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The PPP has an irreversible oxidative phase that converts glucose-6-phosphate through 6-phosphogluconate to ribulose-5-phosphate, producing NADPH and carbon dioxide. A reversible non-oxidative phase interconverts sugar phosphates, including ribose-5-phosphate, fructose-6-phosphate, and glyceraldehyde-3-phosphate. Glucose-6-phosphate dehydrogenase is the rate-limiting enzyme; its activity responds to NADP⁺ and NADPH, and its expression is increased by insulin. NADPH supports reductive biosynthesis and helps maintain reduced glutathione, especially important in red blood cells. G6PD deficiency can leave red cells vulnerable to oxidative damage and hemolysis. The pathway’s tissue distribution, enzyme cofactors, clinical correlations, and links to other metabolic pathways help explain its physiological and diagnostic relevance.