23
Carbohydrate
digestion
Protein
digestion
Fat
digestion
Nucleic acid
digestion
Glucose and galactose are absorbed
via cotransport with sodium ions.
Fructose passes via facilitated
diffusion.
All monosaccharides leave the
epithelial cells via facilitated diffusion,
enter the capillary blood in the
villi, and are transported to the liver
via the hepatic portal vein.
Amino acids are absorbed via
cotransport with sodium ions.
Some dipeptides and tripeptides are
absorbed via cotransport with H
+
and
hydrolyzed to amino acids within the
cells.
Infrequently, transcytosis of small
peptides occurs.
Amino acids leave the epithelial cells
by facilitated diffusion, enter the
capillary blood in the villi, and are
transported to the liver via the
hepatic portal vein.
Units enter intestinal cells by active
transport via membrane carriers.
Units are absorbed into capillary
blood in the villi and transported to
the liver via the hepatic portal vein.
Starch and disaccharides
Oligosaccharides
and disaccharides
Lactose
Maltose
Sucrose
Glucose
Fructose
Salivary amylase
Mouth
Pancreatic
amylase
Brush border
enzymes in
small intestine
(dextrinase, gluco-
amylase, lactase,
maltase, and sucrase)
Small
intestine
Small
intestine
Small
intestine
Small
intestine
Small
intestine
Small
intestine
Small
intestine
Small
intestine
Stomach
Stomach
Mouth
Foodstuff
Proteins
Large polypeptides
Pepsin
(stomach glands)
in presence
of HCl
Unemulsified triglycerides
Emulsification by
the detergent
action of bile
salts ducted
in from the liver
Lingual lipase
Gastric lipase
Pancreatic
lipases
Small polypeptides,
small peptides
Pancreatic
enzymes (trypsin,
chymotrypsin,
carboxypeptidase)
Amino acids
(some dipeptides
and tripeptides)
Monoglycerides (or diglycerides
with gastric lipase) and fatty acids
Brush border
enzymes
(aminopeptidase,
carboxypeptidase,
and dipeptidase)
Galactose
Nucleic acids
Pancreatic ribo-
nuclease and
deoxyribonuclease
Brush border
enzymes
(nucleosidases
and phosphatases)
Pentose sugars,
N-containing bases,
phosphate ions
Path of absorption
Enzyme(s) and source
Site of action
Fatty acids and monoglycerides enter
the intestinal cells via diffusion.
Fatty acids and monoglycerides are
recombined to form triglycerides and
then combined with other lipids and
proteins within the cells. The resulting
chylomicrons are extruded
by exocytosis.
The chylomicrons enter the lacteals
of the villi and are transported to the
systemic circulation via the lymph in
the thoracic duct.
Some short-chain fatty acids are
absorbed, move into the capillary
blood in the villi by diffusion, and are
transported to the liver via the
hepatic portal vein.
Figure 23.32
Flowchart of digestion and absorption of foodstuffs.
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