And the final accept of electrons is diatomic oxygen. And in complex four of the electron transport chain a total of four electrons and four protons are combined into diatomic oxygen to completely reduce that diatomic oxygen into water molecules. And water molecules are harmless to the cell. And the water molecules can be used in a variety of different ways. But under certain conditions, the oxygen is only partially reduced into some type of molecule. So if oxygen, for instance, only gains one electron we form a radical molecule known as the superoxide an ion if it accepts two electrons.