospermic dicots, the cotyledons monocotyledons such as oats and grasses
act as the storage.
This page titled 32.8: Pollination and Fertilization - Development of the
In a seed, the embryo consists of three main parts: the plumule,
Seed is shared under a CC BY-SA 4.0 license and was authored, remixed,
the radicle, and the hypocotyl.
and/or curated by Boundless.

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32.9: POLLINATION AND FERTILIZATION - DEVELOPMENT OF FRUIT AND
FRUIT TYPES
seen in nuts and beans. An aggregate fruit is one that develops from
 LEARNING OBJECTIVES numerous carpels that are all in the same flower; the mature carpels
fuse together to form the entire fruit, as seen in the raspberry. A
Describe the development of a fruit in a flowering plant
multiple fruit develops from an inflorescence or a cluster of flowers.
An example is the pineapple where the flowers fuse together to form
After fertilization, the ovary of the flower usually develops into the the fruit. Accessory fruits (sometimes called false fruits) are not
fruit. Fruits are generally associated with having a sweet taste;
derived from the ovary, but from another part of the flower, such as
however, not all fruits are sweet. The term “fruit” is used for a the receptacle (strawberry) or the hypanthium (apples and pears).
ripened ovary. In most cases, flowers in which fertilization has taken
Fruits generally have three parts: the exocarp (the outermost skin or
place will develop into fruits, while unfertilized flowers will not.
covering), the mesocarp (middle part of the fruit), and the endocarp
The fruit encloses the seeds and the developing embryo, thereby
(the inner part of the fruit). Together, all three are known as the
providing it with protection. Fruits are diverse in their origin and
pericarp. The mesocarp is usually the fleshy, edible part of the fruit;
texture. The sweet tissue of the blackberry, the red flesh of the
however, in some fruits, such as the almond, the seed is the edible
tomato, the shell of the peanut, and the hull of corn (the tough, thin
part (the pit in this case is the endocarp). In many fruits, two, or all
part that gets stuck in your teeth when you eat popcorn) are all fruits.
three of the layers are fused, and are indistinguishable at maturity.
As the fruit matures, the seeds also mature.
Fruits can be dry or fleshy. Furthermore, fruits can be divided into
dehiscent or indehiscent types. Dehiscent fruits, such as peas, readily
release their seeds, while indehiscent fruits, like peaches, rely on
decay to release their seeds.

KEY POINTS
Fruits can be classified as simple, aggregate, multiple, or
accessory.
Simple fruits develop from a single carpel or fused carpels of a
single ovary, while aggregate fruits develop from more than one
carpel found on the same flower.
Multiple fruits develop from a cluster of flowers, while accessory
fruits do not develop from an ovary, but from other parts of a
plant.
The main parts of a fruit include the exocarp (skin), the mesocarp
(middle part), and the endocarp (inner part); these three parts
make up the pericarp.
Dehiscent fruits promptly release their seeds, while indehiscent
fruits rely on decay to release their seeds.

KEY TERMS
exocarp: the outermost covering of the pericarp of fruits; the
skin
simple fruit: fruit that develops from a single carpel or fused
carpels of a single ovary
Figure 32.9.1: Types of fruit: There are four main types of fruits. endocarp: the inner part of the fruit
Simple fruits, such as these nuts, are derived from a single ovary. mesocarp: middle part of the fruit
Aggregate fruits, like raspberries, form from many carpels that fuse accessory fruit: a fruit not derived from the ovary but from
together. Multiple fruits, such as pineapple, form from a cluster of
flowers called an inflorescence. Accessory fruits, like apples, are another part of the flower
formed from a part of the plant other than the ovary.
Fruits may be classified as simple, aggregate, multiple, or accessory, This page titled 32.9: Pollination and Fertilization - Development of Fruit
and Fruit Types is shared under a CC BY-SA 4.0 license and was authored,
depending on their origin. If the fruit develops from a single carpel
remixed, and/or curated by Boundless.
or fused carpels of a single ovary, it is known as a simple fruit, as

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32.10: POLLINATION AND FERTILIZATION - FRUIT AND SEED DISPERSAL
KEY POINTS
 LEARNING OBJECTIVES The means by which seeds are dispersed depend on a seed’s
structure, composition, and size.
Summarize the ways in which fruits and seeds may be
dispersed Seeds dispersed by water are found in light and buoyant fruits,
while those dispersed by wind may have specialized wing-like
appendages.
FRUIT AND SEED DISPERSAL
Animals can disperse seeds by excreting or burying them; other
In addition to protecting the embryo, the fruit plays an important fruits have structures, such as hooks, that attach themselves to
role in seed dispersal. Seeds contained within fruits need to be animals’ fur.
dispersed far from the mother plant so that they may find favorable Humans also play a role as dispersers by moving fruit to new
and less-competitive conditions in which to germinate and grow. places and discarding the inedible portions containing the seeds.
Some fruits have built-in mechanisms that allow them to disperse by Some seeds have the ability to remain dormant and germinate
themselves, whereas others require the help of agents such as wind, when favorable conditions arise.
water, and animals. Modifications in seed structure, composition,
and size aid in dispersal. Wind-dispersed fruit are lightweight and KEY TERMS
may have wing-like appendages that allow them to be carried by the seed dormancy: a seed with the ability to delay germination and
wind. Some have a parachute-like structure to keep them afloat. propagation of the species until suitable conditions are found
Some fruits, such as the dandelion, have hairy, weightless structures dispersal: the movement of a few members of a species to a new
that are suited to dispersal by wind. geographical area, resulting in differentiation of the original
group into new varieties or species

CONTRIBUTIONS AND ATTRIBUTIONS

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Figure 32.10.1: Wind dispersal: Wind is used as a form of dispersal cross-pollination. Provided by: Wiktionary. Located at:
by lightweight seeds, such as those found on dandelions. en.wiktionary.org/wiki/cross-pollination. License: CC BY-SA: Attribution-
Seeds dispersed by water are contained in light and buoyant fruit, ShareAlike
self-pollination. Provided by: Wiktionary. Located at:
giving them the ability to float. Coconuts are well known for their en.wiktionary.org/wiki/self-pollination. License: CC BY-SA: Attribution-
ability to float on water to reach land where they can germinate. ShareAlike
heterostyly. Provided by: Wiktionary. Located at:
Similarly, willow and silver birches produce lightweight fruit that en.wiktionary.org/wiki/heterostyly. License: CC BY-SA: Attribution-
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in their droppings some distance away. Some animals, such as commons.wikimedia.org/wiki/Fi..._Dandelion.JPG. License: CC BY-SA:
squirrels, bury seed-containing fruits for later use; if the squirrel Attribution-ShareAlike
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OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX. coleoptile. Provided by: Wiktionary. Located at:
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32.11: ASEXUAL REPRODUCTION - ASEXUAL REPRODUCTION IN PLANTS

 LEARNING OBJECTIVES

Summarize methods of asexual reproduction in plants

ASEXUAL REPRODUCTION
Many plants are able to propagate themselves using asexual
reproduction. This method does not require the investment required
to produce a flower, attract pollinators, or find a means of seed
dispersal. Asexual reproduction produces plants that are genetically
identical to the parent plant because no mixing of male and female
gametes takes place. Traditionally, these plants survive well under
stable environmental conditions when compared with plants
produced from sexual reproduction because they carry genes
identical to those of their parents.
Plants have two main types of asexual reproduction: vegetative
reproduction and apomixis. Vegetative reproduction results in new
plant individuals without the production of seeds or spores. Many
different types of roots exhibit vegetative reproduction. The corm is
used by gladiolus and garlic. Bulbs, such as a scaly bulb in lilies and
a tunicate bulb in daffodils, are other common examples of this type
of reproduction. A potato is a stem tuber, while parsnip propagates
from a taproot. Ginger and iris produce rhizomes, while ivy uses an
adventitious root (a root arising from a plant part other than the main
or primary root), and the strawberry plant has a stolon, which is also
called a runner.

Figure 32.11.1: Roots: Different types of stems allow for asexual

reproduction. (a) The corm of a garlic plant looks similar to (b) a
tulip bulb, but the corm is solid tissue, while the bulb consists of
layers of modified leaves that surround an underground stem. Both
corms and bulbs can self-propagate, giving rise to new plants. (c)
Ginger forms masses of stems called rhizomes that can give rise to
multiple plants. (d) Potato plants form fleshy stem tubers. Each eye
in the stem tuber can give rise to a new plant. (e) Strawberry plants
form stolons: stems that grow at the soil surface or just below
ground and can give rise to new plants
Some plants can produce seeds without fertilization. Either the ovule
or part of the ovary, which is diploid in nature, gives rise to a new
seed. This method of reproduction is known as apomixis.
An advantage of asexual reproduction is that the resulting plant will
reach maturity faster. Since the new plant is arising from an adult
plant or plant parts, it will also be sturdier than a seedling. Asexual
reproduction can take place by natural or artificial (assisted by
humans) means.

KEY POINTS
Asexual reproduction produces individuals that are genetically
identical to the parent plant.

32.11.1 https://bio.libretexts.org/@go/page/13812
Roots such as corms, stem tubers, rhizomes, and stolon undergo KEY TERMS
vegetative reproduction. stolon: a shoot that grows along the ground and produces roots at
Some plants can produce seeds without fertilization via apomixis its nodes; a runner
where the ovule or ovary gives rise to new seeds. apomixis: process of reproduction in which plants produce seeds
Advantages of asexual reproduction include an increased rate of without fertilization
maturity and a sturdier adult plant.
Asexual reproduction can take place by natural or artificial This page titled 32.11: Asexual Reproduction - Asexual Reproduction in
means. Plants is shared under a CC BY-SA 4.0 license and was authored, remixed,
and/or curated by Boundless.

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32.12: ASEXUAL REPRODUCTION - NATURAL AND ARTIFICIAL METHODS OF
ASEXUAL REPRODUCTION IN PLANTS
together. The vascular systems of the two plants grow and fuse,
 LEARNING OBJECTIVES forming a graft. After a period of time, the scion starts producing
shoots, eventually bearing flowers and fruits. Grafting is widely used
Distinguish between natural and artificial methods of
in viticulture (grape growing) and the citrus industry. Scions capable
asexual reproduction in plants
of producing a particular fruit variety are grafted onto root stock
with specific resistance to disease.
NATURAL METHODS OF ASEXUAL
REPRODUCTION
Natural methods of asexual reproduction include strategies that
plants have developed to self-propagate. Many plants, such as
ginger, onion, gladioli, and dahlia, continue to grow from buds that
are present on the surface of the stem. In some plants, such as the
sweet potato, adventitious roots or runners (stolons) can give rise to
new plants. In Bryophyllum and kalanchoe, the leaves have small
buds on their margins. When these are detached from the plant, they
grow into independent plants; they may also start growing into
independent plants if the leaf touches the soil. Some plants can be
propagated through cuttings alone.

Figure 32.12.1: Grafting: Grafting is an artificial method of asexual

reproduction used to produce plants combining favorable stem
characteristics with favorable root characteristics. The stem of the
plant to be grafted is known as the scion, and the root is called the
stock.

CUTTING
Plants such as coleus and money plant are propagated through stem
cuttings where a portion of the stem containing nodes and internodes
is placed in moist soil and allowed to root. In some species, stems
can start producing a root even when placed only in water. For
example, leaves of the African violet will root if kept undisturbed in
water for several weeks.

LAYERING
Figure 32.12.1: Runners: asexual reproduction: A stolon, or runner,
Layering is a method in which a stem attached to the plant is bent
is a stem that runs along the ground. At the nodes, it forms and covered with soil. Young stems that can be bent easily without
adventitious roots and buds that grow into a new plant. any injury are the preferred plant for this method. Jasmine and
bougainvillea (paper flower) can be propagated this way. In some
ARTIFICIAL METHODS OF ASEXUAL
plants, a modified form of layering known as air layering is
REPRODUCTION
employed. A portion of the bark or outermost covering of the stem is
Artificial methods of asexual reproduction are frequently employed removed and covered with moss, which is then taped. Some
to give rise to new, and sometimes novel, plants. They include
gardeners also apply rooting hormone. After some time, roots will
grafting, cutting, layering, and micropropagation. appear; this portion of the plant can be removed and transplanted
into a separate pot.
GRAFTING
Grafting has long been used to produce novel varieties of roses,
citrus species, and other plants. In grafting, two plant species are
used: part of the stem of the desirable plant is grafted onto a rooted
plant called the stock. The part that is grafted or attached is called
the scion. Both are cut at an oblique angle (any angle other than a
right angle), placed in close contact with each other, and are then
held together. Matching up these two surfaces as closely as possible
is extremely important because these will be holding the plant

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 KEY POINTS
In natural asexual reproduction, roots can give rise to new plants,
or plants can propagate using budding or cutting.
In grafting, part of a plant is attached to the root system of
another plant; the two unite to form a new plant containing the
roots of one and the stem and leaf structure of the other.
Cutting is the process in which the stem of a plant is placed in
moist soil or water to generate a new root system.
In layering, part of the plant’s stem is bent down and covered
with soil; this stem can generate a new root system and,
therefore, an entirely new plant.
Micropropagation is the process in which part of a plant is placed
in plant culture medium and provided with all the hormones and
Figure 32.12.1: Layering: In layering, a part of the stem is buried so nutrients it needs in order to generate new plants.
that it forms a new plant.
When part of a plant is placed in plant culture medium and
MICROPROPAGATION provided with all the hormones and nutrients it needs, it can
Micropropagation (also called plant tissue culture) is a method of generate new plants; this is known as micropropagation.
propagating a large number of plants from a single plant in a short
KEY TERMS
time under laboratory conditions. This method allows propagation of
layering: a method of plant propagation in which a bent stem is
rare, endangered species that may be difficult to grow under natural
conditions, are economically important, or are in demand as disease- covered with soil in order to generate new roots
grafting: process of attaching part of a stem from one plant onto
free plants.
the root of another plant
To start plant tissue culture, a part of the plant such as a stem, leaf, micropropagation: practice of rapidly multiplying plant material
embryo, anther, or seed can be used. The plant material is
to produce a large number of progeny plants using plant tissue
thoroughly sterilized using a combination of chemical treatments culture methods
standardized for that species. Under sterile conditions, the plant
cutting: placing part of a stem containing nodes or internodes in
material is placed on a plant tissue culture medium that contains all water or moist soil in order to produce new plants
the minerals, vitamins, and hormones required by the plant. The
plant part often gives rise to an undifferentiated mass, known as a This page titled 32.12: Asexual Reproduction - Natural and Artificial
callus, from which, after a period of time, individual plantlets begin Methods of Asexual Reproduction in Plants is shared under a CC BY-SA
to grow. These can be separated; they are first grown under 4.0 license and was authored, remixed, and/or curated by Boundless.
greenhouse conditions before they are moved to field conditions.

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32.13: ASEXUAL REPRODUCTION - PLANT LIFE SPANS
trees, are polycarpic; they flower every year. Other polycarpic
 LEARNING OBJECTIVES species, such as perennials, flower several times during their life
span, but not each year. By this method, the plant does not require
Explain the process of aging in plants
all its nutrients to be channeled towards flowering each year.

PLANT LIFE SPANS GENETICS AND ENVIRONMENTAL CONDITIONS

The length of time from the beginning of development to the death As is the case with all living organisms, genetics and environmental
of a plant is called its life span. The life cycle, on the other hand, is conditions have a role to play in determining how long a plant will
the sequence of stages a plant goes through from seed germination to live. Susceptibility to disease, changing environmental conditions,
seed production of the mature plant. Some plants, such as annuals, drought, cold, and competition for nutrients are some of the factors
only need a few weeks to grow, produce seeds, and die. Other plants, that determine the survival of a plant. Plants continue to grow,
such as the bristlecone pine, live for thousands of years. Some despite the presence of dead tissue, such as cork. Individual parts of
bristlecone pines have a documented age of 4,500 years. Even as plants, such as flowers and leaves, have different rates of survival. In
some parts of a plant, such as regions containing meristematic tissue many trees, the older leaves turn yellow and eventually fall from the
(the area of active plant growth consisting of undifferentiated cells tree. Leaf fall is triggered by factors such as a decrease in
capable of cell division) continue to grow, some parts undergo photosynthetic efficiency due to shading by upper leaves or
programmed cell death (apoptosis). The cork found on stems and the oxidative damage incurred as a result of photosynthetic reactions.
water-conducting tissue of the xylem, for example, are composed of The components of the part to be shed are recycled by the plant for
dead cells. use in other processes, such as development of seed and storage.
This process is known as nutrient recycling. However, the complex
pathways of nutrient recycling within a plant are not well understood
The aging of a plant and all the associated processes is known as
senescence, which is marked by several complex biochemical
changes. One of the characteristics of senescence is the breakdown
of chloroplasts, which is characterized by the yellowing of leaves.
The chloroplasts contain components of photosynthetic machinery,
such as membranes and proteins. Chloroplasts also contain DNA.
The proteins, lipids, and nucleic acids are broken down by specific
enzymes into smaller molecules and salvaged by the plant to support
the growth of other plant tissues. Hormones are known to play a role
Figure 32.13.1: Plant life spans: The bristlecone pine, shown here in in senescence. Applications of cytokinins and ethylene delay or
the Ancient Bristlecone Pine Forest in the White Mountains of prevent senescence; in contrast, abscissic acid causes premature
eastern California, has been known to live for 4,500 years. onset of senescence.
ANNUALS, BIENNIAL, AND PERENNIALS
Plant species that complete their life cycle in one season are known
as annuals, an example of which is Arabidopsis, or mouse-ear cress.
Biennials, such as carrots, complete their life cycle in two seasons.
In a biennial’s first season, the plant has a vegetative phase, whereas
in the next season, it completes its reproductive phase. Commercial
growers harvest the carrot roots after the first year of growth and do
not allow the plants to flower. Perennials, such as the magnolia,
complete their life cycle in two years or more.

MONOCARPIC AND POLYCARPIC PLANTS

In another classification based on flowering frequency, monocarpic
plants flower only once in their lifetime; examples of monocarpic
plants include bamboo and yucca. During the vegetative period of
their life cycle (which may be as long as 120 years in some bamboo
species), these plants may reproduce asexually, accumulating a great
Figure 32.13.1: Plant senescence: The autumn color of these Oregon
deal of food material that will be required during their once-in-a- Grape leaves is an example of programmed plant senescence.
lifetime flowering and setting of seed after fertilization. Soon after
flowering, these plants die. Polycarpic plants form flowers many
times during their lifetime. Fruit trees, such as apple and orange

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www.boundless.com//biology/definition/grafting. License: CC BY-SA:
The life span of a plant is the length of time it takes from the Attribution-ShareAlike
micropropagation. Provided by: Wikipedia. Located at:
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layering. Provided by: Wiktionary. Located at:
plant produces its own seeds. en.wiktionary.org/wiki/layering. License: CC BY-SA: Attribution-ShareAlike
Annuals complete their life cycle in one season; biennials cutting. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/cutting.
complete their life cycle in two seasons; and perennials complete License: CC BY-SA: Attribution-ShareAlike
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Monocarpic plants flower only once in their lifetime, while BY: Attribution
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drought, cold, and competition. OpenStax College, Asexual Reproduction. October 17, 2013. Provided by:
Senescence refers to aging of the plant, during which OpenStax CNX. Located at:
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components of the plant cells are broken down and used to Attribution
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KEY TERMS Attribution
annual: a plant which naturally germinates, flowers, and dies in OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44725/latest...ol11448/latest. License: CC
one year BY: Attribution
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for more than two growing seasons en.wiktionary.org/wiki/biennial. License: CC BY-SA: Attribution-ShareAlike
monocarpic. Provided by: Wiktionary. Located at:
monocarpic: a plant that flowers and bears fruit only once en.wiktionary.org/wiki/monocarpic. License: CC BY-SA: Attribution-
before dying ShareAlike
polycarpic: bearing fruit repeatedly, or year after year polycarpic. Provided by: Wiktionary. Located at:
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Botany/Plant reproduction. Provided by: Wikibooks. Located at: Located at: http://cnx.org/content/m44725/latest...ol11448/latest. License: CC
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BY: Attribution
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 CHAPTER OVERVIEW

33: THE ANIMAL BODY- BASIC FORM AND FUNCTION

33.1: Animal Form and Function - Characteristics of the Animal Body
33.2: Animal Form and Function - Body Plans
33.3: Animal Form and Function - Limits on Animal Size and Shape
33.4: Animal Form and Function - Limiting Effects of Diffusion on Size and Development
33.5: Animal Form and Function - Animal Bioenergetics
33.6: Animal Form and Function - Animal Body Planes and Cavities
33.7: Animal Primary Tissues - Epithelial Tissues
33.8: Animal Primary Tissues - Loose, Fibrous, and Cartilage Connective Tissues
33.9: Animal Primary Tissues - Bone, Adipose, and Blood Connective Tissues
33.10: Animal Primary Tissues - Muscle Tissues and Nervous Tissues
33.11: Homeostasis - Homeostatic Process
33.12: Homeostasis - Control of Homeostasis
33.13: Homeostasis - Thermoregulation
33.14: Homeostasis - Heat Conservation and Dissipation

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1
33.1: ANIMAL FORM AND FUNCTION - CHARACTERISTICS OF THE ANIMAL
BODY

 LEARNING OBJECTIVES

Describe how form and function are related in an organism

ANIMAL FORM AND FUNCTION

Animals vary in form and function. From a sponge to a worm to a
goat, an organism has a distinct body plan that limits its size and
shape. The term body plan is the “blueprint” encompassing aspects
such as symmetry, segmentation, and limb disposition. Body plans
have been considered to have evolved in a geologically-sudden flash
during the Cambrian Explosion (roughly 542 million years ago).
However, there is also evidence of a more gradual development of
body plans. With a few exceptions, most notably the sponges and
Placozoa, animals have bodies differentiated into separate tissues, Figure 33.1.1: Arctic fox: An arctic fox is a complex animal, well
which in turn make up more complex organs and organ systems. adapted to its environment. It changes coat color with the seasons
and has longer fur in winter to trap heat.
These include tissues such as muscles, which are able to contract
and control locomotion, and nerves, which send and process signals. KEY POINTS
Typically, there is also an internal digestive chamber with one or two A body plan encompasses symmetry, segmentation, and limb
openings. Animals’ bodies are also designed to interact with their disposition.
environments, whether in the deep sea, a rainforest canopy, or the Almost all animals have bodies made of differentiated tissues,
desert. In addition, animal body plans have evolved in response to which in turn form organs and organ systems.
environmental pressures, as observed in fossil records, in order to Animal bodies have evolved to interact with their environments
enhance survival and reproductive success. Therefore, a large in ways that enhance survival and reproduction.
amount of information about the structure of an organism’s body
(anatomy) and the function of its cells, tissues, and organs KEY TERMS
(physiology) can be learned by studying that organism’s physiology: a branch of biology that deals with the functions and
environment. The arctic fox is an example of a complex animal that activities of life or of living matter (as organs, tissues, or cells)
has adapted to its environment and illustrates the relationships and of the physical and chemical phenomena involved
between an animal’s form and function. body plan: an assemblage of morphological features shared
among many members of a phylum-level group
anatomy: the art of studying the different parts of any organized
body, to discover their situation, structure, and economy;
dissection

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Animal Body is shared under a CC BY-SA 4.0 license and was authored,
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33.2: ANIMAL FORM AND FUNCTION - BODY PLANS
that the terms superior and inferior are usually not used to describe
 LEARNING OBJECTIVES animals. They are only used to describe the position of structures in
the human body (and possibly apes) where the upright posture
Describe the body plan of an animal
means some structures are above or superior to others.

BODY PLANS
Animal body plans follow set patterns related to symmetry. They can
be asymmetrical, radial, or bilateral in form. Asymmetrical animals
are those with no pattern or symmetry, such as a sponge. Radial
symmetry describes an animal with an up-and-down orientation: any
plane cut along its longitudinal axis through the organism produces
equal halves, but not a definite right or left side. This plan is found
mostly in aquatic animals, especially organisms that attach Figure 33.2.1: Directional terms: The table illustrates common
themselves to a base, such as a rock or a boat, and extract their food directional terms that are used to describe the position of body parts
in relation to other body parts.
from the surrounding water as it flows around the organism.
Bilateral symmetry is found in both land-based and aquatic animals; KEY POINTS
it enables a high level of mobility. Bilateral symmetry is illustrated Some animals have a body with no pattern or symmetry, making
in a goat. The goat also has an upper and lower component to it, but them asymmetrical.
a plane cut from front to back separates the animal into definite right Animals (mostly aquatic) with an up-and-down orientation have
and left sides. a radial symmetry in which there is no definite right or left side,
but any longitudinal plane cut produces equal halves.
Animals, either aquatic or terrestrial, that have a high level of
mobility usually have a body plan that is bilaterally symmetric.
Terms such as anterior (front), posterior (rear), dorsal (toward the
back), and ventral (toward the stomach) are used to describe the
position of parts of the body in relation to other parts.

KEY TERMS
asymmetrical: having disproportionate arrangement of parts;
Figure 33.2.1: Body symmetry: Animals exhibit different types of
body symmetry. The sponge is asymmetrical, the sea anemone has
exhibiting no pattern
radial symmetry, and the goat has bilateral symmetry. bilateral symmetry: having equal arrangement of parts
In order to describe structures in the body of an animal it is (symmetry) about a vertical plane running from head to tail
necessary to have a system for describing the position of parts of the radial symmetry: a form of symmetry wherein identical parts
body in relation to other parts. For example, it may be necessary to are arranged in a circular fashion around a central axis
describe the position of the liver in relation to the diaphragm or the
This page titled 33.2: Animal Form and Function - Body Plans is shared
heart in relation to the lungs. The most common terms used when
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
describing positions in the body are anterior (front), posterior (rear),
by Boundless.
dorsal (toward the back), and ventral (toward the stomach). Note

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33.3: ANIMAL FORM AND FUNCTION - LIMITS ON ANIMAL SIZE AND SHAPE
must be increased significantly to accommodate any increase in
 LEARNING OBJECTIVES weight. It is estimated that a doubling of body size increases body
weight by a factor of eight. The increasing thickness of the chitin
Explain how the environment and skeletal structure can put
necessary to support this weight limits most animals with an
limits on the size and shape of animals
exoskeleton to a relatively-small size.

LIMITS ON ANIMAL SIZE AND SHAPE

Animals with bilateral symmetry that live in water tend to have a
fusiform shape: a tubular shaped body that is tapered at both ends.
This shape decreases the drag on the body as it moves through water
and allows the animal to swim at high speeds. Certain types of
sharks can swim at fifty kilometers an hour, while some dolphins
can swim at 32-40 kilometers per hour. Land animals frequently
travel faster (although the tortoise and snail are significantly slower Figure 33.3.1: Apodemes: Apodemes are ingrowths on arthropod
than sharks or dolphins). Another difference in the adaptations of exoskeletons to which muscles attach. The apodemes on this crab
aquatic and land-dwelling organisms is that aquatic organisms are leg are located above and below the fulcrum of the claw. Contraction
of muscles attached to the apodemes pulls the claw closed.
constrained in shape by the forces of drag in the water since water
has higher viscosity than air. However, land-dwelling organisms are The same principles apply to endoskeletons, but they are more
constrained mainly by gravity; drag is relatively unimportant. For efficient because muscles are attached on the outside, making it
example, most adaptations in birds are for gravity, not for drag. easier to compensate for increased mass. An animal with an
endoskeleton has its size determined by the amount of skeletal
system it needs in order to support the other tissues and the amount
of muscle it needs for movement. As the body size increases, both
bone and muscle mass increase. The speed achievable by the animal
is a balance between its overall size and the bone and muscle that
provide support and movement.

KEY POINTS
Aquatic animals tend to have tubular shaped bodies ( fusiform
shape) that decrease drag, enabling them to swim at high speeds.
Terrestrial animals tend to have body shapes that are adapted to
deal with gravity.
Exoskeletons are hard protective coverings or shells that also
provide attachments for muscles.
Before shedding or molting the existing exoskeleton, an animal
Figure 33.3.1: Animal speeds: Land and marine animals travel at
varying speeds. Land animals usually travel at higher speeds, but must first produce a new one.
marine animals such as dolphins and sharks travel relatively fast. The exoskeleton must increase thickness as the animal becomes
Most animals have an exoskeleton, including insects, spiders, larger, which limits body size.
scorpions, horseshoe crabs, centipedes, and crustaceans. Scientists The size of an animal with an endoskeleton is determined by the
estimate that, of insects alone, there are over 30 million species on amount of skeletal system required to support the body and the
our planet. The exoskeleton is a hard covering or shell that provides muscles it needs to move.
benefits to the animal, such as protection against damage from
predators and from water loss (for land animals); it also provides for KEY TERMS
the attachments of muscles. As the tough and resistant outer cover of fusiform: shaped like a spindle; tapering at each end
an arthropod, the exoskeleton may be constructed of a tough exoskeleton: a hard outer structure that provides both structure
polymer, such as chitin, and is often biomineralized with materials, and protection to creatures such as insects, Crustacea, and
such as calcium carbonate. This is fused to the animal’s epidermis. Nematoda
Ingrowths of the exoskeleton called apodemes function as apodeme: an ingrowth of the arthropod exoskeleton, serving as
attachment sites for muscles, similar to tendons in more advanced an attachment site for muscles
animals. In order to grow, the animal must first synthesize a new endoskeleton: the internal skeleton of an animal, which in
exoskeleton underneath the old one and then shed or molt the vertebrates is comprised of bone and cartilage
original covering. This limits the animal’s ability to grow
This page titled 33.3: Animal Form and Function - Limits on Animal Size
continually. It may limit the individual’s ability to mature if molting
and Shape is shared under a CC BY-SA 4.0 license and was authored,
does not occur at the proper time. The thickness of the exoskeleton

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33.4: ANIMAL FORM AND FUNCTION - LIMITING EFFECTS OF DIFFUSION ON
SIZE AND DEVELOPMENT
The solution to producing larger organisms is for them to become
 LEARNING OBJECTIVES multicellular. Specialization occurs in complex organisms, allowing
cells to become less efficient at completing all tasks since they are
Describe how diffusion limits cell size and development
now more efficient at doing fewer tasks. For example, circulatory
systems bring nutrients and remove waste, while respiratory systems
LIMITING EFFECTS OF DIFFUSION ON SIZE AND provide oxygen for the cells and remove carbon dioxide from them.
DEVELOPMENT Other organ systems have developed further specialization of cells
The exchange of nutrients and wastes between a cell and its watery and tissues and efficiently control body functions. Surface-to-
environment occurs through the process of diffusion. All living cells volume ratio also applies to other areas of animal development, such
are bathed in liquid, whether they are in a single-celled organism or as the relationship between muscle mass and cross-sectional surface
a multicellular one. Diffusion is effective over a specific distance area in supporting skeletons or in the relationship between muscle
and limits the size that an individual cell can attain. If a cell is a mass and the generation of dissipation of heat.
single-celled microorganism, such as an amoeba, it can satisfy all of
its nutrient and waste needs through diffusion. If the cell is too large, KEY POINTS
then diffusion is ineffective at completing all of these tasks. The Diffusion is effective over a specific distance, so it’s more
center of the cell does not receive adequate nutrients nor is it able to efficient in small, single-celled microorganisms.
effectively dispel its waste. Diffusion becomes less efficient as the surface-to-volume ratio
An important concept in understanding the efficiency of diffusion as decreases, so diffusion is less effective in larger animals.
a transportation mechanism is the surface-to-volume ratio. Recall To overcome the limitations of diffusion, multicellular organisms
that any three-dimensional object has a surface area and volume; the have developed specialized tissues and systems that are
ratio of these two quantities is the surface-to-volume ratio. Consider responsible for completing a limited number of nutrient and
a cell shaped like a perfect sphere: it has a surface area of 4πr2, and a waste tasks.
volume of (4/3)πr3. The surface-to-volume ratio of a sphere is 3/r; as
KEY TERMS
the cell gets bigger, its surface-to-volume ratio decreases, making
diffusion less efficient. The larger the size of the sphere, or animal, surface-to-volume ratio: the amount of surface area per unit
the less surface area for diffusion it possesses. volume of an object or collection of objects; decreases as volume
increases

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Figure 33.4.1: Surface-to-volume ratio: The image illustrates the

comparison of spheres of one to one thousand volume units. The
surface-to-volume ratio of a sphere decreases as the sphere gets
bigger. The surface area of a sphere is 4πr2 and it has a volume of
(4/3)πr3 which makes the surface-to-volume ratio 3/r. This has an
effect on diffusion because it relies on the surface area of a cell: as a
cell gets bigger, diffusion becomes less efficient.

