carbohydrates chemistry introductory
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Dr. Maha Ismail
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Carbohydrates
Carbohydrates are the most abundant
compounds found in nature.
The name carbohydrates indicatesthat these compounds are
composed of carbon,
hydrogen and oxygen.
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Carbohydrates
Definition:
Arepolyhydroxyl
alcohols havingpotentially active
aldehyde or ketone
group attached to them.
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Function of Carbohydrates
1. Themain source of energy in the diet.2. Storage form of energy.
3. Cell membrane component that mediate
some form of intracellularcommunications.
4. Serve as structural component of manyorganisms including:
The cell wall of bacteria. The fibrous cellulose of plants. The exoskeleton of some insects
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Classification of Carbohydrates
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Classification of Carbohydrates
Monosaccharide
Are sugars which can not be degraded
further into simpler sugars.
Monosaccharides are classified according
to the number of carbon atoms they contain.
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Monosaccharide
Monosaccharides are further classified
according to the nature of the functional
group.
Thus there areAldoses for aldehyde
containing monosaccharides, and Ketoses
for those having a ketone group.
These two classifications may be used
together for exampleAldohexose and
ketohexose.
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Monosaccharide Aldehyde Ketone
Trioses (3 C Glyceraldehydes Dihydroxyacetone
Tetroses (4C Erythrose Erythrulose
Pentose (5 C Ribose Ribulose
Hexoses (6 C Glucose Fructose
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Physical properties of Monosaccharide
1. Soluble in water.
2. Aldose and ketose isomer
3. Contain asymmetric carbon atom. i.e.attached to 4 different groups e.g.
Carbon no.2 in glucose.
4. Optically active compounds.
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2.Aldose and Ketose isomers
Are compounds having the same
molecular formula but differ in the
functional group.
Fructose has a keto group in position 2
whereas glucose has an aldehyde
group in position 1.
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Optically active compounds.
Monosaccharides have the power of
rotating plane of polarized light through
a certain angle, but must contain at least
one asymmetric carbon atom.
Optically active substances are
classified into types according to the
direction in which they rotate thepolarized light.
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When equal amounts of dextro and
levo isomers are present in a solution
the resulting mixture has no optical
activity and the compounds are knownasRacemic Compounds
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Isomers posses the same general
physical and chemical properties.
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L and D configuration or Enantiomers
Enantiomer is a special type of isomer
found in pairs that they are mirror image
of each other.
Any compound containing asymmetric
carbon atom can be present in the space
in 2 different forms which are mirror
image and are called stereo isomers.
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L and D configuration
When the OH group of the carbon atom
adjacent to the last carbon is on the right
side it is called D- sugar, if on the left side
it is called L- sugar.
Nearly all naturally occurring sugars are D-
sugars.
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Carbohydrates
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Epimers
Isomers differ in configuration of the OH
and H around one specific carbon atom, 2,
3, and 4 of glucose.
Glucose and Galactose are epimers in
carbon 4.
Glucose and Mannose are epimers in
carbon 2.
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Cyclization of monosaccharides
The anomeric carbon of aldose or ketose
will react with the alcohol group on the
same sugar to form hemiactal or hemiketal.
If the resulting ring has six members
(5 carbons and 1 oxygen it is a pyranose
ring.
If it is five membered ( 4 carbons and
oxygen it is called furanose ring ( ex.
Fructose
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- Anomers
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- Anomers
Two molecules of D- glucose can differ from
each other, one is the other is .
The two sugars are glucose but they are
anomers of each others.
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Disaccharides
These are glycosides formed by the
condensation of 2 sugars, they
include:
1. Lactose.
2. Maltose.
3. Sucrose.
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Starch
Starch is the most important food source of
carbohydrates.
It is found in cereals, potatoes, grains, tubers &vegetable.
Starch is formed of an glycosidic chain.
It yield only glucose on hydrolysis, as it is ahomoploymer of glucose (Glucosan, or Glucan.
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80-85%
(24-30,
glucose
residues.
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It is called animal starch.
10-12,glucose
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Inulin
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Inulin
Is a starch found in tubers.
Is a homoplymer of fructose ( Fructosan.
This starch is unlike potato starch, is readily
soluble in water & has been used to determine the
glomerular filteration rate.
Chitin:
Is an important structural polysaccharide of
invertebrates.
It is found in the skeleton of insects.
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Isomers
The number of possible isomers of a
compound depends on the number of
asymmetric carbon atoms (n and is
equal to 2 n.Glucose with 4 asymmetric carbon
atoms, so it has 16 isomer