Carbohydrates are an extensive group of organic substances that, together with proteins and fats, form the basis of the human and animal body. Carbohydrates are present in every cell of the body and perform a variety of functions. Small molecules of carbohydrates, represented mainly by glucose, can move throughout the body and perform an energy function. Large molecules of carbohydrates do not move and perform mainly a building function. From food, a person extracts only small molecules, since only they can be absorbed into the intestinal cells. Large molecules of carbohydrates the body has to build itself. The totality of all reactions for the breakdown of food carbohydrates to glucose and the synthesis of new molecules from it, as well as other numerous transformations of these substances in the body, is called carbohydrate metabolism in biochemistry.
Classification
Depending on the structure, there are several groups of carbohydrates.
Monosaccharides are small molecules that are not broken down in the digestive tract. These are glucose, fructose, galactose.
Disaccharides are small carbohydrate molecules that are broken down into two monosaccharides in the digestive tract. For example, lactose - for glucose and galactose, sucrose - for glucose and fructose.
Polysaccharides are large molecules consisting of hundreds of thousands of monosaccharide residues (mainly glucose) linked together. This is starch, meat glycogen.
Carbohydrates and diets
The breakdown time of polysaccharides in the digestive tract is different, depending on their ability to dissolve in water. Some polysaccharides break down quickly in the intestines. Then the glucose obtained during their decay quickly enters the bloodstream. Such polysaccharides are called "fast". Others dissolve worse in the aquatic environment of the intestine, so they break down more slowly, and glucose enters the blood more slowly. Such polysaccharides are called "slow". Some of these elements are not broken down in the intestines at all. They are called insoluble dietary fiber.
Usually, under the name "slow or fast carbohydrates" we mean not the polysaccharides themselves, but foods that contain them in large quantities.
The list of carbohydrates - fast and slow, is presented in the table.
| Fast carbs | Slow carbs |
| fried potatoes | Bran bread |
| White bread | Unprocessed rice grains |
| Mashed potatoes | Peas |
| Honey | Oatmeal |
| Carrots | Buckwheat porridge |
| Corn flakes | Rye bran bread |
| Sugar | Freshly squeezed fruit juice without sugar |
| Muesli | Wholemeal Pasta |
| Chocolate | Red beans |
| Boiled potatoes | Dairy |
| Biscuit | Fresh fruits |
| Corn | Bitter chocolate |
| White Rice | Fructose |
| Black bread | Soybeans |
| Beets | Green vegetables, tomatoes, mushrooms |
| Bananas | - |
| Jam | - |
When choosing products for a diet, a nutritionist always relies on a list of fast and slow carbohydrates. Fast in combination with fats in one product or meal lead to the deposition of fat. Why? The rapid rise in blood glucose stimulates the production of insulin, which provides the body with a store of glucose, including the pathway for the formation of fat from it. As a result, when eating cakes, ice cream, fried potatoes, weight is gained very quickly.
Digestion
From the point of view of biochemistry, the metabolism of carbohydrates takes place in three stages:
- Digestion. It begins in the mouth during chewing food.
- Proper metabolism of carbohydrates.
- Education of end products of exchange.
Carbohydrates are the basis of the human diet. According to the formularational nutrition, in the composition of food they should be 4 times more than proteins or fats. The need for carbohydrates is individual, but, on average, a person needs 300-400 g per day. Of these, about 80% are starch in the composition of potatoes, pasta, cereals and 20% are fast carbohydrates (glucose, fructose).
The exchange of carbohydrates in the body also begins in the oral cavity. Here, the salivary enzyme amylase acts on polysaccharides - starch and glycogen. Amylase hydrolyzes (breaks down) polysaccharides into large fragments - dextrins, which enter the stomach. There are no enzymes that act on carbohydrates, so dextrins in the stomach do not change in any way and pass further along the digestive tract, entering the small intestine. Here, several enzymes act on carbohydrates. Pancreatic juice amylase hydrolyzes dextrins to the disaccharide m altose.
Specific enzymes are secreted by the cells of the intestine itself. The enzyme m altase hydrolyzes m altose to the monosaccharide glucose, lactase hydrolyzes lactose to glucose and galactose, and sucrase hydrolyzes sucrose to glucose and fructose. The resulting monoses are absorbed from the intestines into the blood and through the portal vein enter the liver.
