Chapter 9: Energy in a Cell - Polson Schoolsblogs.polson.k12.mt.us/dobrien/files/2014/02/chap09.pdf · contracting during exercise, your heart pumping, ... Figure 9.3 To access the

226 ENERGY IN A CELL

Energy in a Cell

What You’ll Learn■ You will learn what ATP is.■ You will explain how ATP

provides energy for the cell.■ You will describe how chloro-

plasts trap the sun’s energy tomake ATP and complex car-bohydrates.

■ You will compare ATP pro-duction in mitochondria andchloroplasts.

Why It’s ImportantEvery cell in your body needsenergy in order to function.The energy your cells produceand store is the fuel for basicbody functions such as eatingand breathing.

Scan the chapter, and writedown the titles of the sectionheadings. Compose a few questions about each heading.Look over the headings, andtry to determine general waysin which energy is stored. Forheadings with unfamiliarterms, scan the text and try to find the definitions.

To find out more about howcells use and produce energy,visit the Glencoe Science Web site.science.glencoe.com

READING BIOLOGYREADING BIOLOGY

9ChapterChapter

Energy can be capturedthrough photosynthesisin these cabbage leaves,or burned into electricityat this power plant.

BIOLOGY

http://science.glencoe.com

Section

Cell EnergyEnergy is essential to life. All liv-

ing organisms must be able to pro-duce energy from the environment inwhich they live, store energy forfuture use, and use energy in a con-trolled manner.

Work and the need for energyYou’ve learned about several cell

processes that require energy. Activetransport, cell division, movement offlagella or cilia, and the productionand storage of proteins are someexamples. The transport of proteins isshown in Figure 9.1. You can proba-bly come up with other examples ofbiological work, such as muscles contracting during exercise, yourheart pumping, and your brain

9.1 ATP IN A MOLECULE 227

spring stores energy when it iscompressed. When the compressedspring is released, energy is also

released, energy that sends this smiley-face toy flying into the air. Like this coiledspring, chemical bonds store energy thatcan be released when the bond is broken.Just as some springs are tighter than others, some chemical bonds store more energy than others.

SECTION PREVIEW

ObjectivesExplain why organismsneed a supply ofenergy.Describe how energy isstored and released byATP.

VocabularyATP (adenosine

triphosphate)ADP (adenosine

diphosphate)

9.1 ATP in a Molecule

Stored energyFigure 9.1 Active transportrequires energy tobind and pump thismolecule across theplasma membrane.

Carrier protein shown withtwo binding sites.

AA A molecule binds to thecarrier protein.

BB

Energy released as a phos-phate group from ATP istransferred to the protein.

CC The phosphate group andenergy trigger the proteinto pump the moleculethrough the membrane.

DD

f

ATP (ADP)P P

A

Why is fat the choice?Humans store their excess energy as fat rather than as carbohydrates. Why is this? From an evolutionary and efficiency point of view, fats are better for storage than carbohydrates. Find out why.

AnalysisThe following facts

compare certain characteris-tics of fats and carbohydrates:A. When broken down by the body, each six-carbon mole-

cule of fat yields 51 ATP molecules. Each six-carbon carbo-hydrate molecule yields 38 ATP molecules.

B. Carbohydrates bind and store water. The metabolism ofwater yields zero ATP. Fat has zero grams of water boundto it.

C. An adult who weighs 70 kg can survive on the energyderived from stored fat for 30 days without eating. Thesame person would have to weigh nearly 140 kg to sur-vive 30 days on stored carbohydrates.

Thinking Critically1. From an ATP production viewpoint, use fact B to make a

statement regarding the efficiency of fats vs. carbohy-drates.

2. Explain why the average weight for humans is close to 70 kg and not 140 kg.

Problem-Solving Lab 9-1Problem-Solving Lab 9-1 Recognizing Causeand Effect

controlling your entire body. Thiswork cannot be done without energy.Read the Problem-Solving Lab on thispage and think about how the humanbody stores energy.

When you finish strenuous physi-cal exercise, such as running crosscountry, your body wants a quicksource of energy, so you may eat acandy bar. Similarly, there is a mole-cule in your cells that is a quicksource of energy for any organelle inthe cell that needs it. This energy isstored in the chemical bonds of themolecule and can be used quickly andeasily by the cell.

The name of this energy moleculeis adenosine triphosphate (uh DEN

uh seen • tri FAHS fayt), or ATP forshort. ATP is composed of an adeno-sine molecule with three phosphategroups attached. Recall that phos-phate groups are charged molecules,and remember that molecules withthe same charge do not like being tooclose to each other.

Forming and Breaking Down ATP

The charged phosphate groups actlike the positive poles of two mag-nets. If like poles of a magnet areplaced next to each other, it is diffi-cult to force the magnets together.Likewise, bonding phosphate groupsto adenosine requires considerableenergy. When only one phosphategroup is attached, a small amount ofenergy is required and the chemicalbond does not store much energy. A molecule of this sort is calledadenosine monophosphate (AMP).When a second phosphate is added, a more substantial amount of energyis required to force the two phos-phate groups together. A molecule of this sort is called adenosinediphosphate, or ADP. When thethird phosphate group is added, atremendous amount of energy isrequired to force the third chargedphosphate close to the two otherphosphate groups. The third phos-phate group is so eager to get awayfrom the other two that, when thatbond is broken, a great amount ofenergy is released.

The energy of ATP becomes avail-able when the molecule is brokendown. In other words, when thechemical bond between phosphategroups in ATP is broken, energy isreleased and the resulting molecule isADP. At this point, ADP can reform


OriginWORDWORD

mono-, di-, triFrom the Latinwords mono, di, andtri, meaning “one,”“two,” and “three,”respectively.Adenosine triphos-phate contains threephosphate groups.

ATP by bonding with another phos-phate group. This creates a renew-able cycle of ATP formation andbreakdown. Figure 9.2 illustrates thechemical reactions that are involvedin the cycle.

The formation/breakdown recy-cling activity is important because itrelieves the cell of having to store allof the ATP it needs. As long as phos-phate molecules are available, the cellhas an unlimited supply of energy.Another benefit of the formation/breakdown cycle is that ADP canalso be used as an energy source.Although most cell functions requirethe amount of energy in ATP, somecell functions do not require as muchenergy and can use the energy storedin ADP.

How cells tap into the energystored in ATP

When ATP is broken down andthe energy is released, cells must havea way to capture that energy and useit efficiently. Otherwise, it is wasted.ATP is a small compound. Cellularproteins have a specific site whereATP can bind. Then, when the phos-phate bond is broken and the energyreleased, the cell can use the energyfor activities such as making a proteinor transporting molecules throughthe plasma membrane. This cellularprocess is similar to the way energyin batteries is used by a radio.Batteries sitting on a table are of littleuse if the energy stored within thebatteries cannot be accessed. Whenthe batteries are snapped into theholder on the radio, the radio thenhas access to the stored energy andcan use it. Likewise, when the energyin the batteries has been used, thebatteries can be taken out, recharged,and replaced in the holder. In a simi-lar fashion in a cell, when ATP hasbeen broken down to ADP, ADP is

9.1 ATP IN A MOLECULE 229

+

+

energy released

Adenosine moleculeP P P

P

P

Adenosine P P

Adenosine triphosphate (ATP)

Adenosine diphosphate (ADP) phosphorylatedmolecule

Figure 9.2The addition and release of a phosphate group on adenosine diphosphate creates a cycle of ATP formation and breakdown.

energy

ADP

ADP

ATP

Figure 9.3To access the energy stored in ATP, proteins bind ATP and allow the phosphate group to be released. The ADP that is formed isreleased, and the protein binding site can once again bind ATP.

released from the binding site in theprotein and the binding site may thenbe filled by another ATP molecule.ATP binding and energy release in aprotein is shown in Figure 9.3.

