Like other heterotrophs, the giant panda, Ailuropodamelanoleuca, obtains organic compounds by consumingother organisms. Biochemical pathways within the panda’scells transfer energy from those compounds to ATP.
SECTION 1 Glycolysis and Fermentation
SECTION 2 Aerobic Respiration
Unit 3—Cellular RespirationTopics 1–6
C H A P T E R 7130
7CHAPTER CELLULAR RESPIRATIONCELLULAR RESPIRATION
For project ideas fromScientific American, visitgo.hrw.com and type in the keyword HM6SAA.
G L Y C O L Y S I S A N DF E R M E N T A T I O NMost foods contain usable energy, stored in complex organic
compounds such as proteins, carbohydrates, and fats. All cells
break down organic compounds into simpler molecules, a
process that releases energy to power cellular activities.
HARVESTING CHEMICALENERGY
Cellular respiration is the complex process in which cells makeadenosine triphosphate (ATP) by breaking down organic com-pounds. Recall that autotrophs, such as plants, use photosynthe-sis to convert light energy from the sun into chemical energy,which is stored in organic compounds. Both autotrophs and het-erotrophs undergo cellular respiration to break these organiccompounds into simpler molecules and thus release energy. Someof the energy is used to make ATP. The energy in ATP is then usedby cells to do work.
Overview of Cellular RespirationFigure 7-1 shows that autotrophs and heterotrophs use cellular res-piration to make carbon dioxide (CO2) and water from organiccompounds and oxygen (O2). ATP is also produced during cellularrespiration. Autotrophs then use the CO2 and water to produce O2and organic compounds. Thus, the products of cellular respirationare reactants in photosynthesis. Conversely, the products of pho-tosynthesis are reactants in cellular respiration. Cellular respira-tion can be divided into two stages:
1. Glycolysis Organic compounds are converted into three-carbon molecules of pyruvic (pie-ROO-vik) acid, producing asmall amount of ATP and NADH (an electron carrier molecule).Glycolysis is an anaerobic (AN-uhr-oh-bik) process because itdoes not require the presence of oxygen.
2. Aerobic Respiration If oxygen is present in the cell’s environ-ment, pyruvic acid is broken down and NADH is used to makea large amount of ATP through the process known as aerobic(uhr-OH-bik) respiration (covered later).
Pyruvic acid can enter other pathways if there is no oxygen pre-sent in the cell’s environment. The combination of glycolysis andthese anaerobic pathways is called fermentation.
SECTION 1
O B J E C T I V E S● Identify the two major steps of
cellular respiration.● Describe the major events in
glycolysis.● Compare lactic acid fermentation
with alcoholic fermentation.● Calculate the efficiency of
glycolysis.
V O C A B U L A R Ycellular respirationpyruvic acidNADHanaerobicaerobic respirationglycolysisNAD!
Both autotrophs and heterotrophsproduce carbon dioxide and waterthrough cellular respiration. Manyautotrophs produce organic compoundsand oxygen through photosynthesis.
Many of the reactions in cellular respiration are redox reactions.Recall that in a redox reaction, one reactant is oxidized (loses elec-trons) while another is reduced (gains electrons). Although manykinds of organic compounds can be oxidized in cellular respiration,it is customary to focus on the simple sugar called glucose(C6H12O6). The following equation summarizes cellular respiration:
C6H12O6 ! 6O2 6CO2 ! 6H2O ! energy (ATP)
This equation, however, does not explain how cellular respirationoccurs. It is useful to examine each of the two stages, summarizedin Figure 7-2a. (Figure 7-2b illustrates the differences between cel-lular respiration and fermentation.) The first stage of cellular res-piration is glycolysis.
GLYCOLYSISGlycolysis is a biochemical pathway in which one six-carbon mol-ecule of glucose is oxidized to produce two three-carbon mole-cules of pyruvic acid. Like other biochemical pathways, glycolysisis a series of chemical reactions catalyzed by specific enzymes. Allof the reactions of glycolysis take place in the cytosol and occur infour main steps, as illustrated in Figure 7-3 on the next page.
In step , two phosphate groups are attached to one molecule ofglucose, forming a new six-carbon compound that has two phosphategroups. The phosphate groups are supplied by two molecules of ATP,which are converted into two molecules of ADP in the process.
In step , the six-carbon compound formed in step is splitinto two three-carbon molecules of glyceraldehyde 3-phosphate(G3P). Recall that G3P is also produced by the Calvin cycle inphotosynthesis.
