Part 5 — Factors Affecting Photosynthesis, Blackman’s Law, Light/CO₂ Curves (Q101–125)

Part 5 (Q101–125) of Chapter 13 – Photosynthesis in Higher Plants (Class 11, NEET relevance).

This section covers Factors Affecting Photosynthesis, Blackman’s Law, and Light/CO₂ Response Curves.

Part 5 — Factors Affecting Photosynthesis, Blackman’s Law, Light/CO₂ Curves (Q101–125)

Q101.

Which of the following is not an external factor affecting photosynthesis?
A. Light
B. CO₂ concentration
C. Temperature
D. Chlorophyll content ✅

Explanation:

• A–C. External factors that influence photosynthesis.
• D. (Correct) Chlorophyll is an internal factor.

Q102.

At very low light intensity, photosynthesis is limited by:
A. CO₂ concentration
B. Light ✅
C. Temperature
D. Water

Explanation:

• A. Not limiting if light is low.
• B. (Correct) Low light = insufficient photons → light is limiting.
• C/D. Secondary at this stage.

Q103.

At high light intensity and sufficient CO₂, which factor becomes limiting?
A. Temperature ✅
B. Chlorophyll
C. Oxygen
D. Water

Explanation:

• A. (Correct) At high light + CO₂, enzyme activity depends on temperature.
• B–D. Not immediate limiting factors.

Q104.

Blackman’s Law of Limiting Factors explains:
A. Only one factor affects photosynthesis
B. The factor in shortest supply limits the rate ✅
C. Light is always the limiting factor
D. CO₂ is never limiting

Explanation:

• A/C/D. Misconceptions.
• B. (Correct) The factor in minimum supply controls photosynthesis.

Q105.

In Engelmann’s experiment, the role of aerobic bacteria was to:
A. Fix nitrogen
B. Indicate oxygen evolution ✅
C. Absorb light
D. Produce ATP

Explanation:

• A. Not their role.
• B. (Correct) Bacteria clustered where O₂ evolved most → indicator organisms.
• C/D. Not correct.

Q106.

At very high O₂ concentration, photosynthesis declines due to:
A. Enhanced carboxylation
B. Photorespiration ✅
C. Lack of light
D. Lack of water

Explanation:

• A. O₂ favors oxygenase activity of Rubisco, not carboxylation.
• B. (Correct) High O₂ increases photorespiration, reducing photosynthesis.
• C/D. Not correct.

Q107.

At which CO₂ concentration is photosynthesis in most C₃ plants saturated?
A. 200 ppm
B. 450 ppm ✅
C. 100 ppm
D. 1000 ppm

Explanation:

• A/C. Too low.
• B. (Correct) In C₃ plants, saturation ~450 ppm CO₂.
• D. Beyond saturation, no further increase.

Q108.

At which CO₂ concentration do C₄ plants saturate photosynthesis?
A. 200 ppm
B. 360 ppm
C. 360–400 ppm
D. > 360 ppm ✅

Explanation:

• A–C. Lower values for C₃.
• D. (Correct) C₄ plants saturate at higher CO₂ than C₃.

Q109.

Which is the optimum temperature range for most C₃ plants?
A. 10–15 °C
B. 25–35 °C ✅
C. 40–50 °C
D. 5–10 °C

Explanation:

• A/D. Too low.
• B. (Correct) C₃ plants optimal at 25–35 °C.
• C. C₄ plants thrive better in higher temperature.

Q110.

Which is the optimum temperature for C₄ plants?
A. 10–15 °C
B. 20–25 °C
C. 30–45 °C ✅
D. 50–60 °C

Explanation:

• A/B. Too low.
• C. (Correct) C₄ plants thrive in 30–45 °C, tropical climates.
• D. Too high for enzymes.

Q111.

Which pigment absorbs maximum light for photosynthesis?
A. Chlorophyll a ✅
B. Chlorophyll b
C. Carotenoids
D. Xanthophylls

Explanation:

• A. (Correct) Chl a is the primary pigment in reaction centers.
• B–D. Accessory pigments.

Q112.

In photosynthetic light response curve, at the compensation point:
A. Photosynthesis > respiration
B. Photosynthesis = respiration ✅
C. Photosynthesis < respiration
D. Photosynthesis stops

Explanation:

• A/C. Not compensation point.
• B. (Correct) CO₂ uptake by photosynthesis equals CO₂ release by respiration.
• D. Not true.

