Carbohydrates and biological importance.ppt

BIOMOLECULES
ANITA MITRA
PGT CHEMISTRY
KV No.1 SHIFT 1, Indore

,
Carbohydrates which are sweet in taste, are also
called sugars.
The most common sugar, used in our homes is named as sucrose
whereas the sugar present in milk is known as lactose.
Carbohydrates are also called saccharides (Greek: sakcharon
means sugar).

carbohydrates may
be defined as
optically active
polyhydroxy
aldehydes or
ketones or the
compounds
which produce
such units on
hydrolysis

Carbohydrates which are sweet in taste, are also
called sugars. The most common sugar, used in our homes
is named as sucrose whereas the sugar present in milk is known
as lactose. Carbohydrates are also called saccharides (Greek:
sakcharon means sugar).

SUGAR PRESENTIN MILK IS LACTOSE

Carbohydrates are classified on the basis of their
behaviour on hydrolysis. They have been broadly
divided into following three groups.
(i) Monosaccharides: A carbohydrate that cannot be
hydrolysed further to give simpler unit of polyhydroxy
aldehyde or ketone is called a monosaccharide.
About 20 monosaccharides are known to occur in
nature. examples are glucose, fructose, ribose, etc.

(ii) Oligosaccharides: Carbohydrates that yield two
to ten monosaccharide units, on hydrolysis, are
called oligosaccharides.
They are further classified as disaccharides
trisaccharides,tetrasaccharides, etc., depending
upon the number of monosaccharides, they provide
on hydrolysis. disaccharides. ,
C12H22O11+H2O→ C6H12O6 + C6H12O6
SUCROSE GLUCOSE FRUCTOSE
Lactose
C12H22O11+H2O→ C6H12O6 + C6H12O6
MALTOSE GLUCOSE GLUCOSE
C12H22O11+H2O→ C6H12O6 + C6H12O6
MALTOSE GLUCOSE GALACTOSE

(iii) Polysaccharides: Carbohydrates which yield a large
number of monosaccharide units on hydrolysis are
called polysaccharides.
(C6H10O5)n+n H2O → n C6H12O6
Some common examples are starch, cellulose,
glycogen, gums,
etc. Polysaccharides are not sweet in taste, hence they
are also called non-sugars.

carbohydrates which reduce Fehling’s solution and Tollens’ reagent are
referred to as reducing sugars. All monosaccharides whether aldose or
ketose are reducing sugars.
In disaccharides, if the reducing groups of monosaccharides i.e.,
aldehydic or ketonic groups are bonded, these are non-reducing sugars
e.g. sucrose. On the other hand, sugars in which these functional
groups are free, are called reducing sugars, for example, maltose and
lactose.
TOLLENS REAGENT
FEHILINGS SOLUTION

If a monosaccharide contains an aldehyde group, it is known as
an aldose and if it contains a keto group, it is known as a ketose.

Glucose
Glucose occurs freely in nature as well as in the combined form. It
is present in sweet fruits and honey. Ripe grapes also contain
glucose in large amounts.

Glucose is an aldohexose and is also known as dextrose. It is the
monomer of many of the larger carbohydrates, namely starch,
cellulose. It is probably the most abundant organic compound on
earth. It was assigned the structure given below on the basis of
the following evidences:
1. Its molecular formula was found to be C6H12O6.
2. On prolonged heating with HI, it forms n-hexane, suggesting
that all the six carbon atoms are linked in a straight chain.

Glucose reacts with hydroxylamine to form an oxime and adds a
molecule of hydrogen cyanide to give cyanohydrin. These
reactions confirm the presence of a carbonyl group (>C = 0) in
glucose.
Glucose gets oxidised to six carbon carboxylic acid (gluconic
acid) on reaction with a mild oxidising agent like bromine water.
This indicates that the carbonyl group is present as an
aldehydic group.

Acetylation of glucose with acetic anhydride gives glucose
pentaacetate which confirms the presence of five –OH groups.
Since it exists as a stable compound, five –OH groups should be
attached to different carbon atoms.
On oxidation with nitric acid, glucose as well as gluconic acid both
yield a dicarboxylic acid, saccharic acid. This indicates the
presence of a primary alcoholic (–OH) group in glucose.

The exact spatial arrangement of different —OH groups was
given by Fischer after studying many other properties. Its
configuration is correctly represented as I. So gluconic acid is
represented as II and saccharic acid as III.

Glucose is correctly named as D(+)-glucose. ‘D’ before the name
of glucose represents the configuration whereas ‘(+)’ represents
dextrorotatory nature of the molecule. It may be remembered that ‘D’
and ‘L’ have no relation with the optical activity of the compound.

The structure (I) of glucose explained most of its properties but the following
reactions and facts could not be explained by this structure.

