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)
A hypotonic solution has a lower solute concentration (higher water potential) than the cell contents, so a water‑potential gradient exists across the membrane.
Water moves by osmosis – passive diffusion through the partially permeable phospholipid bilayer (or aquaporins) – down the water‑potential gradient into the cell.
The vacuole (and cytoplasm) takes in water, causing the cell to swell and increase in volume.
The rigid cell wall exerts an inward pressure (wall pressure/pressure potential) on the swollen protoplast, creating turgor pressure; this pressure opposes further water entry, resulting in a turgid cell.
Water moves by osmosis – passive diffusion through the partially permeable phospholipid bilayer (or aquaporins) – down the water‑potential gradient into the cell.
The vacuole (and cytoplasm) takes in water, causing the cell to swell and increase in volume.
The rigid cell wall exerts an inward pressure (wall pressure/pressure potential) on the swollen protoplast, creating turgor pressure; this pressure opposes further water entry, resulting in a turgid cell.
Examiner tips
- Use the term ‘water potential gradient’ early. Mention the membrane’s permeability (phospholipid bilayer or aquaporins). Explain the role of the cell wall and turgor pressure. Keep the answer concise – 4 points, 1–2 sentences each.
Common mistakes
- Confusing hypertonic with hypotonic. Forgetting to mention the cell wall or turgor pressure. Using ‘active transport’ instead of passive osmosis.
Mark scheme (4 marks)
- The hypotonic solution has a lower solute concentration (higher water potential) than the cell contents, so a water potential gradient exists across the membrane.
- Water moves by osmosis — by passive/diffusion movement through the partially permeable (phospholipid bilayer / aquaporins) membrane — down the water potential gradient into the cell.
- The vacuole (and cytoplasm) takes in water, causing the cell to swell / increase in volume.
- The cell wall exerts an inward pressure (wall pressure / pressure potential) on the swollen protoplast, creating turgor pressure; this pressure opposes further water entry, resulting in a turgid cell.
Key terms in this question
Related
- All IB DP Biology Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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