UNIT 1 CARBOHYDRATES

Introduction. Classification, Properties and Biological Importance. Isomers, epimers,

enantiomers, mutarotation, open chain and closed chain structures of glucose.

By Ms. Prajita Kundu

Assistant Professor, GNIPST

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 CARBOHYDRATES

Carbohydrates are the most abundant biomolecules on earth. Oxidation of

carbohydrates is the central energy-yielding pathway in most non-photosynthetic
cells.

Definition:Carbohydrates are polyhydroxy aldehydes or ketones, or substances that

yield such compounds on hydrolysis.

carbohydrates have the empirical formula (CH2O)n.

There are three major classes of carbohydrates:

1. Monosaccharides

Monosaccharides, or simple sugars, consist of a single polyhydroxy aldehyde or

ketone unit. The most abundant monosaccharide in nature is the six-carbon sugar D-
glucose, sometimes referred to as dextrose.

2. Oligosaccharides

Oligosaccharides consist of short chains of monosaccharide units, or residues,

joined by characteristic linkages called glycosidic bonds. The most abundant are the
disaccharides, with two monosaccharide units. Example: sucrose (cane sugar).

3. Polysaccharides

The polysaccharides are sugar polymers containing more than 20 or so

monosaccharide units, and some have hundreds or thousands of units. Example:
starch.

Polysaccharides are of two types based on their function and composition. Based on
function, polysaccharides of two types storage and structural.
A. Storage polysaccharide - starch.
B. Structural polysaccharide - cellulose.

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General properties of carbohydrates

 Carbohydrates act as energy reserves, also stores fuels, and metabolic

intermediates.
 Ribose and deoxyribose sugars forms the structural frame of the genetic
material, RNA and DNA.
 Polysaccharides like cellulose are the structural elements in the cell walls of
bacteria and plants.
 Carbohydrates are linked to proteins and lipids that play important roles in cell
interactions.
 Carbohydrates are organic compounds, they are aldehydes or ketones with
many hydroxyl groups.

Physical Properties of Carbohydrates

 Steroisomerism - Compound shaving same structural formula but they differ in

spatial configuration. Example: Glucose has two isomers with respect to
penultimate carbon atom. They are D-glucose and L-glucose.
 Optical Activity - It is the rotation of plane polarized light forming (+)
glucose and (-) glucose.
 Diastereoisomeers - It the configurational changes with regard to C2, C3, or
C4 in glucose. Example: Mannose, galactose.

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  Annomerism - It is the spatial configuration with respect to the first carbon
atom in aldoses and second carbon atom in ketoses.

Biological Importance

 Carbohydrates are chief energy source, in many animals, they are instant
source of energy. Glucose is broken down by glycolysis/ kreb's cycle to yield
ATP.
 Glucose is the source of storage of energy. It is stored as glycogen in animals
and starch in plants.
 Stored carbohydrates acts as energy source instead of proteins.
 Carbohydrates are intermediates in biosynthesis of fats and proteins.
 Carbohydrates aid in regulation of nerve tissue and is the energy source for
brain.
 Carbohydrates gets associated with lipids and proteins to form surface
antigens, receptor molecules, vitamins and antibiotics.
 They form structural and protective components, like in cell wall of plants and
microorganisms.
 In animals they are important constituent of connective tissues.
 They participate in biological transport, cell-cell communication and activation
of growth factors.
 Carbohydrates that are rich in fibre content help to prevent constipation.
 Also they help in modulation of immune system.

Monosaccharides

 The word “Monosaccharides” derived from the Greek word “Mono” means
Single and “saccharide” means sugar
 Monosaccharides are polyhydroxy aldehydes or ketones which cannot be
further hydrolysed to simple sugar.
 Monosaccharides are simple sugars. They are sweet in taste. They are
soluble in water. They are crystalline in nature.
 They contain 3 to 10 carbon atoms, 2 or more hydroxyl (OH) groups and one
aldehyde (CHO) or one ketone (CO) group.

