CHAPTER 9 CELLULAR RESPIRATION: HARVESTING CHEMICAL …lhsteacher.lexingtonma.org/Pohlman/09B-ProcsCeluarRespiration.pdf · Title: 09B-ProcsCeluarRespiration.ppt Author: Robert Pohlman

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Section B: The Process of Cellular Respiration1. Respiration involves glycolysis, the Krebs cycle, and electron transport: an

overview2. Glycolysis harvests chemical energy by oxidizing glucose to pyruvate: a

closer look3. The Krebs cycle completes the energy-yielding oxidation of organic

molecules: a closer look4. The inner mitochondrial membrane couples electron transport to ATP

synthesis: a closer look5. Cellular respiration generates many ATP molecules for each sugar molecule

it oxidizes: a review

CHAPTER 9CELLULAR RESPIRATION:

HARVESTING CHEMICAL ENERGY

• Respiration occurs in three metabolic stages:glycolysis, the Krebs cycle, and the electrontransport chainand oxidativephosphorylation.

1. Respiration involves glycolysis, the Krebscycle, and electron transport:an overview

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 9.6

• Glycolysis occurs in the cytoplasm.• It begins catabolism by breaking glucose into two

molecules of pyruvate.

• The Krebs cycle occurs in the mitochondrialmatrix.• It degrades pyruvate to carbon dioxide.

• Several steps in glycolysis and the Krebs cycletransfer electrons from substrates to NAD+,forming NADH.

• NADH passes these electrons to the electrontransport chain.


• In the electron transport chain, the electrons movefrom molecule to molecule until they combine withoxygen and hydrogen ions to form water.

• As they are passed along the chain, the energycarried by these electrons is stored in themitochondrion in a form that can be used tosynthesize ATP via oxidative phosphorylation.• Oxidative phosphorylation produces almost 90% of the

ATP generated by respiration.


• Some ATP is also generated in glycolysis and theKrebs cycle by substrate-level phosphorylation.• Here an enzyme

transfers a phosphategroup from anorganic molecule(the substrate)to ADP, formingATP.


Fig. 9.7

• Respiration uses the small steps in the respiratorypathway to break the large denomination of energycontained in glucose into the small change of ATP.• The quantity of energy in ATP is more appropriate for

the level of work required in the cell.

• Ultimately 38 ATP are produced per mole ofglucose that is degraded to carbon dioxide andwater by respiration.


• During glycolysis, glucose, a six carbon-sugar, issplit into two, three-carbon sugars.

• These smaller sugars are oxidized and rearranged toform two molecules of pyruvate.

• Each of the ten steps in glycolysis is catalyzed by aspecific enzyme.

• These steps can be divided into two phases: anenergy investment phase and an energy payoff phase.

2. Glycolysis harvests chemical energy byoxidizing glucose to pyruvate:a closer look


• In the energy investment phase, ATP providesactivation energy by phosphorylating glucose.• This requires 2 ATP per glucose.

• In the energy payoffphase, ATP isproduced bysubstrate-levelphosphorylationand NAD+ isreduced to NADH.

• 4 ATP (net) and2 NADH are producedper glucose.


Fig. 9.8


Fig. 9.9a


Fig. 9.9b

• The net yield from glycolysis is 2 ATP and 2NADH per glucose.• No CO2 is produced during glycolysis.

• Glycolysis occurs whether O2 is present or not.• If O2 is present, pyruvate moves to the Krebs cycle and

the energy stored in NADH can be converted to ATP bythe electron transport system and oxidativephosphorylation.


• More than three quarters of the original energy inglucose is still present in two molecules of pyruvate.

• If oxygen is present, pyruvate enters themitochondrion where enzymes of the Krebs cyclecomplete the oxidation of the organic fuel to carbondioxide.

3. The Krebs cycle completes the energy-yielding oxidation of organic molecules:a closer look


• As pyruvate enters the mitochondrion, amultienzyme complex modifies pyruvate to acetylCoA which enters the Krebs cycle in the matrix.• A carboxyl group is removed as CO2.

• A pair of electrons is transferred from the remainingtwo-carbon fragment to NAD+ to form NADH.

• The oxidizedfragment, acetate,combines withcoenzyme A toform acetyl CoA.


Fig. 9.10

• The Krebs cycle is named after Hans Krebs whowas largely responsible for elucidating itspathways in the 1930’s.• This cycle begins when acetate from acetyl CoA

combines with oxaloacetate to form citrate.

• Ultimately, the oxaloacetate is recycled and the acetateis broken down to CO2.

• Each cycle produces one ATP by substrate-levelphosphorylation, three NADH, and one FADH2(another electron carrier) per acetyl CoA.


• The Krebscycle consistsof eight steps.


Fig. 9.11

• The conversion ofpyruvate and theKrebs cycleproduces largequantities ofelectron carriers.


Fig. 9.12

• Only 4 of 38 ATP ultimately produced by respirationof glucose are derived from substrate-levelphosphorylation.

