Chapter 1: Characteristics & Classification of Living Organisms

Exam Focus: Be able to list and define the seven characteristics, define species, understand and
apply the binomial system, use and construct dichotomous keys, know the main features of the
five kingdoms, and classify vertebrates, arthropods, ferns, and flowering plants (monocots/dicots).
Understand why viruses aren't classified as living.

1. Characteristics of Living Organisms (MRS GREN):

● All living organisms share seven characteristics. You must know these and be able to
briefly define them.
○ Movement: An action by an organism or part of an organism causing a change of
position or place. (Note: Plants move parts, e.g., towards light).
○ Respiration: The chemical reactions in cells that break down nutrient molecules
(like glucose) to release energy for metabolism. This is NOT the same as
breathing.
○ Sensitivity: The ability to detect or sense stimuli (changes) in the internal or external
environment and to make appropriate responses.
○ Growth: A permanent increase in size and dry mass by an increase in cell number
or cell size or both. (Distinguish from temporary swelling like a pufferfish).
○ Reproduction: The processes that make more of the same kind of organism.
(Needed for species survival, not individual survival).
○ Excretion: Removal from organisms of the waste products of metabolism (chemical
reactions in cells including respiration), toxic materials, and substances in excess of
requirements.
○ Nutrition: Taking in of materials for energy, growth, and development; plants require
light, carbon dioxide, water, and ions; animals need organic compounds and ions
and usually water.
●
● Metabolism: The sum of all chemical reactions occurring within a living organism.
Respiration is a key part of metabolism.

2. Classification System:

● Why Classify? To organize the vast diversity of life based on shared features, reflecting
evolutionary relationships (how recently organisms shared a common ancestor).
● Hierarchy: Organisms are grouped into progressively smaller categories. The main ones
for IGCSE are Kingdom, Phylum, Class, Order, Family, Genus, Species. (You primarily
need to know Kingdom, Genus, Species, and specific groups like vertebrates/arthropods).
● Species: The fundamental unit. Defined as a group of organisms that can reproduce
sexually to produce fertile offspring. (Example: Horses are a species, donkeys are a
different species; their offspring, mules, are infertile).
● Binomial System: The universal scientific naming system.
○ Uses two Latin words: Genus name followed by species name.
○ Rules: Genus starts with a capital letter, species starts with a lowercase letter.
Written in italics or underlined if handwritten. (e.g., Homo sapiens or Homo
sapiens).
○ Importance: Avoids confusion caused by common names varying between
languages/regions. Indicates evolutionary relationships (organisms in the same
genus are closely related).
●

3. Dichotomous Keys:

● Purpose: Used to identify unknown organisms based on their visible features.

● Structure: Consists of a series of paired, contrasting statements or questions. Each choice
leads you to another pair or to the identification of the organism.
● Using a Key: Start at statement 1, choose the option that matches your specimen, and
follow the instruction (go to another number or identify the organism).
● Constructing a Key:
 ○ Use easily observable, constant features (e.g., number of legs, presence of wings).
○ Avoid subjective terms (e.g., "large", "small") or features that change (e.g., colour
that fades).
○ Each pair of statements must be opposites (e.g., "has wings" vs. "has no wings")
and refer to the same feature.
○ Start by dividing the group into two, then subdivide those groups until each
organism is isolated.
●

4. The Five Kingdoms:

● You need to know the main distinguishing features of each kingdom. Focus on cell
structure and nutrition.
○ Animal Kingdom: Multicellular; cells have a nucleus but NO cell walls and NO
chloroplasts; nutrition is heterotrophic (ingest organic substances).
○ Plant Kingdom: Multicellular (mostly); cells have a nucleus, cellulose cell walls,
and often chloroplasts; nutrition is autotrophic (photosynthesis). Often have
complex structures like roots, stems, leaves.
○ Fungus Kingdom: Mostly multicellular (made of hyphae forming a mycelium),
some unicellular (yeast); cells have a nucleus and cell walls (made of chitin, not
cellulose); NO chloroplasts; nutrition is saprophytic or parasitic (absorptive
heterotrophs). Reproduce via spores.
○ Protoctist Kingdom: Mostly unicellular, some multicellular (like algae); cells have
a nucleus; a mixed group – some are animal-like (e.g., Amoeba, Paramecium - no
cell wall/chloroplasts), some are plant-like (e.g., Chlamydomonas, algae - have cell
walls/chloroplasts).
○ Prokaryote Kingdom: Unicellular; cells have NO nucleus; have circular DNA loop
and often plasmids in the cytoplasm; have cell walls (not cellulose), cell membrane,
cytoplasm, ribosomes; NO mitochondria. Example: Bacteria.
●

5. Groups within the Animal Kingdom:

● Vertebrates: Animals with a backbone. Know the 5 classes:

