Light reaction

Light Reaction

The Light Reaction or Light-dependent Reactions is the first stage of photosynthesis, the process by which plants, some bacteria, and some protistans use the energy from sunlight to produce sugar. This process converts light energy into chemical energy, which is stored in the bonds of sugar molecules.

Overview

The light-dependent reactions take place in the thylakoid membrane in the chloroplast. They use light energy to make two molecules needed for the next stage of photosynthesis: the energy storage molecule adenosine triphosphate (ATP) and the reduced electron carrier nicotinamide adenine dinucleotide phosphate (NADPH).

Process

The process of light reaction can be divided into two systems: Photosystem I and Photosystem II.

Photosystem II

In Photosystem II, the absorption of light photons by a light-harvesting complex helps charge separation. The excited electrons lost from the reaction center P680 of PSII are replaced by electrons derived from water. The splitting of water is associated with the photolysis of water.

Photosystem I

In Photosystem I, the electrons are used to reduce NADP+ to NADPH. The energy for this comes from the second phase of the photophosphorylation (light reaction).

Significance

The light reactions provide the energy required for the Calvin Cycle, which assembles sugar molecules using CO2 and the energy-rich products of the light-dependent reactions.

See also

• Photosynthesis
• Calvin Cycle
• Chlorophyll
• Photophosphorylation