A student places a piece of potato in a very dilute sugar solution. After 30 minutes, the potato piece becomes firm and slightly heavier. Explain why the mass of the potato piece increased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Water moves into the potato cells by osmosis because the sugar solution outside the cells is more dilute than the cell contents. The higher water concentration outside creates a concentration gradient, so water moves down the gradient into the cells through the partially permeable membrane. This influx of water increases the mass of the potato piece and makes the cells turgid, giving the potato a firm, heavier appearance.
Examiner tips
- Use the word ‘osmosis’ and explain the concentration gradient; link higher external water concentration to lower external solute concentration.
- Show that water enters through the membrane, increasing mass and turgidity; mention the cell’s partially permeable membrane.
Mark scheme (4 marks)
- Water moves into the potato cells by osmosis
- The sugar solution outside is more dilute than the cell contents (higher water concentration outside / lower solute concentration outside)
- Water moves down a concentration gradient (from high water concentration to low water concentration)
- Water enters through the partially permeable membrane, increasing the mass/making cells turgid
Related
- All WJEC GCSE Biology (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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