So these red sections basically are the heme groups. They're a percent of groups that are found within each subunit and each one of these heme groups, so we have 1234. Each one of these contains a single iron atom. And this iron atom can bind oxygen via an oxidation reduction process. So the iron is capable of changing its oxidation state via a redux reaction and that can bind our oxygen. Now, one heme group can bind a single diatomic oxygen molecule.