How does the energy yield of fermentation compare to aerobic respiration?

Fermentation yields much less energy compared to aerobic respiration because the process is less efficient in converting glucose into usable energy. During aerobic respiration, which occurs in the presence of oxygen, glucose is fully oxidized through a series of reactions including glycolysis, the citric acid cycle, and the electron transport chain. This complete oxidation results in a high yield of ATP, typically around 30 to 32 molecules of ATP per glucose molecule.

In contrast, fermentation is an anaerobic process that only partially breaks down glucose. It bypasses the electron transport chain and relies on alternative pathways, such as lactic acid fermentation or alcoholic fermentation, which regenerate NAD+ but do not utilize oxygen. As a result, fermentation yields only 2 ATP molecules per glucose molecule, significantly less than aerobic respiration. This fundamental difference in the substrate breakdown and energy production mechanisms explains why fermentation yields much less energy overall.