The rate-limiting step of photosynthesis under normal conditions is:
Answer: B
The carboxylation of RuBP by RuBisCO is typically the slowest step in photosynthesis under normal light conditions.
Q.62Hard
Which of the following best describes the Pasteur effect?
Answer: B
The Pasteur effect describes how aerobic respiration inhibits the rate of fermentation, as aerobic respiration is more efficient.
Q.63Hard
Which statement about ATP synthase is correct?
Answer: D
ATP synthase works in both processes using chemiosmotic gradient; F0 is the membrane-spanning unit; F1 synthesizes ATP from ADP.
Q.64Hard
A plant is kept in a chamber with elevated CO2 concentration (800 ppm) and saturating light. What will be the most likely effect on the RuBP level in the stroma?
Answer: B
Higher CO2 increases the carboxylation of RuBP to form 3-PG faster, consuming RuBP more rapidly. As long as the reduction phase can regenerate RuBP, levels may remain relatively stable, but the consumption rate increases.
Q.65Hard
In the absence of light, what happens to the pH gradient across the thylakoid membrane?
Answer: B
Without light-driven electron transport, no new protons are pumped into the thylakoid lumen. The existing gradient dissipates as protons leak back through ATP synthase or other pathways.
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Q.66Hard
Which photosystem has a lower reduction potential for its reaction center chlorophyll (P700 vs P680)?
Answer: A
P700 (PSI) has a lower (more negative) reduction potential than P680 (PSII), making it easier to reduce and better suited for final electron acceptance and NADP+ reduction.
Q.67Hard
The Emerson effect demonstrates that photosynthesis efficiency increases when plants are exposed to both red and far-red light together compared to either alone. This is explained by:
Answer: B
The Emerson effect revealed the existence of two photosystems. Both PSI and PSII need to function sequentially for optimal electron transport and ATP/NADPH generation.
Q.68Hard
During the S-state transitions in the oxygen-evolving complex, the manganese cluster undergoes sequential oxidation. How many oxidation steps occur before O2 is released?
Answer: C
The Kok cycle involves S0→S1→S2→S3→S4 states. Four electrons are removed sequentially from the Mn4CaO5 cluster before O2 is released at the S4→S0 transition.
Q.69Hard
A research study on stomatal regulation shows that ABA (abscisic acid) concentration increases 100-fold in guard cells during drought stress. Which of the following correctly describes the mechanism?
Answer: C
ABA stress signaling involves activation of phospholipase C → IP3 generation → Ca2+ release from vacuoles → opening of anion channels (Cl-, malate) → efflux of anions and K+ → loss of turgor → stomatal closure. This is the canonical ABA-signaling pathway in guard cells.
Q.70Hard
Which of the following statements about pollen grains is INCORRECT?
Answer: C
Mature pollen grains are bicellular (or sometimes tricellular), not unicellular. They contain vegetative and generative cells.
Q.71Hard
In apomixis, seeds are produced without:
Answer: B
Apomixis is asexual reproduction through seeds, occurring without meiosis and fertilization, producing clones of the parent.
Q.72Hard
The phenomenon where a single plant produces both male and female flowers is called:
Answer: B
Monoecious plants have both male and female flowers on the same plant (e.g., maize, coconut). Dioecious plants have them on different plants.
Q.73Hard
Which stage of meiosis in microsporogenesis ensures genetic variation in pollen grains?
Answer: B
Anaphase I involves separation of homologous chromosomes, which along with crossing over in Prophase I ensures genetic variation in resulting microspores.
Q.74Hard
Identify the correct sequence in the development of male gametophyte:
Answer: B
The correct sequence is: Microsporocyte undergoes meiosis to form microspores, which undergo mitosis to form mature pollen grain (male gametophyte).
Q.75Hard
The endothecium in the anther wall is important for:
Answer: B
The endothecium contains fibrous thickenings on cell walls that help in anther dehiscence by creating tension during drying, aiding pollen dispersal.
Q.76Hard
The phenomenon where pollen grains retain viability after being kept in dry conditions for extended periods is called:
Answer: A
Pollen dormancy refers to the reduced metabolic activity of pollen that allows it to maintain viability over extended storage periods in dry, cool conditions.
Q.77Hard
In a typical angiosperm flower, after dehiscence, the mature pollen is released at which stage?
Answer: B
Most angiosperms release pollen at the binucleate stage (containing generative and vegetative nuclei). Some release at the trinucleate stage after generative nucleus division.
Q.78Hard
In orchids, the lack of endosperm development is compensated by:
Answer: B
Many orchids have seeds without endosperm (lacking nutritious reserve tissue). They require mycorrhizal fungal associations to obtain nutrients from the soil for germination and growth.
Q.79Hard
The phenomenon of pollen dimorphism is observed in which type of flowers?
Answer: B
Pollen dimorphism is associated with heterostyly, where plants like Primula produce different-sized pollen grains corresponding to different floral forms to promote cross-pollination.
Q.80Hard
Which of the following represents the correct ploidy level progression during the life cycle of a typical angiosperm?
Answer: B
Angiosperm life cycle: diploid sporophyte (2n) → haploid spores (n) → haploid gametophyte (n) → diploid zygote (2n) after fertilization, and triploid endosperm (3n) after double fertilization.
