MPSC 2023 Prelims Paper-I · Question 68 of 100

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.

• (1)To transfer the NADH₂ and ATP — 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.
• (2)To utilize the NADPH and ATP — 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.
• (4)All of the above — 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.

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.

Photosynthesis is examined in MPSC's science section chiefly through the division between the two stages, because that division supports many short questions with unambiguous answers: which stage occurs where, which requires light, which produces oxygen, which consumes ATP, which enzyme belongs to which. This item is a clean example, and the option set shows the Commission's usual construction — one option with the right molecules and the wrong verb, one with the right verb and the wrong molecules, and an 'all of the above' that cannot survive the contradiction between the first two. What it rewards is the habit of storing a process as a transaction rather than as a list of nouns: the light stage earns ATP and NADPH, the Calvin cycle spends them, and a candidate holding it in that form answers correctly without hesitation while one holding only the vocabulary sees four familiar-looking sentences. The second lesson is about escape options. This paper offers 'all of the above' ten times, and reading the options against one another before reading them against your knowledge often settles the matter faster, since two options that contradict each other rule the escape out immediately. Note that the chemical abbreviations here are printed in Latin capitals inside the Marathi column too, so the two language columns pose an identical question.

• The light-dependent reactions occur on the thylakoid membranes and generate ATP, NADPH and oxygen; the light-independent reactions of the Calvin cycle occur in the stroma and consume the ATP and NADPH to fix carbon dioxide into carbohydrate.
• The oxygen released in photosynthesis comes from the splitting of water at photosystem II, a process called photolysis, and not from the carbon dioxide taken in.
• Photosynthesis reduces NADP⁺ to NADPH, whereas respiration reduces NAD⁺ to NADH; NADPH supplies reducing power for biosynthesis while NADH feeds the respiratory electron transport chain.
• Non-cyclic photophosphorylation involves both photosystems and yields ATP, NADPH and oxygen, while cyclic photophosphorylation uses photosystem I alone and yields ATP only, without NADPH or oxygen.
• The Calvin cycle spends three molecules of ATP and two of NADPH for every molecule of carbon dioxide fixed, the fixation being catalysed by the enzyme RuBisCO acting on ribulose bisphosphate.

• Confusing NADP with NAD, which separates the photosynthetic and respiratory coenzymes and is the single commonest slip in this topic
• Attaching the consumption of ATP and NADPH to the light reactions, when the light reactions generate them and the Calvin cycle consumes them
• Believing the oxygen released in photosynthesis comes from carbon dioxide, when it comes from the photolysis of water
• Selecting 'all of the above' without checking the options against one another, when two of them here make contradictory claims and cannot both be true
• Calling the Calvin cycle the dark reaction and inferring that it happens at night, when it is light-independent only in the sense of not using light directly and its enzymes are light-regulated

MPSC's biology questions on photosynthesis cluster around the two-stage division and the products of each stage: where each stage occurs, what each produces or consumes, the source of the released oxygen, the difference between cyclic and non-cyclic photophosphorylation, and the identity of the primary acceptor and the fixing enzyme. Statement-based variants list three or four assertions about the two stages and ask which are correct, and matching variants pair stages with sites or with products. The Commission relies heavily on near-miss options built by swapping a single word — NAD for NADP, utilise for generate, stroma for thylakoid — so the preparation that pays is a two-column note setting the light stage against the Calvin cycle across site, requirement for light, inputs, outputs and enzymes. A candidate who can also state the ATP and NADPH cost per carbon dioxide fixed covers the quantitative variants as well.

No directly related past PYQ was found.

• practice — not a real PYQ

  In photosynthesis, the oxygen that is released into the atmosphere is derived from which of the following ?

  • (a)Carbon dioxide
  • (b)Water
  • (c)Glucose
  • (d)Ribulose bisphosphate

  Answer(b) Water — oxygen is released when water is split at photosystem II in the process of photolysis, which supplies the electrons that replace those lost by the reaction centre. Carbon dioxide is the source of the carbon in the carbohydrate produced, not of the oxygen given off, and this distinction is one of the most frequently tested points in the topic because the overall equation of photosynthesis shows oxygen and carbon dioxide together and invites the wrong inference.

• practice — not a real PYQ

  Cyclic photophosphorylation differs from non-cyclic photophosphorylation in that it does not produce which of the following ?

  • (a)ATP only
  • (b)NADPH and oxygen
  • (c)ATP and NADPH
  • (d)Carbon dioxide

  Answer(b) NADPH and oxygen — cyclic photophosphorylation uses photosystem I alone and returns the energised electron to its own reaction centre, so no electron is available to reduce NADP⁺ and no water needs to be split to replace one. It therefore yields ATP alone, which the chloroplast uses to meet the Calvin cycle's demand for ATP when that demand exceeds what the non-cyclic route supplies alongside NADPH.