Well, first, NAD+ is the oxidized form of NADH. So, it wouldn't really make sense for NADH to be passing electrons back to NAD+. NADH instead dumps it's electrons into a protein called NADH dehydrogenase in a pathway known as the electron transport chain. This involves other proteins and electron transporters that help to eventually form ATP. However, NADH does not pass its electrons to FAD. That occurs in succinate dehydrogenase when electrons from succinate are moved into FAD (which is reduced to FADH2). Also, 2ATP/FADH2 and 3ATP/NADH2 is a general rule. Try not to get too attached to it.
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