The first step in the light reaction of photosynthesis is

Options

A. Formation of ATP
B. Ionization of water
C. Attachment of `CO_(2)` to a pentose sugar
D. Excitement of electron of chlorophyll by a photon of light.

Answer

**Step-by-Step Solution:**

1. **Understanding Photosynthesis**: Photosynthesis is the process by which plants convert light energy into chemical energy, producing their own food in the form of glucose. This process occurs in two main phases: the light reactions and the dark reactions (Calvin cycle).

2. **Location of Light Reactions**: The light reactions take place in the thylakoid membranes of the chloroplasts, specifically within structures called grana.

3. **Role of Chlorophyll**: Chlorophyll is the pigment responsible for absorbing light energy. It captures sunlight and initiates the light reactions.

4. **Absorption of Light**: When light (photons) strikes chlorophyll, it absorbs specific wavelengths of light, leading to the excitation of electrons within the chlorophyll molecule.

5. **Excitation of Electrons**: The absorbed light energy causes the electrons in chlorophyll to become excited. This is the very first step in the light reactions of photosynthesis.

6. **Release of Electrons**: As a result of this excitation, chlorophyll releases electrons. This process is crucial as it starts the chain of reactions that will ultimately lead to the production of ATP and NADPH.

7. **Photolysis of Water**: To compensate for the loss of electrons from chlorophyll, water molecules are split (photolysis), releasing electrons and hydrogen ions. This occurs simultaneously with the excitation of electrons.

8. **Conclusion**: Therefore, the first step in the light reaction of photosynthesis is the excitation of electrons in chlorophyll due to the absorption of photons of light.

**Final Answer**: The first step in the light reaction of photosynthesis is the excitation of electrons of chlorophyll by a photon of light.

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