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May 2025 Section II

0/34 marks

2025 · 6 questions · 32 parts · 34 marks

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1(A)1(B)(i)1(B)(ii)1(B)(iii)1(C)(i)1(C)(ii)1(C)(iii)1(D)(i)1(D)(ii)2(A)2(B)(i)2(B)(ii)2(C)(i)2(C)(ii)2(D)(i)2(D)(ii)3(A)3(B)3(C)3(D)4(A)4(B)4(C)4(D)5(A)5(B)5(C)5(D)6(A)6(B)6(C)6(D)
Context for question 2
Many insects rely on pheromones (chemical signals) that are released by the females to find mating partners. Scientists hypothesize that, in a certain type of moth, the behavior of male moths in response to pheromones is regulated by the extracellular signaling molecule 20E.
To investigate whether the binding of 20E to its receptor, DopEcR, affects behavior in moths, scientists injected male moths with saline (control solution) or with small interfering RNA molecules (siRNAs) that inhibit the expression of the gene encoding DopEcR. The scientists then exposed the moths to the pheromone and determined the percent of total time observed that the moths engaged in general activity, defined as movement in any direction. The scientists also determined the percent of the general activity time that the moths spent in oriented activity, defined as movement toward an area of high pheromone concentration (Table 1).
Table 1. Average General and Oriented Activity in Male Moths Injected With Saline or siRNA Molecules
TreatmentGeneral Activity (percent of total time observed, average ± 2SE)Oriented Activity (percent of general activity, average ± 2SE)
Male moths injected with saline (control solution)95 ± 560 ± 4
Male moths injected with siRNAs that inhibit expression of the gene encoding DopEcR90 ± 825 ± 6
DopEcR is a G protein-coupled receptor. When 20E binds to DopEcR, GTP displaces the GDP bound to the G protein, and a signaling pathway is activated. The scientists hypothesize that this leads to the transcription of genes associated with the oriented activity observed in the male moths (Figure 1).
Figure 1: a simplified model of a signaling pathway activated by the binding of 20E to its receptor, DopEcR
Figure 1. A simplified model of a signaling pathway activated by the binding of 20E to its receptor, DopEcR.
2(A)Membrane Structure and the Fluid Mosaic ModelEasy1 mark
Many receptors are embedded in the plasma membrane. Describe the polarity of the portion of the receptor that is inside the membrane.
2(B)(i)Cell SignallingEasy3 marks
Using the space provided for your response, construct an appropriate type of graph that represents the data in Table 1. Your graph should be appropriately plotted and labeled.
2(B)(ii)Cell SignallingEasy1 mark
Based on the data in Table 1, determine the type of activity that was affected by inhibiting the expression of the DopEcR receptor.
2(C)(i)Cell SignallingEasy1 mark
Based on Table 1, identify the treatment group in which the oriented activity was greater than 50% of the general activity.
2(C)(ii)Cell SignallingEasy1 mark
The scientists studied some moths with a mutation in the gene encoding the G protein. The mutation prevents GTP from displacing the GDP bound to the G protein. Based on Figure 1, predict the effect of this mutation on the oriented activity in male moths exposed to the pheromone. Expression of the gene encoding DopEcR is low in the male moths during their first few days as adults, when they are sexually immature. Gene expression rapidly increases as the moths reach sexual maturity. The scientists claim that this increase in gene expression increases the likelihood of males finding females with whom to mate.
2(D)(i)Cell SignallingEasy1 mark
Use evidence from the information provided to support the scientists’ claim.
2(D)(ii)Cell SignallingEasy1 mark
Based on Figure 1, explain how an inhibitor of the DopEcR pathway might serve as an effective chemical to protect crops from moth damage.
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