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Polysaccharides are polymers composed of multiple subunits of
monosaccharides (simple sugars). They are formed by condensation
reaction, in the same way as disaccharides are formed, the difference
stands in the larger number of monomer units which they are composed
from . The number of monosacchardies composing the chain is variable,
and there are two types of chains that can be formed: branched or
unbranched. The chains may be folded, thus making them
therefore ideal for storage. The size of the molecule makes them
hydrophopic ( insoluble), which is another feature that makes them
ideal for storage, as they exert no osmotic influence and do not
easily diffuse out of the cell. If undergone to the process of
hydrolysis, polysaccharides can be converted to their constituent
monosaccharides ready for use as respiratory substrates. Examples of
storage polysaccharides are starch and glycogen.
Starch is a mixture of two kinds of
of - glucose, amylose and
amylopectin. In amylose, the glucose units are joined by a linear
succession of (1 => 4) glycosidic links. The lengths of the amylose
chains vary within the same sample, but over 1000 glucose units occur
per amylose molecule. The formula masses range from 150 000 to 600
are found under the form of large grains,
ubicated in the inside of chloroplasts and in storage organs. They are
visible under a light microscope.
The long amylose molecules coil into spiral-like helices , which tuck
a significant fraction of the OH groups inside and away from the
contact with water. Thus amylose is only slightly soluble in water.
Amylopectin molecules have both (1 => 4) and (1=>6) glycosidic links.
The (1=>6) bridges link the C-1 ends of linear amylose-type to to C-6
positions of glycose units in other long amylose chains. There are
hundreds of such links per molecule, so amylopectin is heavily
branched, and the branches prevent any coiling of the polymer. This
leaves many more OH groups exposed to water than in amylose, so