Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

3. Structure and Function of

the Cell

Companies, 2004

Part 1 Organization ofthe Human Body

The cell isthe basic structural and func-

tionalunit of all living organisms. The char-

acteristic functions of cells include DNA

replication, manufacture ofmacromolecules

such asproteins and phospholipids, energy

use, and reproduction. Cellsare like very com-

plex but minute factoriesthat are always active,

carrying outthe functionsof life. These microscopic fac-

toriesare so small that an average-sized cellis only one-ﬁfth

the size ofthe smallestdot you can make on a sheet of paper with a sharp pencil. Each hu-

man bodyis made up of trillions of cells. If each cellwas the size of a standard brick, the

colossalhuman statue made from those brickswould be 5 1/2 miles (10 km) high!

Allthe cells of an individual originate from a single fertilized cell. During develop-

ment, celldivision and specialization give rise to a wide varietyof cell types, such asnerve,

muscle, bone, fat, and blood cells. Each celltype hasimportant characteristicsthat are crit-

icalto the normal function of the body as a whole. One ofthe impor tantreasons for main-

taining homeostasisisto keep the trillions of cells that form the body functioning normally.

Although cellsmay have quite different structures and functions, they share sev-

eralcommon characteristics (ﬁgure 3.1; table 3.1). The plasma (plazma˘), or cell, mem-

brane forms the outer boundary of the cell, through which the cell interacts with its

externalenvironment. The nucleus (nookle¯-u˘s) isusuallylocated centrallyand functions

to directcell activities, most of which take place in the cytoplasm(sı¯to¯-plazm), located

between the plasma membrane and the nucleus. Within cells, specialized structures

calledorganelles (orga˘-nelz) perform speciﬁcfunctions.

Thischapter outlines functions of the cell (59), how we see cells (59), and the com-

position ofthe plasma membrane (61). Then it addresses movement through the plasma

membrane(65) and endocytosis and exocytosis (73). The chapter then addressesthe cy-

toplasm(75), organelles (77), and nucleus (85). Itthen presents an overview of cell me-

tabolism (87), protein synthesis (87), cell life cycle (90), and meiosis (94). Finally, the

cellularaspects of aging are discussed (97).

Structure and

Function of

the Cell

Colorized scanning electron micrograph (SEM) of

a dividing cell.

CHAPTER

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

3. Structure and Function of

the Cell

Companies, 2004

Chapter 3 Structure and Function ofthe Cell 59

Functions of the Cell

Objective

■ Outline the major functions of the cell.

The main functions ofthe cell include

1. Basic unit of life. The cell is the smallest part to which an

organism can be reduced that still retains the characteristics

oflife.

2. Protection and support.Cells produce and secrete various

molecules that provide protection and support ofthe body.

For example,bone cells are surrounded by a mineralized

material,making bone a hard tissue that protects the brain

and other organs and that supports the weight of the body.

3. Movement.All the movements of the body occur because of

molecules located within speciﬁc cells such as muscle cells.

4. Communication.Cells produce and receive chemical and

electrical signals that allow them to communicate with one

another.For example, nerve cells communicate with one

another and with muscle cells,causing them to contract.

5. Cell metabolism and energy release. The chemical reactions

that occur within cells are referred to collectively as cell

metabolism.Energy released during metabolism is used for

cell activities,such as the synthesis of new molecules, muscle

contraction,and heat production, which helps maintain body

temperature.

6. Inheritance.Each cell contains a copy of the genetic

information ofthe individual. Specialized cells are

responsible for transmitting that genetic information to the

next generation.

How We See Cells

Objective

■ Explain the differences between the two types of

microscopes.

Most cells are too small to be seen with the unaided eye.As a

result,it is necessary to use microscopes to study them. Light mi-

croscopes allow us to visualize general features of cells. Electron

microscopes,however, must be used to study the ﬁne structure of

cells.A scanning electron microscope (SEM) allows us to see fea-

tures of the cell surface and the surfaces of internal structures.A

transmission electron microscope (TEM) allows us to see

“through”parts of the cell and thus to discover other aspects of cell

structure.If you are not somewhat familiar with these types of mi-

croscopes,you should turn to the discussion on microscopic imag-

ing on p.107.

1. What are the major functions of the cell?

2. What are the differences between light and electron

microscopes?

Figure 3.1

The Cell

A generalized human cellshowing the plasma membrane, nucleus, and cytoplasm with itsorganelles. Although no single cell contains all these organelles, many

cellscontain a large number of them.

Microvilli

Free

ribosome

Microtubule

network

Lysosome

fusing with

incoming

phagocytic

vesicle

Nucleus

Phagocytic

vesicle

Centrioles

Centrosome

Nuclear

envelope

Peroxisome

Nucleolus

Ribosome

Rough

endoplasmic

reticulum

Smooth

endoplasmic

reticulum

CytoplasmPlasma

membrane

Mitochondrion

Golgi

apparatus

Secretory

vesicles

Cilia

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Anatomy and Physiology,

Sixth Edition

I. Organization of the

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3. Structure and Function of

the Cell

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Part1 Organization of the Human Body60

Table 3.1

CellParts Structure Function

Summary of Cell Parts

Plasma Membrane Lipid bilayer composed of phospholipids and Outer boundary of cells that controls entry and exit of

cholesterol with proteins that extend across substances; receptor molecules function in intercellular

or are buried in either surface of the lipid communication; marker molecules enable cells to

bilayer recognize one another

Cytoplasm: Cytosol

Fluid part Water with dissolved ions and molecules; colloid Contains enzymes that catalyze decomposition and synthesis

with suspended proteins reactions; ATP is produced in glycolysis reactions

Cytoskeleton

Microtubules Hollow cylinders composed of the protein tubulin; Support the cytoplasm and form centrioles, spindle fibers, cilia,

25 nm in diameter and flagella; responsible for cell movements

Actin filaments Small fibrils of the protein actin; 8 nm in diameter Support the cytoplasm, form microvilli, responsible for cell

movements

Intermediate Protein fibers; 10 nm in diameter Support the cytoplasm

filaments

Cytoplasmic Aggregates of molecules manufactured or ingested Function depends on the molecules: energy storage (lipids,

inclusions by the cell; may be membrane-bound glycogen), oxygen transport (hemoglobin), skin color

(melanin), and others

Cytoplasm: Organelles

Centrioles Pair of cylindrical organelles in the centrosome, Centers for microtubule formation; determine cell polarity

consisting of triplets of parallel microtubules during cell division; form the basal bodies of cilia and flagella

Spindle fibers Microtubules extending from the centrosome to Assist in the separation of chromosomes during cell division

chromosomes and other parts of the cell

(i.e., aster fibers)

Cilia Extensions of the plasma membrane containing Move materials over the surface of cells

doublets of parallel microtubules; 10 µm in length

Flagellum Extension of the plasma membrane containing In humans, responsible for movement of spermatozoa

doublets of parallel microtubules; 55 µm in length

Microvilli Extension of the plasma membrane containing Increase surface area of the plasma membrane for absorption

microfilaments and secretion; modified to form sensory receptors

Ribosome Ribosomal RNA and proteinsform large and small Site of protein synthesis

subunits; attached to endoplasmic reticulum or free

ribosomes are distributed throughout the cytoplasm

Rough endoplasmic Membranous tubules and flattened sacs with Protein synthesis and transport to Golgi apparatus

reticulum attached ribosomes

Smooth endoplasmic Membranous tubules and flattened sacs with no Manufactures lipids and carbohydrates; detoxifies harmful

reticulum attached ribosomes chemicals; stores calcium

Golgi apparatus Flattened membrane sacs stacked on each other Modification, packaging, and distribution of proteins and

lipids for secretion or internal use

Secretory vesicle Membrane-bounded sac pinched off Golgi apparatus Carries proteins and lipids to cellsurface for secretion

Lysosome Membrane-bounded vesicle pinched off Golgi apparatus Contains digestive enzymes

Peroxisome Membrane-bound vesicle One site of lipid and amino acid degradation and breaks down

hydrogen peroxide

Proteasomes Tube-like protein complexes in the cytoplasm Break down proteins in the cytoplasm

Mitochondria Spherical, rod-shaped, or threadlike structures; Major site of ATP synthesiswhen oxygen is available

enclosed by double membrane; inner membrane

forms projections called cristae

Nucleus

Nuclear envelope Double membrane enclosing the nucleus; the outer Separates nucleus from cytoplasm and regulates movement of

membrane is continuous with the endoplasmic materials into and out of the nucleus

reticulum; nuclear pores extend through the

nuclear envelope

Chromatin Dispersed thin strands of DNA, histones, and other DNA regulates protein (e.g., enzyme) synthesis and therefore

proteins; condenses to form chromosomes the chemical reactions of the cell; DNA is the genetic or

during cell division hereditary material

Nucleolus One to four dense bodies consisting of ribosomal Assembly site of large and small ribosomal subunits

RNA and proteins

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Anatomy and Physiology,

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3. Structure and Function of

the Cell

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Chapter 3 Structure and Function ofthe Cell 61

The regulation of ion movement by cells results in a charge

difference across the plasma membrane called the membrane po-

tential.The outside of the plasma membrane is positively charged

compared to the inside because there are more positively charged

ions immediately on the outside of the plasma membrane and

more negatively charged ions inside.The membrane potential al-

lows cells to function like tiny batteries with a positive and negative

pole.It is an important feature of a living cell’s normal function,

which will be considered in greater detail in chapters 9 and 11.

The plasma membrane consists of 45%-50% lipids,

45%-50% proteins,and 4%-8% car bohydrates (ﬁgure 3.2).The

carbohydrates combine with lipids to form glycolipids and with

proteins to form glycoproteins.The glycocalyx (gl¯ı-k¯o-k¯aliks) is

the collection of glycolipids, glycoproteins,and carbohydrates on

the outer surface of the plasma membrane. The glycocalyx also

contains molecules absorbed from the extracellular environment,

so there is often no precise boundary where the plasma membrane

ends and the extracellular environment begins.

Membrane Lipids

The predominant lipids ofthe plasma membrane are phospholipids

and cholesterol. Phospholipids readily assemble to form a lipid

bilayer,a double layer of lipid molecules, because they have a polar

Plasma Membrane

Objectives

■ Deﬁne intracellular, extracellular, glycocalyx, lipid bilayer,

hydrophilic, and hydrophobic.

■ Explain how phospholipids are arranged in the lipid bilayer.

Whatis the function of cholesterol, and where is it found in

the plasma membrane?

■ What is the signiﬁcance of the ﬂuid nature of the lipid bilayer?

■ Outline the function of membrane proteins as markers,

attachmentsites, channels, receptors, enzymes, and

carriers.

The plasma membrane is the outermost component ofa cell.

Substances inside the plasma membrane are intracellular and

substances outside the cell are extracellular.Sometimes extracel-

lular substances are referred to as intercellular,meaning between

cells. The plasma membrane encloses and supports the cell con-

tents.It attaches cells to the extracellular environment or to other

cells.The ability of cells to recognize and communicate with each

other takes place through the plasma membrane.In addition, the

plasma membrane determines what moves into and out ofcells. As

a result,the intracellular contents of cells is different from the ex-

tracellular environment.

Cholesterol

Cytoskeleton

Receptor protein

Peripheral protein

Polar regions

of phospholipid

molecules

Nonpolar

regions

of phospholipid

molecules

Membrane channel protein

Carbohydrate chains

Glycoprotein

Glycolipid

Glycocalyx

External

membrane

surface

Phospholipid

bilayer

Internal

membrane

surface

TEM 100,000x

Figure 3.2

Plasma Membrane

(a) Fluid-mosaicmodel of the plasma membrane. The membrane iscomposed

ofa bilayer of phospholipids and cholesterol with proteins “ﬂoating” in the

membrane. The nonpolar hydrophobicregion of each phospholipid molecule

isdirected toward the center of the membrane and the polar hydrophilic

region isdirected toward the water environment either outside or inside the

cell. (b) Transmission electron micrograph ofa plasma membrane, with the

membrane indicated bythe blue arrows. Proteins at either surface ofthe lipid

bilayer stain more readilythan the lipid bilayer does and give the membrane

the appearance ofconsisting of three parts: the two darkouter par ts are

proteinsand the phospholipid heads, and the lighter central part is the

phospholipid tailsand cholesterol.

(a)

(b)

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

3. Structure and Function of

the Cell

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(charged) head and a nonpolar (uncharged) tail (see chapter2). The

polarhydrophilic (water-loving) heads are exposed to water inside

and outside the cell, whereas the nonpolar hydrophobic (water-

fearing) tails face one another in the interior ofthe plasma membrane.

The other major lipid in the plasma membrane is cholesterol(see

chapter 2),which is interspersed among the phospholipids and ac-

counts for about a third ofthe total lipids in the plasma membrane.

The hydrophilic OH group of cholesterol extends between the

phospholipid heads to the hydrophilic surface of the membrane

and the hydrophobic part ofthe cholesterol molecule lies within the

hydrophobic region of the phospholipids. The amount of choles-

terol in a given membrane is a major factor in determining the ﬂuid

nature ofthe membrane, which is critical to its function.

Membrane Proteins

The basic structure ofthe plasma membrane and some of its func-

tions are determined by its lipids, but many functions of the

plasma membrane are determined by its proteins. The modern

concept of the plasma membrane, the ﬂuid-mosaic model, sug-

gests that the plasma membrane is neither rigid nor static in struc-

ture but is highly ﬂexible and can change its shape and

composition through time.The lipid bilayer functions as a liquid

in which other molecules such as proteins are suspended.The ﬂuid

nature of the lipid bilayer has several important consequences.It

provides an important means ofdistr ibuting molecules within the

plasma membrane. In addition, slight damage to the membrane

can be repaired because the phospholipids tend to reassemble

around damaged sites and seal them closed.The ﬂuid nature of the

lipid bilayer also enables membranes to fuse with one another.

Some protein molecules, called integral, or intr insic, pro-

teins,penetrate deeply into the lipid bilayer, in many cases,extend-

ing from one surface to the other (ﬁgure 3.3), whereas other

proteins,called p eripheral, or extrinsic, proteins,are attached to

either the inner or outer surfaces of the lipid bilayer.Integral pro-

teins consist ofregions made up of amino acids with hydrophobic R

groups and other regions ofamino acids with hydrophilic R groups

(see chapter 2).The hydrophobic regions are located within the hy-

drophobic part of the membrane, and the hydrophilic regions are

located at the inner or outer surface ofthe membrane or line chan-

nels through the membrane.Peripheral proteins are usually bound

to integral proteins. Membrane proteins are markers,attachment

sites,channels, receptors, enzymes, or carriers. The ability of mem-

brane proteins to function depends on their three-dimensional

shapes and their chemical characteristics.

MarkerMolecules

Marker molecules are cell surface molecules that allow cells to

identify one another or other molecules. They are mostly glyco-

proteins (proteins with attached carbohydrates) or glycolipids

(lipids with attached carbohydrates).The protein portions of gly-

coproteins may be either integral or peripheral proteins (ﬁgure

3.4).Examples include recognition of the oocyte by the sperm cell

and the ability of the immune system to distinguish between self-

cells and foreign cells,such as bacteria or donor cells in an organ

transplant. Intercellular communication and recognition are im-

portant because cells are not isolated entities and they must work

together to ensure normal body functions.

AttachmentSites

Membrane-bound proteins,such as integrins, function as attach-

ment sites,where cells attach to other cells or to extracellular mol-

ecules (ﬁgure 3.5). These membrane proteins also attach to

intracellular molecules.Integrins function in pairs of two integral

proteins, which interact with both intracellular and extracellular

molecules.

Channel Proteins

Channel proteins are one or more integral proteins arranged so

that they form a tiny channel through the plasma membrane (ﬁg-

ure 3.6).The hydrophobic regions of the proteins face outward to-

ward the hydrophobic part of the plasma membrane, and the

Part1 Organization of the Human Body62

Figure 3.3

Globular Proteinsin the Plasma Membrane

(a) Proteinsare commonly depicted as ribbons (see chapter 2). The domain

occupied bythe protein ribbon can be enclosed by a three-dimensional

shaded region. (b) The shaded region can be depicted asa three-dimensional

globular integralprotein inserted into the plasma membrane.

Glycoprotein

(cell surface marker)

Figure 3.4

CellSurface Marker

Glycoproteinson the cell surface allow cellsto identify one another or other

molecules.

(a)

(b)

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Chapter 3 Structure and Function ofthe Cell 63

hydrophilic regions ofthe protein face inward and line the channel.

Small molecules or ions of the right shape, size, and charge can

pass through the channel. The charges in the hydrophilic part of

the channel proteins determine which types of ions can pass

through the channel.

Some channel proteins,called nongated ion channels, are

always open and are responsible for the permeability of the

plasma membrane to ions when the plasma membrane is at rest.

Other channels can be open or closed. Some channel proteins

open in response to ligands(lı¯gandz,lı¯gandz).Ligands are small

molecules that bind to proteins or glycoproteins.This is called a

ligand-gated ion channel. Other channel proteins open the

channel when there is a change in charge across the plasma mem-

brane.This is called a voltage-gated ion channel.

ReceptorMolecules

Receptor molecules(ﬁgure 3.7) are proteins in the plasma mem-

brane with an exposed receptor site on the outer cell surface,

which can attach to speciﬁc ligand molecules.Some membrane re-

ceptors are part ofligand-gated channels. Many receptors and the

ligands they bind are part of an intercellular communication sys-

tem that facilitates coordination ofcell activities.

For example,a ner ve cell can release a chemical messenger

that diffuses to a muscle cell and binds to its receptor.The binding

acts as a signal that triggers a response,such as contraction in the

muscle cell.The same chemical messenger would have no effect on

other cells that lack the speciﬁc receptor molecule.

ReceptorsLinked to Channel Proteins

Some membrane-bound receptors are protein molecules that are

part of ligand-gated ion channels in the plasma membrane.When

ligands bind to the receptor sites ofthis type of receptor,the com-

bination alters the three-dimensional structure of the proteins of

the ion channels,causing the channels either to open or close. The

result is a change in the permeability of the plasma membrane to

the speciﬁc ions passing through the ion channels (ﬁgure 3.8).For

example, acetylcholine released from nerve cells is a ligand that

combines with membrane-bound receptors of skeletal muscle

cells.The combination of acetylcholine molecules with the recep-

tor sites ofthe membrane-bound receptors for acetylcholine opens



channels in the plasma membrane. Consequently,the ions

diffuse into the skeletal muscle cells and trigger events that cause

them tocontract.

Attachment proteins

(integrins)

Intracellular molecule

Extracellular molecule

Figure 3.5

AttachmentSites

Proteins(integrins) in the plasma membrane attach to extracellular molecules.

1. Some regions of a

protein are helical.

Each helical region can

be depicted as a

cylinder.

2. In some membrane

proteins, the helical

regions form a circle with

a channel in the center.

4. The channel protein can

be depicted cut in half

to show the channel.

5. The cut channel protein

is depicted within the

plasma membrane.

3. The ring of cylinders can

be depicted as a 3-D

globular structure with a

channel in the center.

This is called a channel

protein.

Protein

Figure 3.6

ChannelProtein

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the Cell

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CysticFibrosis

Cysticﬁbrosisis a genetic disorder that affects chloride ion channels.

