📘 Study MCQs
Q1. Photosynthesis is a process that converts light energy into chemical energy.
NCERT Basic
• Light energy into chemical energy
• Chemical energy into light energy
• Heat energy into mechanical energy
• Kinetic energy into potential energy
Answer: Light energy into chemical energy
During photosynthesis, plants use sunlight to convert carbon dioxide and water into glucose and oxygen.
The chlorophyll pigment captures light energy which is then stored as chemical energy in glucose molecules.
This process forms the basis of food chains and oxygen production.
Q2. Which pigment is essential for photosynthesis in green plants?
NCERT 2023
• Chlorophyll
• Hemoglobin
• Melanin
• Xanthophyll
Answer: Chlorophyll
Chlorophyll is the primary pigment that captures light energy for photosynthesis.
It absorbs light in the blue and red regions of the spectrum, reflecting green light.
Found in chloroplasts, it initiates the light-dependent reactions.
Q3. The site of light-dependent reactions in photosynthesis is:
NEET 2021
• Thylakoid membranes
• Stroma
• Cytoplasm
• Mitochondria
Answer: Thylakoid membranes
The thylakoid membranes of chloroplasts contain photosystems I and II where light-dependent reactions occur.
These reactions include photolysis of water and ATP synthesis via chemiosmosis.
Electron transport chain components are embedded in these membranes.
Q4. In photosynthesis, oxygen is released from:
• Photolysis of water
• Breakdown of carbon dioxide
• Reduction of glucose
• Oxidation of chlorophyll
Answer: Photolysis of water
During light reactions, water molecules split into oxygen, protons, and electrons.
This process occurs in photosystem II and is catalyzed by the oxygen-evolving complex.
The oxygen is released as a byproduct into the atmosphere.
Q5. The primary acceptor of CO2 in C3 plants is:
NEET 2024
• RuBP
• PEP
• OAA
• PGA
Answer: RuBP
Ribulose-1,5-bisphosphate (RuBP) is the 5-carbon compound that accepts CO2 in the Calvin cycle.
The enzyme RuBisCO catalyzes this carboxylation reaction.
This forms an unstable 6-carbon intermediate that splits into two molecules of 3-PGA.
Q6. Photosynthesis is maximum in which light?
• Red light
• Green light
• Blue light
• Yellow light
Answer: Red light
Chlorophyll absorbs maximum light in the red and blue regions of the spectrum.
Red light has the optimal wavelength (around 680 nm) for photosystem II activation.
Green light is mostly reflected, which is why plants appear green.
Q7. The enzyme RuBisCO is involved in:
NCERT Important
• Carboxylation
• Decarboxylation
• Phosphorylation
• Hydrolysis
Answer: Carboxylation
RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes the carboxylation of RuBP with CO2.
It is the most abundant enzyme on Earth.
It can also catalyze oxygenation of RuBP in photorespiration.
Q8. The first stable product of C4 pathway is:
NEET 2021
• Oxaloacetic acid (OAA)
• Phosphoglyceric acid (PGA)
• Phosphoenolpyruvate (PEP)
• Malic acid
Answer: Oxaloacetic acid (OAA)
In C4 plants, CO2 is initially fixed by PEP carboxylase to form OAA (4-carbon compound).
This occurs in mesophyll cells to concentrate CO2 around RuBisCO.
OAA is then converted to malate or aspartate for transport to bundle sheath cells.
Q9. The number of ATP molecules required to produce one glucose molecule in C3 plants is:
• 18
• 12
• 30
• 36
Answer: 18
For one glucose molecule (6 turns of Calvin cycle): 12 NADPH and 18 ATP are required.
Specifically: 6 CO2 require 12 NADPH and 18 ATP for reduction to glucose.
This includes ATP for regeneration of RuBP.
Q10. Kranz anatomy is characteristic of:
NCERT 2023
• C4 plants
• C3 plants
• CAM plants
• Parasitic plants
Answer: C4 plants
Kranz anatomy refers to the wreath-like arrangement of bundle sheath cells around vascular bundles.
