Post on 12-Dec-2021
Part 3
Digestive System
Digestion
Ingested materials must be broken down for absorption
Majority of absorption in small intestine
Water and alcohol in stomach mucosa
Some salts and vitamins in large intestine
Non-absorbable materials removed by defecation
Fate of Digested Materials
Carbohydrates
Glucose, fructose, and galactose are directly absorbed
Cellulose metabolized by bacteria
Vitamins K and B12 are byproducts
Figure 23.32 (1 of 4)
Carbohydrate 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.
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
Foodstuff
Galactose
Path of absorption Enzyme(s)
and source
Site of
action
Fate of Digested Materials
Proteins
Amino acids, dipeptides and some tripeptides
Absorbed by active transport
Further metabolism in cells → free amino acids → bloodstream
Figure 23.32 (2 of 4)
Protein digestion
• Amino acids are absorbed
by cotransport with
sodium ions.
• Some dipeptides and
tripeptides are absorbed
via cotransport with H+
and hydrolyzed to amino
acids within the cells.
+
• 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.
Small
intestine
Small
intestine
Stomach
Foodstuff
Protein
Large polypeptides
Pepsin
(stomach glands)
in presence
of HCl
Small polypeptides,
small peptides
Pancreatic
enzymes
(trypsin, chymotrypsin,
carboxypeptidase)
Amino acids
(some dipeptides
and tripeptides)
Brush border
enzymes
(aminopeptidase,
carboxypeptidase,
and dipeptidase)
Path of absorption Enzyme(s)
and source
Site of
action
Figure 23.33
Absorptive
epithelial
cell
Apical membrane (microvilli)
Amino
acid
carrier
Capillary
Lumen of
intestine
Pancreatic
proteases
Amino acids of protein fragments
Brush border enzymes
Na+
Na+
1 Proteins and protein fragments
are digested to amino acids by
pancreatic proteases (trypsin,
chymotrypsin, and carboxy-
peptidase), and by brush border
enzymes (carboxypeptidase,
aminopeptidase, and dipeptidase)
of mucosal cells.
2 The amino acids are then
absorbed by active transport into
the absorptive cells, and move to
their opposite side (transcytosis).
3 The amino acids leave the
villus epithelial cell by facilitated
diffusion and enter the capillary
via intercellular clefts.
Active transport
Passive transport
Fate of Digested Materials
Lipids
Emuslification by bile salts and lipase → micelles move between
microvilli → lipids diffuse into intestinal epithelium
(bile salts later reabsorbed in ileum)
Figure 23.34
Epithelial
cells of
small
intestine
Fat droplets
coated with
bile salts
Fat globule
Lacteal
Bile salts
Micelles made up of fatty
acids, monoglycerides,
and bile salts
1 Large fat globules are emulsified
(physically broken up into smaller fat
droplets) by bile salts in the duodenum.
2 Digestion of fat by the pancreatic
enzyme lipase yields free fatty acids and
monoglycerides. These then associate
with bile salts to form micelles which
“ferry” them to the intestinal mucosa.
3 Fatty acids and monoglycerides leave
micelles and diffuse into epithelial cells.
There they are recombined and packaged
with other lipoid substances and proteins
to form chylomicrons.
4 Chylomicrons are extruded from the
epithelial cells by exocytosis. The
chylomicrons enter lacteals. They are
carried away from the intestine by lymph.
Fate of Digested Materials
Lipids
Within intestinal cells
Triglycerides → formed proteins and cholesterol added → chylomicrons → enter lymphatics
Fate of Digested Materials
Lipids
Plasma enzymes generate FFA’s and glycerol → pass capillary wall → protein cholesterol combo returns to liver →
additional proteins added → HDL and LDL created
LDL accumulates in plasma or on artery walls
Difficult for cells to recognize
Figure 23.32 (3 of 4)
Fat digestion
Small
intestine
Small
intestine
Foodstuff
Unemulsified
fats
Emulsification by
the detergent
action of bile
salts ducted
in from the liver
Pancreatic
lipases
Monoglycerides
and fatty acids
Glycerol
and
fatty acids
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, and 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.
Fate of Digested Materials
Water
After digested nutrients removed, large volumes of salt and
water remain in LI
Active Na+ uptake → passive Cl- and water uptake
Undigested materials (cellulose) cause water to be retained in LI
Antibiotics may kill bacteria → digestion impaired
Disorders of the Digestive System
Colon cancer
Second most common cause of cancer death in U.S. in men
98,000 new cases annually
48,000 deaths/year
Diagnosis
Colonoscopy
Fiberoptic endoscope
Polyps often occur before tumor
Disorders of the Digestive System
Colon cancer
Contributing factors
Contact time with carcinogenic material in colon
Diets high in animal material = slowed motility
P53 (tumor suppressor gene) mutation
Hereditary component
Disorders of the Digestive System
Gallstones
A.K.A Cholelithiasis
Bile salts precipitate
Block bile ducts
Jaundice
Treatment
Lithotripsy
Medications
Cholecystectomy
Questions?