Click here to
Cell Biology
The World of Cells
Energy Conversion Mitochondria are small cytoplasmic organelles found in all eukaryotic cells that use oxygen in the release and conversion of energy. They contain a highly specialized and integrated system of enzymes and other proteins that progressively release energy from fuel molecules and convert it into a form that can be used for anabolism and movement.

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.

Figure legend: Mitochondrion. Internally, the mitochondrion is compartmentalized by a folded inner membrane. In cross section and viewed under an electron microscope, a mitochondrion often looks like a hot dog, but when many of these sections are reassembled into a three dimensional shape, as seen above, they show a much greater diversity of form.

© 2001, Professor John Blamire