Haemoglobin
Biology: AQA A Level, topic 3.4.1
Haemoglobin
Haemoglobin is a globular protein found in red blood cells. Haemoglobin carries oxygen from the lungs around the body.
Quaternary stucture
- Haemoglobin is a protein made from four different polypeptide chains.
- The four chains give haemoglobin a quaternary structure.
Haem groups
- Each polypeptide chain in a haemoglobin molecule has a haem group.
- A haem group is a prosthetic group that is attached to the protein.
- The haem groups contain an iron ion.
- The iron ion is what makes haemoglobin red.
Affinity
- Haemoglobin has a high affinity (attraction) for oxygen.
- When red blood cells reach the lungs, oxygen diffuses into the red blood cells and binds to haemoglobin.
- Four molecules of oxygen bind to one molecule of haemoglobin.
- When oxygen binds to haemoglobin, oxyhaemoglobin is formed.
Dissociation
- When the red blood cells reach the tissues in the body (e.g. muscle cells), oxygen is released from the oxyhaemoglobin in a process called dissociation.
Partial Pressure
Haemoglobin has the ability to bind to oxygen and dissociate from oxygen. This is controlled by the partial pressure (concentration) of oxygen in the surrounding cells.
Partial pressure
- Oxygen partial pressure (pO2) is the concentration of oxygen in the cells.
- Carbon dioxide partial pressure (pCO2) is the concentration of carbon dioxide in the cells.
- pO2 is important in determining whether oxygen binds to haemoglobin.
Affinity
- pO2 determines the affinity of haemoglobin for oxygen.
- If pO2 is high, haemoglobin has a high affinity for oxygen and oxygen binds to haemoglobin.
- If pO2 is low, haemoglobin has a low affinity for oxygen and oxygen dissociates from haemoglobin.
Transport of oxygen
- The effect of pO2 on the affinity of haemoglobin allows oxygen to be transported from the lungs (where there is lots of oxygen) to the respiring tissues (where oxygen is limited).
- This allows oxygen to be transported to the cells where oxygen is needed most.
Types of Haemoglobin
Animals in different environments have evolved different forms of haemoglobin. The haemoglobins are able to transport oxygen in slightly different ways.
High altitude
- Animals living at high altitudes have haemoglobin which has a high affinity for oxygen.
- This is an advantage because the air at higher altitudes has a much lower partial pressure than at sea level.
- Birds also have haemoglobin with a high affinity for oxygen.
High metabolic rate
- Animals with high metabolic rates have haemoglobin that dissociates with oxygen very easily.
- This allows oxygen to be quickly and easily supplied to the cells for use in respiration.
Test yourself on Haemoglobin
What is the name given to haemoglobin that has bound to oxygen?
Which of the following statements about haemoglobin are true?
What is the effect of a high pO2 on oxygen?
Which of the following statements about partial pressure are true?
Which of the following statements about types of haemoglobin are true?