Which statement correctly distinguishes catabolic and anabolic metabolic pathways?

• A. Catabolic break down molecules to release energy; anabolic build complex molecules using energy.
• B. Anabolic break down molecules to release energy; catabolic build complex molecules.
• C. Catabolic and anabolic are the same; both require energy input.
• D. Anabolic reactions release energy to power cellular processes.

Answer: (A)

Energy flow in metabolism: catabolic pathways break down molecules to release energy, while anabolic pathways use energy to build complex molecules. When larger molecules are broken apart, bonds are cleaved and energy is released, which is captured in carriers like ATP and NADH. That stored energy then powers biosynthetic processes, such as making proteins from amino acids or synthesizing nucleic acids, lipids, and polysaccharides. So the description that catabolic reactions release energy and anabolic reactions require energy to synthesize more complex structures captures the fundamental difference between these two types of metabolic pathways. The other statements flip or misstate this relationship: anabolic building up molecules is not about breaking them down, and saying both types require energy or that anabolic releases energy would misrepresent how energy is managed in metabolism.