CHAPTER 3  It acts as a selective barrier

Cell Structures and Their Cell Membrane Structure

Functions  The fluid-mosaic model is the model used to
describe the cell membrane structure.
 The membrane contains phospholipids,
Cell Structure cholesterol, proteins, and carbohydrates.
Organelles:  Phospholipids form a bilayer.
specialized structures in cells that perform  Phospholipids contain 2 regions: polar and
specific functions nonpolar.
Example: nucleus, mitochondria, ribosomes
Cytoplasm: Phospholipid Structure
jelly-like substance that holds organelles  A phospholipid molecule has a polar head
Cell membrane: region that is hydrophilic and a nonpolar tail
also termed the plasma membrane a structure that region that is hydrophobic.
encloses the cytoplasm  The polar region is exposed to water around
the membrane.
 The nonpolar region is facing the interior of
the membrane.

The Cell Membrane

Functions of the Cell

 Smallest units of life
Movement through the Cell Membrane
 Cell metabolism and energy use
 The cell membrane has selective
 Synthesis of molecules
permeability, which allows only certain
 Communication
substances to pass in and out of the cell.
 Reproduction and inheritance
 Substances such as enzymes, glycogen, and
potassium are found in higher
Cell Membrane
concentrations inside the cell.
 The cell membrane, or plasma membrane,
 Substances such as sodium, calcium, and
is the outermost component of a cell.
chloride are found in higher concentrations
 It forms a boundary between material in outside the cell.
inside the cell and the outside.
 Materials inside the cell are intracellular and Cell Membrane Passage 1
those outside are extracellular.
  Some substances, like O2 and CO2, can pass  Solutes are substances dissolved in a
directly through the cell membrane’s predominant liquid or gas, which is called
phospholipid bilayer. the solvent.
 Some substances must pass through  Solutes, such as ions or molecules, tend to
transmembrane move from an area of higher concentration
 protein channels, such as Na+ through its of a solute to an area of lower
channels. concentration of that same solute in
 The route of transport through the solution.
membrane depends on the size, shape, and  This movement from high concentration to
charge of the substance a low concentration is diffusion.

Cell Membrane Passage 2 Concentration Gradient

 Some substances require carrier molecules  A concentration gradient is the difference in
to transport them across the cell the concentration of a solute in a solvent
membrane, such as glucose. between two points divided by the distance
 Some substances require a vesicular between the two points.
transport across the membrane.  The concentration gradient is said to be
 The vesicle must fuse with the cell steeper when the concentration difference
membrane for transport. is large and/or the distance is small.

Active Transport and Passive Transport 1

 Passive membrane transport does not
require the cell to expend energy.
 Active membrane transport does require
the cell to expend energy, usually in the
form of ATP. Osmosis 1
 Osmosis is the diffusion of water (a solvent)
across a selectively permeable membrane
Active Transport and Passive Transport 2 from a region of higher water concentration
 Passive membrane transport mechanisms to one of lower water concentration.
include diffusion, osmosis, and facilitated  Osmosis exerts a pressure, termed osmotic
diffusion. pressure, which is the force required to
 Active membrane transport mechanisms prevent movement of water across cell
include active transport, secondary active membrane
transport, endocytosis, and exocytosis.

Diffusion 1
 Diffusion generally involves movement of
substances in a solution down a
concentration gradient.
 A solution is generally composed of two
major parts, solutes and the solvent.
  The cell will neither shrink nor swell.

Hypertonic
 The cytoplasm of a cell in a hypertonic
solution has a lower solute concentration
and higher water concentration than the
surrounding solution.
 Water moves by osmosis from the cell into
the hypertonic solution, resulting in cell
shrinkage, or crenation.