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33.5: ANIMAL FORM AND FUNCTION - ANIMAL BIOENERGETICS
ENERGY REQUIREMENTS RELATED TO LEVELS
 LEARNING OBJECTIVES OF ACTIVITY
The more active an animal is, the more energy is needed to maintain
Differentiate among the ways in which an animal’s energy
requirements are affected by their environment and level of that activity and the higher its BMR or SMR. The average daily rate
activity of energy consumption is about two to four times an animal’s BMR
or SMR. Humans are more sedentary than most animals and have an
average daily rate of only 1.5 times the BMR. The diet of an
ANIMAL BIOENERGETICS
endothermic animal is determined by its BMR.
All animals must obtain their energy from food they ingest or
absorb. These nutrients are converted to adenosine triphosphate ENERGY REQUIREMENTS RELATED TO
(ATP) for short-term storage and use by all cells. Some animals store ENVIRONMENT
energy for slightly longer times as glycogen, while others store Animals adapt to extremes of temperature or food availability
energy for much longer times in the form of triglycerides housed in through torpor. Torpor is a process that leads to a decrease in activity
specialized adipose tissues. No energy system is one hundred and metabolism, which allows animals to survive adverse
percent efficient as an animal’s metabolism produces waste energy conditions. Torpor can be used by animals for long periods. For
in the form of heat. If an animal can conserve that heat and maintain example, animals can enter a state of hibernation during the winter
a relatively-constant body temperature, it is classified as a warm- months, which enables them to maintain a reduced body
blooded animal: an endotherm. The insulation used to conserve the temperature. During hibernation, ground squirrels can achieve an
body heat comes in the forms of fur, fat, or feathers. The absence of abdominal temperature of 0° C (32° F), while a bear’s internal
insulation in ectothermic animals increases their dependence on the temperature is maintained higher at about 37° C (99° F).
environment for body heat.
If torpor occurs during the summer months with high temperatures
The amount of energy expended by an animal over a specific time is and little water, it is called estivation. Some desert animals estivate
called its metabolic rate. The rate is measured in joules, calories, or to survive the harshest months of the year. Torpor can occur on a
kilocalories (1000 calories). Carbohydrates and proteins contain daily basis; this is seen in bats and hummingbirds. While
about 4.5-5 kcal/g, while fat contains about 9 kcal/g. Metabolic rate endothermy is limited in smaller animals by surface-to-volume ratio,
is estimated as the basal metabolic rate (BMR) in endothermic some organisms can be smaller and still be endotherms because they
animals at rest and as the standard metabolic rate (SMR) in employ daily torpor during the part of the day that is coldest. This
ectotherms. Human males have a BMR of 1600-1800 kcal/day, and allows them to conserve energy during the colder parts of the day
human females have a BMR of 1300 to 1500 kcal/day. Even with when they consume more energy to maintain their body temperature.
insulation, endothermal animals require extensive amounts of energy
to maintain a constant body temperature. An ectotherm such as an KEY POINTS
alligator has an SMR of 60 kcal/day. An animal is endothermic (warm-blooded) if it maintains a
relatively-constant body temperature by conserving heat with the
ENERGY REQUIREMENTS RELATED TO BODY help of insulation.
SIZE An animal is ectothermic if it does not have insulation to
Smaller endothermic animals have a greater surface area for their conserve heat and must rely on its environment for body heat.
mass than larger ones. Therefore, smaller animals lose heat at a Metabolic rate is the amount of energy expended by an animal
faster rate than larger animals and require more energy to maintain a over a specific time; in endotherms, it is described as the basal
constant internal temperature. This results in a smaller endothermic metabolic rate (BMR), while in ectotherms, as the standard
animal having a higher BMR, per body weight, than a larger metabolic rate (SMR).
endothermic animal. Smaller endothermic animals have a higher BMR than larger
endothermic animals because they lose heat at a faster rate and
require more energy to maintain a constant internal temperature.
More active animals have higher BMRs or SMRs and require
more energy to maintain their activity.
A long period of inactivity and decreased metabolism ( torpor )
that occurs in the winter months is hibernation; estivation is
torpor that occurs in the summer months.

Figure 33.5.1: Body size and metabolic rate: The mouse has a much KEY TERMS
higher metabolic rate than the elephant since it has greater surface
area relative to mass. endotherm: a warm-blooded animal that maintains a constant
body temperature

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ectotherm: a cold-blooded animal that regulates its body estivation: to go into a state of inactivity during the summer
temperature by exchanging heat with its surroundings months
hibernation: a state of inactivity and metabolic depression in
animals during winter This page titled 33.5: Animal Form and Function - Animal Bioenergetics is
shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

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33.6: ANIMAL FORM AND FUNCTION - ANIMAL BODY PLANES AND CAVITIES

 LEARNING OBJECTIVES

Describe the major body planes and cavities of animals

ANIMAL BODY PLANES AND CAVITIES

A standing vertebrate animal can be divided by several planes that
can be used to as references to describe locations of body parts or
organs. A sagittal plane divides the body into right and left portions.
A midsagittal plane divides the body exactly in the middle, making
two equal right and left halves. A frontal plane (also called a coronal
plane) separates the front (ventral) from the back (dorsal). A
transverse plane (or, horizontal plane) divides the animal into upper
and lower portions. This is sometimes called a cross section; if the
transverse cut is at an angle, it is called an oblique plane.

Figure 33.6.1: Body cavities: Vertebrate animals have two major

body cavities. The dorsal cavity, indicated in green, contains the
cranial and the spinal cavity. The ventral cavity, indicated in yellow,
contains the thoracic cavity and the abdominopelvic cavity. The
thoracic cavity is separated from the abdominopelvic cavity by the
diaphragm. The abdominopelvic cavity is separated into the
Figure 33.6.1: Body planes: Shown are the planes of a quadruped abdominal cavity and the pelvic cavity by an imaginary line parallel
goat and a bipedal human. The midsagittal plane divides the body to the pelvis bones.
exactly in half into right and left portions. The frontal plane divides The anterior cavity has two main subdivisions: the thoracic cavity
the front and back, while the transverse plane divides the body into
upper and lower portions. and the abdominopelvic cavity. The thoracic cavity is the more
Vertebrate animals have a number of defined body cavities. The superior subdivision of the anterior cavity and is enclosed by the rib
posterior (dorsal) and anterior (ventral) cavities are each subdivided cage. The thoracic cavity contains the pleural cavity around the
into smaller cavities. In the posterior cavity, the cranial cavity lungs and the pericardial cavity, which surrounds the heart. The
houses the brain and the spinal cavity (or vertebral cavity) encloses diaphragm forms the floor of the thoracic cavity, separating it from
the more inferior abdominopelvic cavity. The abdominopelvic cavity
the spinal cord. Just as the brain and spinal cord make up a
continuous, uninterrupted structure, the cranial and spinal cavities is the largest cavity in the body. Although no membrane physically
divides the abdominopelvic cavity, it can be useful to distinguish
that house them are also continuous. The brain and spinal cord are
protected by the bones of the skull and vertebral column and by between the abdominal cavity, the division that houses the digestive
organs from the pelvic cavity, the division that houses the organs of
cerebrospinal fluid, a colorless fluid produced by the brain, which
cushions the brain and spinal cord within the posterior (dorsal) reproduction.
cavity. KEY POINTS
A sagittal plane divides the body into right and left portions; a
midsagittal plane divides the body exactly in the middle.
A frontal or coronal plane separates the front from the back.
A transverse or horizontal plane divides the animal into upper
and lower portions; it is called an oblique plane if it is cut at an
angle.
The posterior (dorsal) cavity is a continuous cavity that includes
the cranial cavity (brain) and the spinal cavity (spinal cord).

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33.7: ANIMAL PRIMARY TISSUES - EPITHELIAL TISSUES

 LEARNING OBJECTIVES

Differentiate among the types of epithelial tissues

Epithelial tissues cover the outside of organs and structures in the

body. They also line the lumens of organs in a single layer or
multiple layers of cells. The types of epithelia are classified by the
shapes of cells present and the number of layers of cells. Epithelia
composed of a single layer of cells is called simple epithelia;
epithelial tissue composed of multiple layers is called stratified
epithelia.

TYPES AND SHAPES OF EPITHELIAL TISSUES

SQUAMOUS EPITHELIA
Squamous epithelial cells are generally round, flat, and have a small,
centrally-located nucleus. The cell outline is slightly irregular; cells
fit together to form a covering or lining. When the cells are arranged
in a single layer (simple squamous epithelia), they facilitate
diffusion in tissues, such as the areas of gas exchange in the lungs or
the exchange of nutrients and waste at blood capillaries.
Figure 33.7.1: Cuboidal epithelia: Simple cuboidal epithelial cells
line tubules in the mammalian kidney where they are involved in
filtering the blood.

COLUMNAR EPITHELIA
Columnar epithelial cells are taller than they are wide: they resemble
a stack of columns in an epithelial layer. They are most-commonly
Figure 33.7.1: Squamous epithelia: Squamous epithelia cells (a)
have a slightly-irregular shape and a small, centrally-located
found in a single-layer arrangement. The nuclei of columnar
nucleus. These cells can be stratified into layers, as in (b) this human epithelial cells in the digestive tract appear to be lined up at the base
cervix specimen. of the cells. These cells absorb material from the lumen of the
digestive tract and prepare it for entry into the body through the
CUBOIDAL EPITHELIA
circulatory and lymphatic systems.
Cuboidal epithelial cells are cube-shaped with a single, central
nucleus. They are most-commonly found in a single layer, such as a
simple epithelia in glandular tissues throughout the body where they
prepare and secrete glandular material. They are also found in the
walls of tubules and in the ducts of the kidney and liver.

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 TRANSITIONAL EPITHELIA
Transitional (or uroepithelial) cells appear only in the urinary
system, primarily in the bladder and ureter. These cells are arranged
in a stratified layer, but they have the capability of appearing to pile
up on top of each other in a relaxed, empty bladder. As the urinary
bladder fills, the epithelial layer unfolds and expands to hold the
volume of urine introduced into it; the lining becomes thinner. In
other words, the tissue transitions from thick to thin.

KEY POINTS
Epithelium composed of only a single layer of cells is called
simple epithelium, while epithelium composed of more than one
layer of cells is called stratified.
Squamous epithelial cells are round, flat, and have an irregular
border; their function is usually to diffuse or filter substances
across tissues.
Cuboidal epithelial cells, as wide as they are tall, are cube
Figure 33.7.1: Columnar epithelia: Simple columnar epithelial cells shaped; they are usually found lining glands where they secrete
absorb material from the digestive tract. The nuclei line up at the substances.
base of the cells. Goblet cells secret mucous into the digestive tract Columnar epithelial cells are taller than they are wide and
lumen.
function mostly in absorption, such as in the digestive tract.
Columnar epithelial cells lining the respiratory tract appear to be Pseudostratified columnar epithelia appear to be stratified
stratified. However, each cell is attached to the base membrane of because there seems to be more than one row of nuclei, but, in
the tissue and, therefore, they are simple tissues. The nuclei are fact, it is a single layer of cells with the nuclei at different levels.
arranged at different levels in the layer of cells, making it appear as Transitional epithelium has the ability to stretch; it usually lines
though there is more than one layer. This is called pseudostratified, the interior of organs such as the bladder.
columnar epithelia. This cellular covering has cilia at the apical, or
free, surface of the cells. The cilia enhance the movement of mucous KEY TERMS
and trapped particles out of the respiratory tract, helping to protect goblet cell: glandular simple columnar epithelial cells whose
the system from invasive microorganisms and harmful material that function is to secrete mucin, which dissolves in water to form
has been breathed into the body. Goblet cells are interspersed in mucus
some tissues (such as the lining of the trachea). The goblet cells lumen: The cavity or channel within a tube or tubular organ.
contain mucous that traps irritants, which, in the case of the trachea,
keep these irritants from getting into the lungs. This page titled 33.7: Animal Primary Tissues - Epithelial Tissues is shared
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

Figure 33.7.1: Pseudostratified columnar epithelia: Pseudostratified

columnar epithelia line the respiratory tract. They exist in one layer,
but the arrangement of nuclei at different levels makes it appear that
there is more than one layer.

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33.8: ANIMAL PRIMARY TISSUES - LOOSE, FIBROUS, AND CARTILAGE
CONNECTIVE TISSUES

 LEARNING OBJECTIVES

Distinguish between the different types of connective tissue

CONNECTIVE TISSUES
Connective tissues are composed of a matrix consisting of living
cells and a non-living substance, called the ground substance. The
ground substance is composed of an organic substance (usually a
protein) and an inorganic substance (usually a mineral or water). The
principal cell of connective tissues is the fibroblast, an immature
connective tissue cell that has not yet differentiated. This cell makes
the fibers found in nearly all of the connective tissues. Fibroblasts
are motile, able to carry out mitosis, and can synthesize whichever Figure 33.8.1: Loose connective tissue: Loose connective tissue is
connective tissue is needed. Macrophages, lymphocytes, and, composed of loosely-woven collagen and elastic fibers. The fibers
and other components of the connective tissue matrix are secreted by
occasionally, leukocytes can be found in some of the tissues, while fibroblasts.
others may have specialized cells. The matrix in connective tissues
gives the tissue its density. When a connective tissue has a high FIBROUS CONNECTIVE TISSUE
concentration of cells or fibers, it has a proportionally-less-dense Fibrous connective tissues contain large amounts of collagen fibers
matrix. and few cells or matrix material. The fibers can be arranged
The organic portion, or protein fibers, found in connective tissues irregularly or regularly with the strands lined up in parallel.
are either collagen, elastic, or reticular fibers. Collagen fibers Irregularly-arranged fibrous connective tissues are found in areas of
provide strength to the tissue, preventing it from being torn or the body where stress occurs from all directions, such as the dermis
separated from the surrounding tissues. Elastic fibers are made of the of the skin. Regular fibrous connective tissue is found in tendons
protein elastin; this fiber can stretch to one and one half of its length, (which connect muscles to bones) and ligaments (which connect
returning to its original size and shape. Elastic fibers provide bones to bones).
flexibility to the tissues. Reticular fibers, the third type of protein
fiber found in connective tissues, consist of thin strands of collagen
that form a network of fibers to support the tissue and other organs
to which it is connected.

LOOSE (AREOLAR) CONNECTIVE TISSUE

Loose connective tissue, also called areolar connective tissue, has a
sampling of all of the components of a connective tissue. Loose
connective tissue has some fibroblasts, although macrophages are
present as well. Collagen fibers are relatively wide and stain a light
pink, while elastic fibers are thin and stain dark blue to black. The
space between the formed elements of the tissue is filled with the
Figure 33.8.1: Fibrous connective tissue: Fibrous connective tissue
matrix. The material in the connective tissue gives it a loose from the tendon has strands of collagen fibers lined up in parallel.
consistency similar to a cotton ball that has been pulled apart. Loose This arrangement helps the tissue resist tension that occurs from all
connective tissue is found around every blood vessel, helping to directions.
keep the vessel in place. The tissue is also found around and
CARTILAGE
between most body organs. In summary, areolar tissue is tough, yet
flexible, and comprises membranes. Cartilage is a connective tissue. The cells, called chondrocytes
(mature cartilage cells), make the matrix and fibers of the tissue.
Chondrocytes are found in spaces within the tissue called “lacunae. ”
A cartilage with few collagen and elastic fibers is hyaline cartilage.
The lacunae are randomly scattered throughout the tissue and the
matrix takes on a milky or scrubbed appearance with routine stains.
Sharks have cartilaginous skeletons, as does nearly the entire human
skeleton during some pre-birth developmental stages. A remnant of
this cartilage persists in the outer portion of the human nose. Hyaline

33.8.1 https://bio.libretexts.org/@go/page/13827
cartilage is also found at the ends of long bones, reducing friction KEY POINTS
and cushioning the articulations of these bones. Fibroblasts are cells that generate any connective tissue that the
body needs, as they can move throughout the body and can
undergo mitosis to create new tissues.
Protein fibers run throughout connective tissue, providing
stability and support; they can be either collagen, elastic, or
reticular fibers.
Loose connective tissue is not particularly tough, but surrounds
blood vessels and provides support to internal organs.
Fibrous connective tissue, which is composed of parallel bundles
of collagen fibers, is found in the dermis, tendons, and ligaments.
Hyaline cartilage forms the skeleton of the embryo before it is
Figure 33.8.1: Hyaline cartilage: Hyaline cartilage consists of a transformed into bone; it is found in the adult body at the tip of
matrix with cells called chondrocytes (shown here) embedded in it.
The chondrocytes exist in cavities in the matrix called lacunae. the nose and around the ends of the long bones, where it prevents
Elastic cartilage has a large amount of elastic fibers, giving it friction at the joints.
Fibrocartilage is the strongest of the connective tissues; it is
tremendous flexibility. The ears of most vertebrate animals contain
this cartilage, as do portions of the larynx, or voice box. In contrast, found in regions of the body that experience large amounts of
stress and require a high degree of shock absorption, such as
fibrocartilage contains a large amount of collagen fibers, giving the
tissue tremendous strength. Fibrocartilage comprises the between the vertebrae.
intervertebral discs in vertebrate animals, which must withstand a
KEY TERMS
tremendous amount of stress. Cartilage can also transform from one
chondrocyte: a cell that makes up the tissue of cartilage
type to another. For example, hyaline cartilage found in movable
motile: having the power to move spontaneously
joints, such as the knee and shoulder, often becomes damaged as a
fibroblast: a cell found in connective tissue that produces fibers,
result of age or trauma. Damaged hyaline cartilage is replaced by
such as collagen
fibrocartilage, resulting in “stiff” joints.
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33.9: ANIMAL PRIMARY TISSUES - BONE, ADIPOSE, AND BLOOD
CONNECTIVE TISSUES

 LEARNING OBJECTIVES

Describe the structure and function of connective tissues

made of bone, fat, and blood

BONE
Bone, or osseous tissue, is a connective tissue that has a large
amount of two different types of matrix material. The organic matrix
is materially similar to other connective tissues, including some
amount of collagen and elastic fibers. This gives strength and
flexibility to the tissue. The inorganic matrix consists of mineral
salts, mostly calcium, that give the tissue hardness. Without
adequate organic material in the matrix, the tissue breaks; without
adequate inorganic material in the matrix, the tissue bends.
There are three types of cells in bone: osteoblasts, osteocytes, and
osteoclasts. Osteoblasts are active in making bone for growth and
remodeling. They deposit bone material into the matrix and, after the
matrix surrounds them, they continue to live, but in a reduced
metabolic state as osteocytes. Osteocytes are found in lacunae of the
bone and assist in maintenance of the bone. Osteoclasts are active in
breaking down bone for bone remodeling, providing access to
calcium stored in tissues in order to release it into the blood.
Osteoclasts are usually found on the surface of the tissue.
Figure 33.9.1: Bone structure: (a) Compact bone is a dense matrix
Bone can be divided into two types: compact and spongy. Compact on the outer surface of bone. Spongy bone, inside the compact bone,
bone is found in the shaft (or diaphysis) of a long bone and the is porous with web-like trabeculae. (b) Compact bone is organized
into rings called osteons. Blood vessels, nerves, and lymphatic
surface of the flat bones, while spongy bone is found in the end (or vessels are found in the central Haversian canal. Rings of lamellae
epiphysis) of a long bone. Compact bone is organized into subunits surround the Haversian canal. Between the lamellae are cavities
called osteons. A blood vessel and a nerve are found in the center of called lacunae. Canaliculi are microchannels connecting the lacunae
together. (c) Osteoblasts surround the exterior of the bone.
the osteon within a long opening called the Haversian canal, with Osteoclasts bore tunnels into the bone and osteocytes are found in
radiating circles of compact bone around it known as lamellae. the lacunae.
Small spaces between these circles are called lacunae. Between the
lacunae are microchannels called canaliculi; they connect the ADIPOSE (FAT) TISSUE
lacunae to aid diffusion between the cells. Spongy bone is made of Adipose tissue, or fat tissue, is considered a connective tissue even
tiny plates called trabeculae, which serve as struts, giving the spongy though it does not have fibroblasts or a real matrix, and has only a
bone strength. few fibers. Adipose tissue is composed of cells called adipocytes
that collect and store fat in the form of triglycerides for energy
metabolism. Adipose tissues additionally serve as insulation to help
maintain body temperatures, allowing animals to be endothermic.
They also function as cushioning against damage to body organs.
Under a microscope, adipose tissue cells appear empty due to the
extraction of fat during the processing of the material for viewing.
The thin lines in the image are the cell membranes; the nuclei are the
small, black dots at the edges of the cells.

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 Different types of lymphocytes make antibodies tailored to the
foreign antigens and control the production of those antibodies.
Neutrophils are phagocytic cells that participate in one of the early
lines of defense against microbial invaders, aiding in the removal of
bacteria that has entered the body. Another leukocyte that is found in
the peripheral blood is the monocyte, which give rise to phagocytic
macrophages that clean up dead and damaged cells in the body,
whether they are foreign or from the host animal. Two additional
leukocytes in the blood are eosinophils and basophils, both of which
help to facilitate the inflammatory response.
The slightly-granular material among the cells is a cytoplasmic
fragment of a cell in the bone marrow. This is called a platelet or
thrombocyte. Platelets participate in the stages leading up to
Figure 33.9.1: Adipose tissue: Adipose (fat) is a connective tissue coagulation of the blood to stop bleeding through damaged blood
composed of cells called adipocytes. Adipocytes have small nuclei
localized at the cell edge and store fat for energy usage. vessels. Blood has a number of functions, but primarily it transports
material through the body to bring nutrients to cells and remove
BLOOD waste material from them.
Blood is considered a connective tissue because it has a matrix. The
living cell types are red blood cells, also called erythrocytes, and KEY POINTS
white blood cells, also called leukocytes. The fluid portion of whole Bone contains three types of cells: osteoblasts, which deposit
blood, its matrix, is commonly called plasma. bone; osteocytes, which maintain the bone; and osteoclasts,
which resorb bone.
The functional unit of compact bone is the osteon, which is made
up of concentric rings of bone called lamellae surrounding a
central opening called a Haversian canal, through which nerves
and blood vessels travel.
Compact bone, made of inorganic material that gives it strength
and stability, is located on the shaft of long bones, while spongy
bone, made of organic material, is found inside the ends of the
long bones.
Adipose (fat) tissue contains cells called adipocytes that store fat
in the form of triglyerides; these can be broken down for energy
Figure 33.9.1: Blood Tissue: Blood is a connective tissue that has a by the organism.
fluid matrix, called plasma, and no fibers. Erythrocytes (red blood
cells), the predominant cell type, are involved in the transport of Blood is composed of erythrocytes (red blood cells), which
oxygen and carbon dioxide. Also present are various leukocytes distribute oxygen throughout the body; leukocytes (white blood
(white blood cells) involved in immune response. cells), which mount immune responses; and platelets, which are
The cell found in greatest abundance in blood is the erythrocyte, involved in blood clotting.
responsible for transporting oxygen to body tissues. Erythrocytes are
consistently the same size in a species, but vary in size between KEY TERMS
species. Mammalian erythrocytes lose their nuclei and mitochondria osteon: any of the central canals and surrounding bony layers
when they are released from the bone marrow where they are made. found in compact bone
Fish, amphibian, and avian red blood cells maintain their nuclei and canaliculi: plural form of canaliculus; any of many small canals
mitochondria throughout the cell’s life. The principal job of an or ducts in bone or in some plants
erythrocyte is to carry and deliver oxygen to the tissues. trabecula: a small mineralized spicule that forms a network in
Leukocytes are white blood cells of the immune system involved in spongy bone
defending the body against both infectious disease and foreign osteoblast: a mononucleate cell from which bone develops
materials. Five different and diverse types of leukocytes exist, but osteoclast: a large multinuclear cell associated with the
they are all produced and derived from a multipotent cell in the bone resorption of bone
marrow known as a hematopoietic stem cell. Leukocytes are found
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33.10: ANIMAL PRIMARY TISSUES - MUSCLE TISSUES AND NERVOUS
TISSUES
The function of muscle tissue (smooth, skeletal, and cardiac) is to Cardiac muscle is not under voluntary control, but is influenced by
contract, while nervous tissue is responsible for communication. the autonomic nervous system to speed up or slow down the heart
beat. An added feature to cardiac muscle cells is a line that extends
 LEARNING OBJECTIVES along the end of the cell as it abuts the next cardiac cell in the row.
This line, an intercalated disc, assists in passing electrical impulses
Describe the structure and function of nervous tissue; efficiently from one cell to the next while maintaining the strong
differentiate among the types of muscle tissue connection between neighboring cardiac cells, allowing the cardiac
muscle cells to synchronize the beating of the heart.
MUSCLE TISSUES
There are three types of muscle in animal bodies: smooth, skeletal, NERVOUS TISSUES
and cardiac. They differ by the presence or absence of striations or Nervous tissues are made of cells specialized to receive and transmit
bands, the number and location of nuclei, whether they are electrical impulses from specific areas of the body and to send them
voluntarily or involuntarily controlled, and their location within the to specific locations in the body organized into structures called
body. nerves. A nerve consists of a neuron and glial cells. The main cell of
the nervous system is the neuron. There is a large structure with a
SMOOTH MUSCLE central nucleus: the cell body (or soma) of the neuron. Projections
Smooth muscle cells have a single, centrally-located nucleus and are from the cell body are either dendrites, specialized in receiving
spindle shaped. Constriction of smooth muscle occurs under input, or a single axon, specialized in transmitting impulses. Glial
involuntary, autonomic nervous control in response to local cells support the neurons. Astrocytes regulate the chemical
conditions in the tissues. Smooth muscle tissue is also called non- environment of the nerve cell, while oligodendrocytes insulate the
striated as it lacks the banded appearance of skeletal and cardiac axon so the electrical nerve impulse is transferred more efficiently.
muscle. The walls of blood vessels, the tubes of the digestive Other glial cells support the nutritional and waste requirements of
system, and the tubes of the reproductive systems are composed the neuron. Some of the glial cells are phagocytic, removing debris
primarily of smooth muscle. Contractions of smooth muscle move or damaged cells from the tissue.
food through the digestive tracts and push blood through the blood
vessels.

Figure 33.10.1: Types of muscle fibers: Smooth muscle cells do not

have striations, while skeletal muscle cells do. Cardiac muscle cells
have striations, but, unlike the multinucleate skeletal cells, they have
only one nucleus. Cardiac muscle tissue also has intercalated discs,
specialized regions running along the plasma membrane that join
adjacent cardiac muscle cells and assist in passing an electrical
impulse from cell to cell.

SKELETAL MUSCLE
Skeletal muscle has striations across its cells caused by the
arrangement of the contractile proteins, actin and myosin, that run
throughout the muscle fiber. Skeletal muscle cells can contract by
the attachment of myosin to actin filaments in the muscle, which Figure 33.10.1: Neuron: The neuron has projections called dendrites
that receive signals and projections called axons that send signals.
then ratchets the actin filaments toward the center of the cells. These
Also shown are two types of glial cells: astrocytes to regulate the
muscle cells are relatively long and have multiple nuclei along the chemical environment of the nerve cell, and oligodendrocytes to
edge of the cell. Skeletal muscle is under voluntary, somatic nervous insulate the axon so the electrical nerve impulse is transferred more
efficiently.
system control and is found in the muscles that move bones.
Stimulation of these cells by somatic motor neurons signals the cells KEY POINTS
to contract.
Smooth muscle cells, spindle shaped with only one nucleus,
CARDIAC MUSCLE contract involuntarily to push food through the digestive tract
and blood through blood vessels.
Cardiac muscle is found only in the heart. Similar to skeletal muscle,
Skeletal muscle cells, long, striated, multinucleate cells under
it has cross striations in its cells, but cardiac muscle has a single,
voluntary control, are responsible for the movement of skeletal
centrally-located nucleus; the muscle branches in many directions.
muscles.

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33.11: HOMEOSTASIS - HOMEOSTATIC PROCESS
set point, the body’s systems will usually attempt to revert to it. A
 LEARNING OBJECTIVES change in the internal or external environment (a stimulus) is
detected by a receptor; the response of the system is to adjust the
Give an example and describe a homeostatic process.
deviation parameter toward the set point. For instance, if the body
becomes too warm, adjustments are made to cool the animal. If the
HOMEOSTATIC PROCESS blood’s glucose rises after a meal, adjustments are made to lower the
The human organism consists of trillions of cells working together blood glucose level by moving the nutrient into tissues in the
for the maintenance of the entire organism. While cells may perform command center that require it, or to store it for later use.
very different functions, the cells are quite similar in their metabolic
requirements. Maintaining a constant internal environment with
everything that the cells need to survive (oxygen, glucose, mineral
ions, waste removal, etc.) is necessary for the well-being of
individual cells and the well-being of the entire body. The varied
processes by which the body regulates its internal environment are
collectively referred to as homeostasis.

HOMEOSTASIS
Homeostasis, in a general sense, refers to stability, balance, or
equilibrium. Physiologically, it is the body’s attempt to maintain a Figure 33.11.1: Blood glucose homeostasis: An example of how
constant and balanced internal environment, which requires homeostasis is achieved by controlling blood sugar levels after a
persistent monitoring and adjustments as conditions change. meal.
Adjustment of physiological systems within the body is called KEY POINTS
homeostatic regulation, which involves three parts or mechanisms:
Homeostasis is the body’s attempt to maintain a constant and
(1) the receptor, (2) the control center, and (3) the effector.
balanced internal environment, which requires persistent
The receptor receives information that something in the environment monitoring and adjustments as conditions change.
is changing. The control center or integration center receives and Homeostatic regulation is monitored and adjusted by the
processes information from the receptor. The effector responds to receptor, the command center, and the effector.
the commands of the control center by either opposing or enhancing The receptor receives information based on the internal
the stimulus. This ongoing process continually works to restore and environment; the command center, receives and processes the
maintain homeostasis. For example, during body temperature information; and the effector responds to the command center,
regulation, temperature receptors in the skin communicate opposing or enhancing the stimulus.
information to the brain (the control center) which signals the
effectors: blood vessels and sweat glands in the skin. As the internal KEY TERMS
and external environment of the body are constantly changing, homeostasis: the ability of a system or living organism to adjust
adjustments must be made continuously to stay at or near a specific its internal environment to maintain a stable equilibrium
value: the set point. effector: any muscle, organ etc. that can respond to a stimulus
from a nerve
PURPOSE OF HOMEOSTASIS
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33.12: HOMEOSTASIS - CONTROL OF HOMEOSTASIS
POSITIVE FEEDBACK LOOP
 LEARNING OBJECTIVES A positive feedback loop maintains the direction of the stimulus and
possibly accelerates it. There are few examples of positive feedback
Discuss the ways in which the body maintains homeostasis
and provide examples of each mechanism loops that exist in animal bodies, but one is found in the cascade of
chemical reactions that result in blood clotting, or coagulation. As
one clotting factor is activated, it activates the next factor in
CONTROL OF HOMEOSTASIS
sequence until a fibrin clot is achieved. The direction is maintained,
When a change occurs in an animal’s environment, an adjustment not changed, so this is positive feedback. Another example of
must be made. The receptors sense changes in the environment, positive feedback is uterine contractions during childbirth. The
sending a signal to the control center (in most cases, the brain), hormone oxytocin, made by the endocrine system, stimulates the
which, in turn, generates a response that is signaled to an effector. contraction of the uterus. This produces pain sensed by the nervous
The effector is a muscle or a gland that will carry out the required system. Instead of lowering the oxytocin and causing the pain to
response. Homeostasis is maintained by negative feedback loops subside, more oxytocin is produced until the contractions are
within the organism. In contrast, positive feedback loops push the powerful enough to produce childbirth.
organism further out of homeostasis, but may be necessary for life to
occur. Homeostasis is controlled by the nervous and endocrine
systems in mammals.

