Over the course of glycolysis' nine steps, the 6-carbon molecule glucose is broken down to two 3-carbon pyruvate molecules. The reaction does not occur spontaneously: 2 ATP molecules must be broken down to drive the splitting of glucose into the 2 pyruvates. However, in the course of the breakdown of glucose, the glycolysis reaction produces four ATP, resulting in a net gain of two ATP for the entire process. Glycolysis also results in the production of 2 NADH molecules, which eventually play an important role in the production of additional ATP in the electron transport chain. Glycolysis itself is an anaerobic process. After a cell has completed glycolysis, and depending on the circumstances in which the cell finds itself, that cell can either move into the process of aerobic respiration and commence the citric acid cycle or continue with less efficient aneorobic respiration in a process called fermentation , covered in the third section of this SparkNote on glycolysis.
The preceding steps in glycolysis have transformed one molecule of glucose into two molecules of glyceraldehyde 3-phosphate, but no energy has yet been extracted. On the contrary, thus far two molecules of ATP have been invested. We come now to a series of steps that harvest some of the energy contained in glyceraldehyde 3-phosphate. The initial reaction in this sequence is the conversion of glyceraldehyde 3-phosphate into 1,3-bisphosphoglycerate ( 1,3-BPG ), a reaction catalyzed by glyceraldehyde 3-phosphate dehydrogenase ( Figure ).