What is haemoglobin?
1. Red blood cells that contain haemoglobin, that carries oxygen around the body.
2. Haemoglobin is a large protein made up from 4 polypeptide chains (2 beta polypeptides and 2 alpha polypeptides, with four haem groups 1 attatched to each polypeptide chain.) Quaternary structure.
3. The haem group contains iron, giving haemoglobin its red colour.
4. Has to readily ASSOCIATE with oxygen at surface where gas exchange takes place (at the lungs oxygen joins to heamoglobin in red blood cells to form oxyhaemoglobin) and readily DISSOCIATE from oxygen at the tissues requiring it.
How affinity for oxygen changes...
Haemoglobin changes its affinity for oxygen under different conditions, it changes its shape in the presence of certain substances. One of these is the partial pressure of oxygen.
When cells respire they use up oxygen so the partial pressure of oxygen decreases, so haemoglobin need to return to the lungs to get more oxygen.
Haemoglobins with a high affinity for oxygen take up oxygen more easily but release it less easily.
Haemoglobgins with a low affinity for oxygen take up oxygen less easily but release it more readily.
Dissociation graphs show how affinity for oxygen varies.
1. When partial pressure is high, haemoglobin has a high affinity for oxygen...so it has a high saturation of oxygen.
2. When partial pressure is low haemoglobin has a low affinity for oxygen...so it has a low saturation of oxygen.
3. When saturation is at 100% it means that every haemoglobin molecule is carrying the maximum of 4 molecules of oxygen.
4. When the 1st haemoglobin combines with 1st oxygen molecule the shape of the haemoglobin alters so it is easier for the next molecules to join, but then as the haemoglobin becomes more saturated it gets harder for molecules to join, explaining the 'S' shape of a dissociation curve.
The Bohr Effect
1. Haemoglobin gives up oxygen more readily at higher partial pressure of carbon dioxide.
2. When cells respire they produce carbon dioxide, which raises the partial pressure of carbon dioxide.
3. This increases the rate of oxygen unloading, so the dissociation curve shifts down, so more oxygen is being released.
The further to the left the curve, the greater the affinity of haemoglobin for oxygen.
The further to the right the curve, the lower the affinity of haemoglobin for oxygen.