Digestive and absorptive
functions of the
gastrointestinal system
1
Objectives
Describe the mechanical and chemical digestion
of the stomach.
Describe the digestion and absorption of
carbohydrates.
Describe the digestion and absorption of protein.
Describe the digestion and absorption of lipids.
Describe the absorption of electrolytes, and
water-soluble vitamins, including the role of
intrinsic factor in the absorption of vitamin B12.
2
Primary foods on which the body
lives:
 Carbohydrates
 Fats
 Proteins
 + vitamins, minerals
Cannot be absorbed in their natural
forms—must be digested
3
Digestion of Carbohydrates
3 major sources of carbohydrate in food
 Sucrose----- disaccharide, cane sugar
 Lactose------disaccharide, milk
 Starch--------polysaccharide, non-animal
food eg. Potato and grains
 Cellulose
Essentially all the carbohydrates in the food are
absorbed as monosaccharide and very small amount
as disaccharide
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Digestion of CHO in mouth and stomach
 Salivary amylase
 hydrolyses starch into maltose
(trisaccharide) and glucose polymers
 In the stomach continue to digest CHO
before mixing with gastric juice
 Accounts for 20-40% of CHO digestion
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Digestion of CHO in small intestine
Digestion by pancreatic amylase
 Very powerful amylase, virtually all the CHO will be digested
into maltose and very small glucose polymers
Digestion by Intestinal Epithelial enzymes
 Intestinal microvilli brush border contain enzymes that split
disaccharides and glucose polymers into monosaccharide.
 Glucose represents more than 80% of the final products of CHO
digestion
 Lactose splits into glucose and galactose by lactase
enzyme
 Sucrose splits into fructose and glucose by sucrase enzyme
 Maltose and glucose polymers splits into glucose by
maltase and dextrinase enzymes
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Digestion of ProteinsProtein digestion in small
Dietary proteins are
chemically long chains of
amino acids bound by
peptide linkages
Protein digestion in the
stomach
 Pepsin (Recall from
previous lecture)
 Pepsin important in
digesting collagen. why?
 10-20% of total protein
digestion
intestine
Protein digestion by
pancreatic enzymes
Most of the protein
digestion
results from
action of pancreatic
enzymes
Trypsin, chemotrypsin,
carboxylpolypeptide,
elastase
Protein digestion by small
intestine enzymes
Digestion at the brush
border of the enterocytes
Aminoplpypeptidase and
dipeptidase
Digestion inside the
enterocytes
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Contain multiple peptidases
Digestion of lipids (fat)
 The most abundant fat in the food is
triglycerides
 Triglycerides composed of glycerol and 3
fatty acid molecules
 Digestion by salivary lipase and stomach
lipase is limited (<10%)and generally
unimportant
 Essentially all fat digestion occur in small
intestine
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Digestion of lipids (fat)
 First step in fat digestion is emulsification of fat with bile
acids and lecithin in the duodenum.
 Lipase enzyme is water soluble and can attack fat
globules on their surfaces.
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Digestion of lipids (triglycerides) by pancreatic
lipase
 The most important enzyme in triglycerides
digestion is the pancreatic lipase enzyme.
 Triglycerides will be digested into free fatty acids
(long and short chain) and 2 monglycerides
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Basic principles of gastrointestinal
absorption
Small intestine is the major area of
absorption (90%)
Stomach is a poor absorptive area (highly
lipid soluble substance and aspirin)
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Absorption of Carbohydrates
 Essentially all CHO are absorbed in the form of
monosaccharides
 80% glucose
 20% fructose and galactose
 All monosaccharides absorbed by active transport.
 Glucose transported by sodium co-transport mechanism
 Galactose similar to glucose
 Fructose transported by facilitated diffusion
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Absorption of Proteins
 Most proteins are absorbed through the
luminal membrane of the intestinal epithelial
cells in the form of dipeptide, tripeptide and
amino acids
 Transported by active transport.
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Absorption of lipids
 All dietary lipids absorbed by simple diffusion
 Short chain free fatty acids
 Hydrophobic and very small size
 Pass through enterocytes by simple diffusion then to
blood
Long chain free fatty acids and
monoglycerides
Hydrophobic, large size
Form micelles with bile salts (Micelles
are composed of central fat globule
(containing monoglycerides and long
chain free fatty acids) with molecules of
bile salts projecting outward to cover
the surface of micelle)
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Absorption of lipids cont..
Micelle carry monoglycerides and free fatty acids to
enterocytes brush border.
Monoglycerides and fatty acids diffuse out of micelle into
the enterocyte.
Inside enterocyte, monoglyceride and fatty acids are
recombined to form triglycerides along with protein,
phospholipids and cholesterol to form chylomicrons.
Chylomicrones leave the enterocytes via exocytosis.
Chylomicrones enter lymph then to the blood
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Blood
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Absorption of water
 Water absorption is entirely by
simple diffusion (osmosis)
 Water can move across
intestinal membrane in both
directions to maintain osmotic
equilibrium with blood
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Absorption of Electrolytes
Sodium is actively transported
Negative charged ions chloride, iodide,
bicarbonate and nitrate passively follow
sodium absorption.
Calcium actively transported in a process
stimulated by vitamin D.
Iron is actively transported.
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Absorption of Vitamins
 Fat soluble vitamins (A, D, E, K)
Included with ingested dietary lipids in the
micelles and absorbed via simple diffusion
 Water soluble vitamins and vitamin C
Absorbed via simple diffusion
 Vitamin B12 combined with intrinsic factor
secreted by stomach and then absorbed in
the ileum via active transport mechanism
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Large Intestine
Absorption of
water and
electrolytes
Storage of
fecal matter
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Large Intestine
•
Contains no Villi: all the work of digestion and absorption is
already accomplished in the small intestine.
•
Secretes mucus: to protect epithelial cells from excoriation
and to provide adherent media for holding feces together.
•
Secrets HCO3-: to neutralize the acid secreted by colonic
bacteria.
•
Its bacteria synthesize Vit K: what we eat is never enough.
•
Absorbs fluid: The proximal part. The longer feces remain the
harder they become.
•
Stores feces: in the distal part until the conditions are right to
defecate
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Constituents of Feces
 Normally 75% water and 25% solid
 Indigestible material
 Some inorganic material (e.g. calcium
 and phosphate)
 Bacteria
 Stercobilin and urobilin → give the color
of feces
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