In C3 plants, the enzyme RuBisCO catalyzes carboxylation. Under what condition does RuBisCO exhibit oxygenase activity instead?
Answer: B
RuBisCO has dual specificity. When O2/CO2 ratio is high (low CO2, high O2), the oxygenase activity dominates, leading to photorespiration.
Q.42Medium
In the light-dependent reactions, non-cyclic photophosphorylation produces ATP and NADPH in a ratio of approximately:
Answer: C
The Z-scheme produces approximately 1.5 ATP per NADPH (or 3 ATP per 2 NADPH) due to the proton gradient and energy requirements in the electron transport chain.
Q.43Medium
A seedling grown under continuous red light shows etiolation symptoms. This is primarily because:
Answer: B
While red light activates phytochrome and initiates de-etiolation, it alone is insufficient for complete chloroplast development and normal growth. Blue light and other wavelengths are also required.
Q.44Medium
The quantum requirement for photosynthesis (minimum number of photons needed to fix one CO2) is approximately:
Answer: B
The quantum requirement is about 8 photons per CO2 fixed (4 for PSII and 4 for PSI to generate sufficient ATP and NADPH for the Calvin cycle).
Q.45Medium
A plant exhibits wilting despite adequate soil moisture. The most likely explanation involves:
Answer: B
Wilting occurs when transpiration demand exceeds water uptake capacity. Root pressure alone (typically 2-10 atm) cannot overcome strong transpiration pull, leading to xylem tension and leaf wilting.
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Q.46Medium
Under conditions of high light intensity and stomatal closure, what is the primary fate of electrons in the thylakoid?
Answer: B
Under stress (high light, closed stomata reducing CO2), cyclic photophosphorylation increases, generating ATP without NADPH consumption. This helps dissipate excess light energy and prevent photoinhibition.
Q.47Medium
In photorespiration, glycolate (2-phosphoglycolate) is initially oxidized in peroxisomes. This process consumes O2 but generates no ATP. What is the primary consequence?
Answer: B
Photorespiration recycles glycolate back to useful 3-PG, but this process consumes ATP and reduces RuBP regeneration efficiency, decreasing net photosynthetic yield by 20-50%.
Q.48Medium
A plant grown under continuous red light shows reduced stem elongation compared to white light. Which photoreceptor is primarily responsible for this response?
Answer: C
Red light converts phytochrome to its active Pfr (far-red absorbing) form, which inhibits stem elongation and promotes photomorphogenesis. Continuous red light maintains high Pfr levels, suppressing etiolation. Cryptochrome responds to blue light, and phototropin detects directional light.
Q.49Medium
In C4 plants, the first stable product of photosynthesis is oxaloacetate (4-carbon compound). Which of the following is the primary advantage of this mechanism in tropical environments?
Answer: B
C4 photosynthesis concentrates CO2 around RuBisCO through the C4 pathway, minimizing photorespiration even at high temperatures and low CO2 availability. This is advantageous in tropical/arid regions where stomata must close frequently to conserve water, improving water-use efficiency (WUE).
Q.50Medium
During seed germination, reserve mobilization occurs through enzyme activation. Which hormone primarily promotes the synthesis of α-amylase in the aleurone layer of cereal grains?
Answer: A
GA3 is the primary hormone regulating α-amylase synthesis in cereal aleurone cells during germination. GA3 inactivates DELLA repressor proteins, allowing transcription factors (GAMYB) to activate amylase genes. This hormone controls starch hydrolysis for energy mobilization in growing seedlings.
