What is carbohydrates, the role of carbohydrates in the human body

The chemical properties of cells that make up living organisms depend primarily on the number of carbon atoms that make up to 50% of the dry mass. Carbon atoms are found in the main organic substances: proteins, nucleic acids, lipids and carbohydrates. The latter group includes compounds of carbon and water corresponding to the formula (CH ₂ O) _n , where n is equal to or greater than three. In addition to carbon, hydrogen and oxygen, the molecules can include atoms of phosphorus, nitrogen, sulfur. In this article, we will study the role of carbohydrates in the human body, as well as the features of their structure, properties and functions.

Classification

This group of compounds in biochemistry is divided into three classes: simple sugars (monosaccharides), polymeric compounds with a glycosidic bond - oligosaccharides and biopolymers with a large molecular weight - polysaccharides. Substances of the above classes are found in different types of cells. For example, starch and glucose are present in plant structures, glycogen in human hepatocytes and cell walls of fungi, chitin in the outer skeleton of arthropods. All of the above substances are carbohydrates. The role of carbohydrates in the body is universal. They are the main supplier of energy for the vital manifestations of plant cells, bacteria, animals and humans.

Monosaccharides

They have the general formula C _n H _{2 n} O _n and are divided into groups depending on the number of carbon atoms in the molecule: triose, tetroz, pentose and so on. In the cell organelles and the cytoplasm, simple sugars have two spatial configurations: cyclic and linear. In the first case, carbon atoms are connected to each other by covalent sigma bonds and form closed cycles, in the second case the carbon skeleton is not closed and can have branching. To determine what the role of carbohydrates in the body, let us consider the most common of them - pentoses and hexoses.

Isomers: glucose and fructose

They have the same molecular formula C ₆ H ₁₂ O ₆ , but different structural types of molecules. Earlier we already called the main role of carbohydrates in a living organism - energy. The above substances are cleaved by a cell. As a result, energy is released (17.6 kJ from one gram of glucose). In addition, 36 ATP molecules are synthesized. The decomposition of glucose occurs on the membranes (crystals) of mitochondria and is a chain of enzymatic reactions - the Krebs cycle. It is the most important link in the dissimilation that occurs in all cells of heterotrophic eukaryotic organisms, without exception.

Glucose is also formed in mammalian myocytes due to the cleavage in the muscle tissue of the glycogen reserve. In the future, it is used as an easily disintegrating substance, since providing cells with energy is the main role of carbohydrates in the body. Plants are phototrophs and independently form glucose in the process of photosynthesis. These reactions are called the Calvin cycle. The starting substance is carbon dioxide, and the acceptor is ribosezodiphosphate. The synthesis of glucose occurs in the matrix of chloroplasts. Fructose, having the same molecular formula as glucose, contains in the molecule a functional group of ketones. It is sweeter than glucose, and is found in honey, as well as the juice of berries and fruits. Thus, the biological role of carbohydrates in the body is primarily to use them as a quick source of energy.

The role of pentoses in heredity

Let us dwell on one more group of monosaccharides - ribose and deoxyribose. Their uniqueness lies in the fact that they are part of the polymers - nucleic acids. For all organisms, including non-cellular forms of life, DNA and RNA are the main carriers of hereditary information. Ribose is included in RNA molecules, and deoxyribose is contained in nucleotides of DNA. Consequently, the biological role of carbohydrates in the human body is that they participate in the formation of heredity units - genes and chromosomes.

Examples of pentoses containing an aldehyde group and common in the plant world are xylose (contained in stems and seeds), alpha-arabinose (found in gum of stone fruit trees). Thus, the distribution and biological role of carbohydrates in the body of higher plants is large enough.

What are oligosaccharides

If the residues of molecules of monosaccharides, for example, such as glucose or fructose, are bound by covalent bonds, then oligosaccharides - polymer carbohydrates are formed. The role of carbohydrates in the body of both plants and animals is diverse. This is especially true of disaccharides. The most common among them are sucrose, lactose, maltose and trehalose. So, sucrose, otherwise called reed or sugar beet, is contained in plants in the form of a solution and is stored in their root crops or stems. As a result of hydrolysis, glucose and fructose molecules are formed. Milk sugar, lactose, is of animal origin. In some people, intolerance to this substance is associated with hypoxecretion of the enzyme lactase, which splits milk sugar into galactose and glucose. The role of carbohydrates is vital. For example, trehalose disaccharide, consisting of two residues of glucose, is a part of the hemolymph of crustaceans, spiders, insects. It also occurs in the cells of fungi and some algae.

