The total breakdown of glucose to carbon dioxide and water requires two major steps: glycolysis and aerobic respiration. Glycolysis produces 2 ATP with an additional 34 ATP created through aerobic respiration. However, if no oxygen is present, fermentation will occur producing alcohol and carbon dioxide in yeast and plants and lactate in animals and bacteria. No additional ATP is produced beyond the 2 ATP gained in glycolysis. Glycolysis does not require oxygen but it does require NAD+. Cells with no oxygen readily available must regenerate NAD+ from NADH so at least the 2 ATP can be made. This happens by NADH electrons combining with pyruvate to produce alcohol (yeast, plants) or lactate (animals, bacteria). The reaction must occur in order to regenerate NAD+ for glycolysis.
lactate or alcohol