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Metabolism of living cells is a complex network of chemical reactions to extract, convert and store energy from nutrients that can be analyzed in self-contained parts called pathways. Metabolic pathways contribute to catabolism - the oxidative degradation of complex molecules to simpler compounds, and - the reductive synthesis of complicated molecules from simpler ones. Catabolic pathways release free energy some of which is conserved in the formation of adenosine triphosphate (ATP) and reduced pyridine nucleotides (NADH, NADPH).