Carbohydrates, a fundamental component of the human diet, serve as a primary source of energy for the body. Starch, a complex carbohydrate derived from plants, is composed of numerous glucose units linked together. For the body to utilize starch, it must be broken down into simpler sugar molecules. This intricate digestive journey allows for the absorption of nutrients necessary for bodily functions.
Initial Breakdown in the Mouth
The chemical digestion of starch commences immediately in the oral cavity. As food is chewed, it mixes with saliva containing salivary amylase (ptyalin). This enzyme initiates the breakdown of long starch molecules by targeting specific bonds within their structure. The action of salivary amylase transforms complex starch into smaller carbohydrate fragments, including shorter chains called dextrins and the disaccharide maltose.
This chemical process is distinct from mechanical digestion, which involves chewing and grinding food. Salivary amylase functions most effectively under specific conditions, particularly a nearly neutral pH. Its optimal activity occurs within a pH range of 6.7 to 7.0, a condition found in the mouth. This initial enzymatic activity begins the digestive process before food moves further along the digestive tract.
Starch in the Stomach
Once swallowed, the food bolus, containing partially digested starch and active salivary amylase, enters the stomach. However, the stomach’s highly acidic environment rapidly alters starch digestion. The stomach produces gastric acid, which significantly lowers the pH to a range where salivary amylase can no longer function.
This low pH causes the inactivation of salivary amylase, effectively halting the chemical digestion of starch in the stomach. While chemical starch breakdown largely ceases, mechanical churning by stomach muscles continues. This physical action helps to mix and liquefy the food, preparing it for the next stages of digestion in the small intestine.
Final Digestion in the Small Intestine
The partially digested food, now a semi-liquid mixture, moves from the stomach into the small intestine, where the chemical digestion of starch resumes and is completed. The pancreas secretes pancreatic amylase into the duodenum (the first part of the small intestine). This enzyme continues the breakdown of remaining starch, dextrins, and other carbohydrate fragments into disaccharides, primarily maltose.
Further digestion occurs at the brush border, a specialized surface of the small intestine lining. Here, enzymes like maltase, embedded in the intestinal wall, complete carbohydrate breakdown. Maltase acts on maltose, converting each disaccharide molecule into two glucose units (single sugar molecules). These monosaccharides, primarily glucose, are then absorbed through the intestinal cells and transported into the bloodstream. From the bloodstream, glucose is delivered to cells throughout the body to be used as an immediate source of energy or stored for later use.