The P/O ratio is determined experimentally and is a number that relates the amount of ATP generated per electron carrier. This number can vary depending on the physiological conditions. P stands for the moles of phosphoryl groups consumed to form ATP, and the O stands for moles of oxygen consumed to be reduced to water. The P/O ratio of one NADH is 2.5 ATP, and the P/O ratio of one FADH2 is 1.5 ATP. FADH2 has a lower P/O ratio because FAD has a higher affinity for electrons and donates them through the electron transport chain at complex II and contributes fewer protons to the gradient. Per glucose, 32 ATP are produced. 4 ATP are produced directly while 25 ATP are produced through NADH and 3 ATP are produced through FADH2. The total number of ATP produced can vary from 28-36 depending on the P/O ratios of NADH and FADH2 dependent on physiological conditions.
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