polysaccharide
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Polysaccharides (), or polycarbohydrates, are the most abundant found in . They are long chain carbohydrates composed of units bound together by . This carbohydrate can react with water () using as catalyst, which produces constituent sugars (, or s). They range in structure from linear to highly branched. Examples include storage polysaccharides such as , and and structural polysaccharides such as and . Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure, these s can have distinct properties from their monosaccharide building blocks. They may be or even in water. When all the monosaccharides in a polysaccharide are the same type, the polysaccharide is called a ''homopolysaccharide'' or ''homoglycan'', but when more than one type of monosaccharide is present they are called ''heteropolysaccharides'' or ''heteroglycans''. Natural saccharides are generally composed of simple carbohydrates called s with general formula (CH2O)''n'' where ''n'' is three or more. Examples of monosaccharides are , , and . Polysaccharides, meanwhile, have a general formula of C''x''(H2O)''y'' where ''x'' is usually a large number between 200 and 2500. When the repeating units in the polymer backbone are , as is often the case, the general formula simplifies to (C6H10O5)''n'', where typically . As a rule of thumb, polysaccharides contain more than ten monosaccharide units, whereas s contain three to ten monosaccharide units; but the precise cutoff varies somewhat according to convention. Polysaccharides are an important class of . Their in living organisms is usually either structure- or storage-related. (a polymer of glucose) is used as a storage polysaccharide in plants, being found in the form of both and the branched . In animals, the structurally similar glucose polymer is the more densely branched , sometimes called "animal starch". Glycogen's properties allow it to be metabolized more quickly, which suits the active lives of moving animals. In , they play an important role in bacterial multicellularity. and are examples of structural polysaccharides. Cellulose is used in the s of plants and other organisms and is said to be the most abundant on Earth. It has many uses such as a significant role in the paper and textile industries, and is used as a feedstock for the production of rayon (via the process), cellulose acetate, celluloid, and nitrocellulose. Chitin has a similar structure, but has -containing side branches, increasing its strength. It is found in s and in the cell walls of some . It also has multiple uses, including s. Polysaccharides also include or , , , , , and .


Function


Structure

Nutrition polysaccharides are common sources of energy. Many organisms can easily break down starches into glucose; however, most organisms cannot metabolize cellulose or other polysaccharides like , and . These carbohydrate types can be metabolized by some bacteria and protists. s and s, for example, use microorganisms to process . Even though these complex polysaccharides are not very digestible, they provide important dietary elements for humans. Called , these carbohydrates enhance digestion among other benefits. The main action of dietary fiber is to change the nature of the contents of the , and to change how other nutrients and chemicals are absorbed. Soluble fiber binds to in the small intestine, making them less likely to enter the body; this in turn lowers levels in the blood. Soluble fiber also attenuates the absorption of sugar, reduces sugar response after eating, normalizes blood lipid levels and, once fermented in the colon, produces s as byproducts with wide-ranging physiological activities (discussion below). Although insoluble fiber is associated with reduced diabetes risk, the mechanism by which this occurs is unknown. Not yet formally proposed as an essential macronutrient (as of 2005), dietary fiber is nevertheless regarded as important for the diet, with regulatory authorities in many developed countries recommending increases in fiber intake.


Storage polysaccharides


Starch

is a polymer in which units are bonded by ''alpha''-linkages. It is made up of a mixture of (15–20%) and (80–85%). Amylose consists of a linear chain of several hundred glucose molecules, and Amylopectin is a branched molecule made of several thousand glucose units (every chain of 24–30 glucose units is one unit of Amylopectin). Starches are in . They can be digested by breaking the ''alpha''-linkages (glycosidic bonds). Both humans and other animals have amylases, so they can digest starches. , , , and are major sources of starch in the human diet. The formations of starches are the ways that plants store .


Glycogen

Glycogen serves as the secondary long-term energy storage in and cells, with the primary energy stores being held in . Glycogen is made primarily by the and the s, but can also be made by within the and . Glycogen is analogous to , a glucose polymer in s, and is sometimes referred to as ''animal starch'', having a similar structure to but more extensively branched and compact than starch. Glycogen is a polymer of α(1→4) glycosidic bonds linked, with α(1→6)-linked branches. Glycogen is found in the form of granules in the /cytoplasm in many types, and plays an important role in the . Glycogen forms an reserve that can be quickly mobilized to meet a sudden need for glucose, but one that is less compact and more immediately available as an energy reserve than (lipids). In the liver s, glycogen can compose up to 8 percent (100–120 grams in an adult) of the fresh weight soon after a meal. Only the glycogen stored in the liver can be made accessible to other organs. In the s, glycogen is found in a low of one to two percent of the muscle mass. The amount of glycogen stored in the body—especially within the , , and —varies with physical activity, , and eating habits such as . Small amounts of glycogen are found in the s, and even smaller amounts in certain cells in the and . The uterus also stores glycogen during pregnancy, to nourish the embryo. Glycogen is composed of a branched chain of glucose residues. It is stored in liver and muscles. *It is an energy reserve for animals. *It is the chief form of carbohydrate stored in animal body. *It is insoluble in water. It turns brown-red when mixed with iodine. *It also yields glucose on . File:Glycogen structure.svg, Schematic 2-D cross-sectional view of glycogen. A core protein of is surrounded by branches of units. The entire globular granule may contain approximately 30,000 glucose units. File:Glycogen spacefilling model.jpg, A view of the ic structure of a single branched strand of units in a glycogen .


