The Oxygen Dissociation Curve (AQA AS Biology): Revision Note
Exam code: 7401
The oxyhaemoglobin dissociation curve
The percentage saturation of haemoglobin with oxygen at different oxygen concentrations is shown on a graph known as the oxyhaemoglobin dissociation curve
Oxygen concentration is given as the partial pressure of oxygen (pO2)
Haemoglobin is saturated when all of its oxygen binding sites are taken up with oxygen, i.e. when it contains four oxygen molecules
Cooperative binding
The oxyhaemoglobin dissociation curve has a distinctive curved shape due to the cooperative nature of oxygen binding
The shape of each part of the curve can be explained as follows:
shallow curve at the bottom left
It is difficult for the first oxygen molecule to bind to haemoglobin, so binding of the first oxygen molecule is slow
steep curve in the central region
After the first oxygen molecule binds to haemoglobin the haemoglobin protein changes conformation, making it easier for the next oxygen molecules to bind; this speeds up binding of the remaining oxygen molecules
This shape change of haemoglobin, leading to easier oxygen binding, is known as cooperative binding
levelling off in the top right
As the haemoglobin molecule approaches saturation it takes longer for the fourth oxygen molecule to bind due to the shortage of remaining binding sites
pO2 and oxygen affinity
The oxyhaemoglobin dissociation curve shows changes in haemoglobin's affinity for oxygen at different partial pressures
At high pO2 haemoglobin has a high affinity for oxygen
At low pO2 haemoglobin has a low affinity for oxygen
The changes in haemoglobin's affinity for oxygen as pO2 changes are biologically important because they influence the ease with which oxygen binds and is released in different parts of the body, e.g.:
pO2 in the lungs is high, so haemoglobin can bind easily
PO2 in the muscles is relatively low due to high rates of respiration, and oxygen dissociates easily from haemoglobin
pO2 | % saturation of haemoglobin | Affinity of haemoglobin for oxygen | Oxygen binding / dissociation | Biological implication |
|---|---|---|---|---|
Low | Low | Low | Oxygen binds slowly to haemoglobin and dissociates easily | Haemoglobin cannot pick up oxygen effectively in oxygen-depleted tissues |
Medium | Increasing | Increasing | A small increase in pO2 causes a large increase in haemoglobin saturation A small decrease in pO2 causes a large decrease in percentage saturation of haemoglobin | Easy release of oxygen to the cells in respiring tissues |
High | High | High | Oxygen binds easily and dissociates slowly | Haemoglobin can pick up oxygen and become saturated as blood passes through the lungs |
Examiner Tips and Tricks
Remember that it is possible to read the oxyhaemoglobin dissociation curve in terms of both binding and dissociation of oxygen; a steep curve indicates both:
increased binding of oxygen as pO2 increases
increased dissociation of oxygen as pO2 decreases
Unlock more revision notes. It’s free!
By signing up you agree to our Terms and Privacy Policy.
Already have an account? Log in
Did this page help you?
