Of all the food substances consumed by humans, carbohydrates are undoubtedly the main source of energy. On average, they account for 50 to 70% of daily caloric intake. Despite the fact that a person consumes significantly more carbohydrates than fats and proteins, their reserves in the body are small. This means that the supply of them to the body must be regular.
The need for carbohydrates is very to a large extent depend on the energy expenditure of the body. On average, in an adult male, engaged mainly in mental or light physical labor, the daily need for carbohydrates ranges from 300 to 500 g. physical labor and athletes, it is much higher. Unlike proteins and, to a certain extent, fats, the amount of carbohydrates in diets can be significantly reduced without harm to health. Those who want to lose weight should pay attention to this: carbohydrates are mainly energy value. When 1 g of carbohydrates are oxidized in the body, 4.0 - 4.2 kcal is released. Therefore, at their expense, it is easiest to regulate the calorie intake.
Carbohydrates(saccharides) - the general name of a large class of natural organic compounds. The general formula of monosaccharides can be written as C n (H 2 O) n. In living organisms, sugars with 5 (pentoses) and 6 (hexoses) carbon atoms are most common.
Carbohydrates are divided into groups:
Simple carbohydrates are readily soluble in water and synthesized in green plants. Except small molecules, large ones are also found in the cell, they are polymers. Polymers are complex molecules that are made up of separate "units" connected to each other. Such "links" are called monomers. Substances such as starch, cellulose and chitin are polysaccharides - biological polymers.Monosaccharides include glucose and fructose, which add sweetness to fruits and berries. The food sugar sucrose consists of covalently attached to each other glucose and fructose. Sucrose-like compounds are called disaccharides. Poly-, di- and monosaccharides are called general term- carbohydrates. Carbohydrates are compounds that have diverse and often completely different properties.
Table: Variety of carbohydrates and their properties.
group of carbohydrates | Examples of carbohydrates | Where do they meet | properties |
monosugar | ribose | RNA | |
deoxyribose | DNA |
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glucose | beet sugar |
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fructose | Fruit, honey |
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galactose | The composition of milk lactose |
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oligosaccharides | maltose | malt sugar | Sweet in taste, soluble in water, crystalline, |
sucrose | Cane sugar |
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Lactose | Milk sugar in milk |
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Polysaccharides (built from linear or branched monosaccharides) | Starch |
Vegetable storage carbohydrate | Not sweet white color, do not dissolve in water. |
glycogen | Reserve animal starch in the liver and muscles |
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Fiber (cellulose) | |||
chitin | |||
murein | water . For many human cells (for example, brain and muscle cells), glucose brought in by the blood serves as the main source of energy. Starch and a very similar substance of animal cells - glycogen - are glucose polymers, they serve to store it inside the cell.
2. structural function, that is, they participate in the construction of various cellular structures. Polysaccharide cellulose forms cell walls plant cells, characterized by hardness and rigidity, it is one of the main components of wood. Other components are hemicellulose, also belonging to polysaccharides, and lignin (it has a non-carbohydrate nature). Chitin also performs structural functions. Chitin performs supporting and protective functions. The cell walls of most bacteria consist of murein peptidoglycan- the composition of this compound includes residues of both monosaccharides and amino acids. 3. Carbohydrates play a protective role in plants (cell walls, consisting of cell walls of dead cells, protective formations - spikes, spines, etc.). The general formula of glucose is C 6 H 12 O 6, it is an aldehyde alcohol. Glucose is found in many fruits, plant juices and flower nectar, as well as in the blood of humans and animals. The content of glucose in the blood is maintained at a certain level (0.65-1.1 g per l). If it is artificially lowered, then brain cells begin to experience acute starvation, which can result in fainting, coma, and even death. A long-term increase in blood glucose is also not at all useful: at the same time, diabetes mellitus develops. Mammals, including humans, can synthesize glucose from certain amino acids and breakdown products of glucose itself, such as lactic acid. They are unable to obtain glucose from fatty acids unlike plants and microbes. Interconversions of substances. Excess protein------carbohydrates Excess fat--------------carbohydrates
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Organic compounds that are the main source of energy are called carbohydrates. Most often sugars are found in food plant origin. A carbohydrate deficiency can cause liver dysfunction, and an excess of carbohydrates causes an increase in insulin levels. Let's talk more about sugars.
What are carbohydrates?
These are organic compounds that contain a carbonyl group and several hydroxyl groups. They are part of the tissues of organisms, and are also an important component of cells. Allocate mono -, oligo - and polysaccharides, as well as more complex carbohydrates such as glycolipids, glycosides and others. Carbohydrates are a product of photosynthesis, as well as the main starting material for the biosynthesis of other compounds in plants. Thanks to great variety This class of compounds is capable of playing multifaceted roles in living organisms. Being oxidized, carbohydrates provide energy to all cells. They are involved in the formation of immunity, and are also part of many cellular structures.
Types of sugars
Organic compounds are divided into two groups - simple and complex. Carbohydrates of the first type are monosaccharides that contain a carbonyl group and are derivatives of polyhydric alcohols. The second group includes oligosaccharides and polysaccharides. The former consist of monosaccharide residues (from two to ten), which are connected by a glycosidic bond. The latter may contain hundreds and even thousands of monomers. The table of carbohydrates that are most often found is as follows:
- Glucose.
- Fructose.
- Galactose.
- Sucrose.
- Lactose.
- Maltose.
- Raffinose.
- Starch.
- Cellulose.
- Chitin.
- Muramin.
- Glycogen.
The list of carbohydrates is extensive. Let's dwell on some of them in more detail.
Simple group of carbohydrates
Depending on the place occupied by the carbonyl group in the molecule, two types of monosaccharides are distinguished - aldoses and ketoses. In the former, the functional group is aldehyde, in the latter, ketone. Depending on the number of carbon atoms in the molecule, the name of the monosaccharide is formed. For example, aldohexoses, aldotetroses, ketotrioses, and so on. These substances are most often colorless, poorly soluble in alcohol, but well in water. Simple carbohydrates in foods are solid, not hydrolyzed during digestion. Some of the representatives have a sweet taste.
Group representatives
What is a simple carbohydrate? First, it is glucose, or aldohexose. It exists in two forms - linear and cyclic. Most accurately describes Chemical properties glucose is the second form. Aldohexose contains six carbon atoms. The substance has no color, but it tastes sweet. It is highly soluble in water. You can find glucose almost everywhere. It exists in the organs of plants and animals, as well as in fruits. In nature, aldohexose is formed during photosynthesis.
Secondly, it is galactose. The substance differs from glucose in the spatial arrangement of the hydroxyl and hydrogen groups at the fourth carbon atom in the molecule. Has a sweet taste. It is found in animals and plant organisms and also in some microorganisms.
And the third representative of simple carbohydrates is fructose. The substance is the sweetest sugar produced in nature. It is present in vegetables, fruits, berries, honey. Easily absorbed by the body, quickly excreted from the blood, which leads to its use by patients with diabetes mellitus. Fructose is low in calories and does not cause cavities.
Foods rich in simple sugars
- 90 g - corn syrup.