12
1
enzymes
ATP
Organic compounds
Pyruvic acid +
+
+
CO2 + H2O
Aerobicrespiration
ATP
(a) CELLULAR RESPIRATION
ATP
Glycolysis Glycolysis
Organic compounds
Pyruvic acid
Anaerobicpathways
(b) FERMENTATION
Lactic acid, ethyl alcohol, or othercompounds
Organisms use cellular respiration toharness energy from organic compoundsin food. (a) Glycolysis, the first stage ofcellular respiration, produces a smallamount of ATP. Most of the ATPproduced in cellular respiration resultsfrom aerobic respiration, which is thesecond stage of cellular respiration.(b) In some cells, glycolysis may result infermentation if oxygen is not present.
from the Latin fermentum,meaning “leaven” or anything
that causes baked goods to rise,such as yeast
Word Roots and Origins
In step , the two G3P molecules are oxidized, and eachreceives a phosphate group. The product of this step is two mol-ecules of a new three-carbon compound. As shown in Figure 7-3,the oxidation of G3P is accompanied by the reduction of two mol-ecules of nicotinamide adenine dinucleotide (NAD!) to NADH.NAD! is similar to NADP!, a compound involved in the light reac-tions of photosynthesis. Like NADP!, NAD! is an organic moleculethat accepts electrons during redox reactions.
In step , the phosphate groups added in step and step are removed from the three-carbon compounds formed in step .This reaction produces two molecules of pyruvic acid. Each phos-phate group is combined with a molecule of ADP to make a mole-cule of ATP. Because a total of four phosphate groups were addedin step and step , four molecules of ATP are produced.
Notice that two ATP molecules were used in step , but fourwere produced in step . Therefore, glycolysis has a net yield oftwo ATP molecules for every molecule of glucose that is convertedinto pyruvic acid. What happens to the pyruvic acid depends onthe type of cell and on whether oxygen is present.
FERMENTATIONWhen oxygen is present, cellular respiration continues as pyruvicacid enters the pathways of aerobic respiration. (Aerobic respira-tion is covered in detail in the next section.) In anaerobic condi-tions (when oxygen is absent), however, some cells can convertpyruvic acid into other compounds through additional biochemicalpathways that occur in the cytosol. The combination of glycolysisand these additional pathways, which regenerate NAD!, is known asfermentation. The additional fermentation pathways do not pro-duce ATP. However, if there were not a cellular process that recy-cled NAD! from NADH, glycolysis would quickly use up all theNAD! in the cell. Glycolysis would then stop. ATP productionthrough glycolysis would therefore also stop. The fermentationpathways thus allow for the continued production of ATP.
There are many fermentation pathways, and they differ in termsof the enzymes that are used and the compounds that are madefrom pyruvic acid. Two common fermentation pathways result inthe production of lactic acid and ethyl alcohol.
Glycolysis takes place in the cytosol ofcells and involves four main steps. A netyield of two ATP molecules is producedfor every molecule of glucose thatundergoes glycolysis.
FIGURE 7-3
C H A P T E R 7134
Lactic Acid FermentationIn lactic acid fermentation, an enzyme converts pyruvic acid madeduring glycolysis into another three-carbon compound, called lac-tic acid. As Figure 7-4 shows, lactic acid fermentation involves thetransfer of one hydrogen atom from NADH and the addition of onefree proton (H!) to pyruvic acid. In the process, NADH is oxidizedto form NAD!. The resulting NAD! is used in glycolysis, where it isagain reduced to NADH. Thus, the regeneration of NAD! in lacticacid fermentation helps to keep glycolysis operating.
Lactic acid fermentation by microorganisms plays an essentialrole in the manufacture of many dairy products, as illustrated inFigure 7-5. Milk will ferment naturally if not refrigerated properly orconsumed in a timely manner. Such fermentation of milk is consid-ered “spoiling.” But ever since scientists discovered the microor-ganisms that cause this process, fermentation has been used in acontrolled manner to produce cheese, buttermilk, yogurt, sour
cream, and other cultured dairy products.Only harmless, active microorganisms areused in the fermentation of dairy products.
Lactic acid fermentation also occurs inyour muscle cells during very strenuousexercise, such as sprinting. During thiskind of exercise, muscle cells use up oxy-gen more rapidly than it can be deliveredto them. As oxygen becomes depleted, themuscle cells begin to switch from cellularrespiration to lactic acid fermentation.Lactic acid accumulates in the musclecells, making the cells’ cytosol more acidic.The increased acidity may reduce thecapacity of the cells to contract, resultingin muscle fatigue, pain, and even cramps.Eventually, the lactic acid diffuses into theblood and is transported to the liver,where it can be converted back intopyruvic acid.
