In oxygenic photosynthesis (as in green plants and cyanobacteria), water is split at Photosystem II. What, specifically, does this photolysis of H2O supply to the photosynthetic apparatus?

Introduction / Context:
In oxygenic photosynthesis, water serves as the ultimate electron donor. When light energizes the reaction center chlorophyll P680 in Photosystem II (PSII), it ejects an electron and becomes oxidized to P680+. The system must quickly replace that electron to keep electron flow and ATP/NADPH production moving. This question tests whether you know exactly what the photolysis of water provides and where it fits in the Z-scheme of photosynthesis.

Given Data / Assumptions:

• Oxygenic photosynthesis employs two photosystems in series (PSII then PSI).
• Light excites P680 at PSII; the excited electron is passed down an electron transport chain.
• Water is split by the oxygen-evolving complex (OEC) associated with PSII.
• The Calvin cycle (dark reactions) uses ATP and NADPH, not O2 as a reactant.

Concept / Approach:
PSII oxidizes water via the OEC (manganese cluster), generating electrons, protons (contributing to the thylakoid proton gradient), and molecular oxygen as a by-product. The critical role of water here is to supply electrons that re-reduce the strong oxidant P680+ back to P680, allowing continuous photo-oxidation cycles and downstream reduction of plastoquinone, cytochrome b6f, plastocyanin, and PSI.

Step-by-Step Solution:

Verification / Alternative check:
O2 evolution strictly correlates with PSII activity and water splitting; inhibitors of PSII halt oxygen production and electron donation to PSI, demonstrating the role of water as the source of electrons for P680+ reduction.

Why Other Options Are Wrong:

• O2 for dark reactions: the Calvin cycle does not require O2; it fixes CO2 using ATP and NADPH.
• Electrons to reduce NADH: photosynthesis reduces NADP+ to NADPH, not NADH.
• Electrons for cyclic photophosphorylation: cyclic flow occurs around PSI and does not require water splitting.
• Protons exclusively for mitochondria: the protons contribute to the thylakoid proton motive force, not mitochondrial ATP synthesis.

Common Pitfalls:
Confusing NADPH with NADH and thinking oxygen produced is immediately “used” in the Calvin cycle. Oxygen is a by-product released to the atmosphere; the Calvin cycle needs ATP and NADPH, not O2.

Final Answer:
electrons needed to reduce P680 (i.e., to re-reduce oxidized P680+ in Photosystem II).