During the light-dependent reactions of photosynthesis, which of the following events does NOT occur?

Introduction / Context:
Light reactions convert photon energy into chemical energy (ATP and NADPH) and release O2 from water splitting. CO2 fixation, by contrast, is part of the Calvin–Benson cycle (often called the dark reactions). This question distinguishes light-stage events from carbon fixation steps.

Given Data / Assumptions:

• Photosystem II oxidizes water and evolves O2.
• Photosystem I reduces NADP+ to NADPH.
• The thylakoid membrane hosts electron carriers that generate a proton motive force.

Concept / Approach:
Light absorption by PSII and PSI drives electron transport, water splitting, proton pumping, ATP synthesis, and NADPH formation. CO2 fixation (carboxylation of RuBP by Rubisco) occurs in the stroma during the Calvin cycle and is not a light reaction event, even though it depends on ATP/NADPH from the light stage.

Step-by-Step Solution:

Verification / Alternative check:
Isolated thylakoids in light synthesize ATP/NADPH and evolve O2 but will not fix CO2 without stromal enzymes and the Calvin cycle machinery, confirming the separation of stages.

Why Other Options Are Wrong:

• Water splitting, O2 release, and light absorption are quintessential light reactions.
• Proton gradient formation is central to photophosphorylation.

Common Pitfalls:
Misinterpreting “dark reactions” as requiring darkness; they are light-independent chemically but rely on light-generated ATP and NADPH.

Final Answer:
Carbon dioxide fixation into organic molecules.