Mitochondrion

In cell biology, a mitochondrion (plural mitochondria) (from Greek μίτος or mitos, thread + χονδρίον or khondrion, granule) is a membrane-enclosed organelle, found in most eukaryotic cells. Mitochondria are sometimes described as "cellular power plants," because they generate most of the cell's supply of ATP, used as a source of chemical energy. The number of mitochondria in a cell varies widely by organism and tissue type. Many cells possess only a single mitochondrion, while others can contain several million mitochondria . Although most of a cell's DNA is contained in the cell nucleus, mitochondria have their own independent genomes. According to the endosymbiotic theory, mitochondria are descended from free-living prokaryotes.