There is no fixed shape for mitochondria and they can often be seen changing shape as the cell moves through different degrees of energy requirement. However, when viewed as a thin, cross section under the electron microscope, the most common shape for a mitochondrion is that of a hot dog. Each mitochondrion has two membranes: an inner membrane which is highly convoluted and folded into finger-like projections called cristae, and a smooth outer membrane that is a selective barrier to molecules from the cytoplasm.

Partially broken-down fuel molecules are transported from the cytoplasm, through the membranes of the mitochondrion, and are broken down further within the inner mitochondrial space. As this happens, energy is released from the disintegrating fuel molecules and then trapped again in molecules called adenosine triphosphate( ATP).

Oxygen is required for the final stages of this breakdown and carbon dioxide and water are the two waste products. ATP molecules, carrying their stored energy, are then transported to other parts of the cell to participate in anabolic reactions, such as protein synthesis. Here the energy in the ATP molecules is released to drive nonspontaneous reactions.