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. So one and two, we form the peroxide molecule. In fact, when we discussed complex four, we said that a peroxide bridge is formed between the copper B atom and the heme A three group. Now, these two types of molecules are very, very reactive.