Carbohydrates are the most abundant organic molecules in nature. They are commonly known as saccharides or sugars. They are primarily composed of the elements carbon, hydrogen and oxygen. The name carbohydrate literally means “hydrates of carbon”. Carbohydrates are widely distributed in nature in plants and animals. The most important carbohydrate found in plants is starch. It occurs abundantly in roots, tubers, vegetables and grains. The carbohydrate found in animals is glycogen. It is a storage form of carbohydrate present in liver and muscles, which serves as important sources of energy for vital activities.

1. Tapeshwar Yadav
(Lecturer, MBAHS)
BMLT, DNHE,
M.Sc. Medical Biochemistry
&
Clinical Diagnostic Laboratory
Incharge
(Hetauda Hospital)
Carbohydrates

2. Student Learning Outcomes
At the end of the lecture students will be able to:
 Define Carbohydrates
 Functions of Carbohydrates
 Classification of Carbohydrates in
details

3. Introduction
 Carbohydrates are the most abundant organic molecules
in nature.
 They are commonly known as saccharides or sugars.
 They are primarily composed of the elements carbon,
hydrogen and oxygen.
 The name carbohydrate literally means “hydrates of
carbon”.
 Carbohydrates are widely distributed in nature in plants
and animals.
 The most important carbohydrate found in plants is
starch.
 It occurs abundantly in roots, tubers, vegetables and
grains. The carbohydrate found in animals is glycogen.

4. Definition
Carbohydrates may be defined as
polyhydroxyaldehydes or ketones or
compounds which produce them on
hydrolysis.

5. Functions
 1. They are the most abundant dietary source of
energy (4cal/g) for all organisms.
 2. Carbohydrates are the major source of energy
for man. For example, glucose is usedin the human
body for energy production.
 3. Carbohydrates are precursors for many organic
compounds (fats, amino acids).
 4. Carbohydrates like glycoprotein and glycolipids
participate in the structure of cell membrane and
cellular functions such as cell growth, adhesion and
fertilization.

6. Contd…
 5. They are structural components of many organisms
e.g: fiber (cellulose) of plants, exoskeleton of some
insects and the cell wall of microorganisms.
 6. Carbohydrates serve as reserve food material in
humans and in plants. For example, glycogen in animal
tissue and starch in plants serves as reserve food
materials.
 7. They are utilised as raw materials for several
industries eg: paper, plastics, textiles, alcohol etc.
 8. Certain carbohydrate derivatives are used as drugs
like cardiac glycosides and antibiotics.
 9. Ascorbic acid, a derivative of carbohydrate is a water-
soluble vitamin.

7. Classifications
They are broadly classified into 4 groups i.e.
 1. Monosaccharides : contain one sugar unit.
 2. Disaccharides : contain two sugar
units.
 3. Oligosaccharides : contain 3 - 10 sugar
units.
 4. Polysaccharides : contain more than 10
sugar units.

8. Monosaccharides:
  (Greek: mono = one) are the simplest group
of carbohydrates which can not be further
hydrolysed into simpler forms.
  General formula: Cn(H2O)n

9. Contd…
Based on the functional group and the numbers of
carbon atom, monosaccharides are divided into
different categories:
General formula Aldose (CHO) Ketose (CO)
Trioses (C3H6O3) Glyceraldehyde Dihydroxyacetone
Tetroses C4H8O4) Erythrose Erythrulose
Pentoses (C5H10O5) Ribose Ribulose
Hexoses (C6H12O6) Glucose Fructose
Heptoses (C7H14O7) Glucoheptose Sedoheptulose

10. Contd…
  Aldoses: When the functional group is in
monosaccharides is an aldehyde (H-C=O).
  Ketoses: When the functional group is in
monosaccharides is a keto (C=O).
Fig. Examples of an aldose (A) and a ketose (B) sugar.

11. Contd…
Fig. Some important monosaccharides (systematic names are given in parenthesis)

13. 2. Disaccharides
  (Greek: Di = two) are those sugars which
produce two molecules of the same or different
monosaccharides on hydrolysis.
 General formula: Cn(H2O)n-1
 When two monosaccharides are combined
together by glycosidic linkage, a disaccharide is
formed.
 They are crystalline, water-soluble and sweet to
taste.

14. Contd…
They are of 2 types:
 i. Reducing disaccharides: with free
aldehyde or keto group eg.-maltose, lactose.
 ii. Non-reducing disaccharides: with no free
aldehyde or keto group eg. - Sucrose.
 Maltose → Glucose + Glucose
 Sucrose → Glucose + Fructose
 Lactose → Glucose + Galactose

15. Maltose: malt sugar
  It is composed of two α-D-glucose units held
together by
α (1 → 4) glycosidic bond.
  Sources are germinating cereals and malt.
  The aldehyde group of the second glucose
molecule is free and available for reduction.
So,maltose is a reducing sugar.
  It is the intermediate product in the breakdown of
starch by the enzyme amylase in the intestinal tract.
  It is hydrolysed to glucose by enzyme maltase
and products are absorbed.
  It forms osazone (sunflower shape) with
phenylhydrazine.

