Final answer:
Digestion involves mechanical and chemical processes to break down food into absorbable units. Mechanical digestion begins in the mouth and stomach, while chemical digestion mainly occurs in the small intestine, breaking down large molecules into absorbable substances like simple sugars and amino acids.
Step-by-step explanation:
Digestion is the chemical and mechanical breakdown of foods into smaller units that can be taken across the intestinal epithelium into the body.
There are two types of digestion: mechanical digestion and chemical digestion. Mechanical digestion refers to the physical breakdown of chunks of food into smaller pieces by organs of the digestive system, which begins in the mouth and continues in the stomach. On the other hand, chemical digestion is the chemical breakdown of large, complex food molecules into smaller, simpler nutrient molecules that can be absorbed by blood or lymph. This type of digestion starts in the mouth as saliva breaks down food, continues in the stomach, and occurs mainly in the small intestine.
The small intestine is the primary site for chemical digestion and almost all absorption. Substances that require chemical digestion include carbohydrates, proteins, lipids, and nucleic acids. Carbohydrates, for instance, must be broken down into simple sugars, proteins into amino acids, and lipids into fatty acids and glycerol, before they can be absorbed across the digestive epithelial cells.