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Date May 2018 Marks available 2 Reference code 18M.3.HL.TZ2.3
Level Higher level Paper Paper 3 Time zone Time zone 2
Command term Explain Question number 3 Adapted from N/A

Question

The graph shows the effect of relative humidity on the transpiration rate in Chinese privet (Ligustrum sinense).

The rate of water uptake was measured using the potometer shown in the diagram. The diameter of the capillary tube is 1 mm.

Outline how the rate of water uptake is recorded in this potometer.

[2]
a.

State a variable that needs to be controlled in this experiment.

[1]
b.

Explain the effect of relative humidity on the rate of water uptake.

[2]
c.

Markscheme

a. measure distance «of movement» of air bubble/water in capillary tube 

b. multiply by cross section of capillary 

c. record/divide by time elapsed

Allow any other valid method.

a.

temperature
OR
atmospheric pressure
OR
solar radiation/light
OR
wind speed
OR
leaf area
OR 
size of plant

Do not accept humidity.

b.

a. increased «relative» humidity decreases water uptake
OR
inverse relationship 

b. increased «relative» humidity lowers transpiration «rate» 

c. diffusion gradient reduced «as humidity increases» 

d. less loss of water through stomata
OR
stomata closed

Accept inverse for dry/low humidity.

c.

Examiners report

[N/A]
a.
[N/A]
b.
[N/A]
c.

Syllabus sections

Additional higher level » Topic 9: Plant biology » 9.1 Transport in the xylem of plants
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Additional higher level » Topic 9: Plant biology
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