• The vast majority of the ATP comes from the energyin the electrons carried by NADH (and FADH2).

• The energy in these electrons is used in the electrontransport system to power ATP synthesis.

4. The inner mitochondrial membranecouples electron transport to ATPsynthesis: a closer look


• Thousands of copies of the electron transport chainare found in the extensive surface of the cristae, theinner membrane of the mitochondrion.• Most components of the chain are proteins that are

bound with prosthetic groups that can alternate betweenreduced and oxidized states as they accept and donateelectrons.

• Electrons drop in free energy as they pass downthe electron transport chain.


• Electrons carried byNADH are transferred tothe first molecule in theelectron transport chain,flavoprotein.• The electrons continue

along the chain whichincludes severalcytochrome proteins andone lipid carrier.

• The electrons carried byFADH2 have lower freeenergy and are added toa later point in the chain.


Fig. 9.13

• Electrons from NADH or FADH2 ultimately passto oxygen.• For every two electron carriers (four electrons), one O2

molecule is reduced to two molecules of water.

• The electron transport chain generates no ATPdirectly.

• Its function is to break the large free energy dropfrom food to oxygen into a series of smaller stepsthat release energy in manageable amounts.

• The movement of electrons along the electrontransport chain does contribute to chemiosmosisand ATP synthesis.


• A protein complex, ATPsynthase, in the cristaeactually makes ATP fromADP and Pi.

• ATP used the energy ofan existing protongradient to power ATPsynthesis.• This proton gradient

develops between theintermembrane spaceand the matrix.


Fig. 9.14

• The proton gradient is produced by the movementof electrons along the electron transport chain.

• Several chain molecules can use the exergonicflow of electrons to pump H+ from the matrix tothe intermembrane space.• This concentration of H+ is the proton-motive force.



Fig. 9.15

• The ATP synthase molecules are the only placethat will allow H+ to diffuse back to the matrix.

• This exergonic flow of H+ is used by the enzyme togenerate ATP.

• This coupling of the redox reactions of the electrontransport chain to ATP synthesis is calledchemiosmosis.


• The mechanism of ATPgeneration by ATPsynthase is still an area ofactive investigation.• As hydrogen ions flow

down their gradient, theycause the cylinder portionand attached rod of ATPsynthase to rotate.

• The spinning rod causes aconformational change inthe knob region, activatingcatalytic sites where ADPand inorganic phosphatecombine to make ATP.


Fig. 9.14

• Chemiosmosis is an energy-coupling mechanismthat uses energy stored in the form of an H+ gradientacross a membrane to drive cellular work.• In the mitochondrion, chemiosmosis generates ATP.

• Chemiosmosis in chloroplasts also generates ATP, butlight drives the electron flow down an electron transportchain and H+ gradient formation.

• Prokaryotes generate H+ gradients across their plasmamembrane.

• They can use this proton-motive force not only togenerate ATP but also to pump nutrients and wasteproducts across the membrane and to rotate theirflagella.


• During respiration, most energy flows from glucose-> NADH -> electron transport chain -> proton-motive force -> ATP.

• Considering the fate of carbon, one six-carbonglucose molecule is oxidized to six CO2 molecules.

• Some ATP is produced by substrate-levelphosphorylation during glycolysis and the Krebscycle, but most comes from oxidativephosphorylation.

5. Cellular respiration generates many ATPmolecules for each sugar molecule itoxidizes: a review


• Each NADH from the Krebs cycle and theconversion of pyruvate contributes enough energyto generate a maximum of 3 ATP (rounding up).• The NADH from glycolysis may also yield 3 ATP.

• Each FADH2 from the Krebs cycle can be used togenerate about 2ATP.

• In some eukaryotic cells, NADH produced in thecytosol by glycolysis may be worth only 2 ATP.• The electrons must be shuttled to the mitochondrion.

• In some shuttle systems, the electrons are passed toNAD+, in others the electrons are passed to FAD.


• Assuming the most energy-efficient shuttle ofNADH from glycolysis, a maximum yield of 34ATP is produced by oxidative phosphorylation.

• This plus the 4 ATP from substrate-levelphosphorylation gives a bottom line of 38 ATP.• This maximum figure does not consider other uses of

the proton-motive force.



Fig. 9.16

• How efficient is respiration in generating ATP?• Complete oxidation of glucose releases 686 kcal per

mole.

• Formation of each ATP requires at least 7.3 kcal/mole.

• Efficiency of respiration is 7.3 kcal/mole x 38ATP/glucose/686 kcal/mole glucose = 40%.

• The other approximately 60% is lost as heat.

• Cellular respiration is remarkably efficient inenergy conversion.


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CHAPTER 9 CELLULAR RESPIRATION: HARVESTING CHEMICAL …lhsteacher.lexingtonma.org/Pohlman/09B-ProcsCeluarRespiration.pdf · Title: 09B-ProcsCeluarRespiration.ppt Author: Robert Pohlman