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

For: Diffusion activityVisit: PHSchool.comWeb Code: cbp-3073

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.