16. Contd…

17. Lactose: milk sugar
  It is present in milk. So, it is commonly called as
milk sugar.
  It is composed of β-D-galactose and β-D-
glucose held together by β (1 → 4) glycosidic
bond.
  The anomeric carbon of C1 glucose is free,
hence lactose exhibits reducing properties and
forms osazones (powder-puff or hedge-hog shape).
  It is the most important carbohydrate in the
nutrition of young mammals.
  It is hydrolysed to glucose and galactose by
enzyme lactase in alimentary canal and products

18. Contd…

19. Sucrose: table sugar
  It is also called as ‘cane sugar’ as it can be
obtained from sugarcane.
  Sources are sugar beet, sugar maple,
pineapples, carrots etc.
  It is composed of α-D-glucose and β-D-fructose,
held together by a glycosidic bond (α1→β2)
between C1 of α-glucose and C2 of β-fructose.
  The reducing groups of glucose and fructose are
involved in glycosidic bond, hence sucrose is a non-
reducing sugar and it cannot form osazones.
  It is hydrolysed to glucose and fructose by
enzyme sucrase in alimentary canal and products are

21. 3. Oligosaccharides
  (Greek: oligo = few) are those sugars which
yield 3 to 10 monosaccharides units on
hydrolysis.
For eg:
  Raffinose (Trisaccharide) → Fructose+
Galactose + Glucose
  Stachyose (Tetrasaccharide) → Galactose (2
moles) + Glucose + Fructose
  Verbascose (Pentasaccharide) → Galactose
(3 moles) + Glucose + Fructose

22. 4. Polysaccharides
  (Greek: Poly = many) are those which yield more
than ten molecules of monosaccharides on hydrolysis.
  General formula: (C6H10O5)n
  Polysaccharides are further divided into two
groups:
 a. Homopolysaccharides: which on hydrolysis yield
only a single type of monosaccharide units.
For eg: starch, glycogen, cellulose, inulin, dextrins
etc.
 b. Heteropolysaccharides: which on hydrolysis yield
different types of monosaccharide units or their
derivatives

23. i. Starch
  It is the reserve carbohydrate of plant
kingdom.
  Sources: Potatoes, cereals (rice, wheat),
roots, tapioca and other food grains.
  It is a homopolymer composed of D-glucose
units held together by α-glycosidic bonds. lt is
known as glucosanor glucan.
  It consists of two polysaccharide
components-water soluble amylose (15-20%)
and a water insoluble amylopectin (80-85%).

24. ii. Dextrins:
  Dextrins are the breakdown products of
starch by the enzyme amylase or dilute acids.
  Starch is sequentially hydrolysed through
different dextrins and finally, to maltose and
glucose.

25. iii. Inulin:
 Inulin is a polymer of fructose i.e., fructosan.
 Sources: Dahlia bulbs, garlic, onion etc.
 lt is a low molecular weight (around 5,000)
 Easily soluble in water.
 It is not utilized by the body.
 Importance: used for assessing kidney function
through measurement of glomerular filtration
rate (GFR).

26. iv. Glycogen:
  It is the reserve carbohydrate in animals.
  It is stored in liver and muscle. About 5% of
weight of liver is made up by glycogen.
  Excess carbohydrates are deposited as
glycogen.
  Structure of glycogen is similar to that of
amylopectin with more no. of branches.
  Composition: It is composed of glucose units
joined by α(1→4) glycosidic bond in straight
chains and α(1→6) glycosidic at the branching
points.
  Molecular weight: about 5 million.

27. v. Cellulose
  It occurs exclusively in plants and it is the
most abundant organic substance in plant
kingdom.
  It is a predominant constituent of plant cell
wall.
  It is totally absent in animal body.
  It is composed of β-D-glucose units linked
by β(1→4)glycosidic bonds.
  Cellulose cannot be digested by mammals
including man-due to lack of the enzyme that
cleaves β –glycosidic bonds (α amylase breaks
α bonds only)

28. Contd…
Importance of Cellulose
  It is not digested by human body has great
importance in human nutrition.
  It is a major constituent of fiber, the non-
digestable carbohydrate.
  The functions of dietary fiber include
decreasing the absorption of glucose and
cholesterol from the intestine, besides
increasing the bulk of feces.

29. vi. Chitin:
  Chitin is composed of N-acetyl-glucosamine
units held together by β(1→4)glycosidic bonds.
  It is a structural polysaccharide found in the
exoskeleton of some invertebrates e.g. insects

30. Heteropolysaccharides (or,
Mucopolysaccharides or,
Glycosaminoglycans):
Glycosaminoglycan (GAG) Tissue distribution Functions
i. Hyaluronic acid
Synovial fluid, Connective
tissue, vitrous humor
Serves as a lubricant and
shock absorber.
Promotes wound healing.
ii. Heparin
Blood, spleen, lung, liver,
kidney
Acts as an anticoagulant
(prevents blood clotting).
iii. Chondroitin sulphate
Cartilage, bone, skin, blood
vessel walls
Helps to maintain the
structure and shapes of
tissues.
iv. Dermatan sulphate
Skin, blood vessel valves,
heart valves
Maintains the shape of
tissues.
v. Keratan sulphate
Cornea, cartilage, connective
tissues
Keeps cornea transparent.

31. "Working hard for something we
don't care about is called stressed;
working hard for something we love is
called passion."
Simon Sinek