Some cells engage in lactic acidfermentation when oxygen is absent.In this process, pyruvic acid is reducedto lactic acid and NADH is oxidized to NAD!.
FIGURE 7-4
In cheese making, fungi or bacteria areadded to large vats of milk. Themicroorganisms carry out lactic acidfermentation, converting some of thesugar in the milk to lactic acid.
FIGURE 7-5
135C E L L U L A R R E S P I R A T I O N
Alcoholic 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 fermenta-tion, 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 fermen-tation 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 per-formed by yeast cells. In this case, the CO2 that is produced by fer-mentation makes the bread rise by forming bubbles inside thedough, and the ethyl alcohol evaporates during baking.
EFFICIENCY OF GLYCOLYSISHow efficient is glycolysis in obtaining energy from glucose and 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 measured in units of kilocalories (kcal). One kilocalorie equals 1,000 calo-ries (cal).
Some cells engage in alcoholicfermentation, converting pyruvic acidinto ethyl alcohol. Again, NADH isoxidized to NAD!.
FIGURE 7-6
The yeast Saccharomyces cerevisiae isused in alcohol production and breadmaking.
FIGURE 7-7
kilocalorie
from the Greek chilioi, meaning“thousand,” and the Latin calor,
meaning “heat”
Word Roots and Origins
C H A P T E R 7136
Scientists have calculated that the complete oxidation of a stan-dard amount of glucose releases 686 kcal. The production of astandard amount of ATP from ADP absorbs a minimum of about 7 kcal, depending on the conditions inside the cell. Recall that twoATP molecules are produced from every glucose molecule that isbroken down by glycolysis.
!
! " 100% ! 2%
You can see that the two ATP molecules produced during gly-colysis 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 gly-colysis 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 photosyn-thesis 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.
2 " 7 kcal##
686 kcal
Energy required to make ATP#####Energy released by oxidation of glucose
Efficiency ofglycolysis
1. Explain the role of organic compounds in cellularrespiration.
2. For each six-carbon molecule that begins glycol-ysis, identify how many molecules of ATP areused and how many molecules of ATP areproduced.
3. Distinguish between the products of the twotypes of fermentation discussed in this section.
4. Calculate the efficiency of glycolysis if 12 kcal of energy are required to transfer energy fromglucose to ATP.
CRITICAL THINKNG5. Applying Information A large amount of ATP in
a cell inhibits the enzymes that drive the firststeps of glycolysis. How will this inhibition ofenzymes eventually affect the amount of ATP inthe cell?
6. Predicting Results How might the efficiency ofglycolysis change if this process occurred in onlyone step? Explain your answer.
7. Relating Concepts In what kind of environ-ment would you expect to find organisms thatcarry out fermentation?
O B J E C T I V E S● Relate aerobic respiration to the
structure of a mitochondrion.● Summarize the events of the
Krebs cycle.● Summarize the events of the
electron transport chain andchemiosmosis.
● Calculate the efficiency of aerobicrespiration.
● Contrast the roles of glycolysis and aerobic respiration in cellularrespiration.
V O C A B U L A R Ymitochondrial matrixacetyl CoAKrebs cycleoxaloacetic acidcitric acidFAD
Matrix
MITOCHONDRION
Innermembrane
Outermembrane
Cristae
A E R O B I C R E S P I R A T I O NIn most cells, glycolysis does not result in fermentation.
Instead, when oxygen is available, pyruvic acid undergoes
aerobic respiration, the pathway of cellular respiration that
requires oxygen. Aerobic respiration produces nearly 20 times
as much ATP as is produced by glycolysis alone.
OVERVIEW OF AEROBICRESPIRATION
Aerobic 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 trans-port 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 gly-colysis diffuses across the double membrane of a mitochondrionand enters the mitochondrial matrix. The mitochondrial matrix isthe space inside the inner membrane of a mitochondrion. Figure 7-8 illustrates the relationships between these mitochondrialparts. The mitochondrial matrix contains the enzymes needed tocatalyze the reactions of the Krebs cycle.
In eukaryotic cells, the reactions of aerobic respiration occur insidemitochondria. The Krebs cycle takesplace in the mitochondrial matrix, andthe electron transport chain is located in the inner membrane.
When pyruvic acid enters the mitochondrial matrix, it reactswith a molecule called coenzyme A to form acetyl (uh-SEET-uhl) coen-zyme 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 con-version of pyruvic acid to acetyl CoA is released in a molecule ofCO2. This reaction reduces a molecule of NAD! to NADH.
THE KREBS CYCLEThe 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 , 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 , 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 hydro-gen atom is transferred to NAD!, reducing it to NADH.
