Chemical digestion transforms complex food molecules into simpler forms the body can use. This breakdown is achieved through enzymes, specialized proteins that accelerate chemical reactions. Without chemical digestion, the body would be unable to absorb essential nutrients from consumed food.
In the Mouth
Chemical digestion begins as food enters the mouth. Salivary glands release saliva, containing salivary amylase. This enzyme starts breaking down complex carbohydrates, such as starches, into smaller sugar units like maltose and dextrins. While chewing mechanically breaks down food, salivary amylase initiates chemical alteration.
The action of salivary amylase is relatively brief because food spends only a short time in the mouth before being swallowed. Once food reaches the stomach, the acidic environment deactivates salivary amylase, halting its activity. Therefore, the mouth serves as the initial, though limited, site for carbohydrate digestion.
Within the Stomach
After leaving the mouth, food travels to the stomach, where a different phase of chemical digestion occurs. The stomach lining produces hydrochloric acid, creating a highly acidic environment. This acidity helps denature proteins, unfolding their complex structures, and activates specific enzymes.
The primary enzyme active in the stomach is pepsin, secreted in an inactive form called pepsinogen and then activated by hydrochloric acid. Pepsin begins the breakdown of large protein molecules into smaller polypeptide chains. Additionally, gastric lipase is present, playing a minor role in the digestion of fats, primarily breaking down triglycerides into fatty acids and monoglycerides.
The Small Intestine’s Crucial Role
The small intestine is where the vast majority of chemical digestion takes place, completing the breakdown of carbohydrates, proteins, and fats into molecules small enough for absorption. As partially digested food, or chyme, enters the small intestine from the stomach, it mixes with digestive enzymes from two sources: the pancreas and the small intestinal wall itself.
The pancreas secretes a rich blend of enzymes into the small intestine. Pancreatic amylase continues the digestion of carbohydrates, breaking down remaining starches and dextrins into disaccharides like maltose. For fats, pancreatic lipase, often aided by bile from the liver, is responsible for breaking down triglycerides into free fatty acids and monoglycerides, making it the primary enzyme for fat digestion.
Protein digestion is further advanced by pancreatic proteases, including trypsin and chymotrypsin. These enzymes break down polypeptides into smaller peptides. Trypsin is initially secreted as inactive trypsinogen and activated by an enzyme in the small intestine, which then activates chymotrypsin.
The final stages of chemical digestion occur at the brush border, which is the surface of the small intestine lining covered with tiny projections called microvilli. Enzymes embedded in this brush border, known as brush border enzymes, complete the digestive process. Disaccharidases, such as lactase, sucrase, and maltase, break down disaccharides into their simplest sugar units (monosaccharides) like glucose, fructose, and galactose, which are then ready for absorption. Peptidases, also located at the brush border, further break down small peptides into individual amino acids, dipeptides, and tripeptides, which can then be absorbed. This comprehensive enzymatic action ensures that nutrients are in their most basic forms, allowing for their efficient uptake into the bloodstream.