Let’s consider glycolysis in more detail. First, the reactions between glucose and pyruvate can be considered as two distinct phases:(1) an energy investment phase and (2) an energy generation phase (Fig.3.13). The first reactions make up the ‘energy investment phase’ where stored ATP must be used to form sugar phosphates. Although the end result of glycolysis is the release of energy, glycolysis must be ‘primed’ by the addition of ATP at two points at the beginning of the pathway(Figs.3.14 and 3.15). The purpose of the ATP priming is to add phosphate groups (called phosphorylation) to glucose and to fructose 6-phospate. Note that if glycolysis begins with glycogen as the substrate, the addition of only one ATP is required .