C1.3 Photosynthesis

The question asks for the correct locations of photolysis, ATP synthesis, and NADP⁺ reduction — all processes within the light-dependent reactions.

Photolysis splits water molecules inside the thylakoid lumen (the inner space enclosed by the thylakoid membrane). The reaction is: $2H_{2}O\to 4H^{+}+4e^{-}+O_2$

Protons ($H^{+}$) flow from the thylakoid lumen back into the stroma through ATP synthase, which is embedded in the thylakoid membrane. This is chemiosmosis — the driving force of ATP synthesis.

Electrons boosted by Photosystem I (PSI), located in the thylakoid membrane, are used to reduce $NAD{P}^{+}$ to reduced NADP. This also occurs at the thylakoid membrane.

The correct combination is: photolysis → thylakoid lumen; ATP synthesis → thylakoid membrane; NADP⁺ reduction → thylakoid membrane (at PSI). This matches option B.

We need to eliminate options that place any of the three processes in an incorrect chloroplast compartment.

Option A states photolysis occurs in the stroma. This is incorrect — photolysis occurs in the thylakoid lumen, not the stroma. Eliminate A.

Option C places photolysis in the thylakoid membrane and ATP synthesis in the stroma. While the thylakoid membrane is close, photolysis specifically takes place in the lumen, and ATP synthesis uses the membrane-embedded ATP synthase, not the stroma directly. Eliminate C.

Option D places photolysis in the stroma and ATP synthesis in the thylakoid lumen. Both are incorrect — photolysis is in the lumen, and ATP synthase is in the membrane. Eliminate D.

Option B correctly places photolysis in the thylakoid lumen, ATP synthesis at the thylakoid membrane, and NADP⁺ reduction at PSI in the thylakoid membrane. This is the correct answer.

The question asks specifically about the enzymatic role of RuBisCO in the Calvin cycle. This is the first (carbon fixation) stage of the Calvin cycle.

Carbon fixation is the first stage of the Calvin cycle, occurring in the stroma. $C{O}_2$ from the atmosphere is attached to the 5-carbon acceptor molecule RuBP (ribulose bisphosphate). The reaction is: $C{O}_2+RuBP\stackrel{\text{RuBisCO}}{\to }2\times GP$

RuBisCO (ribulose bisphosphate carboxylase/oxygenase) is the enzyme that catalyses this carboxylation step — attaching $C{O}_2$ to RuBP to produce two molecules of the 3-carbon compound glycerate 3-phosphate (GP).

The reduction of GP to G3P (stage 2) uses ATP and reduced NADP but is not catalysed by RuBisCO. The regeneration of RuBP (stage 3) uses ATP but is also a different set of enzymes. Photolysis is not part of the Calvin cycle at all.

The correct answer is: RuBisCO catalyses the combination of $C{O}_2$ with RuBP, forming two molecules of GP. This is the definition of carbon fixation.

We need to find the option that correctly describes only what RuBisCO does — not what other enzymes or processes do in photosynthesis.

"It reduces GP to G3P by transferring electrons from reduced NADP" describes stage 2 of the Calvin cycle, which requires ATP and reduced NADP but involves different enzymes, not RuBisCO. Eliminate this option.

"It uses ATP to regenerate RuBP from G3P" describes stage 3 of the Calvin cycle (RuBP regeneration). This is also not RuBisCO's role. Eliminate this option.

"It splits water molecules in the thylakoid lumen" describes photolysis, which is part of the light-dependent reactions and is not carried out by RuBisCO. Eliminate this option.

The remaining option — "It catalyses the combination of $C{O}_2$ with RuBP, forming two molecules of GP" — correctly describes RuBisCO's function as the carbon-fixing enzyme in stage 1 of the Calvin cycle.