microvilli are also the site of the glucose/galactose transporter (answers b
and c).However, the glucose/galactose transporter is notthe site of the
deficiency in lactose intolerance. Other brush border enzymes include the
other monosaccharidases and enterokinase, which is important for cleav-
age of pancreatic zymogens (e.g., trypsinogen) to their active form.
Digestion of lipids occurs through the action of bile (from the liver and
bile duct) and lipase (from the pancreas). Bile serves to emulsify the lipid
to form micelles, whereas lipase breaks down the lipid from triglycerides to
fatty acids, glycerol, and monoglycerides (answers d and e).Those three
breakdown products diffuse freely across the microvilli to enter the apical
portion of the enterocyte by passive diffusion. Triglycerides are resynthe-
sized in the smooth endoplasmic reticulum. Proteins are synthesized in the
RER and are combined with sugar and lipid portions in the Golgi to form
glycoproteins and lipoproteins. Those two types of molecules form the cov-
erings of the triglyceride cores of the chylomicra. The chylomicra are
released at the basolateral membranes by exocytosis into the lacteals. From
the lacteals, the chylomicra travel into the cisterna chyli and eventually into
the venous system by way of the thoracic duct. Digestion of fat occurs to a
greater extent in the duodenum and jejunum than in the ileum.
Sugars are broken down by amylase in the oral cavity, with continued
digestion by brush border monosaccharidases. Proteins are broken down
by pepsinogen in the stomach with continued breakdown in the small
intestine by the enzymes of the pancreatic juice (e.g., trypsin, chy-
motrypsin, and carboxypeptidases). The products of protein digestion are
amino acids that are actively transported by transporters also located in the
brush border.
340 Anatomy, Histology, and Cell Biology