Factors Affecting Photosynthesis — NEET Biology MCQs with Solutions
Free NEET Biology Factors Affecting Photosynthesis MCQs with step-by-step solutions (48 questions). Part of Photosynthesis in Higher Plants. Practise online on Prepizo — no login needed.
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Questions with solutions
Q1 — Factors Affecting Photosynthesis · easy · theory
Blackman's law of limiting factors states that when several factors affect a process, its rate is limited by the factor that is:
A. At its maximum
B. Nearest to its minimum value ✓ Correct
C. Constant
D. Unrelated to the rate
Solution: The pace of the process is set by the most limiting (scarcest) factor at that time.
Q2 — Factors Affecting Photosynthesis · easy · theory
Which is NOT an external factor affecting photosynthesis?
A. Temperature
B. CO₂ concentration
C. Light
D. Number of chloroplasts (internal) ✓ Correct
Solution: External factors: light, CO₂, temperature, water. Internal factors include chlorophyll amount, leaf age, chloroplast number.
Q3 — Factors Affecting Photosynthesis · medium · theory
At light saturation, further increase in light intensity does not raise the rate because:
A. Temperature falls
B. Chlorophyll is destroyed
C. Another factor (e.g. CO₂) becomes limiting ✓ Correct
D. Light is no longer absorbed
Solution: Beyond the saturation point, light is no longer limiting; some other factor caps the rate.
Q4 — Factors Affecting Photosynthesis · medium · theory
For most C3 plants, increasing CO₂ concentration up to ______ increases photosynthesis, and this is used commercially in greenhouses:
A. ~78%
B. ~0.05% (higher than atmospheric) ✓ Correct
C. ~21%
D. ~0.001%
Solution: Raising CO₂ (to ~0.05%) boosts C3 photosynthesis (CO₂ fertilisation); C4 plants saturate at lower CO₂.
Q5 — Factors Affecting Photosynthesis · hard · theory
Very high light intensity can DECREASE the photosynthetic rate due to:
A. Excess CO₂
B. Photo-oxidation (breakdown) of chlorophyll ✓ Correct
C. Increased water
D. Low temperature
Solution: Excessive light causes photo-oxidative damage (photoinhibition) to the pigments and photosynthetic apparatus.
Q6 — Factors Affecting Photosynthesis · hard · theory
The temperature response of photosynthesis differs between C3 and C4 plants in that:
A. Both have identical optima
B. Temperature has no effect on C4
C. C4 plants have a higher temperature optimum than C3 plants ✓ Correct
D. C3 plants tolerate higher temperatures
Solution: C4 plants are adapted to warmer, high-light environments and photosynthesise optimally at higher temperatures than C3 plants.
Q7 — Factors Affecting Photosynthesis · medium · numerical
Blackman's law of limiting factors is illustrated when a leaf in bright light and warm temperature stops responding to more light but responds at once to added CO₂. This shows that at that moment:
A. temperature has become the limiting factor for photosynthesis
B. CO₂ has become the limiting factor for photosynthesis ✓ Correct
C. no single factor is limiting the rate
D. light has become the limiting factor for photosynthesis
Solution: When raising light gives no effect but raising CO₂ does, CO₂ is the factor in shortest supply and therefore limits the rate — the essence of Blackman's law.
Q8 — Factors Affecting Photosynthesis · medium · numerical
Assertion (A): Increasing light intensity beyond a certain point no longer raises the photosynthetic rate. Reason (R): At very high light intensity, some other factor such as CO₂ becomes limiting, or photo-oxidation (breakdown of pigments) begins.
A. Both A and R are true and R is the correct explanation of A ✓ Correct
B. A is false but R is true
C. A is true but R is false
D. Both A and R are true but R is not the correct explanation of A
Solution: Light saturation is reached at about 10% of full sunlight; beyond it, either another factor limits the rate or excess light causes photo-oxidative damage — R explains A.
Q9 — Factors Affecting Photosynthesis · medium · numerical
Raising atmospheric CO₂ from the current level to a higher concentration boosts photosynthesis much more in C3 plants than in C4 plants. The reason is that:
A. C4 plants already have CO₂-concentrating machinery and are near-saturated, whereas C3 plants are not ✓ Correct
B. C4 plants lack RuBisCO and so cannot use the extra CO₂
C. CO₂ is toxic to the RuBisCO of C4 plants at high concentration
D. C3 plants close their stomata at high CO₂ while C4 plants open theirs
Solution: C4 plants concentrate CO₂ in the bundle sheath and are already saturated, so extra CO₂ helps little; C3 plants, limited by CO₂, respond strongly — the basis of CO₂ enrichment in greenhouses.