2
1
C C C
C C
Pyruvic acid
CoA
NAD+
NADH + H+
CO2
Acetyl CoA
C
Glycolysis 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.
FIGURE 7-9
C CC C C C C C
C C C C C
C C C C
C C C C
C
C
C C C C
Acetyl CoA Citric acid
5-carboncompound
Oxaloaceticacid
CoA
NAD+
NADH + H+
NAD+
NAD+NADH + H+
NADH + H+
ADP + phosphate
ATP
FADFADH2
CO2
CO2
4-carboncompound 4-carbon
compound
MITOCHONDRION
Mitochondrialmatrix
1 2
4
5 3
THEKREBSCYCLE
The Krebs cycle takes place in themitochondrial matrix and involves five main steps.
FIGURE 7-10
138
139C E L L U L A R R E S P I R A T I O N
In step , the five-carbon compound formed in step alsoreleases a CO2 molecule and a hydrogen atom, forming a four-carbon compound. Again, NAD! is reduced to NADH. Notice that inthis step a molecule of ATP is also synthesized from ADP.
In step , the four-carbon compound formed in step releases 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 , the four-carbon compound formed in step releases 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 pyru-vic 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 mole-cules, 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 mole-cule 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 oxi-dized. 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 actu-ally occurs.
ELECTRON TRANSPORTCHAIN AND CHEMIOSMOSIS
The electron transport chain, linked with chemiosmosis, consti-tutes 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, regenerat-ing NAD! and FAD. To understand how ATP is produced, you mustfollow what happens to the electrons and protons that make upthese hydrogen atoms.
1. Put on your disposable gloves,lab apron, and safety goggles.
2. Add 50 mL of water and fourdrops of phenolphthalein to the 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 drops you 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 min-utes, and repeat steps 3 and 4.
Analysis Which trial produced themost carbon dioxide? Which trialused the most energy?
The electrons in the hydrogen atoms from NADH and FADH2 areat a high energy level. In the electron transport chain, these elec-trons are passed along a series of molecules embedded in the innermitochondrial membrane, as shown in Figure 7-11. In step , 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 , the electrons are passed down the chain. Asthey move from molecule to molecule, they lose energy. In step ,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 , 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 , 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
Electron transport and chemiosmosistake place along the inner mitochondrialmembrane and involve five steps.
FIGURE 7-11
S C I E N C ET E C H N O L O G Y
S O C I E T Y
www.scilinks.orgTopic: Cancer CellsKeyword: HM60209
MITOCHONDRIA: Many Roles in Disease
Every cell contains verysmall organelles that are known as mitochondria.
Mitochondria generate almostall of the ATP that fuels the activ-ity in living organisms. Scientistshave known for years that cer-tain diseases are directly causedby mitochondrial dysfunction.However, new research showsthat mitochondria may play rolesin the symptoms of aging andmay contribute to the develop-ment of Alzheimer’s disease and cancer.
Mitochondrial Diseases
Mitochondria are very unusualorganelles, because they havetheir own DNA. Mutations inmitochondrial DNA are respon-sible 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 pan-creatic failure.
Mitochondria in Aging
Mitochondria may play a role incausing some problems associ-ated with aging. Chemical reac-tions of the Krebs cycle andelectron transport chain some-times 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 dam-age caused by free radicals.
Mitochondria in Other Diseases
Recent research also shows thatmitochondria may be importantin diseases related to apoptosis,or programmed cell death.Scientists have shown that
signals from the mitochondriaare instrumental in startingand/or continuing the apoptosisprocess. Yet sometimes, mito-chondria mistakenly push or failto push the “self-destruct but-ton” 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 Treatments
Researchers are now investigat-ing mitochondria as targets fordrug treatments to prevent ortreat a variety of conditions.Conversely, researchers arealso studying how certain con-ditions impair mitochondrialfunction. One day, scientistsmay use knowledge aboutmitochondria to help ease thesymptoms of aging and to cureor prevent many diseases.
1. How do mitochondria contributeto free radical formation?
2. How could research on mito-chondria be helpful to society?
3. Critical Thinking Evaluatethe following statement:Mitochondria—we can’t live withthem; we can’t live without them.
R E V I E W
141
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!)
