NADH is used for oxidative metabolism. NADH is used for catabolic pathways. In the citric acid cycle you use NAD+ as an electron carrier for the three of the reactions. NAD+ grabs the electrons from the metabolite and is converted to NADH + H. NAD+ wasn't there to take the electrons from the reaction the citric acid cycle could very well stop because you need those electrons picked up to make the next metabolite so the next metabolite can be converted. The NADH + H goes to the electron transport chain (ETC) and donates the electrons to Complex I. It's used for fatty acid-breakdown in the same way, NAD+ takes the electrons from an H-C-OH group to make it a C=O group.
However, NADH can also be used as an electron donor such as the electron transport chain or when lactate is converted back to pyruvate.
NADPH is used for reductive metabolism. NADPH is used for anabolic reactions. NADPH is used for biosynthesis. NADPH is used as reductive power in pathways such as cholesterol synthesis and fatty acid synthesis. The way that helped me learn this at first was that while NADH was for electrons, NADPH was used for the hydrogens for reactions such as reducing a double bond, CH=CH to CH2-CH2; or it can reduce a C=O to CHOH or CH2OH. Once, I learned it that way it was much easier to separate the two concepts.
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