Factors affecting photosynthesis MCQs With Answer

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Understanding Factors affecting photosynthesis MCQs With Answer is essential for B.Pharm students studying plant physiology, pharmacognosy, and drug-plant interactions. This concise, informative introduction covers key determinants—light intensity and quality, CO2 concentration, temperature, water availability, mineral nutrients (N, Mg), stomatal conductance, chlorophyll content, photoinhibition, and limiting factors like Rubisco activity and mesophyll diffusion. These concepts link directly to experimental measurements (gas exchange, chlorophyll fluorescence) and practical applications such as greenhouse CO2 enrichment or stress responses that influence secondary metabolite production. Clear mastery of these topics strengthens your analytical skills for research and quality control in pharmaceutical contexts. ‘Now let’s test your knowledge with 50 MCQs on this topic.’

Q1. Which factor most directly determines the initial rate of the light reactions in photosynthesis?

  • Leaf nitrogen content
  • Light intensity
  • Soil pH
  • Root hydraulic conductivity

Correct Answer: Light intensity

Q2. The point at which CO2 uptake by photosynthesis equals CO2 release by respiration is called the:

  • Light saturation point
  • Quantum yield
  • Compensation point
  • Saturation deficit

Correct Answer: Compensation point

Q3. Which wavelength ranges are most efficiently absorbed by chlorophyll a for photosynthesis?

  • Green (500–570 nm)
  • Yellow (570–590 nm)
  • Blue (430–450 nm) and red (660–680 nm)
  • Far-red (700–750 nm)

Correct Answer: Blue (430–450 nm) and red (660–680 nm)

Q4. A limiting factor concept in photosynthesis means:

  • All factors increase photosynthesis equally
  • The factor at suboptimal level restricts the overall rate
  • Only light intensity matters
  • Only internal chlorophyll content matters

Correct Answer: The factor at suboptimal level restricts the overall rate

Q5. Which enzyme catalyzes the first major step of carbon fixation in the Calvin cycle?

  • ATP synthase
  • Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase)
  • Phosphofructokinase
  • Nitrate reductase

Correct Answer: Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase)

Q6. How does stomatal closure affect photosynthesis during water stress?

  • Increases CO2 uptake and raises photosynthesis
  • Reduces transpiration but increases chlorophyll synthesis
  • Decreases CO2 diffusion into the leaf and limits photosynthesis
  • Enhances mesophyll conductance

Correct Answer: Decreases CO2 diffusion into the leaf and limits photosynthesis

Q7. Which nutrient is central to chlorophyll structure and thus affects photosynthetic capacity?

  • Calcium (Ca)
  • Magnesium (Mg)
  • Potassium (K)
  • Silicon (Si)

Correct Answer: Magnesium (Mg)

Q8. Photoinhibition occurs primarily when:

  • Plants receive low night temperatures only
  • Photosystems absorb more light energy than can be used, damaging PSII
  • CO2 concentration is extremely high
  • Soil moisture is optimal

Correct Answer: Photosystems absorb more light energy than can be used, damaging PSII

Q9. Which process increases photorespiration, reducing net photosynthesis in C3 plants?

  • High atmospheric CO2 concentration
  • Low oxygen concentration
  • High temperature increasing Rubisco oxygenase activity
  • Increased leaf nitrogen content

Correct Answer: High temperature increasing Rubisco oxygenase activity

Q10. C4 plants have an advantage under which conditions compared to C3 plants?

  • Low temperature and high humidity
  • High CO2 and low light
  • High temperature, high light, and low atmospheric CO2
  • Waterlogged soils

Correct Answer: High temperature, high light, and low atmospheric CO2

Q11. Which measurement technique directly assesses net photosynthetic CO2 assimilation in leaves?

  • Chlorophyll fluorescence (Fv/Fm) only
  • Gas-exchange analysis (infrared gas analyzer)
  • Soil moisture probe
  • Leaf colorimetry

Correct Answer: Gas-exchange analysis (infrared gas analyzer)

Q12. Which pigment class primarily provides photoprotection by quenching excess light?

  • Anthocyanins
  • Carotenoids
  • Cellulose
  • Starch granules

Correct Answer: Carotenoids

Q13. The light saturation point refers to:

  • The light intensity beyond which photosynthetic rate no longer increases
  • The minimum light required for any photosynthesis
  • The point of maximum photoinhibition
  • The night-day transition

Correct Answer: The light intensity beyond which photosynthetic rate no longer increases

Q14. Which factor alters the activity of enzymes in the Calvin cycle most directly?