Section AssessmentSection AssessmentUnderstanding Main Ideas1. What processes in the cell need energy from ATP?2. How does ATP store energy?3. How can ADP be “recycled” to form ATP again?4. How do proteins in your cells access the energy

stored in ATP?

Thinking Critically5. Phosphate groups in ATP repel each

other because they have negative charges.

What charge might be present in the ATP binding site of proteins to attract another ATPmolecule?

6. Observing and Inferring When animals shiverin the cold, muscles move almost uncontrollably.Suggest how shivering helps an animal survive inthe cold. For more help, refer to ThinkingCritically in the Skill Handbook.

SKILL REVIEWSKILL REVIEW

Uses of Cell EnergyYou can probably think of hun-

dreds of physical activities thatrequire energy, but energy is equallyimportant at the cellular level fornearly all of the cell’s activities.

Making new molecules is one waythat cells use energy. Some of thesemolecules are enzymes, which carry

out chemical reactions. Other mole-cules build membranes and cellorganelles. Cells use energy to main-tain homeostasis. Kidneys use energyto move molecules and ions in orderto eliminate waste substances whilekeeping needed substances in thebloodstream. Figure 9.4 shows sev-eral organisms and activities thatillustrate ways that cells use energy.


Figure 9.4ATP fuels the cellular activity thatdrives the organism.

Nerve cells transmitimpulses by using ATP topower the active trans-port of certain ions.

AA

Fireflies, some caterpillars, such asthe one shown here, and manymarine organisms produce light bya process called bioluminescence.The light results from a chemicalreaction that is powered by thebreakdown of ATP.

CC

Some organismswith cilia or flagella(left) use energyfrom ATP to move.

BB

Section

Trapping Energy fromSunlight

To use the energy of the sun’slight, plant cells must trap lightenergy and store it in a form that isreadily usable by cell organelles—that form is ATP. However, lightenergy is not available 24 hours a day,so the plant cell must also store someof the energy for the dark hours.Photosynthesis is the process plantsuse to trap the sun’s energy and buildcarbohydrates, called glucose, thatstore energy. To accomplish this, pho-tosynthesis happens in two phases.The light-dependent reactionsconvert light energy into chemicalenergy. The molecules of ATP

produced in the light-dependentreactions are then used to fuel thelight-independent reactions thatproduce glucose. The general equa-tion for photosynthesis is written as follows:

6CO2 + 6H2O ➜ C6H12O6 + 6O2

The BioLab at the end of this chap-ter describes an experiment you canperform to investigate the rate ofphotosynthesis.

The chloroplast and pigmentsRecall that the chloroplast is the

cell organelle where photosynthesisoccurs. It is in the membranes of thethylakoid discs in chloroplasts that thelight-dependent reactions take place.

9.2 PHOTOSYNTHESIS: TRAPPING THE SUN’S ENERGY 231

Have you ever admired a beau-tiful rainbow shimmering inthe air after a rainstorm? You

may not have realized that the colorsyou saw represent the sun’s energybroken into different wavelengths,which our eyes interpret as color. Toleaves on a tree, the wavelengths of light represent a source of energy to be stored and used.

SECTION PREVIEW

ObjectivesRelate the structure of chloroplasts to theevents in photosynthesis.Describe the light-dependent reactions.Explain the reactionsand products of thelight-independentCalvin cycle.

Vocabularyphotosynthesislight-dependent

reactionslight-independent

reactionspigmentschlorophyllelectron transport chainNADP+photolysisCalvin cycle

9.2 Photosynthesis: Trappingthe Sun’s Energy

Photosynthesisrequires sunlight.

OriginWORDWORD

photosynthesisFrom the Greekwords photo, mean-ing “light,” and syn-tithenai, meaning“to put together.”Photosynthesis putstogether sugar mol-ecules using energyfrom light, water,and carbon dioxide.

How does photosyn-thesis vary with lightintensity? Photo-synthesis is the processby which green plantssynthesize organiccompounds from waterand carbon dioxideusing energy absorbedby chlorophyll fromsunlight.

AnalysisGreen plants were exposed to increasing light intensity,

as measured in candelas, and the rate of photosynthesis wasmeasured. The temperature of the plants was kept constantduring the experiment. The graph shown here depicts thedata obtained.

Thinking CriticallyConsidering the overall equation for photosynthesis,

make a statement summarizing what the graph shows. Undernormal field conditions, what factors may limit the relativerate of photosynthesis when the light intensity is increasedand temperature remains constant?

Problem-Solving Lab 9-2Problem-Solving Lab 9-2 Drawing ConclusionsTo trap the energy in the sun’s

light, the thylakoid membranes con-tain pigments, molecules that absorbspecific wavelengths of sunlight. Themost common pigment in chloro-plasts is chlorophyll. Chlorophyll informs a and b absorbs most wave-lengths of light except for green.Because chloroplasts have no meansto absorb this wavelength, it isreflected, giving leaves a greenappearance. In the fall, trees reabsorbchlorophyll from the leaves and otherpigments are visible, giving leaves likethose in Figure 9.5 a wide variety ofcolors. Read the Chemistry Connectionat the end of this chapter to find outmore about biological pigments.

Light-DependentReactions

The first phase of photosynthesisrequires sunlight. As sunlight strikesthe chlorophyll molecules in the thy-lakoid membrane, the energy in thelight is transferred to electrons.These highly energized, or excited,electrons are passed from chlorophyllto an electron transport chain, aseries of proteins embedded in thethylakoid membrane. Use theProblem-Solving Lab shown here toconsider how light intensity affectsphotosynthesis.

Each protein in the chain passesenergized electrons along from protein to protein, similar to a bucketbrigade in which a line of people passa bucket of water from person to person to fight a fire. At each stepalong the transport chain, the electron loses energy, just as some ofthe water might be spilled frombuckets in the fire-fighting chain.The electron transport chain allowssmall amounts of the electron’senergy to be released at a time. Thisenergy can be used to form ATP


Figure 9.5The red, yellow, andpurple pigments arevisible in the autumnwhen trees reabsorbchlorophyll.

Rate

of p

hoto

synt

hesis

500 1000 1500 2000Light intensity (candelas)

80

40

20

60

0

Light Intensity and Photosynthesis

OriginWORDWORD

chlorophyllFrom the Greekwords chloros, mean-ing “pale yellowish-green,” and phyllon,meaning “leaf.”Chlorophyll is agreen pigmentfound in leaves.

Electroncarrier

proteins

Light

Chlorophyll

Chlorophyll

NADP+

+H+

ADP+

ATPNADPH

P

Electron Transport Chains

from ADP, or to pump hydrogen ionsinto the center of the thylakoid disc.

After the electron has traveleddown the first electron transportchain, it is passed down a secondelectron transport chain. Followingthe second electron transport chain,the electron is still very energized. Sothat this energy is not wasted, theelectron is transferred to the stromaof the chloroplast. To do this, anelectron carrier molecule calledNADP+ (nicotinamide adenine dinu-cleotide phosphate) is used. Whencarrying the excited electron, NADP+

combines with a hydrogen ion andbecomes NADPH.