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 elec-trons on to a final electron acceptor. Otherwise, the electron trans-port 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 CELLULARRESPIRATION
How 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 mol-ecule that supplies the electron transport chain can generate threeATP molecules, and each FADH2 molecule can generate two ATPmolecules. Thus, the 10 NADH and two FADH2 molecules madethrough glycolysis, conversion of pyruvic acid to acetyl CoA, andthe Krebs cycle can produce up to 34 ATP molecules by the elec-tron transport chain and chemiosmosis. Adding the four ATP mol-ecules from glycolysis and the Krebs cycle gives a maximum yieldof 38 ATP molecules per molecule of glucose.Glucose
2 NADH
+
+
+
=
+
+
2 ATPproduced directly
6 ATP throughelectron transportand chemiosmosis
6 ATP throughelectron transportand chemiosmosis
2 ATPproduced directly
18 ATP throughelectron transportand chemiosmosis
4 ATP throughelectron transportand chemiosmosis
38 ATP
2 NADH
6 NADH
2 FADH2
Krebs cycle
Glycolysis
Pyruvic acid
Acetyl CoA
CO2, H2O
Follow each pathway to see how oneglucose molecule can generate up to 38 ATP molecules in cellular respirationwhen oxygen is present.
FIGURE 7-12
C H A P T E R 7142
143C E L L U L A R R E S P I R A T I O N
The actual number of ATP molecules generatedthrough cellular respiration varies from cell to cell. Inmost eukaryotic cells, the NADH that is made in thecytosol during glycolysis cannot diffuse through theinner membrane of the mitochondrion. Instead, itmust be actively transported into the mitochondrialmatrix. The active transport of NADH consumes ATP.As a result, most eukaryotic cells produce only about36 ATP molecules per glucose molecule.
The efficiency of cellular respiration can vary depending on con-ditions in the cell. In general, the efficiency when 38 ATP moleculesare generated can be estimated as shown below:
!
! " 100% ! 39%
Thus, cellular respiration is nearly 20 times more efficientthan glycolysis alone. In fact, the efficiency of cellular respiration isquite impressive compared with the efficiency of machines thathumans have designed, such as the car shown in Figure 7-13. Anautomobile engine, for example, is only about 25 percent efficientin extracting energy from gasoline to move a car. Most of theremaining energy released from gasoline is lost as heat.
A SUMMARY OF CELLULARRESPIRATION
Cellular respiration occurs in two stages, as listed below andshown in Figure 7-14:
1. Glycolysis—Glucose is converted into pyruvic acid, producinga small amount of ATP and NADH.
2. Aerobic respiration—Pyruvic acid is converted into CO2 andwater in the presence of oxygen, producing a large amount of ATP.
38 " 7 kcal##
686 kcal
Energy required to make ATP#####Energy released by oxidation of glucose
Cellular respiration occurs in two stages: glycolysis and aerobic respiration(which includes the conversion ofpyruvic acid to acetyl CoA, the Krebscycle, the electron transport chain, andchemiosmosis).
FIGURE 7-14
Through cellular respiration, cells aremore efficient at generating energy thanmany machines—including cars.
FIGURE 7-13
C H A P T E R 7144
The following equation summarizes the complete oxidation ofglucose in cellular respiration:
C6H12O6 ! 6O2 6CO2 ! 6H2O ! energy (ATP)
In addition to glucose, many other compounds can be used as fuelin cellular respiration. Molecules derived from the breakdown offats, proteins, and carbohydrates can enter glycolysis or the Krebscycle at various points in order to yield more energy to an organism.
The equation above can be considered the opposite of the over-all equation for photosynthesis, if glucose is considered to be aproduct of photosynthesis:
6CO2 ! 6H2O C6H12O6 ! 6O2
That is, the products of photosynthesis are reactants in celluar res-piration, and the products of cellular respiration are reactants inphotosynthesis. However, cellular respiration is not the reverse ofphotosynthesis. These two processes involve different biochemi-cal pathways and occur at different sites inside cells.
Another Role of Cellular RespirationCellular respiration provides the ATP that all cells need to supportthe activities of life. But providing cells with ATP is not the onlyimportant function of cellular respiration. Cells also need specificorganic compounds from which to build the macromolecules thatcompose their own structures. Some of these specific compoundsmay not be contained in the food that a heterotroph consumes.
The molecules formed at different steps in glycolysis and theKrebs cycle are often used by cells to make the compounds thatare missing in food. Compounds formed during glycolysis and theKrebs cycle can be diverted into other biochemical pathways inwhich the cell makes the molecules it requires. For example,approximately 10 of the amino acids needed by the human bodycan be made with compounds diverted from the Krebs cycle.
light energy
1. In what part of a mitochondrion does the Krebscycle occur?
2. In what part of a mitochondrion is the electrontransport chain located?
3. What four-carbon compound is regenerated atthe end of the Krebs cycle?
4. What molecule does oxygen become a part of atthe end of the electron transport chain?
5. Is cellular respiration more or less efficient thanfermentation?
6. List the two processes that together result incellular respiration.
CRITICAL THINKING7. Predicting Results Sometimes protons leak out
of a cell or are used for purposes other than ATPproduction. How would this loss of protons affectthe production of ATP in aerobic respiration?
