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 formula (CH2O)n 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.
When the repeating units in the polymer backbone are six-carbon
monosaccharides, as is often the case, the general formula simplifies
to (C6H10O5)n, where typically 40≤n≤3000.
As a rule of thumb, polysaccharides contain more than ten
monosaccharide units, whereas oligosaccharides contain three to ten
monosaccharide units; but the precise cutoff varies somewhat according
to convention. Polysaccharides are an important class of biological
polymers. Their function in living organisms is usually either
structure- or storage-related.
Starch (a polymer of glucose) 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
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
(via the viscose process), cellulose acetate, celluloid, and
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.
2 Storage polysaccharides
3 Structural polysaccharides
4 Acidic polysaccharides
5 Bacterial capsular polysaccharides
6 Chemical identification tests for polysaccharides
Periodic acid-Schiff stain
Periodic acid-Schiff stain (PAS)
7 See also
9 External links
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 carbohydrate types can be metabolized by some
bacteria and protists. Ruminants and termites, for example, use
microorganisms to process cellulose.
Even though these complex polysaccharides 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 (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.
Starch is a glucose polymer in which glucopyranose units are bonded by
alpha-linkages. It is made up of a mixture of amylose (15–20%) and
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 insoluble in water.
They can be digested by breaking the alpha-linkages (glycosidic
bonds). 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
Glycogen serves as the secondary long-term energy storage in animal
and fungal cells, with the primary energy stores being held in adipose
Glycogen is made primarily by the liver and the muscles, but
can also be made by glycogenesis within the brain and stomach.
Glycogen is analogous to 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
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 (lipids).
In the liver hepatocytes, glycogen can compose up to eight percent
(100–120 g 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 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 brown-red when mixed with iodine.
It also yields glucose on hydrolysis.
Schematic 2-D cross-sectional view of glycogen. A core protein of
glycogenin is surrounded by branches of glucose units. The entire
globular granule may contain approximately 30,000 glucose units.
A view of the atomic structure of a single branched strand of glucose
units in a glycogen molecule.
Arabinoxylans are found in both the primary and secondary cell walls
of plants and are the copolymers of two sugars: arabinose and xylose.
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
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 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 (a more
water-soluble derivative of chitin). 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 are a family of complex polysaccharides that contain
1,4-linked α-D-galactosyl uronic acid residues. They are present in
most primary cell walls and in the non-woody parts of terrestrial
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.
Viscosity of Welan gum
Shear Rate (rpm)
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 (called nucleotide sugars) 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
The enzymes that make the A-band (homopolymeric) and B-band
(heteropolymeric) 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
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
Periodic acid-Schiff stain (PAS)
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Polysaccharides with unprotected vicinal diols or amino sugars (i.e.
some OH groups replaced with amine) give a positive periodic
acid-Schiff stain (PAS). 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.
Polysaccharide encapsulated bacteria
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