23% of candidates passed this question.

Both are globular proteins that bind and deliver O2. Due to myoglobin containing a single globin chain its dissociation curve is hyperbolic in shape. Haemoglobin contains 4 globin chains and is a quaternary structure which exhibits cooperatively resulting in a sigmoid shaped dissociation curve. The differing dissociation curves mean that when the PO2 is high, as in the lungs, both myoglobin and haemoglobin are saturated with oxygen. However, at the lower levels of PO2 in the tissues, haemoglobin cannot bind oxygen as well as myoglobin. Myoglobin can bind the O2 released by haemoglobin, which it stores to meet the demands of muscle contraction. This means haemoglobin (with its higher p50) can offload O2 to myoglobin. Comments on the synthesis and degradation gained additional marks but were a common omission.

The physiological relevance was poorly explained. The similarities and differences mean that haemoglobin is the primary means of O2 transport from the lungs to the tissues and myoglobin is the primary O2 carrying pigment of skeletal muscle and acts as local O2 reserve for times of intense muscle activity.