A byproduct of cellular respiration is carbon dioxide, which is produced alongside energy and water during the process of converting glucose into usable energy. In aerobic respiration, glucose is oxidized in the presence of oxygen; this reaction releases energy while generating carbon dioxide as waste. In the case of anaerobic respiration, such as in fermentation, other byproducts like ethanol or lactic acid may be produced instead. Overall, these byproducts are essential for understanding metabolic processes.