Explain how the ultrastructure of the rough endoplasmic reticulum (rER) is related to its function in protein secretion.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The rough endoplasmic reticulum is a network of membrane‑bound cisternae that is continuous with the outer nuclear envelope, so ribosomes can bind directly to its cytosolic surface. Ribosomes translate mRNA and the nascent polypeptide is co‑translationally threaded into the lumen of the rER as it is synthesised. The lumen provides a specialised environment where the protein can fold correctly and undergo post‑translational modifications such as glycosylation, isolated from the cytoplasm. Once processed, proteins are packaged into transport vesicles that bud from the rER membrane and travel to the Golgi apparatus for further modification and eventual secretion.
Examiner tips
- Mention continuity with nuclear envelope to justify direct entry of nascent chains.
- Explain co‑translational import and lumenal modifications.
- Show that vesicle budding links rER to Golgi and secretion.
Common mistakes
- Confusing smooth ER with rough ER; forgetting ribosome attachment.
- Failing to state that modifications occur in the lumen, not cytoplasm.
- Over‑generalising that all proteins are secreted; some are retained or degraded.
Mark scheme (4 marks)
- 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.
- Ribosomes are bound to the cytosolic face of the rER membrane, so polypeptides are co-translationally threaded into the lumen as they are synthesised.
- The enclosed lumen isolates/sequesters the protein from the cytoplasm, allowing folding and glycosylation / post-translational modification within the lumen.
- The rER membrane can bud off transport vesicles that carry proteins to the Golgi apparatus for further processing and eventual secretion.
Key terms in this question
rough endoplasmic reticulum (rER)
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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