53107590 cell biology cell structures and their functions

8/6/2019 53107590 Cell Biology Cell Structures and Their Functions

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Chapter ThrChapter ThrCell Structures and Their Functions

active transportCarrier-mediated process thatrequires ATP and can movesubstances against aconcentration gradient.

cell membrane[plasma (plazma) membrane]Outermost component of the cell,surrounding and binding the restof the cell contents.

diffusion(di-fuzhun) [L. diffundo, to pourin different directions] Tendencyfor solute molecules to move froman area of higher concentration toan area of lower concentration ina solution.

endoplasmic reticulum (ER)(endo-plasmik re-tiku-lum)[endo Gr. plastos, formed]Membranous network inside thecytoplasm; rough ER hasribosomes attached to the surface;smooth ER does not.

facilitated diffusion(fa-sili-ta-td di-fuzhun) Carrier-mediated process that does notrequire ATP and moves substancesinto or out of cells from a higherto a lower concentration.

Golgi apparatus(golje) Stacks of flattened,membrane-bound sacks thatcollect, modify, package, anddistribute proteins and lipids.

meiosis(m-osis)[Gr., a lessening] Processof cell division that results ingametes. Consists of two celldivisions that result in four cells,each of which contains half the

number of chromosomes as theparent cell.

mitochondrion, pl. mitochondria(mto-kondre-on, m to-kondre-a)[Gr. mitos, thread chandros,granule] Small, bean-shaped orrod-shaped structures in thecytoplasm that are sites of ATPproduction.

mitosis(m-tosis) [Gr., thread] Division ofthe nucleus. Process of celldivision that results in two

daughter cells with exactly thesame number and type ofchromosomes as the parent cell.

nucleus, pl. nuclei(nookle-us, nookle-) [L., inside ofa thing] Cell organelle containingmost of the cells genetic material.

osmosis(os-mosis) [Gr. osmos, thrusting oran impulsion] Diffusion of solvent(water) through a selectivelypermeable membrane from aregion of higher water

concentration to one of lowerwater concentration.

ribosome(r bo-som) Small, spherical,cytoplasmic organelle whereprotein synthesis occurs.

ObjectivesAfter reading this chapter, you should be able to:

1. Describe the structure of the cell membrane.

2. Describe the structure and function of the nucleus

nucleoli.

3. Compare the structure and function of rough and s

endoplasmic reticulum.

4. Describe the roles of the Golgi apparatuses and sec

vesicles in secretion.5. Explain the role of lysosomes in digesting material

into cells by phagocytosis.

6. Describe the structure and function of mitochondri

7. Compare the structure and function of cilia, flagell

microvilli.

8. List four ways by which substances cross the cell m

9. Explain the role of osmosis and that of osmotic pre

controlling the movement of water across the cell

membrane. Compare hypotonic, isotonic, and hyper

solutions.

10. Define mediated transport, and compare the proc

facilitated diffusion, active transport, and secondar

transport.

11. Describe endocytosis and exocytosis.

12. Describe the process of protein synthesis.

13. Explain what is accomplished during mitosis and m

14. Define differentiation, and explain how it occurs.


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The cell is the basic living unit of all organisms. The simplestorganisms consist of a single cell, whereas humans are com-posed of trillions of cells. If each of these cells was about thesize of a standard brick, we could build a colossal structure inthe shape of a human over 5 12 miles (10 km) high! Obviously,there are many differences between a cell and a brick. Cellsare much smaller than bricks: An average-sized cell is one fifththe size of the smallest dot you can make on a sheet of paperwith a sharp pencil! In spite of their extremely small size, cellsare complex living structures.

Cells of the human body have many characteristics incommon. However, most cells are also specialized to performspecific functions. The human body is made up of popula-tions of these specialized cells. Communication and coordi-nation between these populations are critical for a complexorganism, such as a human, to survive.

The study of cells is an important link between the studyof chemistry in chapter 2 and tissues in chapter 4. A knowl-edge of chemistry makes it possible to understand cells be-cause cells are composed of molecules that are responsible formany of the characteristics of cells. Cells, in turn, determinethe form and functions of the tissues of the body. It is alsoimportant to understand that a great many diseases and other

human disorders have a cellular basis. This chapter considersthe structure of cells and how cells perform the activities nec-essary for life.

Functions of the CellThe main functions of the cell include

1. Basic unit of life. The cell is the smallest part to whichan organism can be reduced that still retains thecharacteristics of life.

2. Protection and support. Cells produce and secretevarious molecules that provide protection and supportof the body. For example, bone cells are surrounded bya mineralized material, making bone a hard tissue thatprotects the brain and other organs and that supportsthe weight of the body.

3. Movement.All the movements of the body occurbecause of molecules located within specific cells suchas muscle cells.

4. Communication. Cells produce and receive chemicaland electrical signals that allow them to communicatewith one another. For example, nerve cellscommunicate with one another and with muscle cells,causing them to contract.

5. Cell metabolism and energy release. The chemicalreactions that occur within cells are referred to

collectively as cell metabolism. Energy released duringmetabolism is used for cell activities, such as thesynthesis of new molecules, muscle contraction, and heatproduction, which helps maintain body temperature.

6. Inheritance. Each cell contains a copy of the geneticinformation of the individual. Specialized cells areresponsible for transmitting that genetic information tothe next generation.

Cell StructureEach cell is a highly organized unit. Within cells, specializstructures called organelles (orga-nelz, little organs) pform specific functions (figure 3.1 and table 3.1). The nucleis an organelle containing the cells genetic material. The ing material surrounding the nucleus is called cytopla(s to-plazm), which contains many other types of organelThe cytoplasm is enclosed by the cell, or plasma, membra

The number and type of organelles within each determine the cells specific structure and functions. For ample, cells secreting large amounts of protein contain wdeveloped organelles that synthesize and secrete protewhereas muscle cells have organelles that enable the cellscontract. The following sections describe the structure amain functions of the major organelles found in cells.

Cell MembraneThe cell membrane, or plasma (plazma) membrane, is outermost component of a cell. The cell membrane enclothe cytoplasm and forms the boundary between material side the cell and material outside it. Substances outside

cell are called extracellular substances, and substances insthe cell are called intracellular substances. The cell membraencloses the cell, supports the cell contents, is a selective brier that determines what moves into and out of the cell, aplays a role in communication between cells.

The major molecules that make up the cell membraare phospholipids and proteins. In addition, the membracontains other molecules, such as cholesterol, carbohydra

water, and ions. The phospholipids form a double layermolecules. The polar, phosphate-containing ends of the phpholipids are hydrophilic (water loving) and therefore face water inside and outside the cell. The nonpolar, fatty acid enof the phospholipids are hydrophobic (water fearing) atherefore face away from the water on either side of the mebrane, toward the center of the double layer of phospholip(figure 3.2). The double layer of phospholipids forms a lipbarrier between the inside and outside of the cell.

Studies of the arrangement of molecules in the cell mebrane have given rise to a model of its structure called the flmosaic model. The double layer of phospholipid molecules a liquid quality. Cholesterol within the membrane giveadded strength and flexibility. Protein molecules float amothe phospholipid molecules and, in some cases, may extefrom the inner to the outer surface of the cell membrane. Carhydrates may be bound to some protein molecules, modifytheir functions. The proteins function as membrane channcarrier molecules, receptor molecules, enzymes, or structu

supports in the membrane. Membrane channels and carrmolecules are involved with the movement of substanthrough the cell membrane. Receptor molecules are part ofintercellular communication system that enables coordinatof the activities of cells. For example, a nerve cell can releaschemical messenger that moves to a muscle cell and temporily binds to its receptor. The binding acts as a signal that tgers a response such as contraction of the muscle cell.


