And once these two groups are in their reduced form, only then can they actually bind oxygen. So in the next step, in diagram two, we have an oxygen, and the oxygen is the same oxygen molecule that we essentially breathe in from the environment. The oxygen is basically used to form something called a peroxide bridge between this heme A three and this copper B. So once the heme A three and the copper B are in their fully reduced form, and a diatomic oxygen molecule is actually abstracted, and it is used to actually build a peroxide bridge between this structure and this structure here. Now, once we form this bridge, what happens next is, from the matrix of the mitochondria, two protons are abstracted, and those two protons are actually used to help break this bond. But before the two protons are used, two more of these reduced cytochrome C molecules are actually oxidized by protein complex four.