Explain how the addition of solutes to a plant cell's vacuole affects the water potential of that cell and the direction of water movement across its tonoplast.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Adding solutes to the vacuole lowers the solute potential (ψs) of the vacuolar solution, making it more negative.
This reduces the overall water potential (ψ) of the vacuole relative to the cytoplasm or surrounding solution.
Water therefore moves by osmosis across the tonoplast from the region of higher ψ (cytoplasm) to the lower ψ (vacuole).
As water enters the vacuole, the pressure potential (turgor) rises, which in turn raises the vacuolar ψ until equilibrium is reached and net water movement stops.
This reduces the overall water potential (ψ) of the vacuole relative to the cytoplasm or surrounding solution.
Water therefore moves by osmosis across the tonoplast from the region of higher ψ (cytoplasm) to the lower ψ (vacuole).
As water enters the vacuole, the pressure potential (turgor) rises, which in turn raises the vacuolar ψ until equilibrium is reached and net water movement stops.
Examiner tips
- Use the term ‘solute potential’ and ‘water potential’ explicitly. Show the direction of movement (cytoplasm → vacuole) and link it to osmosis. Mention the rise in turgor pressure as the balancing factor.
- Include the key phrase ‘until equilibrium (equal water potentials) is reached’ to demonstrate full understanding.
Common mistakes
- Confusing solute potential with pressure potential. Failing to state that water moves from higher to lower water potential. Omitting the role of turgor pressure in stopping net movement.
Mark scheme (4 marks)
- Adding solutes to the vacuole lowers (makes more negative) the solute potential (ψs) of the vacuolar solution.
- This lowers the overall water potential (ψ) of the vacuole relative to the cytoplasm / surrounding solution.
- Water moves by osmosis across the tonoplast from the higher water potential (cytoplasm) to the lower water potential (vacuole).
- As water enters the vacuole, pressure potential (turgor) increases, which raises the water potential of the vacuole, until equilibrium (equal water potentials) is reached and net water movement stops.
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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