Photosynthesis in Higher Plants — Class 11 MCQs with Answers

Photosynthesis in Higher Plants — important questions & MCQs with answers (Class 11 Biology)

6 solved questions from this chapter's difficulty levels, each with its answer and explanation. Tap an option to check yourself.

0 of 6 answered

1. Q1 of 6Easy

   Site of photosynthesis in cell:

   ANucleusBMitochondrionCChloroplastDRibosome
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   Answer: Chloroplast

   Explanation: Chloroplasts hold thylakoids for light reactions and stroma for the Calvin cycle; mitochondria generate ATP via respiration, not photosynthesis.

2. Q2 of 6Easy

   Photosynthesis converts light energy primarily into which form?

   AHeat energyBChemical energy in carbohydratesCMechanical energyDElectrical energy
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   Answer: Chemical energy in carbohydrates

   Explanation: Photosynthesis converts light energy primarily into chemical energy stored in C-C and C-H bonds of carbohydrates, not heat, mechanical or electrical forms.

3. Q3 of 6Medium

   First stable product of C₃ pathway:

   AGlucoseBOAAC3-PGA (3-phosphoglycerate)DPyruvate
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   Answer: 3-PGA (3-phosphoglycerate)

   Explanation: The Calvin cycle's CO2 fixation by RuBisCO first yields two molecules of 3-PGA, a 3-carbon acid, hence the C3 pathway, distinct from C4's 4-carbon OAA.

4. Q4 of 6Medium

   O₂ released in photosynthesis comes from:

   ACO₂BWater (photolysis)CGlucoseDAir
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   Answer: Water (photolysis)

   Explanation: Ruben and Kamen's 18O-labelled water experiment showed O2 comes from photolysis of water at PSII, not from CO2 reduction in the Calvin cycle.

5. Q5 of 6Hard

   Assertion: Green plants release O₂ in light. Reason: O₂ originates from CO₂ during photosynthesis. Choose the correct option.

   AAssertion true, Reason falseBBoth true and Reason correctCBoth falseDAssertion false, Reason true
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   Answer: Assertion true, Reason false

   Explanation: O2 released in photosynthesis originates from water split at PSII, not from CO2, so the assertion is true but the reason is false, as Ruben and Kamen's isotope tracing proved.

6. Q6 of 6Hard

   First stable product of C₄ pathway:

   APyruvateB3-PGACOAA (4-carbon)DGlucose
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   Answer: OAA (4-carbon)

   Explanation: PEP carboxylase fixes CO2 onto PEP in mesophyll cells, forming the 4-carbon oxaloacetate (OAA), the C4 pathway's signature first product, unlike C3's 3-PGA.

Photosynthesis in Higher Plants notes: key concepts (Class 11 Biology)

Green plants convert light energy into chemical energy in food. This chapter explains where photosynthesis happens, how the light reaction splits water to make ATP and NADPH, how the Calvin cycle fixes CO₂ into sugar, why C4 plants beat photorespiration, and which external factors limit the overall rate.

Site and pigments

Occurs in chloroplasts; chlorophyll a is the primary pigment while chlorophyll b, carotenoids and xanthophylls act as accessory pigments harvesting light.

Two stages

The light reaction in thylakoid membranes traps energy as ATP and NADPH; the light-independent Calvin cycle in the stroma then uses these to fix carbon.

Photosystems

Pigment assemblies PS I (P700) and PS II (P680) absorb light at their reaction centres and pass excited electrons into the electron transport chain.

Photolysis of water

PS II splits water into protons, electrons and O₂; this is the source of the oxygen released and replenishes electrons lost by the P680 reaction centre.

Non-cyclic photophosphorylation

Electrons flow PS II → PS I → NADP⁺, building a proton gradient across the thylakoid that ATP synthase uses; it makes ATP, NADPH and releases O₂.

Cyclic photophosphorylation

Only PS I is active; electrons cycle back producing ATP but neither NADPH nor O₂, operating when the cell needs extra ATP for the Calvin cycle.

Calvin cycle (C3)

CO₂ is fixed onto RuBP by RuBisCO forming two 3-carbon PGA molecules; its carboxylation, reduction and RuBP-regeneration steps build sugar.

RuBisCO's dual role

The most abundant enzyme binds both CO₂ and O₂ at the same active site; a high O₂ level triggers photorespiration instead of productive carbon fixation.

C4 pathway

In plants like maize, PEP carboxylase first fixes CO₂ into 4-carbon oxaloacetate in mesophyll, ferrying it to bundle-sheath cells for the Calvin cycle.

Kranz anatomy

C4 leaves have large, chloroplast-rich bundle-sheath cells wrapped around veins; this concentrates CO₂ around RuBisCO and suppresses photorespiration.

Photorespiration

In C3 plants RuBisCO fixes O₂ to make phosphoglycolate; it wastes energy, releases CO₂ and yields no ATP, NADPH or sugar — a costly, wasteful process.

C3 vs C4 efficiency

C4 plants lack photorespiration, show higher productivity and tolerate hot, dry conditions, while C3 plants dominate cooler, temperate climates.

Limiting factors

Blackman's law of limiting factors: at any moment the factor nearest its minimum caps the rate; light, CO₂, temperature and water all influence it.