You have now seen that pyruvate, a three-carbon molecule, is completely oxidized to CO₂ by pyruvate dehydrogenase and the citric acid cycle. For the cycle to continue, the starting molecules—acetyl CoA and oxidized electron carriers—must all be replenished. The electron carriers are reduced during the citric acid cycle and in Step 6 of glycolysis (see Figure 9.5), and they must be reoxidized:

NADH → NAD⁺ + H⁺ + 2 e^–

FADH₂ → FAD + 2 H⁺ + 2 e^–

These oxidation reactions are coupled to reactions in which other molecules get reduced. When it is present, O₂ is the molecule that eventually accepts these electrons, and it is reduced to form H₂O.