Final answer:
Digestion is the process of breaking down food into smaller structures and molecules through mechanical and chemical reactions primarily in the mouth, stomach, and small intestine.
Step-by-step explanation:
The breakdown of large food items into smaller structures and molecules is known as digestion. This biological process includes both mechanical and chemical reactions that help break down food into its basic components, primarily in the mouth, stomach, and small intestine. Digestion is an integral part of the digestive system, which also involves other processes such as ingestion, propulsion, absorption, and defecation. Not only does digestion reduce complex molecules like proteins, polysaccharides, and lipids to simpler particles like sugars, but it also prepares these nutrients for absorption by the digestive epithelial cells.