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CHAPTER 3
CARBOHYDRATES
Introduction occurrence, classification, biochemical functions and
physiological importance of carbohydrates.
INTRODUCTION
Carbohydrates occur widely in nature. Cellulose of wood and paper,
starches of cereals, roots and tubers, sugars of fruits and milk are examples of
carbohydrates. Animal tissues contain glycogen (animal starch). They are the
major sources of energy and play key role in many biochemical processes
including energy generation.
DEFINITION
Chemically all the carbohydrates are aldehyde or ketone derivatives of
polyhydroxy alcohol or their condensation products. They contain C, H and O. The
general (empirical) formula is Cn (H2O)n . However, there are many exception to
this general formula such as deoxy ribose ( C5 H10 O4 ), formaldehyde (HCHO),
acetic acid (C2 H4O2 ) (not carbohydrates).
CLASSIFICATION
The carbohydrates are classified in four different groups.
1. Monosaccharides: Those sugars which can't be hydrolysed to smaller
units.
2. Disaccharides: Those sugars which are composed of two mono
saccharides.
3. Oligosaccharides: Upon hydrolysis can yield 3 to 10 monosaccharides.
4. Polysaccharides:Simple polysaccharides and Complex polysaccharides
PROPERTIES :
(A)
PHYSICAL PROPERTIES OF MONOSACCHRIDIES:
1. Colourless, odourless, neutral, sweet in taste
2. Soluble in water, insoluble in ether
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3. Optically active
4. Melting point indefinite
5. Heating effect : Melt on heating, on more heating , they are converted
to black carbon.
CARBOHYDRATES
▼
SUGARS
Mono
Saccharides
Disaccharide
(C12H22 O11)
NON SUGAR
Oligo Simple poly
Saccharides Saccharide
Complex Poly
Saccharides
Pentose----Hexose
(C5H10O5) (C6H12O6)
Sucrose * Trisaccharides Pentosan Hemicellulose
(Glu+Fru)
Pectic substances
Maltose
Raffinose
Gum
Arabinose Glucose
(Glu+Glu)
(Glu+Fru+Gala) # Araban Mucilage
Xylose
Galactose Lactose
*Tetrasaccharide # Xylan
Lignin
Ribose
Fructose (Glu+Gala)
Stachyose
# Riban
Glycosides
(2Gala+Glu+Fru)
Hexosans
1.Glucosan
(Starch,dextrin,
cellulose, Glycogen)
2. Fructosan (Inulin)
3. Galactan and
4. Mannan
(B)
CHEMICAL PROPERTIES :
1. Oxidation with mild oxidising agent : In presence of weak oxidising
agent such as alkaline CuSO4, Ammonical AgNO3 or bromine water give
acid ( Gluconic acid), Glucosaccharic acid, Tri-hydroxy butyric acid .
R-CHO + CuSO4 ------------ R-COOH
Oxidation
Red ppt.
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2. Reduction : When monosaccharides are heated in presence of catalyst
with hydrous solution, they give sorbitol or mannitol.
R-CHO + Reducing agent ------------- R-OH
3. Reaction with HCN : Upon reaction with HCN, they produce
corresponding cyanohydrins
4. Reaction with hydrazine : Monosaccharides produce corresponding
hydrazone with phenyl hydrazine
5. Reducing properties
: Some monosaccharides are strong reducing
agents and they reduce oxidising agents such as
CuSO4 and they are converted to acid
OTHER PROPERTIES OF MONOSACCHARIDES:
1. ACETYLATION : When reacted with acetyl chloride, give acetyl derivative
2. FERMENTATION
: Fermentation take place with the help of enzymes of
microorgs to yield alcohol, acetic acid, putrefaction of
meat, sourness of milk etc
a. FERMENTATION OF SUCROSE FROM MOLASSES:
C12H22O11 + H2O
Sucrose
--------------->
Invertase
C6H12O6 +
Glucose
C6H12O6
Fructose
from yeast
C6H12O6 ---------------------------------> 2C2H5OH + 2CO2
Zymase from yeast
ethanol
b) MALTING OF BARLEY (BEER FERMENTATION):
n ( C6H12O5 ) n + H2O
Starch from barley
----------------------->
Hydrolysis by amylase
n ( C12H22O11 )
Maltose
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C12H22O11 + H2O -------->
Maltose
C6H12O6 +
Maltase
C6H12O6
Glucose
Glucose
C6H12O6 ---------------------------------> 2C2H5OH + 2CO2
Glucose
Zymase from yeast
ethanol
3. ESTERIFICATION : When monosaccharides react with acid they yield
Esters.
