CARBOHYDRATE
INTRODUCTION
Carbohydrates are one of the three major classes
of biological molecules.
 Carbohydrates are also the most abundant
biological molecules.
 Carbohydrates derive their name from the
general formula (CH2O)n.
 Carbohydrates are carbon compounds that
contain large quantities of hydroxyl groups.

FUNCTIONS
o
o
o
Nutritional (act as energy storage)
As cell membrane components mediate some
forms of intercellular communication.
Also serve as a structural component of many
organisms, including the cell walls of bacteria,
the exoskeleton of many insects, and the fibrous
cellulose of plants.
Carbohydrates are chemically
characterized as:
 Poly
hydroxy aldehydes or
 Poly
hydroxy ketones.
CLASSIFICATION
All carbohydrates can be classified as
either:
 Monosaccharides
 Disaccharides
 oligosaccharides or Polysaccharides.
 Monosaccharides-
one unit of
carbohydrate
 Disaccharides- Two units of
carbohydrates.
 Oligosaccharide-Two to ten
monosaccharide units.
 Polysaccharides are much larger,
containing hundreds of monosaccharide
units.
MONOSACCHARIDES


Classified according to the number of carbon atoms or
the type of carbonyl group they contain.
Carbohydrates with an aldehyde as their carbonyl
group are called aldoses, whereas those with a keto
as their carbonyl group are called ketoses
Example
Glyceraldehyde is an aldose
Dihydroxyacetone is a ketose.
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Carbohydrates that have a free carbonyl group have
the suffix –ose.
Monosaccharides can be linked by glycosidic bonds to
create larger structures
# Carbons
Category Name
Relevant examples
3
Triose
4
Tetrose
Erythrose
5
Pentose
Ribose, Ribulose,
Xylulose
6
Hexoses
Glucose, Galactose,
Mannose, Fructose
7
Heptoses
Sedoheptulose
9
Nonoses
Glyceraldehyde,
Dihydroxyacetone
Neuraminic acid, also
called sialic acid
Disaccharides

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
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Three common disaccharides:
Sucrose — common table sugar = glucose +
fructose
Lactose — major sugar in milk = glucose +
galactose
Maltose — product of starch digestion = glucose +
glucose

The resulting linkage between the sugars is called a
glycosidic bond. The molecular formula of each of
these disaccharides isC12H22O11 = 2 C6H12O6 − H2O
Polysaccharides

2 types:
HOMO polysaccharides (all 1 type of
monomer)
glycogen, starch, cellulose, chitin


HETERO polysaccharides (different types
of monomers)
peptidoglycans, glycosaminoglycans
If the anomeric hydroxyl is in the α configuration, the
linkage is an α-bond. If it is in the β configuration, the
linkage is a β-bond.
Lactose, for example, is synthesized by forming a glycosidic
bond between carbon 1 of β-galactose and carbon 4 of
glucose. The linkage is, therefore, a β(1→4) glycosidic bond
COMPLEX CARBOHYDRATES
 Carbohydrates
can be attached by
glycosidic bonds to non carbohydrate
structures, including
 purine and pyrimidine bases (found in
nucleic acids)
 aromatic rings (such as those found in
steroids and bilirubin)
 proteins (found in glycoproteins and
proteoglycans),
 lipids (found in glycolipids), to form
glycosides.
ISOMERS
 Isomers
are molecules that have the same
molecular formula, but have a different
structures.
ISOMER 1
ISOMER 2
Examples of isomers:
1. Glucose
2. Fructose
3. Galactose
4. Mannose
Same chemical formula C6 H12 O6
EPIMERS

EPIMERS are sugars that differ in
configuration at ONLY 1 POSITION.
 Examples
of epimers :
 D-glucose & D-galactose (epimeric at
C4)
 D-glucose & D-mannose (epimeric at
C2)
 D-idose & L-glucose (epimeric at C5)
ENANTIOMERS

A special type of isomerism is found in the pairs
of structures that are mirror images of each
other.
 The
two members of the pair are
designated as D and L forms.
 In
D form the OH group on the
asymmetric carbon is on the right.
 In
L form the OH group is on the left
side.
 D-glucose
and L-glucose are
enantiomers:
ASYMMETRIC CARBON
A
carbon linked to four different atoms or
groups farthest from the carbonyl carbon
 Also
called Chiral carbon
CYCLIZATION
 Less
then 1%of CHO exist in an open
chain form.
 Predominantly
 In
found in ring form.
which the aldehyde (or keto) group has
reacted with an alcohol group on the same
sugar, making the carbonyl carbon
(carbon 1 for an aldose, carbon 2 for a
ketose) asymmetric.
 Six
membered ring structures are called
Pyranoses .

five membered ring structures are called
Furanoses .
ANOMERIC CARBON


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The carbonyl carbon after cyclization becomes the
anomeric carbon.
This creates α and β configuration.
In α configuration the OH is on the same of the
ring in fischer projection. In Haworths it is on
the trans side of CH2OH.

