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Carbohydrates are classified on the basis
of their complexity into three basic categories, namely
monosaccharides, oligosaccharides and polysaccharides.
Monosaccharides constitute the basic
units of carbohydrates and cannot be further hydrolyzed
(“hydrolysis” is a term that refers to the addition of water
molecules) to simpler compounds. Examples include glucose and
fructose.
Oligosaccharides hydrolyze to give
several monosaccharides, usually from two to ten. They may be
further classified as disaccharides (yielding two molecules
of monosaccharides on hydrolysis) such as sucrose or maltose,
trisaccharides such as raffinose and tetrasaccharides
such as stachyose, which hydrolyze to give three and four molecules
of monosaccharides respectively.
Polysaccharides are those
carbohydrates, which on hydrolysis yield a large number of
monosaccharide molecules. Examples of polysaccharides include
starch, cellulose and dextrin.
Glucose and glycogen
Glucose, also referred to as grape sugar or
blood sugar, is a product of the physiological process called
photosynthesis performed by all green plants. It is the chief source
of energy for most living organisms. Several fruits such as mangoes,
grapes and figs contain large amounts of glucose.
Glucose is prepared in the laboratory by
the hydrolysis of cane sugar (sucrose) with concentrated
hydrochloric acid in an alcoholic solution in a water bath. Glucose,
being almost insoluble in alcohol, separates and is obtained in the
form of white crystals on cooling. Glucose is produced on a
commercial scale by the hydrolysis of starch with dilute sulphuric
acid under pressure of about 5 atmospheres.
As mentioned before, glucose belongs to the
simplest class of carbohydrates called monosaccharides and is
in fact, the most abundant of this class. It is a well-known fact
that glucose is the chief source of energy required by the body for
its various metabolic activities and therefore, it is imperative
that most complex carbohydrates are broken down into monosaccharides
before they can be absorbed by the blood. Glucose, on account of its
simple chemical structure is directly absorbed into the blood from
the intestine and is taken to various organ systems in the body.
After a meal rich in carbohydrates, the
glucose content in the blood becomes higher than normal and this
condition persists for a brief period. The extra glucose is rapidly
removed from the blood and taken to the liver by a special blood
vessel called the hepatic portal vein. Here, soluble glucose
is converted to an insoluble complex carbohydrate called glycogen
with the help of a hormone called insulin, produced in
the pancreas.
Glycogen is an insoluble complex
carbohydrate made up of several units of glucose, linked together in
branched chains. It is often called animal starch. It is a
reserve food and is easily reconverted to soluble glucose by another
hormone called glucagon also produced in the pancreas, when
the body needs energy. In other words, liver glycogen regulates the
blood sugar level.
Too much or too little of glucose in the
blood leads to two conditions called diabetes mellitus and
hypoglycemia respectively. Insufficient secretion of insulin
causes diabetes mellitus, resulting in the body’s inefficiency in
absorbing glucose from the blood, due to which, the amount of
glucose in the blood remains abnormally high. As the blood
circulates through the kidneys (excretory organs), excess glucose
passes into the urine and is eventually excreted.
The lack of glucose in the blood leads to
another disorder called hypoglycemia, in which the body is deprived
of its source of energy. Some cells of the body require a continuous
supply of glucose. The nerve cells, especially those of the brain,
are most seriously affected by too little glucose. |
