Q.35 The water splitting in photo system-II (PSD) is mediated by metal

1. Mn
2. Mg
3. Fe
4. Ni

   Mn (Manganese) is the metal mediating water splitting in Photosystem II (PSII) (Option A).

   In PSII’s oxygen-evolving complex (OEC), a Mn4CaO5 cluster catalyzes water oxidation (2H₂O → O₂ + 4H⁺ + 4e⁻) during the S-state cycle, driven by light-induced P680⁺ oxidation.

   Option Breakdown

   • Mn: Correct. Four Mn ions (Mn³⁺/Mn⁴⁺ states) form a cubane-like cluster with Ca²⁺, bridging oxo ligands to enable four-electron water splitting without harmful radicals.​

   • Mg: Central to chlorophyll (Mg-porphyrin) for light harvesting in both photosystems; no role in PSII water oxidation.​

   • Fe: Key in PSI (Fe-S clusters for electron transfer), cytochromes (e.g., cyt b6f), and ferredoxin; absent from OEC.​

   • Ni: Found in urease/hydrogenases, not photosynthetic complexes; irrelevant to PSII photochemistry.​

   Water splitting in photosystem II (PSII) is mediated by Mn in the oxygen-evolving complex (OEC), producing all atmospheric O₂ via the S0-S4 cycle. This Mn4Ca cluster accumulates four oxidizing equivalents from P680 to oxidize two H₂O molecules.

   OEC Mechanism Overview

   Light excites P680 → P680⁺, oxidizing Yz (tyrosine) which extracts electrons from Mn cluster over four flashes (Kok cycle). Mn oxidation states shift (e.g., Mn³⁺ → Mn⁴⁺), culminating in O-O bond formation and O₂ release.​

   Metal Role Comparison

   Metal	Role in PSII Water Splitting?	Primary Function
   Mn	Yes (OEC cluster)	Catalyzes 2H₂O → O₂ ​
   Mg	No	Chlorophyll core ​
   Fe	No	PSI electron carriers ​
   Ni	No	Non-photosynthetic enzymes

   Critical for exam prep on light reactions and bioinorganic catalysis.​