Kaycie Nabeta, a 16 year old female from the Internet asks on May 10, 1999,Can you explain where the "extra" ATP comes from in photosynthesis? As I understand it:
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There is not a fixed number of ATP molecules made in the light reactions of photosynthesis because both non-cyclic electron transport (through both photosystems I and II) and cyclic electron transport (though only photosystem I) can occur. The extent of cyclic (ATP forming) and non-cyclic (ATP and NADPH forming) is regulated to balance the amount of ATP and NADPH needed.
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