Which of the following the chlorophyll in photosynthesis is used for ?

Question

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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.

Why the other options are wrong — (a) 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) 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) 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.

Concept: 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.

State PCS 2024 Q105 — Science & Technology PYQ with Answer

State PCS 2024 Q105 — Science & Technology PYQ with Answer