Final answer:
Glycolysis produces a net gain of two ATP molecules because although four ATP are produced, two are used at the beginning of the process to split glucose into pyruvate. The net ATP production from glycolysis is essential for cells, especially under anaerobic conditions or in cells lacking mitochondria, like red blood cells.
Step-by-step explanation:
Net ATP Production in Glycolysis
Glycolysis is an essential metabolic pathway that occurs in the cytoplasm of cells. Although four ATP molecules are produced during this process, two ATP molecules are consumed at the start of glycolysis to split the glucose molecule into two three-carbon pyruvate molecules. Therefore, the net gain of ATP during glycolysis is two ATP molecules. Another important aspect of glycolysis is the formation of NADH, an energy-carrying molecule that will be utilized in later stages of cellular respiration to generate more ATP.
Under anaerobic conditions, cells rely on glycolysis as their sole source of ATP production. Mature mammalian red blood cells are an example of cells that depend exclusively on glycolysis for ATP, since they lack mitochondria and thus cannot perform aerobic respiration.
It is important to note that the two ATP molecules produced in glycolysis also play a role in transporting the NADH from the cytoplasm into the mitochondria, where it can be used in the electron transport chain (ETC). However, the energy cost for this transport should not be confused with the net ATP production of glycolysis, which remains at two ATPs before considering the energy required for NADH transport.