NEGATIVE FEEDBACK MECHANISMS

Any homeostatic process that changes the direction of the stimulus
is a negative feedback loop. It may either increase or decrease the
stimulus, but the stimulus is not allowed to continue as it did before
the receptor sensed it. In other words, if a level is too high, the body
does something to bring it down; conversely, if a level is too low, the
body does something to raise it; hence, the term: negative feedback.
An example of negative feedback is the maintenance of blood
glucose levels. When an animal has eaten, blood glucose levels rise,
which is sensed by the nervous system. Specialized cells in the
pancreas (part of the endocrine system) sense the increase, releasing
the hormone insulin. Insulin causes blood glucose levels to decrease,
as would be expected in a negative feedback system. However, if an Figure 33.12.1: Positive feedback loop: The birth of a human infant
is the result of positive feedback.
animal has not eaten and blood glucose levels decrease, this is
sensed in a different group of cells in the pancreas: the hormone SET POINT
glucagon is released, causing glucose levels to increase. This is still Homeostasis is performed so the body can maintain its internal set
a negative feedback loop, but not in the direction expected by the point. However, there are times when the set point must be adjusted.
use of the term “negative.” Another example of an increase as a When this happens, the feedback loop works to maintain the new
result of a feedback loop is the control of blood calcium. If calcium
setting. An example of changes in a set point can been seen in blood
levels decrease, specialized cells in the parathyroid gland sense this pressure. Over time, the normal or set point for blood pressure can
and release parathyroid hormone (PTH), causing an increased
increase as a result of continued increases in blood pressure. The
absorption of calcium through the intestines and kidneys. The effects body no longer recognizes the elevation as abnormal; there is no
of PTH are to raise blood levels of calcium. Negative feedback loops
attempt made to return to the lower set point. The result is the
are the predominant mechanism used in homeostasis. maintenance of an elevated blood pressure which can have harmful
effects on the body. Medication can lower blood pressure and lower
the set point in the system to a more healthy level through a process
of alteration of the set point in a feedback loop.
Changes can be made in a group of body organ systems in order to
maintain a set point in another system. This is called acclimatization.
This occurs, for instance, when an animal migrates to a higher
altitude than one to which it is accustomed. In order to adjust to the
lower oxygen levels at the new altitude, the body increases the
number of red blood cells circulating in the blood to ensure adequate
oxygen delivery to the tissues. Another example of acclimatization
Figure 33.12.1: Negatie feedback loop: Blood sugar levels are
controlled by a negative feedback loop. is animals that have seasonal changes in their coats: a heavier coat in

33.12.1 https://bio.libretexts.org/@go/page/13832
the winter ensures adequate heat retention, while a light coat in Acclimatization is characterized by the ability to change systems
summer assists in keeping body temperature from rising to harmful within an organism to maintain a set point in a different
levels. environment.

KEY POINTS KEY TERMS

Negative feedback loops are used to maintain homeostasis and acclimatization: the climatic adaptation of an organism that has
achieve the set point within a system. been moved to a new environment
Negative feedback loops are characterized by their ability to endocrine: Producing internal secretions that are transported
either increase or decrease a stimulus, inhibiting the ability of the around the body by the bloodstream.
stimulus to continue as it did prior to sensing of the receptor.
Positive feedback loops are characterized by their ability to This page titled 33.12: Homeostasis - Control of Homeostasis is shared
maintain the direction of a stimulus and can even accelerate its under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.
effect.

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33.13: HOMEOSTASIS - THERMOREGULATION

 LEARNING OBJECTIVES

Outline the various types of processes utilized by animals to

ensure thermoregulation.

Internal thermoregulation contributes to animal’s ability to maintain

homeostasis within a certain range of temperatures. As internal body
temperature rises, physiological processes are affected, such as
enzyme activity. Although enzyme activity initially increases with
temperature, enzymes begin to denature and lose their function at
higher temperatures (around 40-50 C for mammals). As internal
body temperature decreases below normal levels, hypothermia
occurs and other physiological process are affected. There are
various thermoregulation mechanisms that animals use to regulate
their internal body temperature.

TYPES OF THERMOREGULATION (ECTOTHERMY

VS. ENDOTHERMY)
Thermoregulation in organisms runs along a spectrum from Figure 33.13.1: Ectotherm: The Common frog is an ecotherm and
regulates its body based on the temperature of the external
endothermy to ectothermy. Endotherms create most of their heat via environment.
metabolic processes, and are colloquially referred to as “warm-
blooded.” Ectotherms use external sources of temperature to regulate ENDOTHERMS
their body temperatures. Ectotherms are colloquially referred to as In contrast to ectotherms, endotherms regulate their own body
“cold-blooded” even though their body temperatures often stay temperature through internal metabolic processes and usually
within the same temperature ranges as warm-blooded animals. maintain a narrow range of internal temperatures. Heat is usually
generated from the animal’s normal metabolism, but under
ECTOTHERM conditions of excessive cold or low activity, an endotherm generate
An ectotherm, from the Greek (ektós) “outside” and (thermós) “hot,” additional heat by shivering. Many endotherms have a larger number
is an organism in which internal physiological sources of heat are of of mitochondria per cell than ectotherms. These mitochondria
relatively small or quite negligible importance in controlling body enables them to generate heat by increasing the rate at which they
temperature. Since ectotherms rely on environmental heat sources, metabolize fats and sugars. However, endothermic animals must
they can operate at economical metabolic rates. Ectotherms usually sustain their higher metabolism by eating more food more often. For
live in environments in which temperatures are constant, such as the example, a mouse (endotherm) must consume food every day to
tropics or ocean. Ectotherms have developed several behavioral sustain high its metabolism, while a snake (ectotherm) may only eat
thermoregulation mechanisms, such as basking in the sun to increase once a month because its metabolism is much lower.
body temperature or seeking shade to decrease body temperature.
HOMEOTHERMY VS. POIKILOTHERMY

33.13.1 https://bio.libretexts.org/@go/page/13833
 Figure 33.13.1: Homeotherm vs. Poikilotherm: Sustained energy
output of an endothermic animal (mammal) and an ectothermic
animal (reptile) as a function of core temperature. In this scenario,
the mammal is also a homeotherm because it maintains its internal Figure 33.13.1: Mechanisms for heat exchange: Heat can be
body temperature in a very narrow range. The reptile is also a exchanged by four mechanisms: (a) radiation, (b) evaporation, (c)
poikilotherm because it can withstand a large range of temperatures. convection, or (d) conduction.
A poikilotherm is an organism whose internal temperature varies KEY POINTS
considerably. It is the opposite of a homeotherm, an organism which
In response to varying body temperatures, processes such as
maintains thermal homeostasis. Poikilotherm’s internal temperature
enzyme production can be modified to acclimate to changes in
usually varies with the ambient environmental temperature, and
the temperature.
many terrestrial ectotherms are poikilothermic. Poikilothermic
Endotherms regulate their own internal body temperature,
animals include many species of fish, amphibians, and reptiles, as
regardless of fluctuating external temperatures, while ectotherms
well as birds and mammals that lower their metabolism and body
rely on the external environment to regulate their internal body
temperature as part of hibernation or torpor. Some ectotherms can
temperature.
also be homeotherms. For example, some species of tropical fish
Homeotherms maintain their body temperature within a narrow
inhabit coral reefs that have such stable ambient temperatures that
range, while poikilotherms can tolerate a wide variation in
their internal temperature remains constant.
internal body temperature, usually because of environmental
MEANS OF HEAT TRANSFER variation.
Heat can be exchanged between environment and animals via
Heat can be exchanged between an animal and its environment
radiation, evaporation, convection, or conduction processes.
through four mechanisms: radiation, evaporation, convection, and
conduction. Radiation is the emission of electromagnetic “heat” KEY TERMS
waves. Heat radiates from the sun and from dry skin the same
ectotherm: An animal that relies on external environment to
manner. When a mammal sweats, evaporation removes heat from a
regulate its internal body temperature.
surface with a liquid. Convection currents of air remove heat from
endotherm: An animal that regulates its own internal body
the surface of dry skin as the air passes over it. Heat can be
temperature through metabolic processes.
conducted from one surface to another during direct contact with the
homeotherm: An animal that maintains a constant internal body
surfaces, such as an animal resting on a warm rock.
temperature, usually within a narrow range of temperatures.
poikilotherm: An animal that varies its internal body
temperature within a wide range of temperatures, usually as a
result of variation in the environmental temperature.

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33.14: HOMEOSTASIS - HEAT CONSERVATION AND DISSIPATION
Many animals, especially mammals, use metabolic waste heat as a
 LEARNING OBJECTIVES heat source. When muscles are contracted, most of the energy from
the ATP used in muscle actions is wasted energy that translates into
Describe some of the changes animals use in order to
heat. In cases of severe cold, a shivering reflex is activated that
maintain body temperature
generates heat for the body. Many species also have a type of
adipose tissue called brown fat that specializes in generating heat.
HEAT CONSERVATION AND DISSIPATION
Ecothermic animals use changes in their behavior to help regulate
Animals conserve or dissipate heat in a variety of ways. In certain body temperature. For example, a desert ectothermic animal may
climates, endothermic animals have some form of insulation, such as simply seek cooler areas during the hottest part of the day in the
fur, fat, feathers, or some combination thereof. Animals with thick desert to keep from becoming too warm. The same animals may
fur or feathers create an insulating layer of air between their skin and climb onto rocks to capture heat during a cold desert night. Some
internal organs. Polar bears and seals live and swim in a subfreezing animals seek water to aid evaporation in cooling them, as seen with
environment, yet they maintain a constant, warm, body temperature. reptiles. Other ectotherms use group activity, such as the activity of
The arctic fox uses its fluffy tail as extra insulation when it curls up bees to warm a hive to survive winter.
to sleep in cold weather. Mammals have a residual effect from
shivering and increased muscle activity: arrector pili muscles create KEY POINTS
“goose bumps,” causing small hairs to stand up when the individual Heat conservation is characterized by the ability to ensure blood
is cold; this has the intended effect of increasing body temperature. remains in the core by undergoing vasoconstriction, reducing
Mammals use layers of fat to achieve the same end; the loss of blood flow to the periphery (also known as peripheral
significant amounts of body fat will compromise an individual’s vasoconstriction).
ability to conserve heat. Heat dissipation is characterized by the ability to undergo
Endotherms use their circulatory systems to help maintain body vasodilation which increases blood flow to the periphery,
temperature. For example, vasodilation brings more blood and heat resulting in evaporative heat loss.
to the body surface, facilitating radiation and evaporative heat loss, Endothermic animals are defined by their ability to utilize both
which helps to cool the body. However, vasoconstriction reduces vasoconstriction and vasodilation to maintain internal body
blood flow in peripheral blood vessels, forcing blood toward the temperature.
core and the vital organs found there, conserving heat. Some animals Ectothermic animals are defined by their change in behavior
have adaptions to their circulatory system that enable them to (lying in sunlight to warm up, hiding in shade to cool down) to
transfer heat from arteries to veins, thus, warming blood that returns regulate body temperature.
to the heart. This is called a countercurrent heat exchange; it
prevents the cold venous blood from cooling the heart and other KEY TERMS
internal organs. This adaption, which can be shut down in some endotherm: a warm-blooded animal that maintains a constant
animals to prevent overheating the internal organs, is found in many body temperature
animals, including dolphins, sharks, bony fish, bees, and ectotherm: a cold-blooded animal that regulates its body
hummingbirds. In contrast, similar adaptations (as in dolphin flukes temperature by exchanging heat with its surroundings
and elephant ears) can help cool endotherms when needed.
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 CHAPTER OVERVIEW

34: ANIMAL NUTRITION AND THE DIGESTIVE SYSTEM

34.1: Digestive Systems - Introduction
34.2: Digestive Systems - Herbivores, Omnivores, and Carnivores
34.3: Digestive Systems - Invertebrate Digestive Systems
34.4: Digestive Systems - Vertebrate Digestive Systems
34.5: Digestive Systems - Digestive System- Mouth and Stomach
34.6: Digestive Systems - Digestive System- Small and Large Intestines
34.7: Nutrition and Energy Production - Food Requirements and Essential Nutrients
34.8: Nutrition and Energy Production - Food Energy and ATP
34.9: Digestive System Processes - Ingestion
34.10: Digestive System Processes - Digestion and Absorption
34.11: Digestive System Processes - Elimination
34.12: Digestive System Regulation - Neural Responses to Food
34.13: Digestive System Regulation - Hormonal Responses to Food

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1
34.1: DIGESTIVE SYSTEMS - INTRODUCTION

 LEARNING OBJECTIVES

Summarize animal nutrition and the digestive system

INTRODUCTION TO ANIMAL NUTRITION

All living organisms need nutrients to survive. While plants can
obtain the molecules required for cellular function through the
process of photosynthesis, most animals obtain their nutrients by the
consumption of other organisms. At the cellular level, the biological
molecules necessary for animal function are amino acids, lipid
molecules, nucleotides, and simple sugars. The food consumed
consists of protein, fat, and complex carbohydrates, but the
requirements of each are different for each animal.

Figure 34.1.1: Generalized animal digestive system: This diagram

shows a generalized animal digestive system, detailing the different
organs and their functions.
The digestive system consists of a group of organs that form a
closed tube-like structure called the gastrointestinal tract (GI tract)
or the alimentary canal. For convenience, the GI tract is divided into
upper GI tract and lower GI tract. The organs that make up the GI
tract include the mouth, the esophagus, the stomach, the small
Figure 34.1.1: Balanced human diet: For humans, fruits and intestine, and the large intestine. There are also several accessory
vegetables are important in maintaining a balanced diet. Both of organs that secrete various enzymes into the GI tract. These include
these are an important source of vitamins and minerals, as well as
carbohydrates, which are broken down through digestion for energy. the salivary glands, the liver, and the pancreas.
Animals must convert these macromolecules into the simple CHALLENGES TO HUMAN NUTRITION
molecules required for maintaining cellular functions, such as
One of the challenges in human nutrition is maintaining a balance
assembling new molecules, cells, and tissues. The conversion of the
between food intake, storage, and energy expenditure. Imbalances
food consumed to the nutrients required is a multi-step process
can have serious health consequences. For example, eating too much
involving digestion and absorption. During digestion, food particles
food while not expending much energy leads to obesity, which in
are broken down to smaller components which will later be absorbed
turn will increase the risk of developing illnesses such as type-2
by the body.
diabetes and cardiovascular disease. The recent rise in obesity and
DIGESTIVE SYSTEM related diseases means that understanding the role of diet and
nutrition in maintaining good health is more important than ever.
The digestive system is one of the largest organ systems in the
human body. It is responsible for processing ingested food and KEY POINTS
liquids. The cells of the human body all require a wide array of
Animals obtain lipids, proteins, carbohydrates, essential
chemicals to support their metabolic activities, from organic
vitamins, and minerals from the food they consume.
nutrients used as fuel to the water that sustains life at the cellular
The digestive system is composed of a series of organs, each
level. The digestive system not only effectively chemically reduces
with a specific, yet related function, that work to extract nutrients
the compounds in food into their fundamental building blocks, but
from food.
also acts to retain water and excrete undigested materials. The
Organs of the digestive system include the mouth, esophagus,
functions of the digestive system can be summarized as follows:
stomach, small intestine, and the large intestine.
ingestion (eat food), digestion (breakdown of food), absorption
Accessory organs, such as the liver and pancreas, secrete
(extraction of nutrients from the food), and defecation (removal of
digestive juices into the gastrointestinal tract to assist with food
waste products).
breakdown.

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KEY TERMS proteins)
digestion: the process, in the gastrointestinal tract, by which alimentary canal: the organs of a human or an animal through
food is converted into substances that can be utilized by the body which food passes; the digestive tract
macromolecule: a very large molecule, especially used in
This page titled 34.1: Digestive Systems - Introduction is shared under a CC
reference to large biological polymers (e.g. nucleic acids and
BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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34.2: DIGESTIVE SYSTEMS - HERBIVORES, OMNIVORES, AND CARNIVORES

 LEARNING OBJECTIVES

Differentiate among herbivores, omnivores, and carnivores

HERBIVORES, OMNIVORES, AND CARNIVORES

Herbivores are animals whose primary food source is plant-based.
Examples of herbivores include vertebrates like deer, koalas, and
some bird species, as well as invertebrates such as crickets and
Figure 34.2.1: Examples of omnivores: Omnivores such as the (a)
caterpillars. These animals have evolved digestive systems capable bear and (b) crayfish eat both plant- and animal-based food. While
of digesting large amounts of plant material. The plants are high in their food options are greater than those of herbivores or carnivores,
fiber and starch, which provide the main energy source in their diet. they are still limited by what they can find to eat, or what they can
catch.
Since some parts of plant materials, such as cellulose, are hard to
Carnivores are animals that eat other animals. The word carnivore is
digest, the digestive tract of herbivores is adapted so that food may
derived from Latin and means “meat eater.” Wild cats, such as lions
be digested properly. Many large herbivores have symbiotic bacteria
and tigers, are examples of vertebrate carnivores, as are snakes and
within their guts to assist with the breakdown of cellulose. They
sharks, while invertebrate carnivores include sea stars, spiders, and
have long and complex digestive tracts to allow enough space and
ladybugs. Obligate carnivores are those that rely entirely on animal
time for microbial fermentation to occur. Herbivores can be further
flesh to obtain their nutrients; examples of obligate carnivores are
classified into frugivores (fruit-eaters), granivores (seed eaters),
members of the cat family. Facultative carnivores are those that also
nectivores (nectar feeders), and folivores (leaf eaters).
eat non-animal food in addition to animal food. Note that there is no
clear line that differentiates facultative carnivores from omnivores;
dogs would be considered facultative carnivores.

Figure 34.2.1: Examples of carnivores: Carnivores such as the (a)

lion eat primarily meat. The (b) ladybug is also a carnivore that
consumes small insects called aphids.

Figure 34.2.1: Examples of herbivores: Herbivores, such as this (a) KEY POINTS
mule deer and (b) monarch caterpillar, eat primarily plant material. Herbivores are those animals, such as deer and koalas, that only
Some herbivores contain symbiotic bacteria within their intestines to
aid with the digestion of the cellulose found in plant cell walls. eat plant material.
Omnivores are animals that eat both plant- and animal- derived food. Omnivores are those animals, such as bears and humans, that can
Although the Latin term omnivore literally means “eater of eat a variety of food sources, but tend to prefer one type to
everything”, omnivores cannot really eat everything that other another.
animals eat. They can only eat things that are moderately easy to While most carnivores, such as cats, eat only meat, facultative
acquire while being moderately nutritious. For example, most carnivores, such as dogs, behave more like omnivores as they
omnivores cannot live by grazing, nor are they able to eat some can eat plant matter along with meat.
hard-shelled animals or successfully hunt large or fast prey. Humans, Facultative carnivores can eat meat as well as plant material
bears, and chickens are examples of vertebrate omnivores; while obligate carnivores eat meat all the time.
invertebrate omnivores include cockroaches and crayfish.
KEY TERMS
omnivore: an animal which is able to consume both plants (like
a herbivore) and meat (like a carnivore)
obligate carnivore: an animal that necessarily subsists on a diet
consisting mainly of meat because it does not possess the
physiology to digest vegetable matter

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herbivore: any animal that eats only vegetation (i.e. that eats no This page titled 34.2: Digestive Systems - Herbivores, Omnivores, and
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34.3: DIGESTIVE SYSTEMS - INVERTEBRATE DIGESTIVE SYSTEMS

 LEARNING OBJECTIVES

Explain the digestive process in invertebrates

INVERTEBRATE DIGESTIVE SYSTEMS

Animals have evolved different types of digestive systems break
down the different types of food they consume. Invertebrates can be
classified as those that use intracellular digestion and those with
extracellular digestion.

Figure 34.3.1: Invertebrates with Extracellular Digestion:

Invertebrates like grasshoppers have alimentary canals with
specialized compartments for digestion. Their food is broken down
in their digestive tract (extracellular digestion), rather than inside
their individual cells (intracellular digestion).
Ingested material enters the mouth and passes through a hollow,
tubular cavity. The food particles are engulfed by the cells lining the
gastrovascular cavity and the molecular are broken down within the
Figure 34.3.1: Invertebrate digestive systems: (a) A gastrovascular cytoplasm of the cells (intracellular).
cavity has a single opening through which food is ingested and
waste is excreted, as shown in this hydra and in this jellyfish
medusa. (b) An alimentary canal has two openings: a mouth for EXTRACELLULAR DIGESTION
ingesting food and an anus for eliminating waste, as shown in this The alimentary canal is a more advanced digestive system than a
nematode.
gastrovascular cavity and carries out extracellular digestion. Most
INTRACELLULAR DIGESTION other invertebrates like segmented worms (earthworms), arthropods
(grasshoppers), and arachnids (spiders) have alimentary canals. The
The simplest example of digestion intracellular digestion, which
alimentary canal is compartmentalized for different digestive
takes place in a gastrovascular cavity with only one opening. Most
functions and consists of one tube with a mouth at one end and an
animals with soft bodies use this type of digestion, including
anus at the other.
Platyhelminthes (flatworms), Ctenophora (comb jellies), and
Cnidaria (coral, jelly fish, and sea anemones). The gastrovascular Once the food is ingested through the mouth, it passes through the
cavities of these organisms contain one open which serves as both a esophagus and is stored in an organ called the crop; then it passes
“mouth” and an “anus”. into the gizzard where it is churned and digested. From the gizzard,
the food passes through the intestine and nutrients are absorbed.
Because the food has been broken down exterior to the cells, this
type of digestion is called extracellular digestion. The material that
the organism cannot digest is eliminated as feces, called castings,
through the anus.
Most invertebrates use some form of extracellular digestion to break
down their food. Flatworms and cnidarians, however, can use both
types of digestion to break down their food.

KEY POINTS
The simplest invertebrate digestive system in a gastrovascular
cavity consists of only one opening that serves as both the mouth
for taking in food and the anus for excretion.
The gastrovascular cavity has cells lining it that secrete digestive
enzymes to break down the food particles through a process
called intracellular digestion.

34.3.1 https://bio.libretexts.org/@go/page/13842
 An alimentary canal is a long tube that begins with a mouth, then extracellular digestion: Extracellular digestion is a process in
goes to the esophagus, then to the crop, gizzard, intestine, and which animals feed by secreting enzymes through the cell
finally, to an anus; this is used in the process of extracellular membrane onto the food. The enzymes break the food into
digestion. molecules small enough to be taken pass through the cell
Most invertebrates use extracellular digestion; however, there are membrane into the cell. These nutrients are transferred into the
a few phyla that can use both intracellular and extracellular blood or other body fluids and distributed to the rest of the body.
digestion. extracellular: occurring or found outside of a cell
casting: the excreta of an earthworm or similar creature
KEY TERMS intracellular: Intracellular digestion is a form of digestion which
alimentary canal: the organs of a human or an animal through takes place within the cytoplasm of the organism. Intracellular
which food passes; the digestive tract digestion takes place in animals without a digestive tract, in
intracellular digestion: Intracellular digestion is a form of which food items are brought into the cell for digestion.
digestion which takes place within the cytoplasm of the
organism. Intracellular digestion takes place in animals without a This page titled 34.3: Digestive Systems - Invertebrate Digestive Systems is
digestive tract, in which food items are brought into the cell for shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
digestion. curated by Boundless.

34.3.2 https://bio.libretexts.org/@go/page/13842
34.4: DIGESTIVE SYSTEMS - VERTEBRATE DIGESTIVE SYSTEMS
AVIAN
 LEARNING OBJECTIVES Birds face special challenges when it comes to obtaining nutrition
from food. They do not have teeth, so their digestive system must be
Differentiate among the types of vertebrate digestive
systems able to process un-masticated food. Birds have evolved a variety of
beak types that reflect the vast variety in their diet, ranging from
seeds and insects to fruits and nuts. Because most birds fly, their
VERTEBRATE DIGESTIVE SYSTEMS
metabolic rates are high in order to efficiently process food while
Vertebrates have evolved more complex digestive systems to adapt keeping their body weight low. The stomach of birds has two
to their dietary needs. Some animals have a single stomach, while chambers: the proventriculus, where gastric juices are produced to
others have multi-chambered stomachs. Birds have developed a digest the food before it enters the stomach, and the gizzard, where
digestive system adapted to eating un-masticated (un-chewed) food. the food is stored, soaked, and mechanically ground. The undigested
material forms food pellets that are sometimes regurgitated. Most of
MONOGASTRIC: SINGLE-CHAMBERED
the chemical digestion and absorption happens in the intestine, while
STOMACH
the waste is excreted through the cloaca.
As the word monogastric suggests, this type of digestive system
consists of one (“mono”) stomach chamber (“gastric”). Humans and
many animals have a monogastric digestive system. The process of
digestion begins with the mouth and the intake of food. The teeth
play an important role in masticating (chewing) or physically
breaking down food into smaller particles. The enzymes present in
saliva also begin to chemically break down food. The esophagus is a
long tube that connects the mouth to the stomach. Using peristalsis,
the muscles of the esophagus push the food towards the stomach. In
order to speed up the actions of enzymes in the stomach, the
stomach has an extremely acidic environment, with a pH between
1.5 and 2.5. The gastric juices, which include enzymes in the
stomach, act on the food particles and continue the process of
digestion. In the small intestine, enzymes produced by the liver, the
small intestine, and the pancreas continue the process of digestion.
The nutrients are absorbed into the blood stream across the epithelial
cells lining the walls of the small intestines. The waste material
travels to the large intestine where water is absorbed and the drier
waste material is compacted into feces that are stored until excreted
through the rectum.

Figure 34.4.1: Mammalian digestive system (non-ruminant): (a)

Humans and herbivores, such as the (b) rabbit, have a monogastric
digestive system. However, in the rabbit, the small intestine and
cecum are enlarged to allow more time to digest plant material. The
enlarged organ provides more surface area for absorption of
nutrients.

34.4.1 https://bio.libretexts.org/@go/page/13843
 chamber provides larger space and the microbial support necessary
to digest plant material in ruminants. The fermentation process
produces large amounts of gas in the stomach chamber, which must
be eliminated. As in other animals, the small intestine plays an
important role in nutrient absorption, while the large intestine aids in
the elimination of waste.

Figure 34.4.1: Ruminant mammal digestive system: Ruminant

animals, such as goats and cows, have four stomachs. The first two
stomachs, the rumen and the reticulum, contain prokaryotes and
protists that are able to digest cellulose fiber. The ruminant
regurgitates cud from the reticulum, chews it, and swallows it into a
third stomach, the omasum, which removes water. The cud then
passes onto the fourth stomach, the abomasum, where it is digested
by enzymes produced by the ruminant.

PSEUDO-RUMINANTS
Some animals, such as camels and alpacas, are pseudo-ruminants.
They eat a lot of plant material and roughage. Digesting plant
material is not easy because plant cell walls contain the polymeric
Figure 34.4.1: Bird digestive system: The avian esophagus has a sugar molecule cellulose. The digestive enzymes of these animals
pouch, called a crop, which stores food. Food passes from the crop
to the first of two stomachs, called the proventriculus, which cannot break down cellulose, but microorganisms present in the
contains digestive juices that break down food. From the digestive system can. Since the digestive system must be able to
proventriculus, the food enters the second stomach, called the handle large amounts of roughage and break down the cellulose,
gizzard, which grinds food. Some birds swallow stones or grit,
which are stored in the gizzard, to aid the grinding process. Birds do pseudo-ruminants have a three-chamber stomach. In contrast to
not have separate openings to excrete urine and feces. Instead, uric ruminants, their cecum (a pouched organ at the beginning of the
acid from the kidneys is secreted into the large intestine and large intestine containing many microorganisms that are necessary
combined with waste from the digestive process. This waste is
excreted through an opening called the cloaca. for the digestion of plant materials) is large. This is the site where
the roughage is fermented and digested. These animals do not have a
RUMINANTS rumen, but do have an omasum, abomasum, and reticulum.
Ruminants are mainly herbivores, such as cows, sheep, and goats,
whose entire diet consists of eating large amounts of roughage or KEY POINTS
fiber. They have evolved digestive systems that help them process Monogastric animals have a single stomach that secretes
vast amounts of cellulose. An interesting feature of the ruminants’ enzymes to break down food into smaller particles; additional
mouth is that they do not have upper incisor teeth. They use their gastric juices are produced by the liver, salivary glands, and
lower teeth, tongue, and lips to tear and chew their food. From the pancreas to assist with the digestion of food.
mouth, the food travels through the esophagus and into the stomach. The avian digestive system has a mouth (beak), crop (for food
To help digest the large amount of plant material, the stomach of the storage), and gizzard (for breakdown), as well as a two-
ruminants is a multi-chambered organ. The four compartments of chambered stomach consisting of the proventriculus, which
the stomach are called the rumen, reticulum, omasum, and releases enzymes, and the true stomach, which finishes the
abomasum. These chambers contain many microbes that break down breakdown.
cellulose and ferment ingested food. The abomasum, the “true” Ruminants, such as cows and sheep, are those animals that have
stomach, is the equivalent of the monogastric stomach chamber. This four stomachs; they eat plant matter and have symbiotic bacteria
is where gastric juices are secreted. The four-compartment gastric living within their stomachs to help digest cellulose.

34.4.2 https://bio.libretexts.org/@go/page/13843
 Pseudo-ruminants (such as camels and alpacas) are similar to proventriculus: the part of the avian stomach, between the crop
ruminants, but have a three-chambered stomach; the symbiotic and the gizzard, that secretes digestive enzymes
bacteria that help them to break down cellulose is found in the cellulose: a complex carbohydrate that forms the main
cecum, a chamber close to the large intestine. constituent of the cell wall in most plants

KEY TERMS This page titled 34.4: Digestive Systems - Vertebrate Digestive Systems is
peristalsis: the rhythmic, wave-like contraction and relaxation of shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
muscles which propagates in a wave down a muscular tube curated by Boundless.

34.4.3 https://bio.libretexts.org/@go/page/13843
34.5: DIGESTIVE SYSTEMS - DIGESTIVE SYSTEM- MOUTH AND STOMACH
into the esophagus, not into the trachea, preventing food from
 LEARNING OBJECTIVES reaching the lungs.

Describe the parts of the digestive system from the oral ESOPHAGUS
cavity through the stomach The esophagus is a tubular organ connecting the mouth to the
stomach. The chewed and softened food passes through the
PARTS OF THE DIGESTIVE SYSTEM esophagus after being swallowed. The smooth muscles of the
The vertebrate digestive system is designed to facilitate the esophagus undergo a series of wave like movements called
transformation of food matter into the nutrient components that peristalsis that push the food toward the stomach. The peristalsis
sustain organisms. The upper gastrointestinal tract includes the oral wave is unidirectional: it moves food from the mouth to the
cavity, esophagus, and stomach. stomach; reverse movement is not possible. The peristaltic
movement of the esophagus is an involuntary reflex, taking place in
ORAL CAVITY response to the act of swallowing.
The oral cavity, or mouth, is the point of entry of food into the
digestive system. The food is broken into smaller particles by
mastication, the chewing action of the teeth. All mammals have
teeth and can chew their food.

Figure 34.5.1: Esophagus: The esophagus transfers food from the

mouth to the stomach through peristaltic movements.
Figure 34.5.1: Digestion begins in the oral cavity: Digestion of food
begins in the (a) oral cavity. Food is masticated by teeth and STOMACH
moistened by saliva secreted from the (b) salivary glands. Enzymes
in the saliva begin to digest starches and fats. With the help of the A large part of digestion occurs in the stomach. The stomach, a
tongue, the resulting bolus is moved into the esophagus by saclike organ, secretes gastric digestive juices. The pH in the
swallowing. stomach is between 1.5 and 2.5. This highly- acidic environment is
The extensive chemical process of digestion begins in the mouth. As required for the chemical breakdown of food and the extraction of
food is chewed, saliva, produced by the salivary glands, mixes with nutrients. When empty, the stomach is a rather small organ;
the food. Saliva is a watery substance produced in the mouths of however, it can expand to up to 20 times its resting size when filled
many animals. There are three major glands that secrete saliva: the with food. This characteristic is particularly useful for animals that
parotid, the submandibular, and the sublingual. Saliva contains need to eat when food is available.
mucus that moistens food and buffers the pH of the food. Saliva also
contains immunoglobulins and lysozymes, which have antibacterial
action to reduce tooth decay by inhibiting growth of some bacteria.
In addition, saliva contains an enzyme called salivary amylase that
begins the process of converting starches in the food into a
disaccharide called maltose. Another enzyme, lipase, is produced by
the cells in the tongue. It is a member of a class of enzymes that can
break down triglycerides. Lingual lipase begins the breakdown of fat
components in the food. The chewing and wetting action provided
by the teeth and saliva shape the food into a mass called the bolus
for swallowing. The tongue aids in swallowing by moving the bolus
from the mouth into the pharynx. The pharynx opens to two
passageways: the trachea, which leads to the lungs, and the
esophagus, which leads to the stomach. The tracheal opening, the
Figure 34.5.1: Stomach digestion: The human stomach has an
glottis, is covered by a cartilaginous flap, the epiglottis. When extremely acidic environment where most of the protein gets
swallowing, the epiglottis closes the glottis, allowing food to pass digested.

