A mitochondrion is a small, membrane-bound organelle found in eukaryotic cells. It is often referred to as the powerhouse of the cell because it generates most of the cell's supply of adenosine triphosphate (ATP), which is used as a source of chemical energy. Mitochondria are believed to have originated from ancient bacteria that were engulfed by a primitive eukaryotic cell, and they still contain their own small circular DNA.