Chapter 8: Cellular Transport and the Cell...

194Jean Claude Revy/Phototake, NYC

What You’ll Learn■ You will discover how mole-

cules are transported acrossthe plasma membrane.

■ You will sequence the stagesof cell division.

■ You will identify the relation-ship between the cell cycleand cancer.

Why It’s ImportantTransportation of molecules andparticles through the plasmamembrane and cell reproductionare two important functionsthat help cells maintain homeo-stasis and keep you healthy.

Cellular Transport and the Cell CycleCellular Transport and the Cell Cycle

Visit to• study the entire chapter

online• access Web Links for more

information and activities onthe cell cycle

• review content with theInteractive Tutor and self-check quizzes

This photo shows a cell in aplant’s root tip in one stage ofthe cell cycle. Color enhance-ment helps distinguish the chro-mosomes, which appear yellowin this photo.

Understandingthe Photo

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Color-enhanced TEM Magnification: 1600�

Cell wall

Chromosomes

Spindle fibers

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Cellular Transport

Osmosis: Diffusion of WaterAlthough the plasma membrane of a cell can act as a dam or pump for

water-soluble molecules that cannot pass freely through the membrane, itdoes not limit the diffusion of water. Recall that diffusion is the move-ment of particles from an area of higher concentration to an area of lowerconcentration. In a cell, water always moves to reach an equal concentra-tion on both sides of the membrane. The diffusion of water across a selec-tively permeable membrane is called osmosis (ahs MOH sus). Regulatingthe water flow through the plasma membrane is an important factor inmaintaining homeostasis within the cell.

What controls osmosis?If you add sugar to water, the water becomes sweeter as you add more

sugar. If a strong sugar solution and a weak sugar solution are placed indirect contact, water molecules diffuse in one direction and sugar mole-cules diffuse in the other direction until all molecules are evenly distrib-uted throughout.

osmosis from theGreek word osmos,meaning “push-ing”; Osmosis canpush out a cell’splasma membrane.

8.1 CELLULAR TRANSPORT 195

Answer Questions Before you read Chapter 8, write under each tab what you already know about how osmosis affects cells. After you read the chapter, list what you learned about how osmosis affects cells in each type of solution listed on your Foldable.

Fold a vertical sheet of paper from side to side. Make the back edge about 2 cm longer than the front edge.

Turn lengthwise and fold into thirds.

Unfold and cut only the top layer along both folds to make three tabs.

Label each tab.

Osmosis Make the following Foldable to help identify what you already know about osmosis, and what you learned about how osmosis affects cells.

STEP 1

STEP 3

STEP 2

STEP 4

IsotonicSolution

HypotonicSolution

HypertonicSolution

How osmosis affects cells in...

SECTION PREVIEWObjectivesExplain how the processesof diffusion, passive trans-port, and active transportoccur and why they areimportant to cells.Predict the effect of ahypotonic, hypertonic, orisotonic solution on a cell.

Review Vocabularyplasma membrane: the

boundary between thecell and its environment(p. 175)

New Vocabularyosmosisisotonic solutionhypotonic solutionhypertonic solutionpassive transportfacilitated diffusionactive transportendocytosisexocytosis

8.1

If the two solutions are separated bya selectively permeable membranethat allows only water to diffuse acrossit, water flows to the side of the mem-brane where the water concentrationis lower. The water continues to dif-fuse until it is in equal concentrationon both sides of the membrane, asshown in Figure 8.1. Therefore, weknow that unequal distribution of par-ticles, called a concentration gradient,is one factor that controls osmosis.

Cells in an isotonic solution It is important to understand how

osmosis affects cells. Most cells,whether in multicellular or unicellu-lar organisms, are subject to osmosisbecause they are surrounded by watersolutions. In an isotonic solution,the concentration of dissolved sub-stances in the solution is the same asthe concentration of dissolved sub-stances inside the cell. Likewise, theconcentration of water in the solutionis the same as the concentration ofwater inside the cell.

Cells in an isotonic solution doexperience osmosis, but because waterdiffuses into and out of the cells at thesame rate, the cells retain their normalshape, as shown in Figure 8.2.

Cells in a hypotonic solutionIn the hypotonic solution in

Figure 8.3A, the concentration of dissolved substances is lower in the solution outside the cell than the concentration inside the cell.Therefore, there is more water out-side the cell than inside. Cells in ahypotonic solution experience osmo-sis. Water moves through the plasmamembrane into the cell. The cellswells and its internal pressureincreases.

As the pressure increases inside animal cells, the plasma membraneswells, like the red blood cells shownin Figure 8.3B. If the solution isextremely hypotonic, the plasma mem-brane may be unable to withstand thispressure and may burst.

Because plant cells contain a rigidcell wall that supports the cell, theydo not burst when in a hypotonicsolution. As the pressure increasesinside the cell, the plasma membraneis pressed against the cell wall, asshown in Figure 8.3C. Instead ofbursting, the plant cell becomes morefirm. Grocers keep produce lookingfresh by misting the fruits and vegeta-bles with water.

Cells in a hypertonic solutionIn a hypertonic solution, the

concentration of dissolved sub-stances outside the cell is higher thanthe concentration inside the cell.Cells in a hypertonic solution experi-ence osmosis that causes water toflow out.

Animal cells in a hypertonic solu-tion shrivel because of decreasedpressure in the cells.

196 CELLULAR TRANSPORT AND THE CELL CYCLE

Water moleculeSugar molecule

Selectivelypermeablemembrane

Before osmosis After osmosis

Figure 8.1During osmosis, water diffuses across a selectively permeable membrane.Notice that the number of sugar molecules did not change on each sideof the membrane, but the number of water molecules on either side ofthe membrane did change.

iso-, hypo-, hyper-from the Greekwords isos, mean-ing “equal,” hypo,meaning “under,”and hyper, mean-ing “over,” respectively.

8.1 CELLULAR TRANSPORT 197Joseph Kurantsin-Mills, M.Sc.Ph.D

Water moleculeDissolved molecule

H2OH2O

Figure 8.2In an isotonic solu-tion, water mole-cules move into andout of the cell at thesame rate, and cellsretain their normalshape (A). Noticethe concave discshape of a red bloodcell (B). A plant cellhas its normal shapeand pressure in anisotonic solution (C).

