1 FOCUSObjectives7.1.1 Explain what the cell theory is.7.1.2 Describe how researchers

explore the living cell.7.1.3 Distinguish between

eukaryotes and prokaryotes.

Vocabulary PreviewHave students write the Vocabularyterms, dividing each into its separatesyllables as best they can. Remindstudents that each syllable usuallyhas only one vowel sound. The cor-rect syllabications are: cell the•o•ry,nu•cle•us, pro•kar•y•ote,eu•kar•y•ote

Reading StrategyAsk students to make an outline ofthe section, using the blue heads astheir first level of the outline.

2 INSTRUCT

The Discovery of the CellBuild Science SkillsAsking Questions Divide the classinto small groups, and give eachgroup a variety of drawings and pho-tos of cells taken from numeroussources, such as college textbooksand Internet sites. These cells shouldinclude both unicellular organismsand cells from plants and animals.Have each group make drawings ofthe various cells, labeling any struc-ture they recognize. Then, ask eachgroup to brainstorm a list of ques-tions about these cells that theyexpect will be answered as they readthe chapter.

Cell Structure and Function 169

SECTION RESOURCES

Print:

• Teaching Resources, Section Review 7–1• Reading and Study Workbook A,

Section 7–1• Adapted Reading and Study Workbook B,

Section 7–1• Lesson Plans, Section 7–1

Technology:

• iText, Section 7–1• Transparencies Plus, Section 7–1

Tim

eSaver

7–1 Life Is Cellular

Key Concepts• What is the cell theory? • What are the characteristics of

prokaryotes and eukaryotes?

Vocabularycellcell theorynucleuseukaryoteprokaryote

Reading Strategy: Finding Main IdeasAs you read, look for evidence to support the statement “Thecell theory revolutionized howbiologists thought about livingthings.”

Look closely at a part of a living thing, and what do you see? Hold a blade of grass up against the light, and you see tiny

lines running the length of the blade. Examine the tip of yourfinger, and you see the ridges and valleys that make up finger-prints. Place an insect under a microscope, and you see theintricate structures of its wings and the spikes and bristles thatprotect its body. As interesting as these close-up views may be,however, they’re only the beginning of the story. Look closer anddeeper with a more powerful microscope, and you’ll see thatthere is a common structure that makes up every living thing—the cell.

The Discovery of the Cell“Seeing is believing,” an old saying goes. It would be hard to finda better example of this than the discovery of the cell. Withoutthe instruments to make them visible, cells remained out ofsight and, therefore, out of mind for most of human history. Allof this changed with a dramatic advance in technology—theinvention of the microscope.

Early Microscopes It was not until the mid-1600s thatscientists began to use microscopes to observe living things. In1665, Englishman Robert Hooke used an early compoundmicroscope to look at a thin slice of cork, a plant material.Under the microscope, cork seemed to be made of thousands oftiny, empty chambers. Hooke called these chambers“cells” because they reminded him of a monastery’stiny rooms, which were called cells. One of Hooke’sillustrations of cells is shown in Figure 7–1. The termcell is used in biology to this day. We now know, how-ever, that cells are not empty but contain livingmatter.

In Holland around the same time, Anton vanLeeuwenhoek used a single-lens microscope to observepond water and other things. To his amazement, themicroscope revealed a fantastic world of tiny livingorganisms that seemed to be everywhere, even in thevery water he and his neighbors drank.

� Figure 7–1 Using an early microscope, Hooke made thisdrawing of cork cells. In Hooke’s drawings, the cells look likeempty chambers because he was looking at dead plantmatter. Today, we know that living cells are made up of manystructures.

Section 7–1

170 Chapter 7

The Cell Theory Soon, numerous observations made it clearthat were the basic units of life. In 1838, German botanistMatthias Schleiden concluded that all plants were made of cellslike the one in Figure 7–2. The next year, German biologistTheodor Schwann stated that all animals were made of cells.In 1855, the German physician Rudolf Virchow concluded thatnew cells could be produced only from the division of existingcells. These discoveries, confirmed by other biologists, aresummarized in the cell theory, a fundamental concept ofbiology. The cell theory states:

• All living things are composed of cells.• Cells are the basic units of structure and function

in living things.• New cells are produced from existing cells.

Exploring the CellFollowing in the footsteps of Hooke, Virchow, and others, modernbiologists still use microscopes to explore the cell. However,today’s researchers use microscopes and techniques morepowerful than the pioneers of biology could have imagined.Researchers can use fluorescent labels and light microscopy tofollow molecules moving through the cell. Confocal lightmicroscopy, which scans cells with a laser beam, makes itpossible to build three-dimensional images of cells and theirparts. High-resolution video technology makes it easy to pro-duce movies of cells as they grow, divide, and develop.

cells

� Figure 7–2 The cell theorystates that cells are the basic units of all living things. This cell is from aplant leaf. Compare this micrographwith Hooke’s drawing in Figure 7–1.

(magnification: 12,000�)

The History of the CellThe observations and conclusions of many scientists helped todevelop the current understanding of the cell.

1600 1700 1800

1674Anton van LeeuwenhoekLeeuwenhoek observes tiny living organisms indrops of pond water through his simple microscope.

1665Robert HookeHooke publishes his book Micrographia, whichcontains his drawings ofsections of cork as seenthrough one of the firstmicroscopes.

7–1 (continued)

Discuss the contributions of eachof the scientists leading up to theformulation of the cell theory. Besure that students understand theimportance of the individualaccomplishments. It is also impor-tant that students understand thetime span involved. Ask: How dideach observation influence thenext scientist’s work? (Answerswill vary, but generally one clearlyinfluenced the next. The microscopewas essential to the discovery of thecell. Both van Leeuwenhoek andHooke used early microscopes toobserve cells and unicellular organ-isms. Schwann elaboratedSchleiden’s findings, and so on.)

Help students find appropriatebooks and Web sites as they inves-tigate new discoveries about cells.Explain that the branch of biologythat focuses on cells and theirstructures is called cytology, andthe branch that is concerned withone-celled organisms is calledmicrobiology. Encourage studentsto find out what scientist discov-ered each of the organelles theywill learn about in Section 7–2and when the discovery wasmade.

Less Proficient ReadersFocus students’ attention on thecell theory by calling on stu-dents to read aloud the bulletedsentences on page 170. Foreach statement read aloud, callon other students to rephrasethe concept in their own wordsand then explain the concept’ssignificance.

English Language LearnersHelp students add to their per-sonal science glossaries withVocabulary words from this andsubsequent sections of thischapter. For each word, encour-age students to recorddefinitions, note pronunciations,and perhaps add translationsusing their first languages.

Advanced LearnersReview the Writing Activity onpage 171. Determine whichdiscovery each student willresearch and present to theclass. Emphasize that theymust find more informationthan is given in the textbook.Encourage advanced studentsto pursue the more complicateddiscoveries.

Exploring the CellAddress MisconceptionsMany students have the misconcep-tion that a common egg, such as achicken egg, is one cell. Some stu-dents might believe that the egg yolkis the nucleus and the egg white isthe cytoplasm, while others maythink that the yolk is the completecell. Explain that an unfertilized eggdoes have only one cell, but that cellconsists of a small, whitish disk at thetop of the yolk. This disk can be seenif the yolk is carefully separated fromthe rest of the egg. When an egg isfertilized, that cell begins to divideand multiply. The yolk serves as nour-ishment for the developing embryo.

Make ConnectionsMathematics Explain to studentsthat cells are so small that the basicmetric unit of length, the meter, hasno usefulness in measuring cells. Ask:What is one thousandth of a metercalled? (1 millimeter) Explain that 1 micrometer (�m) is one thou-sandth of 1 millimeter. Ask: Howlong is 1 micrometer in terms ofmeters? (1 �m � 1/1000 � 1/1000 m� 1/1,000,000 m) Thus, a micro-meter is one millionth of a meter.Scientists use a unit called thenanometer (nm) to measure cellstructures. A nanometer is 1/1000 ofa micrometer.

Cell Structure and Function 171

Hooke’s observationsRobert Hooke (1635–1703) was the son of anEnglish minister and a graduate of OxfordUniversity. He did important work in mechanicsand physics, and he became one of the mostprominent microscopists of his time. The cork heobserved through a compound microscope hehad built was taken from the bark of an oak tree.

Rectangular and boxlike cork cells are producedin woody plants as the woody stem increases itsgirth. Cork cells are dead at maturity, and thusHooke was not looking at living cells when hegave them a name. Hooke never fully understoodthe significance of his findings, and it was some150 years before the term cell took on its currentmeaning.

HISTORY OF SCIENCE

These new technologies make it possible for researchers tostudy the structure and movement of living cells in great detail.Unfortunately, light itself limits the detail, or resolution, ofimages that can be made with the light microscope. Like all formsof radiation, light waves are diffracted, or scattered, as they passthrough matter, making it impossible to visualize tiny structuressuch as proteins and viruses with light microscopy.

By contrast, electron microscopes are capable of revealingdetails as much as 1000 times smaller than those visible in lightmicroscopes because the wavelengths of electrons are muchshorter than those of light. Transmission electron microscopes(TEMs) make it possible to explore cell structures and largeprotein molecules. Because beams of electrons can only passthrough thin samples, cells and tissues must be cut first intoultrathin slices before they can be examined under a microscope.

With scanning electron microscopes (SEMs), a pencillikebeam of electrons is scanned over the surface of a specimen. ForSEM images, specimens do not have to be cut into thin slices tobe visualized. The scanning electron microscope producesstunning three-dimensional images of cells. Because electronsare easily scattered by molecules in the air, samples examinedin both types of electron microscopes must be placed in a vac-uum in order to be studied. As a result, researchers chemicallypreserve their samples first and then carefully remove all of thewater before placing them in the microscope. This means thatelectron microscopy can be used to visualize only nonliving,preserved cells and tissues.

For: Links on cell theory

Visit: www.SciLinks.orgWeb Code: cbn-3071

NSTA

1838MatthiasSchleidenSchleidenconcludes that all plants aremade up of cells.

Use the library or the Internetto research a new discoveryrelating to the cell or itsstructures. Be sure to includethe scientist(s) responsible forthe discovery. Then, presentyour findings in the form ofan oral report.

1800 1900 2000

1970Lynn Margulis Margulis proposes the ideathat certain organelles, tinystructures within some cells,were once free-living cellsthemselves.

1855RudolphVirchowVirchowproposes that allcells come fromexisting cells,completing thecell theory.

1839TheodorSchwannSchwannconcludesthat allanimals are made up ofcells.

NSTA

Download a worksheeton cell theory for students to com-plete, and find additional teachersupport from NSTA SciLinks.

172 Chapter 7

Use VisualsFigure 7–3 After students havestudied the photos in the figure andread the caption, have them turnback to Section 1–4 and review thetext and figures that describe the dif-ferent kinds of microscopes. Askvolunteers to read aloud the descrip-tions of the SEM and the TEM. Then,ask: What advantage do the scan-ning probe microscopes have overthe SEM and the TEM? (With scan-ning probe microscopes, scientists canstudy parts of living organisms. Withthe SEM and the TEM, only nonlivingsamples can be studied.)

Build Science SkillsClassifying Photocopy images pro-duced by the various kinds ofmicroscopes. Find these images incollege-level textbooks, especiallythose that focus on cell biology andmolecular biology. Then, divide theclass into small groups and give eachgroup a set of these images. Each setshould include at least one imageassociated with a light microscope, aTEM, an SEM, and a scanning probemicroscope. Challenge the groups toclassify the images according towhich microscope was responsiblefor each. When groups have finishedtheir classifications, have a memberof each group present its conclusionsto the class. Invite any objections to agroup’s classification, and emphasizethe differences among the differenttypes of microscopes.