  • Soil texture
  • Leaf surface area
  • Temperature
  • Wind speed

Correct Answer: Temperature

Q15. Elevated atmospheric CO2 commonly affects photosynthesis by:

  • Reducing carbon fixation due to enzyme inhibition
  • Enhancing carboxylation rates and sometimes increasing water-use efficiency
  • Directly increasing stomatal density always
  • Destroying chlorophyll molecules

Correct Answer: Enhancing carboxylation rates and sometimes increasing water-use efficiency

Q16. Which leaf internal feature facilitates rapid CO2 diffusion to chloroplasts?

  • Thick cuticle
  • High mesophyll surface area and intercellular airspace
  • Large vascular bundles only
  • Rigid epidermis

Correct Answer: High mesophyll surface area and intercellular airspace

Q17. Which environmental pollutant is known to impair photosynthesis by entering stomata and damaging chloroplasts?

  • Nitrogen gas (N2)
  • Sulfur dioxide (SO2)
  • Argon
  • Helium

Correct Answer: Sulfur dioxide (SO2)

Q18. In chlorophyll fluorescence, a reduced Fv/Fm ratio commonly indicates:

  • Optimal photosynthetic efficiency
  • Photoinhibition or stress reducing PSII efficiency
  • Excess nitrogen availability
  • Increased stomatal conductance

Correct Answer: Photoinhibition or stress reducing PSII efficiency

Q19. Which herbicide type directly blocks electron flow at photosystem II?

  • PSI electron acceptor enhancers
  • PSII inhibitors like DCMU
  • Auxin analogues
  • Cellulose synthesis inhibitors

Correct Answer: PSII inhibitors like DCMU

Q20. How does salinity primarily reduce photosynthetic rates?

  • By increasing chlorophyll synthesis
  • Through osmotic stress reducing water uptake and stomatal opening
  • By directly converting Rubisco to inactive form via phosphorylation
  • By increasing CO2 diffusion into leaves

Correct Answer: Through osmotic stress reducing water uptake and stomatal opening

Q21. The quantum yield of photosynthesis refers to:

  • The number of photons required to produce one molecule of O2 or fixed CO2
  • The leaf area index in a canopy
  • The amount of chlorophyll per unit leaf area
  • The stomatal aperture at midday

Correct Answer: The number of photons required to produce one molecule of O2 or fixed CO2

Q22. Which change would most likely increase mesophyll conductance to CO2?

  • Increased cell wall thickness
  • Reduced intercellular air spaces
  • Higher chloroplast surface area facing intercellular airspaces
  • Increased cuticular waxes

Correct Answer: Higher chloroplast surface area facing intercellular airspaces

Q23. In CAM plants, when is CO2 primarily fixed into organic acids?

  • During the day via stomatal opening
  • At night when stomata open, storing CO2 as malic acid
  • Only during drought conditions
  • In high humidity afternoons

Correct Answer: At night when stomata open, storing CO2 as malic acid

Q24. Which factor directly affects the activation state of Rubisco?

  • ATP/ADP ratio and Rubisco activase activity linked to light and stromal conditions
  • Soil particle size alone
  • Leaf color only
  • Relative humidity at night only

Correct Answer: ATP/ADP ratio and Rubisco activase activity linked to light and stromal conditions

Q25. Which effect is expected when leaf temperature exceeds the optimal range for photosynthesis?

  • Increased Rubisco specificity for CO2 and higher rates
  • Denaturation of proteins, increased photorespiration, and reduced net photosynthesis
  • Complete cessation of transpiration
  • Immediate increase in leaf chlorophyll

Correct Answer: Denaturation of proteins, increased photorespiration, and reduced net photosynthesis

Q26. Which structural adaptation reduces overheating and helps maintain photosynthesis in high light?

  • Thin, broad leaves with dark pigments
  • Leaf rolling, reflective surfaces, or pubescence to reduce incident radiation
  • Increased stomatal density regardless of water status
  • Removal of the cuticle

Correct Answer: Leaf rolling, reflective surfaces, or pubescence to reduce incident radiation

Q27. How does nitrogen deficiency commonly manifest with respect to photosynthesis?

  • Increased chlorophyll and higher photosynthetic rate
  • Lower chlorophyll content, reduced Rubisco, and decreased photosynthesis
  • Higher mesophyll conductance
  • Increased stomatal opening at night

Correct Answer: Lower chlorophyll content, reduced Rubisco, and decreased photosynthesis

Q28. Which statement about the chloroplast thylakoid membranes is true regarding photosynthesis?

