Which of the following statements is true with regard to the light reaction of Photosynthesis ?

Options

A. In PS-II the reaction centre Chlorophyll-a has an absorption peak at 700 nm hence , is called `P_(700)`
B. In PS-II the reaction centre Chlorophyll-a has an absorption maxima at 680 nm and is called `P_(680)`
C. The splitting of water molecule is associated with PS-I
D. Photosystem-I and II are involved in Z scheme

Answer

To determine which statement is true regarding the light reaction of photosynthesis, we can analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Photosystems**:
- Photosynthesis involves two main photosystems: Photosystem I (PSI) and Photosystem II (PSII). Each has a specific role in the light-dependent reactions.

2. **Analyzing the Statements**:
- **Statement 1**: "Photosystem 1 is a reaction center of chlorophyll a has an absorption peak at 700 nanometers."
- This statement is incorrect. Photosystem I (PSI) does have an absorption peak at 700 nm, but it is Photosystem II (PSII) that has an absorption peak at 680 nm. Therefore, this statement is false.

- **Statement 2**: "Photosystem 2 reaction center chlorophyll a has an absorption maximum of 680 nanometers."
- This statement is true. Photosystem II does indeed have an absorption peak at 680 nm, which is why it is referred to as P680.

- **Statement 3**: "The splitting of water molecule is associated with Photosystem 1."
- This statement is incorrect. The splitting of water (photolysis) occurs in Photosystem II, not Photosystem I. Therefore, this statement is false.

- **Statement 4**: "Photosystem 1 and 2 both are involved in the light reaction."
- This statement is true. Both Photosystem I and Photosystem II are involved in the light reactions of photosynthesis, specifically in the non-cyclic photophosphorylation process.

3. **Conclusion**:
- The true statements regarding the light reaction of photosynthesis are:
- Photosystem II has an absorption maximum of 680 nm (Statement 2).
- Both Photosystem I and II are involved in the light reaction (Statement 4).

Thus, the correct answer to the question is that **Statement 4 is true**: Photosystem 1 and 2 both are involved in the light reaction.