Computer graphic representation of the haemoglobin molecule, the oxygen-carrying substance of human blood. The roughly tubular features (in orange and blue) are polypeptide (protein) chains, of which there are four in two identical pairs. The protein component combines with four discs (in red) which contain the iron pigment, haem. Red blood cells (erythrocytes) are literally packed with haemoglobin, containing only very small amounts of other substances.
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