Three typesof cystic ﬁbrosis exist. In about 70% of cases, a defective

channelprotein is produced that failsto reach the plasma membrane

from itssite of production inside the cell. In the remaining cases, the

channelprotein is incorporated into the plasma membrane butdoes not

function normally. In some cases, the channelprotein failsto bind ATP. In

others, ATP bindsto the channel protein, butthe channel does not open.

Failure ofthese ion channels to function results in the affected cells

producing thick, viscoussecretions. Although cystic ﬁbrosisaffects many

celltypes, its most profound effects are in the pancreas, causing an

inabilityto digest certain types of food, and in the lungs, where it causes

extreme difﬁcultyin breathing.

ReceptorsLinked to G Proteins

Some membrane-bound receptor molecules function by altering

the activity ofa G proteincomplex located on the inner surface of

the plasma membrane (ﬁgure 3.9).The G protein complex consists

of three proteins, called the alpha, beta,and gamma proteins. A

Gprotein attached to a receptor that does not have a ligand bound

to it is inactive and has guanosine diphosphate (GDP) attached to

it (ﬁgure 3.91). When a ligand attaches to the receptor,the G pro-

tein complex binds guanosine triphosphate (GTP) and is activated

(ﬁgure 3.9 2).The activated G protein stimulates a cell response,

often by means of intracellular chemical signals.Some G proteins

open channels in the plasma membrane and others activate en-

zymes associated with the plasma membrane.

Drugsand Receptors

Drugswith structures similar to speciﬁc ligands may compete with those

ligandsfor their receptor sites. Depending on the exact characteristicsof

a drug, itmay either bind to a receptor site and activate the receptor or

bind to a receptor site and inhibitthe action of the receptor. For

example, drugsexist that compete with the ligand epinephrine for its

receptor sites. Some ofthese drugs activate epinephrine receptors and

othersinhibit them.

Enzymesin the Plasma Membrane

Some membrane proteins function as enzymes, which can cat-

alyze chemical reactions on either the inner or outer surface ofthe

plasma membrane.For example, some enzymes on the surface of

cells in the small intestine break the peptide bonds of dipeptides

(molecules consisting of two amino acids connected by a peptide

bond) to form two single amino acids (ﬁgure 3.10). Some

membrane-associated enzymes are always active. Others are

activated by membrane-bound receptors or G proteins.

CarrierProteins

Carrier proteins are integral membrane proteins that move ions

or molecules from one side ofthe plasma membr ane to the other.

The carrier proteins have speciﬁc binding sites to which ions or

molecules attach on one side ofthe plasma membrane. The carrier

proteins change shape to move the bound ions or molecules to the

other side of the plasma membrane where they are released (ﬁg-

ure3.11)

Part1 Organization of the Human Body64

Ligand

Receptor protein

Receptor site

Figure 3.7

Receptor Protein

A protein in the plasma membrane with an exposed receptor site, which can

attach to speciﬁcligands.

Acetylcholine

Receptor sites

for acetylcholine

Closed Na

channel

can diffuse

through the open channel

Acetylcholine bound

to receptor sites

Open Na

channel

(1) The Na

channel has receptor sites for the

ligand, acetylcholine. When the receptor sites

are not occupied by acetylcholine, the Na

channel remains closed.

(2) When two acetylcholine molecules bind to their

receptor sites on the Na

channel, the channel

opens to allow Na

to diffuse through the

channel into the cell.

ProcessFigure 3.8

ReceptorsLinked to a Channel Protein

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Chapter 3 Structure and Function ofthe Cell 65

3. Deﬁne glycolipid and glycoprotein. Describe the difference

between integral and peripheral proteinsin the plasma

membrane.

4. List two functions of marker molecules.

5. Describe and give the function of integrins.

6. Deﬁne nongated ion channel, ligand-gated ion channel,

and voltage-gated ion channel. Whatdetermines the

function of a channel protein?

7. To whatpart of a receptormolecule does a ligand attach? Give

two examplesof how a ligand molecule can bind to a receptor

in the plasma membrane and cause a response in the cell.

8. Give an example of the action of an enzyme in the plasma

membrane.

Movement Through the Plasma

Membrane

Objectives

■ Describe the four ways by which substances can move

through the plasma membrane.

■ Describe the factors that affect the rate and the direction of

diffusion of a solute in a solvent.

■ Describe diffusion, osmosis, and ﬁltration.

■ Describe the processes of facilitated diffusion, active

transport, and secondaryactive transport.

Ligand

Membrane-bound

receptor

G protein

GDP

GTP

βα

Ligand

Membrane-bound

receptor

Stimulates

a cell response

GDP

GTP

(1)

A G protein attached to a receptor without a bound ligand has

guanosine diphosphate (GDP) bound to it and is inactive.

(2)

When a ligand attaches to the receptor, guanosine triphosphate

(GTP) replaces GDP on the α-subunit of the G protein, which

separates from the other subunits. The α-subunit, with GTP

attached, stimulates a cell response.

ProcessFigure 3.9

A Receptor Linked to a G Protein

Membrane-bound

enzyme

Amino acids

Dipeptide

Figure 3.10

Enzyme in the Plasma Membrane

Thisenzyme in the plasma membrane breaks the peptide bond of a dipeptide

to produce two amino acids.

Carrier protein

Transported molecule

The carrier protein binds with a molecule on one side

of the plasma membrane.

The carrier protein changes shape and releases the

molecule on the other side of the plasma membrane.

ProcessFigure 3.11

Carrier Protein

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The plasma membrane separates the extracellular material

from the intracellular material and is selectively permeable, that

is,it allows only certain substances to pass through it. The intracel-

lular material has a different composition from the extracellular

material,and the surv ival of the cell depends on the maintenance

ofthese differences. Enzymes, other proteins, glycogen, and potas-

sium ions are found in higher concentrations intracellularly; and

sodium,calcium, and chloride ions are found in greater concentra-

tions extracellularly.In addition, nutrients must continually enter

the cell,and waste products must exit,but the volume of the cell re-

mains unchanged.Because of the plasma membrane’s permeability

characteristics and its ability to transport molecules selectively,the

cell is able to maintain homeostasis.Rupture of the membrane, al-

teration of its permeability characteristics, or inhibition of trans-

port processes can disrupt the normal concentration differences

across the plasma membrane and lead to cell death.

Molecules and ions can pass through the plasma membrane

in four ways:

1. Directly through the phospholipid membrane.Molecules that

are soluble in lipids,such as oxygen, carbon dioxide, and

steroids,pass through the plasma membrane readily by

dissolving in the lipid bilayer.The phospholipid bilayer acts

as a barrier to most substances that are not lipid-soluble;

but certain small,nonlipid-soluble molecules, such as water,

carbon dioxide,and urea, can diffuse between the

phospholipid molecules ofthe plasma membrane.

2. Membrane channels.There are several types of protein

channels through the plasma membrane.Each channel type

allows only certain molecules to pass through it.The size,

shape,and charge of molecules determines whether they

can pass through a given channel.For example, sodium ions

pass through sodium channels,and potassium and chloride

ions pass through potassium and chloride channels,

respectively.Rapid movement of water across the cell

membrane apparently occurs through membrane channels.

3. Carrier molecules. Large polar molecules that are not lipid-

soluble,such as glucose and amino acids, cannot pass

through the cell membrane in signiﬁcant amounts unless

they are transported by carrier molecules.Substances that

are transported across the cell membrane by carrier

molecules are said to be transported by carrier-mediated

processes.Carrier proteins bind to speciﬁc molecules and

transport them across the cell membrane.Carrier molecules

that transport glucose across the cell membrane do not

transport amino acids,and carrier molecules that transpor t

amino acids do not transport glucose.

4. Vesicles.Large nonlipid-soluble molecules, small pieces of

matter,and even whole cells can be transported across the

cell membrane in a vesicle,which is a small sac surrounded

by a membrane.Because of the ﬂuid nature of membranes,

the vesicle and the cell membrane can fuse,allowing the

contents ofthe vesicle to cross the cell membrane.

Diffusion

A solution consists ofone or more substances called solutes dis-

solved in the predominant liquid or gas, which is called the

solvent. Diffusion is the movement of solutes from an area of

higher concentration to an area of lower concentration in solu-

tion (ﬁgure 3.12).Diffusion is a product of the constant random

motion of all atoms, molecules, or ions in a solution. Because

more solute particles exist in an area of higher concentration

than in an area oflower concentration and because the particles

move randomly,the chances are greater that solute particles will

move from the higher to the lower concentration than in the op-

posite direction.Thus the overall, or net, movement is from the

area of higher concentration to that of lower concentration.At

equilibrium, the net movement of solutes stops, although the

random molecular motion continues, and the movement of

solutes in any one direction is balanced by an equal movement in

the opposite direction. The movement and distribution of

smoke or perfume throughout a room in which no air currents

exist or of a dye throughout a beaker of still water are examples

ofdiffusion.

A concentration difference exists when the concentration of

a solute is greater at one point than at another point in a solvent.

The concentration difference between two points is called the con-

centration,or density gradient. Solutes diffuse with their concen-

tration gradients (from a higher to a lower concentration) until an

equilibrium is achieved. For a given concentration difference be-

tween two points in a solution,the concentration gradient is larger

ifthe distance between the two points is small, and the concentra-

tion gradient is smaller if the distance between the two points

islarge.

The rate of diffusion is inﬂuenced by the magnitude of the

concentration gradient, the temperature of the solution,the size

of the diffusing molecules, and the viscosity of the solvent. The

greater the concentration gradient, the greater is the number of

solute particles moving from a higher to a lower concentration.As

the temperature of a solution increases, the speed at which all

molecules move increases, resulting in a greater diffusion rate.

Small molecules diffuse through a solution more readily than do

large ones. Viscosity is a measure of how easily a liquid ﬂows;

thick solutions,such as syrup, are more viscous than water. Diffu-

sion occurs more slowly in viscous solvents than in thin,

waterysolvents.

Diffusion ofmolecules is an important means by which sub-

stances move between the extracellular and intracellular ﬂuids in

the body.Substances that can diffuse through either the lipid bi-

layer or the membrane channels can pass through the plasma

membrane. Some nutrients enter and some waste products leave

the cell by diffusion,and maintenance of the appropriate intracel-

lular concentration of these substances depends to a large degree

on diffusion. For example, if the extracellular concentration of

oxygen is reduced, inadequate oxygen diffuses into the cell,and

normal cell function cannot occur.Some lipid-soluble ligands can

diffuse through the plasma membrane and attach to receptors in-

side the cell (ﬁgure 3.13).

PREDICT

Urea isa toxic waste produced inside cells. Itdiffuses from the cells

into the blood and iseliminated from the body by the kidneys. What

would happen to the intracellular and extracellular concentration of

urea ifthe kidneys stopped functioning?

Part1 Organization of the Human Body66

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Chapter 3 Structure and Function ofthe Cell 67

Osmosis

Osmosis(os-mo¯sis) is the diffusion ofwater (solvent) across a se-

lectively permeable membrane,such as a plasma membrane. A se-

lectively permeable membrane is a membrane that allows water

but not all the solutes dissolved in the water to diffuse through the

membrane. Water diffuses from a solution with proportionately

more water,across a selectively permeable membrane, and into a

solution with proportionately less water.Because solution concen-

trations are deﬁned in terms of solute concentrations and not in

terms ofwater content (see chapter 2), water diffuses from the less

concentrated solution (fewer solutes, more water) into the more

concentrated solution (more solutes, less water).Osmosis is im-

portant to cells because large volume changes caused by water

movement disrupt normal cell function.

Osmotic pressureis the force required to prevent the move-

ment of water by osmosis across a selectively permeable mem-

brane. The osmotic pressure of a solution can be determined by

placing the solution into a tube that is closed at one end by a selec-

tively permeable membrane (ﬁgure 3.14). The tube is then im-

mersed in distilled water. Water molecules move by osmosis

through the membrane into the tube,forcing the solution to move

up the tube.As the solution rises into the tube, its weight produces

hydrostatic pressure that moves water out ofthe tube back into the

distilled water surrounding the tube. At equilibrium,net move-

ment ofwater stops, which means the movement of water into the

tube by osmosis is equal to the movement ofwater out of the tube

caused by hydrostatic pressure.The osmotic pressure of the solu-

tion in the tube is equal to the hydrostatic pressure that prevents

net movement ofwater into the tube.

The osmotic pressure of a solution provides information

about the tendency for water to move by osmosis across a

selectively permeable membrane. Because water moves from less

Concentration

gradient for red

molecules

Concentration

gradient for blue

molecules

1. One solution (

red balls

representing one type of solute

molecule) is layered onto a

second solution (

blue balls

represent a second type of solute

molecule). There is a

concentration gradient for the red

molecules from the red solution

into the blue solution because

there are no red molecules in the

blue solution. There is also a

concentration gradient for the blue

molecules from the blue solution

into the red solution because

there are no blue molecules in the

red solution.

2. Red molecules move down

their concentration gradient

into the blue solution (

red

arrow

), and the blue

molecules move down their

concentration gradient into

the red solution (

blue

arrow

3. Red and blue molecules are

distributed evenly throughout

the solution. Even though the

red and blue solute molecules

continue to move randomly, an

equilibrium exists, and no net

movement occurs because no

concentration gradient exists.

ProcessFigure 3.12

Diffusion

Ligand

Receptor site

Intracellular

receptor

Figure 3.13

Intracellular Receptor

Thissmall, lipid-soluble ligand diffuses through the plasma membrane and

combineswith the receptor site of an intracellular receptor.

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concentrated solutions (fewer solutes,more water) into more con-

centrated solutions (more solutes, less water), the greater the

concentration of a solution (the less water it has), the greater

the tendency for water to move into the solution, and the

greaterthe osmotic pressure to prevent that movement. Thus, the

greater the concentration of a solution, the greater the osmotic

pressure ofthe solution, and the greater the tendency for water to

move into the solution.

PREDICT

Given the demonstration in ﬁgure 3.14, whatwould happen to

osmoticpressure if the membrane were not selectively permeable but

instead allowed allsolutes and water to pass through it?

Three terms describe the osmotic pressure of solutions.So-

lutions with the same concentration ofsolute particles (see chapter

2) have the same osmotic pressure and are referred to as isosmotic

Part1 Organization of the Human Body68

Because the tube contains salt ions*

(

green and red spheres

)as well

as water molecules (

blue spheres

)

the tube has proportionately less

water than is in the beaker,

which contains only water. The

water molecules diffuse with their

concentration gradient into the

tube (

blue arrows

)

Because the

salt ions cannot leave the

tube, the total fluid volume inside

the tube increases, and fluid moves

up the glass tube (

black arrow

)as a

result of osmosis.

Water

3% salt solution

Selectively

permeable

membrane

Salt solution

rising

Distilled

water

3. Water continues to move into

the tube until the weight of

the column of water in the

tube (hydrostatic pressure)

exerts a downward force

equal to the osmotic force

moving water molecules into

the tube. The hydrostatic

pressure that prevents net

movement of water into the

tube is equal to the osmotic

pressure of the solution in

the tube.

2. The tube is immersed in

distilled water. Water

moves into the tube by

osmosis (see inset above*).

The concentration of salt in

the tube decreases as

water rises in the tube

(

lighter green color

1. The end of a tube

containing a 3% salt

solution (

green

) is closed

at one end with a

selectively permeable

membrane, which allows

water molecules to pass

through it but retains the

salt ions within the tube.

Solution stops

rising when weight

of water column

equals osmotic

force.

Weight

of water

column

Osmotic

force

ProcessFigure 3.14

Osmosis

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because crenation or swelling of cells disrupts their normal func-

tion and can lead to cell death.

The -osmotic terms refer to the concentration of the solu-

tions,and the -tonic terms refer to the tendency of cells to swell or

shrink. These terms should not be used interchangeably.Not all

isosmotic solutions are isotonic.For example, it is possible to pre-

pare a solution ofglycerol and a solution of mannitol that are isos-

motic to the cytoplasm of the cell. Because the solutions are

isosmotic,they have the same concentration of solutes and water as

the cytoplasm. Glycerol,however, can diffuse across the plasma

membrane,and mannitol cannot. When glycerol diffuses into the

cell, the solute concentration of the cytoplasm increases, and its

water concentration decreases.Therefore, water moves by osmosis

into the cell,causing it to swell, and the glycerol solution is both

isosmotic and hypotonic. In contrast,mannitol cannot enter the

cell,and the isosmotic mannitol solution is also isotonic.

Filtration

Filtrationresults when a partition containing small holes is placed

in a stream of moving liquid. The partition works like a minute

sieve. Particles small enough to pass through the holes move

through the partition with the liquid,but par ticles larger than the

holes are prevented from moving beyond the partition.In contrast

to diffusion,ﬁltr ation depends on a pressure difference on either

side of the partition. The liquid moves from the side of the parti-

tion with the greater pressure to the side with the lower pressure.

Filtration occurs in the kidneys as a step in urine formation.

Blood pressure moves ﬂuid from the blood through a partition,or

filtration membrane. Water,ions, and small molecules pass

through the partition, whereas most proteins and blood cells re-

main in the blood.

(ı¯sos-motik).The solutions are still isosmotic even ifthe t ypes of

solute particles in the two solutions differ from each other.If one

solution has a greater concentration of solute particles and there-

fore a greater osmotic pressure than another solution,the ﬁrst so-

lution is said to be hyperosmotic(hı¯per-oz-motik) compared to

the more dilute solution.The more dilute solution, with the lower

osmotic pressure,is hyposmotic (hı¯-pos-motik) compared to the

more concentrated solution.

Three additional terms describe the tendency of cells to

shrink or swell when placed into a solution.If a cell is placed into a

solution in which it neither shrinks nor swells,the solution is said

to be isotonic (ı¯-so¯-tonik).If a cell is placed into a solution and

water moves out ofthe cell by osmosis, causing the cell to shrink,

the solution is called hypertonic(hı¯-per-tonik).If a cell is placed

into a solution and water moves into the cell by osmosis,causing

the cell to swell, the solution is called hypotonic (hı¯-po¯-tonik)

(ﬁgure 3.15a).

An isotonic solution may be isosmotic to the cytoplasm.Be-

cause isosmotic solutions have the same concentration of solutes

and water as the cytoplasm of the cell,no net movement of water

occurs,and the cell neither swells nor shrinks (ﬁgure 3.15b). Hy-

pertonic solutions can be hyperosmotic and have a greater concen-

tration of solute molecules and a lower concentration of water

than the cytoplasm of the cell.Therefore water moves by osmosis

from the cell into the hypertonic solution, causing the cell to

shrink, a process called crenation (kre¯-na¯shu˘n) (ﬁgure 3.15c).

Hypotonic solutions can be hyposmotic and have a smaller con-

centration ofsolute molecules and a greater concentration of water

than the cytoplasm of the cell.Therefore water moves by osmosis

into the cell,causing it to swell. If the cell swells enough, it can rup-

ture,a process called lysis (lı¯sis) (see ﬁgure 3.15a).Solutions in-

jected into the circulatory system or the tissues must be isotonic

Red blood cell

Hypotonic solution

Isotonic solution Hypertonic solution

(a) A hypotonic solution with a

low solute concentration

results in swelling (

black

arrows

) and lysis (

puff of red

in the lower left part of the

cell

) of a red blood cell placed

into the solution.