Figure 8.3In a hypotonic solu-tion, water enters acell by osmosis, caus-ing the cell to swell(A). Animal cells,like these red bloodcells, may continueto swell until theyburst (B). Plant cellsswell beyond theirnormal size as pres-sure increases (C).

Figure 8.4In a hypertonic solu-tion, water leaves acell by osmosis, caus-ing the cell to shrink(A). Animal cells likethese red blood cellsshrivel up as theylose water (B). Plantcells lose pressure asthe plasma mem-brane shrinks awayfrom the cell wall (C).

H2OH2O


H2OH2O


Magnification: unavailable


AA BB CC

AA BB CC


AA BB CC

Plant cells in a hypertonic environ-ment lose water, mainly from the cen-tral vacuole. The plasma membraneand cytoplasm shrink away from thecell wall, as shown in Figure 8.4C. Lossof water in a plant cell results in a dropin pressure and explains why plants wilt.

Passive TransportSome molecules, like water, can pass

through the plasma membrane by sim-ple diffusion, as shown in Figure 8.5A.The cell uses no energy to move theseparticles; therefore, this movement ofparticles across the membrane is classi-fied as passive transport. You caninvestigate passive transport by per-forming the MiniLab on this page.

Passive transport by proteinsRecall that transport proteins help

substances move through the plasmamembrane. Passive transport of mate-rials across the membrane using trans-port proteins is called facilitateddiffusion.

Some transport proteins, calledchannel proteins, form channels thatallow specific molecules to flowthrough, as illustrated in Figure 8.5B.

198 CELLULAR TRANSPORT AND THE CELL CYCLEKS Studio

Figure 8.5Passive transportcan occur by (A)simple diffusion,(B) facilitated dif-fusion by channelproteins, and (C)facilitated diffusionby carrier proteins.

Plasmamembrane

Concentrationgradient

Plasmamembrane


Carrier proteins

Step 1 Step 2

Plasmamembrane


Channelproteins

AA BB

CC

Formulate ModelsCell Membrane Simulation In thisexperiment, a plastic bag is used tomodel a selectively permeable mem-brane. Starch is placed inside of thebag. When iodine and starch mole-cules come in contact with oneanother, a dark purple color results.

Procedure! Fill a plastic bag with 50 mL of

starch. Seal the bag with a twist tie.

@ Fill a beaker with 50 mL of iodine solution. CAUTION:Rinse with water if iodine gets on skin. Iodine is toxic.

# Note and record the color of the starch and iodine.$ Place the bag into the beaker. CAUTION: Wash your hands

with soap after handling lab materials.% Note and record the color of the starch and iodine

24 hours later.

Analysis1. Describe Compare the color of the iodine and starch at

the start and at the conclusion of the experiment.2. Observe Which molecules crossed the membrane? What

is your evidence?3. Think Critically Evaluate whether or not a plastic bag is

an adequate model of a selectively permeable membrane.

The movement is with the concentra-tion gradient, and requires no energyinput from the cell.

Carrier proteins are another typeof transport protein. Carrier proteinschange shape to allow a substance topass through the plasma membrane,as shown in Figure 8.5C. In facili-tated diffusion by carrier protein, themovement is with the concentrationgradient and requires no energy inputfrom the cell.

Active TransportA cell can move particles from a

region of lower concentration to aregion of higher concentration, but itmust expend energy to counteract theforce of diffusion that is moving theparticles in the opposite direction.Movement of materials through amembrane against a concentrationgradient is called active transportand requires energy from the cell.

How active transport occursIn active transport, a transport

protein called a carrier protein firstbinds with a particle of the substanceto be transported. In general, eachtype of carrier protein has a shapethat fits a specific molecule or ion.When the proper molecule bindswith the protein, chemical energy

allows the cell to change the shape ofthe carrier protein so that the particleto be moved is released on the otherside of the membrane, something likethe opening of a door. Once the par-ticle is released, the protein’s originalshape is restored, as illustrated inFigure 8.6. Active transport allowsparticle movement into or out of acell against a concentration gradient.

Transport of substances across thecell membrane is required for cells tomaintain homeostasis. The types oftransport are summarized in Table 8.1.

Compare and contrast active and passive trans-port across the cell membrane.

8.1 CELLULAR TRANSPORT 199

Carrier proteins

Cellularenergy

Step 1 Step 2

Plasmamembrane


+

Table 8.1 Transport Through the Cell Membrane

Type of Transport Direction of Requires Energy ClassificationTransport Protein Used? Movement Input from Cell? of Transport

Simple No With No PassiveDiffusion concentration

gradient

Facilitated Yes—channel With No PassiveDiffusion proteins or concentration

carrier proteins gradient

Active Yes—carrier Against Yes ActiveTransport proteins concentration

gradient

Figure 8.6Carrier proteins are used in active transport to pickup ions or molecules from near the cell membrane,carry them across the membrane, and release themon the other side. Think Critically Why does activetransport require energy?

Transport of LargeParticles

Some cells can take in large mole-cules, groups of molecules, or evenwhole cells. Endocytosis is a processby which a cell surrounds and takes inmaterial from its environment asshown in Figure 8.7. This materialdoes not pass directly through themembrane. Instead, it is engulfed andenclosed by a portion of the cell’splasma membrane. That portion of themembrane then breaks away, and theresulting vacuole with its contentsmoves to the inside of the cell.

Figure 8.7 also shows the reverseprocess of endocytosis, called exocy-tosis. Exocytosis is the expulsion orsecretion of materials from a cell.Cells use exocytosis to expel wastes.They also use this method to secretesubstances, such as hormones pro-duced by the cell. Because endocyto-sis and exocytosis both move massesof material, they both requireenergy.

With the various mechanisms thecell uses to transport materials in andout, cells must also have mechanismsto regulate size and growth.

Understanding Main Ideas1. What factors affect the diffusion of water

through a membrane by osmosis?

2. How do animal cells and plant cells react differently in a hypotonic solution?

3. Compare and contrast active transport and facilitated diffusion.

4. How do carrier proteins facilitate passive transportof molecules across a membrane?

Thinking Critically5. A paramecium expels water when it is in fresh-

water. What can you conclude about the concen-tration gradient in the organism’s environment?

6. Observe and Infer What effect do you think atemperature increase has on osmosis? For morehelp, refer to Observe and Infer in the SkillHandbook.

