Part 8 — Case-based & Higher-order NEET Revision MCQs (Q176–200)
Part 8 (Q176–200) of Chapter 13 – Photosynthesis in Higher Plants (Class 11 NCERT, NEET relevance).
This final set contains Case-based, Analytical, and Higher-order NEET-style Revision Questions, integrating all subtopics: experiments, pigments, light reactions, Calvin cycle, C₄/CAM pathways, photorespiration, efficiency, and limiting factors.
Part 8 – Photosynthesis in Higher Plants – Case-based & Higher-order NEET Revision MCQs (Q176–200)
Q176.
A C₃ plant was kept in continuous red light of 660 nm. Photosynthesis slowed drastically. The reason is:
A. PSII inactive
B. PSI inactive
C. Red drop effect (only PSI active at >680 nm) ✅
D. Rubisco inhibited
Explanation:
- A/B. Both initially active, but >680 nm favors PSI only.
- C. (Correct) Red drop effect → low efficiency beyond 680 nm.
- D. Rubisco not inhibited directly.
Q177.
A plant exposed to light of both 660 nm and 700 nm showed enhanced photosynthesis compared to 660 nm alone. This demonstrates:
A. Blackman’s Law
B. Emerson enhancement effect ✅
C. Warburg effect
D. Kranz anatomy
Explanation:
- A. Limiting factor concept.
- B. (Correct) Emerson effect: synergy of PSI + PSII when both wavelengths used.
- C/D. Not related.
Q178.
If a C₄ plant is genetically engineered to lack PEP carboxylase, what will happen?
A. No photolysis of water
B. No initial CO₂ fixation in mesophyll ✅
C. No ATP synthesis
D. No NADPH generation
Explanation:
- A/C/D. Still functional.
- B. (Correct) PEP carboxylase fixes CO₂ → OAA. Without it, no C₄ cycle.
Q179.
A plant has CO₂ compensation point near 50 ppm. It is most likely:
A. C₄ plant
B. C₃ plant ✅
C. CAM plant
D. Algal plant
Explanation:
- A. C₄ compensation <10 ppm.
- B. (Correct) ~50 ppm = C₃.
- C/D. Variable.
Q180.
Which feature is common to C₄ and CAM plants?
A. Kranz anatomy
B. Initial CO₂ fixation by PEP carboxylase ✅
C. Only PSI present
D. Absence of Calvin cycle
Explanation:
- A. Only C₄.
- B. (Correct) Both fix CO₂ initially via PEP carboxylase.
- C/D. False.
Q181.
A leaf disc experiment shows starch formation only on illuminated parts. This supports:
A. CO₂ is essential
B. Light is essential ✅
C. Chlorophyll is not essential
D. O₂ is essential
Explanation:
- A. Tested separately.
- B. (Correct) Light required for photosynthesis.
- C/D. Incorrect here.
Q182.
When O₂ concentration is doubled, C₃ plants show decline in photosynthesis due to:
A. Reduced ATP
B. Photorespiration ✅
C. Low NADPH
D. Stomatal closure
Explanation:
- A/C/D. Not direct cause.
- B. (Correct) High O₂ favors oxygenase activity of Rubisco.
Q183.
In desert succulents, stomata open at night. This adaptation minimizes:
A. Photorespiration
B. Transpiration ✅
C. CO₂ uptake
D. ATP loss
Explanation:
- A. Secondary effect.
- B. (Correct) Stomata open at cooler nights → conserve water.
- C/D. Not true.
Q184.
If Engelmann’s experiment is repeated with green light only, what will happen?
A. Bacteria cluster in middle
B. No bacterial clustering ✅
C. Bacteria cluster in red zone
D. O₂ released abundantly
Explanation:
- A/D. False.
- B. (Correct) Green light poorly absorbed → no O₂ release → no clustering.
- C. No red zone provided.
Q185.
If a plant uses 12 ATP and 8 NADPH for fixing 4 CO₂, which pathway is it using?
A. C₃ Calvin cycle ✅
B. C₄ pathway
C. CAM pathway
D. Photorespiration
Explanation:
- A. (Correct) Each CO₂ requires 3 ATP + 2 NADPH → 4 CO₂ = 12 ATP + 8 NADPH.
- B/C. More ATP required.
- D. No CO₂ fixation.
Q186.
In C₄ plants, Calvin cycle occurs in:
A. Mesophyll cells
B. Bundle sheath cells ✅
C. Guard cells
D. Root cortex
Explanation:
- A. CO₂ initially fixed here.
- B. (Correct) Calvin cycle restricted to bundle sheath cells.
- C/D. Not involved.
Q187.
Which experimental finding directly supported the presence of two photosystems?
A. Blackman’s law
B. Emerson enhancement effect ✅
C. Warburg effect
D. Sachs’ starch test
Explanation:
- A/C/D. Other aspects.
