DP Biology (first assessment 2025)
Question 21M.2.HL.TZ1.b
| Date | May 2021 | Marks available | [Maximum mark: 5] | Reference code | 21M.2.HL.TZ1.b |
| Level | HL | Paper | 2 | Time zone | TZ1 |
| Command term | Outline | Question number | b | Adapted from | N/A |
b.
[Maximum mark: 5]
21M.2.HL.TZ1.b
Outline how the hormone auxin controls phototropism in plant shoots.
[5]
Markscheme
a. grows/bends towards (brightest) light/sun ✔
b. auxin moved from lighter to shadier side (of shoot/stem tip/apex) ✔
c. moved by auxin efflux pumps ✔
d. auxin promotes cell elongation/cell growth / auxin causes cell wall acidification/loosening ✔
e. more growth on shady side of stem (due to auxin concentration gradient) ✔
f. binds to auxin receptors (in target cells) ✔
g. auxin/auxin receptors promote expression of genes (for growth)/for H+ secretion into wall ✔
Examiners report
Candidates tended either to know the mechanism of phototropism very well, or not at all. It wasn't always clear that the terms shoot, stem and leaf were correctly distinguished in some candidates' minds.
