Question 1
Cylinders of potato were placed in sugar solutions of different concentrations. The percentage change in mass was measured after 60 minutes. (a) Use the data to estimate the concentration of sugar solution that has no effect on the mass of the potato. [1] (b) Explain why the potato cylinder in 0.0 mol/dm³ increased in mass. [2] (c) State one variable that must be kept constant in this investigation. [1]
| Concentration / mol/dm³ | Change in mass / % |
|---|---|
| 0.0 | +18 |
| 0.2 | +7 |
| 0.4 | −4 |
| 0.6 | −13 |
| 0.8 | −20 |
Show mark scheme for question 1
- between 0.30 and 0.35 mol/dm³ (1) allow 0.3 mol/dm³
- water entered the potato cells (1)
- by osmosis through the (partially permeable) cell membrane (1)
- any one from: temperature / volume of solution / time in solution / size (surface area) or mass of cylinder / same type of potato (1) ignore 'amount of potato'
Question 2
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanTwo solutions are separated by a partially permeable membrane. Solution X is dilute. Solution Y is concentrated. What is the net movement of water?
- From X to Y, because X has a lower water potential
- From X to Y, because X has a higher water potential
- From Y to X, because Y has a higher water potential
- There is no net movement because the membrane is partially permeable
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Answer: B (1)
Question 3
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanA piece of onion epidermis is placed in a concentrated sugar solution. When viewed under a microscope the cell membranes have pulled away from the cell walls. Use the terms plasmolysis, flaccid and water potential to explain what has happened.
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- the sugar solution has a lower water potential than the cell contents (1)
- water leaves the cell by osmosis (1)
- through the partially permeable cell membrane (1)
- the cell loses turgor pressure and becomes flaccid (1)
- the cytoplasm shrinks and the cell membrane pulls away from the cell wall: this is plasmolysis (1)
Question 4
Supplement (what this means)
Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels meanA red blood cell and a plant cell are each placed in distilled water. The red blood cell bursts but the plant cell does not. Explain these results.
Show mark scheme for question 4
- distilled water has a higher water potential than the cells / cell contents (1)
- water enters both cells by osmosis (1)
- the red blood cell has no cell wall, so it swells and bursts (1)
- the plant cell wall resists the expansion, so the cell becomes turgid (1) allow 'turgor pressure stops more water entering'