Describe how osmosis causes a plant cell to become turgid when it is placed in a solution that is more dilute than the cell sap inside the vacuole.

Pearson Edexcel International GCSE Biology (4BI1) — 2.4 Movement of substances into and out of cells · Describe · 4 marks · View as Markdown

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

Model answer (4 marks)

The solution outside the cell is more dilute than the cell sap, so there is a higher concentration of water molecules outside the cell.
Water moves into the cell by osmosis across the partially permeable cell membrane.
Water enters the vacuole, causing it to expand and the cell sap to increase in volume.
The vacuole pushes the cytoplasm against the cell wall, making the cell firm and turgid; the cell wall resists further expansion.

Examiner tips

  • Use the word ‘osmosis’ and mention the cell membrane as partially permeable.
  • Show the direction of water flow (outside to inside).
  • Explain the role of the vacuole and cell wall in turgidity.
  • Keep the answer concise – 4 points only.

Common mistakes

  • Confusing ‘more dilute’ with ‘more concentrated’. Failing to mention the cell membrane. Describing water moving out of the cell instead of in. Omitting the role of the cell wall in resisting expansion.

Mark scheme (4 marks)

  1. The solution outside the cell is more dilute than the cell sap, so there is a higher concentration of water molecules outside the cell (than inside)
  2. Water moves into the cell by osmosis across the partially permeable cell membrane
  3. Water enters the vacuole, causing it to expand / the cell sap increases in volume
  4. The vacuole pushes the cytoplasm against the cell wall, making the cell firm / turgid; the cell wall resists further expansion

Key terms in this question

osmosis · turgid · vacuole

Related

More Movement of substances into and out of cells questions

▶ Try answering this question with AI marking (free) →