A student investigates osmosis using pieces of carrot. She places one piece of carrot into distilled water and another piece into a concentrated sugar solution for one hour. After one hour, the piece in distilled water feels firm, while the piece in the concentrated sugar solution feels soft and flexible. Explain why the two pieces of carrot feel different after one hour.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Osmosis is the movement of water molecules across a partially permeable membrane from an area of higher water concentration to an area of lower water concentration.
Model answer (5 marks)
Osmosis is the movement of water molecules across a partially permeable membrane.
In distilled water the water concentration outside the carrot cells is higher than inside, so water moves into the cells.
The influx of water makes the cells turgid and the carrot feels firm.
In the concentrated sugar solution the water concentration inside the cells is higher than outside, so water leaves the cells.
Loss of water causes the cells to plasmolyse, reducing turgor and making the carrot soft and flexible.
In distilled water the water concentration outside the carrot cells is higher than inside, so water moves into the cells.
The influx of water makes the cells turgid and the carrot feels firm.
In the concentrated sugar solution the water concentration inside the cells is higher than outside, so water leaves the cells.
Loss of water causes the cells to plasmolyse, reducing turgor and making the carrot soft and flexible.
Examiner tips
- Use the definition of osmosis first, then explain the direction of water movement for each solution, linking it to turgor and the feel of the carrot.
- Show the cause–effect chain: water concentration → water movement → cell turgor → physical property.
Common mistakes
- Confusing osmotic pressure with hydrostatic pressure; not stating that the membrane is partially permeable.
- Saying the carrot becomes firm because it swells, but not mentioning turgor or plasmolysis.
Mark scheme (5 marks)
- Osmosis is the movement of water molecules across a partially permeable membrane
- Water moves into the carrot in distilled water because the water concentration outside is higher than inside the carrot cells
- Water entering the cells causes them to become turgid / swell, making the carrot firm
- Water moves out of the carrot cells in concentrated sugar solution because the water concentration inside the carrot cells is higher than in the surrounding solution
- Loss of water causes the carrot cells to become plasmolysed / lose turgor, making the carrot soft and flexible
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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