The ten-step catabolic pathway of glycolysis is the initial phase of free-energy release in the breakdown of  glucose and can be split into two phases, the preparatory phase and payoff phase. ADP and phosphate are needed as precursors to synthesize ATP in the payoff reactions of the TCA cycle and oxidative phosphorylation mechanism.  During the payoff phase of glycolysis, the enzymes phosphoglycerate kinase and pyruvate kinase facilitate the addition of a phosphate group to ADP by way of substrate-level phosphorylation.