GI Slides 2010 - Glucose monomers'” goum Starch glycogen...

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Unformatted text preview: Glucose monomers / \ '” goum Starch, glycogen Polysaccharides an digestive- tract lumen H u M In brush Lactose Maltose Sucrose Disaccharides border of small- a I I I epithelial cell 2; :ytiglseol'ial . . . . . . Monosaccharides ce" p Galactose Glucose Glucose Glucose Glucose Fructose (absorbable units) (a) Carbohydrate digestion Digestive_tract Glucose or n The dietary polysaccharides starch and glycogen are lumen galactose Fructose converted into the disaccharide maltose through the action of salivary and pancreatic amylase. 9 Maltose and the dietary disaccharides lactose and sucrose are converted to their respective monosaccharides by the disaccharidases (maltase, lactase, and sucrase) located in the brush borders of the small-intestine epithelial cells. Glucose or _ I Fructose ga'a‘m’se > a The monosaccharides glucose and galactose are absorbed into the epithelial cells by Na+- and energy-dependent secondary active transport (via the symporter SGLT) located at the luminal membrane. a The monosaccharide fructose enters the cell by passive facilitated diffusion via GLUT-5. a Glucose, galactose, and fructose exit the cell at the basal membrane by passive facilitated diffusion via GLUT-2. B These monosaccharides enter the blood by simple diffusion. v Glucose,galactose , _; ‘ ‘ KEY or fructose or»: m = Active transport M = Symport ca illa " D W q =Facilitated diffusion —> = Simple diffusion (b) Carbohydrate absorption lib-W ---- --El - Different amino acids In digestive- tract lumen II Ell-CI In brush m border of small- intestine epithelial cell In cytosol of epithelial cell DIED-DI (3) Protein digestion Digestive4ract Dietary and endogenous proteins are hydrolyzed into their ~ constituent amino acids and a few small peptide fragments by gastric pepsin and the pancreatic proteolytic enzymes. 8 Many small peptides are converted into their respective amino acids by the aminopeptidases located in the brush borders of the small—intestine epithelial cells. Amino acids are absorbed into the epithelial cells by means of Na+- and energy—dependent secondary active transport via a symporter. Various amino acids are transported by carriers specific for them. (I Some small peptides are absorbed by a different type of symporter driven by H“, Na+-, and energy-dependent tertiary active transport. 3 Most absorbed small peptides are broken down into their amino acids by intracellular peptidases. 8 Amino acids exit the cell at the basal membrane via various passive carriers. Amino acids enter the blood by simple diffusion. (A small percentage of di- and tripeptides also enter the blood intact) KEY = Active transport —> = Simple diffusion M = Symport fl = Antiport Q = Facilitated diffusion —> Chemical reaction Blood capillary (b) Protein absorption Triglycerides as large ' .Iy ' " Bile salts {Q} {Q} {Q} {0:- Lipid emulsion Lumen Micelles ,, Chylomicrons .1 Dietary fat in the form of large fat globules com- posed of triglycerides is emulsified by the detergent action of bile salts into a suspension of smaller fat droplets. This lipid emulsion prevents the fat droplets from coalescing and thereby increases the surface area available for attack by pancreatic lipase. a Lipase hydrolyzes the triglycerides into monoglycerides and free fatty acids. These water-insoluble products are carried to the luminal surface of the small-intestine epithelial cells within water-soluble micelles, which are formed by bile salts and other bile constituents. a When a micelle approaches the absorptive epithelial surface, the monoglycerides and fatty acids leave the micelle and passively diffuse through the lipid bilayer of the luminal membranes. , Lumen Micellar diffusion Micelle Microvillus Passive absorption The monoglycerides and free fatty acids are resyn- thesized into triglycerides inside the epithelial cells. B These triglycerides aggregate and are coated with a layer of lipoprotein from the endoplasmic reticulum to form water-soluble Chylomicrons. Chylomicrons are extruded through the basal membrane of the cells by exocytosis. B Chylomicrons are unable to cross the basement membrane of capillaries, so instead they enter the lymphatic vessels, the central lacteals. Stomach r Duodenum Alcohol Fat, fat-soluble vitamins, cholesterol sleaze-1 Left subclavian and left internal jugular veins Vitamin B12 Vitamin K (bacteria) WATER ...
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GI Slides 2010 - Glucose monomers'” goum Starch glycogen...

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