What occurs in the absence of oxygen during cellular respiration?

In the absence of oxygen during cellular respiration, fermentation occurs as a way for cells to continue producing energy. This process allows for the regeneration of NAD+ from NADH, which is crucial for glycolysis to continue. In glycolysis, glucose is broken down into pyruvate, yielding a small amount of ATP. When oxygen is not available, the pyruvate cannot enter the mitochondria for further oxidation through aerobic respiration. Instead, it is converted into either lactic acid (in animal cells) or ethanol and carbon dioxide (in yeast and some bacteria). This enables the cell to continue producing ATP, albeit at a much lower efficiency compared to aerobic respiration.

In contrast, cellular respiration does not completely stop, since glycolysis can still function, leading to fermentation. Photosynthesis is unrelated to cellular respiration, as it is a process that occurs in plants and some bacteria to convert light energy into chemical energy. Lastly, oxidative phosphorylation requires oxygen as the final electron acceptor in the electron transport chain, and it cannot be enhanced without the presence of oxygen.