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The main transport form of carbohydrates in the plant is. Functions of carbohydrates in plants

Plan:

1. The meaning of carbohydrates. General characteristics.

2. Classification of carbohydrates.

3. Structure of carbohydrates.

4. Synthesis, breakdown and transformation of carbohydrates in the plant.

5. Dynamics of carbohydrates during SOM maturation.

The meaning of carbohydrates. General characteristics.

Carbohydrates are the main nutritional and main supporting material of plant cells and tissues.

They make up up to 85-90% of the total mass of the plant organism.

Formed during the process of photosynthesis.

Carbohydrates contain C, H and O.

Representatives: glucose С6Н12О6, sucrose С12Н22О11, fructose, rhamnose, starch, fiber, hemicelluloses, pectin substances, agar-agar.

Sucrose is a carbohydrate synthesized only in the plant body and plays a very important role in the metabolism of plants. Sucrose is the sugar most easily absorbed by the plant. In some plants, sucrose can accumulate in extremely large quantities (sugar beets, sugar cane).

SOMs differ greatly in carbohydrate composition:

Potatoes - most of the carbohydrates are starch;

Green vegetable peas (harvested at the stage of technical maturity) - the bulk of carbohydrates consists of almost equal parts of starch and sugars;

Ripe apples - there is practically no starch, and carbohydrates are represented by glucose, fructose, sucrose;

Persimmon – glucose and fructose, almost no sucrose;

Grapes – glucose and fructose.

Different composition of carbohydrates in individual tissues of the SOM:

The peel contains fiber and pectin substances (protecting the fruit pulp from adverse effects);

The pulp contains starch and sugars (glucose, fructose, sucrose).

Classification of carbohydrates.

All carbohydrates are divided into two groups - Monosas(monosaccharides) and Polioses(polysaccharides)

Several molecules of monosaccharides combine with each other to release water to form a polysaccharide molecule.

Monosaccharides: They can be considered as derivatives of polyhydric alcohols.

Representatives: glucose, fructose, galactose, mannose.

Disaccharides: sucrose (cane sugar), maltose (malt sugar) and cellobiose.

Trisaccharides: Raffinosa et al.

Tetrasaccharides: stachyosis, etc.

Di-, tri- and tetrasaccharides (up to 10 monosaccharides) form the group First order polysaccharides. All representatives of this group are easily soluble in water and in their pure form are crystalline substances (oligosaccharides).

Oligosaccharides (oligosaccharides) can be homo- and heterosaccharides. Sucrose consists of glucose and fructose - furan (heterosugar). Lactose– galactose + glucose. Maltose, trehalose, cellobiose – Glucose + glucose (homosaccharides) differ in the arrangement of the carbon atoms involved in the bond between monosaccharide molecules.

More complex carbohydrates - Second order polysaccharides. Complex substances with a very large molecular weight. They either do not dissolve at all in water or give viscous, colloidal solutions.

Representatives: mucus, starch, dextrins, glycogen, fiber, hemicelluloses, pectin substances, inulin, callose, etc.

The structure of carbohydrates.

Monosaccharides containing three carbon atoms belong to the group Triose, with four – Tetroz, with five - Pentosis, six - Hexose and family - Heptosis.

The most important and widespread in nature are pentoses and hexoses.

Monosaccharides, derivatives of polyhydric alcohols, contain in their molecule, along with alcohol groups –OH, an aldehyde or keto group.

Trioses:

Right-handed Left-handed

D-glyceraldehyde L-glyceraldehyde

Fructose belongs to pentoses, glucose to hexoses.

It has been established that in solutions D-glucose exists in three interconvertible forms, two of which are cyclic.


Similar interconversions of the three forms have also been established for other monosaccharides.

Disaccharides:


Polysaccharides:

They have a linear or branched structure; their polymer molecules consist of monomers (monosaccharides) connected to each other in long chains.

Synthesis, breakdown and transformation of carbohydrates in plants.

Synthesis.

The primary product of photosynthesis is Phosphoglyceric acid. With further transformations it gives various Monosaccharides– glucose, fructose, mannose and galactose (they are formed without the participation of light, as a result of “dark” enzymatic reactions). The formation of hexoses from phosphoglyceric acid or phosphoglyceraldehyde (triose) occurs due to the action of the enzyme Aldolase.


Formation of glucose and fructose from sorbitol.

Along with monosaccharides, sucrose (disaccharide) and starch (polysaccharide) are also formed extremely quickly in the leaves in the light, but this is a secondary process of enzymatic transformations of previously formed monosaccharides (can occur in complete darkness). Sucrose is synthesized from glucose and fructose, as well as from other hexoses. Sucrose is not synthesized from pentoses (arabinose, xylose).

Decay.

Most monosaccharides are fermented by yeast.

Oligosugars break down under the action of appropriate enzymes and during hydrolysis (heating in the presence of acids).

Second order polysaccharides:

Starch(consists of amylose and amylopectin, their ratio in the starch of different plants is different) - decomposes under the action of an enzyme Glucoseamylase and during hydrolysis into glucose molecules; Glycogen(similar).

Fiber (cellulose)– digested only in ruminants by bacteria containing the enzyme Cellulase.

Hemicelluloses hydrolyzed by acids more easily than fiber.

Interconversions.

In plants, saccharides are extremely easily converted into each other.

Interconversions of monosaccharides occur as a result of the action of appropriate enzymes that catalyze the reactions of phosphorylation and the formation of phosphorus esters of sugars.

Under the action of isomerases, monosaccharides are converted into each other.

Enzymes have also been discovered in plant organisms that catalyze the formation of phosphorus esters of sugars and their mutual transformations.

Starch that accumulates in leaves during photosynthesis can very quickly be converted into sucrose (the most important transport form of carbohydrates), flow in the form of sucrose into seeds, fruits, tubers, roots and bulbs, where sucrose is again converted into starch and inulin. Amylase does not take any part in these processes (other enzymes and hydrolysis work).

