Download The mechanism of monosaccharide absorption by intestinal

Absorption of carbohydrates:
The mechanism of monosaccharide absorption by intestinal
epithelial cells Glucose and galactose are absorbed in separate
mechanisms involving Na-depend cotransport.
Fructose is absorbed by facilitated diffusion. Glucose and
galactose are transported against an electrochemical gradient by
secondary acv e trans por t, is shown in figur e (14) .
Na-glucose cotransport and Na-galactose cotransport, using a
gradient as the energy source on basolateral membrane becouse
the intracellular Na+ concentration is low in intestinal cells ,as it
in other cells ,Na+ moves into the cell along gradiant. Glucose and
galactose are extracted across the basolateral membrane into
blood by facilitated diffusion. Fructose is transported across
luminal membrane by facilitated diffusion, then extruded into
blood. Figure(12)
Epithelial cell of the small intestine
Figure(12)absorption of monosaccharide
blood
Absorption of protein:
 Amino acid are transported from the lumen into the cell by Naamino acid cotransporter in apical membrane energy by a
gradient.
 There are four separation cotransporters; each one for neutral,
acidic, basic, and other amino acids.
 The amino acids then are transported across the basolateral
membrane into the blood by facilitated diffusion.
 Again by separation mechanism most ingested protein is
absorbed by intestinal epithelial cells in dipeptides and tripeptide
form and free amino acids.

Inside the cell, most of dipeptides and tripeptides are hydrolyzed
to amino acid by cystosolic peptidases,

the remaining dipeptides and tripeptides are Absorbe
unchanged. Figure(13).
blood
Epithelial cell of the small intestine
blood
Figure(13)absorption of protein
Figure(15)absorption of protein
Absorption of lipid
1- the product of lipid digestion are solubilized in the intestinal lumen in
mixed micelles except glycerol, which is water soluble.
The outer layer of micelle which is cylindrical shape is composed of bile
acids.
2- The micelles diffuse to apical membrane of the intestinal epithelial
cells. The lipids are released from the micelle diffuse down
concentration gradients into the cell. The micelles do not enter the cell.
The bile acid are left behind in the intestinal lumen. Most of the
digested lipid is absorbed by the mid jejunum. The work of bile acid is
completed in side the intestine before they are returned to the liver via
entrohepatic circulation .
3- The product of lipid (fatty acids) are re-esterified on the smooth
endoplasmic reticulum to form the original ingested lipid, triglyceride,
cholesterol ester and phospholipids.
4- Inside cells, lipids are packaged with lipid-carrying particle called
chylornicrons. The chylornicrons , with an average diameter of 1000 Aº,
phospholipid cover 80 % of outside of it, and remaining 20 % of surface
umen
covered with apoproteins which are synthesized by intestinal epithelial
cells, are essential for absorption of it.
5- The chylomicrons are packaged in secretary vesicles on the Golgi
apparatus. The secretary vesicles migrate to the basolateral membranes
and there is exocytosis of the chylornicrons. They are too large to enter
vascular capillaries, they can enter the lymphatic capillaries. The
lymphatic circulation carries them to the thoracic duct which empties
blood
lumen
into the blood
stream. Figure
Epithelial
(14) cell of the small intestine
Epithelial cell of the small intestine
Figure (14)absorption of lipid
Figure (13)absorption of lipid
blood