Description
14.1. INTRODUCTION
Carbohydrates, as the name suggest, were defined as a
group of compounds composed of carbon, hydrogen and
oxygen in which the latter two elements are in the same
proportion as in water and were expressed by a formula
(CH2
O)n
, that is, hydrates of carbon.
The term ‘carbohydrates’ arose from the mistaken belief
that substances of this kind were hydrates of carbon,
because the molecular formula of many substances could
be expressed in the form CX(H2
O)Y
, for example, glucose
(C6
H12 O6
), sucrose (C12 H22 O11), etc. In these examples,
the hydrogen and oxygen are present in the same ratio
as in water. But this definition has certain drawbacks as
given below:
–
It should be kept in mind that all organic compounds
containing hydrogen and oxygen in the proportion
found in water are not carbohydrates. For example,
formaldehyde HCHO for the present purpose written
as C(H2
O); acetic acid CH3
COOH written as C3
(H2
O)2
;
and lactic acid CH3
CHOHCOOH written as C3
(H2
O)3
are not carbohydrates.
–
Also, a large number of carbohydrates such as rhamnose
(C6
H12O5
), cymarose (C7
H14O4
), digitoxose (C6
H12O4
),
etc., are known which do not contain the usual propor-
tions of hydrogen to oxygen.
–
Finally, certain carbohydrates are also known which
contain nitrogen or sulphur in addition to carbon, hydro-
gen and oxygen.
From the above discussion, it can be concluded that
the definitions described above are not correct; however,
carbohydrates are now defined chemically as polyhydroxy
aldehyde or polyhydroxy ketones or compound that on
hydrolyses produce either of the above.
Carbohydrates are among the first products to arise as a
result of photosynthesis. They constitute a large proportion
of the plant biomass and are responsible, as cellulose, for
the rigid cellular framework and, as starch, for providing
an important food reserve. Of special pharmacognostical
importance is the fact that sugars unites with a wide variety
of other compounds to form glycosides and secondary
metabolites. Mucilage, as found in marshmallow root and
psyllium seeds, act as water-retaining vehicles, where as
gums and mucilage, which are similar in composition and
properties, are formed in the plant by injury or stress and
usually appear as solidified exudates; both are typically
composed of uronic acid and sugar units. The cell walls
of the brown seaweeds and the middle lamellae of higher
plant tissues contain polysaccharides consisting almost
entirely of uronic acid components.
Low molecular weight carbohydrates are crystalline,
soluble in water and sweet in taste, for example, glucose,
fructose, sucrose, etc. The high molecular weight carbo-
hydrates (polymers) are amorphous, tasteless and relatively
less soluble in water, for example, starch, cellulose, inulin,
etc.
14.2. CLASSIFICATION
Carbohydrates
Simple sugar (Saccharides) Polysaccharides
Monosaccharides Disaccharides Trisaccharides Tetrasaccharides
Monosaccharides
The term ‘monosaccharides’ is employed for such sugars
that on hydrolysis yield no further, lower sugars. The
general formula of monosaccharides is
monosaccharides are subdivided as bioses, trioses, tetroses,
pentoses, hexoses, heptoses, depending upon the number
of carbon atoms they possess.
Production of sucrose
Sucrose is of considerable metabolic importance in higher
plants. Studies have shown that sucrose is not only the
first sugar formed in photosynthesis but also the main
transport material. Newly formed sucrose is, therefore,
probably the usual precursor for polysaccharide synthesis.
Although an alternative pathway consisting of a reaction
between glucose 1-phosphate and fructose is responsible
for sucrose production in certain microorganisms, the
biosynthesis of this important metabolite in higher plants
apparently occurs as shown in Figure 14.2.
Fructose 6-phosphate, derived from the photosynthetic
cycle, is converted to glucose 1-phosphate, which, in turn,
reacts with UTP to form UDP-glucose. UDP-glucose either
reacts with fructose 6-phosphate to form first sucrose phos-
phate and ultimately sucrose, or with fructose to form sucrose
directly. Once formed, the free sucrose may either remain in
situ or may be translocated via the sieve tubes to various parts
of the plants. A number of reactions, for example, hydrolysis
by invertase or reversal of the synthetic sequence, convert
sucrose to monosaccharides from which other oligosaccha-
rides or polysaccharides may be derived.
UDP
+
Sucrose – P Sucrose + P
Photosynthesis
Fructose + P
UTP
Sucrose + UDP
Fructose-6- P Glucose-6- P Glucose – 1 – P UDP – Glucose + P
Fig. 14.2 Pathways of sucrose biosynthesis
ACACIA GUM
Synonyms
Acacia gum, Acacia vera, Egyptian thorn, Gummi africanum,
Gum Senegal, Gummae mimosae, Kher, Sudan gum arabic,
Somali gum, Yellow thorn, Indian Gum and Gum Arabic.
Biological Source
According to the USP, acacia is the dried gummy exuda-
tion obtained from the stems and branches of Acacia senegal
(L.) Willd or other African species of Acacia. In India, it is
found as dried gummy exudation obtained from the stems
and branches of Acacia arabica Willd, belonging to family
Leguminosae
Geographical Source
Acacia senegal is the characteristic species in the drier parts
of Anglo-Egyptian Sudan and the northern Sahara, and is
to be found throughout the vast area from Senegal to the
Red Sea and to eastern India. It extends southwards to
northern Nigeria, Uganda, Kenya, Tanzania and southern
Africa. The plant is extensively found in Arabia, Kordofan
(North-East Africa), Sri Lanka and Morocco. In India it
is found chiefly in Punjab, Rajasthan and Western Ghats.
Sudan is the major producer of this gum and caters for
about 85% of the world supply.
Cultivation and Collection
Acacia is a thorny tree up to 6 m in height. In Sudan, gum
is tapped from specially cultivated trees while in Senegam-
bia, because of extremes of climate; cracks are produced
on the tree and the gum exudes and is collected from the
wild plants. Acacia trees can be cultivated by sowing the
seeds in the poor, exhausted soil containing no minerals.
The trees also grow as such by seed-dispersal.
Gum is collected by natives from 6 to 8 years old trees,
twice a year in dry weather in November or in February—
March. Natives cut the lower thorny branches to facilitate
the working and by means of an axe make 2–3 ft long and
2–3 inches broad incision on the stem and branches, loosen
the bark by axe and remove it, taking care not to injure
the cambium and xylem. Usually they leave a thin layer of
bark on xylem. If xylem is exposed, white ant enters the
plant and gum is not produced. After injury in winter gum
exudes after 6–8 weeks while in summer after 3–4 weeks.
It is believed that bacteria finding their way through the
incision are more active in summer and gum is produced
quickly. The exuded gum is scraped off, collected in leather
bags and then is cleaned by separating debris of bark and
wood and separating sand, etc., by sieving.
Gum is dried in the sun by keeping it in trays in thin
layers for about 3 weeks when bleaching takes place and
it becomes whiter. This result in uneven contraction and
cracks and fissures are formed on its outer surface and as
a result original transparent gum becomes opaque. This
process is called ripening of the gum.
Subject:- Pharmacognosy 1
Semester:- Sem 3
Course:- Bachelor of pharmacy