Just as proteins contain a bindingsite where they can bind ATP, so theNADP+ molecule and other electroncarrier molecules like it have a bind-ing site for energized electrons.However, in this case, NADPH doesnot use the energy present in theenergized electron; it simply storesthe energy until it can transfer it toanother series of reactions that willtake place in the stroma. There,NADPH will play an important rolein the formation of carbohydrates.The light-dependent reactions aresummarized in Figure 9.6.


Figure 9.6The Light Reactions

Light-Dependent Reactions

Light energy

Chlorophyllpasses energy

down through theelectron transport chain,

providing energy that

transfers to chlorophyll

Sun

splitsH2O

bonds to ADP

formingATPH+

oxygenreleased

for use in thelight-independent reactions

NADP+

NADPH

P

View an animation of the light-dependent reactions in the PresentationBuilder of the InteractiveCD-ROM.

CD-ROM

This expanded viewshows energized elec-trons lose some energyas they are passed fromprotein to proteinthrough the electrontransport chain. Thisenergy can be used toform ATP or NADPH.

BB

Chlorophyll molecules absorb light energy and energizeelectrons for producing ATP or NADPH.

AA

Sun

O2 + 4H+

2H2O

4e–

2H2O 4H+ + O2 + 4e–

Use Isotopes to UnderstandPhotosynthesis C. B. van Niel discov-ered the steps of photosynthesis whenhe used radioactive isotopes of oxygenas tracers. Radioactive isotopes areused to follow a particular moleculethrough a chemical reaction.

Procedure! Study the following equation for photosynthesis

that resulted from the van Niel experiment:6CO2 + 6H2O* ➜ C6H12O6 + 6O2*

@ Radioactive water, water tagged with an isotope of oxy-gen as a tracer (shown with the *), was used. Note wherethe tagged oxygen in water ends up on the right side ofthe chemical reaction.

# Assume that van Niel repeated his experiment, but thistime he put a radioactive tag on the oxygen in CO2.

$ Using materials provided by your teacher, model what youwould predict the appearance of his results would be.Your model must include a “tag” to indicate the oxygenisotope on the left side of the arrow as well as where itends up on the right side of the arrow.

% You must use labels or different colors in your model toindicate also the fate of carbon and hydrogen.

Analysis1. Explain how an isotope can be used as a tag.2. Using your model, predict:

a. the fate of all oxygen molecules that originated fromcarbon dioxide.

b. the fate of all carbon molecules that originated fromcarbon dioxide.

c. the fate of all hydrogen molecules that originatedfrom water.

MiniLab 9-1MiniLab 9-1Restoring electrons to chlorophyll

Although some of the light-ener-gized electrons may be returned tochlorophyll after they’ve movedthrough the electron transport chain,many leave with NADPH for thelight-independent reactions. If theseelectrons are not replaced, the chloro-phyll will be unable to absorb lightand the light-dependent reactions willstop, as will the production of ATP.

To replace the lost electrons, mol-ecules of water are split. Each watermolecule produces one-half moleculeof oxygen gas, two hydrogen ions,and two electrons. This reaction isshown in Figure 9.7 and is calledphotolysis (FO tohl ih sis). The oxy-gen of photolysis supplies the oxygenwe breath. The MiniLab on this pagedescribes how scientists traced oxygenthrough photosynthesis.

Light-IndependentReactions

The second phase of photosynthe-sis does not require light. It is calledthe Calvin cycle, which is a series ofreactions that use carbon dioxide toform carbohydrates. The Calvin cycletakes place in the stroma of thechloroplast. What are the stages of theCalvin cycle? To find out, read theInside Story.


Figure 9.7In photolysis, two mole-cules of water are splitto replace electrons lostfrom chlorophyll.

van Niel

OriginWORDWORD

photolysisFrom the Greekwords photos, mean-ing “light,” andlyein, meaning “tosplit.” Light energysplits water mole-cules in photolysis.

Formulating Models

C C C C C

C

C C C C C C

C C C

C C C

C C CC C C

C C CC C C

C C C C C C

C C C

(Glucose)

(PGA)ADP +

ADP + ATP

ATP

NADP+

NADPH

(RuBP)

(CO2)

(Unstable intermediate)

(PGAL)(PGAL)

(PGAL)

CalvinCycle

P

P


Magnification: 13 450�

The stroma in chloroplastshost the Calvin cycle

INSIDESSTORTORYY

INSIDE

The Calvin Cycle

The Calvin cycle takes the carbon in CO2 and forms carbohydrates through a series of reactions in the

stroma of the chloroplast. NADPH and the ATP produced during the earlier light-dependent reactions are importantmolecules for this series of reactions.

Critical Thinking Environmentalists are concerned about the increasing loss of Earth’s forests. How is the Calvin cycleconnected to this concern?

PGA formation The six-carbonsugar formed in Step 1 is then splitto form two molecules of phos-phoglyceric acid (PGA).

22

Use of ATP and NADPHA series of reactions involv-ing ATP and NADPH fromthe light-dependent reac-tions, converts a moleculeof PGA into phosphoglycer-aldehyde (PGAL), anotherthree-carbon molecule.

33

Glucose productionAfter several rounds of theCalvin cycle, two moleculesof PGAL leave the cycle toform glucose.

44

ATP and PGALreplenish RuBPSome PGAL moleculesreform the five-car-bon sugar with thehelp of energy fromATP. Each five-carbonRuBP is ready tobegin the cycle again.

55

Carbon fixation One carbonatom from CO2 is added to afive-carbon sugar called ribu-lose biphosphate (RuBP).

11

the products can be used again to initiate the cycle.

In the electron transport chain,you learned that an energized elec-tron is passed from protein to proteinand the energy is slowly released. Youcan imagine that making a complexcarbohydrate from a molecule ofCO2 would be a large task for a cell,so the light-independent reactions inthe stroma of the chloroplast breakdown the complicated process intosmall steps. Each sugar moleculemade by the Calvin cycle contains sixcarbon atoms, and because only onemolecule of CO2 is added to the cycleeach time, it takes a total of sixrounds of the cycle to form one sugar.

CAREERS IN BIOLOGY

Biochemist

If you are curious about what makesplants and animals grow and

develop, consider a career as a bio-chemist. The basic research of bio-chemists is to understand howprocesses in an organism work toensure the organism’s survival.

Skills for the JobA bachelor’s degree in chemistry or biochemistry will

qualify you to be a lab assistant. For a more involved position,you will need a master’s degree; advanced research requires aPh.D. Some biochemists work with genes to create new plantsand new chemicals from plants. Others research the causesand cures of diseases or the effects of poor nutrition. Still oth-ers investigate solutions for urgent problems, such as findingbetter ways of growing, storing, and caring for crops.

To find out more about careers inbiology and related fields, visit theGlencoe Science Web site. science.glencoe.com

The Calvin cycleThe Calvin cycle, named after

Melvin Calvin shown in Figure 9.8,is called a cycle because one of thelast molecules formed in the series ofchemical reactions is also one of themolecules needed for the first reac-tion of the cycle. Therefore, one of


Section AssessmentSection AssessmentUnderstanding Main Ideas1. Why do you see green when you look at a leaf

on a tree? Why do you see other colors in thefall?

2. How do the light-dependent reactions of photosynthesis relate to the Calvin cycle?

3. What is the function of water in photosynthesis?Explain the reaction that achieves this function.

4. How does the electron transport chain transferlight energy in photosynthesis?

Thinking Critically5. In photosynthesis, is chlorophyll considered a

reactant, a product, or neither? How does therole of chlorophyll compare with those of CO2and H2O?

