Distinguish turgid flaccid and plasmolysed plant cells
Explain changes in water potential, turgor pressure and membrane position.
Section 3.2 · Osmosis
Exam scope and assessment notes
Extended only (Supplement) · Biology 0970, 2026–2028. Use the stated specimen features and evidence. Diagrams are schematic. Written practice is self-checked; human subject-teacher review, supervised practical competence and cumulative assessment remain pending. Cambridge states that IGCSE 0970 has the same subject content as IGCSE 0610, with 9–1 grading. This StudyForA pathway maps the 2026–2028 syllabus only. This 9–1 graded qualification is available only in a limited number of administrative zones. Confirm that your centre can enter you for this code and exam series with Cambridge International. Cambridge has published a separate syllabus for 2029. StudyForA has not reconciled that version yet; this page supports exams in 2026, 2027 and 2028 only.
Before you begin
- Recall water potential and supporting walls.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What directly indicates plasmolysis in the model?
What you will learn
- Explain changes in water potential, turgor pressure and membrane position.
- Apply the idea to a supplied biological example or investigation.
Describe the cell states
A turgid plant cell contains enough water for its contents to exert substantial outward turgor pressure against the wall. Water entering from higher external water potential increases this pressure; the wall resists expansion. The pressure increase can reduce the overall water-potential difference and limit further net entry.
A flaccid cell has reduced turgor pressure and is less firm. Losing some water does not automatically mean its membrane has visibly pulled away from the wall. Turgid and flaccid describe pressure/support states, not whether the wall exists.
Recognise plasmolysis
In sufficiently low external water potential, water leaves by osmosis, the vacuole and cytoplasm shrink, and the cell membrane can pull away from the cell wall. This separation is plasmolysis. The wall remains outside; it does not shrink inward with the membrane in this model.
Not every wilted tissue is visibly plasmolysed. A supplied microscope observation of membrane separation supports that label more directly than mass loss alone. Replacing a concentrated surrounding solution with a more suitable dilute solution may reverse separation in living, undamaged cells, but recovery is not guaranteed for damaged tissue.
Pause and explain
What is turgor pressure?
Worked example
A plant micrograph shows the membrane and contents pulled inward from an unchanged cell wall after immersion. Identify the state.
Show the worked solution
- The wall remains outside.
- The membrane has separated from it.
- This is stronger evidence than tissue mass loss alone.
Answer Plasmolysis.
Try a topic test
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why a turgid plant cell is firm.
Give me a hint
Connect water, pressure and wall.
Compare my reasoning
- Water-filled contents press outward.
- The wall resists expansion.
Turgor pressure against the resisting wall gives firmness.
Look for these in your work
- Use turgor pressure correctly.
A cell loses some water and has low turgor, but no visible membrane separation. Which term fits best?
Give me a hint
Do not overclaim plasmolysis.
Compare my reasoning
- Pressure has fallen.
- Separation is not observed.
Flaccid.
Look for these in your work
- Distinguish the two water-loss states.
Explain a membrane pulling away from the wall in a concentrated external solution.
Give me a hint
Use water potential and osmosis.
Compare my reasoning
- Outside water potential is lower in the stated model.
- Water leaves; vacuole and cytoplasm shrink.
- The membrane separates from the unchanged wall.
Water loss by osmosis causes plasmolysis.
Look for these in your work
- Link gradient, water loss and membrane position.
Common mistakes
- Flaccid does not automatically mean visibly plasmolysed.
- The wall does not pull inward with the membrane.
What can you explain now?
What is turgor pressure?
Compare with the explanation
Outward pressure of cell contents against the wall.
It contributes to plant firmness.
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Make it stick
Sketch the same plant cell in three states and keep the wall fixed.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.