When cellular respiration occurs in the absence of oxygen, cells engage in a process known as anaerobic respiration or fermentation. Unlike aerobic respiration, which efficiently produces energy by using oxygen to ultimately convert glucose into carbon dioxide and water, anaerobic respiration leads to different end products.
In the case of animal cells, such as muscle cells, the primary byproduct of anaerobic respiration is lactic acid. This occurs when glucose is partially broken down, allowing cells to generate ATP (the energy currency of the cell) but in a much less efficient manner compared to aerobic processes. The accumulation of lactic acid can lead to muscle fatigue and will eventually require oxygen to convert back to pyruvate or glucose.
In yeast and some types of bacteria, anaerobic respiration results in the production of ethanol and carbon dioxide, a process known as alcoholic fermentation. This method is commonly exploited in brewing and baking industries.
Overall, while anaerobic respiration allows cells to produce some energy in low-oxygen environments, it is significantly less efficient than aerobic respiration. This inefficiency can result in the buildup of byproducts that can be harmful to cells if not managed.