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Chapter Three Cell Structures and Thei

Figure 3.1 Generalized Cell Showing the Major Organelles

No single cell contains all organelle types. In addition, some kinds of cells contain many organelles of one type, and another kind of cell contains v

Table 3.1 Organelles and Their Locations and Functions

Organelles Location and Function(s)

Nucleus Usually near center of the cell; contains genetic material of cell (DNA) and nucleoli; sribosome and messenger RNA synthesis

Nucleolus In the nucleus; site of ribosomal RNA and ribosomal protein synthesis

Rough endoplasmic reticulum (rough ER) In cytoplasm; many ribosomes attached to ER; site of protein synthesis

Smooth endoplasmic reticulum (smooth ER) In cytoplasm; site of lipid synthesis

Golgi apparatus In cytoplasm; modifies protein structure and packages proteins in secretory vesicles

Secretory vesicle In cytoplasm; contains materials produced in the cell; formed by the Golgi apparatussecreted by exocytosis

Lysosome In cytoplasm; contains enzymes that digest material taken into the cell

Mitochondrion In cytoplasm; site of aerobic respiration and the major site of ATP synthesisMicrotubule In cytoplasm; supports cytoplasm; assists in cell division and forms components of ci

and flagella

Cilia On cell surface with many on each cell; cilia move substances over surface of certain

Flagella On sperm cell surface with one per cell; propels the sperm cells

Microvilli Extensions of cell surface with many on each cell; increase surface area of certain cel


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Cell Structure

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NucleusThe nucleus (nookle-us) is a large organelle usually locatednear the center of the cell (see figure 3.1). All cells of the bodyhave a nucleus at some point in their life cycle, although somecells, such as red blood cells, lose their nuclei as they mature.Other cells, such as osteoclasts (a type of bone cell) and skele-

tal muscle cells, contain more than one nucleus.The nucleus is bounded by a nuclear envelope,whichconsists of outer and inner membranes with a narrow spacebetween them (figure 3.3). At many points on the surface ofthe nucleus, the inner and outer membranes come together toform nuclear pores, through which materials can pass into orout of the nucleus.

The nucleus contains loosely coiled fibers called chro-matin consisting of deoxyribonucleic acid (DNA) and proteins(see figures 2.17 and 3.3b). During cell division, the chromatinfibers become more tightly coiled to form the 23 pairs ofchro-mosomes (kromo-somz) characteristic of human cells (see thesection on Cell Division on p. 59). The genes that influencethe structural and functional features of every individual are

made up of DNA molecules. The DNA molecules store infor-mation that allows the genes to determine the structure of pro-teins.

Nucleoli and RibosomesNucleoli (noo-kleo-l ) number from one to four per nucleus.They are rounded, dense, well-defined nuclear bodies with no

surrounding membrane (see figure 3.3). The subunits of risomes are formed within a nucleolus. Proteins producedthe cytoplasm move through the nuclear pores into nucleus and to the nucleolus. These proteins are joinedribosomal ribonucleic (r bo-noo-kleik) acid (rRNA), pduced within the nucleolus, to form large and small ribosomsubunits (figure 3.4). The ribosomal subunits then move fr

the nucleus through the nuclear pores into the cytoplawhere one large and one small subunit join to form a ribosomRibosomes (r bo-somz) are the organelles where p

teins are produced (see section on Protein Synthesis on p. 5Free ribosomes are not attached to any other organelles in cytoplasm, whereas other ribosomes are attached to a mebrane called the endoplasmic reticulum.

Rough and SmoothEndoplasmic ReticulumThe endoplasmic reticulum (endo-plasmik re-tiku-lum) (is a series of membranes that extends from the outer nucl

membrane into the cytoplasm (figure 3.5). Rough ER is with ribosomes attached to it. A large amount of rough ERa cell indicates that it is synthesizing large amounts of protfor export from the cell. On the other hand, ER without ribsomes is called smooth ER. Smooth ER is a site for lipid sthesis in cells. Smooth ER also participates in detoxificationchemicals within the cell. In skeletal muscle cells, the smoER stores calcium ions.

Figure 3.2 The Cell Membrane

The cell membrane is composed of a double layer of phospholipid molecules with proteins floating in the membrane. The nonpolar end of eachphospholipid molecule is directed toward the center of the membrane, and the polar end of each phospholipid molecule is directed toward the waterenvironment either outside or inside the cell. Cholesterol molecules are interspersed among the phospholipid molecules. Groups of proteins can formmembrane channels, carrier molecules, receptor molecules, enzymes, or structural supports.


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cell (see figure 3.6). Their membranes then fuse withmembrane, and the contents of the vesicles are relthe exterior of the cell. In many cells, secretory vescumulate in the cytoplasm and are released to the

when the cell receives a signal. For example, secretcles containing the hormone insulin remain in the cyof pancreatic cells until rising blood levels of glucoa stimulus for their re lease .

LysosomesLysosomes (l so-somz) (see figure 3.1) are membranvesicles formed from the Golgi apparatus. They contaety of enzymes that function as intracellular digestive

Particulate material taken into a cell is contained withinthat fuse with lysosomes. The enzymes within the lybreak down the ingested materials. For example, whcells take up bacteria, which the enzymes within lydestroy. Also, when tissues are damaged, ruptured lywithin the damaged cells release their enzymes and dighealthy and damaged cells. The released enzymes aresible for part of the resulting inflammation (see chapte

The Golgi ApparatusThe G ol gi ( g olje) a p p a ra tu s (named for Camillo Golgi[ 18 431926], an Italian histologist) consists of closely packedstacks of curved, membrane-bound sacs (figure 3.6). It col-lects, modifies, packages, and distributes proteins and lipidsm a n u f a c t u red by the ER. For example, proteins produced atthe ribosomes enter the Golgi apparatus from the ER. Insome cases, the Golgi apparatus chemically modifies theproteins by attaching carbohydrate or lipid molecules tothem. The proteins then are packaged into membrane sacsthat pinch off from the margins of the Golgi apparatus (seesection on Secretory Vesicles below). The Golgi apparatus ispresent in larger numbers and is most highly developed in

cells that secrete protein, such as the cells of the salivaryglands or the pancreas .

Secretory VesiclesAvesi cl e (ve si-kl) is a small, membrane-bound sac that trans-ports or stores materials within cells. Se c re to ry vesicl es p inc hoff from the Golgi apparatus and move to the surface of the

Figure 3.3 The Nucleus

(a) The nuclear envelope consists of inner and outer membranes that become fused at the nuclear pores. The nucleolus is a condensed region ofnucleus not bounded by a membrane and consisting mostly of RNA and protein. (b) Transmission electron micrograph of the nucleus. (c) Scannielectron micrograph showing the inner surface of the nuclear envelope and the nuclear pores.

(a)

(b) (c)


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Cell Structure

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Figure 3.4 Production of Ribosomes

PeroxisomesPeroxisomes (per-oksi-somz) are small, membrane-bouvesicles containing enzymes that break down fatty acids aamino acids. Hydrogen peroxide (H2O2), which can be toto the cell, is a by-product of that breakdown. Peroxisomalso contain an enzyme that breaks down hydrogen peroxto water and oxygen. Cells that are active in detoxificatisuch as liver and kidney cells, have many peroxisomes.

Figure 3.5 The Endoplasmic Reticulum

The outer membrane of the nuclear envelope is continuous with theendoplasmic reticulum (ER). Rough ER has ribosomes attached to itsmembrane, and smooth ER has no ribosomes attached to it. Some cellscontain predominantly smooth ER, and others contain predominantlyrough ER.

Did You Know?

Some diseases result from nonfunctional lysosomal enzymes. Forexample, Pompes disease results from the inability of lysosomalenzymes to break down the carbohydrate glycogen produced incertain cells. Glycogen accumulates in large amounts in the heart,liver, and skeletal muscles. Glycogen accumulation in the heartmuscle cells often leads to heart failure. Lipid storage disorders areoften hereditary and are characterized by the accumulation of largeamounts of lipid in phagocytic cells. These cells take up the lipid byphagocytosis, but they lack the enzymes required to break down thelipid droplets. Symptoms include enlargement of the spleen and liveand replacement of bone marrow by lipid-filled phagocytes.


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CytoskeletonThe cytoskeleton (s -to-skele-ton) consists of protsupport the cell, hold organelles in place, and enableto change shape. The cytoskeleton consists of micromicrofilaments, and intermediate filaments (figure 3.

Microtubules are hollow structures formed fromsubunits that perform a variety of roles, such as heprovide support to the cytoplasm of cells, assistinprocess of cell division, and forming essential compo

certain organelles such as cilia and flagella.Microfilaments are small fibrils formed from pro

units that structurally support the cytoplasm. Some mments are involved with cell movements. For examplfilaments in muscle cells enable the cells to shorten or

Intermediate filaments are fibrils formed fromsubunits that are smaller in diameter than microtublarger in diameter than microfilaments. They provchanical support to the cell.

