Explain how the addition of solutes to a plant cell affects its water potential and the direction of osmosis between that cell and an adjacent cell with a higher water potential.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A mesophyll cell in a leaf accumulates dissolved sugars following active loading of sucrose from the phloem. The cell is surrounded by neighbouring mesophyll cells that have not accumulated solutes to the same extent.
Model answer (4 marks)
Adding solutes lowers the solute potential (Ψs) of the sugar‑loaded mesophyll cell, making its overall water potential (Ψ) more negative than that of the neighbouring cell.
Because the sugar‑loaded cell now has a lower Ψ, water moves by osmosis from the adjacent cell (higher Ψ) into the sugar‑loaded cell.
The influx of water increases the cell’s turgor pressure (Ψp), which raises its Ψ until the two cells reach equilibrium.
Because the sugar‑loaded cell now has a lower Ψ, water moves by osmosis from the adjacent cell (higher Ψ) into the sugar‑loaded cell.
The influx of water increases the cell’s turgor pressure (Ψp), which raises its Ψ until the two cells reach equilibrium.
Examiner tips
- Use the terms solute potential, water potential and turgor pressure explicitly.
- Show the direction of water movement (higher to lower Ψ).
- Explain that the rise in Ψp brings the two cells to equilibrium.
Common mistakes
- Confusing solute potential with osmotic potential or pressure potential.
- Saying water moves from the sugar‑loaded cell to the neighbour.
- Omitting the role of turgor pressure in restoring equilibrium.
Mark scheme (4 marks)
- Adding solutes lowers (makes more negative) the solute potential (Ψs) of the cell
- This lowers the overall water potential (Ψ) of the sugar-loaded cell, making it more negative than the adjacent cell
- Water moves by osmosis from the region of higher water potential (adjacent cell) to the region of lower water potential (sugar-loaded cell)
- As water enters, turgor pressure / pressure potential (Ψp) increases, which raises the water potential of the sugar-loaded cell until equilibrium is reached
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 →
More Water potential questions
- Explain how water potential gradients drive the movement of water from the soil …
- Explain the relationship between turgor pressure, solute potential, and water po…
- Explain how the water potential of a plant cell changes as it undergoes plasmoly…
- Explain how the water potential of red blood cells changes when they are placed …