**2.1 Cellulose**

Cellulose is a linear polymer of β-D-glucopyranose units linked to each other by 1,4-glycosidic bonds. The linear cellulose chain has a very strong tendency to form intra and inter-molecular hydrogen bonds, which promotes the collection of parallel chains into basic microfibrils. Most wood species contain 40–45% cellulose based on oven-dry (OD) wood. Compression wood of softwoods contains less crystalline cellulose than non-compression wood [13]. The chemical structure of cellulose is shown in **Figure 1**.


**431**

**Figure 2.**

*Chemical structure of hemicellulose [14].*

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

[22]. The schematic diagram is shown in **Figure 2**.

This is the second most abundant structural component of a typical plant cell wall after cellulose [21]. Cellulose microfibrils are thus linked together in a hydrogen bond with hemicellulose forming cellulose microfibrils (fibers). Hemicellulose has a random, amorphous structure and can be easily hydrolyzed by dilute acid or alkaline of various 5 and 6 carbon sugars, including arabion-xylans glucomannans, and galactans. Xylan is a family of polysaccharides most common to hemicellulose

Lignin is the third most abundant structural component in nature of a typical plant cell wall after cellulose and hemicellulose [14]. This amorphous heteropolymer consists of three different phenylpropane units, namely, p-coumaryl, coniferyl and sinapyl alcohol joined by different linkages (presented in **Figure 3**). Lignin was first discovered in 1813 by a Swiss botanist, A. P de Candolle who described it as fibrous, tasteless and insoluble in water and alcohol, but soluble in a weakly basic solution, thus, making it difficult for biodegradation [15]. It is a class of complex organic polymer forming structural materials for supporting tissues of vascular plants and offers impermeability and resistance to microbial attacks [16]. It strengthens stems and vascular tissue, allowing upward growth and permits

**2.2 Hemicellulose**

*Chemical structure of cellulose [21].*

**Figure 1.**

**2.3 Lignin**

#### **Table 1.**

*Chemical composition of raw sugarcane bagasse (%w/w, dry basis).*

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

**Figure 1.** *Chemical structure of cellulose [21].*

#### **2.2 Hemicellulose**

*Biotechnological Applications of Biomass*

**2. Lignocellulose biomass (LB)**

cellulose more available to the enzymes that convert the carbohydrate polymers into fermentable sugars [5]. Other studies have reported that pretreatment of lignocellulosic biomass (LB) aids to overcome recalcitrance through the combination of chemical and structural changes to the lignin and carbohydrates. Some of the different methods of pretreatment include physical; namely, mechanical pretreatment, physicochemical; namely, steam explosion, chemical; namely, alkali and acidic pretreatment, and biological; namely, manure addition or mixed microorganisms [6, 7]. Nonetheless, these traditional methods of pretreatment are cost-intensive, as additional chemicals or energy are required [8]. Also, useful information for policymakers and researchers on lignin biorefinery is presented in this chapter.

LB is a composite, based on intertwined biopolymers on a dry basis, consisting of 35–45% cellulose, 25–30% hemicellulose, and 25–30% lignin [9]. These are classified into four major proportions based on their source, namely, woody biomass, agricultural residues (for example, rice/wheat/barley straws, corn stover, sugarcane bagasse), energy crops (switchgrass, *Miscanthus* and short-rotation hardwood is specifically grown for biofuel production) and a group of cellulosic wastes (for example, municipal solid waste, pulp mill and lumber mill wastes) [10]. Cellulose and hemicellulose are broken down by enzymatic saccharification into simple sugars which are further digested by microorganisms through the anaerobic digestion process to produce bioenergy such as biogas [11]. Nonetheless, the application of LB for the net reduction of CO2 emissions from the transport sector is considered environmentally benign [12]. As a result, pretreatment becomes very important to improve the digestibility of the LB [5, 13]. **Table 1** shows the various chemical compositions of sugarcane bagasse, a lignin-rich residue obtained from the sugar industry.

Cellulose is a linear polymer of β-D-glucopyranose units linked to each other by 1,4-glycosidic bonds. The linear cellulose chain has a very strong tendency to form intra and inter-molecular hydrogen bonds, which promotes the collection of parallel chains into basic microfibrils. Most wood species contain 40–45% cellulose based on oven-dry (OD) wood. Compression wood of softwoods contains less crystalline cellulose than non-compression wood [13]. The chemical structure of cellulose is

**Components (%) References**

47.0 27.0 23.0 [14] 38.8 26.0 32.4 [15] 45.5 27.0 21.1 [16] 38.4 23.2 25.0 [17] 45.0 25.8 19.1 [18] 39.5 25.6 30.4 [19] 43.6 17.2 22.0 [20]

**Cellulose Hemicellulose Lignin**

*Chemical composition of raw sugarcane bagasse (%w/w, dry basis).*

**430**

**Table 1.**

**2.1 Cellulose**

shown in **Figure 1**.

This is the second most abundant structural component of a typical plant cell wall after cellulose [21]. Cellulose microfibrils are thus linked together in a hydrogen bond with hemicellulose forming cellulose microfibrils (fibers). Hemicellulose has a random, amorphous structure and can be easily hydrolyzed by dilute acid or alkaline of various 5 and 6 carbon sugars, including arabion-xylans glucomannans, and galactans. Xylan is a family of polysaccharides most common to hemicellulose [22]. The schematic diagram is shown in **Figure 2**.

#### **2.3 Lignin**

Lignin is the third most abundant structural component in nature of a typical plant cell wall after cellulose and hemicellulose [14]. This amorphous heteropolymer consists of three different phenylpropane units, namely, p-coumaryl, coniferyl and sinapyl alcohol joined by different linkages (presented in **Figure 3**). Lignin was first discovered in 1813 by a Swiss botanist, A. P de Candolle who described it as fibrous, tasteless and insoluble in water and alcohol, but soluble in a weakly basic solution, thus, making it difficult for biodegradation [15]. It is a class of complex organic polymer forming structural materials for supporting tissues of vascular plants and offers impermeability and resistance to microbial attacks [16]. It strengthens stems and vascular tissue, allowing upward growth and permits

**Figure 2.** *Chemical structure of hemicellulose [14].*

#### **Figure 3.**

*Chemical structure of the main monolignols of lignin.*

water and minerals to be conducted through the xylem under negative pressure without collapse of the tissue. In addition to mechanical support, lignin contributes to protective functions in plants by, for example, increasing resistance to biodegradation and environmental stresses, such as changes in humidity and water balance.
