Glucose is a simple sugar that is used as an energy source by many living cells. Glycolysis (the breakdown of glucose into pyruvate) is the same under fermentation and respiration, but the fate of pyruvate, the product of glycolysis, is different. Whether glucose is respired or fermented depends on whether there is oxygen (O2) present.
Under fermentation, the breakdown of glucose for energy yields two ATP molecules and pyruvate, which is then reduced to end products such as lactate and ethanol. In the presence of O2, glucose is respired to make 38 ATP molecules and CO2. Respiration results in more ATP than fermentation because the energy remaining in lactate and ethanol can only be extracted by reducing them to CO2 in the presence of O2.
Regardless of whether by respiration or fermentation, glycolysis requires three stages:
Stage 1: Preparatory enzymatic reactions lead to a key intermediate. This stage does not involve redox reactions and no energy is released.
Stage 2: This is when the redox reactions occur, producing ATP and pyruvate. At this point, the breakdown of glucose is done, but because the redox reactions aren’t balanced, another stage is required.
Stage 3: The redox reactions needed to balance the reactions in Stage 2 happen in Stage 3. In fermentation, this is the reduction of pyruvate to its fermentation products that are then excreted as waste and in respiration this is the reduction of pyruvate to CO2.