Scanning probe microscopyScanning probe microscopes include a number ofinstruments that use a sharp point called a probeto scan a sample. These include the scanning tun-neling microscope (STM) and the atomic forcemicroscope (AFM). With the STM, the probe doesnot quite touch the sample. An electric currentbetween the probe and the surface of the speci-men is utilized to track the topography of asurface. The problem with an STM is that thesample must be a good conductor, and that limits

the kinds of samples that can be studied. Thisproblem was overcome with the development of the AFM, which utilizes a probe that gently touches the sample surface. The probe of an AFMis similar to the stylus (needle) on a turntable thatplays LP records—that is, an AFM is like a recordplayer. Like a stylus on a record, the probe slidesover the surface of the sample and sends dataabout that surface back to a processor. The forceis so slight that the probe usually causes no dam-age to the sample.

FACTS AND FIGURES

Confocal Light MicrographConfocal light microscopesconstruct images by scanning cellswith a computer-controlled laserbeam. In this fluorescent confocallight micrograph of HeLa cells,researchers attached fluorescentlabels to the different molecules.By doing this, researchers canfollow molecules as they movethrough a living cell. (magnification: 500X)

FIGURE 7–3 VARIETY OF MICROGRAPHS

Different types of microscopes produce a variety of images of cells and cell parts.

Scanning Electron MicrographScanning electron microscopesproduce three-dimensional imagesof the surfaces of cells, such asthese neurons, and tissues.(magnification: 8900X)

In the 1990s, researchers perfected a new class of micro-scopes that produce images by tracing the surfaces of sampleswith a fine probe. These scanning probe microscopes haverevolutionized the study of surfaces and made it possible toobserve single atoms. Unlike electron microscopes, scanningprobe microscopes can operate in ordinary air and can evenshow samples in solution. Researchers have already usedscanning probe microscopes to image DNA and protein mol-ecules as well as a number of important biological structures.

Prokaryotes and EukaryotesCells come in a great variety of shapes and an amazing rangeof sizes. Although typical cells range from 5 to 50 micrometersin diameter, the tiniest mycoplasma bacteria are only 0.2micrometers across, so small that they are difficult to see undereven the best light microscopes. In contrast, the giant amoebaChaos chaos may be 1000 micrometers in diameter, largeenough to be seen with the unaided eye as a tiny speck in pondwater. Despite their differences, all cells have two characteris-tics in common. They are surrounded by a barrier called a cellmembrane; and, at some point in their lives, they contain themolecule that carries biological information—DNA.

Scanning Probe MicrographA scanning probe microscope scansa tiny probe just above the surfaceof a sample and produces an imageby recording the position of theprobe. These powerful instrumentscan even visualize single molecules,such as DNA, on carefully preparedsurfaces. (magnification: 320,000X)

For: Articles on cellsVisit: PHSchool.comWeb Code: cbe-3071

7–1 (continued)

Science News provides studentswith the most current information on cells.

Cell Structure and Function 173

Cells fall into two broad categories, depending onwhether they contain a nucleus. The (plural:nuclei) is a large membrane-enclosed structure that containsthe cell’s genetic material in the form of DNA. (A membraneis a thin layer of material that serves as a covering orlining.) The nucleus controls many of the cell’s activities.

(yoo-KAR-ee-ohts) are cells that containnuclei. (pro-KAR-ee-ohts) are cells that do notcontain nuclei. Both words derive from the Greek wordskaryon, meaning “kernel,” or nucleus, and eu, meaning“true,” or pro, meaning “before.” These words reflect the ideathat prokaryotic cells evolved before nuclei developed.

Prokaryotes Prokaryotic cells are generally smaller andsimpler than eukaryotic cells, although there are manyexceptions to this rule. Prokaryotic cells havegenetic material that is not contained in a nucleus.Some prokaryotes contain internal membranes, but prokary-otes are generally less complicated than eukaryotes. Despitetheir simplicity, prokaryotes carry out every activity associ-ated with living things. They grow, reproduce, respond to theenvironment, and some can even move by gliding alongsurfaces or swimming through liquids. The organisms wecall bacteria are prokaryotes.

Eukaryotes Eukaryotic cells are generally larger andmore complex than prokaryotic cells. As you can see inFigure 7–4, eukaryotic cells generally contain dozens ofstructures and internal membranes, and many are highlyspecialized. Eukaryotic cells contain a nucleus inwhich their genetic material is separated from the restof the cell. Eukaryotes display great variety. Some eukary-otes live solitary lives as single-celled organisms. Others formlarge, multicellular organisms. Plants, animals, fungi, andprotists are eukaryotes.

ProkaryotesEukaryotes

nucleus

Figure 7–4 The cells of eukaryoteshave a nucleus, but the cells ofprokaryotes do not. Notice how manymore structures are located in theeukaryotic cell (bottom) as comparedwith the prokaryotic cell (top).

(magnification: 18,300�)

(magnification: 350�)

1. Key Concept What threestatements describe the celltheory?

2. Key Concept What arethe differences between prokary-otic cells and eukaryotic cells?

3. Compare the processes used toproduce a TEM and an SEM.

4. What structures do all cells have?

5. Critical Thinking InferringHow did the invention of themicroscope help the develop-ment of the cell theory?

7–1 Section AssessmentConstructing a ChartMake a three-column chartcomparing prokaryotes witheukaryotes. In the firstcolumn, list the traits found inall cells. In the secondcolumn, list the features ofprokaryotes. In the thirdcolumn, list the features ofeukaryotes.

Prokaryotes andEukaryotesUse VisualsFigure 7–4 Ask students: What isthe main difference betweenprokaryotic cells and eukaryoticcells? (Eukaryotic cells contain a nu-cleus; prokaryotic cells do not.) Dobacterial cells contain a nucleus?(No. All bacteria are prokaryotes.)What else do eukaryotic cells con-tain that prokaryotic cells don’t?(Eukaryotic cells generally containdozens of structures and internal mem-branes.)

3 ASSESSEvaluate UnderstandingCall on students at random todescribe how scientists use micro-scopes and other techniques toexplore the living cell. Then, ask vol-unteers to explain the differencebetween prokaryotes and eukaryotes.

ReteachReinforce students’ understanding ofthe three ideas that make up the celltheory by having them write state-ments that apply those ideas tospecific living things.

Students’ charts will vary, but theyshould include most of the infor-mation contained in thesubsection Prokaryotes andEukaryotes. Encourage students toadd to their charts as they con-tinue reading the chapter.

If your class subscribes to theiText, use it to review the KeyConcepts in Section 7–1.

7–1 Section Assessment1. All living things are composed of cells. Cells

are the basic units of structure and functionin living things. New cells are produced onlyfrom existing cells.

2. Prokaryotic cells are generally smaller andsimpler than eukaryotic cells. Eukaryotic cellscontain a nucleus; prokaryotic cells do not.Eukaryotic cells generally contain dozens ofstructures and internal membranes.

3. A TEM is produced by passing electronsthrough an extremely thin sample. An SEM isproduced by scanning a pencillike beam ofelectrons over the surface of an object.

4. A cell membrane and DNA5. The microscope was essential in that devel-

opment because it allowed biologists toobserve cells in living things.

174 Chapter 7

At first glance, a factory is a puzzling place. A bewilderingvariety of machines buzz and clatter, people move quickly

in different directions, and the sheer diversity of so muchactivity can be confusing. However, if you take your time andwatch carefully, before long you will begin to identify patterns.What might at first have seemed like chaos begins to makesense.

Comparing the Cell to a FactoryIn some respects, the eukaryotic cell is like a factory. The firsttime you look at a microscope image of a cell, such as the one inFigure 7–5, the cell seems impossibly complex. Look closely at aeukaryotic cell, however, and patterns begin to emerge. To seethose patterns more clearly, we’ll look at some structures thatare common to eukaryotic cells, shown in Figure 7–6. Becausemany of these structures act as if they are specialized organs,these structures are known as literally “littleorgans.”

Cell biologists divide the eukaryotic cell into two majorparts: the nucleus and the cytoplasm. The is theportion of the cell outside the nucleus. As you will see, thenucleus and cytoplasm work together in the business of life.

cytoplasm

organelles,

7–2 Eukaryotic Cell Structure

Key Concept • What are the functions of the

major cell structures?

Vocabularyorganellecytoplasmnuclear envelopechromatinchromosomenucleolusribosomeendoplasmic reticulumGolgi apparatuslysosomevacuolemitochondrionchloroplastcytoskeletoncentriole

Reading Strategy: Building VocabularyBefore you read, preview thevocabulary by skimming thesection and making a list of thehighlighted boldface terms.Leave space to make notes asyou read.

� Figure 7–5 This electron micrographof a plant cell shows many of the differenttypes of structures that are found ineukaryotic cells. The cell has been artifi-cially colored so that you can distinguishone structure from another. (magnification: 1500�)

1 FOCUSObjectives7.2.1 Describe the function of the

cell nucleus.7.2.2 Describe the functions of the

major cell organelles.7.2.3 Identify the main roles of the

cytoskeleton.

Vocabulary PreviewPronounce each Vocabulary wordand have students repeat the pro-nunciation as a class. Pay specialattention to words that are difficultfor English language learners.

Reading StrategyTo help students begin their under-standing of the differences betweenplant cells and animal cells, havethem preview Figure 7–6 and answerthe caption question.

2 INSTRUCT

Comparing the Cellto a FactoryBuild Science SkillsUsing Models Divide the class intosmall groups, and have groups makea labeled, two-dimensional drawingof a typical cell. First, have groupsmeet before reading the section todiscuss what the inside of a cellmight contain. Then, ask groups tomeet again after learning about thestructures of a cell to make thelabeled drawing.

SECTION RESOURCES

Print:

• Teaching Resources, Section Review 7–2• Reading and Study Workbook A,

Section 7–2• Adapted Reading and Study Workbook B,

Section 7–2• Lesson Plans, Section 7–2

Technology:

• iText, Section 7–2• Transparencies Plus, Section 7–2

Tim

eSaver

Section 7–2

Cell Structure and Function 175

Build Science SkillsPredicting Ask students what spe-cific functions a unicellular organismwould need to carry out in order tolive. Then, divide the class into smallgroups, and ask each group to makea table of predictions about whatstructures would likely be foundinside a unicellular organism. Thetable should have two columns:Necessary Function and StructureNeeded to Carry Out Function.

Use VisualsFigure 7–6 Encourage students tomake copies of these labeled illustra-tions in their notebooks. As theylearn about the various structuresthat make up a cell, they can adddefinitions and descriptions of func-tions for each of the labels. Point outthat when they have completed thistask, they will have made the bestpossible tool for review.

Build Science SkillsComparing and ContrastingSet up microscope stations at severallocations around the room, and pro-vide prepared slides of an animal celland a plant cell at each location.Have students make labeled drawingsof each and write a paragraph com-paring and contrasting the two typesof cells.

Answer to . . . Figure 7–6 Plant cells have a cellwall and chloroplasts. Many plant cellsalso have a large, central vacuole.

Vacuole

Chloroplast

Cell wall

Cell membrane Golgi apparatus

Mitochondrion

NucleusNucleolus Nuclear envelope

Ribosome (attached)

Ribosome (free)Smooth endoplasmic reticulum

Plant Cell

Rough endoplasmic reticulum

Cell membrane

Golgi apparatus

Mitochondrion

Centrioles

Nucleus

Nucleolus

Nuclear envelope

Ribosome (attached)

Ribosome (free)

Smooth endoplasmic reticulum

Rough endoplasmic reticulum

Animal Cell

Figure 7–6 Both plant and animal cells contain a variety of organelles.Some structures are specific to either plant cells or animal cells only.Interpreting Graphics What structures do plant cells have that animalcells do not?

