The net ATP-yield in eukaryotes from glycolysis, TCA cycle, and electron transport and oxidative phosphorylation can be readily calculated.
Before the general acceptance of the chemiosmotic hypothesis for oxidative phosphorylation, this calculation was based on phosphate/oxygen ratio (P/O ratio). Most experiments yielded P/O (ATP to ½ O2) ratio of more than two when NADH was the electron donor, and more than one when succinate was the electron donor.
Given the assumption that P/O ratio should have an integral value most experimenters agreed that the P/O ratios must be 3 for NADH and 2 for succinate (FADH)2.
On the basis of these P/O ratios (the number of ATPs formed per oxygen atom and reduced by 2 electrons in electron transport chain), the total ATP yield from oxidation of one glucose molecule in aerobic respiration was calculated to be a maximum of 36 ATPs. The number goes to 38 when malate-aspartate suttle rather than the glycerol 3- phosphate suttle is used.
With the general acceptance of the chemiosmotic hypothesis for coupling ATP synthesis to oxidative phosphorylation, there was no theoretical requirement for P/O ratio to be integral.
The relevant question now became how many protons (H+) are pumped outward by electron transport chain from one NADH to oxygen, and how many protons (H+) must flow inward through the F1/F0 ATPase complex to drive the synthesis of one ATP? The best current estimates for protons pumped out per pair of electrons arc 10 for NADH and 6 for succinate (FADH2).
The most widely accepted experimental value for number of protons required to drive the synthesis of an ATP molecule is 4, of which one is used in transporting Pi (inorganic phosphate), ATP, and ADP across the mitochondrial membrane. If 10 protons are pumped out per NADH and 4 must flow in to produce one ATP, the proton-based P/O ratio is 2.5 (10/4) for NADH and 1.5 (6/4) for succinate (FADH2).
Hence, as given in Table 24.3, 30 molecules of ATP are synthesized when glucose in completely oxidised to CO2. This number goes to 32 when malate-aspartate suttle rather than the glycerol 3-phosphate suttle is used.
ATP-yields in bacteria in aerobic conditions can be less because the bacterial electron transport systems often possess lower P/O ratios than the eukaryotic system. For instance, Escherichia coli with its branched electron transport chains has a P/O ratio around 1.3 when respiring at high oxygen levels and only a ratio of about 0.67 when respiring at low oxygen levels.
In this case ATP synthesis varies with environmental conditions. Perhaps because E. coli normally grows in habitats rich in nutrients it does not have to be particularly efficient in ATP synthesis. Presumably the electron transport chain functions when E. coli is in an aerobic fresh water environment between hosts.