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Absorption of Water Soluble Materials

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BIOL 1840U
Peter Cheung

Absorption of Water Soluble Materials I I. Introduction a. Main fxn of intestinal secretions is to maintain acid/base balance b. Intestines are presented w/ a considerable absorptive load every day i. Highly efficient—absorbs ALL calories that pass (almost) ii. Even absorbs the considerable fluid volume from endogenous sources c. Functional Unit: villus & its associated crypt i. cells at tip are enriched in surface-tethered hydrolytic enzymes & carriers ii. cells in crypts secrete fluid to lumen d. Main divisions of SI i. Duodenum—receives gastric chime, pancreatic acinar secretions, pancreatic ductal secretions, & bile; little absorption of nutrients but lots of H2O absorption & pH is returned to 6-7 ii. Jejunum—major site of absorption iii. Ileum—sim. to jejunum (absorption); at terminus is highly specialized for absorption of ascorbic acid, bile salts, & IF/cobalamin complex iv. Colon—begins at ileocecal valve; reclaims salt & H2O & stores feces e. Epithelial jxns i. Leaky epithelium—handle large volumes at high rates, less selective ii. Tight epithelium—work more slowly, highly selective, & often show absorptive/secretive behavior that’s under hormonal control. II. Carbohydrate Digestion / Absorption a. Salivary & pancreatic amylases—cleave α-1,4 linkages but cannot cleave α-1,6 linkages b. Digestion is usually completed w/in the duodenum b/c it’s very rapid & simple c. Sucrase & lactase hydrolyze sucrose & lactose to give monosaccharides d. Hydrolases are usually in excess => rate limiting step is sugar transport e. Glucose, Galactose, Fructose—products of carbohydrate metabolism f. SGLT1—Na-coupled cotransport system present on brush border, transports Glucose & Galactose g. GLUT transporter—facilitated diffusion transporter; located on apical membrane to transport fructose intracellularly h. GLUT-related carrier—present on basolateral membrane & exports glucose to blood *Fructose & galactose are converted to glucose in enterocytes* III. Protein Digestion /Absorption a. Pepsin—released in stomach; commences limited hydrolysis of protein b. Pancreatic Proteases—main protein digesters; secreted in zymogen i. Trypsinogen activated by enterokinase => trypsin
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