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Evansand PeterTotonoz.)o 1 , 6 - 9 l y c o s i dbi co n dat branch point/o-cH2OHl;------.,Quantitatively, fat is far more important than glycogen as an energy store foranimals, presumably becauseit provides for more efficient storage.The oxidationof a gram of fat releasesabout twice as much energy as the oxidation of a gramof glycogen. Moreover, glycogen differs from fat in binding a great deal of water,producing a sixfold difference in the actual mass of glycogen required to storethe same amount of energy as fat.
An averageadult human stores enough glycogen for only about a day of normal activities but enough fat to last for nearly amonth. If our main fuel reservoir had to be carried as glycogen instead of fat,body weight would increase by an averageof about 60 pounds.Although plants produce NADPH and Arp by photosynthesis,this importantprocess occurs in a specialized organelle, called a chloroplast, which is isolatedfrom the rest of the plant cell by a membrane that is impermeable to both typesof activated carrier molecules.
Moreover, the plant contains many other cellssuch as those in the roots-that lack chloroplasts and therefore cannot producetheir or,rmsugars.Therefore, for most of its ATP production, the plant relies on an50u.HOWCELLSOBTAINENERGYFROMFOOD.95Figure 2-76 How the ATPneeded formost plant cell metabolismis made.Inplants,the chloroplastsand mitochondriato supplycellswithcollaboratemetabolitesand ATP.(Fordetails,seeChapter14.)lightchloroplastmetabolitesexport of sugars from its chloroplasts to the mitochondria that are located in allcells of the plant. Most of the AIP needed by the plant is synthesized in thesemitochondria and exported from them to the rest of the plant cell, using exactlythe same pathways for the oxidative breakdor,rrnof sugars as in nonphotosynthetic organisms (Figure 2-76).During periods of excessphotosynthetic capacity during the day, chloroplasts convert some of the sugars that they make into fats and into starch, apolgner of glucose analogous to the glycogen of animals.
The fats in plants aretriacylglycerols, just like the fats in animals, and differ only in the types of fattyacids that predominate. Fat and starch are both stored in the chloroplast asreservoirs to be mobilized as an energy source during periods of darkness (seeFigure 2-75C).The embryos inside plant seedsmust live on stored sources of energy for aprolonged period, until they germinate to produce leaves that can harvest theenergy in sunlight. For this reason plant seeds often contain especially largeamounts of fats and starch-which makes them a malor food source for animals,including ourselves (Figare 2-7 7),MostAnimalCellsDeriveTheirEnergyfrom FattyAcidsBetweenMealsAfter a meal, most of the energy that an animal needs is derived from sugarsderived from food.
Excesssugars,if any, are used to replenish depleted glycogenstores,or to synthesizefats as a food store. But soon the fat stored in adipose tissue is called into play, and by the morning after an overnight fast, fatty acid oxidation generatesmost of the ATP we need.Low glucose levels in the blood trigger the breakdown of fats for energy production.
As illustrated in Figure 2-78, the triacylglycerols stored in fat dropletsin adipocl'tes are hydrolyzed to produce fatty acids and glycerol, and the fattyacids released are transferred to cells in the body through the bloodstream.\.\hile animals readily convert sugars to fats, they cannot convert fatty acids tosugars.Instead, the fatty acids are oxidized directly.Figure2-77 SomePlant seedsthatserveas important foods for humans.Corn,nuts,and Peasall containrichstoresof starchand fat that providetheyoungplantembryoin the seedwithenergyand buildingblocksfor(Courtesyof the JohnInnesbiosynthesis.Foundation.)96Chapter2:CellChemistryand Biosynthesisstored fatbloodstreamglycerolMUSCLECELLfatty acidso x i d a t i o ni nmitochondriaFigure 2-78 How stored fats aremobilized for energy production inanimals.Low glucoselevelsin the bloodtriggerthe hydrolysisof thetriacylglycerolmoleculesin fat dropletsto free fatty acidsand glycerol,asillustrated.Thesefatty acidsenter thebloodstream,wherethey bind to theabundantblood protein,serumalbumin.Specialfatty acidtransportersin theplasmamembraneof cellsthat oxidizefatty acids,suchas musclecells,then passthesefatty acidsinto the cytosol,fromwhichthey aremovedinto mitochondriafor energyproduction(seeFigure2-80).)Sugarsand FatsAre Both Degradedto AcetylCoAinMitochondriaThe fatty acids imported from the bloodstream are moved into mitochondria, where all of their oxidation takes place).
