Explain the role of auxin in cell elongation.

Mark scheme

Marking points from the official mark scheme (Feb/Mar 2018). The mark or mark type is shown in brackets.

1. [8] any eight from:
2. 1 auxin binds to receptor;
3. 2 in cell surface membrane ;
4. 3 (auxin) increases proton pump activity / described ;
5. 4 (more) protons enter cell wall ;
6. 5 cell wall, becomes more acidic / has reduced pH ;
7. 6 expansins activated;
8. 7 (expansins) loosen / break, bonds between (cellulose) microfibrils ;
9. 8 K⁺ enters cell ;
10. 9 water potential of cell decreases ;
11. 10 more water can enter cell by osmosis / AW ;
12. 11 turgor pressure / described ;
13. 12 ref. to acid growth hypothesis ;
14. [7] any seven from:
15. 1 idea that phenotype results from interaction of genotype and environment ;
16. 2 environment may, limit / modify, expression of gene(s) / AW ;
17. 3 continuous variation example ; e.g. size / mass / height
18. 4 qualified; e.g. because, food / nutrients / ions, missing or malnutrition occurs
19. 5 environment may, trigger / switch on, gene ;
20. 6/7 two named examples ; ;
21. e.g. temperature and change in animal colour
22. high temperature and gender in crocodiles
23. UV light and melanin production
24. wavelength of light and, flowering / fruit colour
25. 8 environment effect usually greater on polygenes ;
26. 9 environment may induce mutation (affecting phenotype);
27. 10 AVP;