The ascent of sap in tall trees is best explained by:
Answer: C
The cohesion-tension theory explains water transport in tall trees through cohesive forces between water molecules and tension created by transpiration.
Q.2Hard
Which of the following is a characteristic of phloem transport?
Answer: C
Phloem transport is bidirectional and can move sugars upwards or downwards depending on source-sink relationships and requires metabolic energy.
Q.3Hard
The ratio of photosynthesis to respiration in a plant is represented by:
Answer: D
Apparent photosynthesis (net photosynthesis) represents the actual photosynthesis minus respiration measured as the gaseous exchange in the plant.
Q.4Hard
Which enzyme catalyzes the fixation of CO2 in the Calvin cycle?
Answer: B
RuBisCO catalyzes the carboxylation of RuBP with CO2 to form 3-phosphoglycerate, the first stable product of dark reactions in C3 plants.
Q.5Hard
The process of photorespiration in C3 plants is initiated when:
Answer: C
Photorespiration occurs when RuBisCO catalyzes oxygenation of RuBP instead of carboxylation, which happens under high O2 and low CO2 conditions.
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Q.6Hard
The Km value of an enzyme indicates:
Answer: B
Km (Michaelis constant) is the substrate concentration at half Vmax, and it inversely represents enzyme affinity for substrate. Lower Km means higher affinity.
Q.7Hard
During the Z-scheme of photosynthesis, electrons move from PSII to PSI. What happens to electrons after they leave PSI?
Answer: B
In the Z-scheme, electrons from PSI reduce NADP+ to NADPH with the help of ferredoxin and NADP reductase enzyme.
Q.8Hard
In the electron transport chain of photosynthesis, the role of cytochrome b6f complex is to:
Answer: B
The cytochrome b6f complex transfers electrons between PSII and PSI while simultaneously pumping H+ ions into thylakoid lumen for chemiosmosis.
Q.9Hard
Which of the following C3 plants can show characteristics similar to CAM plants under water stress?
Answer: C
Some C3 plants show facultative CAM characteristics under severe drought stress, temporarily shifting to CAM metabolism to conserve water.
Q.10Hard
A researcher observes that a plant's net photosynthesis decreases with increase in temperature above 35°C. This is primarily because:
Answer: B
At high temperatures, Rubisco and other enzymes denature reducing their activity. Additionally, stomata close to prevent excessive water loss, reducing CO2 availability and thus net photosynthesis.
Q.11Hard
In the Hatch and Slack pathway, malic acid is synthesized in the mesophyll cells and transported to bundle sheath cells. This adaptation is particularly advantageous because it:
Answer: D
The C4 pathway's spatial separation of initial CO2 fixation (PEP carboxylase in mesophyll) and Calvin cycle (in bundle sheath) concentrates CO2 around Rubisco, reducing photorespiration and increasing CO2 fixation efficiency.
Q.12Hard
A plant exhibits high transpiration rate during midday despite adequate soil moisture. Which mechanism BEST explains stomatal opening under these conditions?
Answer: D
During midday, increased light stimulates photosynthesis in guard cells, producing sugars and activating K+ uptake pumps, while ABA levels decrease, both promoting stomatal opening.
Q.13Hard
Which of the following correctly represents the sequence of electron carriers in the electron transport chain of photosynthesis (from PSII)?
Answer: A
The correct sequence after PSII photolysis is: Plastoquinone accepts electrons → Cytochrome b6f complex → Plastocyanin → PSI, following the Z-scheme.
Q.14Hard
The apparent photosynthesis is ALWAYS greater than true photosynthesis because:
Answer: B
Apparent (net) photosynthesis = Gross photosynthesis - Respiration. True (gross) photosynthesis is always greater since some fixed CO2 is released during concurrent respiration.
Q.15Hard
The proton gradient across thylakoid membrane in photosynthesis is maintained primarily by:
Answer: B
The proton gradient (pH gradient) is established by water photolysis at PSII and the Q-cycle in cytochrome b6f complex, which pumps H+ into the thylakoid lumen.
Q.16Hard
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.17Hard
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.18Hard
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.19Hard
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.20Hard
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.
