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The Bohr Effect
What is it?
The Bohr effect is a physiological phenomenon that Christian Bohr (a physiologist) first came up with in 1904.
Bohr stated that hemoglobin's oxygen binding affinity is inversely related to the concentration of carbon dioxide in the blood.
This means that an increased blood CO2 concentration will cause the haemoglobin molecules to release or dissociate with their oxygen molecules and that a decrease in CO2 concentration will result in the haem groups associating easier with the oxygen molecules.
Haemoglobin is a complex protein with 4 subunits.
- Each subunit within the haemoglobin contains a polypeptide chain and a non-protein haem group.
- Each haem group contains a single iron ion in the form of Fe2+.
- Each iron ion can attract and hold one oxygen molecule.
- The haem group is said to have an affinity for oxygen and each haemoglobin molecule can carry up to 4 oxygen molecules.
- Oxygen is transported to the erythrocytes (red blood cells) which contain haemoglobin.
- When the haemoglobin takes up oxygen is becomes oxyhaemoglobin.
The Oxyhaemoglobin Dissociation Curve
- The ability of haemoglobin to take up and release oxygen (dissociation) relies on how much oxygen there is in the surrounding tissues.
- The amount of oxygen is measured in partial pressure (kPa) which means how much relative pressure the oxygen contributes to a mixture of gases.
- With normal liquid the amount of oxygen that is absorbed is directly proportional to the partial pressure of the oxygen in the air surrounding the liquid.
- This means that on a graph of percentage saturation against partial pressure the line would be straight.
- However at low partial pressure, haemoglobin does not readily take up oxygen molecules because the haem groups that the oxygen are attracted to are in the center of the molecule.
- This makes it hard for the oxygen molecules to reach and associate with the haem groups.
- As the partial pressure increases the diffusion gradient between the haemoglobin molecule and the oxygen molecule rises and eventually an oxygen molecule will associate with a haem group.
- When this happens something known as a conformational change occurs which is when the shape of the haemoglobin molecule changes slightly due to the oxygen associating with the haem group.
- This conformational change allows the oxygen molecules to diffuse into the haemoglobin molecule and associate with the haem groups more easily.
- However, once the haemoglobin molecule contains 3 oxygen molecules it is very hard for the fourth one to associate which means that it's difficult to achieve 100% saturation if the haemoglobin even if the partial pressure is very high.
- Mammalian haemoglobin can achieve 100% saturation because the lungs are adapted so that the partial pressure/oxygen tension in the lungs is sufficient to allow this.
- The way that haemoglobin takes up oxygen creates an S shaped curve on a graph of percentage saturation against partial pressure and this is called the oxyhaemoglobin disassociation curve.
- The haemoglobin of a mammalian fetus has a higher affinity for oxygen than that of an adults.
- This is because the fetal haemoglobin must be able to take in oxygen from an environment that makes the adult haemoglobin release oxygen.
- The fetal haemoglobin absorbs oxygen from the mother's blood which reduces the partial pressure of oxygen within the blood which makes the haemoglobin release more oxygen.
- This makes the oxyhaemoglobin disassociation curve of a fetus to the left of the curve for adult haemoglobin.
The Bohr Shift
The Bohr shift refers to the change in shape of the oxyhaemoglobin dissociation curve when carbon dioxide is present.
Carbon dioxide is released from respiring tissues and when blood enteres these respiring tissues the partial pressure of the respiring tissues is lower than that of the lungs because the oxygen has been used in respiration.
The oxyhaemoglobin then begins to dissociate and the oxygen is released into the tissue (which is beneficial because respiring tissue needs oxygen!).
When tissues (such as contracting muscles) are respiring more there will be more carbon dioxide present and as a result of this the oxyhaemoglobin will dissociate and release more oxygen.
This makes the oxyhaemoglobin dissociation curve shift to the right, this is called the Bohr Shift.