Glycolysis, the first process in cell respiration, produces four ATP, but it uses two of the ATP molecules, therefore producing a net two ATP molecules. The process also yields two molecules of NADH.
Glycolysis begins with glucose and breaks it down into two molecules of phosphoglyceraldehyde. This process utilizes two molecules of ATP for energy. Next, the phosphoglyceraldehyde molecules are converted into two molecules of pyruvate, which also produces four ATP molecules. However, since the first step of glycolysis utilizes two molecules of ATP, the net production of ATP from glycolysis is only two molecules. Once the process of glycolysis is complete, the pyruvate can be oxidated and placed into the Krebs cycle (the next process in cell respiration) to create even more ATP and other high-energy compounds.