Water uptake & transpiration: IGCSE Biology questions with answers
Free Water uptake & transpiration questions for IGCSE Biology (0610), part of Transport. Each question has the answer and the marking points an examiner would look for.
Any 4 of: root hair cells (osmosis); across the root cortex; up the xylem; to mesophyll cells; evaporates into air spaces and diffuses out through stomata B4
4
(b)
loss of water vapour from leaves
loss of water vapour from the leaves B1 by evaporation from mesophyll surfaces and diffusion through stomata B1
2
Question 4Structured · 4 marks
Transpiration has advantages for plants.
(a)
State two functions of transpiration.
[2]
(b)
Explain why plants may wilt on hot, dry, windy days.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
transport of water and minerals; cooling; turgidity
Any 2 of: transport of water and mineral ions; cooling the leaf; keeping cells turgid B2
2
(b)
transpiration faster than uptake
water is lost by transpiration faster than it is absorbed by the roots B2
2
Question 5Structured · 13 marks
A student uses a potometer to measure water uptake by a leafy shoot. The air bubble moves 37.5 mm in 5 minutes. The capillary tube has a radius of 0.50 mm.
(a)
Define transpiration.
[2]
(b)
Calculate the rate of movement of the bubble in mm per minute.
[1]
(c)
Calculate the volume of water taken up in this time, using volume = πr²l.
[2]
(d)
Explain why the shoot is cut under water when setting up the potometer.
[1]
(e)
A fan is switched on near the shoot. Predict and explain the effect on the bubble.
[3]
(f)
State two other environmental factors that increase the rate of transpiration.
[2]
(g)
Explain why a potometer measures water uptake rather than transpiration rate.
[2]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
loss of water vapour from leaves; evaporation from mesophyll surfaces then diffusion out through stomata
loss of water vapour from the leaves B1 by evaporation from the surfaces of mesophyll cells and diffusion through the stomata B1
2
(b)
7.50 mm/min
37.5/5 = 7.50 mm/min B1
1
(c)
29.5 mm³
π × 0.50² × 37.5 M1 29.5 mm³ A1
2
(d)
to stop air entering the xylem
to prevent air bubbles entering / blocking the xylem B1
1
(e)
moves faster; wind removes water vapour, steeper concentration gradient, more diffusion
bubble moves faster B1 moving air removes water vapour from around the leaf B1 maintaining a steep concentration / diffusion gradient so more water vapour diffuses out B1
some of the water taken up is used in photosynthesis / to keep cells turgid B1 so uptake is not exactly equal to the water lost B1
2
Question 6Structured · 14 marks
A student uses a potometer to measure water uptake by a leafy shoot. The distance moved by an air bubble in 5 minutes is recorded under three conditions. Table 6.1 shows the results.
condition
still air, 20 °C
moving air (fan), 20 °C
still air, high humidity, 20 °C
distance moved in 5 min / mm
20
42
9
Table 6.1
(a)
Calculate the rate of water uptake in still air, in mm per minute.
[1]
(b)
The capillary tube has a cross-sectional area of 0.80 mm². Calculate the volume of water taken up per minute in moving air.
[2]
(c)
Explain why water uptake is greater in moving air.
[2]
(d)
Explain why water uptake is lowest in high humidity.
[2]
(e)
A potometer measures water uptake, not transpiration. Suggest why these are not exactly the same.
[1]
(f)
State two precautions needed when setting up a potometer, and explain one of them.
[3]
(g)
Describe the pathway of water from the soil to the air around the leaf.
[3]
Show answers and mark scheme
Part
Answer and marking points
Marks
(a)
4.0 mm/min
20 ÷ 5 = 4.0 B1
1
(b)
6.72 mm³ per minute
42/5 = 8.40 mm per min C1 × 0.80 = 6.72 mm³ per min A1
2
(c)
water vapour blown away; steeper concentration gradient
water vapour is removed from around the leaf B1 maintaining a steep water vapour concentration (diffusion) gradient so more water evaporates / diffuses out of stomata B1
2
(d)
air already moist; gradient shallow; less diffusion
air outside the leaf already contains much water vapour B1 so the concentration gradient is smaller and less water vapour diffuses out B1
2
(e)
some water used in photosynthesis / kept in cells
some water taken up is used in photosynthesis or to keep cells turgid B1
1
(f)
cut shoot under water; seal joints with vaseline; dry leaves
two precautions from: cut the shoot under water / joints sealed or airtight / leaves dry / leave the shoot to adjust before reading B2 one correct explanation, e.g. stops air entering the xylem and blocking water flow / prevents leaks B1
Any 3 of: enters root hair cells by osmosis; moves through cortex to xylem; up the xylem in the stem to the leaf; evaporates from mesophyll cells; diffuses out through stomata B3
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