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Digestion: Carbs, Proteins & Lipids (Lipids (digestion (lingual…
Digestion:
Carbs, Proteins & Lipids
Carbohydrates
absorption
apical
(2) Na-coupled cotransport (SGLT1) of glucose & galactose
(3) facilitated diffusion of fructose (GLUT5)
basolateral
(4) facilitated diffusion of all 3 monosaccharides (GLUT2) >> capillary >> portal system
(1) Na-K pump >> est. gradient
digestion
salivary & pancreatic amylases
intraluminal hydrolysis of starch >> oligosaccharides
endoenzymes: break down internal linear, alpha 1,4 linkages; but cannot break down terminal linkages
BB di-/oligo-saccharidases
oligo >> monosaccharides
lactase, maltase, sucrase, isomaltase
general pattern
BB enzymes hydrolysis >> monosaccharides absorbed
Proteins
general pattern
luminal hydrolysis of polymer (polypeptide) >>monomers (a.a.) absorbed
oligopeptides directly absorbed into enterocytes & specialized M cells >> broken down inside by lysosomal proteases
absorption
apical
(3) secondary active transport via
H+/oligopeptide contransporter (PepT1)
of di-/tri-/tetra-peptides >> peptidases hydrolyze to a.a.
Hartnup's disease
impaired neutral a.a. transport d/t defective PepT1 transporter in SI & proximal tubule of kidney
diminished a.a. & nitrogen supply
symptoms
manifests during infancy
photosensitivity
nystagmus
tremor
failure to thrive
intermittent ataxia
cerebellar ataxia
pellagra-like skin eruptions
gross aminoaciduria
Tx: high-protein diet
autosomal recessive
(2) free a.a. transport >> Na-contransport via specific a.a. carriers
basolateral
(4) facilitated or simple diffusion of a.a. >> portal system
(1) Na-K pump >> est. gradient
digestion
pepsin in stomach
proteins >> smaller proteins, polypeptides
pancreatic enzymes in duo
endopeptidase
trypsin
chymotrypsin
elastase
exopeptidase
carboxypeptidases (A & B)
large polypeptides >> small polypeptides & peptides
BB enzymes in duo & jejunum
small polypeptides >> a.a. or di/tripeptides
dipeptidase
exopeptidase
carboxypeptidases
aminopeptidase
Lipids
general pattern
luminal digestion >> absorpbed >> intracellular resynthesis
TAGs >> monoglycerides or glycerol + FAs >> chylomicrons
digestion
lingual lipase (minimal effect)
gastric lipase (minimal effect)
bile salts >> emulsified fats
pancreatic lipase & cholesterol esterase >> FAs, glycerols, glycerides
reassembled into chylomicrons >> absorbed into lymph of villus
absorption
LCFAs & other lipid products
pancreatic lipase converts TAGs >> monoglycerides & FFAs >> assoc w/ bile salts >> micelles >> ferried to intestinal mucosa
FFAs & monoglycerides leave micelles to diffuse into enterocyte >> recombined & packaged w/ other fatty substances & proteins >> chylomicrons
chylomicrons exocytosed into lacteals >> enter lymph >> carried to R. atrium
once in blood, lipoprotein lipase on surface of cells hydorlyzes TAGs >> FFAs + glycerol
cholesterol taken to liver >> form VLDLs >> take glycerides to cells
glycerides removed from VLDLs >> LDLs >> take cholesterol to organs & blood vessels
XS cholesterol taken back to liver in form of HDL
bile salt emulsify fat globules >> smaller fat globules w/ inc surface area
glycerol, SCFAs & MCFAs: transported directly into portal system, not into lacteals
passive diffusion
see Digestion/Absorption of Lipids map
General
4 functions of GI
motility
secretion
mouth: lingual lipase & salivary alpha amylase
stomach: pepsin, HCl, IF, HCO3, gastric lipase
pancreas: amylases, peptidases, lipase, nucleases, HCO3, insulin, glucagon, somatostatin, etc.
gallbladder: bile
SI: disaccharidases, trypsin, other protein digesting enzymes, colipase
digestion
create smaller absorbable units
via enzymatic hydrolysis
carbs >> monosaccharides
proteins >> a.a.
lipids >> glycerol & FAs
mostly ileum & LI
absorption of
carbs & proteins
primary Na cotransport
dipeptides/tripeptides
a.a.
secondary active transport
glucose/galactose via Na-glucose linked transporter (SGLT1)
insulin has no effect
mainly acts on adipose tissue & resting muscles
on glucose transport in stomach & kidney
on secondary active transport
proteins
fructose
has its own channel (
GLUT5
) via facilitated diffusion;
does not depend on Na-gradient or ATPase
from GIT >> blood or lymph
driven by Na+ gradient est. by Na/K-ATPase
all physiologically significant absorption of nutrients occurs in SI
Small Intestine
Site of all chemical digestion & nutrient absorption
Inc. surface area for absorption
microvilli
villi
plicae circularis (mucosa & submucosa)
circular folds of mucosa
synthesized enzymes act w/in BB
enterokinase: trypsinogen >> trypsin >> (+) zymogens
disaccharidases (sucrase, maltase, lactase)
aminopeptidases
Secretions
lipase: TAG -> FAs + monoglyceride or glycerol
somatostatin: (-) gastric acid secretion
CCK: (+) pancreatic enzyme secretions & gall bladder contraction
Secretin: (+) pancreatic ductal & cholangiocyte secretion of HCO3
does not contain any digestive enzymes
Absorption
proteins
di-/tri-peptides
a.a.
carbs
monosaccharides >> capillary blood in villus >> hepatic portal v. >> liver
active/facilitated transport
80% ingested water
electrolytes
vitamins
minerals
lipids
monoglycerides
FAs
micelles
chylomicrons
mostly from
ileum
except proteins (duo & jejunum)
nucleic acids
Large Intestine
Absorption
completion of fluid absorption >>
mainly in R. colon (cecum, ascending colon & proximal 1/2 of trans. colon)
Na (actively)
bile salts
vitamins from bacteria
cofactors & coenzymes
Secretion
K (active)
HCO3 (active)
Storage & elimination of fecal waste
stored mainly in L colon (distal half of trans. colon, descending colon & sigmoid colon)
fecal content: small volume of K-rich fluid containing undigested plant matter, bacteria, & inorganic material
5 Patterns of Digestion & Absorption
glucose: does not require digestion >> absorbed
protein: digested in lumen >> a.a. absorbed
sucrose: digested by BB enzymes >> monosaccharides absorbed
oligopeptide: directly absorbed >> digested inside cell
TAG: digested in lumen >> absorbed >> resynthesized