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Oxygen is carried around the body as oxyhaemoglobin

  • Red blood cells contain haemoglobin; a large protein with a quaternary structure
  • each of the 4 polypeptide chains in haemoblobin has a haem group containing iron to give haemoglobin its red colour.
  • Haemoglobin has a high affinity for oxygen - each molecule can carry 4 oxygen molecules
  • only in the lungs does the oxygen join to the iron in haemoglobin to form oxyhaemoglobin - this is a reversible reaction

Haemoglobin saturation depends on the partial pressure of oxygen

  • the partial pressure of oxygen just means a measure of oxygen concentration - greater partial pressure means greater concentration
  • partial pressure for carbon dioxide is a measure of CO2 concentration
  • Haemoglobins affinity for oxygen also depends on the partial pressure of oxygen
  • Oxygen loads onto haemoglobin when there is high PO2, and unloads when low PO2

The Haemoglobin journey: 

1) oxygen enters blood capilaries at the alveoli in the lungs. Alveoli have high PO2 so oxygen is loaded onto the Haemoglobin = Oxyhaemoglobin

2) cells respire using up oxygen, lowering the PO2. Red blood cells deliver oxyhaemoglobin to respiring tissues to unload oxygen.

3) Haemoglobin returns to the lungs to pick up more oxygen

Dissociation curves show how affinity for oxygen vaires


  • 0% means no haemoglobin moleclues are carrying oxygen.


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