6. Designing an Experiment Design an experi-ment that would simulate photosynthesis. Formore help, refer to Practicing Scientific Methodsin the Skill Handbook.


Figure 9.8Melvin Calvin

BIOLOGY


Section

Cellular RespirationThe process by which mitochon-

dria break down food molecules toproduce ATP is called cellular respiration. There are three stages ofcellular respiration: glycolysis, the cit-ric acid cycle, and the electron trans-port chain. The first stage, glycolysis,is anaerobic—no oxygen is required.The last two stages are aerobic andrequire oxygen to be completed.

GlycolysisGlycolysis (gli KOL ih sis) is a

series of chemical reactions in the

cytoplasm of a cell that break downglucose, a six-carbon compound, intotwo molecules of pyruvic (pie RUE vik)acid, a three-carbon compound.Because two molecules of ATP areused to start glycolysis, and only fourATP molecules are produced, glycol-ysis is not very efficient, giving a netprofit of only two ATP molecules foreach glucose molecule broken down.

In the electron transport chain ofphotosynthesis, an electron carriercalled NADP+ was described as car-rying energized electrons to anotherlocation in the cell for further chemi-cal reactions. Glycolysis also uses an

9.3 GETTING ENERGY TO MAKE ATP 237

You know that the chlorophyll ingreen plants is the key to photo-synthesis. You also know that

your body needs energy to survive.What would it be like if you hadchlorophyll in your skin and could con-vert light energy to carbohydrates toproduce ATP? People would look quitedifferent. Fortunately, the mitochon-dria in your cells can convert the carbo-hydrates that green plants formed bythe Calvin cycle into ATP. This allowsyou to access the sun’s energy through the work done by plant cells.

SECTION PREVIEW

ObjectivesCompare and contrastcellular respiration andfermentation.Explain how cellsobtain energy from cellular respiration.

Vocabularycellular respirationanaerobicaerobicglycolysiscitric acid cyclelactic acid fermentationalcoholic fermentation

9.3 Getting Energy to Make ATP

OriginWORDWORD

anaerobicFrom the Greekwords an, meaning“without,” andaeros, meaning “air.”Anaerobic organ-isms can live with-out oxygen.

Magnification:221 875�

Human cells produceenergy in mitochondria(inset).

electron carrier called NAD+ (nicotin-amide adenine dinucleotide). NADforms NADH when it is carrying anelectron. Glycolysis is an anaerobicprocess as you can see by the absenceof oxygen in the equation shown inFigure 9.9.

Following glycolysis, the pyruvicacid molecules move to the mito-chondria, the organelles that produceATP for the cell. In the presence ofoxygen, two more stages completecellular respiration: the citric acidcycle (also known as the Krebs cycle)and the electron transport chain ofthe mitochondrion. Before these twostages can begin, however, pyruvicacid undergoes a series of reactions inwhich it loses a molecule of CO2 andcombines with coenzyme A to formacetyl-CoA. The reaction with coen-zyme A produces a molecule ofNADH and H+. These reactions areshown in Figure 9.10.

The citric acid cycleThe citric acid cycle is a series of

chemical reactions similar to theCalvin cycle in that one of the mole-cules needed for the first reaction isalso one of the end products.However, in the Calvin cycle, glucosemolecules are formed; in the citricacid cycle, glucose is broken down.What is the first compound thatacetyl-CoA combines with in the cit-ric acid cycle? To learn the answer,look at the Inside Story.

In the process of breaking downglucose, one molecule of ATP is pro-duced for every turn of the cycle. Twoelectron carriers are used, NAD+ andFAD (flavin adenine dinucleotide). Atotal of three NADH + H+ moleculesand one FADH2 molecule areformed. These electron carriers passthe energized electrons along to theelectron transport chain in the innermembrane of the mitochondrion.


C C C CC C C P

C C C

C C C

C C CPC C

ENERGY ENERGY

2ATP2ADP

4ADP + 4 P

2NAD+2PGAL

Glucose

4ATP

2NADH + 2H+

2 Pyruvicacid

Figure 9.9Glycolysis breaksdown a molecule ofglucose into twomolecules of pyruvicacid. In the process, itforms a net profit oftwo molecules ofATP, two moleculesof NADH , and twohydrogen ions.

C C C C C C C C C CC

NAD+

CO2

NADH + H+

Pyruvicacid

Outside themitochondrion

Mitochondrialmembrane

Coenzyme A

Pyruvicacid

Inside themitochondrion

Intermediateby-product

- CoAAcetyl-CoA

+

Figure 9.10As a result of thereactions that con-vert pyruvic acid toacetyl-CoA within themitochondrion, amolecule of NADHand H+ are formed.

Glycolysis

Recycling of oxaloacetic acidSuccinic acid under-goes a series of reactions inwhich FADH and NADH areformed and oxaloacetic acid is again made available for the cycle.

44

P

NAD+

NAD+

NAD+

C C

NADH + H+

ATP

FADH

NADH + H+

ADP +

Acetyl CoA

Malic acid

Fumaric acid

Oxaloacetic acid

Succinic acid

Citric acid

Ketoglutaric acid

CitricAcidCycle

O O

O O

NADH + H+

C C C C

C C C C

C C C C

C C C C

C C C C C C

C C C C C

C

C



The mitochondria host the citric acid cycle.

INSIDESSTORTORYY

INSIDE

Citric acid The two-carboncompound acetyl-CoA reacts witha four-carbon compound calledoxaloacetic acid to form citric acid, a six-carbon molecule.

11

Formation of the second CO2 Another moleculeof CO2 is released, forming a four-carbon compound,succinic acid. One molecule of ATP and a molecule of NADH are also produced.

33

The Citric Acid Cycle

The citric acid cycle takes a molecule of acetyl-CoA and breaks it down, forming ATP and CO2.

The electron carriers NAD+ and FAD pick up energizedelectrons and pass them to the electron transport chainin the inner mitochondrial membrane.

Critical Thinking How many ATP molecules are pro-duced by a single turn of the citric acid cycle?

Formation of CO2 A moleculeof CO2 is formed, reducing theeventual product to a five-carboncompound, ketoglutaric acid.

In the process, a molecule ofNADH is produced.

22

H+ H+

H+

H+

H+

H+

P

Electroncarrier proteins

Electronpathway

Intermembranespace

ATPsynthase

ADP + ATP

Innermembrane

Matrix

NAD+

FAD

NADH

FADH2

4 H+ + O2 H2O

H2O

The electron transport chainThe electron transport chain in

the inner membrane of the mito-chondrion is very similar to the elec-tron transport chain of the thylakoidmembrane in the chloroplast of plantcells during photosynthesis. NADHand FADH2 pass energized electronsfrom protein to protein within themembrane, slowly releasing smallamounts of the energy containedwithin the electron. Some of thatenergy is used directly to form ATP;some is used to pump H+ ions intothe center of the mitochondrion.Consequently, the mitochondrioninner membrane becomes positivelycharged because of the high concen-tration of positively charged hydro-gen ions. At the same time, the exte-rior of the membrane is negativelycharged, which further attractshydrogen ions. The gradient thatresults is both electrical and chemi-cal: electrical because of the chargedifference; chemical because of theconcentration difference.

The inner membrane of the mito-chondrion forms ATP from this elec-

trochemical gradient of H+ ionsacross the membrane, just as the thy-lakoid membranes did in the chloro-plasts. The electron transport chainand the formation of ATP are shownin Figure 9.11.

