<P> Glucose reacts with oxygen in the following redox reaction, C H O + 6O → 6CO + 6H O, Carbon dioxide and water are waste products, and the overall reaction is exothermic . </P> <P> The breakdown of glucose into energy in the form of molecules of ATP is therefore one of the most important biochemical pathways found in living organisms . </P> <P> Glycolysis, which means "sugar splitting," is the initial process in the cellular respiration pathway . Glycolysis can be either an aerobic or anaerobic process . When oxygen is present, glycolysis continues along the aerobic respiration pathway . If oxygen is not present, then ATP production is restricted to anaerobic respiration . The location where glycolysis, aerobic or anaerobic, occurs is in the cytosol of the cell . In glycolysis, a six - carbon glucose molecule is split into two three - carbon molecules called pyruvate . These carbon molecules are oxidized into NADH and ATP . For the glucose molecule to oxidize into pyruvate, an input of ATP molecules is required . This is known as the investment phase, in which a total of two ATP molecules are consumed . At the end of glycolysis, the total yield of ATP is four molecules, but the net gain is two ATP molecules . Even though ATP is synthesized, the two ATP molecules produced are few compared to the second and third pathways, Krebs cycle and oxidative phosphorylation . </P> <P> Even if there is no oxygen present, glycolysis can continue to generate ATP . However, for glycolysis to continue to produce ATP, there must be NAD+ present, which is responsible for oxidizing glucose . This is achieved by recycling NADH back to NAD+ . When NAD+ is reduced to NADH, the electrons from NADH are eventually transferred to a separate organic molecule, transforming NADH back to NAD+ . This process of renewing the supply of NAD+ is called fermentation, which falls into two categories . </P>

The breakdown of glucose to atp and pyruvic acid is called