Briefly describe the two phases of glycolysis (energy investment and energy capture). Explain why glycolysis requires an initial investment of energy.

In glycolysis, an initial investment of energy, in the form of two ATP molecules, is required to phosphorylate glucose, which eventually yields two three-carbon intermediates. In the following energy capture phase, two ATP molecules are generated from each of the three-carbon intermediate. Because the glucose molecule is split into two intermediates, a total of four ATP molecules are generated in latter phase of glycolysis, which is double the amount of ATP than if glucose had remained intact.