Dynamics of carbohydrates during SOM maturation

1. During the period of ripening on the plant and storage, the starch content of most fruits and vegetables decreases and the sugar content increases.

2. Having reached a certain maximum, the sugar level also begins to decrease.

Green bananas - more than 20% starch and less than 1% sugar;

In ripe bananas, the level of starch decreases to 1%, and the level of sugar increases to 18%.

Most of the sugars are sucrose, but in the optimal ripeness of fruits, sugars are represented by equal parts of sucrose, fructose and glucose.

The same changes are characteristic of apples, although much less pronounced.

If, during ripening on the mother plant, the amount of sugars increases due to mono- and disaccharides, then during their subsequent storage, an increase in the level of sugars, if observed, occurs due to monosaccharides. The amount of disaccharides decreases; under the action of enzymes and hydrolysis (under the influence of acids), they decompose into monosaccharides, as a result of which the number of the latter increases.

In fruits and vegetables that do not contain starch at all, an increase in sugars is also observed during storage. And also in fruits containing starch, the content of sugars formed during storage exceeds the content of starch from which they can be formed. A study of the dynamics of various polysaccharide fractions showed that during post-harvest ripening of fruits, not only hydrolysis of starch occurs, but also pectin substances, hemicelluloses and even celluloses.

U Vegetable peas, vegetable beans and sweet corn during ripening and storage, it is not the conversion of starch into sugar, but, on the contrary, of sugars into starch (when stored at 00C, the transition processes occur more slowly, but in the same order). When legumes are stored in leaves, the time it takes for sugar to transform into starch doubles.

IN Potato tubers Both the processes of synthesis of starch from sugars and the processes of transformation of starch into sugars occur.

As they grow, starch accumulates in the tubers. The higher the ratio of starch to sugars, the higher the quality of potato tubers.

When stored at 00C, starch turns into sugars, but this temperature is optimal for stopping the development of pathogenic microflora (potato rotting).

When the temperature drops from 20 to 00C:

Starch Þ sugar – reduced by 1/3;

Sugar Þ starch – reduced by 20 times;

The rate of sugar consumption during respiration (sugar Þ CO2 + H2O) decreases by 3 times.

Due to this, sugars accumulate during storage. Moreover, in wild forms of potatoes and in the northern regions, the majority of sugars that accumulate during storage are monosaccharides. In our zone, during storage, the same amount of mono- and disaccharides accumulates.

To consume tubers for food and to use them for seeds, it is necessary to reduce the sugar content and increase the starch content; for this you need to keep the tubers at 200C.

Long-term storage of potato tubers at 00C leads to the fact that the time required to convert sugars into starch increases so much that during this period diseases and pests completely affect the tubers.

When stored at 100C, potatoes retain almost the native level of starch, but this temperature does not control the disease. Therefore, it is more economical to store potatoes at 40C, in well-ventilated areas (active ventilation conditions); the tubers must be intact, dry; to prevent germination and diseases, additional means are needed - chemicals.

    Plastic. Carbohydrates are formed in plants during the process of photosynthesis and serve as the starting material for the synthesis of all other organic substances;

    Structural. This role is performed by cellulose or fiber, pectin substances, hemicellulose;

    Storage. Spare nutrients: starch, inulin, sucrose...

    Protective. Sucrose is the main protective nutrient in wintering plants.

    Energy. Carbohydrates are the main substrate of respiration. When 1 g of carbohydrates is oxidized, 17 kJ of energy is released.

2.2. Proteins (B).

Proteins, or proteins, are high-molecular compounds built from amino acids.

Among organic substances, in terms of quantity in plants, carbohydrates and fats are in first place, not proteins. But it is B. that play a decisive role in metabolism.

Functions of proteins in plants.

    Structural. In the cytoplasm of cells, the proportion of proteins is 2/3 of the total mass. Proteins are an integral part of membranes;

    Storage. Plants contain less protein than animal organisms, but quite a lot. So, in cereal seeds - 10-20% of dry weight, in seeds of legumes and oilseeds - 20-40%;

    Energy. Oxidation of 1 g of protein gives 17 kJ;

    Catalytic. Cell enzymes that perform a catalytic function are protein substances;

    Transport. Transport substances through membranes;

    Protective. Proteins are like antibodies.

Proteins perform a number of other specific functions.

2.2.1. Amino acids (A),

A are the basic structural units from which the molecules of all protein substances are built. Amino acids are derivatives of fatty or aromatic acids, containing both an amino group (-NH 2) and a carboxyl group (-COOH). Most natural A. has a general formula

There are about 200 A. in nature, but only 20, as well as two amides, asparagine and glutamine, are involved in the construction of B. The remaining A. are called free.

In B. only left-handed amino acids are present.

From the chemical properties of A. we note them amphotericity. Due to the amphoteric nature of A. in aqueous solutions, depending on the pH of the solution, the dissociation of –COOH or –NH 2 groups is suppressed and A. exhibits the properties of an acid or alkali.

(-) alkaline environment acidic environment charge “+”

H 2 O +R-CH-COO - ← OH- +R-CH-COO- + H+ →R-CH-COOH

H 2 NH 3 N + H 3 N +

The reaction of a solution of A., in which equality of “+” and “-” charges is observed, is called the isoelectric point (IEP). In IET, the A molecule is electrically neutral and does not move in an electric field.

B.'s composition includes 20 A. and two amides—asparagine and glutamine. Of the 20 A., 8 are essential, since they cannot be synthesized in the body of humans and animals, but are synthesized by plants and microorganisms. Essential amino acids include: valine; lysine; methionine; threonine; leucine; isoleucine; tryptophan; phenylalanine.

Representatives A.

Alanine CH 3 -CH-COOH (6.02)

Cysteine ​​CH 2 -CH-COOH (5.02)

Aspartic COOH-CH 2 -CH-COOH (2.97)

acid |

Glutamic COOH-CH 2 -CH 2 -CH-COOH (3.22)

acid |

Lysine CH 2 -CH 2 -CH 2 -CH 2 -CH-COOH (9.74)

2.2.2. Composition and general properties of proteins.

The elemental composition of B. is quite constant and almost all of them contain 50-60% C, 20-24% O, 6-7% H, 15-19% N, and the amount of sulfur is from 0 to 3%. In complex bacteria, phosphorus, iron, zinc, copper are present in small quantities.....

