Haemoglobin subunit. Computer artwork of the molecular structure of a subunit of haemoglobin. Red blood cells (erythrocytes) are seen in the background. Haemoglobin is the oxygen-carrying molecule found in red blood cells. The folded coil represents the globulin polypeptide chain. The width of the chain indicates flexibility; the broader the more flexible. The chain carries a haem group, which contains the iron atom (yellow) that binds oxygen. Each haemoglobin molecule consists of four subunits, two alpha chains and two beta chains. Haemoglobin carries oxygen from the lungs to the tissues and also carbon dioxide from the tissues to the lungs for exhalation.