Metabolic oxidations generate reduced electron carriers, such as NADH and FADH2. Oxidation of these electron carriers in the mitochondrion generates most of the energy needed for ATP synthesis. Most vertebrate cells contain several hundred mitochondria, but the number can be as low as 1 and as high as 100,000.

Figure 15.2a schematically illustrates mitochondrial structure. Note that it has an outer membrane, an inner membrane, an intermembrane space, and a matrix, located within the inner membrane. The outer membrane is porous, whereas the inner membrane is much tighter, serving as a barrier to many biological metabolites. Note, too, that the inner membrane is highly folded into cristae, which project into, and often nearly through the interior of the mitochondrion. Understanding the function of the inner mitochondrial membrane is a key to understanding how the mitochondrion works.

Processes occurring inside the mitochondrial matrix include pyruvate oxidation, fatty acid oxidation, amino acid metabolism, and the citric acid cycle. Furthermore, respiratory proteins are bound to the inner membrane, so the density of cristae corresponds to the respiratory activity of a cell. For example, mitochondria in heart muscle cells (high rates of respiration) are densely packed with cristae, whereas mitochondria in liver cells (low rates of respiration) have more sparsely distributed cristae.

Embedded within the inner membrane are the protein electron carriers (primarily) cytochromes, which constitute the respiratory chain. They are assembled in the form of five multiprotein complexes, named I, II, III, IV, and V. Smaller carriers, such as coenzyme Q and cytochrome c, also participate in carrying electrons. Figure 15.2b shows metabolic reactions occuring inside the mitochondrial matrix and movement of electrons through the complexes in the inner mitochondrial membrane. Note that electrons are ultimately donated with protons to oxygen to form water and that Complex V does not function in transport of electrons.