Instructions

Simple carbohydrates include fructose and glucose; they are quickly broken down and absorbed in the body. These substances lead to a spike in blood sugar, which increases insulin production. As a result, appetite increases and the risk of developing excess weight. Simple carbohydrates are found in berries, vegetables, sweets, pasta, and flour products. Complex carbohydrates contain structurally more complex chains of molecules. The body needs more time to absorb them. Complex carbohydrates are absorbed gradually, while glucose slowly enters the bloodstream, and a person’s appetite stabilizes. The result is a reduction in the amount of excess calories that can be stored as fat. Complex carbohydrates are present in potatoes, nuts, legumes, grains, and plant fibers. Indigestible carbohydrates (dietary fiber) cannot be absorbed by the body. However, when they enter the intestines, they have a positive effect on the digestion process by creating an environment for beneficial bacteria.

Products containing simple carbohydrates are classified as quickly digestible foods. Fruit, vegetable juices and broths are digested in 15-20 minutes. Semi-liquid dishes (vegetables, fruits, salad) are digested in 20-30 minutes. Fruits will be digested in 20-40 minutes, of which grapes, grapefruits, oranges - in 30 minutes, pears, peaches, apples and other semi-sweet fruits - in 40 minutes. Vegetable salads consisting of tomatoes, leafy greens, cucumbers, green or red peppers can be digested within 30-40 minutes. When added to salad vegetable oil this time increases to more than an hour. Vegetables boiled steamed or in water are digested within 40 minutes, broccoli, zucchini, green beans, cauliflower, pumpkin - 45 minutes. Root vegetables take up to 50 minutes to digest.

Complex carbohydrates take longer to digest. In particular, starches are absorbed by the body within an hour. These products include: potatoes, corn, chestnuts. Concentrated carbohydrates are digested in 1 hour 30 minutes. These include: brown rice, oats, buckwheat, millet, beans, lentils, beans. Digestion of carbohydrates occurs in oral cavity and stomach. When chewing food mixes with saliva containing digestive enzyme amylase. This substance hydrolyzes starch into the disaccharide maltose and other glucose polymers. Salivary amylase is blocked in the stomach hydrochloric acid. Digestion of carbohydrates occurs in the small intestine with the help of amylase produced by the pancreas. As a result, they are almost completely converted to maltose and/or other small glucose polymers. They are then broken down into numerous molecules, which dissolve in water and are absorbed into the bloodstream.

These are carbohydrates in which the number of monosaccharide residues exceeds ten and can reach tens of thousands. If a complex carbohydrate consists of identical monosaccharide residues, it is called a homosaccharide, if it consists of different ones, it is called a heterosaccharide.

2.3.1. Homopolysaccharides

Hard, do not have a sweet taste. The main representatives of homopolysaccharides are starch and glycogen.

Starch.

Consists of amylose and amylopectin, is a reserve nutrient in plants (starch grains in potato tubers, cereal grains). The amylose content in starch is 15-20%, amylopectin 75-85%. Amylose contains about 100 - 1000, amylopectin - 600 - 6000 glucose residues.

Glycogen

Animal starch.Contains from 6,000 to 300,000 glucose residues. Can be stored in reserve as a backup source of energy. The largest amount of glycogen is stored in liver cells (7%), in skeletal muscles(1-3%), in the heart (0.5%). Starch and glycogen are broken down in the gastrointestinal tract by the enzyme amylase; in animal cells, glycogen is broken down by glycogen phosphorylase.

Fiber (cellulose).

The main component of the plant cell wall, insoluble in water, consists of 2000-11000 glucose residues connected by a beta-glycosidic bond. Plays in the body important role in stimulating intestinal motility.

Fig. 1. Scheme of the structure of starch chains - amylose (a), amylopectin (b) and a section of the glycogen molecule (c).

2.3.2. Heteropolysaccharides

These are complex carbohydrates, consisting of two or more monosaccharides, most often associated with proteins or lipids.

