Life Processes in Plants Part 3: MCQs (Q51–Q75)

Part 3 — Photosynthesis (advanced) & Transportation (Q51–Q75)

51. Which light color is most effective for photosynthesis?

A. Green
B. Blue and red wavelengths ← Correct answer: B
C. Yellow only
D. Infrared only
Explanations:
A) Green is mostly reflected, not absorbed — incorrect.
B) Correct. Chlorophyll absorbs blue and red best.
C/D) Less effective for chlorophyll absorption — incorrect.

52. Which gas exchange occurs when stomata open?

A. Intake of O₂ only
B. Release of CO₂ only
C. Intake of CO₂ and release of O₂ and water vapor ← Correct answer: C
D. No exchange occurs
Explanations:
A/B/D) Partial or false statements — incorrect.
C) Correct. During photosynthesis, stomata allow CO₂ in, O₂ out; transpiration also occurs.

53. Which mineral deficiency would most directly affect chlorophyll formation and photosynthesis?

A. Potassium (K)
B. Magnesium (Mg) ← Correct answer: B
C. Sodium (Na) only
D. Silicon (Si)
Explanations:
A/C/D) Minerals have roles but Mg is central to chlorophyll molecule — B correct.
B) Correct. Mg is central atom in chlorophyll ring; deficiency reduces greening.

54. Which of the following is TRUE about phloem loading?

A. Sugars move only by diffusion into phloem
B. Sugars can be actively loaded into sieve tubes creating osmotic gradient ← Correct answer: B
C. Phloem loading happens in roots only
D. Phloem loading transports only water
Explanations:
A) Can involve active transport too — incomplete.
B) Correct. Active loading concentrates sucrose in phloem, driving mass flow.
C/D) Incorrect.

55. Root pressure can cause:

A. Leaves to expand only at night
B. Exudation of sap and guttation in some plants ← Correct answer: B
C. Formation of phloem only
D. Xylem collapse
Explanations:
A/C/D) Not direct root pressure effects — incorrect.
B) Correct. Positive hydrostatic pressure in roots can push water out of leaf margins (guttation).

56. The primary function of stomata is to:

A. Absorb water from atmosphere
B. Secrete sugars
C. Permit gas exchange and transpiration ← Correct answer: C
D. Fix nitrogen
Explanations:
A/B/D) Not stomatal roles — incorrect.
C) Correct. Stomata are pores controlled by guard cells facilitating gas exchange and water vapor loss.

57. Which process describes movement of sugars from leaves to roots?

A. Transpiration stream
B. Translocation via phloem ← Correct answer: B
C. Diffusion through xylem
D. Active pumping by guard cells
Explanations:
A) Transpiration involves water in xylem — incorrect.
B) Correct. Phloem translocates organic solutes source → sink.
C/D) Incorrect processes.

58. During the night, photosynthesis:

A. Continues at maximum rate
B. Ceases (no light), but respiration continues ← Correct answer: B
C. Produces more oxygen than during day
D. Is independent of light
Explanations:
A/C/D) Wrong.
B) Correct. Light reactions stop at night; dark reactions need ATP/NADPH produced in light; respiration continues.

59. Which factor would NOT increase photosynthesis rate under otherwise optimal conditions?

A. Increased CO₂ concentration
B. Increased light intensity
C. Increased chlorophyll content
D. Decrease in water availability (drought) ← Correct answer: D
Explanations:
A–C) Generally increase photosynthesis — incorrect choices.
D) Correct. Drought causes stomatal closure reducing CO₂ uptake, reducing photosynthesis.

60. Which cell feature is common to both guard cells and mesophyll cells?

A. Thick cell wall only in guard cells
B. Presence of chloroplasts (in many guard cells and mesophyll) ← Correct answer: B
C. Both are dead at maturity
D. Presence of sieve plates
Explanations:
A/C/D) Incorrect statements.
B) Correct. Guard cells have chloroplasts (unlike epidermal cells) and mesophyll cells are chloroplast-rich.

61. Which part of plant conducts sugars from leaves to growing tissues?

A. Xylem tracheids
B. Phloem sieve tubes
C. Phloem (sieve tubes and companion cells) ← Correct answer: C
D. Root hairs
Explanations:
A) Xylem transports water — incorrect.
B/C) Phloem conducts sugars; C is full mention — correct.
D) Root hairs absorb water/minerals — incorrect.

62. Water is lost from leaves mainly through:

A. Cuticle only
B. Xylem transpiration only
C. Stomata (major pathway) ← Correct answer: C
D. Phloem sieve plates
Explanations:
A) Cuticular transpiration occurs but is minor compared to stomatal — incomplete.
B/D) Not correct.
C) Correct. Stomata open for gas exchange causing most transpiration.

