BIOL 2P93 Lecture Notes - Ileocecal Valve, Cholecystokinin, Transport Protein

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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
ii. Trypsin activates the other proenzymes
iii. Pancreatic proteases convert peptides to free aa
c. Amino acid transport systems
i. Not well understood; 6 have been classified
ii. Na-coupled AA transporters & Facilitated Diffusion transporters
IV. H2O & Salt Reabsorption
a. Salt reabsorption
1
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