Photosystem II
In the photosystem II (PSII) reaction center, energy from sunlight is used to extract electrons from water. The electrons travel through the chloroplast electron transport chain to photosystem I (PSI), which reduces NADP+ to NADPH. The electron transport chain moves protons across the thylakoid membrane into the lumen. At the same time, splitting of water adds protons to the lumen while reduction of NADPH removes protons from the stroma. The net result is a low pH in the thylakoid lumen and a high pH in the stroma. ATP synthase uses this electrochemical gradient to make ATP.
Source
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"OpenStax College, The Light-Dependent Reactions of Photosynthesis. October 16, 2013."
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OpenStax CNX
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