II.

Mitochondrial Structure

A mitochondrion is typically long and slender, but it can appear bean-shaped or oval-shaped under the electron microscope. Ranging in size from 0.5 micrometer (0.00005 in) to 1 micrometer (0.0001 in) in length, a mitochondrion has a double membrane that forms a sac within a sac. The smooth outer membrane holds numerous transport proteins, which shuttle materials in and out of the mitochondrion. The region between the outer and inner membranes, which is filled with liquid, is known as the outer compartment. The inner membrane has numerous folds called cristae. Cristae are the sites of ATP synthesis, and their folded structure greatly increases the surface area where ATP synthesis occurs. Transport proteins, molecules called electron transport chains, and enzymes that synthesize ATP are among the molecules embedded in the cristae. The cristae enclose a liquid-filled region known as the inner compartment, or matrix, which contains a large number of enzymes that are used in the process of aerobic respiration.