Dr. Herat D. Soni
Rural medical college
Carbohydrates may be defined as
polyhydroxy aldehydes or
ketones or compounds which
produce them on hydrolysis.
Formula = (C.H2O)n
Most abundant dietary source of energy. Brain cells
and RBCs are almost wholly dependent on
carbohydrates as the energy source.
Also serve as storage form of energy –Glycogen.
Carbohydrates are precursors for many organic
compounds (fats, amino acids).
Participate in the structure of cell membrane &
cellular functions (cell growth, adhesion and
Certain carbohydrate derivatives are used as
drugs, like cardiac glycosides / antibiotics.
DM (diabetes mellitus)
Same chemical formula but different
Example = Glucose and fructose
Same chemical and structural formula but
differ in spatial configuration.
Asymmetric carbon atom
Asymmetric carbon means that four
different groups are attached to the same
The reference molecule is glyceraldehyde
which has a single asymmetric carbon
The number of possible stereoisomer
depends on the number of asymmetric
carbon atoms by the formula 2
where n is
the number of asymmetric carbon atoms.
Reference Carbon Atom of
All monosaccharide can be considered as molecules
derived from glyceraldehyde by successive addition of
carbon atoms. Therefore, penultimate carbon atom is the
reference carbon atom for naming the mirror images
D and L isomerism(Enantiomers)
• D-sugars are naturally occurring sugars and
body can metabolize only D-sugars.
• D-glucose is dextrorotatory. In clinical practice,
it is often called as dextrose
Optical isomerism(d and l)
The presence of asymmetrical carbon atom
causes optical activity. When a beam of
plane-polarized light is passed through a
solution of carbohydrates, it will rotate the
light either to right or to left.
Right = dextrorotatory (+) (d)
Left = levorotatory (-) (l)
D-glucose is dextrorotatory but D-fructose
Equimolecular mixture of optical isomers
has no net rotation (racemic mixture)
When sugars are different from one
another, only in configuration with
regard to a single carbon atom,
other than the reference carbon
atom, they are called Epimers.
To understand this we first understand
Three Representations of Glucose
The 1st carbon, aldehyde group is
condensed with the hydroxyl group of the
5th carbon to form a ring. Ring structure
represents hemi acetal form.
Glucose exists in biological systems not
as a rectangle, but as a pyranose ring.
b-D-glucopyranose is the predominant
D-glucose has two anomers, alpha and
These anomers are produced by the
spatial configuration with reference to the
first carbon atom in aldoses and second
carbon atom in ketoses.
These carbon atoms are known as
anomeric carbon atoms.
When D glucose is crystallized at
room temperature, and a fresh
solution is prepared, its specific
rotation of polarized light is +112o
but after 12–18 hours it changes
This change in rotation with time
is called mutarotation.
Rotation of 112
Rotation of 19
1/3 are alpha type and 2/3rd are beta
variety to get the specific rotation of +52.5o
• It is the sweetening agent known as cane
• It is present in sugarcane and various fruits.
Hydrolysis of sucrose (optical rotation
+66.5°) will produce one molecule of
glucose (+52.5°) and one molecule of
Therefore, the products will change the
dextrorotation to levorotation, or the plane
of rotation is inverted.
Equimolecular mixture of glucose and
fructose thus formed is called invert sugar.
The enzyme producing hydrolysis of sucrose
is called sucrase or invertase.
Honey contains invert sugar.
Invert sugar is sweeter than sucrose.
The principle of Benedict's test is that
when reducing sugars are heated in the
presence of an alkali(pH 10.6), they get
converted to powerful reducing
compounds known as enediols.
Enediols reduce the cupric ions (Cu2+)
present in the Benedict's reagent to
cuprous ions (Cu+) which get precipitated
as insoluble red copper oxide.
Detect the presence of glucose in urine
It is a standard laboratory test employed
for follow-up of diabetes mellitus in PHC.
Benedict's reagent contains sodium
carbonate, copper sulfate and sodium
Any sugar with free aldehyde/keto group
will reduce the Benedict's reagent.
Therefore, this is not specific for glucose.
Carbohydrates giving positive
Benedict ’ s test:
Glucose, Fructose, Galactose
Starches do not react or react very poorly
with Benedict's reagent, due to the
relatively small number of reducing sugar
moieties, which occur only at the ends of
positive Benedict ’ s test
High concentration of Uric acid and
Homogentisic acid (solution turns black
due to black colored oxidized
Vitamin C (even without Boiling)
Certain drugs like aspirin, cephalosporins
Reagents for this test are present on a strip
of paper in solid form.
