When two photosystems work in a series, first PS-II and then PS-I, which process occurs?

Options

A. Cyclic photophorylation
B. Non-cyclic photophosphoylation
C. Both (1) and (2)
D. none of these

Answer

To answer the question about the process that occurs when two photosystems (PS-II and PS-I) work in series, we can break down the explanation into clear steps:

### Step-by-Step Solution:

1. **Understanding Photosystems**:
- Photosynthesis involves two main photosystems: Photosystem II (PS-II) and Photosystem I (PS-I). These photosystems are essential for the light-dependent reactions of photosynthesis.

2. **Light Reactions Overview**:
- The light reactions of photosynthesis occur in the thylakoid membranes of chloroplasts. They convert light energy into chemical energy in the form of ATP and NADPH.

3. **Types of Photophosphorylation**:
- There are two types of photophosphorylation: cyclic and non-cyclic.
- **Cyclic Photophosphorylation**: Involves only Photosystem I and produces ATP but not NADPH.
- **Non-Cyclic Photophosphorylation**: Involves both Photosystem II and Photosystem I, producing both ATP and NADPH.

4. **Sequence of Events in Non-Cyclic Photophosphorylation**:
- The process begins with PS-II, which absorbs light and excites electrons.
- These electrons are transferred through an electron transport chain to PS-I.
- During this transfer, ATP is produced via chemiosmosis.
- PS-I then absorbs light, re-exciting the electrons, which are used to reduce NADP+ to NADPH.
- Water is split (photolysis) to provide electrons to PS-II, releasing oxygen as a byproduct.

5. **Conclusion**:
- When both photosystems work in series (first PS-II and then PS-I), the process that occurs is **Non-Cyclic Photophosphorylation**. This is characterized by the production of both ATP and NADPH, which are essential for the subsequent dark reactions of photosynthesis.

### Final Answer:
The process that occurs when two photosystems work in series (first PS-II and then PS-I) is **Non-Cyclic Photophosphorylation**.

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