Q113.

At light saturation, if CO₂ concentration is further increased:
A. Rate of photosynthesis increases ✅
B. Rate decreases
C. Remains constant
D. Photosynthesis stops

Explanation:

• A. (Correct) More CO₂ supply enhances carboxylation rate until CO₂ saturation point.
• B/C/D. Not correct.

Q114.

At CO₂ saturation point, increasing light intensity further:
A. Increases photosynthesis
B. No effect on photosynthesis ✅
C. Decreases photosynthesis
D. Stops respiration

Explanation:

• A. Not true.
• B. (Correct) At CO₂ saturation, further increase in light has no effect.
• C/D. Incorrect.

Q115.

Water stress mainly reduces photosynthesis because of:
A. Lack of ATP
B. Stomatal closure reducing CO₂ entry ✅
C. Chlorophyll degradation
D. Rubisco inactivation

Explanation:

• A. Secondary effect.
• B. (Correct) Water deficit closes stomata → less CO₂ available.
• C/D. May occur but not primary cause.

Q116.

In which range of visible light does photosynthesis occur most efficiently?
A. Green (500–570 nm)
B. Blue and red (400–500, 600–700 nm) ✅
C. Yellow (570–590 nm)
D. Infrared (>700 nm)

Explanation:

• A/C/D. Poor absorption.
• B. (Correct) Blue and red light best for photosynthesis.

Q117.

Who first proposed the concept of limiting factors in photosynthesis?
A. Calvin
B. Blackman ✅
C. Ingenhousz
D. Priestley

Explanation:

• A/C/D. Other discoveries.
• B. (Correct) Blackman in 1905 → Law of Limiting Factors.

Q118.

When CO₂ concentration is doubled under light saturation, photosynthesis rate:
A. Increases ✅
B. Decreases
C. Remains unchanged
D. Stops

Explanation:

• A. (Correct) More CO₂ enhances carboxylation by Rubisco until saturation.
• B–D. Incorrect.

Q119.

C₃ plants respond to increased CO₂ with:
A. Decreased rate of photosynthesis
B. Increased rate up to saturation ✅
C. Constant rate
D. Immediate decline

Explanation:

• A/C/D. Not correct.
• B. (Correct) Photosynthesis increases until CO₂ saturation (~450 ppm).

Q120.

In light response curves, C₄ plants show:
A. Lower CO₂ compensation point ✅
B. Higher CO₂ compensation point
C. Same as C₃
D. Zero photorespiration

Explanation:

• A. (Correct) C₄ plants have very low CO₂ compensation point (<10 ppm).
• B. C₃ plants higher (~50 ppm).
• C/D. Not correct fully.

Q121.

In photosynthesis vs light intensity curve, the plateau indicates:
A. Chlorophyll degradation
B. Light saturation point ✅
C. CO₂ compensation
D. Dark reaction only

Explanation:

• A/D. Incorrect.
• B. (Correct) Plateau occurs when increasing light no longer increases photosynthesis.
• C. Different concept.

Q122.

In photosynthesis, the quantum yield decreases in:
A. Blue light
B. Red light
C. Far-red light (>680 nm) ✅
D. Green light

Explanation:

• A/B. Efficient.
• C. (Correct) Beyond red limit, photosynthesis drops (red drop effect).
• D. Poor absorption but not yield decrease.

Q123.

In shade plants, the photosynthetic rate saturates at:
A. Very low light intensity ✅
B. Very high light intensity
C. Only at midday
D. No saturation point

Explanation:

• A. (Correct) Shade plants adapted to low light → saturation achieved early.
• B–D. Not correct.

Q124.

The main internal factor influencing photosynthesis is:
A. Light
B. CO₂ concentration
C. Leaf anatomy and chlorophyll content ✅
D. Water availability

Explanation:

• A/B/D. External factors.
• C. (Correct) Internal factors like chlorophyll and leaf anatomy affect capacity.

Q125.

Which factor is most likely to limit photosynthesis in desert plants during midday?
A. Light intensity
B. CO₂ concentration
C. Stomatal closure due to water stress ✅
D. Temperature

Explanation:

• A/B/D. Light and temp are high, CO₂ is present.
• C. (Correct) Water stress causes stomata to close → CO₂ limitation.

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