Example of Photochemical Reaction: Chlorophyll Absorption in Photosynthesis

Photosynthesis is a vital photochemical process that occurs in plants, algae, and some bacteria. It is the process by which these organisms convert light energy from the sun into chemical energy in the form of glucose. Chlorophyll, the primary pigment responsible for capturing light energy, plays a crucial role in this photochemical reaction.

1. Light Absorption by Chlorophyll
- In the first step of photosynthesis, chlorophyll molecules present in the chloroplasts of plant cells absorb light energy from the sun.
- Chlorophyll molecules contain a porphyrin ring structure that allows them to absorb light in the visible spectrum, mainly in the red and blue regions.
- The absorption of light by chlorophyll molecules excites electrons within the molecule to higher energy levels.

2. Electron Transfer and Energy Conversion
- The absorbed light energy is used to transfer excited electrons from chlorophyll molecules to other electron acceptor molecules present in the thylakoid membrane of the chloroplast.
- This electron transfer is facilitated by a series of protein complexes called the electron transport chain.
- As the excited electrons move through the electron transport chain, they lose energy, which is utilized to pump protons (H+) across the thylakoid membrane from the stroma to the thylakoid lumen.
- This creates a proton gradient, which drives the synthesis of ATP (adenosine triphosphate), a molecule that stores chemical energy.

3. Water Splitting and Oxygen Release
- Another crucial step in photosynthesis is the splitting of water molecules (H2O) into oxygen (O2), protons (H+), and electrons (e^-).
- This process, known as photolysis, occurs in the presence of light and is facilitated by a complex enzyme called photosystem II (PSII).
- The released oxygen molecules are a byproduct of photosynthesis and are essential for sustaining life on Earth.

4. Carbon Dioxide Fixation and Sugar Formation
- In the final step of photosynthesis, the energy stored in ATP and the electrons from the electron transport chain are used to convert carbon dioxide (CO2) into glucose (C6H12O6).
- This process, known as the Calvin cycle or light-independent reactions, takes place in the stroma of chloroplasts.
- The energy from ATP and the high-energy electrons (carried by NADPH) are utilized to power a series of enzyme-catalyzed reactions that convert CO2 into glucose.

Conclusion
Photosynthesis is a prime example of a photochemical reaction, where light energy is converted into chemical energy. Through the absorption of light by chlorophyll, electron transfer, water splitting, and carbon dioxide fixation, plants and other photosynthetic organisms are able to produce glucose and release oxygen into the atmosphere. This process is crucial for the sustenance of life on Earth as it forms the basis of the food chain and provides oxygen for respiration.

Photochemical reaction is a reaction which takes place in the presence of light...

For example ---
2AgCl ---------> 2Ag + Cl in the presence of sunlight

The silver Chloride turns grey...