In plants, the light-dependent reactions of photosynthesis occur in the thylakoid membrane of chloroplasts which houses photosystems PSII and PSI. PSII, the first multi-protein complex of photosynthesis, absorbs photons and passes their energy through the light-harvesting complex. This complex encompasses many bound pigment molecules such as chlorophyll a, chlorophyll b and carotenoids. This active photon absorption generates energy that is transferred to a special pair of chlorophyll a molecules known as the P680 reaction center. This special chlorophyll pair gives up an excited electron to the primary electron acceptor molecule in a process called a photoact. Now the electrons are ready to travel through the electron transport chain to photosystem I. Simultaneously, the enzymatic splitting of water into hydrogen and oxygen releases two electrons that replace the missing electrons from P680. The released hydrogen ions enter the thylakoid space while the oxygen atom combines with another oxygen atom to form O2 gas that is released into the atmosphere.