First reaction in photosynthesis is

Options

A. Photolysis of water
B. Excitation of chlorophyll molecules
C. Formation of `ATP`
D. Fixation of `CO_(2)`.

Answer

**Step-by-Step Solution:**

1. **Understanding Photosynthesis**: Photosynthesis consists of two main stages: the light reactions (or light-dependent reactions) and the dark reactions (or light-independent reactions). The light reactions occur in the thylakoid membranes of the chloroplasts and require light energy.

2. **Identifying the First Reaction**: The very first reaction in the light-dependent stage of photosynthesis is the absorption of light by chlorophyll molecules. This is crucial as it initiates the entire process of photosynthesis.

3. **Excitation of Chlorophyll**: Once the chlorophyll absorbs light, it becomes excited. This excitation is the first significant reaction in photosynthesis, as it leads to subsequent reactions, including the photolysis of water.

4. **Photolysis of Water**: Following the excitation of chlorophyll, water molecules are split (photolysis) to release oxygen, protons, and electrons. This step is essential for providing the electrons needed for the electron transport chain.

5. **Formation of ATP and NADPH**: The energy generated from the excited electrons is used to form ATP and NADPH, which are then utilized in the dark reactions for carbon fixation.

6. **Conclusion**: Among the options provided, the correct answer to the question "First reaction in photosynthesis is" is the **excitation of chlorophyll molecules**.

**Final Answer**: The first reaction in photosynthesis is the excitation of chlorophyll molecules.

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