The final electron acceptor in thechain is oxygen, which reacts withfour hydrogen ions (4H+) to form twomolecules of water (H2O). This is whyoxygen is so important to our bodies.Without oxygen, the proteins in theelectron transport chain cannot passalong the electrons. If a protein can’tpass an electron along, it cannotaccept another electron either. Veryquickly, the entire chain becomesblocked and the aerobic processes ofcellular respiration cannot occur.

Overall, the electron transportchain produces 32 ATP molecules.Obviously, the aerobic method ofATP production is very effective.However, an anaerobic process canproduce ATP for short periods oftime in the absence of oxygen, but itis not efficient enough to generatesufficient quantities of ATP for all ofthe cell’s needs.


Figure 9.11In the electron trans-port chain, the carriermolecules NADH and FADH2 give upelectrons that passthrough a series ofreactions. Oxygen is the final electronacceptor.

Fermentation

There are times when your cellsare without oxygen for a short periodof time. When this happens, ananaerobic process called fermenta-tion follows glycolysis and provides ameans to continue producing ATPuntil oxygen is available again. Someorganisms exist in anaerobic environ-ments and use fermentation to pro-duce energy. There are two majortypes of fermentation: lactic acid fer-mentation and alcoholic fermenta-tion. Figure 9.12 and Table 9.1 com-pare the two processes. Perform theProblem-Solving Lab shown here tofurther compare and contrast cellularrespiration and fermentation.


Is cellular respiration better than fermentation? Themethods by which organisms derive ATP from their food maydiffer; however, the result, the production of ATP molecules,is similar.

AnalysisStudy Table 9.1 and evaluate cellular respiration, lactic

acid fermentation, and alcoholic fermentation.

Thinking Critically1. Describe some of the reasons why cellular respiration pro-

duces so much more ATP than does fermentation.2. Describe a situation when a human would use more than

one of the above processes.3. Think of an organism that might generate ATP only by

fermentation and consider why fermentation is the bestprocess for the organism.

Problem-Solving Lab 9-3Problem-Solving Lab 9-3 AcquiringInformation

Figure 9.12Fermentation produces ATP when oxygenfor respiration is scarce.

Lactic acid

glucose

glycolysis (pyruvic acid)

lactic acid+

2 ATP

Alcoholic

glucose


carbon dioxide+

alcohol+

2 ATP

Cellular respiration

glucose


carbon dioxide+

water+

38 ATP

Table 9.1 Comparison of lactic acid and alcoholic fermentation

Lactic acid and alcoholic fermentation are compa-rable in the production of ATP, but compared tocellular respiration, it is obvious that fermentationis far less efficient in ATP production.

AA

Lactic acid fermenta-tion occurs in somebacteria, in plants,and in most animals,including humans.

BB

Determine if Apple JuiceFerments Organisms such asyeast have the ability to breakdown food molecules and syn-thesize ATP when no oxygen isavailable. When the appropri-ate food is available, yeast cancarry out alcoholic fermenta-tion, producing CO2. Thus, theproduction of CO2 can be usedto judge whether alcoholic fer-mentation is taking place.

Procedure! Carefully study the diagram

and set up the experimentas shown.

@ Hold the test tube in abeaker of warm (not hot)water and observe.

Analysis1. What were the gas bubbles that came from the plastic

pipette?2. Predict what would happen to the rate of bubbles given

off if more yeast were present in the mixture.3. Why was the test tube placed in warm water?4. On the basis of your observations, was this process aerobic

or anaerobic?

MiniLab 9-2MiniLab 9-2 PredictingLactic acid fermentation

Lactic acid fermentation is one ofthe processes that supplies energywhen oxygen is scarce. You know thatunder anaerobic conditions, the elec-tron transport chain backs up becauseoxygen is not present as the final electron acceptor. As NADH andFADH2 arrive with energized elec-trons from the citric acid cycle andglycolysis, they cannot release theirenergized electrons to the electrontransport chain. The citric acid cycleand glycolysis cannot continue with-out a steady supply of NAD+ andFAD.

The cell does not have a methodto replace FAD during anaerobicconditions; however, NAD+ can bereplaced through lactic acid fermen-tation. In lactic acid fermentation,two molecules of pyruvic acid useNADH to form two molecules oflactic acid. This releases NAD+ to beused in glycolysis, allowing two ATPmolecules to be formed for each glu-cose molecule. The lactic acid istransferred from muscle cells, whereit is produced during strenuous exer-cise, to the liver that converts it backto pyruvic acid. The lactic acid thatbuilds up in muscle cells results inmuscle fatigue.

Alcoholic fermentation

Another type of fermentation,alcoholic fermentation, is used by, among others, yeast cells to pro-duce CO2 and ethyl alcohol. Whenmaking bread, like that shown in Figure 9.13, yeast cells produce CO2that forms bubbles in the dough.Eventually the heat of baking thebread kills the yeast and the bubblepockets are left to lighten the bread.You can do the activity in theMiniLab on this page to examine fer-mentation in apple juice.


Figure 9.13Alcoholic fermentation bythe yeast in a bread recipeproduces CO2 bubbles thatraise the bread dough.

Yeast andapple juice

Plastic pipette

Water

Test tube

Metalwashers

ComparingPhotosynthesis andCellular Respiration

The production and breakdown offood molecules are accomplished bydistinct processes that bear certainsimilarities. Both photosynthesis andcellular respiration use electron carri-ers and a cycle of chemical reactionsto form ATP. Both use an electrontransport chain to form ATP and tocreate a chemical and a concentrationgradient of H+ within a cell. Thishydrogen gradient can be used toform ATP by an alternative process.

However, despite using such simi-lar tools, the two cellular processesaccomplish quite different tasks.Photosynthesis produces high-energycarbohydrates and oxygen from thesun’s energy, whereas cellular respira-tion uses oxygen to break down carbo-hydrates to form ATP and compoundswith a much lower level of energy.Also, one of the end products of cellu-lar respiration is CO2, which is one ofthe beginning products for photosyn-thesis. The oxygen produced duringphotosynthesis is a critical moleculenecessary for cellular respiration.Table 9.2 in Figure 9.14 comparesthese complementary processes.


Section AssessmentSection AssessmentUnderstanding Main Ideas1. Compare the ATP yields of glycolysis and aerobic

respiration.2. How do alcoholic and lactic acid fermentation differ?3. How is most of the ATP from aerobic respiration

produced?4. When is lactic acid fermentation important to the

cell?

Thinking Critically5. Compare the energy-producing processes in a

jogger’s leg muscles with those of a sprinter’s

leg muscles. Which is likely to build up more lactic acid? Which runner is more likely to be outof breath after running? Explain.

6. Making and Using Tables Use the sectioncalled Comparing Photosynthesis and CellularRespiration to make a set diagram summarizingthe similarities and differences between the two processes. For more help, refer toOrganizing Information in the Skill Handbook.


Photosynthesis

food accumulated

energy from sun stored in glucose

carbon dioxide taken in

oxygen given off

produces glucose from PGAL

goes on only in light

occurs only in presence of chlorophyll

Cellular Respiration

food broken down

energy of glucose released

carbon dioxide given off

oxygen taken in

produces CO2 and H2O

goes on day and night

occurs in all living cells

Table 9.2 Comparison of photosynthesis and cellular respirationFigure 9.14Photosynthesis andcellular respirationare complementaryprocesses. Therequirements of oneprocess are the prod-ucts of the other.


