Explain how the oxygen dissociation curve of haemoglobin enables efficient loading of oxygen in the lungs and unloading of oxygen in respiring tissues.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Haemoglobin has a very high affinity for O₂ at the high PO₂ in the lungs, so it becomes almost fully saturated and loads oxygen readily.
In respiring tissues the PO₂ is low, so haemoglobin releases O₂ readily.
The sigmoidal (S‑shaped) curve means that a small drop in PO₂ in the tissues causes a large release of O₂ – this is due to cooperative binding.
The Bohr effect shifts the curve to the right in the presence of high CO₂ / low pH, further promoting O₂ unloading where it is most needed.
In respiring tissues the PO₂ is low, so haemoglobin releases O₂ readily.
The sigmoidal (S‑shaped) curve means that a small drop in PO₂ in the tissues causes a large release of O₂ – this is due to cooperative binding.
The Bohr effect shifts the curve to the right in the presence of high CO₂ / low pH, further promoting O₂ unloading where it is most needed.
Examiner tips
- Use the term ‘high affinity’ for lung loading and ‘low affinity’ for tissue unloading. Mention the sigmoidal shape and cooperative binding. Include the Bohr effect as a right‑shift. Keep each point concise and use the exact wording from the mark scheme.
Common mistakes
- Confusing the direction of the Bohr shift (left vs right). Forgetting to link the sigmoidal shape to cooperative binding. Using vague terms like ‘good’ or ‘efficient’ without specific mechanisms.
Mark scheme (4 marks)
- Haemoglobin has a high affinity for oxygen at the high partial pressure of oxygen found in the lungs, so haemoglobin becomes almost fully saturated / loads oxygen readily in the lungs.
- In respiring tissues the partial pressure of oxygen is low, so haemoglobin releases / unloads oxygen readily.
- The sigmoidal (S-shaped) shape of the curve means a small drop in partial pressure of oxygen in the tissues causes a large release of oxygen (cooperative binding / cooperativity).
- The Bohr effect / shift means that the higher CO2 / lower pH in respiring tissues shifts the dissociation curve to the right, further promoting oxygen unloading where it is most needed.
Key terms in this question
oxygen dissociation curve · haemoglobin
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Transport questions
- Explain how the structure of a capillary wall facilitates the exchange of substa…
- Explain how the structure of a semilunar valve prevents the backflow of blood in…
- Explain how the structure of an artery is adapted to withstand and regulate the …
- Explain how the structural features of the atrioventricular (AV) valves prevent …
- Explain how the cardiac cycle produces the pressure changes that drive blood thr…
- Explain how the Bohr shift enables active tissues to obtain more oxygen from hae…