In fact, what we also see in nature is when one of the heme groups binds that oxygen, the adjacent heme groups are also affected. They increase their affinity for oxygen because they change their shape as well. And this is something that we don't see according to this model. 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.