The Digestion and Absorption of Food
537
by the enzymes in the small intestine and pass on to the large
intestine, where they are partially metabolized by bacteria.
The digestion of starch by salivary amylase begins in the
mouth and briefl y continues in the upper part of the stomach
before gastric acid destroys the amylase. Starch digestion is com-
pleted in the small intestine by pancreatic amylase. The products
of both amylases are the disaccharide maltose and a mixture of
short, branched chains of glucose molecules. These products,
along with ingested sucrose and lactose, are broken down
into monosaccharides—glucose, galactose, and fructose—by
enzymes located on the luminal membranes of the small intes-
tine epithelial cells (brush border). These monosaccharides
are then transported across the intestinal epithelium into the
blood. Fructose enters the epithelial cells by facilitated diffu-
sion, whereas glucose and galactose undergo secondary active
transport coupled to sodium. These monosaccharides then
leave the epithelial cells and enter the blood by way of facili-
tated diffusion transporters in the basolateral membranes of
the epithelial cells. Most ingested carbohydrates are digested
and absorbed within the fi rst 20 percent of the small intestine.
Protein
A normal adult requires only 40 to 50 g of protein per day
to supply essential amino acids and replace the nitrogen con-
tained in amino acids that is converted to urea. A typical
American diet contains about 70 to 90 g of protein per day.
This represents about one-sixth of the average daily caloric
intake. In addition, a large amount of protein, in the form of
enzymes and mucus, is secreted into the gastrointestinal tract
or enters it via the disintegration of epithelial cells. Regardless
of source, most of the protein in the lumen is broken down
into amino acids and absorbed by the small intestine.
Proteins are broken down to peptide fragments in the
stomach by pepsin, and in the small intestine by
trypsin
and
chymotrypsin,
the major proteases secreted by the pancreas.
These fragments are further digested to free amino acids by
carboxypeptidase
from the pancreas and
aminopeptidase,
located on the luminal membranes of the small intestine epi-
thelial cells. These last two enzymes split off amino acids from
the carboxyl and amino ends of peptide chains, respectively.
At least 20 different peptidases are located on the luminal
membrane of the epithelial cells, with various specifi cities for
the peptide bonds they attack.
The free amino acids then enter the epithelial cells by
secondary active transport coupled to sodium. There are mul-
tiple transporters with different specifi cities for the 20 types
of amino acids. Short chains of two or three amino acids
are also absorbed by a secondary active transport coupled to
the hydrogen ion gradient. This contrasts with carbohydrate
absorption, in which molecules larger than monosaccharides
are not absorbed. Thus, as was the case for carbohydrates,
luminal absorption of amino acids is a process that requires
energy (ATP). Within the epithelial cell, these di- and tripep-
tides are hydrolyzed to amino acids, which then leave the cell
and enter the blood through a facilitated diffusion carrier in
the basolateral membranes. As with carbohydrates, protein
digestion and absorption are largely completed in the upper
portion of the small intestine.
Very small amounts of intact proteins are able to cross
the intestinal epithelium and gain access to the interstitial fl
uid.
They do so by a combination of endocytosis and exocytosis. The
absorptive capacity for intact proteins is much greater in infants
than in adults, and antibodies (proteins involved in the immu-
nological defense system of the body) secreted into the moth-
er’s milk can be absorbed by the infant, providing some passive
immunity until the infant begins to produce its own antibodies.
Fat
The average daily intake of fat is 70 to 100 g per day in a typi-
cal American diet. This represents about one-third of the aver-
age daily caloric intake. Most is in the form of triglycerides.
Fat digestion occurs almost entirely in the small intestine. The
major digestive enzyme in this process is pancreatic
lipase,
which catalyzes the splitting of bonds linking fatty acids to the
fi rst and third carbon atoms of glycerol, producing two free
fatty acids and a monoglyceride as products:
lipase
Triglyceride
⎯→
Monoglyceride + 2 Fatty acids
The fats in the ingested foods are insoluble in water and
aggregate into large lipid droplets in the upper portion of the
stomach. This is like a mixture of oil and vinegar after shak-
ing. Because pancreatic lipase is a water-soluble enzyme, its
digestive action in the small intestine can take place only at
the
surface
of a lipid droplet. Therefore, if most of the ingested
fat remained in large lipid droplets, the rate of lipid digestion
would be very slow. The rate of digestion is, however, substan-
tially increased by division of the large lipid droplets into a
number of much smaller droplets, each about 1 mm in diam-
eter, thereby increasing their surface area and accessibility to
lipase action. This process is known as
emulsifi
cation,
and
the resulting suspension of small lipid droplets is an emulsion.
The emulsifi cation of fat requires (1) mechanical disrup-
tion of the large fat droplets into smaller droplets, and (2) an
emulsifying agent, which acts to prevent the smaller droplets
from reaggregating back into large droplets. The mechanical
disruption is provided by contractile activity, occurring in the
lower portion of the stomach and in the small intestine, which
grinds and mixes the luminal contents. Phospholipids in food
along with phospholipids and bile salts secreted in the bile
provide the emulsifying agents.
Phospholipids are amphipathic molecules (Chapter 2) con-
sisting of two nonpolar fatty acid chains attached to glycerol,
with a charged phosphate group located on glycerol’s third car-
bon. Bile salts are formed from cholesterol in the liver and are
also amphipathic (
Figure 15–9
). The nonpolar portions of the
phospholipids and bile salts associate with the nonpolar interior
of the lipid droplets, leaving the polar portions exposed at the
water surface. There they repel other lipid droplets that are simi-
larly coated with these emulsifying agents, thereby preventing
their reaggregation into larger fat droplets (
Figure 15–10
).
The coating of the lipid droplets with these emulsifying
agents, however, impairs the accessibility of the water-soluble
lipase to its lipid substrate. To overcome this problem, the pan-
creas secretes a protein known as
colipase,
which is amphipathic
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