Another disaccharide, maltose, or malt sugar, is contained in grains of rye or barley during their germination, is a molecule consisting of two residues of glucose. It is formed as a result of the decay of vegetable or animal starch. In the small intestine of humans and mammals, maltose is cleaved by the action of an enzyme called maltase. If it is absent in pancreatic juice, a pathology arises due to intolerance in foodstuffs of glycogen or vegetable starch. In this case, use a special diet and add to the diet of the enzyme itself.

Complex carbohydrates in nature

They are distributed very widely, especially in the plant world, are biopolymers and have a large molecular mass. For example, in starch it is equal to 800 000, and in cellulose - 1 600 000. Polysaccharides differ in the composition of monomers, the degree of polymerization, and also the length of chains. Unlike simple sugars and oligosaccharides that dissolve well in water and have a sweetish taste, polysaccharides are hydrophobic and tasteless. Consider the role of carbohydrates in the human body using the example of glycogen - animal starch. It is synthesized from glucose and is reserved in hepatocytes and skeletal muscle cells, where its content is two times higher than in the liver. The formation of glycogen is also capable of subcutaneous fatty tissue, neurocytes and macrophages. Another polysaccharide, vegetable starch, is a product of photosynthesis and is formed in green plastids.

From the very beginning of human civilization, the main suppliers of starch were valuable agricultural crops: rice, potatoes, corn. They are still the basis of the dietary diet of the vast majority of the inhabitants of the Earth. That is why carbohydrates are so valuable. The role of carbohydrates in the body is, as we see, in their use as energy-intensive and rapidly digestible organic substances.

There is a group of polysaccharides whose monomers are the residues of hyaluronic acid. They are called pectins and are structural substances of plant cells. Especially rich are the rind of apples, the beet pulp. Cellular substances of pectin regulate intracellular pressure - turgor. In the confectionery industry, they are used as gelling agents and thickeners in the production of high-quality varieties of marshmallow and marmalade. In dietary nutrition, biologically active substances that excrete toxins from the large intestine are used.

What are glycolipids

This is an interesting group of complex compounds of carbohydrates and fats, which are in the nervous tissue. It consists of the head and spinal cord of mammals. Glycolipids are also found in cell membranes. For example, in bacteria they participate in intercellular contacts. Some of these compounds are antigens (substances that detect blood groups of the Landsteiner system AB0). In the cells of animals, plants and humans, in addition to glycolipids, there are also independent molecules of fats. They fulfill primarily the energy function. When splitting one gram of fat, 38.9 kJ energy is released. Lipids are also characterized by a structural function (they are part of the cell membranes). Thus, these functions are performed by carbohydrates and fats. Their role in the body is exceptionally high.

The role of carbohydrates and lipids in the body

In the cells of man and animals, there can be observed mutual conversions of polysaccharides and fats, which occur as a result of metabolism. Scientists, nutritionists found that excessive consumption of starchy food leads to the accumulation of fat. If a person has impaired pancreas in terms of allocation of amylase or leads a sedentary lifestyle, its weight can greatly increase. It is worth remembering that carbohydrate-rich food is split mainly in the duodenum to glucose. It is absorbed by capillaries of small intestinal villi and is deposited in the liver and muscles in the form of glycogen. The more intensive metabolism in the body, the more active it is cleaved to glucose. Then it is used by cells as the main energy material. This information serves as a response to the question of what role carbohydrates play in the human body.

The importance of glycoproteins

The compounds of this group of substances are represented by the complex carbohydrate + protein. They are also called glycoconjugates. These are antibodies, hormones, membrane structures. The latest biochemical studies have established that if glycoproteins begin to change their native (natural) structure, this leads to the development of such complex diseases as asthma, rheumatoid arthritis, cancer. The role of glycoconjugates in cell metabolism is great. So, interferons suppress the reproduction of viruses, immunoglobulins protect the body from pathogenic agents. Blood proteins also belong to this group of substances. They provide protective and buffer properties. All of the above functions confirms the fact that the physiological role of carbohydrates in the body is diverse and extremely important.