  • They are site of the Calvin cycle carbon fixation reactions
  • They house the light-harvesting complexes and electron transport chain generating ATP and NADPH
  • They are primarily storage for starch
  • They control stomatal aperture

Correct Answer: They house the light-harvesting complexes and electron transport chain generating ATP and NADPH

Q29. Which experimental manipulation is most likely to directly demonstrate light limitation in a leaf gas-exchange experiment?

  • Increasing chamber CO2 while holding light constant
  • Decreasing light intensity and measuring proportional decline in assimilation rate
  • Adding excess soil nitrogen
  • Applying foliar ABA

Correct Answer: Decreasing light intensity and measuring proportional decline in assimilation rate

Q30. The compensation point for light is best described as the light intensity where:

  • Photosynthesis equals respiration in terms of carbon balance
  • Chlorophyll is synthesized fastest
  • Stomata are fully closed
  • Leaf temperature equals ambient

Correct Answer: Photosynthesis equals respiration in terms of carbon balance

Q31. Which factor will most directly shift the CO2 compensation point to a higher value in C3 plants?

  • Lower leaf temperature
  • Higher oxygen concentration increasing photorespiration
  • Decreased light intensity
  • Increased soil phosphorus

Correct Answer: Higher oxygen concentration increasing photorespiration

Q32. Which leaf anatomical trait reduces internal CO2 diffusion resistance and can improve photosynthesis?

  • Smaller mesophyll surface area exposed to airspaces
  • Thicker palisade layer without increased airspace
  • Greater intercellular airspace and thin cell walls
  • Heavier cuticle deposition

Correct Answer: Greater intercellular airspace and thin cell walls

Q33. Which describes the effect of high light combined with low CO2 on photosystem function?

  • Balanced ATP/NADPH consumption and enhanced Calvin cycle throughput
  • Excess excitation energy leading to reactive oxygen species and photodamage if not dissipated
  • Immediate increase in Rubisco activation state
  • Complete prevention of photorespiration

Correct Answer: Excess excitation energy leading to reactive oxygen species and photodamage if not dissipated

Q34. Which of the following is a rapid non-invasive indicator of PSII efficiency used in research and diagnostics?

  • Leaf dry weight
  • Chlorophyll fluorescence parameter Fv/Fm
  • Root length measurement
  • Soil electrical conductivity

Correct Answer: Chlorophyll fluorescence parameter Fv/Fm

Q35. How do heavy metals such as cadmium typically impair photosynthesis?

  • By increasing stomatal conductance substantially
  • By substituting magnesium in chlorophyll and disrupting electron transport and enzymes
  • By boosting Rubisco activity
  • By increasing leaf water potential

Correct Answer: By substituting magnesium in chlorophyll and disrupting electron transport and enzymes

Q36. LED light spectra for growth chambers can influence photosynthesis by:

  • Delivering only green light which is most absorbed
  • Allowing customization of red:blue ratios to optimize photosystem excitation and photomorphogenesis
  • Increasing soil CO2 availability
  • Eliminating the need for nutrients

Correct Answer: Allowing customization of red:blue ratios to optimize photosystem excitation and photomorphogenesis

Q37. Which physiological regulator causes stomatal closure under drought, thereby limiting photosynthesis?

  • Cytokinin
  • Abscisic acid (ABA)
  • Gibberellin
  • Auxin

Correct Answer: Abscisic acid (ABA)

Q38. Which statement about light quality and photosynthesis is correct?

  • Green light is the most effective for driving photosynthesis at all canopy depths
  • Blue light influences stomatal opening and photomorphogenic responses, while red light is efficient for driving photosynthesis
  • Far-red light has no effect on plant photoreceptors
  • UV light universally increases photosynthetic CO2 fixation

Correct Answer: Blue light influences stomatal opening and photomorphogenic responses, while red light is efficient for driving photosynthesis

Q39. Which condition will most likely decrease chlorophyll content per unit leaf area and reduce photosynthesis?