Classification of Monosaccharides

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 Monosaccharides are classified in two ways. (a) First of all, based on the
number of carbon atoms present in them and (b) secondly based on the presence of
carbonyl group.

The naturally occurring monosaccharides contain three to seven carbon

atoms per molecule. Monosaccharides of specific sizes may be indicated by names
composed of a stem denoting the number of carbon atoms and the suffix -ose. For
example, the terms triose, tetrose, pentose, andhexose signify monosaccharides
with, respectively, three, four, five, and six carbon atoms. Monosaccharides are also
classified as aldoses or ketoses. Those monosaccharides that contain an aldehyde
functional group are called aldoses; those containing a ketone functional group on
the second carbon atom are ketoses. Combining these classification systems gives
general names that indicate both the type of carbonyl group and the number of
carbon atoms in a molecule. Thus, monosaccharides are described as aldotetroses,
aldopentoses, ketopentoses, ketoheptoses, and so forth. Glucose and fructose are
specific examples of an aldohexose and a ketohexose, respectively.

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Trioses
Trioses are “Monosaccharides” containing 3 carbon atoms. The molecular formula of
triose is C3H6O3

Characteristics
 Trioses are simple sugars

 They are soluble in water

 They are sweet in taste.

 The triose may contain an aldehyde group (aldotriose) or a ketone group

(ketotriose). Example Glycerose and Dehydroxyacetone

Tetroses
Tetroses are “Monosaccharides” containing 4 carbon atoms. The molecular formula
of tetrose is C4H8O4

Characteristics

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  Tetroses are simple sugars

 Tetroses are soluble in water

 They are sweet in tast .

 They are crystalline forms.

 The tetroses may contain an aldehyde group (aldotetrose) or a ketone group

(ketotetrose).

Erythrose

Pentoses
Pentoses are “Monosaccharides” containing 5 carbon atoms. It is an important
component of “nucleic acid”. The molecular formula of Pentose is C5H10O5
Characteristics
 Pentoses are simple sugars
 Pentoses are soluble in water

 They are sweet in tast .

 They are crystalline forms.

 The pentoses may contain an aldehyde group (aldopentose) or a ketone

group (ketopentose).

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Hexoses

Hexoses are “Monosaccharides” containing 6 carbon atoms. The molecular formula

of Hexose is C6H12O6
Characteristics
 Hexoses are simple sugars

 Hexoses are soluble in water

 They are sweet in tast .

 They are crystalline forms.

 The pentoses may contain an aldehyde group (aldohexose) or a ketone group

(ketohexose).
Structure of Monosaccharides
1. Straight or Open Chain Structure: Here 6 carbon atoms of glucose are
arranged in a straight line. It is also called open chain structure because the two
ends remain separate and they are not linked. Open chain structure are of two
types –
 (a)Structure proposed by Fittig and Baeyer

 (b)Structure proposed by Fischer known as Fischer’s Projection Formula.

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2. Cyclic or Ring Structure: Here the atoms are arranged in the form of a ring.
Haworth (1929) proposed this formula and hence the name Haworth’s Projection
Formula. The sugar molecules exist in two type of rings which are as follows –
(a)Furanose Ring – 5 membered ring

(b)Pyranose Ring- 6 membered ring

Properties of Monosaccharides
1.Colour - colourless

2.Shape - crystalline

3.Solubility – water soluble

4.Taste - sweet

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5. Optical activity – Optically active. (a) Dextrorotatory (‘d’ form) and (b) Levorotatory
(‘l’ form)

6. Mutarotation – The change in specific rotation of an optically active compound is

called mutarotation. +1120 +52.50 +190 α-D-glucose β -D-glucose

7. Glucoside formation -

Glucose + Methyl alcohol = Methyl glucoside

8. Esterification –

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9. Reducing agents –
Monosaccharides reduce oxidizing agent such as hydrogen peroxide. In such
reaction, sugar is oxidized at the carbonyl group and oxidizing agent becomes
reduced.
C6H12O6 + 2 Cu(OH)2→C6H12O7 + Cu2O + 2H2O

Glucose Fehling’s GluconicCuprous

solutionacid oxide

10. Formation of Osazone –

Disaccharides

Disaccharides consist of two sugars joined by an O-glycosidic bond.

