The answer is; glycolysis
This process converts glucose molecule to pyruvate. It is an oxygen-independent pathway, unlike the Krebs cycle. Glycolysis occurs in the cell cytoplasm while the Krebs cycle (aerobic pathway) occurs in the mitochondria. In the presence of oxygen, the product of glycolysis, i.e pyruvate, is fed to the Krebs cycle. If oxygen is unavailable the pyruvate is converted to lactate.