Which molecule provides NADPH for reductive biosynthesis and how is it generated in the cytosol?

• A. The PPP generates NADPH and provides reducing power for biosynthesis.
• B. The TCA cycle generates NADPH in the mitochondria.
• C. Fatty acid oxidation generates NADPH directly.
• D. ATP synthase generates NADPH.

Answer: (A)

NADPH provides the reducing power for reductive biosynthesis in the cytosol, and the main source in that compartment is the oxidative branch of the pentose phosphate pathway. In this pathway, glucose-6-phosphate is oxidized by glucose-6-phosphate dehydrogenase, generating NADPH, and a second NADPH is produced by 6-phosphogluconate dehydrogenase as the pathway progresses to ribulose-5-phosphate. This route not only supplies NADPH for fatty acid, cholesterol, and nucleotide synthesis but also connects to ribose-5-phosphate production for nucleotide biosynthesis, all in the cytosol. The other options don’t fit because the TCA cycle operates in mitochondria and mainly yields NADH (not NADPH); fatty acid oxidation generates acetyl-CoA with NADH and FADH2, not NADPH directly; ATP synthase makes ATP rather than NADPH. The pentose phosphate pathway is the classic cytosolic source of NADPH for biosynthetic reactions.