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

Difficulty: Easy

Water (H2O)
Oxygen (O2)
NADH
NADPH
Carbon dioxide (CO2)

Correct Answer: Water (H2O)

Explanation:

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.

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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