P/O ratio (phosphorus:oxygen ratio) The number of atoms of phosphorus (i.e. as phosphate) incorporated as ATP per molecule of oxygen (O2) consumed during oxidative phosphorylation in aerobically respiring cells. Traditionally, calculations of the yield of ATP from reduced coenzymes generated by the Krebs cycle have used integral P/O values for NADH and FADH2 of 3.0 and 2.0 respectively. This assumption has given a net ATP yield of 38 ATP per glucose molecule. However, more recently, reinterpretation of experimental evidence has suggested nonintegral values of 2.5 and 1.5, which give a net yield of only 31 ATP per glucose molecule. The picture is further complicated by which of two mechanisms is used to transport cytosolic NADH (generated by glycolysis) into the mitochondrial matrix. The yield of 38 ATP (or 31 ATP) applies only if the malate/aspartate shuttle is used; if an alternative mechanism, known as the glycerol phosphate shuttle, is used, then the net yield is reduced to 36 ATP (or 29.5 ATP using the ‘modern’ P/O ratios). The latter mechanism is found, for example, in insect flight muscle.
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