What factors influence photosynthesis?

INTERNETINTERNET

Oxygen is one of the products of photosynthesis. Because oxygen is only slightly soluble in water, aquatic plants such as Elodea

give off visible bubbles of oxygen as they carry out photosynthesis. By measuring the rate at which bubbles form, you can measure therate of photosynthesis.

ProblemHow do different wavelengths of

light a plant receives affect its rate ofphotosynthesis?

ObjectivesIn this BioLab, you will:■ Observe photosynthesis in an

aquatic organism.

■ Measure the rate of photosynthesis.■ Observe how various wavelengths

of light influence the rate of photo-synthesis.

■ Use the Internet to collect and com-pare data from other students.

Materials1000-mL beakerthree Elodea plantsstringwasherscolored cellophane, assorted colorslamp with reflector and 150-watt

bulb0.25% sodium hydrogen

carbonate (baking soda) solution

watch with second hand

Safety PrecautionsAlways wear goggles in the lab.

Skill HandbookUse the Skill Handbook if you need

additional help with this lab.

PREPARATIONPREPARATION


1. Interpreting Observations Fromwhere did the bubbles of oxygenemerge? Why?

2. Making Inferences Explain howcounting bubbles measures therate of photosynthesis.

3. Using the Internet Make a graphof your data and data posted byother students with the rate ofphotosynthesis per minute plot-ted against the wavelength oflight you tested for both the con-trol and experimental setups.

Write a sentence or two explain-ing the graph.

ANALYZE AND CONCLUDEANALYZE AND CONCLUDE

Sharing Your DataSharing Your Data

Find this BioLab on the Glencoe Science Web site

at science.glencoe.com. Post your data inthe data table provided for this activity. Usethe additional data from other studentswho tested wavelengths other than thoseyou tested to expand your graph.

1. Construct a basic setup like theone shown opposite.

2. Create a data table to record your measurements. Be sure toinclude a column for each colorof light you will investigate and a column for the control experi-ment.

3. Place the Elodea plants in thebeaker, then completely cover the plants with water. Add someof the baking soda solution. The solution provides CO2 forthe aquarium plants. Be sure to use the same amount ofwater and solution for eachtrial.

4. Conduct a control experiment bydirecting the lamp (without colored cellophane) on the plant and notice when you see the bubbles.

5. Observe and record the numberof oxygen bubbles that Elodeagenerates in five minutes.

6. Repeat steps 4 and 5 with a pieceof colored cellophane. Recordyour observations.

7. Repeat steps 4 and 5 with a different color of cellophane andrecord your observations.

8. Go to the Glencoe Science Web Site at the address shownbelow to post your data.

PROCEDUREPROCEDURE

Bubbles observed in five minutes

Color 1 Color 2Control

Data Table

BIOLOGY


In photosynthesis, light energy is converted into chemical energy. To begin the process,

light is absorbed by colorful pigment molecules contained in chloroplasts.

Apigment is a substance that can absorb thevarious wavelengths of visible light. You

can observe the colors of these wavelengths byletting sunlight pass through a prism to create a“rainbow,” or spectrum, that has red light on oneend, violet on the other, and orange, yellow,green, and blue light in between.

Every photosynthetic pigment is distinctivein that it absorbs certain wavelengths in the visi-ble light spectrum.

Chlorophylls a and b The principal pigmentof photosynthesis is chlorophyll. Chlorophyllexists in two forms designated as a and b. Chloro-phyll a and b both absorb light in the violet toblue and red to red-orange parts of the spectrum,although at somewhat different wavelengths.These pigments also reflect green light, which iswhy plant leaves appear green.

When chlorophyll b absorbs light, it transfersthe energy it acquires to chlorophyll a, whichthen feeds that energy into the chemical reac-tions that lead to the production of ATP andNADP. In this way, chlorophyll b acts as an“accessory” pigment by making it possible forphotosynthesis to occur over a broader spectrumof light than would be possible with chlorophylla alone.

Carotenoids and phycobilins Carotenoids and phycobilins are other kinds of accessory pig-ments that absorb wavelengths of light differentfrom those absorbed by chlorophyll a and b, andso extend the range of light that can be used forphotosynthesis.

Carotenoids are yellow-orange pigments.They are found in all green plants, but theircolor is usually masked by chlorophyll.


ConnectionChemistryChemistry

Connection Plant Pigments

How do you think accessory pigments may haveinfluenced the spread of photosynthetic organ-isms into diverse habitats such as the deep sea?

To find out more about pigments, visit the Glencoe

Science Web site. science.glencoe.com

CONNECTION TO BIOLOGYCONNECTION TO BIOLOGY

Carotenoids are also found in cyanobacteria andin brown algae. A particular carotenoid calledfucoxanthin gives brown algae their characteris-tic dark brown or olive green color.

Phycobilins are blue and red. Red algae gettheir distinctive blood-red coloration from phy-cobilins. Some phycobilins can absorb wave-lengths of green, violet, and blue light that pene-trate into deep water. One species of red algaethat contains these pigments is able to live atocean depths of 269 meters (884 feet). Thealgae’s pigments absorb enough of the incrediblyfaint light that penetrates to this depth—only0.0005 percent of what is available at the water’ssurface—to power photosynthesis.

Pigments color cyanobacteria(above) and red algae (inset).

Magnification: 1630�

BIOLOGY


Chapter 9 AssessmentChapter 9 Assessment

SUMMARYSUMMARY

Section 9.1

Section 9.2

Section 9.3

Main Ideas■ ATP is the molecule that stores energy for easy

use within the cell.■ ATP is formed when a phosphate group is

added to ADP. When ATP is broken down,ADP and phosphate are formed and energy isreleased.

■ ATP is the main link between energy-releasingand energy-using reactions.

Vocabulary(ADP) adenosine

diphosphate (p. 228)(ATP) adenosine

triphosphate (p. 228)

ATP in aMolecule

Main Ideas■ Photosynthesis is the process by which cells use

light energy to make carbohydrates. ■ Chlorophyll in the chloroplasts of plant cells

traps light energy needed for photosynthesis.■ The light reactions of photosynthesis produce

ATP and result in the splitting of water molecules.

■ The reactions of the Calvin cycle make carbohydrates using CO2 along with ATP andhydrogen from the light reactions.

VocabularyCalvin cycle (p. 234)chlorophyll (p. 232)electron transport chain

(p. 232)light-dependent

reactions (p. 231)light-independent

reactions (p. 231)NADP+ (p. 233)photolysis (p. 234)photosynthesis (p. 231)pigments (p. 232)

Main Ideas■ Cellular respiration is the process by which cells

break down carbohydrates to release energy. ■ Cellular respiration takes place in mitochondria,

uses oxygen, and yields many more ATPs thando anaerobic processes.

■ Energy can be released anaerobically byglycolysis followed by alcoholic or lacticacid fermentation.

Vocabulary aerobic respiration

(p. 237)alcoholic fermentation

(p. 242)anaerobic respiration

(p. 237)cellular respiration

(p. 237)citric acid cycle (p. 238)glycolysis (p. 237)lactic acid fermentation

(p. 242)

Getting Energyto Make ATP

CHAPTER 9 ASSESSMENT 247

1. Which of the following is a product of theCalvin cycle?a. carbon dioxide c. oxygenb. NADP+ d. FADH2

UNDERSTANDING MAIN IDEASUNDERSTANDING MAIN IDEAS 2. ________ processes require oxygen, whereas________ processes do not.a. anaerobic—aerobicb. aerobic—anaerobicc. photolysis—aerobicd. aerobic—respiration

Photosynthesis:Trapping theSun’s Energy


3. During all energy conversions, some of theenergy is converted to ________.a. carbon dioxide c. heatb. water d. sunlight

4. Four molecules of glucose would give a netyield of ________ ATP following glycolysis.a. 8 c. 4b. 16 d. 12

5. In which of the following structures do thelight-independent reactions of photosynthe-sis take place?a. c.

b. d.

