Explain how the structure of the Golgi apparatus is related to its function in the processing and secretion of proteins.

IB DP Biology Higher Level (2023 syllabus) — A2.2 Cell structure · 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 Golgi apparatus is a stack of flattened, membrane‑bound cisternae, giving a series of distinct compartments.
1. Vesicles from the rough ER fuse with the cis face, delivering newly synthesised proteins and ensuring a directional flow.
2. As proteins move through the cisternae, enzymes in each compartment add or modify carbohydrate chains (glycosylation) and cleave signal peptides – the stack provides spatially separated enzymatic environments.
3. At the trans face, the modified proteins are packaged into secretory vesicles that bud off and fuse with the plasma membrane (exocytosis), allowing export from the cell.
4. The membrane‑bound nature of the vesicles protects the proteins during transit and prevents degradation.

Examiner tips

  • Use the word "cis" and "trans" to show directionality; mention ER‑derived vesicles and exocytosis; link structure (stack of cisternae) to sequential modification; include glycosylation and signal‑peptide cleavage.
  • Keep the answer concise – 4 marks, so one sentence per point is enough. Use exact terminology: cisternae, cis/trans face, vesicle fusion, glycosylation, exocytosis.

Common mistakes

  • Confusing the Golgi with the ER or mitochondria; not mentioning the directional flow from cis to trans; omitting the role of glycosylation or signal‑peptide cleavage.

Mark scheme (4 marks)

  1. The Golgi apparatus is composed of a stack of flattened membrane-bound cisternae, providing a series of distinct compartments in which sequential modification of proteins can occur.
  2. Vesicles arriving from the rough endoplasmic reticulum fuse with the cis face of the Golgi, delivering proteins for processing, ensuring a directional flow through the organelle.
  3. As proteins pass through the cisternae, enzymes within each compartment add or modify carbohydrate chains (glycosylation) and cleave signal peptides, so the stack provides spatially separated enzymatic environments.
  4. At the trans face, modified proteins are packaged into secretory vesicles that bud off and fuse with the plasma membrane (exocytosis), enabling export from the cell; the membrane-bound nature of the vesicles prevents degradation of proteins during transit.

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