Osmotic Pressure and the Cell

 Osmotic pressure depends on the difference
of solution concentrations inside a cell
relative to outside the cell.
 A cell may be placed in solutions that are Facilitated Diffusion 1
either hypotonic, isotonic, or hypertonic  Lipid soluble substances such as oxygen,
compared to the cell cytoplasm. carbon dioxide, and steroids can diffuse
directly through the phospholipid bilayer.
Hypotonic  Water-soluble substances, such as ions, can
 A hypotonic solution has a lower diffuse across the cell membrane only by
concentration of solutes and a higher passing through cell membrane channels or
concentration of water relative to the through carrier molecules.
cytoplasm of the cell.
 The solution has less tone, or osmotic Facilitated Diffusion 2
pressure, than the cell.  Facilitated diffusion is a carrier-mediated
 Water moves by osmosis into the cell, transport process that moves substances
causing it to swell. across the cell membrane from an area of
 If the cell swells enough, it can rupture, a higher concentration to an area of lower
process called lysis. concentration of that substance.
 Because movement is with the
concentration gradient, metabolic energy in
Isotonic the form of ATP is not required.
 A cell immersed in an isotonic solution has
the same solute concentrations inside and
outside the cell.
 the cell membrane involved in facilitated
diffusion.
 Move water soluble molecules or ions
across the membrane.
 They exhibit specificity; only specific
molecules are transported by the carriers.

Active Transport
 Active transport is a carrier-mediated
process, requiring ATP, that moves
substances across the cell membrane from
regions of lower concentration to those of
Leak and Gated Channels 1 higher concentration against a
 Two classes of cell membrane channels concentration gradient.
include leak channels and gated channels.  Active transport processes accumulate
 Leak channels constantly allow ions to pass necessary substances on one side of the cell
through. membrane at concentrations many times
 Gated channels limit the movement of ions greater than those on the other side.
across the membrane by opening and
closing. Sodium-Potassium Pump 1
 A major example of active transport is the
action of the sodium-potassium pump
present in cell membranes.
 The sodium-potassium pump moves Na+
out of cells and K+ into cells.
 The result is a higher concentration of Na+
outside cells and a higher concentration of
K+ inside cells.

Sodium-Potassium Pump 2

Carrier Molecules 1
 Carrier
molecules are
proteins within
  Pinocytosis has much smaller vesicles
Secondary Active Transport 1 formed, and they contain liquid rather than
 Secondary active transport uses the energy solid particles.
provided by a concentration gradient
established by the active transport of one
substance, such as Na+ to transport other
substances.
 No additional energy is required above the
energy provided by the initial active
transport pump. Receptor-Mediated Endocytosis

Secondary Active Transport 2

 In cotransport, the diffusing substance
moves in the same direction as the initial
active transported substance.
 In countertransport, the diffusing substance
moves in a direction opposite to that of the
initial active transported substance.

Secondary Active Transport

Exocytosis 1
 Exocytosis involves the use of membrane-
bound sacs called secretory vesicles that
accumulate materials for release from the
cell.
 The vesicles move to the cell membrane and
fuse, ultimately releasing the material by
Endocytosis
exocytosis.
 Endocytosis is a process that that brings
 Examples of exocytosis are the secretion of
materials into cell using vesicles.
digestive enzymes.
 Receptor-mediated endocytosis occurs
when a specific substance binds to the
receptor molecule and is transported into
the cell.
 Phagocytosis is often used for endocytosis
when solid particles are ingested.
 membranes with a narrow space between
them.
 The nuclear membrane contains nuclear
pores, through which materials can pass
into or out of the nucleus.

Cell Nucleus 2
 The nuclei of human cells contain 23 pairs of
chromosomes which consist of DNA and
proteins.
 During most of a cell’s life, the
chromosomes are loosely coiled and
collectively called chromatin.
 When a cell prepares to divide, the
chromosomes become tightly coiled and are
visible when viewed with a microscope.
General Cell Structure
 The interior of a cell is composed of the Cell Nucleus 3
cytoplasm, which a jelly-like fluid that  Within the nucleus are Nucleoli, which are
surrounds the organelles. diffuse bodies with no surrounding
 Organelles are specialized structures that membrane. that are found within the
perform certain functions. nucleus
 Organelles include the nucleus, ribosomes,  There are usually one to several nucleoli
endoplasmic reticulum, Golgi apparatus, within the nucleus.
lysosomes, peroxisomes, mitochondria,  The subunits of ribosomes, a type of
cytoskeleton, centrioles, cilia, flagella, and cytoplasmic organelle, are formed within a
microvilli. nucleolus.
 These ribosomal components exit the
nucleus through nuclear pores.

