Digestion explains how our bodies take complex foods and convert them into smaller components that our cells can use to produce energy and carryout essential functions. We cannot simply break down food and absorb it one step. Food is made up of complex proteins, carbohydrates, and fats which must first be dismantled into smaller components namely amino acids, glucose and fatty acids. These nutrients are obtained through sophisticated chemical reactions that rely heavily on compounds called enzymes. Enzymes are catalysts, they speed up reactions. Without them, digestion would take days or weeks. To further complicate the matter, enzymes only work under limited conditions of temperature and acidity (pH). For example, enzymes that break down proteins work at a much lower pH (more acidic environment) than those that work on fat. Fortunately the body has a system that addresses these challenges.
Beginning in the mouth, an almost neutral pH environment, food is broken down mechanically, with the teeth and tongue, and chemically with enzymes found in saliva. Fats and carbohydrates are the primary target. Once the food has been adequately softened, it moves to the highly acidic stomach, an ideal environment for digesting proteins and carbohydrates. Food is also further broken down mechanically through churning. Food then moves into the small intestine, the least acidic stage, where fats are further digested and the majority of nutrient absorption occurs. In the large intestine a final stage of reactions and absorption happens before waste products are eliminated.
To summarize, the digestive system provides the different environments required to chemically process food. Chewing, churning, and other mechanical activities further assist the break down of complex carbohydrates, proteins, and fats into simple sugars, amino acids, and fatty acids that cells can use to produce energy and carryout essential functions.