- 50 g - refined sugar.
- 40.5 g - honey.
- 24 g - figs.
- 13 g - dried apricots.
- 4 g - peaches.
daily intake given substance should not exceed 50 g. As for glucose, in this case the ratio will be slightly different:
- 99.9 g - refined sugar.
- 80.3 g - honey.
- 69.2 g - dates.
- 66.9 g - pearl barley.
- 61.8 g - oatmeal.
- 60.4 g - buckwheat.
To calculate the daily intake of a substance, you need to multiply the weight by 2.6. Simple sugars provide energy to the human body and help to cope with various toxins. But we must not forget that with any use there must be a measure, otherwise serious consequences will not be long in coming.
Oligosaccharides
The most common species in this group are disaccharides. What are carbohydrates containing multiple monosaccharides? They are glycosides containing monomers. Monosaccharides are linked by a glycosidic bond, which is formed as a result of the combination of hydroxyl groups. Based on the structure, disaccharides are divided into two types: reducing and non-reducing. The first is maltose and lactose, and the second is sucrose. The reducing type has good solubility and a sweet taste. Oligosaccharides may contain more than two monomers. If monosaccharides are the same, then such a carbohydrate belongs to the group of homopolysaccharides, and if different, then to heteropolysaccharides. An example of the latter type is the trisaccharide raffinose, which contains residues of glucose, fructose and galactose.
lactose, maltose and sucrose
The latter substance dissolves well, has a sweet taste. Sugar cane and beets are a source of disaccharide. In the body, hydrolysis breaks down sucrose into glucose and fructose. The disaccharide is found in large quantities in refined sugar (99.9 g per 100 g of product), in prunes (67.4 g), in grapes (61.5 g) and in other products. With an excess intake of this substance, the ability to turn almost all nutrients into fat increases. It also increases the level of cholesterol in the blood. A large amount of sucrose negatively affects the intestinal flora.
Milk sugar, or lactose, is found in milk and its derivatives. The carbohydrate is broken down into galactose and glucose by a special enzyme. If it is not in the body, then milk intolerance occurs. Malt sugar or maltose is an intermediate breakdown product of glycogen and starch. In foods, the substance is found in malt, molasses, honey and sprouted grains. The composition of lactose and maltose carbohydrates is represented by monomer residues. Only in the first case they are D-galactose and D-glucose, and in the second case the substance is represented by two D-glucoses. Both carbohydrates are reducing sugars.
Polysaccharides
What are complex carbohydrates? They differ from each other in several ways:
1. According to the structure of the monomers included in the chain.
2. By the order of finding monosaccharides in the chain.
3. According to the type of glycosidic bonds that connect the monomers.
As with oligosaccharides, homo- and heteropolysaccharides can be distinguished in this group. The first includes cellulose and starch, and the second - chitin, glycogen. Polysaccharides are an important source of energy, which is formed as a result of metabolism. They are involved in immune processes, as well as in the adhesion of cells in tissues.
The list of complex carbohydrates is represented by starch, cellulose and glycogen, we will consider them in more detail. One of the main suppliers of carbohydrates is starch. These are compounds that include hundreds of thousands of glucose residues. Carbohydrate is born and stored in the form of grains in the chloroplasts of plants. Through hydrolysis, starch is converted into water-soluble sugars, which facilitates free movement through the parts of the plant. Once in the human body, carbohydrate begins to break down already in the mouth. The largest amount of starch contains grains of cereals, tubers and bulbs of plants. In the diet, it accounts for about 80% of the total amount of carbohydrates consumed. The largest amount of starch, per 100 g of product, is found in rice - 78 g. Slightly less in pasta and millet - 70 and 69 g. One hundred grams of rye bread includes 48 g of starch, and in the same serving of potatoes its amount reaches only 15 g. Daily requirement human body in this carbohydrate is 330-450 g.
Grain products also contain fiber or cellulose. Carbohydrate is part of the cell walls of plants. His contribution is 40-50%. A person is not able to digest cellulose, so there is no necessary enzyme that would carry out the hydrolysis process. But the soft type of fiber, such as potatoes and vegetables, can be well absorbed in the digestive tract. What is the content of this carbohydrate in 100 g of food? Rye and wheat bran are the most fiber-rich foods. Their content reaches 44 g. Cocoa powder includes 35 g of nutritious carbohydrate, and dried mushrooms only 25. Rosehips and ground coffee contain 22 and 21 g. Some of the richest fruits in fiber are apricot and figs. The carbohydrate content in them reaches 18 g. A person needs to eat up to 35 g of cellulose per day. Moreover, the greatest need for carbohydrate occurs at the age of 14 to 50 years.
Glycogen polysaccharide is used as an energy material for the good functioning of muscles and organs. It has no nutritional value, since its content in food is extremely low. The carbohydrate is sometimes called animal starch because of the similarity in structure. In this form, glucose is stored in animal cells (in the largest amount in the liver and muscles). In the liver in adults, the amount of carbohydrate can reach up to 120 g. The leaders in glycogen content are sugar, honey and chocolate. Dates, raisins, marmalade, sweet straws, bananas, watermelon, persimmons and figs can also boast of a high carbohydrate content. The daily norm of glycogen is 100 g per day. If a person is actively involved in sports or performs great job associated with mental activity, the amount of carbohydrate should be increased. Glycogen refers to easily digestible carbohydrates that are stored in reserve, which indicates its use only in case of a lack of energy from other substances.
Polysaccharides also include the following substances:
1. Chitin. It is part of the corneas of arthropods, is present in fungi, lower plants and invertebrates. The substance plays the role of a support material, and also performs mechanical functions.
2. Muramine. It is present as a support-mechanical material of the bacterial cell wall.
3. Dextrans. Polysaccharides act as substitutes for blood plasma. They are obtained by the action of microorganisms on a solution of sucrose.
4. pectin substances. Together with organic acids, they can form jelly and marmalade.
Proteins and carbohydrates. Products. List
The human body needs a certain amount of nutrients every day. For example, carbohydrates should be consumed at the rate of 6-8 g per 1 kg of body weight. If a person leads an active lifestyle, then the number will increase. Carbohydrates are almost always found in foods. Let's make a list of their presence per 100 g of food:
- The largest amount (more than 70 g) is found in sugar, muesli, marmalade, starch and rice.
- From 31 to 70 g - in flour and confectionery products, in pasta, cereals, dried fruits, beans and peas.
- Bananas, ice cream, rose hips, potatoes, tomato paste, compotes, coconut, sunflower seeds and cashew nuts contain 16 to 30 g of carbohydrates.
- From 6 to 15 g - in parsley, dill, beets, carrots, gooseberries, currants, beans, fruits, nuts, corn, beer, pumpkin seeds, dried mushrooms and so on.
- Up to 5 g of carbohydrates are found in green onions, tomatoes, zucchini, pumpkins, cabbage, cucumbers, cranberries, dairy products, eggs, and so on.
Nutrient should not enter the body less than 100 g per day. Otherwise, the cell will not receive the energy it needs. The brain will not be able to perform its functions of analysis and coordination, therefore, the muscles will not receive commands, which will eventually lead to ketosis.
