The chloroplast is the organelle where photosynthesis happens — and NEET questions on this topic test whether you actually know its internal architecture or are guessing from vague diagrams.
Structure. Chloroplasts are double-membrane-bound organelles found in mesophyll cells. The outer membrane is permeable to small molecules; the inner membrane is selective. Inside lies the stroma — the aqueous matrix — and suspended within the stroma is a third membrane system: the thylakoid membrane. Thylakoids are flattened, disc-like sacs. Where thylakoids stack into columns, the stack is called a granum (plural: grana). Thylakoids connecting adjacent grana are called stroma lamellae (or intergranal thylakoids). The space enclosed within each thylakoid is the lumen (NCERT Class 11 Biology Chapter 11, page 140).
Pigments. Photosynthetic pigments are embedded in the thylakoid membrane. The primary pigment is chlorophyll a — the only pigment that directly participates in the light reactions by donating an excited electron to the reaction centre. Chlorophyll b, xanthophylls, and carotenoids are accessory pigments: they absorb wavelengths that chlorophyll a cannot and transfer that energy to chlorophyll a. Carotenoids also protect chlorophyll from photo-oxidation.
Where the confusion hits. A common NEET trap is confusing the site of the light reactions (thylakoid membrane, specifically the grana thylakoids) with the site of the Calvin cycle (stroma). Another frequent error: calling stroma lamellae the same thing as grana — they are NOT stacked, and they lack PS II.
Watch out: Questions may ask what is present in stroma lamellae versus grana thylakoids. Stroma lamellae contain PS I but lack PS II, meaning they carry out only cyclic photophosphorylation.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
How many membranes form the ENVELOPE that bounds a chloroplast?
Answer: B. B is correct. NCERT states that, like mitochondria, chloroplasts are double membrane bound: the envelope is an outer membrane and an inner membrane. Keep that separate from what lies INSIDE — the thylakoids, grana and stroma lamellae form a membranous system within the stroma, and it is a system of flattened sacs, not a third bounding layer. Counting a thylakoid as part of the envelope is the usual way this question is lost (NCERT Class 11 Biology Chapter 11, page 140).
Why A is wrong: A is wrong because a single bounding membrane describes organelles such as the lysosome or vacuole. A chloroplast, like a mitochondrion and a nucleus, is double membrane bound.
Why C is wrong: C is wrong because four would require two extra bounding layers that no chloroplast has. Note that some algal plastids acquired by secondary endosymbiosis do have three or four envelope membranes, but that is outside the NCERT syllabus and does not apply to plant chloroplasts.
Why D is wrong: D is wrong because it adds the thylakoid membrane to the two envelope membranes. The thylakoid is an internal membranous system suspended in the stroma — it does not bound the organelle.
Which pigment directly participates in the photochemical reaction at the reaction centre?
Answer: C. Chlorophyll a is the only pigment that donates its excited electron to the reaction centre in the light reactions. All other pigments are accessory and transfer absorbed energy to chlorophyll a (NCERT Class 11 Biology Chapter 11, page 138).
Why A is wrong: A — Chlorophyll b is an accessory pigment. It absorbs light energy and transfers it to chlorophyll a but does not itself participate in the photochemical reaction.
Why B is wrong: B — Xanthophylls are accessory pigments belonging to the carotenoid family. They broaden the absorption spectrum but do not function at the reaction centre.
Why D is wrong: D — Carotenoids serve as accessory pigments and photoprotective molecules. They do not directly participate in the reaction centre chemistry.
The space enclosed within the thylakoid membrane is called the:
Answer: C. The internal space of a thylakoid is termed the lumen (or thylakoid lumen). The stroma is the fluid outside the thylakoids but inside the chloroplast envelope (NCERT Class 11 Biology Chapter 11, page 140).
Why A is wrong: A — Stroma is the aqueous matrix surrounding the thylakoids inside the chloroplast envelope. It is NOT the space inside the thylakoid.
