Cytochrome c oxidase. Molecular model of a cytochrome c oxidase enzyme from the mitochondria of a cow's heart. Cytochrome molecules perform oxidation and reduction reactions for electron transport, a chain of reactions used to power cellular processes that require energy. This molecule is the last enzyme in that reaction. It sits in the mitochondrial membrane, where it transfers electrons to oxygen molecules. With the addition of hydrogen ions water is formed, releasing energy. The movement of the hydrogen ions also creates a proton gradient across the membrane that is used to form ATP (adenotriphosphate).