Part 7 — Critical & Complex NEET-style Integrative Questions (Q151–175)

Part 7 (Q151–175) of Chapter 13 – Photosynthesis in Higher Plants (Class 11 NCERT, NEET relevance).

This part focuses on Critical & Complex NEET-style Integrative Questions — combining experiments, pathways (C₃, C₄, CAM), efficiency, and limiting factors.

Part 7 — Photosynthesis in Higher Plants – Critical & Complex NEET Questions (Q151–175)

Q151.

In Engelmann’s experiment, if the aerobic bacteria failed to aggregate near red and blue regions, the most likely reason is:
A. CO₂ was absent
B. O₂ was absent ✅
C. Chlorophyll was destroyed
D. Starch synthesis was stopped

Explanation:

• A. CO₂ not tested here.
• B. (Correct) Bacteria cluster where O₂ is released; without O₂ release, no clustering.
• C/D. Would indirectly affect but main factor is O₂.

Q152.

A scientist illuminated a C₃ plant at high light and temperature. Photosynthesis declined. Which process explains this?
A. Calvin cycle shut down
B. Photorespiration increased ✅
C. ATP synthase failed
D. Photolysis of water stopped

Explanation:

• A. Calvin cycle still active.
• B. (Correct) High light + high temp → O₂ accumulates → oxygenase activity of Rubisco ↑ → photorespiration.
• C/D. Not true under high light.

Q153.

Maize plants show high photosynthetic efficiency in hot, arid regions because:
A. They lack Rubisco
B. They have CAM metabolism
C. They are C₄ plants with Kranz anatomy ✅
D. They use only cyclic photophosphorylation

Explanation:

• A. Rubisco present in bundle sheath.
• B. CAM is different (succulents).
• C. (Correct) C₄ pathway avoids photorespiration.
• D. False.

Q154.

In CAM plants, CO₂ is initially fixed at night into:
A. Oxaloacetate, stored as malate ✅
B. Phosphoglycerate
C. Pyruvate
D. Sucrose

Explanation:

• A. (Correct) CAM: night fixation → OAA → malate stored in vacuoles.
• B/C/D. Not initial storage.

Q155.

CAM metabolism provides advantage in:
A. Cold regions
B. High CO₂ areas
C. Hot, arid deserts ✅
D. Aquatic habitats

Explanation:

• A/B/D. Not main adaptation.
• C. (Correct) CAM plants open stomata at night → reduce water loss.

Q156.

If O₂ concentration rises significantly, which plants will maintain maximum photosynthetic efficiency?
A. Rice (C₃)
B. Wheat (C₃)
C. Maize (C₄) ✅
D. Barley (C₃)

Explanation:

• A/B/D. All are C₃ → high photorespiration under O₂.
• C. (Correct) C₄ plants suppress photorespiration.

Q157.

Which of the following experimental results proved that O₂ comes from water, not CO₂?
A. Blackman’s Law
B. Van Niel’s work on purple bacteria ✅
C. Engelmann’s action spectrum
D. Sachs’ starch test

Explanation:

• A/C/D. Other contributions.
• B. (Correct) Van Niel: bacteria used H₂S instead of H₂O → no O₂ evolved → O₂ comes from donor (H₂O).

Q158.

Quantum yield of photosynthesis falls drastically in far-red light because:
A. Both PSI and PSII are inactive
B. Only PSI operates ✅
C. Only PSII operates
D. Photophosphorylation stops completely

Explanation:

• A. Wrong.
• B. (Correct) At >680 nm, only PSI works → no O₂ or NADPH → yield falls.
• C/D. Incorrect.

Q159.

If a C₃ plant is given labeled CO₂ (¹⁴CO₂), the first stable compound labeled would be:
A. Oxaloacetate
B. 3-PGA ✅
C. Malate
D. Pyruvate

Explanation:

• A/C/D. First products of C₄.
• B. (Correct) In C₃ cycle, CO₂ fixed into 3-PGA.

Q160.

If the same is done with a C₄ plant, the first labeled compound would be:
A. 3-PGA
B. Oxaloacetate ✅
C. G3P
D. RuBP

Explanation:

• A. Later product.
• B. (Correct) C₄: CO₂ + PEP → OAA.
• C/D. Not immediate.

Q161.

Why do C₄ plants require more ATP than C₃?
A. To reduce more NADPH
B. For additional steps of CO₂ concentration ✅
C. For excess glucose production
D. Because they lack Calvin cycle

Explanation:

• A. NADPH requirement same.
• B. (Correct) Extra ATP needed to regenerate PEP in mesophyll.
• C/D. Wrong.

Q162.

Which statement is true about CAM plants?
A. They open stomata during day
B. They fix CO₂ at night into malate ✅
C. They lack Calvin cycle
D. They do not respire

Explanation:

• A. Opposite.
• B. (Correct) CAM → CO₂ fixed at night → malate stored.
• C/D. False.

