Haemoglobin and haem, molecular models. Haemoglobin (left) is the molecule that transports oxygen around the body in red blood cells, and carries waste carbon dioxide back to the lungs. It consists of four globin proteins (amino acid chains; blue stick model). Each globin protein is wrapped around a haem group (red and purple), shown in the inset at upper right. Each haem group consists of a porphyrin heterocyclic organic ring (purple) and an iron ion (yellow, Fe2+). It is this iron centre that reversibly bonds to oxygen (O2) and carbon dioxide (CO2) in the blood. The double electric charge on the iron ion is shown here in green.