Mitochondria are membrane-enclosed organelles distributed through the cytosol of most eukaryotic cells. Their number within the cell ranges from a few hundred to, in very active cells, thousands. Their main function is the conversion of the potential energy of food molecules into ATP.Mitochondria have:an outer membrane that encloses the entire structurean inner membrane that encloses a fluid-filled matrixbetween the two is the intermembrane spacethe inner membrane is elaborately folded with shelflike cristae projecting into the matrix.a small number (some 5–10) circular molecules of DNA.This electron micrograph (courtesy of Keith R. Porter) shows a single mitochondrion from a bat pancreas cell. Note the double membrane and the way the inner membrane is folded into cristae. The dark, membrane-bounded objects above the mitochondrion are lysosomes.