Red blood cells are placed into three different solutions. In solution A, the cells swell and burst. In solution B, the cells remain the same size and shape. In solution C, the cells shrink and become crenated. Explain these three observations in terms of water potential and osmosis.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Red blood cells have a partially permeable membrane. The cytoplasm of a red blood cell has a specific water potential.
Model answer (5 marks)
Osmosis is the net movement of water across a partially permeable membrane from a region of higher water potential to a region of lower water potential.
In solution A the water potential of the solution is higher than that of the red‑cell cytoplasm, so water enters the cell by osmosis. The influx of water causes the cell to swell until it bursts.
In solution B the water potential of the solution equals that of the cytoplasm, so there is no net movement of water and the cells keep the same size and shape.
In solution C the water potential of the solution is lower than that of the cytoplasm, so water leaves the cell by osmosis. Loss of water makes the cytoplasm and membrane contract, so the cells shrink and become crenated.
In solution A the water potential of the solution is higher than that of the red‑cell cytoplasm, so water enters the cell by osmosis. The influx of water causes the cell to swell until it bursts.
In solution B the water potential of the solution equals that of the cytoplasm, so there is no net movement of water and the cells keep the same size and shape.
In solution C the water potential of the solution is lower than that of the cytoplasm, so water leaves the cell by osmosis. Loss of water makes the cytoplasm and membrane contract, so the cells shrink and become crenated.
Examiner tips
- Define osmosis and water potential first. Show the direction of water movement for each solution. Explain the physical effect (swelling, no change, crenation) in one sentence per solution. Use the exact wording from the mark scheme where possible.
Common mistakes
- Confusing higher water potential with higher solute concentration. Forgetting to state that the membrane is partially permeable to water. Using ‘osmosis’ without explaining the direction of water movement.
Mark scheme (5 marks)
- Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential
- In solution A, the solution has a higher water potential than the cell cytoplasm, so water enters the cell by osmosis, causing it to swell and burst
- In solution B, the water potential of the solution is equal to the water potential of the cytoplasm, so there is no net movement of water
- In solution C, the solution has a lower water potential than the cell cytoplasm, so water leaves the cell by osmosis
- Loss of water from the cell in solution C causes the cell to shrink / become crenated because the cytoplasm / cell membrane contracts
Key terms in this question
osmosis · water potential · crenated
Related
- All Edexcel A-Level Biology A: Salters-Nuffield (9BN0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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