. "The sequence of reactions by which arginine is synthesized from ornithine, then cleaved to yield urea and regenerate ornithine. The overall reaction equation is NH3 + CO2 + aspartate + 3 ATP + 2 H2O = urea + fumarate + 2 ADP + 2 phosphate + AMP + diphosphate." . . . "urea cycle" . "ornithine cycle" . "urea biosynthesis" . "urea biosynthetic process" . . . . .