How do heterotrophs obtain both energy and building blocks?

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Heterotrophs obtain both energy and building blocks by breaking down preformed organic molecules through central metabolic pathways. As sugars and other organics are metabolized (via glycolysis), ATP is produced and reducing equivalents like NADH are generated. Pyruvate from glycolysis then enters the TCA cycle, yielding more ATP along with key metabolic intermediates. Those intermediates are siphoned off to make biosynthetic precursors for amino acids, nucleotides, lipids, and cell wall components, providing the building blocks needed for growth. The pentose phosphate pathway also supplies ribose-5-phosphate for nucleotides and NADPH for biosynthesis, linking energy production with anabolic supply. In short, degrading organic nutrients fuels energy production and supplies the carbon skeletons and reducing power required for macromolecule synthesis.