Part 6 — Advanced Concepts: Red Drop, Emerson Effect, Quantum Yield, Photorespiration (Q126–150)

Part 6 (Q126–150) of Chapter 13 – Photosynthesis in Higher Plants (Class 11 NCERT, NEET relevance).

This section covers Advanced Concepts: Red Drop, Emerson Effect, Quantum Yield, and Photorespiration vs Photosynthesis Efficiency.

Part 6 — Advanced Concepts: Red Drop, Emerson Effect, Quantum Yield, Photorespiration Efficiency (Q126–150)

Q126.

The “red drop” refers to:
A. Drop in chlorophyll content in red light
B. Sharp decrease in photosynthetic efficiency beyond 680 nm ✅
C. Sudden fall in CO₂ fixation at night
D. Rapid fall in O₂ release under blue light

Explanation:

• A/D. Not true.
• B. (Correct) At wavelengths >680 nm, quantum yield of photosynthesis decreases → “red drop effect”.
• C. Not related.

Q127.

The red drop was first discovered by:
A. Emerson ✅
B. Blackman
C. Calvin
D. Priestley

Explanation:

• A. (Correct) Emerson observed decline in efficiency beyond red light.
• B–D. Other contributions.

Q128.

The Emerson enhancement effect is observed when:
A. Green and yellow light are combined
B. Red and far-red light are supplied together ✅
C. Blue and green light are combined
D. Only far-red light is used

Explanation:

• A/C/D. Not correct.
• B. (Correct) Photosynthesis rate increases synergistically when red + far-red used together → evidence for two photosystems.

Q129.

The Emerson effect provided evidence for:
A. Existence of one photosystem
B. Existence of two photosystems (PSI and PSII) ✅
C. Photorespiration
D. CAM cycle

Explanation:

• A. Opposite.
• B. (Correct) Enhancement with dual wavelengths → two photosystems.
• C/D. Not related.

Q130.

Quantum yield is defined as:
A. Total photons absorbed per CO₂ fixed
B. Number of O₂ molecules released per photon absorbed ✅
C. ATP generated per photon
D. NADPH generated per photon

Explanation:

• A/C/D. Not precise definitions.
• B. (Correct) Quantum yield = O₂/CO₂ molecules produced per photon.

Q131.

The average quantum yield of photosynthesis under optimal conditions is about:
A. 0.1
B. 1.0
C. 0.25 ✅
D. 2.0

Explanation:

• A. Too low.
• B. Idealized value.
• C. (Correct) About 0.25 O₂ per photon absorbed.
• D. Not correct.

Q132.

The minimum number of photons required to evolve one O₂ molecule in photosynthesis is:
A. 2
B. 4
C. 8 ✅
D. 16

Explanation:

• A/B. Too few.
• C. (Correct) 8 photons needed (4 each for PSI & PSII) to release one O₂.
• D. Too high.

Q133.

The “red drop” is overcome by:
A. Increasing CO₂ concentration
B. Simultaneous illumination with red and far-red light ✅
C. Supplying excess ATP
D. Reducing O₂

Explanation:

• A/C/D. Not the cause.
• B. (Correct) Red + far-red → Emerson enhancement effect → no drop.

Q134.

Why does quantum yield decrease in far-red light?
A. Only PSI absorbs far-red light ✅
B. Only PSII absorbs far-red
C. Both PSI and PSII inactive
D. ATP synthesis increases too much

Explanation:

• A. (Correct) At >680 nm, only PSI works → no water splitting, no NADPH.
• B–D. Incorrect explanations.

Q135.

Which photosystem absorbs maximally at 700 nm?
A. PSII
B. PSI ✅
C. Both PSI and PSII
D. None

Explanation:

• A. PSII absorbs at 680 nm.
• B. (Correct) PSI (P700) absorbs maximally at 700 nm.
• C/D. Wrong.

Q136.

Which photosystem absorbs maximally at 680 nm?
A. PSI
B. PSII ✅
C. Both PSI and PSII
D. None

Explanation:

• A. PSI = 700 nm.
• B. (Correct) PSII (P680).
• C/D. Wrong.

Q137.

Why is PSII absent in stroma lamellae?
A. It requires special pigments
B. To prevent O₂ production near PSI ✅
C. To reduce CO₂ concentration
D. Because it works only at night

Explanation:

• A/C/D. Not correct.
• B. (Correct) Prevents O₂ accumulation near PSI (would inhibit its functioning).

Q138.

The energy equivalence of 1 mole of photons (at 680 nm) is approximately:
A. 175 kJ
B. 175 kcal
C. 40 kcal
D. 42 kcal ✅

Explanation:

• A/B. Too high.
• C/D. (Correct) ~42 kcal/mole at 680 nm.

