A student investigates the effect of placing plant cells in solutions of different concentrations. When cells are placed in a very concentrated sugar solution, plasmolysis occurs. Explain what happens to a plant cell during plasmolysis and why this results in cell death.

OCR A-Level Biology B — Advancing Biology (H422) — 2.2 Biological molecules including biochemistry · Explain · 5 marks · View as Markdown

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

Plant cells behave differently depending on the concentration of the solution surrounding them. When a plant cell is placed in a concentrated sugar solution, the net movement of water out of the cell causes a sequence of observable changes.

Model answer (5 marks)

The external solution is hypertonic, having a higher solute concentration and lower water concentration than the cytoplasm. Water therefore leaves the cell by osmosis through the semi‑permeable membrane. As water exits the vacuole, it shrinks and the overall cell volume decreases. The plasma membrane pulls away from the rigid cell wall, a process called plasmolysis. Once the membrane detaches, it can no longer maintain essential functions such as nutrient transport and ion balance, leading to cell death.

Examiner tips

  • Use the term ‘hypertonic’ and ‘osmosis’ to show understanding of the driving force.
  • Explain the sequence: water loss → vacuole shrinkage → membrane detachment → loss of function.
  • Mention that the cell wall remains intact but the membrane cannot sustain cellular processes.

Common mistakes

  • Confusing ‘hypotonic’ for the external solution.
  • Saying the cell wall pulls away instead of the membrane.
  • Failing to link membrane detachment to loss of cellular function.

Mark scheme (5 marks)

  1. The solution outside the cell is more concentrated than inside the cell (higher solute concentration / lower water concentration outside)
  2. Water moves out of the cell by osmosis through the partially permeable membrane
  3. The vacuole shrinks / the cell loses water causing the cell contents to decrease in volume
  4. The cell membrane pulls away from the cell wall
  5. Cell death results because the cell membrane can no longer function / maintain cellular processes once it has detached from the wall

Key terms in this question

plasmolysis

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