Where and how carbohydrates form

The main suppliers of simple and complex sugars are green plants: algae, higher spore, gymnosperms and flowering plants. They all contain pigment chlorophyll in the cells. It is a part of the thylakoids - chloroplast structures. Russian scientist K. A Timiryazev studied the process of photosynthesis, as a result of which carbohydrates are formed. The role of carbohydrates in the body of the plant is to accumulate starch in fruits, seeds and bulbs, that is, in vegetative organs. The mechanism of photosynthesis is quite complex and consists of a series of enzymatic reactions that occur both in the light and in the dark. Glucose is synthesized from carbon dioxide under the action of enzymes. Heterotrophic organisms use green plants as a source of food and energy. Thus, plants are the first link in all trophic chains and are called producers.

In cells of heterotrophic organisms, carbohydrates are synthesized on the channels of the smooth (agranular) endoplasmic reticulum. Then they are used as energy and building materials. In plant cells, carbohydrates are further formed in the Golgi complex, and then go on to form a cellulose cell wall. In the process of digestion of vertebrates, compounds rich in carbohydrates are partially broken down in the oral cavity and stomach. The main reactions of dissimilation occur in the duodenum. It is secreted pancreatic juice, containing an enzyme amylase, splitting starch to glucose. As already mentioned, glucose is absorbed into the blood in the small intestine and spreads throughout the cells. Here it is used as a source of energy and a structural substance. This explains the role of carbohydrates in the body.

Supramembrane complexes of heterotrophic cells

They are typical for animals and mushrooms. The chemical composition and molecular organization of these structures are represented by compounds such as lipids, proteins and carbohydrates. The role of carbohydrates in the body is participation in energy metabolism and membrane construction. In cells of man and animals there is a special structural component called glycocalysis. This thin surface layer consists of glycolipids and glycoproteins linked to the cytoplasmic membrane. It provides a direct connection of cells with the external environment. Here, there is a perception of irritation and extracellular digestion. Due to its carbohydrate membrane, the cells adhere to each other, forming tissues. This phenomenon is called adhesion. We also add that the "tails" of carbohydrate molecules are above the surface of the cell and are directed into the interstitial fluid.

Another group of heterotrophic organisms - fungi, also has a surface device called a cell wall. It includes complex sugars - chitin, glycogen. Some species of fungi also contain soluble carbohydrates, for example trehalose, called mushroom sugar.

In unicellular animals, such as ciliates, the surface layer - the pelicle, also contains complexes of oligosaccharides with proteins and lipids. Some of the simplest pellicles are thin enough and do not interfere with the change in the shape of the body. And in others it thickens and becomes firm, like a shell, performing a protective function.

Cellular wall of plants

It also contains a large amount of carbohydrates, especially cellulose, collected in the form of fiber bundles. These structures form a skeleton immersed in a colloidal matrix. It consists mainly of oligo- and polysaccharides. Cell walls of plant cells can be lignified. In this case, the intervals between the bundles of cellulose are filled with another carbohydrate - lignin. It strengthens the supporting functions of the cell membrane. Often, especially in perennial woody plants, the outer layer consisting of cellulose is covered with a fatty substance - suberin. It prevents water from entering the plant tissues, so the underlying cells die quickly and are covered with a plug.

Summarizing the above, we see that carbohydrates and fats are closely interrelated in the cell wall of plants. Their role in the body of phototrophs is difficult to underestimate, since glycolipid complexes provide support and protective functions. Let's study the variety of carbohydrates, characteristic for the organisms of the kingdom of Drobyanka. It includes prokaryotes, in particular bacteria. Their cell wall contains carbohydrate - murine. Depending on the structure of the surface apparatus, bacteria are divided into gram-positive and gram-negative.

The structure of the second group is more complex. These bacteria have two layers: plastic and rigid. The first contains mucopolysaccharides, for example, murine. Its molecules look like large mesh structures that form a capsule around the bacterial cell. The second layer consists of peptidoglycan, a compound of polysaccharides and proteins.

Lipopolysaccharides of the cell wall allow bacteria to firmly adhere to various substrates, for example, to the tooth enamel or to the membrane of eukaryotic cells. In addition, glycolipids promote the cohesion of bacterial cells among themselves. In this way, for example, chains of streptococci, clusters of staphylococci are formed, moreover, some types of prokaryotes have an additional mucosa - peplos. It contains polysaccharides in its composition and is easily destroyed by the action of hard radiation or by contact with some chemicals, for example antibiotics.