Explain how the water potential of a plant cell changes as the cell absorbs water by osmosis and approaches full turgor.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
As water enters the cell by osmosis, the solute concentration of the cytoplasm/vacuole decreases, so the solute potential (ψs) becomes less negative.
The inflow of water increases the volume of the vacuole/cell, generating an increasing pressure (turgor pressure / pressure potential, ψp) against the cell wall.
Water potential (ψ) is the sum of solute potential and pressure potential (ψ = ψs + ψp); as both ψs becomes less negative and ψp increases, the overall water potential of the cell rises.
At full turgor the water potential of the cell equals the water potential of the surrounding solution, so net osmosis stops and the water potential gradient is zero.
The inflow of water increases the volume of the vacuole/cell, generating an increasing pressure (turgor pressure / pressure potential, ψp) against the cell wall.
Water potential (ψ) is the sum of solute potential and pressure potential (ψ = ψs + ψp); as both ψs becomes less negative and ψp increases, the overall water potential of the cell rises.
At full turgor the water potential of the cell equals the water potential of the surrounding solution, so net osmosis stops and the water potential gradient is zero.
Examiner tips
- Use the formula ψ = ψs + ψp to link the two components. Show the direction of change for ψs (less negative) and ψp (more positive). Explain that the net change in ψ drives water into the cell until equilibrium is reached.
Common mistakes
- Confusing ψs becoming more negative instead of less negative. Forgetting to mention the increase in pressure potential. Stating that water potential always decreases rather than rises towards equilibrium.
Mark scheme (4 marks)
- As water enters the cell by osmosis, the solute concentration of the cytoplasm/vacuole decreases, so the solute potential (ψs) becomes less negative.
- The inflow of water increases the volume of the vacuole/cell, generating an increasing pressure (turgor pressure / pressure potential, ψp) against the cell wall.
- Water potential (ψ) is the sum of solute potential and pressure potential (ψ = ψs + ψp); as both ψs becomes less negative and ψp increases, the overall water potential of the cell rises.
- At full turgor the water potential of the cell equals the water potential of the surrounding solution, so net osmosis stops / water potential gradient is zero.
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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