1. Despite having the aldehyde
group, glucose does not give
2,4-DNP test, Schiff’s test and it
does not form the
hydrogensulphite addition
product with NaHSO3.

2. The pentaacetate of glucose does not react with hydroxylamine
indicating the absence of free —CHO group.
NH2OH
No reaction

Glucose is found to exist in two different
crystalline forms which are named as α
and β. The α-form of glucose (m.p. 419 K)
is obtained by crystallisation from
concentrated solution of glucose at 303 K
while the β-form (m.p. 423 K) is obtained
by crystallisation from hot and saturated
aqueous solution at 371 K.
This behaviour could not be explained by
the open chain structure
(I) for glucose. It was proposed that one of
the —OH groups may add to the —CHO
group and form a cyclic hemiacetal
structure. It was found that glucose forms
a six-membered ring in which —OH at C-5
is involved in ring formation. This explains
the absence of —CHO group and also
existence of glucose in two forms as
shown below. These two cyclic forms exist in
equilibrium with open chain structure.

The two cyclic hemiacetal forms of glucose differ only in the
configuration of the hydroxyl group at C1, called anomeric carbon
(the aldehyde carbon before cyclisation). Such isomers, i.e., α-
form and β-form, are called anomers. The six membered cyclic
structure of glucose is called pyranose structure (α– or β–), in
analogy with pyran. Pyran is a cyclic organic compound with one
oxygen atom and five carbon atoms in the ring. The cyclic
structure of glucose is more correctly represented by Haworth
structure as given below.

Penta acetate of α –D-
glucose acetate
Penta acetate of β –D-
glucose acetate

The structure (I) of glucose explained most of its properties but the following
reactions and facts could not be explained by this structure.
1. Despite having the aldehyde group, glucose does not give 2,4-
DNP test, Schiff’s test and it does not form the hydrogensulphite
addition product with NaHSO3.
2. The pentaacetate of glucose does not react with hydroxylamine
indicating the absence of free —CHO group.
3. Glucose is found to exist in two different crystalline forms which
are named as α and β. The α-form of glucose (m.p. 419 K) is
obtained by crystallisation from concentrated solution of glucose
at 303 K while the β-form (m.p. 423 K) is obtained by
crystallisation from hot and saturated aqueous solution at 371 K.
This behaviour could not be explained by the open chain structure
(I) for glucose. It was proposed that one of the —OH groups may
add to the —CHO group and form a cyclic hemiacetal structure. It
was found that glucose forms a six-membered ring in which —OH
at C-5 is involved in ring formation. This explains the absence of
—CHO group and also existence of glucose in two forms as

Fructose also has the molecular formula C6H12O6 and on the
basis of its reactions it was found to contain a ketonic functional
group at carbon number 2 and six carbons in straight chain as in
the case of glucose. It is a laevorotatory compound.

It also exists in two cyclic forms which
are obtained by the addition of
—OH at C5 to the (C=O ) group. The
ring, thus formed is a five membered ring
and is named as furanose with analogy
to the compound furan. Furan is a five
membered cyclic compound with one
oxygen and four carbon atoms.
It belongs to D-series . It is appropriately written as D-(–)-fructose.
Its open chain structure is as shown.

Di sacharides : Which on hydrolysis gives two monosacharides.
Sucrose: One of the common disaccharides is sucrose which on
hydrolysis gives equimolar mixture of D-(+)-glucose and D-(-)
fructose.

Polysaccharides contain a large number of monosaccharide units
joined together by glycosidic linkages. These are the most
commonly encountered carbohydrates in nature. They mainly act
as the food storage or structural materials.
Starch: Starch is the main storage polysaccharide of plants. It is
the most important dietary source for human beings. High
content
of starch is found in cereals, roots, tubers and some vegetables.
(C6H10O5)n+n H2O → n C6H12O6
It is a polymer of α-glucose and consists of two components—
Amylose and Amylopectin. Amylose is water soluble
componentwhich constitutes about 15-20% of starch.
Amylopectin is insoluble in water and constitutes about 80-
85% of starch.

Cellulose: Cellulose occurs exclusively in plants and it is the most
abundant organic substance in plant kingdom. It is a predominant
constituent of cell wall of plant cells. Cellulose is a straight chain
polysaccharide composed only of β-D-glucose units which are
joined by glycosidic linkage between C1 of one glucose unit and
C4 of the next glucose unit.

Glycogen: The carbohydrates are stored in animal body as
glycogen.
It is also known as animal starch because its structure is similar
to amylopectin and is rather more highly branched. It is present
in liver, muscles and brain. When the body needs glucose,
enzymes break the glycogen down to glucose. Glycogen is also
found in yeast and fungi.

Carbohydrates and biological importance.ppt

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