This illustration summarizes the ATP yield per molecule of glucose at each stage of cellular respiration. Glycolysis and the citric acid cycle each produce two molecules of ATP via substrate-level phosphorylation. During glycolysis, pyruvate oxidation, and the citric acid cycle, electrons are also transferred from substrates to NAD+ and FAD, forming NADH and FADH2. Electrons move from these electron carriers down the electron transport chain to oxygen. The energy released at each step of the chain is stored in a form the mitochondrion can use to make about 26 or 28 molecules of ATP via oxidative phosphorylation. Thus, the total yield from each molecule of glucose is about 30 or 32 molecules of ATP.