Cell Nucleus 4

Cell Nucleus 1
 The nucleus is a large organelle usually
located near the center of the cell.
 The nucleus is bounded by a nuclear
envelope, which consists of outer and inner
 Ribosome Production

Endoplasmic Reticulum 1
 The endoplasmic reticulum (ER) is a series
of membranes forming sacs and tubules
that extends from the outer nuclear
membrane into the cytoplasm.

 The rough ER is involved in protein

synthesis and is rough due to attached
ribosomes.
 The smooth ER has no attached ribosomes
and is a site for lipid synthesis, cellular
detoxification, and it stores calcium ions in
skeletal muscle cells.
Ribosomes
 Ribosome components are produced in the Endoplasmic Reticulum 2
nucleolus.
 Ribosomes are the organelles where
proteins are produced.
 Ribosomes may be attached to other
organelles, such as the endoplasmic
reticulum.
 Ribosomes that are not attached to any
other organelle are called free ribosomes.
  They contain a variety of enzymes that
function as intracellular digestive systems.

 Vesicles formed by endocytosis may fuse

with lysosomes in order to breakdown
materials in the endocytotic vesicles.
 One example is white blood cells
phagocytizing bacteria.

Lysosome Action

Golgi Apparatus 1
 The Golgi apparatus, also called the Golgi
complex, consists of closely packed stacks of
curved,membrane-bound sacs.
 It collects, modifies, packages, and
distributes proteins and lipids manufactured
by the ER. Peroxisomes
 The Golgi apparatus forms vesicles, some of  Peroxisomes are small, membrane-bound
which are secretory vesicles, lysosomes, and vesicles containing enzymes that break
other vesicles. down fatty acids, amino acids, and hydrogen
peroxide (H2O2).
Golgi Apparatus 2  Hydrogen peroxide is a by-product of fatty
acid and amino acid breakdown and can be
toxic to a cell.
 The enzymes in peroxisomes break down
hydrogen.

Mitochondria 1
 Mitochondria (singular mitochondrion) are
small organelles responsible for producing
considerable amounts of ATP by aerobic
(with O2) metabolism.
 They have inner and outer membranes
separated by a space.
Lysosomes  The outer membranes have a smooth
contour, but the inner membranes have
 Lysosomes are membrane-bound vesicles
numerous folds, called cristae, which
formed from the Golgi apparatus.
 project into the interior of the  Microfilaments are small fibrils formed from
mitochondria. protein subunits that structurally support
the cytoplasm, determining cell shape.
Mitochondria 2  Some microfilaments are involved with cell
 The material within the inner membrane is movement.
the mitochondrial matrix and contains  Microfilaments in muscle cells enable the
enzymes and mitochondrial DNA (mtDNA). cells to shorten,or contract.
 Cells with a large energy requirement have
more mitochondria than cells that require Intermediate Filaments
less energy.  Intermediate filaments are fibrils formed
from protein subunits that are smaller in
A Mitochondrion diameter than microtubules but larger in
diameter than microfilaments.
 They provide mechanical support to the cell.
 A specific type of intermediate filament is
keratin, a protein associated with skin cells.

The Cytoskeleton 1

The Cytoskeleton 2
 The cytoskeleton gives internal framework
to the cell.
 It consists of protein structures that support
the cell, hold organelles in place, and enable
the cell to change shape.
Centrioles
 These protein structures are microtubules,
microfilaments, and intermediate filaments.  The centrosome is a specialized area of
cytoplasm close to the nucleus where
Microtubules microtubule formation occurs.
 Microtubules are hollow structures formed  It contains two centrioles, which are
from protein subunits. normally oriented perpendicular to each
other.
 The microtubules perform a variety of roles,
including helping to support the cytoplasm  Each centriole is a small, cylindrical
of cells, assisting in cell division, and organelle composed of microtubules.
forming essential components of certain  The centriole is involved in the process of
organelles, such as cilia and flagella. mitosis.

