CHAPTER 7 CELLULAR RESPIRATION

FIGURE 7-6ALCOHOLIC FERMENTATIONGlucoseGlycolysisCCCCCCPyruvic acidCCCNAD Some cells engage in alcoholicfermentation, converting pyruvic acidinto ethyl alcohol. Again, NADH isoxidized to NAD .CO2NADH H CEthyl alcohol2-carboncompoundCCCCAlcoholic FermentationSome plant cells and unicellular organisms, such as yeast, use aprocess called alcoholic fermentation to convert pyruvic acid intoethyl alcohol. After glycolysis, this pathway requires two steps,which are shown in Figure 7-6. In the first step, a CO2 molecule isremoved from pyruvic acid, leaving a two-carbon compound. In thesecond step, two hydrogen atoms are added to the two-carboncompound to form ethyl alcohol. As in lactic acid fermentation,these hydrogen atoms come from NADH and H , regeneratingNAD for use in glycolysis.Alcoholic fermentation by yeast cells such as those in Figure7-7 is the basis of the wine and beer industry. Yeasts are a typeof fungi. These microorganisms cannot produce their own food.But supplied with food sources that contain sugar (such as fruitsand grains), yeast cells will perform the reactions of fermentation, releasing ethyl alcohol and carbon dioxide in the process.The ethyl alcohol is the ‘alcohol’ in alcoholic beverages. To maketable wines, the CO2 that is generated in the first step of fermentation is allowed to escape. To make sparkling wines, such aschampagne, CO2 is retained within the mixture, “carbonating”the beverage.Bread making also depends on alcoholic fermentation performed by yeast cells. In this case, the CO2 that is produced by fermentation makes the bread rise by forming bubbles inside thedough, and the ethyl alcohol evaporates during baking.FIGURE 7-7The yeast Saccharomyces cerevisiae isused in alcohol production and breadmaking.EFFICIENCY OF GLYCOLYSISHow efficient is glycolysis in obtaining energy from glucoseand using it to make ATP from ADP? To answer this question,one must compare the amount of energy available in glucosewith the amount of energy contained in the ATP that is producedby glycolysis. In such comparisons, energy is often measuredin units of kilocalories (kcal). One kilocalorie equals 1,000 calories (cal).Word Roots and Originskilocaloriefrom the Greek chilioi, meaning“thousand,” and the Latin calor,meaning “heat”C E L L U L A R R E S P I R AT I O NCopyright by Holt, Rinehart and Winston. All rights reserved.135

www.scilinks.orgTopic: FermentationKeyword: HM60568Scientists have calculated that the complete oxidation of a standard amount of glucose releases 686 kcal. The production of astandard amount of ATP from ADP absorbs a minimum of about7 kcal, depending on the conditions inside the cell. Recall that twoATP molecules are produced from every glucose molecule that isbroken down by glycolysis.Efficiency ofglycolysisEnergy required to make ATP"""""Energy released by oxidation of glucose2 ! 7 kcal"" ! 100%686 kcal2%You can see that the two ATP molecules produced during glycolysis receive only a small percentage of the energy that could bereleased by the complete oxidation of each molecule of glucose.Much of the energy originally contained in glucose is still held inpyruvic acid. Even if pyruvic acid is converted into lactic acid orethyl alcohol, no additional ATP is synthesized. It’s clear that glycolysis alone or as part of fermentation is not very efficient intransferring energy from glucose to ATP.Organisms probably evolved to use glycolysis very early in thehistory of life on Earth. The first organisms were bacteria, andthey produced all of their ATP through glycolysis. It took morethan a billion years for the first photosynthetic organisms toappear. The oxygen they released as a byproduct of photosynthesis may have stimulated the evolution of organisms that makemost of their ATP through aerobic respiration.By themselves, the anaerobic pathways provide enough energyfor many present-day organisms. However, most of these organismsare unicellular, and those that are multicellular are very small. All ofthem have limited energy requirements. Larger organisms have muchgreater energy requirements that cannot be satisfied by glycolysisalone. These larger organisms meet their energy requirements withthe more efficient pathways of aerobic respiration.SECTION 1 REVIEW1. Explain the role of organic compounds in cellularrespiration.2. For each six-carbon molecule that begins glycol-5. Applying Information A large amount of ATP inysis, identify how many molecules of ATP areused and how many molecules of ATP areproduced.a cell inhibits the enzymes that drive the firststeps of glycolysis. How will this inhibition ofenzymes eventually affect the amount of ATP inthe cell?3. Distinguish between the products of the two6. Predicting Results How might the efficiency oftypes of fermentation discussed in this section.4. Calculate the efficiency of glycolysis if 12 kcalof energy are required to transfer energy fromglucose to ATP.136CRITICAL THINKNGCHAPTER 7glycolysis change if this process occurred in onlyone step? Explain your answer.7. Relating Concepts In what kind of environment would you expect to find organisms thatcarry out fermentation?

