Explain the relationship between turgor pressure, solute potential, and water potential in a fully turgid plant cell.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water potential (Ψ) is the sum of solute potential (Ψs) and pressure potential (Ψp): Ψ = Ψs + Ψp.
In a fully turgid cell the pressure potential (turgor pressure) is at its maximum positive value, pushing the cell membrane outward against the rigid cell wall.
The solute potential remains negative because dissolved solutes lower the free energy of water; however, in a turgid cell it is less negative than in a flaccid cell because the cell has expanded and the solutes are diluted.
Because the positive pressure potential exactly balances the negative solute potential, the overall water potential of the cell is zero (or equal to that of pure water in the surroundings), so there is no net movement of water into or out of the cell.
In a fully turgid cell the pressure potential (turgor pressure) is at its maximum positive value, pushing the cell membrane outward against the rigid cell wall.
The solute potential remains negative because dissolved solutes lower the free energy of water; however, in a turgid cell it is less negative than in a flaccid cell because the cell has expanded and the solutes are diluted.
Because the positive pressure potential exactly balances the negative solute potential, the overall water potential of the cell is zero (or equal to that of pure water in the surroundings), so there is no net movement of water into or out of the cell.
Examiner tips
- Use the equation Ψ = Ψs + Ψp to link the three terms.
- Show that Ψp is positive and maximal in a turgid cell.
- Explain that Ψs is still negative but less so due to dilution.
- State that Ψ = 0 (or equals the surrounding water potential) to finish the explanation.
Common mistakes
- Confusing pressure potential with solute potential; they are separate terms.
- Claiming that Ψs becomes positive in a turgid cell; it remains negative.
- Saying that water potential is still negative in a turgid cell; it is zero or equal to the surroundings.
Mark scheme (4 marks)
- Water potential (Ψ) equals solute potential (Ψs) plus pressure potential (Ψp) / Ψ = Ψs + Ψp
- In a fully turgid cell, turgor pressure (pressure potential) is at its maximum positive value, pushing outward against the cell wall
- Solute potential remains negative (due to dissolved solutes lowering the free energy of water), and in a fully turgid cell solute potential becomes less negative than in a flaccid cell because the cell volume has increased / solutes are diluted
- In a fully turgid cell, water potential equals zero (or is equal to the water potential of pure water / surroundings) because the positive pressure potential exactly offsets the negative solute potential, so there is no net movement of water
Key terms in this question
water potential · solute potential · turgor pressure · fully turgid
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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