Any of a large group of organic compounds occurring in foods and living tissues and including sugars, starch, and cellulose. They contain hydrogen and oxygen in the same ratio as water (2:1) and typically can be broken down to release energy in the animal body. Chemically, carbohydrates are defined as “optically active polyhydroxy aldehydes or ketones or the compounds which produce units of such type on hydrolysis”.

1. Carbohydrates
Subject: BIO-104
Lecturer: ALIA RAFIQUE

2. Definition
• Any of a large group of organic compounds occurring in foods and living
tissues and including sugars, starch, and cellulose. They contain hydrogen
and oxygen in the same ratio as water (2:1) and typically can be broken
down to release energy in the animal body.
• Chemically, carbohydrates are defined as “optically active polyhydroxy
aldehydes or ketones or the compounds which produce units of such type
on hydrolysis”.

3. Classification of Carbohydrates

4. Types of carbohydrates
Simple carbohydrates:
• Simple carbohydrates are the basic type of carbohydrates. Soft drinks,
candy, cookies and other sweet snacks contain simple carbohydrates. These
foods are often made with white sugar, a form of processed sugar.
• Simple carbohydrates are also found in natural sugars. Fruit, milk and
vegetables contain natural sugars. Honey is a natural sugar.

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Complex carbohydrates:
• Complex carbohydrates represent an important energy source for your
body. They provide the sustained fuel your body needs for exercise, daily
living activities and even rest.
• Complex carbohydrates are often single units (monosaccharides), which are
bound together. The oligosaccharides contain two to ten simple units of
sugar. Polysaccharides contain hundreds and thousands of monosaccharides
which are related. Complex carbohydrates have fairly long-lasting energy.

6. Classification of carbohydrates
The different types of carbohydrates can be classified on the basis of their
behaviour in hydrolysis.They are mainly classified into three groups:
• Monosaccharides
• Disaccharides
• Polysaccharides

7. Monosaccharides
• Monosaccharide carbohydrates are those carbohydrates that cannot be hydrolyzed
further to give simpler units of polyhydroxy aldehyde or ketone.
• If a monosaccharide contains an aldehyde group then it is called aldose and on the
other hand, if it contains a keto group then it is called a ketose.
• They are classified according to the number of carbon atoms in each molecule
as trioses (3C), tetroses (4C), pentoses (5C), hexoses (6C), heptoses (7) and so
on.
• The names of all sugars end with -ose.
• Examples: Glyceraldehyde (triose), Erythrose (tetrose), Ribose (pentose), Glucose
(hexose), Fructose (hexose), Galactose (hexose), Sedoheptulose (heptose), etc.

8. Structure of Carbohydrates-Glucose
One of the most important monosaccharides is glucose. The two commonly
used methods for the preparation of glucose are:
• From Sucrose: If sucrose is boiled with dilute acid in an alcoholic solution
then we obtain glucose and fructose.
• From Starch: We can obtain glucose by hydrolysis of starch and by boiling it
with dilute H2SO4( sulfuric acid) at 393K under elevated pressure.

9. Properties of monosaccharides
• Monosaccharides are reducing sugars.
• The test for reducing sugar is called Benedict’s test.
• They are sugars, which taste sweet, are soluble in water.
• Monosaccharides are crystalline solids at room temperature and quite
soluble in water.They exist in straight chains or in the ring or cyclic forms.

10. Disaccharides
• On hydrolysis, disaccharides yield two molecules of either the same or
different monosaccharides.
• The two monosaccharide units are joined by oxide linkage which is formed
by the loss of water molecule and this linkage is called glycosidic linkage.
• Sucrose is one of the most common disaccharides which on hydrolysis gives
glucose and fructose.
• Maltose and Lactose (also known as milk sugar) are the other two
important disaccharides.
• In maltose, there are two α-D-glucose and in lactose, there are two β-D-
glucose which are connected by an oxide bond.

11. Structure of disaccharides

12. Properties of disaccharides
• They are polar compounds.
• They are readily soluble in water due to hydrogen bonding.
• They have sweet taste.
• They cannot diffuse through cellular membranes.
• Sucrose is a non-reducing sugar.
• Sucrose is the transport sugar and Lactose is the sugar found in milk which
an important constituent of the diet of young mammals.

13. Polysaccharides
• Polysaccharides contain long monosaccharide units joined together by
glycosidic linkage.
• Most of them act as food storage for e.g. Starch. Starch is the main storage
polysaccharide for plants.
• It is a polymer of α glucose and consists of two components-Amylose and
Amylopectin.
• Cellulose is also one of the polysaccharides that are mostly found in plants.
• It is composed of β-D- glucose units joined by a glycosidic linkage between
C1 of one glucose unit and C4 of the next glucose unit.

14. Properties of polysaccharides
• They are not sweet in taste.
• Many are insoluble in water.
• They are hydrophobic in nature.
• They do not form crystals on desiccation.
• Can be extracted to form a white powder.
• They are high molecular weight carbohydrates.

15. Isomers
• Compounds that have the same chemical
formula but have different structures are
called isomers.
• For example, fructose, glucose, mannose,
and galactose are all isomers of each
other, having the same chemical formula,
C6H12O6.

16. Epimers
• Carbohydrate isomers that differ in configuration around only one specific
carbon atom are defined as epimers of each other.
• For example, glucose and galactose are C-4 epimers because their
structures differ only in the position of the –OH group at carbon 4.

17. Enantiomers
• These stereo isomers are mirror images of
each other.
• Example: D-Glucose & L-Glucose
• D=OH on the right side at asymmetric
carbon L=OH on left side at asymmetric
carbon

18. Biological Significance of carbohydrates
• It serves as storage molecules, for example, glycogen in animals and starch
in plants.
• It is used as the main source of energy. For example, fiber is a carbohydrate
that aids in digestion.
• Carbohydrates associated with protein: Glycoproteins.
• Carbohydrates associated with lipid: Glycolipids.
• They form the constituents of nucleic acids.

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• Cell walls of plants and bacteria are composed of cellulose.
• Carbohydrates like cellulose , hemicellulose and lignin provide a mechanical
and protective function to the cell wall of plants .
• Some carbohydratesare necessary in thediet so that oxidation of
fats can proceed normally .