Haemoglobin

• Mammalian circulatory system


  Transports oxygen
• Red blood cells (RBC)

  • Carry the vast majority of oxygen transported by the blood
  • Also called erythrocytes
• Red blood cells (RBC)

  • Have 3 adaptations for transporting oxygen
• Adaptations of RBC for transporting oxygen

  1. Biconcave structure gives large surface area to volume ratio, allowing rapid diffusion of oxygen

  2. Each RBC contains around 300 million molecules of the oxygen carrying protein haemoglobin

  3. Absence of nucleus means more volume available for haemoglobin
• Haemoglobin


  Oxygen carrying protein in RBC
• Haemoglobin


  • Has four polypeptide chains
  • Each polypeptide chain is bound to a prosthetic group called haem
  • Haem contains the ion iron Fe2+
• Conjugated protein


  Haemoglobin is an example, containing the haem prosthetic group
• Haemoglobin


  • Each haem group can combine with one molecule of oxygen
  • One haemoglobin molecule can combine with four oxygen molecules
• Oxyhaemoglobin


  Haemoglobin when bound to oxygen
• Oxyhaemoglobin


  Reaction with oxygen is reversible, so oxygen can be released when required
• Oxygen dissociation curve


  Measures the amount of oxygen that combines with haemoglobin
• Oxygen dissociation curve


  • Y-axis shows percentage saturation of haemoglobin with oxygen
  • X-axis shows partial pressure of oxygen
  • Has an S-shaped (sigmoid) curve
• Oxygen dissociation curve


  1. At low partial pressures, haemoglobin has low affinity for oxygen

  2. As one oxygen molecule binds, affinity for further oxygen increases (positive cooperativity)

  3. At high partial pressures, most haem groups are already filled so chance of oxygen colliding with remaining groups is low
• In alveoli


  Partial pressure of oxygen is high, haemoglobin is around 97% saturated
• In body tissues


  Partial pressure of oxygen decreases, haemoglobin unloads oxygen
• In active tissues


  Partial pressure of oxygen is very low, final oxygen molecule unloads from haemoglobin
• Higher P50 values indicate lower oxygen affinity, while lower P50 values indicate higher oxygen affinity.