Chapter 23
The Digestive System
895
23
Absorption
Tere is much more flowing through the alimentary tube than
food monomers. Indeed, up to 10 L of food, drink, and GI se-
cretions enter the alimentary canal daily, but only 1 L or less
reaches the large intestine. Virtually all of the foodstuffs, 80% of
the electrolytes, and most of the water (remember water follows
salt) are absorbed in the small intestine. Although absorption
occurs all along the length of the small intestine, most of it is
completed by the time chyme reaches the ileum.
Te major absorptive role of the ileum is to reclaim bile salts to
be recycled back to the liver for resecretion. It is virtually impos-
sible to exceed the absorptive capacity of the human gut, and at
the end of the ileum, all that remains is some water, indigestible
Protein fragments entering the small intestine are greeted
by a host of proteolytic enzymes (
Figure 23.33
1
).
Trypsin
and
chymotrypsin
secreted by the pancreas cleave the proteins
into smaller peptides, which in turn become the grist for other
enzymes. Te pancreatic and brush border enzyme
carboxy-
peptidase
splits off one amino acid at a time from the end of the
polypeptide chain that bears the carboxyl group. Other brush
border enzymes such as
aminopeptidase
and
dipeptidase
liber-
ate the final amino acid products. Aminopeptidase digests a pro-
tein, one amino acid at a time, by working from the amine end.
Both carboxypeptidase and aminopeptidase can indepen-
dently dismantle a protein, but the teamwork between these
enzymes and between trypsin and chymotrypsin, which attack
the more internal parts of the protein, speeds up the process
tremendously.
Digestion of Lipids
±riglycerides (neutral fats) are the most abundant fats in the
diet. Te small intestine is the primary site of lipid digestion be-
cause the pancreas is the major source of fat-digesting enzymes,
or
lipases
(see Figure 23.32).
Because triglycerides and their breakdown products are in-
soluble in water, fats need special “pretreatment” with bile salts
to be digested and absorbed in the watery environment of the
small intestine. In aqueous solutions, triglycerides aggregate
to form large fat globules, and only the triglyceride molecules
at the surfaces of such fatty masses are accessible to the water-
soluble lipase enzymes. However, this problem is quickly re-
solved because as the fat globules enter the duodenum, they are
coated with detergent-like bile salts.
Bile salts have both nonpolar and polar regions. Teir nonpo-
lar (hydrophobic) parts cling to the fat molecules, and their polar
(ionized hydrophilic) parts allow them to repel each other and in-
teract with water. As a result, fatty droplets are pulled off the large
fat globules, forming a stable
emulsion
—an aqueous suspension of
fatty droplets, each about 1 mm in diameter (
Figure 23.34
1
).
Emulsification does
not
break chemical bonds. It just reduces
the attraction between fat molecules so they can be more widely
dispersed. Tis process vastly increases the number of triglyc-
eride molecules exposed to the pancreatic lipases. Without bile,
lipids could not be completely digested during the time food
spends in the small intestine.
Te pancreatic lipases catalyze the breakdown of fats by
cleaving off two of the fatty acid chains, yielding free
fatty ac-
ids
and
monoglycerides
—glycerol with one fatty acid chain at-
tached (Figure 23.34
2
). Fat-soluble vitamins that ride with
fats require no digestion.
Digestion of Nucleic Acids
Te nuclei of the cells of ingested foods contain DNA and RNA.
Pancreatic nucleases
in pancreatic juice hydrolyze the nucleic
acids to their
nucleotide
monomers. Intestinal brush border
enzymes (
nucleosidases
and
phosphatases
) then break the nu-
cleotides apart to release their free bases, pentose sugars, and
phosphate ions (see Figure 23.32).
Epithelial
cells of
small
intestine
Fat droplets
coated with
bile salts
Fat globule
Lacteal
Bile salts
Micelles made
up of fatty acids,
monoglycerides,
and bile salts
1
Bile salts in the duodenum
emulsify large fat globules
(physically break them up into
smaller fat droplets).
2
Digestion of fat by the
pancreatic enzyme lipase
yields free fatty acids and
monoglycerides. These then
associate with bile salts to
form micelles which “ferry”
them to the intestinal
mucosa.
3
Fatty acids and
monoglycerides leave
micelles and diffuse into
epithelial cells. There they
are recombined and
packaged with other fatty
substances and proteins to
form chylomicrons.
4
Chylomicrons are
extruded from the epithelial
cells by exocytosis. The
chylomicrons enter lacteals
and are carried away from
the intestine in lymph.
Figure 23.34
Emulsification, digestion, and absorption of fats.
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