Slide 1Slide 2Slide 3AbsorptionSlide 5Slide 6Slide 7Location of Digestive EnzymesSlide 9Disorders of Carbohydrate Digestion and AbsorptionSlide 11Slide 12Location and Activation of GI ProteasesSlide 14Disorders of Protein Digestion and AbsorptionLipid DigestionSlide 17Slide 18Slide 19Slide 20Slide 21Fluid and Electrolyte AbsorptionSlide 23Slide 24Slide 25The Digestive System:6. Digestion and AbsorptionDigestion The mechanical and chemical breakdown of macromolecules into their component building blocks.•Mechanical breakdown accomplished through motility•Chemical breakdown accomplished by digestive enzymesDigestive EnzymesAmylase and Disaccharidases -> CarbohydratesProteases -> ProteinsLipases -> Fatssecreted in saliva, gastric ,and pancreatic juicesbrushborder enzymes at apical side of epithelial cells in small intestineHydrolysisthe cleavage of chemical bonds by the addition of waterDigestionThe chemical breakdown of macromolecules (Carbohydrates, Proteins, Fat)Absorption•Movement of nutrients, water, and electrolytes from the lumen of the GI tract across the epithelial layer into the interstitial fluid (and further into blood and lymph).•Two paths of absorption: transcellular and paracellular.•transcellular: transport across luminal membrane, enter the intestinal epithelial cell, and then be extruded from the cell across the basolateral membrane.•paracellular: transport between two cells along gradients (osmotic, electrochemical)CarbohydratesStarch, Glycogen, and Cellulose are the most common carbohydrate polymers in our dietsDietary Carbohydrates:Starch/Glycogen (60% of diet)Sucrose (20%)(1glucose +1 fructose)Lactose (6%)(1 glucose + 1 galactose)GlucoseGalactoseFructoseGlucoseGalactose (12%)FructoseCarbohydrates are absorbed as MonosaccharidesDietary FiberPolysaccarides that cannot be digested by our enzymes(e.g., plant cellulose, hemicellulose and pectins)In the colon:1. Induce osmotic water flow (less likely to be constipated)2. Fermented by bacteria to organic anions and “gas”Amylose, a(1->4) linkagea-AmylaseCellulose, b(1->4) linkageCellulaseFigure 21.17aCarbohydrate DigestionLocation of Digestive Enzymes•Lumenal–a-Amylase•oral cavity (salivary glands)•duodenum (pancreas)•Brush border of enterocytes–Maltase, Sucrase, LactaseFigure 21.11Figure 21.17b• Glucose and galactose absorption is through secondary active transport.• Fructose absorption is not secondary active transport! Fructose is absorbed through facilitated diffusion along its concentration gradient.Disorders of Carbohydrate Digestion and Absorption•Failure to break down ingested carbohydrate to an absorbable form.•Non-absorbed carbohydrate remains in the lumen with water.-> Osmotic Diarrheae.g.: Lactose intolerance (75% of adults world wide)Figure 21.18ProteinsFigure 21.18Enzymes for Protein DigestionProteins are digested into amino acids and peptidesLocation and Activation of GI ProteasesPepsinogen Pepsin (Endopeptidase)Low pHStomach (Lumen)Trypsinogen is initially converted to Trypsin by brushborder enteropeptidaseSmall Intestine (Lumen and Brushborder)Figure 21.14bFigure 21.18Protein Digestion and Absorption in Small Intestine• Amino acid absorption is also through secondary active transport.• Peptide absorption is through active transport.Disorders of Protein Digestion and Absorption•Deficiency of pancreatic enzymes or defect in transporters of the intestinal epithelial cells.Chronic pancreatitis and Cystic fibrosis Food allergies - absorption of proteins in infant gut causes early immune reaction ???Lipid Digestion•Dietary Lipids include triglycerides, cholesterol, and phospholipids•Hydrophobic•Lingual and gastric lipase begins digestion (10%), but bulk of the work is done in the small intestine.•Most important role of stomach in the digestion of lipid is to control release of chyme into duodenum.Emulsification of Lipids to Facilitate Digestion of Lipids in the Aqueous Environment of the Small IntestineFigure 21.16Figure 21. 16dBile salt recyclingVitamin Absorption• fat-soluble vitamins A, D, E, and K are absorbed in small intestine with fats -> consumption of olestra “fake fat” and orlistat (“Alli”, lipase inhibitor) leads to vitamin A, D, E and K deficiencies• water soluble vitamins (C and most B vitamins) are absorbed by mediated transport•Vitamin B12-Intrinsic Factor absorbed in terminal IleumIron and Calcium AbsorptionFigure 21. 19Iron and calcium are two of very few substances whose intestinal absorption is actively regulated.-> decrease in body concentration leads to enhanced absorption Ingestion: - heme iron -> meat - ionized iron (Fe2+ )-> some plant productsAbsorption: - apical co-transport with H+ on DMT1 (divalent metal transporter 1) - Fe2+ leaves cell on ferroportin transporterRegulation: -secretion of peptide hormone hepcidin by liver -> binds to ferroportin -> degradationAbsorption: - passive paracellular transport in jejunum and ileum - active transport through ion channels in duodenumFluid and Electrolyte Absorption•Small intestine and large intestine have “leaky” epithelium•Transepithelial transport of electrolytes and nutrients is followed by osmotic movement of water from lumen to blood.Figure 21.19c• Na+ /K+ is absorbed along with water.• Undigested polysaccharides (fiber) are metabolized to short-chain fatty acids by microorganisms and these are then absorbed by diffusion. small amount of vitamin Kmixture of acids and gases (flatus)Digestion and Absorption in the Large IntestineA &D(Folic acid)Vitamins (K) from bacterial digestionDuodenum JejunumIleumFigure 21.6+ Fats (minimal)Primary and secondary