Which statement correctly explains the roles of NAD+ and NADP+/NADPH in metabolism?

Which statement correctly explains the roles of NAD+ and NADP+/NADPH in metabolism?

• A. NAD+ is used for reductive biosynthesis.
• B. NADPH is used for ATP production in mitochondria.
• C. NAD+ is used in catabolic pathways to harvest energy, while NADPH is used in reductive biosynthesis and antioxidant defense.
• D. NAD+ and NADPH have identical roles.

Answer: (C)

The main idea is that NAD+ and NADPH serve distinct roles in metabolism based on whether a reaction is oxidizing fuel to harvest energy or providing reducing power for biosynthesis and defense. NAD+ acts as the oxidizing partner in catabolic pathways, accepting electrons as fuels are oxidized, which drives ATP production later via the electron transport chain. The reduced form, NADH, then donates those electrons to generate ATP. NADPH, on the other hand, supplies reducing power for reductive biosynthesis—such as fatty acid, cholesterol, and nucleotide synthesis—and for antioxidant defenses (keeping glutathione and related systems in a reduced, active state). So NAD+ is tied to energy harvesting, while NADPH is tied to building molecules and protecting the cell from oxidative stress. The other statements mix up these roles: NAD+ is not used for reductive biosynthesis, NADPH is not the main donor for ATP production in mitochondria, and the two cofactors do not have identical roles.

The main idea is that NAD+ and NADPH serve distinct roles in metabolism based on whether a reaction is oxidizing fuel to harvest energy or providing reducing power for biosynthesis and defense. NAD+ acts as the oxidizing partner in catabolic pathways, accepting electrons as fuels are oxidized, which drives ATP production later via the electron transport chain. The reduced form, NADH, then donates those electrons to generate ATP. NADPH, on the other hand, supplies reducing power for reductive biosynthesis—such as fatty acid, cholesterol, and nucleotide synthesis—and for antioxidant defenses (keeping glutathione and related systems in a reduced, active state). So NAD+ is tied to energy harvesting, while NADPH is tied to building molecules and protecting the cell from oxidative stress. The other statements mix up these roles: NAD+ is not used for reductive biosynthesis, NADPH is not the main donor for ATP production in mitochondria, and the two cofactors do not have identical roles.