Hyaluronic acid.

A linear polymer consisting of glucuronic acid and acetylglucosamine. It is part of cell walls, synovial fluid, vitreous body, envelops internal organs, and is a jelly-like bactericidal lubricant.

Chondroitin sulfates.

Branched polymers consist of glucuronic acid and N-acetylglucosamine. Serve as the main structural components of cartilage tissue, tendons, and the cornea of ​​the eye; also found in bones and skin.

3. The norm of carbohydrates in the diet

Carbohydrate reserves in the body do not exceed 2-3% of body weight. Due to them, energy reserves untrained person can be covered for no more than 12 hours, and for athletes even less. With normal carbohydrate consumption, the athlete’s body works more economically and gets less tired. Therefore, a constant supply of carbohydrates from food is necessary. The body's need for glucose depends on the level of energy expenditure. As the intensity and severity of physical labor increases, the need for carbohydrates increases. The norm of carbohydrates in the daily diet is 400 grams. for people who do not play sports; for athletes from 600 to 1000 gr. 64% of carbohydrates enter the body in the form of starch (bread, cereals, pasta), 36% in the form of simple sugars (sucrose, fructose, honey, pectin substances).

4. Digestion of carbohydrates in the gastrointestinal tract

When studying the process of carbohydrate digestion, you should remember the enzymes involved in it, find out the conditions of their action in various parts of the digestive tract, and know the intermediate and final products of hydrolysis.

Complex carbohydrates in food entering the human body have a different structure than carbohydrates in the human body. Thus, the polysaccharides that make up plant starch - amylose and amylopectin - are linear or weakly branched polymers of glucose, and the starch of the human body - glycogen - based on the same glucose residues, forms from them a different - highly branched - polymer structure. Therefore, the absorption of food oligo- and polysaccharides begins with their hydrolytic (under the influence of water) splitting into monosaccharides during digestion.

Hydrolytic breakdown of carbohydrates during digestion occurs under the action of glycosidase enzymes, which break down 1-4 and 1-6 glycosidic bonds in complex carbohydrate molecules. Simple carbohydrates do not undergo digestion; only some of them can be fermented in the large intestine under the influence of microbial enzymes.

Glycosidases include amylase of saliva, pancreatic and intestinal juices, maltase of saliva and intestinal juice, terminal dextrinase, sucrase and lactase of intestinal juice. Glycosidases are active in a slightly alkaline environment and are inhibited in an acidic environment, with the exception of salivary amylase, which catalyzes the hydrolysis of polysaccharides in a slightly acidic environment and loses activity with increasing acidity.

In the oral cavity, starch digestion begins under the influence of salivary amylase, which breaks down 1-4 glycosidic bonds between glucose residues inside the amylose and amylopectin molecules. In this case, dextrins and maltose are formed. Saliva also contains small amounts of maltase, which hydrolyzes maltose to glucose. Other disaccharides are not broken down in the mouth

Most of the polysaccharide molecules do not have time to hydrolyze in the mouth. A mixture of large molecules of amylose and amylopectin with smaller ones - dextrins. Maltose and glucose enter the stomach. The highly acidic environment of gastric juice inhibits salivary enzymes, so further transformations of carbohydrates occur in the intestine, the juice of which contains bicarbonates that neutralize the hydrochloric acid of gastric juice. Amylases from pancreatic and intestinal juices are more active than salivary amylase. Intestinal juice also contains terminal dextrinase, which hydrolyzes 1-6 bonds in the molecules of amylopectin and dextrins. These enzymes complete the breakdown of polysaccharides into maltose. The intestinal mucosa also produces enzymes that can hydrolyze disaccharides: maltase, lactase, sucrase. Under the influence of maltase, maltose is split into two glucoses; sucrose, under the influence of sucrase, is split into glucose and fructose; lactase splits lactose into glucose and galactose.

