Light dependent reaction
The light dependent reaction is the first stage of photosynthesis. The process begins when a chlorophyll pigment in photosystem 2 (PS2) in the thylakoid membrane absorbs a photon from light. This causes the PS2 to be in an unstable state which causes an electron to pass from PS2 to cytochrome b6f to photosystem 1 (PS1) in a series of redox reactions (electron transport chain) down the thylakoid membrane. Energy is lost at stages during the redox reaction, this energy is used to pump H+ ions across the thylakoid membrane from the stroma creatining an electrochemical gradient. H+ ions then flow back across membrane through ATP synthase which catalyses the reaction of ADP and Pi to form ATP.