6. What is the first process in the cell to beaffected by anaerobic conditions?a. citric acid cycleb. fermentationc. glycolysisd. electron transport chain

7. Which molecule provides the most accessiblesource of energy for cell organelles?a. glucose c. starchb. ATP d. carbon dioxide

8. Which of the following uses no energy?a. glycolysis c. light reactionsb. Calvin cycle d. muscle contraction

9. Which of the following transports high-energy electrons in photosynthesis?

a. NADP+ c. FADb. NAD+ d. ATP

10. When yeast ferments the sugar in a breadmixture, what is produced that causes thebread dough to rise?a. carbon dioxide c. ethyl alcoholb. water d. oxygen

11. Plants must have a constant supply of________ for photosynthesis, but they pro-vide ________ for cellular respiration.

12. ________ is the first molecule to provide elec-trons for photosynthesis.

13. The ________ membrane is the site of photo-synthesis, whereas the ________ is the site forcellular respiration.

14. ________ is the process by which electronsare restored to chlorophyll after photosyn-thesis.

15. ________ acts as a source of CO2 in cellularrespiration.

16. ________ occurs in the inner membrane ofthe mitochondrion.

17. In fermentation, pyruvic acid can form________ or ________ in addition to NAD+.

18. ATP stores energy because the three________ have a negative charge and they________ each other.

19. ________ produces muscle soreness, whereas________, another type of fermentation, pro-duces ethyl alcohol.

20. ________ such as chlorophyll are the chemi-cal substances that give color to plants.

21. Why would human muscle cells containmany more mitochondria than skin cells?

22. How are cellular respiration and photosyn-thesis complementary processes?

23. What happens to sunlight that strikes a leafbut is not trapped by photosynthesis?

24. What might happen to Earth’s atmosphere ifphotosynthesis suddenly stopped?

APPLYING MAIN IDEASAPPLYING MAIN IDEAS

248 CHAPTER 9 ASSESSMENT

TEST–TAKING TIPTEST–TAKING TIP

Take 5 and Stay Sharp Wanting to perform well on your exam is praise-worthy. But if you study for long periods of time,you could actually end up hurting your chances fora good score. Remember to take frequent shortbreaks in your studies to keep your mind fresh.


CHAPTER 9 ASSESSMENT 249

25. If you were planning on studying the com-pounds that could possibly be the source ofthe oxygen released during photosynthesis,which compounds would you need to consider?

26. Formulating Hypotheses Elodea sprigs wereplaced under a white light, and the rate ofphotosynthesis was measured by counting thenumber of oxygen bubbles per minute for tenminutes. Predict the rate of photosynthesis ifa piece of red cellophane were placed overthe white light.

27. Observing and Inferring Yeast cells must beforced to ferment by placing them in an envi-ronment without any oxygen. Why wouldthe yeast cells carry out aerobic respirationrather than fermentation when oxygen is pre-sent?

28. Recognizing Cause and Effect A windowplant native to the desert of South Africa hasleaves that grow almost entirely undergroundwith only the transparent tip of the leaf pro-truding above the soil surface. Suggest howthis adaptation aids the survival of this plant.

29. Concept Mapping Make a concept mapusing the following vocabulary terms: cellularrespiration, glycolysis, citric acid cycle, electron transport chain.

THINKING CRITICALLYTHINKING CRITICALLY

ASSESSING KNOWLEDGE & SKILLSASSESSING KNOWLEDGE & SKILLS

Yeast cells and sucrose were placed in a testtube, and the tube was then plugged. Theyeast–sucrose mixture incubated for 24hours. Gas bubbles began to rise to the topof the tube. After 24 hours, no sucrose wasleft in the solution.

Interpreting Data Use the graph to answerthe questions below.1. What process was the yeast using to

digest the sucrose at the beginning ofthe experiment?a. photosynthesisb. anaerobic respirationc. aerobic respirationd. light-dependent reactions

2. Which of the following would be left inthe solution after 24 hours?a. sucrose c. oxygenb. lactic acid d. ethyl alcohol

3. What gas would be found in the top ofthe tube after the incubation period?a. carbon dioxide c. hydrogenb. oxygen d. nitrogen

4. Making a Table Construct a table fromthe graph showing the oxygen levels andcarbon dioxide levels.

For additional review, use the assessmentoptions for this chapter found on the Biology: TheDynamics of Life Interactive CD-ROM and on theGlencoe Science Web site.science.glencoe.com

CD-ROM

Gas

leve

l (m

L)0 6 12

Hours

18 24

10

5

O2 CO2

Yeast Cell Respiration

are the steps of

2.

4.

1. 3.

which takes place in mitochondria


The Chemistry of LifeAlthough you are studying biology, chemistry

is fundamental to all biological functions.Understanding some of the basic concepts ofchemistry will enhance your understanding of thebiological world.

Elements and AtomsEvery substance in and on Earth is a combina-

tion of elements. An atom, the smallest compo-nent of an element, is formed by layers of elec-trons around a nucleus made of protons and neu-trons. Atoms come together to form molecules.

FOCUS ON HISTORYFOCUS ON HISTORY

In the 1600s, Anton van Leeuwenhoekwas the first person to view living

organisms through a microscope.Another scientist, Robert Hooke, namedthe structures cells. Two hundred yearslater, several scientists, includingMatthias Schleiden, Theodor Schwann,and Rudolph Virchow continued tostudy animal and plant tissues under themicroscope. Conclusions from all scien-tists were combined to form the celltheory:

1. All organisms are composed of one ormore cells.

2. The cell is the basic unit of organization oforganisms.

3. All cells come from preexisting cells.

The Cell Theory

250

The Life of a CellAll organisms are made up of cells, and

each cell is like a complex, self-contained machine that can perform all of the life func-tions of the cell. Yet as small as they are, all of the mechanisms and processes of these little machines are not fully known, and scientists continue to unravel the marvelous mysteries of the living cell.

For a preview of the cell unit, study this BioDigest before you read the chapters.After you have studied the cell chapters, you can use the BioDigest to review the unit.

BIODIGESTBIODIGEST

VITAL STATISTICSVITAL STATISTICS

Carbon IsotopesIsotopes of carbon contain different numbersof neutrons.Carbon 12: six protons and six neutronsCarbon 13: six protons and seven neutronsCarbon 14: six protons and eight neutrons

van Leeuwenhoek mighthave viewed microorgan-isms like these found in adroplet of pond water.

Cells are microscopic machines

Magnification:200�

Organic CompoundsCarbohydrates are chemical compounds

made up of carbon, hydrogen, and oxygen mole-cules. Common carbohydrates include sugars,starches, and cellulose. Lipids, known as fats andoils, contain a glycerol backbone and three fattyacid chains. Proteins are a combination of aminoacids connected by peptide bonds.

Eukaryotes andProkaryotes

All cells are surrounded by a plasma mem-brane. Eukaryotic cells contain membrane-boundstructures within the cell called organelles. Cellswithout internal membrane-bound structures arecalled prokaryotic cells.

