Glucose is oxidized in two ways to provide energy.
(a) Aerobic respiration: Glucose is completely oxidized to carbon dioxide and water in the presence of oxygen, with the release of a considerable amount of energy. This type of oxidation occurs in most living organisms such as human beings, birds, snakes, frogs, fish, etc.
(b) Anaerobic respiration: This type of respiration occurs in the absence of oxygen. In this type of oxidation, glucose is partially oxidized to ethanol and carbon dioxide (yeast and other bacteria) or lactic acid (muscles during physical exercise and some bacteria) with the release of a small amount of energy.