This document provides information on the composition and metabolism of carbohydrates. It begins by defining carbohydrates and describing their biological significance as fuels, catalysts, and starting materials for other compounds. It then discusses the occurrence of carbohydrates such as starch, glycogen, and simple sugars in plants and animals. The document proceeds to classify carbohydrates and describe important types such as monosaccharides, disaccharides, oligosaccharides, polysaccharides, and mucopolysaccharides. It also summarizes key pathways in carbohydrate metabolism including glycolysis, glycogenesis, and the metabolism of glucose.

1. UNIT – III
COMPOSITION
&
METABOLISM
OF
CARBOHYDRATES
Ms. Kalaivani Sathish

2. INTRODUCTION
 Carbohydrates includes large group of compounds
commonly known as starches or sugars.
 Chemically, they are described as polyhydric alcohols,
having potentially active aldehyde and ketone groups.
 Carbohydrates are in general white soluble in water
with the exception of certain polysacchraides.
 Lower molecular weight and sweet taste.

3. Biological Significance
 Fuel: when carbohydrates oxidised it gives carbon
dioxide, water and energy.
 Catalysts: Promotes oxidation of food stuffs.
 Starting Materials: certain carbohydrates are
the starting materials for the biological synthesis if
componds such as fatty acids and amino acids.

4. Occurrence
 Distributed in plants and animals.
 In form of starch in plants & in glycogen form
in animals.
 Found in roots. Tubers, leaves, vegetables and
grains in plants.
 In animals muscles and liver it is present in a
huge amount.
 Starch and glycogen are polysaccharides having
high molecular weight.

5. Occurrence (Cont..........)
 Lower molecular weight carbohydrates are present
in nature and they comes under the group
monosaccharides and oligosaccharides. (Crystalline
substances, Sweeter in taste).
 Sucrose is obtained from sugarcane, nectar of
flowers.
 Glucosides are the derivatives of carbohydrates
which are used for the treatment of heart diseases.
 Glucose and fructose are simple sugars are widely
distributed in plans.
 Glucose is present in blood and other body fluids.
Lactose is present in milk.

6. Chemical Characteristics
 Carbon. Hydrogen. Oxygen are present.
 Molecular formula for glucose is C6 H12 O6.
 All simple sugars contains a potential aldehyde and ketone
group.

7. Classification
Monosaccharides
Disaccharides
Oligosaccharides
Polysaccharides

19.  On hydrolysis these molecules yields 2
molecules of monosaccharides.
 E.g., Sucrose, Lactose and Maltose
 C12 H22 O11

20.  On Hydrolysis they yields 2 to 10
monosaccharide units.
 Disaccharides also comes under this
category.
 Examples: Raffinose – Tri Saccharide
 Stachyose – Tetra Saccharide
 Verbascose – Penta Saccharide

21. Polysaccharides
 They yields more than ten molecules of
monosaccharides on hydrolysis.
 E.g., Starch, Glycogen, Dextrin, Cellulose and
Inulin.
 (C6 H10 O5)x

22. Mucopolysaccharides
 Mucopolysaccharides are long chains of sugar
molecules that are found throughout the body,
often in mucus and in fluid around the joints.
They are more commonly called
glycosaminoglycans.

23. Mucopolysaccharides
Importances
 It is an essential component of tissue structure.
 Hyaluronic acid, Heparin, dermatan sulfate and
keratan sulfate are important mucopolysachrides.
 Hyaluronic acid serves as a lubricant and shock
absorbent in joints.
 Heparin is an anticoagulants, which prevents blood
clotting.
 Dermatan sulafate is mostly present in skin.

24. DISACCHARIDES
 Sugars containing 2 molecules of
monosaccharides.
 On condensation between 2 monosaccharides
the union takes place between C- 1 of Second
monosaccharide and C – 4 of First
monosaccharide.
 The C- 1 of first monosaccharide is free and
has its potential alsehyde intact.
 Examples: Maltose, Lactose and Sucrose.

25. Maltose
 Maltose is composed of two glucose units.
 Maltose is the end product of digestion of
starch by the action of salivary amylase Ptyalin
in the mouth and Pancreatic amylase in the
intestines.

26. Lactose
 Also known as milk sugar occurs in the milk of
mammals.
 On hydrolysis by enzyme lactase it forms one
molecule of glucose and one molecule of
galactose.
 Lactose is present in the urine of pregnant
women and lactating mother.

27. Sucrose
 Sucrose is a cane sugar.
 Used as sugar in food.
 Obtained from sugarcane, beetroot, pineapple,
honey, fruits and certain palms.
 It is non – reducing sugar.
 It contains one molecule of fructose and one
molecules of glucose.

28. POLYSACCHARIDES

29. STARCH

30. GLYCOGEN

31. CELLULOSE

32. METABOLISM PATHWAYS
OF
GLUCOSE

33. INTRODUCTION
 The metabolism of glucose or glycogen may
be considered under the following sub
divisions.
 Glycolysis – The oxidation of glucose or
glycogen to pyruvate and lactate by the
Embden Myerhof Pathway.
 Glycogenesis – The synthesis of glycogen
from glucose.
 Glycogenolysis – Breakdown of liver
glycogen.

