mitochondrion - (n.)
Plural; mitochondria : An endosymbiont-derived organelle present in most eukaryotic cells and playing a major role in respiratory metabolism and energy retrieval. They are the site of oxidative respiration, i.e., where organic compounds are oxidised to produce energy. Mitochondria are relatively dense, oblong particles, about one twenty-thousandth of an inch in size, surrounded by two membranes, of which the inner one, a site of actively phosphorylating respiratory chains, forms many infoldings, or cristae. ~ See Also: Endosymbiont, Phosphorylation, Respiration.