The most abundant disaccharides are sucrose,lactose and maltose.

Other disaccharides include isomaltose, cellobiose and trehalose.

The disaccharides can be classified into:

1. Homodisaccharides

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2. Heterodisaccharides.

Hommodisaccharides Maltose Isomaltose Celebiose

(malt sugar )

structure 2α-glucose 2 α-glucose 2β-D-glucose

Type of bond α-1-4 α1-6 β1-4

glucosidicbond glucosidicbond glucosidicbond.

Anomeric Carbon Free Free Free

Reducing Property Reducing Reducing Reducing

Produced by It is produced by the hydrolysis by the acid
from starch by the of some hydrolysis of
action of amylase polysaccharides cellulose
such as dextran

Heterodisaccharides: are formed of 2 different monosaccharide units

Heterodisaccharides Sucrose Lactose

Composition α-D-glucose+ β–D-fructose β-D-galactoseand β-D-

glucose
Type of bond α-1-β-2 glucosidic bond OR a β (1 4)
β 2-α-1 fructosidic bond galactosidicbond
AnomericC no free aldehydeor free

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 ketonegroup
Reducing property is not a reducing sugar Reducing
Composition α-D-glucose+β–D-fructose β-D-galactoseandβ-D-
glucose
AnomericC nofreealdehydeorketonegroup free
Effectofhydrolysis The hydrolysis of sucrose to Hydrolysed by the
glucose and fructose is intestinal lactase
catalysed by sucrose (also enzyme into galactose
called invertase), and glucose
Present in Table sugar Milk sugar
Cane sugar, It may appear in urine in
beet sugar late pregnancy and
during lactation

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Polysaccharides

Polysaccharides contain hundreds or thousands of carbohydrate units.

 Polysaccharides are not reducing sugars, since the anomeric carbons are
connected through glycosidic linkages.
 Nomenclature:

Homopolysaccharide- a polysaccharide is made up of one type of

monosaccharide unit

Heteropolysaccharide- a polysaccharide is made up of more than one type of

monosaccharide unit

Starch

 Starch is a polymer consisting of D-glucose units.

 Starches (and other glucose polymers) are usually insoluble in water because
of the high molecular weight, but they can form thick colloidal suspensions
with water.
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  Starch is a storage compound in plants, and made of glucose units
 It is a homopolysaccharide made up of two components: amylose and
amylopectin.
 Most starch is 10-30% amylose and 70-90% amylopectin.
 Amylose – a straight chain structure formed by 1,4 glycosidic bonds
between α-D-glucose molecules.

6CH OH CH2OH
C H2OH 2 CH2OH CH2OH
O 5 O H O O H O H
H H H H H H H
H H H H H
OH H 1 4 OH H 1 OH H OH H OH H
O O O O OH
OH 2
3
H OH H OH H OH H OH H OH
amylose

Structure of Amylose Fraction of Starch

• The amylose chain forms a helix.

• This causes the blue colour change on reaction with iodine.

• Amylose is poorly soluble in water, but forms micellar suspensions

• Amylopectin-a glucose polymer with mainly α -(14) linkages, but it also has
branches formed by α -(16) linkages. Branches are generally longer than
shown above.
CH2OH CH2OH
H O H H O H amylopectin
H H
OH H OH H 1
O
OH O
H OH H OH
CH2OH CH2OH 6CH2 CH2OH CH2OH
H O H H O H H 5 O H H O H H O H
H H H H H
OH H OH H OH H 1 4 OH H OH H
4 O O
O O OH
OH 3 2
H OH H OH H OH H OH H OH

Structure of Amylopectin Fraction of Starch

• Amylopectin causes a red-violet colour change on reaction with iodine.

• This change is usually masked by the much darker reaction of amylose to

iodine.

