PSL201 – March 23 – GI 3&4.docx

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21 Apr 2012
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PSL201 March 23 GI 3&4
Carbohydrates
- most in the form of polysaccharides (starch and cellulose)
- cellulose cannot be digested (excreted in feces)
o aids in intestinal motility
- monosaccharide’s are the absorbable forms
o most carbs must be digested before they can be absorbed
Digestion of Carbohydrates
- polysaccharides are digested by amylases
o located in saliva and pancreatic juice
o salivary amylase is inactivated by stomach acid
o cannot break bonds at branch pints or between glucose monomers
o reduce starch or glycogen to maltose or short branched
polysaccharides called limit dextrins
- brush border enzymes complete the digestion of monosaccharaides
o include dextrins and glucoamylase
o break limit dextrins into sucrase, lactase, and maltase
hydrolysis sucrose into glucose and fructose
lactose into to glucose and galactose
maltose to two glucose
- monosaccarides then transported into epithelial cells
Absorption of Carbohydrates
- glucose and galactose enter the epithelial cells via cotransport w/ Na and
then cross the basal membranse by facilitated diffusion
- fructose is absorbed by facilitated diffusion
Proteins
- ingestion, secretion and the turnover of enterocytes
- digested to tripeptides, dipeptides and amino acids before absorbtion
- endopeptidases split polypeptides at interior peptide bonds (create
smaller peptides)
- exopeptidases cleave of AA at one end (create amino acids)
Digestion of Proteins to Small Peptides and Amino Acids
- begins in stomach with pepsin
- pepsinogen precursor for pepsin is secreted by chief cells of the stomach
o partially activated by interactin with hydrogen ions secreited by
parietal cells of the stomach
o can the FULLY activate other pepsinogens by proteolytic activation
- pesisn then begins digestion but splits bonds between certain AAs
o becomes inactive in alkaline SI
- pancrease release zymogens (trypsinogen, chymotripsinogen, and
procarboxypeptidase)
- enterokinase: brush border enzyme that proteolytically activates trypsinogen
to trypsin
o trypsin activates chymotrypsin and carboxypeptidase
only break doen bonds between certain AAs
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