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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 body—especially within the muscles, liver, and red blood cells—varies
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
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多糖 (Polysaccharide)

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