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    I've been revising photosynthesis and aerobic respiration for my A-level biology, and am pretty sure I understand it in enough detail for the purpose of A2 level. However I am a little confused about the actual mechanism by which NADP in photosynthesis and NAD in cellar respiration are reduced to NADPH and NADH respectively.

    I'll concentrate on NADP/NADPH to make this post less messy.

    As I understand it, NADP is itself a positive molecule, and is reduced to the neutral NADPH by gaining 2 electrons and a Hydrogen proton, and in the process another H+ ion is released as a by-product? Is this right so far?

    If it is, then my next question is do the Hydrogen atoms come from a molecular H2? Or are they individual protons on their own? I had thought that NADP+ first gains 2 electrons (from excited chloropyhll), which would leave it with a negative charge, allowing it to pick up the Hydrogen proton, thus generating NADPH.

    So if I wanted to draw out the mechanism for the reduction, would the reactants include a molecular H2 which splits in two, or just 2 H+ protons? If its the latter (which sounds more likely) then where do these protons come from?

    That's correct. In photosynthesis, the protons are generated from photolysis of water molecules. This splitting generates two protons, two electrons and oxygen. In respiration, the protons that reduce NAD+ are derived from the oxidation of the substrate being catabolised.
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