Q163.

At low CO₂ concentration, which plants are more efficient?
A. C₃
B. C₄ ✅
C. Both equal
D. CAM

Explanation:

• A. Inefficient at low CO₂ due to high compensation point.
• B. (Correct) C₄ can fix CO₂ efficiently even at low concentrations.
• C/D. Not true.

Q164.

If CO₂ concentration is doubled, photosynthesis increases more in:
A. C₃ plants ✅
B. C₄ plants
C. CAM plants
D. Both equally

Explanation:

• A. (Correct) C₃ limited by CO₂ → strong response to doubling.
• B. C₄ already saturated.
• C/D. Not correct.

Q165.

At which CO₂ concentration do C₃ and C₄ plants show equal photosynthetic rates?
A. Very low CO₂
B. Intermediate CO₂ (~360 ppm) ✅
C. High CO₂ (>600 ppm)
D. Compensation point

Explanation:

• A. C₄ better at low CO₂.
• B. (Correct) Around ambient (~360 ppm), rates converge.
• C. C₃ surpasses C₄.
• D. Wrong.

Q166.

Which factor explains the greater productivity of sugarcane than wheat in tropics?
A. Higher chlorophyll content
B. C₄ metabolism with no photorespiration ✅
C. Lower ATP requirement
D. Reduced transpiration

Explanation:

• A/D. Secondary.
• B. (Correct) Sugarcane (C₄) highly productive in hot climates.
• C. ATP requirement is higher in C₄.

Q167.

Which of the following plants is CAM?
A. Maize
B. Wheat
C. Opuntia ✅
D. Rice

Explanation:

• A/D. C₄/C₃ respectively.
• B. C₃.
• C. (Correct) Opuntia (succulent) = CAM metabolism.

Q168.

The main difference between C₄ and CAM pathways is:
A. Initial fixation by Rubisco
B. Temporal vs spatial separation of CO₂ fixation ✅
C. Use of pyruvate
D. Absence of Calvin cycle

Explanation:

• A. Both use PEP initially.
• B. (Correct) C₄ = spatial separation; CAM = temporal (night/day).
• C. Used in both.
• D. Both use Calvin cycle.

Q169.

If Rubisco is inhibited, which pathways would stop immediately?
A. Calvin cycle ✅
B. C₄ pathway entirely
C. CAM cycle entirely
D. Light reaction

Explanation:

• A. (Correct) Calvin cycle needs Rubisco for CO₂ fixation.
• B/C. C₄ & CAM still fix CO₂ initially with PEP, but Calvin cycle halts.
• D. Independent.

Q170.

The photosynthetic efficiency of C₄ plants over C₃ is about:
A. 10% higher
B. 20–30% higher ✅
C. 50% lower
D. Equal

Explanation:

• A/D. Underestimation.
• B. (Correct) C₄ plants show ~20–30% higher efficiency due to suppressed photorespiration.
• C. Wrong.

Q171.

In Warburg’s effect, high O₂ concentration inhibits photosynthesis due to:
A. Inhibition of ATP synthase
B. Competition between O₂ and CO₂ for Rubisco ✅
C. Loss of chlorophyll
D. Proton gradient collapse

Explanation:

• A/C/D. Not primary cause.
• B. (Correct) High O₂ favors oxygenase activity of Rubisco → photorespiration.

Q172.

If the CO₂ compensation point of a plant is near zero, it must be:
A. C₃
B. C₄ ✅
C. CAM
D. Non-photosynthetic

Explanation:

• A. ~50 ppm.
• B. (Correct) C₄ plants have compensation point near 0 ppm.
• C. Variable.
• D. Irrelevant.

Q173.

CAM plants conserve water by:
A. Closing stomata permanently
B. Opening stomata only at night ✅
C. Using Rubisco only
D. Using PSI only

Explanation:

• A. Would stop CO₂ intake.
• B. (Correct) CO₂ fixed at night → malate stored → day release.
• C/D. Incorrect.

Q174.

In desert plants like Opuntia, photosynthetic stems are green because:
A. Leaves are absent, chlorophyll present in stems ✅
B. They store anthocyanin
C. They fix N₂ in stems
D. CAM plants lack chlorophyll in leaves

Explanation:

• A. (Correct) Succulents have reduced leaves → stems photosynthetic.
• B–D. False.

Q175.

The Kranz anatomy adaptation in C₄ plants ensures:
A. More chlorophyll
B. Spatial separation of initial CO₂ fixation and Calvin cycle ✅
C. Night fixation of CO₂
D. Higher stomatal density

Explanation:

• A. Not main role.
• B. (Correct) Mesophyll (PEP fixation) vs bundle sheath (Calvin cycle).
• C. CAM plants.
• D. Not necessarily.

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