PLANT AND ANIMAL CELLS

For: Cell Structure activityVisit: PHSchool.comWeb Code: cbp-3072

For: Cell Structure activityVisit: PHSchool.comWeb Code: cbe-3072Students can interact with thecell art online.

Less Proficient ReadersTo reinforce students’ understanding of cell struc-tures, draw an animal cell on the board. Includeand label the nucleus, the cell membrane, and thecytoplasm. Have students make a copy of thedrawing on a sheet of paper. Then, as eachorganelle is studied and discussed, add labeledstructures to the cell on the board, and have stu-dents add these structures to their own drawings.

English Language LearnersWhen students read about the cytoskeleton onpage 181, explain that cyto- means “cell,” andthus cytoskeleton can be thought of as the “skel-eton of the cell.” Explain that this is an analogy,since the cytoskeleton is not like an animal skel-eton. Also, explain that a filament is a threadlikematerial and a tubule is a “very slender tube.”Thus, the cytoskeleton can be thought of as com-posed of threads and slender tubes.

176 Chapter 7

NucleusUse VisualsFigure 7–7 Ask students: What isthe nucleolus? (It is a small, denseregion of the nucleus where the assem-bly of ribosomes begins.) Where is theDNA that a nucleus contains? (TheDNA is part of the chromatin, which isspread throughout the nucleus most ofthe time.) Why is DNA important?(It holds coded instructions for makingproteins and other important mol-ecules.) Point out that the geneticinformation is the coded instructionsfor making molecules.

Build Science SkillsInferring Remind students thatprokaryotes do not contain a nucleus.Then, ask: If the nucleus controlsmost cell processes in eukaryotes,how can prokaryotes live without anucleus? (Some students might sug-gest that the lives of prokaryotes aren’tas complex as those of eukaryotes.Others might correctly infer that themost important part of a nucleus is theDNA it contains, and prokaryotes haveDNA without having a nucleus.)

The nucleus controls the cellDuring the 1930s and 1940s, researchers per-formed a series of experiments that demon-strated the link between a cell’s nucleus and thephysical characteristics of the cell. Two speciesof Acetabularia algae were used in the experi-ments. This marine alga, though 5 cm long,consists of a single cell. Each cell includes aholdfast at the bottom, a stalk, and a cuplikecap at the top, and the cell’s nucleus is in the

holdfast. The two species that were used haddifferent-shaped caps. Researchers cut the capoff one cell, removed the nucleus from its hold-fast, and transplanted a nucleus from a cell ofthe second species into the holdfast of the firstcell. The cell regenerated a new cap, andresearchers cut off that one. Eventually, the capthat grew was the shape of the cap from thesecond species, from which the transplantednucleus came, and not the shape of the first cap.

HISTORY OF SCIENCE

NucleusIn the same way that the main office controls a large factory, thenucleus is the control center of the cell. The nucleuscontains nearly all the cell’s DNA and with it the codedinstructions for making proteins and other importantmolecules. The structure of the nucleus is shown in Figure 7–7.

The nucleus is surrounded by a composed of two membranes. The nuclear envelope is dottedwith thousands of nuclear pores, which allow material to moveinto and out of the nucleus. Like messages, instructions, andblueprints moving in and out of a main office, a steady stream ofproteins, RNA, and other molecules move through the nuclearpores to and from the rest of the cell.

The granular material you can see in the nucleus is calledChromatin consists of DNA bound to protein.

Most of the time, chromatin is spread throughout the nucleus.When a cell divides, however, chromatin condenses to form

(KROH-muh-sohms). These distinct, threadlikestructures contain the genetic information that is passed fromone generation of cells to the next. You will learn more aboutchromosomes in later chapters.

Most nuclei also contain a small, dense region known as the(noo-KLEE-uh-lus). The nucleolus is where the

assembly of ribosomes begins.

What kind of information is contained in chromosomes?

nucleolus

chromosomes

chromatin.

nuclear envelope

The nucleus controls most cell processes and contains thehereditary information of DNA. The DNA combines with proteinto form chromatin, which is found throughout the nucleus. Thesmall, dense region in the nucleus is the nucleolus.

Nuclear envelope

Nuclear pores

Chromatin

Nucleolus

FIGURE 7–7 THE NUCLEUS7–2 (continued)

Cell Structure and Function 177

ribosomes. are small particles of RNA and pro-tein found throughout the cytoplasm. They produce proteins byfollowing coded instructions that come from the nucleus. Eachribosome, in its own way, is like a small machine in a factory,turning out proteins on orders that come from its “boss”—thecell nucleus. Cells that are active in protein synthesis are oftenpacked with ribosomes.

Endoplasmic ReticulumEukaryotic cells also contain an internal membrane systemknown as the (en-doh-PLAZ-mik endoplasmic reticulum

Ribosomes

The endoplasmic reticulum synthesizes proteins forexport from the cell. The rough endoplasmic reticulum,shown here, gets its name from the “rough” appearance of theribosomes on its surface.

Ribosomes

Endoplasmic reticulum(magnification: about 40,000�)

Ribosomes(magnification: 160,000�)

FIGURE 7–8 ENDOPLASMIC RETICULUM

RibosomesOne of the most important jobs carried out in the cellular“factory” is making proteins. Proteins are assembled on

rih-TIK-yuh-lum), or ER. The endoplasmic reticulum isthe site where lipid components of the cell membraneare assembled, along with proteins and other materialsthat are exported from the cell.

The portion of the ER involved in the synthesis of proteins iscalled rough endoplasmic reticulum, or rough ER. It is giventhis name because of the ribosomes found on its surface. Newlymade proteins leave these ribosomes and are inserted into therough ER, where they may be chemically modified.

RibosomesBuild Science SkillsUsing Analogies Read aloud thesentence in the text that compares aribosome to a machine. Use thiscomparison to discuss how a eukary-otic cell is like a factory. Then,encourage students who need anextra challenge to work together inwriting a short play based on theanalogy of the cell as a factory.Explain that a good play needs someconflict or danger. The “factory”might be under economic threat orsome environmental threat. Advisestudents to include the functions ofas many parts of the factory—cellorganelles—as possible. Once theplay has been written, encourage the“playwrights” to recruit class mem-bers to act out the drama.

EndoplasmicReticulumUse VisualsFigure 7–8 Ask students: What areribosomes composed of? (RNA andprotein) Where are ribosomes pro-duced? (In the nucleolus) What doribosomes produce? (Proteins) Whathappens to these proteins afterthey’re produced by ribosomes?(Membrane proteins are inserteddirectly into the ER membrane. Manyof the proteins produced on the roughER are released or secreted from thecell.) If this were an illustration ofsmooth ER, how would it be differ-ent? (The ER would not haveribosomes on its surface.) What is thefunction of smooth ER? (The smoothER contains enzymes that help synthe-size lipids, such as steroids. Smooth ERalso helps to detoxify and processchemicals.)

Answer to . . . Chromosomes contain

the genetic information that is passedfrom one generation to the next.

Learning from sea urchin nucleiThe German cytologist Theodor Boveri(1862–1915) performed an experiment beforethe invention of microdissection that demon-strated the importance of the nucleus. By vigorousshaking, Boveri removed the nuclei from the eggsof sea urchins of the genus Sphaerechinus. He then

fertilized the eggs (which had no nuclei) withsperm from sea urchins of the genus Echinus. In apractical sense, fertilization resulted in the substi-tution of one nucleus for another. The larvae thatdeveloped had only the traits of Echinus, eventhough the sperm contributed little more than atiny bit of nucleus to the developing organism.

HISTORY OF SCIENCE

178 Chapter 7

Golgi ApparatusBuild Science SkillsComparing and ContrastingStudents often confuse the Golgiapparatus with the endoplasmicreticulum, because both are usuallyrepresented as folded membraneswithin the cytoplasm. Have studentscompare the illustrations in Figure7–8 with those in Figure 7–9. Then,call on students at random to explainthe differences in functions betweenER and the Golgi apparatus.

LysosomesBuild Science SkillsObserving Divide the class intosmall groups, and give each groupaccess to a paramecium culture and a yeast suspension, as well as to amicroscope slide, coverslip, tooth-pick, dropper pipette, andmicroscope. (Prepare the yeast sus-pension by adding a pinch of Congored indicator to a thick mixture ofyeast and water. Then, bring it to agentle boil for 5 minutes. Cool beforeusing. Transfer some parameciumculture from the stock culture at leasta day ahead of time, and then limitthe food supply to the transferredculture.) Have each group prepare aslide of live paramecia using thedropper pipette. Students shouldfocus the slide under the low-powerobjective of the microscope. Theyshould then obtain a small sample of the yeast solution. The indicator in the solution is red above pH 5 andblue below pH 3. The next step is to use a toothpick to transfer a smalldrop of yeast suspension to the edge of the slide and observe theparamecia under the microscope for5 minutes. (Students should observethat the paramecia sweep the yeastthrough their oral grooves and formvacuoles to enclose it. The vacuolesbecome blue at first and eventually red,as lysosomes fuse with the vacuole andrelease acids that digest the yeast.)

Important products of the Golgi apparatusOne of the most important cell componentspackaged and distributed by the Golgi apparatusis material for the membranes of the cell and itsorganelles. Lysosomes, which are essentiallymembranous bags filled with enzymes, areproducts of the Golgi apparatuses. Theseenzymes would destroy the cell if they were

not surrounded by membrane. An example ofhow lysosomes function in cells can be seen inthe way paramecia digest their food. Upon contact with a food organism or some otherparticle, the paramecium envelops the food in avacuole. Lysosomes then fuse with the vacuoleand release acids. The acids quickly digest the contents of the vacuole.

FACTS AND FIGURES

FIGURE 7–9 GOLGI APPARATUS

Proteins that are released, or exported, from the cell aresynthesized on the rough ER, as are many membrane proteins.Rough ER is abundant in cells that produce large amounts ofprotein for export. Other cellular proteins are made on “free”ribosomes, which are not attached to membranes.

The other portion of the ER is known as smooth endoplas-mic reticulum (smooth ER) because ribosomes are not found onits surface. In many cells, the smooth ER contains collections ofenzymes that perform specialized tasks, including the synthesisof membrane lipids and the detoxification of drugs. Liver cells,which play a key role in detoxifying drugs, often contain largeamounts of smooth ER.

Golgi ApparatusProteins produced in the rough ER move next into an organellecalled the discovered by the Italian scientistCamillo Golgi. As you can see in Figure 7–9, Golgi appears as astack of closely apposed membranes. The function of theGolgi apparatus is to modify, sort, and package proteinsand other materials from the endoplasmic reticulum forstorage in the cell or secretion outside the cell. The Golgiapparatus is somewhat like a customization shop, where thefinishing touches are put on proteins before they are ready toleave the “factory.” From the Golgi apparatus, proteins are then“shipped” to their final destinations throughout the cell oroutside of the cell.

Golgi apparatus,

The Golgi apparatus modifies, sorts, and packages proteins.Notice the stacklike membranes that make up the Golgi apparatusin this transmission electron micrograph.

(magnification: about 45,700�)

7–2 (continued)

Cell Structure and Function 179

LysosomesEven the neatest, cleanest factory needs a cleanup crew, andthat’s what lysosomes (LY-suh-sohmz) are. are smallorganelles filled with enzymes. One function of lysosomes is thedigestion, or breakdown, of lipids, carbohydrates, and proteinsinto small molecules that can be used by the rest of the cell.

