Explain what happens to the water potential and volume of a flaccid plant cell when it is placed in pure water, until equilibrium is reached.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Pure water has a water potential of 0 kPa, which is higher than the negative water potential of a flaccid cell (due to its solutes). Water therefore moves by osmosis into the cell down the water‑potential gradient. As water enters, the vacuole expands and the cell increases in volume; the cell wall is stretched, producing a pressure potential (turgor pressure). Equilibrium is reached when the cell’s water potential (now the sum of solute and pressure potentials) equals 0 kPa, so there is no net water movement.
Examiner tips
- Use the terms ‘water potential’, ‘osmosis’, ‘pressure potential’ and ‘turgor pressure’.
- Show the direction of water movement (from pure water into the cell).
- Explain that equilibrium occurs when the cell’s water potential equals that of the surrounding water.
Common mistakes
- Confusing solute potential with pressure potential; not recognising that the cell’s water potential becomes zero at equilibrium.
- Failing to mention that the cell wall is stretched and turgor pressure is generated.
Mark scheme (4 marks)
- Pure water has a water potential of zero, which is higher than the water potential of the flaccid cell (which is negative due to dissolved solutes).
- Water moves by osmosis into the cell down the water potential gradient (from pure water into the cell).
- As water enters, the cell vacuole expands / the cell increases in volume and the cell wall is stretched, generating a pressure potential / turgor pressure.
- Equilibrium is reached when the water potential of the cell equals zero (equals that of pure water) / when the rising pressure potential fully offsets the solute potential, so there is no net movement of water.
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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