SECTION 2A E RO B I C R E S P I R AT I O NOBJECTIVESRelate aerobic respiration to thestructure of a mitochondrion. Summarize the events of theKrebs cycle. Summarize the events of theelectron transport chain andchemiosmosis. Calculate the efficiency of aerobicrespiration. Contrast the roles of glycolysisand aerobic respiration in cellularrespiration. In most cells, glycolysis does not result in fermentation.Instead, when oxygen is available, pyruvic acid undergoesaerobic respiration, the pathway of cellular respiration thatrequires oxygen. Aerobic respiration produces nearly 20 timesas much ATP as is produced by glycolysis alone.OVERVIEW OF AEROBICRESPIRATIONVOCABULARYAerobic respiration has two major stages: the Krebs cycle and theelectron transport chain, which is associated with chemiosmosis(using the energy released as protons move across a membrane tomake ATP). In the Krebs cycle, the oxidation of glucose that beganwith glycolysis is completed. As glucose is oxidized, NAD isreduced to NADH. In the electron transport chain, NADH is used tomake ATP. Although the Krebs cycle also produces a small amountof ATP, most of the ATP produced during aerobic respiration ismade through the activities of the electron transport chain andchemiosmosis. The reactions of the Krebs cycle, the electron transport chain, and chemiosmosis occur only if oxygen is present inthe cell.In prokaryotes, the reactions of the Krebs cycle and the electrontransport chain take place in the cytosol of the cell. In eukaryoticcells, however, these reactions take place inside mitochondriarather than in the cytosol. The pyruvic acid that is produced in glycolysis diffuses across the double membrane of a mitochondrionand enters the mitochondrial matrix. The mitochondrial matrix isthe space inside the inner membrane of a mitochondrion. Figure7-8 illustrates the relationships between these mitochondrialparts. The mitochondrial matrix contains the enzymes needed tocatalyze the reactions of the Krebs cycle.OutermembraneInnermembranemitochondrial matrixacetyl CoAKrebs cycleoxaloacetic acidcitric acidFADFIGURE 7-8In eukaryotic cells, the reactions ofaerobic respiration occur insidemitochondria. The Krebs cycle takesplace in the mitochondrial matrix, andthe electron transport chain is locatedin the inner membrane.Cristae MatrixMITOCHONDRIONC E L L U L A R R E S P I R AT I O NCopyright by Holt, Rinehart and Winston. All rights reserved.137