34. INTRODUCTION (Cont ...)
 Oxidation of Pyruvate to acetyl – CoA -
Oxidation of pyruvate, prior to entry into the
citric acid cycle.
 The Hexose Monophosphate Shunt
Pathway – This is an alternate aerobic
pathway for oxidation of glucose.
 Gluconeogenesis – Formation of glucose or
glycogen from non carbohydrate sources.

35. GLYCOLYSIS
 Glycolysis = “Glyco” – Glucose. “Lysis” – Split.
 Embden Meyerhof Pathway
 In this process we Starts with Glucose (6 C)
and it will ends with 2 X Pyruate (3 C).
 Two Phases of Glycolysis:
1. Phase – I. Energy Investment Phase
2. Phase – 2. Energy Generation Phase

36.  Glycolysis process is a 10 step process.
 First 5 Steps in the glycolysis Process is
Energy Investment Phase. Second 5 Steps of
the glycolysis process is energy generation
phase.
 In each phases of the glycolysis process 5
enzymes are involved.
 Totally 10 Enzymes are involved in the
glycolysis process.

39.  Out of 5 enzyme reactions in energy
investment phase 2 reactions are irreversible
and 3 are reversible.
 Out of 5 enzyme reactions in energy
generation phase 1 reaction is irreversible and
4 are reversible.

40. Enzymes Involved in Glycolysis
 Enzyme Investment
Phase
 Hexokinase
 Phosphohexose
Isomerase
 Phospho fructokinase
– 1
 Aldolase
 Triose Phosphate
Isomerase
 Energy Generation
Phase
 Glceraldhyde 3 –
Phosphate
Dehydrogenase
 Phospho Glycerate
Kinase
 Phospho Glycerate
Mutase
 Enolase
 Pyruvate Kinase

43. Four Stages
of
Embden Meyerhof Pathway
 Initial Phosphorylation
 Conversion to Trioses
 Oxidative Step
 Formulation of Pyruvic Acid and Lactic
Acid

44. Initial Phosphorylation
 Glucose enters into the glycolytic pathway by
phosphorylation to glucose – 6 – phosphate.
 Hexokinase enzyme catalyses the
phosphorylation.
 Glucose + ATP Glucose-6-P +
ADP

45. Conversion to Trioses
 First, Glucose-6-Phosphate is converted to
Fructose-6-Phosphate. (Enzyme –
Phosphohexose Isomerase).
 Secondly Fructose-6-Phosphate is
phosphorylated by ATP, to form Fructose-1-6-
diphosphate.
Fructose-6-P + ATP PFK Fructose-1-6 Di+ADP
Mg ++

46. Conversion to Trioses (Cont....)
 In the next step Fructose 1-6-diphosphate is split by an
enzyme aldolase, into 2 molecules of Triose
phosphates namely Glyceraldehyde-3-Phosphateand
Dihydroxyacetone Phosphate.
Fructose 1-6-Di-P
Glyceraldehyde – 3-P
Dihydroxyacetone - P

47. Conversion to Trioses (Cont....)
Fructose 1-6-Di-P
By the action of triose phosphate isomerase interconversion
of Glyceraldehyde 3-P and Dihydroxyacetone –P occurs.
(DHAP) can be changed into G3P, There will be 2
molecules of G3P for the oxidation in next step.
Glyceraldehyde – 3-P
Dihydroxyacetone - P

48. Oxidation
 Very imporatnt step because ATP is
generated.
 G3P is oxidised into 1-3 diphosphoglyceric
acid, this reaction is catalysed by an enzyme
Glyceraldehyde-3-Phosphate dehydrogenase.
 1-3 diphosphoglyceric acid is converted into 3
phosphoglyceric acid with the help of an
enzyme Phosphoglycerate kinase, during this
process 2 ADP Molecules are joined with 2
phosphate molecule to form 2 ATP Molecules.

49. Oxidation (Cont.....)
 At this stage the 3 Phosphoglycerate is
converted into 2-phosphoglyceric acid by
phosphoglyceromutase.
 In the next stage the 2-Phosphoglyceric acid is
dehydrated by enolase to form
phosphophenolpyruvate.

50. Formation of Pyruvic Acid
 From the high energy phosphate of
phosphophenolpyruvate 2 ADP is transferred
to produce 2 molecules of ATP. With the help
of an enzyme pyruvate kinase 2 Pyruvates are
formed from Phosphophenolpyruvate.

51. CONCLUSION
 Total amount of energy production during glycolysis
given below.
Reactions ATP Used ATP Produced
First Step (First Phase) 1 -
Third Step (First Phase) 1 -
Second Step (Second
Phase)
- 2
Fifth Step (Second
Phase)
2
For every glucose molecule entering glycolysis. 2 ATP & 2 NAD+ Used.
4 ATP & 4 NADH Is produced. (Net Gain is 2 ATP and 2 NADH)