Cilia, Flagella, and MicrovilliCilia (s le-a) project from the surface of cells, are ca

moving (see figure 3.1), and vary in number from thousands per cell. Cilia have a cylindrical shape, concialized microtubules, and are enclosed by the cell meCilia are numerous on surface cells that line the retract. Their coordinated movement moves mucus, i

MitochondriaM ito ch ondri a (mto -kondre-a; sing. mitochondrion) aresmall, bean-shaped or rod-shaped organelles with inner andouter membranes separated by a space (figure 3.7 and seefi gure 3.1). The outer membranes have a smooth contour, butthe inner membranes have numerous infoldings called c ri s t a e( k r iste), which project like shelves into the interior of themi t o ch ond r ia .

Mitochondria are the major sites of adenosine triphos-

phate (ATP) production within cells. ATP is the major energysource for most chemical reactions within the cell, and cells

with a large energy requirement have more mitochondria thancells that require less energy. Mitochondria carry out aerobicrespiration (discussed in greater detail in the section CellMetabolism on p. 54) in which oxygen is required to allow thereactions that produce ATP to proceed. Cells that carry out ex-tensive active transport, which is described on p. 54, containmany mitochondria, and, when muscles enlarge as a result ofexercise, the mitochondria increase in number within themuscle cells and provide the additional ATP required for mus-cle contraction.

Increases in the number of mitochondria result from the

division of preexisting mitochondria. The information formaking some mitochondrial proteins and for mitochondrialdivision is contained in a unique type of DNA within the mi-tochondria. This DNA is more like bacterial DNA than that ofthe cells nucleus.

Figure 3.6 The Golgi Apparatus


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Movement Throughthe Cell MembraneThe cell membrane is selectively permeable, allowing sosubstances, but not others, to pass into or out of the cIntracellular material has a different composition from extcellular material, and the survival of cells depends on mataining the difference. Substances such as enzymes, gly

gen, and potassium ions are found at higher concentratiointracellularly; and sodium, calcium, and chloride ions found in greater concentrations extracellularly. In additinutrients must enter cells continually, and waste produmust exit. Because of the permeability characteristics of cell membrane and its ability to transport certain molecucells are able to maintain proper intracellular concentratioof molecules. Rupture of the membrane, alteration of its pmeability characteristics, or inhibition of transport procescan disrupt the normal intracellular concentration of mocules and lead to cell death.

Molecules can pass through the cell membrane in foways:

1. Directly through the phospholipid membrane. Mo lec uthat are soluble in lipids, such as oxygen, carbondioxide, and steroids, pass through the cell membranreadily by dissolving in the lipid bilayer. Thephospholipid bilayer acts as a barrier to mostsubstances that are not lipid-soluble; but certain smalnon-lipid-soluble molecules, such as water, and ure a ,can diffuse between the phospholipid molecules of thcell membrane.

2. Membrane channels. Cell membrane ch a n n e ls ,consisting of large protein molecules, extend fromone surface of the cell membrane to the other (seefi g u re 3.2). There are several channel types, each of

which allows only certain molecules to pass throughit. The size, shape, and charge of moleculesd e t ermines whether they can pass through each kindof channel. For example, sodium ions pass throughsodium channels, and potassium and chloride ions

Movement Through the Cell Membrane

48

Figure 3.7 Mitochondria(a) Typical mitochondrion structure. (b) Electron micrograph of mitochondria in longitudinal and cross sections.

dust particles are embedded, upward and away from thelungs. This action helps keep the lungs clear of debris.

Flagella (fla-jela) have a structure similar to that of ciliabut are much longer, and usually occur only one per cell.Sperm cells each have one flagellum, which functions to pro-pel the sperm cell.

Microvilli (mkro-vil ) are specialized extensions of thecell membrane that are supported by microfilaments (see fig-ure 3.1), but they do not actively move like cilia and flagella.

Microvilli are numerous on cells that have them and functionto increase the surface area of those cells. They are abundanton the surface of cells that line the intestine, kidney, and otherareas in which absorption is an important function.

(a) (b)

P R E D I C T1

List the organelles that are common in cells that (a) synthesize andsecrete proteins, (b) actively transport substances into cells, and(c) ingest foreign substances. Explain the function of each organelleyou list.

Answer on page 00

Whole-Cell ActivityTo understand how a cell functions, the interactions betweenthe organelles must be considered. For example, the transportof many food molecules into the cell requires ATP and cellmembrane proteins. Most ATP is produced by mitochondria.The production of cell membrane proteins requires aminoacids that are transported into the cell across the cell mem-brane by transport proteins. Information contained in DNAwithin the nucleus determines which amino acids are com-bined at ribosomes to form proteins. The mutual interdepen-

dence of cellular organelles is coordinated to maintain homeo-stasis within the cell and the entire body. The followingsections, Movement Through the Cell Membrane, Cell Metab-olism, Protein Synthesis, and Cell Division, illustrate the inter-actions of organelles that result in a functioning cell.


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Table 3.2 Types and Characteristics of Movement Across Cell Membranes

Type Transport Requires ATP Examples

Diffusion With the concentration gradient through No Oxygen, carbon dioxide, chloride ions,the lipid portion of the cell membrane and ureaor through membrane channels

Osmosis With the concentration gradient (for No Waterwater) through the lipid portion of thecell membrane or through membranechannels

Filtration Movement of liquid and substances No In the kidneys, filtration of everythingby pressure through a partition blood except proteins and blood celcontaining holes

Facilitated diffusion With the concentration gradient by No Glucose in most cellscarrier molecules

Active transport Against the concentration gradient* by Yes Sodium, potassium, calcium, andcarrier molecules hydrogen ions; amino acids

Secondary active transport Against the concentration gradient by Yes Glucose, amino acidscarrier molecules; the energy forsecondary active transport of one

substance comes from theconcentration gradient of another

Endocytosis Movement into cells by vesicles Yes Ingestion of particles by phagocytosisand liquids by pinocytosis

Exocytosis Movement out of cells by vesicles Yes Secretion of proteins

*Active transport normally moves substances against their concentration gradient, but it can also move substances with their concentration gradient.

Figure 3.9 Diffusion

(a) (b) (c)


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concentration gradient, describes the diffusion of solutes from

a higher toward a lower concentration of solutes. Movementup, or against, a concentration gradient, describes the move-ment of solutes from a lower toward a higher concentrationof solutes. This second type of movement does not occur bydiffusion and requires energy to move solutes against theirconcentration gradient. The concentration gradient is said tobe steeper when the concentration gradient is large.

Diffusion is an important means of transporting sub-stances through the extracellular and intracellular fluids in thebody. In addition, substances that can pass either through thelipid layers of the cell membrane or through membrane chan-nels diffuse through the cell membrane. Some nutrients enterand some waste products leave the cell by diffusion. The nor-mal intracellular concentrations of many substances depend

on diffusion. For example, if the extracellular concentration ofoxygen is reduced, not enough oxygen diffuses into the cell,and normal cell function cannot occur.

OsmosisOsmosis (os-mosis) is the diffusion of water (a solvena selectively permeable membrane, such as the cebrane, from a region of higher water concentration lower water concentration (see table 3.2). Osmosis tant to cells because large volume changes caused movement can disrupt normal cell functions. Osmoswhen the cell membrane is either less permeable ormeable to solutes anda concentration gradient for wists across the cell membrane. Water diffuses from awith a higher concentration of water across the cell minto a solution with a lower water concentration. Thto predict the direction of water movement depends oing which solution on either side of a membrane has

est water concentration.The concentration of a solution, however, is

pressed in terms of water, but in terms of solute cotion. For example, if sugar solution A is twice as concas sugar solution B, then solution A has twice as mu(solute) as solution B. As the concentration of a solcreases, the amount of water (solvent) proportioncreases. Thus water diffuses from the less concentration, which has fewer solute molecules and momolecules, into the more concentrated solution wsolute molecules and fewer water molecules.

