C h a p t e r 2 8
Structure and Function of the Gastrointestinal System
691
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
TABLE 28-3
Enzymes Used in Digestion of Carbohydrates
Dietary Carbohydrates
Enzyme
Monosaccharides Produced
Lactose
Lactase
Glucose and galactose
Sucrose
Lactase
Fructose and glucose
Starch
Amylase
Maltose, maltotriose, and
α
-dextrins
Maltose and maltotriose
Maltase
Glucose and glucose
α
-Dextrins
α
-Dextrinase
Glucose and glucose
K +
Blood
ATP
H
2
O
H
2
O
Intestinal
lumen
Basolateral
membrane
Luminal
membrane
Glucose/
galactose
Glucose/
galactose
Glucose/
galactose
Fructose
Fructose
Fructose
K +
Na +
Na +
Na +
Na +
GLUT-2
transporter
SGLT-1
transporter
Enterocyte
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.
