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I know NADH is used in cellular respiration and NADPH is used in photosynthesis. What difference does the phosphate group make that the same one isn't or can't be used for both? Is there a greater reason for this separation or is it just coincidental? Why can't the two be interchanged?

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NADH provides energy for Catabolic reactions and as for NADPH it provides energy for anabolic reactions (Synthesis ) –  user3293 Mar 26 '13 at 1:45
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NADPH is found in the cytosol and stroma (chloroplast) of eukaryotes. NADH is more ubiquitous, but mostly found in bacteria and in mitichondria, possibly evidence for the endosymbosis of bacteria in eukaryotes. Neither can pass easily through a membrane.

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The phosphate group in NADPH doesn't affect the redox abilities of the molecule, it is too far away from the part of the molecule involved in the electron transfer. What the phosphate group does is to allow enzymes to discriminate between NADH and NADPH, which allows the cell to regulate both independently.

The ratio of NAD+ to NADH inside the cell is high, while the ratio of NADP+ to NADPH is kept low. The role of NADPH is mostly anabolic reactions, where NADPH is needed as a reducing agent, the role of NADH is mostly in catabolic reactions, where NAD+ is needed as a oxidizing agent.

You'll find some more information about this in chapter 2 of "Molecular Biology of the Cell by Alberts et al.

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too bad I only have one upvote to give. A direct link to the section in Alberts that talks about NADH/NADPH was great! –  Steve Lianoglou Jan 26 '12 at 13:50
    
Great use of the link to the chapter - I wish more authors did this. –  Mac Cowell Sep 18 '13 at 2:30
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