**2. Lignocellulosic biomass**

Lignocellulosic biomass is consist of cellulose, hemicelluloses, lignin, water, protein and other compounds (**Table 2**). Cellulose and hemicelluloses provide strength to fiber and lignin act as the concrete which hold the fibers [9].


#### **Table 2.**

*Composition of lignocellulosic materials [10].*

About 50% of the CO2 fixed by plants through photosynthesis get stored in cell wall in the form of cellulose [19]. It is a homo-polysaccharide of glucose residues connected by β-1,4 linkages in linear un-branched fashion (**Figure 2**). Basic repeating unit of the cellulose polymer is a cellobiose unit, made up of two glucose anhydride [20]. The long-chain cellulose polymers are attached to each other by van der Waals and hydrogen bonds which results in packing cellulose chains into microfibrils [21, 22]. Overall structure is found to be consisted of two different types of regions: region where the chains are highly ordered is crystalline and the region with less ordered chain is amorphous [23]. The crystalline regions of cellulose are highly stiff thus these are not easily reachable to endo-cellulases [24]. Amorphous region is more readily hydrated and more accessible to enzyme.

Other significant component of lignocellulose is hemicellulose (**Figure 3**). Hemicellulose usually contributes for about 25–35% of the mass in dry wood, about 28% of softwoods, and 35% of hardwoods [26]. As compared to cellulose these possesses low molecular weight. These are found to consist of comparatively shorter chains of about 500–3000 monosaccharide units as compared to 7000– 15,000 glucose residues cellulose [27]. The monosaccharides of hemicelluloses

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**Figure 4.**

**Figure 3.**

*Overview of the Process of Enzymatic Transformation of Biomass*

include pentoses (arabinose, rhamnose and xylose,), hexoses (glucose, galactose and mannose), and uronic acids (d-glucuronic, d-galacturonic acids and 4-o-methylglucuronic). The backbone of hemicelluloses can be a homopolymer or a heteropolymer having β-1,4 or sometimes β-1,3 glycosidic linkages. In hardwood, xylose is the principal pentose sugar but in various agricultural residues and other herbaceous, arabinose is the chief pentose sugar of hemicelluloses [28]. Lignocellulosic microfibrils are found to be surrounded by a complex aromatic heteropolymer known as lignin which provides a tough protective shield to highly energetic cellulose fibers [30]. Lignin comprises of β-aryl ether, biaryl ether, phenylcoumaran, pinoresinol, or diaryl propane linked p-coumaryl, coniferyl

*Chemical structure of lignin (https://en.wikipedia.org/wiki/Lignin).*

*DOI: http://dx.doi.org/10.5772/intechopen.85036*

*Xyloglucan: a component of hemicelluloses [29].*

**Figure 2.** *Structure of cellulose [25].*

*Overview of the Process of Enzymatic Transformation of Biomass DOI: http://dx.doi.org/10.5772/intechopen.85036*

#### **Figure 3.**

*Elements of Bioeconomy*

**Table 2.**

*Composition of lignocellulosic materials [10].*

About 50% of the CO2 fixed by plants through photosynthesis get stored in cell wall in the form of cellulose [19]. It is a homo-polysaccharide of glucose residues connected by β-1,4 linkages in linear un-branched fashion (**Figure 2**). Basic repeating unit of the cellulose polymer is a cellobiose unit, made up of two glucose anhydride [20]. The long-chain cellulose polymers are attached to each other by van der Waals and hydrogen bonds which results in packing cellulose chains into microfibrils [21, 22]. Overall structure is found to be consisted of two different types of regions: region where the chains are highly ordered is crystalline and the region with less ordered chain is amorphous [23]. The crystalline regions of cellulose are highly stiff thus these are not easily reachable to endo-cellulases [24]. Amorphous

**Lignocellulosic materials Cellulose (%) Hemicelluloses (%) Lignin (%) Reference** Sugar cane bagasse 42 25 20 [11] Sweet sorghum 45 27 21 [11] Hard wood 40–55 24–40 18–25 [12] Soft wood 45–50 25–35 25–35 [12] Corn cobs 45 35 15 [13] Corn stover 38 26 19 [14] Rice straw 32.1 24 18 [13] Nut shells 25–30 25–30 30–40 [15] Newspaper 40–55 25–40 18–30 [16] Grasses 25–40 25–50 10–30 [12] Wheat straw 29–35 26–32 16–21 [17] Bagasse 54.87 16.52 23–33 [18]

Other significant component of lignocellulose is hemicellulose (**Figure 3**). Hemicellulose usually contributes for about 25–35% of the mass in dry wood, about 28% of softwoods, and 35% of hardwoods [26]. As compared to cellulose these possesses low molecular weight. These are found to consist of comparatively shorter chains of about 500–3000 monosaccharide units as compared to 7000– 15,000 glucose residues cellulose [27]. The monosaccharides of hemicelluloses

region is more readily hydrated and more accessible to enzyme.

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**Figure 2.**

*Structure of cellulose [25].*

*Xyloglucan: a component of hemicelluloses [29].*

**Figure 4.** *Chemical structure of lignin (https://en.wikipedia.org/wiki/Lignin).*

include pentoses (arabinose, rhamnose and xylose,), hexoses (glucose, galactose and mannose), and uronic acids (d-glucuronic, d-galacturonic acids and 4-o-methylglucuronic). The backbone of hemicelluloses can be a homopolymer or a heteropolymer having β-1,4 or sometimes β-1,3 glycosidic linkages. In hardwood, xylose is the principal pentose sugar but in various agricultural residues and other herbaceous, arabinose is the chief pentose sugar of hemicelluloses [28].

Lignocellulosic microfibrils are found to be surrounded by a complex aromatic heteropolymer known as lignin which provides a tough protective shield to highly energetic cellulose fibers [30]. Lignin comprises of β-aryl ether, biaryl ether, phenylcoumaran, pinoresinol, or diaryl propane linked p-coumaryl, coniferyl

and sinapyl alcohol units (**Figure 4**). It is categorized as softwood lignin when the coniferyl alcohol derivatives predominant, hardwood lignin where both coniferyl and synapyl alcohol derivatives exist together and grass lignin where it chiefly consisted of p-coumaryl alcohol derivatives [31].
