Explain why water moves from the soil into the root hair cells of a plant.

WJEC GCSE Biology (Wales) — 1.1.2 Movement of substances · Explain · 4 marks · View as Markdown

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

Root hair cells are in direct contact with soil water. The cell contents of root hair cells contain dissolved mineral ions and sugars.

Model answer (4 marks)

Osmosis is the process by which water moves.
The root hair cell has a lower water potential (higher solute concentration) than the soil water.
Water therefore moves down a water‑potential gradient, from high to low water potential.
The movement occurs through the partially permeable membrane of the root hair cell.

Examiner tips

  • Use the term osmosis and explain the water‑potential gradient.
  • Show that the cell has a lower water potential than the soil.
  • Mention the membrane is partially permeable.
  • Keep the answer concise and use correct terminology.

Common mistakes

  • Confusing water potential with solute concentration alone.
  • Forgetting to mention the membrane is partially permeable.
  • Using vague terms like ‘water flows’ without explaining the gradient.

Mark scheme (4 marks)

  1. The process by which water moves is osmosis
  2. The root hair cell has a lower water potential / higher solute concentration than the soil water
  3. Water moves down a water potential gradient / from high to low water potential
  4. Water moves through the partially permeable membrane of the root hair cell

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