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Biology · Oct / Nov 2025 · Paper 41 · Question 5

0610

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A student investigated the effect of humidity on water vapour loss in plant leaves. As water vapour was lost, the mass of the leaves decreased. Similar-sized leaves were collected from one species of plant. The leaves were divided into two groups, A and B. The leaves in group A were placed in a low humidity environment and the leaves in group B were placed in a high humidity environment. The leaves in each group were kept in their environment for 12 hours. The temperature in each environment was maintained at 20 °C. The mass of the leaves in each group was measured at the start of the investigation and then every hour throughout the 12-hour period. The student calculated the mass of water vapour lost as a percentage of the starting mass of the leaves for each group. Fig. 5.1 shows the student’s results. 0 0 5 10 15 20 25 30 35 1 2 3 4 5 6 time / hours mass of water vapour lost from the leaves as a percentage of the starting mass 7 8 9 10 11 12 A key B low humidity environment high humidity environment Fig. 5.1

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  • 5(a)(i)[1]

    transpiration / diffusion ;

  • 5(a)(ii)[5]

    any five from:

    comparison

    1

    both lose mass ;

    2

    higher humidity, less water vapour is lost ;

    3

    ref. to steeper rate / greater water lost, initially (for both groups) ; 4

    comparative data quote with units ;

    explanation for high humidity

    5

    (higher humidity) greater amount of water vapour in the atmosphere ; 6

    reduced water potential, gradient ;

    7

    less water vapour lost, through stomata / between guard cells ;

  • 5(a)(iii)[1]

    line drawn on the graph above the low humidity line ;

  • 5(b)(i)[3]

    any three from:

    glands in the skin produce sweat ;

    sweat on (the surface of) the skin evaporates ;

    using / AW, heat / energy (from the body) ;

    (internal) body temperature, decreases / returns, to a set point ; ref. to homeostasis / negative feedback ;

  • 5(b)(ii)[1]

    expiration / AW ;

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