Lysosomes are also involved in breaking down organellesthat have outlived their usefulness. Lysosomes perform the vitalfunction of removing “junk” that might otherwise accumulateand clutter up the cell. A number of serious human diseases,including Tay-Sachs disease, can be traced to lysosomes that failto function properly.

What is the role of lysosomes?

VacuolesEvery factory needs a place to store things, and cells containplaces for storage as well. Some kinds of cells contain saclikestructures called (VAK-yoo-ohlz) that store materialssuch as water, salts, proteins, and carbohydrates. In many plantcells there is a single, large central vacuole filled with liquid. Thepressure of the central vacuole in these cells makes it possible forplants to support heavy structures such as leaves and flowers.

Vacuoles are also found in some single-celled organisms andin some animals. The paramecium in Figure 7–10 contains avacuole called a contractile vacuole. By contracting rhythmically,this specialized vacuole pumps excess water out of the cell. Thecontrol of water content within the cell is just one example of animportant process known as homeostasis. Homeostasis is themaintenance of a controlled internal environment.

Mitochondria and ChloroplastsAll living things require a source of energy. Factories are hookedup to the local power company, but what about cells? Most cells getenergy in one of two ways—from food molecules or from the sun.

Mitochondria Nearly all eukaryotic cells, including plants,contain (myt-oh-KAHN-dree-uh; singular: mito-mitochondria

vacuoles

Lysosomes

Figure 7–10 Vacuoles have a varietyof functions. In the Coleus plant cell(top), the large blue structure is thecentral vacuole that stores salts,proteins, and carbohydrates. Theparamecium (bottom) containscontractile vacuoles that fill with waterand then pump the water out of thecell. Applying Concepts How dovacuoles help support plant structures?

Contractilevacuole

Vacuole

(magnification: about 3000�)

chondrion). Mitochondria are organelles that convertthe chemical energy stored in food into compounds thatare more convenient for the cell to use. Mitochondria areenclosed by two membranes—an outer membrane and an innermembrane. The inner membrane is folded up inside the organelle.

One of the most interesting aspects of mitochondria is theway in which they are inherited. In humans, all or nearly all ofour mitochondria come from the cytoplasm of the ovum, or eggcell. This means that when your relatives are discussing whichside of the family should take credit for your best characteris-tics, you can tell them that you got your mitchondria from Mom!

VacuolesUse VisualsFigure 7–10 After students havestudied the figure and read the cap-tion, explain that the Coleus cell isfrom a multicellular, leafy plant,whereas the paramecium is a micro-scopic unicellular organism that ispart of the kingdom Protista, whichstudents will learn about in Chapter20. Then, ask: How is the functionof a vacuole in a plant cell differentfrom that in a unicellular organ-ism? (A vacuole in a plant cell storesmaterials such as water, salts, proteins,and carbohydrates. It also helps sup-port plant structures. A vacuole in aunicellular organism is specialized topump water out of the cell.)

Mitochondria andChloroplastsBuild Science SkillsUsing Analogies Explain to stu-dents that mitochondria have longbeen called the “powerhouses” ofcells. Ask: What is a “powerhouse”?(A powerhouse is another name for apower plant, which produces electricityfor cities and regions.) How is a mito-chondrion like a powerhouse?Powerhouses convert one source ofenergy to another more useful form.For example, energy from coal, oil, orgas is often converted to electricity, amore useful form for homes and indus-try. A mitochondrion also convertsenergy to a more useful form. It usesenergy from food to make high-energycompounds that the cell can use ingrowth, development, and movement.

When I introduce the structure of the cell, I tryto analogize the cell with the students’ city.Taking this analogy a step further, I organize acooperative learning activity in which I askteams of students to “create” an imaginary citythat correlates cell structures with city compo-nents. Teams should include most of theorganelles in their city. For example, theymight use a mitochondrion as the local powerplant or microtubules as major thoroughfares.

For this activity, each team will need a largesheet of paper or poster board for drawing thecity, as well as colored pencils or similar materi-als. If possible, have teams display their workand explain their “creations” to the class.

—Jorge E. SanchezBiology TeacherGreen Valley High SchoolHenderson, NV

TEACHER TO TEACHER

Answers to . . . Lysosomes break down

lipids, carbohydrates, and proteins.They also break down organelles thathave outlived their usefulness in the cell.

Figure 7–10 The pressure exerted by the liquid in the vacuole makes it possible for plants to support heavystructures.

The origin of eukaryotes?The idea that chloroplasts and mitochondriaoriginated in symbiotic relationships withprokaryotic cells is called the endosymbionthypothesis. According to this hypothesis, theancestors of eukaryotic cells were smallerspecies of prokaryotes living within largerspecies of prokaryotes. Chloroplasts possiblyoriginated when cyanobacteria became

established in larger prokaryotes either as parasites or as prey that were not digested.Mitochondria were once possibly anaerobicheterotrophs that found “safe harbor” insidelarger prokaryotes as the world becameincreasingly aerobic. As the host and symbiontsover time became more and more interdepend-ent, the organisms merged to become a singleorganism.

FACTS AND FIGURES

Chloroplasts Plants and some other organisms containChloroplasts are organelles that

capture the energy from sunlight and convert it into

chemical energy in a process called photosynthesis.

Chloroplasts are the biological equivalents of solar powerplants. Like mitochondria, chloroplasts are surrounded by twomembranes. Inside the organelle are large stacks of othermembranes, which contain the green pigment chlorophyll.

Organelle DNA Unlike other organelles that contain noDNA, chloroplasts and mitochondria contain their own geneticinformation in the form of small DNA molecules. LynnMargulis, an American biologist, has suggested that mitochon-dria and chloroplasts are actually the descendants of ancientprokaryotes. Margulis suggests that the prokaryotic ancestors ofthese organelles evolved a symbiotic relationship with earlyeukaryotes, taking up residence within the eukaryotic cell. Onegroup of prokaryotes had the ability to use oxygen to generateATP. These prokaryotes evolved into mitochondria. Otherprokaryotes that carried out photosynthesis evolved into chloro-plasts. This idea is called the endosymbiotic theory.

chloroplasts.

How can you make a model of a cell?

Materials variety of craft supplies, index cards

Procedure

1. Your class is going to make a model of a plant cellusing the whole classroom. Work with a partner orin a small group to decide what cell part ororganelle you would like to model. (Use Figure7–6 as a starting point. It will give you an idea ofthe relative sizes of various cell parts and theirpossible positions. Figures 7–7 through 7–10 can provide additional information.)

2. Using materials of your choice, make a three-dimensional model of the cell part or organelle youchose. Make the model as complete and as accu-rate as you can.

3. Label an index card with the name of your cell partor organelle and list its main features and func-tions. Attach the card to your model.

4. Attach your model to an appropriate place in theroom. If possible, attach your model to anotherrelated cell part or organelle.

Analyze and Conclude1. Inferring What are the functions of the different

organelles in plant cells?2. Calculating Assume that a typical plant cell is 50

micrometers wide. Calculate the scale of yourclassroom cell model. (Hint: Divide the width of theclassroom by the width of a cell, making sure touse the same units.)

3. Comparing and Contrasting How is yourmodel cell part or organelle similar to the real cellpart or organelle? How is it different?

4. Evaluating Based on your work with this model,describe how you could make a better model.Specify what new information the improved modelwould demonstrate.

For: Cell structure activity

Visit: PHSchool.comWeb Code: cbd-3072

180 Chapter 7

7–2 (continued)

Objective Students will makemodels of cell organelles and a largeclass model of a cell.Skill Focus Using modelsMaterials craft supplies, indexcardsTime 20 minutesAdvance Prep Collect a variety ofcraft supplies, including scissors,construction paper, cardboardtubes, plastic bags, yarn, glue, andbeads.Safety Caution students about theuse of pins and about standing onchairs as they hang up their models.Supervise them as they do so.Strategies• You may want students to build a

model of a different kind of cell. Amodel of a plant cell is suggestedbecause plant cells have a greatvariety of structures and organelles.

• Make sure at least one group isworking on these major structures:cell wall, cell membrane, nucleus,microtubules, microfilaments, ribo-somes, smooth ER, rough ER, Golgiapparatus, lysosomes, vacuoles,mitochondria, and chloroplasts.

Expected Outcomes Studentswill make models of plant-cell struc-tures and arrange them to form acomplete cell.Analyze and Conclude1. Students’ answers should reflectan understanding of the functions ofplant cell organelles.2. Scales will vary depending on thesize of the cell model. A typical scale,assuming that the classroom is 5 macross, would be 5/0.00005 (50 micrometers � 0.00005 meters),or 100,000 : 1.3. The model should be similar inshape and structure to a real cellpart. The model is different in that itis much larger, is made of differentmaterials, and does not function.4. Students should explain howtheir model would be an improve-ment on their previous model.

Cell Structure and Function 181

CytoskeletonBuild Science SkillsUsing Analogies Show students aphoto of a house that’s being built,with only the foundation laid and thebasic frame constructed. Builders callthis initial stage “framing” the house.Ask: How is this house frame like acell’s cytoskeleton? (Just as acytoskeleton is a network of protein fil-aments that helps a cell maintain itsshape, the frame of a house is a net-work of boards and timbers that formsthe shape of the house.)

3 ASSESSEvaluate UnderstandingHave students make a Venn diagramto show organelles that are foundonly in prokaryotic cells, those thatare found only in eukaryotic cells,and those that are found in bothtypes of cells.

ReteachAsk students to make a compare/contrast table that lists all the parts ofa typical cell. Column heads mightinclude Name, Structure, andFunction.

CytoskeletonA supporting structure and a transportation systemcomplete our picture of the cell as a factory. As youknow, a factory building is supported by steel orcement beams and by columns that support its wallsand roof. Eukaryotic cells have a structure—the

—that helps support the cell. Thecytoskeleton is a network of protein filamentsthat helps the cell to maintain its shape. Thecytoskeleton is also involved in movement.Microfilaments and microtubules are two of the princi-pal protein filaments that make up the cytoskeleton.

Microfilaments are threadlike structures made ofa protein called actin. They form extensive networksin some cells and produce a tough, flexible frameworkthat supports the cell. Microfilaments also help cellsmove. Microfilament assembly and disassembly isresponsible for the cytoplasmic movements that allowcells, such as amoebas, to crawl along surfaces.

Microtubules, as shown in Figure 7–11, are hollowstructures made up of proteins known as tubulins. Inmany cells, they play critical roles in maintaining cellshape. Microtubules are also important in cell divi-sion, where they form a structure known as themitotic spindle, which helps to separate chromosomes.In animal cells, tubulin is also used to form a pair ofstructures known as centrioles. arelocated near the nucleus and help to organize celldivision. Centrioles are not found in plant cells.

Microtubules also help to build projections fromthe cell surface, which are known as cilia (singular:cilium) and flagella (singular: flagellum), that enablecells to swim rapidly through liquids. Cilia and fla-gella can produce considerable force; and in some cellsthey move almost like the oars of a boat, pulling orpushing cells through the water. You will learn moreabout cilia and flagella in later chapters.

Centrioles

cytoskeleton

Cell membrane

Endoplasmicreticulum

Microtubule

Microfilament

Ribosomes Mitochondrion

� Figure 7–11 The cytoskeleton is anetwork of protein filaments that helps thecell to maintain its shape and is involved inmany forms of cell movement. The micro-graph shows the microtubules of kidney cells.Microtubules are part of the cytoskeleton thathelp maintain cell shape.

(magnification: 1000�)

1. Key Concept Describe the functions of the endoplasmicreticulum, Golgi apparatus,chloroplast, and mitochondrion.

2. Describe the role of the nucleusin the cell.

3. What are two functions of thecytoskeleton?

4. How is a cell like a factory?5. Critical Thinking Inferring

You examine an unknown cellunder the microscope anddiscover that the cell containschloroplasts. What type oforganism could you infer that the cell came from?