○ Fish: Aquatic; scales; gills (for breathing); fins (for movement/stability); external
fertilization (mostly); eggs without shells.
○ Amphibians: Smooth, moist skin; larvae (tadpoles) are aquatic with gills, adults
often terrestrial with lungs; lay eggs without shells in water; undergo
metamorphosis.
○ Reptiles: Dry, scaly skin; lungs; lay eggs with soft, leathery waterproof shells on
land.
○ Birds: Feathers; beak; forelimbs adapted as wings (most fly); scales on legs; lay
eggs with hard shells; endothermic ('warm-blooded').
○ Mammals: Hair/fur; mammary glands (produce milk); diaphragm; external ears
(pinnae); different types of teeth; endothermic ('warm-blooded'); most have live birth
and placenta.
●
● Arthropods: Animals without a backbone (invertebrates), but with a hard exoskeleton and
jointed legs. Know the 4 classes:
○ Insects: 3 body parts (head, thorax, abdomen); 3 pairs of legs (6 total); 1 pair of
antennae; often 2 pairs of wings.
○ Crustaceans: More than 4 pairs of legs; 2 pairs of antennae; often have gills
(mostly aquatic).
○ Arachnids: 2 body parts (cephalothorax, abdomen); 4 pairs of legs (8 total); NO
antennae.
○ Myriapods: Body consists of many segments; each segment has jointed legs (1 or
2 pairs per segment); 1 pair of antennae.
●

6. Groups within the Plant Kingdom:

 ● Ferns: Have roots, stems (rhizomes), and leaves (fronds); do NOT produce flowers;
reproduce by spores found on the underside of fronds.
● Flowering Plants: Have roots, stems, and leaves; DO produce flowers for reproduction;
seeds are enclosed within an ovary (part of the flower).
○ Monocotyledons (Monocots): One cotyledon (seed leaf); leaves have parallel
veins; fibrous (adventitious) root system; flower parts usually in multiples of 3;
scattered vascular bundles in the stem.
○ Dicotyledons (Dicots): Two cotyledons; leaves have branching (net-like) veins;
usually a tap root system; flower parts usually in multiples of 4 or 5; vascular
bundles arranged in a ring in the stem.
●

7. Viruses:

● Structure: Genetic material (DNA or RNA) enclosed in a protein coat (capsid). They are
not cells.
● Classification: Not classified in the five kingdoms because they are not considered living.
● Why Not Living? They do not show all seven characteristics of life. They can only
reproduce inside living host cells by hijacking the cell's machinery. They have no
metabolism of their own.

Chapter 2: Cells

Exam Focus: Know the structure and function of typical animal, plant, and bacterial cells. Be able
to identify organelles on diagrams/micrographs and state their functions. Understand the concepts
of specialised cells, tissues, organs, and organ systems. Be able to calculate magnification and
actual size using the formula and handle unit conversions (mm to µm).

1. Cell Structure and Function:

● Cells are the basic units of life.

● Microscopes: Essential for viewing cells.
○ Light Microscope: Uses light, lower magnification/resolution, can view living cells.
○ Electron Microscope: Uses electrons, much higher magnification/resolution, views
dead/preserved specimens, reveals ultrastructure.
●
● Key Organelles and their Functions (Crucial for exams):
○ Cell Membrane: Controls passage of substances into and out of the cell (partially
permeable). Present in ALL cells.
○ Cytoplasm: Jelly-like substance where most chemical reactions (metabolism) occur.
Present in ALL cells.
○ Nucleus: Contains chromosomes (made of DNA carrying genetic information) and
controls cell activities. Present in plant, animal, fungal, protoctist cells.
○ Mitochondrion (plural: mitochondria): Site of aerobic respiration, releasing energy
(ATP). Present in plant, animal, fungal, protoctist cells. More numerous in active
cells.
○ Ribosomes: Site of protein synthesis. Very small dots. Present in ALL cells
(including prokaryotes).
○ Cell Wall (Plants): Made of cellulose; provides structural support, defines shape,
prevents bursting due to water uptake (fully permeable). Also present in fungi
(chitin) and prokaryotes (peptidoglycan), but NOT animals.
○ Chloroplasts (Plants): Contain chlorophyll; site of photosynthesis. Found in green
plant cells (leaves, some stems) and some protoctists. NOT in animal, fungal, or
prokaryotic cells.
○ Vacuole (Plants): Large, central, permanent sac containing cell sap (solution of
sugars, salts, etc.); helps maintain cell shape and turgor. Animal cells have small,
temporary vacuoles or vesicles.
●

2. Comparing Cell Types:

 ● Plant vs. Animal: Plant cells have a cell wall, chloroplasts (usually), and a large central
vacuole. Animal cells lack these.
● Prokaryotic (Bacteria) vs. Eukaryotic (Plant/Animal/Fungi/Protoctist): Prokaryotes
lack a true nucleus (DNA is a loop in cytoplasm), lack mitochondria, and have smaller
ribosomes. Eukaryotes have a nucleus and membrane-bound organelles like mitochondria.

3. Levels of Organisation:

● Specialised Cells: Cells adapted (structure relates to function) for a specific job (e.g.,
muscle cells for contraction, nerve cells for transmission, root hair cells for absorption).
● Tissues: A group of similar specialised cells working together to perform a specific function
(e.g., muscle tissue, xylem tissue).
● Organs: A structure made up of different tissues working together to perform specific
functions (e.g., heart, leaf, stomach).
● Organ Systems: A group of organs working together to perform a major function in the
body (e.g., circulatory system, digestive system).
● Organism: An individual living thing made up of organ systems (in multicellular
organisms).
● Hierarchy: Cells → Tissues → Organs → Organ Systems → Organism.

