Question 19M.2.HL.TZ2.1e
| Date | May 2019 | Marks available | [Maximum mark: 3] | Reference code | 19M.2.HL.TZ2.1e |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Discuss | Question number | e | Adapted from | N/A |
Organisms often release chemicals when attacked as part of their defence system. Scientists studied lima bean plants (Phaseolus lunatus) infested with either an armyworm, Spodoptera exigua, or a herbivorous mite, Tetranychus urticae. Both organisms feed on lima bean leaves, causing the leaves to release chemicals.
[Source: https://commons.wikimedia.org/wiki/File:Phaseolus_lunatus_Blanco2.369.png]
![https://commons.wikimedia.org/wiki/File:Phaseolus_lunatus_Blanco2.369.png]](https://commons.wikimedia.org/wiki/File:Phaseolus_lunatus_Blanco2.369.png%5D){kind=link}
The study was conducted to see which defence chemicals were produced by lima bean leaves when infested by armyworms or herbivorous mites. The scientists identified a mixture of compounds (C) released by the plant when attacked. Two of the chemicals in this mixture were identified (C1 and C2).
The scientists hypothesized that the defence chemicals in C act as signals to produce other chemicals (X, Y and Z) that are also involved in the defence of the plant.
The graphs show the amounts of chemicals X, Y and Z produced when the plants were infested by either one of the two herbivores or treated with the different chemicals C1 or C2.
[Source: R Ozawa and G Arimura, Involvement of Jasmonate- and Salicylate-Related Signaling Pathways for the
Production of Specific Herbivore-Induced Volatiles in Plants, Plant and Cell Physiology, 2000, 41, 4, 391–398,
by permission of Oxford University Press]
RNA was collected from leaves of the plants after each treatment (armyworm, herbivorous mite and the chemicals C1 and C2). DNA copies of the extracted RNA were made by a process called reverse transcription. Targeted genes in the DNA were then amplified.
The scientists then used the transcribed DNA of each treatment to study the activation of three genes of the plants’ defence system. The DNA was separated by gel electrophoresis. The activation was tested one hour after treatment and again after 24 hours.
[Source: R Ozawa and G Arimura, Involvement of Jasmonate- and Salicylate-Related Signaling Pathways for the
Production of Specific Herbivore-Induced Volatiles in Plants, Plant and Cell Physiology, 2000, 41, 4, 391–398,
by permission of Oxford University Press]
Each gene is activated by one or more of the treatments. From the gel electrophoresis data, discuss the impact of the herbivorous mite infestation on gene activation compared to treatment with C1 and C2.
[3]
a. herbivorous mites induce activation of gene 2 first «at 1 hour» and then also gene 1 and gene 3 «at 24 hours»
OR
herbivorous mite «infestation» is the only treatment to affect all three genes/leads to greater gene expression overall ✔
b. gene 2 activation similar for mite and C1 «at both 1 and 24 hours» ✔
c. gene 3 activation similar for mite and C2 «both at 24 hours» ✔
d. gene 1 activation slower for mite compared to C2 but more intense (than C2 at 24 hours) ✔ Both parts OWTTE required for mpd.
e. gene 1 and gene 3 expressed in higher amounts «after 24 hours» in mite infestation compared to C2 ✔
For this question, students were commonly able to earn marks as there were a number of things for students to notice in the data.
