On the other hand, hemoglobin is a protein that consists of four individual polypeptide chains. We have alpha one and alpha two, which are two identical alpha chains. And we have beta one and beta two subunits, which are also identical subunits. 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.