For each mol of substrate oxidized Complexes I, III, and IV in the respiratory chain (ie, NADH), 2.5 mol of ATP are formed per 0.5 mol of O2 consumed; ie, the P:O ratio = 2.5. On the other hand, when 1 mol of the substrate (eg, succinate or 3-phosphoglycerate) is oxidized Complexes II, III, and IV, only 1.5 mol of ATP are formed; that is, P:O = 1.5. These reactions are known as oxidative phosphorylation at the respiratory chain level. Reference: Harper; 30th edition; Page no: 131