Mitochondria Possess Two Structurally and Functionally Distinct Membranes

Mitochondria are among the larger organelles in the cell, each one being about the size of an E. coli bacterium. Most eukaryotic cells contain many mitochondria, which collectively can occupy as much as 25 percent of the volume of the cytoplasm. They are large enough to be seen under a light microscope, but the details of their structure can be viewed only with the electron microscope (see Figure 5-26). The outer membrane defines the smooth outer perimeter of the mitochondrion. In contrast, the inner membrane has numerous invaginations called cristae. These membranes define two submitochondrial compartments: the intermembrane space between the outer membrane and the inner membrane with its cristae, and the matrix, or central compartment (Figure 8-6). The fractionation and purification of these membranes and compartments have made it possible to determine their protein and phospholipid compositions and to localize each enzyme-catalyzed reaction to a specific membrane or space.

The outer membrane contains mitochondrial porin, a transmembrane channel protein similar in structure to

Intermembrane space

Cristae

Intermembrane space

Cristae

▲ FIGURE 8-6 Internal structure of a mitochondrion.

(a) Schematic diagram showing the principal membranes and compartments. The cristae form sheets and tubes by invagination of the inner membrane and connect to the inner membrane through relatively small uniform tubular structures called crista junctions. The intermembrane space appears continuous with the lumen of each crista. The F0F1 complexes (small red spheres), which synthesize ATP; are intramembrane particles that protrude from the cristae and inner membrane into the matrix. The matrix contains the mitochondrial DNA (blue strand), ribosomes (small blue spheres), and granules (large yellow spheres).

(b) Computer-generated model of a section of a mitochondrion from chicken brain. This model is based on a three-dimensional electron tomogram calculated from a series of two-dimensional electron micrographs recorded at regular angular intervals. This technique is analogous to a three-dimensional X-ray tomogram or CAT scan. Note the tightly packed cristae (yellow-green), the inner membrane (light blue), and the outer membrane (dark blue). [Part (a) courtesy of T Frey; part (b) from T Frey and C. Mannella, 2000, Trends Biochem. Sci. 25:319.]

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