1. The Nature Of Carbohydrates
P A JOY
© SSER Ltd.

2. Cx(H2O)y 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. CARBOHYDRATES SUGARS POLYSACCHARIDES
THE CLASSIFICATION OF CARBOHYDRATES
Carbohydrates are classified as either sugars or polysaccharides
CARBOHYDRATES
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 representation of the glucose
molecule shows how the
carbon atoms are numbered
ALDEHYDE GROUP
Glucose, in common with
many other hexose sugars
has an aldehyde group
as part of the structure
The carbon atom
of the carbonyl group
is referred to as the
ANOMERIC CARBON
ATOM and, for glucose,
this is carbon 1
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. six-membered ring when the hydroxyl group (OH)
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. The ring structure of glucose is usually represented in Howarth
projection

8. 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

9. MALTOSE = GLUCOSE + GLUCOSE LACTOSE = GLUCOSE + GALACTOSE
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

10. 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
MALTOSE 1
4a glycosidic bond
MALTOSE IS A DISACCHARIDE FORMED WHEN TWO ALPHA
GLUCOSE MOLECULES ARE COVALENTLY BONDED TOGETHER

11. Benedicts solution is a turquoise liquid
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
Benedicts solution is a turquoise liquid
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

12. REDUCING SUGARS SUCROSE RESULT MALTOSE Sucrose is a non-reducing sugar
When Benedicts 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

13. REDUCING SUGARS Why is sucrose a non-reducing sugar?
MALTOSE
SUCROSE
Sugars reduce Benedicts 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

14. REDUCING SUGARS Why is sucrose a non-reducing sugar?
MALTOSE
SUCROSE
Sugars reduce Benedicts 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
Fructose bonds to glucose to form
sucrose

15. 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
MALTOSE
SUCROSE
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 Benedicts 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

16. Sucrose is formed when glucose forms a glycosidic bond with fructose
The anomeric carbon
atom for fructose
is carbon 2
SUCROSE
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

17. following procedure is performed;
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

18. STARCH POLYSACCHARIDES AMYLOSE – long unbranched chain of glucose
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

19. 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

20. AMYLOSE STRUCTURE
THE AMYLOSE HELIX

21. AMYLOPECTIN STRUCTURE
Amylopectin consists of a straight chain of alpha glucose units with branch points
occurring at approximately every twelth 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

22. make up the final starch molecule
AMYLOPECTIN STRUCTURE
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

23. pack inside the amylose helix to give a blue-black colour
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 bread, the blue-black colour
develops

24. GLYCOGEN 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

25. GLUCOSE IS STORED AS GLYCOGEN IN LARGE AMOUNTS IN BOTH THE LIVER
AND SKELETAL MUSCLES

26. STRUCTURAL POYSACCHARIDES
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 4
glycosidic bonds O
CH OH 2 H OH
GLUCOSE 3 5 6 HO
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

27. STRUCTURAL POYSACCHARIDES
The bundles of parallel chains forming hydrogen bonds
with each other creates 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

28. STRUCTURAL POYSACCHARIDES
CHITIN