<P> In photosynthesis, the light - dependent reactions take place on the thylakoid membranes . The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light - independent reactions take place . The thylakoid membrane contains some integral membrane protein complexes that catalyze the light reactions . There are four major protein complexes in the thylakoid membrane: Photosystem II (PSII), Cytochrome b6f complex, Photosystem I (PSI), and ATP synthase . These four complexes work together to ultimately create the products ATP and NADPH . </P> <P> The four photosystems absorb light energy through pigments--primarily the chlorophylls, which are responsible for the green color of leaves . The light - dependent reactions begin in photosystem II . When a chlorophyll a molecule within the reaction center of PSII absorbs a photon, an electron in this molecule attains a higher energy level . Because this state of an electron is very unstable, the electron is transferred from one to another molecule creating a chain of redox reactions, called an electron transport chain (ETC). The electron flow goes from PSII to cytochrome b6f to PSI . In PSI, the electron gets the energy from another photon . The final electron acceptor is NADP . In oxygenic photosynthesis, the first electron donor is water, creating oxygen as a waste product . In anoxygenic photosynthesis various electron donors are used . </P> <P> Cytochrome b6f and ATP synthase work together to create ATP . This process is called photophosphorylation, which occurs in two different ways . In non-cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from PSII to pump protons from the stroma to the lumen . The proton gradient across the thylakoid membrane creates a proton - motive force, used by ATP synthase to form ATP . In cyclic photophosphorylation, cytochrome b6f uses the energy of electrons from not only PSII but also PSI to create more ATP and to stop the production of NADPH . Cyclic phosphorylation is important to create ATP and maintain NADPH in the right proportion for the light - independent reactions . </P>

When do the light reactions of photosynthesis occur