Photosystem II is the first protein complex in the light-dependent reactions of photosynthesis. This protein complex is located within the thylakoid membrane of a plant cell's chloroplast. Light photons enter the photosystem where they are captured by enzymes and used to energize electrons that will be used during photosynthesis.
The energized electrons are then transferred through a series of cofactors and coenzymes also located in the thylakoid membrane. These electrons are replaced at photosystem II with electrons from the splitting of water molecules. When the water molecules separate, lone hydrogen ions concentrate inside the thylakoid space. As the hydrogen level rises, these ions are pumped out of the thylakoid space by the enzyme ATP synthase. This action produces ATP energy, which the cell will later use during the Calvin Cycle to convert carbon dioxide into glucose.