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CAIE A-Level Biology A2 14.2.4 Abscisic Acid and Stomata Questions

Explain abscisic acid and its effects on stomata using evidence from each question and mark scheme.

Syllabus
2028–2030
Course
Biology 9700
Level
A2

Exam points

  • Explain abscisic acid and its effects on stomata.

CAIE A-Level Biology A2 14.2.4 Abscisic Acid and Stomata Questions question 1

[Maximum number: 6]

Stomata in leaves respond to changes in environmental conditions by opening and closing. This regulates carbon dioxide uptake and water loss.

Question (a)

(a)

Stomatal conductance is a measure of the rate of water vapour loss from the intercellular air spaces of leaves to the atmosphere through the stomata.

A student investigated the effect of different concentrations of abscisic acid (ABA) on stomatal conductance in Helianthus annuus, the common sunflower.

The student treated the roots of 15 sunflower plants with different concentrations of ABA solution. The plants were left in standardised conditions for 24 hours.

After 24 hours, the student measured:
- the concentration of ABA in the xylem sap of each plant
- the stomatal conductance of the leaves of each plant.

The results are shown in Fig. 3.1.

Fig. 3.1

Fig. 3.1

[ 6 ]

Question (i)

(i)

With reference to Fig. 3.1, describe the relationship between the concentration of A B A in the xylem sap and stomatal conductance.

[ 2 ]

Question (ii)

(ii)

Suggest explanations for the results shown in Fig. 3.1.

[ 4 ]
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