Which coenzyme is primarily involved in redox reactions during glycolysis and oxidative phosphorylation?

• A. FAD
• B. NADP+
• C. CoA
• D. NAD+

Answer: (D)

NAD+/NADH acts as the main electron carrier in these energy-yielding pathways. In glycolysis, a key step uses NAD+ to accept electrons, forming NADH as a molecule called 1,3-bisphosphoglycerate is produced. Later, during oxidative phosphorylation, the NADH produced donates its electrons to the mitochondrial electron transport chain, returning to the oxidized form NAD+. This cycle of NAD+ accepting electrons and NADH passing them to the chain drives the production of ATP. Other coenzymes play roles in specific steps (FAD participates in some redox reactions, but not as the primary carrier here; NADP+/NADPH is more about biosynthesis and antioxidant work; CoA carries acyl groups), so NAD+/NADH is the best fit for redox reactions in both glycolysis and oxidative phosphorylation.