Flexi Says: Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules. These two molecules go on to stage II of cellular respiration. The energy to split glucose is provided by two molecules of ATP. The very first step involves the phosphorylation of glucose in which the glucose molecule reacts with ATP in presence of hexokinase enzyme. In the reaction, the H of -CH2OH group in glucose is replaced by the phosphate group of ATP to produce glucose-6-phosphate and ADP.