Which statement defines osmosis?
- The movement of particles from a low to a high concentration using energy
- The spreading out of particles of a gas
- The diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
- The movement of any particles from a high to a low concentration
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Answer: C C (1)
Oxygen enters a cell by diffusion. Explain how each of these changes would affect the rate of diffusion: (a) increasing the temperature, (b) increasing the concentration of oxygen outside the cell, (c) increasing the surface area of the membrane.
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- (a) rate increases because particles have more energy / move faster (1)
- (b) rate increases because the concentration gradient is steeper / difference in concentration is bigger (1)
- (c) rate increases because more particles can pass through at once (1)
A cube has sides of 5 cm. Calculate its surface area to volume ratio. Give your answer in the form X : 1.
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- surface area = 6 × 5 × 5 = 150 cm² (1)
- volume = 5 × 5 × 5 = 125 cm³ (1)
- 150 ÷ 125 = 1.2, so 1.2 : 1 (1)
- Correct answer gains all 3 marks.
A single-celled organism does not need a transport system or special exchange surfaces. A large multicellular organism does. Explain why.
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- single-celled organism has a large surface area to volume ratio (1)
- so enough substances can diffuse in and out across the surface to meet its needs (1)
- multicellular organism has a small surface area to volume ratio (1)
- so diffusion across the outside is not enough / diffusion distance too far for inner cells (1)
- so exchange surfaces and a transport system are needed to move molecules in and out of cells (1)
- Max 4.
Explain how the gills of a fish are adapted for exchanging materials.
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- many gill filaments / lamellae give a large surface area (1)
- thin (membrane) gives a short diffusion path (1)
- good blood supply to maintain the concentration gradient (1)
- water flows over the gills / ventilated, keeping the concentration gradient (1)
- Max 3.
A potato cylinder has a mass of 5.0 g. After 30 minutes in a sugar solution its mass is 5.6 g. (a) Calculate the percentage change in mass. (b) Calculate the mean rate of water uptake in g per minute.
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- (a) change in mass = 5.6 − 5.0 = 0.6 g (1)
- (a) 0.6 ÷ 5.0 × 100 (1)
- (a) = 12% (increase) (1) allow +12%
- (b) 0.6 ÷ 30 = 0.02 g/min (1)
A student puts identical potato cylinders into sugar solutions of different concentrations. The table shows the results. Explain why the mass increased in the 0.0 mol/dm³ solution, and estimate the concentration at which there would be no change in mass.
| Concentration of sugar solution (mol/dm³) | Percentage change in mass (%) |
|---|---|
| 0.0 | +18 |
| 0.2 | +9 |
| 0.4 | +1 |
| 0.6 | −7 |
| 0.8 | −14 |
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- water moves into the cells by osmosis (1)
- from the dilute solution to the more concentrated solution (inside the cells) through the partially permeable membrane (1)
- estimate of 0.4 to 0.45 mol/dm³ (1)
Root hair cells contain many mitochondria. Explain how this helps the root hair cell to absorb mineral ions from the soil.
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- mitochondria release energy by respiration (1)
- needed for active transport of mineral ions (against a concentration gradient) (1)
Describe how substances are transported into and out of cells by diffusion, osmosis and active transport. Explain the differences between the three processes.
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| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A clear description of all three processes, with accurate explanation of the differences in direction, what moves and energy. Examples and key terms are used correctly. |
| 2 | 3–4 | Two or three processes described correctly, with some attempt to compare them, but with some gaps. |
| 1 | 1–2 | Some correct statements about one or two processes, with little or no comparison. |
Indicative content
- diffusion: net movement of particles (gas or in solution) from a higher to a lower concentration (down a gradient)
- examples of diffusion: oxygen and carbon dioxide in gas exchange, urea from cells into blood plasma
- osmosis: diffusion of water from a dilute solution to a concentrated solution through a partially permeable membrane
- active transport: moves substances from a more dilute to a more concentrated solution (against the concentration gradient)
- active transport requires energy from respiration, whereas diffusion and osmosis do not
- examples of active transport: mineral ions into root hairs, sugar from the gut into the blood
- osmosis only involves water