Chemistry Question #11

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:
  • 24 ATP come out of the light reaction (12 water molecules times 2 ATP -- one from the pair of hydrogens from photolysis, the other from the pair transported by plastoquinone)
  • 18 ATP are used in the Calvin cycle/dark reaction/carbon fixation (6 cycles times 2 ATP when PGA changes into PGAL plus 6 cycles times 1 ATP when ribulose phosphate changes into ribulose diphosphate)
  • Therefore, 24-18=8 ATP
But 36 ATP are supposed to come out of photosynthesis (because 36 ATP are formed in cellular respiration) Where did the rest of the ATP come from?

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The answer

Gary Brudvig answered on May 10, 1999

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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