Digestive juices lack the enzyme cellulase, which hydrolyzes cellulose supplied with plant foods. However, there are microorganisms in the intestines whose enzymes can break down some cellulose. In this case, the disaccharide cellobiose is formed, which then breaks down to glucose.

Uncleaved cellulose is a mechanical irritant of the intestinal wall, activates its peristalsis and promotes the movement of food mass.

Under the influence of microbial enzymes, the breakdown products of complex carbohydrates can undergo fermentation, resulting in the formation of organic acids, CO 2, CH 4 and H 2. The diagram of carbohydrate transformations in the digestive system is presented in the diagram.

The monosaccharides formed as a result of hydrolysis of carbohydrates are the same in structure in all living organisms. Among the products of digestion, glucose predominates (60%), it is also the main monosaccharide circulating in the blood. In the intestinal wall, fructose and galactose are partially converted into glucose, so that its content in the blood flowing from the intestine is greater than in its cavity.

Absorption of monosaccharides is an active physiological process that requires energy consumption. It is provided by oxidative processes occurring in the cells of the intestinal wall. Monosaccharides obtain energy by interacting with the ATP molecule in reactions whose products are phosphorus esters of monosaccharides. When passing from the intestinal wall into the blood, phosphorus esters are broken down by phosphatases, and free monosaccharides enter the bloodstream. Their entry from the blood into cells various organs is also accompanied by their phosphorylation.

However, the rate of transformation and appearance of glucose in the blood from different products is different. The mechanism of these biological processes is reflected in the concept of “glycemic index” (GI), which shows the rate of conversion of food carbohydrates (starch, glycogen, sucrose, lactose, fructose, etc.) into blood glucose.

The carbohydrate requirement of an adult body is 350-400 g per day, while cellulose and other dietary fiber should be at least 30-40 g.

Food mainly supplies starch, glycogen, cellulose, sucrose, lactose, maltose, glucose and fructose, ribose.

Digestion of carbohydrates in the gastrointestinal tract

Oral cavity

The calcium-containing enzyme α-amylase enters here with saliva. Its optimum pH is 7.1-7.2, activated by Cl – ions. Being endoamylase, it randomly cleaves internal α1,4-glycosidic bonds and does not affect other types of bonds.

In the oral cavity, starch and glycogen can be broken down by α-amylase to dextrins– branched (with α1,4- and α1,6-linkages) and unbranched (with α1,4-linkages) oligosaccharides. Disaccharides are not hydrolyzed by anything.

Stomach

Due to the low pH, amylase is inactivated, although the breakdown of carbohydrates continues for some time inside the bolus.

Intestines

Pancreatic α-amylase works in the cavity of the small intestine, hydrolyzing internal α1,4 bonds in starch and glycogen to form maltose, maltotriose and dextrins.

Dear students, doctors and colleagues.
As for the digestion of homopolysaccharides (starch, glycogen) in the gastrointestinal tract...
In my lectures ( pdf-format) is written about three enzymes secreted with pancreatic juice: α-amylase, oligo-α-1,6-glucosidase, isomaltase.
HOWEVER, upon re-checking it was discovered that not a single caught to me (November 2019) publications on the English-language Internet there is no mention of pancreatic Oligo-α-1,6-glucosidase And isomaltase. At the same time, in RuNet such references are found regularly, although with discrepancies - either these are pancreatic enzymes, or are located on the intestinal wall.
Thus, the data is insufficiently confirmed or mixed up or even erroneous. Therefore, for now I am removing mention of these enzymes from the site and will try to clarify the information.

In addition to cavity digestion, there is also parietal digestion, which is carried out by:

• sucrase-isomaltase complex (working title sucrase) - V jejunum hydrolyzes α1,2-, α1,4-, α1,6-glycosidic bonds, breaks down sucrose, maltose, maltotriose, isomaltose,
• β-glycosidase complex (working title lactase) – hydrolyzes β1,4-glycosidic bonds in lactose between galactose and glucose. In children, lactase activity is very high even before birth and persists for high level up to 5-7 years, after which it decreases,
• glycoamylase complex - located in the lower parts of the small intestine, cleaves α1,4-glycosidic bonds and cleaves off terminal glucose residues in oligosaccharides from the reducing end.