Pyruvic acidC C CCoANAD NADH H When pyruvic acid enters the mitochondrial matrix, it reactswith a molecule called coenzyme A to form acetyl (uh-SEET-uhl) coenzyme A, abbreviated acetyl CoA (uh-SEET-uhl KOH-AY). This reactionis illustrated in Figure 7-9. The acetyl part of acetyl CoA containstwo carbon atoms, but as you learned earlier, pyruvic acid is athree-carbon compound. The carbon atom that is lost in the conversion of pyruvic acid to acetyl CoA is released in a molecule ofCO2. This reaction reduces a molecule of NAD to NADH.CO2 CTHE KREBS CYCLEAcetyl CoAC CFIGURE 7-9Glycolysis yields two molecules ofpyruvic acid. In aerobic respiration, eachmolecule of pyruvic acid reacts withcoenzyme A (CoA) to form a molecule ofacetyl CoA. Notice that CO2, NADH, andH are also produced in this reaction.The Krebs cycle is a biochemical pathway that breaks down acetylCoA, producing CO2, hydrogen atoms, and ATP. The reactions thatmake up the cycle were identified by Hans Krebs (1900–1981), aGerman biochemist. The Krebs cycle has five main steps. Ineukaryotic cells, all five steps occur in the mitochondrial matrix.Examine Figure 7-10 as you read about the steps in the Krebs cycle.In step 1 , a two-carbon molecule of acetyl CoA combines witha four-carbon compound, oxaloacetic (AHKS-uh-loh-uh-SEET-ik) acid,to produce a six-carbon compound, citric (SI-trik) acid. Notice thatthis reaction regenerates coenzyme A.In step 2 , citric acid releases a CO2 molecule and a hydrogenatom to form a five-carbon compound. By losing a hydrogen atomwith its electron, citric acid is oxidized. The electron in the hydrogen atom is transferred to NAD , reducing it to NADH.CoAFIGURE 7-10The Krebs cycle takes place in themitochondrial matrix and involvesfive main steps.Citric acidAcetyl CoAC CCO2 CNAD C C C C C C5-carboncompoundOxaloaceticacidC C C CC C C C CTHEKREBSCYCLEMitochondrialmatrixNADH H 53CO2 CNAD NADH H NAD MITOCHONDRIONNADH H 21ADP phosphate4-carboncompound44-carboncompoundC C C CATPC C C CFADH2138FADCHAPTER 7Copyright by Holt, Rinehart and Winston. All rights reserved.

In step 3 , the five-carbon compound formed in step 2 alsoreleases a CO2 molecule and a hydrogen atom, forming a fourcarbon compound. Again, NAD is reduced to NADH. Notice that inthis step a molecule of ATP is also synthesized from ADP.In step 4 , the four-carbon compound formed in step 3releases a hydrogen atom to form another four-carbon compound.This time, the hydrogen atom is used to reduce FAD to FADH2. FAD,or flavin adenine dinucleotide, is a molecule very similar to NAD .Like NAD , FAD accepts electrons during redox reactions.In step 5 , the four-carbon compound formed in step 4releases a hydrogen atom to regenerate oxaloacetic acid, whichkeeps the Krebs cycle operating. The electron in the hydrogenatom reduces NAD to NADH.Recall that in glycolysis one glucose molecule produces two pyruvic acid molecules, which can then form two molecules of acetylCoA. Thus, one glucose molecule is completely broken down in twoturns of the Krebs cycle. These two turns produce four CO2 molecules, two ATP molecules, and hydrogen atoms that are used tomake six NADH and two FADH2 molecules. The CO2 diffuses out ofthe cells and is given off as waste. The ATP can be used for energy.But note that each glucose molecule yields only two molecules ofATP through the Krebs cycle—the same number as in glycolysis.The bulk of the energy released by the oxidation of glucose stillhas not been transferred to ATP. Glycolysis of one glucose molecule produces two NADH molecules, and the conversion of the tworesulting molecules of pyruvic acid to acetyl CoA produces twomore. Adding the six NADH molecules from the Krebs cycle gives atotal of 10 NADH molecules for every glucose molecule that is oxidized. These 10 NADH molecules and the two FADH2 moleculesfrom the Krebs cycle drive the next stage of aerobic respiration.That is where most of the energy transfer from glucose to ATP actually occurs.Quick LabComparing CO2 ProductionMaterials 250 mL flask, 100 mLgraduated cylinder, phenolphthaleinsolution, pipet, drinking straw, water,clock, sodium hydroxide solutionProcedure1. Put on your disposable gloves,lab apron, and safety goggles.2. Add 50 mL of water and fourdrops of phenolphthalein tothe flask.3. Use the straw to gently blow intothe solution for 1 minute. Add thesodium hydroxide one drop at atime, and gently swirl the flask.Record the number of dropsyou use.4. When the liquid turns pink, stopadding drops.5. Empty and rinse your flask asyour teacher directs, and repeatstep 2. Walk vigorously for 2 minutes, and repeat steps 3 and 4.Analysis Which trial produced themost carbon dioxide? Which trialused the most energy?ELECTRON TRANSPORTCHAIN AND CHEMIOSMOSISThe electron transport chain, linked with chemiosmosis, constitutes the second stage of aerobic respiration. Recall that theelectron transport chain is a series of molecules in a membrane thattransfer electrons from one molecule to another. In eukaryoticcells, the electron transport chain and the enzyme ATP synthaseare embedded in the inner membrane of the mitochondrion infolds called cristae. In prokaryotes, the electron transport chain isin the cell membrane. ATP is produced by the electron transportchain when NADH and FADH2 release hydrogen atoms, regenerating NAD and FAD. To understand how ATP is produced, you mustfollow what happens to the electrons and protons that make upthese hydrogen atoms.www.scilinks.orgTopic: Krebs CycleKeyword: HM60842C E L L U L A R R E S P I R AT I O N139