SKILL REVIEWSKILL REVIEW


Exocytosis

Nucleus

Wastes

Digestion

Endocytosis

Figure 8.7Some unicellular organisms ingest food byendocytosis and release wastes or cell prod-ucts from a vacuole by exocytosis.

endo-, exo- fromthe Greek wordsendon, meaning“within,” and exo,meaning “out”;Endocytosis movesmaterials into thecell; exocytosismoves materialsout of the cell.

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8.2 SECTION PREVIEWObjectivesSequence the events ofthe cell cycle.Relate the function of acell to its organization intissues, organs, and organsystems.

Review Vocabularyorganelle: the membrane-

bound structures withineukaryotic cells (p. 173)

New Vocabularychromosomechromatincell cycleinterphasemitosisprophasesister chromatidcentromerecentriolespindlemetaphaseanaphasetelophasecytokinesistissueorganorgan system

8.2 CELL GROWTH AND REPRODUCTION 201

Cell Size LimitationsThe cells that make up a multicellular organism come in a wide variety

of sizes and shapes. Some cells, such as red blood cells, measure only8 �m (micrometers) in diameter. Other cells, such as nerve cells in largeanimals, can reach lengths of up to 1 m but have small diameters. The cellwith the largest diameter is the yolk of an ostrich egg measuring 8 cm.Most living cells, however, are between 2 and 200 µm in diameter.Considering this wide range of cell sizes, why then can’t most organismsbe just one giant cell?

Diffusion limits cell sizeYou know that the plasma membrane allows nutrients to enter the

cell and wastes to leave. Within the cell, nutrients and wastes move bydiffusion.

Although diffusion is a fast and efficient process over short distances, itbecomes slow and inefficient as the distances become larger. Imagine amitochondrion at the center of a cell with a diameter of 20 cm. It wouldhave to wait months before receiving molecules entering the cell. Becauseof the slow rate of diffusion, organisms can’t be just one giant-sized cell.

What makes up your body?Using an Analogy Where do you live? This question sounds simpleenough, but it has many answers. You live at a certain address, whichis a part of a city. Many cities and towns form the state in which youlive. The states form a country. Some tasks are performed by thecountry as a whole, while others are performed by states, cities, orindividuals. In thesame way, your bodycells are parts of tis-sues, organs, organsystems, and the bodyas a whole. Compare and ContrastCells in multicellular andunicellular organismsundergo cell division.Which type of cells do youthink is more specialized?

Cell Growth andReproduction

DNA limits cell sizeYou have learned that the nucleus

contains blueprints for the cell’s pro-teins. Proteins are used throughoutthe cell by almost all organelles toperform critical cell functions. Butthere is a limit to how quickly theblueprints for these proteins can becopied in the nucleus and made intoproteins in the cytoplasm. The cellcannot survive unless there is enoughDNA to support the protein needs ofthe cell.

What happens in larger cells wherean increased amount of cytoplasmrequires increased supplies of enzymes?In many large cells, such as the giantamoeba Pelomyxa shown in Figure 8.8,more than one nucleus is present.Large amounts of DNA in many nucleiensure that cell activities are carried outquickly and efficiently.

Surface area-to-volume ratioAnother size-limiting factor is the

cell’s surface area-to-volume ratio. Asa cell’s size increases, its volumeincreases much faster than its surfacearea. Picture a cube-shaped cell likethose shown in Figure 8.9. The small-est cell has 1 mm sides, a surface areaof 6 mm2, and a volume of 1 mm3. Ifthe side of the cell is doubled to 2 mm,the surface area will increase fourfoldto 6 � 2 � 2 � 24 mm2. Observe whathappens to the volume; it increaseseightfold to 8 mm3.

What does this mean for cells?How does the surface area-to-volumeratio affect cell function? If cell sizedoubled, the cell would require eighttimes more nutrients and would haveeight times more waste to excrete.

202 CELLULAR TRANSPORT AND THE CELL CYCLEMichael Abbey/Visuals Unlimited

4 mm

4 mm4 mm

2 mm

2 mm2 mm

1 mm

1 mm1 mm

Surface area = 6 mm2

Volume = 1 mm3

Surface area = 24 mm2

Volume = 8 mm3

Figure 8.9Surface area-to-volume ratio is one of the factors that limits cell size.Note how the surface area and the volume change as the sides of a celldouble in length from 1 mm to 2 mm.

Figure 8.8This giant amoeba isonly several millimetersin diameter, but it canhave up to 1000 nuclei.Explain How doesthis benefit theorganism?

LM Magnification: 100�


The surface area, however, wouldincrease by a factor of only four.Thus, the plasma membrane wouldnot have enough surface areathrough which oxygen, nutrients, andwastes could diffuse. The cell wouldeither starve to death or be poisonedfrom the buildup of waste products.You can investigate surface area-to-volume ratios yourself in theProblem-Solving Lab shown here.

Because cell size can have dramaticand negative effects on a cell, cellsmust have some method of maintain-ing optimum size. In fact, cells dividebefore they become too large to func-tion properly. Cell division accom-plishes other purposes, too, as youwill read next.

Cell ReproductionRecall that the cell theory states

that all cells come from preexistingcells. Cell division is the process bywhich new cells are produced fromone cell. Cell division results in twocells that are identical to the origi-nal, parent cell. Right now, as youare reading this page, many of thecells in your body are growing,dividing, and dying. Old cells on the soles of your feet and on thepalms of your hands are being shedand replaced, cuts and bruises arehealing, and your intestines are pro-ducing millions of new cells eachsecond. New cells are produced astadpoles become frogs, and as an ivyvine grows and wraps around agarden trellis. All organisms growand change; worn-out tissues arerepaired or are replaced by newlyproduced cells.

Explain two reasonswhy cell division is a required cellprocess.

The discovery of chromosomesEarly biologists observed that just

before cell division, several short,stringy structures suddenly appearedin the nucleus. Scientists also noticedthat these structures seemed to vanishsoon after division of a cell. Thesestructures, which contain DNA andbecome darkly colored when stained,are called chromosomes (KROH muhsohmz).

Eventually, scientists learned thatchromosomes are the carriers of thegenetic material that is copied andpassed from generation to generationof cells. This genetic material is cru-cial to the identity of the cell.Accurate transmission of chromo-somes during cell division is critical.

chromosome fromthe Greek wordschroma, meaning“colored,” andsoma, meaning“body”; Chromo-somes are dark-staining structuresthat containgenetic material.