The role of cellulose in digestion

Cellulose is not digested by human enzymes, because the corresponding enzymes are not formed. But in the large intestine under the influence microflora enzymes some of it can be hydrolyzed to form cellobiose and glucose. Glucose is partially used by the microflora itself and is oxidized to organic acids(oil, milk), which stimulate intestinal motility. Small part glucose can be absorbed into the blood.

Metabolism and functions of carbohydrates.

The human body contains several dozen different monosaccharides and many different oligo- and polysaccharides. The functions of carbohydrates in the body are as follows:

1) Carbohydrates serve as a source of energy: due to their oxidation, approximately half of all human energy needs are satisfied. In energy metabolism the main role belongs to glucose and glycogen.

2) Carbohydrates are part of the structural and functional components of cells. These include pentoses of nucleotides and nucleic acids, carbohydrates of glycolipids and glycoproteins, heteropolysaccharides of the intercellular substance.

3) Compounds of other classes, in particular lipids and some amino acids, can be synthesized in the body from carbohydrates.

Thus, carbohydrates perform multiple functions, and each of them is vital for the body. But if we talk about the quantitative side, then the first place belongs to the use of carbohydrates as an energy source.

The most common carbohydrate in animals is glucose. It plays the role of a link between the energetic and plastic functions of carbohydrates, since all other monosaccharides can be formed from glucose, and vice versa - different monosaccharides can be converted into glucose.

The body's source of carbohydrates is food carbohydrates - mainly starch, as well as sucrose and lactose. In addition, glucose can be formed in the body from amino acids, as well as from glycerol, which is part of fats.

Digestion of carbohydrates

Food carbohydrates in the digestive tract break down into monomers under the action of glycosidases - enzymes that catalyze the hydrolysis of glycosidic bonds.

Digestion of starch begins in the oral cavity: saliva contains the enzyme amylase (α-1,4-glycosidase), which breaks down α-1,4-glycosidic bonds. Since food does not stay in the mouth for long, starch is only partially digested here. The main site of starch digestion is small intestine, where amylase enters as part of pancreatic juice. Amylase does not hydrolyze the glycosidic bond in disaccharides.

Maltose, lactose and sucrose are hydrolyzed by specific glycosidases - maltase, lactase and sucrase, respectively. These enzymes are synthesized in intestinal cells. The products of carbohydrate digestion (glucose, galactose, fructose) enter the blood.

Fig.1 Digestion of carbohydrates

Maintaining a constant concentration of glucose in the blood is the result of the simultaneous occurrence of two processes: the entry of glucose into the blood from the liver and its consumption from the blood by tissues, where it is used as energy material.

Let's consider glycogen synthesis.

Glycogen– a complex carbohydrate of animal origin, a polymer whose monomer is α-glucose residues, which are interconnected through 1-4, 1-6 glycosidic bonds, but have a more branched structure than starch (up to 3000 glucose residues). The molecular weight of glycogen is very large - OH ranges from 1 to 15 million. Purified glycogen is a white powder. It is highly soluble in water and can be precipitated from solution with alcohol. With “I” it gives a brown color. In the liver it is found in the form of granules in combination with cell proteins. The amount of glycogen in the liver can reach 50-70 g - this is general reserve glycogen; constitutes from 2 to 8% of the liver mass. Glycogen is also found in muscles, where it forms local reserve, it is found in small quantities in other organs and tissues, including adipose tissue. Glycogen in the liver is a mobile reserve of carbohydrates; fasting for 24 hours completely depletes it. According to White et al., skeletal muscle contains approximately 2/3 of the total body glycogen (due to the large mass of muscles, most of the glycogen is located in them) - up to 120 g (for a man weighing 70 kg), but in skeletal muscles its content is from 0 .5 to 1% by weight. Unlike liver glycogen, muscle glycogen is not depleted as easily when fasting, even for long periods of time. The mechanism of glycogen synthesis in the liver from glucose has now been elucidated. In liver cells, glucose undergoes phosphorylation with the participation of an enzyme hexokinase with the formation of glucose-6-P.

