☀️ Photosynthesis in Higher Plants – MCQs

Part 1 (Q1–Q25)

Q1. Photosynthesis is essentially a process of

A) Energy release
B) Energy conversion ✅
C) Energy storage only
D) Energy destruction

Explanation: Light energy is converted into chemical energy (ATP, NADPH; stored in sugars).

Q2. The site of light reactions is the

A) Stroma
B) Thylakoid membranes (grana) ✅
C) Cytosol
D) Matrix

Explanation: Photosystems, ETC, ATP synthase are embedded in thylakoid membranes.

Q3. The site of dark reactions (Calvin cycle) is

A) Grana
B) Stroma ✅
C) Thylakoid lumen
D) Outer membrane

Explanation: Calvin cycle enzymes (Rubisco, etc.) are located in the stroma.

Q4. The primary CO₂ acceptor in C₃ plants is

A) PEP
B) RuBP (ribulose-1,5-bisphosphate) ✅
C) PGA
D) OAA

Explanation: Rubisco carboxylates RuBP to form 2 PGA in C₃ cycle.

Q5. The primary CO₂ acceptor in C₄ plants is

A) RuBP
B) PEP (phosphoenolpyruvate) ✅
C) OAA
D) Malate

Explanation: PEP carboxylase fixes CO₂ (as HCO₃⁻) to PEP → OAA.

Q6. The main product of the Calvin cycle that exits the cycle is

A) RuBP
B) PGA
C) Triose phosphate (G3P) ✅
D) Glucose

Explanation: G3P is exported to cytosol for sucrose or retained for starch.

Q7. The enzyme that fixes CO₂ in C₃ plants is

A) PEP carboxylase
B) Rubisco ✅
C) Malate dehydrogenase
D) Carbonic anhydrase

Explanation: Rubisco catalyzes carboxylation of RuBP in the Calvin cycle.

Q8. The pigment that initiates the light reaction is

A) Xanthophyll
B) β-carotene
C) Chlorophyll a ✅
D) Chlorophyll b

Explanation: Chlorophyll a is the reaction center pigment in both photosystems.

Q9. PS II reaction center absorbs maximally at

A) 700 nm
B) 680 nm (P680) ✅
C) 650 nm
D) 720 nm

Explanation: PS II reaction center is P680; PS I is P700.

Q10. Splitting of water (photolysis) occurs in

A) PS I
B) PS II ✅
C) Stroma
D) Cytosol

Explanation: OEC (Mn complex) of PS II splits water → O₂, e⁻, H⁺.

Q11. The immediate electron donor to P700⁺ is

A) Plastocyanin ✅
B) Ferredoxin
C) Cytochrome b₆f
D) Plastoquinone

Explanation: In the Z-scheme, plastocyanin reduces P700⁺ of PS I.

Q12. Non-cyclic photophosphorylation produces

A) ATP only
B) NADPH only
C) ATP + NADPH + O₂ ✅
D) O₂ only

Explanation: Linear electron flow (PS II → PS I) yields ATP, NADPH, and O₂.

Q13. Cyclic photophosphorylation involves

A) PS I only ✅
B) PS II only
C) Both PS I & II
D) No photosystem

Explanation: Electrons cycle around PS I → ATP only; no NADPH/O₂.

Q14. The “Z-scheme” represents

A) Calvin cycle steps
B) Electron transport in light reactions ✅
C) Photorespiration
D) CAM pathway

Explanation: Z-scheme shows flow of electrons from water (PS II) → NADP⁺ via PS I.

Q15. The first stable product of C₃ cycle is

A) OAA
B) PGA (3-phosphoglycerate) ✅
C) Malate
D) G3P

Explanation: Carboxylation of RuBP gives two PGA (C₃) molecules.

Q16. The first stable product of C₄ cycle is

A) PGA
B) OAA (C₄) ✅
C) G3P
D) RuBP

Explanation: PEP carboxylase forms OAA (C₄) which is converted to malate/aspartate.

Q17. Kranz anatomy is characteristic of

A) C₃ plants
B) C₄ plants ✅
C) CAM plants
D) Aquatic plants

Explanation: C₄ leaves show Kranz (wreath) anatomy with bundle sheath chloroplasts.

