Composition of Biomass
Plant cell wall is constituted by mainly 6 components: (i)
water soluble sugars, amino acids and aliphatic acids, (v)
ether and alcohol-soluble constituents (e.g.
fats, oils, waxes, resin and many pigments), and (iv)
proteins. These components build up plant biomass. Proportion of these constituents vary in different groups of plants and even in the same group. If the concentration of sugar is high, the biomass will be sugary e.g.
sugarcane, and sugar beet. Similarly, high amount of starch present in biomass yields the starchy biomass e.g.
potato and tapioca. Variation in chemical constituents (cellulose, hemicellulose and lignin) in some plants is shown in Table 19.4.
Table 19.4. Chemical composition of lignocellulosic material, and cotton (on % dry weight basis).
|Sources of biomass
(N x 6.25)
|Crop residues **
: * Cowling(1975); ** Solneker (1976); *** Wenzel (1970).
Fig. 19.1. Structure of cellulose chain.
Cellulose constitutes the major portion of plant cell wall, the fundamental unit of which is glucose. Formation of cellulose is a complex process. From each glucose unit, one molecule of water is removed to yield an anhydrous glucose:
O = C6
The anhydrous glucose units are linked end to end with b-1,4 - linkage to form the long chain polymer of cellulose (C6
)n. Here n represents the degree of polymerization, the number of which varies from 5000 to 10,000 (Fig. 19.1). Enzymatic hydrolysis of cellulose and production of glucose are given under Enzymatic digestion
Fig. 19.2. Structure of constituents of hemicellulose.
Hemicellulose is also constituted by sugars (xylans) which comprises of 20-25 per cent plant biomass on dry weight basis.
Fig. 19.3. Structure of p-hydroxy cinnomyl alcohol unit forming lignin.
In addition, it also contains glucose and several other hexoses (galactose and mannose) and pentoses (xylose and arabinose) (Fig. 19.2). The proportion of these constituents varies plant to plant. Degree of polymerization to yield hemicellulose does not exceed beyond 50. The polymers has branched chains. It occurs as amorphous mass around the cellulose strands. Hemicellulose are insoluble in water but easily solubilized in alkali.
Fig. 19.4. Structure of lignin.
The third constituent, lignin, is a complex and high molecular weight polymer (Fig. 19.4). It is formed by de-hydrogenation of p-hydroxy-cinnamyl alcohols (Fig. 19.3) such as p-coumaryl (I), coniferyl (II), and sinapyl (III) alcohols. Presence of these alcohols differs in different plant groups. For example, gymnosperm lignin is formed from coniferyl alcohols, angiosperm lignin is formed from the mixture of coniferyl and cinapyl alcohols, and grass lignin from mixtures of coniferyl, sinapyl and coumaryl alcohols (Bhushan, 1985). Lignin is phenolic in nature; it is very stable and difficult to isolate. It occurs between the cells and cell walls.
It is deposited during lignification of the plant tissue and gets intimately associated within the cell walls with cellulose and hemicellulose and imparts the plant an excellent strength and rigidity.
As a result of photosynthesis, an enormous amount of plant biomass is accumulated in terrestrial and aquatic systems, which are then utilized into different ways as the source of energy.