Galactogen

Galactogen is a polysaccharide of that functions as energy storage in snails and some . This polysaccharide is exclusive of the reproduction and is only found in the albumen gland from the female snail reproductive system and in the of eggs. Galactogen serves as an energy reserve for developing embryos and hatchlings, which is later replaced by in juveniles and adults.


Inulin

is a naturally occurring polysaccharide composed of , a plant-derived food that cannot be completely broken down by human digestive enzymes.


Structural polysaccharides


Arabinoxylans

s are found in both the primary and secondary cell walls of plants and are the copolymers of two sugars: and . They may also have beneficial effects on human health.


Cellulose

The structural components of s are formed primarily from . Wood is largely cellulose and , while and are nearly pure cellulose. Cellulose is a made with repeated glucose units bonded together by ''beta''-linkages. Humans and many animals lack an enzyme to break the ''beta''-linkages, so they do not digest cellulose. Certain animals such as s can digest cellulose, because bacteria possessing the enzyme are present in their gut. Cellulose is insoluble in water. It does not change color when mixed with iodine. On hydrolysis, it yields glucose. It is the most abundant carbohydrate in nature.


Chitin

is one of many naturally occurring . It forms a structural component of many animals, such as s. Over time it is in the natural environment. Its breakdown may be catalyzed by s called s, secreted by microorganisms such as and and produced by some plants. Some of these microorganisms have to simple from the decomposition of chitin. If chitin is detected, they then produce s to digest it by cleaving the s in order to convert it to simple sugars and . Chemically, chitin is closely related to (a more water-soluble derivative of chitin). It is also closely related to in that it is a long unbranched chain of derivatives. Both materials contribute structure and strength, protecting the organism.


Pectins

s are a family of complex polysaccharides that contain 1,4-linked α--galactosyl uronic acid residues. They are present in most primary cell walls and in the nonwoody parts of terrestrial plants.


Acidic polysaccharides

Acidic polysaccharides are polysaccharides that contain s, phosphate groups and/or groups.


Bacterial polysaccharides

commonly produce a thick, mucous-like, layer of polysaccharide. This "capsule" cloaks ic s on the bacterial surface that would otherwise provoke an immune response and thereby lead to the destruction of the bacteria. Capsular polysaccharides are water-soluble, commonly acidic, and have s on the order of 100,000 to 2,000,000 . They are linear and consist of regularly repeating subunits of one to six s. There is enormous structural diversity; nearly two hundred different polysaccharides are produced by ' alone. Mixtures of capsular polysaccharides, either or native, are used as s. Bacteria and many other microbes, including and , often secrete polysaccharides to help them adhere to surfaces and to prevent them from drying out. Humans have developed some of these polysaccharides into useful products, including , , , , diutan gum and . Levan-type exopolysaccharide produced by ''Pantoea agglomerans'' ZMR7 was reported to decrease the viability of rhabdomyosarcoma (RD) and breast cancer (MDA) cells compared with untreated cancer cells. In addition, it has high antiparasitic activity against the promastigote of ''Leishmania tropica.'' Most of these polysaccharides exhibit useful properties when dissolved in water at very low levels. This makes various liquids used in everyday life, such as some foods, lotions, cleaners, and paints, viscous when stationary, but much more free-flowing when even slight shear is applied by stirring or shaking, pouring, wiping, or brushing. This property is named pseudoplasticity or ; the study of such matters is called . : Aqueous solutions of the polysaccharide alone have a curious behavior when stirred: after stirring ceases, the solution initially continues to swirl due to momentum, then slows to a standstill due to viscosity and reverses direction briefly before stopping. This recoil is due to the elastic effect of the polysaccharide chains, previously stretched in solution, returning to their relaxed state. Cell-surface polysaccharides play diverse roles in bacterial and . They serve as a barrier between the and the environment, mediate host-pathogen interactions. Polysaccharides also play an important role in formation of and the structuring of complex life forms in bacteria like .'' These polysaccharides are synthesized from -activated precursors (called s) and, in most cases, all the enzymes necessary for biosynthesis, assembly and transport of the completed polymer are encoded by genes organized in dedicated clusters within the genome of the . is one of the most important cell-surface polysaccharides, as it plays a key structural role in outer membrane integrity, as well as being an important mediator of host-pathogen interactions. The enzymes that make the ''A-band'' (homopolymeric) and ''B-band'' (heteropolymeric) O-antigens have been identified and the s defined. The exopolysaccharide alginate is a linear copolymer of β-1,4-linked -mannuronic acid and -guluronic acid residues, and is responsible for the mucoid phenotype of late-stage cystic fibrosis disease. The ''pel'' and ''psl'' loci are two recently discovered gene clusters that also encode s found to be important for biofilm formation. is a biosurfactant whose production is tightly regulated at the al level, but the precise role that it plays in disease is not well understood at present. Protein , particularly of and , became a focus of research by several groups from about 2007, and has been shown to be important for adhesion and invasion during bacterial infection.


Chemical identification tests for polysaccharides


Periodic acid-Schiff stain (PAS)

Polysaccharides with unprotected or amino sugars (where some groups are replaced with s) give a positive (PAS). The list of polysaccharides that stain with PAS is long. Although s of epithelial origins stain with PAS, mucins of connective tissue origin have so many acidic substitutions that they do not have enough glycol or amino-alcohol groups left to react with PAS.


See also

* * *


References


External links




European Polysaccharide Network of Excellence
{{Authority control Carbohydrate chemistry