What are carbohydrates, we told, but, in addition to them, proteins are an indispensable substance for life. They are a chain of amino acids linked peptide bond. Depending on the composition, proteins differ in their properties. For example, these substances play the role of a building material, since each cell of the body includes them in its composition. Some types of proteins are enzymes and hormones, as well as a source of energy. They influence the development and growth of the body, regulate the acid-base and water balance.
The table of carbohydrates in food showed that in meat and fish, as well as in some types of vegetables, their number is minimal. What is the content of proteins in food? The richest product is food gelatin, it contains 87.2 g of the substance per 100 g. Next comes mustard (37.1 g) and soy (34.9 g). The ratio of proteins and carbohydrates in daily intake per 1 kg of weight should be 0.8 g and 7 g. For better absorption of the first substance, it is necessary to take food in which it takes light form. This applies to proteins that are present in dairy products and eggs. Proteins and carbohydrates do not combine well in one meal. The table on separate nutrition shows which variations are best avoided:
- Rice with fish.
- Potatoes and chicken.
- Pasta and meat.
- Sandwiches with cheese and ham.
- Breaded fish.
- Walnut cakes.
- Omelet with ham.
- Flour with berries.
- Melon and watermelon should be eaten separately an hour before the main meal.
Match well:
- Meat with salad.
- Fish with vegetables or grilled.
- Cheese and ham separately.
- Nuts in general.
- Omelet with vegetables.
The rules of separate nutrition are based on knowledge of the laws of biochemistry and information about the work of enzymes and food juices. For good digestion, any kind of food requires an individual set of gastric fluids, a certain amount of water, an alkaline or acidic environment, and the presence or absence of enzymes. For example, a meal rich in carbohydrates, for better digestion, requires digestive juice with alkaline enzymes that break down these organic substances. But food rich in proteins already requires acidic enzymes... By following the simple rules of food compliance, a person strengthens his health and maintains a constant weight, without the help of diets.
"Bad" and "good" carbohydrates
"Fast" (or "wrong") substances are compounds that contain a small number of monosaccharides. Such carbohydrates are able to be quickly digested, increase blood sugar levels, and also increase the amount of insulin secreted. The latter lowers blood sugar levels by converting it into fat. The use of carbohydrates after dinner for a person who monitors his weight is the greatest danger. At this time, the body is most predisposed to an increase in fat mass. What exactly contains the wrong carbohydrates? Products listed below:
1. Confectionery.
3. Jam.
4. Sweet juices and compotes.
7. Potatoes.
8. Pasta.
9. White rice
10. Chocolate.
Basically, these are products that do not require long preparation. After such a meal, you need to move a lot, otherwise excess weight will let you know.
"Proper" carbohydrates contain more than three simple monomers. They are absorbed slowly and do not cause a sharp rise in sugar. This type of carbohydrate contains a large number of fiber, which is practically indigestible. In this regard, a person remains full for a long time, additional energy is required for the breakdown of such food, in addition, a natural cleansing of the body occurs. Let's make a list of complex carbohydrates, or rather, the products in which they are found:
- Bread with bran and whole grains.
- Buckwheat and oatmeal.
- Green vegetables.
- Coarse pasta.
- Mushrooms.
- Peas.
- Red beans.
- Tomatoes.
- Milk products.
- Fruits.
- Bitter chocolate.
- Berries.
- Lentils.
To keep yourself in good shape, you need to eat more "good" carbohydrates in foods and as few "bad" ones as possible. The latter are best taken in the first half of the day. If you need to lose weight, it is better to exclude the use of "wrong" carbohydrates, since when using them, a person receives food in more. The "right" nutrients are low in calories and can keep you feeling full for a long time. This does not mean a complete rejection of "bad" carbohydrates, but only their reasonable use.
CARBOHYDRATES
Carbohydrates are part of the cells and tissues of all plant and animal organisms and, by mass, make up the bulk of the organic matter on Earth. Carbohydrates account for about 80% of the dry matter of plants and about 20% of animals. Plants synthesize carbohydrates from inorganic compounds - carbon dioxide and water (CO 2 and H 2 O).
Carbohydrates are divided into two groups: monosaccharides (monoses) and polysaccharides (polyoses).
Monosaccharides
For a detailed study of the material related to the classification of carbohydrates, isomerism, nomenclature, structure, etc., you need to watch the animated films "Carbohydrates. Genetic D - a series of sugars" and "Construction of Haworth's formulas for D - galactose" (this video is only available on CD-ROM ). The texts accompanying these films, in in full moved to this subsection and follow below.
Carbohydrates. Genetic D-series of sugars
"Carbohydrates are widely distributed in nature and perform various important functions in living organisms. They supply energy for biological processes, and are also the starting material for the synthesis of other intermediate or final metabolites in the body. Carbohydrates have general formula C n (H 2 O ) m from which the name of these natural compounds originated.
Carbohydrates are divided into simple sugars or monosaccharides and polymers of these simple sugars or polysaccharides. Among polysaccharides, a group of oligosaccharides containing from 2 to 10 monosaccharide residues in a molecule should be distinguished. These include, in particular, disaccharides.
Monosaccharides are heterofunctional compounds. Their molecules simultaneously contain both carbonyl (aldehyde or ketone) and several hydroxyl groups, i.e. monosaccharides are polyhydroxycarbonyl compounds - polyhydroxyaldehydes and polyhydroxyketones. Depending on this, monosaccharides are divided into aldoses (the monosaccharide contains an aldehyde group) and ketoses (the keto group is contained). For example, glucose is an aldose and fructose is a ketose.
(glucose (aldose))
(fructose (ketose))
Depending on the number of carbon atoms in the molecule, the monosaccharide is called tetrose, pentose, hexose, etc. If we combine the last two types of classification, then glucose is aldohexose, and fructose is ketohexose. Most naturally occurring monosaccharides are pentoses and hexoses.
Monosaccharides are depicted in the form of Fisher projection formulas, i.e. in the form of a projection of the tetrahedral model of carbon atoms on the plane of the drawing. The carbon chain in them is written vertically. In aldoses, the aldehyde group is placed at the top, in ketoses, the primary alcohol group adjacent to the carbonyl group. The hydrogen atom and the hydroxyl group at the asymmetric carbon atom are placed on a horizontal line. An asymmetric carbon atom is located in the resulting crosshairs of two straight lines and is not indicated by a symbol. From the groups located at the top, the numbering of the carbon chain begins. (Let's define an asymmetric carbon atom: it is a carbon atom bonded to four different atoms or groups.)
Establishing an absolute configuration, i.e. the true arrangement in space of substituents at an asymmetric carbon atom is a very laborious, and until some time it was even an impossible task. It is possible to characterize compounds by comparing their configurations with those of reference compounds, i.e. define relative configurations.
