A desert plant maintains stomata closure during the day and opens them at night, fixing CO2 into malic acid. This adaptation is characteristic of which photosynthetic pathway?
Answer: C
CAM (Crassulacean Acid Metabolism) plants fix CO2 at night into malate and release it during the day for photosynthesis, reducing water loss in arid environments.
Q.282Medium
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.283Easy
Stomatal opening is primarily regulated by the accumulation of which osmotically active solute in guard cells?
Answer: C
K+ accumulation in guard cells lowers the water potential, causing water influx and turgor increase, leading to stomatal opening. This is the primary mechanism.
Q.284Medium
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.285Medium
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).
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Q.286Hard
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.
Q.287Hard
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.288Medium
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.
Q.289Hard
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.290Easy
Which of the following correctly describes the role of carotenoids in photosynthesis?
Answer: B
Carotenoids absorb light energy in blue and green regions, transfer it to chlorophyll, and provide photoprotection by quenching singlet oxygen and excess excitation energy.
Q.291Medium
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.292Easy
The photoperiodic response in plants (flowering) is primarily detected by which photoreceptor?
Answer: C
Phytochrome exists in two interconvertible forms (Pr and Pfr) and acts as a red/far-red light sensor, measuring day length for photoperiodic flowering responses.
Q.293Medium
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.294Easy
A variegated plant leaf shows white and green patches. Which organelle dysfunction is responsible for white patches?
Answer: B
White patches result from chloroplasts lacking chlorophyll or being non-functional, typically due to mutations in chloroplast DNA (maternal inheritance). Green patches have functional chloroplasts.
Q.295Hard
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.296Easy
Which of the following statements about transpiration pull is correct?
Answer: A
Transpiration pull is the tension/negative pressure developed in xylem vessels due to continuous water loss (transpiration) from leaves. This creates a suction force that pulls water upward from roots. It can occur both day and night through lenticels and cuticles, though primarily during day.
Q.297Medium
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.298Medium
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.299Hard
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.300Medium
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.
