Explain how osmosis causes a plant cell to become turgid when placed in a hypotonic solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water moves by osmosis from the hypotonic solution (higher water potential) into the plant cell (lower water potential) across the selectively permeable membrane.
The vacuole swells as it absorbs water, increasing cell volume and pushing the cytoplasm against the rigid cell wall.
The inextensible cell wall generates a pressure potential (turgor pressure) that opposes further water entry.
Equilibrium is reached when the internal water potential equals the external one, so the cell is fully turgid and net water movement stops.
The vacuole swells as it absorbs water, increasing cell volume and pushing the cytoplasm against the rigid cell wall.
The inextensible cell wall generates a pressure potential (turgor pressure) that opposes further water entry.
Equilibrium is reached when the internal water potential equals the external one, so the cell is fully turgid and net water movement stops.
Examiner tips
- Use the term ‘osmosis’ and ‘water potential’ early. Show the sequence: water → vacuole → cell wall → turgor pressure. Mention the balance of pressure and osmotic potentials at equilibrium.
- common_mistakes
- :
- Confusing hypotonic with hypertonic. Forgetting to mention the cell wall’s role. Using vague terms like ‘pressure’ without specifying turgor pressure.
Mark scheme (4 marks)
- Water moves by osmosis from the hypotonic solution (higher water potential) into the cell (lower water potential) across the partially/selectively permeable membrane.
- The vacuole enlarges as it absorbs water, increasing the volume of the cell and pushing the cytoplasm against the cell wall.
- The inextensible (rigid) cell wall generates a pressure potential (turgor pressure / wall pressure) that opposes further entry of water.
- Equilibrium is reached when the water potential inside the cell equals that of the external solution (i.e. when pressure potential balances the solute/osmotic potential), at which point the cell is fully turgid and net water movement ceases.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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