Clinical Focus Relationships Between Cell Structure and Cell FunctionEach cell is well adapted for the functionsit performs, and the abundance of or-ganelles in each cell reflects the functionof the cell. For example, epithelial cellsthat line the larger-diameter respiratorypassages secrete mucus and transport ittoward the throat, where it is either swal-

lowed or expelled from the body bycoughing. Particles of dust and other de-bris suspended in the air become trappedin the mucus. The production and trans-port of mucus from the respiratory pas-sages function to keep these passagesclean. Cells of the respiratory system haveabundant rough ER, Golgi apparatuses,

secretory vesicles, and cilia. The ribosomeson the rough ER are the sites where pro-teins, a major component of mucus, areproduced. The Golgi apparatuses packagethe proteins and other components ofmucus into secretory vesicles, which moveto the surface of the epithelial cells. The

contents of the secretory vesicles are re-leased onto the surface of the epithelialcells. Cilia on the cell surface then propelthe mucus toward the throat.

In people who smoke, the prolongedexposure of the respiratory epithelium tothe irritation of tobacco smoke causes therespiratory epithelial cells to change in

structure and function. The cells and form several layers of epitheliThese flattened epithelial cells nocontain abundant rough ER, Golgi tuses, secretory vesicles, or cilia. Thratory epithelium is adapted to protunderlying cells from irritation, bu

altered by smoking it can no longetion to secrete mucus and transpoward the throat to clean the respassages. Extensive replacement ofepithelial cells in respiratory passagsociated with chronic inflammationrespiratory passages (bronchitis), wcommon in people who smoke h e a

P R E D I C T

Urea is a toxic waste produced inside liver cells. It diffuses from thosecells into the blood and is eliminated from the body by the kidneys.What would happen to the intracellular and extracellularconcentration of urea if the kidneys stopped functioning?

Answer on page 00

2


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less concentrated solutions (less solute, more water) into moreconcentrated solutions (more solute, less water). The greater theconcentration of a solution (the less water it has), the greaterthe tendency for water to move into the solution, and thegreater the osmotic pressure must be to prevent that movement.

Cells will either swell, remain unchanged, or shrinkwhen placed into a solution. When a cell is placed into a hy-potonic (hpo-tonik) solution, the solution usually has alower concentration of solutes and a higher concentration ofwater than the cytoplasm of the cell. Water moves by osmosisinto the cell, causing it to swell. If the cell swells enough, itcan rupture, a process called lysis (l sis) (figure 3.11a). When

a cell is immersed in an isotonic (so-tonik) solution, theconcentrations of various solutes and water are the same onboth sides of the cell membrane. The cell therefore neithershrinks nor swells (figure 3.11b). When a cell is immersed ina hypertonic (h per-tonik) solution, the solution usually hasa higher concentration of solutes and a lower concentration ofwater than the cytoplasm of the cell. Water moves by osmosisfrom the cell into the hypertonic solution, resulting in cellshrinkage, or crenation (kre-nashun) (figure 3.11c). Solutionsinjected into the circulatory system or into tissues must be iso-tonic because swelling or shrinking disrupts the normal func-tion of cells and can lead to cell death.

FiltrationFiltration is the movement of fluid through a partition con-taining small holes (see table 3.2). The fluid movement resultsfrom the pressure or weight of the fluid pushing against thepartition. The fluid and substances small enough to passthrough the holes move through the partition, but substanceslarger than the holes do not pass through it. For example, in

a car, oil but not dirt particles passes through an oilthe body, filtration occurs in the kidneys as a step in uduction. Blood pressure moves fluid from the blooda partition, or filtration membrane. Water, ions, and smecules pass through the filtration membrane as a stepformation, whereas larger substances, such as protblood cells, remain in the blood (see chapter 18).

Mediated Transport MechanismMany nutrient molecules, such as amino acids and cannot enter the cell by the process of diffusion, and m

stances, such as proteins, produced in cells cannot lcell by diffusion. Carrier molecules within the cell mare involved in carrier-mediated transport mechanismfunction to move large, water-soluble molecules or elcharged ions across the cell membrane. After a molectransported binds to a carrier molecule on one side of tbrane, the three-dimensional shape of the carrier changes, and the transported molecule is moved to thsite side of the cell membrane (figure 3.12). The tramolecule is then released by the carrier molecule, wsumes its original shape and is available to transportmolecule. There are three kinds of mediated transpotated diffusion, active transport, and secondary active t

Facilitated DiffusionFacilitated diffusion is a mediated transport procmoves substances into or out of cells from a higher toconcentration (see table 3.2). Because movement is concentration gradient, metabolic energy in the formis not required.

Figure 3.11 Effects of Hypotonic, Isotonic, and Hypertonic Solutions on Red Blood Cells


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54

Active TransportActive transport is a carrier-mediated process that moves sub-stances from regions of lower concentration to ones of higherconcentration against a concentration gradient (see table 3.2).Consequently, active transport processes accumulate sub-stances on one side of the cell membrane at concentrationsmany times greater than those on the other side. Active trans-port requires energy in the form of ATP, and if ATP is not avail-able, active transport stops. Examples of active transport in-clude the movement of amino acids from the small intestineinto the blood.

In some cases, the active transport mechanism can ex-

change one substance for another. For example, the sod iu m

potassium ex ch ange pump moves sodium ions out of cellsand potassium ions into cells (figure 3.13). The result is ahigher concentration of sodium ions outside the cell and ahigher concentration of potassium ions inside the cell. Theconcentration gradients for sodium and potassium ions, es-tablished by the sodiumpotassium exchange pump, are es-sential in maintaining the resting membrane potential (seechapter 8).

Cystic fibrosis is a genetic disorder that affects the ac-tive transport of chlorine ions into cells. This disorder is dis-cussed in the Systems Pathologyessay on p. 66.

Secondary Active TransportS econ d ary active tran spo rt involves the active transport ofone substance, such as an ion, out of a cell, establishing aconcentration gradient. The diffusion of the substance back

into the cell, down its concentration gradient, provides ene rgy to transport a diffe rent substance, such as glucointo the cell (figure 3.14).

Endocytosis and ExocytosisEndocytosis (endo-s -tosis) is the uptake of material throuthe cell membrane by the formation of a membrane-bousac called a vesicle (see table 3.2). The two types of endo

tosis are phagocytosis and pinocytosis.Phagocytosis (fago-s -t osis) means cell eating and plies to endocytosis when solid particles are ingested. A pof the cell membrane extends around a particle and fusesthat the particle is surrounded by the membrane. That parthe membrane then pinches off to form a vesicle containthe particle. The vesicle is within the cytoplasm of the cand the cell membrane is left intact (figure 3.15). White blocells and some other cell types phagocytize bacteria, cell bris, and foreign particles. Phagocytosis is an important meby which white blood cells take up and destroy harmful sstances that have entered the body.

Pinocytosis (pino-s -t osis) means cell drinking. Idistinguished from phagocytosis in that much smaller vesic

are formed, they contain liquid rather than particles, and cell membrane invaginates to form the vesicles that are takinto the cell. Pinocytosis is a common transport mechaniand occurs in certain kidney cells, epithelial cells of the inttine, liver cells, and cells that line capillaries.

In some cells, secretions accumulate within vesicThese secretory vesicles then move to the cell membrawhere the vesicle membrane fuses with the cell membraand the content of the vesicle is eliminated from the cell (figure 3.6). This process is called exocytosis (ekso-s-to(figure 3.16 and see table 3.2). Secretion of digestive enzymby the pancreas, of mucus by the salivary glands, and of mfrom the mammary glands are examples of exocytosis. In ma

respects the process is similar to that of endocytosis, but it curs in an opposite direction. Endocytosis results in the uptaof materials by cells, and exocytosis in the release of materfrom cells. Both endocytosis and exocytosis require energythe form of ATP to form vesicles.

Cell MetabolismCell metabolism is the sum of all chemical reactions in the cell (fig3.17). The breakdown of food mocules releases energy that is used

synthesize ATP (see chapter 1When ATP is broken down, energyreleased which can be used to drother chemical reactions or processuch as active transport. The bredown of the sugar glucose, suchthe sugar from a candy bar, by a ries of reactions within the cytopla

Figure 3.12 Mediated Transport Mechanism

(a) A molecule binds to a protein carrier molecule on one side of the cell membrane. (b) The carriermolecule changes shape and releases the molecule on the other side of the cell membrane.

P R E D I C T

The transport of glucose into most cells occurs by facilitated diffusion.Because diffusion occurs from a higher to a lower concentration, glucosecannot accumulate within these cells at a higher concentration than isfound outside the cell. Once glucose enters a cell, it is rapidly convertedto other molecules, such as glucose phosphate or glycogen. What effectdoes this conversion have on the ability of the cell to transport glucose?

