Chemical Digestion Results in the Reduction of Complex Nutrients into Simpler Molecules

Chemical digestion of each major nutrient is accomplished by the process of hydrolysis, the splitting of a chemical bond by the insertion of a water molecule. Glycosidic linkages in carbohydrates, peptide bonds in proteins, ester bonds in fats, and phosphodiester bonds in nucleic acids are all cleaved by hydrolysis during digestion.

Hydrolysis in the digestive tract is catalyzed by the action of enzymes. There are two general classes of digestive enzymes: those that act within the lumen of the gut, and those that act at the membrane surface of the epithelium. Enzymes acting within the lumen originate from the major GI glands, including the salivary gland, gastric glands, and especially the pancreas. The secretions of these glands become thoroughly mixed with ingesta and exert their actions throughout the lumen of their associated gut segments; thus the actions they catalyze are referred to as the Iimiinal phase of digestion. In general, luminal-phase digestion results in the incomplete hydrolysis of nutrients, resulting in the formation of short­chain polymers from the original macromolecules (Figure 30-6).

The hydrolytic process is completed by enzymes that are Chemicallybound to the surface epithelium of the small intes­tine. These enzymes break the short-chain polymers resulting from luminal-phase digestion into monomers that can be absorbed across the epithelium. This final phase, which occurs at the epithelial membrane surface, is referred to as the mem­branous phase of digestion. Membranous-phase digestion is followed closely by absorption.