Transcript Digestion processes

Major food groups
• Carbohydrates = sugars = saccharides
• Lipids = fats
• Proteins
• nucleic acids
Carbohydrates, proteins and nucleic acids (but not
fats) are long chains of smaller subunits, which are
monomers. The combinations are polymers.
Polymers in which the order of monomers provides
information: proteins and nucleic acids.
Nucleic acid digestion- pancreatic ribonuclease & deoxyribonuclease; nucleosidases
& phosphatases; act in small intestines
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Composition and Function of Pancreatic Juice
• Many enzymes are secreted in inactive
form:
Examples include:
– Trypsinogen is activated to trypsin
– Pepsinogen activated to pepsin by HCl
– Procarboxypeptidase is activated to
carboxypeptidase
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• Digestive enzymes that are produced
by the small intestine are all brush
border enzymes (membrane-bound)
• Advantages and disadvantages of
having membrane-bound enzymes
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Figure 23.27 Activation of pancreatic proteases in the small intestine.
Stomach
Pancreas
Epithelial
cells
Membrane-bound
enteropeptidase
Trypsinogen
Trypsin
(inactive)
Chymotrypsinogen
Chymotrypsin
(inactive)
Procarboxypeptidase
Carboxypeptidase
(inactive)
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Protein digestion
Foodstuff
Protein
Large polypeptides
Small polypeptides,
small peptides
Amino acids
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Enzymes
and source
Pepsin
(stomach glands)
in presence
of HCl
Pancreatic
enzymes
(trypsin, chymotrypsin,
carboxypeptidase)
Brush border
enzymes
(small intestine)
Site of
action
Path of absorption
Stomach
Small
intestine
+
• Amino acids enter
the capillary blood in the
Small
villi, and are transported
intestine
to the liver via the hepatic
portal vein.
Figure 23.33 Protein digestion and absorption in the small intestine.
Amino acids of protein fragments
Brush border enzymes
Apical membrane (microvilli)
Lumen of
intestine
Pancreatic
proteases
1
Protein
fragments (peptides)
are digested to amino acids by
brush border enzymes of
mucosal cells.
Absorptive
epithelial
cell
2 The amino acids are then
absorbed by active transport into
the absorptive cells
Amino
acid
carrier
3 The amino acids leave the
Active transport
Passive transport
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Capillary
villus epithelial cell by facilitated
diffusion and enter the capillary
via intercellular clefts.
Carbohydrate digestion
Foodstuff
Enzyme(s)
and source
Site of
action
Path of absorption
Starch and disaccharides
Oligosaccharides
and disaccharides
Lactose Maltose Sucrose
Galactose Glucose Fructose
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Salivary
amylase
Pancreatic
amylase
Brush border
enzymes in
small intestine
Mouth
Small
intestine
Small
intestine
• All monosaccharides
enter the capillary
blood in the villi, and
are transported to the
liver via the hepatic
portal vein.
Note: fats are NOT made of a long chain of monomers (unlike proteins and polysaccharides)
Fat digestion
Foodstuff
Enzyme(s)
and source
Site of
action
Unemulsified
fats
Emulsification by
the detergent
action of bile
salts from the
liver
Pancreatic
lipases
fatty acids
glycerol
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Small
intestine
Path of absorption
• Fatty acids and glycerol
enter the intestinal cells via
diffusion.
• Fatty acids and monoglycerides
are recombined to form
triglycerides and then
combined with other lipids and
Small
proteins within the cells to make
intestine chylomicrons, which 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.
Figure 23.34 Emulsification, digestion, and absorption of fats.
Fat globule
1 Large fat globules are emulsified
(physically broken up into smaller fat
droplets) by bile salts in the duodenum.
Bile salts
Fat droplets
coated with
bile salts
2 Digestion of fat by the pancreatic
enzyme lipase yields free fatty acids and
glycerol, still associated
with bile salts
Fatty acids
and bile salts
3 Fatty acids and glycerol diffuse into epithelial cells.
There they are recombined and packaged
with other lipids and proteins
to form chylomicrons.
4 Chylomicrons are extruded from the
Epithelial
cells of
small
intestine
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Lacteal
epithelial cells by exocytosis. The
chylomicrons enter lacteals. They are
carried away from the intestine by lymph.
Nucleic acid digestion
Foodstuff
Enzyme(s)
and source
Site of
action
Path of absorption
Nucleic acids
Pentose sugars,
N-containing bases,
phosphate ions
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Pancreatic ribonuclease and
deoxyribonuclease
Brush border
enzymes
(nucleosidases
and phosphatases)
Small
intestine
• Subunits are absorbed into
capillary blood in the villi
Small
and transported to the
intestine
liver via the hepatic portal
vein.
In the large intestine:
• Other than digestion by enteric bacteria, no
further digestion takes place
– Bacteria synthesize B complex vitamins and vitamin K
• water, and electrolytes are absorbed
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