(b) An isotonic solution with a

concentration of solutes

equal to that inside the cell

results in a normally shaped

red blood cell. Water moves

into and out of the cell in

equilibrium (

black arrows

but there is no net water

movement.

high solute concentration,

causes shrinkage (crenation)

of the red blood cell as water

moves out of the cell and into

the hypertonic solution (

black

arrows

Figure 3.15

Effectsof Hypotonic, Isotonic, and Hypertonic Solutions on Red Blood Cells

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9. List four ways that substances move across the plasma

membrane.

10. Deﬁne solute, solvent, and concentration gradient. Do

solutesdiffuse with or against their concentration gradient?

11. How is the rate of diffusion affected by an increased

concentration gradient? Byincreased temperature of a

solution? Byincreased viscosity of the solvent?

12. Deﬁne osmosis and osmotic pressure. As the concentration

of a solution increases, whathappens to its osmotic

pressure and to the tendencyfor water to move into it?

13. Compare isosmotic, hyperosmotic, and hyposmotic

solutionsto isotonic, hypertonic, and hypotonic solutions.

Whattype of solution causes crenation of a cell? What type

of solution causeslysis of a cell?

14. Deﬁne ﬁltration and give an example of where it occurs in

the body.

Mediated TransportMechanisms

Many essential molecules,such as amino acids and glucose, cannot

enter the cell by simple diffusion,and many products, such as pro-

teins,cannot exit the cell by diffusion. Mediated transport mecha-

nisms involve carrier proteins within the plasma membrane that

move large, water-soluble molecules or electrically charged mole-

cules across the plasma membrane. Once a molecule to be trans-

ported binds to the carrier protein on one side ofthe membrane, the

three-dimensional shape of the carrier protein changes, and the

transported molecule is moved to the opposite side of the mem-

brane (see ﬁgure 3.11).The carrier protein then resumes its original

shape and is available to transport other molecules.

Mediated transport mechanisms have three characteristics:

speciﬁcity, competition,and saturation. Speciﬁcity means that

each carrier protein binds to and transports only a single type of

molecule.For example, the carrier protein that transports glucose

does not bind to amino acids or ions.The chemical structure of the

binding site determines the speciﬁcity of the carrier protein (see

ﬁgure 3.11). Competition is the result of similar molecules bind-

ing to the carrier protein.Although the binding sites of carrier pro-

teins exhibit speciﬁcity,closely related substances may bind to the

same binding site. The substance in the greater concentration or

the substance that binds to the binding site more readily is trans-

ported across the plasma membrane at the greater rate (ﬁgure

3.16b).Satur ation means that the rate of transport of molecules

across the membrane is limited by the number ofavailable carrier

proteins. As the concentration of a transported substance in-

creases, more carrier proteins have their binding sites occupied.

The rate at which the substance is transported increases;however,

once the concentration ofthe substance is increased so that all the

binding sites are occupied,the rate of transport remains constant,

even though the concentration of the substance increases further

(ﬁgure 3.17).

Three kinds ofmediated transport exist: facilitated diffusion,

active transport,and secondary active transport.

Facilitated Diffusion

Facilitated diffusion is a carrier-mediated process that moves

substances into or out ofcells from a higher to a lower concentra-

tion. Facilitated diffusion does not require metabolic energy to

transport substances across the plasma membrane. The rate at

which molecules are transported is directly proportional to their

concentration gradient up to the point of saturation,when all the

carrier proteins are occupied.Then the rate of transport remains

constant at its maximum rate.

PREDICT

The transportof glucose into and out of most cells, such asmuscle

and fatcells, occurs by facilitated diffusion. Once glucose entersa

cell, itis rapidly converted to other molecules, such asglucose-6-

phosphate or glycogen. Whateffect does this conversion have on the

abilityof the cell to acquire glucose? Explain.

Active Transport

Active transportis a mediated transport process that requires en-

ergy provided by ATP (ﬁgure 3.18).Movement of the transported

substance to the opposite side ofthe membrane and its subsequent

release from the carrier protein are fueled by the breakdown of

ATP.The maximum rate at which active transport proceeds de-

pends on the number ofcar rier proteins in the plasma membrane

and the availability ofadequate ATP.Active-transport processes are

important because they can move substances against their concen-

tration gradients,that is, from lower concentrations to higher con-

centrations. Consequently,the y have the ability to accumulate

Part1 Organization of the Human Body70

Competition. Similarly shaped molecules can

compete for the same binding site.

Specificity. Only molecules that are the right shape

to bind to the binding site are transported.

Yes Yes

Yes

(b)

Binding site

(a)

Figure 3.16

Mediated Transport: Speciﬁcityand

Competition

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substances on one side ofthe plasma membrane at concentrations

many times greater than those on the other side.Active transport

can also move substances from higher to lower concentrations.

Some active-transport mechanisms exchange one sub-

stance for another. For example, the sodium–potassium ex-

change pump moves sodium out ofcells and potassium into cells

(ﬁgure 3.18).The result is a higher concentration of sodium out-

side the cell and a higher concentration of potassium inside

thecell.

15. What is mediated transport? What types of molecules are

moved through the plasma membrane bymediated

transport?

16. Describe speciﬁcity, competition, and saturation as

characteristicsof mediated transport mechanisms.

17. Contrast facilitated diffusion and active transport in relation

to energyexpenditure and movement of substances with or

againsttheir concentration gradients.

18. What are secondary active transport, cotransport, and

countertransport?

SecondaryActive Transport

Secondary active transport involves the active transport of an

ion such as sodium out ofa cell, establishing a concentration gra-

dient,with a higher concentration of the ions outside the cell. The

tendency for the ions to move back into the cell,down their con-

centration gradient, provides the energy necessary to transport a

different ion or some other molecule into the cell.For example,

glucose is transported from the lumen ofthe intestine into epithe-

lial cells by secondary active transport (ﬁgure 3.19).This process

requires two carrier proteins:(1) a sodium–potassium exchange

pump actively transports Na



out ofthe cell, and (2) the other car-

rier protein facilitates the movement ofNa



and glucose into the

cell. Both Na



and glucose are necessary for the carrier protein

tofunction.

The movement of Na



down their concentration gradient

provides the energy to move glucose molecules into the cell against

their concentration gradient. Thus glucose can accumulate at

concentrations higher inside the cell than outside. Because the

movement of glucose molecules against their concentration

The rate of transport of molecules into a

cell is plotted against the concentration

of those molecules outside the cell. As

the concentration increases, the rate of

transport increases and then levels off.

1. When the concentration of molecules

outside the cell is low, the transport

rate is low because it is limited by the

number of molecules available to be

transported.

2. When more molecules are present

outside the cell, as long as enough

carrier proteins are available, more

molecules can be transported, and

therefore the transport rate increases.

3. The transport rate is limited by the

number of carrier proteins and the

rate at which each carrier protein can

transport solutes. When the number

of molecules outside the cell is so

large that the carrier proteins are all

occupied, the system is saturated and

the transport rate cannot increase.

Extracellular fluid

Cytoplasm

Concentration of molecules

outside the cell

Rate of

molecule

transport

Carrier protein

Molecule to be transported

ProcessFigure 3.17

Saturation ofa Carrier Protein

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gradient results from the formation ofa concentration gr adient of



by an active transport mechanism,the process is called sec-

ondary active transport.

The ions or molecules moved by secondary active transport

can move in the same direction as or in a different direction across

the membrane than the ion that enters the cell by diffusion down

its concentration gradient.Cotransp ort, or sympor t, is a type of

secondary active transport where movement is in the same direc-

tion. For example,glucose, fructose, and amino acids move with



into cells ofthe intestine and kidneys. Countertransp ort, or

antiport, is a type of secondar y active transport where ions or

molecules move in opposite directions.For example, the internal

pH of cells is maintained by countertransport, which moves H



out ofthe cell as Na



move into the cell.

Part1 Organization of the Human Body72

Extracellular fluid

Cytoplasm

ATP binding site

Carrier protein

1. Three sodiumions (Na

) and adenosine triphosphate

(ATP) bind to the carrier protein.

ADP

Breakdown of ATP

(releases energy)

Carrier protein changes

shape (requires energy)

2. The ATP breaks down to adenosine diphosphate

(ADP) and a phosphate (P) and releases energy.

3. The carrier protein changes shape, and the Na

are

transported across the membrane.

4. The Na

diffuse away from the carrier protein.

5.Two potassium ions (K

) bind to the carrier protein.

6. The phosphate is released.

Carrier protein resumes

original shape

7. The carrier protein changes shape, transporting K

across the membrane, and the K

diffuse away from

the carrier protein. The carrier protein can again

bind to Na

and ATP.

ATP

ProcessFigure 3.18

Sodium-Potassium Exchange Pump

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Chapter 3 Structure and Function ofthe Cell 73

PREDICT

In cardiac(heart) muscle cells, the concentration of intracellular

2

affectsthe force of heart contraction. The higher the intracellular

2

concentration, the greater the force ofcontraction. Na



/Ca

2

countertransporthelps to regulate intracellular Ca

2

levelsby

transporting Ca

2

outof cardiac muscle cells. Given that digitalis

slowsthe transport of Na



, should the heartbeat more or less

forcefullywhen exposed to this drug? Explain.

Endocytosisand Exocytosis

Objective

■ Describe the processes of endocytosis and exocytosis.

Endocytosis (endo¯-sı¯-to¯sis), or the internalization of sub-

stances,includes both phagocy tosis and pinocytosis and refers to

the bulk uptake ofmaterial through the plasma membr ane by the

formation ofa vesicle. A vesicle is a membrane-bounded sac found

within the cytoplasm of a cell.A portion of the plasma membrane

wraps around a particle or droplet and fuses so that the particle or

droplet is surrounded by a membrane.That portion of the mem-

brane then “pinches off”so that the particle or droplet, surrounded

by a membrane,is within the cytoplasm of the cell, and the plasma

membrane is left intact.

Phagocytosis (f a¯g-o¯ -sı¯-to¯sis) literally means cell-eating

(ﬁgure 3.20) and applies to endocytosis when solid particles are in-

gested and phagocytic vesicles are formed.White blood cells and

some other cell types phagocytize bacteria,cell debris, and foreign

particles.Phagocytosis is therefore important in the elimination of

harmful substances from the body.

Pinocytosis(pino¯-sı¯-to¯sis) means cell-drinking and is dis-

tinguished from phagocytosis in that smaller vesicles are formed

and they contain molecules dissolved in liquid rather than particles

(ﬁgure 3.21).Pinocytosis often forms vesicles near the tips of deep

invaginations of the plasma membrane.It is a common transport

Extracellular fluid

Sodium–

potassium

exchange

pump

Glucose

Carrier

molecule

A sodium–potassium exchange pump maintains a

concentration of Na

that is higher outside the cell

than inside.

move back into the cell by a carrier protein

that also moves glucose. The concentration gradient

for Na

provides energy required to move glucose

against its concentration gradient.

This example shows cotransport of Na

and glucose.

Cytoplasm

ProcessFigure 3.19

SecondaryActive Transport

Cell

processes

Particle

Phagocytic

vesicle

SEM 7,000x

Figure 3.20

Endocytosis

(a) Phagocytosis. (b) Transmission electron micrograph ofphagocytosis.

(a)

(b)

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the cells.Cholesterol and growth factors are examples of molecules

that can be taken into a cell by receptor-mediated endocytosis.

Both phagocytosis and pinocytosis require energy in the form of

ATP and therefore are active processes.Because they involve the

bulk movement of material into the cell, however,phagocytosis

and pinocytosis do not exhibit either the degree of speciﬁcity or

saturation that active transport exhibits.

Hypercholesterolemia

Hypercholesterolemiais a common geneticdisorder affecting 1 in every

500 adultsin the United States. It consists ofa reduction in or absence

oflow-density lipoprotein (LDL) receptors on cell surfaces. Thisinterferes

with receptor-mediated endocytosisof LDL cholesterol. Asa result of

inadequate cholesteroluptake, cholesterol synthesiswithin these cells

isnot regulated, and too much cholesterol is produced. The excess

cholesterolaccumulates in blood vessels, resulting in atherosclerosis.

Atherosclerosiscan result in heartattacks or strokes.

In some cells, secretions accumulate within vesicles.These

secretory vesicles then move to the plasma membrane,where the

membrane of the vesicle fuses with the plasma membrane and the

content ofthe vesicle is expelled from the cell. This process is called

exocytosis(ekso¯-sı¯-to¯sis) (ﬁgure 3.23).Secretion of digestive en-

zymes by the pancreas,of mucus by the salivary glands, and of milk

by the mammary glands are examples of exocytosis. In some re-

spects the process is similar to phagocytosis and pinocytosis but

occurs in the opposite direction.

Table 3.2 summarizes and compares the mechanisms by

which different kinds of molecules are transported across the

plasma membrane.

19. Deﬁne endocytosis and vesicle. How do phagocytosis and

pinocytosisdiffer from each other?

20. What is receptor-mediated endocytosis?

21. Describe and give examples of exocytosis.

Part1 Organization of the Human Body74

phenomenon in a variety ofcell ty pes and occurs in certain cells of

the kidneys,epithelial cells of the intestines, cells of the liver,and

cells that line capillaries.

Endocytosis can exhibit speciﬁcity. For example,cells that

phagocytize bacteria and necrotic tissue do not phagocytize

healthy cells.The plasma membrane may contain speciﬁc receptor

molecules that recognize certain substances and allow them to be

transported into the cell by phagocytosis or pinocytosis. This is

called receptor-mediated endocytosis, and the receptor sites

combine only with certain molecules (ﬁgure 3.22). This mecha-

nism increases the rate at which speciﬁc substances are taken up by

Pinocytotic

vesicles

Capillary

wall

Interior of

capillary

Exterior of

capillary

Exterior of capillary

Endothelial

cell of capillary

Pinocytosis

Exocytosis

Interior of capillary

Red blood

cell

TEM 72,000x

Figure 3.21

Pinocytosis

(a) Pinocytosisis much like phagocytosis, exceptthe cell processes and

therefore the vesiclesformed are much smaller and the material inside the

vesicle isliquid rather than particulate. Pinocytotic vesicles form on the internal

side ofa capillary, are transported across the cell, and open byexocytosis

outside the capillary. (b) Transmission electron micrograph ofpinocytosis.

1.Receptor molecules on the cell

surface bind to molecules to be

taken into the cell.

2.The receptors and the bound

molecules are taken into the

cell as a vesicle is formed.

3.A vesicle is formed.

Vesicle

Molecules to be transported

ProcessFigure 3.22

Receptor-Mediated Endocytosis

(a)

(b)

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Cytoplasm

Objective

■ Describe the cytosol and cytoskeleton of the cell.

Cytoplasm,the cellular material outside the nucleus but inside

the plasma membrane,is about half cytosol and half organelles.

Cytosol

Cytosol(sı¯to¯-sol) consists of a ﬂuid portion, a cytoskeleton,and cy-

toplasmic inclusions.The ﬂuid portion of cytosol is a solution w ith

dissolved ions and molecules and a colloid with suspended mole-

cules, especially proteins.Many of these proteins are enzymes that

catalyze the breakdown of molecules for energy or the synthesis of

sugars,fatty acids, nucleotides, amino acids, and other molecules.

Cytoskeleton

Thecy toskeleton supports the cell and holds the nucleus and or-

ganelles in place.It is also responsible for cell movements, such as

changes in cell shape or movement of cell organelles. The cy-

toskeleton consists ofthree groups of proteins: microtubules, actin

ﬁlaments,and intermediate ﬁlaments (ﬁgure 3.24).

Microtubules are hollow tubules composed primarily of

protein units called tubulin. The microtubules are about 25

nanometers (nm) in diameter,with walls about 5 nm thick. Micro-

tubules vary in length but are normally several micrometers (m)

long. Microtubules play a variety of roles within cells. They help

provide support and structure to the cytoplasm of the cell, much

like an internal scaffolding.They are involved in the process of cell

division, transport of intracellular materials, and form essential

components of certain cell organelles, such as centrioles,spindle

ﬁbers,cilia, and ﬂagella.

Actin ﬁlaments,or microﬁlaments, are small ﬁbrils about

8nm in diameter that form bundles, sheets, or networks in the cy-

toplasm ofcells. These ﬁlaments have a spiderweb-like appearance

within the cell.Actin ﬁlaments provide structure to the cytoplasm

and mechanical support for microvilli.Actin ﬁlaments support the

plasma membrane and deﬁne the shape ofthe cell. Changes in cell

shape involve the breakdown and reconstruction of actin ﬁla-

ments. These changes in shape allow some cells to move about.

Muscle cells contain a large number ofhighly organized actin ﬁla-

ments responsible for the muscle’s contractile capabilities (see

chapter 9).

Intermediate ﬁlaments are protein ﬁbers about 10 nm in

diameter.They provide mechanical strength to cells. For example,

intermediate ﬁlaments support the extensions ofnerve cells, which

have a very small diameter but can be a meter in length.

CytoplasmicInclusions

The cytosol also contains cytoplasmic inclusions, which are ag-

gregates ofchemicals either produced by the cell or taken in by the

cell.For example, lipid droplets or glycogen granules store energy-

rich molecules; hemoglobin in red blood cells transports oxygen;

melanin is a pigment that colors the skin, hair, and eyes;and

lipochromes(lipo¯-kro¯mz) are pigments that increase in amount

with age. Dust, minerals, and dyes can also accumulate in the

cytoplasm.

22. Deﬁne cytoplasm and cytosol.

23. What are the two general functions of the cytoskeleton?

24. Describe and list the functions of microtubules, actin

ﬁlaments, and intermediate ﬁlaments.

25. Deﬁne and give examples of cytoplasmic inclusions. What

are lipochromes?

1. The Golgi apparatus concentrates and, in

some cases, modifies protein molecules

produced by the rough endoplasmic

reticulum and then packages them in

secretory vesicles.

3. In exocytosis, the vesicle moves to

the plasma membrane, fuses with

the membrane, opens to the

outside, and releases its contents

into the extracellular space.

2. A secretory vesicle is pinched off

the Golgi apparatus.

Plasma membrane

Released contents

of secretory vesicle

Secretory vesicle from

Golgi apparatus

Secretory vesicle fused

to the plasma membrane

Golgi apparatus

TEM 30,000x

ProcessFigure 3.23

Exocytosis

(a) Example ofexocytosis. (b) Transmission electron micrograph ofexocytosis.

(a)

(b)

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Table 3.2

Transport Substances

Mechanism Description Transported Example

Comparison of Membrane Transport Mechanisms

Diffusion Random movement of molecules Lipid-soluble molecules dissolve in Oxygen, carbon dioxide, and lipids

results in net movement from the lipid bilayer and diffuse such as steroid hormones

areas of higher to lower through it; ions and small dissolve in the lipid bilayer; Cl



concentration. molecules diffuse through and urea move through

membrane channels. membrane channels.

Osmosis Water diffuses across a selectively Water diffuses through the Water moves from the stomach

permeable membrane. lipid bilayer. into the blood.

Filtration Liquid moves through a partition Liquid and substances pass through Filtration in the kidneys allows

that allows some, but not all, of holes in the partition. removal of everything from the

the substances in the liquid to blood except proteins and

pass through it; movement is due blood cells.

to a pressure difference across

the partition.

Facilitated diffusion Carrier molecules combine with Some substances too large to pass Glucose moves by facilitated

substances and move them through membrane channels and diffusion into muscle cells and

across the plasma membrane; no too polar to dissolve in the lipid fat cells.

