Red blood cells contain hemoglobin, which is responsible for both oxygen and carbon dioxide transport in the blood. Hemoglobin is a complex protein made of four polypeptide chains, each of which has the unique ability to bind oxygen. Amazingly enough, there are about 280 million molecules of hemoglobin in each red blood cell with the potential ability to carry a billion molecules of oxygen. Each polypeptide chain has a single molecule of heme, which has an iron ion at its center. It is the iron ion that interacts with oxygen.