The sites of oxidative phosphorylation coupling in the respiratory chain are NADH → ubiquinone, ubiquinone → cytochrome c, and cytochrome c → O2. The site of coupling of oxidative phosphorylation can be determined by calculating the free energy change and the P/O ratio, which is the number of moles of inorganic phosphorus atoms consumed for every 1 mole of oxygen atoms consumed in the oxidation of a metabolite, i.e., the number of moles of ATP synthesized. The coupling site of oxidative phosphorylation is within complexes I, III, and IV, and the two respiratory chains pass through the coupling site to produce ATP differently; NADH oxidation generates 2.5 molecules of ATP; FADH2 oxidation generates 1.5 molecules of ATP. Oxidative phosphorylation is a biochemical process that occurs in the inner mitochondrial membrane of eukaryotic cells or in the cytoplasm of prokaryotic organisms, and it is a coupling reaction in which the energy released from the oxidation of substances in the body is supplied to ADP and inorganic phosphate through the respiratory chain to synthesize ATP. Ninety-five percent of the ATP in organisms comes from this way.