**The Splitting of Water Molecules**

The splitting of water molecules is a crucial step in the process of photosynthesis. It occurs during the light-dependent reactions, specifically in Photosystem II (PS II).

**Photosystem II (PS II)**

Photosystem II is one of the two photosystems involved in the light-dependent reactions of photosynthesis. It is located in the thylakoid membrane of the chloroplasts. PS II primarily functions to capture light energy and use it to split water molecules, releasing oxygen gas, protons (H+), and electrons (e-) in the process.

**Process of Water Splitting in PS II**

1. Absorption of Light Energy: PS II contains chlorophyll pigments and other accessory pigments that are capable of absorbing light energy. When light is absorbed by these pigments, it excites the electrons, raising them to a higher energy level.

2. Electron Transport: The excited electrons are transferred from one molecule to another in a series of electron carriers within the thylakoid membrane. This process creates a flow of electrons, known as the electron transport chain.

3. Photolysis of Water: As the excited electrons are transferred through the electron transport chain, they are replaced by electrons derived from the splitting of water molecules. This process is called photolysis, and it occurs in the oxygen-evolving complex (OEC) of PS II.

4. Oxygen Release: During photolysis, water molecules are split, resulting in the release of oxygen gas (O2). This oxygen is the byproduct of photosynthesis and is essential for sustaining life on Earth.

5. Proton Accumulation: In addition to oxygen release, the splitting of water also leads to the accumulation of protons (H+) within the thylakoid lumen. These protons contribute to the establishment of a proton gradient, which is essential for ATP synthesis during the subsequent process of chemiosmosis.

6. Electron Donation: The electrons derived from water splitting are passed along the electron transport chain, eventually reaching Photosystem I (PS I). In PS I, these electrons are re-energized by light and used to reduce NADP+ to NADPH, which is a high-energy electron carrier used in the Calvin cycle (the light-independent reactions of photosynthesis).

Hence, the splitting of water molecules is associated with Photosystem II (PS II) during the light-dependent reactions of photosynthesis.

Photolysis of Water is needed when there is a complete usage of the electron i.e the electron does not return to the photosystem ( Non cyclic photophosphorylation ) This cycle is associated with Photosystem 1and 2 and Water splitting complex is associated with photosystem 2, which is associated with inner surface of thylakoid.