Draw ConclusionsWhat happens to the surface area of a cell as its volumeincreases? One reason cells are small is that they need alarge surface area as compared to volume so nutrients can dif-fuse in and wastes can diffuse out.

Solve the Problem Look at the cubes shown below. Note the size and magnitudeof difference in surface area and volume.

Thinking Critically1. Estimate How many small cubes (1 mm) do you think it

would take to fill the largest cube (4 mm)?2. Use Models Using the cubes as models, describe how a

cell is affected by its size. 3. Infer Explain how a small change in cell size can have a

huge impact on cellular processes.

4 mm4 mm

4 mm

2 mm

2 mm

2 mm1 mm

1 mm

1 mm

Surface area � 6 mm2

Volume � 1 mm3Surface area � 24 mm2

Volume � 8 mm3

The structure of eukaryotic chromosomes

For most of a cell’s lifetime, chro-mosomes exist as chromatin, longstrands of DNA wrapped around

proteins called histones. Under anelectron microscope, chromatinlooks like beads on a string. Eachbead is a group of histones called anucleosome. Before a cell can divide,the long strands of chromatin mustbe reorganized, just as you would coila long strand of rope before storingit. As the nucleus begins to divide,chromosomes take on a differentstructure in which the chromatinbecomes tightly packed. Look atFigure 8.10 for more information onchromosome structure.

The Cell CycleFall follows summer, night follows

day, and low tide follows high tide.Many events in nature follow a recur-ring, cyclical pattern. Living organismsare no exception. One cycle commonto most living things is the cycle of thecell. The cell cycle is the sequence ofgrowth and division of a cell.

As a cell proceeds through its cycle,it goes through two general periods: aperiod of growth and a period of divi-sion. The majority of a cell’s life isspent in the growth period known asinterphase. During interphase, a cellgrows in size and carries on metabo-lism. Also during this period, chromo-somes are duplicated in preparationfor the period of division.

Following interphase, a cell entersits period of nuclear division calledmitosis (mi TOH sus). Mitosis is theprocess by which two daughter cellsare formed, each containing a com-plete set of chromosomes. Interphaseand mitosis make up the bulk of thecell cycle. Following mitosis, thecytoplasm divides, separating the twodaughter cells. You can use theProblem-Solving Lab on this page andthe BioLab at the end of this chapterto investigate the rate of mitosis.


Observe and InferHow does the length of the cell cycle vary? The cell cyclevaries greatly in length from one kind of cell to another. Somekinds of cells divide rapidly, while others divide more slowly.

Solve the ProblemExamine the cell cycle diagrams of two different types of cells.Observe the total length of each cell cycle and the length oftime each cell spends in each phase of the cell cycle.

Thinking Critically1. Make and Use Graphs Which part of the cell cycle is

most variable in length? 2. Infer What can you infer about the functions of these

two types of cells? 3. Think Critically Why do you think the cycle of some

types of cells is faster than in others? Explain your answer.

7 hours 3 hours

Mitosis1 hour

11 hours

Interphase

Total = 22 hours

37 hours

7 hours

Mitosis1 hourInterphase

Total = 48 hours

3 hours

Histone H1

Nucleosome

DNA

Centromere

Sisterchromatids

Chromosome Supercoil within chromosome

Continued coilingwithin supercoil

Chromosome StructureFigure 8.10The chromosomes of a eukaryotic cell undergo changes in shapeand structure during the different phases of the cell cycle. Ametaphase chromosome is a compact arrangement of DNA andproteins. During interphase, the chromosomes are long and tan-gled, resembling a plate of spaghetti. Critical Thinking Why is itimportant for the chromosomes to be compact and untan-gled during mitosis?


Interphase: A Busy Time

Interphase, the busiest phase of thecell cycle, is divided into three partsas shown in Figure 8.11. During thefirst part, the cell grows and proteinproduction is high. In the next part ofinterphase, the cell copies its chro-mosomes. DNA synthesis does notoccur all through interphase but is confined to this specific time. After the chromosomes have been

duplicated, the cell enters anothershorter growth period in which mito-chondria and other organelles aremanufactured and cell parts neededfor cell division are assembled.Following this activity, interphaseends and mitosis begins.

The Phases of MitosisCells undergo mitosis as they

approach the maximum cell size atwhich the nucleus can provide blue-prints for proteins, and the plasmamembrane can efficiently transportnutrients and wastes into and out ofthe cell.

Although cell division is a continu-ous process, biologists recognize fourdistinct phases of mitosis—each phasemerging into the next. The four phasesof mitosis are prophase, metaphase,anaphase, and telophase. Refer toFigure 8.13 to help you understandthe process as you read about mitosis.

Prophase: The first phase of mitosis

During prophase, the first andlongest phase of mitosis, the long,stringy chromatin coils up into visiblechromosomes. As you can see inFigure 8.12, each duplicated chro-mosome is made up of two halves.The two halves of the doubled struc-ture are called sister chromatids.Sister chromatids and the DNA theycontain are exact copies of each otherand are formed when DNA is copiedduring interphase. Sister chromatidsare held together by a structure calleda centromere, which plays a role inchromosome movement during mito-sis. By their characteristic location,centromeres also help scientists iden-tify and study chromosomes.

As prophase continues, the nucleusbegins to disappear as the nuclear enve-lope and the nucleolus disintegrate.


DNA synthesisand replication

Rapid growthand metabolic activity

Centrioles replicate; cell preparesfor division

Interphase

Mitosis

CytokinesisFigure 8.11In preparation formitosis, most of thetime spent in the cellcycle is in interphase.The process of mitosis,represented here bythe yellow wedge, isshown in detail inFigure 8.13.

Sister chromatids

Centromere

Figure 8.12The two sister chromatidsare held together by acentromere.

8.2 CELL GROWTH AND REPRODUCTION 207(t ct b)Ed Reschke/Peter Arnold, Inc., (cb)Carolina Biological Supply/Phototake, NYC

Figure 8.13Mitosis begins after interphase. Follow the stagesof mitosis as you read the text. The diagrams andthe photos show mitosis in plant cells.

AnaphaseThe centromeressplit and the sisterchromatids arepulled apart toopposite poles ofthe cell.

CC

Stained LM Magnification: 500�


Spindle fibers

Disappearingnuclear envelope

Doubled chromosome

Centromere

Sister chromatids

Nuclear envelopereappears

Two daughtercells are formed

ProphaseThe chromatin coilsto form visiblechromosomes.

