Which metabolic pathways primarily rely on NADPH for reducing power?

• A. Fatty acid synthesis, cholesterol synthesis, and reductive biosynthesis; PPP provides NADPH.
• B. Glycolysis, TCA cycle, and urea cycle.
• C. Oxidative phosphorylation and glycolysis.
• D. DNA replication and transcription.

Answer: (B)

NADPH is the main source of reducing power for anabolic, biosynthetic processes rather than for energy production. The most prominent use is in fatty acid synthesis and cholesterol synthesis, where NADPH provides the electrons needed to drive reductive steps in building complex lipids. This is why the pentose phosphate pathway is key: it generates NADPH (and also supplies ribose-5-phosphate for nucleotide synthesis), making NADPH widely available for reductive biosynthesis.

By contrast, pathways like glycolysis and the TCA cycle are geared toward energy capture and mainly shuttle electrons through NADH and FADH2 to the electron transport chain; the urea cycle also relies on different cofactors for its reactions. Oxidative phosphorylation uses NADH-derived electrons, not NADPH, for ATP production. DNA replication and transcription depend on nucleotide substrates and polymerization chemistry rather than NADPH as the direct reducing power source.