The Physiology of Flowering Plants - KHAM PHA MOI

ANAEROBIC RESPIRATION 49Table 2.3 The balance sheet for respiratory production of molecules of ATPper 1 molecule of glucose, assuming complete coupling and complete oxidationvia glycolysis, Krebs cycle and cytochrome oxidase system.Substrate level oxidation in glycolysis 4Substrate level oxidation in Krebs cycle 2Terminal oxidation, 2 NADH from glycolysis 4Terminal oxidation, 8 NADH from Krebs cycle 24Terminal oxidation, 2 succinate 4Total produced 38Used up in glycolysis, priming reactions 2Net gain 36the Krebs cycle, with terminal oxidation through cytochrome oxidase,one can add up the ATP molecules per 1 molecule glucose(Table 2.3).The NADH from glycolysis, reacting with mitochondria from theoutside, is oxidized with the production of only 2 ATP per NADH,whilst mitochondrially produced coenzyme oxidation yields 3 ATPper 1 NADH. The free energy of complete oxidation of glucose is2880 kJ mol –1 . The free energy of hydrolysis of ATP is highly dependenton factors such as ATP concentration and pH, but under cellularconditions is at least 42 kJ mol –1 . Assuming this value, a gain of36 ATP per molecule of glucose is equivalent to an energy conservationof [36 42], 1512 kJ mol –1 , or 52% of the total available, a veryhigh degree of energy conservation. (The value would be pushedeven higher if phosphorylated sugars enter the glycolytic pathway,Section 2.8.2.)The calculation in Table 2.3 is, however, based on an assumptionof complete coupling of substrate breakdown and terminal oxidation toATP synthesis. This is unlikely to be the situation in a cell, at least notin all circumstances. Any NADH which is used in reductive reactionsdoes not yield ATP, and terminal oxidation by the alternative oxidaseproduces only 1 ATP per molecule of NADH oxidized. The 12 NADPHformed per molecule of glucose in the PPP could theoretically beoxidized by the mitochondria with the production of 24 ATP, but,as stated, the PPP probably provides NADPH mainly for reductivereactions rather than for terminal oxidation. It is therefore not possibleto say exactly how much ATP is produced per glucose moleculein a particular situation.2.10 Anaerobic respiration2.10.1 Occurrence and endurance of anaerobiosis in plantsFlowering plants are obligate aerobes: no flowering plant can completeits life cycle without O 2 . Most flowering plant organs, except dormant