And notice that because the oxygen is not attached to our iron atom, that iron atom is found below the plane of that protoporphine. Because when it's not balanced as an oxygen, the iron is simply too large of an atom. It contains a large electron density. Now, this green structure is simply the surface of the adjacent polypeptide subunit. And notice there's a certain distance between these two subunits and they interact in a certain way. Now, what happens when oxygen actually binds onto the iron of this heme group?