Final answer:
NAD is used in catabolic reactions, while NADP is used in anabolic reactions.
Step-by-step explanation:
The reason NAD is primarily used in catabolic reactions and NADP is primarily used in anabolic reactions is because catabolic reactions involve the breakdown of larger molecules into smaller ones, releasing energy in the process. NAD is involved in carrying the electrons produced during these reactions. On the other hand, anabolic reactions involve the synthesis of larger molecules from smaller ones, requiring energy. NADP is used as an electron carrier in these reactions, providing the energy needed for the synthesis of complex molecules.