63. Which substance is transported in phloem?

A. Mineral ions only
B. Water only
C. Sucrose and other organic molecules ← Correct answer: C
D. Oxygen only
Explanations:
A/B/D) Not accurate for phloem function.
C) Correct. Phloem translocates assimilates (sugars, amino acids).

64. The light-independent reactions (Calvin cycle) occur in:

A. Thylakoid membrane
B. Stroma of chloroplast ← Correct answer: B
C. Mitochondrial matrix
D. Cytoplasm only
Explanations:
A) Light reactions occur in thylakoid membranes — incorrect.
B) Correct. Calvin cycle enzymes are in stroma; use ATP/NADPH to fix CO₂.
C/D) Not location for Calvin cycle in plants — incorrect.

65. Which sequence is correct for movement of water from soil to atmosphere?

A. Atmosphere → stomata → xylem → root → soil
B. Soil → root → xylem → leaf mesophyll → stomata → atmosphere ← Correct answer: B
C. Soil → phloem → atmosphere
D. Soil → stomata → root
Explanations:
A/C/D) Incorrect orderings.
B) Correct. Water uptake in roots → transport via xylem → evaporation via stomata.

66. Which mechanism explains water rise in tall trees?

A. Root pressure alone sufficient for all heights
B. Capillarity alone explains it
C. Cohesion-tension theory (transpiration pull, cohesion, adhesion) ← Correct answer: C
D. Active pumping of water by stem cells
Explanations:
A/B) Insufficient alone for tall trees — incorrect.
C) Correct. Combined cohesion of water molecules and tension from transpiration drives ascent.
D) Not a plant mechanism — incorrect.

67. Which of the following is NOT a product of photosynthesis?

A. Glucose
B. Oxygen
C. Nitrogen ← Correct answer: C
D. Starch (derived from glucose)
Explanations:
A/B/D) Products or derivatives — incorrect choices.
C) Correct. Nitrogen is not produced by photosynthesis.

68. In CAM plants, stomata open at:

A. Day only
B. Night only (to reduce water loss) ← Correct answer: B
C. Always closed
D. Randomly
Explanations:
A/C/D) Not CAM strategy.
B) Correct. CAM plants (e.g., succulents) fix CO₂ at night to minimize water loss.

69. Which component of chlorophyll captures light energy?

A. Central magnesium ion and porphyrin ring
B. Iron atom only
C. Porphyrin ring with Mg at centre ← Correct answer: C
D. Protein core only
Explanations:
A) Correct concept, but C phrasing complete — C chosen.
B/D) Incorrect.

70. Which condition fosters maximum transpiration?

A. High humidity, low wind
B. Hot, dry, windy, bright sunlight ← Correct answer: B
C. Nighttime damp conditions
D. Waterlogged soil only
Explanations:
A/C/D) Conditions that reduce transpiration — incorrect.
B) Correct. Environmental factors increasing vapor pressure deficit raise transpiration.

71. Which of these statements about phloem is FALSE?

A. Phloem conducts in both directions depending on source-sink dynamics
B. Phloem loading can be active and energy-dependent
C. Phloem consists of dead hollow tubes like xylem vessels ← Correct answer: C
D. Companion cells assist sieve tube elements
Explanations:
A/B/D) True statements — incorrect to choose.
C) Correct. Phloem sieve elements are living (though enucleate in some cases) and associated with companion cells; xylem elements are dead.

72. Which hormone increases cell division and is used in tissue culture to promote callus formation?

A. Auxin alone
B. Ethylene only
C. Cytokinin (often with auxin) ← Correct answer: C
D. Abscisic acid only
Explanations:
A/B/D) Not primarily used alone for callus induction — incorrect.
C) Correct. Cytokinins (with auxins) promote cell division and shoot formation in culture.

73. Which of the following best describes transpiration pull strength?

A. Independent of humidity and temperature
B. Increased with higher humidity
C. Increased with lower humidity and higher temperature ← Correct answer: C
D. Caused by phloem loading only
Explanations:
A/B/D) Incorrect relations.
C) Correct. Greater vapor pressure deficit increases transpiration pull.

74. Where is chlorophyll located within the chloroplast?

A. In the stroma only
B. Embedded in thylakoid membranes (grana) ← Correct answer: B
C. In mitochondrial matrix
D. In cytosol only
Explanations:
A/C/D) Incorrect locations.
B) Correct. Light-harvesting complexes in thylakoid membranes contain chlorophyll.

75. Which of these is TRUE about C₃ and C₄ plants?

A. C₃ plants have spatial separation of steps like C₄ — incorrect
B. C₄ plants minimize photorespiration by concentrating CO₂ in bundle sheath cells ← Correct answer: B
C. C₃ plants are always desert succulents — incorrect
D. C₄ plants cannot perform Calvin cycle — incorrect
Explanations:
A/C/D) False statements.
B) Correct. C₄ pathway is adaptation to hot/arid conditions. (more…)