Glycolysis consists of two distinct phases: the energy investment phase and the energy harvesting phase (see figure below). In the first phase, glucose is broken down into 2 molecules of fructose 1,6-bisphosphate at the cost of 2 ATP molecules. The second step gives rise to 1 pyruvate molecule, 2 ATP molecules, and 1 NADH molecule per molecule of fructose 1,6-bisphosphate (double products per glucose molecule).

A digram outlining the steps in glycolysis. Glucose is the starting point. First, an arrow leads from glucose to fructose 1 6 bisphosphate. This arrow is labelled to show an energy investment was required for the reaction. The investment required 2 molecules of ATP, which were de-phosphorylated to ADP. Next, an arrow leads from fructose 1 6 bisphosphate to 2 molecules of pyruvate. This reaction generates energy, with 4 molecules of ADP becoming phosphorylated to produce 4 molecules pf ATP. There is a net gain of 2 molecules of ATP for glycolysis as a whole. Additionally, during the second reaction, 2 molecules of NA Dplus are reduced to 2 molecules of NADH. The two molecules of pyruvate are the final product of glycolysis.

Figure 1: The overall reaction of glycolysis.