Microfilaments Centriole
  The proteins produced are in turn
determined by the genetic information in
the nucleus.
 Information in DNA provides the cell with a
code for its cellular processes.

DNA 1
 DNA contains the information that directs
protein synthesis; a process called gene
Cilia expression.
 Cilia project from the surface of certain  A DNA molecule consists of nucleotides
cells. joined together to form two nucleotide
 They are responsible for the movement of strands.
materials over the top of cells, such as  The two strands are connected and
mucus. resemble a ladder that is twisted around its
 Cilia are cylindrical structures that extend long axis.
from the cell and are composed of  Each nucleotide consists of a 5-carbon
microtubules. sugar, a phosphate group, and a nitrogenous
base.
Flagella
 Flagella have a structure similar to that of DNA 2
cilia but are much longer, and they usually  each nucleotide on one DNA strand has a
occur only one per cell. specific bonding pattern to another
 Sperm cells each have one flagellum, which nucleotide on the opposite strand.
propels the sperm cell.  A gene is a sequence of nucleotides that
provides a chemical set of instructions for
Microvilli making a specific protein.
 Microvilli are specialized extensions of the
cell membrane that are supported by Gene Expression
microfilaments.  Gene expression, which is protein synthesis,
 They do not actively move as cilia and involves transcription and translation.
flagella do.  Transcription involves copying DNA into
 Microvilli are numerous on cells that have messenger RNA.
them and they increase the surface area of  Translation involves messenger RNA being
those cells. used to produce a protein.
 They are abundant on the surface of cells
that line the intestine, kidney, and other Overview of Gene Expression
areas in which absorption is an important
function.

Whole Cell Activity

 A cell’s characteristics are determine by the
type of proteins produced.
 Transcription 4

Transcription 1
 Transcription takes place in the nucleus of
the cell.
 DNA determines the structure of mRNA
through transcription.
 During transcription, the double strands of a
DNA segment separate, and DNA Translation 1
nucleotides of the gene pair with RNA  Translation occurs in the cell cytoplasm after
nucleotides that form the mRNA. mRNA has exited the nucleus through the
nuclear pores.
Transcription 2  The mRNA attaches to a ribosome.
 DNA contains one of the following organic  Codons (3 nucleotide bases) on the mRNA
bases: thymine, adenine, cytosine, or are read by anticodons (3 nucleotide bases)
guanine. on transfer RNA (tRNA).
 Messenger RNA (mRNA) contains uracil,
adenine, cytosine, or guanine. Translation 2
 Transfer RNA transports specific amino acids
Transcription 3 from the cytoplasm to the ribosome-mRNA
 DNA nucleotides pair only with specific RNA complex and initiates formation of the
nucleotides. polypeptide chain.
 DNA’s thymine pairs with RNA’s adenine.  The process continues until the entire
 DNA’s adenine pairs with RNA’s uracil. polypeptide is completely formed.
 DNA’s cytosine pairs with RNA’s guanine
 DNA’s guanine pairs with RNA’s cytosine. The Cell Cycle 1
  During growth and development, cell Mitosis
division occurs to increase the number of  Mitosis involves formation of 2 daughter
cells or replace damaged or dying ones. cells from a single parent cell.
 This cell division involves a cell cycle.  Mitosis is divided into four phases:
 The cell cycle includes two major phases: a prophase, metaphase, anaphase, and
nondividing phase, called interphase, and a telophase.
cell dividing phase, termed mitosis.
Prophase
The Cell Cycle 2  During prophase the chromatin condenses
 A cell spends most of its life cycle in to form visible chromosomes.
interphase which is divided into three  Microtubules, termed spindle fibers, form
phases: to assist in breaking the centromere
 G1 phase, during which the cell carries out between the chromatids and move the
normal metabolic activity chromosomes to opposite sides of the cell.
 S phase, during which the DNA is replicated;  The nuclear membrane dissolves.
and
 G2 phase, during which the cell prepares to Metaphase
divide.  During metaphase, the chromosomes align
 At the end of interphase, a cell has two near the center of the cell.
complete sets of genetic material  The movement of the chromosomes is
regulated by the attached spindle fibers.
The Cell Cycle 3
Anaphase
 At the beginning of anaphase, the
chromatids separate and each chromatid is
called a chromosome.
 Each of the two sets of 46 chromosomes is
moved by the spindle fibers toward the
centriole at one of the poles of the cell.
 At the end of anaphase, each set of
chromosomes has reached an opposite pole
of the cell, and the cytoplasm begins to
divide.
Cell Genetic Content
 Each human cell (except sperm and egg) Telophase
contains 23 pairs of chromosomes, a total of  During telophase, the chromosomes in each
46. of the daughter cells become organized to
 The sperm and egg contain 23 form two separate nuclei, one in each newly
chromosomes total. formed daughter cell.
 One pair of chromosomes are the sex  The chromosomes begin to unravel and
chromosomes, which consist of two X resemble the genetic material during
chromosomes if the person is a female or an interphase.
X and Y chromosome if the person is a male.
  Following telophase, cytoplasm division is Diversity of Cell Types
completed, and two separate daughter cells
are produced.