Persuasive WritingImage that you are LynnMargulis. Write a persuasiveletter to the editor of amagazine, explaining youridea. Your explanation shouldbe clear to people who do nothave a biology background.Hint: Review the concept ofsymbiosis in Section 4–2.

7–2 Section Assessment

Your students can extend theirknowledge of cell structurethrough this online experience.

If your class subscribes to theiText, use it to review the KeyConcepts in Section 7–2.

7–2 Section Assessment1. Rough ER makes membranes and secretory

proteins. Smooth ER makes lipids and helps indetoxification. The Golgi apparatus modifies,sorts, and packages proteins and other materials from the ER for storage or secretion.Chloroplasts capture the energy of sunlightand convert it into chemical energy.Mitochondria convert stored chemical energyinto compounds that the cell can use.

2. It is the control center of the cell.

3. It helps the cell maintain its shape and also isinvolved in movement.

4. Answers may vary. A typical response willcompare ribosomes to factory machines andthe cytoskeleton to a supporting structure.Students should also compare otherorganelles to various parts of a factory.

5. Students should infer that the organismwould either be a plant or some other organ-ism that carries out photosynthesis.

Students’ letters may vary. Thefocus of all letters, though, shouldbe to explain the endosymbiotictheory. Because this explanationmust be clear to people without abiology background, studentsshould explain in simple but accu-rate terms what mitochondria,chloroplasts, and prokaryotes are,as well as how such symbiotic rela-tionships could evolve.

182 Chapter 7

When you first study a country, you may begin by examininga map of the country’s borders. Before you can learn

anything about a nation, it’s important to understand where itbegins and where it ends. The same principle applies to cells.Among the most important parts of a cell are its borders, whichseparate the cell from its surroundings. All cells are surroundedby a thin, flexible barrier known as the Manycells also produce a strong supporting layer around the mem-brane known as a

Cell MembraneThe cell membrane regulates what enters and leaves

the cell and also provides protection and support. Thecomposition of nearly all cell membranes is a double-layeredsheet called a As you can see in Figure 7–12,there are two layers of lipids, hence the name bilayer. The lipidbilayer gives cell membranes a flexible structure that forms astrong barrier between the cell and its surroundings.

In addition to lipids, most cell membranes contain proteinmolecules that are embedded in the lipid bilayer. Carbohydratemolecules are attached to many of these proteins. In fact, there areso many kinds of molecules in cell membranes that scientistsdescribe the membrane as a “mosaic” of different molecules. Amosaic is a work of art made of individual tiles or other piecesassembled to form a picture or design. As you will see, some of theproteins form channels and pumps that help to move materialacross the cell membrane. Many of the carbohydrates act likechemical identification cards, allowing individual cells to identifyone another.

lipid bilayer.

cell wall.

cell membrane.

7–3 Cell Boundaries

Key Concepts• What are the main functions of

the cell membrane and thecell wall?

• What happens during diffusion?

• What is osmosis?

Vocabularycell membrane • cell walllipid bilayer • concentrationdiffusion • equilibriumosmosis • isotonichypertonic • hypotonicfacilitated diffusionactive transport • endocytosisphagocytosis • pinocytosisexocytosis

Reading Strategy:Summarizing As you read,make a list of the ways in whichsubstances can move through thecell membrane. Write onesentence describing each process.

Proteins

Carbohydratechains

Lipid bilayerProteinchannel

Inside of Cell(cytoplasm)

CellMembrane

Outsideof Cell

� Figure 7–12 The cellmembrane regulates what entersand leaves the cell. This illustrationof the cell membrane shows that itis made up of a lipid bilayer inwhich proteins are embedded.

CC

1 FOCUS

Objectives7.3.1 Identify the main functions of

the cell membrane and thecell wall.

7.3.2 Describe what happens dur-ing diffusion.

7.3.3 Explain the processes of osmosis, facilitated diffusion,and active transport.

Vocabulary PreviewSuggest that students preview themeaning of the Vocabulary terms inthe section by skimming the text tofind the highlighted boldface wordsand their meanings.

Reading StrategyBefore students read, have them skim the section to identify and makea list of the main ideas. Then, as theyread the section they should writedown supporting details for eachmain idea.

2 INSTRUCT

Cell MembraneUse VisualsFigure 7–12 Ask students: Whatdoes it mean that a cell membranehas a “lipid bilayer”? (A cell mem-brane is composed of two layers of lipidmolecules.) What do the blue mol-ecules represent in the illustrationof the cell membrane? (They repre-sent carbohydrate chains attached tothe outside of the protein moleculesembedded in the lipid bilayer.) Explainthat these carbohydrate moleculesare particularly important in cellrecognition. Nearly all cells have spe-cial carbohydrate molecules on theirsurfaces—cell markers—that areunique.

SECTION RESOURCES

Print:

• Laboratory Manual A, Chapter 7 Lab• Teaching Resources, Section Review 7–3,

Chapter 7 Real-World Lab• Reading and Study Workbook A,

Section 7–3• Adapted Reading and Study Workbook B,

Section 7–3• Lesson Plans, Section 7–3

Technology:

• iText, Section 7–3• Animated Biological Concepts Videotape

Library, 5 Diffusion and Osmosis, 6 Passiveand Active Transport, 7 Endocytosis andExocytosis

• Transparencies Plus, Section 7–3• Lab Simulations CD-ROM, Biomembranes 1:

Membrane Structure and Transport• Virtual Labs, Lab 3, Lab 4, Lab 5

Tim

eSaver

Section 7–3

Cell Structure and Function 183

Cell WallsCell walls are present in many organisms, including plants,algae, fungi, and many prokaryotes. Cell walls lie outside thecell membrane. Most cell walls are porous enough to allowwater, oxygen, carbon dioxide, and certain other substances topass through easily. The main function of the cellwall is to provide support and protection for the cell.

Most cell walls are made from fibers of carbohydrate andprotein. These substances are produced within the cell andthen released at the surface of the cell membrane where theyare assembled to form the wall. Plant cell walls are composedmostly of cellulose, a tough carbohydrate fiber. Cellulose isthe principal component of both wood and paper, so everytime you pick up a sheet of paper, you are holding the stuff ofcell walls in your hand.

Diffusion Through Cell BoundariesEvery living cell exists in a liquid environment that it needs tosurvive. It may not always seem that way; yet even in the dustand heat of a desert like the one in Figure 7–13, the cells ofcactus plants, scorpions, and vultures are bathed in liquid.One of the most important functions of the cell membrane is toregulate the movement of dissolved molecules from the liquidon one side of the membrane to the liquid on the other side.

Measuring Concentration The cytoplasm of a cell con-tains a solution of many different substances in water. Recallthat a solution is a mixture of two or more substances. Thesubstances dissolved in the solution are called solutes. The

of a solution is the mass of solute in a givenvolume of solution, or mass/volume. For example, if you dis-solved 12 grams of salt in 3 liters of water, the concentrationof the solution would be 12 g/3 L, or 4 g/L (grams per liter). Ifyou had 12 grams of salt in 6 liters of water, the concentra-tion would be 12 g/6 L, or 2 g/L. The first solution is twice asconcentrated as the second solution.

concentration

� Figure 7–13 The cells of livingthings are bathed in liquid even in dryenvironments. When it rains, thesecactus plants store the water in theirstems. Applying Concepts Which cellstructure could serve as a storage locationfor water?

For: Links on cell membranes

Visit: www.SciLinks.orgWeb Code: cbn-3073

NSTA

Cell WallsAddress MisconceptionsSome students may have the miscon-ception that a cell wall takes theplace of a cell membrane. Emphasizethat all cells have a cell membrane.Some cells have the added character-istic of having a cell wall outside thecell membrane. Some students mayalso have the misconception that acell wall is impenetrable, like the wallof a building. Read aloud the sen-tence in their text that explains thatcell walls are porous. Point out that ifcell walls weren’t porous, nothingcould pass into or out of plant cells,for instance.

Diffusion ThroughCell BoundariesMake ConnectionsMathematics Reinforce students’understanding of concentration bybuilding on the example in the text.Ask: If you dissolved 12 grams ofsalt in 3 liters of water, what is theconcentration of salt in the solu-tion? (4 g/L) Suppose you added 12more grams of salt to the solution.What would be the resulting con-centration? (24 g/3 L = 8 g/L) Whatif you then added another 3 litersof water to that solution. Whatwould be the resulting concentra-tion? (24 g/6 L = 4 g/L) Whichsolution of the ones discussedwould be called the most concen-trated? (The solution in which theconcentration is 8 g/L) Point out thatin each case the solute has a rela-tively small volume compared to thevolume of the solvent. Yet, small dif-ferences in solute can have a greateffect in living things.

To help students remember the structures andfunctions of an animal cell, give each pair of stu-dents an acetate sheet and a marking pen.Assign each student pair a different cell structure,and ask them to draw the cell structure in a spe-cific place on their acetate sheet. As you discussthe parts of the cell, place all the sheets on anoverhead projector. Then, draw a cell membranesurrounding all the cell structures on the top

acetate sheet. Have students identify each struc-ture and tell what its function is, including thecell membrane.

—Beverly CeaBiology TeacherGrimsley High SchoolGreensboro, NC

TEACHER TO TEACHER

Answer to . . . Figure 7–13 Vacuoles

NSTA

Download a worksheeton cell membranes for students tocomplete, and find additionalteacher support from NSTASciLinks.

184 Chapter 7

Build Science SkillsUsing Models Have students actout the process of diffusion. Tobegin, group class members at theclassroom door. Then, tell them tospread out through the classroom insuch a way that no two students arecloser to each other than to anyother students. Discuss how mol-ecules randomly spread out througha liquid or a gas.

Build Science SkillsObserving Students can observethe action of diffusion through thissimple activity. Divide the class intosmall groups, and give each grouptwo beakers, salt, a teaspoon, andfood coloring, as well as access towater. Have groups follow this pro-cedure. Fill one beaker aboutone-third full of water, and then addabout a half teaspoon of salt and afew drops of food coloring. Fill thesecond beaker about half full ofwater. Then, pour the contents of thefirst beaker into the second beaker,and observe what happens. (Studentsshould observe that the colored salt-water will sink to the bottom of thebeaker.) Let the second beaker standovernight, and observe any changesthat have occurred. (The next day,students should observe that the liquidin the beaker will be uniformly col-ored throughout.) Ask students:What process occurred thatchanged the mixtureovernight? (Diffusion)

Diffusion In a solution, particles move constantly. Theycollide with one another and tend to spread out randomly. As aresult, the particles tend to move from an area where they aremore concentrated to an area where they are less concentrated,a process known as (dih-FYOO-zhun). When theconcentration of the solute is the same throughout a system, thesystem has reached

What do diffusion and equilibrium have to do with cell mem-branes? Suppose a substance is present in unequal concentra-tions on either side of a cell membrane, as shown in Figure 7–14.If the substance can cross the cell membrane, its particles willtend to move toward the area where it is less concentrated untilequilibrium is reached. At that point, the concentration of thesubstance on both sides of the cell membrane will be the same.

Because diffusion depends upon random particlemovements, substances diffuse across membranes with-out requiring the cell to use energy. Even when equilibriumis reached, particles of a solution will continue to move acrossthe membrane in both directions. However, because almostequal numbers of particles move in each direction, there is nofurther change in concentration.

