Which pathway provides most NADPH in a typical cell?

• A. Glycolysis provides most NADPH
• B. The pentose phosphate pathway oxidative branch provides most NADPH
• C. Citric acid cycle provides NADPH
• D. Beta-oxidation provides NADPH

Answer: (B)

NADPH serves as the reducing power for biosynthesis and antioxidant defense, and the oxidative branch of the pentose phosphate pathway is the primary source of cellular NADPH. In this branch, glucose-6-phosphate is oxidized by glucose-6-phosphate dehydrogenase, producing NADPH and 6-phosphoglucono-δ-lactone. The next oxidative step further oxidizes 6-phosphogluconate to ribulose-5-phosphate, generating a second molecule of NADPH. This sequence provides a substantial amount of NADPH for anabolic reactions and maintaining redox balance. Other pathways mainly generate NADH or FADH2 for energy production rather than NADPH, such as glycolysis (which yields NADH in the glyceraldehyde-3-phosphate dehydrogenase step), the citric acid cycle, and beta-oxidation.