Haemoglobin, molecular model. This molecule transports oxygen around the body in red blood cells. It consists of four globin proteins (amino acid chains; blue stick model). Each globin protein is wrapped around a haem group (red stick model). Haemoglobin also carries waste carbon dioxide back to the lungs. Each haem area consists of a porphyrin heterocyclic organic ring and an iron ion (yellow, Fe2+). It is this iron centre that reversibly bonds to oxygen (O2) and carbon dioxide (CO2) in the blood.