Fig.2 Glycogen synthesis scheme

1. Glucose + ATP hexoxynase Glucose-6-P + ADP

2. Glucose-6-P phosphoglucomutase Glucose-1-P

(involved in synthesis)

3. Glucose-1-P + UTP glucose-1-P uridyl transferase UDP-1-glucose + H 4 P 2 O 7

4. UDP-1-glucose + glycogen glycogen synthase Glycogen + UDP

(seed)

The resulting UDP can be phosphorylated again by ATP and the entire cycle of glucose-1-P transformations is repeated again.

The activity of the glycogen synthase enzyme is regulated by covalent modification. This enzyme can be found in two forms: glycogen synthase I (independent - independent of glucose-6-P) and glycogen synthase D (dependent - dependent on glucose-6-P).

Protein kinase phosphorylates with the participation of ATP (does not phosphorylate the form of the I-enzyme, converting it into the phosphorylated form of the D-enzyme, in which the hydroxyl groups of serine are phosphorylated).

ATP + GS – OH protein kinase ADP + GS – O – P – OH

Glycogen synthase I Glycogen synthase D

The I-form of glycogen synthase is more active than the D-form, however, the D-form is an allosteric enzyme activated by a specific provider – glucose-6-P. IN at rest muscle enzyme is located in The I-form is not phosphorylated. active form, V reducing muscle, the enzyme is phosphorylated in the D-form and is almost inactive. In the presence of a sufficiently high concentration of glucose-6-phosphate, the D form is fully active. Hence, phosphorylation and dephosphorylation glycogen synthase plays a key role in fine regulation glycogen synthesis.

Regulation of glycogen synthesis:

A number of endocrine glands, in particular the pancreas, play an important role in the regulation of blood sugar.

Insulin is produced in the B cells of the islets of Langerhans of the pancreas in the form proinsulin. When converted into insulin, the polypeptide chain of proinsulin is split at two points, and the middle inactive fragment of 22 amino acid residues is isolated.

Insulin lowers blood sugar, delays the breakdown of glycogen in the liver and promotes glycogen deposition in the muscles.

Hormone glucagon acts in contrast to insulin as hyperglycemic.

Adrenal glands also take part in the regulation of blood sugar. Impulses from the central nervous system cause additional release of adrenaline produced in the adrenal medulla. Adrenaline increases enzyme activity phosphohylases, which stimulates the breakdown of glycogen. As a result, blood sugar levels increase. The so-called hyperglykelin(emotional excitement before the start, before the exam).

Corticosteroids unlike adrenaline, they stimulate the formation of glucose from nitrogen-free amino acid residues.

Glycogenolysis

Due to the ability to deposit glycogen mainly in the liver and muscles, and to a lesser extent in other organs and tissues, conditions are created for the normal accumulation of carbohydrate reserves. With an increase in energy consumption, the breakdown of glycogen into glucose increases.

Glycogen mobilization can occur in two ways: 1st – phosphorolytic and 2nd – hydrolytic.

Phosphorolysis plays a key role in the mobilization of glycogen, converting it from a storage form to a metabolically active form in the presence of the enzyme phosphorylase.

Fig.3 Hormonal regulation of phosphorolytic cleavage of glucose residue from glycogen.

The process of glycogen breakdown begins with the action of the hormones adrenaline and glucagon, which convert inactive adenylate cyclase into active one. It, in turn, promotes the formation of cAMP from ATP. Under the action of active protein kinase and phosphorylase kinase “b”, inactive phosphorylase “b” is converted into active “a”.

