Transcript Digestion & Absorption

SSN SBPM Block 5:
Digestion & Absorption
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Overview
Carbohydrate digestion and absorption
Protein digestion and absorption
Fat digestion and absorption
Cobalamin (B12) and others
Celiac sprue
Immunity
Shadi (csc43)
Overview: Modifications of the gut
Glycocalyx
for digestion and absorption
and enzymes
and transporters
on surface of
microvilli
Microvilli
Length
(with cores of
microfilaments)
on surface of
ENTEROCYTE
Villi
Plicae
Carbohydrate digestion and
absorption
In lumen
 Starch  maltose + maltriose +
α-limit dextrins
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Amylase
At brush-border
 Maltose/maltriose  glucose
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SGLT-1
• Na+ coupled
(Na/K ATPase for
gradient)
• D-hexoses w/
pyranose ring
Glucoamylase (maltase)
Sucrase-isomaltase
Sucrase-isomaltase
Sucrose  glucose + fructose
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basolateral
α-limit dextrins  glucose
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apical
Sucrase-isomaltase
Lactose  glucose + galactose
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Lactase
GLUT-5
• Fructose absorption
• Jejunum
• Facilitated diffusion
GLUT-2
Protein digestion and
absorption
In lumen
 Protein  AA, di- & tripeptides, &
(AA)n via pancreatic PROTEASES
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Endopeptidases (chymotrypsin,
elastin, trypsin)
Exopeptidases (carboxypeptidases)
Secreted as zymogens by pancreas
Activated by TRYPSIN
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PepT1
• H+/oligopeptide
Trypsinogen  trypsin via
ENTEROPEPTIDASE (brush border)
At brush-border
 Oligopeptides  AA, di- &
tripeptides via PEPTIDASES
Also cytoplasmic peptidases
Apical AA
transporter
• Na+ coupled
Basolateral AA
transporter
• Na+ independent
Fat digestion and
absorption
TG, DG, CE
EMULSION DROPLET 
MIXED MICELLE
Surface TGs are
thinned by digestion by
pancreatic lipase & bile
salts; replaced by
TGs from core of
droplet. Lamellae decrease
& are ultimately lost.
Na+/H+ exchanger
creates acidic
microenvironment 
protonated FFA
uncharged (HA) 
absorption
AND THEN…
• Glycerol, short- &
medium-chain FA
pass through
straight to
capillaries
• Enterocytes
reesterify lipids
(long-chain FA, etc)
back to TGs & PLs
 chylomicrons
(packaged in Golgi)
secreted into
lymphatics
Cobalamin (B12)
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Cobalamin (B12) bound to protein in food
Stomach: Haptocorrin binds B12
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Small intestine: IF binds B12
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Haptocorrin secreted in stomach by gastric glands
Haptocorrin degraded in intestine
IF secreted in stomach by parietal cells
Ileum: enterocytes bind IF-B12 complex
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In ILEUM, by receptor-mediated endocytosis
B12 needs to bind transcobalamine II to exit cells
Absorption of different
nutrients (TESTABLE)
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Carbohydrates, proteins, lipids:
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Calcium, iron, folate:
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Active absorption duodenum
Bile acids:
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Maximum duodenum
Maximum ileum
Cobalamin (B12):
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Ileum only
Celiac Sprue
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Duodenum and jejunum
Gluten hypersensitivity
Defects in fat and protein
absorption and lactase deficiency
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Excessive turnover of gut’s lining  epithelium becomes
lined by crypt cells (no enterocyte with brush border
enzymes), flat villi  malabsorption
 Protein malabsorption leads to deficiency in essential AAs
 no albumin  low oncotic pressure  edema
 Fat malabsorption (steatorrhea) leads to fatty stools
Lactase deficiency detected by measuring H2 levels in
breath (produced by lactose metabolism by bacteria in
colon)
Immunity
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Cf. HISTOLOGY
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Paneth cells  storage of α-defensins in zymogen
granules
M cells (overlying Peyer’s patches)  transfer of
antigens by transcytosis
IgA  secretion by transcytosis into lumen
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IgA receptor is internalized with IgA, and a part of it
becomes secretory component (secreted with IgA)