4. Magnification Calculations:

● The formula triangle (I AM): Image size = Actual size × Magnification.

○ Magnification = Image size / Actual size
○ Actual size = Image size / Magnification
●
● Units: MUST be the same for image size and actual size before calculating. Convert mm
to µm (or vice versa) first.
○ 1 mm = 1000 micrometres (µm)
○ 1 cm = 10 mm = 10 000 µm
●
● Magnification is written with a '×' sign and has NO units.
● Measure image size accurately from diagrams/micrographs using a ruler.

Chapter 3: Movement into and out of Cells

Exam Focus: Define diffusion, osmosis, and active transport. Explain the factors affecting the rate
of diffusion. Understand water potential and how osmosis affects animal and plant cells (turgid,
flaccid, plasmolysis, lysis). Explain active transport, including the role of carrier proteins and
energy from respiration. Be able to interpret experimental data related to these processes.

1. Diffusion:

● Definition: The net movement of particles (molecules or ions) from a region of their higher
concentration to a region of their lower concentration (down a concentration gradient) due
to the particles' random motion and kinetic energy.
● Key Concepts:
○ Net movement: More particles move down the gradient than up it.
○ Concentration Gradient: The difference in concentration between two areas.
○ Random Movement: Particles move randomly; diffusion is the overall statistical
outcome.
○ Kinetic Energy: Particles are always moving; higher temperature means more
kinetic energy and faster movement.
●
● Importance: Gas exchange (O₂, CO₂ in lungs and leaves), absorption of some digested
food, movement of neurotransmitters.
● Factors Affecting Rate:
○ Concentration Gradient: Steeper gradient → faster rate.
○ Temperature: Higher temperature → faster rate.
○ Surface Area: Larger surface area (of the membrane/exchange surface)
→ faster rate.
 ○ Diffusion Distance: Shorter distance → faster rate.
●

2. Osmosis:

● Definition: The net movement of water molecules from a region of higher water potential
to a region of lower water potential through a partially permeable membrane.
● Key Concepts:
○ Water Molecules Only: Osmosis specifically describes the movement of water.
○ Water Potential (Ψ): The 'free energy' or effective concentration of water. Pure
water has the highest Ψ (zero). Adding solutes lowers (makes more negative) the
Ψ. Water moves down a water potential gradient (from high Ψ to low Ψ).
○ Partially Permeable Membrane: Allows water molecules through but restricts solute
movement (e.g., cell membrane, Visking/dialysis tubing). Cell walls are fully
permeable.
●
● Osmosis in Animal Cells:
○ In pure water (higher external Ψ): Water enters, cell swells and bursts (lysis)
because there is no cell wall.
○ In concentrated solution (lower external Ψ): Water leaves, cell shrinks (crenation).
●
● Osmosis in Plant Cells:
○ In pure water (higher external Ψ): Water enters, vacuole swells, cytoplasm pushes
against the cell wall. Cell becomes turgid. The strong cell wall prevents bursting.
Turgor provides support.
○ In concentrated solution (lower external Ψ): Water leaves, vacuole/cytoplasm
shrink, cell membrane pulls away from the cell wall. Cell becomes flaccid, may lead
to plasmolysis. Causes wilting.
●

3. Active Transport:

● Definition: The movement of particles through a cell membrane against a concentration

gradient (from a region of lower concentration to a region of higher concentration), using
energy released during respiration.
● Key Features:
○ Moves against the concentration gradient.
○ Requires energy (ATP) from respiration (occurs in mitochondria).
○ Requires specific carrier proteins embedded in the cell membrane. These proteins
bind to the substance, change shape using energy, and transport it across the
membrane.
●
● Importance: Uptake of mineral ions (e.g., nitrates) by root hair cells from the soil; uptake of
glucose by cells lining the small intestine and kidney tubules.

Summary for Exam Success:

● Definitions are Key: Be precise with definitions for diffusion, osmosis, active transport,
species, binomial system, etc. Use the keywords highlighted in the definitions.
● Compare and Contrast: Be ready to compare animal vs. plant cells, prokaryotes vs.
eukaryotes, the different vertebrate/arthropod groups, monocots vs. dicots, diffusion vs.
osmosis vs. active transport. Use tables to organize comparisons.
● Explain Processes: Understand how processes work – how a key functions, how osmosis
affects cells differently, how active transport uses carrier proteins and energy.
● Structure and Function: Always link the structure of a cell/organelle/tissue to its function.
● Apply Knowledge: Be able to use keys, interpret diagrams/graphs/tables of results from
experiments (like the agar jelly or potato strip experiments), and perform magnification
calculations accurately (watch units!).
● Kingdom Features: Memorize the core features (cell wall presence/type, nucleus,
chloroplasts, nutrition) for the five kingdoms and viruses.
Good luck with your revision! Remember to practice past paper questions to test your
understanding and application of these concepts.