Answer on page 00

3


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Figure 3.13 SodiumPotassium Exchange Pump

of a cell is called gly c olys i s ( g l - k o li-s i s ) . Glucose is con-verted to pyruvic acid, which can enter alternative biochemi-cal pathways, depending on oxygen availability.

Aerobic (ar-obik) re s p i ra t i on occurs when oxygen isavailable. Pyruvic acid molecules enter mitochondria and,th rough a series of chemical reactions, called the citric acidcycle and the electron-transport chain, are converted to car-bon dioxide and water. Aerobic respiration can produce 36

to 38 ATP molecules from each glucose molecule. respiration re quires oxygen because the last reactioseries is the combination of oxygen with hydrogenwater. If this reaction does not take place, the reactmediately preceding it do not occur either. This explbreathing oxygen is necessary for animal life: withgen, aerobic respiration is inhibited, and the cells doduce enough ATP to sustain life. During aerobic res



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Protein Synthesis

56

the carbon atoms of food molecules are broken down to car-bon dioxide. Thus, the carbon dioxide humans breathe outcomes from the food they eat.

A n a e robic re s p i ra t i o n occurs without oxygen and in-cludes the conversion of pyruvic acid to lactic acid. There isa net production of two ATP molecules for each glucose

molecule. Anaerobic respiration does not produce as muchATP as aerobic respiration, but it allows cells to function forshort periods when oxygen levels are too low for aero b i crespiration to provide all the needed AT P. For example, dur-ing intense exercise, when aerobic respiration has depletedthe oxygen supply, anaerobic respiration can provide addi-tional AT P.

Protein SynthesisDNA contains the information that directs protein synthesis.The proteins produced in a cell are structural components in-side the cell, structural proteins secreted to the outside of the

cell, and enzymes that regulate chemical reactions in the cell.DNA influences the structural and functional characteristics ofthe entire organism because it directs protein synthesis.

Whether an individual has blue eyes, brown hair, or other in-herited traits is determined ultimately by DNA.

A DNA molecule consists of nucleotides joined togetherto form two nucleotide strands (see figure 2.17). The twostrands are connected and resemble a ladder that is twistedaround its long axis. The nucleotides function as chemicalletters that form chemical words. A gene is a sequence ofnucleotides (making a word) providing a chemical set of in-structions for making a specific protein. Each DNA moleculecontains many different genes.

Recall from chapter 2 that proteins consist of aminoacids. The unique structural and functional characteristics ofd ifferent proteins are determined by the kinds, numbers, andarrangement of their amino acids. The nucleotide sequenceof a gene determines the amino acid sequence of a specificprotei n.

DNA directs the production of proteins in two stepstranscription and translationwhich can be illustrated wan analogy. Suppose a chef wants a recipe that is found oin a reference book in the library. Because the book canbe checked out, the chef makes a copy, or transcriptionthe recipe. Later, in the kitchen the information containedthe copied recipe is used to prepare a meal. The changingsomething from one form to another (from recipe to mealcalled translation.

In terms of this analogy, DNA (the reference book) ctains many genes (recipes) for making different prote(meals). DNA, however, is too large a molecule to pass throuthe nuclear pores to go to the ribosomes (kitchen) where

Figure 3.14 Secondary Active Transport

Figure 3.15 Phagocytosis

Cell processes extend from the cell and surround the particle to be takinto the cell by phagocytosis. The cell processes surround the particle fuse to form a vesicle that contains the particle. The vesicle then isinternalized within the cell.


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Protein Synthesis

58

TranslationTranslation, the synthesis of proteins based on the inform a-tion in mRNA, occurs at ribosomes. The mRNA molecules

p roduced by transcription pass through the nuclear pores tothe ribosomes. The information in mRNA is carried ing roups of three nucleotides called c o d o n s , which code forspecific amino acids. For example, the nucleotide sequenceuracil, cytosine, and adenine (UCA) of mRNA codes for theamino acid serine. There are 64 possible mRNA codons, butonly 20 amino acids are in proteins. As a result, more thanone codon can code for the same amino acid. For example,CGA, CGG, CGT, and CGC code for the amino acid alanine,and UUU and UAC code for phenylalanine. Some codons donot code for amino acids but perf o rm other functions. Forexample, UAA acts as a signal for stopping the pro d u c t i o nof a pro t e i n .

Protein synthesis requires two types of RNA in additionto mRNA: tRNA and ribosomal RNA (rRNA). There is one typeof tRNA for each mRNA codon. A series of three nucleotidesof each tRNA molecule, the anticodon, pairs with the codonof the mRNA. Another part of each tRNA molecule binds to aspecific amino acid. For example, the tRNA that pairs with the

Figure 3.18 Overview of Protein Synthesis

P R E D I C T

Explain how changing one nucleotide within a DNA molecule of a cecould change the structure of a protein produced by the cell.

Answer on page 00

4

UUU codon of mRNA has the anticodon AAA and binds oto the amino acid phenylalanine.

The ribosomes, which consist of ribosomal RNA aproteins, align mRNA with tRNA molecules so that the a

codons of tRNAs pair with the codons of mRNA while mRNA is attached to a ribosome (figure 3.20). The amacids bound to the tRNAs are then joined to one another an enzyme associated with the ribosome. The enzyme cauthe formation of a chemical bond, called a peptide bond,tween the adjacent amino acids to form a po lypeptide chaconsisting of many amino acids bound together by peptbonds. The polypeptide chain then becomes folded to fothe three-dimensional structure of the protein moleculep rotein can consist of a single polypeptide chain or twomo re polypeptide chains that are joined after each chainp roduced on separate ribosomes.


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for sex cells, contains 46 chromosomes. Sex cells hthe number of chromosomes as other cells (see theon Meiosis on p. 61). The 46 chromosomes are dip lo i d ( diployd) number of chromosomes and anized to form 23 pairs of chromosomes. Of the 23 ppair is the sex chromosomes, which consist of twom o s omes if the person is a female or an X chromosa Y ch romo s o m e if the person is a male. The remapairs of chromosomes are called auto s o m e s ( a wtThe combination of sex chromosomes determines

viduals sex, and the autosomes determine most otha c t e r i stic s .

MitosisAll cells of the body, except those that give rise to sdivide by mitosis (m -tosis). Mitosis involves tw(1) the genetic material within a cell is replicated,cated, and (2) the cell divides to form two daughter cthe same amount and type of DNA as the parent cell.DNA determines the structure and function of cdaughter cells, which have the same DNA as the pacan have the same structure and perform the same f

as the parent cell.The period between active cell divisions is ct e rphase, during which DNA is replicated. The twoof DNA separate from each other, and each strand sa template for the production of a new strand of Dure 3.21). Nucleotides found in the DNA of a templapair with nucleotides that are subsequently joinedzymes to form a new strand of DNA. The sequenccleotides in the DNA template determines the sequnucleotides in the new strand of DNA because adenwith thymine, and cytosine pairs with guanine. Tstrand of DNA combines with the template strand tdouble strand of DNA.

At the end of interphase, each cell has two c

sets of genetic material. The DNA is dispersed thrthe nucleus as thin threads called ch ro m a t i n ( kro(f i g ure 3.22a).

Mitosis follows interphase. For convenience, mdivided into four stages. Although each stage represjor events, the process of mitosis is continuous. Learnof the stages is helpful, but the most important concederstand is how each of the two cells produced by mtains the same number and type of chromosomes asent cell. There are four stages in mitosis:

1. P ro p h a s e . During p ro phase ( f i g u re 3.22b), thec h romatin condenses to form visible chro mo s omAfter interphase, each chromosome is made upseparate but genetically identical strands of chrcalled ch ro m a t i d s ( kroma-tidz), which are linkone point by a specialized region called thec e n t ro m e re ( se ntro -mer). Replication of the gematerial during interphase results in the two id

Cell DivisionCell division is the formation of two daughter cells from a sin-gle parent cell. The new cells necessary for growth and tissuerepair are formed through mitosis, and the sex cells necessaryfor reproduction are formed through meiosis.

