Respiration completes the process begun by glycolysis and the citric acid cycle, because it provides a way of reoxidizing the carrier molecules, NADH and FADH₂. So far there has been no reason to call the reactions that we have discussed "aerobic" because no oxygen has been involved. The oxidative steps have involved only the transfer of H atoms from the molecule being oxidized to a carrier molecule. The respiratory chain provides the means of finally linking these reactions to the use of oxygen.

We again are faced with the dilemma of having the energy available (52.7 kcal per NADH) larger than the amount that can be received and stored in one step (7.3 kcal per ATP). The answer, as before, is a series of smaller free energy steps, at three of which ATP is synthesized. The steps are the successive reduction and reoxidation of the members of the respiratory chain, shown opposite. Incoming NADH is oxidized to NAD⁺ in the process of reducing a flavoprotein, an enzyme that has attached to it a flavin group similar to that found in FAD. This flavoprotein is reoxidized as it reduces a small organic molecule, ubiquinone, shown on the next page. (The name means “everywhere-quinone," because the molecule is found universally in all eucaryotic cells.) Ubiquinone then reduces cytochrome b, which is the first of a series of related proteins that contain iron in a heme group, as do myoglobin and hemoglobin, discussed in Chapter 22. In the subsequent ladder of cytochromes, b reduces cytochrome c₁, c₁ reduces cytochrome c, c reduces cytochrome a, a reduces cytochrome a₃, and the respiratory chain comes to an end when cytochrome a₃ reduces oxygen to H₂0.