Haemoglobin and Bohr effect

• Haemoglobin


  • Has four polypeptide chains
  • Each polypeptide chain contains a haem group
  • Each of the four haem groups combined to one oxygen molecule
  • 1 molecule of haemoglobin combined to a maximum of four oxygen molecules
• Oxygen dissociation curve


  Plot of percentage saturation of haemoglobin with oxygen against the partial pressure of oxygen
• Affinity of haemoglobin for oxygen

  How strongly haemoglobin binds to oxygen, depends on the partial pressure of oxygen
• In the alveoli, the partial pressure of oxygen is high
• Under high oxygen partial pressure in alveoli, haemoglobin has a high affinity for oxygen and has an oxygen saturation of around 97%
• Almost all of the haemoglobin molecules have loaded 4 oxygen molecules under high oxygen partial pressure
• As red blood cells move from lungs to tissues


  1. Partial pressure of oxygen decreases

  2. Tissue cells take up oxygen for aerobic respiration
• At a certain partial pressure, an average of 1 oxygen molecule now unloads from each haemoglobin molecule
• Unloading of oxygen


  • Changes the quaternary structure of haemoglobin
  • Reduces the affinity of the haem groups for oxygen
• In very active tissue, the partial pressure of oxygen falls even further because the haemoglobin has reduced affinity for oxygen, making it easier for more oxygen molecules to unload
• Bohr effect


  • Increased carbon dioxide partial pressure shifts the oxygen dissociation curve to the right
  • Reduces the oxygen affinity of haemoglobin
• Low carbon dioxide partial pressure


  Haemoglobin 75% saturated at 7 kPa oxygen partial pressure
• High carbon dioxide partial pressure

  Haemoglobin only 25% saturated at 7 kPa oxygen partial pressure
• In the lungs, haemoglobin has a high affinity for oxygen due to low carbon dioxide partial pressure so has a high level of oxygen saturation
• In active tissues undergoing aerobic respiration with high carbon dioxide partial pressure, haemoglobin has lower oxygen affinity so more likely to unload oxygen
• Carbon dioxide forms carbonic acid in blood

  1. Carbonic acid releases hydrogen ion H+

  2. Carbonic acid combines with haemoglobin

  3. Changes haemoglobin quaternary structure

  4. Reduces haemoglobin's affinity for oxygen

  5. Causes haemoglobin to unload oxygen more easily