Polysaccharides are high molecular weight optically active carbohydrates (molecular weight from several thousand to millions), the molecules of which consist of monosaccharide residues (see). These are colorless, amorphous substances, most of which easily swell in water, forming viscous colloidal solutions. Polysaccharides are widely distributed in nature (the most common is cellulose - an integral part of wood). Starch and some other polysaccharides are formed in plants in the process of photosynthesis. In acid or enzymatic hydrolysis, polysaccharides break down into simple sugars - monosaccharides.
In living organisms, polysaccharides serve as an energy reserve (glycogen in animals, starch in plants), perform the functions of supporting elements (chitin in insects and crustaceans, cellulose in plants). Such polysaccharides as mucopolysaccharides (see) are natural anticoagulants (see) (for example, heparin) or perform some special functions. Polysaccharides, especially starch, are important constituents of food . Many polysaccharides serve as raw materials: starch - in the food, pharmaceutical industry , etc., cellulose - for the production of fibers. Physiologically active polysaccharides - heparin (see), dextrins, gums - are used in medicine.
See also Mucopolysaccharides, Carbohydrates .The
What are polysaccharides (synonyms: complex sugars, polyols, glycans) are carbohydrates, the molecules of which consist of several residues (from two to several thousand) of identical or different monosaccharides or substances close to them (deoxisaccharides, amino sugars, uronic acids, etc. .).
The general formula for the most common polysaccharides is: C n H 2m O m
. All polysaccharides are constructed according to the type of glycosides (see): the hydrogen atom in the hemiacetal hydroxyl of one monosaccharide molecule is replaced by the second monosaccharide molecule, the hydrogen atom in the hemiacetal hydroxyl of the second molecule is replaced by a third molecule, etc.
As a result, for any number of monosaccharide residues in the polysaccharide molecule, there is usually only one free half-acetal hydroxyl ("aldehyde", or restoring "start" of the polyglycoside chain).
One polyglycoside chain can be attached through the oxygen of its hemiacetal hydroxyl to any of the intermediate monosaccharide residues of another polyglycoside chain; thus, branching polysaccharides arise.
Different polysaccharides differ in the degree of polymerization, that is, in the number of monosaccharide residues in the molecule; Depending on this, they distinguish: a) oligosaccharides containing from 2 to 9 monosaccharide residues (disaccharides, trisaccharides, etc.) with a small mol. weight, well soluble in water, with a sweet taste - polysaccharides are sugar-like; b) higher polioes, usually containing several hundreds and even thousands of residues, high molecular substances, poorly soluble or insoluble in water, not having a sweet taste.
Polysaccharides differ in the presence of identical or different monosaccharide residues [homopolysaccharides (for example, glycogen, cellulose, otherwise cellulose, amylose consist of glucose residues, chitin from glucosamine, pectic acid from galacturonic acid) and heteropolysaccharides (for example, hemicelluloses, acacia, many bacterial polysaccharides)].
The presence of a straight polyglycoside chain (as in amylose, cellulose) and to some extent branched (amylopectin, glycogen) also serves as a sign of the difference between polysaccharides. Finally, polysaccharides are distinguished by the presence of pyranose or furanose rings (inulin), by the presence of α-configuration of monosaccharide residues (amylose), β-configuration (cellulose) or both of these configurations (guaran) and by the presence of certain glycosidic bonds, which connect the first carbon atom of one residue to the fourth or other carbon atoms of the other residue, for example, the α-1,4 (amylose), β-1,4 (cellulose), α-1,6 (dextran) bonds,
In many cases, the molecules of polysaccharides have different glycosidic bonds. By origin, polysaccharides are divided into vegetable, animal and polysaccharide microorganisms (bacteria and fungi).
As polyglycosides, polysaccharides undergo hydrolysis - acid or enzymatic. Since free alcohol hydroxyls remain in each monosaccharide residue, the polysaccharides can form compounds of the type of ethers and esters that are important for identification, establishment of the structure (methyl esters), as well as important substances in practice (eg, cellulose esters).
Such higher polysaccharides, as starch and a number of oligosaccharides (sucrose, lactose) have an important nutritional value. Many polysaccharides play the role of energy reserves of organisms: glycogen (see) in animals, starch and other polysaccharides in plants.
A number of polysaccharides [cellulose (cellulose) in plants and chitin in some animals - crustaceans, insects] plays an important supporting role. Many polysaccharides, especially muco-polysaccharides (see), containing the remains of amino sugars and often uronic acids, perform important highly specialized functions [for example, heparin is a natural anticoagulant, hyaluronic acid (see) carries barrier functions, mucopolysaccharides of the blood group (so-called group-specific polysaccharides ) and tissues determine their specificity]. Many polysaccharides possess antigenic (immunity-inducing) properties (immunospecific polysaccharides). A number of polysaccharides are used as honey. preparations: dextran (see), heparin (see), etc.
Many polysaccharides are of great technical value, for example cellulose, dextrins, pectic substances, which are derivatives of polygalacturonic acid.