Often the biological energy storage systems are also based on carbohydrates like sucrose (saccharose), starch and inulin, which will not be discussed here. In this overview we will only focus on the structural carbohy­drates of terrestrial biomass.

Structural Carbohydrates

Cellulose and hemicellulose can be found in the cell wall of all terrestrial plant cells. In terrestrial biomass the combined cellulose and hemicellulose fraction represents almost always more than 50% of the total biomass based on dry weight. Cellulose is a linear poly­mer composed of b-D-glucopyranose (glucose) units forming microfibrils that give strength and resistance to the cell wall. The hemicellulose consists of a wide variety of polysaccharides (composed of pentoses, hexoses, hexuronic acids), which are interspersed with the microfibrils of cellulose, conferring consistency and flexibility to the structure of the cell wall (Spiridon and Popa, 2008).

CELLULOSE

Cellulose is the basic structural component of plant cell walls and comprises about a third of all vegetable materials. Cellulose is a complex polysaccharide, con­sisting of 3000 or more b-(1 / 4) linked D-glucose units (Table 17.2). It is present in wood in quantities between 40% and 50% on dry matter basis (Table 17.1). It is the most abundant of all naturally occurring organic compounds, comprising over 50% of all the carbon in vegetation. Cellulose is a straight-chain polymer where no coiling or branching occurs, and the molecule adopts an extended and rather stiff rodlike conformation. Cellulose consists of crystalline parts together with some amorphous regions. The chains can stack together to form larger microfibrils, which make cellulose highly insoluble in water. Cellulose microfibrils may also associate with water and matrix polysaccharides, such as the (1 / 3, 1 / 4)-b-D-glucans, heteroxylans (arabinoxylans (AXs)) and glucomannans (GMs) (Sinha et al., 2011; Fengel and Wegener, 1984).

HEMICELLULOSES

Hemicelluloses are the world’s second most abun­dant renewable polymers after cellulose in lignocellu — losic materials. Hemicelluloses are a heterogeneous class of polymers representing, in general, 15—35% of plant biomass and which may contain pentoses (b-D-xylose and a-L-arabinose), hexoses (b-D-mannose, b-D-glucose, and a-D-galactose) and/or uronic acids (a-D-glucuronic, a-D-4-O-methylgalacturonic and

a-D-galacturonic acids). Other sugars such as a-L-rham — nose and a-L-fucose may also be present in small amounts and the hydroxyl groups of sugars can be partially substituted with acetyl groups (Ebringerova et al., 2005; Peng et al., 2012; Girio et al., 2010). Compo­sition and amounts strongly depend on plant source, plant tissue and geographical location. Hemicelluloses are usually bonded to other cell wall components such as cellulose, cell wall proteins, lignin, and phenolic com­pounds by covalent and hydrogen bonds, and by ionic and hydrophobic interactions. The most relevant hemi — celluloses are the xylans and the GMs, with xylans being the most abundant. Xylans are the main hemicellulose components of secondary cell walls constituting about 20—30% of the biomass of hardwoods (angiosperms) and herbaceous plants. In some tissues of grasses and cereals xylans can account up to 50% (Ebringerova et al., 2005). Xylans are usually available in large amounts as by-products of forest, agriculture, agroin­dustries, wood and pulp and paper industries. Mannan-type hemicelluloses such as GMs and galacto — glucomannans (GGMs) are the major hemicellulosic components of the secondary wall of softwoods (gymnosperms), whereas in hardwoods they occur in minor mounts. Depending on their biological origin, different hemicellulose structures can be found (Table 17.2). Upon hydrolysis, the hemicelluloses are converted into the corresponding monosaccharides (Table 17.1). The major hemicelluloses are discussed below.