Where carbohydrates enter when absorbed in the small intestine



The absorption of carbohydrates in the small intestine is a complex biochemical process. Glucose, galactose and fructose are absorbed in the intestinal cavity with high efficiency, but at different speeds, namely

( descending absorption rate): galactose glucose fructose mannose arabinose xylose.

Simple sugars can enter the blood and lymph through the cell membranes of the small intestine by simple sugars in a concentration gradient. But now it is proved that simple sugars are absorbed by the secondary active transport with the help of Na + ions.

It is believed that the carrier proteins on the outer surface of the membranes of small intestine cells are combined with certain monosaccharides( glucose or galactose) and Na + ions, forming a mobile complex of the Ka + transfer-monosaccharide. The transport process requires energy and is essentially an enzymatic reaction. This is confirmed by the fact that absorption improves with an increase in the activity of the enzyme adenosine triphosphatase, which cleaves ATP with the formation of ADP and phosphate, accompanied by the release of energy to activate the transport mechanism.

The enzymatic nature of the carbohydrate transport process is also evidenced by the conjugacy of carbohydrate digestion( in particular, of the wall surface) to monosaccharides with their absorption. It is supposed( OMUgolev) that there is a single catalytic system in the membrane of the microvilli, which combines the functions of the enzyme and the carrier and ensures the transfer of the monosaccharide to the carrier. Thanks to such a system, Nike-rotary through and fast absorption.

An important link in the mechanism of carbohydrate absorption is the transmembrane transfer of Na + and K + ions. In particular, Na + ions are necessary for the transport of a monosaccharide against a concentration gradient. In different contact areas of the carrier protein, the Na + ion and the monosaccharide join, and in the form of such a complex they pass through the membrane into the cytoplasm of the small intestine cell. In the cytoplasm, the complex breaks down, the monosaccharide is used in the cell or transported into the blood, and the Na + ion is “pumped out” of the Na +, K + -ATPase cells by a complex energy-dependent enzyme system. At the same time, K ions enter( “pumped”) into the cell.

Thus, the absorption of carbohydrates is coupled with certain electrolyte metabolism units — Na + and K + ions.


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Absorption atnk intestine

Nutrients enter the blood and lymphatic capillaries through the epithelial sheath of the digestive tract. This mainly occurs in the small intestine, which is adapted to ensure that the absorption is as efficient as possible.

Inside the intestine is lined with mucous membrane with a huge number of outgrowths: more than 2500 fibers are placed on each square centimeter of the inner surface of this organ. Each cell of the villus forms up to 3000 microvilli. Thanks to the villi and microvilli, the inner surface of the small intestine exceeds the area of ​​the football field. So, for parietal digestion in the body there is a huge surface # 8212;substances are absorbed through it.

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The structure of the large intestine

The villous cavities accommodate blood and lymphatic capillaries, elements of smooth muscle tissue, and nerve fibers. Villi and microvilli is the main "device" that provides nutrient absorption.

How is the absorption of substances?

There are two ways to transport substances through the intestinal epithelium: through the gaps between the cells and through the epithelial cells themselves. In the first case, it is carried out by diffusion. In this way, water and some mineral salts and organic compounds come to the internal environment. However, by diffusion to the internal environment of the villus, only a small part of the nutrients gets. Many molecules have to penetrate the villi through the most epithelial cells. First of all, these molecules must overcome their plasma membranes. Special carrier molecules help them in this. Once in the cell, the nutrient molecules move in the cytoplasm to another cell and out through the membrane into the extracellular fluid. The overcoming of these barriers by molecules of substances, absorbed, usually requires a large amount of energy.

Digestion in the colon

What happens to substances that have got to the villous intercellular fluid?their molecules are directed into the blood or lymphatic capillaries of the villi. Glucose, dissolved in water, amino acids, salts of mineral substances pass directly into the blood. The products of the breakdown of fats( glycerin and fatty acids) first enter the lymph, and with it enter the blood circulatory system.

Digestion in the large intestine

The human large intestine is 1.2–1.5 m long, its diameter reaches 9 cm. Digestion and absorption of food is mainly completed in the small intestine. The only exceptions are some substances, such as cellulose. It is partially digested in the large intestine by numerous lactic acid bacteria. These bacteria, , mutants, , synthesize substances that are beneficial to humans: certain amino acids, vitamin K, and B vitamins that enter the blood and are transported to every cell in the human body.

Digestive juice, which is produced by the glands of the colon walls, contains almost no enzymes. Its main component is # 8212;undigested mucus, and they become like oils.

