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.