As food travels through the digestive system of animals, it undergoes a series of physical and chemical changes that break it down to provide energy for cells. Physical changes simply alter the appearance of something. For example, chewing breaks large food molecules into smaller ones. In contrast, chemical changes occur when the chemical make-up of the food particle is changed to create a new substance. During the digestive process, enzymes change carbohydrates, proteins, fats and nucleic acids into substances that can be absorbed by cells.