ATP is used; substances are bilayer are transported.

always moved from areas of

higher to lower concentration; it

exhibits the characteristics of

specificity, saturation, and

competition.

Active transport Carrier molecules combine with Substances too large to pass through Ions such as Na



, K



, and Ca

2

substances and move them channels and too polar to are actively transported.

across the plasma membrane; dissolve in the lipid bilayer are

ATP is used; substances can be transported; substances that are

moved from areas of lower to accumulated in concentrations

higher concentration; it exhibits higher on one side of the

the characteristics of specificity, membrane than on the other are

saturation, and competition. transported.

Secondary active Ions are moved acrossthe plasma Some sugars, amino acids, and ions A concentration gradient for Na



transport membrane by active transport, are transported. exists in intestinal epithelial

which establishes a concentration cells. This gradient provides

gradient; ATP is required; ions the energy for the cotransport

then move back down their of glucose. As Na



enter the

concentration gradient by cell, down their concentration

facilitated diffusion, and another gradient, glucose also enters

ion or molecule moves with the the cell. In many cells, H



diffusion ion (cotransport) or in countertransported (in the

the opposite direction opposite direction) with Na



(countertransport).

Endocytosis The plasma membrane forms a Phagocytosis takes in cells and solid Immune system cells called

vesicle around the substances to particles; pinocytosis takes in phagocytes ingest bacteria and

be transported, and the vesicle is molecules dissolved in liquid. cellular debris; most cells take

taken into the cell; this requires in substances through

ATP; in receptor-mediated pinocytosis.

endocytosis specific substances

are ingested.

Exocytosis Materials manufactured by the cell Proteins and other water-soluble Digestive enzymes, hormones,

are packaged in secretory vesicles molecules are transported neurotransmitters, and

that fuse with the plasma out of cells. glandular secretions are

membrane and release their transported, and cell waste

contents to the outside of the products are eliminated.

cell; this requires ATP.

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Chapter 3 Structure and Function ofthe Cell 77

Organelles

Objectives

■ Describe centrioles, spindle ﬁbers, cilia, ﬂagella, and

microvilli.

■ Explain the structure and function of ribosomes, rough

endoplasmicreticulum, smooth endoplasmic reticulum,

Golgi apparatus, and secretoryvesicles.

■ Distinguish between lysosomes, peroxisomes, and

proteasomes.

■ Describe the structure and function of mitochondria.

Organelles are small structures within cells that are special-

ized for particular functions,such as manufacturing proteins or pro-

ducing ATP.Organelles can be thought ofas individual workstations

within the cell,each responsible for performing speciﬁc tasks. Most,

but not all organelles have membranes that are similar to the plasma

membrane. The membranes separate the interior of the organelles

from the rest ofthe cy toplasm,creating a subcellular compartment

with its own enzymes that is capable of carrying out its own unique

chemical reactions.The nucleus is an example of an organelle.

The number and type of cytoplasmic organelles within each

cell are related to the speciﬁc structure and function of the cell.

Cells secreting large amounts of protein contain well-developed

organelles that synthesize and secrete protein,whereas cells actively

transporting substances such as sodium ions across their plasma

membrane contain highly developed organelles that produce ATP.

The following sections describe the structure and main functions

ofthe major cy toplasmic organelles found in cells.

Centriolesand Spindle Fibers

Thecentrosome (sentro¯-so¯m) is a specialized zone ofcytoplasm

close to the nucleus that is the center ofmicrotubule formation. It

contains two centrioles (sentre¯-o¯lz). Each centriole is a small,

cylindrical organelle about 0.3–0.5 m in length and 0.15 m in

diameter,and the two centrioles are normally oriented perpen-

dicular to each other within the centrosome (see ﬁgure 3.1).The

wall ofthe centriole is composed of nine evenly spaced,longitudi-

nally oriented,parallel units, or triplets. Each unit consists of three

parallel microtubules joined together (ﬁgure 3.25).

Microtubules appear to inﬂuence the distribution of actin

and intermediate ﬁlaments.Through its control of microtubule for-

mation,the centrosome is therefore closely involved in determining

cell shape and movement. The microtubules extending from the

centrosomes are very dynamic—constantly growing andshrinking.

Before cell division,the two centrioles double in number,the

centrosome divides into two,and one centrosome, containing two

centrioles,moves to each end of the cell. Microtubules called spin-

dle ﬁbersextend out in all directions from the centrosome. These

microtubules grow and shrink even more rapidly than those of

nondividing cells.If the extended end of a spindle ﬁber comes in

contact with a kinetochore(ki-ne¯to¯-ko¯r,ki-neto¯-ko¯ r), a special-

ized region on each chromosome,the spindle ﬁber attaches to the

kinetochore and stops growing or shrinking. Eventually spindle

ﬁbers from each centromere bind to the kinetochores of all the

chromosomes.During cell division, the microtubules facilitate the

movement of chromosomes toward the two centrosomes (see the

section on “Cell Division”near the end of the chapter).

Plasma membrane

Mitochondrion

Microtubules are composed

of tubulin protein subunits.

Microtubules are 25 nm

diameter tubes with 5 nm

thick walls.

Intermediate filaments are

protein fibers 10 nm in diameter.

Intermediate

filament

Microtubule

Actin filaments (microfilaments) are

composed of actin subunits and are

about 8 nm in diameter.

Ribosomes

Endoplasmic

reticulum

Nucleus

Protein subunits

10 nm

8 nm

25 nm

5 nm

SEM 60,000x

Figure 3.24

Cytoskeleton

(a) Diagram ofthe cytoskeleton. (b) Scanning electron micrograph of the cytoskeleton.

(a) (b)

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Cilia and Flagella

Cilia(sile¯-a˘) are appendages that project from the surface of cells

and are capable ofmovement. They are usually limited to one sur-

face ofa given cell and vary in number from one to thousands per

cell. Cilia are cylindrical in shape, about 10 m in length and

0.2m in diameter, and the shaft of each cilium is enclosed by the

plasma membrane. Two centrally located microtubules and nine

peripheral pairs offused microtubules, the so-called 92 arrange-

ment,extend from the base to the tip of each cilium (ﬁgure 3.26).

Movement ofthe microtubules past each other, a process that re-

quires energy from ATP,is responsible for movement of the cilia.

Dynein arms,proteins connecting adjacent pairs of microtubules,

push the microtubules past each other.A basal body (a modiﬁed

centriole) is located in the cytoplasm at the base ofthe cilium. Cilia

are numerous on surface cells that line the respiratory tract and the

female reproductive tract.In these regions cilia move in a coordi-

nated fashion,with a power stroke in one direction and a recovery

stroke in the opposite direction (ﬁgure 3.27).Their motion moves

materials over the surface ofthe cells. For example, cilia in the tra-

chea move mucus embedded with dust particles upward and away

from the lungs.This action helps keep the lungs clear of debris.

Flagella (ﬂa˘-jela˘) have a structure similar to cilia but are

longer (55 m), and usually only one exists per sperm cell.Fur-

thermore,whereas cilia move small particles across the cell surface,

ﬂagella move the cell.For example, each sperm cell is propelled by

a single ﬂagellum. In contrast to cilia,which have a power stroke

and a recovery stroke,ﬂagella move in a wavelike fashion.

Microvilli

Microvilli(mı¯-kro¯-vilı¯) (ﬁgure 3.28) are cylindrically shaped exten-

sions of the plasma membrane about 0.5–1.0 m in length and 90

nm in diameter.Normally many microvilli are on each cell,and they

function to increase the cell surface area.A student looking at photo-

graphs may confuse microvilli with cilia.Microvilli, however,are only

one-tenth to one-twentieth the size ofcilia. Individual microvilli can

usually only be seen with an electron microscope,whereas cilia can be

seen with a light microscope. Microvilli do not move,and they are

supported with actin ﬁlaments, not microtubules. Microvilli are

found in the intestine,kidney, and other areas in which absorption is

an important function.In certain locations of the body, microvilli are

highly modiﬁed to function as sensory receptors.For example, elon-

gated microvilli in hair cells ofthe inner ear respond to sound.

26. Deﬁne organelles.

27. Describe and list the functions of centrosomes. Explain the

structure of centrioles.

28. What are spindle ﬁbers? Explain the relationship between

centrosomes, spindle ﬁbers, and the kinetochoresof

chromosomesduring cell division.

29. Contrast the structure and function of cilia and ﬂagella.

30. Describe the structure and function of microvilli. How are

microvilli differentfrom cilia?

Ribosomes

Ribosomes(rı¯bo¯-so¯ms) are the sites ofprotein synthesis.Each r i-

bosome is composed ofa large subunit and a smaller one. The ri-

bosomal subunits,which consist of ribosomal RNA (rRNA) and

proteins,are produced separately in the nucleolus of the nucleus.

The ribosomal subunits then move through the nuclear pores into

the cytoplasm, where they assemble to form the functional ribo-

some during protein synthesis (ﬁgure 3.29). Ribosomes can be

found free in the cytoplasm or associated with a membrane called

the endoplasmic reticulum.Free rib osomes primarily synthesize

proteins used inside the cell,whereas endoplasmic reticulum ribo-

somes can produce proteins that are secreted from the cell.

EndoplasmicReticulum

The outer membrane of the nuclear envelope is continuous with

a series of membranes distributed throughout the cytoplasm of

the cell, collectively referred to as the endoplasmic reticulum

(endo¯-plasmik re-tiku¯-lu˘m;network inside the cytoplasm) (ﬁg-

ure 3.30).The endoplasmic reticulum consists of broad, ﬂattened,

interconnecting sacs and tubules.The interior spaces of those sacs

and tubules are called cisternae(sis-terne¯) and are isolated from

the rest ofthe cytoplasm.

Rough endoplasmic reticulum is endoplasmic reticulum

with attached ribosomes.The ribosomes of the rough endoplasmic

reticulum are sites where proteins are produced and modiﬁed for

Part1 Organization of the Human Body78

Microtubule

triplet

Centriole

TEM 60,000x

Figure 3.25

Centriole

(a) Structure ofa centriole, which comprisesnine triplets of microtubules.

Each tripletcontains one complete microtubule fused to two incomplete

microtubules. (b) Transmission electron micrograph ofa pair ofcentrioles,

which are normallylocated together near the nucleus. One is shown in cross

section and one in longitudinalsection.

(a)

(b)

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Microtubules

Basal body

Plasma

membrane

Dynein arm

Microtubule

Plasma

membrane

Microtubules

TEM 100,000x

Figure 3.26

Cilia and Flagella

(a) Ciliaryor ﬂagellar structures. The shaftis composed of nine microtubule doublets around its periphery and two in the center. Dynein arms are proteinsthat connect

one pair ofmicrotubules to another pair. Dynein arm movement, which requiresATP, causesthe microtubules to slide past each other, resulting in bending or movement

ofthe cilium or ﬂagellum. A basal bodyattaches the cilium or ﬂagellum to the plasma membrane. (b) TEM through cilium. (c) TEM through basal body of cilium.

secretion and for internal use. The amount and conﬁguration of

the endoplasmic reticulum within the cytoplasm depend on the

cell type and function. Cells with abundant rough endoplasmic

reticulum synthesize large amounts ofprotein that are secreted for

use outside the cell.

Smooth endoplasmic reticulum, which is endoplasmic

reticulum without attached ribosomes, manufactures lipids,such

as phospholipids, cholesterol, steroid hormones, and carbohy-

drates like glycogen.Many phospholipids produced in the smooth

endoplasmic reticulum help form vesicles within the cell and con-

tribute to the plasma membrane. Cells that synthesize large

amounts of lipid contain dense accumulations of smooth

endoplasmic reticulum. Enzymes required for lipid synthesis are

Power

stroke

Recovery

stroke

Figure 3.27

CiliaryMovement

(a) Power and (b) recoverystrokes.

(a)

(b)

(c)

(a) (b)

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Plasma membrane

Microvillus

Actin filaments

Cytoplasm

TEM 60,000x

Figure 3.28

Microvillus

(a) A microvillusis a tiny tubular extension of the celland contains cytoplasm and some actin ﬁlaments (microﬁlaments). (b) Transmission electron micrograph

ofmicrovilli.

1. Ribosomal proteins, produced in the

cytoplasm, are transported through

nuclear pores into the nucleolus.

3. The small and large ribosomal subunits

leave the nucleolus and the nucleus

through nuclear pores.

2. rRNA, most of which is produced in the

nucleolus, is assembled with ribosomal

proteins to form small and large ribosomal

subunits.

4. The small and large subunits, now in the

cytoplasm, combine with each other and

with mRNA.

Ribosomal

proteins from

cytoplasm

Small

ribosomal

unit

Large

ribosomal

unit

Nuclear pore

Nucleus

Nucleolus

DNA

(chromatin)

mRNA

Ribosome

rRNA

ProcessFigure 3.29

Production ofRibosomes

(a) (b)

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associated with the membranes ofthe smooth endoplasmic reticu-

lum.Smooth endoplasmic reticulum also participates in the detox-

iﬁcation processes by which enzymes act on chemicals and drugs to

change their structure and reduce their toxicity.The smooth endo-

plasmic reticulum ofskeletal muscle stores calcium ions that func-

tion in muscle contraction.

Golgi Apparatus

The Golgi (go¯lje¯) apparatus (ﬁgure 3.31) is composed of ﬂat-

tened membranous sacs,containing cisternae, that are stacked on

each other like dinner plates.The Golgi apparatus can be thought

ofas a packaging and distribution center because it modiﬁes, pack-

ages, and distributes proteins and lipids manufactured by the

rough and smooth endoplasmic reticula (ﬁgure 3.32). Proteins

produced at the ribosomes of the rough endoplasmic reticulum

enter the endoplasmic reticulum and, then,are surrounded by a

vesicle (vesi-kl), or little sac, that forms from the membrane of

the endoplasmic reticulum.This vesicle, called a transport vesicle,

moves to the Golgi apparatus,fuses with its membrane, and releases

the protein into its cisterna.The Golgi apparatus concentrates and,

in some cases,chemically modiﬁes the proteins by synthesizing and

attaching carbohydrate molecules to the proteins to form glycopro-

teins or attaching lipids to proteins to form lipoproteins.The pro-

teins are then packaged into vesicles that pinch offfrom the margins

of the Golgi apparatus and are distributed to various locations.

Some vesicles carry proteins to the plasma membrane, where the

proteins are secreted from the cell by exocytosis;other vesicles con-

tain proteins that become part of the plasma membrane; and still

other vesicles contain enzymes that are used within the cell.

The Golgi apparatuses are most numerous and most highly

developed in cells that secrete large amounts of protein or glyco-

proteins,such as cells in the salivary glands and the pancreas.

31. What kinds of molecules are in ribosomes? Where are

ribosomal subunitsformed and assembled?

32. Compare the functions of free ribosomes and endoplasmic

reticulum ribosomes.

33. How is the endoplasmic reticulum related to the nuclear

envelope? Howare the cisternae of the endoplasmic

reticulum related to the restof the cytoplasm?

34. What are the functions of smooth endoplasmic reticulum?

35. Describe the structure and function of the Golgi apparatus.

36. Describe the production of a protein at the endoplasmic

reticulum and itsdistribution to the Golgi apparatus. Name

three waysin which proteins are distributed from the Golgi

apparatus.

SecretoryVesicles

The membrane-bounded secretory vesicles (see ﬁgure 3.31) that

pinch offfrom the Golgi apparatus move to the surface of the cell,

their membranes fuse with the plasma membrane,and the contents

ofthe vesicle are released to the exterior by exocytosis.The membranes

ofthe vesicles are then incorporated into the plasma membrane.

Secretory vesicles accumulate in many cells,but their con-

tents frequently are not released to the exterior until a signal is re-

ceived by the cell.For example, secretory vesicles that contain the

hormone insulin do not release it until the concentration of glu-

cose in the blood increases and acts as a signal for the secretion of

insulin from the cells.

Lysosomes

Lysosomes (lı¯so¯-so¯mz) are membrane-bound vesicles that pinch

off from the Golgi apparatus (see ﬁgure 3.31).They contain a vari-

ety ofhydrolytic enzymes that function as intracellular digestive sys-

tems.Vesicles taken into the cell fuse with the lysosomes to form one

vesicle and to expose the phagocytized materials to hydrolytic en-

zymes (ﬁgure 3.33).Various enzymes within lysosomes digest nu-

cleic acids,proteins, polysaccharides,and lipids. Certain white blood

cells have large numbers oflysosomes that contain enzymes to digest

phagocytized bacteria. Lysosomes also digest organelles of the cell

that are no longer functional in a process called autophagia(aw-to¯-

fa¯je¯-a˘; self-eating). Furthermore, when tissues are damaged,

Smooth

endoplasmic

reticulum

Cytoplasm

Rough

endoplasmic

reticulum

Outer membrane

of nuclear envelope

Nuclear pore

Ribosomes

Cisternae of

endoplasmic

reticulum

Nucleus

Ribosome

Nucleus

Rough

endoplasmic

reticulum

TEM 30,000x

Figure 3.30

The EndoplasmicReticulum

(a) The endoplasmicreticulum is continuous with the nuclear envelope and

can existas either rough endoplasmic reticulum (with ribosomes) or smooth

endoplasmicreticulum (without ribosomes). (b) Transmission electron

micrograph ofthe rough endoplasmic reticulum.

(a)

(b)

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Secretory

vesicle

Golgi

apparatus

Mitochondrion

Secretory vesicles

Cisterna

Transfer

vesicle

TEM 40,000x

Figure 3.31

Golgi Apparatus

(a) The Golgi apparatusis composed of ﬂattened membranoussacs, containing cisternae, and resembles a stack of dinner plates or pancakes. (b) Transmission

electron micrograph ofthe Golgi apparatus.

1. Some proteins are produced at ribosomes on the surface

of the rough endoplasmic reticulum and are transferred

into the cisterna as they are produced.

2. The proteins are surrounded by a vesicle that forms from

the membrane of the endoplasmic reticulum.

3. The vesicle moves from the endoplasmic reticulum to the

Golgi apparatus, fuses with its membrane and releases

the proteins into its cisterna.

4. The Golgi apparatus concentrates and, in some cases,

modifies the proteins into glycoproteins or lipoproteins.

5. The proteins are packaged into vesicles that form from

the membrane of the Golgi apparatus.

6. Some vesicles, such as lysosomes, contain enzymes

that are used within the cell.

7. Secretory vesicles carry proteins to the plasma

membrane, where the proteins are secreted from the cell

by exocytosis.

8. Some vesicles contain proteins that become part of the

plasma membrane.

mRNA

Protein

Cisterna

Ribosome

Exocytosis

Vesicles

Endoplasmic

reticulum

Vesicle

within cell

Secretory

vesicles

Golgi

apparatus

Proteins

incorporated

into membrane

Vesicle

ProcessFigure 3.32

Function ofthe Golgi Apparatus

(a) (b)

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ruptured lysosomes within the damaged cells release their enzymes,

which digest both damaged and healthy cells.In other cells,the lyso-

somes move to the plasma membrane,and the enzymes are secreted

by exocytosis.For example, the normal process of bone remodeling

involves the breakdown ofbone tissue by specialized bone cells. En-

zymes responsible for that degradation are released into the extra-

cellular ﬂuid from lysosomes produced by those cells.

