Name: 多糖類 (Polysaccharide)
Uploaded: 2021-01-14T06:00:51.000Z
Duration: 14 min 51 s
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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.

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.

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

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.

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.