During mitosis and meiosis the DNA within the pare n tcell is distributed to the daughter cells. The DNA is foundwithin chromosomes. Each cell of the human body, except

Figure 3.19 Transcription

Formation of mRNA by transcription of DNA chains in the cell nucleus. Asegment of the DNA chain is opened, and RNA polymerase (an enzyme)assembles nucleotides into mRNA according to the base pair combinationsshown in the inset. Thus the sequence of nucleotides in DNA determinesthe sequence of nucleotides in mRNA. As nucleotides are added, an mRNAchain is formed.


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Cell Division

60

Figure 3.20 Translation of mRNA to Produce a Protein


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Each of the two sets of 46 chromosomes is movethe spindle fibers toward the centriole at one of poles of the cell. At the end of anaphase, each schromosomes has reached an opposite pole of tand the cytoplasm begins to divide.

4. Telophase. During telophase (figure 3.22e), the csomes in each of the daughter cells become orgto form two separate nuclei. The chromosomes unravel and resemble the genetic material durininterphase.

Following telophase, the cytoplasm of the tcompletes division, and two separate daughter cellsduced (figure 3.22f).

MeiosisThe formation of all body cells, excepcells, occurs by mitosis. Sex cells arebym e i osis ( m - osis), a process in wnucleus of a sex cell precursor cell untwo divisions, resulting in (1) four nuccontaining half as many chromosome

pa rent cell and (2) one chromosomeach of the chromosome pairs. The cells that are produced diffe rentiate m e t e s ( g a metz), or s ex cells. The sexs p e rm cells in males and oo c y t e s (o ofemales (see chapter 19). Each gameh a p l o i d ( haployd) number of chro m

which is half the number of chro mfound in other body cells. The haploidof chromosomes in humans is 23 somes. Sperm cells have 22 autosommosomes and either an X or Y chromand oocytes contain 22 autosomal somes and an X chromosome. Duringt i on ,when a sperm cell fuses with anthe normal number of 46 chromosompairs, is re e s tab l i sh ed .

Meiosis involves two divisions. division during meiosis is divided istages: prophase I, metaphase I, anaand telophase I (figure 3.23). As in pof mitosis, during prophase I of menuclear envelope degenerates, spindf o rm, and the already duplicated somes become visible. Each chroconsists of two chromatids joined bt ro m e re. In prophase I, however, th

bers of each pair of chromosomes lie g e t h e r. Because each chromosome cotwo chromatids, the four chromatic h romosome pair is called a t emetaphase I the tetrads align near thof the cell, and in anaphase I eachc h romosomes separates and movesopposite poles of the cell. For each

c h romatids of each chromosome. Also duringp rophase, microtubules called spindle fib e rs e x t e ndf rom the ce n t rio l e s (s entre-olz) to the centro m e re s(see figure 3.1 and 3.22b). Centrioles are smallorganelles that divide and migrate to each pole of thecell. In late prophase, the nucleolus and nuclearenvelope disappear.

2. Metaphase. In metaphase (figure 3.22c), thechromosomes align near the center of the cell.

3. Anaphase.At the beginning ofanaphase (figure 3.22d),

the centromeres separate. When this happens, eachchromatid is then referred to as a chromosome. Thus,when the centromeres divide, the chromosome numberdoubles to form two identical sets of 46 chromosomes.

Figure 3.21 Replication of DNA

The strands of DNA separate from each other, and each strand functions as a template forthe production of a new strand. The base-pairing relationship between nucleotides (seeinset) determines the sequence of nucleotides in the newly formed strand. Two identicalmolecules of DNA are produced, each with one new strand and one old, template strand ofthe original DNA molecule.


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Cell Division

62

Figure 3.22 Mitosis


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Figure 3.23 Meiosis


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P R E D I C T

Cancer cells divide continuously. The normal mechanisms that

regulate whether cell division occurs or ceases do not functioproperly in cancer cells. Cancer cells, such as breast cancer cenot look like normal, mature cells. Explain.

Answer on page 00

5

Table 3.3 Comparison of Mitosis and Meiosis

Feature Mitosis Meiosis

Time of DNA replication Interphase Interphase

Number of cell divisions One Two; there is no replication of DNA between thmeiotic divisions

Cells produced Two daughter cells genetically identical to the parent Gametes, each different from the parent cell acell; each daughter cell has the diploid number other; the gametes have the haploid numbof chromosomes. chromosomes; in males, four gametes (sper

in females, 1 gamete (oocyte) and two or thpolar bodies

Function New cells are formed during growth or tissue repair; Gametes are produced for reproduction; durinnew cells have identical DNA and can perform the fertilization the haploid number of chromosame functions as the parent cells in each gamete unites to restore the diploi

number typical of most cells; genetic variabincreased because of crossing over and randistribution of chromosomes

Did You Know?

A tumor (toomor; a swelling) is any swelling that occurs within thebody, usually involving cell proliferation. A tumor can be eithermalignant (ma-lignant, meaning with malice or intent to causeharm), able to spread and become worse, or benign (be-nn, meaningkind), not inclined to spread and not likely to become wors e.

Cancer (kanser) refers to a malignant, spreading tumor and theillness that results from such a tumor. Benign tumors are usually lessdangerous than malignant tumors, but they can cause problems. As abenign tumor enlarges, it can compress surrounding tissues and impairtheir functions. In some cases (e.g., brain tumors), the results can be death.

Malignant tumors can spread by local growth and expansion or bymetastasis (me-tasta-sis, meaning moving to another place), whichresults from tumor cells separating from the main neoplasm and beingcarried by the lymphatic or circulatory system to a new site, where asecond tumor forms.

Cancers lack the normal growth control that is exhibited by mostother adult tissues. Cancer results when a cell or group of cells, forsome reason, breaks away from the normal control of growth anddifferentiation. This breaking loose involves the genetic machinery andcan be induced by viruses, environmental toxins, and other causes.

The illness associated with cancer usually occurs as the tumor invadesand destroys the healthy surrounding tissues, eliminating their functions.

Promising anticancer therapies are being developed in which cellsresponsible for immune responses can be stimulated to recognizetumor cells and destroy them. A major advantage in such anticancertreatments is that the cells of the immune system can specificallyattack the tumor cells and not other, healthy tissues. Other therapiescurrently under investigation include techniques to starve a tumor todeath by cutting off its blood supply. Drugs that can inhibit bloodvessel development are currently under investigation.

Did You Know?

A tumor (toomor; a swelling) is any swelling that occurs within thebody, usually involving cell proliferation. A tumor can be eithermalignant (ma-lignant, meaning with malice or intent to causeharm), able to spread and become worse, or benign (be-nn, meaningkind), not inclined to spread and not likely to become wors e.

Cancer (kanser) refers to a malignant, spreading tumor and theillness that results from such a tumor. Benign tumors are usually lessdangerous than malignant tumors, but they can cause problems. As abenign tumor enlarges, it can compress surrounding tissues and impairtheir functions. In some cases (e.g., brain tumors), the results can be death.

Malignant tumors can spread by local growth and expansion or bymetastasis (me-tasta-sis, meaning moving to another place), whichresults from tumor cells separating from the main neoplasm and beingcarried by the lymphatic or circulatory system to a new site, where asecond tumor forms.

Cancers lack the normal growth control that is exhibited by mostother adult tissues. Cancer results when a cell or group of cells, forsome reason, breaks away from the normal control of growth anddifferentiation. This breaking loose involves the genetic machinery andcan be induced by viruses, environmental toxins, and other causes.

The illness associated with cancer usually occurs as the tumor invadesand destroys the healthy surrounding tissues, eliminating their functions.

Promising anticancer therapies are being developed in which cellsresponsible for immune responses can be stimulated to recognizetumor cells and destroy them. A major advantage in such anticancertreatments is that the cells of the immune system can specificallyattack the tumor cells and not other, healthy tissues. Other therapiescurrently under investigation include techniques to starve a tumor todeath by cutting off its blood supply. Drugs that can inhibit bloodvessel development are currently under investigation.


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The Current First Level Head

Systems PathologySystems Pathologys y s t e m s p a t h o l o g y

CYSTIC FIBROSIS

c y s t i c f i b r o s i s

Tim S. is a 5-year-old white male. He is small for his ageand has had frequent bouts of pulmonary infections all hislife. Tim always seemed to have a runny nose. None ofthe infections were very serious, mostly just irritating. Thistime, however, his congestion became so extreme that he was unable to breathe and was rushed to the hospital.T h e re, a series of tests demonstrated that Tim suff e red fro mcystic fibro s i s .

