Haemoglobin

• Red blood cells are flexible

  • can pass through narrow capillaries so they touch the sides
  • can be folded
  • reduces the length of diffusion pathway of oxygen
  • cell forms a biconcave disc shape
• Biconcave disc shape increases the surface area: volume ratio so increases the area which oxygen can diffuse.

  Haemoglobin molecules closer to the cell membrane so reduce the length of the diffusion pathway
• Red blood cells have no nucleus or other organelles so only contains haemoglobin
• Haemoglobin contains two alpha and two beta polypeptide chains
• Each polypeptide chain in haemoglobin is folded into a helix
• All four polypeptides are linked to form a spherical shape, each chain associating with a haem group containing a ferrous ion
• Each iron ion can combine with a single O2 molecule
• Affinity is the attraction between molecules that results in the formation of a new molecule
• Haemoglobin needs to be able to readily load oxygen at the site of gas exchange so must have a high affinity
• Haemoglobin needs to unload its oxygen at respiring tissues so the affinity must decrease
• If the metabolic rate of an organism is high, its haemoglobin tends to have a lower affinity for oxygen so it unloads more readily
• If low oxygen is available, haemoglobin has a higher affinity for oxygen
• Associating is the process by which haemoglobin binds with oxygen
• Dissociating is the process by which haemoglobin releases its oxygen