And so these four polypeptide chains give the hemoglobin molecule quantumary structure. And that's exactly what gives that hemoglobin the ability to bind oxygen cooperatively. And we'll see what that means in the next lecture. So what hemoglobin does is it essentially continually delivers the oxygen from the lungs and to the tissues and cells of our body. And it also binds CO2 and brings the carbon dioxide back to the lungs so that the carbon dioxide can be expelled by our body. Now, in this lecture, what I'd like to focus on is how these two proteins actually are capable of binding to oxygen in the first place.