Water plays an important role in the light reaction of photosynthesis. It is the source of electrons and hydrogen ions that are used to reduce NADP+ to NADPH, which is a high-energy molecule that carries electrons for the dark reaction²³. Water also provides oxygen as a by-product when it is split by the light energy absorbed by chlorophyll in PSII¹². Oxygen is released into the atmosphere and can be used by other living organisms for respiration²³. Water also helps to maintain the pH and osmotic balance of the chloroplasts by regulating the flow of protons across the thylakoid membrane⁵⁶..

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