A student places a piece of potato tissue into a concentrated sugar solution and leaves it for 30 minutes. When removed, the potato tissue is noticeably smaller and has become flaccid. Explain why the potato tissue changed in this way.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Potato cells contain a vacuole filled with cell sap. The cell membrane acts as a selectively permeable barrier. When potato tissue is placed in a concentrated sugar solution, water moves across the cell membrane.
Model answer (5 marks)
1. The sugar solution has a lower water concentration than the cell sap inside the potato cells.
2. Water therefore moves out of the cells by osmosis.
3. Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration.
4. The vacuole loses water, so the cell contents shrink away from the cell wall – the cell becomes plasmolysed.
5. This loss of water reduces turgor pressure, so the tissue decreases in size and becomes flaccid.
2. Water therefore moves out of the cells by osmosis.
3. Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration.
4. The vacuole loses water, so the cell contents shrink away from the cell wall – the cell becomes plasmolysed.
5. This loss of water reduces turgor pressure, so the tissue decreases in size and becomes flaccid.
Examiner tips
- Use the word ‘osmosis’ and explain its direction; link water concentration to movement. Show the sequence: lower concentration → water leaves → plasmolysis → loss of turgor → flaccidity. Include the term ‘turgor pressure’ to demonstrate understanding of cell support.
Common mistakes
- Confusing the direction of water flow (claiming water moves into the cells). Forgetting to mention plasmolysis or turgor pressure. Using vague terms like ‘shrinks’ without explaining the osmotic mechanism.
Mark scheme (5 marks)
- The concentrated sugar solution has a lower water concentration than the cell sap inside the potato cells
- Water moves out of the potato cells by osmosis
- Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration
- The vacuole loses water / the cell contents shrink away from the cell wall (the cell becomes plasmolysed)
- This loss of water causes the tissue to decrease in size / become flaccid because the cells no longer have turgor pressure supporting them
Related
- All Eduqas A-Level Biology revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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