These plants have two types of photosynthetic cells: mesophyll and bundle sheath.
This anatomy minimizes photorespiration by spatially separating initial and final CO2 fixation.
Q11. The phenomenon of photorespiration occurs in:
• C3 plants
• C4 plants
• CAM plants
• All plants
Answer: C3 plants
Photorespiration occurs when RuBisCO acts as an oxygenase rather than carboxylase.
It is more pronounced in C3 plants under conditions of high O2 and low CO2.
Leads to loss of fixed carbon and consumption of ATP without sugar production.
Q12. Which metal is present in chlorophyll?
NEET 2021
• Magnesium
• Iron
• Copper
• Zinc
Answer: Magnesium
Chlorophyll contains a magnesium ion at the center of its porphyrin ring.
Magnesium is essential for chlorophyll synthesis and photosynthesis.
Deficiency causes chlorosis (yellowing of leaves).
Q13. The pigment that protects chlorophyll from photo-oxidation is:
NCERT 2023
• Carotenoids
• Phycobilins
• Anthocyanins
• Flavonoids
Answer: Carotenoids
Carotenoids absorb excess light energy that could damage chlorophyll.
They dissipate energy as heat through the xanthophyll cycle.
Also extend the range of light absorption for photosynthesis.
Q14. The C4 pathway is also called:
• Hatch and Slack pathway
• Calvin pathway
• EMP pathway
• TCA pathway
Answer: Hatch and Slack pathway
Discovered by Marshall Hatch and Roger Slack in the 1960s.
This pathway minimizes photorespiration by concentrating CO2.
Involves initial fixation in mesophyll cells and final fixation in bundle sheath cells.
Q15. The wavelength of light most effective in photosynthesis is:
NEET 2024
• 400-700 nm
• 300-400 nm
• 700-800 nm
• 200-300 nm
Answer: 400-700 nm
This range is called Photosynthetically Active Radiation (PAR).
Blue (400-500 nm) and red (600-700 nm) are most effective.
Green light (500-600 nm) is least absorbed.
Q16. Photosystem I is associated with:
NCERT 2023
• P700
• P680
• P650
• P720
Answer: P700
Photosystem I has reaction center chlorophyll a with absorption maximum at 700 nm.
It is involved in cyclic and non-cyclic photophosphorylation.
Receives electrons from plastocyanin and reduces NADP+ to NADPH.
Q17. During cyclic photophosphorylation, the electron acceptor is:
• Ferredoxin
• NADP+
• Oxygen
• Water
Answer: Ferredoxin
In cyclic photophosphorylation, electrons from PS I return to the electron transport chain via ferredoxin.
Only ATP is produced, no NADPH or oxygen evolution occurs.
This helps maintain ATP/NADPH balance for Calvin cycle.
Q18. The chemiosmotic mechanism in chloroplasts involves:
NCERT Important
• Proton gradient across thylakoid membrane
• Electron gradient across stroma
• Sodium-potassium pump
• Calcium ion channels
Answer: Proton gradient across thylakoid membrane
Light-driven electron transport pumps protons from stroma into thylakoid lumen.
This creates a proton gradient and pH difference across the membrane.
ATP synthase uses this gradient to produce ATP (chemiosmosis).
Q19. CAM plants are adapted to:
• Arid conditions
• Aquatic conditions
• Temperate regions
• Tundra regions
Answer: Arid conditions
Crassulacean Acid Metabolism (CAM) plants open stomata at night to minimize water loss.
They fix CO2 into malic acid at night and release it for photosynthesis during day.
Examples: cacti, pineapple, bryophyllum.
Q20. Which is the correct sequence of electron acceptors in Z-scheme?
NCERT 2023
• PQ → Cyt b6f → PC
• PC → Cyt b6f → PQ
• Cyt b6f → PC → PQ
• PQ → PC → Cyt b6f
Answer: PQ → Cyt b6f → PC
In non-cyclic electron flow: PS II → Pheophytin → PQ → Cyt b6f → PC → PS I → Ferredoxin → NADP+.
Plastoquinone (PQ) carries electrons to cytochrome b6f complex.
