In plants, stomata open and close in response to changes in environmental conditions. Explain why stomata need to open and close according to environmental conditions.

Mark scheme

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

1. [3] any three from:
2. [3] 1 need to open to obtain carbon dioxide for, photosynthesis / Calvin cycle ;
3. [3] 2 need to close to prevent water loss by, transpiration / described ;
4. [3] 3 can close, at night / when dark, (to save water) as no photosynthesis ; AW
5. [3] 4 must close to prevent water loss when, high light intensity / high temperatures / high wind speed / water stress / drought conditions ;
6. [6] any six from:
7. [6] 1 H+, pumped / moved by active transport, out (of guard cells);
8. [6] 2 decrease in H+ inside (of guard cells) / proton gradient set up / AW ;
9. [6] 3 K+ channels open / K+ enters (guard cells);
10. [6] 4 water potential (in guard cells), decreases / lowers / becomes more negative;
11. [6] 5 water enters (guard cells) by, osmosis / down the water potential gradient ;
12. [6] 6 cells, expand / become (more) turgid ;
13. [6] 7 outer wall thinner to allow (more), stretching / bending or inner wall thicker to allow less, stretching / bending;
14. [6] 8 AVP; e.g. light activates, ATP production / proton pump sentry of, chloride ions / Cl- (in addition to K+) / entry of water via aquaporins
15. [6] max 3 if stomatal closure described