The relative configuration of monosaccharides is determined by the configuration standard - glyceraldehyde, to which, at the end of the last century, certain configurations were arbitrarily assigned, designated as D- and L - glyceraldehydes. The configuration of the asymmetric carbon atom of the monosaccharide furthest from the carbonyl group is compared with the configuration of their asymmetric carbon atoms. In pentoses, this atom is the fourth carbon atom ( From 4 ), in hexoses - the fifth ( From 5 ), i.e. penultimate in the chain of carbon atoms. If the configuration of these carbon atoms coincides with the configuration D - glyceraldehyde monosaccharide is classified as D - in a row. And vice versa, if it matches the configuration L - glyceraldehyde consider that the monosaccharide belongs to L - row. Symbol D means that the hydroxyl group at the corresponding asymmetric carbon atom in the Fischer projection is located to the right of vertical line, and the symbol L - that the hydroxyl group is located on the left.
Genetic D-series of sugars
The ancestor of aldose is glyceraldehyde. Consider the genetic relationship of sugars D - row with D - glyceraldehyde.
In organic chemistry, there is a method for increasing the carbon chain of monosaccharides by successively introducing a group
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N– |
I |
-IS HE |
between the carbonyl group and the adjacent carbon atom. Introduction of this group into the molecule D - glyceraldehyde leads to two diastereomeric tetroses - D - erythrosis and D - treose. This is due to the fact that a new carbon atom introduced into the monosaccharide chain becomes asymmetric. For the same reason, each tetrose obtained, and then pentose, when one more carbon atom is introduced into their molecule, also gives two diastereomeric sugars. Diastereomers are stereoisomers that differ in the configuration of one or more asymmetric carbon atoms.
This is how D is obtained - a series of sugars from D - glyceraldehyde. As can be seen, all members of the above series, being obtained from D - glyceraldehyde, retained its asymmetric carbon atom. This is the last asymmetric carbon atom in the chain of carbon atoms of the presented monosaccharides.
Each aldose D -number corresponds to a stereoisomer L - a series whose molecules relate to each other as an object and an incompatible mirror image. Such stereoisomers are called enantiomers.
It should be noted in conclusion that the above series of aldohexoses is not limited to the four shown. As shown above, from D - ribose and D - xylose, you can get two more pairs of diastereomeric sugars. However, we focused only on aldohexoses, which are most common in nature.
Construction of Haworth formulas for D-galactose
"At the same time as introducing organic chemistry ideas about the structure of glucose and other monosaccharides as polyhydroxy aldehydes or polyhydroxy ketones, described by open-chain formulas, facts began to accumulate in the chemistry of carbohydrates that were difficult to explain from the standpoint of such structures. It turned out that glucose and other monosaccharides exist in the form of cyclic hemiacetals formed as a result of an intramolecular reaction of the corresponding functional groups.
Ordinary hemiacetals are formed by the interaction of molecules of two compounds - an aldehyde and an alcohol. During the reaction, the double bond of the carbonyl group is broken, at the place of the break, the hydrogen atom of the hydroxyl and the remainder of the alcohol are added to it. Cyclic hemiacetals are formed due to the interaction of similar functional groups belonging to the molecule of one compound - a monosaccharide. The reaction proceeds in the same direction: the double bond of the carbonyl group is broken, the hydrogen atom of the hydroxyl is added to the carbonyl oxygen, and a cycle is formed due to the binding of carbon atoms of the carbonyl and oxygen of the hydroxyl groups.
The most stable hemiacetals are formed by hydroxyl groups at the fourth and fifth carbon atoms. The resulting five-membered and six-membered rings are called the furanose and pyranose forms of monosaccharides, respectively. These names come from the names of five- and six-membered heterocyclic compounds with an oxygen atom in the cycle - furan and pyran.
Monosaccharides that have a cyclic form are conveniently represented by Haworth's promising formulas. They are idealized planar five- and six-membered rings with an oxygen atom in the ring, making it possible to see the mutual arrangement of all substituents relative to the plane of the ring.
Consider the construction of Haworth formulas using the example D - galactose.
To construct the Haworth formulas, it is first necessary to number the carbon atoms of the monosaccharide in the Fisher projection and turn it to the right so that the chain of carbon atoms takes a horizontal position. Then the atoms and groups located in the projection formula on the left will be at the top, and those located on the right - below the horizontal line, and with a further transition to cyclic formulas - above and below the plane of the cycle, respectively. In reality, the carbon chain of a monosaccharide is not located in a straight line, but takes a curved shape in space. As can be seen, the hydroxyl at the fifth carbon atom is significantly removed from the aldehyde group; occupies a position unfavorable for closing the ring. To bring the functional groups closer together, a part of the molecule is rotated around the valence axis connecting the fourth and fifth carbon atoms counterclockwise by one valence angle. As a result of this rotation, the hydroxyl of the fifth carbon atom approaches the aldehyde group, while the other two substituents also change their position - in particular, the CH 2 OH group is located above the chain of carbon atoms. At the same time, the aldehyde group, due to rotation around s - the bond between the first and second carbon atoms approaches the hydroxyl. The approached functional groups interact with each other according to the above scheme, leading to the formation of a hemiacetal with a six-membered pyranose ring.
The resulting hydroxyl group is called a glycosidic group. The formation of a cyclic hemiacetal leads to the appearance of a new asymmetric carbon atom, called anomeric. As a result, two diastereomers are formed - a-and b - anomers differing only in the configuration of the first carbon atom.
The various configurations of the anomeric carbon atom result from the fact that the aldehyde group, which has a planar configuration, due to rotation around s - links between lanes with the first and second carbon atoms refers to the attacking reagent (hydroxyl group) both on one and opposite sides of the plane. The hydroxyl group attacks the carbonyl group from either side. double bond, leading to hemiacetals with different configurations of the first carbon atom. In other words, the main reason for the simultaneous formation a-and b -anomers lies in the non-stereoselectivity of the discussed reaction.
a - anomer, the configuration of the anomeric center is the same as the configuration of the last asymmetric carbon atom, which determines belonging to D - and L - in a row, and b - anomer - opposite. In aldopentosis and aldohexosis D - series in Haworth's formulas glycosidic hydroxyl group y a - anomer is located under the plane, and y b - anomers - above the plane of the cycle.
According to similar rules, the transition to the furanose forms of Haworth is carried out. The only difference is that the hydroxyl of the fourth carbon atom is involved in the reaction, and for the convergence of functional groups, it is necessary to rotate part of the molecule around s - bonds between the third and fourth carbon atoms and clockwise, as a result of which the fifth and sixth carbon atoms will be located under the plane of the cycle.
The names of the cyclic forms of monosaccharides include indications of the configuration of the anomeric center ( a - or b -), the name of the monosaccharide and its series ( D - or L -) and cycle size (furanose or pyranose). For example , a , D - galactopyranose or b, D - galactofuranose."
Receipt
Glucose is predominantly found in free form in nature. She is also structural unit many polysaccharides. Other monosaccharides in the free state are rare and are mainly known as components of oligo- and polysaccharides. In nature, glucose is obtained as a result of photosynthesis reaction:
6CO 2 + 6H 2 O ® C 6 H 12 O 6 (glucose) + 6O 2
For the first time, glucose was obtained in 1811 by the Russian chemist G.E. Kirchhoff during the hydrolysis of starch. Later, the synthesis of monosaccharides from formaldehyde in an alkaline medium was proposed by A.M. Butlerov.
