GATE 2026 BT Q.25: solution and answer

Official answer: CSource: Official answer key, GATE 2026 BT (IIT Guwahati)

1. The light reactions take place in the thylakoid membranes. Photosystem II splits water (photolysis), releasing ${\text{O}}_2$, protons and electrons.

2. Electrons pass along the chain to photosystem I and finally reduce ${\text{NADP}}^{+}$ to $\text{NADPH}$ (ferredoxin-NADP reductase).

3. The proton gradient across the thylakoid membrane drives ATP synthase, so ATP is made by photophosphorylation.

4. The products of the light reactions are therefore ${\text{O}}_2$, $\text{NADPH}$ and ATP, which is option C.

5. Option A is wrong. Plants make $\text{NADPH}$, not $\text{NADH}$, and no ${\text{H}}_2$ is released.

6. Option B is wrong. ${\text{NADP}}^{+}$ is the electron acceptor, so it is used up rather than made. ${\text{CO}}_2$ is a substrate of the Calvin cycle, not a product of the light reactions.

7. Option D is wrong. ${\text{NADP}}^{+}$ is the reactant. The reduced form $\text{NADPH}$ is the product, and it is used with ATP in the Calvin cycle.