Properties of proteins.

    Amphoteric. B. contain free NH 2 and COOH groups and can dissociate as acids and bases (see example A.). They have IET. When a solution reaction is equal to or close to the IET, proteins are characterized by extreme instability and easily precipitate from solutions under the weakest external influences. This is used to isolate proteins.

    Denaturation. This is the loss of protein’s biological properties under the influence of various external influences - high temperature, the action of acids, heavy metal salts, alcohol, acetone, etc. (see colloid coagulation factors). As a result of exposure, a change in the structure of polypeptide chains occurs in the protein molecule, the spatial structure is disrupted, but decomposition into amino acids does not occur. For example, when heating a chicken egg, the white coagulates. This is irreversible denaturation; or completely dried seeds.

    Biological nutritional value of proteins (BNV). It is determined by the content of essential A. in B. For this, the B. studied is compared with standard B., approved by the FAO (International Food and Agricultural Organization). The amino acid score of each essential amino acid is calculated and expressed as % content of essential A. in the protein under study (mg) x 100%

Those A., whose amino acid score is less than 100%, are called limiting. In many proteins there are no individual essential proteins at all. For example, tryptophan is absent in apple proteins; in many plant bacteria, the limiting ones are most often the four essential amino acids - lysine, tryptophan, methionine and threonine. B. that do not contain some essential A. are called defective. Plant B. are considered inferior, and animal B. are considered inferior. full-fledged. To create 1 kg of animal food, 8-12 kg of vegetable food is consumed. Based on the BOC of protein, one can estimate: 100% - milk and egg proteins; other animals B – 90-95%; B. legumes – 75-85%; B. grain crops - 60-70%.

2.2.3. The structure of proteins.

According to the polypeptide theory of the structure of B. (Danilevsky, Fischer), amino acids interact with each other to form a peptide bond - CO-NH-. Di-, tri-, pento- and polypeptides are formed.

The B. molecule is constructed from one or more interconnected polypeptide chains consisting of amino acid residues.

CH 3 CH 2 CH CH 3 CH 2 CH

H 2 N-CH-COOH + H 2 N-CH-COOH →H 2 N-CH-CO-NH-CH-COOH + H 2 O

Alanine cysteine ​​alanylcysteine

(dipeptide)

Structure B.

There are different levels of organization of a protein molecule and each molecule has its own spatial structure. The loss or disruption of this structure causes a disruption in the function performed (denaturation).

There are different levels of organization of a protein molecule.

    Primary structure. Determined by the number and sequence of amino acids in the B molecule. The primary structure is fixed genetically. With this structure, the B. molecule has a thread-like shape. …….

The primary structure of homologous proteins is used, in particular, as a criterion for establishing the relationship between individual species of plants, animals and humans.

    Secondary structure. It is a helical configuration of polypeptide chains. The decisive role in its education belongs to hydrogenconnections...... However, disulfide bonds (-S-S-) can also appear between individual points of the helix, which disrupt the typical helical structure.

    Tertiary structure. This is an even higher level of organization B. It characterizes the spatial configuration of the molecule. It is due to the fact that free carboxyl, amine, hydroxyl and other groups of side radicals of amino acid molecules in polypeptide chains interact with each other to form amide, ester and salt bonds. Due to this, the polypeptide chain, which has a certain secondary structure, is further folded and packed and acquires a specific spatial configuration. Hydrogen and disulfide bonds also play a significant role in its formation. A globular (spherical) form of proteins is formed.

    Quaternary structure. It is formed by the combination of several proteins with a tertiary structure. It should be noted that the functional activity of a particular protein is determined by all four levels of its organization.

2.2.4. Protein classification.

Based on their structure, proteins are divided into proteins, or simple proteins, built only from amino acid residues, and proteids, or complex proteins, consisting of a simple protein and some other non-protein compound tightly bound to it. Depending on the nature of the non-protein part, proteids are divided into subgroups.

    Phosphoproteins - proteins are combined with phosphoric acid.

    Lipoproteins - proteins are combined with phospholipids and other lipids, for example, in membranes.

    Glycoproteins - protein is combined with carbohydrates and their derivatives. For example, as part of plant mucilages.

    Metalloproteins – contain metals, g.o. microelements: Fe, Cu, Zn….. These are mainly metal-containing enzymes: catalase, cytochromes, etc.

    Nucleoproteins are one of the most important subgroups. Here the protein combines with nucleic acids.

The classification of proteins according to solubility in various solvents is of great practical importance. The following are distinguished: faction B. by solubility:

    Albumins are water soluble. A typical representative is chicken egg albumin, many proteins are enzymes.

    Globulins are proteins that are soluble in weak solutions of neutral salts (4 or 10% NaCl or KCl).

    Prolamins - dissolve in 70% ethyl alcohol. For example, gliadins of wheat and rye.

    Glutelins - dissolve in weak alkali solutions (0.2-2%).

    Histones are low-molecular alkaline bacteria contained in the nuclei of cells.

Fractions of B. differ in amino acid composition and biological nutritional value (BNC). According to BPC, the fractions are arranged in the sequence: albumins › globulins ≈ glutelins › prolamins. The content of fractions depends on the type of plant; it is not the same in different parts of the grain. (see private biochemistry of agricultural crops).

      Lipids (L).

Lipids are fats (F) and fat-like substances (lipoids) that are similar in their physicochemical properties, but differ in their biological role in the body.

Lipids are generally divided into two groups: fats and lipoids. Typically, fat-soluble vitamins are also classified as lipids.

Carbohydrates are a group of organic substances with the general formula (CH2O)n, i.e. they contain only oxygen, carbon and hydrogen. Carbohydrates have a much simpler structure than proteins. Carbohydrates are divided into 3 large classes: monosaccharides, disaccharides and polysaccharides.