- B. (Correct) Dual-wavelength enhancement proved PSI & PSII.
Q188.
If 6 CO₂ molecules are fixed in Calvin cycle, the net gain of G3P molecules is:
A. 1
B. 2 ✅
C. 3
D. 6
Explanation:
- A/D. Miscounts.
- B. (Correct) 6 CO₂ → 12 G3P → 10 used to regenerate RuBP, 2 net.
- C. Not correct.
Q189.
Which statement about CAM plants is true?
A. They lack Rubisco
B. They use temporal separation of CO₂ fixation ✅
C. They have Kranz anatomy
D. They perform Calvin cycle at night
Explanation:
- A. Rubisco present.
- B. (Correct) CO₂ fixation at night; Calvin cycle during day.
- C. Kranz is C₄.
- D. False.
Q190.
At CO₂ saturation, increasing light further does not enhance photosynthesis. This illustrates:
A. Quantum yield
B. Limiting factor concept ✅
C. Compensation point
D. Emerson effect
Explanation:
- A/C/D. Not correct.
- B. (Correct) When one factor saturates, another becomes limiting.
Q191.
If a C₄ plant requires 30 ATP and 12 NADPH for producing 1 glucose, why is it still advantageous?
A. Because NADPH is recycled
B. Because photorespiration is suppressed ✅
C. Because less O₂ is needed
D. Because no Calvin cycle is required
Explanation:
- A. Not relevant.
- B. (Correct) Higher ATP cost is offset by efficient CO₂ fixation without photorespiration.
- C/D. Wrong.
Q192.
If PSI alone is illuminated and PSII is inactivated, what will occur?
A. ATP formed, no NADPH or O₂ ✅
B. Both ATP and NADPH formed
C. Only O₂ released
D. CO₂ fixation increases
Explanation:
- A. (Correct) Cyclic photophosphorylation only → ATP formed.
- B–D. Not correct.
Q193.
A scientist observes that in a plant, the rate of photosynthesis continues to increase even at high CO₂ and high temperature. This plant is likely:
A. C₃
B. C₄ ✅
C. CAM
D. Non-green
Explanation:
- A. Saturates earlier.
- B. (Correct) C₄ plants continue efficient photosynthesis at high temp & CO₂.
- C. Slower response.
- D. Irrelevant.
Q194.
Why is the compensation point lower in C₄ plants?
A. No Rubisco
B. High CO₂ concentration in bundle sheath ✅
C. Lack of PSII
D. No stomata
Explanation:
- A. Rubisco present.
- B. (Correct) CO₂ concentrating mechanism reduces compensation point.
- C/D. Wrong.
Q195.
If Rubisco activity is completely inhibited, what immediate product formation halts?
A. OAA
B. 3-PGA ✅
C. Malate
D. G3P
Explanation:
- A/C. C₄ cycle intermediates.
- B. (Correct) Rubisco catalyzes RuBP + CO₂ → 3-PGA.
- D. Later product.
Q196.
During photorespiration, glycolate is converted to glycine in:
A. Chloroplast
B. Peroxisome ✅
C. Mitochondria
D. Cytoplasm
Explanation:
- A. Glycolate formed here.
- B. (Correct) In peroxisome, glycolate → glyoxylate → glycine.
- C. Later step.
- D. Not involved.
Q197.
In CAM pathway, CO₂ fixed at night is released during the day for:
A. Photorespiration
B. Calvin cycle ✅
C. C₄ cycle
D. Respiration
Explanation:
- A/C/D. Not correct.
- B. (Correct) Stored malate decarboxylated → CO₂ enters Calvin cycle in day.
Q198.
Which of the following is not an adaptation to minimize photorespiration?
A. Kranz anatomy in C₄ plants
B. Temporal separation in CAM plants
C. Stomatal closure during day in C₃ plants ✅
D. CO₂ concentrating mechanisms
Explanation:
- A/B/D. True adaptations.
- C. (Correct) Stomatal closure reduces CO₂ uptake → increases photorespiration.
Q199.
In an experiment, a plant was illuminated with red light only (>680 nm). No O₂ was evolved. Why?
A. Both PSI & PSII active
B. Only PSI active ✅
C. Only PSII active
D. No chlorophyll present
Explanation:
- A/C/D. Not true.
- B. (Correct) Far-red excites only PSI → no water splitting → no O₂.
Q200.
Which combination of features gives maximum photosynthetic efficiency in hot, dry climates?
A. C₃ metabolism + high Rubisco
B. C₄ metabolism + Kranz anatomy ✅
C. CAM metabolism + no Calvin cycle
D. C₃ metabolism + high O₂
Explanation:
- A/D. Photorespiration high.
- B. (Correct) C₄ with Kranz anatomy → suppress photorespiration, conserve water.
- C. CAM still uses Calvin cycle.
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