And the production of ATP molecules without using the proteins of the electron transport chain is known as substrate level phosphorylation. Now, the production of ATP molecules using the electron transport chain, as we'll see in just a moment, is known as oxidative phosphorylation. So substrate level phosphorylation is the production of ATP in glycolysis. And substrate level phosphorylation also occurs in the citric acid cycle, as we'll see in just a moment. Now, although aerobic respiration means we have oxygen present in the cell, glycolysis takes place regardless of whether or not we have oxygen present inside the cell. So that implies glycolysis is the first step of aerobic as well as anaerobic respiration.