GASTROINTESTINAL PHYSIOLOGY
Chapter-II
(DIGESTION and ABSORPTION)
Ass. Prof. Dr. Emre Hamurtekin
EMU Faculty of Pharmacy
INTRODUCTION
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CARBOHYDRATES
PROTEINS
NUCLEIC ACIDS
LIPIDS
VITAMINS & MINERALS
CARBOHYDRATES
• The principal dietary carbohydrates are:
– Polysaccharides (Starches)
• Amylopectin***
• Amylose
– Disaccharides (lactose & sucrose)
– Monosaccharides (fructose & glucose)
• Salivary and pancreatic α-amylases hydrolyze 1:4 α bonds.
• The end products of α-amylase digestion are:
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Maltose
Maltotriose
α- limit dextrin
Glucose oligomers
• Oligosaccharidases are located in the brush border of small intestinal
epithelial cells.
• Isomaltase is responsible for hydrolysis of 1:6 α bonds
CARBOHYDRATES
glucose
fructose
Sucrase
Sucrose
glucose
Lactose
Lactase
galactose
CARBOHYDRATES
• SGLT-1 transports glucose & galactose into the
intestinal epithelial cell.
• Transport of glucose and galactose is
dependent on Na in the intestinal lumen.
• Fructose absorption is independent of Na; it is
transported by facilitated diffusion.
• GLUT 5 and GLUT 2 is responsible for the
absorption of fructose.
PROTEINS
• Protein digestion begins in the stomach
(pepsins).
• Pepsins are secreted as proenzymes and
activated by gastric acid.
• Pepsinogen I
• Pepsinogen II
• Products of peptic digestion are polypeptides.
PROTEINS
• Polypeptides are further digested in the small
intestine by pancreatic and intestinal proteolytic
enzymes.
• Endopeptidases:
– Trypsin
– Chymotrypsin
– Elastase
• Exopeptidases:
– Carboxypeptidase A
– Carboxypeptidase B
PROTEINS
PROTEINS
PROTEINS
• Brush border of the intestinal mucosal cell:
– Aminopeptidases
– Carboxypeptidases
– Endopeptidases
– Dipeptidases
• Intracellular peptidases hydrolyzes some diand tripeptides that enters into the intestinal
epithelium.
PROTEINS
• At least seven different transport systems
transport a.a. into enterocytes.
• Five of them cotransport a.a. and Na.
• Two of them are Na independent.
• Di- and tripeptides are transported into
enterocytes by PepT1 system (requires H).
• A.a absorption is rapid in duodenum and
jejunum.
NUCLEIC ACIDS
Pancreatic nucleases
nucleotides
Intestinal enzymes
nucleosides
phosphoric acid
blood
sugars
purine / pyrimidine
bases
active transport
LIPIDS
• Most of the fat digestion begins in the duodenum (pancreatic
lipase*)
• The products are:
– FFA
– 2-monoglycerides
• Pancreatic lipase acts on emulsified fats and activitiy is facilitated
by colipase
LIPIDS
• Steatorrhea:
– Fatty, bulky stool because of the impaired
digestion and absorption of the fat.
– Mostly due to the lipase deficiency,
– Gastric acid hypersecretion & defective
reabsorption of bile salts are the other causes.
– Malabsorption of fat-soluble vitamins results.
LIPIDS
• Transport of lipids into the enterocytes occurs
mainly by passive diffusion, but also carriers
may be involved.
no modification
FA < 10-12 C atoms
active transport
FFA
FA
ENTEROCYTE
BLOOD
esterified
FA > 10-12 C atoms
Chylomicrons
TG + ChE
Chylomicrons
exocytosis
LYMPHATIC
CIRCULATION
VITAMINS
• Fat-soluble vitamins:
Vitamin A, D, E and K
Most vitamins
absorbed
Esterase
Incorporation into micelles
Upper small intestine
Vit B12
Absorption
Ileum
VITAMINS
• Vit B12 and folate absorption are Na-independent.
• Remaining water-soluble vitamins
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Thiamin
Riboflavin
Niacin
Pyridoxine
Pantothenate
Biotin
Ascorbic acid
Na cotransporters
THE END