What is Light Reaction?

The light reaction is the first stage of photosynthesis, occurring in the thylakoid membranes of chloroplasts. It uses sunlight energy to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate), which are essential for the next phase—the dark reaction or Calvin cycle.

When light hits the chlorophyll pigments, it excites electrons. These energized electrons travel through a chain of proteins known as the electron transport chain. During this process, water (H₂O) molecules split—a reaction called photolysis—releasing oxygen (O₂) as a byproduct, protons (H⁺), and electrons.

The electrons move through Photosystem II and Photosystem I, generating energy to convert ADP into ATP (through photophosphorylation) and reduce NADP⁺ to NADPH. Both ATP and NADPH carry chemical energy and reducing power required to convert carbon dioxide (CO₂) into glucose during the dark reaction.

In summary, the light reaction converts solar energy into chemical energy, produces oxygen, and forms ATP and NADPH, which drive the second stage of photosynthesis.