Starch is a polysaccharide made up of alpha glucose monosaccharides. However the alpha glucose in starch forms two compounds; amylopectin and amlylose. Amylopectin makes up 70 % of starch, it consists of chains of glucose monomers linked with 1,4 glycosidic bonds. However about every 20-30 residus there is a 1,6 bond which results in branches being fromed.
Amylose makes up the other 30% of starch, however it is an unbranched linear polymer unlike amylopectin. The monomers are joined in1,4 glycosidic bonds. The chains usually have around 300 alpha glucose molecules and form a stiff left handed single helix shape.
Starch is the main carbohydrate food store in plants. It is well suited to this because; it is compact, insoluble so cannot move out of cells, has no osmotic effects, it doesn’t react within the cell and it can easily be turned into a sugar by hydrolysing it.
Glycogen is the polysaccharide that is used by animals for storing glucose for future use. Like amylopectin, it also consists of a 1,4 linkages and a 1,6 linkages, however they branch far more often. One of the functions of having many branches in the molecule is that it gives a lot more ends for enzymes to work on to hydrolyse the glycogen to form glucose. Thus, glucose can be released much more quickly than if there was just one end for the enzymes to work on, this is important as it can be used as a respiratory substrate.. It is a very compact structure therefore saving space by concentrating the alpha glucose.
The major component in the rigid cell walls in plants is cellulose. Cellulose is a linear polysaccharide with many beta glucose monosaccharides linked in a long chain by 1,4 glycosidic bonds. It has no branches or helical shape. Hydrogen bonding occurs between different chains forming cross links which hold the chains together. When a large amount of chains come together, say 2000, they form a microfibril which is very strong.
It is good for making rigid cell walls because of this strength. Cellulose is a readily available raw material making it very useful. It’s used in paper, cellophane food packaging, photographic film and even cotton clothes and tea towels.
There are a few other important polysaccharides such as Chitin. Chitin is very similar to cellulose, it is a polysaccharide with added amino acids to form a mucopolysaccharide. It is used in the forming the exoskeletons of insects and other arthropods, as it is strong lightweight and waterproof.
Callose is another polymer of glucose found in plants. The monomers are linked by1,3 glycosidic bonds. It is used to line the inside of a plants phloem. Murein is a polysaccharide found in the cell walls of prokaryotes, a type of bacteria in plant roots.
In conclusion I have found that polysaccharides serve a number of biological services do to their structure and the type of monomer they are made of. This can bee seen for example with starch which is compact to allow more energy to be stored.