4. METHYLATION : Methylating agents such as CH3I + Ag2O or CH3OH +
HCl, CH3Cl to yield glycosides.
C6H11 O5 OH + CH3OH ------
HCl
C6H12O5-OCH3
+
H2O
Methyl glycoside
5. OPTICAL ACTIVITY: Many of the soluble carbohydrates are
optically active
when present in solution. Compounds which in solution, rotate the plane of
polarised light to the right are called dextro rotatory (clock wise) and to left side
are called leavo rotatory (anti-clock wise).
e.g.
1. Glucose = (+) 52.5 0 (dextro-rotatory)
2. Fructose = (-) 133.5 0 (Leavo-rotatory)
Glucose exists in two isomeric forms α - d-glucose and β -d-glucose.
6. ISOMERISM: It is a phenomenon in which substances have same molecular
formula but different structural formulas are called isomerism.
There are two types of isomerism:
a. CHEMICAL ISOMERISM: Where the molecular formula is same but the
atomic groupings are different.e.g. Glucose (aldehyde) and fructose (keto).
Both have molecular formula = C6H12O6
.
Glucose ------------------ Fructose
(Aldehyde) --------------- ( Keto )
-CHO -------------------- C = 0
C6 H12 O6 ---------------
C6 H12 O6
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GLUCOSE
FRUCTOSE
CHO
|
H – C - OH
|
OH – C – H
|
H – C – OH
|
H – C – OH
|
CH2OH
CH2OH
|
C=O
|
OH-C-H
|
H-C-OH
|
H-C-OH
|
CH2OH
b. STEREOISOMERISM: Molecules that have exactly the same atomic
grouping but arranged in different patterns.
Glucose
D-GLUCOSE
↔
Mannose
D-MANNOSE
CHO
|
H – C - OH
|
OH –C – H
|
H – C – OH
|
H– C – OH
|
CH2OH
CHO
|
HO - C- H
|
OH-C-H
|
H-C-OH
|
H-C-OH
|
CH2OH
(7) RING STRUCTURE:
Haworth proposed a simple ring or cyclic hemi acetal structure for glucose,
which is called Pyranose structure
H
|
OH –C--------------|
H – C – OH
|
OH—C – H
O
|
H – C – OH
|
H –C --------------|
CH2OH
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CH2OH
|
H
H
|
OH OH
|
H
O
O
H
|
H OH
|
OH
CH2OH
|
H
OH
|
|
OHCH OH
2
H
|
OH
Pyranose structure
|
H
Furanose structure
(8) Polymerisation and Condensation:
a) Polymerisation: It is a process in which two or more same monomers are
united together to form a complex polymer without evolution of water, NH3 or
other compound. The total number of elements remains unaltered.
e.g.
CO2 + H2O --------------- HCHO +
Photosynthesis
6(HCHO)
b) Condensation:
H2O
Formaldyhyde
------------------- C6H12O6 (glucose)
It is a process in which two or more same or different
compounds are united together to form a complex polymer with or without
evolution of water, NH3 or other compound. The total number of elements may not
remain same because of loss of H2O,NH3 etc.
