Porth's Essentials of Pathophysiology, 4e

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Structure and Function of the Gastrointestinal System

C h a p t e r 2 8

TABLE 28-3 Enzymes Used in Digestion of Carbohydrates Dietary Carbohydrates Enzyme

Monosaccharides Produced

Lactose Sucrose

Lactase Lactase Amylase Maltase

Glucose and galactose Fructose and glucose

Maltose, maltotriose, and α -dextrins

Starch

Maltose and maltotriose

Glucose and glucose Glucose and glucose

α -Dextrins

α -Dextrinase

Protein Digestion and Absorption Protein digestion begins in the stomach with the action of pepsin. Pepsinogen, the enzyme precursor of pepsin, is secreted by the chief cells in response to a meal and acid pH. Acid in the stomach is required for the conversion of pepsinogen to pepsin. Pepsin is inactivated when it enters the intestine by the alkaline pH. Proteins are broken down further by pancreatic enzymes, such as trypsin, chymotrypsin, carboxypepti- dase, and elastase. As with pepsin, the pancreatic enzymes are secreted as precursor molecules. Trypsinogen, which lacks enzymatic activity, is activated by an enzyme located on the brush border cells of the duodenal enterocytes. Activated trypsin activates additional tryp- sinogen molecules and other pancreatic precursor pro- teolytic enzymes. The amino acids are then liberated on the surface of the mucosal surface of the intestine by brush border enzymes that degrade proteins into peptides that are one, two, or three amino acids long.

As with glucose, many amino acids are transported across the mucosal membrane in a sodium-linked pro- cess that uses ATP as an energy source. Some amino acids are absorbed by facilitated diffusion processes that do not require sodium. Fat Digestion and Absorption The average adult eats approximately 60 to 100 g of fat daily, principally as triglycerides. The first step in diges- tion of lipids is to break the large globules of dietary fat into smaller particles so that water-soluble digestive enzymes can act on the surface molecules. This emulsifi- cation process begins in the stomach with agitation of the globules and continues in the duodenum under the influ- ence of bile from the liver (Fig. 28-12). Emulsification greatly increases the number of triglyceride molecules exposed to pancreatic lipase, which splits triglycerides into free fatty acids and monoglycerides. Bile salts play an additional role by forming micelles that transport these

Enterocyte

Blood

Intestinal lumen

SGLT-1 transporter

K +

K +

ATP

Na +

Na +

Na +

Na +

Glucose/ galactose

Glucose/ galactose

Glucose/ galactose

GLUT-2 transporter

Fructose

Fructose

Fructose

FIGURE 28-11. Intestinal transport of glucose, galactose, and fructose. Glucose and galactose are transported across the apical membrane by the sodium–glucose cotransporter (SGLT-1). Glucose moves out of the intestinal cell and into the blood using a glucose transporter-2 (GLUT-2) protein. Sodium is transported out of the cell by the Na + /K + -ATPase sodium pump.This creates the gradient needed to operate the transport system. Fructose is passively transported across the apical and basolateral membranes of the intestinal cell.

H 2 O

H 2 O

Basolateral membrane

Luminal membrane