Question 21N.2.HL.TZ0.5
| Date | November 2021 | Marks available | [Maximum mark: 7] | Reference code | 21N.2.HL.TZ0.5 |
| Level | HL | Paper | 2 | Time zone | TZ0 |
| Command term | Compare and contrast, Explain, Outline | Question number | 5 | Adapted from | N/A |
Compare and contrast the mode of nutrition of detritivores and saprotrophs.
[2]
Accept not autotrophic/not photosynthetic instead of heterotrophic.
Do not accept that both groups are decomposers or consumers for the similarity.
About half of candidates answered correctly and there were some well-informed answers, but also many that showed a lack of familiarity with nutrition in detritivores and saprotrophs.
Explain how some plant species are able to respond to changes in their abiotic environment and flower at a precise time of the year.
[3]
a. genes for flowering are activated/gene activation/changes to gene expression;
b. shoot apex changes from producing leaves/stem to producing flowers;
c. daylength/duration of the day/night length/photoperiod measured/detected/responded to;
d. short day plants flower when they have a long night/period of darkness
OR
long day plants only flower when they have a short night/period of darkness;
e. so short day plants/SDPs flower in late summer/fall/autumn/winter
OR
so long day plants/LDPs flower in spring/(early) summer;
The only relevant changes in the abiotic environment were night length variation over the seasons of the year, which determines when flowering should occur. There were many complicated answers describing the interconversion of the forms of phytochrome, but according to the syllabus this level of detail is not expected and often the simpler ideas that plants can measure night length and respond by the timing of flowering in the year were omitted. Also mostly missing, were the idea of changes to gene expression in the shoot apex, so floral organs start to develop instead of leaves. The average score for this question was only slightly higher than one mark, but the correlation coefficient was high.
Outline the extension of the stem in plants.
[2]
a. apical meristem (of shoot/stem) produces cells/elongates the stem
OR
cell division/mitosis in tip/apex of shoot/stem;
b. auxin stimulates cell/stem growth/extension/enlargement;
c. elongation of cells causes stem to grow (in length);
Again, accounts were varied, with stronger ones clearly explaining how the shoot apical meristem generates cells by mitosis and how elongation of these cells, stimulated by auxin, causes stem elongation. Some candidates were side-tracked by phototropism but were able to score some marks from among irrelevant ideas.