The chemical reaction that occurs during fermentation is primarily the conversion of glucose into lactic acid or ethanol and carbon dioxide, depending on the type of organism. In the absence of oxygen, certain organisms like yeast undergo alcoholic fermentation, breaking down glucose into ethanol and carbon dioxide. In contrast, animal cells, such as those in muscles, can produce lactic acid through lactic acid fermentation when oxygen levels are low. Fermentation allows organisms to generate energy when aerobic respiration is not possible, making it an important metabolic pathway for survival in anaerobic conditions.