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Haem group. Computer graphic of the molecular structure of a haem group, part of the blood protein haemoglobin which carries oxygen and carbon dioxide around the body. The atoms, represented as spheres, are colour-coded: iron (red), nitrogen (yellow), carbon (grey) and hydrogen (dark blue). Haemoglobin consists of four chains of the protein globin, each of which contains a haem group. Haem is an iron-containing porphyrin molecule. The flat group of four rings around the iron atom give oxygen and carbon dioxide easy access to the iron, allowing them to bind to it and be released without undergoing any chemical change. (KEYSTONE/SCIENCE PHOTO LIBRARY/PASIEKA)