Diseasesof Lysosomal Enzymes

Some diseasesresult from nonfunctional lysosomal enzymes. For

example, Pompe’sdisease results from the inability of lysosomal

enzymesto break down glycogen. The glycogen accumulatesin large

amountsin the heart, liver, and skeletal muscles, an accumulation that

often leadsto heart failure. Familial hyperlipoproteinemia is a group of

geneticdisorders characterized by the accumulation oflarge amounts

oflipids in phagocytic cells that lackthe normal enzymes required to

breakdown the lipid droplets. Symptoms include abdominal pain,

enlargementof the spleen and liver, and eruption of yellow nodules in the

skin ﬁlled with the affected phagocyticcells. Mucopolysaccharidoses,

such asHurler’s syndrome, are diseases in which lysosomalenzymes

are unable to breakdown mucopolysaccharides(glycosaminoglycans),

so these moleculesaccumulate in the lysosomes of connective tissue

cellsand nerve cells. People affected by these diseases suffer mental

retardation and skeletaldeformities.

Peroxisomes

Peroxisomes(per-oksi-so¯ mz) are membrane-bounded vesicles that

are smaller than lysosomes.Peroxisomes contain enzymes that break

down fatty acids and amino acids.Hydrogen peroxide (H

),which

can be toxic to the cell,is a by-product of that breakdown. Peroxi-

somes also contain the enzyme catalase, which breaks down

hydrogen peroxide to water and oxygen.Cells that are active in detox-

iﬁcation,such as liver and kidney cells, have many peroxisomes.

Proteasomes

Proteasomes (pro¯te¯-a˘-so¯mz) consist oflarge protein complexes,

including several enzymes that break down and recycle proteins

within the cell. Proteasomes are not surrounded by membranes.

They are tunnel-like structures, similar to channel protein com-

plexes;the inner surfaces of the tunnel have enzymatic regions that

break down proteins.Smaller protein subunits close the ends of the

tunnel and regulate which proteins are taken into it for digestion.

Mitochondria

Mitochondria (mı¯-to¯-kondre¯-a˘) provide energy for the cell. Con-

sequently,they are often called the cell’s power plants. Mitochon-

dria are usually illustrated as small, rod-shaped structures (ﬁgure

3.34).In living cells, time-lapse photomicrography reveals that mi-

tochondria constantly change shape from spherical to rod-shaped

or even to long,threadlike structures. Mitochondria are the major

sites ofATP production, which is the major energy source for most

energy-requiring chemical reactions within the cell.Each mitochon-

drion has an inner and outer membrane separated by an intermem-

branous space.The outer membrane has a smooth contour, but the

inner membrane has numerous infoldings called cristae (kriste¯)

that project like shelves into the interior ofthe mitochondria.

A complex series ofmitochondrial enzymes forms two major

enzyme systems that are responsible for oxidative metabolism and

most ATP synthesis (see chapter 25).The enzymes of the citric acid

(or Krebs) cycle are found in the matrix,which is the substance lo-

cated in the space formed by the inner membrane.The enzymes of

the electron transport chain are embedded within the inner mem-

brane. Cells with a greater energy requirement have more mito-

chondria with more cristae than cells with lower energy

requirements.Within the cytoplasm of a given cell, the mitochon-

dria are more numerous in areas in which ATP is used.For example,

Vesicle taken

into the cell

Fusion of vesicle

with lysosome

Lysosome

Golgi

apparatus

Plasma membrane

Vesicle forming

Cytoplasm

1. A vesicle forms around material

outside the cell.

2. The vesicle is pinched off from

the plasma membrane and

becomes a separate vesicle

inside the cell.

3. A lysosome is pinched off the

Golgi apparatus.

4. The lysosome fuses with the vesicle.

5. The enzymes from the lysosome mix with

the material in the vesicle, and the

enzymes digest the material.

ProcessFigure 3.33

Action ofLysosomes

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mitochondria are numerous in cells that perform active transport

and are packed near the membrane where active transport occurs.

Increases in the number ofmitochondria result from the divi-

sion ofpreexisting mitochondria. When muscles enlarge as a result of

exercise, the number of mitochondria within the muscle cells in-

creases to provide the additional ATP required for muscle contraction.

The information for making some mitochondrial proteins is

stored in DNA contained within the mitochondria themselves,and

those proteins are synthesized on ribosomes within the mitochon-

dria.The structure of many other mitochondrial proteins is deter-

mined by nuclear DNA,however,and these proteins are synthesized

on ribosomes within the cytoplasm and then transported into the

mitochondria.Both the mitochondrial DNA and mitochondrial ri-

bosomes are very different from those within the nucleus and cyto-

plasm ofthe cell, respectively. Mitochondrial DNA is a closed circle

ofabout 16,500 base pairs (bp) coding for 37 genes, compared with

the open strands ofnuclear DNA, which is composed of 3 billion bp

coding for 30,000 genes. In addition, unlike nuclear DNA,mito-

chondrial DNA does not have associatedproteins.

PREDICT

Describe the structuralcharacteristics ofcells that are highly

specialized to do the following: (a) synthesize and secrete proteins,

(b) activelytransport substances into the cell, (c) synthesize lipids,

and (d) phagocytize foreign substances.

MitochondrialDNA

Halfof the nuclear DNA of an individual is derived from the mother, and

halfis derived from the father; but all, or nearly all, mitochondrialDNA

comesfrom the mother. The mitochondria of the sperm cellfrom the

father are notincorporated into the oocyte at the time of fertilization.

Because onlythe mother’s mitochondrial DNA is passed down from

generation to generation, maternalpedigrees are much easier to trace

using mitochondrialDNA than with nuclear DNA. This unique qualityof

mitochondria hasbeen used in a number of studies, from reuniting

mothersor grandmothers with lost children to searching for the originsof

the human species. A number ofdegenerative disorders affecting the

nervoussystem, heart, or kidneys have been linked to mutations in

mitochondrialDNA. The study of these disorders is providing valuable

cluesto the aging process.

37. Deﬁne secretory vesicles.

38. Describe the process by which lysosomal enzymes digest

phagocytized materials. Deﬁne autophagia.

39. What is the function of peroxisomes? How does catalase

protectcells?

40. Describe the structure and function of proteasomes.

41. What is the function of mitochondria? What enzymes are

found on the cristae and in the matrix? Howcan the number

of mitochondria in a cell increase?

Part1 Organization of the Human Body84

TEM 30,000x

Outer membrane

Intermembrane space

Inner membrane

Matrix

Enzymes

Cross section

Longitudinal section

Crista

Figure 3.34

Mitochondrion

(a) Typicalmitochondrion structure. (b) Transmission electron micrograph ofmitochondria in longitudinal and cross section.

(b)

(a)

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Nucleolus

Ribosomes

Nuclear pores

Outer membrane

Inner membrane

Nuclear

envelope

Nucleus

Inner membrane

of nuclear envelope

Nuclear pores

Outer membrane

of nuclear envelope

Nucleolus

Chromatin

Interior of

nucleus

Nuclear

envelope

Space

TEM 20,000x

SEM 50,000x

Figure 3.35

The Nucleus

(a) The nuclear envelope consistsof inner and outer membranesthat become fused at the nuclear pores. The nucleolus is a condensed region of the nucleus not

bounded bya membrane and consisting mostly of RNA and protein. (b) Transmission electron micrograph ofthe nucleus. (c) Scanning electron micrograph showing

the inner surface ofthe nuclear envelope and the nuclear pores.

Chapter 3 Structure and Function ofthe Cell 85

Nucleus

Objective

■ Describe the structure and function of the nucleus and

nucleolus.

Thenucleus, which contains most of the genetic information

ofthe cell, is a large, membrane-bounded structure usually located

near the center ofthe cell. It may be spherical, elongated, or lobed,

depending on the cell type.All cells of the body have a nucleus at

some point in their life cycle,although some cells, such as red blood

cells (also called red blood corpuscles or erythrocytes), lose their

nuclei as they develop.Other cells, such as skeletal muscle cells and

certain bone cells, called osteoclasts, contain more than one nu-

cleus. The nucleus consists of nucleoplasm surrounded by a nu-

clear envelope (ﬁgure 3.35) composed of two membranes

separated by a space.At many points on the surface of the nuclear

envelope, the inner and outer membranes fuse to form porelike

structures,the nuclear pores. Molecules move between the nucleus

and the cytoplasm through these nuclear pores.

Deoxyribonucleic acid (DNA) and associated proteins are

dispersed throughout the nucleus as thin strands about 4–5 nm in

diameter.The proteins include histones (histo¯nz) and other pro-

teins that play a role in the regulation ofDNA function. The DNA

and protein strands can be stained with dyes and are called chro-

matin (kro¯ma-tin; colored material) (ﬁgure 3.36).Chromatin is

distributed throughout the nucleus but is more condensed and

more readily stained in some areas than in others.The more highly

condensed chromatin apparently is less functional than the more

evenly distributed chromatin,which stains lighter.During cell divi-

sion, the chromatin condenses to form the more densely coiled

bodies called chromosomes(colored bodies).

DNA ultimately determines the structure of proteins (pro-

tein synthesis is described later in this chapter). Many structural

components of the cell and all the enzymes, which regulate most

chemical reactions in the cell,are proteins. By determining protein

structure, DNA therefore ultimately controls the structural and

functional characteristics of the cell. DNA does not leave the nu-

cleus but functions by means ofan intermediate, ribonucleic acid

(RNA),which can leave the nucleus.DNA determines the structure

ofmessenger RNA (mRNA), ribosomal RNA (rRNA), and transfer

RNA (tRNA) (all described in more detail later).mRNA moves out

ofthe nucleus through the nuclear pores into the cytoplasm, where

it determines the structure ofproteins.

(a)

(b) (c)

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cells with nuclei,such as nerve and skeletal muscle cells, survive as

long as the individual person survives.

A nucleolus (noo-kle¯o¯-lu˘s) is a somewhat rounded dense

region within the nucleus that lacks a surrounding membrane (see

ﬁgure 3.35).Usually one nucleolus exists per nucleus, but several

smaller,accessory nucleoli may also be seen in some nuclei, espe-

cially during the latter phases ofcell division. The nucleolus incor-

porates portions of 10 chromosomes (ﬁve pairs),called nucleolar

organizer regions.These regions contain DNA from which rRNA

is produced.Within the nucleolus, the subunits of ribosomes are

manufactured (see preceding section on “Ribosomes”).

42. Describe the structure of the nucleus and nuclear envelope.

Whatis the function of the nuclear pores?

43. What molecules are found in chromatin? How does

chromatin become a chromosome?

44. List the types of RNA whose structure is determined by DNA.

Howcan DNA control the structural and functional

characteristicsof the cell without leaving the nucleus?

45. Describe the nucleolus. Deﬁne and give the function of the

nucleolarorganizer regions.

Part1 Organization of the Human Body86

Chromosome

Chromatin

Nucleotides

Cytosine

Guanine

AdenineThymine

Globular histone

proteins

Segment of

DNA molecule

Figure 3.36

Chromosome Structure

DNA isassociated with globular histone proteins. Usually the DNA molecule isstretched out, resembling a string of beads, and is called chromatin. During cell

division, however, the chromatin condensesto become bodiescalled chromosomes.

Human Genome Project

TheHuman Genome Project is an ambitious international project, which

began in 1990, with the 15-year goalof mapping and sequencing the

entire human genome. The genomeis the total of allthe genes contained

within each cell. One goalof the Human Genome Projectis to construct a

map indicating where each ofthe approximately27,000–30,000 genes

islocated on the human chromosomes. The other major goalof the

projectis to determine the sequence of the estimated 3 billion base

pairs(bp) that make up the human DNA molecules. The sequencing is

now complete, and the mapping continues. Itis hoped that by knowing

for whatproteins the genes implicated in genetic disorders are coded,

and bydetermining the functions of those proteins, we will be able to

more effectivelytreat these disorders.

Because mRNA synthesis occurs within the nucleus, cells

without nuclei accomplish protein synthesis only as long as the

mRNA produced before the nucleus degenerates remains func-

tional.The nuclei of de veloping red blood cells are expelled from

the cells before the red blood cells enter the blood, where they

survive without a nucleus for about 120 days.In comparison, many

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Chapter 3 Structure and Function ofthe Cell 87

Overview of CellMetabolism

Objective

■ Deﬁne cell metabolism, and contrast aerobic and anaerobic

respiration.

Cell metabolismis the sum of all the catabolic (decomposi-

tion) and anabolic (synthesis) reactions in the cell.The breakdown

of food molecules such as carbohydrates, lipids,and proteins re-

leases energy that is used to synthesize ATP.Each ATP molecule

contains a portion of the energy originally stored in the chemical

bonds of the food molecules. The ATP molecules are smaller

“packets”of energy that, when released, can be used to drive other

chemical reactions or processes such as active transport.

The production ofATP takes place in the cytosol and in mi-

tochondria through a series of chemical reactions (see chapter 25

for details).Energy from food molecules is transferred to ATP in a

controlled fashion.If the energy in food molecules were released all

at once,the cell literally would burn up.

The breakdown of the sugar glucose, such as from sugar

found in a candy bar,is used to illustrate the production of ATP

from food molecules.Once glucose is transported into a cell, a se-

ries of reactions takes place within the cytosol.These chemical re-

actions, collectively called glycolysis (glı¯-koli-sis),convert the

glucose to pyruvic acid.Pyruvic acid can enter different biochemi-

cal pathways,depending on oxygen availability (ﬁgure 3.37).

Aerobic (a¯r-o¯bik) respiration occurs when oxygen is

available. The pyruvic acid molecules enter mitochondria and,

through another series of chemical reactions, collectively called

the citric acid cycle and the electron-transport chain, are con-

verted to carbon dioxide and water.Aerobic respiration can pro-

duce up to 38 ATP molecules from the energy contained in each

glucose molecule.

Several important points should be noted about aerobic res-

piration.First, the quantities of ATP produced through aerobic res-

piration are absolutely necessary to maintain the energy-requiring

chemical reactions of life in human cells.Second, aerobic respira-

tion requires oxygen because the last chemical reaction that takes

place in aerobic respiration is the combination ofoxygen with hy-

drogen to form water.If this reaction does not take place, the reac-

tions immediately preceding it do not occur either.This explains

why breathing oxygen is necessary for human life:without oxygen,

aerobic respiration is inhibited, and the cells do not produce

enough ATP to sustain life.Finally, during aerobic respiration the

carbon atoms offood molecules are separated from one another to

form carbon dioxide.Thus the carbon dioxide humans breathe out

comes from the food they eat.

Anaerobic (an-a¯r-o¯bik) respiration occurs without oxy-

gen and includes the conversion ofpyruvic acid to lactic acid. A net

production oftwo ATP molecules occurs for each glucose molecule

used.Anaerobic respiration does not produce as much ATP as aer-

obic respiration,but it does allow the cells to function for short pe-

riods when oxygen levels are too low for aerobic respiration to

provide all the needed ATP.For example,during intense exercise,

when aerobic respiration has depleted the oxygen supply,anaero-

bic respiration can provide additional ATP.

46. Deﬁne cell metabolism. What molecule is synthesized using

the energyreleased by the breakdown of food molecules?

47. Deﬁne glycolysis, aerobic respiration, and anaerobic

respiration.

48. How many ATP molecules are produced from one glucose

molecule in aerobicand anaerobic respiration?

49. During aerobic respiration, what happens to the oxygen we

breathe in? Where doesthe carbon dioxide we breathe out

come from?

50. Besides ATP, what molecule is produced asa result of

anaerobicrespiration? Under what conditions is anaerobic

respiration necessary?

Protein Synthesis

Objective

■ Describe the process of protein synthesis.

Normal cell structure and function would not be possible

without proteins (ﬁgure 3.38), which form the cytoskeleton and

other structural components of cells and function as transport

molecules, receptors,and enzymes. In addition, proteins secreted

from cells perform vital functions:collagen is a structural protein

that gives tissues ﬂexibility and strength, enzymes control the

chemical reactions of food digestion in the intestines,and protein

hormones regulate the activities ofmany tissues.

Ultimately,the production of all the proteins in the body is

under the control ofDNA. Recall from chapter 2 that the building

blocks of DNA are nucleotides containing adenine (A), thymine

(T),cy tosine (C),and guanine (G). The nucleotides form two an-

tiparallel strands ofnucleic acids. The term antiparallel means that

the strands are parallel but extend in opposite directions. Each

strand has a 5 (phosphate) end and a 3 (hydroxyl) end. The

Glucose

)

Glycolysis

Pyruvic

acid

Cytoplasm

Mitochondrion

2 lactic acid+2ATP

Citric acid cycle

Electron-transport chain

6CO

+6H

O+38ATP

Anaerobic

respiration

Aerobic

respiration

Figure 3.37

Overview ofCell Metabolism

Aerobicrespiration requires oxygen and produces more ATP per glucose

molecule than doesanaerobic metabolism.

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sequence ofthe nucleotides in the DNA is a method of storing in-

formation. Every three nucleotides, called a triplet, code for an

amino acid, and amino acids are the building blocks of proteins.

All of the triplets required to code for the synthesis of a speciﬁc

protein are called a gene.

The production of proteins from the stored information in

DNA involves two steps:transcription and translation, which can

be illustrated with an analogy.Suppose a cook wants a recipe that

is found only in a reference book in the library.Because the book

cannot be checked out, the cook makes a handwritten copy,or

transcription, of the recipe. Later,in the kitchen the information

contained in the copied recipe is used to prepare the meal.The

changing of something from one form to another (from recipe to

meal) is called translation. In this analogy, DNA is the reference

book that contains many recipes for making different proteins.

DNA,however, is too large a molecule to pass through the nuclear

envelope to go to the ribosomes (the kitchen),where the proteins

are prepared.Just as the reference book stays in the library, DNA

remains in the nucleus.Therefore, through transcription, the cell

makes a copy ofthe information in DNA (the recipe) necessary to

make a particular protein (the meal). The copy,which is called

messenger RNA (mRNA),travels from the nucleus to ribosomes

in the cytoplasm,where the information in the copy is used to con-

struct a protein (i.e.,translation). Of course, to turn a recipe into a

meal,the actual ingredients are needed. The ingredients necessary

to synthesize a protein are amino acids. Specialized transport

molecules,called transfer RNA (tRNA), carry the amino acids to

the ribosome (ﬁgure 3.39).

In summary,the synthesis of proteins involves transcription,

making a copy of part of the stored information in DNA, and

translation,converting that copied information into a protein. The

details oftranscr iption and translation are considerednext.

Transcription

Transcription is the synthesis of mRNA on the basis of the se-

quence of nucleotides in DNA.It occurs when the double strands

ofa DNA segment separate, one of its strands ser ves as a template,

and RNA nucleotides pair with DNA nucleotides of the template

(ﬁgure 3.39).Nucleotides pair with each other according to the fol-

lowing rule: adenine pairs with thymine or uracil, and cytosine

pairs with guanine. DNA contains thymine,but ur acil replaces

thymine in RNA. Adenine,thymine, cytosine, and guanine nu-

cleotides ofDNA therefore pair with uracil, adenine, guanine, and

cytosine nucleotides ofmRNA, respectively.

