A-LevelBiologyTransport in plants9700/21 · Oct/Nov 2021 · Q59 Marks

A student investigated the effects of air movement on the rate of transpiration by using a potometer.

(a)Define the term transpiration.[2]
(b)The potometer used by the student is shown in Fig. 5.1. [Figure 5.1] The student recorded the distance the air bubble in the apparatus had moved after ten minutes with no air movement. The rate of water uptake was used as a measure of the rate of transpiration. The apparatus was then reset using the reservoir, and the experiment was repeated a further two times. All other variables were standardised during the three experiments. Table 5.1 shows the results recorded by the student. [Table 5.1] Use Table 5.1 to calculate the mean rate of movement of the bubble.[1]
(c)The student carried out another experiment using the same plant. In this experiment a fan was used to blow air across the leaves of the plant. All other variables were standardised. The results showed that the bubble moved further in 10 minutes. The student concluded that air movement increases the rate of transpiration. Explain why air movement increases the rate of transpiration.[2]
(d)Explain how water moves up through a xylem vessel in the stem of the plant in the potometer, shown in Fig. 5.1.[4]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2021). The mark or mark type is shown in brackets.

[2] in context of from a plant loss of water vapour ; I evaporation unqualified
from, the aerial parts / leaves ;
[1] 1.2 (mm min-1) ;
[2] any two from: humid air/water vapour, moves away from the leaves ;
water potential gradient is steeper ;
higher rate of evaporation into the air spaces in the leaf;
[4] any four from: movement by, cohesion-tension;
cohesion / hydrogen bonds, between water molecules ;
A cohesive force
(water moves as an) unbroken column / AW ;
transpiration pull
or column of water, under tension / pulled up;
ref. to adhesion of water molecules to, cellulose / lining / walls (of xylem vessels) ;
A adhesive force
ref. to hydrophilic / polar, property of cellulose (fibres) ;
A hydrophilic / polar, parts of lignin
ref. to water potential;
e.g. (evaporation results in) lower water potential at the top of the xylem vessel
(water moves) down a water potential gradient/from a region of high water potential to a region of lower water potential

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