Which molecule provides reducing power for anabolic (biosynthetic) reactions?

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Reducing power for biosynthesis comes from NADPH, the donor of hydride electrons to anabolic reactions. In anabolic pathways, enzymes use NADPH to reduce substrates—think of ribonucleotide reductase converting ribonucleotides to deoxyribonucleotides and fatty acid synthase steps that require NADPH to build long chains. NADPH is generated by NADP+-dependent dehydrogenases, notably in the pentose phosphate pathway, and its structure (the extra phosphate) keeps it separate from the energy-producing NADH pool, directing reducing power specifically to biosynthetic work. In contrast, NADH mainly feeds the electron transport chain to drive ATP production, not biosynthesis. So NADPH is the molecule that provides the reducing power for anabolic reactions.