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Although the citric acid cycle itself does not use 02, itrequires 02 in order to proceed because there is no other efficient way for theNADH to get rid of its electrons and thus regeneratethe NAD+ that is needed tokeep the cycle going.The citric acid cycle takes place inside mitochondria in eucaryotic cells. Itresults in the complete oxidation of the carbon atoms of the acetyl groups inacetyl CoA, converting them into CO2. But the acetyl group is not oxidizeddirectly. Instead, this group is transferred from acetyl CoA to a larger, four-carbon molecule, oxaloacetate,to form the six-carbon tricarboxylic acid, citric acid,for which the subsequent cycle of reactions is named. The citric acid molecule isthen gradually oxidized, allowing the energy of this oxidation to be harnessedtoproduce energy-rich activated carrier molecules. The chain of eight reactionsforms a cycle because at the end the oxaloacetate is regenerated and enters anew turn of the cycle, as shown in outline in Figure 2-82.we have thus far discussed only one of the three types of activated carriermolecules that are produced by the citric acid cycle, the NAD+-NADH pair (seeFigure 2-60).
In addition to three molecules of NADH, each turn of the cycle alsoproduces one molecule of FADH2 (reduced flavin adenine dinucleotide) fromFAD and one molecule of the ribonucleotide GTP (guanosine triphosphate)from GDP The structures of these two activated carrier molecules are illustratedin Figure 2-83.
GTP is a close relative of ATB and the transfer of its terminalphosphate group to ADP produces one ATP molecule in each cycle. Like NADH,FADHz is a carrier of high-energy electrons and hydrogen. As we discussshortly,the energy that is stored in the readily transferred high-energy electrons ofNADH and FADH2will be utilized subsequently for Arp production through theprocess of oxidatiue phosphorylation, the only step in the oxidative catabolismof foodstuffs that directly requires gaseousoxygen (oz) from the atmosphere.Panel 2-9 (pp.
122-123)presents the complete citric acid cycle.Water, ratherthan molecular oxygen, supplies the extra oxygen atoms required to make co2from the acetyl groups entering the citric acid cycle.As illustrated in the panel,o-t- s-coAHrcacetylCoA2Ctoxd toacelale4C+H*/I4CSTEP1*{srEP2-)lu4lllil-,,/./VsrEP85C\srEP3r*E!E-f.-co,I5CN E TR E S U L OT N ET U R NO FT H EC Y C L EP R O D U C ETSH R E EN A D H ,O N EG T BA N DO N EF A D H 2A, N D R E L E A S ETSW O M O L E C U L EOSF C O IFigure2-82 Simpleoverviewof thecitric acid cycle.<TAGT>The reactionofacetylcoA with oxaloacetatestartsthecycleby producingcitrate(citricacid).Ineachturn of the cycle,two moleculesofCO2are producedas wasteproducts,plusthreemoleculesof NADH,one moleculeof GTP,and one moleculeof FADH2.Thenumberof carbonatomsin eachintermediateis shown in a yellowbox.Fordetails,see Panel2-9 (pp.
122-123).guanineo-ollc2H-???\'.,-i2eOHill- N - - c- t - r ncllrtllHrC-9t-r-t--N-C\OH3C-COHOHtl,rN '' c,.,rC'-N.'nCH,I'IH-C-OHIH-C -OHH-C-OH(B)three moleculesof water are split in each cycle,and the orygen atoms of someof them are ultimately used to make CO2.In addition to pyruvate and fatty acids, some amino acids pass from thecytosolinto mitochondria, where they are alsoconvertedinto acetylCoAor oneof the other intermediatesof the citric acid cycle.Thus,in the eucaryoticcell,themitochondrion is the center toward which all energy-yieldingprocesseslead,whether they begin with sugars,fats,or proteins.Both the citric acid cycle and glycolysisalso function as starting points forimportant biosynthetic reactionsby producing vital carbon-containing intermediates,such as oxaloacetateand a-ketoglutarate.Someof these substancesproduced by catabolism are transferredback from the mitochondrion to thecytosol,where they servein anabolicreactionsasprecursorsfor the synthesisofmany essentialmolecules,such as amino acids (Figure244).H2C-O--O-Figure2-83 The structuresof GTPandFADHz.(A)GTPand GDPare closerelativesof ATPand ADP,respectively.(B)FADH2is a carrierof hydrogensandhigh-energyelectrons,like NADHandNADPH.lt is shown here in its oxidizedform (FAD)with the hydrogen-canyingatoms h ighlightedin yellow.nucleotidesglucose6-phosp nur.