MITOCHONDRIONInnermembraneElectron transport chainH H 3H (highconcentration)32e–2H e–31Innermitochondrialmembrane412e–FADFADH2H ATPsynthaseNADHATPADP phosphateNAD MITOCHONDRIALMATRIXO2 4e– 4H 2H2O5FIGURE 7-11Electron transport and chemiosmosistake place along the inner mitochondrialmembrane and involve five steps.140The electrons in the hydrogen atoms from NADH and FADH2 areat a high energy level. In the electron transport chain, these electrons are passed along a series of molecules embedded in the innermitochondrial membrane, as shown in Figure 7-11. In step 1 , NADHand FADH2 give up electrons to the electron transport chain. NADHdonates electrons at the beginning, and FADH2 donates them fartherdown the chain. These molecules also give up protons (hydrogenions, H ). In step 2 , the electrons are passed down the chain. Asthey move from molecule to molecule, they lose energy. In step 3 ,the energy lost from the electrons is used to pump protons from thematrix, building a high concentration of protons between the innerand outer membranes. Thus, a concentration gradient of protons iscreated across the inner membrane. An electrical gradient is alsocreated, as the protons carry a positive charge.In step 4 , the concentration and electrical gradients of protonsdrive the synthesis of ATP by chemiosmosis, the same process thatgenerates ATP in photosynthesis. ATP synthase molecules areembedded in the inner membrane, near the electron transportchain molecules. As protons move through ATP synthase anddown their concentration and electrical gradients, ATP is madefrom ADP and phosphate. In step 5 , oxygen is the final acceptorof electrons that have passed down the chain. Oxygen also acceptsprotons that were part of the hydrogen atoms supplied by NADHand FADH2. The protons, electrons, and oxygen all combine to formwater, as shown by the equation in step 5 .CHAPTER 7Copyright by Holt, Rinehart and Winston. All rights reserved.

