Computer graphics space-filling representation of the haem group of the protein haemoglobin, the oxygen- and carbon dioxide-transporting pigment in blood. The site of interraction between oxygen or carbon dioxide and haemoglobin is an atom of iron (the central purple sphere), held in the centre of a perfectly flat porphyrin ring. The flat shape of the porphyrin allows oxygen and carbon dioxide access to the central iron atom to bind, be carried and ultimately be released without undergoing any chemical change. Image modelled using Chemical Design's Chem-X software.