The Cell Cycle

Apoptosis
 Apoptosis, termed programmed cell death,
is a normal process by which cell numbers
within various tissues are adjusted and
controlled.
 In the developing fetus, apoptosis removes
extra tissue, such as cells between the
developing fingers and toes.
 In some adult tissues, apoptosis eliminates
excess cells to maintain a constant number
of cells within the tissue.

Cellular Aspects of Aging

 There are various causes for cellular aging.
 Existence of a cellular clock
 Presence of death genes
Tumors  DNA damage
 Tumors are abnormal proliferations of cells.
 They are due to problems occurring in the
cell cycle. Accessibility Content: Text Alternatives for Images
 Some tumors are benign and some are
malignant (cancer). Generalized Cell - Text Alternative
 Malignant tumors can spread by a process,
termed metastasis. The cell shows an outer cell membrane made of a
phospholipid bilayer and embedded protein. The
Differentiation cutaway view of the nucleus, located in the center
 A sperm cell and an oocyte unite to form a of the
single cell, then a great number of mitotic cell, shows the outer nuclear envelope, nucleolus,
divisions occur to give the trillions of cells of nuclear pore, nucleoplasm, and chromatin. The
the body. nucleolus is the diffuse body of the nucleus. The
 The process by which cells develop with passageways of the nucleus are labeled the nuclear
specialized structures and functions is called pores. Outside the nucleus lies the rough
differentiation. endoplasmic reticulum that is attached with
ribosomes. The smooth endoplasmic reticulum lacks
 During differentiation of a cell, some
ribosomes. The specialized area with centrosomes is
portions of DNA are active, but others are
labeled the centriole. The centrosomes are
inactive.
cylindrical organelles with perpendicular
orientation. The cytoskeleton is the internal permeable membrane. Water moves into the tube
framework. The mitochondria have separated inner by osmosis. Because the tube contains salt ions as
and outer membranes. A membrane-bound vesicle well as water molecules, there is
formed from the Golgi apparatus is labeled proportionately less water in the tube than in the
lysosome. The lysosome beaker, which contains only water. The water
fusing with incoming phagocytic vesicle is shown. molecules diffuse with their concentration gradient
Another membrane-bound vesicle is labeled into the tube. Because the salt ions cannot leave the
peroxisome. The packed stacks of membrane-bound tube, the total fluid
sacs are labeled Golgi apparatus. The secretory volume inside the tube increases, and fluid moves
vesicles, a phagocytic vesicle, cilia, and microvilli are up the glass tube as a result of osmosis. The
also shown. Cilia and microvilli are extended concentration of salt in the tube decreases as water
projections of the cell. rises in the tube. Water moves by osmosis into the
tube until hydrostatic pressure prevents further
movement of water into the tube. The hydrostatic
pressure is equal to the osmotic pressure of the
solution in the tube. The solution stops rising when
the weight of the water column prevents further
The Cell Membrane - Text Alternative movement of water into the tube by osmosis.

