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. The yield of ATP from reduced coenzymes generated by the Krebs cycle depends on how many hydrogen ions (H⁺) are transported across the inner mitochondrial membrane for each molecule of NADH or FADH₂ entering the electron transport chain. It is now assumed that 10 H⁺ ions are transported across for each molecule of NADH, and that four H⁺ must re-enter the mitochondrial matrix via ATP synthase to produce one molecule of ATP. This equates to 2.5 ATP per NADH, and 1.5 ATP per molecule of FADH₂. These values give a net P/O yield of 31 ATP per glucose molecule. The picture is further complicated by which mechanism is used to transport cytosolic NADH (generated by glycolysis) into the mitochondrial matrix. The yield of 31 ATP applies only if the malate/aspartate shuttle is used, as in liver cells and heart cells; if an alternative mechanism, known as the glycerol phosphate shuttle, is used, then the net yield is reduced to 29.5 ATP. The latter mechanism is found, for example, in muscle cells.