1. © SSER Ltd.

2. THE NATURE OF CARBOHYDRATES
Carbohydrates are compounds of great importance in both the biological and commercial world
They are used as a source of energy in all organisms and as structural materials in membranes, cell walls and the exoskeletons of many arthropods
All carbohydrates contain the elements carbon (C), hydrogen (H) and oxygen (O) with the hydrogen and oxygen being present in a 2 : 1 ratio
The general formula of a carbohydrate is:
Cx(H2O)y
EXAMPLES
The formula for glucose is C6H12O6
The formula for sucrose is C12H22O11

3. THE CLASSIFICATION OF CARBOHYDRATES
Carbohydrates are classified as either sugars or polysaccharides
Sugars
Polysaccharides
Monosaccharides
Disaccharides
Storage
Structural
Monosaccharides are single sugar units that include:
Glucose
Fructose
Galactose
Disaccharides are
double sugar units that include:
Sucrose
Maltose
Lactose
Glycogen
and Starch
are storage carbohydrates;
animal cells store
glucose as glycogen and plant cells store glucose as starch
Cellulose and Chitin
are important structural carbohydrates;
cellulose forms the fabric of many cells walls and chitin is a major component of the exoskeletons of many arthropods
Glucose

4. MONOSACCHARIDES
Monosaccharides are single sugar units that form the building blocks for the larger carbohydrates
There are many different monosaccharides; they vary according to the number of carbon atoms that they possess and in the way the atoms are arranged in the molecules
Glucose, the main source of energy for most organisms, is a hexose sugar with six carbon atoms and the formula C6H12O6
Glucose exists in both straight chain and ring form with rings forming when glucose is dissolved in water
chain structure
ring structure

5. GLUCOSE This straight chain
Aldehyde group
This straight chain
representation of the glucose molecule shows how the carbon atoms are numbered
The carbon atom
of the carbonyl group
is referred to as the
anomeric carbon
atom and for glucose,
this is carbon 1
Glucose, in common with
many other hexose sugars,
has an aldehyde group
as part of the structure
The carbon atom that
forms part of this aldehyde
group is always carbon 1
The C = O carbonyl group
has reducing properties such
that all monosaccharides
are reducing sugars
The remainder of the molecule is a series of bonded carbon atoms with attached hydrogen atoms and hydroxyl (OH) groups

6. GLUCOSE In solution glucose exists in ring form Glucose forms a
six-membered ring when the hydroxyl group (OH) on carbon 5 adds to the aldehyde group on carbon 1

7. GLUCOSE
The ring structure of glucose is usually represented in Howarth projection

8. MOLECULAR MODELS The relative positions of the atoms in molecules
can be demonstrated through model building
Glucose
Carbon
Oxygen
Hydrogen

9. Each hexose sugar exists in both alpha and beta forms
ISOMERS
Each hexose sugar exists in both alpha and beta forms
These Isomers can be distinguished by the arrangement of the OH and H groups about the extreme right carbon atom
IN the ring (Carbon 1)

10. GLUCOSE
Simplified views of a and b glucose are often used for representing these molecules

11. DISACCHARIDES
Disaccharides are sugars composed of two monosaccharides covalently bonded
together by a glycosidic linkage
Maltose, also known as malt sugar, is formed from two glucose molecules
Lactose, or milk sugar, is a disaccharide formed when
the monosaccharides glucose and galactose bond
Sucrose is common household sugar and is formed when the monosaccharides glucose and fructose bond
maltose = glucose + glucose
lactose = glucose + galactose
sucrose = glucose + fructose

12. THE FORMATION OF MALTOSE
Maltose forms when two alpha glucose molecules undergo
a condensation reaction and form
a glycosidic bond between the two
molecules
- H2O
condensation reaction C1 C4 1
4a glycosidic bond
Maltose is a disaccharide, formed when two alpha
Glucose molecules are covalently bonded together

13. REDUCING SUGARS
All the monosaccharides and many of the disaccharides are reducing sugars
Benedict’s test is used to determine the reducing properties of the different sugars
Benedict’s solution is a turquoise/blue
solution containing copper ions and sodium
hydroxide; the copper ions exist as Cu2+
in this reagent
If a sugar is a reducing sugar then the Cu2+
ions are reduced to Cu+ which, in the
presence of alkaline sodium hydroxide,
form copper oxide
Copper oxide is insoluble and precipitates
out of the solution as a brick-red
precipitate

14. REDUCING SUGARS Sucrose Result Maltose Sucrose is a non-reducing sugar
When Benedict's test is performed with the disaccharides maltose and sucrose, the following result is obtained
Sucrose
Result
Maltose
Sucrose is a
non-reducing sugar
Maltose is a
reducing sugar

15. The anomeric carbon atom for glucose is carbon 1
REDUCING SUGARS
Why is sucrose a non-reducing sugar?
Sugars reduce Benedict's solution when the anomeric
carbon atom is made available to reduce the copper
ions in the solution
The anomeric carbon atom is the carbon of the
carbonyl group present in the straight chain form
of the sugar
The anomeric carbon atom for glucose is carbon 1

16. The anomeric carbon atom for fructose is carbon 2
REDUCING SUGARS
Why is sucrose a non-reducing sugar?
Glucose
Fructose
Sugars reduce Benedict's solution when the anomeric
carbon atom is made available to reduce the copper
ions in the solution
The anomeric carbon atom for fructose is carbon 2
Ketone group
Fructose is a ketose sugar and bonds to glucose to form sucrose