This pairing relationship between nucleotides ensures that

the information in DNA is transcribed correctly to mRNA. The

RNA nucleotides combine through dehydration reactions cat-

alyzed by RNA polymerase enzymes to form a long mRNA seg-

ment. The elongation of all nucleic acids, both DNA and RNA,

occurs in the same chemical direction:from the 5 to the 3 end of

Part1 Organization of the Human Body88

DNA strand

mRNA strand

Cytoplasm

Nucleus

Nucleolus

Transcription

mRNA strand

tRNA

Ribosome

Translation

Amino acid

pool

Protein chain

1. DNA contains the

information

necessary to

produce proteins.

2. Transcription of

DNA results in

mRNA, which is a

copy of the

information in DNA

needed to make a

protein.

3. The mRNA leaves

the nucleus and

goes to a ribosome.

4. Amino acids, the

building blocks of

proteins, are

carried to the

ribosome by tRNAs.

5. In the process of

translation, the

information

contained in mRNA

is used to

determine the

number, kinds, and

arrangement of

amino acids in the

protein.

ProcessFigure 3.38

Overview ofProtein Synthesis

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Chapter 3 Structure and Function ofthe Cell 89

the molecule. The mRNA molecule contains the information

required to determine the sequence of amino acids in a protein.

The information, called the genetic code, is carried in groups of

three nucleotides called codons.

The number and sequence ofcodons in the mRNA are deter-

mined by the number and sequence of sets of three nucleotides,

called triplets,in the segments of DNA that were transcribed. For ex-

ample,the triplet code of CTA in DNA results in the codon GAU in

mRNA,which codes for aspartic acid. Each codon codes for a spe-

ciﬁc amino acid.Sixty-four possible mRNA codons exist,but only 20

amino acids are in proteins.As a result,the genetic code is redundant

because more than one codon codes for some amino acids. For

example,CGA, CGG, CGT,and CGC all code for the amino acid ala-

nine,and UUU and UAC both code for phenylalanine.Some codons

do not code for amino acids but perform other functions.AUG and

sometimes GUG act as signals for starting the transcription of a

stretch ofDNA to RNA. Three codons, UAA,UGA, and UAG, act as

signals for stopping the transcription of DNA to RNA.

The region of a DNA molecule between the codon starting

transcription and the codon stopping transcription is transcribed

into a stretch of RNA and is called a transcription unit. A tran-

scription unit codes for a protein or part ofa protein. A transcrip-

tion unit is not necessarily a gene.A gene is a functional unit, and

some regulatory genes don’t code for proteins.A molecular deﬁni-

tion ofa gene is all of the nucleic acid sequences necessary to make

a functional RNA or protein.

Not all ofa continuous stretch of DNA may code for parts of

a protein.Regions of the DNA that code for parts of the protein are

called exons, whereas those regions of the DNA that do not code

for portions of the protein are called introns. Both the exon and

intron regions of the DNA may be transcribed into mRNA. An

mRNA containing introns is called a pre-mRNA.After a stretch of

pre-mRNA has been transcribed,the introns can be removed and

the exons spliced together by enzyme complexes called spliceo-

somes to produce the functional mRNA (ﬁgure 3.40). These

changes in the mRNA are called posttranscriptional processing.

DNA

Guanine

Adenine

Thymine

Cytosine

Uracil

DNA strands

separate

Nucleotides

align

Nucleotides

mRNA is

formed

Figure 3.39

Formation ofmRNA by Transcription ofDNA

A segmentof the DNA molecule is opened, and RNA polymerase (an enzyme

thatis not shown) assembles nucleotides into mRNA according to the base-

pair combinationsshown in the inset. Thus the sequence of nucleotidesin

DNA determinesthe sequence of nucleotides in mRNA. As nucleotides are

added, an mRNA molecule isformed.

Transcription

Pre-mRNA

mRNA

Processing

DNA

Exon 1 Intron

Specific RNA regions

CutCut

Intron

Exon 2

Exon 1 Exon 2

Splice

Figure 3.40

PosttranscriptionalChange in mRNA

An intron iscleaved from between two exons and is discarded. The exonsare

spliced together byspliceosomes to make the functionalmRNA.

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Thalassemia

Hemoglobin isan oxygen-carrying protein molecule composed of four

polypeptides.Thalassemia is a group of genetic disorders in which one

or more ofthe polypeptides of hemoglobin is produced in decreased

amountsas the result of defective posttranscriptional processing. The

decreased amountof hemoglobin in the blood causes anemia, which

reducesthe oxygen-carrying capacity ofthe blood.

Translation

The synthesis of a protein at the ribosome in response to the

codons ofmRNA is called translation. In addition to mRNA, trans-

lation requires ribosomes and tRNA.Ribosomes consist of riboso-

mal RNA (rRNA) and proteins.Like mRNA, tRNA and rRNA are

produced in the nucleus by transcription.

The function of tRNA is to match a speciﬁc amino acid to a

speciﬁc codon ofmRNA. To do this,one end of each kind oftRNA

combines with a speciﬁc amino acid.Another part of the tRNA has

ananticodon, which consists ofthree nucleotides. On the basis of

the pairing relationships between nucleotides, the anticodon can

combine only with its matched codon.For example, the tRNA that

binds to aspartic acid has the anticodon CUA,which combines

with the codon GAU ofmRNA. Therefore the codon GAU codes

for aspartic acid.

Ribosomes align the codons of the mRNA with the anti-

codons of tRNA and then join the amino acids of adjacent tRNA

molecules.As the amino acids are joined together,a chain of amino

acids,or a protein, is formed. The step-by-step process of protein

synthesis at the ribosome is described in detail in ﬁgure 3.41.

Many proteins are longer when ﬁrst made than they are in

their ﬁnal,functional state. These proteins are called proproteins,

and the extra piece of the molecule is cleaved off by enzymes to

make the proprotein into a functional protein.Many proteins are

enzymes,and the proproteins of those enzymes are called proen-

zymes. If many proenzymes were made within cells as functional

enzymes, they could digest the cell that made them.Instead, they

are made as proenzymes and are not converted to active enzymes

until they reach a protected region ofthe body, such as inside the

small intestine,where they are functional. Many proteins have side

chains,such as polysaccharides, added to them following transla-

tion. Some proteins are composed of two or more amino acid

chains that are joined after each chain is produced on separate ri-

bosomes.These various modiﬁcations to proteins are referred to as

posttranslational processing.

After the initial part ofmRNA is used by a ribosome, another

ribosome can attach to the mRNA and begin to make a protein.The

resulting cluster of ribosomes attached to the mRNA is called a

polyribosome. Each ribosome in a poly ribosome produces an

identical protein,and poly ribosomes are an efﬁcient way to use a

single mRNA molecule to produce many copies ofthe same protein.

PREDICT

Explain how changing one nucleotide within a DNA molecule ofa cell

could change the structure ofa protein produced bythat cell. What

effectwould this change have on the protein’s function?

Regulation ofProtein Synthesis

All of the cells in the body,except for sex cells, have the same DNA.

The transcription of mRNA in cells is regulated,however, so that all

portions of all DNA molecules are not continually transcribed.The

proteins associated with DNA in the nucleus play a role in regulating

the transcription. As cells differentiate and become specialized for

speciﬁc functions during development, part of the DNA becomes

nonfunctional and is not transcribed, whereas other segments of

DNA remain very active.For example,in most cells the DNA coding

for hemoglobin is nonfunctional,and little if any hemoglobin is syn-

thesized.In developing red blood cells, however,the DNA coding for

hemoglobin is functional,and hemoglobin synthesis occurs rapidly.

Protein synthesis in a single cell is not normally constant,but

it occurs more rapidly at some times than others.Regulatory mol-

ecules that interact with the nuclear proteins can either increase or

decrease the transcription rate of speciﬁc DNA segments. For ex-

ample,thyroxine, a hormone released by cells of the thyroid gland,

enters cells such as skeletal muscle cells,interacts with speciﬁc nu-

clear proteins,and increases speciﬁc types of mRNA transcription.

Consequently,the production of certain proteins increases.As a re-

sult,an increase in the number of mitochondria and an increase in

metabolism occur in these cells.

51. What type of molecule is produced as a result of

transcription? Of translation? Where do these eventstake

place?

52. In what molecules are triplets, codons, and anticodons

found? Whatis the genetic code?

53. How are triplets, transcription units, and genes related?

54. Describe the role of mRNA, rRNA, and tRNA in the production

of a protein ata ribosome. What is a polyribosome?

55. What are exons and introns? How are they related to pre-

mRNA and posttranscriptional processing?

56. Deﬁne proprotein, proenzyme, and posttranslational

processing.

57. State two ways the cell controls what DNA is transcribed.

CellLife Cycle

Objective

■ Explain what is accomplished during mitosis and

cytokinesis.

Thecell life cycle includes the changes a cell undergoes from

the time it is formed until it divides to produce two new cells.The

life cycle of a cell has two stages,an interphase and a cell division

stage (ﬁgure 3.42).

Interphase

Interphase is the phase between cell divisions. Ninety percent or

more ofthe life cycle of a ty pical cell is spent in interphase.During

this time the cell carries out the metabolic activities necessary for

life and performs its specialized functions such as secreting diges-

tive enzymes.In addition, the cell prepares to divide. This prepara-

tion includes an increase in cell size, because many cell

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Chapter 3 Structure and Function ofthe Cell 91

tRNA

Anticodon

Codon

Amino acid

Open tRNA

binding site

mRNA strand

Ribosome

Ribosome moves

to next codon of

mRNA strand

1.To start protein synthesis a ribosome binds to

mRNA. The ribosome also has two binding sites

for tRNA, one of which is occupied by a tRNA with

its amino acid. Note that the codon of mRNA and

the anticodon of tRNA are aligned and joined. The

other tRNA binding site is open.

2. By occupying the open tRNA binding site the next

tRNA is properly aligned with mRNA and with the

other tRNA.

3. An enzyme within the ribosome catalyzes a

synthesis reaction to form a peptide bond

between the amino acids. Note that the amino

acids are now associated with only one of the

tRNAs.

4. The ribosome shifts position by three nucleotides.

The tRNA without the amino acid is released from

the ribosome, and the tRNA with the amino acids

takes its position. A tRNA binding site is left open

by the shift. Additional amino acids can be added

by repeating steps

through

.Eventually a stop

codon in the mRNA ends the production of the

protein, which is released from the ribosome.

5. Multiple ribosomes attach to a single mRNA. As

the ribosomes move down the mRNA, proteins

attached to the ribosomes lengthen and

eventually detach from the mRNA.

ProcessFigure 3.41

Translation ofmRNA to Produce a Protein

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components double in quantity, and a replication of the cell’s

DNA. The centrioles within the centrosome are also duplicated.

Consequently,when the cell divides, each new cell receives the or-

ganelles and DNA necessary for continued functioning.

Interphase can be divided into three subphases,called G

,S,

and G

. During G

(the ﬁrst gap phase) and G

(the second gap

phase),the cell carries out routine metabolic activities. During the

S phase (the synthesis phase), the DNA is replicated (new DNA is

synthesized). Many cells in the body do not divide for days,

months,or even years. These “resting” cells exit the cell cycle and

enter what is called the G

phase,in which they remain unless stim-

ulated to divide.

DNA Replication

DNA replicationis the process by which two new strands of DNA

are made,using the two existing strands as templates. During in-

terphase, DNA and its associated proteins appear as dispersed

chromatin threads within the nucleus.When DNA replication be-

gins, the two strands of each DNA molecule separate from each

other for some distance (ﬁgure 3.43).Each strand then functions as

a template,or pattern, for the production of a new strand of DNA,

which is formed as new nucleotides pair with the existing nu-

cleotides of each strand of the separated DNA molecule.The pro-

duction of the new nucleotide strands is catalyzed by DNA

polymerase,which adds new nucleotides at the 3 end of the grow-

ing strands.One strand, called the leading strand, is formed as a

continuous strand, whereas the other strand, called the lagging

strand, is formed in short segments going in the opposite direc-

tion.The short segments are then spliced by DNA ligase. As a re-

sult of DNA replication, two identical DNA molecules are

produced.Each of the two new DNA molecules has one strand of

nucleotides derived from the original DNA molecule and one

newly synthesized strand.

PREDICT

Suppose a molecule ofDNA separates, forming strands 1 and 2. Part

ofthe nucleotide sequence in strand 1 is ATGCTA. From thistemplate,

whatwould be the sequence of nucleotides in the DNA replicated from

strand 1 and strand 2?

CellDivision

New cells necessary for growth and tissue repair are produced by

cell division.A parent cell divides to form two daughter cells, each

ofwhich has the same amount and ty pe of DNA as the parent cell.

Because DNA determines cell structure and function,the daughter

cells have the same structure and perform the same functions as

the parent cell.

Cell division involves two major events:the division of the

nucleus to form two new nuclei,and the division of the cytoplasm

to form two new cells.Each of the new cells contains one of the

newly formed nuclei.The division of the nucleus occurs by mito-

sis,and the division of the cytoplasm is called cytokinesis.

Mitosis

Mitosis (mı¯-to¯sis) is the division of the nucleus into two nuclei,

each ofwhich has the same amount and ty pe of DNA as the origi-

nal nucleus.The DNA, which was dispersed as chromatin in inter-

phase, condenses in mitosis to form chromosomes.All human

somatic (so¯-matik) cells,which include all cells except the sex

cells,contain 46 chromosomes, which are referred to as a diploid

(diployd) number ofchromosomes. Sex cells have half the num-

ber of chromosomes as somatic cells (see section on “Meiosis”).

The 46 chromosomes in somatic cells are organized into 23 pairs of

chromosomes. Twenty-two ofthese pairs are called autosomes.

Each member of an autosomal pair of chromosomes looks struc-

turally alike, and together they are called a homologous (ho˘-

molo¯-gu˘s) pair of chromosomes.One member of each autosomal

pair is derived from the person’s father,and the other is derived

from the mother.The remaining pair of chromosomes are the sex

chromosomes. In females,the sex chromosomes look alike, and

each is called an X chromosome.In males, the sex chromosomes

do not look alike.One chromosome is an X chromosome, and the

other is smaller and is called a Y chromosome. One X chromo-

some of a female is derived from her mother and the other is de-

rived from her father.The X chromosome of a male is derived from

his mother and the Y chromosome is derived from his father.

Part1 Organization of the Human Body92

Mitosis

(M phase)

Interphase

(b)

(a)

Cytokinesis

S phase

(synthesis phase)

DNA replication

phase

(second gap phase)

Routine metabolism

phase

(first gap phase)

Routine metabolism

Telophase

Anaphase

Metaphase

Prophase

Figure 3.42

Cell Cycle

The cellcycle is divided into interphase and mitosis. Interphase isdivided into

, S, and G

subphases. During G

and G

, the cellcarries out routine

metabolicactivities. During the S phase DNA is replicated. (a) Following

mitosis, two cellsare formed by the processof cytokinesis. Each new cell

beginsa new cell cycle. (b) Many cells exitthe cell cycle and enter the G

phase, where theyremain until stimulated to divide, at which pointthey

reenter the cellcycle.

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For convenience of discussion,mitosis is divided into four

phases: prophase, metaphase, anaphase, and telophase (telo¯-

fa¯z). Although each phase represents major events, mitosis is a

continuous process,and no discrete jumps occur from one phase

to another.Learning the characteristics associated with each phase

is helpful,but a more important concept is how each daughter cell

obtains the same number and type of chromosomes as the parent

cell.The major events of mitosis are summarized in ﬁgure 3.44.

Cytokinesis

Cytokinesis(sı¯to¯-ki-ne¯sis) is the division ofthe cytoplasm of the cell

to produce two new cells.Cytokinesis begins in anaphase, continues

through telophase, and ends in the following interphase (see ﬁgure

3.45).The ﬁrst sign of cytokinesis is the formation of a cleavage fur-

row,or puckering of the plasma membrane, which forms midway be-

tween the centrioles.A contractile ring composed primarily of actin

ﬁlaments pulls the plasma membrane inward,dividing the cell into

two halves.Cytokinesis is complete when the membranes of the two

halves separate at the cleavage furrow to form two separate cells.

58. Deﬁne interphase. What percent of the cell life cycle is

typicallyspent in interphase?

59. Describe the cell’s activities during G

, S, and G

phasesof

the cell life cycle.

60. Describe the process of DNA replication. What are the

functionsof DNA polymerase and DNA ligase?

61. Deﬁne mitosis. How do the two nuclei thatare produced in

mitosiscompare to the original nucleus?

62. How many chromosomes are contained in a human somatic

cell? Howare the chromosomes of males and females the

same? Howare they different?

63. List the events that occur during interphase, prophase,

metaphase, anaphase, and telophase of mitosis.

64. Describe cytokinesis.

DNA molecule unwinds

Old strand

New strand

New DNA molecule

Old strand

Original DNA molecule

Nucleotide

Guanine

AdenineThymine

Cytosine

Figure 3.43

Replication ofDNA

Replication ofDNA during interphase produces two identicalmolecules of DNA. The strands of the DNA molecule separate from each other, and each strand functions asa

template on which another strand isformed. The base-pairing relationship between nucleotidesdetermines the sequence of nucleotides in the newly formed strands.

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Nucleus

Centriole

Spindle

fiber

Astral fiber

Centriole

Centromere

Spindle fiber

Chromatin

Chromatid

Chromosome

Chromosomes

Chromatid

LM 1,000x

ProcessFigure 3.44

Mitosis

(1)Interphase. DNA, which is dispersed as chromatin, replicates. The two strandsof each DNA molecule separate, and a copy of each strand ismade. Consequently,

two identicalDNA molecules are produced. The pair of centriolesreplicates to produce two pairs of centrioles.

(2)Prophase. Chromatin strands condense to form chromosomes. Each chromosome iscomposed of two identical strands of chromatin called chromatids,which

are joined together atone point by a specialized region called the centromere.Each chromatid contains one of the DNA molecules replicated during interphase. One

pair ofcentrioles movesto each side, or pole, of the cell. Microtubulesform near the centrioles and project in all directions. Some of the microtubules end blindly and

are calledastral ﬁbers. Others, known as spindle ﬁbersproject toward an invisible line, called the equator, and either overlap with ﬁbersfrom other centrioles or

attach to the centromeresof the chromosomes. At the end of prophase the nuclear envelope degenerates, and the nucleoli disappear.

(3)Metaphase. The chromosomes align along the equator with spindle ﬁbers from each pair of centrioles, located at opposite poles ofthe cell, attached to their

centromeres.

Cloning

Through the processof differentiation, cells become specialized to

certain functionsand are no longer capable of producing an entire

organism ifisolated. Over 30 years ago, however, it wasdemonstrated in

frogsthat if the nucleus is removed from a differentiated celland is

transferred to an oocyte with the nucleusremoved, a complete, normal

frog can develop from thatoocyte. This process, called cloning,

demonstrated thatduring differentiation, genetic information is not

irrevocablylost. Because mammalian oocytes are considerablysmaller

than frog oocytes, cloning ofmammalian cells has been technically

much more difﬁcult. Dr. Ian Wilmutand his colleaguesat the Roslin

Institute in Edinburgh, Scotland, overcame those technicaldifﬁcultiesin

1996, when theysuccessfully cloned the ﬁrst mammal, a sheep. Since

thattime, several other mammalian species have been cloned.

Meiosis

Objective

■ Describe the events of meiosis, and explain how they result

in the production of geneticallyunique individuals.

All cells of the body,except sex cells, are formed by mitosis.

