Comparing and contrasting functions and features of carbohydrates cellulose, chitin,starch and glycogen - page 2
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: 2 of 4 pages: 1 2 3 4
the creation of a branch. Glycogen does not possess a reducing end: the 'reducing end' glucose residue is not free but is covalently bound to a protein termed glycogenin as a beta-linkage to a surface tyrosine residue. (2) The glycogen granules contain both glycogen and the enzymes of glycogen synthesis (glycogenesis) and degradation (glycogenolysis). The enzymes are nested between the outer branches of the glycogen molecules and act on the non-reducing ends. Therefore, the many non-reducing end-branches of glycogen facilitate its rapid synthesis and catabolism.(2)
Cellulose is synthesized in higher plants by enzyme complexes localized at the cell membrane called cellulose synthase.(2), Cellulose is also a polysaccharide carbohydrate, related to starch, however, rather than forming α-1,4 glycosidic bonds, cellulose has β-1,4 bonds (2), formed by a condensation reaction, making it a long-chain polymeric polysaccharide carbohydrate, of beta-glucose (5). The primary cell wall of green plants is made primarily of cellulose; the secondary wall contains cellulose with variable amounts of lignin. Lignin and cellulose, considered together, are termed lignocellulose, which is argued to be one of the most common biopolymers on Earth(2). Cellulose is a straight chain polymer: unlike starch, no coiling occurs, and the molecule adopts an extended rod-like conformation. In microfibrils, the multiple hydroxyl groups on the glucose residues hydrogen bond with each other, holding the chains firmly together and contributing to their high tensile strength. This strength is important in cell walls, where they are meshed into a carbohydrate matrix, helping keep plant cells rigid. (1)
Another area in which cellulose contrasts to starch is that cellulose is also much more crystalline. Whereas starch has an crystalline to amorphous transition at 60 -70 °C in water as in cooking, it takes 320°C and 25 MPa for cellulose to become amorphous in water.(4)
Chitin is one of the main components in the cell walls of fungi, linking it to cellulose, the exoskeletons of insects and other arthropods, and in some other animals. Chitin is also a polysaccharide, analogous in chemical structure to cellulose. It consists of units of a glucose derivative ( N -acetyl- d -glucosamine) joined to form a long, unbranched chain.(1) These are linked together in β-1,4 fashion (in a similar manner to the glucose units which form cellulose). In effect chitin may be described as cellulose with one hydroxyl group on each monomer replaced by an acetylamine group. This allows for increased hydrogen bonding between adjacent polymers, giving the
Last 5 comments…
There have been no comments posted for this article, but you need to register if you want to be the first!