Comparing and contrasting functions and features of carbohydrates cellulose, chitin,starch and glycogen - page 1
Keywords: carbohydrates structure function features starch chitin cellulose glycogen essay
By xbryonyx on 13/02/2007
Level: A Level (Year 12) / AS Level
Page Number: 1 of 4 pages: 1 2 3 4
Starch and glycogen are both storage compounds. Cellulose and chitin also have similar functions and structure. Write a structured essay comparing and contrasting the features and functions of these carbohydrates.
In green plants starch is produced by photosynthesis ; it is one of the chief forms in which plants store food. It is stored most abundantly in tubers (e.g., the white potato), roots (e.g., the sweet potato), seeds, and fruits; it appears in the form of grains that differ in size, shape, and markings in various plants.(1) Biochemically, starch is a combination of two polymeric carbohydrates (polysaccharides) called amylose and amylopectin. Amylose is constituted by glucose monomer units joined to one another head-to-tail forming α-1,4 glycosidic bonds . Amylopectin differs from amylose in that branching occurs, with an α-1,6 linkage every 24-30 glucose monomer units. The overall structure of amylopectin is not that of a linear polysaccharide chain since two glucose units frequently form a branch point, so the result is the coiled molecule most suitable for storage in starch grains. Both amylopectin and amylose are polymers of glucose, and a typical starch polymer chain consists of around 2500 glucose molecules in their varied forms of polymerisation. In general, starches have the formula (C6H10O5)n, where "n" denotes the total number of glucose monomer units.(2)
Structurally, the starch forms clusters of linked linear polymers, where the α-1,4 linked chains form columns of glucose units which branch regularly at the alpha-1,6 links. Starch is semi-crystalline and has a crystalline to amorphous transition at 60-70°C in water in a process called gelatinization. (2)
The starch obtained by animals from plants is stored in the animal body in the form of glycogen.(1) Glycogen, also known as ‘animal starch’ is a starch-like polysaccharide that is found in the liver and muscles of humans and the higher animals and in the cells of the lower animals. Like starch, it is a polysaccharide and is the principal storage form of glucose (Glc) in animal and human cells. Glycogen is found in the form of granules in the cytosol in many cell types. It forms a highly branched polymer that is better described as a dendrimer of about 60,000 glucose residues and has a molecular weight between 106 and 107 daltons (4.8 million approx.).(2) Most of Glc units are linked by α-1,4 glycosidic bonds, approximately 1 in 12 Glc residues also makes -1,6 glycosidic bond with a second Glc, which results in
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