Placeholder Image

字幕表 動画を再生する

  • Polysaccharides are polymeric carbohydrate molecules composed of long chains of monosaccharide

  • units bound together by glycosidic linkages and on hydrolysis give the constituent monosaccharides

  • or oligosaccharides. They range in structure from linear to highly branched. Examples include

  • storage polysaccharides such as starch and glycogen, and structural polysaccharides such

  • as cellulose and chitin. Polysaccharides are often quite heterogeneous,

  • containing slight modifications of the repeating unit. Depending on the structure, these macromolecules

  • can have distinct properties from their monosaccharide building blocks. They may be amorphous or

  • even insoluble 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 of simple carbohydrates called monosaccharides with

  • general formulan where n is three or more. Examples of monosaccharides are glucose, fructose,

  • and glyceraldehyde Polysaccharides, meanwhile, have a general formula of Cx(H2O)y where x

  • is usually a large number between 200 and 2500. Considering that the repeating units

  • in the polymer backbone are often six-carbon monosaccharides, the general formula can also

  • be represented asn where 40≤n≤3000. Polysaccharides contain more than ten monosaccharide

  • units. Definitions of how large a carbohydrate must be to fall into the categories polysaccharides

  • or oligosaccharides vary according to personal opinion. Polysaccharides are an important

  • class of biological polymers. Their function in living organisms is usually either structure-

  • or storage-related. Starch is used as a storage polysaccharide in plants, being found in the

  • form of both amylose and the branched amylopectin. In animals, the structurally similar glucose

  • polymer is the more densely branched glycogen, sometimes called 'animal starch'. Glycogen's

  • properties allow it to be metabolized more quickly, which suits the active lives of moving

  • animals. Cellulose and chitin are examples of structural

  • polysaccharides. Cellulose is used in the cell walls of plants and other organisms,

  • and is said to be the most abundant organic molecule 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, cellulose acetate, celluloid, and nitrocellulose. Chitin has

  • a similar structure, but has nitrogen-containing side branches, increasing its strength. It

  • is found in arthropod exoskeletons and in the cell walls of some fungi. It also has

  • multiple uses, including surgical threads. Polysaccharides also include callose or laminarin,

  • chrysolaminarin, xylan, arabinoxylan, mannan, fucoidan and galactomannan.

  • 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 chitin and arabinoxylans. These carbohydrates types can be metabolized

  • by some bacteria and protists. Ruminants and termites, for example, use microorganisms

  • to process cellulose. Even though these complex carbohydrates are

  • not very digestible, they provide important dietary elements for humans. Called dietary

  • fiber, these carbohydrates enhance digestion among other benefits. The main action of dietary

  • fiber is to change the nature of the contents of the gastrointestinal tract, and to change

  • how other nutrients and chemicals are absorbed. Soluble fiber binds to bile acids in the small

  • intestine, making them less likely to enter the body; this in turn lowers cholesterol

  • 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

  • short-chain fatty acids as byproducts with wide-ranging physiological activities. 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, dietary fiber is nevertheless regarded as important for the diet, with regulatory

  • authorities in many developed countries recommending increases in fiber intake.

  • Storage polysaccharides Starches

  • Starches are glucose polymers in which glucopyranose units are bonded by alpha-linkages. It is

  • made up of a mixture of amylose and amylopectin. Amylose consists of a linear chain of several

  • hundred glucose molecules and Amylopectin is a branched molecule made of several thousand

  • glucose units. Starches are insoluble in water. They can be digested by hydrolysis, catalyzed

  • by enzymes called amylases, which can break the alpha-linkages. Both humans and animals

  • have amylases, so they can digest starches. Potato, rice, wheat, and maize are major sources

  • of starch in the human diet. The formations of starches are the ways that plants store

  • glucose Glycogen

  • Glycogen serves as the secondary long-term energy storage in animal and fungal cells,

  • with the primary energy stores being held in adipose tissue. Glycogen is made primarily

  • by the liver and the muscles, but can also be made by glycogenesis within the brain and

  • stomach. Glycogen is the analogue of starch, a glucose

  • polymer in plants, and is sometimes referred to as animal starch, having a similar structure

  • to amylopectin 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 cytosol/cytoplasm in many cell types, and plays an important

  • role in the glucose cycle. Glycogen forms an energy 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 triglycerides. In the liver hepatocytes, glycogen can compose

  • up to eight percent of the fresh weight soon after a meal. Only the glycogen stored in

  • the liver can be made accessible to other organs. In the muscles, glycogen is found

  • in a low concentration of one to two percent of the muscle mass. The amount of glycogen

  • stored in the bodyespecially within the muscles, liver, and red blood cellsvaries

  • with physical activity, basal metabolic rate, and eating habits such as intermittent fasting.

