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• The phosphate/oxygen ratio, also known as the P/O ratio, refers to the amount of ATP produced by the transportation of two electrons via a specific electron transport chain, which is followed by oxygen atom reduction.
• It shows how well the consumption of oxygen in electron transport and the synthesis of ATP in oxidative phosphorylation are linked.
• As a result, the P/O ratio plays an important role in ATP metabolism.

• The P/O ratio is determined by the number of hydrogen ions delivered externallywithrespect to the mitochondrial membrane via an electrochemical gradient and the number of protons returned inward through the membrane by an enzyme-like ATP synthase.
• Although, due to proton leakage, not all transferred protons may be utilized for ATP production. However, protons are also used for other purposes like transporting inorganic phosphate into the mitochondrial matrix.

Thus, the P/O ratio is not always an integer.