S C I E N C ETECHNOLOGYSOCIETYMITOCHONDRIA: Many Roles in DiseaseEvery cell contains verysmall organelles that areknown as mitochondria.Mitochondria generate almostall of the ATP that fuels the activity in living organisms. Scientistshave known for years that certain diseases are directly causedby mitochondrial dysfunction.However, new research showsthat mitochondria may play rolesin the symptoms of aging andmay contribute to the development of Alzheimer’s diseaseand cancer.Mitochondrial DiseasesMitochondria are very unusualorganelles, because they havetheir own DNA. Mutations inmitochondrial DNA are responsible for several rare but seriousdisorders. Examples includeLeigh’s syndrome, a potentiallydeadly childhood disease thatcauses loss of motor and verbalskills, and Pearson’s syndrome,which causes childhood bonemarrow dysfunction and pancreatic failure.Mitochondria in AgingMitochondria may play a role incausing some problems associated with aging. Chemical reactions of the Krebs cycle andelectron transport chain sometimes release stray electronsthat “leak out” of mitochondriainto the cell. These electrons cancombine with oxygen to formfree radicals. Free radicals areespecially reactive atoms orgroups of atoms with one ormore unpaired electrons. Freeradicals quickly react with othermolecules, such as DNA andprotein; these reactions maydisrupt cell activity. Biologiststhink that many characteristicsof human aging, from wrinklesto mental decline, may bebrought on partly by the damage caused by free radicals.Mitochondria in Other DiseasesRecent research also shows thatmitochondria may be importantin diseases related to apoptosis,or programmed cell death.Scientists have shown thatsignals from the mitochondriaare instrumental in startingand/or continuing the apoptosisprocess. Yet sometimes, mitochondria mistakenly push or failto push the “self-destruct button” in cells. In cases of strokeand Alzheimer’s disease, forexample, mitochondria maycause too many cells to die,which may lead to mental lapsesand other symptoms. In the caseof cancer, mitochondria may failto initiate apoptosis. This failurecould allow tumor cells to growand invade healthy tissues.Promise of New TreatmentsResearchers are now investigating mitochondria as targets fordrug treatments to prevent ortreat a variety of conditions.Conversely, researchers arealso studying how certain conditions impair mitochondrialfunction. One day, scientistsmay use knowledge aboutmitochondria to help ease thesymptoms of aging and to cureor prevent many diseases.REVIEW1. How do mitochondria contributeto free radical formation?2. How could research on mitochondria be helpful to society?3. Critical Thinking Evaluatethe following statement:Mitochondria—we can’t live withthem; we can’t live without them.Mitochondria may play a role in programmed cell death, or apoptosis. Awhite blood cell undergoing apoptosis (right) looks very different from anormal white blood cell (left). (SEM 2,600 )www.scilinks.orgTopic: Cancer CellsKeyword: HM60209141

The Importance of OxygenATP can be synthesized by chemiosmosis only if electrons continueto move from molecule to molecule in the electron transport chain.The last molecule in the electron transport chain must pass electrons on to a final electron acceptor. Otherwise, the electron transport chain would come to a halt. Consider what would happen ifcars kept entering a dead-end, one-way street. At some point, nomore cars could enter the street. Similarly, if the last molecule couldnot “unload” the electrons it accepts, then no more electrons couldenter the electron transport chain and ATP synthesis would stop. Byaccepting electrons from the last molecule in the electron transportchain, oxygen allows additional electrons to pass along the chain. Asa result, ATP can continue to be made through chemiosmosis.EFFICIENCY OF CELLULARRESPIRATIONFIGURE 7-12Follow each pathway to see how oneglucose molecule can generate up to38 ATP molecules in cellular respirationwhen oxygen is present.GlucoseHow many ATP molecules are made in cellular respiration? Refer toFigure 7-12 as you calculate the total. Recall that glycolysis and theKrebs cycle each produce two ATP molecules directly for everyglucose molecule that is oxidized. Furthermore, each NADH molecule that supplies the electron transport chain can generate threeATP molecules,

ATP is also produced during cellular respiration. Autotrophs then use the CO 2 and water to produce O 2 and organic compounds. Thus, the products of cellular respiration are reactants in photosynthesis. Conversely, the products of pho-tosynthesis are reactants in cellular respiration. Cellular respira-tion can be divided into two stages: