7.6: Digestion of Carbohydrates
- Describe digestion of carbohydrates.
Introduction to Metabolism
Metabolism is all reactions that take place in a living thing. It is divided into two parts, catabolism and anabolism . Catabolism involves compounds being broken down into smaller ones. This results in release of energy (exothermic). Anabolism involves synthesis of larger compounds from smaller ones. This requires energy (endothermic).
ATP is a key player in metabolism. Energy released during catabolism is used to drive the formation of ATP. ATP can be hydrolyzed to ADP and a phosphate ion. The energy released during the hydrolysis of ATP is used for anabolism. Digestion of food is an example of catabolism. Starch is hydrolyzed with water and the aid of enzymes to split into its building blocks glucose.
Digestion of Carbohydrates
The first stage of catabolism is digestion , in which food molecules are broken down into smaller molecules. Carbohydrate digestion begins in the mouth (Figure \(\PageIndex{1}\)) where salivary \(\alpha\)-amylase attacks the \(\alpha\)-glycosidic linkages in starch, the main carbohydrate ingested by humans. Hydrolysis of the glycosidic linkages with water produces a mixture of dextrins, maltose, and glucose. The \(\alpha\)-amylase mixed into the food remains active as the food passes through the esophagus, but it is rapidly inactivated in the acidic environment (low pH) of the stomach.
The primary site of carbohydrate digestion is the small intestine. Starch is digested to glucose by the action of amylase [hydrolyzes the α-(1 4) glycosidic bond] and debranching enzymes [hydrolyzes the α-(1 6) glycosidic bond]. Dextrins too are converted to maltose. Maltose is then hydrolyzed into two glucose molecules with the help of the enzyme maltase. Disaccharides such as sucrose and lactose are digested/hydrolyzed in the small intestine, where they are acted on by the enzymes sucrase and lactase, respectively. The major products of the complete hydrolysis of disaccharides and polysaccharides are three monosaccharide units: glucose, fructose, and galactose. These are absorbed through the wall of the small intestine into the bloodstream.