8. Inferring Relationships How does the arrange-ment of the cristae in the inner membrane ofmitochondria affect the rate of aerobic respira-tion? Explain your answer.
9. Making Calculations Calculate the efficiency ofcellular respiration if a cell generates 32 ATPmolecules per molecule of glucose.
● Cellular respiration is the process by which cells breakdown organic compounds to produce ATP.
● Cellular respiration begins with glycolysis, which takesplace in the cytosol of cells. During glycolysis, oneglucose molecule is oxidized to form two pyruvic acidmolecules. Glycolysis results in a net production of twoATP molecules and two NADH molecules.
● If oxygen is not present, glycolysis may lead to anaerobicpathways in which pyruvic acid is converted into otherorganic molecules in the cytosol. Glycolysis combined
with these anaerobic pathways is called fermentation.Fermentation does not produce ATP, but it doesregenerate NAD!, which helps keep glycolysis operating.
● In lactic acid fermentation, an enzyme converts pyruvicacid into lactic acid.
● In alcoholic fermentation, other enzymes convert pyruvicacid into ethyl alcohol and CO2.
● Through glycolysis, only about 2 percent of the energyavailable from the oxidation of glucose is captured as ATP.
mitochondrial matrix (p. 137)acetyl CoA (p. 138)
Krebs cycle (p. 138)oxaloacetic acid (p. 138)
citric acid (p. 138)FAD (p. 139)
Vocabulary
Aerobic RespirationSECTION 2
● In eukaryotic cells, the processes of aerobic respirationoccur inside the mitochondria. The Krebs cycle occurs inthe mitochondrial matrix. The electron transport chain isembedded in the inner mitochondrial membrane.
● In the mitochondrial matrix, pyruvic acid produced inglycolysis is converted into acetyl CoA. Then, acetyl CoA enters the Krebs cycle. Each turn of the Krebs cyclegenerates three NADH, one FADH2, one ATP, and two CO2 molecules.
● NADH and FADH2 donate electrons to the electrontransport chain in the inner mitochondrial membrane.These electrons are passed from molecule to molecule inthe transport chain.
● As electrons pass along the electron transport chain,protons donated by NADH and FADH2 are pumped intothe space between the inner and outer mitochondrialmembranes. This pumping creates a concentration
gradient of protons and a charge gradient across theinner mitochondrial membrane. As protons move throughATP synthase, down their concentration and chargegradients, and back into the mitochondrial matrix, ATP is produced.
● During aerobic respiration, oxygen accepts both protonsand electrons from the electron transport chain. As aresult, oxygen is converted to water.
● Cellular respiration can produce up to 38 ATP moleculesfrom the oxidation of a single molecule of glucose. Thus,up to 39 percent of the energy released by the oxidationof glucose can be transferred to ATP. However, mosteukaryotic cells produce only about 36 ATP molecules per molecule of glucose.
● Cellular respiration uses the processes of glycolysis andaerobic respiration to obtain energy from organiccompounds.
USING VOCABULARY1. For each pair of terms, explain the relationship
between the terms.a. alcoholic fermentation and lactic acid
fermentationb. glycolysis and pyruvic acidc. mitochondrial matrix and Krebs cycle
2. Use the following terms in the same sentence:acetyl CoA, citric acid, and oxaloacetic acid.
3. Explain the difference between the termsfermentation and cellular respiration.
4. Word Roots and Origins The word glycolysis isderived from the Greek words glykys, whichmeans “sweet,” and lysis, which means “loosen-ing.” Using this information, explain why the termglycolysis is a good name for the biologicalprocess it describes.
UNDERSTANDING KEY CONCEPTS5. Compare the two stages of cellular respiration.6. Explain why the net yield of ATP molecules in gly-
colysis is two, even though four ATP moleculesare produced.
7. Describe what causes your muscles to becomefatigued and sometimes develop cramps whenyou exercise too strenuously.
8. Calculate the efficiency of glycolysis if the num-ber of ATP molecules produced during glycolysiswere 5 times greater.
9. Name the two areas of the mitochondrion wherethe major stages of aerobic respiration occur.
10. Explain the importance of the cyclical nature ofthe Krebs cycle.
11. Define the specific role that oxygen plays in theelectron transport chain.
12. Summarize the process of electron transport andchemiosmosis, including where the electrontransport chain is located, what structures makeup the electron transport chain, where the elec-trons come from and their characteristics, whathappens to them as they move along the electrontransport chain and as they reach the end of thechain, and what is accomplished by this process.
