Final answer:
In fermentation, either in muscle or yeast cells, only two ATP molecules are produced per glucose molecule due to the reliance on glycolysis in the absence of oxygen.
Step-by-step explanation:
In the case of fermentation in muscle cells or yeast cells, only two ATP molecules are generated from each molecule of glucose. Fermentation occurs under anaerobic conditions, where oxygen is absent, and relies solely on glycolysis for ATP production. Though four ATP molecules are produced during glycolysis, two are consumed in the initial steps, resulting in a net yield of two ATP molecules. By contrast, aerobic respiration can yield a much larger amount of ATP, around 36-38 ATP molecules per glucose molecule, due to the additional stages of the citric acid cycle and electron transport chain that are operational when oxygen is available.