Street trees in a heatwave
339 words over 31 numbered lines, then 7 parts worth 12 marks (Modular Unit 2 (4WBI2) style, 12–13 marks). Read the article, answer each part, then open its mark scheme and set your mark.
Illustrative article written for Scholar Lab, not a Pearson text. The biology is real; the studies, people and figures are invented or rounded for practice.
Street trees in a heatwave
During a long summer heatwave, gardeners in many cities watched young
street trees droop, with leaves turning brown at the edges. A tree has
no pump. Instead, water is pulled up through the plant as water
evaporates from its leaves, a process called transpiration.
Water enters a plant through its root hair cells. Each root hair is a
long, thin extension of a cell that grows between soil particles, so
the root has a very large surface area. Water moves into a root hair
by osmosis, because the cell sap is a more concentrated solution than
the water in the soil. Mineral ions such as nitrate are taken up as
well.
From the roots, water travels up the stem in xylem vessels, which are
long hollow tubes made from dead cells with walls strengthened by
lignin. In the leaves, water evaporates from the surfaces of mesophyll
cells into the air spaces, and water vapour then diffuses out through
the stomata. As water leaves the top of the plant, more is drawn up
behind it.
In a heatwave the rate of transpiration rises, because water molecules
have more kinetic energy and evaporate faster. Dry air and wind make
the loss even greater. If the roots cannot replace the water quickly
enough, cells lose their turgor and the leaves wilt. Many plants
respond by closing their stomata, which saves water but also stops
carbon dioxide from entering the leaf.
A group of students used a potometer to investigate the effect of wind
on a leafy shoot. They cut the shoot under water and fitted it into
the apparatus, then placed a small electric fan at different distances
to give different wind speeds. They timed how far an air bubble moved
along a capillary tube and used this to work out the volume of water
taken up. Their illustrative results are shown in Figure 1.
Some city councils are now planting tree species with smaller leaves,
thicker waxy cuticles and fewer stomata in streets that are expected
to become hotter and drier.
Figure 1 Rate of water uptake by a leafy shoot at different wind speeds
Read the article and study the figure. Use the information in the article and your own knowledge to answer the questions. The lines in brackets show where to look; select them to highlight the text.
Use the information in the article and your own knowledge to answer the questions. The lines in brackets show you where to look; select them to highlight the text.
State what is meant by the term transpiration.
(1)Explain how water enters a root hair cell from the soil.
(2)Describe the effect of wind speed on the rate of water uptake shown in Figure 1.
(2)Calculate the percentage increase in the rate of water uptake when the wind speed rises from 0 to 3 m s⁻¹ in Figure 1.
(2)Explain why wind increases the rate of transpiration.
(2)A potometer measures the rate of water uptake. Suggest why this is not exactly the same as the rate of transpiration.
(2)Give one control variable the students should have kept the same in this investigation.
(1)Total for this question: 12 marks
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