So as one of the heme group becomes occupied with oxygen, it causes the other heme groups to increase their affinity for oxygen. And likewise, when one of the heme groups releases the oxygen, the other occupied heme groups become much more likely to unload and release that oxygen. So this is what we mean by the cooperative nature and the cooperative behavior of hemoglobin molecules. Now, once again, what this implies is that the heme groups, the four heme groups in hemoglobin somehow interact with one another. They cooperate with one another. But if we examine the structure of hemoglobin, we'll see that the four different hemoglobin molecule, the four different heme groups, these red structures within each polypeptide chain are actually separated by a certain distance.