Explain how the water potential of red blood cells changes when they are placed in a hypertonic solution, and describe the consequences for cell structure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The hypertonic solution has a lower (more negative) water potential than the cytoplasm of the red blood cell.
Water moves out of the red blood cell by osmosis down the water potential gradient.
Loss of water reduces the volume of the cell, causing the cell membrane to shrink inward / the cell to crenate.
As water leaves, the solute concentration inside the cell increases, lowering the water potential of the cytoplasm until equilibrium is reached (water potentials equalise).
Water moves out of the red blood cell by osmosis down the water potential gradient.
Loss of water reduces the volume of the cell, causing the cell membrane to shrink inward / the cell to crenate.
As water leaves, the solute concentration inside the cell increases, lowering the water potential of the cytoplasm until equilibrium is reached (water potentials equalise).
Examiner tips
- Use the word ‘osmosis’ and ‘water potential gradient’ to show understanding of the driving force.
- Explain the sequence: lower external potential → water leaves → volume decreases → crenation.
- Mention that the cell’s internal water potential becomes more negative until it matches the external.
- Show the final equilibrium state where potentials are equalised.
Common mistakes
- Confusing hypertonic with hypotonic; writing that water enters the cell.
- Forgetting to mention the shrinkage (crenation) of the cell membrane.
- Failing to state that the internal water potential becomes more negative until equilibrium is reached.
Mark scheme (4 marks)
- The hypertonic solution has a lower (more negative) water potential than the cytoplasm of the red blood cell.
- Water moves out of the red blood cell by osmosis down the water potential gradient.
- Loss of water reduces the volume of the cell, causing the cell membrane to shrink inward / the cell to crenate.
- As water leaves, the solute concentration inside the cell increases, lowering the water potential of the cytoplasm until equilibrium is reached (water potentials equalise).
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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