Explain what happens to a plant cell when it is placed in pure water, including the role of the cell wall.

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).

A student places a flaccid plant cell into a beaker of pure water and observes changes over time.

Model answer (5 marks)

Pure water has a higher water concentration than the cell sap, so water moves into the cell by osmosis through the partially permeable membrane.
The vacuole and other cell contents swell, making the cell turgid.
The swollen contents push against the rigid cell wall, generating turgor pressure.
Because the cell wall is inelastic, it resists further expansion and prevents the cell from bursting.

Examiner tips

Use the term ‘osmosis’ and ‘turgor pressure’
Show the sequence: high water outside → water in → swelling → pressure on wall
Mention the cell wall’s rigidity to justify no lysis

Common mistakes

Confusing the cell wall with the plasma membrane
Forgetting that the cell becomes turgid before the wall is pushed
Using ‘burst’ instead of ‘lysis’ or ‘plasmolysis’

Mark scheme (5 marks)

Pure water has a higher water concentration than the cell contents / cell sap
Water moves into the cell by osmosis (through the partially permeable membrane)
The vacuole / cell contents swell and the cell becomes turgid
The cell contents push against the cell wall, generating pressure / the cell wall is pushed outward
The cell wall is inelastic / rigid, so it resists further expansion and prevents the cell from bursting

Key terms in this question

cell wall