Digestion is the mechanical and chemical break down of food into small organic fragments . It is important to break down macromolecules into smaller fragments that are of suitable size for absorption across the digestive system. Large, complex molecules of proteins, polysaccharides, and lipids must be reduced to simpler particles before they can be absorbed by the digestive epithelial cells. Different organs play specific roles in the digestive process. The animal diet needs carbohydrates, protein, and fat.