In industry, glucose is obtained by hydrolysis of starch in the presence of sulfuric acid.
(C 6 H 10 O 5) n (starch) + nH 2 O -– H 2 SO 4,t ° ® nC 6 H 12 O 6 (glucose)
Physical properties
Monosaccharides - solids, easily soluble in water, poorly - in alcohol and completely insoluble in ether. Aqueous solutions have a neutral reaction to litmus. Most monosaccharides have a sweet taste, but less than beet sugar.
Chemical properties
Monosaccharides exhibit the properties of alcohols and carbonyl compounds.
I. Reactions at the carbonyl group
1. Oxidation.
a) As with all aldehydes, oxidation of monosaccharides leads to the corresponding acids. So, when glucose is oxidized with an ammonia solution of silver hydroxide, gluconic acid is formed (the "silver mirror" reaction).
b) The reaction of monosaccharides with copper hydroxide when heated also leads to aldonic acids.
c) Stronger oxidizing agents oxidize not only the aldehyde group, but also the primary alcohol group into the carboxyl group, leading to dibasic sugar (aldaric) acids. Typically, concentrated nitric acid is used for this oxidation.
2. Recovery.
The reduction of sugars leads to polyhydric alcohols. Hydrogen in the presence of nickel, lithium aluminum hydride, etc. are used as a reducing agent.
3. Despite the similarity of the chemical properties of monosaccharides with aldehydes, glucose does not react with sodium hydrosulfite ( NaHSO3).
II. Reactions on hydroxyl groups
Reactions on the hydroxyl groups of monosaccharides are carried out, as a rule, in the hemiacetal (cyclic) form.
1. Alkylation (formation of ethers).
Under the action of methyl alcohol in the presence of gaseous hydrogen chloride, the hydrogen atom of the glycosidic hydroxyl is replaced by a methyl group.
When using stronger alkylating agents, such as For example , methyl iodide or dimethyl sulfate, such a transformation affects all hydroxyl groups of the monosaccharide.
2. Acylation (formation esters).
When acetic anhydride acts on glucose, an ester is formed - pentaacetylglucose.
3. Like all polyhydric alcohols, glucose with copper hydroxide ( II ) gives an intense blue color (qualitative reaction).
III. Specific reactions
In addition to the above, glucose is also characterized by some specific properties- fermentation processes. Fermentation is the breakdown of sugar molecules under the influence of enzymes (enzymes). Sugars with a multiple of three carbon atoms are fermented. There are many types of fermentation, among which the most famous are the following:
a) alcoholic fermentation
C 6 H 12 O 6 ® 2CH 3 -CH 2 OH (ethyl alcohol) + 2CO 2
b) lactic fermentation
c) butyric fermentation
C6H12O6® CH 3 -CH 2 -CH 2 -COOH(butyric acid) + 2 H 2 + 2CO 2
The mentioned types of fermentation caused by microorganisms are of wide practical importance. For example, alcohol - for the production of ethyl alcohol, in winemaking, brewing, etc., and lactic acid - for the production of lactic acid and fermented milk products.
disaccharides
Disaccharides (bioses) upon hydrolysis form two identical or different monosaccharides. To establish the structure of disaccharides, it is necessary to know: from which monosaccharides it is built, what is the configuration of the anomeric centers in these monosaccharides ( a - or b -), what are the sizes of the ring (furanose or pyranose) and with the participation of which hydroxyls two monosaccharide molecules are linked.
Disaccharides are divided into two groups: reducing and non-reducing.
Reducing disaccharides include, in particular, maltose (malt sugar) contained in malt, i. sprouted, and then dried and crushed grains of cereals.
(maltose)
Maltose is made up of two residues D - glucopyranoses, which are linked by a (1–4) -glycosidic bond, i.e. the glycosidic hydroxyl of one molecule and the alcohol hydroxyl at the fourth carbon atom of another monosaccharide molecule participate in the formation of an ether bond. An anomeric carbon atom ( From 1 ) participating in the formation of this bond has a - configuration, and an anomeric atom with a free glycosidic hydroxyl (indicated in red) can have both a - (a - maltose) and b - configuration (b - maltose).
Maltose is a white crystal, highly soluble in water, sweet in taste, but much less than that of sugar (sucrose).
As can be seen, maltose contains a free glycosidic hydroxyl, as a result of which the ability to open the ring and transfer to the aldehyde form is retained. In this regard, maltose is able to enter into reactions characteristic of aldehydes, and, in particular, to give the "silver mirror" reaction, therefore it is called a reducing disaccharide. In addition, maltose enters into many reactions characteristic of monosaccharides, For example , forms ethers and esters (see chemical properties of monosaccharides).
Non-reducing disaccharides include sucrose (beet or canesugar). It is found in sugar cane, sugar beets (up to 28% of dry matter), plant juices and fruits. The sucrose molecule is made up of a , D - glucopyranose and b, D - fructofuranoses.
(sucrose)
In contrast to maltose, the glycosidic bond (1–2) between monosaccharides is formed at the expense of the glycosidic hydroxyls of both molecules, that is, there is no free glycosidic hydroxyl. As a result, there is no reducing ability of sucrose, it does not give the "silver mirror" reaction, therefore it is referred to as non-reducing disaccharides.
Sucrose - white crystalline substance, sweet in taste, well soluble in water.
Sucrose is characterized by reactions on hydroxyl groups. Like all disaccharides, sucrose in acid or enzymatic hydrolysis turns into monosaccharides, of which it is composed.
Polysaccharides
The most important polysaccharides are starch and cellulose (fiber). They are built from glucose residues. The general formula for these polysaccharides ( C 6 H 10 O 5 n . Glycosidic (at C 1 -atom) and alcohol (at C 4 -atom) hydroxyls usually take part in the formation of polysaccharide molecules, i.e. a (1–4)-glycosidic bond is formed.
Starch
Starch is a mixture of two polysaccharides built from a , D - glucopyranose links: amylose (10-20%) and amylopectin (80-90%). Starch is formed in plants during photosynthesis and is deposited as a "reserve" carbohydrate in roots, tubers and seeds. For example, grains of rice, wheat, rye and other cereals contain 60-80% starch, potato tubers - 15-20%. A related role in the animal world is played by the polysaccharide glycogen, which is "stored" mainly in the liver.
Starch is a white powder consisting of small grains, insoluble in cold water. When processing starch warm water it is possible to isolate two fractions: the fraction soluble in warm water and consisting of amylose polysaccharide, and a fraction that only swells in warm water to form a paste and consists of amylopectin polysaccharide.
Amylose has a linear structure, a , D - glucopyranose residues are linked by (1–4)-glycosidic bonds. The elemental cell of amylose (and starch in general) is represented as follows:
The amylopectin molecule is built In a similar way, however, has branching in the chain, which creates spatial structure. At branch points, monosaccharide residues are linked by (1–6)-glycosidic bonds. Between the branch points are usually 20-25 glucose residues.
