2.18 Transpiration and translocation
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Define transpiration.
Mark scheme
- loss of water (vapour) from the leaves / plant (1)
- by evaporation and diffusion through the stomata (1)
Model answer
Transpiration is the loss of water vapour from a plant's leaves, by evaporation and diffusion out through the stomata.
Describe how water moves from the soil to the air around a leaf.
Mark scheme
- water enters root hair cells by osmosis (1)
- moves up the xylem (1)
- evaporates from (mesophyll) cells inside the leaf (1)
- (water vapour) diffuses out through the stomata (1)
Model answer
Water enters the root hair cells from the soil by osmosis. It travels up the stem in the xylem to the leaves. There it evaporates from the surfaces of the mesophyll cells and the water vapour diffuses out of the leaf through the stomata.
Explain why the rate of transpiration increases on a windy day.
Mark scheme
- wind removes / blows away water vapour from around the leaf (1)
- maintains a steep concentration gradient (so faster diffusion) (1)
Model answer
Wind blows the water vapour away from the leaf surface, keeping a steep concentration gradient, so water vapour diffuses out faster.
Explain why the rate of transpiration is lower when the air is humid.
Mark scheme
- there is more water vapour in the air (1)
- so the concentration gradient is shallower / less diffusion out of the leaf (1)
Model answer
Humid air already holds a lot of water vapour, so the concentration gradient between the inside of the leaf and the air is smaller, and diffusion is slower.
Through which tissue does translocation take place?
Choose one answer.
Correct Not quite: the answer is Phloem
Mark scheme
B: phloem
Model answer
B: phloem, which carries dissolved sugars.
Give two reasons why transpiration is useful to a plant.
Mark scheme
Any two from:
- supplies water for photosynthesis (1)
- carries mineral ions from the roots to the leaves (1)
- keeps cells turgid / supports the plant (1)
- cools the leaves (1)
Model answer
It carries mineral ions from the roots to the leaves, and it cools the leaves as water evaporates.
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