E.g. Synthesis of Starch: n(C6H12O6) ----------- n (C6H10O5 ) + n (H2O)
Glucose
Starch
PENTOSES
There are 5 carbon sugars such as xylose, arabinose and ribose in a
complex polymeric form which has importance is agriculture.
1. D-Xylose
: Wood gum (xylan)  Wheat bran oat hulls, cotton seed hull.
2. L-Arabinose : Vegetable gum Gum- -
It has a industrial importance
arabic, cherry tree gum
3. D- Ribose
: Nucleic acid
------- Part of DNA and RNA structure.
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HEXOSES
GLUCOSE
1. Aldehyde sugar
2. C6 H12 O6
FRUCTOSE
GALACTOSE
Keto sugar
Aldehyde sugar
C6 H12 O6
3. Grape sugar
C6 H12 O6
Fruit sugar
Brain sugar
4. Structure
(5) Reducing sugar
Reducing sugar
Reducing sugar
(6) Osazone +Ve
Osazone +Ve
Osazone +Ve
(7) Can occur freely
Doesn’t occur freely
Occur in fruit and honey
in nature. Can be
obtained only from
lactose.
GLUCOSE
CHO
|
H – C - OH
|
OH – C – H
|
H – C – OH
|
H – C – OH
|
CH2OH
CH2OH
|
H
H
|
OH OH
|
H
FRUCTOSE
CH2OH
|
C=O
|
OH-C-H
GALACTOSE
CHO
|
H-C-OH
|
OH-C-H
|
H-C-OH
|
H-C-OH
|
CH2OH
O
O
H
|
H OH
|
OH
Pyranose structure
CH2OH
|
H
H
|
OH
OH
|
|
OH CH2OH
|
H
Furanose structure
|
OH-C-H
|
H-C-OH
|
CH2OH
CH2OH
|
H
OH
|
H OH
|
H
O
H
|
OH
H
|
OH
Pyranose structure
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DISACCHARIDES
The disaccharides are group of compounds having two Monosaccharides
linked with a glycosidic bond with elimination of water.
Molecular formula: C12H22O11
C6 H12 O6
+ C6 H12 O6
--------- C12 H22 O11 + H2O
Two types of disaccharides: 1. Reducing sugars. 2. Non reducing sugar.
REDUCING SUGARS: Those sugars which have either free aldehyde or keto
group for the reaction.
NON REDUCING SUGAR: Those sugars which do not have either free aldehyde
or keto group for the reaction.
1. SUCROSE: It is also called cane sugar composed of glucose and fructose.
Glucose + Fructose ------- Sucrose + H2O
CH2OH
|
H
H
|
OH OH
|
H
O
O
H
|
OH
H
|
OH
+
CH2OH
|
H
OH
|
|
OH CH OH
2
H
|
OH
Glucose
------
|
H
Fructose
CH2OH
|
H
OH
|
H OH
|
H
O
H
H
|
OH
SUCROSE
O
O
CH2OH
|
H
H
|
OH
OH CH2OH
|
H
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The linkage in the sucrose involves the C1 of glucose with the C2 of
fructose with a oxygen bond. During the formation of sucrose, the aldehyde group
of glucose and keto group of fructose is lost so the sucrose becomes nonreducing.
It is hydrolysed to monosaccharide (glucose and fructose with the enzyme
invertase (sucrase). It gives Osazone negative reaction. It does not show
mutarotation and can be crystallised. It is also called invert sugar.
2. MALTOSE : Malt sugar is composed of glucose + glucose with α 1,4 linkage.
It is a reducing sugar.
Molecular formula : C12 H22 O11
C6 H12 O6
+
Glucose
C6 H12 O6
--------- C12 H22 O11 + H2O
Glucose
Maltose
It is a product formed from starch during malting process. The hydrolysis of
starch by diastase enzyme yields maltose.
Maltose is hydrolysed to two glucose by maltase enzyme. It is also called
glucose glucoside. Specific rotation is 138.5 0 and forms characteristic osazone.
