Carbohydrates are an extensive group of organic substances that, together with proteins and fats, form the basis of the human body and animals. Carbohydrates are present in every cell of the body, 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. A person extracts only small molecules from food, since only they can be absorbed into the intestinal cells. Large molecules of carbohydrates, the body has to build itself. The set of all reactions for the breakdown of carbohydrates of food into glucose and the synthesis of new molecules from it, as well as other numerous transformations of these substances in the body, are called carbohydrate metabolism in biochemistry.
Classification
Depending on the structure, several groups of carbohydrates are distinguished.
Monosaccharides are small molecules that do not break down in the digestive tract. These are glucose, fructose, galactose.
Disaccharides are small carbohydrate molecules that break 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), interconnected. This is starch, glycogen meat.
Carbohydrates and diets
The digestion time of polysaccharides in the digestive tract is different, which depends on their ability to dissolve in water. Some polysaccharides break down quickly in the intestines. Then the glucose obtained by their breakdown quickly enters the bloodstream. Such polysaccharides are called "fast." Others dissolve worse in the aquatic environment of the intestine, therefore, they break down more slowly, and glucose enters the bloodstream more slowly. Such polysaccharides are called "slow." Some of these elements do not break down at all in the intestines. They are called insoluble dietary fiber.
Usually under the name "slow or fast carbohydrates" are meant not the polysaccharides themselves, but products that contain them in large quantities.
The list of carbohydrates - fast and slow, is presented in the table.
| Fast carbohydrates | Slow carbohydrates |
| fried potatoes | Bran bread |
| White bread | Unprocessed rice grains |
| Mashed potatoes | Peas |
| Honey | Oat flakes |
| Carrot | Buckwheat porridge |
| Cornflakes | Rye bread with bran |
| Sugar | Sugar-Free Fresh Fruit Juice |
| Muesli | Wholemeal pasta |
| Chocolate | Red beans |
| Boiled potatoes | Dairy |
| Biscuit | Fresh fruits |
| Corn | bitter chocolate |
| White rice | Fructose |
| Black bread | Soybean |
| Beet | Green vegetables, tomatoes, mushrooms |
| Bananas | - |
| Jam | - |
When choosing foods for making 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 fat deposition. Why? A rapid increase in blood glucose stimulates the production of insulin, which provides a supply of glucose in the body, including the way fat is formed from it. As a result, when eating cakes, ice cream, fried potatoes, the weight gains very quickly.
Digestion
From the point of view of biochemistry, carbohydrate metabolism takes place in three stages:
- Digestion: It begins in the oral cavity while chewing food.
- Actually carbohydrate metabolism.
- Education end products exchange.
Carbohydrates are the basis of the human diet. According to the formula of a balanced diet, there should be 4 times more of them in food 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% is in starch in potatoes, pasta, cereals and 20% in fast carbohydrates (glucose, fructose).
The metabolism of carbohydrates in the body also begins in the oral cavity. Here, an amylase saliva enzyme acts on polysaccharides - starch and glycogen. Amylase hydrolyzes (breaks down) polysaccharides into large fragments - dextrins that enter the stomach. There are no enzymes acting on carbohydrates, so the 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 maltose disaccharide.
Specific enzymes secrete cells of the intestine itself. The maltase enzyme hydrolyzes maltose to glucose monosaccharide, lactase to lactose to glucose and galactose, and sucrose to sucrose to glucose and fructose. The resulting monoses are absorbed from the intestine into the bloodstream and enter the liver through the portal vein.
The role of the liver in carbohydrate metabolism
This organ ensures the maintenance of a certain level of glucose in the blood due to the reactions of the synthesis and breakdown of glycogen.
In the liver, monosaccharides interconvert reactions occur - fructose and galactose turn into glucose, and glucose can turn into fructose.
In this organ there are reactions of gluconeogenesis - the synthesis of glucose from non-carbohydrate precursors - amino acids, glycerin, lactic acid. Also, the hormone insulin is neutralized by the enzyme insulinase.
Glucose metabolism
Glucose plays a key role in the biochemistry of carbohydrate metabolism and in the general metabolism of the body, since 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:
- Carbohydrate food.
- Liver glycogen.
- Amino acids.
Consumes glucose in the body to:
- energy production
- glycogen synthesis in the liver and muscles,
- amino acid synthesis
- 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 production. The sequence of glucose conversion reactions leading to the release of energy is called glycolysis. Glycolysis reactions can occur in the presence of oxygen, then they talk about aerobic glycolysis, or under oxygen-free conditions, then the process is anaerobic.
During the anaerobic process, one glucose molecule is converted into two lactic acid (lactate) molecules and energy is released. Anaerobic glycolysis produces little energy: two molecules of ATP are obtained from one glucose molecule - a substance whose chemical bonds accumulate energy. This method of generating energy is used for short-term work of skeletal muscles - from 5 seconds to 15 minutes, that is, at that time, while the mechanisms of supplying muscles with oxygen do not have time to turn on.
