For simplicity, the molecules in the ETC are shown as four "complexes." Complex II is represented here by "coenzyme Q". The enzyme ATP synthase is responsible for the phosphorylation of ADP to form ATP in a process called chemiosmosis.

In this module, fully reduced molecules are shown in blue, while molecules in their oxidized state are shown in red. At any one time, some of the molecules in the three complexes will be reduced and others oxidized.