STRUCTURE :
CH2OH
|
H
H
|
OH OH
|
H
O
H
H
|
OH
H
O
CH2OH
|
H
OH
|
H
O
H
|
OH
H
|
OH
3. LACTOSE :It is called milk sugar and composed of glucose + galactose
Molecular formula :
C12 H22 O11
It is joined with the β 1,4 linkage.
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It is a reducing sugar giving a characteristic osazone positive reaction.
It does not ferment easily hence ideal constituent of milk. It furnishes
galactose (brain sugar) requirement for brain function. It is hydrolysed by βgalactosidase (old name lactase) to
glucose + galactose. It is also hydrolysed by emulsion enzyme.
Structure of Lactose :
CH2OH
|
H
OH
|
H OH
|
H
O
H
H
|
OH
O
H
CH2OH
|
H
OH
|
H
O
H
|
OH
H
|
OH
POLYSACCHARIDES :
The polysaccharides are complex carbohydrates, which are polymerised
anhydride of a large but undetermined number of simple sugar.
Empirical formula : ( C6 H10 O5 ) n
PROPERTIES OF POLYSACCHARIDES:
High molecular weight, unreactive, form colloidal solution, insoluble in
water. Upon hydrolysis, it yields their constituent monosaccharides,. They are
most important nutrients of foods of plant origin, Their main function is storage of
reserve energy and structure.
STARCH :
It’s a glucose polymer with α 1,4, and α 1,6 linkage (branching). It occurs in
leaves, seeds, fruits and tubers. It is found in leaf chloroplast wherein they are
accumulated in daytime and hydrolysed and trans-located in night. It consists of
amylose and amylopectin. Amylose occurs inside the starch granule while
amylopectin is found in outer layer of starch grain. There are basic differences
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between Amylose and Amylopectin. It is a white amorphous powder, insoluble in
water.
AMYLOSE
AMYLOPECTIN
1. Inner layer of starch
1. Outer layer of starch
2. Straight chain
2. Branched
3. Alpha 1, 4, linkage
3. Alpha 1, 6 linkages
4. Mol.wt. 1 K – 50 K
4. mol.wt. 500 K – 1000 K
5. Blue color with I2
5. Purple colour wilth I2
6. Forms gel
6. Doesn’t form gel
7. 20% in starch
7. 80% in starch
PROPERTIES OF STARCH:
a) Action of enzyme:
Starch ------------ Amylodextrin ----------- Arithrodextrin
Diastase
V
Glucose-------- Maltose ------------Achrodextrin
Maltase
b) ACTION OF ACIDS : When boiled with dilute acid, it is hydrolysed to dextrin,
maltose and finally to glucose. When treated with concentrated H2SO4 + Conc.
HNO3, it gives nitro starch which is used as blasting explosive.
c) GEL FORMATION
: It is a characteristic used in a dry cell and sizing
operations.
d) It reacts with iodine to give blue colour.
USES :
1. It is used as food. It provides glucose upon hydrolysis to give the energy
from the food of plant origin such as cereals and tubers.
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2. Used in cloth washing
3. Used as explosive
4. Used as indicator in the lab.
5. Used in the preparation of gum
6. Used as electrolyte medium in dry cell.
CELLULOSE :
Cellulose is the most abundant organic substance found in nature. It is the
principal constituent of cell walls in higher plants. It occurs in almost pure form
(98%) in cotton fibres and to a lessor extent in flax (80%), jute (60-70%), wood
(40-50%) and cereal straws (30-43%).
