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.142Medium
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.143Medium
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.144Medium
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.145Medium
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.146Medium
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.147Medium
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.148Medium
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.149Medium
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.150Medium
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.
Q.151Medium
Which of the following represents the correct sequence of events in double fertilization?
Answer: A
In double fertilization, one sperm fuses with the egg to form zygote, and the other fuses with two polar nuclei to form primary endosperm nucleus.
Q.152Medium
The embryo sac in flowering plants develops from:
Answer: C
The functional megaspore undergoes mitotic divisions to develop into the embryo sac (female gametophyte).
Q.153Medium
In a typical angiosperm, how many mitotic divisions occur in the male gametophyte after meiosis?
Answer: A
After meiosis produces microspores, one mitotic division produces a two-celled pollen grain (mature male gametophyte).
Q.154Medium
Which of the following is NOT a method of vegetative propagation?
Answer: C
Pollination is part of sexual reproduction, not vegetative propagation. It involves transfer of pollen.
Q.155Medium
The endosperm in angiosperms is:
Answer: C
Endosperm is triploid (3n), formed by fusion of one sperm nucleus (n) with two polar nuclei (2n).
Q.156Medium
Which stage represents the gametophytic phase in the life cycle of angiosperms?
Answer: B
Pollen grain (male gametophyte) and embryo sac (female gametophyte) represent the gametophytic phase in angiosperms.
Q.157Medium
The synergid cells in the embryo sac function to:
Answer: B
Synergids guide the pollen tube towards the egg apparatus and help in pollen tube entry through the filiform apparatus.
Q.158Medium
Which of the following correctly describes the ploidy level changes during sexual reproduction?
Answer: B
Meiosis reduces ploidy from diploid (2n) to haploid (n), while fertilization restores diploid (2n) condition.
Q.159Medium
The primary endosperm nucleus in angiosperms is formed by:
Answer: B
The primary endosperm nucleus is formed by the fusion of one sperm cell with the two polar nuclei (or secondary nucleus), making it triploid.
Q.160Medium
During megasporogenesis in angiosperms, a megaspore mother cell undergoes meiosis to produce four megaspores. In most flowering plants, how many of these megaspores degenerate?
Answer: C
In typical monosporic development, the megaspore mother cell produces four megaspores through meiosis, but only one (usually the chalazal megaspore) survives and develops into the female gametophyte. The other three degenerate.
