The exact physiochemical mechanism by which the digested food materials are absorbed is not known so far.
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It is believed that phenomena such as passive transported active transport (diffusion) are involved in the absorption of digested food materials. The movements of villi also cause some absorption.
There are some evidences that the leucocytes (white blood corpuscles) found in the intestinal mucosa, actually pass through the intestinal wall and pick up the food particles and carry them back into the blood and lymph and thus help in absorption.
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1. Passive transport:
In passive transport materials move through cells, membranes and intercellular spaces because of differences in the concentration gradient.
The substances are in aqueous solutian and they move either due to random motion of solute molecules, or due to osmotic properties of the cell.
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The extent of passive transport varies with the type of cell, its physiological condition and the properties of the penetrating molccules. The absorption of most vitamins, purines and pyrimidines is due to passive transport.
2. Active transport:
During active transport, molecules move from the alimentary canal against a chemical potential gradient or diffusion gradient. The absorption of sodium ions, fat, glucose, amino- acids is due to active transport.
Current views:
According to Anderson and Ussing, 1963 and Holter, 1961 special carrier molecules are found in the alimentary’ canal.
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These molecules form a complex with the transported material on one side of the membrane and release it on the other side where they dissociate and set free.
3. Absorption of carbohydrates:
The intestinal epithelium is practically impermeable to polysaccharides but recent findings have shown that measurable amounts of disaccharides are absorbed through the intestinal epithelium when they are fed in sufficient amounts, some disaccharides normally seen to pass into the cells of the intestinal epithelium where they are split by disaccharases such as maltase and lactase.
The greatest amount of carbohydrate, however, seems not to be absorbed before being split into monosaccharides.
During passive diffusion monosaccharides such as fructose, mannose and most pentoses are absorbed, where as hexoses, glucose and galactose are absorbed as a result of active diffusion.
All the carbohydrates are absorbed in the form of monosaccharides directly into the blood stream. Blood carries these sugars into the liver where the excess of sugar is converted into glycogen in which form they are stored.
4. Absorption of proteins:
Proteins, as a result of digestion, are hydrolyzed into a-amino-acids.. Phosphoric acid and nucieic acids may also be formed but the a-amino-acids are the primary units liberated.
The early workers thought that all amino-acids are absorbed by diffusion (active) but later Gibson and Weisman, 1951, proved that the absorption of amino-acid is a selective chemical process as the natural a-amino-acids are more rapidly absorbed than their optical mates, D-amirfo-acids.
Some of the a-amino-acids are absorbed against a concentration gradient, e.g., actively absorbed while others are not.
In few cases, as with glutamic acid and aspartic acid considerable transamination occurs in the intestinal mucosa but others are absorbed without transformation.
5. Absorption of fats:
In the process of digestion the fats are hydrolyzed into fatty acids. Fat absorption is an active process.
The end products of fat digestion are absorbed by the lacteals c f the intestinal villi. From the lacteals they pass into the lymph vessels and finally reach into the blood stream via thoracic duct.
Various theories have been formulated about the nature of fat absorption. The most important are as follows:
(i) Old views:
Prior to 1900 it was believed that fats are absorbed by the intestinal wall only after they had been hydrolyzed into fatty acids and glycerol. It was further believed that fats could be absorbed as finely emulsified particles.
(ii) Pfluger’s views:
According to Pfluger no emulsified fats could be absorbed by the intestinal cells and all the fats must be hydrolyzed before absorption into the body.
(iii) Lipolytic hypothesis:
According to this hypothesis the fats are absorbed in the form of soaps. The natural fats are hydrolyzed into fatty acids and glycerol by the action of pancreatic lipase.
The fatty acids, thus formed, are made water soluble by saponification or by hydrotropic action of bile salts.
These saponified fatty acids along with glycerine are absorbed by the cells of intestinal epithelium where they are said to combine to yield first phospholipids and then triglycerides what are known as complex fats. These complex fats pass to lacteals for transport via thoracic duct to systemic blood.
(iv) Partition hypothesis:
Frazer formulated this hypothesis, according to which the fats are absorbed both in the hydrolyzed as well as in the emulsified form. Frazer believed that the tiny droplets of fats enter the intestinal cells through the pores present in their outer boundaries.
(v) Recent views:
According to recent findings the complex fats, that are found in the lacteals, are formed after absorption.
6. Absorption of water:
The absorption of water begins in the stomach, but, since it passes rapidly into the intestine it is mainly absorbed in it.
Enormous quantity of water can be absorbed by the intestine (in man 15 to 20 litres per day). Process of osmosis form the principal mechanism of water absorption since the osmotic pressure of the food is usually higher than that of the chyme.
7. Absorption of mineral salts:
In addition to the organic food materials certain inorganic salts are also absorbed by the cells of the intestinal epithelium.
The salts of alkaline metals are absorbed into the blood through cells of the intestinal epithelium and not through the intercellular spaces.
The salts of haloid acids are absorbed better than sulphates and carbonates. Na, K and CI ions are absorbed rapidly but Mg and SO4 ions are absorbed with difficulty.
Absorption in large intestine:
In the large intestine practically no absorption of food materials takes place because the chyle contains no absorbable substance by the time it reaches the large intestine. In the large intestine simply the absorption of water takes place.