The Life of a Cell

251

BIODIGESTBIODIGEST

Polysaccharides

A polysaccharide is a type of carbohydratemade up of a chain of monosaccharides.

Amino acids canbe joined withpeptide bonds toform proteins.

A eukaryoticcell containsmembrane-boundorganelles.

A prokaryotic celldoes not containmembrane-boundorganelles.

Organelles

Lipids are made up of aglycerol backbone and threefatty acid chains.

Glycerol

Fatty acid chains



Carbohydrate

Phospholipidbilayer

ProteinFatty acid tail

Phosphate head

Cell Organelles The organelles of a cell work together to

carry out the functions necessary for cell survival.

Plasma Membrane According to the fluid mosaic model, the

plasma membrane is formed by two layers ofphospholipids with the fatty acid chains back-to-back; the phosphate groups face the externalenvironment, and proteins are embedded in themembrane.

Control of Cell FunctionsThe nucleus contains the master plans for

proteins, which are then produced by organellescalled ribosomes. The nucleus also controls cellu-lar functions.

Assembly, Transport, and Storage

The cytoplasm suspends the cell’s organelles,including endoplasmic reticulum, Golgi appara-tus, vacuoles, and lysozomes. The endoplasmicreticulum and Golgi apparatus transport andmodify proteins.

Energy Transformers Chloroplasts are found in plant cells and

capture the sun’s light energy so it can be transformed into useable chemical energy.Mitochondria are found in both animal and plant cells and transform the food you eat into a useable energy form.

Support and LocomotionA network of microfilaments and micro-

tubules attach to the plasma membrane to givethe cell structure. Cilia are short, numerous pro-jections that move like a wave. Flagella are longerprojections that move in a whiplike fashion topropel a cell.

The Life of a Cell

252

BIODIGESTBIODIGEST

The plasma membrane is composed ofa lipid bilayer with embedded proteins.

Plasma membrane

Mitochondrion

Nucleus

Chloroplast

Cell wall

Vacuole

Golgi apparatus

Ribosomes

Roughendoplasmic

reticulum

Magnification: 46 000�Magnification: 11 250�

Plant cell Animal cell

ProphaseTelophase

Anaphase

Interphase

Metaphase

Diffusion and OsmosisThe selectively permeable plasma membrane

allows only certain substances to cross. Diffusionis the movement of a substance from an area ofhigher concentration to an area of lower concen-tration. Diffusion of water across a selectively per-meable membrane is called osmosis.

Simple diffusion across a membrane occurs by random movement. Facilitated diffusionrequires proteins to bind and help move mole-cules across the membrane. Active transportrequires energy to move molecules across a concentration gradient.

MitosisAs cells grow, they reach a size

where the plasma membrane can-not transport enough nutrientsand wastes to maintain cellgrowth. At this point, the cellundergoes mitosis and divides.

Prior to mitosis is a period ofintense metabolic activity calledinterphase. The first stage of mito-sis is prophase, when the dupli-cated chromosomes condense andthe mitotic spindle forms on thetwo opposite ends of the cell. Thechromosomes line up in the centerof the cell during metaphase andslowly separate during anaphase.In telophase the nucleus dividesfollowed by cytokinesis, which sep-arates the daughter cells.

The Life of a Cell

253

BIODIGESTBIODIGEST

Carrier protein

Higher concentrationof ions

Ions Lower concentrationof ions Energy

Membrane proteins can transport substances against the concentrationgradient in active transport.

During the stages of mitosis, chromosomes are replicated and separatedinto two daughter cells.


Cellular EnvironmentsIsotonic solution: same number of mole-cules inside and outside the cellHypotonic solution: more molecules insidethe cell; water enters the cellHypertonic solution: fewer moleculesinside the cell; water leaves the cell


Energy in a Cell Adenosine triphosphate (ATP) is the most

commonly used source of energy in a cell. Twoorganelles are involved in forming ATP fromother sources of energy.

Chloroplasts in plant cells harvest energyfrom the sun’s rays and convert it to ATP using

light-dependent reactions. Light-independentreactions convert energy into carbohydrates suchas starch through the Calvin cycle.

Mitochondria, found in both plant and ani-mal cells, convert food energy into ATP through aseries of chemical reactions that include glycolysis,the citric acid cycle, and the electron transportchain.

FOCUS ON HEALTHFOCUS ON HEALTH

Cancer is one condition that can result when acell can no longer control the rate of mitosis.

Cancer may be due to factors inside the cell, such asenzyme overproduction or production at the wrongtime. Cancer can also be a result of environmentalfactors.

In recent years, much emphasis has been placedon healthy lifestyles that can help prevent cancer.Eating diets rich in fruits and vegetables, exercisingregularly, and quitting or avoiding smoking can allhelp reduce the incidence of cancer.

Cancer

The Life of a Cell

254

BIODIGESTBIODIGEST

In chloroplasts, the Calvin cycle allows thesun’s energy to be stored as carbohydratesfor later use.

In mitochondria, the citric acidcycle allows the breakdown offood sources to form ATP.

This melanoma is an exampleof skin cancer.


Magnification:70 875�

The Life of a Cell

255

BIODIGESTBIODIGEST


ATP production for each molecule of glucoseGlycolysis: produces a net gain of two ATP, two NADH, and

two H+Acetyl-CoA formation: produces two NADHCitric acid cycle (Krebs cycle): produces two ATP, six NADH,

and two FADHElectron transport chain: produces 32 ATP from NADH and

FADHLactic acid fermentation: produces two ATP

BIODIGEST ASSESSMENTBIODIGEST ASSESSMENT

Understanding Main Ideas1. What is a starch?

a. a lipid c. a proteinb. a carbohydrate d. a peptide

2. The building blocks of proteins are________.a. fatty acids c. amino acidsb. monosaccharides d. glycerol

3. A cell that does not contain internal, mem-brane-bound structures is a ________.a. prokaryoteb. eukaryotec. yeast celld. a cell with organelles

4. The organelle that produces proteins is the________.a. nucleus c. ribosomesb. lysozome d. vacuole

5. What structure is part of the cell’s skele-ton?a. Golgi apparatusb. nucleusc. endoplasmic reticulumd. microfilaments

6. The movement of a substance across theplasma membrane against the concentra-tion gradient by binding to a protein is________.a. facilitated transportb. osmosisc. simple diffusiond. active transport

7. What structure is involved in movement?a. cilia c. vacuoleb. lysozomes d. chloroplasts

8. The stage of mitosis when the chromatidsseparate and move to opposite sides ofthe cell is ________.a. prophase c. anaphaseb. metaphase d. telophase

9. The organelle that transforms light energyto ATP is ________.a. mitochondrionb. endoplasmic reticulumc. chloroplastd. nucleus

10. Which is involved in capturing light energyfrom the sun?a. light-dependent reactionsb. Citric acid cyclec. Calvin cycled. light-independent reactions

Thinking Critically1. Compare and contrast the functions of

chloroplasts and mitochondria in energytransformation.

2. Describe the organelles of a eukaryoticcell.

3. Relate transport across the plasma mem-brane in isotonic, hypertonic, and hypo-tonic solutions to changes in cell shapedue to water movement across the mem-brane.

Energy from ATP

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Chapter 9: Energy in a Cell - Polson Schoolsblogs.polson.k12.mt.us/dobrien/files/2014/02/chap09.pdf · contracting during exercise, your heart pumping, ... Figure 9.3 To access the