In the Calvin cycle, which enzyme catalyzes the fixation of CO2?
Answer: A
RuBisCO is the most abundant enzyme on Earth and catalyzes the carboxylation of RuBP with CO2 in the dark reactions.
Q.262Medium
Which of the following statements about respiration is correct?
Answer: C
In plants, anaerobic conditions lead to fermentation producing ethanol and CO2. Aerobic respiration yields ~30-32 ATP; Krebs cycle produces limited ATP directly; Glycolysis is cytoplasmic.
Q.263Easy
The Q10 value for enzyme-catalyzed reactions is typically:
Answer: B
Q10 of 2.0-3.0 indicates that enzyme activity doubles or triples with every 10°C increase in temperature, typical for biological systems.
Q.264Medium
Which statement about transpiration is INCORRECT?
Answer: C
Transpiration is highly dependent on atmospheric humidity. Lower humidity increases transpiration rate due to greater water vapor gradient.
Q.265Medium
In the electron transport chain, which complex pumps the most protons across the inner mitochondrial membrane?
Answer: A
Complex I (NADH dehydrogenase) pumps 4 protons, Complex III pumps 4, Complex IV pumps 2, making Complex I the most significant proton pump.
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Q.266Medium
Which of the following factors does NOT directly affect photosynthesis?
Answer: B
While soil pH affects nutrient availability, it does not directly affect the photosynthetic process itself. Chlorophyll, CO2, and light wavelength directly influence photosynthesis.
Q.267Easy
The grana in chloroplasts are composed of:
Answer: A
Grana are stacks of thylakoids arranged vertically, while stroma lamellae are unstacked thylakoids connecting different grana.
Q.268Medium
In photorespiration, the primary substrate is:
Answer: B
RuBisCO catalyzes oxygenation of RuBP instead of carboxylation under high O2/CO2 ratios, initiating the photorespiratory pathway.
Q.269Medium
Which organelle is responsible for amino acid synthesis in plants?
Answer: B
Chloroplasts synthesize amino acids using carbon skeletons from the Calvin cycle and nitrogen from nitrite reduction.
Q.270Hard
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.271Hard
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.272Medium
At saturating light intensity, photosynthesis is primarily limited by:
Answer: C
When light is saturating, CO2 becomes the limiting factor as RuBisCO activity depends on substrate availability.
Q.273Easy
The phenomenon where plants lose water in liquid form through hydathodes is called:
Answer: B
Guttation occurs when root pressure forces water out through hydathodes (water pores) as liquid water droplets, typically at night.
Q.274Hard
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.275Medium
In C4 plants, the first stable product of CO2 fixation is:
Answer: B
In the Hatch-Slack pathway, PEP carboxylase catalyzes formation of oxaloacetate (4-carbon compound) in mesophyll cells.
Q.276Medium
Which of the following is a characteristic feature of bundle sheath cells in C4 plants?
Answer: A
Bundle sheath cells in C4 plants contain large chloroplasts with well-developed grana for the Calvin cycle, unlike mesophyll cells.
Q.277Medium
During the Calvin cycle, the reduction phase requires ATP and NADPH. How many molecules of ATP are consumed to fix one molecule of CO2?
Answer: C
The Calvin cycle requires 3 ATP and 2 NADPH for each CO2 fixed. One ATP is used in the carboxylation step, and 2 ATP are used in the reduction phase to regenerate RuBP.
Q.278Easy
Which of the following enzyme complexes in the thylakoid membrane is responsible for water splitting and oxygen evolution?
Answer: B
Photosystem II contains the oxygen-evolving complex (OEC) with a manganese cluster that catalyzes water splitting, releasing O2, H+, and electrons.
Q.279Hard
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.280Medium
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.