Each molecule offatty acid (as the activated molecule /a tty acyl coA) is broken down completelyby a cycle of reactions that trims two carbons at a time from its carboxyl end,generating one molecule of acetyl coA for each turn of the cycle. A molecule ofNADH and a molecule of FADH2 are also produced in this procesSugars and fats are the major energy sources for most nonorganisms, including humans. However, most of the useful ene8 t r i m e r so flipoamide reductasetransacetylase+6 dimersofdihydrolipoyldehydrogenase+ 1 2 d i m e r so fpyruvatedecarboxylaseo,//cH.csi*ht{iiiiacetyl coA(B)Figure 2-79 The oxidation of pyruvateto acetylCoA and COz.(A)The structureof the pyruvatedehydrogenasecomplex,whichcontains60 polypeptidechains.Thisis an exampleof a largemultienzymecomplexin which reactionintermediatesare passeddirectlyfromone enzymeto another.In eucaryoticcellsit is locatedin the mitochondrion.(B)The reactionscarriedout by thepyruvatedehydrogenasecomplex.Thecomplexconvertspyruvateto acetylcoAin the mitochondrialmatrix;NADHis alsoproducedin this reaction.A, B,and C arethe three enzymespyruvatedecarboxylase,Iipoam ide reductasetronsacetylose,and dihydrolipoyIdehydrogenase,respectively.Theseenzymesareillustratedin (A);theiractivitiesarelinkedas shown.97HOWCELL5OBTAINENERGYFROMFOODS u g a r sa n dpolysaccharidesFats+fatty acidsCYTOSOLFigure2-80 Pathwaysfor the production of acetyl CoAfrom sugarsand fats.
The mitochondrioninlt iseucaryoticcellsis the placewhereacetylCoAis producedfrom both typesof majorfood molecules.occurand wheremostof its ATPis made.thereforethe olacewheremostof the cell'soxidationreactionsin detailin Chapter14.arediscussedThestructureand functionof mitochondriaextracted from the oxidation of both types of foodstuffs remains stored in theacetyl CoA molecules that are produced by the two t)?es of reactions justdescribed. The citric acid cycle of reactions, in which the acetyl group in acetylCoA is oxidized to CO2and H2O,is therefore central to the energy metabolism ofaerobic organisms. In eucaryotesthese reactions all take place in mitochondria.We should therefore not be surprised to discover that the mitochondrion is theplace where most of the ATP is produced in animal cells. In contrast, aerobicbacteria carry out all of their reactions in a single compartment, the cytosol, andit is here that the citric acid cycle takes place in these cells.TheCitricAcidCycleGeneratesNADHby OxidizingAcetylGroupsto COzIn the nineteenth century, biologists noticed that in the absence of air (anaerobic conditions) cells produce lactic acid (for example, in muscle) or ethanol (forexample, in yeast), while in its presence (aerobic conditions) they consume 02and produce CO2and H2O.Efforts to define the pathways of aerobic metabolismFigure2-81 The oxidation of fatty acidsto acetyl CoA.(A)Electronmicrographofa lipid droplet in the cytoplasm(top),andthe structureof fats (bottom).FatsareThe glycerolportion,totriacylglycerols.whichthreefatty acidsarelinkedthroughesterbonds,is shownhereinareinsolublein waterand formblue.Fatsfatlargelipiddropletsin the specializedin whichthey arecells(calledadipocytes)stored.(B)The fatty acid oxidationcycle.The cycleis catalyzedby a seriesof fourEachenzymesin the mitochondrion.turn of the cycleshortensthe fattyacidchain by two carbons(shownin red)andgeneratesone moleculeof acetylCoAand one moleculeeachof NADHandThe structureof FADHzisFADHz.presentedin Figure2-838.(4, courtesyof Daniel5.
Friend.)(B)fatty acyl CoARr,-CH2-CH2-CH2-Chydrocarbontailfatty acyl CoAshortenedby .:fl,- CH2 - Ctwo carDons//o\s-coA/C H'?\ S-C1pm@oC-acetylCoAhydrirclrrbohtailhydrocarbontail\?R-CH2-C-CH2-C.,,oOHHr$,-CHr- Ct l- C HHotlCe s t e rb o n d6itHzooC-//o-ccHi$tct-tr-cu:'shydrocarbontail2Co.5{6A98Chapter2: CellChemistryand Biosynthesiseventually focused on the oxidation ofpyruvate and led in 1937to the discoveryof the citric acid cycle, also knoum as the tricarboxylic acid cycle or the Krebscycle.Thecitric acid cycle accounts for about two-thirds of the total oxidation ofcarbon compounds in most cells, and its major end products are CO2and highenergy electrons in the form of NADH. The CO2 is released as a waste product,while the high-energy electrons from NADH are passed to a membrane-boundelectron-transport chain (discussedin Chapter 14), eventually combining with02 to produce H2O.
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