The process of photosynthesis involves two steps–
(i) Light dependent phase or photochemical reaction.
(ii) Light independent these or dark reaction.
Light reaction occurs in grana fraction of chloroplast and in this reaction are included those activities, which are dependent on light. The grana of chloroplasts contains many collaborating molecules of pigment. A quantum of light is absorbed by a single antenna chlorophyll, then it migrates from one molecule to the other till it reaches the reaction center. This quantum of light is used for generating ATP and NADPH, which is later consumed in dark reactions produce sugars by fixing CO₂ molecules.