1 (a) Fig. 1.1 shows part of a guard cell. [Figure 1.1] (b) With reference to Fig. 1.1, outline the events that occur in a guard cell during times of water stress.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2019). The mark or mark type is shown in brackets.

1. [1] Q - receptor;
2. [1] R - proton / hydrogen ion / H+, pump / carrier ;
3. [1] S - calcium (ion) / Ca2+, channel (protein);
4. [1] T - cell surface membrane / plasma membrane / plasmalemma ;
5. [1] ABA / abscisic acid, binds / attaches, to receptor;
6. [1] calcium ions enter cell ;
7. [1] (Ca2+ is) second messenger;
8. [1] proton / hydrogen ion / H+, exit, stops / is inhibited ;
9. [1] potassium ions / K+, leave (cell) ;
10. [1] water potential / ψ, increases (in, cell / cytoplasm);
11. [1] water leaves (cell), by osmosis OR down, ψ / osmotic, gradient ;
12. [1] cell, volume decreases / becomes flaccid / loses turgidity / loses turgor;