# Explain how haemoglobin's quaternary structure enables it to transport oxygen efficiently in the blood.

> IB DP Biology Standard Level (2023 syllabus) — B1.2 Proteins · Explain · 4 marks

## Mark scheme (4 marks)

1. Haemoglobin is made up of four polypeptide subunits (quaternary structure) held together by interactions between the subunits.
2. Each subunit contains a haem (prosthetic) group with a central iron ion that binds one oxygen molecule.
3. Having four subunits allows haemoglobin to carry up to four oxygen molecules simultaneously, increasing the oxygen-carrying capacity of the blood.
4. Cooperative binding occurs because binding of oxygen to one subunit changes the shape of the other subunits, increasing their affinity for oxygen, so haemoglobin loads oxygen efficiently at the lungs and unloads it in respiring tissues.

## Key terms

- [quaternary structure](https://www.gradenine.co.uk/glossary/quaternary-structure)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-haemoglobin-s-quaternary-structure-enables-ce6bf5f5) · Published by Druglandscape Ltd.