Why B is wrong: B — Matrix refers to the interior of the mitochondrion, not the chloroplast thylakoid.
Why D is wrong: D — Cisterna is a term used for flattened sacs of the Golgi apparatus or endoplasmic reticulum, not thylakoids.
A student observes that stroma lamellae can perform cyclic photophosphorylation but not non-cyclic photophosphorylation. Which of the following best explains this observation?
Answer: A. Stroma lamellae (intergranal thylakoids) contain PS I but lack PS II. Non-cyclic photophosphorylation requires both PS II and PS I, so only cyclic electron flow (PS I alone) can occur in stroma lamellae (NCERT Class 11 Biology Chapter 11, page 140).
Why B is wrong: B — Stroma lamellae do contain chlorophyll a (as part of PS I). The absence is specifically of PS II, not of chlorophyll a itself.
Why C is wrong: C — Stroma lamellae do possess ATP synthase complexes, which is precisely why they can carry out cyclic photophosphorylation and generate ATP.
Why D is wrong: D — Stroma lamellae are membrane structures (they are thylakoid extensions connecting grana). They are membrane-bound by definition.
A chloroplast extract is treated so that all carotenoid pigments are selectively removed. Which of the following consequences is most likely under high light intensity?
Answer: B. Carotenoids protect chlorophyll from photo-oxidation by quenching triplet-state chlorophyll and scavenging reactive oxygen species. Without them, excess light energy damages chlorophyll molecules (NCERT Class 11 Biology Chapter 11, page 138).
Why A is wrong: A — The Calvin cycle depends on ATP and NADPH supply, not directly on carotenoids. It would not stop immediately upon carotenoid removal, though it would be affected over time as chlorophyll degrades.
Why C is wrong: C — Stroma pH is influenced by proton pumping during the light reactions. Removing carotenoids does not directly cause a dramatic stroma pH shift; the primary effect is photoprotective loss.
Why D is wrong: D — There is no mechanism by which removing carotenoids causes thylakoid membrane fusion. Membrane structure is maintained by lipids and integral proteins, not by carotenoids.
Which of the following correctly matches the chloroplast compartment with a process that occurs there?
Answer: B. Photolysis of water (splitting of H₂O to release O₂, protons, and electrons) occurs at the PS II complex, which is located in the grana thylakoid membrane (NCERT Class 11 Biology Chapter 11, pages 139–140).
Why A is wrong: A — CO₂ fixation (Calvin cycle) occurs in the stroma, not in the thylakoid lumen. The lumen accumulates H⁺ ions to drive chemiosmotic ATP synthesis.
Why C is wrong: C — Light-dependent electron transport chains are embedded in the thylakoid membrane, not free in the stroma. The stroma is the site of the Calvin cycle.
Why D is wrong: D — The outer membrane of the chloroplast is a permeable envelope layer. No photosynthetic electron transport or phosphorylation occurs there.
A researcher isolates intact chloroplasts and separates the grana fraction from the stroma fraction. The grana fraction is illuminated but provided with no CO₂. Which products will this fraction generate?
Answer: D. Intact grana thylakoids with both PS II and PS I, when illuminated, carry out non-cyclic photophosphorylation: water is split (releasing O₂), electrons flow through the chain, NADP⁺ is reduced to NADPH, and the proton gradient drives ATP synthesis. G3P requires the Calvin cycle (stroma + CO₂), which is absent. So the fraction produces ATP, NADPH and O₂, which is option D; NCERT notes that grana lamellae carry both PS I and PS II (NCERT Class 11 Biology Chapter 11, page 140).
Why A is wrong: A — G3P (glyceraldehyde-3-phosphate) is a product of the Calvin cycle, which requires stroma enzymes and CO₂. Neither is present in the isolated grana fraction.
Why B is wrong: B — O₂ is released from water splitting at PS II, but the electron transport chain also generates ATP (via chemiosmosis) and NADPH (via NADP⁺ reductase). O₂ alone does not account for the full output.
