The function of red blood cells is to transport oxygen to tissues. This is accomplished by the intracellular protein
. The quaternary hemoglobin protein is composed of two
subunits. Each of the subunits contains a central iron containing protein called a
protein. It is the iron atom in the heme protein that binds to molecular oxygen. As a result of the four iron-containing heme groups, each molecule of hemoglobin can carry four atoms of oxygen.