What conditions are present when equilibrium is reachedin a solution?

equilibrium.

diffusion

� Figure 7–14 Diffusion is theprocess by which molecules of asubstance move from areas of higherconcentration to areas of lowerconcentration. Diffusion doesnot require the cell to use energy.

CellMembrane

Solute

A

There is a higher concentration of solute on one side of the membrane as compared to theother side of the membrane.

B

Solute particles movefrom the side of the membrane with a higher concentration of solute to the side of the membrane with a lower concentration of solute. The solute particles will continue to diffuse across the membrane until equilibrium is reached.

C

When equilibrium is reached, solute particles continue todiffuse across the membrane in both directions.

DIFFUSION

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7–3 (continued)

For: Diffusion activityVisit: PHSchool.comWeb Code: cbe-3073Students explore the process ofdiffusion online.

Inclusion/Special NeedsTo help students understand the differencebetween facilitated diffusion and active transport,use the analogy of going through a fence at anopen gate or going through a turnstile. Walkingthrough an open gate is like facilitated diffusion—it takes no energy to pass through that “carrierprotein” in the fence. Moving through a turnstileis like active transport—a person has to use energyto move through that “molecular pump.”

Advanced LearnersPoint out that the human digestive system isunable to digest cellulose, and thus cellulose passes through the digestive tract without beingbroken down. Explain that cellulose is neverthelessan important part of a healthy diet. Cellulose inthe diet is called fiber, and consuming enoughfiber may help prevent some forms of cancer.Encourage students who need a challenge to findout how cellulose aids in digestion and helps pre-vent disease.

Cell Structure and Function 185

Osmosis

A hypodermic injection is one that isadministered under the skin.

DemonstrationDemonstrate the concept of selectivepermeability by using a kitchenstrainer. As students observe, pourdifferent sorts of materials throughthe strainer, making sure to choosesome materials that will go throughthe strainer and some that will not.Materials might include water, sugar,sand, small stones, marbles, andpaper clips. Students will observethat the strainer is selectively permeable.

Use VisualsFigure 7–15 Have students studythe figure and read the caption.Then, ask: In the beaker on the left,which solution is hypertonic andwhich is hypotonic? (The solution onthe left side of the membrane is hyper-tonic, and the solution on the right sideis hypotonic.) In this model, towhich material is the membranepermeable, water or sugar? (Water)Emphasize that the membrane allowsone material—water—to passthrough it, while blocking the othermaterial—sugar. This makes themembrane selectively permeable.Finally, ask students to draw a thirdbeaker that would model a situationin which the two solutions on eitherside of the membrane are isotonic.(Students should make a drawing inwhich the same number of sugar mol-ecules are evenly distributed on bothsides of the membrane.)

Water finds its wayOsmosis is easy to observe in cells, yet it was longa mystery as to how water can cross membranesso quickly. Water is a polar molecule, and as suchit is not lipid soluble and should not be expectedto cross a lipid bilayer. Many puzzled physicalchemists suggested that biochemists should lookfor some kind of channels in the membrane. In

the late twentieth century, such channels were infact discovered. These channels, which are membrane-spanning proteins, are named aqua-porins. They have been found in scores of cells,and some scientists think that they might be pres-ent in nearly all cells. Their discovery is so recentthat a detailed analysis of how they work may beyears away.

SCIENCE UPDATE

Answer to . . . Equilibrium is reached

when the concentration of the solute isthe same throughout the solution.

OsmosisAlthough many substances can diffuse across biological mem-branes, some are too large or too strongly charged to cross thelipid bilayer. If a substance is able to diffuse across a membrane,the membrane is said to be permeable to it. A membrane isimpermeable to substances that cannot pass across it. Mostbiological membranes are selectively permeable, meaning thatsome substances can pass across them and others cannot.

Water passes quite easily across most membranes, eventhough many solute molecules cannot. An important processknown as is the result. Osmosis is the diffusionof water through a selectively permeable membrane.

How Osmosis Works Look at the beaker on the left inFigure 7–15. There are more sugar molecules on the left side ofthe selectively permeable membrane than on the right side.That means that the concentration of water is lower on the leftthan it is on the right. The membrane is permeable to water butnot to sugar. This means that water can cross the membrane inboth directions, but sugar cannot. As a result, there is a netmovement of water from the area of high concentration to thearea of low concentration.

Water will tend to move across the membrane until equilib-rium is reached. At that point, the concentrations of water andsugar will be the same on both sides of the membrane. When thishappens, the two solutions will be which means “samestrength.” When the experiment began, the more concentratedsugar solution was which means “above strength,”as compared to the dilute sugar solution. The dilute sugarsolution was or “below strength.”hypotonic,

hypertonic,

isotonic,

osmosis

Hypotonic comes from theGreek word hupo, meaning“under,” and the New Latinword tonicus, meaning “tension”or “strength.” So a hypotonicsolution is less strong, or lessconcentrated, than anothersolution of the same type. Ifderma means “skin,” howwould you describe a hypo-dermic injection?

Selectively permeablemembrane

Concentratedsugar solution

(Water lessconcentrated)

Sugarmolecules

Movementof water

Dilute sugarsolution(Water moreconcentrated)

� Figure 7–15 Osmosis is the diffusion of water through a selectively permeablemembrane. In the first beaker, water is more concentrated on the right side of the membrane. As a result, the water diffuses (as shown in the second beaker) to the area of lower concentration.

For: Osmosis activityVisit: PHSchool.comWeb Code: cbp-3075

For: Osmosis activityVisit: PHSchool.comWeb Code: cbe-3075Students interact with the art ofosmosis online.

186 Chapter 7

Build Science SkillsApplying Concepts Ask studentsto consider this real-life circumstance.A homeowner contracts a lawn com-pany to add fertilizer to the lawn inorder to make the grass grow better.This process is normally done byspraying a mixture of fertilizer andwater onto the lawn. Ask: Whatwould happen if too much fertil-izer and too little water weresprayed onto the lawn? (Studentsmay know that the grass would appearto be burned.) Can you suggestwhat happened to the cells of thegrass? (They lost water because of theconcentrated solution of fertilizeraround them.) In that case, was the fertilizer-water mixture hypotonicor hypertonic compared to thegrass cells? (The mixture was hyper-tonic compared to the grass cells.)

DemonstrationPlace a small number of paramecia ina petri dish on a microprojector.Have students observe the parameciaas you discuss contractile vacuoles,which some unicellular organismshave to pump water out of the cell.Flood the environment of the para-mecia with distilled water. Asstudents continue to observe, pointout the action of the contractile vacu-oles. Ask: What was added to thedish? (Pure water) How do youknow? (The action of the contractilevacuoles increased.) What will even-tually happen to the paramecia?(They will explode.) Why? (The vacu-oles cannot keep up with the inwardmovement of water because of osmo-sis.) What would happen if a smallamount of salt water were added?(Vacuole action would probably returnto normal.)

Penicillin works by osmosisPenicillin, one of the most important antibioticdrugs in the history of medicine, depends onosmosis for its killing action. Penicillin inhibits anenzyme with which many bacteria producechemical cross-links in their cell walls. This leads to

the formation of a weakened cell wall that cannotstand the stress of osmotic pressure. Gradually,the cell wall becomes weaker and weaker until itbreaks, and the bacterium bursts under the inrushof water caused by osmosis.

FACTS AND FIGURES

Osmotic Pressure For organisms to survive, they must havea way to balance the intake and loss of water. Osmosis exerts apressure known as osmotic pressure on the hypertonic side of aselectively permeable membrane. Osmotic pressure can causeserious problems for a cell. Because the cell is filled with salts,sugars, proteins, and other molecules, it will almost always behypertonic to fresh water. This means that osmotic pressureshould produce a net movement of water into a typical cell thatis surrounded by fresh water. If that happens, the volume of acell will increase until the cell becomes swollen. Eventually, thecell may burst like an overinflated balloon.

Fortunately, cells in large organisms are not in danger ofbursting. Most cells in such organisms do not come in contactwith fresh water. Instead, the cells are bathed in fluids, such as blood, that are isotonic. These isotonic fluids have concentra-tions of dissolved materials roughly equal to those in the cellsthemselves.

Other cells, such as plant cells and bacteria, which do comeinto contact with fresh water, are surrounded by tough cellwalls. The cell walls prevent the cells from expanding, evenunder tremendous osmotic pressure. However, the increasedosmotic pressure makes the cells extremely vulnerable toinjuries to their cell walls.

What structures protect plant and bacterial cells frompotential damage resulting from osmotic pressure?

Solution Animal Cell Plant Cell

Isotonic: The concentration ofsolutes is the same insideand outside the cell.

Hypertonic:Solution has a highersolute concentrationthan the cell.

Hypotonic:Solution has a lowersolute concentrationthan the cell.

Water inWater in

Vacuole

Water out Water out

Water in

Water out

Water in Water out

Cell wall Cellmembrane

The Effects of Osmosis on Cells

� Figure 7–16 Cells placed in anisotonic solution neither gain nor losewater. In a hypertonic solution,animal cells shrink, and plant cellvacuoles collapse. In a hypotonicsolution, animal cells swell and burst.The vacuoles of plant cells swell,pushing the cell contents out againstthe cell wall. Predicting Whatwould happen to the animal cell in theisotonic solution if it were placed in purewater?

7–3 (continued)

Facilitated Diffusion

Cell Structure and Function 187

Facilitated DiffusionA few molecules, such as the sugar glucose, seem to passthrough the cell membrane much more quickly than theyshould. One might think that these molecules are too large ortoo strongly charged to cross the membrane, and yet theydiffuse across quite easily.

How does this happen? The answer is that cell membraneshave protein channels that make it easy for certain molecules tocross the membrane. Red blood cells, for example, have a cellmembrane protein with an internal channel that allows glucoseto pass through it. Only glucose can pass through this channel,and it can move through in either direction. This cell membraneprotein is said to facilitate, or help, the diffusion of glucoseacross the membrane. The process, shown in Figure 7–17, isknown as (fuh-SIL-uh-tayt-ud) Hundreds of different protein channels have been found thatallow particular substances to cross different membranes.

Although facilitated diffusion is fast and specific, it is stilldiffusion. Therefore, a net movement of molecules across a cellmembrane will occur only if there is a higher concentration ofthe particular molecules on one side than on the other side. Thismovement does not require the use of the cell’s energy.

diffusion.facilitated

How can you model permeabilityin cells?

Materials graduated cylinder, plastic sandwichbag, starch, twist tie, 500-mL beaker, iodinesolution

Procedure

1. Pour about 50 mL of water into a plastic sandwichbag. Add 10 mL of starch. Secure the bag with atwist tie, and shake it gently to mix in the starch.

2. Put on your goggles, plastic gloves, and apron.3. Pour 250 mL of water into a 500-mL beaker.

CAUTION: Handle the beaker carefully. Add 15drops of iodine. CAUTION: Iodine is corrosive andirritating to the skin and can stain skin and clothing.Be careful not to spill it on yourself.

4. Place the sandwich bag of water and starch intothe beaker of water and iodine.

5. After 20 minutes, look at the sandwich bag in thebeaker. Observe and record any changes thatoccurred.

Analyze and Conclude1. Using Models What cell structure does the

sandwich bag represent?2. Observing What did you see inside the sandwich

bag? Outside the sandwich bag?3. Inferring Iodine turns blue-black in the presence

of starch. What process do you think occurred thatcaused the results you observed? Explain.

Glucosemolecules

Proteinchannel

� Figure 7–17 During facilitateddiffusion, molecules, such as glucose,that cannot diffuse across the cellmembrane’s lipid bilayer on theirown move through protein channelsinstead. Applying Concepts Doesfacilitated diffusion require the cell touse energy?

