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? So because of the electronegative character of oxygen, when diatomic oxygen shown here actually binds onto this heme group, more specifically, onto the iron of that heme group, it pulls away some of that electron density from that hein group, it pulls away some of that electron density from that iron atom. And because some of the electrons move away from that iron atom, it decreases the electron density and the size of that iron atom. And now, because the iron atom is smaller, it is able to move into the plane, into the center of the plane of that protoporphy and as shown in the following diagram.