Monosaccharides are simple carbohydrates that do not have a polymer structure. Monosaccharide molecules can contain a different number of carbon atoms: 3 (m 434h71fe rhiose), 4 (tetroses), 5 (pentoses), 6 (hexoses), 7 (hexoses), of which trioses, pentoses and hexoses are the most common in plants.

Trioses have the general formula C3H6O3; There are only two trioses - glyceraldehyde and dihydroxyacetone. These sugars are intermediate products in the process of glycolysis during respiration.

Pentoses have the general formula C5H10O5. Of the pentoses, ribose and deoxyribose are the most important, because they are part of nucleic acids: deoxyribose - in DNA, ribose - in RNA, as well as some other important substances - NAD, NADP, FAD and ATP.

Hexoses have the general formula C6H12O6. Of the hexoses in plants, the most common are glucose and, to a lesser extent, fructose. Glucose and fructose have different important functions in the cell. They serve as a source of energy for the cell, which is released when they are oxidized during respiration. The most common disaccharide, sucrose, is formed from glucose and fructose. Glucose serves as a monomer for the formation of the most common plant polysaccharides - starch and glucose. In juicy fruits, glucose and fructose serve as reserve substances.

Disaccharides are sugars whose molecules are formed from 2 molecules of monosaccharides as a result of a condensation reaction, i.e. combination of monosaccharide molecules with the release of water. For example, the sucrose disaccharide molecule consists of a glucose residue and a fructose residue:

С6Н12О6 + С6Н12О6 → С12Н22О11 + Н2О

Sucrose has an interesting property: it is as soluble in water as glucose, but chemically much less active. Therefore, carbohydrates are transported through the phloem precisely in the form of sucrose: due to its high solubility, it can be transported in the form of a fairly concentrated solution, and due to its chemical inertness, it does not enter into any reactions along the way. In some plants, sucrose serves as a reserve substance - for example, in carrots, sugar beets and sugar cane.

Polysaccharides are polymers formed by the condensation of many monosaccharide molecules. In plants, polysaccharides perform 2 functions - structural and storage.

1.Structural polysaccharides - Polysaccharides are convenient for use as structural substances for 2 reasons:


They have long, strong molecules

Polysaccharides are chemically inactive, therefore the structures formed from them are resistant to various external influences.

There are 2 main types of structural polysaccharides - cellulose and hemicelluloses. Cellulose is formed from β-glucose residues; it has very long branched molecules, insoluble in water and resistant to various chemical influences. Cellulose is contained in the cell wall and plays the role of a rigid, strong reinforcement in it. Hemicelluloses are formed from residues of various monosaccharides - arabinose, mannose, xylose, etc. Hemicelluloses are part of the cell wall matrix.

2. Reserve polysaccharides - Polysaccharides are convenient for use as reserve substances for 2 reasons:

The large size of polysaccharide molecules makes them insoluble in water, which means they do not have a chemical or osmotic effect on the cell;

Polysaccharides are easily converted to monosaccharides by hydrolysis

The main storage polysaccharide in plants is starch. Starch is a polymer of α-glucose. Strictly speaking, starch is a mixture of 2 polysaccharides: amylose, which has linear molecules, and amylopectin, which has branched molecules. If necessary, starch is easily hydrolyzed to glucose. It is starch that is a reserve substance in most plants - grains, corn, potatoes, etc. In cells, starch is contained in the form of starch grains in chloroplasts or cytoplasm.

Let's consider carbohydrates in plants, which, like fats, organic acids and tannins, are important and are constantly found in both vegetative and reproductive organs.

Carbohydrates are made up of carbon, hydrogen and oxygen. The last two elements are in the same quantitative combination with each other as in water (H 2 O), that is, for a certain number of hydrogen atoms there are half as many oxygen atoms.

Carbohydrates make up up to 85-90% of the substances included in the plant body.

Carbohydrates are the main nutritional and supporting material in plant cells and tissues.

Carbohydrates are divided into monosaccharides, disaccharides and polysaccharides.

Of the monosaccharides in plants, hexoses with the composition C 6 H 12 O 6 are common. These include glucose, fructose, etc.

Glucose (otherwise called dextrose or grape sugar) is found in grapes - about 20%, in apples, pears, plums, cherries and wine berries. Glucose has the ability to crystallize.

Fructose (otherwise called levulose or fruit sugar) crystallizes with difficulty and is found together with glucose in fruits, nectaries, bee honey, bulbs, etc. (Fructose is called levulose because when a polarized beam of light passes through it, the latter deviates to the left. In The opposite of fructose, grape sugar deflects a polarized beam to the right. Polarized light is light transmitted through Iceland spar prisms, which are birefringent. These prisms are an integral part of the polarizing apparatus.)

The properties of hexoses are as follows. They have a particularly sweet taste and are easily soluble in water. The primary formation of hexoses occurs in the leaves. They are easily converted into starch, which, in turn, can easily be converted into sugar with the participation of the enzyme diastase. Glucose and fructose have the ability to easily penetrate from cell to cell and quickly move throughout the plant. In the presence of yeast, hexoses easily ferment and turn into alcohol. A characteristic and sensitive reagent for hexoses is blue Fehling's liquid; with its help you can easily open the smallest quantities of them: when heated, a brick-red precipitate of cuprous oxide forms.

Sometimes hexoses are found in plants in combination with aromatic alcohols, bitter or caustic substances. These compounds are then called glucosides, for example amygdalin, which imparts bitterness to the seeds of almonds and other stone fruits. Amygdalin contains a toxic substance - hydrocyanic acid. Glucosides not only protect seeds and fruits from being eaten by animals, but also protect the seeds of juicy fruits from premature germination.

Disaccharides are carbohydrates with the composition C 12 H 22 O 11. These include sucrose, or cane sugar, and maltose. Sucrose is formed in plants from two hexose particles (glucose and fructose) with the release of a water particle:

C 6 H 12 O 6 + C 6 H 12 O 6 = C 12 H 22 O 11 + H 2 O.