(amylopectin)
Starch easily undergoes hydrolysis: when heated in the presence of sulfuric acid, glucose is formed.
(C 6 H 10 O 5 ) n (starch) + nH 2 O –– H 2 SO 4, t ° ® nC 6 H 12 O 6 (glucose)
Depending on the reaction conditions, hydrolysis can be carried out stepwise with the formation of intermediate products.
(C 6 H 10 O 5 ) n (starch) ® (C 6 H 10 O 5 ) m (dextrins (m< n )) ® xC 12 H 22 O 11 (мальтоза) ® nC 6 H 12 O 6 (глюкоза)
A qualitative reaction to starch is its interaction with iodine - an intense blue color is observed. Such staining appears if a drop of iodine solution is placed on a slice of potato or a slice of white bread.
Starch does not enter into the "silver mirror" reaction.
Starch is a valuable food product. To facilitate its absorption, products containing starch are subjected to heat treatment, i.e. potatoes and cereals are boiled, bread is baked. The processes of dextrinization (the formation of dextrins) carried out in this case contribute to better absorption of starch by the body and subsequent hydrolysis to glucose.
In the food industry, starch is used in the production of sausages, confectionery and culinary products. It is also used to obtain glucose, in the manufacture of paper, textiles, adhesives, medicines, etc.
Cellulose (fiber)
Cellulose is the most common plant polysaccharide. It has great mechanical strength and acts as a supporting material for plants. Wood contains 50-70% cellulose, cotton is almost pure cellulose.
Like starch, the structural unit of cellulose is D - glucopyranose, the links of which are connected by (1-4) -glycosidic bonds. However, cellulose is different from starch. b - the configuration of glycosidic bonds between cycles and a strictly linear structure.
Cellulose consists of filamentous molecules, which are assembled into bundles by hydrogen bonds of hydroxyl groups within the chain, as well as between adjacent chains. It is this chain packing that provides high mechanical strength, fiber content, water insolubility, and chemical inertness, which makes cellulose an ideal material for building cell walls.
b - The glycosidic bond is not destroyed by human digestive enzymes, so cellulose cannot serve as food for him, although in a certain amount it is a ballast substance necessary for normal nutrition. Ruminant animals have cellulose-digesting enzymes in their stomachs, so ruminant animals use fiber as a food component.
Despite the insolubility of cellulose in water and common organic solvents, it is soluble in Schweitzer's reagent (a solution of copper hydroxide in ammonia), as well as in concentrated solution zinc chloride and concentrated sulfuric acid.
Like starch, cellulose acid hydrolysis gives glucose.
Cellulose is a polyhydric alcohol; there are three elements per unit cell of the polymer. hydroxyl groups. In this regard, cellulose is characterized by esterification reactions (the formation of esters). Reactions with nitric acid and acetic anhydride.
Fully esterified fiber is known as pyroxylin, which, after appropriate processing, turns into smokeless powder. Depending on the nitration conditions, cellulose dinitrate can be obtained, which is called colloxylin in the technique. It is also used in the manufacture of gunpowder and solid propellants. In addition, celluloid is made on the basis of colloxylin.
Triacetylcellulose (or cellulose acetate) is a valuable product for the manufacture of non-combustible film and acetate silk. To do this, cellulose acetate is dissolved in a mixture of dichloromethane and ethanol, and this solution is forced through spinnerets into a stream of warm air. The solvent evaporates and the streams of the solution turn into the thinnest threads of acetate silk.
Cellulose does not give a "silver mirror" reaction.
Speaking about the use of cellulose, one cannot but say that a large amount of cellulose is consumed for the manufacture of various papers. Paper is a thin layer of fiber fibers, glued and pressed on a special paper machine.
From the above, it is already clear that the use of cellulose by humans is so wide and varied that an independent section can be devoted to the use of products of chemical processing of cellulose.
END OF SECTION
The chemical properties of the cells that make up living organisms depend primarily on the number of carbon atoms that make up up to 50% of the dry mass. Carbon atoms are in the main organic matter: proteins, nucleic acids, lipids and carbohydrates. The latter group includes carbon and water compounds 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 phosphorus, nitrogen, and sulfur atoms. 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 various types of cells. For example, starch and glucose are found in plant structures, glycogen in human hepatocytes and fungal cell walls, and chitin in the external skeleton of arthropods. All of the above are carbohydrates. The role of carbohydrates in the body is universal. They are the main supplier of energy for the vital manifestations of 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: trioses, tetroses, pentoses, and so on. In the composition of cell organelles and 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 may have branches. To determine the role of carbohydrates in the body, 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 views molecules. We have previously called leading role carbohydrates in a living organism - energy. The above substances are broken down by the cell. As a result, energy is released (17.6 kJ from one gram of glucose). In addition, 36 ATP molecules. The breakdown of glucose occurs on the membranes (cristae) 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 breakdown of glycogen stores in muscle tissue. In the future, it is used as an easily decomposing substance, since providing cells with energy is the main role of carbohydrates in the body. Plants are phototrophs and produce their own glucose during photosynthesis. These reactions are called the Calvin cycle. starting material is carbon dioxide, and the acceptor is ribolesodiphosphate. Glucose synthesis occurs in the chloroplast matrix. Fructose, having the same molecular formula as glucose, contains in the molecule functional group ketones. It is sweeter than glucose and is found in honey, as well as the juice of berries and fruits. Thus, biological role 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 polymers - nucleic acids. For all organisms, including non-cellular life forms, DNA and RNA are the main carriers hereditary information. Ribose is found in RNA molecules, while deoxyribose is found in DNA nucleotides. Consequently, the biological role of carbohydrates in the human body is that they are involved in the formation of units of heredity - genes and chromosomes.
Examples of pentoses containing an aldehyde group and common in flora, are xylose (found in stems and seeds), alpha-arabinose (found in the gum of stone fruit trees). Thus, the distribution and biological role of carbohydrates in the body higher plants are big enough.
What are oligosaccharides
If the residues of monosaccharide molecules, such as glucose or fructose, are linked covalent bonds, then oligosaccharides are formed - polymeric carbohydrates. The role of carbohydrates in the body of both plants and animals is diverse. This is especially true for disaccharides. The most common among them are sucrose, lactose, maltose and trehalose. So, sucrose, otherwise called cane, or is found in plants in the form of a solution and is stored in their roots or stems. As a result of hydrolysis, glucose and fructose molecules are formed. is of animal origin. Some people have intolerance to this substance, associated with hyposecretion of the enzyme lactase, which breaks down milk sugar into galactose and glucose. The role of carbohydrates in the life of the body is diverse. For example, the disaccharide trehalose, consisting of two glucose residues, is part of the hemolymph of crustaceans, spiders, and insects. It is also found in the cells of fungi and some algae.
Another disaccharide - maltose, or malt sugar, is found in rye or barley grains during their germination, is a molecule consisting of two glucose residues. It is formed as a result of the breakdown of vegetable or animal starch. In the small intestine of humans and mammals, maltose is broken down by the action of the enzyme maltase. In its absence in the pancreatic juice, a pathology occurs due to intolerance to glycogen or vegetable starch in foods. In this case, a special diet is used and the enzyme itself is added to the diet.
