Explain how the structure of the rough endoplasmic reticulum (rER) is related to its function in the production and processing of proteins destined for secretion.

IB DP Biology Standard 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 rER is studded with ribosomes on its cytoplasmic surface, the sites of translation. Newly synthesised polypeptides are threaded into the lumen as they are made, creating a separate compartment that isolates them from the cytosol. Within the lumen the proteins fold and undergo post‑translational modifications such as N‑glycosylation, which are required for their correct structure and function. The extensive network of membrane‑enclosed cisternae provides a large surface area and a continuous transport system, so processed proteins are packaged into vesicles that bud off and are transported to the Golgi apparatus for further processing or secretion.

Examiner tips

  • Mention ribosomes on the cytoplasmic surface as the site of translation
  • Explain that polypeptides enter the lumen during synthesis
  • Include folding and glycosylation as key modifications
  • Show how the cisternal network allows vesicle budding to the Golgi

Common mistakes

  • Confusing the rough ER with the smooth ER
  • Omitting the role of ribosomes or the lumen
  • Failing to mention glycosylation or other post‑translational modifications

Mark scheme (4 marks)

  1. The rER is studded with ribosomes on its cytoplasmic surface, which are the sites of translation/protein synthesis.
  2. Newly synthesised polypeptides are threaded into the lumen of the rER as they are made, providing a separate compartment that isolates them from the cytosol.
  3. Within the lumen, proteins undergo folding and post-translational modifications such as glycosylation, which are required for their correct structure and function.
  4. The extensive network of membrane-enclosed cisternae provides a large surface area and continuous transport system, so processed proteins are packaged into vesicles that bud off and are transported to the Golgi apparatus for further processing or secretion.

Key terms in this question

rough endoplasmic reticulum (rER)

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