mitochondrion
mitochondrion An oval, or occasionally round or thread-shaped, organelle, whose length averages 2 μm and that occurs in large numbers in the cytoplasm of eukaryotic cells. It is a double-membrane-bound structure in which the inner membrane is thrown into folds (the cristae) that penetrate the inner matrix to varying depths. It is a semiautonomous organelle containing its own DNA and 70S ribosomes, and reproducing by binary fission, partly under nuclear control. It is the major site of ATP production and thus of oxygen consumption in cells, and houses the enzymes involved in the citric-acid cycle and in oxidative phosphorylation. Ribosomes have a sedimentation factor; those with a factor of 70 are known as 70S ribosomes. Bacteria have 70S ribosomes, but the cytoplasmic ribosomes of eukaryotic cells are 80S. Endosymbiotic bacteria are believed to have evolved into mitochondria.
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mitochondrion
