Figure 235

Structure of the mitochondrion, a. This electron micrograph shows a mitochondrion in a pancreatic acinar cell. Note that the inner mitochondrial membrane forms the cristae (C) through a series of infold-ings, as is evident in the region of the arrow. The outer mitochondrial membrane is a smooth continuous envelope that is separate and distinct from the inner membrane, x200,000. b. Schematic diagram showing the components of a mitochondrion. Note the location of the elementary particles (inset), the shape of which relates to the three-dimensional structure of ATP synthase.

trix. The outer mitochondrial membrane is in close contact with the cytoplasm. The space between the two membranes is called the intermembrane space. The structural components of mitochondria possess specific characteristics related to their function:

• Outer mitochondrial membrane. This 6- to 7-nm-thick smooth membrane contains many voltage-dependent anion channels (also called mitochondrialporins). These large channels (approximately 3 nm in diameter) are permeable to uncharged molecules up to 5,000 Da. Thus small molecules, ions, and metabolites can enter the intermembrane space but cannot penetrate the inner membrane. The environment of the intermembrane space is therefore similar to that of cytoplasm with respect to ions and small molecules. The outer membrane possesses receptors for proteins and polypeptides that translocate into the intermembrane space. It also contains several enzymes, including phospholipase A2, monoamine oxidase, and acetyl coenzyme A (CoA) synthase.

• Inner mitochondrial membrane. The TEM reveals that this membrane is thinner than the outer mitochondrial membrane. It is arranged into numerous folds ('cristaej that significantly increase the inner membrane surface area (see Fig. 2.35). These folds project into the matrix that constitutes the inner compartment of the organelle. In some cells involved in steroid metabolism, the inner membrane may form tubular or vesicular projections into the matrix. The inner membrane is rich in the phospholipid cardiolipin, 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. 2.35, 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 45).

• 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 /3-oxidation. The major products of the matrix are C02 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 /3-oxidation of fatty acids. The energy generated from these reactions, which take place in the mitochondrial matrix, is represented by hydrogen ions (H1) 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 inter-membrane space (Fig. 2.36). These pumps constitute the

0 0

Post a comment