But according to this diagram here, if three of these groups are filled with oxygen, our molecule still exists in some intermediate state. It will not exist in the r state. So that is the limitation of the sequential model. And notice that this is compensated by the concerted model. And that's exactly why we have to use both of these models to basically correctly describe the mechanism by which hemoglobin actually binds oxygen. So, in conclusion, neither model can describe the cooperative nature of hemoglobin alone.