250+ TOP MCQs on Factors Affecting Photosynthesis and Answers

Botany Online Quiz on “Factors Affecting Photosynthesis”.

1. Which of these is not an internal factor affecting photosynthesis?
a) Size
b) Age
c) Number
d) Water
Answer: d
Clarification: Various internal and external factors affect the rate of photosynthesis, which in turn affects the overall productivity and yield of the plant. Size, age and number are internal factors while water is an external factor.

2. What do the internal factors affecting photosynthesis dependent on?
a) External factors
b) Geographical area
c) Genetic predisposition
d) Species and sub-species
Answer: c
Clarification: The genetic predisposition or the genetic make up of a plant is an internal factor that affects the rate of photosynthesis. The arrangement of genes present in the plant is expressed in a certain way in different plants.

3. Which of these is not an external factor affecting photosynthesis?
a) Sunlight
b) Temperature
c) Leaf orientation
d) CO₂ concentration
Answer: c
Clarification: Various internal and external factors affect the rate of photosynthesis, which in turn affects the overall productivity and yield of the plant. Sunlight, temperature and CO₂ concentration are external factors.

4. Which of these statements is false regarding the factors affecting photosynthesis?
a) The leaf orientation of a plant affects photosynthesis
b) Both internal and external factors affect photosynthesis
c) Photosynthetic rate is determined by a factor at sub-optimal level
d) Both CO₂ concentration in the environment and inside the plant affect photosynthesis
Answer: a
Clarification: The rate of photosynthesis is affected by both external and internal factors. The photosynthetic rate is determined by a factor at sub-optimal level which is known as the limiting factor.

5. Which scientist gave the Law of Limiting Factors?
a) Hooke
b) Blackman
c) Fleming
d) Mendel
Answer: b
Clarification: Blackman gave the Law of Limiting Factors in 1905. It states that if multiple factors affect a process, then the rate of the process is determined by the factor which is the closest to its minimal value.

6. At what percentage of sunlight does light saturation occur?
a) 30
b) 20
c) 10
d) 15
Answer: c
Clarification: Light is an external factor that affects the rate of photosynthesis. Light saturation occurs at 10% of full sunlight. At high light intensities, the rate of photosynthesis does not increase linearly.

7. A portion of the graph ‘Rate of photosynthesis vs. Light intensity’ is labelled ‘A’. What causes ‘A’?

a) Increase in light intensity
b) Decrease in light intensity
c) Opening of stomata
d) Closing of stomata
Answer: a
Clarification: In the graph ‘Rate of photosynthesis vs. Light intensity’, the portion labelled ‘A’ implies that with an increase in the light intensity, the rate remains constant after some time and does not increase.

8. What happens when the incident light on a plant is excessive?
a) RuBP undergoes oxidation
b) Vascular bundles lose functionality
c) Mesophyll cells are destroyed
d) Chlorophyll breaks down
Answer: d
Clarification: Light is rarely a limiting factor for photosynthesis, but excess amount of light reaching the photosynthetic parts of plant causes the breakdown of chlorophyll and hence, reduces the rate of photosynthesis.

9. What is the atmospheric concentration of carbon dioxide?
a) 0.3 – 0.4%
b) 3 – 4%
c) 0.03 – 0.04%
d) 0.003 – 0.004%
Answer: c
Clarification: The atmospheric concentration of carbon dioxide is 0.03 – 0.04%. Carbon dioxide concentration is an important external factor that affects the rate of photosynthesis in all plants.

10. Increase in CO₂ concentration up to 0.5% causes an increase in photosynthesis. True or false?
a) True
b) False
Answer: b
Clarification: Increase in CO₂ concentration up to 0.05% causes an increase in photosynthesis. If the carbon dioxide concentration increases beyond this level, it can become damaging for the plant over long periods.

11. At what concentration do C₄ plants show saturation?
a) 430 μlL^-1
b) 450 μlL^-1
c) 340 μlL^-1
d) 360 μlL^-1
Answer: d
Clarification: C₃ plants and C₄ plants show different carbon dioxide concentration saturations. At high intensities of light, C₄ plants show carbon dioxide saturation at 360 μlL^-1 while C₃ plants show saturation at 450 μlL^-1.

12. Which of these plants are grown in carbon dioxide enriched atmosphere?
a) Maize
b) Sugarcane
c) Tomato
d) Millets
Answer: c
Clarification: Maize, sugarcane and millets are C₄ plants. Since C₃ plants reach saturation at higher carbon dioxide concentrations, plants such as tomatoes and bell peppers are grown in carbon dioxide enriched atmosphere.

13. Which of these plants have the highest optimum temperature?
a) Tropical C₃
b) Temperate C₃
c) Tropical C₄
d) Temperate C₄
Answer: c
Clarification: Tropical plants have a higher optimum temperature than temperate plants. C₄ plants show a higher rate of photosynthesis at higher temperatures than C₃ plants. Hence, tropical C₄ plants have the highest optimum temperature.

14. Why do stomata close during water shortage?
a) To reduce CO₂ availability
b) To trap water
c) To increase metabolic activity
d) To increase the surface area of leaves
Answer: a
Clarification: During water shortage in plants, the stomata close in order to reduce the availability of carbon dioxide for carbon dioxide fixation, which is the first part of the Calvin cycle of photosynthesis.

15. Which of these is not an effect of water stress?
a) Closing of stomata
b) Increase in metabolism
c) Wilting of leaves
d) Reduction in surface area of leaves
Answer: b
Clarification: When there is a water stress, its effects can be seen on the plant. Water stress closes stomata hence carbon dioxide availability reduces. The leaves wilt to reduce surface area and metabolism.

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