This is a mitochondrion.The outer membrane is believed to be eukaryotic in origin. The inner membrane is, according to the endosymbiont theory, the cell membrane of an ancient prokaryote that became an endosymbiont. This inner membrane is the location of the electron transport system. The infoldings of this membrane are called cristae. The space between the two membranes is the location for proton accumulation. This accumulation represents the conservation of energy that can ultimately be used in oxidative phosphorylation. This phosphorylation briefly traps the energy in a high- energy phosphate bond in the molecule, ATP.

The fluid inside the inner membrane, the mitochondrial matrix, is the cytoplasm of the original endosymbiont. This area is responsible for the Kreb's cycle. This cycle is also known as the TCA (tricarboxylic acid) cycle and the citric acid cycle.

The mitochondrion has its own naked, circular DNA, and its own 70s ribosomes. This is further evidence for the origin of the mitochondrion as a prokaryotic endosymbiont. The genetic code used in the mitochondrion is slightly different than that of eukaryotic cells as well.

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