# Explain how the oxygen dissociation curve of haemoglobin enables efficient loading of oxygen in the lungs and unloading of oxygen in respiring tissues.

> IB DP Biology Higher Level (2023 syllabus) — B3.2 Transport · Explain · 4 marks

## Mark scheme (4 marks)

1. 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.
2. In respiring tissues the partial pressure of oxygen is low, so haemoglobin releases / unloads oxygen readily.
3. 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).
4. 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

- [oxygen dissociation curve](https://www.gradenine.co.uk/glossary/oxygen-dissociation-curve)
- [haemoglobin](https://www.gradenine.co.uk/glossary/haemoglobin)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-oxygen-dissociation-curve-7813293d) · Published by Druglandscape Ltd.