So the problem with this model is if we examine this particular diagram here, even though we have one oxygen bound, and even though this is the state that will predominate, this structure here should technically have a slightly different shape, a slightly different conformation, which is not something that we see based on this model. And so, to compensate for that limitation, we came up with a second model that we call the sequential model. So let's take a look at what the sequential model actually tells us. So, in the Sequential model, what we see happen is as soon as one of these heme groups, for example, this one binds the oxygen, that heme group completely changes its shape, and that changes the shape of the entire polypeptide that contains the heme group. And that's why, when going from this structure to this structure, this entire square becomes a circle, because it changes its conformation. And not only that, but the nearby polypeptide chains will also change their confirmation slightly.