Mitochondrial origins

The eukaryotic mitochondrion (pl. mitochondria) is the 'power house of the cell. An outer membrane, similar in composition to the plasma membrane surrounds the organelle. The inner membrane is contiguous, at membrane junctions, with the inner membrane that forms the walls of cristae. The inner mitochondrial membrane contains more than 100 different polypeptides. The protein to phospolipid ratio is very high – more than 3:1 by weight, having about 1 protein for 15 phospholipids. The inner membrane is also rich in an unusual phospholipid, cardiolipin, which is usually characteristic of bacterial plasma membranes. This composition, along with other evidence, has led to the assumption that the inner membrane is derived from endosymbiotic prokaryotes.

Mitochondria are believed to have developed from an endosymbiotic union with alpha-proteobacteria, specifically the Rickettsiales. Phylogenetic analyses indicate that genome of R. prowazekii is more closely related to that of mitochondria than is any other microbe yet analyzed. Neither genome contains genes required for anaerobic glycolysis. R. prowazekii does contain a complete set of genes encoding components of the tricarboxylic acid cycle and the respiratory-chain complex, so ATP production in Rickettsia is the same as that in mitochondria. The genes from Rickettsia prowazekii encoding cytochrome b (cob) and cytochrome c oxidase subunit I (cox1) provide further phylogenetic evidence for a link with mitochondrial origins.

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