Question 1
Paper 1A style
A cell is placed in a solution that is isotonic to its cytoplasm. Which statement is correct?
- No water molecules cross the cell membrane
- Water moves into and out of the cell at equal rates
- Water moves into the cell faster than it leaves
- Water moves out of the cell faster than it enters
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Answer: B
Question 2
Paper 2A style
Explain how sodium chloride and glucose dissolve in water.
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- water molecules are polar / have δ+ hydrogen and δ− oxygen [1]
- Na⁺ ions are attracted to the δ− oxygen of water molecules [1]
- Cl⁻ ions are attracted to the δ+ hydrogen of water molecules [1]
- glucose is polar and forms hydrogen bonds with water molecules [1]
- each solute particle becomes surrounded by water molecules / solvation [1]
- max 3
Question 3
Paper 1B style
Potato cylinders were placed in sucrose solutions for 60 minutes. Five repeats were done at each concentration (the data are for practice). (a) Calculate the percentage change in mass of a cylinder that went from 1.60 g to 1.76 g. [1] (b) Using the means, estimate the isotonic sucrose concentration of the potato tissue, showing how you did this. [2] (c) State what the standard error indicates and deduce which concentration gave the least reliable mean. [2] (d) Suggest one variable that should be controlled. [1]
| Sucrose concentration / mol dm⁻³ | Mean percentage change in mass / % | Standard error / % |
|---|---|---|
| 0.0 | +18.0 | 1.1 |
| 0.2 | +7.5 | 0.9 |
| 0.4 | −3.0 | 2.6 |
| 0.6 | −11.5 | 1.0 |
| 0.8 | −17.0 | 1.2 |
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- (a) (1.76 − 1.60) ÷ 1.60 × 100 = +10% [1]
- (b) isotonic point is where percentage change is zero / line crosses the x-axis [1]
- between 0.2 and 0.4 mol dm⁻³, about 0.34 mol dm⁻³ [1] Accept 0.32–0.36
- (c) standard error shows how precisely the mean is known / how close the sample mean is likely to be to the true mean [1] Do not accept spread of the data alone (that is standard deviation)
- 0.4 mol dm⁻³, as it has the largest standard error [1]
- (d) one of: temperature / size or surface area of cylinders / source of potato / time / volume of solution / blotting method [1]
Question 4
Paper 2A style
Compare and contrast the effects of a hypotonic medium on a red blood cell and on a plant cell, and explain why fluids given intravenously are isotonic.
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- both: water enters the cell by osmosis [1]
- red blood cell swells and may burst / haemolysis, as it has no cell wall [1]
- plant cell becomes turgid / develops turgor pressure, but the cell wall prevents bursting [1]
- isotonic IV fluids cause no net water movement into or out of blood cells [1]
- so blood cells do not burst or crenate / shrink [1]
- max 4