Overview of Carbohydrates

Overview of Carbohydrates
Dr. Anil V Dusane
Sir Parashurambhau College
Pune, India
anildusane@gmail.com
1

Introduction
• Carbohydrates are carbon, hydrogen and oxygen containing
compounds. These are also referred as “saccharides”.
• These are the most abundant organic molecules in nature.
• Carbohydrates are soluble in water.
• These are sweet in taste so called as as “sugars”.
• Definition: Carbohydrates are defined as the polyhydroxy alcohols
with aldehydes or ketones and their derivatives. These are the
hydrates of carbon.
• Most of these compounds fit in empirical formula Cn (H2O)n.
• Carbohydrates serve as energy sources, essential structural
components in organisms and part of the structure of nucleic acids,
which contain genetic information. 2

Physical properties of carbohydrates
1. Stereoisomerism: Compounds have same structural
formula but differ in spatial configuration. E.g. Glucose
has two isomers D-glucose and L-glucose.
2. Optical Activity: It is the rotation of plane-polarized
light forming (+) glucose and (-) glucose.
3. Diastereoisomers: It the configurational change with
regard to C2, C3, or C4 in glucose. E.g. Mannose,
galactose.
4. Annomerism: It is the spatial configuration with
respect to the first carbon atom in aldoses and second
carbon atom in ketoses.
3
Stereoisomerism
Annomerism
Diastereoisomers

Chemical properties of Carbohydrates
1. Benedict’s test: Reducing sugars when heated in the presence of an
alkali gets converted to powerful reducing species known as enediols
and changes its coluor to orange-red/brick red.
2. Oxidation: Carbonyl groups of monosaccharides (reducing
sugars)oxidize to give carboxylic acids. In Benedict’s test, D-glucose is
oxidized to D-gluconic acid. Therefore glucose is considered a
reducing sugar.
3. Reduction to alcohols: The C=O groups in open-chain forms of
carbohydrates is reduced to alcohols by sodium borohydride(NaBH4 or
catalytic hydrogenation, H2, Ni, EtOH/H2O). The products are known
as “alditols”.
4. Osazone formation: Osazone are carbohydrate derivatives when
sugars are reacted with an excess of phenylhydrazine. eg. Glucosazone
4

Properties of Monosaccharides
Physical properties of monosaccharides:
1. Taste and colour: Most monosaccharides have a sweet taste (fructose is sweetest;
73% sweeter than sucrose) and colourless.
2. Physical nature: Solids at room temperature.
3. Solubility: These are highly soluble in water despite of their high molecular weights
(due to presence of large numbers of OH groups) than most molecules of similar
Molecular weight. These are also soluble in water, however insoluble in ether.
4. Optical activity: Monosaccharides exhibit optical activity.
Chemical properties:
1. Oxidation: These can be oxidized by various oxidizing agents.
2. Reduction: The free aldehyde and ketone groups may be reduced to alcoholic
hydroxyl groups by sodium mercury amalgam and water.
3. Schiff base :Aldehyde groups of monosaccharide may condense with primary
amines to form Schiff base (is a nitrogen analog of an aldehyde or ketone in which
the C=O group is replaced by C=N-R group)
5

Properties of Monosaccharides
Important tests for monosaccharides:
1. Fehlings test: Fehlings solution contains cupric sulphite, sodium hydroxide and
sodium potassium tartarate. When this solution is mixed with glucose and boiled,
the aldehyde group of glucose is oxidized, and cupric salt is reduced to cuprous
oxide giving red precipitate. C6H12O6 + 2Cu (OH)2 ===== C6H12O7 + Cu2O + H2O.
2. Benedict’s test: Benedict’s solution contains blue coloured copper sulphite. When
alkaline solution of the glucose is added to this and warmed, the aldehyde group of
glucose is oxidized and blue coloured copper sulphite is changed to green colour
copper hydroxide. Ultimately Cu(OH2) is changed to reddish brown cuprous oxide.
3. Molisch test: Sugars and their polymers give characteristic colour in presence of
strong sulphuric acid and α-napththol. The sugars in presence of acids undergo
dehydration to form furfural. The colour is produced by the condensation of
aldehyde and phenol.
4. Seliwanoff’s test: Sugars containing keto group when heated with HCl and
resorcinol produce bright red colour. This distinguishes between aldose and ketose
sugars. This test relies on the principle that, when heated, ketoses are more rapidly
dehydrated than aldoses.
6

Classification of Carbohydrates
Classification based on chemical structure and degree of
polymerization. Carbohydrates are divided into four major
groups
1. Monosaccharides
2. Disaccharides
3. Oligosaccharides
4. Polysaccharides
Monosaccharides:
• These are often called simple sugars, which can not be
further hydrolyzed into a simpler form with general formula
Cn (H2O)n. 8