AA

TelophaseTwo distinctdaughter cells areformed. The cellsseparate as the cellcycle proceeds intothe next interphase.

DD

MetaphaseThe chromosomesmove to the equatorof the spindle.

BB



208 CELLULAR TRANSPORT AND THE CELL CYCLEBarry King, University of California, School of Medicine/Biological Photo Service

Figure 8.14Centrioles duplicate during interphase. In the pho-tomicrograph, one centriole is cut crosswise and theother longitudinally.

Microtubule


By late prophase, these structures arecompletely absent. In animal cells,two important pairs of structures, thecentrioles, begin to migrate to oppo-site ends of the cell. Centrioles aresmall, dark, cylindrical structures thatare made of microtubules and arelocated just outside the nucleus, asshown in Figure 8.14. Centriolesplay a role in chromatid separation.

As the pairs of centrioles move toopposite ends of the cell, anotherimportant structure, called the spindle,begins to form between them. Thespindle is a football-shaped, cagelikestructure consisting of thin fibers madeof microtubules. In plant cells, thespindle forms without centrioles. Thespindle fibers play a vital role in theseparation of sister chromatids duringmitosis.

Metaphase: The second stage of mitosis

During metaphase, the short sec-ond phase of mitosis, the doubled

chromosomes become attached to thespindle fibers by their centromeres.The chromosomes are pulled by thespindle fibers and begin to line up onthe midline, or equator, of the spin-dle. Each sister chromatid is attachedto its own spindle fiber. One sisterchromatid’s spindle fiber extends toone pole, and the other extends to theopposite pole. This arrangement isimportant because it ensures thateach new cell receives an identical andcomplete set of chromosomes.

Anaphase: The third phase of mitosis

The separation of sister chro-matids marks the beginning ofanaphase, the third phase of mito-sis. During anaphase, the cen-tromeres split apart and chromatidpairs from each chromosome sepa-rate from each other. The chro-matids are pulled apart by theshortening of the microtubules inthe spindle fibers.

Centriole

Telophase: The fourth phase of mitosis

The final phase of mitosis istelophase. Telophase begins as thechromatids reach the opposite poles ofthe cell. During telophase, many of thechanges that occurred during prophaseare reversed as the new cells prepare fortheir own independent existence. Thechromosomes, which had been tightlycoiled since the end of prophase, nowunwind so they can begin to direct themetabolic activities of the new cells.The spindle begins to break down, thenucleolus reappears, and a new nuclearenvelope forms around each set of chro-mosomes. Finally, a new double mem-brane begins to form between the twonew nuclei.

CytokinesisFollowing telophase, the cell’s cyto-

plasm divides in a process calledcytokinesis (si toh kih NEE sus).Cytokinesis differs between plants andanimals. Toward the end of telophasein animal cells, the plasma membranepinches in along the equator as shownin Figure 8.15. As the cell cycle pro-ceeds, the two new cells are separated.Find out more about mitosis in animalcells in the MiniLab.

Plant cells have a rigid cell wall, so the plasma membrane does notpinch in. Rather, a structure known asthe cell plate is laid down across thecell’s equator. A cell membrane formsaround each cell, and new cell wallsform on each side of the cell plate untilseparation is complete.

8.2 CELL GROWTH AND REPRODUCTION 209(t)Ed Reschke/Peter Arnold, Inc., (b)David M. Phillips/Visuals Unlimited

Compare and ContrastSeeing Asters The result of theprocess of mitosis is similar in plantand animal cells. However, animalcells develop structures calledasters that are thought to serve asa brace for the spindle fibers,while plant cells do not developasters.

Procedure! Examine a slide showing fish mitosis

under low- and high-power magnification. CAUTION: Use care when handling prepared slides.

@ Find cells that are undergoing mitosis. You will be able tosee dark-stained rodlike structures within certain cells.These structures are chromosomes.

# Note the appearance and location of asters. They willappear as ray or starlike structures at opposite ends ofcells that are in metaphase.

$ Asters may also be observed in cells that are in otherphases of the cell cycle.

Analysis1. Describe What is the location of asters in cells that are in

prophase?2. Infer How do you know that asters are not critical to

mitosis?3. Use Models Sketch and label a plant cell and an animal

cell in prophase.

Asters


Figure 8.15The furrow, created when proteins positioned underthe plasma membrane at the equator of this frogcell contracted and slid past each other, will deepenuntil the cell is pinched in two.

Color-enhanced SEM Magnification: 25�

Understanding Main Ideas1. Describe how a cell’s surface area-to-volume ratio

limits its size.

2. Why is it necessary for a cell’s chromosomes to bedistributed to its daughter cells in such a precisemanner?

3. Relate cells to each level of organization in a multicellular organism.

4. In multicellular organisms, describe two cellularspecializations that result from mitosis.

Thinking Critically5. At one time, interphase was referred to as the

resting phase of the cell cycle. Why do you thinkthis description is no longer used?

6. Get the Big Picture Make a table sequencingthe phases of the cell cycle. Mention one impor-tant event that occurs at each phase. For morehelp, refer to Get the Big Picture in the SkillHandbook.



Tissue (muscle tissue)

Cell (muscle cell)

Organ (stomach)

Organ system (digestive system)

Organism (Florida panther)

Figure 8.16Cells of complex multicellular organisms are organized into tissues,organs, and organ systems. Sequence What levels of organizationis a human blood cell a part of?

Results of MitosisMitosis is a process that guarantees

genetic continuity, resulting in theproduction of two new cells withchromosome sets that are identical tothose of the parent cell. These newdaughter cells will carry out the samecellular processes and functions asthose of the parent cell and will growand divide just as the parent cell did.

When mitosis is complete, unicellu-lar organisms remain as single cells—the organism simply multiplied. Inmulticellular organisms, cell growthand reproduction result in groups ofcells that work together as tissue toperform a specific function. Tissuesorganize in various combinations toform organs that perform more com-plex roles within the organism. Forexample, cells make up muscle tissue,

then muscle tissue works with othertissues in the organ called the stomachto mix up food. Multiple organs thatwork together form an organ system.The stomach is one organ in the diges-tive system, which functions to breakup and digest food.

