NADH and NADPH are equivalent in terms of their standard redox potentials, but because redox enzymes are usuallyselective for one or the other of them, two distinct pools of reductants exist. NADH is used as a source of reducing
equivalents for the electron transport chain (ETC) while NADPH provides reducing equivalents for many biosyntheticreactions. Hence, even within a single spatial compartment such as the cytoplasm, the NADH to NAD+ ratio can be
very low, favoring oxidation of fuels, while simultaneously the NADPH to NADP+ ratio can be very high, facilitating
biosynthesis.