Chemical energy that is stored in food molecules is released through cellular respiration. Part of this energy is kept as adenosine triphosphate, or ATP. Although fats, proteins and carbohydrates can also be used as fuel for cellular respiration, glucose is most commonly used as an example.
Food molecules generally undergo three stages of breakdown for ATP production. It starts with digestion, where enzymes act on food molecules to break them down. This is followed by glycolysis, which changes one molecule of glucose into two smaller molecules of pyruvate. This substance then enters the cell's mitochondria, where the oxidative breakdown of food takes place. NADH, carbon dioxide and ATP are formed during this stage.