Question 19M.2.HL.TZ2.1b
| Date | May 2019 | Marks available | [Maximum mark: 3] | Reference code | 19M.2.HL.TZ2.1b |
| Level | HL | Paper | 2 | Time zone | TZ2 |
| Command term | Compare and contrast | Question number | b | 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]
Compare and contrast the effects of treatment of the leaves using C1 and C2 with the effects of infestation.
[3]
a. C1 caused the leaf to produce two of the same chemicals/Y and Z as the attack of herbivorous mites in a similar pattern «but in lower quantities» ✔ OWTTE
b. C1 produces the least «total» amount of chemicals of all the treatments ✔
c. C2 has very similar pattern to those caused by the armyworms «but in lower quantities» ✔ OWTTE
d. both herbivores caused a greater production of chemicals/all three chemicals compared to either C1 or C2 ✔
e. armyworms cause the greatest total amount of chemical production of any of the other treatments ✔
f. other valid comparison of chemical effect versus herbivore effect ✔
Clear comparison required between herbivore infestation and chemical treatment not simple lists of values.
This question was less well answered. A number of students misinterpreted the question by comparing the results of treating with C1 and C2 only and not comparing these to the effects of infestation.
