And in each one of these polypeptide chains, we have this rat structure. And these red structures are the heme groups. They contain the iron and the protoporphy that are responsible for binding diatomic oxygen. And because one hein group binds 01:02 molecule, one hemoglobin molecule can bind in total a maximum of four diatomic oxygen molecules. Now, usually, instead of picturing our hemoglobin as consisting of four individual chains, we look at the hemoglobin as consisting of two individual dimers. One of these dimers is the alpha one, beta one dimer, and the other dimer is the alpha two beta two dimer.