Actual site of dark reaction in photosynthesis inside cytoplast is

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UnlockIAS · Accepted Answer

Correct — A, Stroma. Photosynthesis runs in two linked phases inside the chloroplast, and they happen in two different compartments of it. The LIGHT reaction takes place on the thylakoid membranes — the flattened sacs that are stacked into grana — because that is where the photosystems, the chlorophyll pigments, the electron transport chain and the ATP synthase are embedded. It splits water, releases oxygen and produces the two energy carriers, ATP and NADPH. The DARK reaction, properly called the biosynthetic phase or the Calvin cycle, then takes those carriers and uses them to fix carbon dioxide into carbohydrate. Its enzymes — above all RuBisCO, the most abundant protein on Earth — are dissolved not in a membrane but in the STROMA, the colourless fluid matrix that fills the chloroplast and surrounds the grana. So the actual site of the dark reaction is the stroma, and option (a) is right. Note two things the name conceals. The dark reaction is not a reaction that needs darkness; it is called dark only because it does not itself require light, and in a living leaf it runs in daylight, driven by the ATP and NADPH the light reaction is producing next door. And the stem's word 'cytoplast' is a misprint for chloroplast — the paper is printed that way and the card does not repair it, but no reader should be misled: the dark reaction happens inside the chloroplast, not in the cell's general cytoplasm. Options (b) and (c) both name parts of the thylakoid system, which is the light reaction's territory, and since (a) is correct the escape option cannot stand.

Why the other options are wrong — (b) A granum is a stack of thylakoids — the coin-pile structures visible in an electron micrograph of a chloroplast — and it is where the LIGHT reaction happens, not the dark one. The photosystems and the electron transport chain sit in those membranes because light capture needs an organised array of pigment molecules held in place, which a fluid cannot provide.  (c) The grana lamellae are the thylakoid membranes themselves, the individual discs that make up a granum. Same compartment as option (b) and the same answer: light reaction, not dark. Offering the stack and the individual membrane as two separate options is a way of making the light-reaction site look like two chances out of three.  (d) Fails because the stroma is precisely the right answer and is on the list. The escape option is only correct when each named alternative can be shown to be wrong, and here the standard NCERT attribution of the biosynthetic phase to the stroma is the first option offered.

Concept: A chloroplast is a double-membraned organelle with a third, internal membrane system. Inside the two envelope membranes lies the stroma, a protein-rich fluid that contains the chloroplast's own circular DNA, its ribosomes, starch grains and the whole enzyme machinery of carbon fixation. Suspended in that fluid is the thylakoid system: flattened membrane sacs stacked into grana and connected between stacks by stroma lamellae. The division of labour follows the physics. Capturing light requires pigments and electron carriers arranged in a fixed spatial order and a membrane across which protons can be pumped, so the light reaction is membrane-bound. Fixing carbon requires enzymes meeting substrates by diffusion, so the Calvin cycle is soluble. The cycle itself has three stages — carboxylation, in which RuBisCO adds carbon dioxide to the five-carbon sugar RuBP; reduction, which consumes ATP and NADPH to make triose phosphate; and regeneration of RuBP so the cycle can turn again. Six turns are needed to make one molecule of glucose.

State PCS 2025 Q31 — Science & Technology PYQ with Answer

State PCS 2025 Q31 — Science & Technology PYQ with Answer