Q10 — Factors Affecting Photosynthesis · medium · numerical
The four factors listed by the book that influence the rate of photosynthesis are light, CO₂ concentration, temperature and:
A. the concentration of atmospheric oxygen only
B. the amount of chlorophyll (an internal/plant factor) ✓ Correct
C. the wind speed over the leaf surface
D. the soil pH around the roots
Solution: Besides the three external factors (light, CO₂, temperature), the internal factor is the amount of chlorophyll (a plant factor tied to leaf number/age and growth).
Q11 — Factors Affecting Photosynthesis · medium · numerical
The temperature optimum of photosynthesis is not the same for all plants. C4 plants respond to higher temperatures with a higher rate of photosynthesis than C3 plants because C4 plants:
A. depend on light rather than enzymes, which are temperature-insensitive
B. carry out photosynthesis only below 15 °C
C. are adapted to warmer, tropical habitats and their enzymes work best there ✓ Correct
D. lack any temperature optimum and photosynthesise equally at all temperatures
Solution: Because the dark reactions are enzyme-controlled and temperature-sensitive, and C4 plants are typically tropical, they show a higher temperature optimum than temperate C3 plants.
Q12 — Factors Affecting Photosynthesis · easy · numerical
Which of the following is an INTERNAL (plant) factor, rather than an external factor, affecting the rate of photosynthesis?
A. the ambient temperature around the plant
B. the number and age of leaves and the amount of chlorophyll ✓ Correct
C. the concentration of carbon dioxide in the air
D. the intensity of the incident light
Solution: Leaf number, leaf age, chloroplast and chlorophyll content are internal (plant) factors; light, CO₂ and temperature are the external factors.
Q13 — Factors Affecting Photosynthesis · easy · numerical
Of the wavelengths of light, photosynthesis is driven most effectively by:
A. blue and red light ✓ Correct
B. green and yellow light
C. ultraviolet light alone
D. infrared light alone
Solution: Chlorophyll absorbs and uses blue and red light most effectively, which is why the action spectrum of photosynthesis peaks there; green light is largely reflected.
Q14 — Factors Affecting Photosynthesis · easy · numerical
At any given moment, Blackman's principle holds that the rate of photosynthesis is governed by:
A. the average of all the factors acting together
B. only the light intensity, regardless of other factors
C. the factor that is present in the greatest amount
D. the factor that is present in the least (limiting) amount ✓ Correct
Solution: By the law of limiting factors, when several factors affect a process its rate is set by the one closest to its minimum value; correcting that factor raises the rate.
Q15 — Factors Affecting Photosynthesis · easy
Photosynthesis is dependent on
A. internal factors of plant
B. external factors of environment
C. both (1) and (2) ✓ Correct
D. None of these
Solution: The rate of photosynthesis is controlled by both internal (plant) factors such as chlorophyll, number and age of leaves, and mesophyll, and external (environmental) factors such as light, CO2, temperature and water.
Q16 — Factors Affecting Photosynthesis · medium
How many of the following factors affect photosynthesis? Age of leaves, orientation of leaf, internal CO2 concentration, amount of chlorophyll, number of leaves
A. 4
B. 3
C. 2
D. 5 ✓ Correct
Solution: All five listed factors (age of leaves, orientation, internal CO2, chlorophyll, number of leaves) are internal plant factors that affect photosynthesis.
Q17 — Factors Affecting Photosynthesis · medium
Blackman's which law comes into effect when several factors affect any biochemical process?
A. Law of Limited Components (1905)
B. Law of Limited Components (1910)
C. Law of Limiting Factors (1905) ✓ Correct
D. Law of Limiting Factors (1910)
Solution: Blackman (1905) proposed the Law of Limiting Factors governing biochemical processes affected by multiple factors.
Q18 — Factors Affecting Photosynthesis · medium
According to Blackman's Law, the rate of a chemical process is determined by the factor which
A. both (1) and (2)
B. Is nearest to its maximum value
C. Is nearest to its minimum value ✓ Correct
D. none of these
Solution: By the Law of Limiting Factors, the rate is limited by the factor that is nearest its minimum value (in shortest supply).