13. Explain why most eukaryotic cells produce fewerthan 38—the maximum number possible—ATPmolecules for every glucose molecule that is oxi-dized by cellular respiration.
14. Compare the efficiency of glycolysis alone withthe efficiency of cellular respiration.
15. Unit 3—Cellular RespirationWrite a report summarizing howexercise physiologists regulate thediets and training of athletes. Find
out how diets vary according to the needs ofeach athlete. Research the relationship betweenexercise and metabolism.
16. CONCEPT MAPPING Use the following terms to create a concept map that
shows the activities of fermentation: alcoholicfermentation, anaerobic pathway, fermentation,glycolysis, lactic acid fermentation, and pyruvicacid.
CRITICAL THINKING17. Evaluating Results Some yeast can use fermenta-
tion or cellular respiration. If oxygen is present,these yeast cells consume glucose much moreslowly than if oxygen is absent. Explain thisobservation.
18. Inferring Relationships How does cellular respira-tion ultimately depend on photosynthesis?
19. Predicting Results Some eukaryotic cells mustuse ATP to move NADH into the mitochondrialmatrix. Would you expect cellular respiration tobe more or less efficient in prokaryotic cells thanin eukaryotic cells? Explain your answer.
20. Interpreting Graphics The graph below shows therate of ATP production by a culture of yeast cellsover time. At the time indicated by the dashedline, cyanide was added to the culture. Cyanideblocks the flow of electrons to O2 from the elec-tron transport chain in mitochondria. Explainwhy adding cyanide affects ATP production in the way indicated by the graph.
Standardized Test PreparationDIRECTIONS: Choose the letter of the answer choicethat best answers the question.
1. Which of the following must pyruvic acid be con-verted into before the Krebs cycle can proceed?A. NADHB. glucoseC. citric acidD. acetyl CoA
2. Which of the following occurs in lactic acid fermentation?F. Oxygen is consumed.G. Lactic acid is converted into pyruvic acid.H. NAD! is regenerated for use in glycolysis.J. Electrons pass through the electron
transport chain.3. Which of the following is not a product of the
Krebs cycle?A. CO2B. ATPC. FADH2D. ethyl alcohol
4. In which way is cellular respiration similar tophotosynthesis?F. They both make G3P.G. They both involve ATP.H. They both involve chemiosmosis.J. all of the above
5. ATP is synthesized in chemiosmosis when which of the following moves across the innermitochondrial membrane?A. NADHB. oxygenC. protonsD. citric acid
INTERPRETING GRAPHICS: The illustration showspart of a biochemical pathway. Use the illustration toanswer the question that follows.
6. This reaction occurs during which of the following processes?F. Krebs cycleG. acetyl CoA formationH. alcoholic fermentationJ. lactic acid fermentation
DIRECTIONS: Complete the following analogy.7. glycolysis : pyruvic acid :: Krebs cycle :
A. O2B. ATPC. lactic acidD. acetyl CoA
INTERPRETING GRAPHICS: The illustration belowshows some stages and reactants of cellular respira-tion. Use the illustration to answer the question that follows.
8. At which of the points is ATP, the main energycurrency of the cell, produced?F. 1 onlyG. 2 onlyH. 1 and 3J. 1, 2, and 3
SHORT RESPONSEThe inner membrane of a mitochondrion is folded;these folds are called cristae.
How might cellular respiration be different if the innermitochondrial membrane were not folded?
EXTENDED RESPONSEOxygen is produced during the reactions of photo-synthesis, and it is used in the reactions of cellularrespiration.
Part A How does oxygen get into or out of chloro-plasts and mitochondria?
Part B What are the roles of oxygen in the processesof photosynthesis and cellular respiration,and how are the roles similar?
Make sure you get plenty of restthe night before the test. Eat a healthy breakfast theday of the test and wear comfortable clothing.
■ Measure the rate of cellular respiration in germinat-ing seeds.
■ Compare cellular respiration rates in germinating andnongerminating seeds.
■ experimenting■ collecting data■ analyzing data
■ volumeter jar and screw-on lid
■ tap water at room temperature
■ ring stand with support ring
■ 8.5 in. ! 11 in. piece of cardboard
■ 8.5 in. ! 11 in. sheet of white paper
■ cellophane tape■ 40 germinating corn or
pea seeds■ 40 nongerminating corn
or pea seeds
Background
1. When glucose is oxidized in cellular respiration,what other substance is consumed and what sub-stances are produced?
2. Write the balanced equation for the complete oxida-tion of glucose in cellular respiration.
Setting Up the Apparatus1. Pour water into the volumeter jar until the jar
is about two-thirds full. Screw on the lid.2. Place the cardboard on top of the ring stand sup-
port ring, and tape a sheet of white paper to thecardboard. Adjust the support ring so that the card-board is level, as shown in the illustration below.
