BPSC 70th CCE Re-exam Prelims General Studies · Question 31 of 142

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.

• (b)Granum — 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)Grana Lamella — 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)None of the above — 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.

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.

Answer this by asking one question of each option: is it a membrane or a fluid? Granum and grana lamella are membrane structures; stroma is the fluid. Then attach the rule — light reaction on the membrane, dark reaction in the fluid — and the answer follows without recalling any enzyme name. The rule generalises usefully: respiration's electron transport chain is likewise on a membrane, the inner mitochondrial membrane, while the Krebs cycle's enzymes are soluble in the mitochondrial matrix, which is the mitochondrion's equivalent of the stroma. Two vocabulary traps are worth defusing in advance. 'Dark reaction' does not mean it happens at night; the modern name, biosynthetic phase, was adopted precisely to stop that misreading. And the printed word 'cytoplast' in this stem is the Commission's own slip for chloroplast — a reminder that a candidate must read a defective stem for what it evidently means rather than be thrown by it, since none of the four options would make sense for the general cytoplasm of a plant cell.

• The light reaction occurs on the thylakoid membranes stacked as grana and produces ATP and NADPH while splitting water and releasing oxygen; the dark reaction, or Calvin cycle, occurs in the stroma.
• RuBisCO — ribulose bisphosphate carboxylase-oxygenase — catalyses the first step of carbon fixation in the stroma and is the most abundant protein on Earth.
• The Calvin cycle has three stages: carboxylation, reduction and regeneration of RuBP; six turns of the cycle are required to build one molecule of glucose.
• A chloroplast is bounded by two envelope membranes; the stroma inside contains circular chloroplast DNA, ribosomes and starch grains as well as the carbon-fixing enzymes.
• The same membrane-versus-fluid division holds in the mitochondrion: the electron transport chain is on the inner membrane, while the Krebs cycle enzymes are soluble in the matrix.

• Reading 'dark reaction' as a reaction that happens in the dark. It is light-independent, and in a living leaf it runs during the day on the ATP and NADPH the light reaction supplies.
• Choosing granum or grana lamella because they sound like the working parts of the chloroplast. They are, but for the light reaction.
• Being derailed by the printed word 'cytoplast'. The Commission's own slip for chloroplast; none of the options would make sense for the general cytoplasm of a plant cell.

BPSC asks cell biology as single-site identification — which organelle, which compartment, which enzyme — with an escape option to punish guessing, and does not always print the terms correctly. UPSC asks the same material as a matching list of process against organelle, or as a conceptual question about what photosynthesis does to energy, so a candidate has to know both where each process happens and what it accomplishes.

• practice — not a real PYQ

  The enzyme RuBisCO, which catalyses the first step of carbon fixation, is located in the

  • (a)Thylakoid membrane
  • (b)Stroma of the chloroplast
  • (c)Mitochondrial matrix
  • (d)Cytoplasm of the cell

  Answer(b) Stroma of the chloroplast — where the whole soluble enzyme machinery of the Calvin cycle sits. RuBisCO is the most abundant protein on Earth.

• practice — not a real PYQ

  In cellular respiration, the enzymes of the Krebs cycle are located in the

  • (a)Inner mitochondrial membrane
  • (b)Mitochondrial matrix
  • (c)Cytoplasm
  • (d)Outer mitochondrial membrane

  Answer(b) Mitochondrial matrix — the same membrane-versus-fluid division as in the chloroplast. The electron transport chain sits on the inner membrane, just as the light reaction sits on the thylakoid membrane.