# Explain how the structural organisation of the rough endoplasmic reticulum and the Golgi apparatus enables post-translational modification of proteins to occur in a regulated, sequential manner.

> IB DP Biology Higher Level (2023 syllabus) — B2.2 Organelles and compartmentalisation · Explain · 4 marks

## Mark scheme (4 marks)

1. The rough ER is studded with ribosomes on its cytosolic face, so newly synthesised polypeptides are co-translationally threaded into the lumen, isolating them from cytosolic enzymes.
2. Initial modifications (e.g. addition of N-linked oligosaccharide chains / disulfide bond formation / correct folding assisted by chaperones) occur within the ER lumen, providing a distinct chemical environment for these early steps.
3. Vesicles bud from the ER and fuse with the cis face of the Golgi, delivering proteins to successive cisternae (cis → medial → trans), each containing different sets of modifying enzymes, so modifications are applied in a fixed sequence.
4. Membrane-bound compartmentalisation throughout prevents modified proteins from re-mixing with unmodified ones or with cytosolic contents, ensuring fidelity and correct targeting of the finished protein (e.g. via sorting signals recognised at the trans-Golgi network).

## Key terms

- [rough endoplasmic reticulum](https://www.gradenine.co.uk/glossary/rough-endoplasmic-reticulum)
- [Golgi apparatus](https://www.gradenine.co.uk/glossary/golgi-apparatus)
- [post-translational modification](https://www.gradenine.co.uk/glossary/post-translational-modification)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-structural-organisation-of-36b793db) · Published by Druglandscape Ltd.