/amrnosugars+fructose6-phosphate'glycolipidsglycoproteins+/\I+++serined i h y d r o x y a c e t o n+ePhosPhate3-phosphoglyceratelipidsa m i n oa c i d spyrimidinesphosphoenolpyruvatealanine .*.-orrrlura"andthe citricFigure2-84Glycolysisacidcycleprovidethe precursorsmanyimportantneededto synthesizeTheaminoacids,biologicalmolecules.andotherlipids,sugars,nucleotides,hereasproducts-inmolecules-shownfor the manyturnserveasthe precursorsthe cell.Eachb/ackmacromoleculesofarrowinthisdiagramdenotesa singletheredreaction;enzyme-catalyzedwitharrowsgenerallyrepresentpathwaysto producemanystepsthatarerequiredproducts.the indicated100Chapter2:CellChemistryand BiosynthesisElectronTransportDrivesthe Synthesisof the Majorityof the ATPin MostCellsMost chemical energy is released in the last step in the degradation of a foodmolecule.
In this final process the electron carriers NADH and FADH2 transferthe electrons that they have gained when oxidizing other molecules to the electron-transport chain, which is embedded in the inner membrane of the mitochondrion (seeFigure 14-10).As the electrons pass along this long chain of specialized electron acceptor and donor molecules, they fall to successivelylowerenergy states.The energy that the electrons release in this process pumps H+ions (protons) across the membrane-from the inner mitochondrial compartment to the outside-generating a gradient of H+ ions (Figure 2-85). This gradient servesas a source of energy,being tapped like a battery to drive a variety ofenergy-requiring reactions.The most prominent of these reactions is the generation of ATP by the phosphorylation of ADPAt the end of this series of electron transfers, the electrons are passed tomolecules of oxygen gas (Oz) that have diffused into the mitochondrion, whichsimultaneously combine with protons (H*) from the surrounding solution toproduce water molecules.
The electrons have now reached their lowest energyIevel, and therefore all the available energy has been extracted from the oxidizedfood molecule. This process, termed oxidative phosphorylation (Figure 2-86),also occurs in the plasma membrane of bacteria. As one of the most remarkableachievements of cell evolution, it is a central topic of Chapter 14.In total, the complete oxidation of a molecule of glucose to H2O and CO2isused by the cell to produce about 30 molecules of ATP In contrast, only 2molecules of ATP are produced per molecule of glucose by glycolysis alone.AminoAcidsand NucleotidesArePartof the NitrogenCycleSo far we have concentrated mainly on carbohydrate metabolism and have notyet considered the metabolism of nitrogen or sulfur. These two elements areimportant constituents of biological macromolecules. Nitrogen and sulfuratoms pass from compound to compound and between organisms and theirenvironment in a seriesof reversible cycles.Although molecular nitrogen is abundant in the Earth's atmosphere, nitrogen is chemically unreactive as a gas.Only a few living speciesare able to incorporate it into organic molecules, a process called nitrogen fixation.
Nitrogenfixation occurs in certain microorganisms and by some geophysical processes,such as lightning discharge.It is essentialto the biosphere as a whole, for without it life could not exist on this planet. Only a small fraction of the nitrogenouscompounds in today's organisms, however, is due to fresh products of nitrogenfixation from the atmosphere. Most organic nitrogen has been in circulation forpyruvatefromgl y c o l y s i sICozIN A D Hf r o mglycolysisIOzIpyruvate:*Hl * PIItacetyl CoActTRtcACIDCYCLECoA2eIu^rDAilvE-PHOSPHORYLATIONtMITOCHONDRIONHzoFigure2-85 The generationof anH+gradientacrossa membranebyelectron-transportreactions.A high-energyelectron(derived,forexample,from the oxidationof ametabolite)is passedsequentiallybycarriersA, B,and C to a lowerenergystate.In this diagramcarrierB is arrangedin the membranein sucha way that ittakesup H+from one sideand releasesitto the otherasthe electronpasses.Theresultis an H+gradient.As discussedinChapter14,this gradientis an importantform of energythat is harnessedby othermembraneoroteinsto drivetheformationof ATP.Figure2-86 Thefinal stagesof oxidationof food molecules.Moleculesof NADH(FADHzand FADH2is not shown)areproducedby the citricacidcycle.Theseactivatedcarriersdonatehigh-energyelectronsthat areeventuallyusedtoreduceoxygengasto water.A majorportionof the energyreleasedduringthe transferof theseelectronsalongan electron-transferchainin themitochondrialinnermembrane(or in theplasmamembraneof bacteria)isharnessedto drivethe synthesisof ATPhencethe nameoxidative(discussedphosphorylationin Chapter14).101HOW CELLSOBTAINENERGYFROMFOODsome time, passing from one living organism to another.
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