Osmosis is an important process that occurs across cell membranes. Explain what happens to a plant cell when it is placed in a solution that has a lower water concentration than the cell's contents.

WJEC A-Level Biology (Wales) — 1.3 Cell membranes and transport · Explain · 5 marks · View as Markdown

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

A student placed a piece of plant tissue into a concentrated salt solution and left it for 30 minutes. When the tissue was removed, it felt limp and flexible rather than firm.

Model answer (5 marks)

Water moves out of the cell by osmosis. It moves from a higher water concentration inside the cell to a lower concentration in the solution across the selectively permeable cell membrane. The vacuole and cytoplasm lose water, causing the cell to shrink. The cell membrane pulls away from the cell wall – plasmolysis occurs. The cell becomes flaccid, loses turgor pressure, so the tissue feels limp.

Examiner tips

  • Use the word ‘osmosis’ and state the direction of water movement. Include ‘plasmolysis’ and ‘turgor pressure’.
  • Show the sequence: water loss → shrinkage → membrane pulls away → loss of firmness.

Common mistakes

  • Confusing water moving into the cell instead of out.
  • Forgetting to mention plasmolysis or turgor pressure.

Mark scheme (5 marks)

  1. Water moves out of the cell by osmosis
  2. Water moves from a higher water concentration (inside the cell) to a lower water concentration (in the solution) across the selectively permeable cell membrane
  3. The vacuole and cytoplasm lose water, causing the cell to shrink
  4. The cell membrane pulls away from the cell wall (plasmolysis occurs)
  5. The cell becomes flaccid / loses turgor pressure, so the tissue feels limp

Key terms in this question

osmosis · cell membrane

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