Q18. Photorespiration is high when

A) CO₂ is high
B) O₂ is high and temperature is high ✅
C) Light is low
D) Water is abundant

Explanation: Rubisco oxygenase activity rises with high O₂/low CO₂, high temperature.

Q19. Photorespiration involves these organelles

A) Chloroplast–Mitochondria–Peroxisome ✅
B) Chloroplast–Vacuole–ER
C) Mitochondria–Golgi–ER
D) Peroxisome–Lysosome–Golgi

Explanation: The glycolate pathway spans chloroplasts, peroxisomes, mitochondria.

Q20. The Hill reaction demonstrates

A) CO₂ fixation
B) O₂ evolution from water ✅
C) Starch formation
D) Photorespiration

Explanation: Hill showed that isolated chloroplasts evolve O₂ in light without CO₂.

Q21. The “red drop” (decrease in quantum yield >680 nm) and its enhancement effect were shown by

A) Blackman
B) Calvin
C) Emerson ✅
D) Hill

Explanation: Emerson discovered red drop and enhancement effect, implying two photosystems.

Q22. The law of limiting factors in photosynthesis was proposed by

A) Sachs
B) Blackman ✅
C) Liebig
D) van Niel

Explanation: Blackman’s law: the slowest (limiting) factor controls the rate.

Q23. The Calvin cycle is also known as

A) Hatch–Slack pathway
B) Reductive pentose phosphate pathway ✅
C) C₂ cycle
D) Glyoxylate cycle

Explanation: Calvin–Benson cycle reduces CO₂ via the reductive pentose phosphate route.

Q24. The key carboxylating enzyme in C₄ mesophyll cells is

A) Rubisco
B) PEP carboxylase ✅
C) Carbonic anhydrase
D) Malic enzyme

Explanation: PEPC fixes HCO₃⁻ to PEP producing OAA (C₄ acid).

Q25. C₄ plants show high photosynthetic efficiency primarily due to

A) More chlorophyll a
B) CO₂ concentrating mechanism ✅
C) Larger leaves
D) More stomata

Explanation: Bundle sheath decarboxylation elevates CO₂ near Rubisco, suppressing photorespiration.

☀️ Photosynthesis in Higher Plants

Part 2 (Q26–Q50)

Q26. The Hatch–Slack pathway operates in

A) C₃ plants
B) C₄ plants ✅
C) CAM plants
D) Algae

Explanation: The C₄ pathway was described by Hatch & Slack in sugarcane, maize, etc.

Q27. CAM plants open stomata

A) During day
B) During night ✅
C) Both day and night
D) Never

Explanation: CAM plants (cacti, pineapple) open stomata at night to minimize water loss.

Q28. The first stable product in CAM pathway is

A) PGA
B) OAA ✅
C) G3P
D) RuBP

Explanation: Like C₄ plants, CAM plants first form OAA at night.

Q29. The C₂ cycle is another name for

A) Calvin cycle
B) Photorespiration ✅
C) CAM pathway
D) Glyoxylate cycle

Explanation: Photorespiration is also called the C₂ cycle, producing 2-carbon glycolate.

Q30. The oxygenase activity of Rubisco leads to

A) CO₂ fixation
B) Photorespiration ✅
C) CAM pathway
D) Dark respiration

Explanation: Rubisco has dual function—carboxylase (CO₂ fixation) and oxygenase (photorespiration).

Q31. Which plant shows highest photosynthetic efficiency?

A) Rice
B) Wheat
C) Sugarcane ✅
D) Cotton

Explanation: Sugarcane (C₄ plant) has high efficiency due to no photorespiration.

Q32. Which light is least effective in photosynthesis?

A) Red
B) Blue
C) Green ✅
D) Violet

Explanation: Green light is reflected, not absorbed → least effective.

Q33. Chlorophyll a absorbs maximally at

A) 680 nm and 700 nm ✅
B) 400 nm and 500 nm
C) 650 nm and 720 nm
D) 600 nm only

Explanation: Chlorophyll a peaks at blue (~430 nm) and red (~680–700 nm).

