And each one of these polypeptide subunits contains a single Hen group that is capable of binding oxygen molecules. Now, because these four polypeptides can interact with one another our hemoglobin displays something called Cooperativity. And what that means is if we take a fully unsaturated deoxy hemoglobin protein and one of the heme groups accepts an oxygen molecule then that will create a conformational change in the entire structure and that will cause the other three unoccupied hein groups to become much more likely to accept additional oxygen molecules. Likewise, if we have a fully saturated oxymoglobin molecule and one of the hein groups unloads or releases an oxygen molecule then that will create a conformational change in the structure of the four polypeptides and that will make it more likely for the other three occupied hein groups to actually release an oxygen molecule. So this is what we mean by Cooperativity. Now, this leads to the sigmoidal shape the S shape of the oxygen hemoglobin dissociation curve that can be seen in the following diagram.