Explain why water moves out of a red blood cell when it is placed in a concentrated salt solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A red blood cell is placed into a concentrated salt solution. After a short time, the cell shrinks.
Model answer (4 marks)
Water moves by osmosis.
The salt solution has a lower water concentration than the inside of the cell, so the water concentration is higher inside the cell than outside.
Water moves through the partially permeable membrane.
Water moves from an area of higher water concentration to an area of lower water concentration (down its concentration gradient).
The salt solution has a lower water concentration than the inside of the cell, so the water concentration is higher inside the cell than outside.
Water moves through the partially permeable membrane.
Water moves from an area of higher water concentration to an area of lower water concentration (down its concentration gradient).
Examiner tips
- Use the term ‘osmosis’ first. State the concentration difference clearly. Mention the semi‑permeable membrane. Show the direction of movement (high to low).
Common mistakes
- Confusing solute concentration with water concentration. Saying water moves ‘into’ the cell instead of ‘out’. Omitting the membrane’s role.
Mark scheme (4 marks)
- Water moves by osmosis
- The salt solution has a lower water concentration than the inside of the cell / the water concentration is higher inside the cell than outside
- Water moves through the partially permeable membrane
- Water moves from an area of higher water concentration to an area of lower water concentration (down its concentration gradient)
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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