A piece of celery is left out of water for several hours and becomes limp. It is then placed in distilled water for one hour. Explain why the celery becomes firm again after being placed in distilled water.

Cambridge International IGCSE Biology (0610) — 3.2 Osmosis · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Distilled water has a higher water concentration (higher water potential) than the celery cells. Water therefore moves into the cells by osmosis through the partially permeable membrane. The cells take in water and swell, increasing in volume. The increased volume raises turgor pressure, which pushes the cell membrane against the cell wall and makes the celery firm again.

Examiner tips

  • Use the term ‘water potential’ and ‘turgor pressure’
  • Show the direction of water movement (from high to low water potential)
  • Explain that swelling is due to osmosis, not just water absorption

Common mistakes

  • Confusing ‘osmotic pressure’ with ‘turgor pressure’
  • Failing to mention the higher water potential of distilled water
  • Not linking swelling to increased turgor pressure

Mark scheme (4 marks)

  1. Distilled water has a higher water concentration / water potential than the celery cells
  2. Water enters the celery cells by osmosis (through the partially permeable membrane)
  3. The cells take in water and swell / increase in volume
  4. The cells become turgid / turgor pressure increases, pushing the cell membrane against the cell wall, making the celery firm

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