When boiled with sulfuric acid, a particle of water is added to cane sugar, and the disaccharide breaks down into glucose and fructose:

C 12 H 22 O 11 + H 2 O = C 6 H 12 O 6 + C 6 H 12 O 6.

The same reaction occurs when the enzyme invertase acts on cane sugar, therefore the conversion of cane sugar into hexoses is called inversion, and the resulting hexoses are called invert sugar.

Cane sugar- This is the sugar that is consumed in food. It has long been extracted from the stems of cereal - sugar cane (Saccharum officinarum), growing in tropical countries. It is also found in the roots of many root vegetables, of which the largest amount is found in the roots of sugar beets (from 17 to 23%). Cane sugar is extracted from sugar beets at beet sugar factories. Sucrose easily dissolves in water and crystallizes well (granulated sugar). It does not reduce cuprous oxide from feling liquid.

Maltose is formed from starch under the action of the enzyme diastase:

2(C 6 H 10 O 5)n + nH 2 O = nC 12 H 22 O 11.

When a maltose molecule is broken down (hydrolyzed) by the enzyme maltase, two hexose molecules are formed:

C 12 H 22 O 11 + H 2 O = 2C 6 H 12 O 6.

Maltose reduces cuprous oxide from fehling liquid.

In some plants (cotton in the seeds, eucalyptus in the leaves, sugar beets in the roots, etc.) the trisaccharide raffinose (C 18 H 32 O 16) is also found.

Polysaccharides are carbohydrates with the composition (C 6 H 10 O 5) n Polysaccharides can be considered as several particles of monosaccharides, from which the same number of water particles have separated:

NC 6 H 12 O 6 - nH 2 O = (C 6 H 10 O 5)n.

In living plant tissues, polysaccharides (or polyoses) include starch, inulin, fiber, or cellulose, hemicellulose, pectin substances, etc. Mushrooms contain glycogen, a carbohydrate characteristic of animal organisms and therefore sometimes called animal starch.

Starch is a high-molecular carbohydrate found in plants as a reserve substance. Primary starch is formed in the green parts of the plant, such as leaves, as a result of the process of photosynthesis. In the leaves, starch is converted into glucose, which in the phloem of the veins is converted into sucrose and flows out of the leaves and goes to the growing parts, plants, or to places where reserve substances are deposited. In these places, sucrose is converted into starch, which is deposited in the form of tiny grains. This starch is called secondary starch.

The places where secondary starch is deposited are leukoplasts located in the cells of tubers, roots and fruits.

The main properties of starch are the following: 1) it does not dissolve in cold water; 2) when heated in water, it turns into a paste; 3) starch grains have a cryptocrystalline structure; 4) from the action of the iodine solution it turns blue, dark blue, violet and black (depending on the strength of the solution); 5) under the influence of the enzyme diastase, starch is converted into sugar; 6) in polarized light, starch grains glow and a characteristic figure of a dark cross is visible on them.

Starch consists of several components - amylose, amylopectin, etc., differing in solubility in water, reaction with iodine solution and some other characteristics. Amylose dissolves in warm water and turns bright blue from iodine; amylopectin is slightly soluble even in hot water and gets a red-violet color from iodine.

The amount of starch in plants varies greatly: cereal grains contain 60-70%, legume seeds - 35-50%, potatoes - 15-25%.

Inulin is a polysaccharide found in the underground organs of many plants of the Asteraceae family as a reserve nutrient carbohydrate. Such plants are, for example, elecampane (lnula), dahlia, earthen pear, etc. Inulin is found in cells in dissolved form. When roots and tubers of Asteraceae plants are kept in alcohol, inulin crystallizes in the form of spherocrystals.

Fiber or cellulose, like starch, does not dissolve in water. Cell membranes are made of fiber. Its composition is similar to starch. An example of pure fiber is cotton wool, which is made up of the hairs that cover cotton seeds. Good quality filter paper is also pure fiber. Fiber dissolves in an ammonia solution of copper oxide. When exposed to sulfuric acid, fiber turns into amyloid, a colloidal substance resembling starch and turning blue from iodine. In strong sulfuric acid, fiber dissolves, turning into glucose. The reagent for fiber is chlorine-zinc-iodine, which gives it a purple color. Zinc chloride, like sulfuric acid, first converts fiber into amyloid, which is then stained with iodine. Pure iodine turns fiber yellow. Under the influence of the enzyme cytase, fiber turns into sugar. Fiber plays an important role in industry (fabrics, paper, celluloid, pyroxylin).

In plants, cell membranes consisting of fiber are often subject to lignification and suberization.

The amount of cellulose and wood varies greatly in different plants and different parts of them. For example, the grains of naked cereals (rye, wheat) contain 3-4% cellulose and wood, and the grains of filmy cereals (barley, oats) contain 8-10%, hay - 34%, oat straw - 40%, rye straw - up to 54%.

Hemicellulose, a substance similar to fiber, is deposited as a reserve nutrient. It is not soluble in water, but weak acids easily hydrolyze it, while fiber is hydrolyzed by concentrated acids.

Hemicellulose is deposited in the cell walls of cereal grains (corn, rye, etc.), in the seeds of lupine, date and palm tree Phytelephas macrocarpa. Its hardness is such that palm seeds are used to make buttons called “vegetable ivory.” When seeds germinate, hemicellulose dissolves, turning into sugar with the help of enzymes: it goes to nourish the embryo.

Pectic substances- high molecular weight compounds of carbohydrate nature. Contained in significant quantities in fruits, tubers and plant stems. In plants, pectic substances are usually found in the form of water-insoluble protopectin. When fruits ripen, water-insoluble protopectin contained in the cell walls is converted into soluble pectin. During the process of flax retting, under the influence of microorganisms, pectin substances are hydrolyzed - maceration and separation of the fibers from each other occurs. (Maceration (from the Latin “maceration” - softening) is the natural or artificial separation of tissue cells as a result of the destruction of the intercellular substance.)