Cystic fibrosis is a genetic disorder that occurs at a rateof approximately one per 2000 births and currently affects

33,000 people in the United States. It is the most commonlethal genetic disorder among whites. The diagnosis is basedon the existence of recurrent respiratory disease, increasedsodium in the sweat, and high levels of unabsorbed fats in thestool. Approximately 98% of all cases of cystic fibrosis are di-agnosed before the patient is 18 years old.

At the molecular level, cystic fibrosis results from an ab-normality in chloride ion channels. There are three types ofcystic fibrosis: (1) In about 70% of cases, a defective channelprotein fails to reach the cell membrane from its site of pro-duction inside the cell. (2) In the second group, the channelprotein is incorporated into the cell membrane but fails tobind ATP. (3) In the final category, the channel protein is in-

corporated into the cell membrane and ATP is bound to thechannel protein, but the channel does not open. The result ofany of these defects is that chloride ions do not exit cells at anormal rate.

Normally, as chloride ions move out of cells lining tubes,such as ducts or respiratory passages in the body, water fol-lows by osmosis. In cystic fibrosis, chloride ions do not exitthese cells at normal rates and, therefore less water moves intothe tubes. With less water present, the mucus produced bycells lining those tubes is thick and cannot be readily movedover the surface of the cells by their cilia. As a result, the tubesbecome clogged with mucus, and much of their normal func-tion is lost.

The most critical effects of cystic fibrosis, accounting for

90% of the deaths, are on the respiratory system. Cystic fibro-sis also affects the secretory cells lining ducts of the pancreas,sweat glands, and salivary glands.

In normal lungs, a thin fluid layer of mucus is movedciliated cells. In people with cystic fibrosis, the viscous muresists movement by cilia and accumulates in the lung psages. The mucus accumulation obstructs the passagewand increases the likelihood of infections. This resultschronic airflow obstruction, difficulty in breathing, and recrent respiratory infections. Chronic coughing occurs as thefected person attempts to remove the mucus.

Cystic fibrosis was once fatal during early childhobut many patients are now surviving into young adultho

because of modern medical treatment. Currently, appromately 80% of people with cystic fibrosis live past age Pulmonary therapy consists of supporting and enhancing isting respiratory functions, and infections are treated wa nt ib ioti cs.

The buildup of thick mucus in the pancreatic and hpatic ducts blocks them so that pancreatic digestive enzymand bile salts are prevented from reaching the small intestiAs a result, fats and fat-soluble vitamins, which require bsalts for absorption, and which cannot be adequately digeswithout pancreatic enzymes, are not taken up by intesticells in normal amounts. The patient suffers from deficiencof vitamins A, D, E, and K, which result in conditions such

night blindness, skin disorders, rickets, and excessive bleing. Therapy includes administering the missing vitaminsthe patient and reducing dietary fat intake.

Future treatments could include the developmentdrugs that correct or assist chloride ion transpAlternatively, cystic fibrosis may some day be cured througene therapy; that is, inserting a functional copy of the deftive gene into the cells of people with the disease.

P R E D I C T

Predict the effect of cystic fibrosis on the concentrations of chlorideions inside and outside the cell. In normal muscle and nerve cells atrest, many potassium ion channels are open and potassium ions tendto flow out of the cell down their concentration gradient. How is thflow of potassium ions affected in cells of people with cystic fibrosis

Answer on page 00

6

66


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Systems Interactions

System Interactions

Integumentary Cystic fibrosis is characterized by increased perspiration with abnormally high quantities of sodium in the sweat whlead to decreased blood sodium levels. A number of skin rashes and other disorders can develop as a result of the abnperspiration.

Nervous Night blindness can develop as a result of vitamin A deficiency caused by insufficient absorption of the vitamin in tgestive tract.

Endocrine Diabetes mellitus resulting from decreased production of the hormone insulin may develop because blockage of the

atic duct by mucus results in pancreatic digestive enzymes, retained within the pancreas, destroying the pancreatic (pancreatic islets), which produce insulin.

Cardiovascular Fragile blood vessels can develop, resulting in excessive bleeding. Decreased blood clotting results from insufficient vK absorption from the digestive tract. Erythrocyte (red blood cell) membranes become fragile because of inadequate E absorption.

Respiratory The respiratory passages become clogged with viscous mucus, which blocks the airways and inhibits respiration. Recrespiratory infections also occur. Decreased airflow into and out of the lungs results in reduced oxygen flow to the tRespiratory complications account for most deaths.

Digestive Pancreatic ducts and ducts from the liver and salivary glands are blocked with thick mucus. Fats and the fat-solublemins, A, D, E, and K, are poorly absorbed. Deficiencies in fat-soluble vitamins result that affect many other systems. Ttestine can become impacted with dehydrated stool. Gallstones can form in the gallbladder or liver ducts.

Reproductive Reproductive ability is greatly decreased. In 95% of males with cystic fibrosis, there is an absence of living sperm cesemen. Viscous secretions in the male or female reproductive tracts decrease fertility.

Cell Structure and Function

Cells are highly organized units composed of living material. The nucleus contains genetic material, and cytoplasm is

living material outside the nucleus.

Cell Membrane

The cell membrane forms the outer boundary of the cell. Itdetermines what enters and leaves the cell.

The cell membrane is composed of a double layer of lipidmolecules in which proteins float. The proteins function asmembrane channels, carrier molecules, receptor molecules,enzymes, and structural components of the membrane.

Nucleus

The nuclear envelope consists of two separate membraneswith nuclear pores.

DNA and associated proteins are found inside the nucleusas chromatin. DNA is the hereditary material of the cell andcontrols the activities of the cell.

Nucleoli and Ribosomes Nucleoli consist of RNA and proteins and are the sites of

ribosomal subunit assembly. Ribosomes are the sites of protein synthesis.

Rough and Smooth Endoplasmic Reticulum

Rough ER is ER with ribosomes attached. It is a major site ofprotein synthesis.

Smooth ER does not have ribosomes attached and site of lipid synthesis.

The Golgi Apparatus

The Golgi apparatus is a series of closely packed msacs that function to collect, modify, package, and

proteins and lipids produced by the ER.

Secretory Vesicles

Secretory vesicles are membrane-bound sacs that csubstances from the Golgi apparatus to the cell me

where the vesicle contents are released.

Lysosomes

Membrane-bound sacs containing enzymes are calllysosomes. Within the cell the lysosomes break dowphagocytized material.

Mitochondria

Mitochondria are the major sites of ATP productioncells use as an energy source. Mitochondria carry oaerobic respiration (requires oxygen).

Cytoskeleton

The cytoskeleton supports the cytoplasm and organis involved with cell movements.

The cytoskeleton is composed of microtubules,microfilaments, and intermediate filaments.

Summary


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Summary

68

Cilia, Flagella, and Microvilli

Cilia move substances over the surface of cells. Flagella are much longer than cilia and propel sperm cells. Microvilli increase the surface area of cells and aid in absorption.

Whole-Cell Activity

The interactions between organelles must be considered forcell function to be fully understood.


Lipid-soluble molecules pass through the cell membranereadily by dissolving in the lipid portion of the membrane.

Small molecules can pass through membrane channels. Large molecules that are not lipid-soluble can be

transported through the membrane by carrier molecules. Large molecules that are not lipid-soluble, particles, and

cells can be transported across the membrane by vesicles.

Diffusion

Diffusion is the movement of a solute from an area of higherconcentration to an area of lower concentration within a solvent.At equilibrium, there is a uniform distribution of molecules.

For a given distance, a concentration gradient is equal tothe higher concentration minus the lower concentration of a

solute in a solution.

Osmosis

Osmosis is the diffusion of a solvent (water) across aselectively permeable membrane.

Osmotic pressure is a measure of the tendency of water tomove across the selectively permeable membrane.

In a hypotonic solution, cells swell (and can undergo lysis);in an isotonic solution, cells neither swell nor shrink; and ina hypertonic solution, cells shrink and undergo crenation.

Filtration

Filtration is the passage of a solution through a partition inresponse to a pressure difference. Some materials in the

solution do not pass through the partition.

Mediated Transport Mechanisms

Mediated transport is the movement of a substance across amembrane by means of a carrier molecule. The substancestransported tend to be large, water-soluble molecules.

