Explain what happens to a plant cell when it is placed in a solution that has a lower water concentration than the cell contents, and explain why the cell does not burst.

AQA A-Level Biology (7402) — 3.2.3 Transport Across Membranes · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Water moves out of the cell by osmosis, from an area of higher water concentration (inside the cell) to an area of lower water concentration (in the solution). The cell becomes plasmolysed: the vacuole and cytoplasm shrink and the cell becomes flaccid. The cell membrane pulls away from the cell wall (plasmolysis). The cell wall is rigid and inelastic, so it prevents the cell from bursting; it supports the cell and resists changes in volume.

Examiner tips

  • Use the word ‘osmosis’ and explain direction of water movement; mention plasmolysis and the role of the rigid cell wall; keep answer concise and use correct terminology.
  • Structure answer in clear points to show each mark point.

Mark scheme (5 marks)

  1. Water moves out of the cell by osmosis
  2. Water moves from an area of higher water concentration (inside the cell) to an area of lower water concentration (in the solution)
  3. The cell becomes plasmolysed / the vacuole and cytoplasm shrink / the cell becomes flaccid
  4. The cell membrane pulls away from the cell wall (plasmolysis)
  5. The cell wall is rigid / inelastic, so it prevents the cell from bursting / the cell wall supports the cell and resists changes in volume

Key terms in this question

water concentration

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