30 free MCQs on Photosynthesis in Higher Plants with worked answers and explanations. Light reactions, the Calvin cycle, C3/C4/CAM pathways, photorespiration, and factors affecting the rate of photosynthesis. One of the highest-yield NEET chapters.
Take the timed Photosynthesis in Higher Plants quiz →Below are 30 practice questions on Photosynthesis in Higher Plants, sorted Easy → Hard. Tap “Show answer & explanation” under any question to check yourself. Want the full theory first? Read the Photosynthesis in Higher Plants notes.
Simplified photosynthesis diagram: light energy, CO2, and H2O enter the leaf and are converted in the chloroplast into glucose and O2.
Q1.
Photosynthesis takes place in the:
Answer: C. Chloroplast
Why: Photosynthesis occurs in chloroplasts, specifically in the thylakoid membranes (light reactions) and stroma (dark reactions).
Q2.
The overall equation for photosynthesis is:
Answer: B. 6CO2 + 6H2O → C6H12O6 + 6O2
Why: 6CO2 + 6H2O + light energy → C6H12O6 + 6O2. Carbon dioxide and water are converted to glucose and oxygen using light.
Q3.
Chlorophyll absorbs which colors of light most strongly?
Answer: B. Red and blue-violet
Why: Chlorophyll absorbs red and blue-violet light. Green light is reflected: this is why plants appear green.
Q4.
The dark reaction (Calvin cycle) occurs in the:
Answer: C. Stroma of chloroplast
Why: The Calvin cycle (dark reactions) occurs in the stroma of the chloroplast. It uses ATP and NADPH to fix CO2 into sugars.
Q5.
Plants need which element for chlorophyll synthesis?
Answer: B. Magnesium
Why: Chlorophyll has magnesium (Mg) at its center. Magnesium deficiency causes yellowing (chlorosis) in plants.
Q6.
The Calvin cycle is also called:
Answer: B. C3 cycle or dark reaction
Why: Calvin cycle = C3 pathway = dark reaction (or light-independent reaction). First stable product is 3-carbon PGA (3-phosphoglycerate).
Q7.
Photorespiration is the process in which:
Answer: B. Rubisco fixes O2 instead of CO2, reducing efficiency
Why: Photorespiration: at high O2/low CO2, Rubisco adds O2 instead of CO2 to RuBP. This wasteful process reduces photosynthesis efficiency in C3 plants.
Q8.
In C4 plants, CO2 is initially fixed in mesophyll cells to form:
Answer: B. Oxaloacetate (4-carbon compound)
Why: C4 plants first fix CO2 into oxaloacetate (4-carbon) in mesophyll cells via PEP carboxylase, then transfer CO2 to bundle sheath for Calvin cycle.
Q9.
The first product of the Calvin cycle (C3 pathway) is:
Answer: C. PGA (3-phosphoglycerate)
Why: In the Calvin cycle, CO2 + RuBP → 2 molecules of PGA (3-phosphoglycerate, a 3-carbon compound). Hence called C3 pathway.
Q10.
The green pigment chlorophyll, essential for photosynthesis, is located within which cell organelle?
Answer: D. Chloroplast
Why: Chlorophyll is contained within the chloroplasts of plant cells, specifically embedded in the membranes of thylakoids, where it captures light energy for photosynthesis.
Q11.
C4 plants like maize have an advantage over C3 plants because:
Answer: B. PEP carboxylase does not fix O2, reducing photorespiration in hot climates
Why: C4 plants use PEP carboxylase (high affinity for CO2, no affinity for O2) in mesophyll, concentrating CO2 for Rubisco in bundle sheath: minimizing photorespiration.
Q12.
The compensation point in photosynthesis is where:
Answer: B. Photosynthesis rate equals respiration rate (net gas exchange = 0)
Why: Compensation point: light intensity at which CO2 fixed by photosynthesis = CO2 released by respiration. Net gas exchange is zero.
Q13.
The ratio of cyclic to non-cyclic photophosphorylation differs in:
Answer: A. Amount of ATP and NADPH produced (cyclic makes only ATP; non-cyclic makes ATP + NADPH + O2)
Why: Cyclic photophosphorylation (PS I only): produces ATP only. Non-cyclic (PS II + PS I): produces ATP + NADPH, and water is oxidized releasing O2.
Q14.
In the dark, CAM plants open their stomata to:
Answer: B. Fix CO2 at night as malate (stored in vacuole) to reduce water loss during day
Why: CAM plants (cacti, succulents): stomata open at night to fix CO2 (as malate). Stomata closed during hot day to prevent water loss. CO2 released from malate for Calvin cycle during day.
Q15.
ATP and NADPH from light reactions are used in the Calvin cycle to:
Answer: C. Reduce CO2 to produce G3P (glyceraldehyde-3-phosphate)
Why: Calvin cycle: ATP and NADPH (from light reactions) provide energy and electrons to convert CO2 into G3P, which is used to make sugars.
Q16.
Rubisco catalyzes in the Calvin cycle:
Answer: B. CO2 fixation: CO2 + RuBP → 2 PGA
Why: Rubisco (RuBisCO = ribulose bisphosphate carboxylase/oxygenase) catalyzes fixation of CO2 onto RuBP to form 2 molecules of 3-PGA.
Q17.
