CARBOHYDRATES

Carbohydrates are the polyhydroxy aldehydes or ketones & their derivatives. They consist of carbon,
hydrogen and oxygen. The hydrogen and oxygen are in the ratio 2:1. They are essential for the body & are
the principal source of energy for the body. They are widely distributed both in animal and plant tissues. In
animal cells, they occur chiefly in the form of glucose and glycogen. In plants, cellulose and starch are the
main forms of carbohydrates present. Empirical formula of carbohydrates: (CH2O)n. This formula holds true
for only monosaccharides. Carbohydrates basically contain only carbon, hydrogen & oxygen atoms.
Classification: Carbohydrates are usually classified as 3 groups based on their hydrolysis:
Monosaccharides, oligosaccharides & polysaccharides.
Monosaccharides: These are “simple sugars”
General formula: (CH2O)n. Simplest sugars and cannot be hydrolyzed into smaller units. If a
monosaccharide contains an aldehyde group then it is called aldose. Eg:- Glyceraldehyde, Glucose etc. If a
monosaccharide contains a keto group then it is called ketose. Eg:- dihydroxyacetone, fructose etc. The most
abundant monosaccharide is D-Glucose also called dextrose. The monosaccharides may be subdivided into
trioses, tetroses, pentoses, hexoses, heptoses, depending upon the no. of carbon atoms they possess.
Oligosaccharides: These are compound sugars that yield 2 to 10 molecules of the same or different
monosaccharide on hydrolysis. An oligosaccharide yielding 2 molecules of monosaccharide on hydrolysis is
designated as a disaccharide. An oligosaccharide yielding 3 molecules of monosaccharide on hydrolysis is
designated as a trisaccharide.
Polysaccharides: These are also compound sugars and yield more than 10 molecules of monosaccharide on
hydrolysis. These may be further classified depending on whether the monosaccharide molecules produced
as a result of the hydrolysis of polysaccharide are of the same type i.e., homopolysaccharides or of different
types i.e., heteropolysaccharides.
Homopolysaccharides: if they contain only one type of monosaccharide as, glycogen starch and chitin.
Heteropolysaccharides: contains 2 or more different kinds of monosaccharides. Eg:- hyaluronic acid,
chondroitin sulfate.
Polysaccharides are further classified as storage (starch, glycogen…) and structural polysaccharides
(Celluluse, chitin…) based on their function.
Configuration relationship of D-aldoses and D-ketoses
Configuration: The configuration of a molecule is the permanent geometry that results from the spatial
arrangement of its bonds.

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 Haworth structure of glucopyranose and Fructofuranose

Fischer structure of glucose and fructose

Amino sugars: Amino sugars are sugar molecules in which 2' hydroxyl group has been replaced with an
amine group. Amino sugars are important constituents of many polysaccharides, including glycoproteins &
of glycosaminoglycans. Examples of Amino sugars: - N-Acetylglucosamine, Sialic acid (N acetyl
neuraminic acid), N acetyl muramic acid.
 Sialic acid uses: It is necessary for mental & physical well being.
 Sialic acid is normally found on the surface of cell membranes.
 N-acetyl D-glucosamine is the main component of the polysaccharide in chitin. It is a tough outer
skeleton of arthropods & insects. Chitin is present in the exoskeleton of crustaceans & other
arthropods as well as the cell walls of fungi & many higher organisms.

Deoxy sugars: Deoxy sugars are carbohydrate derivatives that are lacking an –OH hydroxyl group at the 2-
carbon of the sugar cyclic ring. Biologically important deoxy sugars are: 1) 2-deoxy ribose, the component
of DNA, 2) L-fucose or 6-deoxy-L-galactose, 3) 6-deoxy-L-mannose or L-rhamnose. Oxygen of the
hydroxyl group may be removed to form deoxy sugars.
Rhamnose: is also a component of the outer cell membrane of acid fast bacteria in the mycobacterium genus,
which causes tuberculosis. Rhamnose shown to have anti-inflammatory & skin soothing properties. L-
Fucose has application in cosmetics pharmaceutical & dietary supplements.

OLIGOSACCHARIDES
Oligosaccharides are formed by joining two to ten monosaccharides. Disaccharides are the most common
oligosaccharide. These sugars are produced when two monosaccharides are linked by an "oxygen bridge"
called an O-glycosidic bond. Maltose a malt sugar is formed from two alpha-D-glucose molecules. It is a
disaccharide linked by an alpha (1, 4) glycosidic bond. The first carbon of one molecule is bonded to the 4th
carbon of the other molecule. Maltose is a reducing sugar.

