A structure within the cytoplasm of eukaryotic cells that carries out aerobic respiration: it is the site of the Krebs cycle and electron transport chain, and therefore the cell's energy production. Mitochondria vary greatly in shape, size, and number but are typically oval or sausage-shaped and bounded by two membranes, the inner one being folded into finger-like projections (cristae); they contain their own DNA (see mitochondrial DNA). Components of the electron transport chain and, in plants, the alternative respiratory pathway are located in the inner mitochondrial membrane. They are most numerous in cells with a high level of metabolic activity.
http://cellbio.utmb.edu/cellbio/mitoch1.htm#Menu Overview of mitochondrial structure and function, produced for the University of Texas Cell Biology Graduate Program