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Polysaccharides are the polymers or long chains of monosaccharides like glucose, sucrose and lactose. Those monomers are bond together via glycosidic bonds.
In nature, polysaccharides have three important roles. Some types of polysaccharides are used as storage molecules, some are used as structural polysaccharides and most obvious importance of polysaccharides is their use as energy sources.
Starch is the most common polysaccharide in human life. Starch is a polymer of glucose bonded together via alpha-glycosidic bonds. Starch is widely found in plant materials like wheat, corn and rice and many more others like potatoes and cassava. There are two types of molecules in starch, amylose and amylopectin. Amylocpectin is branched polymer while amylose is unbranched. The most important use of starch is as an energy source for human and a lot of other animals. Also it is used as a storage molecule in plants. Starch can be broken down by amylase enzyme in abdomens of most of animals.
Glycogen is the storage molecule in animals and it's analogous to starch in plants. It has a similar structure to amylopectin. Glycogen is made in liver and muscles and mainly stored in muscle tissues as a long term energy storage for human and animals.
Cellulose and chitin are both structural polymers of monosaccharides. Cellulose is glucose monomers combined via beta-glycosidic bonds in contrast to alpha-glycosidic bonds in starch. It is the primary component in plant cell walls and make up the most of wood in nature. Therefore wood is the most common source for cellulose in industry. Cellulase can be digested by cellulase enzymes and it is found in most of the ruminants like cows and other animals dependent on plant materials.
Chitin is a main component of cell walls of fungi and found on crusts of crabs, shrimps and lobsters. It acts as a structural polymer like cellulose, and chitin is reported to have a lot of medicinal values.
You need to remember that glicosidic bonds link the units of monosaccharides to form the chains of polysaccharides.
You need to remember that D-glucose dehydrates to starch and cellulose polymers, hence, the glicosidic bonds links the units of glucose to form the carbohydrate starch.
The starch and cellulose are like polymers but cellulose is stronger than starch since the glucose units in starch are linked by alpha bonds, while the glucose units in cellulose are linked by beta bonds.
You also need to know that starch and glycogen are storage polysaccharides, while chitin and cellulose are structural pollysaccharides.
The human storage polysaccharide is called glycogen and the storage polysaccharide in plants is called starch.
The structural polysaccharides provide the rigidity to the cells, hence cellulose is the structural polysaccharide found in plants, while chitin is found in some insects or crustaceans.
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