The site of aerobic cellular respiration, mitochondria produce most of the useful chemical energy of eukaryotic cells. Mitochondria have a double-membrane system in which the inner membrane is highly folded, extending into the inner matrix. In these interior compartments are the proteins and enzymes necessary to generate ATP, the chemical energy that fuels most cellular reactions. Thought to have evolved by endosymbiosis from early bacterial cells, mitochondria have their own genetic system. Mitochondria contain circular DNA molecules coding for small numbers of proteins, rRNAs and tRNAs, and within the inner compartment these genes can be transcribed and translated. The number of mitochondria in a cell varies widely, with some cells housing a single large mitochondrion, and other cells containing many thousands of them. In mammalian heart muscle cells, mitochondria can account for almost 50% of the total cell volume. Extremely dynamic organelles, mitochondria can change shape, fuse, and divide.

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