During aerobic glycolysis reactions, one glucose molecule is converted into two pyruvic acid (pyruvate) molecules. The process, taking into account the energy spent on its own reactions, gives 8 ATP molecules. Pyruvate enters further oxidation reactions - oxidative decarboxylation and the citrate cycle (Krebs cycle, tricarboxylic acid cycle). As a result of these transformations, 30 ATP molecules will be released onto the glucose molecule.
Glycogen metabolism
The function of glycogen is the storage of glucose in the cells of an animal organism. The same function in plant cells is performed by starch. Glycogen is sometimes called animal starch. Both substances are polysaccharides built from repeatedly repeating glucose residues. A glycogen molecule is more branched and compact than a starch molecule.
The processes of metabolism in the body of glycogen carbohydrate are especially intense in the liver and skeletal muscles.
Glycogen is synthesized within 1-2 hours after a meal, when the blood glucose level is high. To form a glycogen molecule, a primer is needed - a seed consisting of several glucose residues. At the end of the primer, new residues in the form of UTP-glucose are sequentially attached. 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 growth of the glycogen molecule. This molecule will elongate and branch many times until a high concentration of glucose in the blood remains.
Between meals, glycogen breaks down (glycogenolysis), releasing glucose.
Obtained by the breakdown of liver glycogen, it goes into the bloodstream and is used for the needs of the whole organism. Glucose obtained by the breakdown of glycogen in the muscles is spent on the needs of only muscles.
Glucose formation from non-carbohydrate precursors - gluconeogenesis
The body has enough energy stored in the form of glycogen for only a few hours. After a day of starvation of this substance in the liver does not remain. Therefore, with carbohydrate-free diets, complete starvation, or with prolonged physical work, the normal level of glucose in the blood is maintained due to its synthesis from non-carbohydrate precursors - amino acids, glycerol of lactic acid. 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 fasting. In the absence of food , tissue proteins break down to amino acids, fats - to fatty acids and glycerol.
From lactic acid, glucose is synthesized after intense physical activity, 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 again returns to the working muscle.
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. This can be achieved in two ways: by increasing the penetration of glucose from the blood into the cells of the body and increasing its use in them.
- Insulin provides the penetration of glucose into the cells of certain tissues - muscle and fat. They are called insulin-dependent. In the brain, lymphatic tissue, red blood cells, glucose enters 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).
- Strengthening the inclusion of glucose in the pentose phosphate cycle.
All other hormones that regulate carbohydrate metabolism are glucagon, adrenaline, glucocorticoids, thyroxine, growth hormone, ACTH. They increase blood glucose. Glucagon activates the breakdown of glycogen in the liver and the synthesis of glucose from non-carbohydrate precursors. Adrenaline activates the breakdown of glycogen in the liver and muscles.
Metabolic disorders. Hypoglycemia
The most common disorders of carbohydrate metabolism are hypo- and hyperglycemia.
Hypoglycemia - a condition of the body caused by a low level of glucose in the blood (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 - a violation of the synthesis of glycogen or the synthesis of glucose from non-carbohydrate precursors.
- Carbohydrate starvation.
- Long physical activity.
- Kidney pathology - a violation of the reverse absorption of glucose from primary urine.
- Digestive disorders - pathologies of the breakdown of carbohydrates in food or the process of absorption of glucose.
- Pathologies of the endocrine system - an excess of insulin or a lack of thyroid hormones, glucocorticoids, growth hormone (STH), glucagon, catecholamines.
The extreme manifestation of hypoglycemia is a 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 you do not take the necessary actions within a few minutes, a person will lose consciousness and may die. Typically, patients with diabetes can recognize signs of a drop in blood glucose 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, an organism condition caused by persistent high blood glucose (above 10 mmol / l). The reasons may be:
- pathology of the endocrine system. The most common cause of hyperglycemia is diabetes. There are diabetes mellitus type I and II. In the first case, the cause of the disease is insulin deficiency caused by damage to pancreatic cells secreting this hormone. The defeat of the gland most often has an autoimmune character. Type II diabetes mellitus develops during the normal production of insulin, therefore it is called insulin-independent; but insulin does not fulfill its function - it does not conduct glucose into the cells of muscle and adipose tissue.
- neurosis, stress activate the production of hormones - adrenaline, glucocorticoids, the thyroid gland, which enhance the breakdown of glycogen and glucose synthesis from non-carbohydrate precursors in the liver, inhibit glycogen synthesis;
- liver pathology;
- binge eating.
In biochemistry, carbohydrate metabolism is one of the most interesting and extensive topics for study and research.