Such α and β configuration are called
diastereomers and they are not mirror images.
Enzymes can distinguished between these two
forms:
 Glycogen is synthesized from α-D
glucopyranose

Cellulose is synthesized from β -D
glucopyranose
REDUCING SUGAR
 Sugars
in which the oxygen of the
anomeric carbon is free and not attached
to any other structure, such sugars can
act as reducing agents and are called
reducing sugars.
DIGESTION OF DIETARY CARBOHYDRATES
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Principal sites are the mouth and intestinal lumen.
This digestion is rapid and catalyzed by enzymes
known as glycoside hydrolases (glycosidases)
that hydrolyze glycosidic bonds.
Little monosaccharide present in diets of mixed
animal and plant origin, the enzymes are primarily
endoglycosidases that hydrolyze polysaccharides and
oliosaccharides, and disaccharidases that hydrolyse
tri- and disaccharides into their reducing sugar
components.
Glycosidases are usually for glycosyl residue to be
removed as well as for the type of bond to be broken.
The final products of carbohydrate digestion are the
monosaccharides, glucose, galactose, and fructose that
are absorbed by cells of the small intestine.
In mouth:
 The major dietary polysaccharides are of plant
(starch, composed of amylose and amylopectin) and
animal (glycogen) origin.
 During mastication, salivary α-amylase acts briefly
on dietary starch and glycogen, hydrolyzing random
α(1→4) bonds.
 Branched amylopectin and glycogen contain α(1→6)
bonds, which α-amylase cannot hydrolyze, the digest
resulting from its action contains a mixture of short,
branched and unbranched oligosaccharides known as
dextrins
 Disaccharides are also present as they, too, are
resistant to amylase.
In stomach:
Carbohydrate digestion halts temporarily in the
stomach, because the high acidity inactivates salivary
α-amylase.
In small intestine
pancreatic α-amylase
When the acidic stomach contents reach the small
intestine, they are neutralized by bicarbonate
secreted by the pancreas, and pancreatic αamylase continues the process of starch digestion.
Intestinal disaccharidases
The final digestive processes occur primarily at the
mucosal lining of the upper jejunum and include
the action of several disaccharidases
For example
 Isomaltase cleaves the α(1→6) bond in isomaltose
 Maltase cleaves the α(1→4) bond in maltose and
maltotriose, each producing glucose.
 Sucrase cleaves the α(1→2) bond in sucrose,
producing glucose and fructose
 lactase (β-galactosidase) cleaves the β(1→4) bond
in lactose, producing galactose and glucose.
ABNORMAL DEGRADATION OF
DISACCHARIDES
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The overall process of carbohydrate digestion and
absorption is so efficient in healthy individuals that
ordinarily all digestible dietary carbohydrate is absorbed
by the time the ingested material reaches the lower
jejunum
However, because only monosaccharides are absorbed, any
deficiency (genetic or acquired) in a specific disaccharidase
activity of the intestinal mucosa causes the passage of
undigested carbohydrate into the large intestine.
As a consequence of the presence of this osmotically active
material, water is drawn from the mucosa into the large
intestine, causing osmotic diarrhea.
This is reinforced by the bacterial fermentation of the
remaining carbohydrate to two- and three-carbon
compounds (which are also osmotically active) plus large
volumes of CO2 and H2 gas, causing abdominal cramps,
diarrhea, and flatulence.
DIGESTIVE ENZYME DEFICIENCIES
Genetic deficiencies of the individual
disaccharidases result in disaccharide
intolerance.
 Alterations in disaccharide degradation can also
be caused by a variety of intestinal diseases,
malnutrition, and drugs that injure the mucosa
of the small intestine.
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LACTOSE INTOLERANCE:
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More than 70% of the world’s adults are lactose
intolerant manifest in certain populations.
90% of adults of African or Asian descent are lactasedeficient therefore less able to metabolize lactose than
individuals of Northern European origin.
The age-dependent loss of lactase activity represents
a reduction in the amount of enzyme produced. It is
thought to be caused by small variations in the DNA
sequence of a region on chromosome 2.
Treatment for this disorder is to reduce consumption
of milk and eat yogurts and some cheeses as well as
green vegetables, such as broccoli, to ensure adequate
calcium intake; to use lactase-treated products; or to
take lactase in pillform prior to eating.
CONGENITAL SUCRASE- ISOMALTASE
DEFICIENCY
This autosomal recessive disorder results in an
intolerance of ingested sucrose.
 Treatment includes the dietary restriction of
sucrose and enzyme replacement therapy.
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