34.5.1 https://bio.libretexts.org/@go/page/13844
The stomach is also the major site for protein digestion in animals The epiglottis covers the trachea so the bolus (ball of chewed
other than ruminants. Protein digestion is mediated in the stomach food) does not go down into the trachea or lungs, but rather into
chamber by an enzyme called pepsin, which is secreted by the chief the esophagus.
cells in the stomach in an inactive form called pepsinogen. Another The tongue positions the bolus for swallowing and then
cell type, parietal cells, secrete hydrogen and chloride ions, which peristalsis pushes the bolus down the esophagus into the
combine in the lumen to form hydrochloric acid, the primary acidic stomach.
component of the stomach juices. Hydrochloric acid helps to convert In the stomach, acids and enzymes are secreted to break down
the inactive pepsinogen to pepsin. The highly-acidic environment food into its nutrient components.
also kills many microorganisms in the food and, combined with the The churning of the stomach helps to mix the digestive juices
action of the enzyme pepsin, results in the hydrolysis of protein in with the food, turning it into a substance called chyme.
the food. Chemical digestion is facilitated by the churning action of
the stomach. Contraction and relaxation of smooth muscles mixes KEY TERMS
the stomach contents about every 20 minutes. The partially-digested bolus: a round mass of something, especially of chewed food in
food and gastric juice mixture is called chyme. Chyme passes from the mouth or alimentary canal
the stomach to the small intestine. Further protein digestion takes peristalsis: the rhythmic, wave-like contraction and relaxation of
place in the small intestine. Gastric emptying occurs within two to muscles which propagates in a wave down a muscular tube
six hours after a meal. Only a small amount of chyme is released pepsin: a digestive enzyme that chemically digests, or breaks
into the small intestine at a time. The movement of chyme from the down, proteins into shorter chains of amino acids
stomach into the small intestine is regulated by the pyloric sphincter. chyme: the thick semifluid mass of partly digested food that is
passed from the stomach to the duodenum
KEY POINTS
Mechanical and chemical digestion begin in the mouth with the This page titled 34.5: Digestive Systems - Digestive System- Mouth and
chewing of food and the release of saliva, which starts Stomach is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.
carbohydrate digestion.

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34.6: DIGESTIVE SYSTEMS - DIGESTIVE SYSTEM- SMALL AND LARGE
INTESTINES
food is sent from the ileum to the colon through the ileocecal valve
 LEARNING OBJECTIVES via peristaltic movements of the muscle. The vermiform, “worm-
like,” appendix is located at the ileocecal valve. The appendix of
Describe the parts of the digestive system from the small
humans secretes no enzymes and has an insignificant role in
intestine through the accessory organs
immunity.

PARTS OF THE DIGESTIVE SYSTEM LARGE INTESTINE

The vertebrate digestive system is designed to facilitate the The large intestine reabsorbs water from undigested food material
transformation of food matter into the nutrient components that and processes waste material; although it is also capable of
sustain organisms. The lower gastrointestinal tract includes the small absorbing vitamins that are synthesized by the normal microflora
and large intestines, rectum, anus, and accessory organs. housed herein. The human large intestine is much smaller in length
than the small intestine, but larger in diameter. It has three parts: the
SMALL INTESTINE cecum, the colon, and the rectum. The cecum joins the ileum to the
Chyme moves from the stomach to the small intestine: the organ colon. It is the receiving pouch for the waste matter. The colon,
where the digestion of protein, fats, and carbohydrates is completed. home to many bacteria or “intestinal flora” that aid in the digestive
The small intestine is a long tube-like organ with a highly-folded processes, can be divided into four regions: the ascending colon, the
surface containing finger-like projections: the villi. The apical transverse colon, the descending colon, and the sigmoid colon. The
surface of each villus has many microscopic projections: the main functions of the colon are to extract the water and mineral salts
microvilli. These structures are lined with epithelial cells on the from undigested food and to store waste material. Due to their diet,
luminal side to allow the nutrients from the digested food to be carnivorous mammals have a shorter large intestine compared to
absorbed into the blood stream on the other side. The villi and herbivorous mammals.
microvilli, with their many folds, increase the surface area of the
intestine and increase absorption efficiency of the nutrients.

Figure 34.6.1: Villi of the small intestine: Villi are folds on the small Figure 34.6.1: Large intestine: The large intestine reabsorbs water
intestine lining that increase the surface area to facilitate the from undigested food and stores waste material until it is eliminated.
absorption of nutrients.
RECTUM AND ANUS
The human small intestine, over 6 m long, is divided into three parts:
the duodenum, the jejunum, and the ileum. The “C-shaped,” fixed The rectum is the terminal end of the large intestine. Its primary role
part of the small intestine, the duodenum, is separated from the is to store the feces until defecation. The feces are propelled using
stomach by the pyloric sphincter which opens to allow chyme to peristaltic movements during elimination. The anus, an opening at
move from the stomach to the duodenum where it mixes with the far-end of the digestive tract, is the exit point for the waste
pancreatic juices. The alkaline solution is rich in bicarbonate that material. Two sphincters between the rectum and anus control
neutralizes the acidity of chyme and acts as a buffer. Digestive juices elimination: the inner sphincter is involuntary, while the outer
from the pancreas, liver, and gallbladder, as well as from gland cells sphincter is voluntary.
of the intestinal wall itself, enter the duodenum. Absorption of fatty
ACCESSORY ORGANS
acids also takes place in there.
The organs discussed above are those of the digestive tract through
The second part of the small intestine is called the jejunum. Here,
which food passes. Accessory organs are those that add secretions
hydrolysis of nutrients is continued while most of the carbohydrates
(enzymes) that catabolize food into nutrients. Accessory organs
and amino acids are absorbed through the intestinal lining. The bulk
include salivary glands, the liver, the pancreas, and the gallbladder.
of chemical digestion and nutrient absorption occurs in the jejunum.
The liver, pancreas, and gallbladder are regulated by hormones in
The ileum is the last part of the small intestine. It is here that bile response to the food consumed.
salts and vitamins are absorbed into blood stream. The undigested

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The liver, the largest internal organ in humans, plays a very License: CC BY: Attribution
omnivore. Provided by: garrettward Wikispace. Located at:
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the vitamins and fats along with synthesizing many plasma proteins. BY: Attribution
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34.7: NUTRITION AND ENERGY PRODUCTION - FOOD REQUIREMENTS AND
ESSENTIAL NUTRIENTS
calories. Fats are required in the diet to aid the absorption of fat-
 LEARNING OBJECTIVES soluble vitamins and the production of fat-soluble hormones.

Describe the essential nutrients required for cellular function ESSENTIAL NUTRIENTS
that cannot be synthesized by the animal body While the animal body can synthesize many of the molecules
required for function from the organic precursors, there are some
FOOD REQUIREMENTS nutrients that need to be consumed from food. These nutrients are
What are the fundamental requirements of the animal diet? The termed essential nutrients: they must be eaten as the body cannot
animal diet should be well balanced and provide nutrients required produce them.
for bodily function along with the minerals and vitamins required for Vitamins and minerals are substances found in the food we eat. Your
maintaining structure and regulation necessary for good health and body needs them to be able to work properly and for growth and
reproductive capability. development. Each vitamin has its own special role to play. For
example, vitamin D (added to whole milk or naturally-occurring in
sardines), helps make bones strong, while vitamin A (found in
carrots) helps with night vision. Vitamins fall into two categories: fat
soluble and water soluble. The fat-soluble vitamins dissolve in fat
and can be stored in your body, whereas the water-soluble vitamins
need to dissolve in water before your body can absorb them;
therefore, the body cannot store them.
Fat-soluble vitamins are found primarily in foods that contain fat
and oil, such as animal fats, vegetable oils, dairy foods, liver, and
fatty fish. Your body needs these vitamins every day to enable it to
work properly. However, you do not need to eat foods containing
these every day. If your body does not need these vitamins
immediately, they will be stored in the liver and fat tissues for future
use. This means that stores can build up; if you have more than you
Figure 34.7.1: A balanced diet: For humans, a balanced diet need, fat soluble vitamins can become harmful. Some fat-soluble
includes fruits, vegetables, grains, and protein. Each of these food vitamins include vitamin A, vitamin K, vitamin D, and vitamin E.
sources provides different nutrients the body cannot make for itself. Unlike the other fat-soluble vitamins, vitamin D is difficult to obtain
These include vitamins, omega 3 fatty acids, and some amino acids.
in adequate quantities in a normal diet; therefore, supplementation
ORGANIC PRECURSORS may be necessary.
The organic molecules required for building cellular material and Water-soluble vitamins are not stored in the body; therefore, you
tissues must come from food. Carbohydrates or sugars are the need to have them more frequently. If you have more then you need,
primary source of organic carbons in the animal body. During the body rids itself of the extra vitamins during urination. Because
digestion, digestible carbohydrates are ultimately broken down into the body does not store these vitamins, they are generally not
glucose and used to provide energy through metabolic pathways. harmful. Water-soluble vitamins are found in foods that include
The excess sugars in the body are converted into glycogen and fruits, vegetables, and grains. Unlike fat-soluble vitamins, they can
stored in the liver and muscles for later use. Glycogen stores are be destroyed by heat. This means that sometimes these vitamins can
used to fuel prolonged exertions, such as long-distance running, and often be lost during cooking. This is why it is better to steam or grill
to provide energy during food shortage. Excess digestible these foods rather then boil them. Some water-soluble vitamins
carbohydrates are stored by mammals in order to survive famine and include vitamin B6, vitamin B12, vitamin C, biotin, folic acid,
aid in mobility. niacin, and riboflavin.
Another important requirement is that of nitrogen. Protein The omega-3 alpha-linolenic acid and the omega-6 linoleic acid are
catabolism provides a source of organic nitrogen. Amino acids are essential fatty acids needed to synthesize some membrane
the building blocks of proteins and protein breakdown provides phospholipids. Many people take supplements to ensure they are
amino acids that are used for cellular function. The carbon and obtaining all the essential fatty acids they need. Sea buckthorn
nitrogen derived from these become the building block for contains many of these fatty acids and is also high in vitamins. Sea
nucleotides, nucleic acids, proteins, cells, and tissues. Excess buckthorn can be used to treat acne and promote weight loss and
nitrogen must be excreted, as it is toxic. Fats add flavor to food and wound healing.
promote a sense of satiety or fullness. Fatty foods are also
significant sources of energy because one gram of fat contains nine

34.7.1 https://bio.libretexts.org/@go/page/13847
 Figure 34.7.1: Sea buckthorn seed oil: Sea buckthorn seed oil
contains many vital nutrients.
Minerals are inorganic essential nutrients that must also be obtained
from food. Among their many functions, minerals help in cell
structure and regulation; they are also considered co-factors. In
addition to vitamins and minerals, certain amino acids must also be Figure 34.7.1: Amino Acids: There are 20 known amino acids.
Animals can make only 11, so the others must be obtained through
procured from food and cannot be synthesized by the body. These the diet. Meats are the best source of amino acids, although some
amino acids are the “essential” amino acids. The human body can amino acids can also be obtained from vegetables and grains.
synthesize only 11 of the 20 required amino acids. The rest must be
obtained from food.
KEY POINTS
The animal diet needs to be well-balanced in order to ensure that
all necessary vitamins and minerals are being obtained.
Vitamins are important for maintaining bodily health, making
bones strong, and seeing in the dark.
Water-soluble vitamins are not stored by the body and need to be
consumed more regularly than fat-soluble vitamins, which build
up within body tissues.
Essential fatty acids need to be consumed through the diet and
are important building blocks of cell membranes.
Nine of the 20 amino acids cannot be synthesized by the body
and need to be obtained from the diet.

KEY TERMS
nutrient: a source of nourishment, such as food, that can be
metabolized by an organism to give energy and build tissue
catabolism: destructive metabolism, usually including the
release of energy and breakdown of materials
vitamin: any of a specific group of organic compounds essential
in small quantities for healthy human growth, metabolism,
development, and body function

This page titled 34.7: Nutrition and Energy Production - Food Requirements
and Essential Nutrients is shared under a CC BY-SA 4.0 license and was
authored, remixed, and/or curated by Boundless.

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34.8: NUTRITION AND ENERGY PRODUCTION - FOOD ENERGY AND ATP
ATP is required for all cellular functions. It is used to build the
 LEARNING OBJECTIVES organic molecules that are required for cells and tissues. It also
provides energy for muscle contraction and for the transmission of
Summarize the ways in which animals obtain, store, and use
electrical signals in the nervous system. When the amount of ATP
food energy
available is in excess of the body’s requirements, the liver uses the
excess ATP and excess glucose to produce molecules called
FOOD ENERGY AND ATP glycogen (a polymeric form of glucose) that is stored in the liver and
Animals need food to obtain energy and maintain homeostasis. skeletal muscle cells. When blood sugar drops, the liver releases
Homeostasis is the ability of a system to maintain a stable internal glucose from stores of glycogen. Skeletal muscle converts glycogen
environment even in the face of external changes to the to glucose during intense exercise. The process of converting
environment. For example, the normal body temperature of humans glucose and excess ATP to glycogen and the storage of excess
is 37°C (98.6°F). Humans maintain this temperature even when the energy is an evolutionarily-important step in helping animals deal
external temperature is hot or cold. The energy it takes to maintain with mobility, food shortages, and famine.
this body temperature is obtained from food.
The primary source of energy for animals is carbohydrates,
KEY POINTS
primarily glucose: the body’s fuel. The digestible carbohydrates in Animals obtain energy from the food they consume, using that
an animal’s diet are converted to glucose molecules and into energy energy to maintain body temperature and perform other
through a series of catabolic chemical reactions. metabolic functions.
Glucose, found in the food animals eat, is broken down during
Adenosine triphosphate, or ATP, is the primary energy currency in
the process of cellular respiration into an energy source called
cells. ATP stores energy in phosphate ester bonds, releasing energy
ATP.
when the phosphodiester bonds are broken: ATP is converted to
When excess ATP and glucose are present, the liver converts
ADP and a phosphate group. ATP is produced by the oxidative
them into a molecule called glycogen, which is stored for later
reactions in the cytoplasm and mitochondrion of the cell, where
use.
carbohydrates, proteins, and fats undergo a series of metabolic
reactions collectively called cellular respiration. KEY TERMS
glucose: a simple monosaccharide (sugar) with a molecular
formula of C6H12O6; it is a principal source of energy for
cellular metabolism
adenosine triphosphate: a multifunctional nucleoside
triphosphate used in cells as a coenzyme, often called the
“molecular unit of energy currency” in intracellular energy
transfer
phosphodiester: any of many biologically active compounds in
which two alcohols form ester bonds with phosphate

CONTRIBUTIONS AND ATTRIBUTIONS

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commons.wikimedia.org/wiki/Fi...d_oil_gel.JPEG. License: CC BY:
Figure 34.8.1: ATP production pathways: ATP is the energy
Attribution
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of energy. Attribution-ShareAlike

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34.9: DIGESTIVE SYSTEM PROCESSES - INGESTION
Besides nutritional items, other substances may be ingested,
 LEARNING OBJECTIVES including medications (where ingestion is termed oral
administration) and substances considered inedible, such as insect
Describe the process of ingestion and its role in the digestive
shells. Ingestion is also a common route taken by pathogenic
system
organisms and poisons entering the body.
Some pathogens transmitted via ingestion include viruses, bacteria,
Obtaining nutrition and energy from food is a multi-step process.
and parasites. Most commonly, this takes place via the fecal-oral
For animals, the first step is ingestion, the act of taking in food. The
route. An intermediate step is often involved, such as drinking water
large molecules found in intact food cannot pass through the cell
contaminated by feces or food prepared by workers who fail to
membranes. Food needs to be broken into smaller particles so that
practice adequate hand-washing. This is more common in regions
animals can harness the nutrients and organic molecules. The first
where untreated sewage is prevalent. Diseases transmitted via the
step in this process is ingestion: taking in food through the mouth.
fecal-oral route include hepatitis A, polio, and cholera.
Once in the mouth, the teeth, saliva, and tongue play important roles
in mastication (preparing the food into bolus). Mastication, or
KEY POINTS
chewing, is an extremely important part of the digestive process,
Food is ingested through the mouth and broken down through
especially for fruits and vegetables, as these have indigestible
mastication (chewing).
cellulose coats which must be physically broken down. Also,
Food must be chewed in order to be swallowed and broken down
digestive enzymes only work on the surfaces of food particles, so the
by digestive enzymes.
smaller the particle, the more efficient the digestive process. While
While food is being chewed, saliva chemically processes the
the food is being mechanically broken down, the enzymes in saliva
food to aid in swallowing.
begin to chemically process the food as well. The combined action
Medications and harmful or inedible substances may be ingested
of these processes modifies the food from large particles to a soft
as well.
mass that can be swallowed and can travel the length of the
Pathogens, such as viruses, bacteria, and parasites, may be
esophagus.
transmitted via ingestion, causing diseases like hepatitis A, polio,
and cholera.

KEY TERMS
ingestion: consuming something orally, whether it be food,
drink, medicine, or other substance; the first step of digestion
bolus: a round mass of something, especially of chewed food in
the mouth or alimentary canal
mastication: the process of chewing

This page titled 34.9: Digestive System Processes - Ingestion is shared

under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
by Boundless.

Figure 34.9.1: Mastication: The first step in obtaining nutrition is

ingestion. Ingested food must be broken down into small pieces by
mastication, or chewing.

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34.10: DIGESTIVE SYSTEM PROCESSES - DIGESTION AND ABSORPTION

 LEARNING OBJECTIVES

Explain the processes of digestion and absorption

DIGESTION AND ABSORPTION

Digestion is the mechanical and chemical break down of food into
small organic fragments. Mechanical digestion refers to the physical
breakdown of large pieces of food into smaller pieces which can
subsequently be accessed by digestive enzymes. In chemical
digestion, enzymes break down food into the small molecules the
body can use.
It is important to break down macromolecules into smaller
fragments that are of suitable size for absorption across cell
membranes. Large, complex molecules of proteins, polysaccharides,
and lipids must be reduced to simpler particles before they can be
absorbed by the digestive epithelial cells. Different organs play
specific roles in the digestive process. The animal diet needs
carbohydrates, protein, and fat, as well as vitamins and inorganic
components for nutritional balance.
Digestive enzymes are enzymes that break down polymeric
macromolecules into their smaller building blocks, in order to
facilitate their absorption by the body. Digestive enzymes are found
in the digestive tracts of animals. Digestive enzymes are diverse and
are found in the saliva secreted by the salivary glands, in the
stomach secreted by cells lining the stomach, in the pancreatic juice
secreted by pancreatic exocrine cells, and in the intestinal (small and
Figure 34.10.1: Mechanical and chemical digestion: Mechanical and
large) secretions, or as part of the lining of the gastrointestinal tract.
chemical digestion of food takes place in many steps, beginning in
Intestinal microflora benefit the host by gleaning the energy from the mouth and ending in the rectum.
the fermentation of undigested carbohydrates and the subsequent
CARBOHYDRATES
absorption of short-chain fatty acids. Intestinal bacteria also play a
role in synthesizing vitamin B and vitamin K as well as metabolizing The digestion of carbohydrates begins in the mouth. The salivary
bile acids, sterols and xenobiotics. enzyme amylase begins the breakdown of food starches into
maltose, a disaccharide. As the food travels through the esophagus
to the stomach, no significant digestion of carbohydrates takes place.
The acidic environment in the stomach stops amylase from
continuing to break down the molecules.
The next step of carbohydrate digestion takes place in the
duodenum. The chyme from the stomach enters the duodenum and
mixes with the digestive secretions from the pancreas, liver, and
gallbladder. Pancreatic juices also contain amylase, which continues
the breakdown of starch and glycogen into maltose and other
disaccharides. These disaccharides are then broken down into
monosaccharides by enzymes called maltases, sucrases, and lactases.
The monosaccharides produced are absorbed so that they can be
used in metabolic pathways to harness energy. They are absorbed
across the intestinal epithelium into the bloodstream to be
transported to the different cells in the body.

34.10.1 https://bio.libretexts.org/@go/page/13851
 in the digestion of lipids, primarily triglycerides, through
emulsification. Emulsification is a process in which large lipid
globules are broken down into several small lipid globules. These
small globules are widely distributed in the chyme rather than
forming large aggregates. Lipids are hydrophobic substances. Bile
contains bile salts, which have hydrophobic and hydrophilic sides.
Figure 34.10.1: Digestion of carbohydrates: Digestion of The bile salts’ hydrophilic side can interface with water, while the
carbohydrates is performed by several enzymes. Starch and
glycogen are broken down into glucose by amylase and maltase.
hydrophobic side interfaces with lipids, thereby emulsifying large
Sucrose (table sugar) and lactose (milk sugar) are broken down by lipid globules into small lipid globules.
sucrase and lactase, respectively.
Emulsification is important for the digestion of lipids because
PROTEIN lipases can only efficiently act on the lipids when they are broken
into small aggregates. Lipases break down the lipids into fatty acids
A large part of protein digestion takes place in the stomach. The
and glycerides. These molecules can pass through the plasma
enzyme pepsin plays an important role in the digestion of proteins
membrane of the cell, entering the epithelial cells of the intestinal
by breaking them down into peptides, short chains of four to nine
lining. The bile salts surround long-chain fatty acids and
amino acids. In the duodenum, other enzymes – trypsin, elastase,
monoglycerides, forming tiny spheres called micelles. The micelles
and chymotrypsin – act on the peptides, reducing them to smaller
move into the brush border of the small intestine absorptive cells
peptides. These enzymes are produced by the pancreas and released
where the long-chain fatty acids and monoglycerides diffuse out of
into the duodenum where they also act on the chyme. Further
the micelles into the absorptive cells, leaving the micelles behind in
breakdown of peptides to single amino acids is aided by enzymes
the chyme. The long-chain fatty acids and monoglycerides
called peptidases (those that break down peptides). The amino acids
recombine in the absorptive cells to form triglycerides, which
are absorbed into the bloodstream through the small intestine.
aggregate into globules, and are then coated with proteins. These
large spheres are called chylomicrons. Chylomicrons contain
triglycerides, cholesterol, and other lipids; they have proteins on
their surface. The surface is also composed of the hydrophilic
phosphate “heads” of phospholipids. Together, they enable the
chylomicron to move in an aqueous environment without exposing
the lipids to water. Chylomicrons leave the absorptive cells via
exocytosis, entering the lymphatic vessels. From there, they enter
the blood in the subclavian vein.

Figure 34.10.1: Protein digestion and absorption: Protein digestion

is a multistep process that begins in the stomach and continues
through the intestines. Proteins are absorbed into the blood stream Figure 34.10.1: Lipid digestion and absorption: Lipids are digested
by the small intestine. and absorbed in the small intestine.

LIPIDS VITAMINS
Lipid (fat) digestion begins in the stomach with the aid of lingual Vitamins can be either water-soluble or lipid-soluble. Fat-soluble
lipase and gastric lipase. However, the bulk of lipid digestion occurs vitamins are absorbed in the same manner as lipids. It is important to
in the small intestine due to pancreatic lipase. When chyme enters consume some amount of dietary lipid to aid the absorption of lipid-
the duodenum, the hormonal responses trigger the release of bile, soluble vitamins. Water-soluble vitamins can be directly absorbed
which is produced in the liver and stored in the gallbladder. Bile aids into the bloodstream from the intestine.

34.10.2 https://bio.libretexts.org/@go/page/13851
KEY POINTS bloodstream from the intestine.
In the mouth, carbohydrates are broken down by amylase into
KEY TERMS
maltose (a disaccharide ) and then move down the esophagus,
which produces mucus for lubrication, but no digestive enzymes. chemical digestion: The process of enzymes breaking down
In the duodenum, disaccharides are broken down into food into small molecules the body can use.
monosaccharides by enzymes called maltases, sucrases, and lipase: Enzymes in the pancreatic juices that break down lipids.
lactases; the monosaccharides produced are then absorbed into chylomicron: A microscopic globule of triglycerids and other
the bloodstream and transported to cells to be used in metabolic lipids coated with proteins, found in blood and lymphatic
pathways to harness energy. vessels, that is associated with the digestion of fats.
In the stomach, proteins are broken down into peptides, which amylase: Any of a class of digestive enzymes present in saliva
are then broken down into single amino acids that are absorbed that break down complex carbohydrates, such as starch, into
in the bloodstream though the small intestine. simpler sugars like glucose.
Lipids are digested mainly in the small intestine by bile salts mechanical digestion: The physical breakdown of large pieces
through the process of emulsification, which allows lipases to of food into smaller pieces which can subsequently be accessed
divide lipids into fatty acids and monoglycerides. by enzymes.
Monoglycerides and fatty acids enter absorptive cells in the
This page titled 34.10: Digestive System Processes - Digestion and
small intestine through micelles; they leave micelles and Absorption is shared under a CC BY-SA 4.0 license and was authored,
recombine into chylomicrons, which then enter the bloodstream. remixed, and/or curated by Boundless.
Fat-soluble vitamins are absorbed in the same manner as lipids;
water-soluble vitamins can be directly absorbed into the

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34.11: DIGESTIVE SYSTEM PROCESSES - ELIMINATION
KEY POINTS
 LEARNING OBJECTIVES Water is reabsorbed in the colon after undigested food enters it
from the small intestine.
Describe the process of elimination and problems that can
occur Waste is moved through the colon by peristaltic movements of
the muscle and is stored in the rectum.
The rectum expands in response to the storage of fecal matter;
The final step in digestion is the elimination of undigested food
neural signals are triggered, and the waste is eliminated from the
content and waste products. After food passes through the small
anus by peristaltic movements of the rectum.
intestine, the undigested food material enters the colon, where most
Constipation is a condition where the feces are hardened because
of the water is reabsorbed. Recall that the colon is also home to the
of excess water removal in the colon.
microflora called “intestinal flora” that aid in the digestion process.
Diarrhea results when large amounts of water are not removed
The semi-solid waste is moved through the colon by peristaltic
from the feces.
movements of the muscle and is stored in the rectum. As the rectum
Emesis, or vomiting, is elimination of food by forceful expulsion
expands in response to storage of fecal matter, it triggers the neural
through the mouth caused by the strong contractions produced by
signals required to set up the urge to eliminate. The solid waste is
the stomach muscles.
eliminated through the anus using peristaltic movements of the
rectum. KEY TERMS
emesis: the act or process of vomiting
intestinal flora: the bacterial colonies that normally live in the
digestive tract of animals
constipation: condition where the feces are hardened because of
excess water removal in the colon

CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44742/latest...ol11448/latest. License: CC
BY: Attribution
Medical Physiology/Gastrointestinal Physiology/Motility. Provided by:
Wikibooks. Located at: en.wikibooks.org/wiki/Medical...%23Mastication.
License: CC BY-SA: Attribution-ShareAlike
Ingestion. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/Ingestion. License: CC BY-SA: Attribution-ShareAlike
OpenStax College, Digestive System Processes. November 24, 2013. Provided
by: OpenStax CNX. Located at: http://cnx.org/content/m44742/latest/.
License: CC BY: Attribution
mastication. Provided by: Wiktionary. Located at:
en.wiktionary.org/wiki/mastication. License: CC BY-SA: Attribution-
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species of bacteria present in the human gut. bolus. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/bolus.
License: CC BY-SA: Attribution-ShareAlike
Fork, Woman, Eating, Salad - Free image - 207410. Provided by: Pixabay.
COMMON PROBLEMS WITH ELIMINATION Located at: pixabay.com/en/fork-woman-eat...ose-up-207410/. License: CC
Diarrhea and constipation are some of the most common health BY: Attribution
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
concerns that affect digestion. Constipation is a condition where the Located at: http://cnx.org/content/m44742/latest...ol11448/latest. License: CC
feces are hardened because of excess water removal in the colon. In BY: Attribution
Boundless. Provided by: Boundless Learning. Located at:
contrast, if not enough water is removed from the feces, it results in www.boundless.com//biology/definition/lipase. License: CC BY-SA:
diarrhea. Many bacteria, including the ones that cause cholera, affect Attribution-ShareAlike
chylomicron. Provided by: Wiktionary. Located at:
the proteins involved in water reabsorption in the colon and result in en.wiktionary.org/wiki/chylomicron. License: CC BY-SA: Attribution-
excessive diarrhea. ShareAlike
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en.wiktionary.org/wiki/amylase. License: CC BY-SA: Attribution-ShareAlike
EMESIS Fork, Woman, Eating, Salad - Free image - 207410. Provided by: Pixabay.
Emesis, or vomiting, is elimination of food by forceful expulsion Located at: pixabay.com/en/fork-woman-eat...ose-up-207410/. License: CC
BY: Attribution
through the mouth. It is often in response to an irritant that affects OpenStax College, Digestive System Processes. October 17, 2013. Provided by:
the digestive tract, including, but not limited to, viruses, bacteria, OpenStax CNX. Located at:
http://cnx.org/content/m44742/latest...e_34_03_03.jpg. License: CC BY:
emotions, trauma, and food poisoning. This forceful expulsion of the Attribution
food is due to the strong contractions produced by the stomach OpenStax College, Digestive System Processes. October 17, 2013. Provided by:
OpenStax CNX. Located at:
muscles. The process of emesis is regulated by the medulla. http://cnx.org/content/m44742/latest...e_34_03_04.png. License: CC BY:
Attribution

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OpenStax College, Digestive System Processes. October 17, 2013. Provided by: http://cnx.org/content/m44742/latest...e_34_03_03.jpg. License: CC BY:
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Attribution OpenStax CNX. Located at:
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Attribution OpenStax CNX. Located at:
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BY: Attribution OpenStax College, Digestive System Processes. October 17, 2013. Provided by:
Boundless. Provided by: Boundless Learning. Located at: OpenStax CNX. Located at:
www.boundless.com//biology/de...n/constipation. License: CC BY-SA: http://cnx.org/content/m44742/latest...e_34_03_02.jpg. License: CC BY:
Attribution-ShareAlike Attribution
intestinal flora. Provided by: Wiktionary. Located at: Gut flora. Provided by: Wikipedia. Located at:
en.wiktionary.org/wiki/intestinal+flora. License: CC BY-SA: Attribution- en.Wikipedia.org/wiki/Gut_flora. License: Public Domain: No Known
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License: CC BY-SA: Attribution-ShareAlike This page titled 34.11: Digestive System Processes - Elimination is shared
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34.12: DIGESTIVE SYSTEM REGULATION - NEURAL RESPONSES TO FOOD
secretion in the cephalic phase can also take place at the thought of
 LEARNING OBJECTIVES food. Right now, if you think about a piece of chocolate or a crispy
potato chip, the increase in salivation is a cephalic phase response to
Summarize the neural responses to food
the thought. The central nervous system prepares the stomach to
receive food.
In reaction to the smell, sight, or thought of food, the first hormonal
The gastric phase begins once the food arrives in the stomach. It
response is that of salivation. The salivary glands secrete more saliva
builds on the stimulation provided during the cephalic phase. Gastric
in response to the stimulus presented by food in preparation for
acids and enzymes process the ingested materials. The gastric phase
digestion. Simultaneously, the stomach begins to produce
is stimulated by (1) distension of the stomach, (2) a decrease in the
hydrochloric acid to digest the food. Recall that the peristaltic
pH of the gastric contents, and (3) the presence of undigested
movements of the esophagus and other organs of the digestive tract
material. This phase consists of local, hormonal, and neural
are under the control of the brain. The brain prepares these muscles
responses. These responses stimulate secretions and powerful
for movement as well. When the stomach is full, the part of the brain
contractions.
that detects satiety signals fullness. There are three overlapping
phases of gastric control: the cephalic phase, the gastric phase, and The intestinal phase begins when chyme enters the small intestine,
the intestinal phase. Each requires many enzymes and is under triggering digestive secretions. This phase controls the rate of gastric
neural control as well. emptying. In addition to gastric emptying, when chyme enters the
small intestine, it triggers other hormonal and neural events that
coordinate the activities of the intestinal tract, pancreas, liver, and
gallbladder.

