Oxygenic photosynthesis — Identify the primary electron donor that replaces the photo-excited electrons and ultimately releases O2 as a by-product.

Question

Solve this multiple-choice question and choose the correct option.

Accepted Answer

Introduction / Context:In plants, algae, and cyanobacteria, the light reactions remove electrons from water via the oxygen-evolving complex of Photosystem II. This process generates O2 and feeds electrons into the photosynthetic electron transport chain. Given Data / Assumptions: PSII extracts electrons from H2O, releasing O2 and protons into the thylakoid lumen. Electrons travel through plastoquinone → cytochrome b6f → plastocyanin → PSI → NADP+. NADPH is produced at the end of non-cyclic flow, not used as the initial donor. Concept / Approach:Identify the substrate of the oxygen-evolving complex. Only water can account for both electron donation and O2 evolution in oxygenic photosynthesis. Step-by-Step Solution: Light excites P680 → strong oxidant forms.OEC oxidizes H2O → O2 + 4 H+ + 4 e−.Electrons proceed through the chain to reduce NADP+ to NADPH. Verification / Alternative check:Isotopic tracer experiments with H2^18O demonstrate that the oxygen evolved comes from water. Why Other Options Are Wrong: b) O2 is a product, not an electron donor.c,d) NADH/NADPH are reduced carriers; they are electron donors elsewhere but not the initial donor in oxygenic photosynthesis.e) CO2 is the carbon source in the Calvin cycle, not the electron donor. Common Pitfalls:Confusing the direction of electron flow; in photosynthesis, electrons originate from water and end in NADPH. Final Answer:Water (H2O).

Oxygenic photosynthesis — Identify the primary electron donor that replaces the photo-excited electrons and ultimately releases O2 as a by-product.

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