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. And what that implies is the heme groups do not actually interact with one another in a direct fashion. And as we'll see in just a moment, even though the heme groups do not interact with one directly, the surfaces between the polypeptide chains do interact with one another. And more specifically, it's the surface to surface interaction between the two dimers in this hemoglobin that actually causes that cooperative behavior of hemoglobin.