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

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

> Haemoglobin is a protein found in red blood cells that reversibly binds oxygen. The relationship between the partial pressure of oxygen (pO₂) and the percentage saturation of haemoglobin is represented by a characteristic S-shaped (sigmoidal) curve.

## Mark scheme (4 marks)

1. Haemoglobin becomes almost fully saturated with oxygen at the high pO₂ found in the lungs/alveolar capillaries, enabling maximum oxygen loading.
2. In respiring tissues, the pO₂ is low, so haemoglobin readily releases / dissociates from oxygen, supplying it to cells for aerobic respiration.
3. The sigmoidal/S-shaped nature of the curve means that a small drop in pO₂ in the tissues causes a large release of oxygen (cooperative binding/allostery), making unloading efficient.
4. A shift of the curve to the right (Bohr effect) in tissues with high CO₂ / low pH further promotes oxygen unloading where it is most needed.

## Key terms

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

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-haemoglobin-s-oxygen-dissociation-curve-354cad0a) · Published by Druglandscape Ltd.