Flexi Says: Most of the carbohydrates like starch are broken down to glucose by various enzymes. Glucose is then further broken down to release energy through the process of cellular respiration. In the presence of oxygen i.e. aerobic respiration, glucose is completely oxidized to carbon dioxide and water to release energy. In absence of oxygen i.e anaerobic respiration, incomplete breakdown of glucose occurs to form ethanol or lactic acid and releases less amount of energy compared to aerobic respiration.