Q34. Accessory pigments (chlorophyll b, carotenoids, xanthophylls) mainly function to

A) Fix CO₂
B) Transfer energy to chlorophyll a ✅
C) Absorb green light
D) Produce ATP

Explanation: They broaden absorption spectrum and transfer energy to Chl a.

Q35. The end products of light reaction are

A) ATP + NADPH + O₂ ✅
B) ATP + CO₂
C) NADH + H₂O
D) Glucose + O₂

Explanation: Light reaction yields ATP, NADPH, and O₂.

Q36. Which one is a C₄ plant?

A) Wheat
B) Rice
C) Maize ✅
D) Pea

Explanation: Maize is a typical C₄ plant with Kranz anatomy.

Q37. Blackman’s law states

A) Light is not necessary
B) Photosynthesis is controlled by the slowest factor ✅
C) Only CO₂ affects rate
D) Only temperature affects rate

Explanation: Blackman (1905): rate is determined by the limiting factor.

Q38. The assimilatory power for Calvin cycle is

A) ATP + NADPH ✅
B) ATP + NADH
C) FADH₂ + NADPH
D) Glucose + ATP

Explanation: Dark reactions require ATP + NADPH from light reaction.

Q39. The Calvin cycle was discovered using

A) Oxygen isotopes
B) Carbon-14 tracer ✅
C) Hydrogen isotopes
D) Nitrogen isotopes

Explanation: Calvin & Benson used ¹⁴C labeling to trace CO₂ fixation.

Q40. In photosynthesis, the hydrogen donor is

A) CO₂
B) H₂O ✅
C) NADPH
D) Glucose

Explanation: Water donates electrons/H⁺ in light reactions, releasing O₂.

Q41. Photorespiration is considered a wasteful process because

A) It fixes CO₂
B) It releases CO₂ without ATP gain ✅
C) It uses NADPH efficiently
D) It increases sugar production

Explanation: Photorespiration releases CO₂ and consumes ATP/NADPH, reducing efficiency.

Q42. The main sugar transported in plants after photosynthesis is

A) Glucose
B) Sucrose ✅
C) Maltose
D) Fructose

Explanation: Sucrose is the chief transport form in phloem.

Q43. C₄ plants are more efficient because

A) They have more Rubisco
B) They concentrate CO₂ at Rubisco site ✅
C) They need less light
D) They avoid Calvin cycle

Explanation: Bundle sheath decarboxylation increases CO₂ near Rubisco → suppresses oxygenase activity.

Q44. CAM plants are common in

A) Tropical rain forests
B) Deserts ✅
C) Temperate forests
D) Marshy areas

Explanation: CAM plants (cacti, pineapple) are desert plants with water-saving metabolism.

Q45. Which factor is NOT a raw material for photosynthesis?

A) Water
B) CO₂
C) Sunlight
D) Oxygen ✅

Explanation: O₂ is a by-product, not a raw material.

Q46. During photophosphorylation, protons accumulate in

A) Stroma
B) Thylakoid lumen ✅
C) Cytosol
D) Outer membrane

Explanation: H⁺ ions accumulate in thylakoid lumen, driving ATP synthesis.

Q47. The proton gradient in light reaction is used for

A) Oxygen release
B) NADPH synthesis
C) ATP synthesis ✅
D) CO₂ fixation

Explanation: Chemiosmosis uses proton gradient across thylakoid to make ATP.

Q48. Which factor increases photosynthetic rate up to a certain limit?

A) Light intensity ✅
B) O₂ concentration
C) Waterlogging
D) Salt stress

Explanation: Higher light intensity boosts photosynthesis until saturation.

Q49. Which factor causes photorespiration to increase?

A) High CO₂
B) High O₂ ✅
C) Low temperature
D) Low O₂

Explanation: High O₂/CO₂ ratio enhances oxygenase activity of Rubisco.

Q50. The number of ATP required for fixation of one CO₂ in Calvin cycle is

A) 2
B) 3 ✅
C) 4
D) 5

Explanation: 3 ATP + 2 NADPH are needed to fix 1 CO₂ into carbohydrate.

