Explain how the water potential of a plant cell changes as it undergoes plasmolysis, and describe the state of the cell at the point of incipient plasmolysis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When plant cells are placed in solutions of increasing solute concentration, they lose water by osmosis and may undergo plasmolysis.
Model answer (4 marks)
As the cell loses water by osmosis the internal solute concentration rises, making the solute potential (ψs) more negative.
The loss of water reduces the pressure exerted by the protoplast on the cell wall, so the pressure potential (ψp, or turgor pressure) falls toward zero.
Because ψ = ψs + ψp, both terms becoming more negative means the overall water potential of the cell decreases (becomes more negative) as plasmolysis proceeds.
At incipient plasmolysis the membrane has just begun to separate from the wall; turgor pressure is zero, so ψ = ψs. The cell is flaccid but the protoplast still occupies the cell wall.
The loss of water reduces the pressure exerted by the protoplast on the cell wall, so the pressure potential (ψp, or turgor pressure) falls toward zero.
Because ψ = ψs + ψp, both terms becoming more negative means the overall water potential of the cell decreases (becomes more negative) as plasmolysis proceeds.
At incipient plasmolysis the membrane has just begun to separate from the wall; turgor pressure is zero, so ψ = ψs. The cell is flaccid but the protoplast still occupies the cell wall.
Examiner tips
- Show the change in ψs and ψp separately, then combine them to explain the overall ψ change.
- Mention that at incipient plasmolysis ψp = 0 so ψ = ψs and the cell is flaccid but the protoplast still fills the wall.
Common mistakes
- Confusing ψs with ψp or vice versa.
- Saying the cell becomes rigid instead of flaccid at incipient plasmolysis.
- Ignoring that ψ = ψs + ψp and both terms change.
Mark scheme (4 marks)
- As the cell loses water by osmosis, the solute concentration inside the cell increases, causing the solute potential (ψs) to become more negative.
- As the cell loses water, the cell wall no longer pushes inward on the contents, so pressure potential (ψp / turgor pressure) decreases towards zero.
- Overall water potential of the cell decreases (becomes more negative) as water is lost, because ψ = ψs + ψp and both components decrease.
- At incipient plasmolysis, the cell membrane has just begun to pull away from the cell wall, turgor pressure equals zero, so water potential equals solute potential (ψ = ψs); the cell is flaccid but the protoplast still fills the cell wall.
Key terms in this question
plasmolysis · incipient plasmolysis
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
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