The mitochondria are the power plants of cells. Each sausage-shaped mitochondrion is covered by an outer membrane; the inner membrane of a mitochondrion is folded into compartments called cristae (meaning "box"). The matrix, or inner space created by the cristae, contains the enzymes necessary for the many chemical reactions that eventually transform food molecules into energy.
Cells contain hundreds to thousands of mitochondria. An interesting aspect of mitochondria is that they contain their own DNA sequences, although not in the profusion that the nucleus contains. The presence of this separate DNA, along with the resemblance of mitochondria to single-celled prokaryotes, has led to a theory of eukaryotic evolution called the endosymbiotic theory. This theory postulates that mitochondria were once separate prokaryotes that became engulfed within other prokaryotes. Instead of being digested, the mitochondrial prokaryotes remained within the engulfing cell and performed its energy-releasing functions. Over millions of years, this symbiotic relationship fostered the evolution of the eukaryotic cell.
Science EncyclopediaScience & Philosophy: Categorical judgement to ChimaeraCell - Types Of Cells, Prokaryotes And Eukaryotes, Cell Size And Numbers, The Structure And Function Of Cells - The structure of eukaryotes, Plant organelles, Vacuoles, Cell wall