KEY POINTS
The cephalic phase is controlled by sight, sense, and smell,
which trigger neural responses, including salivation and
hydrochloric acid production, before food has even reached the
mouth.
Once food reaches the stomach, gastric acids and enzymes
process the ingested materials in the gastric phase, which
involves local, hormonal, and neural responses.
The intestinal phase controls the rate of gastric emptying and the
release of hormones needed to digest chyme in the small
intestine.

KEY TERMS
Figure 34.12.1: Salivation: Seeing a plate of food triggers the neural: of, or relating to the nerves, neurons or the nervous
secretion of saliva in the mouth and the production of hydrochloric system
acid in the stomach. salivary gland: any of several exocrine glands that produce
DIGESTIVE PHASES saliva to break down carbohydrates in food enzymatically
peristaltic: of, or pertaining to the rhythmic, wave-like
The response to food begins even before food enters the mouth. The
contraction of the digestive tract that forces food through it
first phase of ingestion, called the cephalic phase, is controlled by
the neural response to the stimulus provided by food. All aspects, This page titled 34.12: Digestive System Regulation - Neural Responses to
such as sight, sense, and smell, trigger the neural responses resulting Food is shared under a CC BY-SA 4.0 license and was authored, remixed,
in salivation and secretion of gastric juices. The gastric and salivary and/or curated by Boundless.

34.12.1 https://bio.libretexts.org/@go/page/13854
34.13: DIGESTIVE SYSTEM REGULATION - HORMONAL RESPONSES TO FOOD
secreted by the small intestine to slow down the peristaltic
 LEARNING OBJECTIVES movements of the intestine to allow fatty foods more time to be
digested and absorbed.
Describe hormonal responses to food
Understanding the hormonal control of the digestive system is an
important area of ongoing research. Scientists are exploring the role
The endocrine system controls the response of the various glands in
of each hormone in the digestive process and developing ways to
the body and the release of hormones at the appropriate times. The
target these hormones. Advances could lead to knowledge that may
endocrine system’s effects are slow to initiate, but prolonged in their
help to battle the obesity epidemic.
response, lasting from a few hours up to weeks. The system is made
of a series of glands that produce chemicals called hormones. These
KEY POINTS
hormones are chemical mediators released from endocrine tissue
The presence and absence of hormones that are released into the
into the bloodstream where they travel to target tissue and generate a
bloodstream generate specific digestive responses; they either
response.
stimulate or discontinue digestive processes.
One of the important factors under hormonal control is the stomach In hormone control, a negative feedback mechanism takes place
acid environment. During the gastric phase, the hormone gastrin is when the stomach is empty and its acidic environment does not
secreted by G cells in the stomach in response to the presence of need to be maintained; as a result, a hormone is released to stop
proteins. Gastrin stimulates the release of stomach acid, or the release of hydrochloric acid, which was previously activated
hydrochloric acid (HCl), which aids in the digestion of the majority to aid digestion.
of proteins. However, when the stomach is emptied, the acidic In some cases, hormones work in tandem and sequentially to
environment need not be maintained and a hormone called achieve important digestive functions, such as in the breakdown
somatostatin stops the release of hydrochloric acid. This is of acidic chyme, where hormones act in releasing the appropriate
controlled by a negative feedback mechanism. secretions in the appropriate stages of digestion.
In the duodenum, digestive secretions from the liver, pancreas, and When digesting certain types of foods, such as ones high in fat,
gallbladder play an important role in digesting chyme during the hormones can control the speed of food digestion and, therefore,
intestinal phase. In order to neutralize the acidic chyme, a hormone absorption.
called secretin stimulates the pancreas to produce alkaline
bicarbonate solution and deliver it to the duodenum. Secretin acts in KEY TERMS
tandem with another hormone called cholecystokinin (CCK). Not endocrine system: a control system of ductless glands that
only does CCK stimulate the pancreas to produce the requisite secrete hormones which circulate via the bloodstream to affect
pancreatic juices, it also stimulates the gallbladder to release bile cells within specific organs
into the duodenum. chyme: the thick semifluid mass of partly digested food that is
passed from the stomach to the duodenum
secretin: a peptide hormone, secreted by the duodenum, that
serves to regulate its acidity
cholecystokinin: any of several peptide hormones that stimulate
the digestion of fat and protein
somatostatin: a polypeptide hormone, secreted by the pancreas,
that inhibits the production of certain other hormones
gastrin: a hormone that stimulates the production of gastric acid
in the stomach

CONTRIBUTIONS AND ATTRIBUTIONS

OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
Located at: http://cnx.org/content/m44744/latest...ol11448/latest. License: CC
BY: Attribution
neural. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/neural.
License: CC BY-SA: Attribution-ShareAlike
peristaltic. Provided by: Wiktionary. Located at:
Figure 34.13.1: Digestive endocrine system: Hormones, such as en.wiktionary.org/wiki/peristaltic. License: CC BY-SA: Attribution-
secretin and cholecystokinin, play important roles in digestive ShareAlike
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generate specific controls in the digestion of chyme. As seen in the en.wiktionary.org/wiki/salivary_gland. License: CC BY-SA: Attribution-
image, hormones are vital players in several bodily functions and ShareAlike
processes. OpenStax College, Digestive System Regulation. October 17, 2013. Provided
by: OpenStax CNX. Located at:
Another level of hormonal control occurs in response to the http://cnx.org/content/m44744/latest...e_34_04_01.jpg. License: CC BY:
composition of food. Foods high in lipids (fatty foods) take a long Attribution
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
time to digest. A hormone called gastric inhibitory peptide is Located at: http://cnx.org/content/m44744/latest...ol11448/latest. License: CC

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BY: Attribution endocrine system. Provided by: Wiktionary. Located at:
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en.Wikipedia.org/wiki/Endocrine_system. License: CC BY-SA: Attribution- ShareAlike
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 CHAPTER OVERVIEW

35: THE NERVOUS SYSTEM

35.1: Neurons and Glial Cells - Introduction
35.2: Neurons and Glial Cells - Neurons
35.3: Neurons and Glial Cells - Glia
35.4: How Neurons Communicate - Nerve Impulse Transmission within a Neuron- Resting Potential
35.5: How Neurons Communicate - Nerve Impulse Transmission within a Neuron- Action Potential
35.6: How Neurons Communicate - Synaptic Transmission
35.7: How Neurons Communicate - Signal Summation
35.8: How Neurons Communicate - Synaptic Plasticity
35.9: The Nervous System
35.10: The Central Nervous System - Cerebral Cortex and Brain Lobes
35.11: The Central Nervous System - Midbrain and Brain Stem
35.12: The Central Nervous System - Spinal Cord
35.13: The Peripheral Nervous System - Autonomic Nervous System
35.14: The Peripheral Nervous System - The Sensory-Somatic Nervous System
35.15: Neurodegenerative Disorders - Introduction
35.16: Nervous System Disorders - Neurodevelopmental Disorders - Autism and ADHD
35.17: Nervous System Disorders - Neurodevelopmental Disorders - Mental Illnesses
35.18: Nervous System Disorders - Other Neurological Disorders

This page titled 35: The Nervous System is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

1
35.1: NEURONS AND GLIAL CELLS - INTRODUCTION
up of a small “brain” and two nerve cords, and a peripheral nervous
 LEARNING OBJECTIVES system containing a system of nerves that extend throughout the
body. The insect nervous system is more complex, but also fairly
Recall the differences in structure and function between the
decentralized. It contains a brain, ventral nerve cord, and ganglia
central and peripheral nervous systems
(clusters of connected neurons). These ganglia can control
movements and behaviors without input from the brain. Octopuses
THE NERVOUS SYSTEM: INTRODUCTION may have the most complicated of invertebrate nervous systems.
The nervous system coordinates the body’s voluntary and They have neurons that are organized in specialized lobes and eyes
involuntary actions and transmits signals between different parts of that are structurally similar to vertebrate species.
the body. Nervous tissue first arose in wormlike organisms
approximately 550 to 600 million years ago. In most types of
vertebrate animals, it consists of two main parts: the central nervous
system (CNS) and the peripheral nervous system (PNS). The CNS
contains the brain and spinal cord. The PNS consists mainly of
nerves, which are long fibers that connect the CNS to every other
part of the body. The PNS includes motor neurons (mediating
voluntary movement), the autonomic nervous system (comprising
the sympathetic nervous system and the parasympathetic nervous
system, which regulate involuntary functions), and the enteric
nervous system (a semi-independent part of the nervous system
whose function is to control the gastrointestinal system).
The nervous system performs several functions simultaneously. For
example, as you are reading, the visual system is processing what is
seen on the page; the motor system controls the turn of the pages (or
click of the mouse); the prefrontal cortex maintains attention. Even
fundamental functions, like breathing and regulation of body
temperature, are controlled by the nervous system. A nervous system
is an organism’s control center: it processes sensory information
from outside (and inside) the body and controls all behaviors, from
Figure 35.1.1: Various nervous systems: (a) In cnidarians, nerve
eating to sleeping to finding a mate. cells form a decentralized nerve net. (b) In echinoderms, nerve cells
are bundled into fibers called nerves. (c) In animals exhibiting
bilateral symmetry, such as planarians, neurons cluster into an
anterior brain that processes information. (d) In addition to a brain,
arthropods have clusters of nerve cell bodies, called peripheral
ganglia, located along the ventral nerve cord. Mollusks, such as
squid and (e) octopuses, which must hunt to survive, have complex
brains containing millions of neurons. In (f) vertebrates, the brain
and spinal cord comprise the central nervous system, while neurons
extending into the rest of the body comprise the peripheral nervous
system.
Compared to invertebrates, vertebrate nervous systems are more
complex, centralized, and specialized. While there is great diversity
among different vertebrate nervous systems, they all share a basic
Figure 35.1.1: Nervous system at work: An athlete’s nervous system structure: a CNS and a PNS. One interesting difference between the
is hard at work during the planning and execution of a movement as
precise as a high jump. Parts of the nervous system are involved in nervous systems of invertebrates and vertebrates is that the nerve
determining how hard to push off and when to turn, as well as cords of many invertebrates are located ventrally (near the
controlling the muscles throughout the body that make this abdomen), whereas the vertebrate spinal cords are located dorsally
complicated movement possible without knocking the bar down; all
in just a few seconds. (near the back). There is debate among evolutionary biologists as to
Nervous systems throughout the animal kingdom vary in structure whether these different nervous system plans evolved separately or
and complexity. Some organisms, such as sea sponges, lack a true whether the invertebrate body plan arrangement somehow “flipped”
nervous system. Others, such as jellyfish, lack a true brain. Instead, during the evolution of vertebrates.
they have a system of separate-but-connected nerve cells (neurons) The nervous system is made up of neurons, specialized cells that can
called a “nerve net.” Echinoderms, such as sea stars, have nerve receive and transmit chemical or electrical signals, and glia, cells
cells that are bundled into fibers called nerves. Flatworms of the that provide support functions for the neurons by playing an
phylum Platyhelminthes have both a central nervous system, made information processing role that is complementary to neurons. A

35.1.1 https://bio.libretexts.org/@go/page/13864
neuron can be compared to an electrical wire: it transmits a signal The functions of the nervous system are performed by two types
from one place to another. Glia can be compared to the workers at of cells: neurons, which transmit signals between them and from
the electric company who make sure wires go to the right places, one part of the body to another, and glia, which regulate
maintain the wires, and take down wires that are broken. Although homeostasis, providing support and protection to the function of
glial cells support neurons, recent evidence suggests they also neurons.
assume some of the signaling functions of neurons.
KEY TERMS
KEY POINTS neuron: cell of the nervous system that conducts nerve impulses;
The central nervous system contains the brain and spinal cord; consisting of an axon and several dendrites
the peripheral nervous system consists of nerves, motor neurons, nervous system: an organ system that coordinates the body’s
the autonomic nervous system, and the enteric nervous system. voluntary and involuntary actions and transmits signals between
The nervous system coordinates the voluntary and involuntary different parts of the body
actions of the body by transmitting signals from the brain to the glial cell: cell in the nervous system that supports and protects
other body parts and listening for feedback. neurons
Nervous systems vary across different animals; some
invertebrates lack a true nervous system or true brain, while This page titled 35.1: Neurons and Glial Cells - Introduction is shared under
other invertebrates have a brain and a system of nerves. a CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Boundless.
Unlike vertebrates, not all invertebrates have both a CNS and
PNS; their nerve cords are located ventrally rather than dorsally.

35.1.2 https://bio.libretexts.org/@go/page/13864
35.2: NEURONS AND GLIAL CELLS - NEURONS
called axon terminals. These terminals, in turn, synapse on other
 LEARNING OBJECTIVES neurons, muscles, or target organs. Chemicals released at axon
terminals allow signals to be communicated to these other cells.
Describe the functions of the structural components of a
Neurons usually have one or two axons, but some neurons, like
neuron
amacrine cells in the retina, do not contain any axons. Some axons
are covered with myelin, which acts as an insulator to minimize
The nervous system of the common laboratory fly, Drosophila
dissipation of the electrical signal as it travels down the axon,
melanogaster, contains around 100,000 neurons, the same number as
greatly increasing the speed on conduction. This insulation is
a lobster. This number compares to 75 million in the mouse and 300 important as the axon from a human motor neuron can be as long as
million in the octopus. A human brain contains around 86 billion
a meter: from the base of the spine to the toes. The myelin sheath is
neurons. Despite these very different numbers, the nervous systems
not actually part of the neuron. Myelin is produced by glial cells.
of these animals control many of the same behaviors, from basic
Along these types of axons, there are periodic gaps in the myelin
reflexes to more complicated behaviors such as finding food and
sheath. These gaps, called “nodes of Ranvier,” are sites where the
courting mates. The ability of neurons to communicate with each
signal is “recharged” as it travels along the axon.
other, as well as with other types of cells, underlies all of these
It is important to note that a single neuron does not act alone.
behaviors.
Neuronal communication depends on the connections that neurons
Most neurons share the same cellular components. But neurons are make with one another (as well as with other cells, such as muscle
also highly specialized: different types of neurons have different cells). Dendrites from a single neuron may receive synaptic contact
sizes and shapes that relate to their functional roles. from many other neurons. For example, dendrites from a Purkinje
PARTS OF A NEURON cell in the cerebellum are thought to receive contact from as many as
200,000 other neurons.
Each neuron has a cell body (or soma) that contains a nucleus,
smooth and rough endoplasmic reticulum, Golgi apparatus, TYPES OF NEURONS
mitochondria, and other cellular components. Neurons also contain
There are different types of neurons; the functional role of a given
unique structures, relative to most cells, which are required for
neuron is intimately dependent on its structure. There is an amazing
receiving and sending the electrical signals that make neuronal
diversity of neuron shapes and sizes found in different parts of the
communication possible. Dendrites are tree-like structures that nervous system (and across species).
extend away from the cell body to receive messages from other
neurons at specialized junctions called synapses. While some
neurons have no dendrites, other types of neurons have multiple
dendrites. Dendrites can have small protrusions called dendritic
spines, which further increase surface area for possible synaptic
connections.

Figure 35.2.1: Neuron diversity: There is great diversity in the size

and shape of neurons throughout the nervous system. Examples
include (a) a pyramidal cell from the cerebral cortex, (b) a Purkinje
cell from the cerebellar cortex, and (c) olfactory cells from the
olfactory epithelium and olfactory bulb.
While there are many defined neuron cell subtypes, neurons are
broadly divided into four basic types: unipolar, bipolar, multipolar,
and pseudounipolar. Unipolar neurons have only one structure that
extends away from the soma. These neurons are not found in
Figure 35.2.1: Cellular structure of neurons: Neurons contain vertebrates, but are found in insects where they stimulate muscles or
organelles common to many other cells, such as a nucleus and glands. A bipolar neuron has one axon and one dendrite extending
mitochondria. They also have more specialized structures, including
dendrites and axons. from the soma. An example of a bipolar neuron is a retinal bipolar
Once a signal is received by the dendrite, it then travels passively to cell, which receives signals from photoreceptor cells that are
sensitive to light and transmits these signals to ganglion cells that
the cell body. The cell body contains a specialized structure, the
axon hillock, that integrates signals from multiple synapses and carry the signal to the brain. Multipolar neurons are the most
common type of neuron. Each multipolar neuron contains one axon
serves as a junction between the cell body and an axon: a tube-like
structure that propagates the integrated signal to specialized endings and multiple dendrites. Multipolar neurons can be found in the

35.2.1 https://bio.libretexts.org/@go/page/13865
central nervous system (brain and spinal cord). The Purkinje cell, a synapses; not all neurons have dendrites.
multipolar neuron in the cerebellum, has many branching dendrites, Synapses enable the dendrites from a single neuron to interact
but only one axon. Pseudounipolar cells share characteristics with and receive signals from many other neurons.
both unipolar and bipolar cells. A pseudounipolar cell has a single Axons are tube-like structures that send signals to other neurons,
structure that extends from the soma (like a unipolar cell), which muscles, or organs; not all neurons have axons.
later branches into two distinct structures (like a bipolar cell). Most Neurons are divided into four major types: unipolar, bipolar,
sensory neurons are pseudounipolar and have an axon that branches multipolar, and pseudounipolar.
into two extensions: one connected to dendrites that receives sensory Unipolar neurons have only one structure extending from the
information and another that transmits this information to the spinal soma; bipolar neurons have one axon and one dendrite extending
cord. from the soma.
Multipolar neurons contain one axon and many dendrites;
pseudounipolar neurons have a single structure that extends from
the soma, which later branches into two distinct structures.

KEY TERMS
dendrite: branched projections of a neuron that conduct the
impulses received from other neural cells to the cell body
axon: long slender projection of a nerve cell that conducts nerve
impulses away from the cell body to other neurons, muscles, and
organs
synapse: the junction between the terminal of a neuron and
either another neuron or a muscle or gland cell, over which nerve
Figure 35.2.1: Types of Neurons: Neurons are broadly divided into
four main types based on the number and placement of axons: (1) impulses pass
unipolar, (2) bipolar, (3) multipolar, and (4) pseudounipolar.
This page titled 35.2: Neurons and Glial Cells - Neurons is shared under a
KEY POINTS CC BY-SA 4.0 license and was authored, remixed, and/or curated by
Dendrites are the tree-like structures in neurons that extend away Boundless.
from the cell body to receive messages from other neurons at

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35.3: NEURONS AND GLIAL CELLS - GLIA

 LEARNING OBJECTIVES

Describe the specific roles that the seven types of glia play
in the nervous systems

While glia (or glial cells ) are often thought of as the supporting cast
of the nervous system, the number of glial cells in the brain actually
outnumbers the number of neurons by a factor of ten. Neurons
would be unable to function without the vital roles that are fulfilled
by these glial cells. Glia guide developing neurons to their Figure 35.3.1: Images of glial cells: (a) Astrocytes and (b)
oligodendrocytes are glial cells of the central nervous system.
destinations, buffer ions and chemicals that would otherwise harm
neurons, and provide myelin sheaths around axons. Scientists have
recently discovered that they also play a role in responding to nerve
activity and modulating communication between nerve cells. When
glia do not function properly, the result can be disastrous; most brain
tumors are caused by mutations in glia.

TYPES OF GLIA
There are several different types of glia with different functions.
Astrocytes make contact with both capillaries and neurons in the
CNS. They provide nutrients and other substances to neurons,
regulate the concentrations of ions and chemicals in the extracellular Figure 35.3.1: Glial cells: Glial cells support neurons and maintain
fluid, and provide structural support for synapses. Astrocytes also their environment. Glial cells of the (a) central nervous system
include oligodendrocytes, astrocytes, ependymal cells, and
form the blood-brain barrier: a structure that blocks entrance of toxic microglial cells. Oligodendrocytes form the myelin sheath around
substances into the brain. They have been shown, through calcium- axons. Astrocytes provide nutrients to neurons, maintain their
imaging experiments, to become active in response to nerve activity, extracellular environment, and provide structural support. Microglia
scavenge pathogens and dead cells. Ependymal cells produce
transmit calcium waves between astrocytes, and modulate the cerebrospinal fluid that cushions the neurons. Glial cells of the (b)
activity of surrounding synapses. Satellite glia provide nutrients and peripheral nervous system include Schwann cells, which form the
structural support for neurons in the PNS. Microglia scavenge and myelin sheath, and satellite cells, which provide nutrients and
structural support to neurons.
degrade dead cells, protecting the brain from invading
microorganisms. Oligodendrocytes form myelin sheaths around KEY POINTS
axons in the CNS. One axon can be myelinated by several Glia guide developing neurons to their destinations, buffer
oligodendrocytes; one oligodendrocyte can provide myelin for harmful ions and chemicals, and build the myelin sheaths around
multiple neurons. This is distinctive from the PNS where a single axons.
Schwann cell provides myelin for only one axon as the entire In the CNS astrocytes provide nutrients to neurons, give
Schwann cell surrounds the axon. Radial glia serve as bridges for synapses structural support, and block toxic substances from
developing neurons as they migrate to their end destinations.
entering the brain; satellite glia provide nutrients and structural
Ependymal cells line fluid-filled ventricles of the brain and the support for neurons in the PNS.
central canal of the spinal cord. They are involved in the production
Microglia scavenge and degrade dead cells, protecting the brain
of cerebrospinal fluid, which serves as a cushion for the brain, from invading microorganisms.
moves the fluid between the spinal cord and the brain, and is a
Oligodendrocytes form myelin sheaths around axons in the CNS;
component for the choroid plexus. Schwann cell forms myelin sheaths around axons in the PNS.
Radial glia serve as bridges for developing neurons as they
migrate to their end destinations.
Ependymal cells line fluid-filled ventricles of the brain and
central canal of the spinal cord which produce cerebrospinal
fluid.

KEY TERMS
satellite glia: glial cell that provides nutrients for neurons in the
PNS

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35.4: HOW NEURONS COMMUNICATE - NERVE IMPULSE TRANSMISSION
WITHIN A NEURON- RESTING POTENTIAL
the most-negative equilibrium potential, usually the resting potential
 LEARNING OBJECTIVES can be no more negative than the potassium equilibrium potential.
A neuron at rest is negatively charged because the inside of a cell is
Explain the formation of the resting potential in neurons
approximately 70 millivolts more negative than the outside (−70
mV); this number varies by neuron type and by species. This voltage
NERVE IMPULSE TRANSMISSION WITHIN A
is called the resting membrane potential and is caused by differences
NEURON
in the concentrations of ions inside and outside the cell. If the
For the nervous system to function, neurons must be able to send membrane were equally permeable to all ions, each type of ion
and receive signals. These signals are possible because each neuron would flow across the membrane and the system would reach
has a charged cellular membrane (a voltage difference between the equilibrium. Because ions cannot simply cross the membrane at will,
inside and the outside). The charge of this membrane can change in there are different concentrations of several ions inside and outside
response to neurotransmitter molecules released from other neurons the cell. The difference in the number of positively-charged
and environmental stimuli. Any voltage is a difference in electric potassium ions (K+) inside and outside the cell dominates the resting
potential between two points; for example, the separation of positive membrane potential. When the membrane is at rest, K+ ions
and negative electric charges on opposite sides of a resistive barrier. accumulate inside the cell due to a net movement with the
To understand how neurons communicate, one must first understand concentration gradient. The negative resting membrane potential is
the basis of charged membranes and the baseline or ‘resting’ created and maintained by increasing the concentration of cations
membrane charge. outside the cell (in the extracellular fluid) relative to inside the cell
(in the cytoplasm). The negative charge within the cell is created by
NEURONAL CHARGED MEMBRANES
the cell membrane being more permeable to K+ movement than
The lipid bilayer membrane that surrounds a neuron is impermeable Na+movement.
to charged molecules or ions. To enter or exit the neuron, ions must
pass through special proteins called ion channels that span the
membrane. Ion channels have different configurations: open, closed,
and inactive. Some ion channels need to be activated in order to
open and allow ions to pass into or out of the cell. These ion
channels are sensitive to the environment and can change their shape
accordingly. Ion channels that change their structure in response to
voltage changes are called voltage-gated ion channels. Voltage-gated
ion channels regulate the relative concentrations of different ions
inside and outside the cell. The difference in total charge between
the inside and outside of the cell is called the membrane potential.

Figure 35.4.1: Ion channel configurations: Voltage-gated ion

channels are closed at the resting potential and open in response to
changes in membrane voltage. After activation, they become
inactivated for a brief period and will no longer open in response to
a signal.

RESTING MEMBRANE POTENTIAL

For quiescent cells, the relatively-static membrane potential is
known as the resting membrane potential. The resting membrane
potential is at equilibrium since it relies on the constant expenditure
of energy for its maintenance. It is dominated by the ionic species in
the system that has the greatest conductance across the membrane.
For most cells, this is potassium. As potassium is also the ion with

35.4.1 https://bio.libretexts.org/@go/page/13868
 possesses potassium and sodium leakage channels that allow the two
cations to diffuse down their concentration gradient. However, the
neurons have far more potassium leakage channels than sodium
leakage channels. Therefore, potassium diffuses out of the cell at a
much faster rate than sodium leaks in. More cations leaving the cell
than entering it causes the interior of the cell to be negatively
charged relative to the outside of the cell. The actions of the sodium-
potassium pump help to maintain the resting potential, once it is
established. Recall that sodium-potassium pumps bring two K+ ions
into the cell while removing three Na+ ions per ATP consumed. As
more cations are expelled from the cell than are taken in, the inside
of the cell remains negatively charged relative to the extracellular
fluid.

KEY POINTS
When the neuronal membrane is at rest, the resting potential is
negative due to the accumulation of more sodium ions outside
the cell than potassium ions inside the cell.
Potassium ions diffuse out of the cell at a much faster rate than
sodium ions diffuse into the cell because neurons have many
more potassium leakage channels than sodium leakage channels.
Sodium-potassium pumps move two potassium ions inside the
cell as three sodium ions are pumped out to maintain the
negatively-charged membrane inside the cell; this helps maintain
the resting potential.

KEY TERMS
ion channel: a protein complex or single protein that penetrates a
cell membrane and catalyzes the passage of specific ions through
that membrane
Figure 35.4.1: Membrane potential: The (a) resting membrane
potential is a result of different concentrations of Na+ and K+ ions membrane potential: the difference in electrical potential across
inside and outside the cell. A nerve impulse causes Na+ to enter the the enclosing membrane of a cell
cell, resulting in (b) depolarization. At the peak action potential, K+ resting potential: the nearly latent membrane potential of
channels open and the cell becomes (c) hyperpolarized.
inactive cells
In neurons, potassium ions (K+) are maintained at high
concentrations within the cell, while sodium ions (Na+) are This page titled 35.4: How Neurons Communicate - Nerve Impulse
maintained at high concentrations outside of the cell. The cell Transmission within a Neuron- Resting Potential is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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35.5: HOW NEURONS COMMUNICATE - NERVE IMPULSE TRANSMISSION
WITHIN A NEURON- ACTION POTENTIAL
HYPERPOLARIZATION AND RETURN TO
 LEARNING OBJECTIVES RESTING POTENTIAL
Action potentials are considered an “all-or nothing” event. Once the
Explain the formation of the action potential in neurons
threshold potential is reached, the neuron completely depolarizes. As
soon as depolarization is complete, the cell “resets” its membrane
ACTION POTENTIAL
voltage back to the resting potential. The Na+ channels close,
A neuron can receive input from other neurons via a chemical called beginning the neuron’s refractory period. At the same time, voltage-
a neurotransmitter. If this input is strong enough, the neuron will gated K+ channels open, allowing K+ to leave the cell. As K+ ions
pass the signal to downstream neurons. Transmission of a signal leave the cell, the membrane potential once again becomes negative.
within a neuron (in one direction only, from dendrite to axon The diffusion of K+ out of the cell hyperpolarizes the cell, making
terminal) is carried out by the opening and closing of voltage-gated the membrane potential more negative than the cell’s normal resting
ion channels, which cause a brief reversal of the resting membrane potential. At this point, the sodium channels return to their resting
potential to create an action potential. As an action potential travels state, ready to open again if the membrane potential again exceeds
down the axon, the polarity changes across the membrane. Once the the threshold potential. Eventually, the extra K+ ions diffuse out of
signal reaches the axon terminal, it stimulates other neurons. the cell through the potassium leakage channels, bringing the cell
from its hyperpolarized state back to its resting membrane potential.

MYELIN AND PROPAGATION OF THE ACTION

POTENTIAL
For an action potential to communicate information to another
neuron, it must travel along the axon and reach the axon terminals
where it can initiate neurotransmitter release. The speed of
conduction of an action potential along an axon is influenced by
both the diameter of the axon and the axon’s resistance to current
leak. Myelin acts as an insulator that prevents current from leaving
the axon, increasing the speed of action potential conduction.
Diseases like multiple sclerosis cause degeneration of the myelin,
Figure 35.5.1: Formation of an action potential: The formation of an which slows action potential conduction because axon areas are no
action potential can be divided into five steps. (1) A stimulus from a longer insulated so the current leaks.
sensory cell or another neuron causes the target cell to depolarize
toward the threshold potential. (2) If the threshold of excitation is
reached, all Na+ channels open and the membrane depolarizes. (3)
At the peak action potential, K+ channels open and K+ begins to
leave the cell. At the same time, Na+ channels close. (4) The
membrane becomes hyperpolarized as K+ ions continue to leave the
cell. The hyperpolarized membrane is in a refractory period and
cannot fire. (5) The K+ channels close and the Na+/K+ transporter
restores the resting potential.

DEPOLARIZATION AND THE ACTION POTENTIAL

When neurotransmitter molecules bind to receptors located on a
neuron’s dendrites, voltage-gated ion channels open. At excitatory
synapses, positive ions flood the interior of the neuron and
depolarize the membrane, decreasing the difference in voltage
between the inside and outside of the neuron. A stimulus from a
sensory cell or another neuron depolarizes the target neuron to its
threshold potential (-55 mV), and Na+ channels in the axon hillock
Figure 35.5.1: Action potential travel along a neuronal axon: The
open, starting an action potential. Once the sodium channels open, action potential is conducted down the axon as the axon membrane
the neuron completely depolarizes to a membrane potential of about depolarizes, then repolarizes.
+40 mV. The action potential travels down the neuron as Na+ A node of Ranvier is a natural gap in the myelin sheath along the
channels open. axon. These unmyelinated spaces are about one micrometer long and
contain voltage gated Na+ and K+ channels. The flow of ions
through these channels, particularly the Na+ channels, regenerates
the action potential over and over again along the axon. Action

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potential “jumps” from one node to the next in saltatory conduction. as potassium ions leave the cell; the cell cannot fire during this
If nodes of Ranvier were not present along an axon, the action refractory period.
potential would propagate very slowly; Na+ and K+ channels would The action potential travels down the axon as the membrane of
have to continuously regenerate action potentials at every point the axon depolarizes and repolarizes.
along the axon. Nodes of Ranvier also save energy for the neuron Myelin insulates the axon to prevent leakage of the current as it
since the channels only need to be present at the nodes and not along travels down the axon.
the entire axon. Nodes of Ranvier are gaps in the myelin along the axons; they
contain sodium and potassium ion channels, allowing the action
potential to travel quickly down the axon by jumping from one
node to the next.

KEY TERMS
action potential: a short term change in the electrical potential
that travels along a cell
depolarization: a decrease in the difference in voltage between
Figure 35.5.1: Nodes of Ranvier: Nodes of Ranvier are gaps in the inside and outside of the neuron
myelin coverage along axons. Nodes contain voltage-gated K+ and
Na+ channels. Action potentials travel down the axon by jumping hyperpolarize: to increase the polarity of something, especially
from one node to the next. the polarity across a biological membrane
node of Ranvier: a small constriction in the myelin sheath of
KEY POINTS axons
Action potentials are formed when a stimulus causes the cell saltatory conduction: the process of regenerating the action
membrane to depolarize past the threshold of excitation, causing potential at each node of Ranvier
all sodium ion channels to open.
When the potassium ion channels are opened and sodium ion This page titled 35.5: How Neurons Communicate - Nerve Impulse
channels are closed, the cell membrane becomes hyperpolarized Transmission within a Neuron- Action Potential is shared under a CC BY-
SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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35.6: HOW NEURONS COMMUNICATE - SYNAPTIC TRANSMISSION

 LEARNING OBJECTIVES

Describe the process of synaptic transmission

SYNAPTIC TRANSMISSION
In a chemical synapse, the pre and post synaptic membranes are
separated by a synaptic cleft, a fluid filled space. The chemical event
is involved in the transmission of the impulse via release, diffusion,
receptor binding of neurotransmitter molecules and unidirectional
communication between neurons.

