Mechanism of Light Reactions.Photosynthesis is the first stage of light-dependent reactions in which the solar energy is converted into chemical energy in the form of ATP and NADPH. ... The protein complexes and the pigment molecules help in the production of NADPH as well as ATP.

**Mechanism of Light Reaction in Photosynthesis**

Photosynthesis is the process by which plants and some other organisms convert light energy into chemical energy in the form of glucose. It involves two main stages: the light reaction and the dark reaction (also known as the Calvin cycle). In this explanation, we will focus on the mechanism of the light reaction.

**1. Conversion of Light Energy:**
During the light reaction, light energy is absorbed by pigments called chlorophylls present in the thylakoid membranes of the chloroplasts. The light energy excites electrons within the chlorophyll molecules, raising them to a higher energy level.

**2. Splitting of Water:**
The excited electrons are then transferred to an electron carrier molecule called NADP+ (nicotinamide adenine dinucleotide phosphate), which gets reduced to NADPH. To replace the lost electrons, water molecules are split into oxygen, hydrogen ions (H+), and electrons. This process is known as photolysis or the splitting of water.

**3. Generation of ATP:**
Simultaneously, the energy released during the transfer of electrons is used to pump protons (H+) across the thylakoid membrane from the stroma to the lumen, creating a proton gradient. This gradient is essential for the synthesis of ATP (adenosine triphosphate), the energy currency of cells. The protons flow back through ATP synthase, driving the synthesis of ATP from ADP (adenosine diphosphate) and inorganic phosphate (Pi) through a process called chemiosmosis.

**4. Electron Transport Chain:**
The energized electrons from chlorophyll are passed through a series of electron carrier molecules embedded in the thylakoid membrane, collectively known as the electron transport chain (ETC). As the electrons move through the ETC, their energy is gradually released and used to pump protons across the membrane.

**5. Final Electron Acceptor:**
The final electron acceptor in the ETC is a molecule called NADP+, which combines with the electrons and H+ ions to form NADPH. This NADPH, along with ATP generated during the light reaction, will be used in the dark reaction to produce glucose.

**6. Oxygen Release:**
The oxygen molecules generated during the splitting of water are released as a byproduct of the light reaction. This oxygen is vital for sustaining life on earth and is released into the atmosphere.

In summary, the light reaction of photosynthesis involves the absorption of light energy by chlorophyll, the splitting of water to release oxygen and electrons, the generation of ATP through chemiosmosis, and the production of NADPH as a reducing agent. These products, along with the ATP generated, are then used in the dark reaction to convert carbon dioxide into glucose, completing the process of photosynthesis.