Mucus and gum are colloidal polysaccharides that are soluble in water. Mucilage is found in large quantities in the peel of flax seeds. Gum can be observed in the form of cherry glue, formed in places of damage to the branches and trunks of cherries, plums, apricots, etc.

Lichenin is a polysaccharide found in lichens (for example, in “Icelandic moss” - Cetraria islandica).

Agar-agar is a high molecular weight polysaccharide found in some seaweeds. Agar-agar dissolves in hot water, and after cooling it solidifies into a jelly. It is used in bacteriology for nutrient media and in the confectionery industry for the production of jelly, marshmallows, and marmalades.

Monosaccharides

Glucose C6H2O6 (structural formulas, see Fig. 2) (monose, hexose, aldose, grape sugar) is the most common of monoses in both the plant and animal world. Contained in free form in all green parts of plants, seeds, various fruits and berries. Glucose is found in large quantities in grapes - hence its name - grape sugar. The biological role of glucose is especially great in the formation of polysaccharides - starch, cellulose, built from D-glucose residues. Glucose is part of cane sugar, glycosides, tannin and other tannins. Glucose is well fermented by yeast.

Fructose C6H12O6 (structural formulas, see Fig. 3) (monose, hexose, ketose, levulose, fruit sugar) is found in all green plants and in the nectar of flowers. There is especially a lot of it in fruits, so its second name is fruit sugar. Fructose is much sweeter than other sugars. It is part of sucrose and high molecular weight polysaccharides, such as inulin. Like glucose, fructose is well fermented by yeast.

Disaccharides

Sucrose С12Н22О11 (disaccharide) is extremely widespread in plants, especially in beet roots (from 14 to 20% of dry weight), as well as in the stems of sugar cane (mass fraction of sucrose from 14 to 25%).

Sucrose consists of -D-glucopyranose and -D-fructofuranose, connected by 1 2 bonds due to glycosidic hydroxyls.

Sucrose does not contain free glycosidic hydroxyl, is a non-reducing sugar, and is therefore relatively chemically inert, except for its extreme sensitivity to acid hydrolysis. Therefore, sucrose is a transport sugar, in the form of which carbon and energy are transported throughout the plant. It is in the form of sucrose that carbohydrates move from places of synthesis (leaves) to places where they are stored (fruits, roots, seeds, stems). Sucrose moves along the conducting bundles of plants at a speed of 2030 cm/h. Sucrose is very soluble in water and has a sweet taste. With increasing temperature, its solubility increases. Sucrose is insoluble in absolute alcohol, but it dissolves better in aqueous alcohol. When heated to 190-200 C and above, sucrose dehydrates with the formation of various colored polymer products - caramels. These products, called kohlers, are used in the cognac industry to color cognacs.

Hydrolysis of sucrose.

When sucrose solutions are heated in an acidic environment or under the action of the enzyme -fructofuranosidase, it hydrolyzes, forming a mixture of equal amounts of glucose and fructose, which is called invert sugar (Fig. 7).


Rice. 7.

The enzyme -fructofuranosidase is widespread in nature; it is especially active in yeast. The enzyme is used in the confectionery industry, since the invert sugar formed under its influence prevents the crystallization of sucrose in confectionery products. Invert sugar is sweeter than sucrose due to the presence of free fructose. This allows you to save sucrose by using invert sugar. Acid hydrolysis of sucrose also occurs when cooking jam and making jam, but enzymatic hydrolysis is easier than acid hydrolysis.

Maltose C12H22O11 consists of two -D-glucopyranose residues connected by a 1 4 glycosidic bond.

Free maltose is found in small quantities in plants, but appears during germination, as it is formed during the hydrolytic breakdown of starch. It is absent in normal grains and flour. Its presence in flour indicates that this flour is obtained from sprouted grain. Malt, which is used in brewing, contains a large amount of maltose, which is why maltose is also called malt sugar. Under the action of the enzyme -glucosidase (maltase), maltose undergoes hydrolysis to D-glucose. Maltose is fermented by yeast.

Lactose C12H22O11 is built from -D-galactopyranose and D-glucopyranose, connected by a 1 4 glycosidic bond. It is rarely found in plants.


Lactose is found in large quantities (45%) in milk, which is why it is called milk sugar. It is a reducing sugar with a faint sweet taste. Fermented with lactose yeast to lactic acid.

Cellobiose C12H22O11 consists of two -D-glucopyranose residues connected by a 1 4 glycosidic bond.


It serves as a structural component of cellulose polysaccharide and is formed from it during hydrolysis under the action of the enzyme cellulase. This enzyme is produced by a number of microorganisms and is also active in germinating seeds.

Non-sugar-like polysaccharides

Storage polysaccharides

Starch (C6H10O5)n is the most important representative of polysaccharides in plants. This storage polysaccharide is used by plants as energy material. Starch is not synthesized in the animal body; glycogen is a similar reserve carbohydrate in animals.

Starch is found in large quantities in the endosperm of cereals - 6585% of its mass, in potatoes - up to 20%.

Starch is not a chemically individual substance. In addition to polysaccharides, its composition includes minerals, mainly represented by phosphoric acid, lipids and high-molecular fatty acids - palmitic, stearic and some other compounds adsorbed by the carbohydrate polysaccharide structure of starch.

In the endosperm cells, starch is found in the form of starch grains, the shape and size of which are characteristic of this type of plant. The shape of starch grains makes it possible to easily recognize the starches of different plants under a microscope, which is used to detect the admixture of one starch in another, for example, when adding corn, oat or potato flour to wheat flour.

In the storage tissues of various organs - tubers, bulbs, larger starch grains are stored in amyloplasts as secondary (reserve) starch. Starch grains have a layered structure.

The structure of carbohydrate components of starch

The carbohydrate part of starch consists of two polysaccharides:

  • 1. Amylose;
  • 2. Amylopectin.
  • 1 The structure of amylose.

In the amylose molecule, glucose residues are linked by glycosidic 1 4 bonds, forming a linear chain (Fig. 8, a).

Amylose has a reducing end (A) and a non-reducing end (B).

