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

Mark scheme

Marking points from the official mark scheme (May/June 2024). The mark or mark type is shown in brackets.

1. [1] (a) protons / H+ ions, pumped, from guard cells / into guard cell wall ;
2. [1] (a) ref. to negative potential (inside cell) or Cl- ions enter (cell);
3. [1] (a) (voltage-gated) K+ channels open or K+ ions enter (guard) cells ;
4. [1] (a) water enters cell, by osmosis / down water potential gradient ;
5. [1] (a) (guard) cells, become turgid / swell / increase in volume ;
6. [1] (b)(i) as abscisic acid concentration increases stomatal conductance decreases or negative correlation / inverse correlation / inversely proportional ;
7. [1] (b)(i) two sets of x y figures to support mp 1 with units (at least once);
8. [1] (b)(i) steep decrease in conductance to 100-150 ABA ;
9. [1] (b)(i) no / little, effect on conductance above 250–425 ABA ;
10. [1] (b)(ii) ABA causes stomata to close;
11. [1] (b)(ii) ABA binds to receptor on guard cell;
12. [1] (b)(ii) Ca2+ ions, enter (guard) cells / act as 2nd messenger ;
13. [1] (b)(ii) K+ ions leave (guard) cells;
14. [1] (b)(ii) water leaves so cells become, flaccid / less turgid ;
15. [1] (b)(ii) AVP;
16. [1] (c) DNA / promoter ;