2.20 Plants in extreme environments
5 exam questions worth 11 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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Explain how having spines instead of leaves helps a cactus survive in the desert.
Mark scheme
- smaller surface area (1)
- so less water lost by transpiration / evaporation (1)
Model answer
Spines have a much smaller surface area than leaves, so less water is lost by transpiration.
Marram grass grows on sand dunes. Its leaves roll up in dry weather and it has hairs and sunken stomata on the inside of the leaf.
Explain how these features reduce water loss.
Mark scheme
- rolled leaf / hairs / sunken stomata trap (moist) air (1)
- air near the stomata has a high concentration of water vapour / is humid (1)
- reduces the concentration gradient (1)
- so less water vapour diffuses out / less transpiration (1)
Model answer
Rolling the leaf, the hairs and the sunken stomata all trap a layer of moist air next to the stomata. The air there has a high concentration of water vapour, so the concentration gradient between the inside of the leaf and the air is smaller. Less water vapour diffuses out, so less water is lost by transpiration.
Give the function of a thick waxy cuticle on a desert plant.
Mark scheme
reduces water loss / evaporation (from the surface) / waterproofing
Model answer
It waterproofs the surface and reduces the water lost by evaporation.
Pine trees grow in cold places where the ground is frozen for months.
Suggest why they need adaptations to reduce water loss.
Mark scheme
- water in the soil is frozen (1)
- so it cannot be taken up by the roots / water is hard to replace (1)
Model answer
When the soil water is frozen, the roots cannot take it up, so any water lost from the leaves is hard to replace.
Some desert plants open their stomata only at night.
Suggest one advantage and one disadvantage of this.
Mark scheme
- advantage: cooler at night so less water lost by evaporation / transpiration (1)
- disadvantage: less CO2 taken in during the day / less photosynthesis (1)
Model answer
It is cooler at night, so less water evaporates. But carbon dioxide can only enter at night, which may limit photosynthesis during the day.
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