Sex cells are formed by meiosis(mı¯-o¯sis). In meiosis the nucleus

undergoes two divisions resulting in four nuclei,each containing

(1) (2) (3)

half as many chromosomes as the parent cell.The daughter cells

that are produced by cytokinesis differentiate into gametes

(game¯ tz),or sex cells. The gametes are reproductive cells—sperm

cells in males and oocytes (egg cells) in females. Each gamete not

only has halfthe number of chromosomes found in a somatic cell

but also has one chromosome from each ofthe homologous pairs

found in the parent cell.The complement of chromosomes in a ga-

mete is referred to as a haploidnumber. Oocytes contain one au-

tosomal chromosome from each ofthe 22 homologous pairs and

an X chromosome.Sperm cells have 22 autosomal chromosomes

and either an X or Y chromosome.Dur ing fertilization,when a

sperm cell fuses with an oocyte,the normal number of 46 chromo-

somes in 23 pairs is reestablished. The sex of the baby is deter-

mined by the sperm cell that fertilizes the oocyte.The sex is male if

a Y chromosome is carried by the sperm cell that fertilizes the

oocyte and female ifthe sperm cell car ries an X chromosome.

The ﬁrst division during meiosis is divided into four phases:

prophase I,metaphase I, anaphase I, and telophase I (ﬁgure 3.45). As

in prophase of mitosis, the nuclear envelope degenerates,spindle

ﬁbers form,and the already duplicated chromosomes become visible.

Each chromosome consists of two chromatids joined by a cen-

tromere.In prophase I, however, the four chromatids of a homolo-

gous pair of chromosomes join together, or synapse, (sin-aps,

sı˘-naps),to form a tetrad (four). In metaphase I the tetrads align at

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the equatorial plane, and in anaphase I each pair of homologous

chromosomes separate and move toward opposite poles ofthe cell.

For each pair of homologous chromosomes, one daughter cell re-

ceives one member ofthe pair,and the other daughter cell receives the

other member.Thus each daughter cell has 23 chromosomes,each of

which is composed oftwo chromatids. Telophase I,w ith cytokinesis,

is similar to telophase ofmitosis, and two daughter cells are produced.

Interkinesis(inter-ki-ne¯sis) is the phase between the for-

mation of the daughter cells and the second meiotic division. No

duplication of DNA occurs during interkinesis. The second divi-

sion of meiosis also has four phases: prophase II, metaphase II,

anaphase II,and telophase II. These stages occur much as they do

in mitosis,except that 23 chromosomes are present instead of 46.

The chromosomes align at the equatorial plane in metaphase II,

and their chromatids split apart in anaphase II. The chromatids

then are called chromosomes,and each new cell receives 23 chro-

mosomes.Table 3.3 compares mitosis and meiosis.

In addition to reducing the number ofchromosomes in a cell

from 46 to 23,meiosis is also responsible for genetic diversity for

two reasons.First, a random distribution of the chromosomes is re-

ceived from each parent.One member of each homologous pair of

chromosomes was derived from the person’s father and the other

member from the person’s mother.The homologous chromosomes

align randomly during metaphase I; when they split apart, each

daughter cell receives some ofthe father’s and some of the mother’s

chromosomes.The number of chromosomes each daughter cell re-

ceives from each parent is determined by chance,however.

Second, when tetrads are formed, some of the chromatids

may break apart,and part of one chromatid from one homologous

pair may be exchanged for part of another chromatid from the

other homologous pair (ﬁgure 3.46). This exchange is called

crossing-over;as a result, chromatids with different DNA content

are formed.

With random assortment ofhomologous chromosomes and

crossing-over,the possible number of gametes with different ge-

netic makeup is practically unlimited.When the different gametes

of two individuals unite, it is virtually certain that the resulting

genetic makeup never has occurred before and never will occur

again.The genetic makeup of each new human being is unique.

65. Compare meiosis and mitosis, including types of cells

involved, numberof divisions, number of nuclei produced,

and numberof chromosomes in each nucleus.

66. Deﬁne gamete, sperm cell, and oocyte.

67. What is a tetrad? Name two processes in meiosis that

increase geneticdiversity.

Centriole

Cleavage

furrow

Nuclear envelope

Nucleoli

Identical

chromosomes

Cleavage

furrow

LM 1,000x

ProcessFigure 3.44 (

continued)

(4) Anaphase. The centromeres separate, and each chromatid isthen referred to as a chromosome. Thus, when the centromeres divide, the chromosome number

doubles, and there are two identicalsets of chromosomes. The two sets ofchromosomes are pulled by the spindle ﬁbers toward the poles of the cell. Separation of

the chromatidssignals the beginning of anaphase, and bythe time anaphase has ended, the chromosomes have reached the poles of the cell. The beginning of

cytokinesisis evident during anaphase; along the equator of the cellthe cytoplasm becomes narrower as the plasma membrane pinches inward.

(5)Telophase.The migration of each set of chromosomes is complete. A new nuclear envelope develops from the endoplasmic reticulum, and the nucleoli

reappear. During the latter portion oftelophase the spindle ﬁbers disappear, and the chromosomesunravel to become less distinct chromatin threads. The nuclei

ofthe two daughter cells assume the appearance ofinterphase nuclei, and the process of mitosis is complete.

(6)Interphase. Cytokinesis, which continued from anaphase through telophase, becomescomplete when the plasma membranes move close enough together at

the equator ofthe cell to fuse, completely separating the two new daughter cells, each ofwhich now has a complete set of chromosomes (a diploid number of

chromosomes) identicalto the parent cell.

(4) (5) (6)

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Early prophase I

The duplicated

chromosomes

become visible

(chromatids are

shown separated

for emphasis, they

actually are so close

together that they

appear as a

single strand).

Middle prophase I

Homologous

chromosomes

synapse to form

tetrads.

Metaphase I

Tetrads align

at the

equatorial

plane.

Anaphase I

Homologous

chromosomes

move apart to

opposite sides

of the cell.

Telophase I

New nuclei form,

and the cell

divides; during

interkinesis

(not shown) there

is no duplication

of chromosomes.

Prophase II

Each

chromosome

consists of

two chromatids.

Prophase II

(top of next column)

Metaphase II

Chromosomes

align at the

equatorial

plane.

Anaphase II

Chromatids

separate and

each is now

called a

chromosome.

Telophase II

New nuclei

form around

the chromosomes.

Haploid cells

The chromosomes

are about to

unravel and

become less

distinct chromatin.

In the male

Meiosis results

in four sperm

cells.

In the female

Meiosis results

in only one

functional cell,

called an oocyte,

and two or three

very small cells,

calledpolar bodies.

Chromosome

Nucleus

Centrioles

Chromatids

Tetrad

Spindle

fibers

Homologous

chromosomes

Centromere

Equatorial

plane

Cleavage

furrow

First division (meiosis I)

Second division (meiosis II)

ProcessFigure 3.45

Meiosis

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Table 3.3

Feature Mitosis Meiosis

Comparison of Mitosis and Meiosis

Time of DNA replication Interphase Interphase

Number of cell divisions One Two; no replication of DNA occurs in between the two meiotic

divisions.

Cellproduced The two daughter cells are genetically identical to the Gametes are each different from the parent cell and from each other;

parent cell; each daughter cell has a diploid the gametes have a haploid number of chromosomes; in males,

number of chromosomes. four gametes (sperm cells); in females, one gamete (oocyte)

and two or three polar bodies, which eventually disintegrate.

Function New cells are formed during growth or tissue repair; Gametes are produced for reproduction; during fertilization the

new cells have identical DNA and can perform chromosomesfrom the haploid gametes unite to restore the

the same functions as the parent cells. diploid number typical of somatic cells; genetic variability is

increased because of random distribution of chromosomes

during meiosis and crossing-over.

Cellular Aspectsof Aging

Objective

■ Outline the major theories of aging.

A number of cellular structures and/or events appear to be

involved in the process ofaging. The major theories of aging con-

centrate on molecules within the cell,such as lipids, proteins, and

nucleic acids.It is estimated that at least 35% of the factors affect-

ing aging are genetic.

1. Cellular clock.One theor y of aging suggests that there is a

cellular clock,which, after a certain passage of time or a

certain number ofcell divisions, results in death of the

cellline.

2. Death genes.Another theor y suggests that there are “death

genes,”which turn on late in life,or sometimes prematurely,

causing cells to deteriorate and die.

3. DNA damage.Other theories suggest that through time,

DNA is damaged,resulting in cell degeneration and death.

It may be that DNA is protected from damage by a speciﬁc

sequence ofnucleotides, TTAGGG, called a telomere

(tel¯o-m¯er),at the end ofchromosomes. Apparently,during

DNA replication,nucleotides are lost at the extreme distal

end ofthe DNA molecule. Telomeres,at this ext reme end,

take the brunt ofthis replicative loss, thereby protecting

regions ofDNA that code for essential proteins. Telomerase

is an enzyme that mediates the repair and maintains the

integrity of the telomeric region of chromosomes.The

enzyme can even add additional nucleotides to the

telomeric region.Telomerase appears to be lost from aging

populations ofsomatic cells. Without telomerase to repair

the telomeres,they tend to degenerate during replication,

and eventually,critical, functional regions of DNA are lost

during replication,resulting in cell death.

4. Free radicals.The DNA in somatic cells may also be

susceptible to more direct damage,resulting in somatic

mutations,which may result in cellular dysfunction and,

ultimately,cell death. One of the major sources of DNA

Tetrad

Chromatids

Centromere

Chromosome

Homologous chromosomes

Figure 3.46

Crossing-Over

Crossing-over mayoccur during prophase I ofmeiosis. (a) A pair of replicated

homologouschromosomes. (b) Chromatids of the homologous chromosomes

form a tetrad. The chromatidsare crossed in two places. The chromatidsmay

breakat the points of crossing and become fused to the opposite

chromosome, resulting in crossing-over. (c) Geneticmaterial is exchanged

following crossing-over ofthe chromatids.

Apoptosis(Programmed Cell Death)

Apoptosis(apop-to¯sis, apo¯-to¯sis), or programmed cell death, is a

normalprocess by which cellnumbers within various tissues are

adjusted and controlled. During development, extra cellsare removed by

apoptosis, such ascells between the developing ﬁngersand toes, to

ﬁne-tune the contoursof the developing fetus. The number of cellsin

mostadult tissues is maintained at a speciﬁclevel . Apoptosiseliminates

excesscells produced byproliferation within some adult tissues to

maintain a constantnumber of cellswithin the tissue. Damaged or

potentiallydangerous cells, virus-infected cells, and potentialcancer

cellsare also eliminated by apoptosis.

Apoptosisis regulated by speciﬁc genes. The proteins coded for

bythose genes initiate events within the cell that ultimately lead to

the cell’sdeath. As apoptosis begins, the chromatin within the

nucleuscondenses and fragments. This is followed by fragmentation

ofthe nucleus and ﬁnally by death and fragmentation of the cell. The

cellfragments are cleaned up by specialized cellscalled

macrophages.

(a) (b) (c)

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Clinical Focus GeneticEngineering

We are living in an exciting era, when the

geneticbases of many human illnesses are

rapidly being revealed. Aswe discover the

defective genesassociated with these dis-

easesand learn the nature and function of

the proteinsthey encode, our ability to un-

derstand and therefore to treat many of

these diseasesis improved. Once we have

learned the basisof a given disease, a num-

ber ofapproaches are possible for treating

it, such asgenetic engineering or other mo-

lecular techniques. For example, the gene

for insulin hasbeen inserted into a bacte-

rial genome, thereby enabling the bac-

terium to produce large quantities of

human insulin, which increases its avail-

ability and functional quality. Antibodies

are being developed thatwill target speciﬁc

cellsor cell surface marker molecules asso-

ciated with diseases such as arthritis or

cancer. Clinicaltrials are underway to test

the efﬁcacyof introducing a functional copy

ofa gene into the cells of a person who has

a defective gene.

A negative side existsto this technol-

ogy, however. Manypeople are concerned

that the introduction of foreign genesinto

bacteria and human cells mayhave unex-

pected side effects. Manygenes have multi-

ple functions, and a danger existsthat we

may begin using gene therapy before we

know allthe ramiﬁcations. Some people are

greatlyconcerned about how far genetic en-

gineering should be allowed to go. What

range of“genetic defects” should humanity

be allowed to change, or should no limitbe

established? For example, when we dis-

cover the genesinvolved in controlling hu-

man height, should parents be allowed to

use gene therapy to increase a child’s

height so that he or she can be better at

basketball? An even more immediate con-

cern is to what extent a person’sgenetic

code should be made public. For example,

should a medical insurance company or

employer be allowed to see a person’sge-

netic proﬁle to set insurance premiumsor

make employment judgments? Ifa person

isshown to have a gene for muscular dys-

trophy, should the person’sinsurance com-

pany be given that information? Also of

concern is whether a person or company

should be able to patentand thus to own a

human gene.

mitochondrial function could result in the loss ofenergy

critical to cell function and,ultimately, to cell death.One

proposal suggests that reduced caloric intake may reduce

free radical damage to mitochondria.

68. How might a cellular clock, death genes, DNA damage, free

radicals, ormitochondrial damage contribute to cellular

aging?

Part1 Organization of the Human Body98

damage is apparently from free radicals,which are atoms

or molecules with an unpaired electron.

5. Mitochondrial damage.It may be that mitochondrial DNA

ismore sensitive to free-radical damage than is nuclear

DNA.Mitochondrial DNA damage may result in loss of

proteins critical to mitochondrial function.Because the

mitochondria are the power plants ofcells, loss of

1. The plasma membrane forms the outer boundary of the cell.

2. The nucleus directs the activities of the cell.

3. Cytoplasm,between the nucleus and plasma membrane, is where

most cell activities take place.

Functionsof the Cell

(p. 59)

1. Cells are the basic unit of life.

2. Cells provide protection and support.

3. Cells allow for movement.

4. Cells provide a means of communication.

5. Cells metabolize and release energy.

6. Cells provide for inheritance.

How We See Cells

(p. 59)

1. Light microscopes allow us to visualize general features of cells.

2. Electron microscopes allow us to visualize the ﬁne structure of cells.

Plasma Membrane

(p. 61)

1. The plasma membrane passively or actively regulates what enters or

leaves the cell.

2. The plasma membrane is composed of a phospholipid bilayer in

which proteins are suspended (ﬂuid-mosaic model).

Membrane Lipids

Lipids give the plasma membrane most of its structure and some of its

function.

Membrane Proteins

1. Membrane proteins function as markers,attachment sites, channels,

receptors,enzymes, and carriers.

2. Some receptor molecules are linked to and control channel proteins.

3. Some receptor molcules are linked to G proteins,which, in turn,

control numerous cellular activities.

MovementThrough the Plasma Membrane

(p. 65)

1. Lipid-soluble molecules pass through the plasma membrane readily

by dissolving in the lipid bilayer.

2. Small molecules pass through membrane channels. Most channels

are positively charged,allowing negatively charged ions and neutral

molecules to pass through more readily than positively charged ions.

3. Large polar substances (e.g., glucose and amino acids) are

transported through the membrane by carrier molecules.

4. Larger pieces of material enter cells in vesicles.

SUMMARY

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Diffusion

1. Diffusion is the movement of a substance from an area of higher

concentration to one oflower concentration (with a concentration

gradient).

2. The concentration gradient is the difference in solute

concentration between two points divided by the distance

separating the points.

3. The rate of diffusion increases with an increase in the concentration

gradient,an increase in temperature, a decrease in molecular size,

and a decrease in viscosity.

4. The end result of diffusion is a uniform distribution of molecules.

5. Diffusion requires no expenditure of energy.

Osmosis

1. Osmosis is the diffusion of water (solvent) across a selectively

permeable membrane.

2. Osmotic pressure is the force required to prevent the movement of

water across a selectively permeable membrane.

3. Isosmotic solutions have the same concentration of solute particles,

hyperosmotic solutions have a greater concentration,and

hyposmotic solutions have a lesser concentration ofsolute particles

than a reference solution.

4. Cells placed in an isotonic solution neither swell nor shrink.In a

hypertonic solution they shrink (crenate),and in a hypotonic

solution they swell and may burst (lyse).

Filtration

1. Filtration is the movement of a liquid through a partition with holes

that allow the liquid,but not everything in the liquid, to pass

through them.

2. Liquid movement results from a pressure difference across the

partition.

Mediated TransportMechanisms

1. Mediated transport is the movement of a substance across a

membrane by means ofa carrier molecule. The substances

transported tend to be large,water-soluble molecules.

• The carrier molecules have binding sites that bind with either a

single transport molecule or a group ofsimilar t ransport

molecules.This selectiveness is called speciﬁcity.

• Similar molecules can compete for carrier molecules, with each

reducing the rate oftransport of the other.

• Once all the carrier molecules are in use, the rate of transport

cannot increase further (saturation).

2. Three kinds of mediated transport can be identiﬁed.

• Facilitated diffusion moves substances with their concentration

gradient and does not require energy expenditure (ATP).

• Active transport can move substances against their concentration

gradient and requires ATP.An exchange pump is an active-

transport mechanism that simultaneously moves two substances

in opposite directions across the plasma membrane.

• In secondary active transport, an ion is moved across the plasma

membrane by active transport,and the energy produced by the ion

diffusing back down its concentration gradient can transport

another molecule,such as glucose, against its concentration gradient.

Endocytosisand Exocytosis

1. Endocytosis is the bulk movement of materials into cells.

• Phagocytosis is the bulk movement of solid material into cells by

the formation ofa vesicle.

• Pinocytosis is similar to phagocytosis, except that the ingested

material is much smaller or is in solution.

2. Exocytosis is the secretion of materials from cells by vesicle

formation.

3. Endocytosis and exocytosis use vesicles,can be speciﬁc (receptor-

mediated endocytosis) for the substance transported,and require

energy.

Cytoplasm

(p. 75)

The cytoplasm is the material outside the nucleus and inside the plasma

membrane.

Cytocol

1. Cytosol consists of a ﬂuid part (the site of chemical reactions), the

cytoskeleton,and cytoplasmic inclusions.

2. The cytoskeleton supports the cell and enables cell movements.It

consists ofprotein ﬁbers.

• Microtubules are hollow tubes composed of the protein tubulin.

They form spindle ﬁbers and are components ofcentrioles, cilia,

and ﬂagella.

• Actin ﬁlaments are small protein ﬁbrils that provide structure to

the cytoplasm or cause cell movements.

• Intermediate ﬁlaments are protein ﬁbers that provide structural

strength to cells.

3. Cytoplasmic inclusions,such as lipochromes, are not surrounded by

membranes.

Organelles

(p. 77)

Organelles are subcellular structures specialized for speciﬁc functions.

Centriolesand Spindle Fibers

1. Centrioles are cylindrical organelles located in the centrosome,a

specialized zone ofthe cytoplasm. The centrosome is the site of

microtubule formation.

2. Spindle ﬁbers are involved in the separation of chromosomes during

cell division.

Cilia and Flagella

1. Movement of materials over the surface of the cell is facilitated by cilia.

2. Flagella, much longer than cilia,propel sperm cells.

Microvilli

Microvilli increase the surface area ofthe plasma membrane for absorp-

tion or secretion.

Ribosomes

1. Ribosomes consist of small and large subunits manufactured in the

nucleolus and assembled in the cytoplasm.