The phosphorylase enzyme exists in two forms: phosphorylase "b" - inactive (dimer), phosphorylase "a" - active (tetramer). Each of the subunits contains a phosphoserine residue, which has important for catalytic activity and a pyridoxal phosphate coenzyme molecule linked by a covalent bond to a lysine residue.

2 m. phosphorylase “b” + 4 ATP Mg ++ 1 m. phosphorylase a + 4 ADP

Active phosphorylase kinase acts on glycogen in the presence of H 3 PO 4, which leads to the formation of glucose-1-phosphate. The resulting glucose-1-phosphate is converted into glucose-6-phosphate by the action of phosphoglucomutase. The formation of free glucose occurs under the action of glucose-6-phosphatase.

Gluconeogenesis

Glycogen synthesis can also be carried out from non-carbohydrate substrates, this process is called gluconeogenesis. Substrate in gluconeogenesis can speak lactate(lactic acid), formed during anaerobic oxidation of glucose

(glycolysis). By simply reversing the reactions of glycolysis, this the process cannot proceed due to a violation of the equilibrium constants catalyzed by a number of enzymes.

Fig.4 Glycolysis and gluconeogenesis

The reversal of these reactions is achieved as a result of the following processes:

The main path of transformation PVA to oxaloacetate is localized in mitochondria. After passing through the mitochondrial membrane

PVK carboxylates to oxaloacetate and leaves the mitochondria in the form malate(this path is quantitatively more important) and again in the cytoplasm turns into oxaloacetate. The resulting oxaloacetate in the cytoplasm is converted to glucose-6-P. Dephosphorylation it is carried out glucose-6-phosphatase in the endoplasmic reticulum, up to glucose.

Glycolysis

Glycolysis- a complex enzymatic process of glucose conversion that occurs with insufficient O 2 consumption. The end product of glycolysis is lactic acid.

Fig.4 Glycolysis and gluconeogenesis

The overall equation of glycolysis can be represented as follows:

C 6 H 12 O 6 + 2ADP + 2PH H 2CH 3 CH(OH)COOH + 2ATP+ 2H 2 O

Biological significance glycolysis:

I. Reversibility of glycolysis - glucose can be formed from lactic acid due to gluconeogenesis.

II. The formation of phosphorylated compounds - hexoses and trioses, which are more easily converted in the body.

III. The process of glycolysis is very important in high altitude conditions, with short-term physical activity, as well as in diseases accompanied by hypoxia.

Biological chemistry Lelevich Vladimir Valeryanovich

Digestion of carbohydrates

Digestion of carbohydrates

Saliva contains the enzyme β-amylase, which breaks down β-1,4-glycosidic bonds inside polysaccharide molecules.

Digestion of the bulk of carbohydrates occurs in duodenum under the influence of pancreatic juice enzymes - β-amylase, amylo-1,6-glycosidase and oligo-1,6-glycosidase (terminal dextrinase).

Enzymes that cleave glycosidic bonds in disaccharides (disaccharidases) form enzymatic complexes localized on outer surface cytoplasmic membrane of enterocytes.

Sucrose-isomaltase complex - hydrolyzes sucrose and isomaltose, cleaving β-1,2 - and β-1,6-glycosidic bonds. In addition, it has maltase and maltotriase activity, hydrolyzing β-1,4-glycosidic bonds in maltose and maltotriose (a trisaccharide formed from starch).

Glycoamylase complex - catalyzes the hydrolysis of β-1,4 bonds between glucose residues in olisaccharides, acting from the reducing end. It also breaks down bonds in maltose, acting like maltase.

Glycosidase complex (lactase) - breaks down ?-1,4-glycosidic bonds in lactose.

Trehalase is also a glycosidase complex that hydrolyzes the bonds between monomers in trehalose, a disaccharide found in mushrooms. Trehalose consists of two glucose residues linked by a glycosidic bond between the first anomeric carbon atoms.

From the book Biology [ Complete guide to prepare for the Unified State Exam] author Lerner Georgy Isaakovich

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