In cell biology, a mitochondrion is a membrane-enclosed organelle, found in most eukaryotic cells. Mitochondria are "cellular power plants," because they generate most of the cell's supply of ATP, used as a source of chemical energy. Each pyruvate molecule produced by glycolysis is actively transported across the inner mitochondrial membrane, and into the matrix where it is oxidized and combined with coenzyme A to form CO₂, acetyl-CoA and NADH.