Overall 'glycolys i s429++enolasephosphoglycerate mutase+OO–CCHOHCH2O3-phosphoglycerateOO–CCHOCH2OH2-phosphoglycerateOO–CCHOCH2OH2-phosphoglycerateOO–CCOCH2H2OphosphoenolpyruvateOO–CCOCH2phosphoenolpyruvateOO–CCOCH3pyruvateOO–CCOCH3OO–CCOCH3+phosphoglycerate kinaseOCCHOHCH2O1,3-bisphosphoglycerate+OO–CCHOHCH2O3-phosphoglycerate The two molecules of glyceraldehyde 3-phosphate are oxidized. The energy-generation phase of glycolysis begins, as NADH and a new high-energy anhydride linkage to phosphate are formed (see Figure 13–5).Step 6H++++glyceraldehyde 3-phosphatedehydrogenaseOHCCHOHCH2Oglyceraldehyde3-phosphateOOCCHOHCH2O1,3-bisphosphoglycerateH++ The transfer to ADP of the high-energy phosphate group that was generated in step 6 forms ATP.Step 7 The remaining phosphate ester linkage in 3-phosphoglycerate, which has a relatively low free energy of hydrolysis, is moved from carbon 3 to carbon 2 to form 2-phosphoglycerate.Step 8 The removal of water from 2-phosphoglycerate creates a high-energy enol phosphate linkage.Step 9 The transfer to ADP of the high-energy phosphate group that was generated in step 9 forms ATP, completing glycolysis.Step 10NET RESULT OF GLYCOLYSIS+pyruvate kinaseCH2OHOOHOHOHHOglucosetwo moleculesof pyruvateIn addition to the pyruvate, the net products aretwo molecules of ATP and two molecules of NADH.123ATPADPATPATP ATPATP ATPATP ATPADPNADHNADHNADHNAD+PiPPPPPPPPPPPOThe Breakdown and Utilization of Sugars and Fats The $ATP$ge ts $us e d$in$anaboli sm.$ But$where$does $the$NADH$go?421polysaccharidesproteinsamino acidsfatssimple sugarsfatty acidsand glycerolGLYCOLYSISCITRICACIDCYCLEOXIDATIVEPHOSPHORYLATIONCO2CO2CO2H2OH2OO2O2STAGE 1:BREAKDOWNOF FOODSTO SIMPLE SUBUNITS (A)(B)STAGE 2:BREAKDOWN OF SIMPLE SUBUNITS TO ACETYL CoA; LIMITED AMOUNTS OF ATP AND NADHPRODUCEDSTAGE 3:COMPLETEOXIDATIONOF ACETYLCoA TO H2OAND CO2; LARGEAMOUNTS OF ATPPRODUCED IN MITOCHONDRIONECB4 e13.02/13.03CYTOSOLouter mitochondrialmembranemitochondrialmatrixplasmamembrane of eukaryoticcellinner mitochondrialmembraneglucoseATPATPATPATPNADHATPNADHNADHNADHpyruvateacetyl CoAFOOD + + ++NET RESULT:The Breakdown and Utilization of Sugars and Fats activated carriers that will subsequently help drive the production of much larger amounts of ATP by oxidative phosphorylation.Food Molecules Are Broken Down in Three StagesThe proteins, fats, and polysaccharides that make up most of the food we eat must be broken down into smaller molecules before our cells can use them—either as a source of energy or as building blocks for making other organic molecules. This breakdown process—in which enzymes degrade complex organic molecules into simpler ones—is called catabolism. The process takes place in three stages, as illustrated in Figure 13–3.Figure 13–3 The breakdown of food molecules occurs in three stages. (A) Stage 1 mostly occurs outside cells in the mouth and the gut—although intracellular lysosomes can also digest large organic molecules. Stage 2 occurs mainly in the cytosol, except for the final step of conversion of pyruvate to acetyl groups on acetyl CoA, which occurs in the mitochondrial matrix. Stage 3 begins with the citric acid cycle in the mitochondrial matrix and concludes with oxidative phosphorylation on the mitochondrial inner membrane. The NADH generated in stage 2—during glycolysis and the conversion of pyruvate to acetyl CoA—adds to the NADH produced by the citric acid cycle to drive the production of ATP by oxidative phosphorylation. (B) The net products of the complete oxidation of food include ATP, NADH, CO2, and H2O. The ATP and NADH provide the energy and electrons needed for biosynthesis; the CO2 and H2O are waste products.Ult i matel y,$ NADHs$are$us e d$during$oxidative$phosphorylation,$ which$takes$the$electrons$in$NADH$into$the$electron$ transport$chain,$conve rting$NADH$back$into$NAD+.$NAD+$is$reus ed$ in$gly coly sis$and$in$citric$acid$cycl e.But'what' if'you' don’t' have'oxygen,'which' is 'ne ede d'for' oxidat ive'phos phorylation?Many$cells $and$microbe s $(yeast$for$example)$ can$s urvive$in$anaerobic$conditions.$How$do$the$NADHs$get$r ecy cled$back$into$NAD+?425lower one. The electrons that are passed along the electron-transport chain are ultimately passed on to O2, forming water.In giving up its electrons, NADH is converted back into NAD+, which is then available to be used again for glycolysis. In the absence of oxygen, NAD+ can be regenerated by an alternate type of energy-yielding reaction called a fermentation, as we discuss next.Fermentations Can Produce ATP in the Absence of OxygenFor most animal and plant cells, glycolysis is only a prelude to the third and final stage of the breakdown of food molecules, in which large amounts of ATP are generated in mitochondria by oxidative phospho-rylation, a process that requires the consumption of oxygen. However, for many anaerobic microorganisms, which can grow and divide in the absence of oxygen, glycolysis is the principal source of ATP. The same is true for certain animal cells, such as skeletal muscle cells, which can continue to function at low levels of oxygen. In these anaerobic conditions, the pyruvate and NADH made by gly-colysis remain in the cytosol. The pyruvate is converted into products that are excreted from the cell: lactate in muscle cells, for example, or ethanol and CO2 in the yeast cells used in brewing and breadmaking. The NADH gives up its electrons in the cytosol, and is converted back to the NAD+ required to maintain the reactions of glycolysis (Figure 13–6). Such energy-yielding pathways that break down sugar in the absence of oxygen are called fermentations. Scientific studies of the commercially important fermentations carried out by yeasts laid the foundations for early biochemistry. Figure 13–6 Pyruvate is broken down in the absence of oxygen by fermentation. (A) When inadequate oxygen is present, for example, in a muscle cell undergoing vigorous contraction, the pyruvate produced by glycolysis is converted to lactate in the cytosol. This reaction restores the NAD+ consumed in step 6 of glycolysis, but the whole pathway yields much less energy overall than if the pyruvate were oxidized in mitochondria. (B) In microorganisms that can grow anaerobically, pyruvate is converted into carbon dioxide and ethanol. Again, this pathway regenerates NAD+ from NADH,