Glycogen

• Storage polysaccharide in animals

• Glycogen constitutes up to 10% of liver mass and 1-2% of muscle mass

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 • Glycogen is stored energy for the organism

• Similar in structure to amylopectin, only difference from starch: number of

branches

• Alpha(1,6) branches every 8-12 residues

• Like amylopectin, glycogen gives a red-violet color with iodine

Cellulose

• The β-glucose molecules are joined by condensation, i.e. the removal of

water, forming β-(1,4) glycosidic linkages.

• The glucose units are linked into straight chains each 100-1000 units long.

• Weak hydrogen bonds form between parallel chains binding them into
cellulose microfibrils.

• Cellulose microfibrils arrange themselves into thicker bundles called

microfibrils. (These are usually referred to as fibres.)

• The cellulose fibres are often “glued” together by other compounds such as
hemicelluloses and calcium pectate to form complex structures such as plant
cell walls.

• Because of the β-linkages, cellulose has a different overall shape from

amylose, forming extended straight chains which hydrogen bond to each
other, resulting in a very rigid structure.
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 • Cellulose is an important structural polysaccharide, and is the single most
abundant organic compound on earth. It is the material in plant cell walls that
provides strength and rigidity; wood is 50% cellulose.
• Most animals lack the enzymes needed to digest cellulose, although it
does provide needed roughage (dietary fiber) to stimulate contraction of the
intestines and thus help pass food along through the digestive system
• Some animals, such as cows, sheep, and horses, can process cellulose
through the use of colonies of bacteria in the digestive system which are
capable of breaking cellulose down to glucose; ruminants use a series of
stomachs to allow cellulose a longer time to digest. Some other animals such
as rabbits reprocess digested food to allow more time for the breakdown of
cellulose to occur.
• Cellulose is also important industrially, from its presence in wood, paper,
cotton, cellophane, rayon, linen, nitrocellulose (guncotton), photographic films
(cellulose acetate), etc.

CHITIN

• Chitin is a polymer that can be found in anything from the shells of beetlesto
webs of spiders. It is present all around us, in plant and animal creatures.

• It is sometimes considered to be a spinoff of cellulose, because the two are

very molecularly similar.

• Cellulose contains a hydroxy group, and chitin contains acetamide.

• Chitin is unusual because it is a "natural polymer," or a combination of

elements that exists naturally on earth.

• Usually, polymers are man-made. Crabs, beetles, worms and mushrooms

contain large amount of chitin.

• Chitin is a very firm material, and it help protect an insect against harm and
pressure
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Inulin

• Inulin is stored in the t ubers of the dahlia and artichoke and in the roots of
dandelion. It is also fo und in onion and garlic.
• Inulin (Fig. 8–4) has a molecular weight of about 5,000 and consists of about
30–35 fructose units per molecule.
• It is formed in the plants by eliminating a molecule of water from t he glycosidic
OH group on carbon atom 2 of one β-D-fructose unit and the alcoholic OH
group on carbon atom 1 of the adjacent β-D-fructose unit.

Pectin

• Pectins are found as intercellular substances in the tissues of young plants

and are especially abundant in ripe fruits such as guava, apples and pears.
• Pectin is a polysaccharide of α-D-galacturonic acid where som e of the free
carboxyl groups are, either partly or completely, esterified with methyl alcohol
and others are combined withcalcium or magnesium ions. Chemically, they
are called polygalacturonides

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Mucopolysaccharides

Polysaccharides that are composed not only of a mixture of simple sugars but
also of derivatives of sugars such as amino sugars and uronic sugars are called
mucopolysaccharides.

Hyaluronic acid

• It is the most abundant member of mucopolysaccharides and is found in

higher animals as a component of various tissues such as the vitreous body
of the eye, the umbilical cord and the synovial fluid of joints.
• It is a straight-chain polymer of D-glucuronic acid and N-acetyl- D-
glucosamine (NAG) alternating in the chain. Its molecular weight approaches
approximately, 5,000,000. linkages invloved, β-1 → 3 and β-1 → 4.