When the paper is wet with urine, the
reagents dissolve in urine on paper and
react with glucose in urine.
The darkness of color can be correlated
with amount of glucose present in urine.
Because Glucose oxidase enzyme can act
only on beta-D Glucose, other reducing
substances do not give this test positive.
Thus, compounds like Vitamin C,
Aspirin utilize H2O2 produced in the
Due to lack of H2O2, Peroxidase can
not oxidize dye. Thus, glucose may
not be detected even if present, if
urine contain Vitamin C or Aspirin in
large amount. This phenomenon is
called false negative result.
All reducing sugars will form
osazones with excess of phenyl
hydrazine when kept at boiling
Glucose, Galactose and Fructose will
produce the same needle-shaped
All carbohydrates when treated with conc.
sulphuric acid undergo dehydration to
give fufural compounds. These
compounds condense with Alpha-napthol
to form colored compounds.
Molish test is given by sugars with at
least five carbons because it involves
furfural derivatives, which are five carbon
Same principle as benedicts test
Fehling’s A contains 7% copper
sulphate and Fehling’s B contains
sodium potassium tartarate.
This test is based on the same principle as
But, the test medium is acidic.
In acidic medium (pH 4.6)
monosaccharides react faster than
Barfoed’s reagent contains copper acetate
in glacial acetic acid.
Seliwanoff’s test is a chemical test
which distinguishes between aldose
and ketose sugars.
Ketohexoses like fructose on
treatment with HCl form 5-
hydroxymethylfurfural, which on
condensation with resorcinol gives a
cherry red complex.
The glucuronic acid is used by the body for
conjugation with insoluble molecules to make them
soluble in water for detoxification purpose and also
for synthesis of heteropolysaccharides.
Reduction to Form Alcohols
When treated with reducing agents
hydrogen can reduce sugars. Aldose yields
Glucose is reduced to sorbitol
mannose to mannitol
fructose becomes sorbitol and mannitol
Galactose is reduced to dulcitol and
ribose to ribitol.
Significance of reduction
Sorbitol, mannitol and dulcitol are
used to identify bacterial colonies.
Mannitol is also used to reduce
intracranial tension by forced
The osmotic effect of sorbitol produces
changes in tissues when they
accumulate in abnormal amounts, e.g.
cataract of lens.
• Lactulose is also known as beta-D-
• Used in constipation
The hydroxyl group of anomeric carbon of a
carbohydrate can join with a hydroxyl group
of another carbohydrate or some other
compound to form a glycoside and the bond
so formed is known as glycosidic bond.
eg. R-OH + HO-R’ R-O-R' + H2O
The non-carbohydrate moiety is known as
aglycone –phenol, sterol, glycerol and
Glycosidic bond can be N-linked or, O-
Biomedical importance of
Cardiac Glycosides –Digoxin, Digitoxin
◦ Used in cardiac insufficiency.
◦ Stimulate cardiac muscle contraction.
◦ Contain steroids as aglycone
Ouabain –Sodium pump inhibitor.
◦ Given in Tuberculosis
◦ cause renal damage, glycosuria.
◦ Blocks the transport of sugar across the
mucosal cells of small intestine & also
renal tubular epithelium.
Formation of Esters
Esterification of alcoholic groups of
monosaccharides with phosphoric
acid is a common reaction in
Examples :Glucose-6-phosphate, and
ATP donates the phosphate moiety.
Amino groups may be substituted for hydroxyl
groups of sugars to give rise to amino sugars
Amino sugars Found in
Glucosamine Hyaluronic acid, heparin and
blood group substances
Galactosamine Chondroitin sulphate of
cartilage, bone and tendons.
Mannosamine constituent of glycoproteins
(GluNac) and N-
Mucopolysaccharide and cell
Oxygen of the hydroxyl group may be removed to
form Deoxy sugars
It is the reserve carbohydrate of plant
Sources: Potatoes, cereals (rice,
wheat) and other food grains.
Starch is composed of amylose and
Amylose is made up of glucose units
with alpha-1,4 glycosidic linkages to
form an unbranched long chain. Water
The insoluble part absorbs water and
forms paste like gel; this is called
Amylopectin is also made up of
glucose units, but is highly branched.
The branching points are made by
Iodine test for starch
Starch will form a blue colored
complex with iodine; this color
disappears on heating and reappears
when cooled. This is a sensitive test
Starch is nonreducing because the
free sugar groups are negligible in
Hydrolysis of starch
Amylodextrin = violet color with
iodine and is non-reducing.
