What happen during the light dependent reaction of photosynthesis?

The plant takes in sunlight energy and uses it to break up H20. It then moves to the next step, where it combines hydrogen, oxygen, and carbon dioxide to make glucose.

During the light dependent reaction :
- Light absorption takes place
- Water splits
- Oxygen is released
- And there is formation of high energy chemical intermediates (ATP and NADPH).

Photosynthesis consists of two parts:
1. Light reaction
2. Dark reaction (calvin-benson cycle)
The whole idea of photosynthesis is to take inorganic carbon dioxide and reduce it to organic sugars.
During the light reaction, light energy (termed photons) is captured by pigment molecules in the chloroplast. This energy then kick out and electron and send it into the electron transport chain. The free energy of the electron, which ultimately came from the light photon is gradullay released during the electron transport chain and this is coupled to the active transport of protons from the chloroplast stroma into the thylakoid lumen. As the protons fllow back thourgh the ATPase enzyme their free energy is used to drive the synthesis of ATP. You can say that this is a form of passive transport, or facilitated diffusion. All passive transport processes release free energy, and in this case that free energy is used to drive the endergonic synthesis of ATP.
Also, the light reaction produces NADPH which function as a reducing agent for the calvin cycle. You know that REDOX reactions are always coupled if something is oxidized, something else HAS TO BE reduced. NADPH provides the reducing power to reduce CO2 in sugar, it, in return becomes oxidized back to NADP+.
The ATP function as the energy currency of the cell and is subsequently used up calvin cycle to drive the synthesis of sugars.

The light-dependent portion of photosynthesis is carried out by two consecutive photosystems (photosystem I and photosystem II) in the thylakoid membrane of the chloroplasts. The photosystems are driven by the excited chlorophyll molecules.

To begin photosynthesis, the chlorophyll molecule in photosystem II is excited by sunlight and the energy produced helps to break down a water molecule (H2O) into ½O2 (with electrons removed) and 2H+. The removed electrons are excited by the light energy. When the electrons prepare to come to their rest state, they go through an oxidative phosphorylation process and produces an ATP molecule.

As the electrons are coming to a resting state, they are excited again in photosystem I and raised to a even higher energy state. The excited electrons are then used to produce NADP+ + H+. The highly energetic NADPH molecule is then fed into the Calvin Cycle to conduct carbon fixation.