The site of the light-dependent reactions of photosynthesis within the chloroplast is the:
Answer: C. Thylakoid membrane, within the grana
Why: Light reactions, including light absorption, water splitting and ATP/NADPH formation, occur on the thylakoid membranes, which contain the photosystems and electron transport chain.
Q18.
Photorespiration in C3 plants occurs mainly because rubisco:
Answer: A. Can bind O2 as well as CO2 when CO2 is low and O2 is high
Why: Rubisco has both carboxylase and oxygenase activity; under high O2/low CO2 conditions, it binds O2 instead, initiating photorespiration, which consumes energy without fixing carbon or producing sugar.
Q19.
Splitting of the water molecule during photosynthesis (photolysis) takes place in association with:
Answer: C. Photosystem II, replacing electrons lost by chlorophyll
Why: Photolysis of water occurs near Photosystem II, supplying electrons to replace those lost by excited chlorophyll molecules, with O2 released as a by-product.
Q20.
In C3 plants, the first stable product formed after CO2 fixation by rubisco is:
Answer: D. 3-phosphoglycerate (PGA), a 3-carbon compound
Why: In C3 plants, rubisco fixes CO2 onto RuBP to form an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglyceric acid (PGA), a 3-carbon compound.
Q21.
In the Q cycle of photosynthesis, the function is to:
Answer: B. Pump protons across thylakoid membrane via plastoquinone (PQ) oxidation/reduction, increasing H+ gradient for ATP synthesis
Why: Q cycle at the cytochrome b6f complex: transfers 2 electrons from plastoquinol (PQH2) to plastocyanin while pumping extra protons into the thylakoid lumen, boosting ATP synthesis.
Q22.
Ferredoxin-NADP+ reductase (FNR) in the light reactions:
Answer: B. Reduces NADP+ to NADPH using electrons from ferredoxin
Why: FNR: enzyme that accepts 2 electrons from 2 reduced ferredoxin molecules and transfers them to NADP+, reducing it to NADPH (final electron acceptor of the light reactions).
Q23.
The oxygen-evolving complex (OEC) in PS II contains:
Answer: B. A manganese cluster (Mn4CaO5) that oxidizes water to release O2
Why: OEC (oxygen-evolving complex) contains a Mn4CaO5 cluster. It cycles through 5 S-states (S0-S4). At S4, two water molecules are oxidized to release O2, 4H+ and 4e-.
Q24.
In the Calvin cycle, 3 turns are needed to produce one net G3P because:
Answer: B. Each turn fixes one CO2 and regenerates RuBP; 3 CO2 are needed to produce 1 net G3P (a 3-carbon molecule)
Why: Calvin cycle: 3 CO2 + 3 RuBP → 6 PGA → 6 G3P. 5 G3P are used to regenerate 3 RuBP, leaving 1 net G3P. Full cycle uses 9 ATP and 6 NADPH for 3 CO2.
Q25.
Rubisco is a poor catalyst because:
Answer: B. It has low catalytic rate (kcat ~3/sec) and reacts with O2 as well as CO2 (oxygenase activity), causing photorespiration
Why: Rubisco: very slow turnover (~3 CO2/sec vs ~1000 for typical enzymes). Cannot fully discriminate between CO2 and O2. At high O2/low CO2, adds O2 (photorespiration), wasting energy.
Q26.
The 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway in plastids produces:
Answer: B. Terpenoids (isoprene, carotenoids, diterpenes) for photosynthesis and plant defense
Why: MEP pathway (methylerythritol phosphate/non-mevalonate pathway) in plastids produces IPP and DMAPP, precursors for carotenoids, chlorophylls, gibberellins, and isoprene.
Q27.
Crassulacean acid metabolism (CAM) evolved as:
Answer: B. Adaptation to arid conditions -- temporally separating CO2 fixation (night) from Calvin cycle (day) to minimize water loss
Why: CAM: evolved in desert plants (cacti, agaves, succulents) for maximal water use efficiency. Night CO2 fixation (PEP carboxylase) stores CO2 as malate; day decarboxylation feeds Calvin cycle with stomata closed.
Q28.
State transition (dark-to-light adaptation) in chloroplasts involves:
Answer: B. Reversible phosphorylation of LHCII, which migrates between PS II and PS I to balance excitation
Why: State transitions: excess excitation of PS II activates STN7 kinase, which phosphorylates LHCII. Phospho-LHCII migrates to PS I (state 2). PPH1/TAP38 phosphatase reverses this, returning to state 1.
Q29.
The photorespiratory salvage pathway (C2 cycle) recycles:
Answer: B. Glycolate (toxic product of Rubisco oxygenase) back to PGA via chloroplast, peroxisome, and mitochondria
Why: C2 photorespiratory cycle: 2-phosphoglycolate from Rubisco oxygenase → glycolate (chloroplast) → glyoxylate (peroxisome) → glycine (mitochondria, releases CO2 and NH3) → serine → PGA.
Q30.
The Hill reaction, a classic experiment demonstrating the light reaction of photosynthesis, established that:
Answer: B. Isolated chloroplasts can evolve oxygen from water in the presence of a suitable electron acceptor, even without CO2 fixation
Why: Robert Hill demonstrated that isolated chloroplasts, supplied with an artificial electron acceptor, could evolve oxygen from water even in the absence of CO2, showing that O2 evolution (water splitting) is independent of CO2 fixation.