And the heme group is responsible for actually binding the oxygen via an oxidation reduction reaction. So we have heme group one, hemegroup two, heme group three and heme group four. And these heme groups can bind a single oxygen molecule each. And that means because we have four subunits and each one of these carries one heme group we can bind four oxygen molecules per hemoglobin molecule. So once again, hemoglobin is a tetrimer that consists of four individual subunits. Each subunit is equipped with a heme group that is capable of binding oxygen molecules.