GLYCOLYSIS
Glycolysis, which takes place in the cytosol, is an anaerobic process—that is, no oxygen is required for this phase of aerobic respiration. Glycolysis apparently evolved early in the evolutionary history of life. All eukaryotes and many prokaryotes have the necessary enzymes to carry out the ten steps of glycolysis.

During glycolysis, each molecule of glucose is broken into two three-carbon molecules of pyruvate. In the process there is a net production of two ATPs. Some of the electrons (as part of hydrogen atoms) stripped off the original glucose molecule are used to reduce two molecules of NAD⁺, forming two NADH; these NADH molecules are used later in aerobic respiration.