Classification of Carbohydrates
• Based on number of carbon atoms, classified as trioses, tetroses,
pentoses, hexoses and heptoses.
I. Trioses (3C): These contain three carbon atoms. Eg. glyceraldehyde
(aldose), Dihydroxyacetone (ketose).
II. Tetroses (4C): These contain four carbon atoms in the chain Eg.
erythrose (A), erythrulose (K)
III. Pentoses (5C): Contain five carbon atom. Ribose and deoxyribose
(constituents of nucleic acids) Eg. ribose, xylose, arabinose (A); ribulose
xylulose (K)
IV. Hexoses (6C): These are six carbon atom sugars. Glucose, galactose
(constituent of glycolipids and glycoproteins), mannose (A) (occurs in
glycoproteins), Fructose (K)
V. Heptoses (7C): Contain seven carbon atom. Glucoheptose,
galactoheptose (A), Sedoheptulose (K) Note: A-Aldose, K-Ketose 9

Classification of carbohydrates
2. Disaccharides:
• These carbohydrates produce two molecules of the same or different
monosaccharides on the hydrolysis.
• Two units of monosaccharides are linked by a glycosidic bond.
• General formula for disaccharide is Cn(H2O)n-1 . E.g. lactose, maltose, sucrose.
i. Maltose (malt sugar): It is hydrolyzed to glucose by maltase and the products
are absorbed. It is reducing sugar and it reacts with hemiacetal. It is colorless,
crystalline, soluble in water and insoluble in ether.
Maltose = Glucose + Glucose.
ii. Lactose (milk sugar): It is hydrolyzed to glucose and galactose by lactase. It is
formed by the mammary glands. Lactose= Glucose + Galactose
iii. Sucrose(cane sugar): It is hydrolyzed to glucose and fructose by invertase
(sucrase). It is widely distributed in plants and occurs especially in sugar-cane,
sugar maple, sugarbeets, pineapple, etc. Sucrose= Glucose+ Fructose.
10

3. Oligosaccharides:
• It yields 3-10 saccharide units of simple sugars on
hydrolysis.
• A large number of oligosaccharides have been prepared by
partially breaking down more complex carbohydrates
(polysaccharides).
• Naturally occurring oligosaccharides in plants.
• Raffinose: It is a trisaccharide found in many plants,
consists of melibiose (galactose and glucose) and fructose.
• Maltotriose: It is a trisaccharide of glucose that occurs in
some plants and in the blood of certain arthropods.
11
Raffinose
Maltotriose

4. Polysaccharides:
• These carbohydrates yield more than 10 molecules of
monosaccharides on hydrolysis.
• The general formula is (C6H10O5)x.
• These may be classified as homopolysaccharides (same
monosaccharides) and heteropolysaccharides (different
monosaccharides).
i. Homopolysaccharides: Starch, cellulose, etc.
ii. Heteropolysaccharides: Mucopolysaccharides.
12

Starch
Homopolysaccharides:
Starch:
• It is white soft amorphous powder and devoid of
sweetness. It is insoluble in water, alcohol and ether
at ordinary temperature.
• It has specific rotation.
• On heating in presence of moisture starch breaks
into large fragments called dextrins.
• It develops blue colour with iodine.
• Glucose units are joined by 1,4 - links to form
unbranched chains which are in the form of a helix
with six glucose units per turn.
13
Starch

Starch
• Starch is made up of amylose and amylopectin. These both
are composed of glucopyranose units.
• Amylopectin is branched and the point of branching is the
sixth carbon atom of glucose. It has high molecular weight of
about 5,00000.
• Amylose is made of α-D-glucose units, bonded to each other
through α(1→4) glycosidic bonds. It is unbranched unlike to
amylopectin. It makes up to 20-30% of the starch. It is a
tightly packed helical structure, so amylose is more resistant
to digestion than amylopectin.
• In plants cells the starch is in the form of granules.
• The microscopic form of the granules is characteristic, and it
serves as identification mark in many plants.
• It is the most important source of carbohydrate in our food
and it is found in cereals, potatoes, legumes and other
vegetables in high concentrations.
14

Cellulose
• Cellulose is tough, white solid, insoluble in water and it gives no colour with
iodine.
• It lacks sweetness and it is chemically inert.
• It has carbon, hydrogen and oxygen with chemical formula (C6H10O5)n.
• It is soluble in ammonical cupric hydroxide solution.
• It is a polysaccharide consisting of a linear chain of several hundred to many
thousands of β(1→4) linked D-glucose units.
Uses:
• It is the main constituent of plant cell.
• Man can not digest cellulose but still cellulose has a considerable human
dietetic value as it adds bulk to the intestinal contents, thereby stimulating
peristalsis and elimination of food residues.
• It is used to produce paperboard and paper products
• It helps as an additive in various food items
• It is used in the production of rayon
• It is used as a preservative in cheese as it an anti-clumping agent
• It is used in making explosives
• It is used in the manufacturing of nitrocellulose 15

