Flow of electrons in non-cyclic photophosphorylation is

Options

A. from PS I to PS II
B. bidirectional
C. unidirectional
D. both (2) and (3)

Answer

To understand the flow of electrons in non-cyclic photophosphorylation, let's break it down step by step:

### Step-by-Step Solution:

1. **Definition of Non-Cyclic Photophosphorylation**:
Non-cyclic photophosphorylation is a process that occurs in the thylakoid membranes of chloroplasts during photosynthesis. It involves the flow of electrons through two photosystems (PS2 and PS1) and ultimately leads to the production of ATP and NADPH.

2. **Electron Flow Direction**:
In non-cyclic photophosphorylation, the flow of electrons is unidirectional. This means that the electrons move in one direction only, from Photosystem II (PS2) to Photosystem I (PS1).

3. **Process Initiation**:
The process begins when light energy is absorbed by chlorophyll in PS2. This energy excites electrons, which are then transferred to an electron transport chain.

4. **Water Splitting**:
To replace the electrons lost by PS2, water molecules are split in a process called photolysis. This reaction produces oxygen, protons, and electrons. The oxygen is released as a byproduct.

5. **Electron Transport Chain**:
The excited electrons from PS2 travel through the electron transport chain, losing energy, which is used to pump protons into the thylakoid lumen, creating a proton gradient.

6. **Formation of ATP and NADPH**:
The electrons eventually reach PS1, where they are re-excited by light energy. These high-energy electrons are then used to reduce NADP+ to NADPH. The proton gradient created earlier is utilized by ATP synthase to generate ATP.

7. **Conclusion**:
Therefore, the flow of electrons in non-cyclic photophosphorylation is unidirectional, moving from PS2 to PS1.

### Final Answer:
The flow of electrons in non-cyclic photophosphorylation is **unidirectional**.

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