During non-cyclic photophosphorylation,when electrons are lost from the reaction centre at PS-II, what is the source which replaces these electrons?

Options

A. Oxygen
B. Water
C. Carbon dioxide
D. Light

Answer

**Step-by-Step Solution:**

1. **Understanding Non-Cyclic Photophosphorylation**: Non-cyclic photophosphorylation is a process that occurs during photosynthesis where light energy is used to generate ATP and NADPH. It involves two photosystems: Photosystem II (PS-II) and Photosystem I (PS-I).

2. **Role of Photosystem II**: In this process, when light strikes PS-II, it excites electrons, which are then lost from the reaction center of PS-II. This is a crucial step in the electron transport chain.

3. **Need for Electron Replacement**: When electrons are lost from PS-II, there is a need to replace these electrons to maintain the flow of the electron transport chain and continue the process of photophosphorylation.

4. **Source of Replacement Electrons**: The electrons lost from PS-II are replaced by electrons derived from the splitting of water molecules. This process is known as photolysis of water.

5. **Photolysis of Water**: Water (H₂O) is split into oxygen (O₂), protons (H⁺), and electrons (e⁻). The reaction can be summarized as:
\[ 2H₂O \rightarrow 4H^+ + 4e^- + O₂ \]
Here, the electrons produced from the splitting of water are used to replace the electrons lost from PS-II.

6. **Conclusion**: Therefore, the source that replaces the electrons lost from the reaction center at PS-II during non-cyclic photophosphorylation is water.

**Final Answer**: Water

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