1
Triple only
[1 mark]What is the function of the air spaces in the spongy mesophyll?
- They absorb light
- They let carbon dioxide diffuse to the cells
- They carry sucrose away
- They stop water loss
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Answer: B B
2
Triple only
[3 marks]Explain how the palisade mesophyll layer is adapted for photosynthesis. (3)
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- near the top / upper surface of the leaf so it receives the most light (1)
- cells contain many chloroplasts (1)
- containing chlorophyll to absorb light for photosynthesis (1)
- cells are tall and tightly packed, so many cells can absorb light (1)
- Max 3
3[2 marks]
Explain why a plant loses water faster on a windy day than on a still day. (2)
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- moving air carries away water vapour from around the stomata / leaf (1)
- so the concentration gradient is steeper / water vapour diffuses out faster (1)
4[3 marks]
A student used a potometer to measure water uptake. In three repeats, the air bubble moved 45 mm, 41 mm and 49 mm in 5 minutes. Calculate the mean rate of water uptake in mm per minute. (3)
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- (45 + 41 + 49) ÷ 3 = 45 mm (1)
- 45 ÷ 5 (1)
- 9 mm per minute (1) allow 9.0; allow correct answer with unit for 3 marks; allow ecf from an incorrect mean
5[2 marks]
A leafy shoot has a mass of 40.0 g. After one day, its mass is 37.0 g. Calculate the percentage loss of mass. (2)
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- (40.0 − 37.0) ÷ 40.0 × 100 (1)
- 7.5% (1) allow correct answer with no working for 2 marks
6
Triple only
[4 marks]Cacti grow in very dry places. Explain how two features of a cactus reduce water loss. (4)
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- leaves are reduced to spines / small leaves (1)
- small surface area so less evaporation (1) allow less water vapour lost
- thick waxy cuticle (1)
- waterproof so less water evaporates (1) allow reduces water loss through the surface
- few stomata / stomata in pits / closed in the day (1) allow stomata open at night
- less water vapour diffuses out (1)
- Max 4
7[6 marks]
Explain how light intensity, air movement and temperature affect the rate of water uptake by a plant. (6)
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| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | All three factors are covered with the correct effect on water uptake and a clear explanation linked to stomata, evaporation or the concentration gradient. Scientific terms are used accurately. |
| 2 | 3–4 | The effect of two or three factors is stated, with some correct explanation. |
| 1 | 1–2 | The effect of one or two factors is stated with little or no explanation. |
Indicative content
- increasing light intensity increases the rate of water uptake because stomata open in the light
- more water vapour diffuses out of the leaf, so more water is pulled up the xylem
- increasing air movement increases the rate because water vapour is carried away from the leaf
- this keeps a steeper concentration gradient so water vapour diffuses out faster
- increasing temperature increases the rate because water evaporates faster from the cells
- water vapour particles also move faster, so diffusion out through the stomata is faster