The role of the liver in carbohydrate metabolism
This organ maintains a certain level of glucose in the blood due to the reactions of synthesis and breakdown of glycogen.
Reactions of interconversion of monosaccharides take place in the liver - fructose and galactose are converted to glucose, and glucose can be converted to fructose.
Gluconeogenesis reactions take place in this organ -synthesis of glucose from non-carbohydrate precursors - amino acids, glycerol, lactic acid. It also neutralizes the hormone insulin with the help of the enzyme insulinase.
Glucose metabolism
Glucose plays a key role in the biochemistry of carbohydrate metabolism and in the overall metabolism of the body, as it is the main source of energy.
The level of glucose in the blood is a constant value and is 4 - 6 mmol / l. The main sources of this element in the blood are:
- Food carbohydrates.
- Liver glycogen.
- Amino acids.
Glucose is consumed in the body for:
- energy generation,
- Glycogen synthesis in the liver and muscles,
- synthesis of amino acids,
- fat synthesis.
Natural source of energy
Glucose is a universal source of energy for all body cells. Energy is needed to build your own molecules, muscle contraction, heat generation. The sequence of glucose conversion reactions leading to the release of energy is called glycolysis. Glycolysis reactions can take place in the presence of oxygen, then they speak of aerobic glycolysis, or in oxygen-free conditions, then the process is anaerobic.
During the anaerobic process, one molecule of glucose is converted into two molecules of lactic acid (lactate) and energy is released. Anaerobic glycolysis provides little energy: from one molecule of glucose, two molecules of ATP are obtained - a substance whose chemical bonds accumulate energy. This way to getenergy is used for short-term work of skeletal muscles - from 5 seconds to 15 minutes, that is, while the mechanisms for supplying muscles with oxygen do not have time to turn on.
During the reactions of aerobic glycolysis, one molecule of glucose is converted into two molecules of pyruvic acid (pyruvate). The process, taking into account the energy spent on its own reactions, gives 8 ATP molecules. Pyruvate enters into further oxidation reactions - oxidative decarboxylation and citrate cycle (Krebs cycle, tricarboxylic acid cycle). As a result of these transformations, 30 ATP molecules will be released per glucose molecule.
Glycogen exchange
The function of glycogen is the storage of glucose in the cells of an animal organism. Starch performs the same function in plant cells. Glycogen is sometimes called animal starch. Both substances are polysaccharides built from multiply repeating glucose residues. The glycogen molecule is more branched and compact than the starch molecule.
The processes of metabolism in the body of carbohydrate glycogen are especially intensive in the liver and skeletal muscles.
Glycogen is synthesized within 1-2 hours after a meal when blood glucose levels are high. For the formation of a glycogen molecule, a primer is needed - a seed consisting of several glucose residues. New residues in the form of UTP-glucose are sequentially attached to the end of the primer. When the chain grows by 11-12 residues, a side chain of 5-6 of the same fragments joins it. Now the chain coming from the primer has two ends - two points of growthglycogen molecules. This molecule will repeatedly elongate and branch as long as a high concentration of glucose in the blood remains.
Between meals, glycogen breaks down (glycogenolysis), releasing glucose.
Obtained from the breakdown of liver glycogen, it goes into the blood and is used for the needs of the whole organism. Glucose obtained from the breakdown of glycogen in the muscles is used only for the needs of the muscles.
Formation of glucose from non-carbohydrate precursors - gluconeogenesis
The body only has enough energy stored in the form of glycogen for a few hours. After a day of starvation, this substance does not remain in the liver. Therefore, with carbohydrate-free diets, complete starvation, or during prolonged physical work, the normal level of glucose in the blood is maintained due to its synthesis from non-carbohydrate precursors - amino acids, lactic acid glycerol. All these reactions occur mainly in the liver, as well as in the kidneys and intestinal mucosa. Thus, the processes of metabolism of carbohydrates, fats and proteins are closely intertwined.
From amino acids and glycerol, glucose is synthesized during starvation. In the absence of food, tissue proteins break down into amino acids, fats into fatty acids and glycerol.