The illustration A shows a cell membrane that is Red Blood Cell Changes in Differing Solutions – Text
made of phospholipid bilayer in which nonpolar Alternative
regions of phospholipid molecules face inside and
polar regions of phospholipid molecules face The illustration and micrograph A show that when a
outside. Carbohydrate red blood cell is placed in a hypotonic solution
chains are located on the external surface water enters the cell by osmosis, causing the cell to
membrane and cytoskeleton are located in the swell or even lyse (puff of cytoplasm in lower part of
internal membrane surface. cell). The illustration and micrograph B show that
when a red blood cell is placed in an isotonic
Diffusion 2 - Text Alternative solution water moves into and out of the cell at the
same rate. No net water movement occurs, and the
The illustration shows a beaker with distilled water cell shape remains normal. The illustration and
to which a salt cube is added. A concentration micrograph C shows When a red blood cell is placed
gradient for salt (cube) exists between the salt from in a hypertonic solution water moves by osmosis out
the salt crystal and the water in the beaker. Salt ions of the cell and into the solution, resulting in
(small dots) crenation.
move down their concentration gradient into the
water. Salt ions and water molecules are distributed Diffusion through the Cell Membrane – Text
evenly throughout the solution. Alternative

Osmosis 2 - Text Alternative The illustration shows a cell membrane that has
specific non-lipid-soluble molecules or ions enter
The illustration shows a tube containing a 3 percent through the membrane channel into the cytoplasm.
salt solution submerged into a beaker of distilled Non-lipid-soluble molecules bounce off the surface
water. The tube is closed with a selectively of plasma membrane. Lipid-soluble molecules pass
through the plasma membrane to the cytoplasm Exocytosis 2 – Text Alternative
causing a change in gradient.
The illustration A shows a cell with secretory vesicle
Leak and Gated Channels 2 – Text Alternative containing vesicle contents. Secretory vesicle is
fused to cell membrane and the contents of the
The illustration shows a plasma membrane that has secretory vesicle are released out of the cell. The
potassium ion leak channel (always open) through micrograph B shows secretory vesicle releasing its
which potassium ion from the cytoplasm leave the contents.
cell, Gated sodium channel (closed), and Gated
sodium ion channel (open) through which sodium Cell Nucleus 4 – Text Alternative
ions enter the cytoplasm.
The illustration A shows an enlarged view of the
Carrier Molecules 2 – Text Alternative nucleus that has a nuclear envelope which is made
of outer membrane with nuclear pore followed by
The illustration shows a carrier molecule on the space, and inner membrane. The nucleus has a
plasma membrane through which glucose enter the central nucleolus. The micrograph B shows nuclear
cell thus changing the concentration gradient. envelope, interior of nucleus, nucleolus, and
Carrier molecule changes shape and releases chromatin. The micrograph C shows outer
glucose. membrane of nuclear envelope followed by inner
membrane of nuclear envelope with nuclear pores.
Secondary Active Transport 3 – Text Alternative
Chromosome Structure – Text Alternative
The illustration shows a phospholipid bilayer with an
embedded sodium-potassium pump and a carrier An enlarged view of a part of a cell shows a pair of
molecule. The sodium-potassium pump moves coiled chromosomes; the loose coils of the
potassium ions inside the cell (cytoplasm) and chromosomes bind to proteins and are labeled the
sodium ions out of the cell. This causes a high chromatin, which makes up a double-stranded D N
concentration of sodium ions outside the cell. A A molecule. The D N A shows paired bases.
concentration gradient is established, which
supplies the energy to transport the glucose Ribosome Production – Text Alternative
molecules to the cytoplasm through the carrier
molecule. Sodium ions outside the cell are also The illustration shows a cell with a nucleus. The
transported to the cytoplasm through the carrier ribosomal protein from cytoplasm enter the nucleus
molecule. where it attaches to the rRNA in the nucleolus and
exits the nucleus as small ribosomal unit and large
ribosomal unit through the nuclear pore. The large
and small ribosomal unit then process the mRNA in
Receptor-Mediated Endocytosis – Text Alternative the cytoplasm.