3. CAUTION Put on a labapron, safety goggles,
and protective gloves. Keep the seeds, which mayhave been treated with a fungicide, away fromyour skin. Place 40 germinating seeds in a gradu-ated cylinder and measure their volume. Do thesame for the 40 nongerminating seeds.
4. Add beads to the seeds that have the smaller vol-ume until the combined volume is the same as thevolume of the other group of seeds. Then transferboth groups to separate cups. To a third cup, add avolume of beads equal to the volume of each of theother two cups.
5. Remove the stopper assemblies from three volume-ter tubes and transfer the contents of the three cupsto separate tubes. Place a 2 cm plug of dry cottoninto each tube, leaving a gap of about 1 cmbetween the cotton and the seeds or beads.
6. CAUTION Soda lime is corrosive. Do nottouch it. If it gets on your skin or clothing,
wash it off at the sink. If it gets in your eyes,immediately flush it out at the eyewash stationwhile calling to your teacher. Using forceps, place a packet of soda lime wrapped in gauze on top ofthe cotton plug in each tube. Soda lime absorbs theCO2 that is produced as a result of respiration.
7. Gently but firmly press the stopper assembly intoeach volumeter tube. Insert the tubes into the vol-umeter jar through the large holes in the lid.
PART A
MATERIALS
PROCESS SKILLS
OBJECTIVES
EXPLORATION LAB
■ 100 mL graduatedcylinder
■ glass or plastic beads■ 3 plastic or paper cups■ 3 volumeters■ cotton■ 3 soda-lime packets■ forceps■ Pasteur pipet■ colored water■ ruler with millimeter
8. Use a Pasteur pipet to place a small drop of coloredwater into the three capillary tubes. Tilt two of thetubes slightly until the drops are lined up with the out-ermost calibration mark. Carefully attach these tubesto the latex tubing on the two volumeter tubes con-taining seeds. Position the drop in the third capillarytube near the middle of the tube. Attach this tube tothe volumeter tube that contains only beads. This vol-umeter is the control volumeter. Tape all three capillarytubes to the paper on the ring stand.
9. Wait 5 min for the temperature to become uniformthroughout the volumeter jar. While you wait, make adata table like the one shown below. Then return thedrops in the capillary tubes to their original positionsby using the syringes to inject air into or withdraw air from the volumeter tubes, if necessary.
Measuring Respiration Rates10. On the paper beneath the capillary tubes, mark the
position of one end of each drop of colored water.Note the time. Repeat this procedure every 5 min for20 min. If respiration is rapid, you may have to reposi-tion the drops as you did in Step 9. In which directionwould you expect a drop to move if respiration in thevolumeter tube were causing it to move?
11. Remove the paper from the ring stand and use a rulerto measure the distance moved by the drops duringeach time interval. If you repositioned any drops in Step 10, be sure to add this adjustment when youmeasure the distances. Enter the measurements in the“Uncorrected” columns in your data table.
12. Clean up your materials and wash yourhands before leaving the lab.
Analysis and Conclusions1. No respiration should have occurred in the control
volumeter, which contained only beads. Therefore, anymovement of the drop in the control volumeter musthave been caused by changes in the temperature ofthe volumeter jar or the air pressure in the classroom.Since these changes would have affected all three vol-umeters to the same extent, you must subtract the dis-tance you measured for the control volumeter from thedistances you measured for the other two volumeters.Do this calculation for each time interval, and enterthe results in the “Corrected” columns in your datatable.
2. Each capillary tube has a capacity of 0.063 mLbetween each 1 cm mark on the tube. Use this infor-mation to calculate the volume of O2 consumed by thegerminating and nongerminating seeds during eachtime interval. Enter these results in your data table.
3. Prepare a graph to show the volume of O2 consumedversus time; use different symbols or colors to distin-guish the points for the germinating seeds from thosefor the nongerminating seeds. Make sure each pointrepresents the cumulative volume of O2 consumed. Forexample, the point plotted for the 15–20 min intervalshould represent the volume consumed during thatinterval plus the volume consumed during all of thepreceding intervals. Draw the best-fit line through thepoints for each group of seeds. From the slope of thisline, calculate the average rate of respiration in milli-liters of O2 per minute for both groups of seeds.
4. Which group of seeds had the higher average rate ofrespiration? What is the significance of this differencein terms of a seed’s ability to survive for long periods?
