First reaction in photosynthesis is

The first reaction in photosynthesis is the Excitation of chlorophyll molecules.
When energy from light is absorbed by proteins called reaction centers that contain green chlorophyll (and other colored) pigments/chromophores. In plants, these proteins are held inside organelles called chloroplasts. In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas.