Chapter 2
Carbohydrates
1- Monosaccharides
2- Disaccharides
3- Polysaccharides
4- Digestion of carbohydrates
Carbohydrates
• Chemically, carbohydrates are the most
abundant organic molecules in nature in
which carbon, hydrogen and oxygen bond
together in the ratio: C x (H2O)y
• The word carbohydrate means "hydrate of
carbon"
• Carbohydrates providing the energy in the
diet of most organisms.
• Humans break down carbohydrates
during the process of metabolism to release
energy.
– For example, the chemical metabolism of
the sugar glucose is shown below:
C6H12O6 + 6 O2
6 CO2 + 6 H2O + energy
• Human obtain carbohydrates by eating
foods that contain them, for example
potatoes, rice, breads,ect.
• Carbohydrates are manufactured by plants
during the process of photosynthesis is a
metabolic pathway that converts carbon dioxide
into organic compounds, especially sugars, using
the energy from sunlight.
Plants harvest (collect) energy from sunlight to
run the reaction described above in reverse
6 CO2 + 6 H2O + energy
from
sunlight
C6H12O6 + 6 O2
Functions of Carbohydrates:
• Acting as a storage form of energy
in the body
• Serving as cell membrane
components.
• Serving as a structural component
of many organisms such as:
1- Cell wall of bacteria.
2- Exoskeleton of insects.
3-Fibrous cellulose of plants.
Classification and structure
of Carbohydrates
• There are four classes of Carbohydrates,
Based on the number of sugar units:




Monosaccharides
Disaccharides.
Oligosaccharides.
Polysaccharides
most carbohydrates are
polyhydroxyaldehydes,
polyhydroxyketones, or compounds
that yield them after hydrolysis.
●The simpler members of the
carbohydrate family are often referred
to as saccharides.
• Carbohydrates are classified as
monosaccharides, disaccharides,
oligosaccharides, or polysaccharides
depending on the number of simple sugars
they contain.
• Monosaccharides (simple sugars) cannot be
broken down into simpler sugars under mild
conditions
• Oligosaccharides = "a few" - usually 2 to 10
• Polysaccharides are polymers of the simple
sugars
I-Monosaccharides:
(simple sugar)
Definition:
– Monosaccharides are colorless
crystalline solids
– Freely soluble in water
– Insoluble in non polar solvents
– It is the basic units of Carbohydrates
– Most have a sweet taste.
Also many of Monosaccaharides are
synthesized from simpler (noncarbohydrate) substances in a
process named gluconeogenesis
(is a metabolic pathway that results in
the generation of glucose from noncarbohydrate carbon substrates such as
lactate, glycerol, and glucogenic amino
acids).
Classification of monosacharides:
a) Monosaccharides are classified according to the
chemical nature of their carbonyl group, which (is
the carbon atom that is doubled- bounded to an
oxygen atom to form a carbonyl group ) while each
of other carbon atoms has a hydroxyl group :
– If the carbonyl group is an aldehyde the sugar is
an aldose as in glyceraldehyde and glucose.
– If the carbonyl group is a ketone the sugar is an
ketose as in Dihydroxyacetone and fructose
●Monosaccharides containing an aldehyde
group are classified as aldoses; those
containing a ketone group are classified as
ketoses.
The suffix –ose indicates that a molecule is a
carbohydrate, and the prefixes tri-, tetr-, and
so forth indicate the number of carbon atoms
in the chain.
●There are only two trioses:
the aldotriose
glyceraldehyde
the ketotriose
dihydroxyacetone.
b) Monosaccharides can be classified
according to the number of carbon
atoms they contain:
– The smallest monosaccharide, those with
three carbon atoms are trioses
– Those with 4,5,6 and 7 carbon atoms are called
tetroses, pentoses, hexoses, and heptoses
respectively.
Each of these monosaccharides exists in two
series :
aldo trioses and
keto trioses
aldo tetroses and
keto tetroses
aldo pentoses and
keto pentoses
aldo hexoses and
keto hexoses
The most abundant monosaccharides in nature are the
hexoses, which
– Include the aldohexose D- glucose and the ketohexose
D- fructose.
A- Fischer Projection Formulas
To draw a Fischer projection, you draw a threedimensional representation of the molecule
oriented so that the vertical bonds from the
stereocenter are directed away from you and
the horizontal bonds are directed towards
you. ( None of the bonds to sterocenter are in
the plane of the paper).
Isomers and epimers
• Isomers:
– Are compounds that have
• The same chemical formula
• Have different structures
– Example :
• Fructose, glucose, mannose, and galactose.
• Are all isomers of each other having the same
chemical formula C6 H12 O6
• Epimers:
– Are compounds that differ in their configuration around
only one specific carbon atom ( with the exception of
the carbonyl carbon ).
– They are also isomers
1) Glucose and galactose are C4 epimers
– Their structures differ only in the position of the - OH
group at carbon 4
2) Glucose and mannose are C2 epimers.
 Their structures differ only in the position of
the - OH group at carbon 2
• Enantiomers :
– A special type of isomers is found in
the pairs of structures that are mirror
images of each other.
– The two members of the pair are
designated as a D- and L- sugar .
– The vast majority of the sugars in humans
are D- sugars.
Enantiomers :
Monosaccharides are chiral
• The number of chiral
carbons present in a
ketose is always one
less than the number
found in the same
length aldose
• Number of possible
steroisomers = 2n (n =
the number of chiral
carbons)
O
H
C
CH2OH
H
C*
OH
HO
C*
H
H
C*
H
C*
C
O
HO
C*
H
OH
H
C*
OH
OH
H
C*
OH
CH2OH
CH2OH
D-glucose
D-fructose
The following table shows the
configuration relationships among DAldotriose, D-Aldotetroses, DAldopentoses, and D-Aldohexose.
B. Haworth Projections
A common way of representing the cyclic structure of
monosaccharides is the Haworth projection, named
after the English chemist Sir Walter N. Haworth
(Nobel Prize for chemistry, 1937).
• In a Haworth projection, a five or six-membered
cyclic hemiacetal is represented as a pentagon or
hexagon,
Six-membered
hemiacetal ring is
indicated by -pyran-,
pyranose
Five membered
hemiacetal ring is
indicated by -furan-,
furanose.
• Cyclization of monosaccharides
– Monosaccharides are predominantly found
in a ring form:
• In which the aldehyde ( or ketone )
group has reacted with an alcohol
group on the same sugar, and form a
covalent bond
– While less than 1 % exists in the
open - chain ( acyclic ) form.
An aldehyde can react with an alcohol to form a hemiacetal
A ketone can react with an alcohol to form a hemiketal.
The glucose ring form is created
when:
• The oxygen on carbon number 5 links with
the carbon having the carbonyl group (carbon
number 1)
• And transfers its hydrogen to the carbonyl
oxygen to create a hydroxyl group.
• The rearrangement produces alpha glucose
when the hydroxyl group is on the opposite side
of the -CH2OH group,
• or beta glucose when the hydroxyl group is on
the same side as the -CH2OH group.
Haworth Projections
O
H
-OH up = beta
C1
H
C2
OH
HO
C3
H
H
C4
OH
H
C5
OH
CH2OH
4
6
5
3
For all non-anomeric carbons, -OH
groups point down in Haworth
projections if pointing right in
Fischer projections
-OH down = alpha
2
1
Anomeric carbon
(most oxidized)
1)Anomeric carbon
• Formation of a ring results in the creation of an
anomeric carbon:
– At carbon 1 of an aldose or
– At carbon 2 of a ketoses
 Haworth projections: Represent the cyclic sugars
as having essentially planar rings, with the OH at
the anomeric C1 extending either: Below the ring
(α) Above the ring (β).
–.
Anomers: α or β
– Are isomers which differ only in their
configuration about their carbonyl
carbon atom.
– These structures are designated the α or β
configuration of the sugar. For example:
– α - D - glucose and β - D – glucose.
These two sugars are both glucose, but
they are anomers of each other.
●the designation ß means that the -OH
on the anomeric carbon of the cyclic
hemiacetal lies on the same side of the
ring as the terminal –CH2OH.
●the designation α means that the -OH
on the anomeric carbon of the cyclic
hemiacetal lies on the side of the ring
opposite from the terminal-CH2OH.
Mutarotation process:
• Is when cyclic α and β anomers of a sugar in
solution are in equilibrium with each other, and
can be spontaneously interconverted.
• The cyclic form of glucose known as
glucopyranose.
• The cyclic form of fructose known as
fructofuranose.
What Are the Characteristic
Reactions of Monosaccharides?
A. Formation of Glycosides
(Acetals)
Treatment of an aldehyde or ketone with one
molecule of alcohol yields a hemiacetal,
and treatment of the hemiacetal with a
molecule of' alcohol yields an acetal.
Treatment of a monosaccharide all forms of
which exist almost exclusively as cyclic
hemiacetals with an alcohol also yields an
acetal.
B. Reduction to Alditols
The carbonyl group of a monosaccharide
can be reduced to a hydroxyl group by a
variety of reducing agents, including
hydrogen in the presence of a transition
metal catalyst and sodium borohydride.
The reduction products are known as
alditols.
• Sorbitol is found in the plant world in
many berries and in cherries, plums,
pears, apples, seaweed, and algae.
• It is about 60% as sweet as sucrose
(table sugar) and is used in the
manufacture of candies and as a
sugar substitute for diabetics.
C.
Oxidation to Aldonic Acids
(Reducing Sugars)
Aldehydes (RCHO) are oxidized to
carboxylic acids (RCOOH) by several
agents, including oxygen, O2 .
Reducing sugars:
• Reducing sugars is when the oxygen on the
anomeric carbon (the carbonyl group) of a sugar is
not attached to any other structure.
. A reducing sugar can react with chemical reagents.
For example,
– (Benedict’s solution) and reduce the reactive
component, with the anomeric carbon becoming
oxidized.
D. Oxidation to Uronic Acids
Enzyme- catalyzed oxidation of D-glucose, yields
D-glucuronic acids
D-Glucuronic acid is widely
distributed in- both the plant and
animal worlds.
In humans, it serves as an important
component of the
■ acidic polysaccharides of connective
tissues,
II. Disaccharides
• Consist of two monosaccharides covalently bound
to each other.
• In disaccharides the chemical bond that joins the two
monosaccharide units is called a glycosidic bond.
• Glycosidic bond :
– Is formed when the hydroxyl group on one of the
sugars reacts with the anomeric carbon on the
second sugar.
– Is readily hydrolyzed by acid.
– It resists cleavage by base.
Naming glycosidic bonds
• Glycosidic bonds between sugars are
named According:
– To the numbers of the connected carbons
– To the position of the anomeric hydroxyl
group of the sugar involved in the bond.
• If this anomeric hydroxyl group 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, β (1- 4)
glycosidic bond
• Disaccharides can be hydrolyzed to yield their free
monosaccharide components by boiling with dilute
acid.
• They are also abundant in nature.
• The most common disaccharides are:
– Maltose
– Lactose
– Sucrose
Maltose
• Contain two D- glucose residues joined by a
glycosidic linkage between carbon atom 1 ( the
anomeric carbon ) of the first glucose residue and
carbon atom 4 of the second glucose.
• The configuration of the anomeric carbon atom in
the glycosidic linkage between the two D-glucose
residues is α
– The linkage is thus symbolized α (1- 4)
• The monosaccharide unit bearing the anomeric
carbon is designated by the first number in this
symbol.
Maltose
• Both the glucose residues of maltose are in
pyranose form
• Maltose is a reducing sugar since it has one
free carbonyl group that can be oxidized.
• Maltose is hydrolyzed to two molecules of Dglucose by the intestinal enzyme Maltase that
is specific for the α (1- 4) bond
Lactose
• Found in milk
• Contain D-galactose and D- glucose joined
by a glycosidics linkage between carbon
atom 1 (the anomeric carbon )of galactose
residue and carbon atom 4 of the glucose.
• The configuration of the anomeric carbon
atom in the glycosidic linkage between the Dgalactose residues and the glucose residue is
– β and the linkage is thus symbolized β (1- 4)
Lactose
• Both the galactose and glucose residues of
lactose are in pyranose form
• Lactose is a reducing sugar since it has one
free carbonyl group on the glucose residue that
can be oxidized.
• During digestion lactose undergoes enzymatic
hydrolysis by the Lactase enzyme of the
intestinal mucosal cells.
Sucrose
• Is a disaccharide of glucose and fructose joined
by a glycosidic linkage between carbon atom 1
(the anomeric carbon )of glucose residue and
carbon atom 2 of the fructose.
• In sucrose:
– The anomeric hydroxyl group of glucose in
the α configuration is Glycosidically linked
to the anomeric hydroxyl group of fructose
in the β configuration.
– The linkage is thus symbolized (α1- β2).
Sucrose
• Sucrose is Not a reducing sugar
– Because it contains No free anomeric carbon
• Since the anomeric carbons of both glucose
and fructose residues are linked to each
other.
• Sucrose is hydrolyzed in the small intestine to
glucose and fructose by action of the enzyme
sucrase (invertase).
• It is the sweetest of the three common
disaccharides.
D. Relative Sweetness
Among the disaccharide sweetening agents,
n-fructose tastes the sweetest even sweeter than
sucrose.
 The sweet taste of honey is due largely to
fructose and D-glucose.
 Lactose has almost no sweetness and is
sometimes added to foods as a filler.
Sugar
fructose
sucrose
glucose
maltose
galactose
Lactose
Sweetness
173%
100%
74%
33%
33%
16%
III. Polysaccharides
• Polysaccharides are polymer of twenty or more
monosaccharide units.
• The polysaccharides can be:
– Homopolymers:
• Composed of a single type of monosaccharide
– Glucans : are homopolymers of glucose
– Mannans: are homopolymers of mannose
– Levans : are homopolymers of fructose
– Hetropolymers:
• Made up from a variety of different
monosaccharides.
• In addition they can have
– Linear
– Branched
• The glycosidic bonds between the monosaccharide units can be
–
–
–
–
–
–
–
–
–
1-2
1-3
1-4
1-6
2 -1
2-1
2-3
2- 4
2- 6
with the anomeric carbon in either the α of β configuration.
CELLULOSE
•
Is the main structural element of plant cells.
–
–
•
•
•
It is part of the cell wall and
Is a major component of wood.
Cellulose is a linear polymer of glucose linked
by β (1- 4) glycosidic bonds.
The glucose β (1 - 4) glucose glycosidic bond in
cellulose results in a rigid linear polymer of
glucose.
The rigid nature of this polymer makes it an
excellent structural element.
•
The glucose β (1- 4) glucose bond are
– Very stable
– Difficult to hydrolyzed by chemical means.
• A very few species of bacteria contain an enzyme
that can hydrolyzed the β (1- 4) bond.
• Animals, such as cow that derive most of their
nutrients from plant material it:
– Contain bacteria in their gut that can
hydrolyze the glucose β (1- 4) glucose bond of
cellulose.
Why cellulose is insoluble in
water?
Cellulose molecules act much like stiff rods, a
characteristic that enables them to align
themselves side by side into well-organized,
water-insoluble fibers in which the OH groups
form numerous intermolecular hydrogen
bonds. This arrangement of parallel chains in
bundles gives cellulose fibers their high
mechanical strength.
When a piece of cellulose-containing
material is placed in water, there
are not enough -OH groups on the
surface of the fiber to pull
individual cellulose molecules away
from the strongly hydrogen-bonded
fiber.
STARCH
•
•
Is the storage form of glucose units in plant cells
When a plant seed, tuber, and / or bulb is in an
energy poor state:
• The starch is broken down and used for
energy and / or precursors.
Starch as isolated from plants is a mixture of two
polymers :
•
–
–
AMYLOSE
AMYLOPECTIN.
AMYLOSE:
–
Is a linear polymer of glucose units liked
α (1- 4) glycosidic bond.
AMYLOPECTIN:
– Is a branched polymer.
– It has a backbone of glucose units linked in
α (1- 4) glycosidic bond.
– Every 24 to 30 residues along a backbone
there is a branch point.
– At the branch points the glucose residues are
linked in and α (1- 6 )configuration.
Complete hydrolysis of both amylose and
amylopectin yields only D-glucose.
Amylose is composed of continuous,
unbranched chains of as many as 4000 nglucose units joined by α-l,4-g1ycosidic bonds.
Amylopectin contains chains of as many as
10,000 D-glucose units also joined by α-l,4g1ycosidic bonds.
GLYCOGEN
• It is a homopolymer of glucose.
• Is the storage form of glucose in animal cells.
• In men, glycogen is stored in the liver and
skeletal muscle.
• When energy poor :
– Glycogen is broken down to glucose and used for
• Energy
• Precursors
•
•
Glycogen is a branched polymer
Similar to amylopectin:
– It has backbone of glucose residues
linked in an α (1- 4) configuration and
– Branches that are linked α (1- 6).
• Different from amylopectin:
– In that it is more highly branched.
– It contains α (1- 6) branch every 8-13
residues along a backbone.
• The branched polysaccharides (Amylopectin
and Glycogen:
– Have only one reducing end and numerous
non reducing ends
DEXTRANS
•
Are an important family of storage
polysaccharides in
– Bacteria and yeast.
• The glycosidic linkage along the backbone
of these polysaccharides is α (1- 6)
– Dextrans are usually branched polysaccharides
– Dextrans produced by oral bacteria are usually
glucans
Other Biologically Important Sugar
Derivatives
• Monosaccharide units, which an OH group is
replaced by H are known as deoxy sugars.
– The biologically most important of these is
• β-D-2-deoxyribose, the sugar component
of DNAs sugar-phosphate backbone.
D-Ribose
2-Deoxy-D-Ribose
L-Ribose
α-D-deoxyribose
• amino sugars:
– One or more OH groups are replaced by an
• amino or acetylamino group:
– D-Glucosamine
– D-galactosamine
- are components of numerous biologically
important polysaccharide
(Chitin , apolysaccharides formed from glucose amin units, isthe
major polysaccaride of the shells of insecrs and custaceans).
Amino Sugars
DIGESTION OF
CARBOHYDRATES
• The principal sites of dietary carbohydrate
digestion are
– The mouth
– Intestinal lumen
• This digestion is rapid and is generally
completed by the time
• There is little monosaccharide present in
diets of mixed animal and plant origin.
• The enzymes needed for degradation of most
dietary carbohydrates are primarily:
– Disaccharidases
– Endoglycosidases:
• That break oligosaccharides and
polysaccharides
• Hydrolysis of glycosidic bond is catalyzed by a
family of glycosidases that
– Degrade carbohydrates into reducing sugar
components.
– These enzymes are usually specific for the:
• Structure and configuration of the glycosyl
residue to be removed
• Type of bond to be broken.
A. Digestion of carbohydrates
begins in the mouth
• The major dietary polysaccharides are of :
– Animal (glycogen)
– Plant origin (starch, composed of amylose and
amylopectin).
• During mastication, salivary α-amylase acts
briefly on dietary starch in a random manner,
breaking some α (1-4) bonds.
• There are both α (1-4)- and β (1-4)endoglucosidases in nature.
• But humans Do not produce and secret β (1-4)endoglucosidases in digestive juices. Therefore,
– They are unable to digest cellulose - a
carbohydrate of plant origin containing β (1-4)
glycosidic bonds between glucose residues.
– Because branched amylopectin and glycogen also
contain α (1-6) bonds,
• The digest resulting from the action of α-amylase
contains a mixture of smaller, branched
oligosaccharide molecules.
• Carbohydrate digestion halts (stop)
temporarily in the stomach,
– Because the high acidity
• Inactivates the salivary α-amylase
B) Further digestion of carbohydrates by
pancreatic enzymes occurs in the small
intestine
• When the acidic stomach contents reach the
small intestine:
– They are neutralized by bicarbonate secreted by
the pancreas
– Pancreatic α-amylase continues the process of
starch digestion.
C. Final carbohydrate digestion by
enzymes synthesized by the intestinal
mucosal Cells
• The final digestive processes occur at the mucosal
lining of the upper jejunum declining as they proceed
down the small intestine, and include the action of
several:
– Disaccharidases
– Oligosaccharidases. For example:
1)Isomaltase cleaves the α (1-6) bond in
isomaltose
2)Maltase cleaves maltose
• Both producing glucose
3) Sucrase cleaves sucrose producing glucose
and fructose
4) Lactase (β-galactosidase) cleaves lactose
producing galactose and glucose.
D. Absorption of monosaccharides by
intestinal mucosal cells
• The duodenum and upper jejunum
– Absorb the bulk of the dietary sugars.
–Insulin is not required for the uptake of
glucose by intestinal cells
–Different sugars have different
mechanisms of absorption.