Q139.

The number of ATP and NADPH used to fix 6 CO₂ in Calvin cycle is:
A. 6 ATP, 6 NADPH
B. 12 ATP, 6 NADPH
C. 18 ATP, 12 NADPH ✅
D. 12 ATP, 18 NADPH

Explanation:

• A/B/D. Incorrect ratios.
• C. (Correct) 6 CO₂ → 18 ATP + 12 NADPH needed.

Q140.

C₄ plants are more efficient than C₃ in photosynthesis because:
A. They require less ATP
B. They suppress photorespiration ✅
C. They lack Rubisco
D. They work in dark

Explanation:

• A. Wrong, they require more ATP.
• B. (Correct) CO₂ concentrating mechanism prevents oxygenase activity.
• C/D. False.

Q141.

Which is the CO₂ compensation point for C₃ plants?
A. 5 ppm
B. 50 ppm ✅
C. 100 ppm
D. 500 ppm

Explanation:

• A. Too low.
• B. (Correct) ~50 ppm for C₃ plants.
• C/D. Too high.

Q142.

Which is the CO₂ compensation point for C₄ plants?
A. <10 ppm ✅
B. 50 ppm
C. 100 ppm
D. 500 ppm

Explanation:

• A. (Correct) C₄ plants ~5–10 ppm.
• B–D. Values for C₃ or beyond.

Q143.

Quantum yield is highest in which light?
A. Green
B. Red ✅
C. Yellow
D. Far-red

Explanation:

• A/C/D. Less effective.
• B. (Correct) Red light most efficient → high quantum yield.

Q144.

The net photosynthetic rate is maximum when:
A. CO₂ is low, O₂ is high
B. CO₂ is high, O₂ is low ✅
C. CO₂ and O₂ are both low
D. CO₂ and O₂ are both high

Explanation:

• A. Increases photorespiration.
• B. (Correct) Maximizes carboxylase activity of Rubisco.
• C/D. Less effective.

Q145.

In which plants does the Warburg effect (inhibition of photosynthesis by high O₂) occur strongly?
A. C₄ plants
B. C₃ plants ✅
C. CAM plants
D. Algae only

Explanation:

• A. C₄ plants avoid this.
• B. (Correct) C₃ plants strongly affected by Warburg effect.
• C/D. Less.

Q146.

In photorespiration, oxygenase activity of Rubisco produces:
A. 1 PGA + 1 phosphoglycolate ✅
B. 2 PGA
C. 1 RuBP + O₂
D. 1 glucose

Explanation:

• A. (Correct) O₂ + RuBP → 1 PGA + 1 phosphoglycolate.
• B–D. Wrong products.

Q147.

Why is photorespiration negligible in C₄ plants?
A. They lack Rubisco
B. Bundle sheath cells have high CO₂ concentration ✅
C. They lack mitochondria
D. They absorb only red light

Explanation:

• A. Rubisco present.
• B. (Correct) CO₂ concentrated near Rubisco → suppresses oxygenase activity.
• C/D. False.

Q148.

Quantum yield of O₂ evolution decreases in:
A. Green light
B. Far-red light (>680 nm) ✅
C. Red light
D. Blue light

Explanation:

• A. Poor absorption but not yield drop specifically.
• B. (Correct) Only PSI active → no O₂ evolution.
• C/D. High efficiency.

Q149.

The main reason for greater efficiency of C₄ plants in tropical climates is:
A. Lower chlorophyll content
B. Higher water-use efficiency ✅
C. Lack of ATP use
D. Absence of Rubisco

Explanation:

• A/C/D. Not correct.
• B. (Correct) C₄ plants conserve water by minimizing photorespiration and closing stomata efficiently.

Q150.

The CO₂ fixation efficiency of Rubisco is low because:
A. It binds both CO₂ and O₂ ✅
B. It requires too much ATP
C. It is located in mitochondria
D. It works only in dark

Explanation:

• A. (Correct) Rubisco is a bifunctional enzyme → wasteful photorespiration occurs.
• B–D. Wrong.

Photosynthesis Class 11, Photosynthesis NEET MCQs, Light reaction Z scheme, Calvin cycle C3 pathway, C4 pathway Kranz anatomy, Photorespiration C2 cycle, Factors affecting photosynthesis, Class 11 Biology MCQs, photosynthesis in higher plants class 11 mcqs, light reaction and z scheme neet questions, calvin cycle c3 pathway notes ncert, c4 pathway and kranz anatomy neet, photorespiration c2 cycle class 11 biology, factors affecting photosynthesis neet questions, photosynthesis experiments class 11 solved mcqs, class 11 biology photosynthesis online learning for neet