All organ systems work togetherfor the survival of the organism,whether the organism is a fly or ahuman. Figure 8.16 shows an exam-ple of cell specialization and organiza-tion for a complex organism. Inaddition to its digestive system, thepanther has a number of other organsystems that have developed throughcell specialization. It is important toremember that no matter how com-plex the organ system or organismbecomes, the cell is still the mostbasic unit of that organization.



8.3SECTION PREVIEWObjectivesDescribe the role ofenzymes in the regulationof the cell cycle.Distinguish between theevents of a normal cellcycle and the abnormalevents that result in cancer.Identify ways to poten-tially reduce the risk ofcancer.

Review Vocabularyprotein: a large complex

polymer composed ofcarbon, hydrogen, oxy-gen, nitrogen, and usu-ally sulfur (p. 160)

New Vocabularycancergene

8.3 CONTROL OF THE CELL CYCLE 211

Normal Control of the Cell CycleWhy do some types of cells divide rapidly, while others divide slowly?

What tells a cell when it is time to leave one part of the cell cycle andbegin the next?Proteins and enzymes control the cell cycle

The cell cycle is controlled by proteins called cyclins and a set ofenzymes that attach to the cyclin and become activated. The interactionof these molecules, based on conditions both in the cell’s environmentand inside the cell, control the cell cycle. Occasionally, cells lose controlof the cell cycle. This uncontrolled dividing of cells can result from thefailure to produce certain enzymes, the overproduction of enzymes, orthe production of other enzymes at the wrong time. Cancer is a malig-nant growth resulting from uncontrolled cell division. This loss of con-trol may be caused by environmental factors or by changes in enzymeproduction.

Enzyme production is directed by genes located on the chromosomes.A gene is a segment of DNA that controls the production of a protein.

Many studies point to the portion of interphase just before DNA repli-cation as being a key control period in the cell cycle. Scientists have iden-tified several enzymes that trigger DNA replication.

Getting ControlFinding Main Ideas As you read through the sec-tion on control of the cellcycle, answer the followingquestions.

Study Organizer 1. Enzymes control the cell

cycle. What controlsenzyme production?

2. What are two environ-mental factors that contribute to the develop-ment of cancer? List anypossible ways you caninfluence these factors.

3. How does a person’s dietrelate to the chances ofgetting cancer?

Luis M. De La Maza, PhD.M.D./Phototake, NYC

Color-enhanced SEM Magnification: 7500�

Control of the Cell Cycle

This tumor is developing due to a mistake inthe cell cycle.

Cancer: A Mistake in the Cell Cycle

Currently, scientists consider can-cer to be a result of changes in one ormore of the genes that produce sub-stances that are involved in control-ling the cell cycle. These changes areexpressed as cancer when somethingprompts the damaged genes intoaction. Cancerous cells form masses

of tissue called tumors that deprivenormal cells of nutrients. In laterstages, cancer cells enter the circula-tory system and spread throughoutthe body, a process called metastasis,forming new tumors that disrupt thefunction of organs, organ systems,and ultimately, the organism.

Cancer is the second leading causeof death in the United States,exceeded only by heart disease. Can-cer can affect any tissue in the body. Inthe United States, lung, colon, breast,and prostate cancers are the mostprevalent types. Use the Problem-Solving Lab on this page to estimatethe number of people in the UnitedStates who will develop these kinds ofcancers in this decade, and how manypeople are expected to die from can-cers. The Connection to Health featureat the end of this chapter further dis-cusses skin cancer.

Infer why cancer isdifficult to treat in later stages.

The causes of cancerThe causes of cancer are difficult to

pinpoint because both genetic andenvironmental factors are involved.The environmental influences of can-cer become obvious when you con-sider that people in differentcountries develop different types of cancers at different rates. Forexample, the rate of breast cancer isrelatively high in the United States,but relatively low in Japan. Similarly,stomach cancer is common in Japan,but rare in the United States.

Other environmental factors, suchas cigarette smoke, air and water pollution, and exposure to ultra-violet radiation from the sun, are allknown to damage the genes thatcontrol the cell cycle. Cancer mayalso be caused by viral infections thatdamage the genes.


Interpret DataHow does the incidence of cancer vary? Cancer affectsmany different body organs. In addition, the same body organ,such as our skin, can be affected by several different types ofcancer. Some types of cancer are more treatable than others.Use the following graph to analyze the incidence of cancer.

Thinking Critically1. Make and Use Graphs Which cancer type is most com-

mon? Least common?2. Interpret Data Which cancer type seems to be least treat-

able? Most treatable?3. Interpret Data Using breast cancer as an example, calcu-

late the percent of survival for this cancer type. 4. Use Numbers Approximately what percentage of new

cancer cases in the United States in 2000 were lung cancer?

20 00060 000100 000140 000180 000220 000260 000300 000340 000760 000800 000840 000

Kind of cancer

Num

ber o

f cas

es

Breas

tLu

ng

Prosta

teSk

in:

bas

al ce

ll and

squam

ous

Skin

:

mela

noma

Colon

Cancer Rates in the United States (2000)

Estimated new casesEstimated deaths

Cancer preventionFrom recent and ongoing investi-

gations, scientists have established aclear link between a healthy lifestyleand the incidence of cancer.

Physicians and dietary expertsagree that diets low in fat and high infiber content can reduce the risk ofmany kinds of cancer. For example,diets high in fat have been linked toincreased risk for colon, breast, andprostate cancers, among others.People who consume only a minimalamount of fat reduce the potentialrisk for these and other cancers andmay also maintain a healthy bodyweight more easily. In addition,recent studies suggest that diets highin fiber are associated with reducedrisk for cancer, especially colon can-cer. Fruits, vegetables, and grainproducts are excellent dietary optionsbecause of their fiber content andbecause they are naturally low in fat.The foods displayed in Figure 8.17illustrate some of the choices that areassociated with cancer prevention.

Vitamins and minerals may alsohelp prevent cancer. Key in this cate-gory are carotenoids, vitamins A, C,and E, and calcium. Carotenoids arefound in foods such as yellow andorange vegetables and green leafyvegetables. Citrus fruits are a great

source of vitamin C, and many dairyproducts are rich in calcium.

In addition to diet, other healthychoices such as daily exercise and notusing tobacco also are known toreduce the risk of cancer.

Understanding Main Ideas1. Do all cells complete the cell cycle in the same

amount of time?2. Describe how enzymes control the cell cycle.3. How can disruption of the cell cycle result in cancer?4. How does cancer affect normal cell functioning?Thinking Critically5. What evidence shows that the environment influ-

ences the occurrence of cancer?

