Explain how the endomembrane system enables a secretory protein to be correctly processed and exported from a eukaryotic cell.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Ribosomes bound to the rough endoplasmic reticulum (RER) translate the mRNA and insert the nascent polypeptide into the RER lumen.
The protein folds and receives early post‑translational modifications, such as N‑linked glycosylation, while still in the RER.
It is then incorporated into transport vesicles that bud from the RER and fuse with the Golgi apparatus.
In the Golgi the protein undergoes further modification and sorting; secretory vesicles bud from the trans face and fuse with the plasma membrane, releasing the protein by exocytosis.
The protein folds and receives early post‑translational modifications, such as N‑linked glycosylation, while still in the RER.
It is then incorporated into transport vesicles that bud from the RER and fuse with the Golgi apparatus.
In the Golgi the protein undergoes further modification and sorting; secretory vesicles bud from the trans face and fuse with the plasma membrane, releasing the protein by exocytosis.
Examiner tips
- Show the sequence of organelles: RER → Golgi → plasma membrane
- Mention key processes: folding, glycosylation, vesicle budding, fusion, exocytosis
- Use correct terminology: rough ER, Golgi, vesicle, exocytosis
Common mistakes
- Confusing rough ER with smooth ER
- Omitting the Golgi step or the exocytosis step
- Using vague terms like ‘transport’ without specifying vesicle budding and fusion
Mark scheme (4 marks)
- Ribosomes on the rough endoplasmic reticulum (RER) synthesise the protein directly into the lumen of the RER.
- The protein undergoes initial folding and post-translational modification (e.g., glycosylation) within the RER lumen.
- The protein is packaged into transport vesicles that bud from the RER and fuse with the Golgi apparatus.
- Further modification and sorting occur in the Golgi apparatus; secretory vesicles bud from the trans face and fuse with the plasma membrane, releasing the protein by exocytosis.
Key terms in this question
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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