And hemoglobin has quadnary structure. In fact, it is a tetromer. It consists of four individual polypeptide subunits. So we have polypeptide subunit one, subunit two, subunit three and subunit four. So we have two alpha and two beta subunits to form this tetromer molecule. Now, inside each one of these subunits we have a helper prosthetic group we call the heme group.