Erythrodextrin = red color with
iodine and mildly reduce the
Achrodextrins = no color with
Maltose = (no color with iodine, but
It is the reserve carbohydrate in animals.
It is stored in liver and muscle.
Liver glycogen stores increase during the
well-fed state , and are depleted during a
Glycogen is composed of glucose units
joined by alpha-1,4 links in straight
chains. It also has alpha-1,6 glycosidic
linkages at the branching points.
Glycogen is more branched and more
compact than amylopectin.
It is made up of glucose units combined
with beta-1,4 linkages.
It has a straight line structure, with no
Beta-1,4 bridges are hydrolyzed by the
enzyme cellobiase. But this enzyme is
absent in animal and human digestive
system, and hence cellulose cannot be
Fiber can absorb 10–15 times its own
weight in water, drawing fluid into
the lumen of the intestine
Increasing bowel motility
1.Decrease the risk for constipation
It is a major constituent of fiber, the
Bile salt Fibers
2. Lower LDL cholesterol levels
Delays gastric emptying and can result in a
sensation of fullness
4. Reduced peaks of blood glucose
following a meal
Can bind various toxic substances
including carcinogens & eliminate them in
3.Decreases chances of some cancers
It is a long chain homoglycan composed
of D-fructose units with repeating beta-1,2
It is the reserve carbohydrate present in
various bulbs and tubers, such as onion,
It is clinically used to find renal
clearance value and glomerular
These are highly branched homopolymers
of glucose units with 1-6, 1-4 and 1-3
linkages. They are produced by micro-
Since they will not easily go out of
vascular compartment, they are used for
intravenous infusion as plasma volume
expander for treatment of hypovolemic
Dextrose, Dextrin and Dextran
D-glucose is otherwise called Dextrose, a
term often used in bed-side medicine, e.g.
Dextrin is the partially digested product of
Dextran is high molecular weight
carbohydrate, synthesized by bacteria.
It is present in exoskeletons of
It is composed of units of N-
acetylglucosamine with beta-1,4
Agar = The linear polysaccharide Agarose
It is dissolved in water at 100ºC, which
upon cooling sets into a gel. Agarose is
used as matrix for electrophoresis.
Agar cannot be digested by bacteria and
hence used widely as a supporting agent to
culture bacterial colonies.
[acidic sugar–amino sugar]n.
Because of their large number of negative
charges, these heteropolysaccharide chains
tend to be extended in solution. They repel
each other, and are surrounded by a shell
of water molecules. When brought
together, they “slip” past each other.
This produces the “slippery” consistency
of mucous secretions and synovial fluid.
This property contributes to
the resilience of synovial fluid
and the vitreous humor of the
GAGs Composition Tissue distribution Functions
and N-acetyl D-
Gel around ovum
lubricant and shock
absorbant in joints
D-glucuronic acid and N-
valves and skin.
Helps to maintain
the structure And
shapes of tissues
D-Iduronic acid and
Skin Helps to maintain
shapes of tissues
galactose and N-acetyl
Heparin sulphated glucosamine
and glucuronic acid or
blood, lung, liver
N-Acetyl-glucosamine → beta-1, 4-
Glucuronic acid → beta-1-3-N-Acetyl
glucosamine and so on.
Breaks b(1-4 linkages) in hyaluronic acid.
Present in high concentration in testes,
seminal fluid, and in certain snake and insect
Hyaluronidase of semen clears the gel
(hyaluronic acid) around the ovum allowing a
better penetration of sperm into the ovum.
Serves important role in fertilization
Hyaluronidase of bacteria helps their invasion
into the animal tissues.
glucuronic acid → beta-1,3-N-acetyl
galactosamine sulfate → beta-1, 4
and so on
Dermatan sulfateDermatan sulfate
L-iduronic acid and N-acetylgalactosamine
in beta-1, 3 linkages
Only GAG not having Uronic acid.
It contains repeating units of sulphated
glucosamine → alpha-1, 4-L-iduronic acid
or glucuronic acid → and so on
Heparin is an anticoagulant( prevents
Heparin helps in the release of the enzyme
lipoprotein lipase which helps in clearing
the turbidity of lipemic plasma.
Lipoprotein lipase breaks TG in glycerol
lipoprotein lipase from
heparan sulphate binding
site hence clearing factor
Proteoglycans: When carbohydrate
chains are attached to a polypeptide chain.
Glycoproteins: Carbohydrate content ≤
Glycoprotein Major function
Glycophorin glycoprotein of
Collagen Structure of cartilage
Ceruloplasmin Transport protein
Immunoglobulin Defense against
Intrinsic factor Absorption of vitamin
Fibrinogen Blood clotting