  • Moderate fertilizer application
  • Severe nitrogen deficiency
  • Optimal watering
  • Mild shading

Correct Answer: Severe nitrogen deficiency

Q40. An increase in boundary layer resistance around a leaf will most likely:

  • Enhance CO2 diffusion into the stomata
  • Reduce gas exchange and can limit photosynthesis under still conditions
  • Increase leaf temperature uniformly without affecting gas exchange
  • Decrease the risk of photoinhibition automatically

Correct Answer: Reduce gas exchange and can limit photosynthesis under still conditions

Q41. Which agricultural practice can directly increase photosynthetic carbon assimilation in greenhouse crops?

  • Reducing light intensity to very low levels
  • Enriching greenhouse air with CO2 to elevate ambient CO2 concentration
  • Applying high-salt irrigation exclusively
  • Blocking all blue light wavelengths

Correct Answer: Enriching greenhouse air with CO2 to elevate ambient CO2 concentration

Q42. Which effect does severe waterlogging (anoxia in roots) typically have on photosynthesis?

  • Enhances root uptake of CO2 and increases photosynthesis
  • Impairs root function, reduces water and nutrient supply, and thereby decreases photosynthesis
  • Always increases stomatal conductance
  • Increases chlorophyll synthesis directly

Correct Answer: Impairs root function, reduces water and nutrient supply, and thereby decreases photosynthesis

Q43. Leaf age affects photosynthetic capacity because:

  • All leaves have equal Rubisco content regardless of age
  • Young expanding leaves may have incomplete chloroplast development; very old leaves show senescence and reduced photosynthetic enzymes
  • Older leaves always perform better than young leaves
  • Leaf age only affects transpiration, not photosynthesis

Correct Answer: Young expanding leaves may have incomplete chloroplast development; very old leaves show senescence and reduced photosynthetic enzymes

Q44. Which describes the relationship between temperature and photosynthetic rate in most plants?

  • Photosynthetic rate is independent of temperature
  • Rate increases with temperature up to an optimum, then declines as enzymes are denatured and photorespiration rises
  • Photosynthesis always increases linearly with temperature
  • Photosynthesis ceases below 50°C only

Correct Answer: Rate increases with temperature up to an optimum, then declines as enzymes are denatured and photorespiration rises

Q45. Which method would help differentiate whether CO2 or light is the limiting factor in a leaf photosynthesis experiment?

  • Simultaneously decrease both CO2 and light
  • Vary CO2 concentration at saturating light and observe assimilation response
  • Only measure soil pH
  • Change leaf color artificially

Correct Answer: Vary CO2 concentration at saturating light and observe assimilation response

Q46. Which is a likely effect of ozone pollution on leaf photosynthesis?

  • Ozone acts as a fertilizer increasing photosynthesis
  • It can oxidatively damage membranes and enzymes, decreasing photosynthetic efficiency
  • Ozone permanently opens stomata increasing CO2 uptake
  • It exclusively increases root growth to compensate

Correct Answer: It can oxidatively damage membranes and enzymes, decreasing photosynthetic efficiency

Q47. The term “light-use efficiency” commonly refers to:

  • The ratio of absorbed photons to biomass or CO2 fixed
  • The amount of light reflected by leaves
  • The leaf thickness at noon
  • The stomatal index of a species

Correct Answer: The ratio of absorbed photons to biomass or CO2 fixed

Q48. Why does canopy structure influence whole-plant photosynthesis?

  • Canopy structure controls distribution of light, leaf angles, and self-shading, affecting integrated photosynthetic rates
  • Canopy structure only affects root growth, not photosynthesis
  • All leaves in canopy receive identical light
  • It only affects plant height but not photosynthetic efficiency

Correct Answer: Canopy structure controls distribution of light, leaf angles, and self-shading, affecting integrated photosynthetic rates

Q49. Which cellular change supports sustained high photosynthetic rates during prolonged high-light exposure?

  • Downregulation of antioxidant systems
  • Upregulation of photoprotective pigments and heat dissipation mechanisms (non-photochemical quenching)
  • Permanent closure of stomata
  • Reduction of chloroplast number per cell

Correct Answer: Upregulation of photoprotective pigments and heat dissipation mechanisms (non-photochemical quenching)

Q50. To enhance photosynthetic productivity in crop breeding, which traits are most promising targets?

  • Increased leaf reflectance and reduced chlorophyll
  • Improved Rubisco specificity or regeneration capacity, enhanced mesophyll conductance, and optimized canopy light distribution
  • Reduced nutrient uptake capacity
  • Increased sensitivity to photoinhibition

Correct Answer: Improved Rubisco specificity or regeneration capacity, enhanced mesophyll conductance, and optimized canopy light distribution

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