During the metabolic process of glycolysis, a net gain of 2 ATP molecules is produced. In glycolysis, one glucose molecule is broken down into two molecules of pyruvate. Although a total of 4 ATP molecules are generated in the process, 2 ATP molecules are utilized in the initial steps of glycolysis to help convert glucose into a usable form. Therefore, when you subtract the 2 ATP used from the 4 ATP produced, the result is a net gain of 2 ATP.
This process takes place in the cytoplasm of the cell and does not require oxygen, making it an essential means of generating energy under anaerobic conditions as well. The two pyruvate molecules generated can then enter further metabolic pathways, such as the citric acid cycle, for additional energy production when oxygen is present.