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PSII absorbs a photon to produce a so-called high energy electron which transfers via an electron transport chain to cytochrome b6f and then to PSI. The then-reduced PSI, absorbs another photon producing a more highly reducing electron, which converts NADP+ to NADPH. In oxygenic photosynthesis, the first electron donor is water, creating oxygen (O2) as a by-product. In anoxygenic photosynthesis, various electron donors are used. Cytochrome b6f and ATP synthase work together to produce ATP (photophosphorylation) in two distinct ways. In non-cyclic photophosphorylation, cytochrome b6f uses electrons from PSII and energy from PSI[citation needed] to pump protons from the stroma to the lumen. The resulting proton gradient across the thylakoid membrane creates a proton-motive force, used by ATP synthase to form ATP. In cyclic photophosphorylation, cytochrome b6f uses electrons and energy from 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.