A student places a piece of potato in a dilute sugar solution. After 30 minutes, the potato becomes firmer. Explain why water moves into the potato cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
The potato cells contain a higher concentration of solutes than the surrounding dilute sugar solution, so the solution is more dilute than the cell contents.
Water moves by osmosis.
Osmosis causes water to move from a region of higher water concentration (the dilute solution) to a region of lower water concentration (inside the cell).
The water passes through the partially permeable membrane of the potato cell.
Water moves by osmosis.
Osmosis causes water to move from a region of higher water concentration (the dilute solution) to a region of lower water concentration (inside the cell).
The water passes through the partially permeable membrane of the potato cell.
Examiner tips
- Use the word ‘osmosis’ explicitly. Show the direction of water flow (outside to inside). Mention the partially permeable membrane. Keep the answer concise and use correct terminology.
Common mistakes
- Confusing solute concentration with water concentration. Forgetting to mention the membrane. Using vague terms like ‘water moves’ without specifying osmosis.
Mark scheme (4 marks)
- The potato cells have a higher concentration of solutes than the dilute sugar solution (or the solution is more dilute than the cell contents)
- Water moves by osmosis
- Water moves from a region of higher water concentration (the dilute solution) to a region of lower water concentration (inside the cell)
- Water moves through the partially permeable membrane (of the potato cell)
Related
- All Eduqas GCSE Biology revision notes →
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- Decode the mark scheme abbreviations →