  • Small amounts of glycogen are found in the kidneys, and even smaller amounts in certain

  • glial cells in the brain and white blood cells. 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 red when

  • mixed with iodine. It also yields glucose on hydrolysis.

  • Structural polysaccharides Arabinoxylans

  • Arabinoxylans are found in both the primary and secondary cell walls of plants and are

  • the copolymers of two pentose sugars: arabinose and xylose.

  • Cellulose The structural component of plants are formed

  • primarily from cellulose. Wood is largely cellulose and lignin, while paper and cotton

  • are nearly pure cellulose. Cellulose is a polymer 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 termites 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

  • Chitin is one of many naturally occurring polymers. It forms a structural component

  • of many animals, such as exoskeletons. Over time it is bio-degradable in the natural environment.

  • Its breakdown may be catalyzed by enzymes called chitinases, secreted by microorganisms

  • such as bacteria and fungi, and produced by some plants. Some of these microorganisms

  • have receptors to simple sugars from the decomposition of chitin. If chitin is detected, they then

  • produce enzymes to digest it by cleaving the glycosidic bonds in order to convert it to

  • simple sugars and ammonia. Chemically, chitin is closely related to chitosan.

  • It is also closely related to cellulose in that it is a long unbranched chain of glucose

  • derivatives. Both materials contribute structure and strength, protecting the organism.

  • Pectins Pectins are a family of complex polysaccharides

  • that contain 1,4-linked α-D-galactosyluronic acid residues. They are present in most primary

  • cell walls and in the non-woody parts of terrestrial plants.

  • Acidic polysaccharides Acidic polysaccharides are polysaccharides

  • that contain carboxyl groups, phosphate groups and/or sulfuric ester groups.

  • Bacterial capsular polysaccharides Pathogenic bacteria commonly produce a thick,

  • mucous-like, layer of polysaccharide. This "capsule" cloaks antigenic proteins 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

  • molecular weights on the order of 100-2000 kDa. They are linear and consist of regularly

  • repeating subunits of one to six monosaccharides. There is enormous structural diversity; nearly

  • two hundred different polysaccharides are produced by E. coli alone. Mixtures of capsular

  • polysaccharides, either conjugated or native are used as vaccines.

  • Bacteria and many other microbes, including fungi and algae, 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 xanthan gum, dextran,

  • welan gum, gellan gum, diutan gum and pullulan. Most of these polysaccharides exhibit useful

  • visco-elastic 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

  • shear thinning; the study of such matters is called rheology.

  • 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 ecology and physiology. They serve as a barrier between the cell wall

  • and the environment, mediate host-pathogen interactions, and form structural components

  • of biofilms. These polysaccharides are synthesized from nucleotide-activated precursors 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 organism. Lipopolysaccharide 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 and B-band

  • O-antigens have been identified and the metabolic pathways defined. The exopolysaccharide alginate

  • is a linear copolymer of β-1,4-linked D-mannuronic acid and L-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 exopolysaccharides found

  • to be important for biofilm formation. Rhamnolipid is a biosurfactant whose production is tightly

  • regulated at the transcriptional level, but the precise role that it plays in disease

  • is not well understood at present. Protein glycosylation, particularly of pilin and flagellin,

  • 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

  • Polysaccharides with unprotected vicinal diols or amino sugars give a positive Periodic acid-Schiff

  • stain. The list of polysaccharides that stain with PAS is long. Although mucins 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 Glycan

  • Oligosaccharide nomenclature Polysaccharide encapsulated bacteria

  • References

  • External links Polysaccharide Structure

  • Applications and commercial sources of polysaccharides European Polysaccharide Network of Excellence

Polysaccharides are polymeric carbohydrate molecules composed of long chains of monosaccharide

字幕と単語

動画の操作 ここで「動画」の調整と「字幕」の表示を設定することができます

B2 中上級

多糖類 (Polysaccharide)

  • 238 6
    VoiceTube に公開 2021 年 01 月 14 日
動画の中の単語