The illustration shows molecules to be transported Endoplasmic Reticulum 2 – Text Alternative

near the receptor molecules on the cell membrane. The illustration A show an enlarged view of the
Receptors and bound molecules taken into the cell endoplasmic reticulum that lies outside the nucleus
as vesicle forms. and is continuation of nuclear pore. The rough
endoplasmic contains ribosomes and extends arginine derived from the amino acid pool that
further as smooth endoplasmic reticulum. translated by the tRNA.

Lysosome Action – Text Alternative Transcription 4 – Text Alternative

The illustration shows foreign particles near the cell The illustration shows DNA helix in which a segment
membrane engulfed by the formation of vesicles. is opened where nucleotide align on the DNA
The vesicles then fuse with the lysosome that are template strand and mRNA is formed by base
released from the Golgi apparatus. The lysosomes pairing of cytosine to guanine, thymine to adenine,
with digestive and uracil to adenine.
enzymes fuse with the vesicles to breakdown
substances.

A Mitochondrion – Text Alternative

The Cell Cycle 3 – Text Alternative
The illustration A shows an enlarged view of the
mitochondria that has an outer membrane followed The cell cycle has an interphase and cytokinesis
by intermembrane and inner membrane that forms stage. It begins with G 1 or the first gap phase,
folds called cristae between which lies the matrix. which is related to routine metabolism. This phase is
An enlarged view shows enzymes on the crista. The followed by the S phase or the synthesis phase
micrograph B shows a longitudinal section and cross when D N A replication
section of mitochondria in cytoplasm. occurs; next is the G 2 phase or the second gap
phase that prepares the cell for cell division. It is
The Cytoskeleton 1 – Text Alternative followed by the M phase or mitosis phase. The M
phase includes prophase, metaphase, anaphase,
The illustration A shows a section of the cell that has and telophase. The cell undergoes cytokinesis after
nucleus, ribosomes, endoplasmic reticulum, which it enters either the G 1 phase or G not phase.
mitochondria, and cell membrane. The cytoskeleton
is made of microfilaments (two chains of protein The Cell Cycle – Text Alternative
subunits
intertwined with 8 nanometer width), microtubules The illustration and micrograph shows interphase,
(coils of protein subunits with 25 nanometer width), prophase, metaphase, anaphase, telophase, and
and intermediate filaments (protein subunits end of mitosis. In interphase the chromatins are
twisted to form a rope like structure with 10 visible in the nucleus. In prophase the centrioles
nanometer width). The micrograph B shows begin to align and the chromosomes with
microtubules and intermediate filaments. chromatids and centromere appear. In metaphase
the centrioles with spindle fiber are on the opposite
Overview of Gene Expression – Text Alternative poles with chromosomes in the equatorial plane. In
anaphase identical chromosomes are pulled apart
The illustration shows transcription of DNA strand in by the spindle fibers. In telophase the nuclear
the nucleus lead to formation of mRNA that envelope appear around the identical chromosomes
attaches to the ribosome where translation occurs. and
The polypeptide chain formed has aspartic acid and cleavage of the cells begins. There are 2 cells at the
end of mitosis.
Diversity of Cell Types – Text Alternative

The illustration shows bones cells from the skeletal

system, nerve cells the nervous system, muscle cells
from the muscle system, and red blood cells from
the cardiovascular system.