Why are catabolism and anabolism interdependent?

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Catabolic pathways break down nutrients to harvest energy (ATP) and to generate the building blocks, or precursors, needed for biosynthesis. Anabolic pathways, in turn, use that ATP and those carbon skeletons to construct macromolecules like proteins, nucleic acids, lipids, and polysaccharides. Because synthesis requires both energy and specific precursor molecules, the cell relies on the outputs of catabolism to drive biosynthesis. If ATP is limited or essential precursors such as acetyl-CoA, oxaloacetate, or ribose-5-phosphate aren’t available, anabolic processes stall. Conversely, the energy carriers (ATP and reducing equivalents) and the precursor pools produced by catabolism fuel the anabolic reactions, creating a tightly connected loop that supports growth and maintenance. The statement that catabolism supplies ATP and precursors and that anabolism can occur only when they’re available captures this interdependence.