Elementary plant Biochemistry and Biotechnology (2+1)

b)Structural polysaccharides

Structural polysaccharides

Cellulose
 
• Cellulose is the most abundant organic substance found in nature. It is the principal constituent of cell walls in higher plants.It occurs in almost pure form (98%) in cotton fibres and to a lessor extent in flax (80%), jute (60-70%), wood (40-50%) and cereal straws (30-43%). It is linear, unbranched homoglycan of 10,000 to 15,000 D-glucose units joined by ß-1—>4 linkages. The structure of cellulose can be represented as a series of glucopyranose rings in the chair conformation.


• The most stable conformation for the polymer is the chair turned 180º relative to the adjacent glucose residues yielding a straight extended chain. Celluose molecules within the plant cell walls are organized into biological units of structure known as microfibrils. A microfibril consists of a bundle of cellulose molecules arranged with its long axis parallel to that of the others. This arrangement permits the formation of intramolecular hydrogen bonding between the hydroxyl group of C-3 of one glucose residue and the pyranose ring oxygen atom of the next glucose residue.




• This hydrogen bond impart a double bond character to the glycosidic bond and impedes the rotation of adjacent glucose residues around the glycosidic bond. Within the microfibril, the adjacent cellulose molecules are linked by intermolecular hydrogen bond between C-6 hydroxyl group of one molecule and the glycosidic bond oxygen atom of adjacent cellulose molecule. the cross section of the microfibril consists of a central crystalline core of about 5–30 nm short diameters.


• The central crystalline core contains around 50-100 cellulose molecules which are arranged in perfect three dimensional array and exhibits a crystalline structure. Surrounding this crystalline core is a region of paracrystalline matrix which contains about 100 polysaccharide molecules of cellulose and hemicellulose. This region does not have perfect three-dimensional order and water molecules are able to penetrate the paracrystalline region but not the crystalline core.