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Carbohydrates like glucose, fructose, cane sugar, starch
and cellulose were earlier considered to be hydrates of
carbon, Cx(H2O)y·However, some carbohydrates cannot
be represented as hydrates of carbon. The term
'carbohydrates' now includes all polyhydroxy-aldehydes
and ketones along with substances they yield on
hydrolysis. In short, compounds containing carbon,
hydrogen and oxygen with a general formula of (CH2O)n
are called carbohydrates.
Plants produce carbohydrates by photosynthesis and
are classified into monosaccharides, oligosaccharides
and polysaccharides.
Monosaccharides are non-hydrolysable sugars, such
as glucose and fructose, which are soluble in
water and sweet in taste. These are the building blocks of
various carbohydrate molecules. Monosaccharides
which contain an aldehyde group are called aldoses,
whereas those containing a ketone group are called
ketoses.
Oligosaccharides are low-molecular-weight
condensation polymers of monosaccharides and may be
further classified as disaccharides (two monosaccharide
units) and trisaccharides (three monosaccharide units).
Sucrose and maltose are examples of disaccharides.
Non-sugars like starch, dextrin and cellulose, which
on hydrolysis yield a large number of monosaccharide
molecules, are called polysaccharides. They
are amorphous, tasteless, non-reducing, mostly insoluble
in water and are further classified as homopolysaccharides
and hetero-polysaccharides. Their
molecular weight is usually very high and many of them
(like starch or glycogen) have molecular weights of
several million.
Carbohydrates perform many vital roles in living
organisms. Sugars, especially glucose and its
derivatives, are essential intermediates in the conversion
of food into energy. Starch and other polysaccharides
serve as energy stores in plants, particularly in seeds,
tubers, bulbs, leaves, etc., and are a major energy source
for animals and including humans. Cellulose, lignin, etc.
form supporting cell walls and woody tissues of plants.
Chitin is a structural polysaccharide found in the body
shells of many invertebrate animals. Carbohydrates also
occur in the surface coat of animal cells and in bacterial
cell walls. Carbohydrates are an important natural source
of ethyl alcohol that is now used extensively in gasohol.