Starch solution

Iodine solution

Answers to . . . Cell walls prevent the

cells from expanding, even undertremendous osmotic pressure.

Figure 7–16 The volume of the cellwould increase until it became swollen.Eventually, it could burst like an over-inflated balloon.

Figure 7–17 No

Rate of facilitated diffusionFacilitated diffusion depends on a difference inconcentration. In simple diffusion, that is theonly factor that affects rate. In facilitated diffu-sion, the rate also depends on the number ofspecific carrier protein molecules in the mem-brane, because the diffusing molecules canmove across the membrane only through thoseproteins. An example is the diffusion of glucose

into cells, as described on this page. Such diffu-sion occurs most of the time as facilitateddiffusion. No matter how much the cell “needs”the glucose—no matter how great the differ-ence is in concentration inside and outside thecell—the rate at which the glucose can diffuseinto the cell has a limit because of the limitednumber of glucose carrier protein molecules inthe lipid bilayer.

FACTS AND FIGURES

Objective Students will be able tomake and investigate a model ofpermeability in cells.Skills Focus Using Models,InferringTime 20 minutesAdvance Prep To save time andreduce possible spills, you may wantto prepare the starch mixture andiodine solution yourself in advance.Safety Read the MSDS on iodine.If you prepare the iodine solutionyourself, be sure to wear goggles,plastic gloves, and an apron.Strategy It will take up to 20 min-utes for the iodine to pass throughthe bag and react with the starch,so you may want to set up this labat the beginning of the class andreturn to it at the end. Expected Outcome Studentsshould observe that the water insidethe sandwich bag turned blue-black.The water outside the sandwich bagdid not change.Analyze and Conclude1. The sandwich bag represents thecell membrane.2. The water inside the sandwichbag turned blue-black. The wateroutside the sandwich bag did notchange.3. Diffusion occurred. The iodinemolecules diffused through thesandwich bag and reacted with thestarch inside. The starch moleculeswere too big to pass through thebag, and so there was no reactionoutside the bag.

188 Chapter 7

Active TransportAs powerful as diffusion is, cells sometimes must movematerials in the opposite direction—against a concentra-tion difference. This is accomplished by a process known as

As its name implies, active transportrequires energy. The active transport of small molecules orions across a cell membrane is generally carried out bytransport proteins or “pumps” that are found in the mem-brane itself. Larger molecules and clumps of material canalso be actively transported across the cell membrane byprocesses known as endocytosis and exocytosis. The trans-port of these larger materials sometimes involves changesin the shape of the cell membrane.

Molecular Transport Small molecules and ions arecarried across membranes by proteins in the membranethat act like energy-requiring pumps. Many cells use suchproteins to move calcium, potassium, and sodium ionsacross cell membranes. Changes in protein shape, as shownin Figure 7–19, seem to play an important role in thepumping process. A considerable portion of the energy usedby cells in their daily activities is devoted to providing theenergy to keep this form of active transport working. Theuse of energy in these systems enables cells to concentratesubstances in a particular location, even when the forces ofdiffusion might tend to move these substances in theopposite direction.

active transport.

Crossing the Cell MembraneThe cell membrane regulates what enters and leavesthe cell and also provides protection and support. Thecore of nearly all cell membranes is a double-layeredsheet called a lipid bilayer. Most materials entering thecell pass across this membrane by diffusion. Thegraph shows the sizes of several molecules that candiffuse across a lipid bilayer.

1. Predicting Which substances do you think willdiffuse across the lipid bilayer most quickly? Mostslowly? Explain your answers.

2. Formulating Hypotheses Formulate a hypothe-sis about the relationship between molecule sizeand rate of diffusion.

3. Designing Experiments Design an experimentto test your hypothesis.

0 100Size (daltons)

50

Mo

lecu

le

150

Size of Molecules

200

Carbondioxide

Ethanol

Glucose

Glycerol

Oxygen

Urea

Water

� Figure 7–18 Phagocytosis is oneform of active transport. Duringphagocytosis, extensions of cytoplasmsurround and engulf large particles.Amoebas are one type of organism thatuses this process to take in food andother materials.

Active TransportUse VisualsFigure 7–18 After students havestudied the image and read the cap-tion, ask: Why is phagocytosis anexample of active transport andnot facilitated diffusion?(Phagocytosis requires the input ofenergy, because the organism usesenergy to surround and engulf a largeparticle.)

Build Science SkillsUsing Analogies Set up a rampusing a board propped up on oneend by a stack of books. Then, as stu-dents observe, roll a ball down theramp and push it back up again. Ask:Which is like facilitated diffusionand which is like active transport,rolling the ball down the ramp orpushing it up? Explain. (Rolling theball down is like facilitated diffusion,because neither requires addition ofenergy. Pushing the ball back up is likeactive transport, because it requiresaddition of energy.)

FACTS AND FIGURES

Protein molecules and active transportOne of the most important examples of activetransport is known as the sodium potassiumpump, in which sodium ions are maintained at alower concentration inside the cell and potassiumions are maintained at a higher concentrationinside the cell. The active transport by proteinmolecules of these ions is central to the produc-tion of electrical impulses by nerve cells. At one

time, scientists thought that the protein mol-ecules actually rotated as they transported substances through the cell membrane, pickingup their parcels on the outside and dumpingthem on the inside. Now, scientists think that thetransported molecules are somehow squeezed through the transport proteins, as the proteinschange their configuration to accommodate theirriders.

7–3 (continued)

As students examine the bar graph ofthe different molecules, point outthat some molecules move throughthe lipid bilayer by simple diffusionand others move through by facili-tated diffusion. In most cases, forexample, water moves through selec-tively permeable membranes bydiffusion, whereas glucose movesthrough by facilitated diffusion.

Answers1. Students may predict that waterwill diffuse most quickly because it isthe smallest and glucose will diffusemost slowly because it is the largest.2. Students’ hypotheses may include:The smaller a molecule is, the faster it diffuses.3. Students’ experiments should bedesigned to test their hypotheses and control variables.

Cell Structure and Function 189

Molecule tobe carried

Moleculebeing carried

Energy

� Figure 7–19 Active transport of particles against aconcentration difference requires transport proteins andenergy. Interpreting Graphics What is happening inthe illustration?

Endocytosis and Exocytosis Larger moleculesand even solid clumps of material may be transported bymovements of the cell membrane. One of these move-ments is called endocytosis (en-doh-sy-TOH-sis).

is the process of taking material into thecell by means of infoldings, or pockets, of the cell mem-brane. The pocket that results breaks loose from theouter portion of the cell membrane and forms a vacuolewithin the cytoplasm. Large molecules, clumps of food,and even whole cells can be taken up in this way. Twoexamples of endocytosis are phagocytosis (fag-oh-sy-TOH-sis) and pinocytosis (py-nuh-sy-TOH-sis).

Phagocytosis means “cell eating.” In extensions of cytoplasm surround a particle and packageit within a food vacuole. The cell then engulfs it.Amoebas use this method of taking in food. Engulfingmaterial in this way requires a considerable amount ofenergy and, therefore, is correctly considered a form ofactive transport.

In a process similar to endocytosis, many cells takeup liquid from the surrounding environment. Tinypockets form along the cell membrane, fill with liquid,and pinch off to form vacuoles within the cell. Thisprocess is known as

Many cells also release large amounts of materialfrom the cell, a process known as exocytosis (ek-soh-sy-TOH-sis). During the membrane of thevacuole surrounding the material fuses with the cellmembrane, forcing the contents out of the cell. Theremoval of water by means of a contractile vacuole is one example of this kind of active transport.

exocytosis,

pinocytosis.

phagocytosis,

Endocytosis

1. Key Concept Describe thefunctions of the cell membraneand cell wall.

2. Key Concept Whathappens during diffusion?

3. Key Concept Describehow water moves during osmosis.

4. What is the basic structure of acell membrane?

5. What is the difference betweenphagocytosis and pinocytosis?

6. Critical Thinking Comparingand Contrasting What is themain way that active transportdiffers from diffusion?

HomeostasisWhat is the relationshipbetween active transport andhomeostasis? Give oneexample of active transport inan organism, and explain howthe organism uses energy tomaintain homeostasis. Youmay want to refer to Section 1–3.

7–3 Section Assessment

For: Active Transport activityVisit: PHSchool.comWeb Code: cbp-3076

3 ASSESSEvaluate UnderstandingMake up a list of fictitious substances.Have students describe the methodof transport a cell would use to moveeach substance through the cellmembrane and why. Students neednot be accurate; what you are look-ing for is correct reasoning.

ReteachHave students write definitions of dif-fusion, osmosis, facilitated diffusion,and active transport. Then, discussways in which they are similar andways in which they are different.

Answer to . . .

Figure 7–19 A molecule is movingacross the cell membrane from an areaof low concentration to an area of highconcentration with the help of a trans-port protein and the use of energy.

If your class subscribes to theiText, use it to review the KeyConcepts in Section 7–3.

Students should state the definitionof both active transport and homeo-stasis, which they learned about inSection 1–3. Examples may vary.Students might suggest that anymaterial taken into a cell by activetransport that is necessary for nor-mal cell activities helps maintainhomeostasis, and active transport isa process that uses energy.

7–3 Section Assessment1. The cell membrane regulates what enters

and leaves the cell and also provides protec-tion and support. The cell wall providessupport and protection for the cell.

2. Particles tend to move from an area wherethey are more concentrated to an areawhere they are less concentrated.

3. Osmosis is the diffusion of water through aselectively permeable membrane.

4. The basic structure is a double-layered sheet

called a lipid bilayer, in which proteins areembedded.

5. In phagocytosis, extensions of cytoplasm sur-round a particle and package it within a foodvacuole. In pinocytosis, tiny pockets formalong the cell membrane, fill with liquid, andpinch off to form vacuoles within the cell.

6. Active transport requires the input of ener-gy, but diffusion does not require additionalenergy.

For: Active Transport activityVisit: PHSchool.comWeb Code: cbe-3076Students interact with the art ofactive transport online.

190 Chapter 7

Earth is sometimes called a living planet, and for good reason.From its simple beginnings, life has spread to every corner of

the globe, penetrating deep into the earth and far beneath thesurface of the seas. The diversity of life is so great that you mighthave to remind yourself that all living things are composed ofcells, use the same basic chemistry, follow the same genetic code,and even contain the same kinds of organelles. This does notmean that all living things are the same. It does mean that theirdifferences arise from the ways in which cells are specialized toperform certain tasks and the ways in which cells associate witheach other to form multicellular organisms.

Unicellular OrganismsCells are the basic living units of all organisms, but sometimes asingle cell is a little more than that. Sometimes, a cell is theorganism. A single-celled organism is also called a unicellularorganism. Unicellular organisms do everything that you wouldexpect a living thing to do. They grow, respond to the environ-ment, transform energy, and reproduce. In terms of their num-bers, unicellular organisms dominate life on Earth. Someexamples of unicellular organisms are shown in Figure 7–20.

Multicellular OrganismsOrganisms that are made up of many cells are called multicellu-lar. There is a great variety among multicellular organisms.However, all multicellular organisms depend on communicationand cooperation among specialized cells. Cells through-out an organism can develop in different ways to performdifferent tasks. This process is called Some examples of specialized cells are shown in Figure 7–21.

cell specialization.

7– 4 The Diversity of Cellular Life

Key Concepts• What is cell specialization? • What are the four levels of

organization in multicellularorganisms?

Vocabularycell specializationtissueorganorgan system

Reading Strategy: Using Visuals Before youread, preview Figure 7–22.As you read, note the differentlevels of organization in thebody.