17. Why is sucrose a non-reducing sugar?
This potential anomeric carbon atom is unavailable
This potential anomeric carbon atom is available to reduce Benedict’s reagent
When maltose is boiled with Benedict’s reagent, the
region of the ring containing the anomeric carbon
atom (carbon 1) may open exposing a carbonyl
group capable of reducing Benedict's reagent –
Only an anomeric carbon atom that is not
involved in the formation of the
glycosidic bond may be exposed
The one available anomeric carbon atom is sufficient
for this molecule to reduce Benedict’s solution and
thus maltose is a reducing sugar

18. Sucrose is a non-reducing sugar
Sucrose is synthesised when glucose forms a glycosidic bond with fructose
The anomeric carbon
atom for fructose
is carbon 2
glucose
fructose
glycosidic
bond
The anomeric carbon
atom for glucose
is carbon 1
As both the anomeric carbon atoms are involved in forming the glycosidic bond when glucose and fructose join, there are no potentially free anomeric carbon atoms available to reduce Benedict’s solution
Sucrose is a non-reducing sugar

19. TEST FOR SUCROSE
In order to determine if sucrose is present in a sample
or solution then the following procedure is performed:
The sample or solution under consideration is boiled for
at least fifteen minutes in hydrochloric acid
Boiling in acid breaks glycosidic bonds – the glycosidic bond is hydrolysed
This procedure is called acid hydrolysis
The solution is then neutralised by adding drops of
alkali while testing with pH paper
Benedict’s test is now performed on the resulting solution
If a brick-red precipitate forms then sucrose was present in the original solution
Acid hydrolysis breaks the glycosidic bonds in the sucrose molecules
releasing free glucose and free fructose into the solution
Glucose and fructose are both monosaccharides and therefore reducing sugars
If no precipitate is obtained then sucrose was not present in the original sample
The need to neutralise the solution following acid hydrolysis is due
to the fact that the Benedict’s test requires an alkaline medium

20. Starch POLYSACCHARIDES
Polysaccharides are large polymers of the monosaccharides
Unlike monosaccharides and disaccharides, polysaccharides are
either insoluble or form colloidal suspensions
The principal storage polysaccharides are starch and glycogen
Starch is a polymer of alpha glucose and is, in fact, a mixture of
two different polysaccharides – amylose and amylopectin
Starch
Amylose – long unbranched chain of glucose units
Amylopectin – highly branched polymer of glucose units

21. The amylose chain, once formed, coils into a helix
AMYLOSE STRUCTURE
Amylose is formed by a series of condensation reactions that bond alpha glucose molecules together into a long chain forming many glycosidic bonds
The amylose chain, once formed, coils into a helix

22. formed from covalently bonded a glucose molecules
The amylose helix
formed from covalently
bonded a glucose molecules

23. AMYLOPECTIN STRUCTURE
Amylopectin consists of a straight chain of alpha glucose units with branch points occurring at approximately every twelfth glucose unit along the straight chain
The branch points form when carbon 6, of a glucose molecule in the straight chain, forms a glycosidic bond with carbon 1 of a glucose molecule positioned above the chain 1 6a
glycosidic bond

24. This highly branched amylopectin molecule is wrapped
around the amylose to make up the final starch molecule
This large insoluble molecule, with branch points that allow for easy
access for enzymes when breaking down the molecule, makes
starch an ideal food storage compound

25. REACTION BETWEEN STARCH AND IODINE SOLUTION
When iodine in potassium iodide solution is added to starch, the iodine molecules pack inside the amylose helix to give a blue-black colour
When iodine reacts with the starch in this piece of potato, the blue-black colour develops

27. Glycogen is often referred to as animal starch
Glycogen has the same overall structure as amylopectin but
there is significantly more branching in this molecule
More of these
branch points form

28. Glucose is stored as glycogen in large amounts in both the liver
and skeletal muscles

29. STRUCTURAL POLYSACCHARIDES
Cellulose is one of the most important structural polysaccharides as it is the major component of plant cell walls
Cellulose is a polymer of beta glucose units where each glucose molecule is inverted with respect to its neighbour 1
4b glycosidic bond
The orientation of the beta glucose units places many hydroxyl (OH) groups on each side of the molecule
Many parallel chains of beta glucose units form and each chain forms hydrogen bonds between the OH groups of adjacent chains

30. STRUCTURAL POLYSACCHARIDES
The bundles of parallel chains, forming hydrogen bonds with each other, create a molecule that confers rigidity and strength to the structures of which they form a part
hydrogen bonds between parallel chains of beta glucose
The rigidity and strength of plant cell walls is a consequence
of the incorporation of cellulose into their structure

31. STRUCTURAL POLYSACCHARIDES
Chitin
Chitin is a polysaccharide forming the exoskeleton of insects and crustaceans and the fabric of fungal cell walls; it is a linear polymer of the sugar derivative called N-acetylglucosamine
N-acetylglucosamine

32. Chitin forms the exoskeleton of insects and crustaceans

33. Chitin forms the fabric of
fungal cell walls

34. Copyright © 2008 SSER Ltd. and its licensors. All rights reserved
Copyright © 2008 SSER Ltd. and its licensors. All rights reserved. All graphics are for viewing purposes only.