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Chapter 15
tion and, therefore, free to diffuse into the intestinal epithelium.
Note that it is not the micelle that is absorbed, but rather the
individual lipid molecules released from the micelle.
Although fatty acids and monoglycerides enter epithe-
lial cells from the intestinal lumen, it is triglycerides that are
released on the other side of the cell into the interstitial fl
uid.
In other words, during their passage through the epithelial
cells, fatty acids and monoglycerides are resynthesized into tri-
glycerides. This occurs in the smooth endoplasmic reticulum,
where the enzymes for triglyceride synthesis are located. This
process lowers the concentration of cytosolic free fatty acids
and monoglycerides and thus maintains a diffusion gradient
for these molecules into the cell. Within this organelle, the
resynthesized fat aggregates into small droplets coated with
amphipathic proteins that perform an emulsifying function
similar to that of bile salts.
The exit of these fat droplets from the cell follows the
same pathway as a secreted protein. Vesicles containing the
droplet pinch off the endoplasmic reticulum, are processed
through the Golgi apparatus, and eventually fuse with the
plasma membrane, releasing the fat droplet into the intersti-
tial fl uid. These 1 micron-diameter, extracellular fat droplets
are known as
chylomicrons.
Chylomicrons contain not only
triglycerides but other lipids (including phospholipids, choles-
terol, and fat-soluble vitamins) that have been absorbed by the
same process that led to fatty acid and monoglyceride move-
ment into the epithelial cells of the small intestine.
Hydroxyl groups
C – NH – CH
2
– COO
O
CH
3
CH
3
OH
CH
3
OH
HO
Bile salt (glycocholic acid)
(a)
(b)
Nonpolar side
Polar side
Peptide bond
Carboxyl group
Figure 15–9
Structure of bile salts. (a) Chemical formula of glycocholic acid,
one of several bile salts secreted by the liver (polar groups in color).
Note the similarity to the structure of steroids (see Figure 11–3).
(b) Three-dimensional structure of a bile salt, showing its polar and
nonpolar surfaces.
Figure 15–10
Emulsifi cation of fat by bile salts and phospholipids. Note that the
nonpolar sides (green) of bile salts and phospholipids are oriented
toward fat, while the polar sides (red) of these compounds are
oriented outward.
Emulsification
Bile salt
Phospholipid
Emulsion droplet
+
Fat globule
and lodges on the lipid droplet surface. Colipase binds the lipase
enzyme, holding it on the surface of the lipid droplet.
Although emulsifi cation speeds up digestion, absorption
of the water-insoluble products of the lipase reaction would
still be very slow if it were not for a second action of the bile
salts, the formation of
micelles,
which are similar in structure
to emulsion droplets but much smaller—4 to 7 nm in diam-
eter. Micelles consist of bile salts, fatty acids, monoglycerides,
and phospholipids all clustered together with the polar ends
of each molecule oriented toward the micelle’s surface and the
nonpolar portions forming the micelle’s core (
Figure 15–11
).
Also included in the core of the micelle are small amounts of
fat-soluble vitamins and cholesterol.
How do micelles increase absorption? Although fatty acids
and monoglycerides have an extremely low solubility in water, a
few molecules do exist in solution and are free to diffuse across
the lipid portion of the luminal plasma membranes of the epi-
thelial cells lining the small intestine. Micelles, containing the
products of fat digestion, are in equilibrium with the small con-
centration of fat digestion products that are free in solution.
Thus, micelles are continuously breaking down and reforming.
When a micelle breaks down, its contents are released into the
solution and they become available to diffuse across the intes-
tinal lining. As the concentrations of free lipids fall, because of
their diffusion into epithelial cells, more lipids are released into
the free phase as micelles break down (see Figure 15–11). Thus,
the micelles provide a means of keeping most of the insoluble
fat digestion products in small, soluble aggregates, while at the
same time replenishing the small amount of products in solu-
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