Final answer:
A net of two ATP are synthesized through substrate level phosphorylation per glucose molecule in glycolysis; four ATPs are produced, but two are used in earlier steps, resulting in a net gain of two ATPs.
Step-by-step explanation:
In the process of glycolysis, a net of two ATP are produced by substrate level phosphorylation per glucose molecule. During glycolysis, two ATPs are used up in the early stages, but four ATPs are produced later on.
This means that four ATPs are produced, but since two are used, there's a net gain of two ATPs by the end. Specifically, these ATP molecules are generated as a result of the direct transfer of a phosphate group from a phosphorylated substrate to ADP, and this occurs at two different steps in the glycolysis pathway—in the seventh and tenth steps involving the enzymes phosphoglycerate kinase and pyruvate kinase, respectively.
Although a total of four ATPs are produced in the energy payoff phase of glycolysis, it's important to note that during the energy-investment phase, two ATPs are consumed, resulting in a net gain of two ATPs for each glucose molecule.
However, these two ATPs are sometimes considered to be used for transporting NADH produced during glycolysis from the cytoplasm into the mitochondria, suggesting that the net production of ATP during glycolysis can effectively be zero in the presence of a mitochondrial membrane.
It is also noteworthy that each NADH from the glycolysis has a potential to produce a further three ATPs in the electron transport chain (ETC), under aerobic conditions.