Glycolysis is a process that occurs in the cytoplasm of cells, breaking down glucose to produce energy in the form of ATP. During glycolysis, one molecule of glucose (which has six carbon atoms) is converted into two molecules of pyruvate (which have three carbon atoms each).
In the initial steps of glycolysis, some ATP is consumed to modify glucose and prepare it for breakdown. Specifically, two ATP molecules are used to phosphorylate glucose and its derivatives, making them more reactive.
However, as the pathway continues, ATP is generated through a series of enzymatic reactions. In the later stages of glycolysis, four ATP molecules are produced by substrate-level phosphorylation, a process where a phosphate group is directly transferred to ADP (adenosine diphosphate) from a phosphorylated intermediate.
Overall, the net gain of ATP from glycolysis is two ATP molecules, since we started with two ATP consumed and ended with four produced. This efficient metabolic pathway allows cells to generate energy quickly even in the absence of oxygen.