Post on 07-Jul-2018
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GASTROINTESTINAL
SYSTEM
Digestion and Absorption of Nutrients
Rondang R. Soegianto2010
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Introduction
Functions of the GI Tract
Digestion
Secretion
Motility Absorption
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Digestion:
hysical process: !he"ing# GI contraction
!he$ical process: Digesti%e en&y$es
Source of digesti%e en&y$es:
'(ocrine glands ) Sali%ary glands
ancreas
Gallbladder
*i%er
Also cells and glands in $ucosa
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Secretion:
During digestion# large %olu$e of fluid
secreted
into lu$en of GI tract fro$ e(ocr
glands and epithelial cells of intestinal
lu$en,
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Motility
Secretions and lu$inal contents $o%e fro$
$outh to anus by $otility .
coordinated contraction of s$ooth $uscle
Absorption
roducts of digestion ta/en into the body
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Dietary !arbohydrate
A, olysaccharide: lant carbohydrate:
) !ellulose# present in diet but no digesti%e
en&y$e cellulase)
) A$ylu$ consists of :
A$ylose 245 6)1#+ lin/s
A$ylopectin 745 6)1#+ and 6)1#0 lin/sfor branching
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Ani$al starch
) Si$ilar to A$ylopectin "ith $ore branches
STA8!9 and G*!;G'< are poly$ers of
glucose
;nly one glucose residue has reducing
group on !1
;ther glucose residues are non-reducing
ends
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>, Disaccharides
Sucrose: Fruc ? Gluc
*actose: Glu/ ? Galac
Maltose: Glu/ ? Gluc
!, Monosaccharides:
Gluc# Fruc# Galac
8educing sugars "ith reducing groupon !1 or !2
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1+1+
1,2 !arbohydrate Digestion Glycoside hydrolysis Monosach Intestinal $ucosa
Digesti%e 'n&y$es: ) Salivary α-amylase
p9 opt ,
) Pancreatic α- amylase
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1-1-
Sali%ary a$ylase stopped
by acidity of sto$ach
ancreatic a$ylase undergoes
buffering action of
ancreatic Buice and >ile inS$all intestine
!ontinues digestion of starch and
glycogen in food
roducts: ;ligosaccharides and
disaccharide $altose5
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) Brush border carbohydrase are:
Sucrase-α-dextrinase, glucoamylase, lactase
In intestinal epithelial cells
Alpha)de(trinase specific for 6)1#0)lin/ages in
a$ylopectin and glycogen
roducts: Gluc# Fruc# Galac
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!*I
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1,3 !arbohydrate Absorption
Transport $echanis$s by $e$brane carriers
Fruc: G*@T-# facilitated diffusionGluc ? Galac : SG*T1# sodium dependent symport
;ne gluc ? 2
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1=1=
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!*I
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2,1 Dietary Fat
9ydrophobic $olecules
MaBor D F : Triacylglycerols .
'sters of an alcohol glycerol5 and FA
In nature: 3 different FA in $olecule structure
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Fatty Acids:
Saturated : al$itate# Stearate
Monounsaturated: ;leate
olyunsaturated: *inoleate# *inolenate
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2+2+
2,2 Fat Digestion
a.Salivary and lingual lipases
For TAG "ith short to$ediu$ FA co"s $il/5
roduct: 2)$onoacylglycerols
9elp e$ulsify other fat in the diet
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2-2-
b, In s$all intestine
>ile salts e$ulsify fats
Pancreatic lipase and co-lipase
secreted fro$ pancreas
roducts:2)$onoacylglycerols ? FA fro$ !1 and !3
c, For$ation of bile salts $icelles Micelles contain: TAG# Monoacylglycerols#
FA# Fat soluble %ita$in
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2020
d, !holesteryl esters in diet are hydroly&ed by
cholesterylester hydrolase
@nesterified cholesterol and
cholesterol esters ta/en up
in bile salt $icelles chylo$icrons
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e, hospholipids in diet hydroly&ed by
pancreatic phospholipase !
8e$o%es fatty acid at !2
roduct:
*ysophospholipid# a po"erful detergent
FA and phospholipid incorporated into$icelles chylo$icron
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f, Digestion and Absorption of TAG
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2=2=
g, Absorption of dietary lipids
FA and 2)MAG pac/aged into micelles
. $icro droplets e$ulsified by bilesalts,
Also included in $icelles: !holesterol
Fat soluble %ita$is
Micelles Micro%illi
Short and $ediu$ FA !+ H !125 do not
need bile salts for absorption,
'nter portal blood directly,
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CLINICAL NOTE
Steatorrhea: lipid $alabsorption
increased lipid including %its A# D# '#
and essential FA5 in feces caused bydisturbed lipid digestion andor absorption
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h, Secretion of lipids fro$ enterocytes
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i. Ch!o"i#ron for"ation
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B, 'nterohepatic circulation of bile salts
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3+3+
$. %i!e a#ids are snthesi&ed fro" #ho!estero!
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CLINICAL NOTE
!holestyra$ine . anion e(change resin
>inds bile acids K 8eabsorption
@sed therapeutically along "ith
#$%-"o reductase Inhibitors in
hypercholesterole$ia
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A#ti'ation of gastri# and pan#reati# &"ogens
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(an#reati# En&"es
a, roteolytic en&y$es
b, ancreatic a$ylasec, ancreatic lipase
the only en&y$e i$portant in fat digestion5
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&ndopeptidase ) pepsin# trypsin
&xopeptidase
) carbo(ypeptidase pancreas5
a$inopeptidase intnal $ucosa
dipeptidase
roduct: Free a$ino acids
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Absorption of a"ino a#ids
)ti!i&e Na*dependent transport proteins
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rotein turno%er
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+astrointestina! hor"ones
%astrin: pyloric glands and pro(i$al
duodenu$
Sti$ulates acid and pepsinogen secretion in
sto$ach
Gastrin secretion sti$ulated by protein
$eals and %agus ner%e,
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Secretin: upper intestine
Sti$ulates "ater and bicarbonate secretion bypancreas to neutrali&e acidification by acid
chy$e,
"holecysto'inin: duodenu$ and pro(i$al
BeBunu$
Sti$ulates pancreatic en&y$es, 9or$one
secretionincreased by fats# peptides# a$ino acids in
lu$en of s$all intestine,
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RE,ERENCES
1, Medical >ioche$istry# A,!, >ro"nie# N,!,ernohan
'*S'OI'8# 2-
2, *ippincotts Illustrated 8e%ie"s: >ioche$istry
,!, !ha$pe# 8,A, 9ar%ey# D,8, Ferrier *ippincott illia$s P il/ins 2-
3, >asic Medical >ioche$istry, A !linical Approach
D,>, Mar/s# A,D, Mar/s# !,M, S$ith
illia$s P il/ins 1==0
+, 9arpers Illustrated >ioche$istry 2th ed, 20