BPSC 70th CCE Prelims General Studies · Question 105 of 143

Correct — C, Absorbing light. Chlorophyll is a pigment, and NCERT's definition of a pigment is the whole answer: 'Pigments are substances that have an ability to absorb light, at specific wavelengths.' Chlorophyll a is described there as 'the chief pigment associated with photosynthesis', and the evidence is the match between two graphs — the absorption spectrum of chlorophyll a, which peaks in the blue and the red, and the action spectrum of photosynthesis, which peaks at the same wavelengths. Green light is largely reflected rather than absorbed, which is why leaves look green: the colour you see is the light chlorophyll rejected. Mechanically, the pigments sit in light-harvesting complexes inside Photosystem I and Photosystem II, hundreds of pigment molecules bound to protein and acting as an antenna; a single chlorophyll a molecule at the centre of each complex is the reaction centre, absorbing at 700 nm in PS I, hence P700, and at 680 nm in PS II, hence P680. Chlorophyll b, xanthophylls and carotenoids are accessory pigments that widen the range of usable wavelengths, hand their energy on to chlorophyll a, and protect it from photo-oxidation. Everything else in photosynthesis is downstream of that single act of absorption. NCERT lists the light reaction as 'light absorption, water splitting, oxygen release, and the formation of high-energy chemical intermediates, ATP and NADPH' — four separate events, of which only the first belongs to the pigment itself. Water splitting is carried out by a water-splitting complex associated with PS II, and the reduction of carbon dioxide happens later, in the stroma, in the Calvin cycle. The question is therefore testing whether you can separate the pigment's job from the reactions its energy pays for.

• (a)Breaking down water molecules — The strongest wrong answer, because photolysis really does happen and really is tied to Photosystem II, of which chlorophyll a is part. But NCERT is careful: 'the water splitting complex is associated with the PS II', and it is that complex, not the pigment, which carries out 2H2O → 4H+ + O2 + 4e−. Chlorophyll supplies the excitation that makes the splitting necessary — PS II must replace the electrons it lost — rather than doing the chemistry.
• (b)Reduction of CO2 — This is the biosynthetic phase, and it happens somewhere else. Carbon dioxide is fixed onto the five-carbon sugar ribulose bisphosphate by the enzyme RuBisCO in the stroma of the chloroplast, in the Calvin cycle, using the ATP and NADPH made earlier. No pigment takes part in that step, and it can run in the dark for as long as the ATP and NADPH last.
• (d)No function — Plainly false, and included only as a fourth body. Without chlorophyll there is no absorption of light energy, no excited electron, no electron transport chain, no ATP or NADPH, and therefore no carbon fixation — which is exactly why a plant deprived of the ability to make chlorophyll turns pale and dies rather than merely losing its colour.

Photosynthesis is two connected processes rather than one, and knowing which half a given event belongs to answers most questions on the topic. The light reaction, or photochemical phase, takes place on the thylakoid membranes and consists of light absorption, water splitting, oxygen release and the making of ATP and NADPH. The biosynthetic phase, still commonly called the dark reaction, takes place in the stroma and uses that ATP and NADPH to fix carbon dioxide into sugar through the Calvin cycle — carboxylation of ribulose bisphosphate by RuBisCO, reduction, and regeneration of the acceptor. Chlorophyll belongs entirely to the first half, and to only the first of its four events. A chromatographic separation of leaf pigments shows four bands, not one — chlorophyll a, bright or blue-green; chlorophyll b, yellow-green; xanthophylls, yellow; and carotenoids, yellow to yellow-orange — and only chlorophyll a occupies the reaction centre. The rest are accessory pigments whose job is to catch wavelengths chlorophyll a misses and pass the energy inward.

The reliable way to reason through a question like this is to ask, for each option, what kind of thing is being described — a physical event, a chemical reaction, or a molecule's property — and then ask whether a pigment can do it. Absorbing light is a property of a pigment; that is the definition of the word. Splitting water and reducing carbon dioxide are chemical reactions, and reactions in a cell are catalysed by enzyme complexes, not by pigments: the water-splitting complex of PS II does the first, RuBisCO does the second. That single test disposes of both (a) and (b) without needing to remember any wavelength. Option (d) is the sort of absurd fourth option that can be discarded on sight, but it is worth noticing that it is not always absurd in exams — 'no function' is occasionally correct for vestigial structures, so read it rather than skipping it. A second useful check is location: chlorophyll and the water-splitting complex sit on the thylakoid membrane, while carbon dioxide reduction happens in the stroma, so an option that moves a process to the wrong compartment is wrong for that reason alone.

• NCERT's definition: 'Pigments are substances that have an ability to absorb light, at specific wavelengths' — and chlorophyll a is 'the chief pigment associated with photosynthesis'
• Most photosynthesis occurs in the blue and red regions of the spectrum, where chlorophyll a's absorption peaks coincide with the action spectrum of photosynthesis; green is largely reflected
• The reaction-centre chlorophyll a absorbs at 700 nm in Photosystem I (P700) and at 680 nm in Photosystem II (P680); hundreds of other pigment molecules form the light-harvesting antenna
• Accessory pigments — chlorophyll b, xanthophylls and carotenoids — widen the usable wavelength range, transfer energy to chlorophyll a, and protect it from photo-oxidation
• Water splitting is performed by a complex associated with PS II: 2H2O → 4H+ + O2 + 4e−, which is the source of the oxygen released
• Carbon dioxide is fixed in the stroma by RuBisCO onto ribulose bisphosphate in the Calvin cycle, using the ATP and NADPH generated by the light reaction

• Attributing photolysis to chlorophyll because it happens at Photosystem II — the water-splitting complex is associated with PS II, not performed by the pigment
• Treating the 'dark reaction' as one that needs darkness; it needs the products of the light reaction, and runs in daylight
• Assuming chlorophyll absorbs green light because leaves are green — green is the band it largely reflects

BPSC asks photosynthesis as one crisp function attached to one named component — what chlorophyll does, what stomata do, where the process occurs — with the wrong options drawn from adjacent steps of the same pathway, so the discrimination has to be at the level of the individual step. UPSC prefers the energetics and the ecology: whether free energy becomes potential energy, which organelle hosts which process, and what share of the world's oxygen phytoplankton contribute.

• practice — not a real PYQ

  In which part of the chloroplast does the Calvin cycle take place ?

  • (a)Thylakoid membrane
  • (b)Thylakoid lumen
  • (c)Stroma
  • (d)Outer membrane

  Answer(c) Stroma — the light reaction occurs on the thylakoid membranes, while carbon fixation by RuBisCO occurs in the stroma using the ATP and NADPH produced there.

• practice — not a real PYQ

  The reaction centre chlorophyll a of Photosystem I is designated

  • (a)P680
  • (b)P700
  • (c)P870
  • (d)P540

  Answer(b) P700 — PS I's reaction centre absorbs at 700 nm; PS II's absorbs at 680 nm and is called P680.