Linear amylose chains containing from 100 to several thousand glucose residues are capable of spiraling and thus taking on a more compact shape (Fig. 8, b). Amylose dissolves well in water, forming true solutions that are unstable and capable of retrogradation - spontaneous precipitation.

Rice. 8.

a - diagram of the connection of glucose molecules in amylose; b - spatial structure of amylose; c -- diagram of the connection of glucose molecules in amylopectin; d -- spatial molecule of amylopectin

2 The structure of amylopectin

Amylopectin is a branched component of starch. It contains up to 50,000 glucose residues, interconnected mainly by 1 4 glycosidic bonds (linear sections of the amylopectin molecule). At each branching point, glucose molecules (-D-glucopyranose) form a 1 6 glycosidic bond, which constitutes about 5% of the total number of glycosidic bonds in the amylopectin molecule (Fig. 8, c, d).

Each amylopectin molecule has one reducing end (A) and a large number of non-reducing ends (B). The structure of amylopectin is three-dimensional, its branches are located in all directions and give the molecule a spherical shape. Amylopectin does not dissolve in water, forming a suspension, but when heated or under pressure it forms a viscous solution - a paste. With iodine, a suspension of amylopectin gives a red-brown color, while iodine is adsorbed on the amylopectin molecule, so the color of the suspension is due to the color of the iodine itself.

As a rule, the amylose content in starch ranges from 10 to 30%, and amylopectin - from 70 to 90%. Some varieties of barley, corn and rice are called waxy. In the grains of these crops, the starch consists only of amylopectin. In apples, starch is represented only by amylose.

Enzymatic hydrolysis of starch

Starch hydrolysis is catalyzed by enzymes - amylases. Amylases belong to the class of hydrolases, a subclass - carbohydrases. There are b- and -amylases. These are single-component enzymes consisting of protein molecules. The role of the active center in them is performed by the groups - NH2 and - SH.

Characteristics of b - amylase

b - Amylase is found in the saliva and pancreas of animals, in molds, in sprouted grains of wheat, rye, barley (malt).

b- Amylase is a thermostable enzyme; its optimum is at a temperature of 700C. The optimal pH value is 5.6-6.0; at pH 3.3-4.0 it quickly collapses.

Characteristics - amylase

Amylase is found in grains of wheat, rye, barley, soybeans, and sweet potatoes. However, the activity of the enzyme in ripened seeds and fruits is low; activity increases during seed germination.

β-amylase breaks down amylose completely, converting it 100% into maltose. Amylopectin breaks down maltose and dextrins, which give a red-brown color with iodine, splitting only the free ends of glucose chains. The action stops when it reaches branches. β-amylase breaks down amylopectin by 54% to form maltose. The resulting dextrins are hydrolyzed by b-amylase to form dextrins of lower molecular weight and which do not stain with iodine. With the subsequent long-term action of b-amylose on starch, about 85% of it is converted into maltose.

Those. the action of β-amylase produces mainly maltose and some high-molecular dextrins. The action of b-amylase produces mainly dextrins of lower molecular weight and a small amount of maltose. Neither b- nor b-amylases alone can completely hydrolyze starch to form maltose. With the simultaneous action of both amylases, starch is hydrolyzed by 95%.

Starch hydrolysis products

As the final products of amylose hydrolysis, not only maltose, but also glucose is usually formed, and during the hydrolysis of amylopectin, maltose, glucose and a small amount of oligosaccharides containing a 6-I6 glycosidic bond are formed. Glycosidic bond b I6 is hydrolyzed by R - enzyme. The main product formed during the hydrolysis of amylose and amylopectin is maltose. Next, maltose under the action of b-glucosidase (maltase) is hydrolyzed to D-glucose.

Amylase preparations are widely used in baking as improvers. The addition of amylases leads to the formation of a softer bread crumb and reduces the rate of bread staling during storage.

Glycogen and phytoglycogen (plant glycogen) are found in corn grains. In structure, phytoglycogen is close to the storage polysaccharide of animal organisms - glycogen, which is called animal starch. Phytoglycogen, like animal glycogen, has a higher degree of branching than amylopectin, about 10% of its bonds are 1 6 bonds, while amylopectin has about 5% of such bonds.

Inulin belongs to the reserve polysaccharides of plants. It represents a group of molecular forms of approximately the same size.

Inulin, as a reserve polysaccharide, is deposited in the underground storage organs of plants - in the tubers of Jerusalem artichoke, dahlia, and artichoke rhizomes. Moreover, as an energy reserve of a substance, it is preferable to starch.

Another reserve polysaccharide, levan, has a structure close to inulin. The number of monosaccharide residues in levan is 78.

Levans are temporary storage polysaccharides of cereal plants. They are found in the leaves, stems and roots of plants and are used during grain ripening for the synthesis of starch. Like inulin, levan contains a terminal sucrose residue. The polysaccharide chain of inulin and levan does not have reducing ends - their anomeric carbon atoms are occupied in the formation of a glycosidic bond.

Other storage polysaccharides include galactomannans in soybean seeds and glucomannans, which are stored by some tropical plants, but their chemical structure has not been fully established.

Structural polysaccharides

Cellulose (C6H10O5) is a second-order polysaccharide and is the main component of cell walls. Cellulose consists of -D-glucose residues connected to each other by 1 4 glycosidic bonds (Fig. 9, a). Among other polysaccharides that make up the plant cell wall, it belongs to microfibrillar polysaccharides, since in cell walls cellulose molecules are connected into structural units called microfibrils. The latter consists of a bundle of cellulose molecules located parallel to each other along its length.

Rice. 9.

a - a connection of glucose molecules; b - structure of microfibrils; c - spatial structure

Pulp Spread

On average, there are about 8,000 glucose residues per cellulose molecule. Hydroxyls at carbon atoms C2, C3 and C6 are not substituted. The repeating unit in the cellulose molecule is a residue of the disaccharide cellobiose.

