# Explain how hydrophobic interactions and disulfide bonds each contribute to the stability of the tertiary structure of a protein.

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

## Mark scheme (4 marks)

1. Hydrophobic R-groups cluster together in the interior of the protein, away from the aqueous environment.
2. This clustering minimises the free energy of the system / is thermodynamically favourable, thereby stabilising the folded conformation.
3. Disulfide bonds are covalent bonds formed between the sulfhydryl (-SH) groups of two cysteine residues.
4. Disulfide bonds covalently cross-link distant parts of the polypeptide chain, holding the tertiary structure in a fixed, stable conformation.

## Key terms

- [tertiary structure](https://www.gradenine.co.uk/glossary/tertiary-structure)
- [hydrophobic interactions](https://www.gradenine.co.uk/glossary/hydrophobic-interactions)
- [disulfide bond](https://www.gradenine.co.uk/glossary/disulfide-bond)

## 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-hydrophobic-interactions-and-disulfide-998cd9b9) · Published by Druglandscape Ltd.