It is linear, unbranched homoglycan of 10,000 to 15,000 D-glucose units
joined by -14 linkages. The structure of cellulose can be represented as a
series of glucopyranose rings in the chair conformation. The most stable
conformation for the polymer is the chair turned 180 relative to the adjacent
glucose residues yielding a straight extended chain. Celluose molecules within
the plant cell walls are organized into biological units of structure known as
microfibrils. A microfibril consists of a bundle of cellulose molecules arranged
with its long axis parallel to that of the others. This arrangement permits the
formation of intramolecular hydrogen bonding between the hydroxyl group of C-3
of one glucose residue and the pyranose ring oxygen atom of the next glucose
residue. This hydrogen bond impart a double bond character to the glycosidic
bond and impedes the rotation of adjacent glucose residues around the glycosidic
bond. Within the microfibril, the adjacent cellulose molecules are linked by
intermolecular hydrogen bond between C-6 hydroxyl group of one molecule and
the glycosidic bond oxygen atom of adjacent cellulose molecule.

40 - 52%
Wool and brans 
30 - 40%
Wood
Flax, Hemp, Cotton  70 – 90%
Hemicellulose : Mixed polysaccharide containing uronic acid, hexose and
pentose units which are soluble in alkali and digested by enzymes. Act as
cementing materials of plant cellulose also utilised in compost making.
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PROPERTIES :
It is a colourless, tasteless, fibrous compound, which is insoluble in water
and organic solvents but soluble in cupric amino.hydroxide prepared in cold HCl
and ZnCl2
Mol. Wt. :
20.000 ----50.000
CHEMICAL PROPERTIES :
1. HYDROLYSIS : It is converted to glucose when either boiled in conc.
H2SO4 for a long time or treated with enzyme cellulase which breaks β 1,4
linkage.
cellulase
Cellulose ------------- Cellobiose (Disaccharide) ------------ Glucose
Glucose+glucose with β 1,4 linkage
2. HYDRATION : When treated with H2O, it gives hydro- cellulose which is a
characteristics of plant sap and important for physiological functions of
plants.
3. NITRATION : When heated with mixture of H2SO4 + HNO3, it is converted
to nitro cellulose.
4. ACTION OF ALKALI : No reaction with dilute alkali but with conc. alkali,
the cellulose occupy the shape of smooth bright fibre which is translucent.
This process is called mercerisation.
5. REACTION WITH LIGNIN : When it reacts with lignin, it gives lignocellulose which is a crusting material in the wood bark. The lingo-cellulose
is not easily broken down.
The amylase enzyme cannot act upon cellulose. The animal gut
(Cows, buffalo etc.) and microorgs such as fungi and bacteria, which
secrete cellulase enzyme which breaks the β 1,4 linkage to yield glucose.
Human cannot digest cellulose because we do not have cellulase enzyme.
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IMPORTANCE
1. Cotton and textile industry : Production of wrapping, gunny bags. Ropes
and netting are made from cellulose. The cotton cloth which involve the use
of pure cellulose.
2. Paper manufacture : The papers of all types are prepared either from low
grade or pure Cellulose, which are used as newspaper wrappers or better
quality papers.
3. Explosive : Fully nitrated cellulose (gum coated) in form of compressed
blocks are used as explosives. When gun cotton + nitro-glycerine +
Vaseline are prepared, it produces smokeless explosive which is called
cordite.
4. Used in making of celluloid: Toys, bangles, combs, knife, garlands etc.
5. Used in preparing celluloid paper for motorcar.
6. Used in artificial silk and in bamboo furniture
7. Used in photographic films.
GLYCOGEN
The storage of carbohydrate in animal is in form of
glycogen (animal
starch). It is similar to amylopectin in its structure except that the mol.wt is lower
and chains are shorter. The branching is extensive. It is found in liver and muscle.
It dissolves in water to yield opalescent solution. It is easy hydrolysed to glucose.
It is non-reducing and gives red colour with iodine.
OTHER POLYSACCHARIDES :
Inulin : Inulin is a non-digestible fructosyl oligosaccharide found naturally in
more than 36000 types of plants. It is a storage polysaccharide found in
onion,garlic,chicory,artichoke,asparagus,banana,wheat and rye.It consists of
mainly,if not exclusively, of
- 21 fructosyl-fructose links.A starting glucose
moiety can be present,but is not necessary.Inulin is asoluble fibre that helps
maintain normal bowel function,decreases constipation,lowers cholesrerol and
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triglycerides.It is used for fat replacement and fibre enrichment in processed
foods.