Why C is wrong: C — ATP alone would be the product of cyclic photophosphorylation only (PS I pathway). Grana thylakoids contain both PS II and PS I, so non-cyclic flow also occurs, producing NADPH alongside ATP.
Consider the following statements about chloroplast pigments:
(i) Chlorophyll a absorbs red and blue-violet light.
(ii) Chlorophyll b absorbs light and transfers energy to the reaction centre.
(iii) Carotenoids absorb green-yellow light and reflect red-orange.
(iv) Xanthophylls are oxygen-containing carotenoids.
How many of the above statements are correct?
Answer: A. Statements (i), (ii), and (iv) are correct. Chlorophyll a has absorption peaks in red (~680 nm) and blue-violet (~430 nm). Chlorophyll b is an accessory pigment that transfers absorbed energy to chlorophyll a at the reaction centre. Xanthophylls are indeed oxygenated derivatives of carotenoids. Statement (iii) is incorrect: carotenoids absorb blue-violet light and reflect/transmit yellow-orange-red wavelengths — they do NOT absorb green-yellow (NCERT Class 11 Biology Chapter 11, pages 137–138).
Why B is wrong: B — Two correct would require two statements to be wrong. Only statement (iii) is incorrect (carotenoids absorb blue-violet, not green-yellow). Three statements are correct.
Why C is wrong: C — Only one correct would mean three statements are wrong. Statements (i), (ii), and (iv) are all verifiable facts from NCERT, so this undercounts.
Why D is wrong: D — Statement (iii) is incorrect. Carotenoids absorb in the blue-violet range (400–500 nm) and appear yellow-orange because they reflect those wavelengths. They do not absorb green-yellow light.
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Given
A question states: "In chloroplasts, the__(A)__ is the site of light reactions, while the__(B)__ is the site of the Calvin cycle. The__(C)__ connect grana stacks and contain PS I but not PS II."
Required
Identify the three chloroplast structures A, B, and C.
Concept
The chloroplast has distinct compartments: the thylakoid membrane system (further divided into grana thylakoids and stroma lamellae) and the stroma. Each compartment hosts specific reactions.
Formula / Principle
No formula applies. The principle is compartment–function mapping: light reactions → thylakoid membrane of grana; Calvin cycle → stroma; stroma lamellae → PS I only (cyclic photophosphorylation).
Substitution / Application
• (A): Light reactions occur on the thylakoid membrane, specifically the grana thylakoids where both PS I and PS II are located.• (B): The Calvin cycle enzymes (including RuBisCO) are dissolved in the stroma.• (C): Stroma lamellae (intergranal thylakoids) connect adjacent grana and contain PS I but lack PS II.
Calculation
No arithmetic required. This is a direct terminology-mapping question.
Final answer
A = grana thylakoid membrane, B = stroma, C = stroma lamellae.
Common trap
Confusing stroma lamellae with grana. NEET may present "stroma lamellae" as a distractor for "site of non-cyclic photophosphorylation." Since stroma lamellae lack PS II, they cannot perform non-cyclic electron flow — only cyclic photophosphorylation generating ATP without NADPH.
Similar NEET-style question
"Which chloroplast structure contains PS I but not PS II, and can carry out only cyclic photophosphorylation?"
Options: (a) Grana thylakoid (b) Stroma lamella (c) Inner membrane (d) Outer membrane
Answer: (b) Stroma lamella.
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Chloroplast: outer + inner membrane; thylakoids stacked into grana; stroma. Pigments: chl a (P680, P700 reaction centres), chl b, xanthophylls, carotenoids. Action spectrum: rate of photosynthesis vs wavelength; matches absorption of chl a.
-- NCERT Class 11 Biology, Chapter 11, p. 137More in Plant Physiology: 5 exam traps and mistakes · 1 question pattern from its other lessons.
3 questions from NEET 2024, 2025, 2026. Answers verified against NTA official keys.
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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