# Explain how the ultrastructure of the rough endoplasmic reticulum (rER) is related to its function in protein secretion.

> IB DP Biology Higher Level (2023 syllabus) — A2.2 Cell structure · Explain · 4 marks

## Mark scheme (4 marks)

1. The rER is continuous with the outer nuclear envelope / forms an interconnected membrane network, allowing newly synthesised proteins to enter the lumen directly from ribosomes.
2. Ribosomes are bound to the cytosolic face of the rER membrane, so polypeptides are co-translationally threaded into the lumen as they are synthesised.
3. The enclosed lumen isolates/sequesters the protein from the cytoplasm, allowing folding and glycosylation / post-translational modification within the lumen.
4. The rER membrane can bud off transport vesicles that carry proteins to the Golgi apparatus for further processing and eventual secretion.

## Key terms

- [rough endoplasmic reticulum (rER)](https://www.gradenine.co.uk/glossary/rough-endoplasmic-reticulum-rer)

## 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-ultrastructure-of-the-e0130242) · Published by Druglandscape Ltd.