☀️ Photosynthesis in Higher Plants

Part 3 (Q51–Q75)

Q51. The energy currency produced in light reaction is

A) NADPH
B) ATP ✅
C) G3P
D) Sucrose

Explanation: ATP formed in light reactions drives the Calvin cycle and other processes.

Q52. Which is called the universal electron donor in photosynthesis?

A) NADPH ✅
B) ATP
C) G3P
D) RuBP

Explanation: NADPH provides reducing power for CO₂ fixation.

Q53. How many CO₂ molecules are required to form one glucose in the Calvin cycle?

A) 3
B) 6 ✅
C) 4
D) 12

Explanation: 6 CO₂ molecules are fixed → one glucose equivalent.

Q54. How many turns of Calvin cycle are needed to synthesize one G3P molecule?

A) 1
B) 2 ✅
C) 3
D) 6

Explanation: Two turns of the Calvin cycle (2 CO₂) yield one G3P exported.

Q55. How many ATP are required to fix 6 CO₂ molecules into glucose?

A) 12
B) 18 ✅
C) 24
D) 30

Explanation: For 1 CO₂ → 3 ATP; thus 6 CO₂ → 18 ATP.

Q56. How many NADPH are required for fixation of 6 CO₂ molecules?

A) 6
B) 12 ✅
C) 18
D) 24

Explanation: For 1 CO₂ → 2 NADPH; thus 6 CO₂ → 12 NADPH.

Q57. Which of the following is a photoautotroph?

A) Fungi
B) Green plants ✅
C) Animals
D) Amoeba

Explanation: Green plants synthesize food using light + CO₂ → photoautotrophs.

Q58. The action spectrum of photosynthesis was demonstrated by

A) Sachs
B) Engelmann ✅
C) Hill
D) Calvin

Explanation: Engelmann’s experiment (1882) showed maximum photosynthesis in red & blue light using oxygen-seeking bacteria.

Q59. The absorption spectrum of chlorophyll a shows peaks in

A) Red and blue regions ✅
B) Green and yellow
C) Infrared only
D) Orange and violet only

Explanation: Chl a absorbs maximally in blue (~430 nm) and red (~680 nm).

Q60. Which light wavelength is least effective in photosynthesis?

A) 400–500 nm
B) 500–600 nm (green) ✅
C) 600–700 nm
D) 700–720 nm

Explanation: Green light is reflected, thus least effective.

Q61. In C₄ plants, Calvin cycle occurs in

A) Mesophyll cells
B) Bundle sheath cells ✅
C) Guard cells
D) Root cells

Explanation: In C₄ plants, Calvin cycle is confined to bundle sheath chloroplasts.

Q62. In C₄ plants, initial CO₂ fixation occurs in

A) Bundle sheath
B) Mesophyll cells ✅
C) Guard cells
D) Phloem

Explanation: Mesophyll cells fix CO₂ by PEP carboxylase → OAA.

Q63. Kranz anatomy refers to

A) Concentric arrangement of bundle sheath cells ✅
B) Stomatal distribution
C) Root anatomy
D) Epidermal tissue

Explanation: Kranz anatomy = wreath-like arrangement of bundle sheath around vascular bundles in C₄ plants.

Q64. Which of the following is a CAM plant?

A) Wheat
B) Rice
C) Pineapple ✅
D) Sugarcane

Explanation: Pineapple uses CAM pathway to minimize water loss.

Q65. In CAM plants, CO₂ is fixed at night into

A) PGA
B) OAA ✅
C) G3P
D) RuBP

Explanation: CO₂ is fixed into OAA at night (via PEP carboxylase).

Q66. In photorespiration, the substrate for Rubisco is

A) CO₂
B) O₂ ✅
C) RuBP
D) Malate

Explanation: Under high O₂/low CO₂, Rubisco oxygenase fixes O₂, initiating photorespiration.

Q67. The toxic compound formed during photorespiration is

A) Glycolate ✅
B) Malate
C) OAA
D) Acetyl-CoA

Explanation: Rubisco oxygenase forms glycolate (2C) which is metabolized via C₂ cycle.