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Lactose, a milk sugar is formed from one ß-galactose (C1) and one glucose (C4, α/ß) molecule. It is a
disaccharide linked by beta (1,4) glycosidic bond. Sucrose, cane sugar or invert sugar is a non reducing
sugar and is formed from one glucose (α-C1) and one fructose (ß-C2) molecule. It is a disaccharide linked by
beta (1,2) glycosidic bond. Other examples are cellobiose (2 glucose beta 1, 4), isomaltose (2 glucose
alpha 1, 6) and trehalose (2 glucose alpha 1, 1).

Polysaccharides are extended polymers of monosaccharide units joined by O-glycosidic linkages.

Polysaccharides can be broadly classified into two classes:
 Homo-polysaccharides – are made up of one repeating monosaccharide units. Ex: cellulose, starch,
glycogen.
 Hetero-polysaccharides – are made up of two repeating monosaccharide units. Ex. hyaluronic acid.
All polysaccharides are formed by the same basic process where monosaccharides are connected via glycosidic
bonds. These glycosidic bonds consist of an oxygen molecule bridging two carbon rings. The bond is formed
when a hydroxyl group is lost from the carbon of one molecule, while the hydrogen is lost by the hydroxyl
group of another monosaccharide. Because two molecules of hydrogen and one of oxygen are expelled, the
reaction is a dehydration reaction. The structure of the molecules being combined determines the structures
and properties of the resulting polysaccharide. A polysaccharide used for energy storage will give easy access
to the constituent monosaccharides whereas a polysaccharide used for support is usually a long chain of
monosaccharides that form fibrous structures.
Functions of Polysaccharides
Polysaccharides form a crucial part of cell function and structure.
 Storage polysaccharides: Polysaccharides such as starch and glycogen are called storage polysaccharides
because they are stored in the liver and muscles to be converted to energy later for body functions. Starch
is found in plants whereas glycogen is found in animals.

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 Structural polysaccharides: Polysaccharides such as cellulose are structural polysaccharides which are
found in the cell walls of plants. Another structural polysaccharide is chitin.

Homopolysaccharides
Starch: It is the storage polysaccharide found in plant cells and exists in two forms: amylose (20%) is the
helical form of starch comprised only of alpha-1,4 linkages and amylopectin (80%) that has a structure with
branched alpha-1,6 linkages and branching for every 24-30 sugars.

Glycogen: This polysaccharide is found in animals to store energy and is composed of alpha-1,4-glycosidic
bonds with branched alpha-1,6 bonds. Its structure is similar to amylopectin, except the branching frequency
(Branching for every 8-10 sugars). It is mainly produced by the liver and muscles, by the process
called glycogenesis.
Cellulose: Is a structural polysaccharide that is found in the cell wall of plants and when consumed, it acts as
a dietary fibre. Cellulose is said to be the most abundant organic molecule on earth. Wood, paper, and cotton
are common forms of cellulose. It is formed from glucose molecules via ß (1, 4) glycosidic bond.
Chitin: It is made up of N acetyl D glucosamine held together by ß (1,4) glycosidic bond.
Inulin: A storage polysaccharide in plants, made up of ß fructose with 1, 4 glycosidic bond. Used in renal
analysis as inulin clearance test.
Heteropolysaccharides

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These are found in different structural and functional roles in the human body. They are the
glycosaminoglycans or mucopolysaccharides that are formed by the endoplasmic reticulum. They mature in
the Golgi apparatus and form important components of connective tissues and are found in collagen and elastin.
 Hyaluronic Acid: Acts as a lubricant in the synovial fluid of joints. It is made up of repeating units of D-
glucuronic acid and N acetyl glucosamine.
 Chondroitin Sulfate A: It contributes to tensile strength and elasticity of cartilages, tendons, ligaments,
and walls of the aorta. It contains alternate units of D glucuronate and N acetyl D galactosamine 4
sulfate.
 Dermatan sulfate (CS-B): It is found mainly in the skin, and also is in vessels, heart, lungs. It may be
related to coagulation and vascular diseases and other conditions. It contains alternate units of L-
iduronate and N acetyl D galactosamine 4 sulfate.
 Keratan sulfate: Present in the cornea, cartilage & bone and a variety of other structures as nails and hair.
It is composed of galacturonate 3 sulfate and N acetyl glucosamine.
 Heparin: Is present as an anticoagulant in the blood. It contains alternate units of N sulfo D glucosamine
6 sulfate and glucuronate 2 sulfate/L-iduronate.
 Pectins: Present in primary cell walls of plants. Rich in fruits. It is made up of galacturonic acid,
substituted form with α 1, 4 glycosidic bond.
 Peptidoglycans: a bacterial cell wall component, contain repeating units of N-acetyl muramic acid and N-
acetyl glucosamine which are interlinked by peptides.
 Teichoic acid: are bacterial copolymers of glycerol and ribitol phosphate and carbohydrates linked
via phosphodiester bonds. Teichoic acids are found within the cell wall of most Gram-positive bacteria