The Physiology of Flowering Plants - KHAM PHA MOI

RESPIRATION: THE OXIDATIVE BREAKDOWN OF ORGANIC COMPOUNDS 43This reduces to:C 6 þ 12 NADP þ ! 6CO 2 þ 12 NADPH þ 12 H þ (2:13)The PPP thus does achieve the complete oxidation of glucose to CO 2,but without ATP formation; there is no substrate-level phosphorylationinvolved. The major function of the PPP is thought to be theprovision of NADPH for reductive biosyntheses, e.g. lipid formation,and for the production of metabolic intermediates; the pentosesugars can be utilized for nucleotide synthesis. (Some of the NADPHmay be oxidized by mitochondria with ATP formation: see Section 2.9,terminal oxidation.)The Krebs cycleThe Krebs cycle is the reaction series which achieves the completeoxidation of pyruvate (coming mainly from glycolysis) to CO 2. It islocated entirely and exclusively in the mitochondria. The reactionsare summarized in Fig. 2.14.The pyruvate (with three C atoms) first loses CO 2 by oxidativedecarboxylation catalysed by the enzyme pyruvate dehydrogenase,which also links the remaining 2-C fragment, an acetyl moiety,to coenzyme A producing acetyl-CoA. This condenses with the 4-Cacid oxaloacetate to form the 6-C acid citrate. Then, as indicated inFig. 2.14, there follows a series of molecular rearrangements, oxidationsteps and oxidative decarboxylation steps, until the equivalentof the pyruvate has been converted to CO 2 and the oxaloacetate hasbeen regenerated. There is one substrate-level phosphorylation producingATP, associated with the oxidation of 2-oxoglutarate. At threesteps, a molecule of water is added to the reactants. The overall finalbalance sheet for respiration shows water as a product (Equation 2.9),but water is also a substrate in respiration. (Compare with photosynthesis,where water is not only consumed as indicated by theoverall equation, but is also a product.) The 5 pairs of H equivalentsremoved from the substrates in the Krebs cycle reduce NAD + , exceptfor the succinate oxidation step, where no coenzyme is involved. Theenzymes are present in the mitochondrial matrix, but succinatedehydrogenase is again an exception, being bound to the cristamembrane of the mitochondrion.Between them the Krebs cycle and glycolysis can carry out thecomplete oxidation of glucose. With 2[H] representing the pairs of Hequivalents removed from substrates, one can writeGlycolysis C 6 H 12 O 6 ! C 3 H 4 O 3 þ 2 2½HŠ (2:14)Krebs cycle 2C 3 H 4 O 3 þ 6H 2 O ! CO 2 þ 10 2½HŠ (2:15)Sum C 6 H 12 O 6 þ 6H 2 O ! CO 2 þ 12 2½HŠ (2:16)