Facilitated diffusion moves substances from a higher to alower concentration and does not require energy in theform of ATP.

Active transport can move substances from a lower to ahigher concentration and requires ATP. An exchange pumpis an active transport mechanism that moves two substancesin opposite directions across the cell membrane.

Secondary active-transport uses the power of one substancemoving down its concentration gradient to move another

substance into the cell.

Endocytosis and Exocytosis

Endocytosis is the movement of materials into cells by thef o rmation of a vesicle. Phagocytosis is the movement of

solid material into cells by the formation of a vesicle.Pinocytosis is similar to phagocytosis, except that thematerial ingested is much smaller and is in solution.

Exocytosis is the secretion of materials from cells by vesiformation.

Cell Metabolism

Aerobic respiration requires oxygen and produces carbondioxide, water, and 36 to 38 ATP molecules from a molecof glucose.

Anaerobic respiration does not require oxygen and produclactic acid and two ATP molecules from a molecule of gluco

Protein Synthesis Cell activity is regulated by enzymes (proteins), and DNA

controls enzyme production.

Transcription

During transcription, the sequence of nucleotides in DNAgene) determines the sequence of nucleotides in mRNA; mRNA moves through the nuclear pores to ribosomes.

Translation

During translation the sequence of codons in mRNA is us

at ribosomes to produce proteins. Anticodons of tRNA binto the codons of mRNA, and the amino acids carried bytRNA are joined to form a protein.

Cell DivisionMitosis

Cell division that occurs by mitosis produces new cells fogrowth and tissue repair.

DNA replicates during interphase, the time between celldivision.

Mitosis is divided into four stages:ProphaseEach chromosome consists of two chromatidjoined at the centromere.MetaphaseChromosomes align at the center of the cell

Anaphase

Chromatids separate at the centromere andmigrate to opposite poles.TelophaseThe two new nuclei assume their normalstructure, and cell division is completed, producing twonew daughter cells.

Meiosis

Meiosis results in the formation of gametes (sperm cells ooocytes). Gametes have half the number (haploid numbeof chromosomes that other (diploid) body cells do.

There are two cell divisions in meiosis. Each division hasfour stages similar to those in mitosis.

During meiosis the processes of crossing over within tetrand random distribution of chromosomes increase geneti

variability.

Differentiation Differentiation, the process by which cells develop

specialized structures and functions, results from theselective activation and inactivation of DNA.


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Content Review

1. Define cytoplasm and cell organelle.2. Describe the structure of the cell membrane. What functions

does it perform?3. Describe the structure of the nucleus and nuclear envelope.

Name the organelles found in the nucleus, and give theirfunctions.

4. Where are ribosomes assembled, and what kinds of molecules

are found in them?5. What is endoplasmic reticulum? Compare the functions ofrough and smooth endoplasmic reticulum.

6. Describe the Golgi apparatus, and state its function.7. Where are secretory vesicles produced? What are their

contents, and how are they released?8. What is the function of the lysosomes?9. Describe the structure and function of mitochondria.

10. Name the components of the cytoskeleton, and give theirfunctions.

11. Describe the structure and function of cilia, flagella, andmicrovilli.

12. How do lipid-soluble molecules, small molecules that are notlipid-soluble, and large molecules that are not lipid-solublecross the cell membrane?

13. Define solution, solute, solvent, diffusion, and concengradient.

14. Define osmosis and osmotic pressure.15. What happens to cells that are placed in isotonic solutio

hypertonic or hypotonic solutions? What are crenation a16. Define filtration.17. What is mediated transport? How are facilitated diffus

active transport similar, and how are they different?18. How does secondary active transport work?19. Describe phagocytosis, pinocytosis, and exocytosis. W

they accomplish?20. Describe how proteins are synthesized and how the s

of DNA determines the structure of proteins.21. Define autosome, sex chromosome, diploid number,

haploid number.22. How do the sex chromosomes of males and females 23. Describe what happens during interphase and each p

mitosis. What kind of tissues undergo mitosis?24. Describe the events of meiosis. What happens during

to increase genetic variability?25. Define differentiation. In general terms, how does diffe

occur?

1. Suppose that a cell has the following characteristics: manymitochondria, well-developed rough ER, well-developed Golgiapparatuses, and numerous vesicles. Predict the major functionof the cell. Explain how each characteristic supports yourprediction.

2. Secretory vesicles fuse with the cell membrane to release theircontents to the outside of the cell. In this process themembrane of the secretory vesicle becomes part of the cell

membrane. Because small pieces of membrane are continuallyadded to the cell membrane, one would expect the cellmembrane to become larger and larger as secretion continues.The cell membrane stays the same size, however. Explain howthis happens.

3. The body of a male was found floating in the salt water ofGrand Pacific Bay, which has a concentration that is slightlygreater than body fluids. When seen during an autopsy, thecells in his lung tissues were clearly swollen. Choose the mostlogical conclusion.a. He probably drowned in the bay.b. He may have been murdered elsewhere.c. He did not drown.

Reasoning

4. Patients with kidney failure can be kept alive by dialyremoves toxic waste products from the blood. In a diamachine, blood flows past one side of a selectively pedialysis membrane, and dialysis fluid flows on the oththe membrane. Small substances, such as ions, glucoseurea, can pass through the dialysis membrane, but largsubstances, such as proteins, cannot. If you wanted todialysis machine to remove only the toxic waste produ

from blood, what could you use for the dialysis fluid?a. A solution that is isotonic and contains only proteib. A solution that is isotonic and contains the same con

of substances as blood, except for having no urea in c. Distilled waterd. Blood

5. In sickle-cell anemia a protein inside red blood cells dfunction normally. Consequently, the red blood cells bsickle-shaped and plug up small blood vessels. It is knthat sickle-cell anemia is hereditary and results from cone nucleotide for a different nucleotide within the geis responsible for producing the protein. Explain howchange results in an abnormally functioning protein.


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Answers to Predict Questions

Answers to Predict Questions

1. p. 00 (a) Cells specialized tosynthesize and secrete proteins haveabundant rough ER, because this is animportant site of protein synthesis.Well-developed Golgi apparatuses existto package proteins in secretoryvesicles, and numerous secretory

vesicles are present.(b) Cells highly specialized to activelytransport substances into the cell havea large surface area exposed to thefluid from which substances areactively transported. Numerousmitochondria are present near themembrane across which activetransport occurs.(c) Cells highly specialized to ingestforeign substances have numerouslysosomes in their cytoplasm andevidence of vesicles containing foreignsubstances.

2. p. 00 Urea is produced continually byliver cells and diffuses from the cellsinto the blood. If the kidneys stopeliminating urea, it begins toaccumulate in the blood and in theliver cells. The urea finally reaches

concentrations high enough to be toxicto cells, causing cell damage followedby cell death.

3. p. 00 Glucose transported byfacilitated diffusion across the cellmembrane moves from a higher to alower concentration. If glucose

molecules are converted quickly tosome other molecule as they enter thecell, a large concentration difference ismaintained, and thus glucose transportinto the cell continues proportional tothe magnitude of the concentrationdifference.

4. p. 00 Changing a single nucleotidewithin a DNA molecule, also changesthe nucleotide sequence of messengerRNA produced from that segment ofDNA. The change in mRNA results in adifferent codon, and a different aminoacid is placed in the amino acid chainfor which the messenger RNA codes.Because a change in the amino acidsequence of a protein can change itsstructure, one substitution of anucleotide in a DNA chain can result inaltered protein structure and function.

5. p. 00 Cancer cells generally appeabe undifferentiated. Instead of dividand then undergoing differentiation,they continue to divide and do notdifferentiate. One measure of theseverity of cancer is related to thedegree of differentiation the cancer

cells have undergone. Those that aremore differentiated divide more slowand are less dangerous than those thdifferentiate little.

6. p. 00 Chloride ions do not move innormal amounts out of the cells ofpeople with cystic fibrosis becausechloride ion channels are defective.Instead, the chloride ions tend toaccumulate inside the cell. Potassiumions tend to move out of muscle andnerve cells down their concentrationgradient. The positively chargedpotassium ions, however, are attractby the negatively charged chloride iaccumulated inside the cell. Thisattraction reduces the movement ofpotassium ions out of the cell andcauses more potassium ions toaccumulate inside the cell.

53107590 cell biology cell structures and their functions

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