CHEMICAL SYNAPSE
Neurotransmission at a chemical synapse begins with the arrival of
an action potential at the presynaptic axon terminal. When an action
potential reaches the axon terminal, it depolarizes the membrane and
opens voltage-gated Na+ channels. Na+ ions enter the cell, further
depolarizing the presynaptic membrane. This depolarization causes
voltage-gated Ca2+ channels to open. Calcium ions entering the cell
initiate a signaling cascade. A calcium sensing protein binds calcium
and interacts with SNARE proteins. These SNARE proteins are
involved in the membrane fusion. The synaptic vesicles fuse with
the presynaptic axon terminal membrane and empty their contents
by exocytosis into the synaptic cleft. Calcium is quickly removed
from the terminal.

Figure 35.6.1: Communication at a chemical synapse:

Communication at chemical synapses requires release of
neurotransmitters. When the presynaptic membrane is depolarized,
voltage-gated Ca2+ channels open and allow Ca2+ to enter the cell.
The calcium entry causes synaptic vesicles to fuse with the
membrane and release neurotransmitter molecules into the synaptic
cleft. The neurotransmitter diffuses across the synaptic cleft and
binds to ligand-gated ion channels in the postsynaptic membrane,
resulting in a localized depolarization or hyperpolarization of the
postsynaptic neuron.

The binding of a specific neurotransmitter causes particular ion

channels, in this case ligand-gated channels, on the postsynaptic
membrane to open. The binding of a neurotransmitter to its receptor
is reversible. As long as it is bound to a post synaptic receptor, a
neurotransmitter continues to affect membrane potential. The effects
of the neurotransmitter generally lasts few milliseconds before being
terminated. The neurotransmitter termination can occur in three
Figure 35.6.1: Synaptic vesicles inside a neuron: This ways. First, reuptake by astrocytes or presynaptic terminal where the
pseudocolored image taken with a scanning electron microscope
shows an axon terminal that was broken open to reveal synaptic neurotransmitter is stored or destroyed by enzymes. Second,
vesicles (blue and orange) inside the neuron. degradation by enzymes in the synaptic cleft such as
Fusion of a vesicle with the presynaptic membrane causes acetylcholinesterase. Third, diffusion of the neurotransmitter as it
neurotransmitters to be released into the synaptic cleft. The moves away from the synapse.
neurotransmitter diffuses across the synaptic cleft, binding to
receptor proteins on the postsynaptic membrane.
KEY POINTS
In a chemical synapse, the pre and post synaptic membranes are
separated by a synaptic cleft, a fluid filled space.
The chemical event is involved in the transmission of the
impulse via release, diffusion, receptor binding of

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neurotransmitter molecules and unidirectional communication
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between neurons. is shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
The neurotransmitter termination can occur in three ways – curated by Boundless.
reuptake, enzymatic degradation in the cleft and diffusion.

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35.7: HOW NEURONS COMMUNICATE - SIGNAL SUMMATION
potential. If the neuron only receives excitatory impulses, it will also
 LEARNING OBJECTIVES generate an action potential. However, if the neuron receives as
many inhibitory as excitatory impulses, the inhibition cancels out the
Describe signal summation
excitation and the nerve impulse will stop there. Spatial summation
means that the effects of impulses received at different places on the
Each neuron connects with numerous other neurons, often receiving
neuron add up so that the neuron may fire when such impulses are
multiple impulses from them. Sometimes, a single excitatory
received simultaneously, even if each impulse on its own would not
postsynaptic potential (EPSP) is strong enough to induce an action
be sufficient to cause firing. Temporal summation means that the
potential in the postsynaptic neuron, but often multiple presynaptic effects of impulses received at the same place can add up if the
inputs must create EPSPs around the same time for the postsynaptic
impulses are received in close temporal succession. Thus, the neuron
neuron to be sufficiently depolarized to fire an action potential.
may fire when multiple impulses are received, even if each impulse
Summation, either spatial or temporal, is the addition of these
on its own would not be sufficient to cause firing.
impulses at the axon hillock. Together, synaptic summation and the
threshold for excitation act as a filter so that random “noise” in the KEY POINTS
system is not transmitted as important information. Simultaneous impulses may add together from different places
on the neuron to reach the threshold of excitation during spatial
summation.
When individual impulses cannot reach the threshold of
excitation on their own, they can can add up at the same location
on the neuron over a short time; this is known as temporal
summation.
The action potential of a neuron is fired only when the net
change of excitatory and inhibitory impulses is non-zero.

KEY TERMS
temporal summation: the effect when impulses received at the
Figure 35.7.1: Signal summation at the axon hillock: A single
neuron can receive both excitatory and inhibitory inputs from
same place on the neuron add up
multiple neurons. All these inputs are added together at the axon spatial summation: the effect when simultaneous impulses
hillock. If the EPSPs are strong enough to overcome the IPSPs and received at different places on the neuron add up to fire the
reach the threshold of excitation, the neuron will fire.
neuron
One neuron often has input from many presynaptic neurons, whether axon hillock: the specialized part of the soma of a neuron that is
excitatory or inhibitory; therefore, inhibitory postsynaptic potentials connected to the axon and where impulses are added together
(IPSPs) can cancel out EPSPs and vice versa. The net change in
postsynaptic membrane voltage determines whether the postsynaptic This page titled 35.7: How Neurons Communicate - Signal Summation is
cell has reached its threshold of excitation needed to fire an action shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
curated by Boundless.

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35.8: HOW NEURONS COMMUNICATE - SYNAPTIC PLASTICITY
for a short time because of either an increase in size of the readily-
 LEARNING OBJECTIVES releasable pool of packaged transmitter or an increase in the amount
of packaged transmitter released in response to each action potential.
Distinguish between long-term potentiation and long-term
Depletion of these readily-releasable vesicles causes synaptic
depression
fatigue. Short-term synaptic depression can also arise from post-
synaptic processes and from feedback activation of presynaptic
Synaptic plasticity is the strengthening or weakening of synapses
receptors.
over time in response to increases or decreases in their activity.
Plastic change also results from the alteration of the number of LONG-TERM POTENTIATION (LTP)
receptors located on a synapse. Synaptic plasticity is the basis of Long-term potentiation (LTP) is a persistent strengthening of a
learning and memory, enabling a flexible, functioning nervous synaptic connection, which can last for minutes or hours. LTP is
system. Synaptic plasticity can be either short-term (synaptic based on the Hebbian principle: “cells that fire together wire
enhancement or synaptic depression) or long-term. Two processes in together. ” There are various mechanisms, none fully understood,
particular, long-term potentiation (LTP) and long-term depression behind the synaptic strengthening seen with LTP.
(LTD), are important forms of synaptic plasticity that occur in
One known mechanism involves a type of postsynaptic glutamate
synapses in the hippocampus: a brain region involved in storing
receptor: NMDA (N-Methyl-D-aspartate) receptors. These receptors
memories.
are normally blocked by magnesium ions. However, when the
postsynaptic neuron is depolarized by multiple presynaptic inputs in
quick succession (either from one neuron or multiple neurons), the
magnesium ions are forced out and Ca2+ ions pass into the
postsynaptic cell. Next, Ca2+ ions entering the cell initiate a
signaling cascade that causes a different type of glutamate receptor,
AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid)
receptors, to be inserted into the postsynaptic membrane. Activated
AMPA receptors allow positive ions to enter the cell.
Therefore, the next time glutamate is released from the presynaptic
membrane, it will have a larger excitatory effect (EPSP) on the
postsynaptic cell because the binding of glutamate to these AMPA
receptors will allow more positive ions into the cell. The insertion of
additional AMPA receptors strengthens the synapse so that the
postsynaptic neuron is more likely to fire in response to presynaptic
neurotransmitter release. Some drugs co-opt the LTP pathway; this
synaptic strengthening can lead to addiction.

LONG-TERM DEPRESSION (LTD)

Figure 35.8.1: Long-term potentiation and depression: Calcium
entry through postsynaptic NMDA receptors can initiate two Long-term depression (LTD) is essentially the reverse of LTP: it is a
different forms of synaptic plasticity: long-term potentiation (LTP) long-term weakening of a synaptic connection. One mechanism
and long-term depression (LTD). LTP arises when a single synapse
is repeatedly stimulated. This stimulation causes a calcium- and
known to cause LTD also involves AMPA receptors. In this
CaMKII-dependent cellular cascade, which results in the insertion of situation, calcium that enters through NMDA receptors initiates a
more AMPA receptors into the postsynaptic membrane. The next different signaling cascade, which results in the removal of AMPA
time glutamate is released from the presynaptic cell, it will bind to
both NMDA and the newly-inserted AMPA receptors, thus
receptors from the postsynaptic membrane. With the decrease in
depolarizing the membrane more efficiently. LTD occurs when few AMPA receptors in the membrane, the postsynaptic neuron is less
glutamate molecules bind to NMDA receptors at a synapse (due to a responsive to the glutamate released from the presynaptic neuron.
low firing rate of the presynaptic neuron). The calcium that does
While it may seem counterintuitive, LTD may be just as important
flow through NMDA receptors initiates a different calcineurin and
protein phosphatase 1-dependent cascade, which results in the for learning and memory as LTP. The weakening and pruning of
endocytosis of AMPA receptors. This makes the postsynaptic neuron unused synapses trims unimportant connections, leaving only the
less responsive to glutamate released from the presynaptic neuron.
salient connections strengthened by long-term potentiation.
SHORT-TERM SYNAPTIC ENHANCEMENT AND
KEY POINTS
DEPRESSION
Short-term synaptic enhancement occurs when the amount of
Short-term synaptic plasticity acts on a timescale of tens of
available neurotransmitter is increased, while short-term synaptic
milliseconds to a few minutes. Short-term synaptic enhancement
depression occurs when the amount of vesicles with
results from more synaptic terminals releasing transmitters in
neurotransmitters is decreased.
response to presynaptic action potentials. Synapses will strengthen

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35.9: THE NERVOUS SYSTEM
the brain and spinal cord like plastic wrap. The space between the
 LEARNING OBJECTIVES arachnoid and pia maters is filled with cerebrospinal fluid (CSF).
CSF is produced by a tissue called the choroid plexus in fluid-filled
Summarize the nervous system
compartments in the CNS called ventricles. The brain floats in CSF,
which acts as a cushion and shock absorber, making the brain
INTRODUCTION TO THE NERVOUS SYSTEM neutrally buoyant. CSF also functions to circulate chemical
The nervous system of higher vertebrates (the group that includes substances throughout the brain and into the spinal cord.
humans) is a widely-distributed communication and processing Skin
network that serves controlling functions over other organ systems. Periosteum
It possesses a key function in the orientation of the individual; Bone
controls its behavior, motor function, and sensory processing; and
Dura mater
contains mechanisms to store information. A classification of the
Arachnoid
nervous system can be performed under different aspects. The
anatomical compartmentalization of its components defines the Pia mater
classical approach. Two major divisions of the nervous system are
the central nervous system (CNS) and the peripheral nervous system
(PNS).
Figure 35.9.1: Meninges of the brain: The outermost layer of the
CENTRAL NERVOUS SYSTEM meninges is the dura mater, which protects the brain and spinal cord.
The innermost layer is the pia mater, which directly covers the brain
The vertebrate central nervous system (CNS) contains the brain and and spinal cord. The arachnoid mater is found between the two.
the spinal cord. The brain contains structurally- and functionally-
defined regions. In mammals, these include the cortex (which can be PERIPHERAL NERVOUS SYSTEM
broken down into four primary functional lobes: frontal, temporal, The peripheral nervous system consists of nerves that are connected
occipital, and parietal), basal ganglia, thalamus, hypothalamus, to the brain (cranial nerves) and nerves that are connected to the
limbic system, cerebellum, and brainstem; although structures in spinal cord (spinal nerves). The main function of the PNS is to
some of these designations overlap. While functions may be connect the central nervous system (CNS) to the limbs and organs,
primarily localized to one structure in the brain, most complex essentially serving as a communication relay between the brain and
functions, such as language and sleep, involve neurons in multiple the extremities. Unlike the CNS, the PNS is not protected by the
brain regions. The spinal cord is the information superhighway, bone of spine and skull. Nor does it have a barrier between itself and
connecting the brain with the rest of the body through the peripheral the blood, leaving it exposed to toxins and mechanical injuries.
nerves. It transmits sensory and motor input and also controls motor The autonomic nervous system, also part of the peripheral nervous
reflexes. system, controls internal body functions that are not under conscious
control. For example, when a prey animal is chased by a predator,
1 the autonomic nervous system automatically increases the rate of
breathing and the heartbeat. It dilates the blood vessels that carry
blood to the muscles, releases glucose from the liver, and makes
2 other adjustments to provide for the sudden increase in activity.
When the animal has escaped and is safe once again, the nervous
system slows down all these processes and resumes all the normal
3
body activities, such as the digestion of food.

KEY POINTS
The central nervous system consists of the brain, which controls
complex body functions, and the spinal cord, which transmits
Figure 35.9.1: Central nervous system: The central nervous system
(2) is a combination of the brain (1) and the spinal cord (3). signals from the brain to the rest of the body.
The CNS is covered with three layers of protective coverings called The brain and spinal cord are covered by three layers of
meninges (from the Greek word for membrane). The outermost layer meninges, or protective coverings: the dura mater, the arachnoid
is the dura mater (Latin for “hard mother”). As the Latin name mater, and the pia mater.
suggests, the primary function for this thick layer is to protect the Cerebrospinal fluid surrounds the brain, cushioning it and
brain and spinal cord. The dura mater also contains vein-like providing shock absorption to prevent damage.
structures that carry blood from the brain back to the heart. The The peripheral nervous system is made up of nerves that
middle layer is the web-like arachnoid mater. The last layer is the pia originate within the brain and spinal cord; it serves to relay
mater (Latin for “soft mother”), which directly contacts and covers

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35.10: THE CENTRAL NERVOUS SYSTEM - CEREBRAL CORTEX AND BRAIN
LOBES
frontal lobes show that parts of this area are involved in personality,
 LEARNING OBJECTIVES socialization, and assessing risk.

Describe the structure and function of the cerebral cortex

The brain is the part of the central nervous system that is contained
in the cranial cavity of the skull. It includes the cerebral cortex,
limbic system, basal ganglia, thalamus, hypothalamus, and
cerebellum.

CEREBRAL CORTEX
The outermost part of the brain is a thick piece of nervous system
tissue called the cerebral cortex, which is folded into hills called gyri
(singular: gyrus) and valleys called sulci (singular: sulcus ). The
cortex is composed of two hemispheres, right and left, which are
separated by a large sulcus. A thick fiber bundle, the corpus
callosum, connects the two hemispheres, allowing information to be
passed from one side to the other. Although there are some brain
functions that are localized more to one hemisphere than the other,
Figure 35.10.1: Motor cortex control of muscle movement:
the functions of the two hemispheres are largely redundant. In fact, Different parts of the motor cortex control different muscle groups.
sometimes (very rarely) an entire hemisphere is removed to treat Muscle groups that are neighbors in the body are generally
controlled by neighboring regions of the motor cortex as well. For
severe epilepsy. While patients do suffer some deficits following the example, the neurons that control shoulder movement are near the
surgery, they can have surprisingly few problems, especially when neurons that control elbow movement, which are themselves next to
the surgery is performed on children who have relatively- those that control wrist movement.
undeveloped nervous systems.
In other surgeries to treat severe epilepsy, the corpus callosum is cut
instead of removing an entire hemisphere. This causes a condition
called split-brain syndrome, which gives insights into unique
functions of the two hemispheres. For example, when an object is
presented to patients’ left visual fields, they may be unable to
verbally name the object (and may claim not to have seen an object
at all). This is because the visual input from the left visual field
crosses and enters the right hemisphere and is unable to signal to the
speech center, which generally is found in the left side of the brain.
Remarkably, if a split-brain patient is asked to pick up a specific
object out of a group of objects with the left hand, the patient will be
able to do so, but will still be unable to vocally identify it. Figure 35.10.1: Lobes of the cerebral cortex: The human cerebral
cortex includes the frontal, parietal, temporal, and occipital lobes,
THE FOUR BRAIN LOBES each of which is involved in a different higher function.

Each hemisphere of the mammalian cerebral cortex can be broken The parietal lobe is located at the top of the brain. Neurons in the
parietal lobe are involved in speech and reading. Two of the parietal
down into four functionally- and spatially-defined lobes: frontal,
parietal, temporal, and occipital. The frontal lobe is located at the lobe’s main functions are processing somatosensation (touch
sensations such as pressure, pain, heat, cold) and processing
front of the brain, over the eyes. This lobe contains the olfactory
bulb, which processes smells. The frontal lobe also contains the proprioception (the sense of how parts of the body are oriented in
space). The parietal lobe contains a somatosensory map of the body
motor cortex, which is important for planning and implementing
movement. Areas within the motor cortex map to different muscle similar to the motor cortex.
groups; there is some organization to this map. For example, the The occipital lobe is located at the back of the brain. It is primarily
neurons that control movement of the fingers are next to the neurons involved in vision: seeing, recognizing, and identifying the visual
that control movement of the hand. Neurons in the frontal lobe also world.
control cognitive functions such as maintaining attention, speech, The temporal lobe is located at the base of the brain by the ears. It is
and decision-making. Studies of humans who have damaged their primarily involved in processing and interpreting sounds. It also
contains the hippocampus (Greek for “seahorse”, which is what it

35.10.1 https://bio.libretexts.org/@go/page/13877
resembles), a structure that processes memory formation. The role of along with sensing where each part of the body is in relation to
the hippocampus in memory was partially determined by studying the others and its environment.
one famous epileptic patient, HM, who had both sides of his The occipital lobe interprets visual cues, such as what we see and
hippocampus removed in an attempt to cure his epilepsy. His recognition of faces and objects.
seizures went away, but he could no longer form new memories The temporal lobe processes and interprets sounds and is also
(although he could remember some facts from before his surgery involved in forming new memories, a task for which the
and could learn new motor tasks). hippocampus, a structure inside the temporal lobe, is responsible.

KEY POINTS KEY TERMS

The cerebral cortex is the outermost layer of the brain; it is easily corpus callosum: in mammals, a broad band of nerve fibers that
recognizable by the grooves (sulci) and “hills” (gyri). connects the left and right hemispheres of the brain
The brain contains two hemispheres, the left and the right, which proprioception: the sense of the position of parts of the body,
are connected by a bundle of nerve fibers called the corpus relative to other neighbouring parts of the body
callosum that transmits information between them. somatosensation: general senses which respond to stimuli like
The frontal lobe houses the olfactory bulb, which processes temperature, pain, pressure, and vibration
smells; the motor cortex, which controls movement; and it gyrus: a ridge or fold on the cerebral cortex
controls cognitive functions such as attention, speech, and sulcus: any of the grooves that mark the convolutions of the
decision-making. surface of the brain
The parietal lobe is involved in speech and reading, as well as
interpreting touch sensations such as pressure, pain, heat, cold, This page titled 35.10: The Central Nervous System - Cerebral Cortex and
Brain Lobes is shared under a CC BY-SA 4.0 license and was authored,
remixed, and/or curated by Boundless.

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35.11: THE CENTRAL NERVOUS SYSTEM - MIDBRAIN AND BRAIN STEM

 LEARNING OBJECTIVES

Explain the structure and function of the non-cerebral cortex

portions of the brain

BASAL GANGLIA
Interconnected brain areas called the basal ganglia (or basal nuclei)
play important roles in movement control and posture. Damage to
the basal ganglia, which occurs in Parkinson’s disease, leads to
motor impairments such as a shuffling gait when walking. The basal
ganglia also regulate motivation. For example, when a wasp sting
led to bilateral basal ganglia damage in a 25-year-old businessman,
he began to spend all his days in bed and showed no interest in
anything or anybody. But when he was externally stimulated, as Figure 35.11.1: The limbic system: The limbic system regulates
emotion and other behaviors. It includes parts of the cerebral cortex
when someone asked to play a card game with him, he was able to located near the center of the brain, including the cingulate gyrus
function normally. Interestingly, he and other similar patients do not and the hippocampus as well as the thalamus, hypothalamus, and
report feeling bored or frustrated by their state. amygdala.

THALAMUS CEREBELLUM
The thalamus (Greek for “inner chamber”) acts as a gateway to and The cerebellum (Latin for “little brain”) sits at the base of the brain
from the cortex. It receives sensory and motor inputs from the body on top of the brainstem. The cerebellum controls balance and aids in
and also receives feedback from the cortex. This feedback coordinating movement and learning new motor tasks.
mechanism can modulate conscious awareness of sensory and motor
BRAINSTEM
inputs depending on the attention and arousal state of the animal.
The thalamus helps regulate consciousness, arousal, and sleep states. The brainstem connects the rest of the brain with the spinal cord. It
A rare genetic disorder, fatal familial insomnia, causes the consists of the midbrain, medulla oblongata, and the pons. Motor
degeneration of thalamic neurons and glia. This disorder prevents and sensory neurons extend through the brainstem, allowing for the
affected patients from being able to sleep, among other symptoms, relay of signals between the brain and spinal cord. Ascending neural
and is eventually fatal. pathways cross in this section of the brain, allowing the left
hemisphere of the cerebrum to control the right side of the body and
HYPOTHALAMUS vice versa. The brainstem coordinates motor control signals sent
Below the thalamus is the hypothalamus. The hypothalamus controls from the brain to the body. It also controls several important
the endocrine system by sending signals to the pituitary gland, a pea- functions of the body including alertness, arousal, breathing, blood
sized endocrine gland that releases several different hormones that pressure, digestion, heart rate, swallowing, walking, and sensory and
affect other glands as well as other cells. This relationship means motor information integration.
that the hypothalamus regulates important behaviors that are
KEY POINTS
controlled by these hormones. The hypothalamus is the body’s
thermostat: it makes sure key functions like food and water intake, The basal ganglia control movement and posture; they also
energy expenditure, and body temperature are kept at appropriate appear to be involved in self-motivation.
levels. Neurons within the hypothalamus also regulate circadian The thalamus communicates information from the cerebral
rhythms, sometimes called sleep cycles. cortex to the rest of the body and also helps regulate the states of
consciousness versus sleep.
The limbic system is a connected set of structures that regulates
The hypothalamus regulates the pituitary gland, which controls
emotion, as well as behaviors related to fear and motivation. It plays
the release of hormones throughout the body; it indirectly
a role in memory formation and includes parts of the thalamus and
regulates functions such as water intake, body temperature, and
hypothalamus as well as the hippocampus. One important structure
sleep cycles.
within the limbic system is a temporal lobe structure called the
The limbic system includes the amygdala, the hippocampus, and
amygdala (Greek for “almond”). The two amygdale are important
parts of the thalamus and hypothalamus; it regulates emotion,
both for the sensation of fear and for recognizing fearful faces. The
fear, and motivation.
cingulate gyrus helps regulate emotions and pain.
The cerebellum controls motor reflexes and is, therefore,
involved in balance and muscle coordination.
The brainstem connects and transmits signals from the brain to
the spinal cord, controlling functions such as breathing, heart

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 rate, and alertness. endocrine system: a control system of ductless glands that
secrete hormones which circulate via the bloodstream to affect
KEY TERMS cells within specific organs
cingulate gyrus: a section of the cerebral cortex, belonging to
the fornicate gyrus, which arches over the corpus callosum This page titled 35.11: The Central Nervous System - Midbrain and Brain
limbic system: part of the human brain involved in emotion, Stem is shared under a CC BY-SA 4.0 license and was authored, remixed,
motivation, and emotional association with memory and/or curated by Boundless.

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35.12: THE CENTRAL NERVOUS SYSTEM - SPINAL CORD
treat because spinal nerves do not regenerate, although ongoing
 LEARNING OBJECTIVES research suggests that stem cell transplants may be able to act as a
bridge to reconnect severed nerves. Researchers are also looking at
Describe the structure and function of the spinal cord
ways to prevent the inflammation that worsens nerve damage after
injury. One such treatment is to pump the body with cold saline to
Connecting to the brainstem and extending down the body through
induce hypothermia. This cooling can prevent swelling and other
the spinal column is the spinal cord: a thick bundle of nerve tissue
processes that are thought to worsen spinal cord injuries.
that carries information about the body to the brain and from the
brain to the body. The spinal cord is contained within the bones of KEY POINTS
the vertebral column, but is able to communicate signals to and from The spinal cord consists of a butterfly-shaped area of grey
the body through its connections with spinal nerves (part of the matter, containing neuronal and glial cell bodies, surrounded by
peripheral nervous system). A cross-section of the spinal cord looks white matter that contains the axons of the neurons.
like a white oval containing a gray butterfly-shape. Myelinated Neurons at the back of the spinal cord ( dorsal ) generally
axons (the part of neurons that send signals) compose the “white transmit information from the body to the brain, while neurons at
matter,” while neuron and glial cell bodies (neuronal “support” cells) the front of the spinal cord ( ventral ) primarily transmit
compose the “grey matter.” Grey matter is also composed of information from the brain to the body.
interneurons, which connect two neurons, each located in different The spinal cord controls reflexes, which are incredibly fast
parts of the body. Axons and cell bodies in the dorsal (facing the reactions to stimuli; the speed at which they operate is due to the
back of the animal) spinal cord convey mostly sensory information fact that they involve only a local connection between neurons
from the body to the brain. Axons and cell bodies in the ventral and are not relayed through the brain.
(facing the front of the animal) spinal cord primarily transmit signals Spinal cord injuries often result in paralysis; they do not heal, as
controlling movement from the brain to the body. spinal nerves lack the ability to regenerate.

KEY TERMS
grey matter: a collection of cell bodies and (usually) dendritic
connections, in contrast to white matter
synapse: the junction between the terminal of a neuron and
either another neuron or a muscle or gland cell, over which nerve
impulses pass
axon: long slender projection of a nerve cell that conducts nerve
impulses away from the cell body to other neurons, muscles, and
organs
Figure 35.12.1: Spinal cord cross-section: A cross-section of the white matter: a region of the central nervous system containing
spinal cord shows grey matter (containing cell bodies and
interneurons) and white matter (containing axons). myelinated nerve fibers and no dendrites
The spinal cord also controls motor reflexes. These reflexes are interneuron: a multipolar neuron that connects afferent and
quick, unconscious movements, such as automatically removing a efferent neurons
hand from a hot object. Reflexes are so fast because they involve
CONTRIBUTIONS AND ATTRIBUTIONS
local synaptic connections. For example, the knee reflex that a
OpenStax College, Biology. October 17, 2013. Provided by: OpenStax CNX.
doctor tests during a routine physical is controlled by a single Located at: http://cnx.org/content/m44749/latest...ol11448/latest. License: CC
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In the United States, there around 10,000 spinal cord injuries each License: CC BY-SA: Attribution-ShareAlike
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lead to paralysis. The extent of the paralysis depends on the location en.wiktionary.org/wiki/corpus_callosum. License: CC BY-SA: Attribution-
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paralysis to the legs. Spinal cord injuries are notoriously difficult to

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35.13: THE PERIPHERAL NERVOUS SYSTEM - AUTONOMIC NERVOUS
SYSTEM

 LEARNING OBJECTIVES

Explain the function of the autonomic nervous system

AUTONOMIC NERVOUS SYSTEM

The autonomic nervous system (ANS) serves as the relay between
the central nervous system (CNS) and the internal organs. It controls
the lungs, the heart, smooth muscle, and exocrine and endocrine
glands, largely without conscious control. It can continuously
monitor the conditions of these different systems and implement
changes as needed. Signaling to the target tissue usually involves
two synapses. A preganglionic neuron (originating in the CNS)
synapses to a neuron in a ganglion that, in turn, synapses on the
target organ. There are two divisions of the autonomic nervous
system that often have opposing effects: the sympathetic nervous
system and the parasympathetic nervous system.

Figure 35.13.1: Autonomic nervous system: Autonomic responses

are mediated by the sympathetic and the parasympathetic systems,
which are antagonistic to one another. The sympathetic system
activates the “fight or flight” response, while the parasympathetic
system activates the “rest and digest” response. In the autonomic
nervous system, a preganglionic neuron of the CNS synapses with a
postganglionic neuron of the parasympathetic nervous system. The
postganglionic neuron, in turn, acts on a target organ.

SYMPATHETIC NERVOUS SYSTEM

Figure 35.13.1: Actions of the SNS and PNS: The sympathetic and The sympathetic nervous system is responsible for the “fight or
parasympathetic nervous systems often have opposing effects on
target organs. flight” response that occurs when an animal encounters a dangerous
situation. One way to remember this is to think of the surprise a
person feels when encountering a snake (“snake” and “sympathetic”
both begin with “s”). Examples of functions controlled by the
sympathetic nervous system include an accelerated heart rate and
inhibited digestion, both of which help prepare an organism’s body
for the physical strain required to escape a potentially dangerous
situation or to fend off a predator.
Most preganglionic neurons in the sympathetic nervous system
originate in the spinal cord. The axons of these neurons release
acetylcholine on postganglionic neurons within sympathetic ganglia
(the sympathetic ganglia form a chain that extends alongside the
spinal cord). The acetylcholine activates the postganglionic neurons.
Postganglionic neurons then release norepinephrine onto target
organs. As anyone who has ever felt a rush before a big test, speech,

35.13.1 https://bio.libretexts.org/@go/page/13882
or athletic event can attest, the effects of the sympathetic nervous acetylcholine release on target organs include slowing of heart rate,
system are quite pervasive. This is both because one preganglionic lowered blood pressure, and stimulation of digestion.
neuron synapses on multiple postganglionic neurons, amplifying the
effect of the original synapse, and because the adrenal gland also KEY POINTS
releases norepinephrine (and the closely-related hormone The autonomic nervous system controls the workings of internal
epinephrine) into the blood stream. The physiological effects of this organs such as the heart, lungs, digestive system, and endocrine
norepinephrine release include dilating the trachea and bronchi systems; it does so without conscious effort.
(making it easier for the animal to breathe), increasing heart rate, The sympathetic nervous system controls the body’s automatic
and moving blood from the skin to the heart, muscles, and brain (so response to danger, increasing the heart rate, dilating the blood
the animal can think and run). The strength and speed of the vessels, slowing digestion, and moving blood flow to the heart,
sympathetic response helps an organism avoid danger. Scientists muscles, and brain.
have found evidence that it may also increase long term potentiation The parasympathetic nervous system works in opposition to the
in neurons, allowing the animal to remember the dangerous situation sympathetic; during periods of rest it slows the heart rate, lowers
and avoid it in the future. the blood pressure, stimulates digestion, and moves blood flow
back to the skin.
PARASYMPATHETIC NERVOUS SYSTEM
While the sympathetic nervous system is activated in stressful KEY TERMS
situations, the parasympathetic nervous system allows an animal to preganglionic: describing the nerve fibres that supply a ganglion
“rest and digest.” One way to remember this is to think that during a sympathetic nervous system: the part of the autonomic nervous
restful situation like a picnic, the parasympathetic nervous system is system that under stress raises blood pressure and heart rate,
in control (“picnic” and “parasympathetic” both start with “p”). constricts blood vessels and dilates the pupils
Parasympathetic preganglionic neurons have cell bodies located in parasympathetic nervous system: one of the divisions of the
the brainstem and in the sacral (toward the bottom) spinal cord. The autonomic nervous system, based between the brain and the
axons of the preganglionic neurons release acetylcholine on the spinal cord, that slows the heart and relaxes muscles
postganglionic neurons, which are generally located very near the acetylcholine: a neurotransmitter in humans and other animals,
target organs. which is an ester of acetic acid and choline
The parasympathetic nervous system resets organ function after the
This page titled 35.13: The Peripheral Nervous System - Autonomic
sympathetic nervous system is activated (the common adrenaline
Nervous System is shared under a CC BY-SA 4.0 license and was authored,
dump you feel after a ‘fight-or-flight’ event). Effects of
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35.14: THE PERIPHERAL NERVOUS SYSTEM - THE SENSORY-SOMATIC
NERVOUS SYSTEM
SPINAL NERVES
 LEARNING OBJECTIVES Spinal nerves transmit sensory and motor information between the
spinal cord and the rest of the body. Each of the 31 spinal nerves (in
Explain the role of the cranial and spinal nerves in the
humans) contains both sensory and motor axons. The sensory
sensory-somatic nervous system
neuron cell bodies are grouped in structures called dorsal root
ganglia. Each sensory neuron has one projection with a sensory
The sensory-somatic nervous system is composed of cranial and
receptor ending in skin, muscle, or sensory organs, and another that
spinal nerves and contains both sensory and motor neurons. Sensory
synapses with a neuron in the dorsal spinal cord. Motor neurons
neurons transmit sensory information from the skin, skeletal muscle,
have cell bodies in the ventral gray matter of the spinal cord that
and sensory organs to the central nervous system (CNS). Motor
project to muscle through the ventral root. These neurons are usually
neurons transmit messages about desired movement from the CNS
stimulated by interneurons within the spinal cord, but are sometimes
to the muscles, causing them to contract. Without its sensory-
directly stimulated by sensory neurons.
somatic nervous system, an animal would be unable to process any
information about its environment (what it sees, feels, hears, etc. )
and could not control motor movements. Unlike the autonomic
nervous system, which has two synapses between the CNS and the
target organ, sensory and motor neurons have only one synapse: one
ending of the neuron is at the organ and the other directly contacts a
CNS neuron. Acetylcholine is the main neurotransmitter released at
these synapses.