Q68. The oxygenase activity of Rubisco occurs in

A) Mesophyll cells
B) Chloroplast stroma ✅
C) Mitochondria
D) Peroxisomes

Explanation: Rubisco is located in the chloroplast stroma.

Q69. Which organelle is NOT involved in photorespiration?

A) Chloroplast
B) Mitochondria
C) Peroxisome
D) Golgi apparatus ✅

Explanation: Golgi is not involved; photorespiration involves chloroplasts, peroxisomes, mitochondria.

Q70. C₄ plants minimize photorespiration because

A) They lack Rubisco
B) They concentrate CO₂ in bundle sheath ✅
C) They have less stomata
D) They absorb more green light

Explanation: C₄ plants elevate CO₂ near Rubisco, preventing oxygenase action.

Q71. The enzyme PEP carboxylase has

A) High affinity for O₂
B) High affinity for CO₂ ✅
C) No affinity for CO₂
D) Dual affinity for O₂ and CO₂

Explanation: PEPC has high CO₂ affinity, no oxygenase activity.

Q72. The productivity of C₄ plants is higher than C₃ plants because

A) They need less light
B) They avoid photorespiration ✅
C) They fix less CO₂
D) They produce less NADPH

Explanation: C₄ plants eliminate photorespiration, thus more productive.

Q73. The wasteful process associated with Rubisco oxygenase activity is

A) CAM cycle
B) Photorespiration ✅
C) Glycolysis
D) Fermentation

Explanation: Rubisco oxygenase activity → photorespiration.

Q74. The function of carotenoids is

A) Absorb green light
B) Photoprotection and accessory pigments ✅
C) Fix CO₂
D) Oxygen release

Explanation: Carotenoids protect from photooxidation and extend absorption spectrum.

Q75. The Calvin cycle was discovered by

A) Blackman
B) Hill
C) Calvin and Benson ✅
D) Emerson

Explanation: Melvin Calvin and Andrew Benson elucidated the Calvin cycle using ¹⁴C tracers.

Great 🌱 Let’s wrap it up with Part 4 (Q76–Q100) of the 100 MCQs on “Plant Physiology – Photosynthesis in Higher Plants.”
Each question has options (A–D), the correct answer marked, and a short NEET + CBSE explanation.

☀️ Photosynthesis in Higher Plants

Part 4 (Q76–Q100)

Q76. Which statement about Rubisco is correct?

A) It is the most abundant protein on earth ✅
B) It fixes nitrogen
C) It is a pigment protein
D) It splits water

Explanation: Rubisco is the most abundant enzyme, catalyzing CO₂ fixation.

Q77. Which event does not occur during cyclic photophosphorylation?

A) ATP formation
B) NADPH formation ✅
C) Electron cycling through PS I
D) Proton gradient formation

Explanation: Cyclic photophosphorylation makes ATP only, no NADPH or O₂.

Q78. Which pigment absorbs maximum blue light?

A) Chlorophyll a
B) Chlorophyll b ✅
C) Carotenoids
D) Xanthophyll

Explanation: Chlorophyll b absorbs strongly in the blue region (450–500 nm).

Q79. In the Z-scheme, the electron ultimately reduces

A) O₂
B) NADP⁺ ✅
C) Water
D) ATP

Explanation: Electrons from water → PS II → PS I → reduce NADP⁺ → NADPH.

Q80. Oxygen in photosynthesis is released from

A) CO₂
B) H₂O ✅
C) NADPH
D) ATP

Explanation: Photolysis of water releases O₂ as a by-product.

Q81. The plant that shows CAM pathway is

A) Maize
B) Wheat
C) Opuntia ✅
D) Rice

Explanation: Opuntia (cactus) is a CAM plant, fixing CO₂ at night.

Q82. The wasteful cycle that decreases photosynthetic yield is

A) Calvin cycle
B) Photorespiration ✅
C) Glycolysis
D) CAM cycle

Explanation: Photorespiration consumes ATP/NADPH and releases CO₂ → wasteful.

