Cellulose, Hemicellulose and Lignin

Cellulose:

Cellulose is the principal constituent of all plant life. It is a linear polymer of anhydroglucose units linked in 1 and 4 position by a alpha-glucoside links. The empirical formula of cellulose (C6H10O¬5)n corresponds to a polyanhydride of glucose. The two terminal glucose residues of cellulose molecule contain two different end groups; one contains a reducing end group, where as the other contains an extra secondary hydroxyl group in the position C4 and is known as the non-reducing end group.

There are two secondary and one primary alcoholic hydroxyl groups in each basic anhydro-D-glucose unit (C6H10O¬5)n which are arranged in positions 2, 3 and 6 respectively, on the basic unit. The reactivity of the hydroxyl groups varies in different reactions. In many reactions the primary hydroxyl groups have a greater reactivity. The two secondary hydroxyls, at the second and third carbon atoms, differ somewhat in their reactivity, the primary hydroxyls of cellulose elementary units are responsible for the sorbability and dye-ability of cellulose materials. The high hydroxyl content of cellulose might suggest high water solubility. The because of stiffness of the chains and hydrogen bonding between hydroxyl groups of adjacent chains.

Besides hydrogen bonding, another type of linkages called semiacetal linkages is present between the adjacent chain molecules of cellulose.

From X-ray diffraction diagram, it has been concluded that cellulose has two regions: crystalline and amorphous. In the amorphous region the polymer chains end to be folded, and consequently, they will have rather different properties than the crystalline region. The most of the chemical reactions take place in disordered region of with cellulose. Again, polymeric fibers are never completely crystalline. This interconnection of crystalline and amorphous regions enhances the strength of the polymer.

Hemicellulose

The isolated hemicelluloses are amorphous substances. A mixture of polysaccharides called hemicellulose closely interpenetrates the cellulose and lignin of plant cell walls. It is a group of cell wall polysaccharides which unlike cellulose, are soluble in dilute alkali and are readily hydrolysed to pentose and hexose with some uronic acids.

Chowdhry and Saha identified a number of simple sugars and galacturonic acid from the hydrolysate of delignified jute hemicellulose. Norman found about 15 to 18 xylan type hemicellulose. He also classified hemicellulose into polyuronides and cellulosan. The xylan is a cellulosan type hemicellulose and is free from uronic acid while polyuronides invariable contain uronic acids. High xylan contend fiber usually means a high lignin content and is a characteristic of a fiber of poor quality and vice-versa. Sarker and others showed xylose linked with methyluronic acid formed the basic building units of hemicellulose in okra. It appeared that six xylose units were linked with one methyl glucouronic acid unit.

Lignin

Most plant tissues contain in addition to carbohydrate and extractives, an amorphous polymeric gummy material is known as lignin. The nature of lignin and its relationship to cellulose and other constituents of jute, banana, PALF, okra bast fiber etc are still uncertain. Unlike cellulose and hemicellulose, lignin gives a series of color reactions that indicate the presence of compounds for which these reactions are typical. Isolated lignin is generally an amorphous material having an average high molecular weight.

Lignin is an insoluble, resin like substance of phenolic character. It is built up to a large extent, of phenyl propane building stones, of ten having a hydroxyl group in the para position and methoxyl group/groups in meta position/positions to the side chain. Besides, there may be carbon to carbon or carbon to oxygen bonds joining the aromatic ring to the other portions of the structure. The recent opinion is that the major portion of the lignin is combined with carbohydrate materials, probably with hemicellulose through two types of linkages, one being alkali sensitive and the other alkali-resistant.

The alkali sensitive linkage is an ester type of combination existing between hydroxyl groups of lignin and carboxyl groups of uronic acids of hemicelluloses. The other one is believed to be of ether or similar type occurs through he hydroxyl groups of lignin. The lignin molecule, thus being polyfunctional due to the presence of alcoholic and phenolic hydroxyl groups, may exist in combination with two more neighbouring chain molecules, cellulose or hemicellulose, serving the function of a cross-linking agent.