CRANIAL NERVES
Humans have 12 cranial nerves, nerves that emerge from or enter the
skull (cranium), as opposed to the spinal nerves, which emerge from
the vertebral column. Each cranial nerve has a name. Some cranial
nerves transmit only sensory information. For example, the olfactory
nerve transmits information about smells from the nose to the Figure 35.14.1: Spinal nerves: Spinal nerves contain both sensory
brainstem. Other cranial nerves transmit almost solely motor and motor axons. The cell bodies of sensory neurons are located in
dorsal root ganglia. The cell bodies of motor neurons are found in
information. The oculomotor nerve controls the opening and closing the ventral portion of the gray matter of the spinal cord.
of the eyelid and some eye movements. Other cranial nerves contain
a mix of sensory and motor fibers. For example, the KEY POINTS
glossopharyngeal nerve has a role in both taste (sensory) and The sensory and motor neurons of the sensory-somatic system
swallowing (motor). have only one synapse between the organ and a neuron of the
CNS; these synapses utilize acetylcholine to transmit signals
across this synapse.
The twelve cranial nerves either enter or exit from the skull;
some transmit only sensory information, some transmit only
motor information, and some transmit both.
There are 31 spinal nerves that convey both sensory and motor
signals between the spinal cord and the rest of the body.

KEY TERMS
cranial nerve: any of the twelve paired nerves that originate
from the brainstem instead of the spinal cord
spinal nerve: one of 31 pairs of nerves that carry motor, sensory,
and autonomic signals between the spinal cord and the body
acetylcholine: a neurotransmitter in humans and other animals,
Figure 35.14.1: Cranial nerves: The human brain contains 12 cranial which is an ester of acetic acid and choline
nerves that receive sensory input and control motor output for the
head and neck. CONTRIBUTIONS AND ATTRIBUTIONS
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35.15: NEURODEGENERATIVE DISORDERS - INTRODUCTION

 LEARNING OBJECTIVES

Distinguish between the neurodegenerative disorders of

Alzheimer’s disease and Parkinson’s disease

Neurodegenerative disorders are illnesses characterized by a loss of

nervous system functioning that are usually caused by neuronal
death. These diseases generally worsen over time as more and more
neurons die. The symptoms of a particular neurodegenerative
disease are related to where in the nervous system the death of Figure 35.15.1: Alzheimer’s disease: Compared to a normal brain
(left), the brain from a patient with Alzheimer’s disease (right)
neurons occurs. Spinocerebellar ataxia, for example, leads to shows a dramatic neurodegeneration, particularly within the
neuronal death in the cerebellum. The death of these neurons causes ventricles and hippocampus.
problems in balance and walking. Neurodegenerative disorders One form of the disease is usually caused by mutations in one of
include Huntington’s disease, amyotrophic lateral sclerosis (ALS), three known genes. This rare form of early-onset Alzheimer’s
Alzheimer’s disease, other dementia disorders, and Parkinson’s disease affects fewer than five percent of patients with the disease
disease. In this section, Alzheimer’s and Parkinson’s disease will be and causes dementia beginning between the ages of 30 and 60. The
discussed in more depth. more prevalent, late-onset form of the disease probably also has a
genetic component. One particular gene, apolipoprotein E (APOE)
ALZHEIMER’S DISEASE has a variant (E4) that increases a carrier ‘s probability of
Alzheimer’s disease is the most common cause of dementia in the developing the disease. Many other genes have been identified that
elderly. In 2012, an estimated 5.4 million Americans suffered from may be involved in the pathology.
Alzheimer’s disease. Payments for their care are estimated at $200 Unfortunately, there is no cure for Alzheimer’s disease. Current
billion. Roughly one in every eight people age 65 or older has the treatments focus on managing the symptoms of the disease. Because
disease. Due to the aging of the baby-boomer generation, there are decrease in the activity of cholinergic neurons (neurons that use the
projected to be as many as 13 million Alzheimer’s patients in the neurotransmitter acetylcholine ) is common in Alzheimer’s disease,
United States in the year 2050. several drugs used to treat the disease work by increasing
Symptoms of Alzheimer’s disease include disruptive memory loss, acetylcholine neurotransmission, often by inhibiting the enzyme that
confusion about time or place, difficulty planning or executing tasks, breaks down acetylcholine in the synaptic cleft. Other clinical
poor judgement, and personality changes. Problems smelling certain interventions focus on behavioral therapies such as psychotherapy,
scents can also be indicative of Alzheimer’s disease and may serve sensory therapy, and cognitive exercises. Since Alzheimer’s disease
as an early warning sign. Many of these symptoms are also common appears to hijack the normal aging process, research into prevention
in people who are aging normally, so it is the severity and longevity is prevalent.
of the symptoms that determine whether a person is suffering from
Alzheimer’s. PARKINSON’S DISEASE
Alzheimer’s disease was named for Alois Alzheimer, a German Parkinson’s disease is also a neurodegenerative disease. It was first
psychiatrist who published a report in 1911 about a woman who characterized by James Parkinson in 1817. Each year, 50,000-60,000
showed severe dementia symptoms. Along with his colleagues, he people in the United States are diagnosed with the disease.
examined the woman’s brain following her death and reported the Parkinson’s disease causes the loss of dopamine neurons in the
presence of abnormal clumps, which are now called amyloid substantia nigra, a midbrain structure that regulates movement. Loss
plaques, along with tangled brain fibers called neurofibrillary of these neurons causes many symptoms including tremor (shaking
tangles. Amyloid plaques, neurofibrillary tangles, and an overall of fingers or a limb), slowed movement, speech changes, balance
shrinking of brain volume are commonly seen in the brains of and posture problems, and rigid muscles. The combination of these
Alzheimer’s patients. Loss of neurons in the hippocampus is symptoms often causes a characteristic slow, hunched, shuffling
especially severe in advanced Alzheimer’s patients. Many research walk. Patients with Parkinson’s disease can also exhibit
groups are examining the causes of these hallmarks of the disease. psychological symptoms, such as dementia or emotional problems.

35.15.1 https://bio.libretexts.org/@go/page/13885
 KEY POINTS
Neural death is the main cause behind neurodegenerative
disorders.
Symptoms of neurodegenerative disorders usually depend on the
area within the nervous system where neuron deaths take place.
Alzheimer’s disease, characterized by severe dementia, can
appear in the form of disruptive memory loss, confusion,
difficulty planning or executing tasks, poor judgement, and
personality changes.
A decrease in the activity of cholinergic neurons is commonly
seen in patients with Alzheimer’s disease.
In Parkinson’s disease, the loss of dopamine neurons results in
symptoms that include tremors, slowed movement, speech
Figure 35.15.1: Parkinson’s disease: Parkinson’s patients often have changes, balance and posture problems, and rigid muscles.
a characteristic hunched walk. The disease is likely the result of a Neither Alzheimer’s nor Parkinson’s disease have cures, but
combination of genetic and environmental factors.
there are drug treatments available to control symptoms.
Although some patients have a form of the disease known to be
caused by a single mutation, for most patients, the exact causes of KEY TERMS
Parkinson’s disease remain unknown. The disease probably results neurodegenerative: of, pertaining to, or resulting in the
from a combination of genetic and environmental factors, similar to progressive loss of nerve cells and of neurologic function
Alzheimer’s disease. Post-mortem analysis of brains from dementia: a progressive decline in cognitive function due to
Parkinson’s patients shows the presence of Lewy bodies, abnormal damage or disease in the brain beyond what might be expected
protein clumps, in dopaminergic neurons. The prevalence of these from normal aging
Lewy bodies often correlates with the severity of the disease. Parkinson’s disease: a degenerative disorder of the central
There is no cure for Parkinson’s disease; treatment is focused on nervous system
easing symptoms. One of the most-commonly prescribed drugs for Alzheimer’s disease: a disorder involving loss of mental
Parkinson’s is L-DOPA, which is a chemical that is converted into functions resulting from brain tissue changes; senile dementia
dopamine by neurons in the brain. This conversion increases the
overall level of dopamine neurotransmission and can help This page titled 35.15: Neurodegenerative Disorders - Introduction is shared
compensate for the loss of dopaminergic neurons in the substantia under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
nigra. Other drugs work by inhibiting the enzyme that breaks down by Boundless.
dopamine.

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35.16: NERVOUS SYSTEM DISORDERS - NEURODEVELOPMENTAL
DISORDERS - AUTISM AND ADHD
ATTENTION DEFICIT HYPERACTIVITY DISORDER
 LEARNING OBJECTIVES (ADHD)
Approximately three to five percent of children and adults are
Distinguish between the neurodevelopmental disorders of
affected by attention deficit/hyperactivity disorder (ADHD). Like
autism and ADHD
ASD, ADHD is more prevalent in males than females. Symptoms of
the disorder include inattention (lack of focus), executive
Neurodevelopmental disorders occur when the development of the
functioning difficulties, impulsivity, and hyperactivity beyond what
nervous system is disturbed. There are several different classes of
is characteristic of the normal developmental stage. Some patients
neurodevelopmental disorders. Some, like Down Syndrome, cause
do not have the hyperactive component of symptoms and are
intellectual deficits, while others specifically affect communication,
diagnosed with a subtype of ADHD: attention deficit disorder
learning, or the motor system. Some disorders, such as autism
(ADD). Many people with ADHD also show comorbidity: they
spectrum disorder and attention deficit/hyperactivity disorder, have
develop secondary disorders in addition to ADHD. Examples
complex symptoms.
include depression or obsessive compulsive disorder (OCD).
AUTISM
Autism spectrum disorder (ASD, sometimes just “autism”) is a
neurodevelopmental disorder in which severity differs from person
to person. Estimates for the prevalence of the disorder have changed
rapidly in the past few decades. Current estimates suggest that one in
88 children will develop the disorder. ASD is four times more
prevalent in males than females.
A characteristic symptom of ASD is impaired social skills. Children Figure 35.16.1: Comorbidity with ADHD: Many people with
with autism may have difficulty making and maintaining eye contact ADHD have one or more other psychological or neurological
disorders.
and reading social cues. They also may have problems feeling
empathy for others. Other symptoms of ASD include repetitive The cause of ADHD is unknown, although research points to a delay
motor behaviors (such as rocking back and forth), preoccupation and dysfunction in the development of the prefrontal cortex and
with specific subjects, strict adherence to certain rituals, and unusual disturbances in neurotransmission. According to some twin studies,
language use. Up to 30 percent of patients with ASD develop the disorder has a strong genetic component. There are several
epilepsy. Patients with some forms of the disorder (e.g., Fragile X candidate genes that may contribute to the disorder, but no definitive
syndrome) also have intellectual disability. Because it is a spectrum links have been discovered. Environmental factors, including
disorder, other ASD patients are very functional and have good-to- exposure to certain pesticides, may also contribute to the
excellent language skills. Many of these patients do not feel that development of ADHD in some patients. Treatment for ADHD often
they suffer from a disorder and instead just believe that they process involves behavioral therapies and the prescription of stimulant
information differently. medications, which, paradoxically, cause a calming effect in these
patients.
Except for some well-characterized, clearly-genetic forms of autism
(e.g., Fragile X and Rett Syndrome), the causes of ASD are largely KEY POINTS
unknown. Variants of several genes correlate with the presence of Disturbances in the development of the nervous system, genetic
ASD, but for any given patient, many different mutations in different
or environmental, may lead to neurodevelopmental diseases.
genes may be required for the disease to develop. At a general level, Individuals affected by autism are believed to have one of many
ASD is thought to be a disease of “incorrect” wiring. Accordingly, different mutations in genes required for the disease to cause
brains of some ASD patients lack the same level of synaptic pruning
disruptions in the nervous system that are generally observed;
that occurs in non-affected people. There has been some however, studies on specifics are still inconclusive.
unsubstantiated controversy linking vaccinations and autism. In the
In ADHD, a strong genetic component may contribute to the
1990s, a research paper linked autism to a common vaccine given to disorder; however, no definitive links have been found.
children. This paper was retracted when it was discovered that the
Individuals with ADHD may experience other psychological or
author falsified data; follow-up studies showed no connection neurological disorders in addition to their ADHD symptoms; this
between vaccines and autism.
experience of having more than one disorder is termed
Treatment for autism usually combines behavioral therapies and comorbidity.
interventions, along with medications to treat other disorders The cause of both autism and ADHD are unknown and cures are
common to people with autism ( depression, anxiety, obsessive unavailable; however, treatments to alleviate symptoms are
compulsive disorder). Although early interventions can help mitigate accessible.
the effects of the disease, there is currently no cure for ASD.

35.16.1 https://bio.libretexts.org/@go/page/13886
KEY TERMS rett syndrome: a neurodevelopmental disorder of the grey
autism: disorder observed in early childhood with symptoms of matter of the brain that almost exclusively affects females, but
abnormal self-absorption, characterised by lack of response to has also been found in male patients
other humans and a limited ability or disinclination to comorbidity: the presence of one or more disorders (or diseases)
communicate and socialize in addition to a primary disease or disorder
attention deficit hyperactivity disorder: a developmental neurodevelopmental disorder: a disorder of brain function that
disorder in which a person has a persistent pattern of affects emotion, learning ability and memoryand that unfolds as
impulsiveness and inattention, with or without a component of the individual grows
hyperactivity
This page titled 35.16: Nervous System Disorders - Neurodevelopmental
fragile X syndrome: a particular, genetic syndrome, caused by
Disorders - Autism and ADHD is shared under a CC BY-SA 4.0 license and
the excessive repetition of a particular trinucleotide was authored, remixed, and/or curated by Boundless.

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35.17: NERVOUS SYSTEM DISORDERS - NEURODEVELOPMENTAL
DISORDERS - MENTAL ILLNESSES

 LEARNING OBJECTIVES

Distinguish between the disorders of schizophrenia and

depression

Mental illnesses are nervous system disorders that result in problems

with thinking, mood, or relating with other people. These disorders
are severe enough to affect a person’s quality of life and often make
it difficult for people to perform the routine tasks of daily living.
Debilitating mental disorders plague approximately 12.5 million
Americans (about 1 in 17 people) at an annual cost of more than
$300 billion. There are several types of mental disorders including
schizophrenia, major depression, bipolar disorder, anxiety disorders,
post-traumatic stress disorder, and many others. The American
Psychiatric Association publishes the Diagnostic and Statistical
Manual of Mental Disorders (or DSM), which describes the Figure 35.17.1: Schizophrenia: The development of schizophrenia is
symptoms required for a patient to be diagnosed with a particular thought to be caused by malfunctioning dopaminergic neurons,
which causes brain dysfunction and an imbalance of chemicals in
mental disorder. Each newly-released version of the DSM contains the brain.
different symptoms and classifications as researchers learn more
about these disorders, their causes, and how they relate to each other. DEPRESSION
A more detailed discussion of two mental illnesses, schizophrenia
Major depression (also referred to as just “depression” or “major
and major depression, is given below.
depressive disorder”) affects approximately 6.7 percent of the adults
SCHIZOPHRENIA in the United States each year and is one of the most common
Schizophrenia is a serious and often-debilitating mental illness mental disorders. To be diagnosed with major depressive disorder, a
affecting one percent of the population in the United States. person must have experienced a severely-depressed mood lasting
Symptoms of the disease include the inability to differentiate longer than two weeks along with other symptoms that may include
between reality and imagination, inappropriate and unregulated a loss of enjoyment in activities that were previously enjoyed,
emotional responses, difficulty thinking, and problems with social changes in appetite and sleep schedules, difficulty concentrating,
situations. Symptoms of schizophrenia may be characterized as feelings of worthlessness, and suicidal thoughts. The exact causes of
either “negative” (deficit symptoms) or “positive”. Positive major depression are unknown and probably include both genetic
symptoms are those that most individuals do not normally and environmental risk factors. Some research supports the “classic
experience, but are present in people with schizophrenia. They can monoamine hypothesis,” which suggests that depression is caused
include delusions, disordered thoughts and speech, and tactile, by a decrease in norepinephrine and serotonin neurotransmission.
auditory, visual, olfactory and gustatory hallucinations, typically One argument against this hypothesis is the fact that some
regarded as manifestations of psychosis. Negative symptoms are antidepressant medications cause an increase in norepinephrine and
deficits of normal emotional responses or of other thought processes, serotonin release within a few hours of beginning treatment, but
and commonly include flat or blunted affect and emotion, poverty of clinical results of these medications are not seen until weeks later.
speech, inability to experience pleasure, lack of desire to form This has led to alternative hypotheses. For example, dopamine may
also be decreased in depressed patients, or it may actually be an
relationships, and lack of motivation.
increase in norepinephrine and serotonin that causes the disease, and
Many schizophrenic patients are diagnosed in their late adolescence antidepressants force a feedback loop that decreases this release.
or early 20s. The development of schizophrenia is thought to involve
Treatments for depression include psychotherapy, electroconvulsive
malfunctioning dopaminergic neurons and may also involve
therapy, deep-brain stimulation, and prescription medications. Most
problems with glutamate signaling. Treatment for the disease usually
commonly, individuals undergo some combination of psychotherapy
requires anti-psychotic medications that work by blocking dopamine
and medication. There are several classes of antidepressant
receptors and decreasing dopamine neurotransmission in the brain.
medications that work through different mechanisms. For example,
This decrease in dopamine can cause Parkinson’s disease-like
monoamine oxidase inhibitors (MAO inhibitors) block the enzyme
symptoms in some patients. While some classes of anti-psychotics
that degrades many neurotransmitters (including dopamine,
can be quite effective at treating the disease, they are not a cure;
serotonin, norepinephrine), resulting in increased neurotransmitter in
most patients must remain medicated for the rest of their lives.
the synaptic cleft. Selective serotonin reuptake inhibitors (SSRIs)
block the reuptake of serotonin into the presynaptic neuron. This

35.17.1 https://bio.libretexts.org/@go/page/13887
blockage results in an increase in serotonin in the synaptic cleft. KEY TERMS
Other types of drugs, such as norepinephrine-dopamine reuptake norepinephrine: a neurotransmitter found in the locus coeruleus
inhibitors and norepinephrine-serotonin reuptake inhibitors, are also which is synthesized from dopamine
used to treat depression. serotonin: an indoleamine neurotransmitter that is involved in
depression, appetite, etc., and is crucial in maintaining a sense of
KEY POINTS
well-being, security, etc.
Complications with thinking, mood, or problems relating to other mental disorder: any of the various diseases affecting the mind
people are issues that are commonly associated with those onset by brain damage or genetics
affected with neurodevelopmental disorders. schizophrenia: a psychiatric diagnosis denoting a persistent,
Malfunctioning dopaminergic neurons and problems with often chronic, mental illness variously affecting behavior,
glutamate signaling are thought to be potential causes of thinking, and emotion
schizophrenia. depression: in psychotherapy and psychiatry, a period of
Although no definitive answer yet exists, genetic and unhappiness or low morale which lasts longer than several weeks
environmental risk factors are believed to be the main causes of and may include ideation of self-inflicted injury or suicide
depression. dopamine: a neurotransmitter associated with movement,
Exact cures do not exist for either schizophrenia or depression; attention, learning, and the brain’s pleasure and reward system
however, schizophrenia may be treated with anti-psychotic
medications while depression treatments include psychotherapy, This page titled 35.17: Nervous System Disorders - Neurodevelopmental
electroconvulsive therapy, deep-brain stimulation, and Disorders - Mental Illnesses is shared under a CC BY-SA 4.0 license and
prescription medications. was authored, remixed, and/or curated by Boundless.

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35.18: NERVOUS SYSTEM DISORDERS - OTHER NEUROLOGICAL DISORDERS

 LEARNING OBJECTIVES

Distinguish between the neurological disorders of epilepsy

and stroke

There are several other neurological disorders that cannot be easily

placed into clean-cut categories. These include chronic pain
conditions, cancers of the nervous system, epilepsy disorders, and
stroke. Epilepsy and stroke are discussed below.

EPILEPSY
Estimates suggest that up to three percent of people in the United
States will be diagnosed with epilepsy in their lifetime. While there
are several different types of epilepsy, all are characterized by
recurrent seizures. Epilepsy itself can be a symptom of a brain
injury, disease, or other illness. For example, people who have Figure 35.18.1: Stroke effects on the brain: A cerebral infarction,
intellectual disability or autism spectrum disorder can experience shaded in blue, occurs after a stroke when blood fails to reach a
portion of the brain long enough to cause damage. The red arrow
seizures, presumably because the developmental wiring depicts the midline shift that occurs in the brain, which is also
malfunctions that caused their disorders also put them at risk for caused by a stroke.
epilepsy. For many patients, however, the cause of their epilepsy is
never identified and is probably a combination of genetic and KEY POINTS
environmental factors. Often, seizures can be controlled with anti- Although all types of epilepsy are characterized by recurrent
convulsant medications. However, for very severe cases, patients seizures, the disorder itself can be a symptom of various factors,
may undergo brain surgery to remove the brain area where seizures both genetic and environmental; the specific causes of epilepsy
originate. remain to be identified.
Neural death, caused by a lack of oxygen for a prolonged period
STROKE of time, is the main cause of stroke.
A stroke results when blood fails to reach a portion of the brain for a Anti-convulsant medications and brain removal surgery are
long enough time to cause damage. Without the oxygen supplied by treatments for epilepsy, while anti-clotting medication and
blood flow, neurons in this brain region die. This neuronal death can physical therapy are used in the treatment of stroke.
cause many different symptoms, depending on the brain area Anti-convulsant medications and brain removal surgery are
affected, including headache, muscle weakness or paralysis, speech treatments for epilepsy while anti-clotting medication and
disturbances, sensory problems, memory loss, and confusion. Stroke physical therapy are used in the treatment of stroke.
is often caused by blood clots, but can also be caused by the bursting
of a weak blood vessel. Strokes are extremely common; they are the KEY TERMS
third most-common cause of death in the United States. On average epilepsy: a medical condition in which the sufferer experiences
one person experiences a stroke every 40 seconds in the United seizures (or convulsions) and blackouts
States. Approximately 75 percent of strokes occur in people older stroke: the loss of brain function arising when the blood supply
than 65. Risk factors for stroke include high blood pressure, to the brain is suddenly interrupted
diabetes, high cholesterol, and a family history of stroke. Smoking
doubles the risk of stroke. Treatment following a stroke can include
CONTRIBUTIONS AND ATTRIBUTIONS
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(sometimes intense) physical therapy. BY: Attribution
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 CHAPTER OVERVIEW

36: SENSORY SYSTEMS

36.1: Sensory Processes - Reception
36.2: Sensory Processes - Transduction and Perception
36.3: Somatosensation - Somatosensory Receptors
36.4: Somatosensation - Integration of Signals from Mechanoreceptors
36.5: Somatosensation - Thermoreception
36.6: Taste and Smell - Tastes and Odors
36.7: Taste and Smell - Reception and Transduction
36.8: Hearing and Vestibular Sensation - Sound
36.9: Hearing and Vestibular Sensation - Reception of Sound
36.10: Hearing and Vestibular Sensation - The Vestibular System
36.11: Hearing and Vestibular Sensation - Balance and Determining Equilibrium
36.12: Vision - Light
36.13: Vision - Anatomy of the Eye
36.14: Vision - Transduction of Light
36.15: Vision - Visual Processing

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1
36.1: SENSORY PROCESSES - RECEPTION
RECEPTION
 LEARNING OBJECTIVES The first step in sensation is reception: the activation of sensory
receptors by stimuli such as mechanical stimuli (being bent or
Explain the process of sensory reception
squished, for example), chemicals, or temperature. The receptor can
then respond to the stimuli. The region in space in which a given
INTRODUCTION TO SENSATION
sensory receptor can respond to a stimulus, be it far away or in
In more advanced animals, the senses are constantly at work, contact with the body, is that receptor’s receptive field. Think for a
making the animal aware of stimuli, such as light or sound or the moment about the differences in receptive fields for the different
presence of a chemical substance in the external environment, while senses. For the sense of touch, a stimulus must come into contact
monitoring information about the organism’s internal environment. with body. For the sense of hearing, a stimulus can be a moderate
All bilaterally symmetric animals have a sensory system. The distance away. For vision, a stimulus can be very far away; for
development of any species ‘ sensory system has been driven by example, the visual system perceives light from stars at enormous
natural selection; thus, sensory systems differ among species distances.
according to the demands of their environments. For example, the
shark, unlike most fish predators, is electrosensitive (i.e., sensitive to
electrical fields produced by other animals in its environment).
While it is helpful to this underwater predator, electrosensitivity is a
sense not found in most land animals.
Senses provide information about the body and its environment.
Humans have five special senses: olfaction (smell), gustation (taste),
equilibrium (balance and body position), vision, and hearing.
Additionally, we possess general senses, also called
somatosensation, which respond to stimuli like temperature, pain,
pressure, and vibration. Vestibular sensation, which is an organism’s
sense of spatial orientation and balance, proprioception (position of
bones, joints, and muscles), and the sense of limb position that is
used to track kinesthesia (limb movement) are part of
somatosensation. Although the sensory systems associated with
these senses are very different, all share a common function: to
Figure 36.1.1: Visual sensory system: This scheme shows the flow
convert a stimulus (light, sound, or the position of the body) into an of information from the eyes to the central connections of the optic
electrical signal in the nervous system. This process is called sensory nerves and optic tracts, to the visual cortex. Area V1 is the region of
transduction. the brain which is engaged in vision.

There are two broad types of cellular systems that perform sensory KEY POINTS
transduction. In one, a neuron works with a sensory receptor, a cell, Reception is the process of activating a sensory receptor by a
or cell process that is specialized to engage with and detect a stimuli.
specific stimulus. Stimulation of the sensory receptor activates the Sensory transduction is the process of converting that sensory
associated afferent neuron, which carries information about the signal to an electrical signal in the sensory neuron.
stimulus to the central nervous system. In the second type of sensory The process of reception is dependent on the stimuli itself, the
transduction, a sensory nerve ending responds to a stimulus in the type of receptor, receptor specificity, and the receptive field,
internal or external environment; this neuron constitutes the sensory which can vary depending on the receptor type.
receptor. Free nerve endings can be stimulated by several different
stimuli, thus showing little receptor specificity. For example, pain KEY TERMS
receptors in your gums and teeth may be stimulated by temperature somatosensation: general senses which respond to stimuli like
changes, chemical stimulation, or pressure. temperature, pain, pressure, and vibration
reception: the act or ability to receive signals from stimuli

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BY-SA 4.0 license and was authored, remixed, and/or curated by Boundless.

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36.2: SENSORY PROCESSES - TRANSDUCTION AND PERCEPTION
Sensory receptors for the various senses work differently from each
 LEARNING OBJECTIVES other. They are specialized according to the type of stimulus they
sense; thus, they have receptor specificity. For example, touch
Explain how stimuli are converted to signals that are carried
receptors, light receptors, and sound receptors are each activated by
to the central nervous system
different stimuli. Touch receptors are not sensitive to light or sound;
they are sensitive only to touch or pressure. However, stimuli may
TRANSDUCTION be combined at higher levels in the brain, as happens with olfaction,
The most fundamental function of a sensory system is the translation contributing to our sense of taste.
of a sensory signal to an electrical signal in the nervous system. This
takes place at the sensory receptor. The change in electrical potential ENCODING AND TRANSMISSION OF SENSORY
that is produced is called the receptor potential. How is sensory INFORMATION
input, such as pressure on the skin, changed to a receptor potential? Four aspects of sensory information are encoded by sensory
As an example, a type of receptor called a mechanoreceptor systems: the type of stimulus, the location of the stimulus in the
possesses specialized membranes that respond to pressure. receptive field, the duration of the stimulus, and the relative intensity
Disturbance of these dendrites by compressing them or bending of the stimulus. Thus, action potentials transmitted over a sensory
them opens gated ion channels in the plasma membrane of the receptor’s afferent axons encode one type of stimulus. This
sensory neuron, changing its electrical potential. In the nervous segregation of the senses is preserved in other sensory circuits. For
system, a positive change of a neuron’s electrical potential (also example, auditory receptors transmit signals over their own
called the membrane potential), depolarizes the neuron. Receptor dedicated system. The electrical activity in the axons of the auditory
potentials are graded potentials: the magnitude of these graded receptors will be interpreted by the brain as an auditory stimulus: a
(receptor) potentials varies with the strength of the stimulus. If the sound.
magnitude of depolarization is sufficient (that is, if membrane The intensity of a stimulus is often encoded in the rate of action
potential reaches a threshold), the neuron will fire an action potentials produced by the sensory receptor. Thus, an intense
potential. In most cases, the correct stimulus impinging on a sensory stimulus will produce a more rapid train of action potentials.
receptor will drive membrane potential in a positive direction, Reducing the stimulus will likewise slow the rate of production of
although for some receptors, such as those in the visual system, this action potentials. A second way in which intensity is encoded is by
is not always the case. the number of receptors activated. An intense stimulus might initiate
action potentials in a large number of adjacent receptors, while a less
intense stimulus might stimulate fewer receptors. Integration of
sensory information begins as soon as the information is received in
the central nervous system.

PERCEPTION
Perception is an individual’s interpretation of a sensation. Although
perception relies on the activation of sensory receptors, perception
happens, not at the level of the sensory receptor, but at the brain
level. The brain distinguishes sensory stimuli through a sensory
pathway: action potentials from sensory receptors travel along
neurons that are dedicated to a particular stimulus.
All sensory signals, except those from the olfactory system, are
transmitted though the central nervous system: they are routed to the
thalamus and to the appropriate region of the cortex. The thalamus is
a structure in the forebrain that serves as a clearinghouse and relay
station for sensory (as well as motor) signals. When the sensory
signal exits the thalamus, it is conducted to the specific area of the
Figure 36.2.1: Mechanoreceptor activation: (a) Mechanosensitive cortex dedicated to processing that particular sense.
ion channels are gated ion channels that respond to mechanical
deformation of the plasma membrane. A mechanosensitive channel
is connected to the plasma membrane and the cytoskeleton by hair-
like tethers. When pressure causes the extracellular matrix to move,
the channel opens, allowing ions to enter or exit the cell. (b)
Stereocilia in the human ear are connected to mechanosensitive ion
channels. When a sound causes the stereocilia to move,
mechanosensitive ion channels transduce the signal to the cochlear
nerve.

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 action potential: a short term change in the electrical potential
that travels along a cell
transduction: the translation of a sensory signal in the sensory
system to an electrical signal in the nervous system

CONTRIBUTIONS AND ATTRIBUTIONS

reception. Provided by: Wiktionary. Located at:
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Located at: http://cnx.org/content/m44754/latest...ol11448/latest. License: CC
Figure 36.2.1: Sensation processing: The brain has dedicated areas BY: Attribution
OpenStax College, Biology. October 23, 2013. Provided by: OpenStax CNX.
to the processing of stimuli, including: (a) thalamus and (b) the
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auditory, visual and somatosensory processing regions. BY: Attribution
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KEY POINTS en.wiktionary.org/wiki/somatosensation. License: CC BY-SA: Attribution-
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stimulus. License: CC BY: Attribution

KEY TERMS This page titled 36.2: Sensory Processes - Transduction and Perception is
membrane potential: the difference in electrical potential across shared under a CC BY-SA 4.0 license and was authored, remixed, and/or
the enclosing membrane of a cell curated by Boundless.

