Among the following which mineral plays an important role in photolysis of water?
(1) Mg⁺⁺ (2) Ca⁺⁺
(3) Na⁺ (4) Mn⁺⁺                

The mineral ion that plays an important role in photolysis of water is (4) Mn++ (manganese). Manganese forms the core of the oxygen-evolving complex in Photosystem II, directly enabling the water-splitting reaction essential for photosynthesis.​

Manganese and Water Photolysis in Photosystem II

Introduction

Key phrase: manganese ion photolysis water photosystem II

Water photolysis at Photosystem II is the foundational biological reaction providing electrons for photosynthetic energy conversion. This process is critically dependent on the manganese cluster that forms the heart of the oxygen-evolving complex (OEC), facilitating the stepwise extraction of electrons from water molecules and releasing oxygen as a byproduct.​

Explanation of Each Option

• (1) Mg++ (Magnesium):

  • Incorrect. Magnesium ions are central to chlorophyll structure and ATP synthesis, but are not directly involved in water splitting at PSII.​

• (2) Ca++ (Calcium):

  • Partially correct. Calcium acts as a cofactor in the OEC, stabilizing the manganese cluster and aiding water oxidation. However, manganese is the catalytically active ion responsible for the actual photolysis function.​

• (3) Na+ (Sodium):

  • Incorrect. Sodium ions do not contribute to water splitting or the function of PSII’s OEC in photosynthesis.​

• (4) Mn++ (Manganese):

  • Correct. Manganese ions are essential cofactors in the OEC, where they participate in electron extraction from water and oxygen production. Without manganese, the plant cannot split water to generate electrons for photosynthetic electron transport and oxygen evolution.​

Quick Facts: Manganese’s Role in Photosynthesis

• The Mn cluster within the OEC of PSII enables stepwise water oxidation.

• Manganese deficiency impairs oxygen evolution and overall photosynthetic capacity.​

• Calcium assists in OEC function but is not the primary catalytic ion.