Haemoglobin and Oxygen Transport

Haemoglobin

• Haemoglobin has four subunits, each with a haem group containing an iron ion to bind to oxygen.
• The four subunits have different affinities for oxygen at different partial pressures of oxygen.
• Haemoglobin is globular, with hydrophilic R groups on outside, to be soluble in cytoplasm of the red blood cell. Never write haemoglobin dissolves in the blood.

1. In the lungs the partial pressure of oxygen is high, so haemoglobin loads oxygen.
2. At the respiring tissues the partial pressure of oxygen is lower, so haemoglobin unloads oxygen.

The dissociation curve

Percentage saturation remains fairly constant at high partial pressures and drops sharply at low partial pressures. Never describe the curve as the rate of saturation of haemoglobin.

• More carbon dioxide in respiring tissue lowers the affinity of haemoglobin for oxygen, so more oxygen is released: the Bohr shift. Carbon dioxide is able to bind to the amino groups of the polypeptide, giving a structure that has higher affinity for carbon dioxide to cause the oxygen to dissociate. Never write carbon dioxide binding to haem groups.
• Fetal haemoglobin has higher affinity than adult haemoglobin at the same partial pressure of oxygen, so that oxygen will leave the adult haemoglobin and bind to fetal haemoglobin. Write fetus. Never write baby haemoglobin.