684
UNIT FIVE
from the intestinal mucosa and the pancreas
(fig. 17.40)
.
Large protein molecules are ultimately broken down into
amino acids, which are then absorbed into the villi by active
transport and enter the circulation.
Fat molecules are digested almost entirely by enzymes
from the intestinal mucosa and pancreas
g. 17.41)
. The
resulting fatty acid molecules are absorbed in the following
steps: (1) The fatty acid molecules dissolve in the epithe-
lial cell membranes of the villi and diffuse through them.
(2) The endoplasmic reticula of the cells use the fatty acids
to resynthesize fat molecules similar to those previously
digested. (3) These fats collect in clusters that become
encased in protein. (4) The resulting large molecules of
lipoprotein are called
chylomicrons,
and they make their
way to the lacteals of the villi. (5) Periodic contractions of
smooth muscles in the villi help empty the lacteals into the
cysterna chyli (see ±
g. 16.6
a
), an expansion of the thoracic
duct. The lymph carries the chylomicrons to the blood-
stream
g. 17.42)
.
Chylomicrons in the blood transport dietary fats to mus-
cle and adipose cells. Similarly, very-low-density lipoprotein
(VLDL) molecules, produced in the liver, transport triglycer-
ides synthesized from excess dietary carbohydrates. As VLDL
molecules reach adipose cells, an enzyme,
lipoprotein lipase,
catalyzes reactions that unload their triglycerides, convert-
ing VLDL to low-density lipoprotein (LDL) molecules. Most
of the triglycerides have been removed, so LDL molecules
have a higher cholesterol content than the original VLDL
molecules. Cells in the peripheral tissues remove LDL from
plasma by receptor-mediated endocytosis, thus obtaining a
supply of cholesterol (see chapter 3, p. 96).
While LDL delivers cholesterol to tissues, high-density
lipoprotein (HDL) removes cholesterol from tissues and
delivers it to the liver. The liver produces the basic HDL
Absorption in the Small Intestine
Villi greatly increase the surface area of the intestinal
mucosa, making the small intestine the most important
absorbing organ of the alimentary canal. The small intestine
is so effective in absorbing digestive products, water, and
electrolytes, that very little absorbable material reaches the
organ’s distal end.
Carbohydrate digestion begins in the mouth with the
activity of salivary amylase and is completed in the small
intestine by enzymes from the intestinal mucosa and pancreas
(± g. 17.39)
. The resulting monosaccharides are absorbed by
facilitated diffusion or active transport into the villi and enter
blood capillaries (see chapter 3, pp. 93 and 95).
Protein digestion begins in the stomach as a result of pep-
sin activity and is completed in the small intestine by enzymes
Many adults do not produce suf
cient lactase to adequately digest
lactose, or milk sugar. In this
lactose intolerance,
the sugar remains
undigested, increasing osmotic pressure oF the intestinal contents
and drawing water into the intestines. At the same time, intestinal
bacteria metabolize undigested sugar, producing organic acids and
gases. The overall result is bloating, intestinal cramps, and diarrhea.
To avoid these symptoms, people with lactose intolerance can take
lactase pills beFore eating dairy products, and inFants can drink For-
mula based on soybeans. Genetic evidence suggests that lactose
intolerance may be the “normal” condition, with the ability to digest
lactose the result oF a mutation that occurred recently in our evolu-
tionary past and became advantageous when the advent oF agricul-
ture brought dairy Foods to human populations. The trait oF ability to
digest lactose has increased in parallel to increased use oF dairy Foods
at least three times in history, in di±
erent populations.
Maltose
Glucose
Monosaccharides
Disaccharide
H
2
O
Glucose
+
H
H
O
Maltase
Water
Maltase
+
HO
H
O
OH
OH
H
O
H
H
HO
H
O
OH
HO
H
O
FIGURE 17.39
Digestion breaks down complex carbohydrates into disaccharides, which are then broken down into monosaccharides, which are
small enough For intestinal villi to absorb. The monosaccharides then enter the bloodstream.
H
NC
H
H
Dipeptide
(from protein digestion)
Amino acid
Amino acid
Dipeptidase
Water
++
R
C
H
N
O
C
R
H
H
NC
H
H
R
C
OH
O
H
NC
H
H
R
C
OH
O
C
OH
O
Dipeptidase
H
2
O
FIGURE 17.40
The amino acids that result From dipeptide digestion are absorbed by intestinal villi and enter the blood.
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