Q19 — Factors Affecting Photosynthesis · medium
The relationship between incident light and CO2 fixation rate at higher light intensity is
A. Rate decreases with increasing intensity
B. None of these
C. Rate increases with increasing intensity
D. Rate is constant with increasing intensity ✓ Correct
Solution: At higher light intensities the rate levels off (light saturation), so it remains roughly constant with further increase in intensity.
Q20 — Factors Affecting Photosynthesis · medium
At lower light intensity, the rate of CO2 fixation
A. Increases with increasing light intensity ✓ Correct
B. Decreases with increasing light intensity
C. None of these
D. Is constant with increasing light intensity
Solution: At low light intensities the rate of CO2 fixation increases linearly with increasing light intensity.
Q21 — Factors Affecting Photosynthesis · medium
Light saturation occurs at ________ of full sunlight.
A. 5%
B. 20%
C. 10% ✓ Correct
D. 40%
Solution: Light saturation of photosynthesis is reached at about 10% of full sunlight; beyond this, light is rarely limiting in nature.
Q22 — Factors Affecting Photosynthesis · medium
Very big increase in light intensity causes
A. None of these
B. Increase in photosynthesis
C. Decrease in photosynthesis ✓ Correct
D. No change in photosynthesis rate
Solution: A very large increase in light intensity causes breakdown of chlorophyll and a decrease in the photosynthetic rate.
Q23 — Factors Affecting Photosynthesis · medium
C4 plants show saturation at CO2 concentration of
A. 360 µL/L ✓ Correct
B. 240 µL/L
C. 540 µL/L
D. 450 µL/L
Solution: C4 plants show CO2 saturation at about 360 µL/L, a much lower concentration than C3 plants.
Q24 — Factors Affecting Photosynthesis · medium
C3 plants show saturation for CO2 concentration at
A. 240 µL/L
B. 450 µL/L ✓ Correct
C. 540 µL/L
D. 360 µL/L
Solution: C3 plants respond to increased CO2 and show saturation only beyond about 450 µL/L.
Q25 — Factors Affecting Photosynthesis · hard
Assertion: Greenhouse crops like tomatoes and bell pepper are grown in CO2 enriched atmosphere for higher yields. Reason: C4 plants respond to higher CO2 concentration by showing increased rate of photosynthesis. Choose the correct option.
A. Assertion and Reason are wrong
B. Both Assertion and Reason are correct but Reason is not the explanation for Assertion
C. Assertion is correct but Reason is wrong ✓ Correct
D. Both Assertion and Reason are correct and Reason is correct explanation for Assertion
Solution: Tomato and bell pepper are C3 plants (not C4) and it is C3 plants that respond to higher CO2; hence the assertion is correct but the reason is wrong.
Q26 — Factors Affecting Photosynthesis · medium
Which of the following is true?
A. Light is rarely a limiting factor for photosynthesis in nature
B. Both (1) and (2) ✓ Correct
C. CO2 is a major limiting factor for photosynthesis in nature
D. None of these
Solution: In nature CO2 is the major limiting factor for photosynthesis, while light is rarely limiting.
Q27 — Factors Affecting Photosynthesis · medium
The concentration of CO2 beyond which the CO2 becomes damaging over longer periods is
A. 0.08%
B. 0.04%
C. 0.05% ✓ Correct
D. 0.03%
Solution: CO2 concentrations higher than about 0.05% can become damaging to plants over longer periods.
Q28 — Factors Affecting Photosynthesis · medium
The CO2 fixation rate increases up to the CO2 concentration of
A. 800 ppm
B. 500 ppm ✓ Correct
C. 400 ppm
D. 300 ppm
Solution: CO2 fixation rate rises with CO2 up to roughly 0.05% (about 500 ppm), beyond which it becomes damaging.
Q29 — Factors Affecting Photosynthesis · hard
At low light conditions, which of the groups respond positively to increase in CO2?
A. Both
B. C3
C. C4
D. None ✓ Correct
Solution: At low light intensities neither C3 nor C4 plants respond to increased CO2, because light itself is limiting.
Q30 — Factors Affecting Photosynthesis · hard
At high light conditions, which of the groups respond positively to increase in CO2?
A. C4
B. Both
C. C3 ✓ Correct
D. None
Solution: At high light intensity C4 plants saturate at low CO2 (~360 µL/L), while C3 plants respond positively to increased CO2, saturating only beyond ~450 µL/L.