From lactic acid, glucose is synthesized after intense exercise, when it accumulates in large quantities in the muscles and liver during anaerobic glycolysis. From the muscles, lactic acid is transferred to the liver, where glucose is synthesized from it, which is returned to the workingmuscle.
Regulation of carbohydrate metabolism
This process is carried out by the nervous system, the endocrine system (hormones) and at the intracellular level. The task of regulation is to ensure a stable level of glucose in the blood. Of the hormones that regulate carbohydrate metabolism, the main ones are insulin and glucagon. They are produced in the pancreas.
The main task of insulin in the body is to lower blood glucose levels. This can be achieved in two ways: by increasing the penetration of glucose from the blood into the cells of the body and by increasing its use in them.
- Insulin ensures the penetration of glucose into the cells of certain tissues - muscle and fat. They are called insulin dependent. Glucose enters the brain, lymphatic tissue, red blood cells without the participation of insulin.
- Insulin enhances the use of glucose by cells by:
- Activation of glycolysis enzymes (glucokinase, phosphofructokinase, pyruvate kinase).
- Activation of glycogen synthesis (due to increased conversion of glucose to glucose-6-phosphate and stimulation of glycogen synthase).
- Inhibition of gluconeogenesis enzymes (pyruvate carboxylase, glucose-6-phosphatase, phosphoenolpyruvate carboxykinase).
- Increase incorporation of glucose into the pentose phosphate cycle.
All other hormones that regulate carbohydrate metabolism are glucagon, adrenaline, glucocorticoids, thyroxine, growth hormone, ACTH. They increase blood glucose levels. Glucagon activates the breakdown of glycogen in the liver and the synthesis of glucose from non-carbohydratepredecessors. Adrenaline activates the breakdown of glycogen in the liver and muscles.
Exchange violations. Hypoglycemia
The most common disorders of carbohydrate metabolism are hypo- and hyperglycemia.
Hypoglycemia is a state of the body caused by low blood glucose levels (below 3.8 mmol/l). The reasons may be: a decrease in the intake of this substance into the blood from the intestine or liver, an increase in its use by tissues. Hypoglycemia can lead to:
- Liver pathology - impaired glycogen synthesis or glucose synthesis from non-carbohydrate precursors.
- Carbohydrate starvation.
- Prolonged physical activity.
- Pathologies of the kidneys - impaired reabsorption of glucose from primary urine.
- Digestion disorders - pathologies of the breakdown of food carbohydrates or the process of glucose absorption.
- Pathologies of the endocrine system - excess insulin or lack of thyroid hormones, glucocorticoids, growth hormone (GH), glucagon, catecholamines.
The extreme manifestation of hypoglycemia is hypoglycemic coma, which most often develops in patients with type I diabetes mellitus with an overdose of insulin. Low blood glucose leads to oxygen and energy starvation of the brain, which causes characteristic symptoms. It is characterized by extremely rapid development - if the necessary actions are not taken within a few minutes, a person will lose consciousness and may die. Typically, diabetic patients are able to recognize signs of a drop in glucose levels.blood and know what to do - drink a glass of sweet juice or eat a sweet bun.
Hyperglycemia
Another type of carbohydrate metabolism disorder is hyperglycemia - a state of the body caused by a persistently high blood glucose level (above 10 mmol/l). Reasons may be:
- pathology of the endocrine system. The most common cause of hyperglycemia is diabetes mellitus. Distinguish between type I and type II diabetes. In the first case, the cause of the disease is insulin deficiency caused by damage to the pancreatic cells that secrete this hormone. The defeat of the gland is most often autoimmune in nature. Type II diabetes mellitus develops with normal insulin production, therefore it is called non-insulin dependent; but insulin does not perform its function - it does not carry glucose into the cells of muscle and adipose tissue.
- neurosis, stress activate the production of hormones - adrenaline, glucocorticoids, thyroid gland, which increase the breakdown of glycogen and glucose synthesis from non-carbohydrate precursors in the liver, inhibit glycogen synthesis;
- liver pathology;
- overeating.
In biochemistry, carbohydrate metabolism is one of the most interesting and extensive topics for study and research.
