Haemoglobin

• Haemoglobin is a water soluble globular protein which consists of twoalpha and two beta polypeptide chains each containing a haem group
• Haemoglobin carries oxygen in the blood as oxygen can bind to the haem group, oxygen can also be released when needed.
• Each molcule of haemoglobin can carry four oxygen molecules.
• The affinity of oxygen for haemoglobin varies depending on the partial pressure of oxygen which is a measure of oxygen concentration. The greater the concentration of dissolved

  oxygen in cells the greater the partial pressure. Therefore, as partial pressure increases, the affinity of haemoglobin for oxygen increases, that is oxygen binds to haemoglobin tightly. This occurs in the lungs in the process known as loading.
• During respiration, oxygen is used up therefore the partial pressure decreases, thus decreasing the affinity of oxygen for haemoglobin. As a result of that, oxygen is released in respiring tissues where it is needed. After the unloading process, the haemoglobin returns to the lungs where it binds to oxygen again.
• Dissociation curves illustrate the change in haemoglobin saturation as partial pressure changes. The saturation of haemoglobin is affected by its affinity for oxygen, therefore in the case where partial pressure is high, haemoglobin has high affinity for oxygen and is therefore highly saturated, and vice versa.
• Saturation can also have an effect on affinity, as after binding to the first oxygen molecule, the affinity of haemoglobin for oxygen increases due to a change in shape, thus making it easier for the other oxygen molecules to bind.
• Fetal haemoglobin has a different affinity for oxygen compared to adult haemoglobin, as in needs to be better at absorbing oxygen because by the time oxygen reaches the placenta, the oxygen saturation of the blood has decreased. Therefore, fetal haemoglobin must have a higher affinity for oxygen in order for the foetus to survive at low partial pressure.
• The affinity of haemoglobin for oxygen is also affected by the partial pressure of carbon dioxide. Carbon dioxide is released by respiring cells which require oxygen for the process to

  occur. Therefore, in the presence of carbon dioxide, the affinity of haemoglobin for oxygen decreases, thus causingoxygen to be released. This is known as the Bohr effect