Figure 7–20 Yeasts, which are oftenused in bread making, are unicellularfungi. The Volvox aureus cells shown areactually individual alga cells that livetogether in a colony. The unicellularspiral-shaped bacterium Leptospirainterrogans causes a serious disease inhumans.

Volvox aureus (magnification: 250�)Yeast (magnification: 3400�) Leptospira interrogans (magnification: 27,000�)

1 FOCUSObjectives7.4.1 Describe cell specialization.7.4.2 Identify the organization lev-

els in multicellular organisms.

Vocabulary PreviewOn the chalkboard, make a conceptmap entitled Cell Specialization,using the section’s Vocabulary terms.The map should show this hierarchy:cell, tissue, organ, organ system.

Reading StrategyHave the students rewrite the sec-tion’s blue headings as questions.Then, have them answer those ques-tions as they read the section.

2 INSTRUCT

UnicellularOrganismsUse VisualsFigure 7–20 Have students read thecaption and study the unicellularorganisms. Explain that the yeastcells are a form of fungi, which stu-dents will study in Chapter 21. Volvoxaureus is a plantlike protist, whichstudents will read about in Chapter20. The disease caused by the bac-terium shown is called leptospirosis,which is mostly a disease of wild ani-mals but can affect humans as well.Then, ask: Which of these organ-isms are prokaryotic, and which areeukaryotic? (The yeast cells and theVolvox aureus are eukaryotic. The bac-terium Leptospira interrogans isprokaryotic). Emphasize that not allunicellular organisms are bacteria orprokaryotic. SECTION RESOURCES

Print:

• Laboratory Manual B, Chapter 7 Lab• Teaching Resources, Section Review 7–4,

Enrichment• Reading and Study Workbook A,

Section 7–4• Adapted Reading and Study Workbook B,

Section 7–4• Lesson Plans, Section 7–4

Technology:

• iText, Section 7–4• Transparencies Plus, Section 7–4

Tim

eSaver

Section 7–4

Cell Structure and Function 191

FIGURE 7–21 CELL SPECIALIZATION

Specialized Animal Cells Animal cells are specialized inmany ways. Red blood cells are specialized to transport oxygen.Red blood cells contain a protein that binds to oxygen in thelungs and transports the oxygen throughout the body where it isreleased. Cells specialized to produce proteins, for example, arefound in the pancreas. The pancreas is a gland that producesenzymes that make it possible to digest food. As you mightexpect, pancreatic cells are packed with ribosomes and roughER, which are where proteins are produced. Pancreatic cells alsopossess large amounts of other organelles needed for proteinexport, including a well-developed Golgi apparatus and clustersof storage vacuoles loaded with enzymes.

The human ability to move is result of the specialized struc-tures of muscle cells. These cells generate force by using a dramat-ically overdeveloped cytoskeleton. Skeletal muscle cells are packedwith fibers arranged in a tight, regular pattern. Those fibers areactin microfilaments and a cytoskeletal protein called myosin.When they contract, muscle cells use chemical energy to pull thesefibers past each other, generating force. Whether your muscles arelarge or small, your muscle cells themselves are “bulked up” withthese specialized cytoskeletal proteins to a degree that makesthem the body’s undisputed heavy-lifting champions.

Cells in multicellular organisms are specialized to perform particular functions within the organism. Red blood cells transport oxygen through-out the body. Pancreatic cells produce compoundssuch as insulin that the body needs. Muscle cellscontract and relax to move parts of the body. Guardcells control the opening and closing of stomata onthe undersides of leaves.

Pancreatic Cell (magnification: 4000�) Muscle Cell (magnification: 350�) Stomata (magnification: 510�)

Red Blood Cells (magnification: 13,000�)

MulticellularOrganismsUse VisualsFigure 7–21 Discuss with studentshow each of the cells shown is spe-cialized, carrying out only one or afew particular functions within theorganism. Have students contrastthese cells with those shown inFigure 7–20. Emphasize that the cellsof unicellular organisms must carryout all the essential functions of life,whereas the cells of multicellularorganisms are specialists.

Build Science SkillsUsing Analogies Introduce stu-dents to an analogy between the waycells evolved toward greater special-ization and the way society evolvedtoward greater specialization duringthe Industrial Revolution. Just as cellsbecame specialized to perform par-ticular functions, people left theirgeneralized lives in rural areas to takejobs in urban industries that pro-duced particular goods or fulfilledparticular functions. Challenge stu-dents to develop this analogy, eitherin a class discussion or in individualessays.

Comprehension: Modified ClozeBeginning Distribute a modified paragraphabout levels of organization, but substituteblanks for some strategic words. For example,“There are four levels of organization in many-celled organisms. Cells that work togetherform _____. Tissues that work together form_____. Organs that work together form _____.”Give students a list of the correct answers, andhave them fill in each blank with one of the

words. Post these terms on a word wall withother Vocabulary terms from the chapter. Intermediate Have the students complete thebeginning-level cloze activity, but add severalsentences and terms. When students havecompleted the exercise, they can work in pairsto correct each other’s answers. The studentsin each pair can also extend the activity bywriting two examples of types of cells, tissues,organs, and organ systems.

SUPPORT FOR ENGLISH LANGUAGE LEARNERS

192 Chapter 7

Specialized Plant Cells A plant basking in the sunlightmay seem quiet and passive, but it is actually interacting withthe environment at every moment. It rapidly exchanges carbondioxide, oxygen, water vapor, and other gases through tinyopenings called stomata on the undersides of leaves. Highlyspecialized cells, known as guard cells, regulate this exchange.Guard cells monitor the plant’s internal conditions, changingtheir shape according to those conditions. For example, when theplant can benefit from gas exchange, the stomata open. Thestomata close tightly when the plant’s internal conditions change.

Levels of OrganizationBiologists have identified levels of organization that make iteasier to describe the cells within a multicellular organism.

The levels of organization in a multicellular organ-ism are individual cells, tissues, organs, and organ sys-tems. These levels of organization are shown in Figure 7–22.

Tissues In multicellular organisms, cells are the first level oforganization. Similar cells are grouped into units called tissues. A

is a group of similar cells that perform a particularfunction. The collection of cells that produce digestive enzymes inthe pancreas makes up one kind of tissue. Most animals have fourmain types of tissue: muscle, epithelial, nervous, and connectivetissue. You will read about these tissues in later chapters.

tissue

Careers in Biology

For: Career linksVisit: PHSchool.comWeb Code: cbb-3074

HistotechnologistJob Description: work in a hospital laboratory,research institution, industrial laboratory, orgovernment agency to prepare slides of bodytissues for microscopic examination usingspecial dyes and more advanced techniques,such as electron microscopy

Education: a bachelor’s degree from a certifiedhistotechnology program or a bachelor'sdegree with emphasis in biology and chem-istry and one year’s experience under a board-certified pathologist to become eligible fornational certification exam, leading to ahistotechnologist (HTL) certification

Skills: background in biology, anatomy,pathology, and/or chemistry; manual dexterity;attention to detail; good organizational skills;strong writing skills; computer literacy

Levels ofOrganizationBuild Science SkillsApplying Concepts Show studentsphotographs, diagrams, or slides ofvarious specialized cells. Discuss theirspecialization and what makes themuniquely suited to their function. Forexample, you might show humanred blood cells. Then, ask: Howwould you describe the shape ofthese cells? (They all have a disklikeshape.) What important functiondo red blood cells have? (Studentsmay know that red blood cells carryoxygen.) Contrast these cells withcells from epithelial tissue. Elicit fromstudents the idea that epithelial cellsare closely packed and regular inappearance, a structure that suitstheir function of protection.

When teaching cell structure, I ask small groupsof students to research the structure and func-tion of a specific organelle, using a variety ofsources, including college texts and relevantInternet sites. The members of each group worktogether to prepare a report to the class, com-plete with a detailed model of the organelle theyresearched. I have groups present their reports inthe form of a justification of the importance oftheir organelles to their “parent organization,”

Cell Industries, with the understanding that theymight be “downsized” if they don’t make theircase convincingly. Groups must cite the source ofany information they present. The models stu-dents make are creative, and they are a stronglearning reinforcement tool.

—Tamsen K. MeyerBiology TeacherBoulder High SchoolBoulder, CO

TEACHER TO TEACHER

7–4 (continued)

This is one of the many careers thatare little known to the public but areessential to the functioning of a goodhealth facility. • Have students find out what kind of

technology is being used by histo-technologists at a local hospital,clinic, or university. Students cangather information by interviewing alocal histotechnician or administrator.

Resources Have interested studentscontact a local college or universityto see if it has a certified histotech-nology program. They might alsogather information about this careerby contacting the National Societyfor Histotechnology.

Careers in Biology

You can have students write amore extensive job description aswell as list the educational require-ments for a career in this field.

Cell Structure and Function 193

Build Science SkillsClassifying To reinforce the conceptof levels of organization, play a gameof Name That Cell, Tissue, Organ, orSystem. Give students as many exam-ples as you can in random order. Askthem to identify whether the exampleis a cell, a tissue, an organ, or anorgan system.

3 ASSESSEvaluate UnderstandingAsk students to explain the levels oforganization involved in touch. (Atypical response might mention individ-ual nerve cells, or receptors; braintissue; the brain; the nervous system.)

ReteachDirect students’ attention to Figure7–21 on page 191. Ask them whichlevel of organization is representedby the cells shown. (Individual cells)Then, for each kind of specializedcell, discuss the tissue, organ, andorgan system of which that cell is apart.

Organs Many tasks within the body are too complicated to becarried out by just one type of tissue. In these cases, many groupsof tissues work together as an For example, each musclein your body is an individual organ. Within a muscle, however,there is much more than muscle tissue. There are nerve tissuesand connective tissues. Each type of tissue performs an essentialtask to help the organ function.

Organ Systems In most cases, an organ completes a seriesof specialized tasks. A group of organs that work together toperform a specific function is called an

The organization of the body’s cells into tissues, organs, andorgan systems creates a division of labor among those cells thatmakes multicellular life possible. Specialized cells such as nerveand muscle cells are able to function precisely because othercells are specialized to obtain the food and oxygen needed bythose cells. This overall specialization and interdependence isone of the remarkable attributes of living things. Appreciatingthis characteristic is an important step in understanding thenature of living things.

organ system.

organ.

Digestive system Stomach Smooth muscle tissue Muscle cell

Figure 7–22 The levels of organization in a multicel-lular organism are individual cells, tissues, organs, andorgan systems. In this example, muscle cells make upsmooth muscle tissue, which, along with other tissues,makes up the stomach, an organ. The stomach, in turn, is part of an organ system, the digestive system.

1. Key Concept In whatkinds of organisms is cell special-ization a characteristic?

2. Key Concept List thelevels of biological organizationin multicellular organisms frommost simple to most complex.

3. How are unicellular organismssimilar to multicellular organisms?

4. Critical Thinking PredictingUsing what you know about theways muscle moves, predictwhich organelles would be mostcommon in muscle cells.

Using AnalogiesUse an organized area in yourlife—such as school, sports, orextracurricular activities—toconstruct an analogy toexplain how the levels oforganization in that chosenarea can be compared withthose of living organisms.

7– 4 Section Assessment

If your class subscribes to theiText, use it to review the KeyConcepts in Section 7–4.

Make sure that each student’sanalogy appropriately relates anarea of the student’s life to the lev-els of organization in living things.

7–4 Section Assessment1. Multicellular organisms have cell special-

ization.2. Individual cells, tissues, organs, and organ

systems3. Both unicellular and multicellular organisms

grow, respond to the environment, transformenergy, and reproduce.

4. Muscle cells have a large number of mito-chondria, because mitochondria releaseenergy from stored food molecules and mus-cle cells need great amounts of energy to dothe tasks they do.