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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?
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    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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