Inside our body, we have two different proteins, myoglobin and hemoglobin, that carry out the function of bringing oxygen to the cells of our body, where that oxygen can then be used in a process called cellular respiration, aerobic cellular respiration, to produce ATP molecules. Now, previously, we said that these two proteins contain heme groups, and it's the heme group group that is responsible for actually binding and holding on to the oxygen. Now, in myoglobin, we have a single polypeptide chain, and so we have a single heme group. But in hemoglobin, because we have four different polypeptide chains, we have four different heme groups. Now, one heme group can bind one oxygen. And what that means is a single hemoglobin can bind four times as many oxygen molecules as myoglobin can.