b. Schematic diagram showing the components of a mitochondrion. Note the location of the elementary particles (inset), the shape of which refects the three-dimensional structure of ATP synthase.
which makes the membrane impermeable to ions. The membrane forming the cristae contains proteins that have three major functions: (1) performing the oxidation reactions of the respiratory electron-transport chain, (2) synthesizing ATP, and (3) regulating transport of metabolites into and out of the matrix. The enzymes of the respiratory chain are attached to the inner membrane and project their heads into the matrix (Fig. 2.37, rectangle). With the TEM, these enzymes appear as tennis racquet–shaped structures called elementary particles. Their heads measure about 10 nm in diameter and contain enzymes that carry out oxidative phosphorylation, which generates ATP.
 Intermembrane space. This space is located between the inner and outer membranes and contains specific enzymes that use the ATP generated in the inner membrane. These enzymes include creatine kinase, adenylate kinase, and cytochrome c. The latter is an important factor in initiating apoptosis (see page 94).
 Matrix. The mitochondrial matrix is surrounded by the inner mitochondrial membrane and contains the soluble enzymes of the citric acid cycle (Krebs cycle) and the enzymes involved in fatty-acid -oxidation. The major products of the matrix are CO2 and reduced NADH, which is the source of electrons for the electron-transport chain.
Mitochondria contain dense matrix granules that store Ca2 and other divalent and trivalent cations. These granules increase in number and size when the concentration of divalent (and trivalent) cations increases in the cytoplasm. Mitochondria can accumulate cations against a concentration gradient. Thus, in addition to ATP production, mitochondria also regulate the concentration of certain ions of the cytoplasmic matrix, a role they share with the sER. The matrix also contains mitochondrial DNA, ribosomes, and tRNAs.
Mitochondria contain the enzyme system that generates ATP by means of the citric acid cycle and oxidative phosphorylation.
Mitochondria generate ATP in a variety of metabolic pathways, including oxidative phosphorylation, the citric acid cycle, and -oxidation of fatty acids. The energy generated from these reactions, which take place in the mitochondrial matrix, is represented by hydrogen ions (H ) derived from reduced NADH. These ions drive a series of proton pumps located within the inner mitochondrial membrane that transfer H from the matrix to the intermembrane space (Fig. 2.38). These pumps constitute the electron-transport chain of respiratory enzymes (see Fig. 2.37). The transfer of H across the inner mitochondrial membrane establishes an
FIGURE 2.38 • Schematic diagram illustrating how mitochondria generate energy. The diagram indicates the ATP synthase complex and the electron-transport chain of proteins located in the inner mitochondrial membrane. The electron-transport chain generates a proton gradient between the matrix and intermembrane space that is used to produce ATP. Numbers represent sequential proteins involved in the electron-transport chain and ATP production: 1, NADH dehydrogenase complex; 2, ubiquinone; 3, cytochrome b-c1 complex; 4, cytochrome c; 5, cytochrome oxidase complex; and 6, ATP synthase complex.