Caption

Molecular computer model of the structure of haemoglobin, the oxygen-carrying substance of human blood. Blue ribbons represent the polypeptide (protein) chains; this protein component combines with four haem groups (in pink), iron-containing porphyrin substances. Haemoglobin has the ability to combine reversibly with oxygen, taking it up in the lungs and releasing it through the tissues. Red blood cells (erythrocytes) are literally packed with haemoglobin, containing only very small amounts of other substances.