Pectin
Heteropolysaccharides:
• Pectin is a heterosaccharide derived from the cell wall of plants.
• Pectin vary in their chain lengths, complexity and the order of each of the
monosaccharide units.
• The characteristic structure of pectin is a linear chain of α (1-4)linked D-galacturonic acid
that forms the pectin-backbone, a homogalacturonan.
• Pectin acid is water soluble. It is present in the cell wall and in the intercellular substance.
• Pectin is methylated pectic acids and insoluble pectic substances are called protopectin.
• It contains arabinose, galactose and galacturonic acid. It is found in middle labella
between the plant cells.
• These are abundant in fruits, particularly in the rind of citrus fruits like oranges and
lemons.
Uses:
i. It is commercially marketed as a gelling agent for fruit jellies.
ii. Pectin is used for high cholesterol and high triglycerides.
iii. It is used to prevent colon cancer and prostate cancer.
iv. It is also used for diabetes and gastroesophageal reflux disease (GERD).
v. Pectin is also used to reduce the skin flushing associated with taking niacin.
vi. Pectin is used as a thickening agent in cooking and baking.
vii. Pectin is an ingredient in some denture adhesives. 16

Chitin
Heteropolysaccharides:
Chitin:
• Chitin (C8H13O5N)n, a long-chain polymer of N-acetylglucosamine
(derivative of glucose).
• The aminosugars which occurs in nature in the combination with
proteins and glucosamine are called chitin.
• It is a fibrous substance generally, it is made up of chitobiose, a
disaccharide which on decomposition yields N-acetyl
glucosamine. On hydrolysis chitin is hydrolyzed by acids and it
yields acetic acid and glucosamine.
Uses:
i. It is a primary component of cell walls in fungi.
ii. Chitin is a structural material for crustaceans and insects. It is
also important in lens formation of eyes.
iii. It is a fibrous substance consisting of polysaccharides, which is
the major constituent in the exoskeleton of arthropods and the
cell walls of fungi.
17

Biological functions of Carbohydrates
• Storage substances of potential energy: About 60% of the total energy
requirement of man is provided by the breakdown of carbohydrates. One
gram of carbohydrate on oxidation gives 4 Kcal energy. Glucose supplies the
instant energy to the tissues. Glucose is the sole form of energy for the brain
and other nervous tissues. Carbohydrate is stored in the body in the form of
glycogen. Glycogen stored in liver and it maintains the glucose level in blood.
• Structural component: Carbohydrates are important structural components
of living organisms. Monosaccharides are the constituents of nucleic acids,
coenzymes, flavoproteins, and blood group substances. Immuno-
polysaccharides play a part in resistance of infections. Heparin prevents the
clotting of blood. Glucuronic acid, which occurs in the liver, acts as a
detoxifying agent. Glycosides are components of steroid hormones. The cell
wall of plants and capsule of bacteria has carbohydrates. 18

Biological functions of Carbohydrates
• Regulation of fat metabolism: Some carbohydrates are essential for normal
oxidation of fats. It prevents ketosis.
• Protein sparing function: Inhibits the breakdown of proteins for energy as
they are the primary source of energy. This spares protein for building of
tissue.
• Role in gastrointestinal function: Indigestible substances like cellulose,
hemicellulose and pectin provide the roughage of food and thus help the
peristaltic movements of the digestive tract. Lactose promotes the growth of
desirable bacteria in the small intestine. Lactose also increases calcium
absorption.
• Basic component of human diet: Carbohydrates are known as one of the
basic component of food including sugars, starch, and fibre which are
abundantly found in grains, fruits, and milk products.
19

Biological significance of monosaccharides
• Biological importance of monosaccharides: Ribose and deoxyribose
(pentoses) are important constituents of nucleic acids and many
coenzymes .e.g. ATP, NAD, NADP. Glucose, fructose, mannose,
galactose, xylulose and ribose are monosaccharides of biological
importance.
• Physiological functions of hexoses:
• D-glucose- It is carried by the blood and used by tissues. Its presence
in the urine causes glycosuria.
• D-Fructose- It can be changed to glucose in the liver and intestine and
thus used in the body.
• D-galactose- it is synthesized in the mammary glands to form lactose
of milk. It is constituent of glycolipids and glycoproteins.
• D-mannose- It frequently occurs in glycoproteins.
20

Questions
1. What are carbohydrates? Classify carbohydrates.
2. Write an account on the functions of carbohydrates.
3. Write short notes
i. Starch
ii. Pectin
iii. Cellulose
iv. Polysaccharides
v. Chemical properties of carbohydrates
21

Thanks
Dr. Anil V Dusane
Sir Parashurambhau College, Pune, India
anildusane@gmail.com
22

Overview of Carbohydrates

More Related Content

What's hot

Similar to Overview of Carbohydrates

More from Sir Parashurambhau College, Pune

Recently uploaded

In this document

Overview of Carbohydrates