Chitin : Structural polysaccharides in invertebrates (Insect wings).
pectin substances.
Pectin substances are natural components of plants and their fruits. They
occur in plants in connection with cellulose and such substances are called
protopectin. Protopectin is the binder of cell walls. Especially large amounts of
pectin substances are present in fruit such as: currant, gooseberry, citrus fruits
and apples. Pectin is a preparation obtained in industrial conditions, containing
pectin substances isolated from plant material and soluble in water. Those
preparations are used as food and medicine additives and they have the ability to
make gels in proper conditions. Raw material for our pectin is dried apple pomace,
containing 8-12 % pectin substances, and dried lemon peel, containing 18-25 %
pectin substances, from where they are extracted by diluted acid solution and
subsequently precipitated by alcohol, purified, dried and crumbled. Being the
substance of plant origin, it is the best gelling agent for jams and fruit jellies
production. Being the naturally compound of fruit, it makes products manufactured
with its addition retain fully organoleptic characteristics
Pectin classification
Depending on the applied raw material the following pectins can be
distinguished:



Apple pectins
Citrus-apple pectins
Citrus pectins
Depending on degree of esterification (DE) they are divided into :


High esterificated pectins (DE above 50 %)
Low esterificated pectins (DE below 50 %)
Agar
Agar is a heteropolysaccharide obtained from red algae.It is composed of
agarose –neutral gelling fraction
Agaropectin – sulfated non-gelling fraction
39
It is the most effective gelling agents known andis soluble in hot water.It melts in
the temperature range of 60 –90oC and sets between 32 and 39oC to form gel.
Uses As Solidifying agent, emulsifier
Pharmaceuticals, cosmetics and food
Laxative
Sizing material in tentile industry
Emulsifierin dairy products
Microbial lab.
Seaweed Polysaccharides
Structurally they are highly branched and composed of many different
monosaccharides.
Alginic acid is obtained from seawood principally from brown algae. It is
composed of 1 4 linked D-mannuronic acid and   4 linked L-guluronic acid
– random or alternating sequence
Uses:
1. Sauces, frozen deserts, fruit pies
2. Tabilize emulsions
3. Soft drinks (gum Arabic)
4. Beer making to stabilize foam (gum arabic0
5. Cosmetics and lotions, paints, ink
6. Adhesive, laxative, foods paper industry (karaya)
7. Food industry-ice creams, salad dressings, pie fillings (locust gum)
8. Ice cream, paper industry, fire hoses, medicines (guar)
9. Salad dressing, cheese (improves sprcading) lower cholesterol, helps
diabetics to control sugar.
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METABOLISM OF CARBOHYDRATES:
Metabolism is comprised of two processes :
a) Anabolism : The process of formation of complex polymeric substances from
simple monomeric units involving the energy is known as
anabolism.
Sun light
CO2 + H2O ------> HCHO -------> C6H12O6 -----> OLIGOSACCHARIDE
photosynthesis
-------------> POLYSACCHARIDE
b) Catabolism: The process of breakdown of complex polymeric substances into
simple monomeric units with release of energy.
Carbohydrate (Polysaccharide) -----> oligosaccharide ------>monosaccharide
Hydrolysis
Glycolysis
--------------> Pyruvic acid --------------> CO2 + H2O + ENERGY
TCA cycle
BIOLOGICAL SIGNIFICANCE OF CARBOHYDRATE :
1. Structural component of cells : Cellulose, chitin etc.
2. Major source of energy
3. Key role in metabolism
: Starch, glycogen.
: Metabolism of amino acids and fatty acids.
4. Specific functions :
Ribose ------------- Nucleic acid and nucleoprotein
Galactose---------- Cerebroside - brain lipid
Lactose ------------ Milk
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