Glycolysis and the citric acid cycle provide the precursors needed to synthesize many important biological molecules. The amino acids, nucleotides, lipids, sugars, and other molecules—shown here as products—in turn serve as the precursors for the many macromolecules of the cell. Each black arrow in this diagram denotes a single enzyme-catalyzed reaction; the red arrows generally represent pathways with many steps that are required to produce the indicated products.  Fair Use and Copyright info