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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