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36.3: SOMATOSENSATION - SOMATOSENSORY RECEPTORS
borders. That makes them very sensitive to edges; they come into
 LEARNING OBJECTIVES use in tasks such as typing on a keyboard.
Meissner’s corpuscles, also known as tactile corpuscles, are found in
Describe the structure and function of mechanoreceptors
the upper dermis, but they project into the epidermis. They are found
primarily in the glabrous skin on the fingertips and eyelids. They
SOMATOSENSORY RECEPTORS
respond to fine touch and pressure, but they also respond to low-
Sensory receptors are classified into five categories: frequency vibration or flutter. They are rapidly- adapting, fluid-
mechanoreceptors, thermoreceptors, proprioceptors, pain receptors, filled, encapsulated neurons with small, well-defined borders which
and chemoreceptors. These categories are based on the nature of the are responsive to fine details. Merkel’s disks and Meissner’s
stimuli that each receptor class transduces. Mechanoreceptors in the corpuscles are not as plentiful in the palms as they are in the
skin are described as encapsulated or unencapsulated. A free nerve fingertips.
ending is an unencapsulated dendrite of a sensory neuron; they are
the most common nerve endings in skin. Free nerve endings are
sensitive to painful stimuli, to hot and cold, and to light touch. They
are slow to adjust to a stimulus and so are less sensitive to abrupt
changes in stimulation.

MECHANORECEPTORS
There are three classes of mechanoreceptors: tactile, proprioceptors,
and baroreceptors. Mechanoreceptors sense stimuli due to physical
deformation of their plasma membranes. They contain
mechanically-gated ion channels whose gates open or close in Figure 36.3.1: Meissner corpuscles: Meissner corpuscles in the
response to pressure, touch, stretching, and sound. There are four fingertips, such as the one viewed here using bright field light
primary tactile mechanoreceptors in human skin: Merkel’s disks, microscopy, allow for touch discrimination of fine detail.
Meissner’s corpuscles, Ruffini endings, and Pacinian corpuscle; two Deeper in the dermis, near the base, are Ruffini endings, which are
are located toward the surface of the skin and two are located deeper. also known as bulbous corpuscles. They are found in both glabrous
A fifth type of mechanoreceptor, Krause end bulbs, are found only and hairy skin. These are slow-adapting, encapsulated
in specialized regions. mechanoreceptors that detect skin stretch and deformations within
joints; they provide valuable feedback for gripping objects and
controlling finger position and movement. Thus, they also contribute
to proprioception and kinesthesia. Ruffini endings also detect
warmth. Note that these warmth detectors are situated deeper in the
skin than are the cold detectors. It is not surprising, then, that
humans detect cold stimuli before they detect warm stimuli.
Pacinian corpuscles, located deep in the dermis of both glabrous and
hairy skin, are structurally similar to Meissner’s corpuscles. They
are found in the bone periosteum, joint capsules, pancreas and other
viscera, breast, and genitals. They are rapidly-adapting
Figure 36.3.1: Primary mechanoreceptors: Four of the primary mechanoreceptors that sense deep, transient (not prolonged)
mechanoreceptors in human skin are shown. Merkel’s disks, which
are unencapsulated, respond to light touch. Meissner’s corpuscles, pressure, and high-frequency vibration. Pacinian receptors detect
Ruffini endings, Pacinian corpuscles, and Krause end bulbs are all pressure and vibration by being compressed which stimulates their
encapsulated. Meissner’s corpuscles respond to touch and low- internal dendrites. There are fewer Pacinian corpuscles and Ruffini
frequency vibration. Ruffini endings detect stretch, deformation
within joints, and warmth. Pacinian corpuscles detect transient endings in skin than there are Merkel’s disks and Meissner’s
pressure and high-frequency vibration. Krause end bulbs detect cold. corpuscles.
Merkel’s disks are found in the upper layers of skin near the base of
the epidermis, both in skin that has hair and on glabrous skin; that is,
the hairless skin found on the palms and fingers, the soles of the feet,
and the lips of humans and other primates. Merkel’s disks are
densely distributed in the fingertips and lips. They are slow-
adapting, unencapsulated nerve endings, which respond to light
touch. Light touch, also known as discriminative touch, is a light
pressure that allows the location of a stimulus to be pinpointed. The
receptive fields of Merkel’s disks are small, with well-defined

36.3.1 https://bio.libretexts.org/@go/page/13951
 skin that has hair or is glabrous.
Meissner’s corpuscles are rapidly-adapting, encapsulated
neurons that responds to low-frequency vibrations and fine
touch; they are located in the glabrous skin on fingertips and
eyelids.
Ruffini endings are slow adapting, encapsulated receptors that
respond to skin stretch and are present in both the glabrous and
hairy skin.
-Pacinian corpuscles are rapidly-adapting, deep receptors that
respond to deep pressure and high-frequency vibration.

Figure 36.3.1: Pacinian corpuscles: Pacinian corpuscles, such as KEY TERMS

these visualized using bright field light microscopy, detect pressure
(touch) and high-frequency vibration. dendrite: branched projections of a neuron that conduct the
impulses received from other neural cells to the cell body
KEY POINTS glabrous: smooth, hairless, bald
The four major types of tactile mechanoreceptors include:
This page titled 36.3: Somatosensation - Somatosensory Receptors is shared
Merkel’s disks, Meissner’s corpuscles, Ruffini endings, and
under a CC BY-SA 4.0 license and was authored, remixed, and/or curated
Pacinian corpuscles.
by Boundless.
Merkel’s disk are slow-adapting, unencapsulated nerve endings
that respond to light touch; they are present in the upper layers of

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36.4: SOMATOSENSATION - INTEGRATION OF SIGNALS FROM
MECHANORECEPTORS
less-precise perception. Touch receptors are denser in glabrous skin
 LEARNING OBJECTIVES (the type found on human fingertips and lips, for example), which is
typically more sensitive and is thicker than hairy skin (4 to 5 mm
Describe how the density of mechanoreceptors affects the
versus 2 to 3 mm). Thus, the fingers, which require the ability to
receptive field
detect fine detail, have many, densely-packed (up to 500 per cubic
cm) mechanoreceptors with small receptive fields (around 10 square
INTEGRATION OF SIGNALS FROM
mm), while the back and legs, for example, have fewer receptors
MECHANORECEPTORS
with large receptive fields. Receptors with large receptive fields
The configuration of the different types of receptors working in
usually have a “hot spot”: an area within the receptive field (usually
concert in the human skin results in a very refined sense of touch.
in the center, directly over the receptor) where stimulation produces
The nociceptive receptors (those that detect pain) are located near
the most intense response. Tactile-sense-related cortical neurons
the surface. Small, finely-calibrated mechanoreceptors (Merkel’s
have receptive fields on the skin that can be modified by experience
disks and Meissner’s corpuscles) are located in the upper layers and
or by injury to sensory nerves, resulting in changes in the field’s size
can precisely localize even gentle touch. The large
and position. In general, these neurons have relatively large
mechanoreceptors (Pacinian corpuscles and Ruffini endings) are
receptive fields (much larger than those of dorsal root ganglion
located in the lower layers and respond to deeper touch. Consider
cells). However, the neurons are able to discriminate fine detail due
that the deep pressure that reaches those deeper receptors would not
to patterns of excitation and inhibition relative to the field, which
need to be finely localized. Both the upper and lower layers of the
leads to spatial resolution.
skin hold rapidly- and slowly-adapting receptors. Both primary
somatosensory cortex and secondary cortical areas are responsible The relative density of pressure receptors in different locations on
for processing the complex picture of stimuli transmitted from the the body can be demonstrated experimentally using a two-point
interplay of mechanoreceptors. discrimination test. In this demonstration, two sharp points, such as
two thumbtacks, are brought into contact with the subject’s skin
(though not hard enough to cause pain or break the skin). The
subject reports if they feel one point or two points. If the two points
are felt as one point, it can be inferred that the two points are both in
the receptive field of a single sensory receptor. If two points are felt
as two separate points, each is in the receptive field of two separate
sensory receptors. The points could then be moved closer and re-
tested until the subject reports feeling only one point. The size of the
receptive field of a single receptor could be estimated from that
distance.

KEY POINTS
The various types of receptors, nociceptors, mechanoreceptors
(both small and large), thermoreceptors, chemoreceptors, and
proprioreceptors, work together to ensure that complex stimuli
are transmitted properly to the brain for processing.
The distribution of mechanoreceptors within the body can affect
how stimuli are perceived; this is dependent on the size of the
receptive field and whether single or multiple sensory receptors
are activated.
A large receptive field allows for detection of stimuli over a wide
Figure 36.4.1: Sensory receptor structure: Structure of four different area, but can result in less precise detection; a small receptive
types of sensory receptors found within the sensory system. field allows for detection of stimuli over a small area, which
DENSITY OF MECHANORECEPTORS results in more precise detection.
In the somatosensory system, receptive fields are regions of the skin The two-point discrimination test can be used to determine the
density of receptors within various locations by measuring
or of internal organs. During the transmission of sensory information
from these fields, the signals must be conveyed to the nervous whether a two-point stimulus (such as thumb tacks) is detected as
one or two points.
system. The mechanoreceptors are activated, the signal is conveyed,
and then processed. Some types of mechanoreceptors have large
receptive fields, while others have smaller ones. Large receptive
fields allow the cell to detect changes over a wider area, but lead to a

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KEY TERMS
This page titled 36.4: Somatosensation - Integration of Signals from
mechanoreceptor: any receptor that provides an organism with Mechanoreceptors is shared under a CC BY-SA 4.0 license and was
information about mechanical changes in its environment, such authored, remixed, and/or curated by Boundless.
as movement, tension and pressure

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36.5: SOMATOSENSATION - THERMORECEPTION
to sustained pressure and show very little adaptation. Ruffinian
 LEARNING OBJECTIVES endings are located in the deep layers of the skin where they register
mechanical deformation within joints as well as continuous pressure
Describe the various types of receptors used for
states.They also act as thermoreceptors that respond for an extended
thermoreception: Krause end bulbs, Ruffini endings, free
period; in case of deep burn, there will be no pain as these receptors
nerve endings
will be burned off.

THERMORECEPTION
Thermoception or thermoreception is the sense by which an
organism perceives temperatures. The details of how temperature
receptors work are still being investigated. Mammals have at least
two types of sensors: those that detect heat (i.e., temperatures above
body temperature) and those that detect cold (i.e., temperatures
below body temperature). A thermoreceptor is a sensory receptor or,
more accurately, the receptive portion of a sensory neuron that codes Figure 36.5.1: Ruffini endings: A drawing of a Ruffini ending
absolute and relative changes in temperature, primarily within the receptor which can detect warmth.
innocuous range. The adequate stimulus for a warm receptor is In addition to Krause end bulbs that detect cold and Ruffini endings
warming, which results in an increase in their action potential that detect warmth, there are different types of cold receptors on free
discharge rate; cooling results in a decrease in warm receptor nerve endings.
discharge rate. For cold receptors, their firing rate increases during
cooling and decreases during warming. The types of receptors TYPES OF THERMORECEPTORS: FREE NERVE
capable of detecting changes in temperature can vary. ENDINGS
There are thermoreceptors that are located in the dermis, skeletal
TYPES OF THERMORECEPTORS: CAPSULE muscles, liver, and hypothalamus that are activated by different
RECEPTORS temperatures. These thermoreceptors, which have free nerve
Some of the receptors that exhibit the ability to detect changes in endings, include only two types of thermoreceptors that signal
temperature include Krause end bulbs and Ruffini endings. Krause innocuous warmth and cooling respectively in our skin. The warm
end bulbs are defined by cylindrical or oval bodies consisting of a receptors show a maximum sensitivity at ~ 45°C, signal
capsule that is formed by the expansion of the connective-tissue temperatures between 30 and 45°C, and cannot unambiguously
sheath, containing an axis-cylinder core. End-bulbs are found in the signal temperatures higher than 45°C; they are unmyelinated. The
conjunctiva of the eye, in the mucous membrane of the lips and cold receptors have their maximum sensitivity at ~ 27°C, signal
tongue, and in the epineurium of nerve trunks. They are also found temperatures above 17°C, and some consist of lightly-myelinated
in the penis and the clitoris; hence, the name of genital corpuscles. fibers, while others are unmyelinated. Our sense of temperature
In these locations, they have a mulberry-like appearance, being comes from the comparison of the signals from the warm and cold
constricted by connective-tissue septa into two to six knob-like receptors. Thermoreceptors are poor indicators of absolute
masses. temperature, but are very sensitive to changes in skin temperature.

THE THERMORECEPTOR PATHWAY

The thermoreceptor pathway in the brain runs from the spinal cord
through the thalamus to the primary somatosensory cortex. Warmth
and cold information from the face travels through one of the cranial
nerves to the brain. You know from experience that a tolerably cold
or hot stimulus can quickly progress to a much more intense
stimulus that is no longer tolerable. Any stimulus that is too intense
can be perceived as pain because temperature sensations are
conducted along the same pathways that carry pain sensations.

KEY POINTS
Figure 36.5.1: Krause end bulb: A drawing of a Krause end bulb
receptor which can detect cold. Thermoreceptors can include: Krause end bulbs, which detect
cold and are defined by capsules; Ruffini endings, which detect
The Ruffini endings, enlarged dendritic endings with elongated
warmth and are defined by enlarged dendritic endings; and warm
capsules, can act as thermoreceptors. This spindle-shaped receptor is
and cold receptors present on free nerve endings which can
sensitive to skin stretch, contributing to the kinesthetic sense of and
detect a range of temperature.
control of finger position and movement. Ruffini corpuscles respond

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 The cold receptors present on free nerve endings, that can be OpenStax College, Somatosensation. October 17, 2013. Provided by: OpenStax
CNX. Located at: http://cnx.org/content/m44757/latest/Figure_36_02_03.jpg.
either lightly-myelinated or unmyelinated, have a maximum License: CC BY: Attribution
sensitivity at ~ 27°C and will signal temperatures above 17°C. Structure of sensory system (4 models) E. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/File:St..._models)_E.PNG. License: CC BY-SA:
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Located at: http://cnx.org/content/m44757/latest/?collection=col11448/latest.
and will signal temperature above 30°C. License: CC BY: Attribution
Ruffini ending. Provided by: Wikipedia. Located at:
KEY TERMS en.Wikipedia.org/wiki/Ruffini_ending. License: CC BY-SA: Attribution-
ShareAlike
thermoreceptor: a nerve cell that is sensitive to changes in Sensory Systems/Somatosensory System. Provided by: Wikibooks. Located at:
temperature en.wikibooks.org/wiki/Sensory_Systems/Somatosensory_System%23Thermo
receptors. License: CC BY-SA: Attribution-ShareAlike
somatosensory: of or pertaining to the perception of sensory Bulboid corpuscle. Provided by: Wikipedia. Located at:
stimuli produced by the skin or internal organs en.Wikipedia.org/wiki/Bulboid_corpuscle. License: CC BY-SA: Attribution-
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epineurium: the connective tissue framework and sheath of a Thermoception. Provided by: Wikipedia. Located at:
nerve which bind together the nerve bundles, each of which has en.Wikipedia.org/wiki/Thermoception. License: CC BY-SA: Attribution-
ShareAlike
its own special sheath, or perineurium Thermoreceptor. Provided by: Wikipedia. Located at:
en.Wikipedia.org/wiki/Thermoreceptor. License: CC BY-SA: Attribution-
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36.6: TASTE AND SMELL - TASTES AND ODORS

 LEARNING OBJECTIVES

Explain the interaction of taste and odor

TASTES AND ODORS

Both taste and odor stimuli are molecules taken in from the
environment. The primary tastes detected by humans are sweet, sour,
bitter, salty, and umami. The first four tastes need little explanation.
The identification of umami as a fundamental taste occurred fairly
recently. It was identified in 1908 by Japanese scientist Kikunae
Ikeda while he worked with seaweed broth, but it was not widely Figure 36.6.1: Uniform distribution of taste receptors (the myth of
accepted as a taste that could be physiologically distinguished until the tongue map): Humans detect taste using receptors called taste
buds. Each of these receptors is specially adapted to determine one
many years later. The taste of umami, also known as savoriness, is type of taste sensation. Recent evidence suggests that taste receptors
attributable to the taste of the amino acid L-glutamate. In fact, are uniformly distributed across the tongue; thus, this traditional
monosodium glutamate, or MSG, is often used in cooking to tongue map is no longer valid.
enhance the savory taste of certain foods. The adaptive value of The senses of smell and taste combine at the back of the throat.
being able to distinguish umami is that savory substances tend to be When you taste something before you smell it, the smell lingers
high in protein. internally up to the nose causing you to smell it. Both smell and taste
use chemoreceptors, which essentially means they are both sensing
the chemical environment. This chemoreception in regards to taste,
occurs via the presence of specialized taste receptors within the
mouth that are referred to as taste cells and are bundled together to
form taste buds. These taste buds, located in papillae which are
found across the tongue, are specific for the five modalities: salt,
sweet, sour, bitter and umami. These receptors are activated when
their specific stimulus (i.e. sweet or salt molecules) is present and
signals to the brain.
In addition to the activation of the taste receptors, there are similar
receptors within the nose that coordinates with activation of the taste
receptors. When you eat something, you can tell the difference
between sweet and bitter. It is the sense of smell that is used to
distinguish the difference. Although humans commonly distinguish
Figure 36.6.1: Uniform Distribution of Taste Receptors: Humans
detect taste using receptors called taste buds. Each of these receptors taste as one sense and smell as another, they work together to create
is specially adapted to determine one type of taste sensation. Recent the perception of flavor. A person’s perception of flavor is reduced if
evidence suggests that taste receptors are uniformly distributed he or she has congested nasal passages.
across the tongue; thus, the traditional tongue map is no longer valid.
All odors that we perceive are molecules in the air we breathe. If a KEY POINTS
substance does not release molecules into the air from its surface, it Humans can taste sweet, sour, bitter, salty, and umami; umami is
has no smell. If a human or other animal does not have a receptor the savoriness of certain foods that are commonly high in
that recognizes a specific molecule, then that molecule has no smell. protein.
Humans have about 350 olfactory receptor subtypes that work in Odors come from molecules in the air that stimulate receptors in
various combinations to allow us to sense about 10,000 different the nose; if an organism does not have a receptor for that
odors. Compare that to mice, for example, which have about 1,300 particular odor molecule, for that organism, the odor has no
olfactory receptor types and, therefore, probably sense many more smell.
odors. The senses of smell and taste are directly related because they
both use the same types of receptors.
If one’s sense of smell is not functional, then the sense of taste
will also not function because of the relationship of the receptors.

KEY TERMS
umami: one of the five basic tastes, the savory taste of foods
such as seaweed, cured fish, aged cheeses and meats

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olfactory: concerning the sense of smell
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receptor: a protein on a cell wall that binds with specific CC BY-SA 4.0 license and was authored, remixed, and/or curated by
molecules so that they can be absorbed into the cell in order to Boundless.
control certain functions

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36.7: TASTE AND SMELL - RECEPTION AND TRANSDUCTION
TASTE AND SMELL
 LEARNING OBJECTIVES Detecting a taste (gustation) is fairly similar to detecting an odor
(olfaction), given that both taste and smell rely on chemical
Describe the process by which tastes and odors are sensed
receptors being stimulated by certain molecules. The primary organ
of taste is the taste bud. A taste bud is a cluster of gustatory
RECEPTION AND TRANSDUCTION
receptors (taste cells) that are located within the bumps on the
Odorants (odor molecules) enter the nose and dissolve in the tongue called papillae (singular: papilla). There are several
olfactory epithelium, the mucosa at the back of the nasal cavity. The structurally-distinct papillae. Filiform papillae, which are located
olfactory epithelium is a collection of specialized olfactory receptors across the tongue, are tactile, providing friction that helps the tongue
in the back of the nasal cavity that spans an area about 5 cm2 in move substances; they contain no taste cells. In contrast, fungiform
humans. Recall that sensory cells are neurons. An olfactory receptor, papillae, which are located mainly on the anterior two-thirds of the
which is a dendrite of a specialized neuron, responds when it binds tongue, each contain one to eight taste buds; they also have receptors
certain molecules inhaled from the environment by sending impulses for pressure and temperature. The large circumvallate papillae
directly to the olfactory bulb of the brain. Humans have about 12 contain up to 100 taste buds and form a V near the posterior margin
million olfactory receptors distributed among hundreds of different of the tongue.
receptor types that respond to different odors. Twelve million seems
like a large number of receptors, but compare that to other animals:
rabbits have about 100 million, most dogs have about 1 billion, and
bloodhounds (dogs selectively bred for their sense of smell) have
about 4 billion.

Figure 36.7.1: Taste buds: (a) Foliate, circumvallate, and fungiform

papillae are located on different regions of the tongue. (b) Foliate
papillae are prominent protrusions on this light micrograph.
In humans, there are five primary tastes; each taste has only one
corresponding type of receptor. Thus, like olfaction, each receptor is
specific to its stimulus ( tastant ). Transduction of the five tastes
happens through different mechanisms that reflect the molecular
composition of the tastant. A salty tastant (containing NaCl)
provides the sodium ions (Na+) that enter the taste neurons, exciting
them directly. Sour tastants are acids which belong to the
thermoreceptor protein family. Binding of an acid or other sour-
tasting molecule triggers a change in the ion channel which
Figure 36.7.1: Human olfactory system: In the human olfactory increases hydrogen ion (H+) concentrations in the taste neurons;
system, (a) bipolar olfactory neurons extend from (b) the olfactory thus, depolarizing them. Sweet, bitter, and umami tastants require a
epithelium, where olfactory receptors are located, to the olfactory
bulb. G-protein-coupled receptor. These tastants bind to their respective
Olfactory neurons are bipolar neurons (neurons with two processes receptors, thereby exciting the specialized neurons associated with
from the cell body). Each neuron has a single dendrite buried in the them.
olfactory epithelium; extending from this dendrite are 5 to 20 Both tasting abilities and sense of smell change with age. In humans,
receptor-laden, hair-like cilia that trap odorant molecules. The the senses decline dramatically by age 50 and continue to decline. A
sensory receptors on the cilia are proteins. It is the variations in their child may find a food to be too spicy, whereas an elderly person may
amino acid chains that make the receptors sensitive to different find the same food to be bland and unappetizing.
odorants. Each olfactory sensory neuron has only one type of
receptor on its cilia. The receptors are specialized to detect specific KEY POINTS
odorants, so the bipolar neurons themselves are specialized. When Odorants are received by receptors in the nose, which send
an odorant binds with a receptor that recognizes it, the sensory signals to the olfactory bulb of the brain to create an appropriate
neuron associated with the receptor is stimulated. Olfactory response; humans have about 12 million receptors.
stimulation is the only sensory information that directly reaches the Taste results when molecules are dissolved in fluid and reach the
cerebral cortex, whereas other sensations are relayed through the gustatory receptors on the tongue; the signals are sent to the
thalamus. brain to determine which flavor (bitter, sour, sweet, salty, umami
) is being consumed.

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License: CC BY: Attribution
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KEY TERMS License: CC BY-SA: Attribution-ShareAlike
tastant: any substance that stimulates the sense of taste papilla. Provided by: Wiktionary. Located at: en.wiktionary.org/wiki/papilla.
License: CC BY-SA: Attribution-ShareAlike
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36.8: HEARING AND VESTIBULAR SENSATION - SOUND

 LEARNING OBJECTIVES

Describe the relationship of amplitude and frequency of a

sound wave to attributes of sound

SOUND
Auditory stimuli are sound waves, which are mechanical pressure
waves that move through a medium, such as air or water. There are
no sound waves in a vacuum since there are no air molecules for the
waves to move through. The speed of sound waves differs based on
altitude, temperature, and medium. At sea level and a temperature of
20º C (68º F), sound waves travel in the air at about 343 meters per Figure 36.8.1: Amplitude: For sound waves, wavelength
second. corresponds to pitch. The amplitude of the wave corresponds to
volume. The sound wave shown with a dashed line is softer in
As is true for all waves, there are four main characteristics of a volume than the sound wave shown with a solid line.
sound wave: frequency, wavelength, period, and amplitude.
Frequency is the number of waves per unit of time; in sound, it is KEY POINTS
heard as pitch. High-frequency (≥15.000Hz) sounds are higher- Sound waves are mechanical pressure waves that must travel
pitched (short wavelength) than low-frequency (long wavelengths; through a medium and cannot exist in a vacuum.
≤100Hz) sounds. Frequency is measured in cycles per second. For There are four main characteristics of a sound wave: frequency,
sound, the most-commonly used unit is hertz (Hz), or cycles per wavelength, period, and amplitude.
second. Most humans can perceive sounds with frequencies between Frequency is the number of waves per unit of time and is heard
30 and 20,000 Hz. Women are typically better at hearing high as pitch; high-frequency sounds are high-pitched, and low-
frequencies, but everyone’s ability to hear high frequencies frequency sounds are low-pitched.
decreases with age. Dogs detect up to about 40,000 Hz; cats, 60,000 Most humans can perceive sounds with frequencies between 30
Hz; bats, 100,000 Hz; dolphins, 150,000 Hz; and the American shad and 20,000 Hz; other animals, such as dolphins, can detect
(Alosa sapidissima), a fish, can hear 180,000 Hz. Those frequencies sounds at far higher frequencies.
above the human range are called ultrasound. Amplitude, the dimension of a wave from peak to trough, is
Amplitude, or the dimension of a wave from peak to trough, in heard as volume; louder sounds have greater amplitudes than
sound is heard as volume. The sound waves of louder sounds have those of softer sounds.
greater amplitude than those of softer sounds. For sound, volume is
KEY TERMS
measured in decibels (dB). The softest sound that a human can hear
is the zero point. Humans speak normally at 60 decibels. frequency: characterized as a periodic vibration that is audible;
property of sound that most determines pitch and is measured in
hertz
amplitude: measure of a wave from its highest point to its
lowest point; heard as volume
ultrasound: sound frequencies above the human detectable
ceiling of approximately 20,000 Hz

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36.9: HEARING AND VESTIBULAR SENSATION - RECEPTION OF SOUND

 LEARNING OBJECTIVES

Explain how animals sense sound

RECEPTION OF SOUND
In order to hear a sound, the auditory system must accomplish three
basic tasks. First, it must deliver the acoustic stimulus to the
receptors; second, it must convert the stimulus from pressure
changes into electrical signals; and third, it must process these
electrical signals so that they can efficiently indicate the qualities of
the sound source, such as frequency (pitch), amplitude (loudness,
volume), and location.
The human ear can be divided into three functional segments:
Figure 36.9.1: Human ear: Sound travels through the outer ear to the
the outer ear: collects sound energy from the environment and middle ear, which is bounded on its exterior by the tympanic
sends it to the eardrum membrane. The middle ear contains three bones called ossicles that
transfer the sound wave to the oval window, the exterior boundary of
the middle ear: transduces the mechanical pressure signals from the inner ear.
the ear drum into electrical signals
the inner ear: interprets the electrical signals from the middle ear KEY POINTS
using hair cells The human ear can be divided into three functional segments: the
outer ear, the middle ear, and the inner ear.
In mammals, sound waves are collected by the external,
Sound waves are collected by the pinna, travel through the
cartilaginous outer part of the ear called the pinna. They then travel
auditory canal, and cause vibration of the tympanum (ear drum).
through the auditory canal, causing vibration of the thin diaphragm
The three ossicles of the middle ear ( malleus, incus, and stapes )
called the tympanum, or ear drum, the innermost part of the outer
transfer energy from the vibrating ear drum to the inner ear.
ear. Interior to the tympanum is the middle ear, which holds three
The incus connects the malleus to the stapes, which allows
small bones called the ossicles (“little bones”), that transfer energy
vibrations to reach the inner ear.
from the moving tympanum to the inner ear. The three ossicles are
the malleus (also known as the hammer), the incus (the anvil), and KEY TERMS
stapes (the stirrup). The three ossicles are unique to mammals; each
malleus: small hammer-shaped bone of the middle ear
plays a role in hearing. The malleus attaches at three points to the
incus: small anvil-shaped bone in the middle ear; connects the
interior surface of the tympanic membrane. The incus attaches the
malleus to the stapes
malleus to the stapes. In humans, the stapes is not long enough to
stapes: small stirrup-shaped bone of the middle ear
reach the tympanum. If we did not have the malleus and the incus,
pinna: the visible, cartilaginous part of the ear that resides
then the vibrations of the tympanum would never reach the inner ear.
outside of the head and collects sound waves
These bones also function to collect force and amplify sounds. The
tympanum: innermost part of the outer ear; the eardrum
ear ossicles are homologous to bones in a fish mouth; the bones that
support gills in fish are thought to be adapted for use in the This page titled 36.9: Hearing and Vestibular Sensation - Reception of
vertebrate ear over evolutionary time. Many animals (frogs, reptiles, Sound is shared under a CC BY-SA 4.0 license and was authored, remixed,
and birds, for example) use the stapes of the middle ear to transmit and/or curated by Boundless.
vibrations to it.

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36.10: HEARING AND VESTIBULAR SENSATION - THE VESTIBULAR SYSTEM
angles to the horizontal axis. When the brain processes input from
 LEARNING OBJECTIVES all three canals together, it can detect angular acceleration or
deceleration in three dimensions. When the head turns, the fluid in
Identify the structures of the vestibular system that respond
the canals shifts, thereby bending stereocilia and sending signals to
to gravity
the brain. Upon cessation of acceleration or deceleration, the
movement of the fluid within the canals slows or stops. For example,
VESTIBULAR INFORMATION imagine holding a glass of water. When moving forward, water may
The stimuli associated with the vestibular system are linear splash backwards onto the hand; when motion has stopped, water
acceleration (gravity) and angular acceleration/deceleration. Gravity, may splash forward onto the fingers. While in motion, the water
acceleration, and deceleration are detected by evaluating the inertia settles in the glass and does not splash. Note that the canals are not
on receptive cells in the vestibular system. Gravity is detected sensitive to velocity itself, but to changes in velocity. In this way,
through head position, while angular acceleration and deceleration moving forward at 60 mph with your eyes closed would not give the
are expressed through turning or tilting of the head. sensation of movement, but suddenly accelerating or braking would
The vestibular system has some similarities with the auditory stimulate the receptors.
system. It utilizes hair cells just like the auditory system, but it
excites them in different ways. There are five vestibular receptor
HIGHER PROCESSING
organs in the inner ear, all of which help to maintain balance: the Hair cells from the utricle, saccule, and semicircular canals also
utricle, the saccule, and three semicircular canals. Together, they communicate through bipolar neurons to the cochlear nucleus in the
make up what is known as the vestibular labyrinth. The utricle and medulla. Cochlear neurons send descending projections to the spinal
saccule are most responsive to acceleration in a straight line, such as cord and ascending projections to the pons, thalamus, and
gravity. The roughly 30,000 hair cells in the utricle and 16,000 hair cerebellum. Connections to the cerebellum are important for
cells in the saccule lie below a gelatinous layer, with their stereocilia coordinated movements. There are also projections to the temporal
(singular: stereocilium) projecting into the gelatin. Embedded in this cortex, which account for feelings of dizziness; projections to
gelatin are calcium carbonate crystals, similar to tiny rocks. When autonomic nervous system areas in the brainstem, which account for
the head is tilted, the crystals continue to be pulled straight down by motion sickness; and projections to the primary somatosensory
gravity, but the new angle of the head causes the gelatin to shift, cortex, which monitors subjective measurements of the external
thereby bending the stereocilia. The bending of the stereocilia world and self-movement. People with lesions in the vestibular area
stimulates specific neurons that signal to the brain that the head is of the somatosensory cortex see vertical objects in the world as
tilted, allowing the maintenance of balance. It is the vestibular being tilted. Finally, the vestibular signals project to certain optic
branch of the vestibulocochlear cranial nerve that deals with muscles to coordinate eye and head movements.
balance.
KEY POINTS
The vestibular system uses hair cells, as does the auditory
system, but it excites them in different ways.
There are five vestibular receptor organs in the inner ear (the
vestibular labyrinth): the utricle, the saccule, and three
semicircular canals; the u