2. Ribosomes are the sites of protein synthesis.

3. Ribosomes can be free or associated with the endoplasmic reticulum.

EndoplasmicReticulum

1. The endoplasmic reticulum is an extension of the outer membrane

ofthe nuclear envelope and forms tubules or sacs (cisternae)

throughout the cell.

2. The rough endoplasmic reticulum has ribosomes and is a site of

protein synthesis and modiﬁcation.

3. The smooth endoplasmic reticulum lacks ribosomes and is involved

in lipid production,detoxiﬁcation, and calcium storage.

Golgi Apparatus

The Golgi apparatus is a series of closely packed,modiﬁed cisternae that

function to modify,package, and distribute lipids and proteins produced

by the endoplasmic reticulum.

SecretoryVesicles

Secretory vesicles are membrane-bound sacs surrounded by membranes

that carry substances from the Golgi apparatus to the plasma membrane,

where the contents ofthe vesicle are released by exocytosis.

Lysosomes

1. Lysosomes are membrane-bounded sacs containing hydrolytic

enzymes.Within the cell, the enzymes break down phagocytized

material and nonfunctional organelles (autophagia).

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

3. Structure and Function of

the Cell

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2. Enzymes released from the cell by lysis or enzymes secreted from the

cell can digest extracellular material.

Peroxisomes

Peroxisomes are membrane-bounded sacs containing enzymes that digest

fatty acids and amino acids and enzymes that catalyze the breakdown of

hydrogen peroxide.

Proteasomes

Proteasomes are large multienzyme complexes,not bound by membranes,

which digest selected proteins within the cell.

Mitochondria

1. Mitochondria are the major sites of the production of ATP,which is

used as an energy source by cells.

2. The mitochondria have a smooth outer membrane and an inner

membrane that is infolded to produce cristae.

3. Mitochondria contain their own DNA,can produce some of their

own proteins,and can replicate independently of the cell.

Nucleus

(p. 85)

1. The nuclear envelope consists of two separate membranes with

nuclear pores.

2. DNA and associated proteins are found inside the nucleus as

chromatin.DNA is the hereditary material of the cell and controls

the activities ofthe cell by producing proteins through RNA.

3. Proteins play a role in the regulation ofDNA activity.

4. Nucleoli consist of RNA and proteins and are the sites ofribosomal

subunit assembly.

Overview ofCell Metabolism

(p. 87)

1. Aerobic respiration requires oxygen and produces carbon dioxide,

water,and up to 38 ATP molecules from a molecule ofglucose.

2. Anaerobic respiration does not require oxygen and produces lactic

acid and two ATP molecules from a molecule ofglucose.

Protein Synthesis

(p. 87)

1. Transcription:information stored in DNA is copied to mRNA.

2. Translation:the mRNA goes to ribosomes, where it directs the

synthesis ofproteins.

Transcription

1. DNA unwinds and,through nucleotide pairing, produces mRNA

(transcription).

2. The genetic code, which codes for amino acids,consists of codons,

which are sequences ofthree nucleotides in mRNA.

3. Introns are removed and exons are spliced by spliceosomes during

posttranscriptional processing.

Translation

1. The mRNA moves through the nuclear pores to ribosomes.

2. Transfer RNA (tRNA),which carries amino acids, interacts at the

ribosome with mRNA.The anticodons of tRNA bind to the codons

ofmRNA, and the amino acids are joined to form a protein

(translation).

3. Proproteins,some of which are proenzymes, are modiﬁed into

proteins,some of which are enzymes, during posttranslational

processing.

Regulation ofProtein Synthesis

1. Cells become specialized because of inactivation of certain parts of

the DNA molecule and activation ofother parts.

2. The level of DNA activity and thus protein production can be

controlled internally or can be affected by regulatory substances

secreted by other cells.

CellLife Cycle

(p. 90)

The cell life cycle has two stages:interphase and mitosis.

Interphase

Interphase is the period between cell divisions.

DNA Replication

DNA unwinds,and each strand produces a new DNA molecule during

replication.

CellDivision

Cell division includes nuclear division and cytoplasmic division.

Mitosis

1. Mitosis is the replication of the nucleus of the cell, and cytokinesis is

division ofthe cytoplasm of the cell.

2. Humans have 22 pairs of homologous chromosomes called

autosomes.Females also have two X chromosomes,and males also

have an X chromosome and a Y chromosome.

3. Mitosis is a continuous process divided into four phases.

• Prophase. Chromatin condenses to become visible as

chromosomes.Each chromosome consists of two chromatids

joined at the centromere.Centrioles move to opposite poles of the

cell,and astral ﬁbers and spindle ﬁbers form. Nucleoli disappear,

and the nuclear envelope degenerates.

• Metaphase. Chromosomes align at the equatorial plane.

• Anaphase. The chromatids of each chromosome separate at the

centromere.Each chromatid then is called a chromosome. The

chromosomes migrate to opposite poles.

• Telophase.Chromosomes unravel to become chromatin. The

nuclear envelope and nucleoli reappear.

Cytokinesis

Cytokinesis begins with the formation of the cleavage furrow during

anaphase.It is complete when the plasma membrane comes together at the

equator,thus producing two new daughter cells.

Meiosis

(p. 94)

1. Meiosis results in the production of gametes (oocytes or sperm cells).

2. All gametes receive one-half of the homologous autosomes (one

from each homologous pair).Oocytes also receive an X

chromosome.Sperm cells have an X or a Y chromosome.

3. Two cell divisions occur in meiosis.Each division has four phases

(prophase,metaphase, anaphase, and telophase) similar to those in

mitosis.

• In the ﬁrst division tetrads form, crossing-over occurs,and

homologous chromosomes are distributed randomly.Two cells are

formed,each with 23 chromosomes. Each chromosome has two

chromatids.

• In the second division, the chromatids of each chromosome

separate,and each cell receives 23 chromatids, which then are

called chromosomes.

4. Genetic variability is increased by crossing-over and random

assortment ofchromosomes.

Cellular Aspectsof Aging

(p. 97)

There are ﬁve major theories ofaging:

1. Cellular clock.A cell line may die out after a certain time or a certain

number ofcell divisions.

2. Death genes. There may be “death genes,”which turn on late in life,

causing cells to die.

3. DNA damage.Telomeres normally protect DNA from damage during

replication,and telomerase protects these telomeres.Aging cells lack

telomerase and telomeres,and other DNA, become open to damage.

4. Free radicals.Free radicals may also damage DNA.

5. Mitochondrial damage. Mitochondrial DNA may be the most sensitive

to free-radical damage.

Part1 Organization of the Human Body100

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

3. Structure and Function of

the Cell

Companies, 2004

Chapter 3 Structure and Function ofthe Cell 101

1. In the plasma membrane, form(s) the lipid bilayer,

determine(s) the ﬂuid nature ofthe membrane,

and mainly determine(s) the function of the

membrane.

a. phospholipids, cholesterol,proteins

b. phospholipids,proteins, cholesterol

c. proteins, cholesterol,phospholipids

d. cholesterol,phospholipids, proteins

e. cholesterol, proteins,phospholipids

2. Which of the following are functions of the proteins found in the

plasma membrane?

a. channel proteins

b. marker molecules

c. receptor molecules

d. enzymes

e. all of the above

3. Integrins in the plasma membrane function as

a. channel proteins.

b. marker molecules.

c. attachment sites.

d. enzymes.

e. receptor molecules.

4. In general, lipid-soluble molecules diffuse through the

;small, water-soluble molecules diffuse through

the .

a. membrane channels, membrane channels

b. membrane channels,lipid bilayer

c. lipid bilayer,carrier molecules

d. lipid bilayer,membrane channels

e. carrier proteins, membrane channels

5. Small pieces of matter,and even whole cells, can be transported

across the plasma membrane in

a. membrane channels.

b. carrier molecules.

c. receptor molecules.

d. marker molecules.

e. vesicles.

6. The rate of diffusion increases if the

a. concentration gradient decreases.

b. temperature ofa solution decreases.

c. viscosity of a solution decreases.

d. all ofthe above.

7. Concerning the process of diffusion, at equilibrium

a. the net movement of solutes stops.

b. random molecular motion continues.

c. there is an equal movement of solute in opposite directions.

d. concentration ofsolute is equal throughout the solution.

e. all of the above.

8. Which of these statements about osmosis is true?

a. Osmosis always involves a membrane that allows water and all

solutes to diffuse through it.

b. The greater the solute concentration,the smaller the osmotic

pressure ofa solution.

c. Osmosis moves water from a greater solute concentration to a

lesser solute concentration.

d. The greater the osmotic pressure ofa solution, the greater the

tendency for water to move into the solution.

e. Osmosis occurs because of hydrostatic pressure outside the cell.

9. If a cell is placed in a solution,lysis of the cell may

occur.

a. hypertonic

b. hypotonic

c. isotonic

d. isosmotic

10. Container A contains a 10% salt solution,and container B contains

a 20% salt solution.If the two solutions are connected, the net

movement ofwater by diffusion is from to

,and the net movement of salt by diffusion is from

to .

a. A,B; A,B

b. A,B;B,A

c. B,A; A,B

d. B,A;B,A

11. Suppose that a woman ran a long-distance race in the summer.

During the race she lost a large amount ofhyposmotic sweat. You

would expect her cells to

a. shrink.

b. swell.

c. stay the same

12. Suppose that a man is doing heavy exercise in the hot summer sun.

He sweats profusely.He then drinks a large amount of distilled

water.After he drank the water,you would expect his tissue cells to

a. shrink.

b. swell.

c. remain the same.

13. Unlike diffusion and osmosis,ﬁltration depends on a

on the two sides ofthe partition.

a. concentration gradient

b. pressure difference

c. difference in electric charge

d. difference in osmotic pressure

e. hyposmotic solution

14. Which of these statements about facilitated diffusion is true?

a. In facilitated diffusion, net movement is with the concentration

gradient.

b. Facilitated diffusion requires the expenditure ofenergy.

c. Facilitated diffusion does not require a carrier protein.

d. Facilitated diffusion moves materials through membrane channels.

e. Facilitated diffusion moves materials in vesicles.

15. Which of these statements concerning contransport of glucose into

cells is true?

a. The sodium-potassium exchange pump moves Na

into cells.

b. The concentration ofNa

outside cells is less than inside cells.

c. A carrier protein moves Na

into cells and glucose out ofcells.

d. The concentration ofglucose can be greater inside cells than

outside cells.

e. As Na

is actively transported into the cell,glucose is carried along.

16. A white blood cell ingests solid particles by forming vesicles.This

describes the process of

a. exocytosis.

b. facilitated diffusion.

c. secondary active transport.

d. phagocytosis.

e. pinocytosis.

17. Given these characteristics:

1. requires energy

2. requires carrier proteins

3. requires membrane channels

4. requires vesicles

Choose the characteristics that apply to exocytosis.

a. 1, 2

b. 1,4

c. 1, 3, 4

d. 1,2, 3

e. 1, 2, 3,4

REVIEW AND COMPREHENSION

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

3. Structure and Function of

the Cell

Companies, 2004

18. Cytoplasm is found

a. in the nucleus.

b. outside the nucleus and inside the plasma membrane.

c. outside the plasma membrane.

d. inside mitochondria.

e. everywhere in the cell.

19. Which of these elements of the cytoskeleton is composed of tubulin

and forms essential components ofcentrioles, spindle ﬁbers, cilia,

and ﬂagella?

a. actin ﬁlaments

b. intermediate ﬁlaments

c. microtubules

20. Cylindrically shaped extensions of the plasma membrane that do

not move,and are supported with actin ﬁlaments; they may

function in absorption or as sensory receptors.This describes

a. centrioles.

b. spindle ﬁbers.

c. cilia.

d. ﬂagella.

e. microvilli.

21. A large structure, normally visible in the nucleus of a cell, where

ribosomal subunits are produced.

a. endoplasmic reticulum.

b. mitochondria.

c. nucleolus.

d. lysosome.

22. A cell that synthesizes large amounts of protein for use outside the

cell has a large

a. number of cytoplasmic inclusions.

b. number ofmitochondria.

c. amount of rough endoplasmic reticulum.

d. amount ofsmooth endoplasmic reticulum.

e. number of lysosomes.

23. Which of these organelles produces large amounts of ATP?

a. nucleus

b. mitochondria

c. ribosomes

d. endoplasmic reticulum

e. lysosomes

24. Mature red blood cells cannot

a. synthesize ATP.

b. transport oxygen.

c. synthesize new protein.

d. use glucose as a nutrient.

25. For each glucose molecule,aerobic respiration may produce up to

ATP and 6 CO

molecules,whereas anaerobic

respiration produces ATP and 2 lactic acid

molecules.

a. 2, 2

b. 2,4

c. 2, 38

d. 38,2

e. 38, 38

26. A portion of an mRNA molecule that determines one amino acid in

a polypeptide chain is called a

a. nucleotide.

b. gene.

c. codon.

d. exon.

e. intron.

27. In which of these organelles is mRNA synthesized?

a. nucleus

b. ribosome

c. endoplasmic reticulum

d. nuclear envelope

e. peroxisome

28. During the cell life cycle,DNA replication occurs during the

a. G

phase.

b. G

phase.

c. M phase.

d. S phase.

29. Given the following activities:

1. repair

2. growth

3. gamete production

4. differentiation

Which ofthe activities are the result of mitosis?

a. 2

b. 3

c. 1, 2

d. 3,4

e. 1, 2, 4

30. Which of these processes does not occur during meiosis?

a. crossing-over

b. interkinesis

c. tetrad formation

d. production ofchromatids

e. production of gametes with the diploid number of chromosomes

Answers in Appendix F

Part1 Organization of the Human Body102

1. Why does a surgeon irrigate a surgical wound from which a tumor

has been removed with sterile distilled water rather than with sterile

isotonic saline?

2. Solution A is hyperosmotic to solution B.If solution A is separated

from solution B by a selectively permeable membrane,does water

move from solution A into solution B or vice versa? Explain.

3. A dialysis membrane is selectively permeable, and substances

smaller than proteins are able to pass through it.If you wanted to

use a dialysis machine to remove only urea (a small molecule) from

blood,what could you use for the dialysis ﬂuid?

a. a solution that is isotonic and contains only large molecules,

such as protein

b. a solution that is isotonic and contains the same concentration

ofall substances except that it has no urea

c. distilled water,which contains no ions or dissolved molecules

d. blood,which is isotonic and contains the same concentration of

all substances,including urea

4. A researcher wants to determine the nature of the transport

mechanism that moved substance X into a cell.She could measure

the concentration ofsubstance X in the extracellular ﬂuid and within

the cell,as well as the rate of movement of substance X into the cell.

She does a series ofexperiments and gathers the data shown in the

graph.Choose the transport process that is consistent with the data.

a. diffusion

b. active transport

c. facilitated diffusion

d. not enough information to make a judgment

CRITICAL THINKING

Seeley−Stephens−Tate:

Anatomy and Physiology,

Sixth Edition

I. Organization of the

Human Body

3. Structure and Function of

the Cell

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Chapter 3 Structure and Function ofthe Cell 103

1. Urea is continually produced by metabolizing cells and diffuses from

the cells into the interstitial spaces and from the interstitial spaces

into the blood.If the kidneys stop eliminating urea, it begins to

accumulate in the blood.Because the concentration of urea increases

in the blood,urea cannot diffuse from the interstitial spaces. As urea

accumulates in the interstitial spaces,the rate of diffusion from cells

into the interstitial spaces slows because the urea must pass from a

higher to a lower concentration by the process ofdiffusion. the urea

ﬁnally reaches concentrations high enough to be toxic to cells,

thereby causing cell damage followed by cell death.

2. If the membrane is freely permeable, the solutes in the tube diffuse

from the tube (higher concentration ofsolutes) into the beaker

(lower concentration ofsolutes) until equal amounts of solutes exist

inside the tube and beaker (i.e.,equilibrium). In a similar fashion,

water in the beaker diffuses from the beaker (higher concentration

ofwater) into the tube (lower concentration of water) until equal

amounts ofwater are inside the tube and beaker. Consequently,the

solution concentrations inside the tube and beaker are the same

because they both contain the same amounts ofsolutes and water.

Under these conditions,no net movement of water into the tube

occurs.This simple experiment demonstrates that osmosis and

osmotic pressure require a membrane that is selectively permeable.

3. Glucose transported by facilitated diffusion across the plasma

membrane moves from a higher to a lower concentration.If glucose

molecules are quickly converted to some other molecule as they

enter the cell,a steep concentration gradient is maintained. The rate

ofglucose transport into the cell is directly proportional to the

magnitude ofthe concentration gradient.

4. Digitalis should increase the force of heart concentration. By

interfering with Na

transport,digitalis decreases the concentration

gradient for Na

because fewer ions are pumped out ofcells by

active transport.Consequently, fewer ions diffuse into cells,and

fewer Ca

ions move out ofthe cells by countertransport. The

higher intracellular levels ofCa

promote more forceful

concentrations.

5. a. Cells highly specialized to synthesize and secrete proteins have

large amounts ofrough endoplasmic reticulum (ribosomes

attached to endoplasmic reticulum) because these organelles are

important for protein synthesis.Golgi apparatuses are well

developed because they package materials for release in secretory

vesicles.Also, numerous secretory vesicles exist in the cytoplasm.

b. Cells highly specialized to actively transport substances into the

cell have a large surface area exposed to the ﬂuid from which

substances are actively transported,and numerous mitochondria

are present near the membrane across which active transport

occurs.

c. Cells highly specialized to synthesize lipids have large amounts of

smooth endoplasmic reticulum.Depending on the kind of lipid

produced,lipid droplets may accumulate in the cytoplasm.

d. Cells highly specialized to phagocytize foreign substances have

numerous lysosomes in their cytoplasm and evidence of

phagocytic vesicles.

6. By changing a single nucleotide within a DNA molecule, a change in

the nucleotide ofmRNA produced from that segment of DNA also

occurs,and a different amino acid is placed in the amino acid chain

for which the mRNA provides direction.Because a change in the

amino acid sequence ofa protein could change its structure, one

substitution ofa nucleotide in a DNA chain could result in altered

protein structure and function.

7. Because adenine pairs with thymine (no uracil exists in DNA) and

cytosine pairs with guanine,the sequence of DNA replicated from

strand 1 is TACGAT.This sequence is also the sequence ofDNA in

the original strand 2.A replicate of strand 2 is therefore ATGCTA,

which is the same as the original strand 1.

ANSWERS TO PREDICT QUESTIONS

Rate of movement

of substance X into

the cell

Concentration of substance X within the cell

minus the concentration outside the cell

–

5. Predict the consequence of a reduced intracellular K

concentration

on the resting membrane potential.

6. If you had the ability to inhibit mRNA synthesis with a drug, explain

how you could distinguish between proteins released from secretory

vesicles in which they had been stored and proteins released from

cells in which they have been newly synthesized.

7. Given the following data from electron micrographs of a cell,

predict the major function ofthe cell:

• moderate number of mitochondria;

• well-developed rough endoplasmic reticulum;

• moderate number of lysosomes;

• well-developed Golgi apparatus;

• dense nuclear chromatin;

• numerous vesicles.

Answers in Appendix G

Graph depicting the rate ofmovement of substance Xfrom a ﬂuid into a cell ( y

axis) versusthe concentration of substance Xwithin the cell (x axis). At point A

the extracellular concentration ofsubstance Xis equal to the intracellular

concentration ofsubstance X(designated 0 on the x axis).

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