12. Which mineral ion play important role in photolysis of water associated with photosystem II?
(1) Manganese     (2) Magnesium
(3) Iron                  (4) Molybdenum

The mineral ion that plays an important role in the photolysis of water associated with photosystem II is (1) Manganese. The oxygen-evolving complex (OEC) of PSII features a unique manganese cluster that is indispensable for the water-splitting reaction, enabling oxygen evolution in photosynthesis.​

Manganese: Key to Water Photolysis in Photosystem II

Introduction

Key phrase: manganese role water photolysis photosystem II

The process of oxygenic photosynthesis relies on the photolysis (splitting) of water at Photosystem II. Only specific mineral ions with the right electronic properties support this reaction—foremost among them is manganese, which forms the catalytic heart of the oxygen-evolving complex.​

Explanation of Each Option

• (1) Manganese

  • Correct. Manganese ions are a fundamental part of the Mn4CaO5 cluster within the oxygen-evolving complex of PSII. This cluster accumulates oxidizing equivalents and catalyzes the multistep four-electron oxidation of water to produce molecular oxygen, protons, and electrons essential for the photosynthetic electron transport chain.​

• (2) Magnesium

  • Incorrect. Magnesium ions are critical for chlorophyll function and ATPase activity but do not play a direct role in the catalytic process of water photolysis at PSII.​

• (3) Iron

  • Incorrect. Iron is necessary for electron transfer within the thylakoid membranes (e.g., cytochromes) but is not central to the water-splitting reaction of PSII OEC.​

• (4) Molybdenum

  • Incorrect. Molybdenum ions are involved in nitrogen metabolism (nitrogenase enzymes) rather than the photolytic activity of water in chloroplasts.​

Key Facts About Manganese in Photosystem II

• The Mn4CaO5 cluster acts as the water oxidation catalyst in PSII.

• Manganese enables oxygen production by facilitating electron extraction from water molecules.

• A deficiency in manganese directly impairs oxygen evolution and limits photosynthetic efficiency.​