What is the principal function of the light-dependent reactions of photosynthesis ?

Question

UnlockIAS · Accepted Answer

Correct — option (3). Photosynthesis is worked in two stages, and the whole of this question is about keeping them apart. The light-dependent reactions take place on the thylakoid membranes of the chloroplast and require light directly; the light-independent reactions, the Calvin cycle, take place in the stroma and do not. The light reactions capture the energy of sunlight and store it in two portable chemical forms — ATP, which carries energy, and NADPH, which carries reducing power — and these two together are often called the assimilatory power of the cell. That is what option (3) states, and it is the answer. The mechanism runs like this. Light absorbed by the pigments of photosystem II energises electrons, which are passed along an electron transport chain to photosystem I, where a second absorption of light raises them again before they are finally used to reduce NADP⁺ to NADPH. The electrons lost by photosystem II are replaced from water, which is split in the process called photolysis, and it is this splitting that releases the oxygen given off by a green plant — the oxygen comes from water and not from carbon dioxide. As the electrons travel down the chain, protons are pumped into the thylakoid lumen, and the gradient so created drives ATP synthase to make ATP, a process called photophosphorylation. So the light stage delivers three products: ATP, NADPH and oxygen. The second stage then spends what the first stage earned. In the Calvin cycle carbon dioxide is fixed onto ribulose bisphosphate by the enzyme RuBisCO, and the ATP and NADPH made in the light are consumed to reduce the resulting three-carbon acid to carbohydrate and to regenerate the acceptor, at a cost of three ATP and two NADPH for every molecule of carbon dioxide fixed. Generating the two carriers is therefore the function of the light stage and consuming them is the function of the dark stage, which is exactly the distinction the four options are built on. Option (3) is the answer.

Why the other options are wrong — (a) Two things are wrong with this option, and either is enough to reject it. The first is the coenzyme: photosynthesis reduces NADP⁺ to NADPH, whereas NAD is the coenzyme of respiration, reduced during glycolysis and the Krebs cycle. The single phosphate group that distinguishes NADP from NAD separates the anabolic economy of the cell, where NADPH supplies reducing power for biosynthesis, from the catabolic economy, where NADH feeds the respiratory electron transport chain — and the paper's printed form 'NADH₂' is in any case a dated way of writing the reduced coenzyme, which is properly NADH together with a free proton. The second defect is the verb. The light reactions do not merely transfer an existing carrier; they generate it, by reducing NADP⁺ with electrons drawn ultimately from water. This option is aimed at the candidate who remembers that a reduced coenzyme and ATP are involved but has not fixed which coenzyme belongs to which process.  (b) This option names the right two molecules and attaches them to the wrong stage, which makes it the most attractive of the three and the one that separates a candidate who has understood the division of labour from one who has merely learnt the vocabulary. Utilising NADPH and ATP is precisely what the light-independent reactions do: in the Calvin cycle they are spent to reduce the fixed carbon to carbohydrate and to regenerate ribulose bisphosphate, three ATP and two NADPH going to each molecule of carbon dioxide fixed. The light reactions stand on the other side of that transaction and produce the two carriers. Note also that this option and option (3) cannot both be true of the same stage, since generating and consuming are opposite operations, and noticing that they conflict is enough to show that the answer lies between them and that 'All of the above' cannot stand.  (d) This escape option is impossible here for a reason visible on the page itself. Options (2) and (3) make contradictory claims about the same stage — one says the light reactions consume NADPH and ATP, the other says they produce them — and a set of statements that contradict one another cannot all be correct together. Option (1) fails independently, since it names NAD rather than NADP. 'All of the above' is offered ten times across the four hundred options of this paper and being offered is not evidence of being keyed; it deserves to be selected only when every individual statement has been separately verified as true. Here the quickest route through the question is to notice the contradiction between options (2) and (3), strike out option (4) on that ground alone, and then decide which of the remaining two describes the light stage.

Concept: Photosynthesis converts light energy into the chemical energy of carbohydrate, and it does so in two coupled stages that differ in location, in requirement for light and in what they produce. The light-dependent stage runs on the thylakoid membranes. Pigments arranged in two photosystems, P680 in photosystem II and P700 in photosystem I, absorb light and pass energised electrons along a transport chain in what is called the Z-scheme; water is split at photosystem II to replace the lost electrons, releasing oxygen and protons; the proton gradient built across the thylakoid membrane drives ATP synthase; and NADP⁺ is reduced to NADPH at the far end of the chain. This route, using both photosystems, is non-cyclic photophosphorylation and yields ATP, NADPH and oxygen. A variant called cyclic photophosphorylation uses photosystem I alone, returning the electron to its source, and so produces ATP only — no NADPH and no oxygen — which the chloroplast uses to top up ATP when the Calvin cycle demands more of it than the non-cyclic route supplies. The light-independent stage runs in the stroma and needs no light directly, though its enzymes are regulated by light, so calling it the 'dark reaction' is misleading. In it the enzyme RuBisCO attaches carbon dioxide to the five-carbon acceptor ribulose bisphosphate, and the products are reduced using NADPH and ATP and partly recycled to regenerate the acceptor, three ATP and two NADPH being spent for each carbon dioxide fixed. The two stages are joined by nothing but these carriers: the light stage charges them and the Calvin cycle discharges them, which is why a plant deprived of light stops fixing carbon within a short time even though the Calvin cycle enzymes remain present.

State PCS 2023 Q68 — Science & Technology PYQ with Answer

State PCS 2023 Q68 — Science & Technology PYQ with Answer