Absorption of Digested Products
Absorption is the process by which nutrient molecules are taken into the cells of a living organism. In higher animals, all absorption takes place in the small intestine.
Absorption of Carbohydrates
All carbohydrates are absorbed in the form of monosaccharides directly into the bloodstream. Blood carries these monosaccharides to the liver where they are converted into glycogen. In this form, they are stored in the liver. The transport of most monosaccharides through the intestinal membrane can occur against large concentrations gradually and therefore requires an active source of energy. Undigested disaccharides are not absorbed.
Absorption of Proteins
Proteins are absorbed as amino acids directly into the bloodstream. Blood carries these amino acids to the tissues where they become part of the metabolic pool, from which body proteins are synthesized. Amino acids are oxidized, producing energy. The energy for most of this transport is supplied by a sodium co-transport mechanism.
Absorption of Fats
Fats are mostly absorbed in the duodenum and jejunum in human beings. They are hydrolyzed giving a mixture of glycerol, diglycerides, monoglycerides, and free fatty acids. Most of the glycerol is absorbed through the blood capillaries.
Since monoglycerides, diglycerides, and fatty acids are insoluble in water, they are not absorbed directly from intestinal contents. They are first incorporated into small, spherical, water-soluble droplets, called micelles, with the help of bile salts and phospholipids in the intestinal lumen. These micelles are then easily absorbed as such as without undergoing hydrolysis. These are accumulated in the spaces of the endoplasmic reticulum of mucosal cells and are subsequently discharged into the spaces of villi. Absorbed fatty acids, diglycerides, and monoglycerides are used in synthesizing fat in the intestinal cells. These fats are then released into lymph in the form of droplets called chylomicrons.
Absorption of Different Nutrients in Digestive System
|Mouth||Drugs coming in contact with the mucosa of the tongue of the mouth and the lower side of the tongue are absorbed into the blood capillaries lining them.|
|Stomach||Absorption of water, simple sugar, and alcohol.|
|Small Intestine||The principal organ for the absorption of nutrients, final products of digestion such as glucose, fructose, fatty acids, glycerol, and amino acids are absorbed through the mucosa into the bloodstream and lymph.|
|Large Intestine||Absorption of water, some minerals, and drugs.|
Mechanism of Absorption
Absorption across the gastro-intestinal mucosa occurred by passive or active transport.
The passage of nutrient molecules from a region of high concentration to a region of low concentration through a membrane called passive absorption. Passive absorption is of two types diffusion and osmosis.
Diffusion may be simple or facilitated. Transport of nutrients across the membrane along the concentration gradient without the use of a carrier molecule is known as simple diffusion.
Several nutrients like glucose, amino acids, etc. which have a large diameter and not lipid-soluble, are absorbed from the intestine by facilitated diffusion.
Osmosis is the diffusion of water molecules through gastro-intestinal mucosa from low solute concentration to a region of high solute concentration.
Inactive absorption, nutrient molecules are usually absorbed from regions of low concentration to high concentration, i.e, molecules move against the concentration gradient. This requires a considerable expenditure of energy that comes from ATP. Inactive transport, a carrier molecule picks up the nutrient molecule and forms a carrier nutrient complex with it. After releasing the nutrient molecule, the carrier utilizes ATP and moves again to pick up another molecule.
Inactive absorption, a carrier molecule picks up the nutrient molecule and forms a carrier nutrient complex with it. After releasing the carrier utilizes ATP and moves back and pick up another molecule.