Pharmacognosy II
Carbohydrates
It is a hydrated carbon compounds. It is defined as a poly hydroxyl
aldehyde or keton, containing carbon, hydrogen and oxygen, the ratio of
hydrogen to oxygen is the same as their ratio in water.
Carbohydrate are divided into:
1- Monosaccharides.
2- Di and oligosaccharides.
3- Polysaccharides.
Monosaccharides
They are the simplest carbohydrates, they consist either of two carbon
atoms and called bios, or three carbon atoms (trios), or tetrose (4 carbon),
pentose (5 carbon), hexose (6 carbon), heptose(7 carbon), up to nine
carbons. Bios are not found freely in the plant.
Examples
Trios: glyceraldehyde
Tetrose :
D-erythrose
D-threose
Pentose:
Others: arabinose, ribulose, xylulose
Hexose:
Others: rhamnose, galactose
Heptose:
Haworth projection formula
This assumes that there is a ring structure between carbon 1 and 5
Each OH group on the left is drawn upward, while the OH group on the
right is drawn downward.
Evidence indicates that glucose and other hexose often exist in cyclic
forms as well as in straight structure.
Glucose is an aldohexose, that is a poly hydroxyl alcohol having an
aldehyde group, whereas fructose, which has a ketone group, is a
ketohexose. These groups explain the reducing properties of the
monosaccharides and account for the commonly applied term "reducing
sugars".
Disaccharides and oligosaccharides
Disaccharides: are compounds that yield two monosaccharide molecules
on hydrolysis.
e.g. sucrose: it yields equimolecular quantities of glucose and fructose. It
is the only disaccharide that occurs abundantly in free state in the plant. It
is a non-reducing sugar.
Maltose: it’s a reducing sugar, on hydrolysis it yields two molecules of
glucose.
Lactose: known as cow milk sugar, has a reducing property and on
hydrolysis yields equimolecular quantities of glucose and galactose.
Oligosaccharide: (Greek oligo means few), is often applied to
saccharides containing 3-10 units of sugar.
Polysaccharides: they are derived from monosaccharides. They are
complex, high molecular weight polysaccharides as starch, inulin and
cellulose. They can usually be hydrolyzed to the component hexose and
therefore are called hexosan.
Starch yield glucose and hence called glucosan, while inulin yields
fructose, therefore known as fructosan.
Carbohydrate utilization
Storage carbohydrate such as the starch of plants or glycogen of animals
is made available for energy production by a process which involves
conversion to pyruvate and then to acetyl-coenzyme A, then to acetate
which will pass to the tricarboxylic acid cycle (TCA).
As a result of this, the energy rich carbohydrate is oxidized to CO 2 and
H2O. Coenzymes in TCA will carry the liberated hydrogen atoms to the
cytochrome system in which the energy is liberated in stages, with the
formation of ATP from ADP. The hydrogen combines with oxygen to
form water.
1- Embden-Meyerhof scheme ( pathway) of glycolysis:
a number of pathways for the initial metabolism of glucose are known to
various living tissues. One of them is the embden-Meyerhof scheme of
glycolysis.
The kinetics and enzyme systems involved in this pathway have been studied
extensively. One molecule of glucose can give rise to two molecules of
pyruvate, each of which is converted to acetate and CO2.
This yields 2 NADH molecules and 4 ATP molecules, leading
to a net gain of 2 NADH molecules and 2 ATP molecules from
the glycolytic pathway per glucose.
2- Tricyclic acid cycle:
TCA cycle represent the oxidation of one acetate to two molecules of
CO2 giving rise to 12 molecules of ATP ( 1 ATP = 8 Kcal).
pyruvate
NAD
NADH
TCA cycle
ADP
ATP
The overall reaction for the metabolism of one molecule of glucose in terms of ADP
and ATP is:
C6H12O6+6O2+38ADP+38 P(inorganic)
glucose
6H2O+6CO2+38ATP
The overall scheme:
Complex polysaccharides
In systematic nomenclature polysaccharides are given the ending "an"
and another word for polysaccharide is the generic term glycan. If only
one type of monosaccharide unit is present, the polysaccharide is a
"homoglycan" but a "heteroglycan" if more than one kind on
monosaccharide is involved.
Examples of homoglycans:
1234-
Starch : composed of glucose
Inulin: fructose
Dextran: polyglucagon formed from sucrose.
Cellulose: consist of several hundred of D-glucose.
Examples of heteroglycans
123456-
Gums.
Tragacanth.
Acacia.
Agar.
Plantago seed (psyllium seed).
Pectin.