Mitochondrion from a heart muscle cell showing numerous cristae, animated coloured transmission electron micrograph (TEM). The mitochondrion is a membrane-enclosed organelle, found in most eukaryotic cells. Mitochondria are sometimes described as the 'power plants' of the cell because they convert NADH and NADPH into energy in the form of ATP (adenosine triphosphate) through the process of oxidative phosphorylation. A typical eukaryotic cell contains about 2, 000 mitochondria. Mitochondria contain DNA (deoxyribonucleic acid) that is independent of the DNA located in the cell nucleus. According to the endosymbiotic theory, mitochondria are descended from free-living prokaryotes.