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. So to see how that takes place, let's take a look at the following diagram. So, in this diagram to the left, we have the alpha helix of one of the polypeptide chains. And this alpha helix is attached to this heme group.