Explain how the structural features of the vacuole in a mature plant cell are related to its functions.

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 vacuole is surrounded by the tonoplast, a selectively permeable membrane that controls the movement of ions and solutes, allowing turgor pressure to be regulated.
The large, central vacuole occupies up to 90 % of the cell’s volume, providing a large hydrostatic pressure against the cell wall, which maintains cell rigidity and support in non‑woody tissues.
The vacuole stores solutes such as ions, sugars, amino acids and secondary metabolites (e.g. anthocyanins, tannins), sequestering them away from the cytoplasm where they could interfere with metabolic reactions.
The vacuole has an acidic lumen (maintained by proton pumps in the tonoplast), which activates hydrolytic enzymes stored within it, enabling the vacuole to function in degradation of waste or damaged cell components (analogous to lysosomes in animal cells).

Examiner tips

  • Use the term ‘tonoplast’ and ‘turgor pressure’ to show understanding of membrane control and mechanical support.
  • Mention the vacuole’s storage role and acidic environment to link structure with degradation function.
  • Keep the answer concise – 4 points, one sentence per point.
  • Use UK spelling (e.g. ‘organelles’, ‘hydrolytic’).

Common mistakes

  • Confusing the vacuole with the cytoplasm or mistaking the tonoplast for the plasma membrane.
  • Failing to mention the acidic lumen or the role of proton pumps.
  • Using vague terms like ‘big’ instead of ‘large, central’ or ‘stores’ without specifying solute types.

Mark scheme (4 marks)

  1. The vacuole is enclosed by the tonoplast (a selectively permeable membrane), which controls the movement of ions and solutes into and out of the vacuole, allowing turgor pressure to be regulated.
  2. The large, central vacuole occupies up to 90% of the cell's volume, providing a large hydrostatic (turgor) pressure against the cell wall, which maintains cell rigidity and support in non-woody tissues.
  3. The vacuole stores solutes such as ions, sugars, amino acids, and secondary metabolites (e.g. anthocyanins, tannins), sequestering them away from the cytoplasm where they could interfere with metabolic reactions.
  4. The vacuole has an acidic lumen (maintained by proton pumps in the tonoplast), which activates hydrolytic enzymes stored within it, enabling the vacuole to function in degradation of waste or damaged cell components (analogous to lysosomes in animal cells).

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