Which statement best describes how energy and building blocks move between catabolism and anabolism?

• A. Anabolism supplies energy and precursors to drive catabolism.
• B. Catabolism provides ATP and precursors, which anabolism uses.
• C. Energy is stored but never exchanged.
• D. Precursor molecules alone drive cellular functions without energy.

Answer: (B)

Energy and building blocks move between catabolic and anabolic processes in a coordinated flow: catabolic reactions break down molecules to harvest energy and produce precursor molecules, and these outputs fuel biosynthesis in anabolic pathways. The statement that catabolism provides ATP and precursors, which anabolism uses, captures this direction of flow. ATP serves as the energy currency, driving energy-demanding steps in building cellular components, while precursor molecules supply the carbon skeletons and reducing power needed for synthesis.

Think of it as energy and building blocks being generated by breaking things down, then reused to build new stuff. The other ideas don’t fit because energy isn’t stuck away permanently or never exchanged—cells continually shuttle energy via ATP and related carriers. And precursors don’t alone drive cellular functions—many biosynthetic steps require energy input to proceed.