Red blood cells are placed into three different solutions. In solution A, the cells remain the same size. In solution B, the cells swell and may burst. In solution C, the cells shrink and become crenated. Explain these observations in terms of osmosis and water potential.
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 water potential of −0.9 kPa when in normal body fluids.
Model answer (5 marks)
Osmosis is the 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 equals that of the red‑blood‑cell cytoplasm (−0.9 kPa). Because the potentials are equal there is no net movement of water and the cell remains the same size.
In solution B the water potential of the solution is higher than that of the cell. Water therefore moves into the cell by osmosis. The influx of water increases the cell volume, and because red‑blood cells lack a rigid cell wall they can burst (lyse) when the internal pressure becomes too great.
In solution C the water potential of the solution is lower than that of the cell. Water therefore leaves the cell by osmosis, causing the cell to lose volume and become crenated (shrunken).
In solution A the water potential of the solution equals that of the red‑blood‑cell cytoplasm (−0.9 kPa). Because the potentials are equal there is no net movement of water and the cell remains the same size.
In solution B the water potential of the solution is higher than that of the cell. Water therefore moves into the cell by osmosis. The influx of water increases the cell volume, and because red‑blood cells lack a rigid cell wall they can burst (lyse) when the internal pressure becomes too great.
In solution C the water potential of the solution is lower than that of the cell. Water therefore leaves the cell by osmosis, causing the cell to lose volume and become crenated (shrunken).
Examiner tips
- Define osmosis and water potential first; this shows understanding. Explain the direction of water flow using the relative potentials. Link the water movement to the physical change in the cell (size, bursting, crenation). Use the exact wording from the mark scheme where possible.
Common mistakes
- Confusing higher water potential with higher solute concentration. Forgetting that red‑blood cells have no cell wall to resist swelling. Not stating that water moves from higher to lower potential.
Mark scheme (5 marks)
- Osmosis is the 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 water potential of the solution equals that of the cell cytoplasm, so there is no net movement of water and the cell stays the same size
- In solution B, the solution has a higher water potential than the cell cytoplasm, so water moves into the cell by osmosis
- The influx of water causes the cell to swell and may cause it to burst (lyse) because red blood cells have no cell wall to resist the pressure
- In solution C, the solution has a lower water potential than the cell cytoplasm, so water moves out of the cell by osmosis, causing the cell to shrink / become crenated
Key terms in this question
osmosis · water potential · crenated
Related
- All Pearson Edexcel A-Level Biology A (9BI0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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