Complex carbohydrates in nature
They are very widespread, especially in the plant kingdom, they are biopolymers and have a large molecular weight. For example, in starch it is 800,000, and in cellulose it is 1,600,000. Polysaccharides differ in their monomer composition, degree of polymerization, and chain length. Unlike simple sugars and oligosaccharides, which 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 stored in hepatocytes and skeletal muscle cells, where its content is twice as high as in the liver. Subcutaneous adipose tissue, neurocytes and macrophages are also capable of forming glycogen. 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 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 quickly digestible organic substances.
There is a group of polysaccharides whose monomers are hyaluronic acid residues. They are called pectins and are structural substances of plant cells. The peel of apples, beet pulp are especially rich in them. Cellular substances pectins regulate intracellular pressure - turgor. In the confectionery industry, they are used as gelling agents and thickeners in the production of high-quality varieties of marshmallows and marmalade. AT diet food used as a biological active substances well removing toxins from the large intestine.
What are glycolipids
This is interesting group complex compounds carbohydrates and fats found in the nervous tissue. It consists of the brain and spinal cord of mammals. Glycolipids are also found in cell membranes. For example, in bacteria, they participate in some of these compounds are antigens (substances that reveal the blood groups of the Landsteiner AB0 system). In the cells of animals, plants and humans, in addition to glycolipids, there are also independent fat molecules. They perform primarily energy function. When splitting one gram of fat, 38.9 kJ of energy is released. Lipids are also characterized by a structural function (they are part of cell membranes). Thus, these functions are performed by carbohydrates and fats. Their role in the body is exceptionally great.
The role of carbohydrates and lipids in the body
In human and animal cells, mutual transformations of polysaccharides and fats occurring as a result of metabolism can be observed. Diet scientists have found that excessive consumption of starchy foods leads to fat accumulation. If a person has a violation of the pancreas in terms of the release of amylase or leads a sedentary lifestyle, his weight can increase greatly. It is worth remembering that carbohydrate-rich foods are broken down mainly in the duodenum to glucose. It is absorbed by the capillaries of the villi of the small intestine and deposited in the liver and muscles in the form of glycogen. The more intense the metabolism in the body, the more actively it breaks down to glucose. Then it is used by cells as the main energy material. This information serves as an answer to the question of what role carbohydrates play in the human body.
The value of glycoproteins
Compounds of this group of substances are represented by a carbohydrate + protein complex. 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, and 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 are confirmed by the fact that the physiological role of carbohydrates in the body is diverse and extremely important.
Where and how are carbohydrates formed?
The main suppliers of simple and complex sugars are green plants: algae, higher spores, gymnosperms and flowering plants. All of them contain the pigment chlorophyll in their cells. It is part of the thylakoids - the structures of chloroplasts. The Russian scientist K. A. Timiryazev studied the process of photosynthesis, which results in the formation of carbohydrates. The role of carbohydrates in the plant body is the accumulation of 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 occurring both in the light and in the dark. Glucose is synthesized from carbon dioxide by the action of enzymes. Heterotrophic organisms use green plants as a source of food and energy. Thus, it is plants that are the first link in all and are called producers.
In the cells of heterotrophic organisms, carbohydrates are synthesized on the channels of the smooth (agranular) endoplasmic reticulum. Then they are used as energy and building material. In plant cells, carbohydrates are additionally formed in the Golgi complex, and then go to the formation of the cellulose cell wall. In the process of digestion of vertebrates, carbohydrate-rich compounds are partially broken down into oral cavity and stomach. The main dissimilation reactions occur in the duodenum. It secretes pancreatic juice, which contains the enzyme amylase, which breaks down starch into glucose. As mentioned earlier, glucose is absorbed into the blood in the small intestine and is carried to all cells. Here it is used as a source of energy and structural substance. This explains the role carbohydrates play in the body.
Supramembrane complexes of heterotrophic cells
They are characteristic of animals and fungi. Chemical composition and molecular organization These structures are represented by compounds such as lipids, proteins and carbohydrates. The role of carbohydrates in the body is participation in and construction of membranes. Human and animal cells have a special structural component called the glycocalyx. This thin surface layer is composed of glycolipids and glycoproteins associated with cytoplasmic membrane. It provides a direct connection of cells with the external environment. This is also where the perception of stimuli and extracellular digestion take place. Thanks to their carbohydrate shell, cells stick together to form tissues. This phenomenon is called adhesion. We also add that the “tails” of carbohydrate molecules are located above the cell surface and are directed into the interstitial fluid.
Another group of heterotrophic organisms, fungi, also has a surface apparatus called a cell wall. It includes complex sugars - chitin, glycogen. Some types of mushrooms also contain soluble carbohydrates, such as trehalose, called mushroom sugar.
In unicellular animals, such as ciliates, the surface layer, the pellicle, also contains complexes of oligosaccharides with proteins and lipids. In some protozoa, the pellicle is quite thin and does not interfere with the change in body shape. And in others, it thickens and becomes strong, like a shell, performing a protective function.
plant cell wall
It also contains a large amount of carbohydrates, especially cellulose, collected in the form of fiber bundles. These structures form a framework embedded in a colloidal matrix. It consists mainly of oligo- and polysaccharides. The cell walls of plant cells can become lignified. In this case, the gaps between the cellulose bundles are filled with another carbohydrate - lignin. It enhances the supporting functions of the cell membrane. Often, especially in perennial woody plants, outer layer, consisting of cellulose, is covered with a fat-like substance - suberin. It prevents water from entering plant tissues, so the underlying cells quickly die and become covered with a layer of cork.
Summarizing the above, we see that carbohydrates and fats are closely interconnected in the cell wall of plants. Their role in the body of phototrophs is difficult to underestimate, since glycolipid complexes provide supporting and protective functions. Let us study the variety of carbohydrates characteristic of the organisms of the Drobyanka kingdom. It includes prokaryotes, in particular bacteria. Their cell wall contains a carbohydrate called murein. 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 former contains mucopolysaccharides such as murein. Its molecules look like large mesh structures that form a capsule around the bacterial cell. The second layer consists of peptidoglycan - a combination of polysaccharides and proteins.
Cell wall lipopolysaccharides allow bacteria to adhere strongly to various substrates, such as tooth enamel or the membrane of eukaryotic cells. In addition, glycolipids promote adhesion of bacterial cells to each other. In this way, for example, chains of streptococci, clusters of staphylococci are formed, moreover, some types of prokaryotes have an additional mucous membrane - peplos. It contains polysaccharides in its composition and is easily destroyed under the action of hard radiation or by contact with certain chemicals, such as antibiotics.
