A student places a piece of potato into a concentrated salt solution and leaves it for 30 minutes. After 30 minutes, the potato piece has decreased in mass. Explain why the mass of the potato has decreased.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A student places a piece of potato into a concentrated salt solution and leaves it for 30 minutes. After 30 minutes, the potato piece has decreased in mass.
Model answer (5 marks)
Water moves out of the potato cells by osmosis.
The potato cells have a higher water potential (lower solute concentration) than the surrounding salt solution.
Because the cell membrane is partially permeable, water flows from the cell into the solution.
This loss of water from the cells reduces the mass of the potato.
The decrease in mass is therefore due to water leaving the cells by osmosis into the concentrated salt solution.
The potato cells have a higher water potential (lower solute concentration) than the surrounding salt solution.
Because the cell membrane is partially permeable, water flows from the cell into the solution.
This loss of water from the cells reduces the mass of the potato.
The decrease in mass is therefore due to water leaving the cells by osmosis into the concentrated salt solution.
Examiner tips
- Use the word "osmosis" explicitly. Mention the cell membrane as partially permeable. Explain that water potential is higher inside the cell. Show the direction of water flow (cell → solution).
Common mistakes
- Forgetting to mention osmosis or the membrane. Saying water moves into the potato instead of out. Using vague terms like "water loss" without linking to osmosis.
Mark scheme (5 marks)
- Water moves out of the potato cells (into the salt solution)
- By osmosis
- The water potential / concentration of water is higher inside the potato cells than in the salt solution
- Water moves through the partially permeable membrane (of the potato cell)
- Loss of water from the cells causes the potato to lose mass
Key terms in this question
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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