6. Recognize Cause and Effect Although not all cancers are preventable, some lifestyle choices, such as a healthy diet and regular exercise, can decrease your cancer risk. Give a summary of how these two lifestyle choices could be implemented by teens. For more help, refer to Recognize Cause and Effect in the Skill Handbook.


8.3 CONTROL OF THE CELL CYCLE 213KS Studios/Bob Mullenix

Figure 8.17A healthy diet mayreduce your risk of cancer. Classify Whattypes of food makeup a diet thatreduces the risk of cancer?




Y

X

AA

Before You BeginMitosis and the resultingmultiplication of cells areresponsible for the growthof an organism. Does mito-sis occur in all areas of anorganism at the same rate,or are there certain areaswithin an organism wheremitosis occurs more often?You will answer this ques-tion in this BioLab. Yourorganism will be an onion,and the areas you aregoing to investigate willbe different locations in its root.

Where is mitosis mostcommon?

ProblemDoes mitosis occur at the same rate in all of the parts of anonion root?

ObjectivesIn this BioLab, you will:■ Observe cells in two different root areas.■ Identify the stages of mitosis in each area.

Materialsprepared slide of onion root tipmicroscope

Skill HandbookIf you need help with this lab, refer to the Skill Handbook.

Safety PrecautionsCAUTION: Report any glass breakage to your teacher.

1. Using diagram A as a guide, locate area X on a prepared slide of onion root tip.

2. Place the prepared slide under your microscope and uselow power to locate area X. CAUTION: Use care when han-dling prepared slides.

3. Switch to high power. 4. Using diagram B as a guide:

a. Identify those cells that are in mitosis and those cellsthat are in interphase.

b. Create a data table. Record the number of cellsobserved in each phase of mitosis and interphase forarea X. Note: It will be easier to count and keep trackof cells by following rows. See diagram C as a guide tocounting.

5. Using diagram A again, locate area Y on the same pre-pared slide.

PROCEDUREPROCEDURE

PREPARATIONPREPARATION

High-power view

Startcountinghere

Endcountinghere

Interphase

Prophase

Metaphase

Anaphase

Telophase

8.3 CONTROL OF THE CELL CYCLE 215

BB

CC

6. Place the prepared slide under your microscope and use low power to locatearea Y.

7. Switch to high power.8. Using diagram B as a guide:

a. Identify those cells that are in mitosis and those that are in interphase. b. Record in the data table the number of cells observed in each phase of

mitosis and interphase for area Y.9. Clean all equipment as instructed by your teacher,

and return everything to its proper place.CLEANUP AND DISPOSAL

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

1. Observe Which area of the onion root tip (X or Y) had the greatest per-centage of cells undergoing mitosis? The lowest? Use specific totals fromyour data table to support your answer.

2. Predict If mitosis is associated with rapidgrowth, where do you believe is the locationof most rapid root growth, area X or Y?Explain your answer.

3. Apply Where might you look for cells in thehuman body that are undergoing mitosis?

4. Think Critically Assume that you were notable to observe cells in every phase of mito-sis. Explain why this might be, consideringthe length of each phase.

5. What factors might causemisleading results? How could you avoidthese problems?

ERROR ANALYSIS

Make and Use Graphs Prepare a circlegraph that shows the total number of cellscounted in area X and the percentage of cellsin each phase of mitosis.

Web Links To find out more about mitosis, visitbdol.glencoe.com/mitosis

http://bdol.glencoe.com/mitosis


Skin Cancer

Skin cancer accounts for one-third of allmalignancies diagnosed in the United States,

and the incidence of skin cancer is increasing.Most cases are caused by exposure to harmfulultraviolet rays emitted by the sun, so skin cancermost often develops on the exposed face or neck.The people most at risk are those whose fair skincontains smaller amounts of a protective pigmentcalled melanin.

Skin is composed of two layers of tissue, theepidermis and the dermis. The epidermis is thepart that we see on the surface of our bodies andis composed of multiple layers of closely packedcells. As the cells reach the surface, they die andbecome flattened. Eventually they flake away. Toreplace the loss, cells on the innermost layer ofthe epidermis are constantly dividing.

Your body has a natural protection system toshield skin cells from potentially harmful raysof the sun. A pigment called melanin is pro-duced by cells called melanocytes and absorbsthe UV rays before they reach basal cells.

Types of skin cancers Uncontrolled divi-sion of epidermal cells leads to skin cancer.Squamous cell carcinoma is a common type ofskin cancer that affects cells throughout theepidermis. Squamous cell cancer takes theform of red or pink tumors that can grow rap-idly and spread. Precancerous growths pro-duced by sun-damaged basal cells can becomebasal cell carcinoma, another common type ofskin cancer. In basal cell carcinoma, the can-cerous cells are from the layer of the epidermisthat replenishes the shed epithelial cells. Bothsquamous cell carcinoma and basal cell carci-noma are usually discovered when they aresmall and can be easily removed in a doctor’soffice. Both types also respond to treatmentsuch as surgery, chemotherapy, and radiationtherapy.

The most lethal skin cancer is malignantmelanoma. Melanomas are cancerous growths ofthe melanocytes that normally protect other cellsin the epithelium from the harmful rays of thesun. An important indication of a melanoma canbe a change in color of an area of skin to a vari-ety of colors including black, brown, red, darkblue, or gray. A single melanoma can have severalcolors within the tumor. Melanomas can alsoform at the site of moles. Melanomas can be dan-gerous because cancerous cells from the tumorcan travel to other areas of the body before themelanoma is detected. Early detection is essen-tial, and melanomas can be surgically removed.

Dixie Knight/Medichrome/The Stock Shop

Describe Scientists know that the UV rays of sunlight can contribute to skin cancer. Write a para-graph describing how you can minimize the risk.

To find out more about skin cancer, visit bdol.glencoe.com/health

Epidermis

Melanocytes

MelaningranulesDermis

Structure of the skin

Melanoma

http://bdol.glencoe.com/health

Section 8.1STUDY GUIDESTUDY GUIDE

CHAPTER 8 ASSESSMENT 217Luis M. De La Maza, PhD.M.D./Phototake, NYC

Section 8.2

Section 8.3

To help you reviewosmosis, use the Organizational StudyFold on page 195.

