Variation in Haemoglobin

Haemoglobin is a protein which carries oxygen round the body. Different species have different types of heamoglobin depending on their environment.

• Haemoglobin is found in the cytoplasm of red blood cells
• It is a large protein with a quaternary structure - made up of four polypeptide chains joined together so can carry a max of four O2 molecules at one time
• Each polypeptide contains a haem group which contains iron - gives haemoglobin red colour
• Oxygen associates with haemoglobin in the lungs to form oxyheamoglobin. When oxyhaemolgobin reaches a site of low oxygen concentration in body cells, O2 dissociates and becomes haemoglobin again

In order to be efficient - easily combine and separate at particular sites - it can change shape

• Where CO2 is low and O2 high, there is a high affinity of haemoglobin for oxygen, so oxygen loads
• Where CO2 is high and O2 low, there is a low affinity of haemoglobin for oxygen, so oxygen unloads

Affintiy for oxygen - tendency to combine with oxygen

Dissociation Curve - Shows how saturated haemoglobin is with oxygen at any given partial pressure

Partial Pressure - Measure of concentration of a gas. pO2 is partial pressure of oxygen. pCO2 is partial pressure of carbon dioxide

• 100% saturation means all 4 haemoglobin molecules are carrying an oxygen. 0% saturation means none of the haemoglobin molecules are carrying oxygen
• The graph forms an S shape because:
  • At very low pO2, the polypeptide chains are closely linked so difficult to absorb oxygen - less steep curve at bottom
  • Once first oxygen molecule loads, the shape changes and the other 3 load easily - steep middle section of curve
  • As haemoglobin becomes saturated it becomes harder for more O2 to join
• The partial pressure of carbon dioxide also affects the dissociation curve to allow O2 to readily unload at areas of increased respiration
• At sites where there is a high pCO2 (respiring cells) the curve moves to the right meaning more oxygen is being released. At sites of low pCO2 (lungs) the curve moves to the left meaning more oxygen is loaded

Different organisms have different types of haemoglobin with different oxygen transporting capacities

Organisms in an environment with a low concentration of oxygen (eg. mountain goat) have haemoglobin with a higher affinity for oxygen. Their dissociation curve moves to the left of ours so can more readily load oxygen

Organisms in an enivronment with a high concentration of oxygen and are very active have a high demand for oxygen so have a lower affinity for oxygen. Their dissociation curve moves to the right of ours so can readily unload oxygen

Another factor which affects the affinity of haemoglobin for oxygen is:

• Size - A samller mammal loses heat at a faster rate so has a faster motabolism. This requires an increased amount of oxygen so their dissociation curve moves to the right so they unload oxygen more readily
• Activity - Swimming or flying require a lot of respiration in muscles so they require more oxygen. Their dissociation curve moves to the right so they unload oxygen more readily at respiring tissues