Chlorophyll is a very important pigment that specializes in absorbing light. It can be found in some bacteria as well as in the chloroplast organelle in plants and green algae. Chlorophyll absorbs red and blue wavelengths most strongly and, to a lesser degree, violet. The green spectrum of light is not absorbed by the chlorophyll and is instead reflected. This green reflection is why leaves tend to look green.
Photosystem II is the first light-dependent reaction that uses chlorophyll to capture light; Photosystem I happens second. There are two main components of these systems: the antenna complex and the reaction center. The antenna complex is where most of the chloropyll is found, and it is arranged to optimize the amount of light absorbed. In Photosystem II, the chlorophyll has an absorption peak of around 680 nm while Photosystem I captures light best around 700 nm.
The reaction center holds more specialized chlorophyll pigments, which help extract excited electrons from the antenna complex, where the light is being absorbed.
Another reaction triggered by the light absorption is photolysis. This reaction splits water molecules, which releases the oxygen back out into the atmosphere. Excited electrons from this reaction, along with the excited electrons from the Photosystem reaction centers, goes through a series of redox reactions in the electron transport chain. The energy released from these reactions helps make ATP, which is one of the main energy molecules of the cell.
The energy generated by the light-dependent reactions helps run the light-independent reactions, which break down carbon dioxide and generate carbohydrate or other nutrients for the plant to use. This series of reactions is also called the Calvin cycle; sometimes it is referred to as the dark cycle since it uses light in an indirect manner.