Gastrointestinal System 2010

8/19/2019 Gastrointestinal System 2010

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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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00


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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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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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3-3-

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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3=


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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

Gastrointestinal System 2010

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