Properties of cellulose

Cellulose does not dissolve in water, but swells in it. Free hydroxyl groups can be replaced by radicals - methyl -CH3 or acetal with the formation of a simple or ester bond. This property plays an important role in the study of the structure of cellulose, and also finds application in industry in the production of artificial fibers, varnishes, artificial leather and explosives.

Cellulose digestibility

In most animals and humans, cellulose is not digested in the gastrointestinal tract, since their bodies do not produce cellulase, an enzyme that hydrolyzes the 4 glycosidic bond. This enzyme is synthesized by various kinds of microorganisms that cause wood rotting. Termites digest cellulose well because symbiotic microorganisms that produce cellulase live in their intestines.

Cattle feed rations include cellulose (as part of straw and other components), since their stomach contains microorganisms that synthesize the enzyme cellulase.

The meaning of cellulose

The industrial importance of cellulose is enormous - the production of cotton fabrics, paper, industrial wood and a number of chemical products based on the processing of cellulose.

Hemicelluloses are second-order polysaccharides that, together with pectin substances and lignin, form the matrix of plant cell walls, filling the space between the frame of the walls, composed of cellulose microfibrils.

Hemicelluloses are divided into three groups:

  • 1. Xylans;
  • 2. Mannans;
  • 3. Galactans.
  • 1. Xylans are formed by D-xylopyranose residues connected by 4 bonds in a linear chain. Seven out of every ten xylose residues are acetylated at C3 and rarely at C2. Some xylose residues have 4-o-methyl-D-glucuronic acid attached via a glycosidic 2 bond.
  • 2. Mannans consist of a backbone formed from -D-mannopyranose and -D-aminopyranose residues linked by glycosidic 4 bonds. Single residues of -D-galactopyranose are attached to some mannose residues of the main chain by 6 bonds. The hydroxyl groups at C2 and C3 of some mannose residues are acetylated.
  • 3. Galactans consist of β-galactopyranose residues connected by 4 bonds into the main chain. At C6 they are joined by disaccharides consisting of D-galactopyranose and L-arabofuranose.

Pectic substances are a group of high molecular weight polysaccharides that, together with cellulose, hemicellulose and lignin, form the cell walls of plants.

The structure of pectin substances

The main structural component of pectin substances is galacturonic acid, from which the main chain is built; The side chains include arabinose, galactose and rhamnose. Some of the acid groups of galacturonic acid are esterified with methyl alcohol (Fig. 10), i.e. the monomer is methoxygalacturonic acid. In the methoxypolygalacturonic chain, the monomer units are connected by 4 glycosidic bonds, the side chains (branches) are attached to the main chain by 2 glycosidic bonds.

Pectin substances from sugar beets, apples, and citrus fruits differ from each other in the composition of the side chains of the polygalacturonic chain and in physical properties.

Depending on the number of methoxy groups and the degree of polymerization, high- and low-esterified pectins are distinguished. In the former, more than 50% of the carboxyl groups are esterified, in the latter, less than 50% of the carboxyl groups.

Pectin substances are physical mixtures of pectins with accompanying substances - pentosans and hexosans. The molecular weight of pectin is from 20 to 50 kDa.

There are apple pectin, which is obtained from apple pomace, citrus pectin - from citrus peels and pomace, beet pectin - from beet pulp. Quince, red currant, dogwood, cherry plum and other fruits and berries are rich in pectin substances.

In plants, pectic substances are present in the form of insoluble protopectin associated with araban or xylan in the cell wall. Protopectin is converted into soluble pectin either by acid hydrolysis or by the action of the enzyme protopectinase. Pectin is isolated from aqueous solutions by precipitation with alcohol or 50% acetone.

Pectic acids and their salts

Pectic acids are high-molecular polygalacturonic acids, a small part of the carboxyl groups of which is esterified with methyl alcohol. Salts of pectic acids are called pectinates. If pectin is completely demethoxylated, then they are called pectic acids, and their salts are called pectates.

Pectolytic enzymes

Enzymes involved in the hydrolysis of pectin substances are called pectolytic. They are of great importance, as they help increase the yield and clarify fruit and berry juices. Pectin substances in plants are usually not found in free form, but in the form of a complex complex - protopectin. In this complex, methoxylated polygalacturonic acid is associated with other carbohydrate components of the cell - araban and galactan. Under the action of the enzyme protopectinase, Araban and galactan are cleaved from protopectin. As a result of the action of this enzyme, methoxylated polygalacturonic acid, or soluble pectin, is formed. Soluble pectin is further broken down by other pectolytic enzymes.

When the enzyme pectin esterase acts on soluble pectin, ester bonds are hydrolyzed, resulting in the formation of methyl alcohol and polygalacturonic acid, i.e. pectin esterase cleaves off the methoxy groups of methoxypolygalacturonic acid.

The enzyme polygalacturonase, when acting on soluble pectin, cleaves the bonds between those regions of polygalacturonic acid that do not contain methoxyl groups.

Technological and physiological significance

An important property of pectin substances is their ability to gel, that is, to form strong jellies in the presence of a large amount of sugar (6570%) and at a pH of 3.13.5. In the resulting jelly, the mass fraction of pectin ranges from 0.2 to 1.5%.

Pectin substances are also capable of forming gels with appropriate treatment - in the presence of hydrogen peroxide and peroxidase, cross-linking of the side chains occurs; in the presence of acid and sugar, as well as calcium salts, pectins also form gels with high water absorption capacity - 1 g of pectin can absorb from 60 to 150 g of water.

Only highly esterified pectins form dense gels. Partial hydrolysis of methyl esters leads to a decrease in gelling ability. With complete hydrolysis of methoxy groups in alkaline solutions or under the action of the enzyme pectinesterase, pectic acids are formed, which are polygalacturonic acid. Polygalacturonic acid is not capable of forming jelly.

The gelling ability of pectin substances is the basis for their use as a gelling component in the confectionery industry for the production of confitures, marmalade, marshmallows, jellies, jams, as well as in the canning industry, bakery and cheese production.

Pectin substances have important physiological properties, removing heavy metals from the body as a result of the combination of multivalent metal ions with non-esterified groups --COO- according to the type of ionic bonds.