Chondroitin

• Chondroitin is of limited distributioin. It is found in cartilage and is also a

component of cell coats. It is a parent substance for two more widely
distributed mucopolysaccharides, chondroitin sulfate A and chondroitin sulfate
B.
• Chondroitin is similar in structure to hyaluronic acid except that it contains
galactosamine rather than glucosamine. It is, thus, a polymer of β-
Dglucuronido- 1, 3-N-acetyl-D-galactosamine joined by β- 1 → 4 linkages.

• The two chondroitin sulfate A and C are widely distributed and form major
structural components of cartilage, tendons and bones.

• Chondroitin sulfates may be regarded as derivatives of chondroitin where, in

thegalactosamine moiety, a sulfate group is esterified either at carbon 4 as in
chondroitin sulfate A or at carbon 6 as in chondroitin sulfate C

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 • The two linkages involved in both types of chondroitin sulfate would,
obviously, be the same. These are β-1 → 3 and β-1 → 4.

Dermatan Sulfate

• Dermatan sulfate is a mucopolysaccharide structurally similar to chondroitin

sulfate A except that the D-glucuronic acid is replaced by L-iduronic acid
• The two linkages involved are α-1 → 3 and β-1 → 4. Dermatan sulfate is also
known byits conventional name, chondroitin sulfate B.

Keratosulfate

• Keratosulfate differs from other mucopolysaccharides in that the uronic acid

component is replaced by D-galactose. Here, the second acetylated amino
sugar component (which is N-acetyl-D-glucosamine in this case) is esterified
by a sulfate group at carbon 6. Although, the two alternating linkages involved
are β-1 → 4 and β-1 → 3, in this case the linkage between the repeating
disaccharide units is β-1 →3 rather than β-1 → 4.

Heparin

• composed of D-glucuronic acid units, most of which (about 7 out of every 8)

are esterified at C2 and D-glucosamine-N-sulfate (= sulfonylaminoglucose)

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 units with an additio nal O-sulfate group at C6. Both the linkages of the
polymer are alternating α-1 → 4. Thus, the sulfate content is v ery high and
corresponds to about 5–6 molecules per tetrasaccharide repeatin g unit.
• Heparin acts as an anticoagulant. It prevents coagulation of blood by inhibiting
the prothrombin thrombin conversion. This eliminates the effect of thrombin on
fibrinogen.

The following table is the list of biologically important polysaccharides and their
functions. Polysaccharides are complex carbohydrates.
Name of the
Composition Occurrence Functions
Polysaccharide
Polymer of glucose
containing a straight In several plant
chain of glucose molecules species as main storage of
Starch
(amylose) and a branched storage reserve food
chain of glucose molecules carbohydrate
(amylopectin)
Animals (equivalent Storage of
Glycogen Polymer of glucose
of starch) reserve food
Different regions of
Cellulose Polymer of glucose plant, in sieve tubes Cell wall matrix
of phloem
In roots and tubers Storage of
Insulin Polymer of fructose
(like Dahlia) reserve food

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 Polymer of galactose and
Pectin Plant cell wall Cell wall matrix
its derivatives
Polymer of pentoses and
Hemi cellulose Plant cell wall Cell wall matrix
sugar acids
Plant cell wall (dead
Lignin Polymer of glucose cells like Cell wall matrix
sclerenchyma)
Bodywall of Exoskeleton
Chitin Polymer of glucose arthropods. In some Impermeable to
fungi also water
Polysaccharide cross Cell wall of Structural
Murein
linked with amino acids prokaryotic cells protection
Connective tissue Ground
Hyaluronic acid Polymer of sugar acids matrix, Outer coat of substance,
mammalian eggs protection
Chrondroitin Connective tissue Ground
Polymer of sugar acids
sulphate matrix substance
Closely related to Connective tissue
Heparin Anticoagulant
chrondroitin cells
Gums - bark or
Gums and Polymers of sugars and Retain water in
trees. Mucilages -
mucilages sugar acids dry seasons
flower

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