Exam questions · Biology · Plant Adaptations and Hormones
Plants in Extreme Environments
- 6 exam questions
- 18 marks
- 9 quick checks
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1 State [1 mark]
State what is meant by an adaptation.
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Model answer
A feature of an organism that helps it to survive in its environment.
Mark scheme
- A feature that helps survival — 1 mark
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2 State [2 marks]
State two ways in which the leaves of a plant from a desert reduce the loss of water.
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Model answer
Any two from: small leaves, or spines, which have a small surface area; a thick waxy cuticle; few stomata, or stomata sunk in pits; stomata open only at night.
Mark scheme
- First — 1 mark
- Second — 1 mark
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3 Explain [3 marks]
Explain how rolled leaves help marram grass to survive on a windy sand dune.
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Model answer
The stomata are on the inside of the roll, where moist air is trapped. This reduces the concentration gradient of water vapour between the leaf and the air, so less water diffuses out of the leaf, and the wind does not blow away the humid air.
Mark scheme
- Stomata are inside the roll — 1 mark
- Humid air trapped — 1 mark
- Smaller gradient, so less water lost — 1 mark
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4 Explain [3 marks]
The stomata of a water lily are on the upper surface of its leaves, but the stomata of a bean plant are mostly on the lower surface. Explain why.
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Model answer
The lower surface of the water lily leaf is in contact with the water, so gases cannot enter there, and the upper surface is in the air. In the bean plant, having the stomata on the lower surface in the shade reduces water loss.
Mark scheme
- Lower surface of the water lily is in water — 1 mark
- Upper surface in contact with air for gas exchange — 1 mark
- Bean stomata on the lower surface lose less water — 1 mark
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5 Calculate [3 marks]
A flat leaf is modelled as a block 5 cm by 5 cm by 0.2 cm. Calculate its surface area to volume ratio. Give your answer to the nearest whole number.
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Model answer
Surface area = 2 × 25 + 4 × (5 × 0.2) = 50 + 4 = 54 cm squared. Volume = 5 × 5 × 0.2 = 5 cm cubed. Ratio = 54 ÷ 5 = 10.8, which is 11 : 1.
Mark scheme
- Surface area 54 — 1 mark
- Volume 5 — 1 mark
- Ratio 11 : 1 — 1 mark
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6 Evaluate [6 marks]
A student suggests that a plant with very large leaves and many stomata would survive well in a desert. Evaluate this idea, using ideas about leaf size and shape, the cuticle and the stomata.
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Model answer
Large leaves have a large surface area, so they would lose a lot of water by transpiration, and in a desert there is very little water to replace it. Many stomata would also let more water vapour escape, especially in the heat of the day. The plant would probably wilt and die. A large surface area does catch a lot of light for photosynthesis, which would be a benefit, and many stomata would let in a lot of carbon dioxide. However, desert plants have small leaves or spines, a thick cuticle, and few stomata, which may be open at night, because the loss of water is a much greater problem than a shortage of light. The idea is therefore a poor one.
Mark scheme
- Large leaves lose a lot of water — 1 mark
- Water is scarce in a desert — 1 mark
- Many stomata let more water vapour escape — 1 mark
- Large surface area gives more light for photosynthesis — 1 mark
- Desert plants have small leaves, a thick cuticle and few stomata — 1 mark
- A reasoned conclusion — 1 mark
Quick check
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1
What are the leaves of a cactus adapted into?
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C: Spines
They are reduced to spines, which have a very small surface area.
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2
Why do many desert plants have a thick waxy cuticle?
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A: To reduce the loss of water
It reduces the loss of water through the surface.
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3
Where are the stomata of a water lily leaf found?
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D: On the upper surface
They are on the upper surface, which is in contact with the air.
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4
Why are sunken stomata useful in a dry environment?
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B: They trap humid air
The pit traps humid air, which slows the diffusion of water vapour out of the leaf.
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5
Which statement about a water lily leaf is correct?
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C: It has a thin cuticle
It has a thin cuticle, because water is not short.
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6
A cube has a surface area of 24 cm squared and a volume of 8 cm cubed. What is the surface area to volume ratio?
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A: 3 : 1
\(24 \div 8 = 3\), so the ratio is 3 : 1.
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7
Why do some cacti open their stomata only at night?
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D: Less water is lost in the cooler air
It is cooler at night, so less water is lost.
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8
Why is a low, rounded cushion shape useful for a plant on a cold, windy mountain?
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B: It reduces the wind and the surface area
It gives shelter from the wind, and has a small surface area for its volume.
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9
A desert plant has 60 stomata per square millimetre and a leaf from a bean plant has 240. What percentage fewer stomata does the desert plant have?
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C: 75%
\((240 - 60) \div 240 \times 100 = 75\%\).