Digestion in the Large Intestine # 8212;main stages of

Why are food debris compacted in the large intestine? It is there that intensive absorption of water into the blood vessels occurs. Because of this, chyme .advancing, gradually turns into dense fecal masses. Fecal masses can remain in the large intestine up to 36 hours, and then move to the rectum. From the rectum, they are brought out through the anus, surrounded by with the sphincter of .This sphincter, unlike those that are placed in the esophagus and stomach, is arbitrarily reduced. This means that the person controls the excretion of feces. Consequently, absorption occurs in all parts of the digestive tract. However, on each of them different substances come to the internal environment. In the oral cavity and esophagus nutrients are almost not absorbed. Water, glucose, amino acids, etc. are absorbed in a small amount in the stomach. Intensive absorption of nutrients occurs in the small intestine. In the large intestine is absorbed mainly water.


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Digestion in the small intestine

In the small intestine, there are 2 interrelated types of digestion - abdominal and parietal( membrane).The most important process in the digestive function is absorption. In the upper parts of the digestive tube( mouth, esophagus, stomach) absorption is negligible. In the stomach, water and alcohol are absorbed, a small amount of salts and carbohydrate breakdown products. Minor absorption occurs in the duodenum. The main mass of nutrients is absorbed in the small intestine, with an unequal speed in different parts of it. Maximum absorption occurs in the upper portions of the small intestine.
The small intestine performs several important functions:
1. mixing the chyme with the secrets of the pancreas, liver and intestinal mucosa;
2. digestion of food;
3. suction of homogenized and digested material;
4. Further promotion of the remaining material on the gastrointestinal tract;
5. secretion of hormones;
6. Immunological protection.
Nutritional masses( chyme) from the duodenum are transferred to the small intestine, where digestion with digestive juices that have been secreted into the duodenum continues. However, its own intestinal juice is starting to operate here.produced by the liberkunov and brunner glands of the mucous membrane of the small intestine. The intestinal juice contains enterokinase .as well as a complete set of enzymes that break down proteins, fats and carbohydrates. These enzymes are involved only in parietal digestion, since they are not excreted into the intestinal cavity. Abdominal digestion in the small intestine is carried out by enzymes that come with food chyme. Abdominal digestion is most effective for the hydrolysis of coarse-molecular substances.

Pristenochny digestion( membrane) .open acad. AMUgolevym, occurs on the surface of the small intestine microvilli. It completes the intermediate and final stages of digestion by hydrolysis of the intermediate cleavage products. Microvilli are cylindrical processes of the intestinal epithelium with a height of 1-2 microns. Their number is enormous - from 50 to 200 million per 1 sq. Km.mm surface of the intestine, which increases the absorption surface of the small intestine 14-30 times. The extensive surface of the microvilli improves the absorption processes. The products of intermediate hydrolysis fall into the zone of the so-called brush border formed by microvilli, where the final stage of hydrolysis occurs and the transition to absorption by the main enzymes involved in parietal digestion are amylase, lipase and protease. Due to this digestion, the cleavage of 80-90% of peptide and glycolysis bonds and 55-60% of triglycerides occurs( see Fig. 1).

Figure1. The cross section of the two villi of the small intestine and the crypt between them, several types of cells of the mucous structure that are inside the villus are visible.

Pristenochnaya digestion is in close interaction with abdominal. Abdominal digestion prepares the original food substrates for parietal digestion, and the latter reduces the amount of chyme processed in abdominal digestion due to the transfer of products of partial hydrolysis to the brush border. These processes contribute to the most complete digestion of all components of food and prepare them for absorption.
The motor activity of the small intestine provides mixing of the chyme with digestive juices and its promotion along the intestine due to the reduction of the circular and longitudinal muscles. With the reduction of the longitudinal fibers of the smooth muscles of the intestine, a section of the intestine shortens, and when relaxed, its lengthening occurs. The duration of the periods of contraction and relaxation of sections of the intestine with pendulum movements is 4-6 s. This periodicity is due to the automatism of the smooth muscles of the intestine - the ability of the muscles to periodically contract and relax without external influences. Contractions of the intestinal circular muscles induce peristaltic movements that promote the movement of food forward. Along the length of the intestine several peristaltic waves move simultaneously.
The contractions of the longitudinal and circular muscles are regulated by the vagus and sympathetic nerves. The vagus nerve stimulates intestinal motor function. The brake signals are transmitted along the sympathetic nerve, which reduce muscle tone and inhibit mechanical bowel movements. The humoral factors also influence the intestinal motor function: serotonin, choline and enterokinin stimulate bowel movement.

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