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Key Concepts■ Osmosis is the diffusion of water through a

selectively permeable membrane.■ Passive transport moves a substance with

the concentration gradient and requires noenergy from the cell.

■ Active transport moves materials againstthe concentration gradient and requiresenergy to overcome the flow of materialsopposite the concentration gradient.

■ Large particles may enter a cell by endo-cytosis and leave by exocytosis.

Vocabularyactive transport (p. 199)endocytosis (p. 200)exocytosis (p. 200)facilitated diffusion (p. 198)hypertonic solution

(p. 196)hypotonic solution

(p. 196)isotonic solution (p. 196)osmosis (p. 195)passive transport

(p. 198)

CellularTransport

Key Concepts■ Cell size is limited largely by the diffusion

rate of materials into and out of the cell,the amount of DNA available to programthe cell’s metabolism, and the cell’s surfacearea-to-volume ratio.

■ The life cycle of a cell is divided into twogeneral periods: a period of active growthand metabolism known as interphase, and aperiod that leads to cell division known asmitosis.

■ Mitosis is divided into four phases:prophase, metaphase, anaphase, andtelophase.

■ The cells of most multicellular organismsare organized into tissues, organs, andorgan systems.

Vocabularyanaphase (p. 208)cell cycle (p. 204)centriole (p. 208)centromere (p. 206)chromatin (p. 204)chromosome (p. 203)cytokinesis (p. 209)interphase (p. 204)metaphase (p. 208)mitosis (p. 204)organ (p. 210)organ system (p. 210)prophase (p. 206)sister chromatid (p. 206)spindle (p. 208)telophase (p. 209)tissue (p. 210)

Cell Growth andReproduction

Key Concepts■ The cell cycle is controlled by key enzymes

that are produced at specific points in thecell cycle.

■ Cancer is caused by genetic and environ-mental factors that change the genes thatcontrol the cell cycle.

Vocabularycancer (p. 211)gene (p. 211)

Color-enhanced SEMMagnification: 7500�

Control of theCell Cycle

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Review the Chapter 8 vocabulary words listed inthe Study Guide on page 217. Determine if each statement is true or false. If false, replace theunderlined word with the correct vocabularyword.

1. Mitosis is the result of uncontrolled divisionof cells.

2. Small, dark cylindrical structures that aremade of microtubules and located just out-side the nucleus are called genes.

3. Diffusion of water across a selectively per-meable membrane is called cytokinesis.

4. In a hypotonic solution, the concentrationof dissolved substances inside cells is higherthan the concentration outside the cell.

5. Cancer is a period of nuclear division in a cell.

6. What kind of environment is described whenthe concentration of dissolved substances isgreater outside the cell than inside?A. hypotonic C. isotonic B. hypertonic D. saline

7. How is osmosis defined?A. as active transportB. as diffusion of water through a selectively

permeable membraneC. as an example of facilitated diffusionD. as requiring a transport protein

8. An amoeba ingests large food particles bywhat process?A. osmosis C. endocytosisB. diffusion D. exocytosis

9. Of what are chromosomes composed?A. cytoplasm C. RNA and proteinsB. centrioles D. DNA and proteins

10. Which of the following does NOT occurduring interphase?A. excretion of wastesB. cell repairC. protein synthesisD. nuclear division

11. During metaphase, the chromosomes move to the equator of what structure (shown here)?A. polesB. cell plateC. centrioleD. spindle

12. All but which of the following factors limitcell size?A. time required for diffusionB. elasticity of the plasma membraneC. presence of only one nucleusD. surface area-to-volume ratio

13. Which of the following is NOT a knowncause of cancer?A. environmental influencesB. certain virusesC. cigarette smokeD. bacterial infections

14. Open Ended How would you expect thenumber of mitochondria in a cell to berelated to the amount of active transport itcarries out?

15. Open Ended Suppose that all of theenzymes that control the normal cell cyclewere identified. Suggest some ways that thisinformation might be used to fight cancer.

16. Open Ended Substance A’s molecules aresmall. Substance B’s molecules, which reactwith substance A to produce a blue-blackcolor, are larger in comparison. If a solutionof substance A is placed inside a selectivelypermeable bag, and the bag is placed in asolution of substance B, what will happen?

17. Predict What do you think will happenwhen a freshwater paramecium is placed insalt water?

218 CHAPTER 8 ASSESSMENTEd Reschke/Peter Arnold, Inc.

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Multiple ChoiceUse the following illustration to answer questions19–23.

19. Which drawing indicates a cell in metaphaseof mitosis?A. A C. CB. B D. D

20. During which stage do the chromatids ofchromosomes separate?A. A C. CB. B D. D

21. Which drawing indicates a cell whosenuclear membrane is dissolving?A. A C. CB. B D. D

22. Which of the following indicates the correctorder of mitosis in animal cells?A. A-B-C-D C. C-A-D-BB. B-C-A-D D. C-B-A-D

23. Which drawing shows a cell in anaphase?A. A C. CB. B D. D

24. A biologist notes that some cells are growingfaster than others in a tissue culture. A weeklater, the fast-growing cells have tripled innumber. This observation is a clue that thefast-growing cells ________.A. have killed the slow-growing cellsB. might be unable to control mitosisC. were exposed to radiation or chemicalsD. contain an unknown enzyme

25. Which parts of the nucleosome are made ofDNA?A. 1 and 2 C. 2 and 3B. 1 and 3 D. 2 and 4

26. Which parts of the nucleosome are made ofprotein?A. 1 and 2 C. 2 and 3B. 1 and 3 D. 2 and 4

2 3

Nucleosome

14

A B C D

18. Cystic fibrosisis a genetic disorder that results from theinability of cells to properly transport somematerials. Visit

to investigate cystic fibrosis. Write an essay that explains what you have learnedabout cystic fibrosis and present it to yourclass.

REAL WORLD BIOCHALLENGE

CHAPTER 8 ASSESSMENT 219

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Constructed Response/Grid InRecord your answers on your answer document.

27. Open Ended The cell cycle can be affected by internal and external factors. Injury to a tissuecan prompt changes in the cell cycle of the cells near the injury site. Formulate a testablehypothesis concerning a specific type of cell’s response to injury. State your hypothesis, planan investigative procedure to test your hypothesis, and list the steps.

28. Open Ended Explain why drinking quantities of ocean water is dangerous to humans.

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