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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The rough ER has ribosomes on its cytosolic surface, so nascent polypeptides are co‑translationally threaded into the ER lumen, isolating them from cytosolic enzymes.
Inside the lumen the proteins undergo initial modifications – e.g. addition of N‑linked oligosaccharides, formation of disulfide bonds and correct folding with chaperones – in a distinct chemical environment.
Vesicles bud from the ER and fuse with the cis face of the Golgi, delivering the proteins to successive cisternae (cis → medial → trans). Each cisterna contains a specific set of enzymes, so the modifications are applied in a fixed, sequential order.
The membrane‑bound compartmentalisation of the ER and Golgi prevents re‑mixing of modified and unmodified proteins and keeps them separate from cytosolic contents, ensuring fidelity and correct targeting of the finished protein (e.g. sorting signals recognised at the trans‑Golgi network).

Examiner tips

  • Mention ribosomes on the ER surface and co‑translational import into the lumen. State the specific early modifications that occur in the ER. Explain the cis‑to‑trans cisternal sequence and the role of different enzymes. Highlight membrane compartmentalisation as the mechanism that keeps the pathway regulated.

Common mistakes

  • Failing to specify that ribosomes are on the cytosolic face of the rough ER. Confusing the order of modifications or omitting the cis‑medial‑trans sequence. Not mentioning that the Golgi cisternae contain distinct enzymes or that compartmentalisation prevents re‑mixing.

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 in this question

rough endoplasmic reticulum · Golgi apparatus · post-translational modification

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