Absorption of Carbohydrate, lipid and Protein
Carbohydrate absorption
Monosaccharides (eg. glucose, fructose) are absorbed by active transport mediated by carrier and sodium pump.
Absorption of glucose
Glucose gets attached to specific transport proteins that lie on the luminal side of the enterocytes. These transport proteins have two binding sites one for glucose and one for sodium.
Once glucose and Na+ occupy the binding sites, the transport protein moves across the cell membrane and unload the glucose and Na+ into the cell. Hence, this process is referred to as sodium co-transport.
The transport of glucose will not occur unless Na+ is present.
Within the cell, glucose moves down by concentration gradient through the basoleteral membrane by facilitated diffusion, to extra-cellular space and then into the blood and finally to liver where the monosaccharides are stored as glycogen.
Galactose is absorbed more rapidly than glucose but fructose absorption is slower than glucose absorption.
Fructose absorption is by facilitated diffusion and not energy dependent. Hence, fructose can not be absorbed against concentration gradient.
Mannose, xylose and arabinose are poorly absorbed by diffusion and Maltose, sucrose and lactose as such are absorbed very slightly.
Disaccharides do not generally enter into the blood stream because of the presence of disaccharidases in the brush border of mucosa, which converts them to monosaccharides.
Lipid absorption
Lipid absorption begins in the dental duodenum and ends in proximal part of the jejunum. As the micelles come in close contact with surface of enterocytes, the lipid components diffuse through the glycocalyx to the apical membrane by a special fatty acid binding proteins which aids the transport the fatty acids across the cell membrane.
Other components in the micelle such as monoglycerides, cholesterol and vitamin A diffuse into the apical membrane.
Bile salts get detached from the micelles during fat absorption and remain in a free state. In the ileum, Na-co transport system function as specific bile acid transport which nearly complete the absorption of bile salts.
After absorption, the bile salts are transported to liver by the portal blood.
The liver extracts bile acids and maintains the normal concentration of bile acids in systemic blood. This process of recirculation of between liver and intestinal lumen is referred as enterohepatic circulation of bile.
Glycerol is absorbed by passive diffusion into the mesenteric venous blood.
Short chain fatty acids up to C10 are water-soluble get into mesenteric portal blood.
Monoglycerides and long chain fatty acids enter the microvilli and pass on to the lacteal by simple diffusion.
Only free form of cholesterol can be absorbed, whereas cholesterol esters must be hydrolyzed by pancreatic brush border hydrolases..
Cholesterol is absorbed less efficiently than triglycerides. Presence of hydro cholesterol or plant sterols inhibits cholesterol absorption.
In the epithelial cells, cholesterol is re-esterified before their transfer to lacteals.
Before absorption phospholipids are hydrolyzed to free fatty acids and lysophospholipids by phospholilpase of pancreas and intestinal epithelium.
Within the epithelial cells, long chain fatty acids are converted into fatty acyl-CoA involving co-enzyme A and ATP.
The fatty acyl co-enzyme reacts with monoglycerides to form di and tri glycerides.
The newly formed triglycerides differ from that of dietary fat. GlycerolPO4 derived from glucose metabolism provides glycerol residue for the triglyceride synthesis.
In addition, phospholipids and cholesterol esters are produced in the epithelial cells. Small amounts of proteins are added to the lipid droplet before their transfer from epithelial cells to lymph.
Chylomicrons are the products containing high amount of triglycerides, low level of phospholipids, cholesterol esters and proteins. They leave the cell by reverse pinocytosis and enter into the lacteals.
Lipid absorption begins in the distal duodenum and completed in the proximal jejunum.
The absorbed fat is in the form of an emulsion and imparts a milky appearance to the lymph, called as “chyle”. This leave the cell by reverse pinocytosis and enter in the lacteal. Though lymphatic channel and thoracic duct it is added into the blood for its distribution to tissues.
Short chain fatty acids are absorbed by blood from colon and caecum in sheep and horse and caecum in pigs.
Protein absorption
The free amino acids are readily absorbed chiefly by active transport requiring Na -co transport system.
Three types of carriers are involved in the transport of acidic, basic and neutral amino acids.
L-isomer forms of plant and animal protein are more readily absorbed than D-isomers, acidic basic and meutral amino acids
Some di and tri peptides are also absorbed. Intracellular peptidase hydrolyses these peptides to amino acids.
Intracellular amino acids diffuse across the basolateral membrane to reach liver via portal blood, whereas intact proteins are absorbed via lymph pathway.
Immediately after birth immunoglobulins from colostrum are absorbed by a process of pinocytosis particularly in lambs, piglets, kids, calves and pups.
The immunoglobulin absorption decreases with time after birth and ceases after 24-36 hours.