Plastocyanin (PC) transfers electrons to PS I.
Q21. The ratio of oxygen produced to CO2 consumed in photosynthesis is:
• 1:1
• 2:1
• 1:2
• 3:1
Answer: 1:1
The overall equation: 6CO2 + 12H2O → C6H12O6 + 6O2 + 6H2O
For each CO2 molecule consumed, one O2 molecule is produced.
This ratio remains constant under normal photosynthetic conditions.
Q22. Action spectrum of photosynthesis was first described by:
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• Engelmann
• Calvin
• Blackman
• Arnon
Answer: Engelmann
Theodor Engelmann used aerobic bacteria to map action spectrum in 1882.
Bacteria clustered in regions of blue and red light where oxygen production was maximum.
This demonstrated chlorophyll’s role in specific light absorption.
Q23. The process that competes with carboxylation in C3 plants is:
NEET 2024
• Photorespiration
• Photophosphorylation
• Photolysis
• Phototropism
Answer: Photorespiration
At high temperatures and high O2 concentrations, RuBisCO acts as oxygenase.
This initiates photorespiration which competes with carboxylation.
Reduces photosynthetic efficiency by up to 25% in C3 plants.
Q24. Bundle sheath cells in C4 plants lack:
• Photosystem II
• RuBisCO
• PEP carboxylase
• Chloroplasts
Answer: Photosystem II
Bundle sheath chloroplasts in C4 plants are agranal (lack grana).
They contain PS I but lack PS II, preventing oxygen evolution in these cells.
This creates a low-oxygen environment favorable for RuBisCO activity.
Q25. The primary function of light reactions is to produce:
NCERT Important
• ATP and NADPH
• Glucose and oxygen
• ADP and NADP+
• Starch and sucrose
Answer: ATP and NADPH
Light reactions convert light energy to chemical energy as ATP and NADPH.
These products drive the dark reactions (Calvin cycle) for CO2 fixation.
Oxygen is released as a byproduct during these reactions.
Q26. The splitting of water during photosynthesis is called:
NEET 2021
• Photolysis
• Hydrolysis
• Pyrolysis
• Electrolysis
Answer: Photolysis
Photolysis of water occurs in PS II and requires light energy.
2H2O → 4H+ + 4e- + O2
Manganese cluster in oxygen-evolving complex catalyzes this reaction.
Provides electrons for the electron transport chain.
Q27. Compensation point refers to the light intensity where:
• Photosynthesis = Respiration
• Photosynthesis > Respiration
• Photosynthesis < Respiration
• No photosynthesis occurs
Answer: Photosynthesis = Respiration
At compensation point, CO2 fixed in photosynthesis equals CO2 released in respiration.
No net gas exchange occurs.
Below this point, plant consumes stored food; above it, accumulates food.
Q28. Who proposed the Law of Limiting Factors in photosynthesis?
NCERT 2023
• Blackman
• Engelmann
• Calvin
• Pasteur
Answer: Blackman
F.F. Blackman (1905) proposed that when a process depends on several factors, its rate is limited by the slowest factor.
For example, at low light, light is limiting; at high light, CO2 or temperature may be limiting.
This principle helps understand photosynthesis optimization.
Q29. In C3 plants, the first product of CO2 fixation is:
NCERT Important
• 3-Phosphoglycerate (3-PGA)
• Oxaloacetate (OAA)
• Phosphoenolpyruvate (PEP)
• Malate
Answer: 3-Phosphoglycerate (3-PGA)
RuBisCO catalyzes carboxylation of RuBP to form unstable 6C intermediate.
This immediately splits into two molecules of 3-PGA (3-carbon compound).
3-PGA is then reduced to G3P in the Calvin cycle using ATP and NADPH.
Q30. The pigment involved in red drop phenomenon is:
• Chlorophyll a
• Chlorophyll b
• Carotenoids
• Phycoerythrin
Answer: Chlorophyll a
Red drop refers to sharp decline in quantum yield beyond 680 nm.
Discovered by Robert Emerson in 1957.
Led to discovery of two photosystems working together for efficient photosynthesis.