A level-readyAll subjects · Biology · WBI11 (Unit 1)
Haemoglobin and the oxygen dissociation curve
Haemoglobin loads oxygen in the lungs and unloads it in respiring tissues. The shape of its dissociation curve explains how.
Haemoglobin
Haemoglobin is a globular protein with four polypeptide chains (two α and two β), each holding a haem group containing an iron ion (Fe2+). Each haem binds one oxygen molecule, so one haemoglobin carries up to four O2 as oxyhaemoglobin.
Carbon dioxide is carried mostly as hydrogencarbonate ions in plasma, some bound to haemoglobin (carbaminohaemoglobin), and a little dissolved.
The S-shaped curve
The oxygen dissociation curve plots % saturation of haemoglobin against partial pressure of oxygen (pO2). It is S-shaped (sigmoid):
- At high pO2 (lungs) haemoglobin is nearly saturated: it loads oxygen.
- At low pO2 (respiring tissues) saturation falls steeply: it unloads oxygen.
The curve is steep in the middle because once the first O2 binds, the shape of the molecule changes and it becomes easier for the next ones to bind (cooperative binding).
Bohr effect and fetal haemoglobin
- Bohr effect: higher pCO2 lowers the pH and reduces haemoglobin's affinity for oxygen, so the curve shifts to the right. In active tissues, more oxygen is released exactly where it is needed.
- Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin, so its curve lies to the left. At the low pO2 of the placenta, fetal haemoglobin can take oxygen from the mother's haemoglobin.
Worked example
Explain why the Bohr effect helps an exercising muscle.
- Muscle respires faster and releases more CO2.
- Higher pCO2 lowers haemoglobin's affinity for O2 (curve shifts right).
- At the same pO2, haemoglobin is less saturated, so more O2 is unloaded.
Answer: More oxygen is released to the muscle that is respiring fastest.
Check yourself
- What is the maximum number of oxygen molecules one haemoglobin molecule can carry?
Show answer
4. Four haem groups, each binding one O2.
- How many oxygen molecules can 250 haemoglobin molecules carry when fully saturated?
Show answer
1000. 250 x 4 = 1000.
- An increase in carbon dioxide concentration shifts the oxygen dissociation curve
- to the right, so more oxygen is unloaded
- to the left, so more oxygen is unloaded
- to the right, so less oxygen is unloaded
- upwards, with no change in unloading
Show answer
to the right, so more oxygen is unloaded. The Bohr effect: CO2 lowers affinity, shifting the curve right.
- Why does fetal haemoglobin have a higher affinity for oxygen than adult haemoglobin?
- So it releases oxygen more easily in the fetus's lungs
- Because the fetus produces more CO2
- So it can carry more CO2
- So it can take oxygen from the mother's blood at the placenta
Show answer
So it can take oxygen from the mother's blood at the placenta. Higher affinity lets it load oxygen at the low pO2 found at the placenta.
- Why is the dissociation curve S-shaped?
- Haemoglobin has only one haem group
- Binding of the first O2 changes haemoglobin's shape, making further binding easier
- Oxygen dissolves directly in plasma
- Iron is released at low pO2
Show answer
Binding of the first O2 changes haemoglobin's shape, making further binding easier. Cooperative binding makes the curve steep in the middle.
- In respiring tissue, pO2 is low. What does haemoglobin do?
- Loads oxygen
- Binds more tightly to oxygen
- Releases its iron
- Unloads oxygen
Show answer
Unloads oxygen. At low pO2 saturation drops, so oxygen is released.
- How is most carbon dioxide carried in the blood?
- Dissolved as CO2 gas in plasma
- As hydrogencarbonate ions in the plasma
- Bound to the iron in haem
- Inside platelets
Show answer
As hydrogencarbonate ions in the plasma. Most CO2 is converted to hydrogencarbonate ions.
Sources
Last updated 7 November 2026. Spotted a mistake? Tell us.