Q83. In C₄ cycle, decarboxylation of malate occurs in

A) Mesophyll
B) Bundle sheath ✅
C) Guard cells
D) Phloem

Explanation: C₄ acids (malate/aspartate) release CO₂ in bundle sheath for Calvin cycle.

Q84. The ATP: NADPH ratio in cyclic photophosphorylation is

A) 1:1
B) More ATP, no NADPH ✅
C) More NADPH, no ATP
D) 2:1

Explanation: Cyclic flow yields ATP only, no NADPH.

Q85. The ATP: NADPH ratio in non-cyclic photophosphorylation is

A) 1:1 ✅
B) 2:1
C) 3:1
D) Variable

Explanation: Linear flow produces equal amounts of ATP and NADPH.

Q86. Which scientist discovered that O₂ comes from water, not CO₂?

A) Hill
B) van Niel ✅
C) Calvin
D) Sachs

Explanation: van Niel proposed O₂ evolves from H₂O splitting, later proven.

Q87. Which is the major site of photosynthesis in plants?

A) Stem
B) Leaf ✅
C) Root
D) Flower

Explanation: Leaves with chloroplast-rich mesophyll are main photosynthetic organs.

Q88. Which plant group lacks Kranz anatomy?

A) C₄ plants
B) C₃ plants ✅
C) CAM plants
D) Maize

Explanation: C₃ plants (e.g., rice, wheat) do not have Kranz anatomy.

Q89. The CO₂ compensation point is lowest in

A) C₃ plants
B) C₄ plants ✅
C) CAM plants
D) Algae

Explanation: C₄ plants have nearly zero photorespiration, very low CO₂ compensation point.

Q90. Which light has maximum photosynthetic efficiency?

A) Green
B) Red ✅
C) Blue
D) Yellow

Explanation: Red light (~680 nm) is most efficient for photosynthesis.

Q91. Which cycle fixes atmospheric CO₂ directly into carbohydrate?

A) C₄ cycle
B) Calvin cycle ✅
C) CAM cycle
D) Glycolate cycle

Explanation: Calvin cycle incorporates CO₂ into sugars.

Q92. The end product of photorespiration is

A) PGA
B) CO₂ ✅
C) Glucose
D) Malate

Explanation: Photorespiration ultimately releases CO₂ without ATP gain.

Q93. Which organelle provides reducing power for Calvin cycle?

A) Mitochondria
B) Chloroplast ✅
C) Peroxisome
D) Cytosol

Explanation: Chloroplast light reactions provide ATP + NADPH for Calvin cycle.

Q94. The dark reaction is called so because

A) It occurs at night only
B) It does not need direct light ✅
C) It requires darkness
D) It happens in mitochondria

Explanation: Dark reactions are light-independent, though they occur in day using ATP/NADPH.

Q95. The number of ATP needed to produce one molecule of O₂ in photosynthesis is

A) 2
B) 3
C) 4 ✅
D) 6

Explanation: 4 electrons are needed to evolve 1 O₂ → linked to 4 ATP synthesis.

Q96. Which compound is the immediate hydrogen donor in Calvin cycle?

A) NADPH ✅
B) FADH₂
C) ATP
D) Water

Explanation: NADPH reduces 3-PGA to G3P in Calvin cycle.

Q97. Which pigment protects chlorophyll from photooxidation?

A) Carotenoids ✅
B) Chlorophyll b
C) Anthocyanins
D) Phytochrome

Explanation: Carotenoids quench excess light energy, preventing oxidative damage.

Q98. Which is the direct product of Calvin cycle?

A) Glucose
B) G3P (triose phosphate) ✅
C) Starch
D) Sucrose

Explanation: Calvin cycle yields G3P, used for glucose, starch, sucrose synthesis.

Q99. The number of Calvin cycle turns required to form one glucose molecule is

A) 3
B) 6 ✅
C) 4
D) 12

Explanation: 6 turns (fixing 6 CO₂) are needed to form one glucose equivalent.

Q100. The main significance of photosynthesis is

A) CO₂ release
B) Conversion of light to chemical energy ✅
C) O₂ consumption
D) Glucose oxidation

Explanation: Photosynthesis sustains life by converting light → chemical energy (glucose, ATP) and producing O₂.

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