§ 1. CLASSIFICATION AND FUNCTIONS OF CARBOHYDRATES
Even in ancient times, mankind got acquainted with carbohydrates and learned how to use them in its Everyday life. Cotton, flax, wood, starch, honey, cane sugar are just some of the carbohydrates that played an important role in the development of civilization. Carbohydrates are among the most common organic compounds in nature. They are integral components of the cells of any organism, including bacteria, plants and animals. In plants, carbohydrates account for 80 - 90% of dry weight, in animals - about 2% of body weight. Their synthesis from carbon dioxide and water is carried out by green plants using energy. sunlight (photosynthesis ). The total stoichiometric equation for this process is:
Glucose and other simple carbohydrates are then converted into more complex carbohydrates such as starch and cellulose. Plants use these carbohydrates to release energy through the process of respiration. This process is essentially the reverse of the process of photosynthesis:
Interesting to know! Green plants and bacteria in the process of photosynthesis annually absorb approximately 200 billion tons of carbon dioxide from the atmosphere. In this case, about 130 billion tons of oxygen are released into the atmosphere and 50 billion tons of organic carbon compounds, mainly carbohydrates, are synthesized.
Animals are unable to synthesize carbohydrates from carbon dioxide and water. By consuming carbohydrates with food, animals spend the energy accumulated in them to maintain vital processes. high content carbohydrates are characterized by such types of our food as bakery products, potatoes, cereals, etc.
The name "carbohydrates" is historical. The first representatives of these substances were described by the summary formula C m H 2 n O n or C m (H 2 O) n . Another name for carbohydrates is Sahara - due to the sweet taste of the simplest carbohydrates. In its own way chemical structure Carbohydrates are a complex and diverse group of compounds. Among them, there are both fairly simple compounds with a molecular weight of about 200, and giant polymers, molecular mass which reaches several million. Along with carbon, hydrogen, and oxygen atoms, carbohydrates can contain atoms of phosphorus, nitrogen, sulfur, and, rarely, other elements.
Classification of carbohydrates
All known carbohydrates can be divided into two large groups – simple carbohydrates and complex carbohydrates. separate group make up carbohydrate-containing mixed polymers, for example, glycoproteins- a complex with a protein molecule, glycolipids - complex with lipid, etc.
Simple carbohydrates (monosaccharides, or monoses) are polyhydroxycarbonyl compounds that are not capable of forming simpler carbohydrate molecules upon hydrolysis. If monosaccharides contain an aldehyde group, then they belong to the class of aldoses (aldehyde alcohols), if ketone - to the class of ketoses (keto alcohols). Depending on the number of carbon atoms in a monosaccharide molecule, trioses (C 3), tetroses (C 4), pentoses (C 5), hexoses (C 6), etc. are distinguished:
The most common in nature are pentoses and hexoses.
Complex carbohydrates ( polysaccharides, or polioses) are polymers built from monosaccharide residues. They hydrolyze to form simple carbohydrates. Depending on the degree of polymerization, they are divided into low molecular weight ( oligosaccharides, the degree of polymerization of which, as a rule, is less than 10) and macromolecular. Oligosaccharides are sugar-like carbohydrates that are soluble in water and have a sweet taste. According to their ability to reduce metal ions (Cu 2+, Ag +), they are divided into regenerating and non-reducing. Polysaccharides, depending on the composition, can also be divided into two groups: homopolysaccharides and heteropolysaccharides. Homopolysaccharides are built from monosaccharide residues of the same type, and heteropolysaccharides are built from residues of different monosaccharides.
What has been said with examples of the most common representatives of each group of carbohydrates can be represented as the following diagram:
Functions of carbohydrates
The biological functions of polysaccharides are very diverse.
Energy and storage function
Carbohydrates contain the main amount of calories consumed by a person with food. Starch is the main carbohydrate in food. It is found in bakery products, potatoes, as part of cereals. The human diet also contains glycogen (in the liver and meat), sucrose (as additives to various dishes), fructose (in fruits and honey), lactose (in milk). Polysaccharides, before being absorbed by the body, must be hydrolyzed with digestive enzymes to monosaccharides. Only in this form they are absorbed into the blood. With the blood flow, monosaccharides enter the organs and tissues, where they are used to synthesize their own carbohydrates or other substances, or undergo splitting in order to extract energy from them.
The energy released from the breakdown of glucose is stored in the form of ATP. There are two processes of glucose breakdown: anaerobic (in the absence of oxygen) and aerobic (in the presence of oxygen). Lactic acid is formed as a result of the anaerobic process
which, with severe physical activity accumulates in the muscles and causes pain.
As a result of the aerobic process, glucose is oxidized to carbon monoxide (IV) and water:
As a result of aerobic breakdown of glucose, much more energy is released than as a result of anaerobic breakdown. In general, the oxidation of 1 g of carbohydrates releases 16.9 kJ of energy.
Glucose can undergo alcoholic fermentation. This process is carried out by yeast under anaerobic conditions:
Alcoholic fermentation is widely used in industry for the production of wines and ethyl alcohol.
Man learned to use not only alcoholic fermentation, but also found the use of lactic acid fermentation, for example, to obtain lactic acid products and pickle vegetables.
In humans and animals there are no enzymes capable of hydrolyzing cellulose; nevertheless, cellulose is the main food component for many animals, in particular, for ruminants. The stomach of these animals contains large quantities of bacteria and protozoa that produce the enzyme cellulase catalyzes the hydrolysis of cellulose to glucose. The latter can undergo further transformations, as a result of which butyric, acetic, propionic acids are formed, which can be absorbed into the blood of ruminants.
Carbohydrates also perform a reserve function. So, starch, sucrose, glucose in plants and glycogen in animals they are the energy reserve of their cells.
Structural, supporting and protective functions
Cellulose in plants and chitin in invertebrates and fungi, they perform supporting and protective functions. Polysaccharides form a capsule in microorganisms, thereby strengthening the membrane. Lipopolysaccharides of bacteria and glycoproteins of the surface of animal cells provide selectivity of intercellular interaction and immunological reactions of the body. Ribose serves building material for RNA and deoxyribose for DNA.
Performs a protective function heparin. This carbohydrate, being an inhibitor of blood clotting, prevents the formation of blood clots. It is found in the blood and connective tissue mammals. The cell walls of bacteria, formed by polysaccharides, are held together by short amino acid chains, protect bacterial cells from adverse influences. Carbohydrates are involved in crustaceans and insects in the construction of the external skeleton, which performs a protective function.
Regulatory function
Fiber enhances intestinal motility, thereby improving digestion.
An interesting possibility is the use of carbohydrates as a source of liquid fuel - ethanol. Since ancient times, wood has been used for heating homes and cooking. AT modern society this type of fuel is being replaced by other types - oil and coal, which are cheaper and more convenient to use. However, vegetable raw materials, despite some inconveniences in use, unlike oil and coal, are a renewable source of energy. But its use in engines internal combustion difficult. For these purposes, it is preferable to use liquid fuel or gas. From low-grade wood, straw or other plant materials containing cellulose or starch, liquid fuel can be obtained - ethanol. To do this, you must first hydrolyze cellulose or starch and get glucose:
and then subject the resulting glucose to alcoholic fermentation and obtain ethyl alcohol. Once refined, it can be used as a fuel in internal combustion engines. It should be noted that in Brazil, for this purpose, billions of liters of alcohol are obtained annually from sugar cane, sorghum and cassava and used in internal combustion engines.
