**Table 3.**

*Extraction, Applications and Characterization of Plant Fibers DOI: http://dx.doi.org/10.5772/intechopen.103093*

**Figure 7.** *Cellulose molecule.*

regions, or randomly in amorphous regions. Mechanical properties of natural fibers depend on their type of cellulose, as each type has its own cellular geometry. If cellulose is a prime structural constituent for the vast majority of plant cell walls, then hemicellulose with lignin acts as binding materials. Properties depend on the fiber cell geometry of each type of cellulose and its degree of polymerization.

Hemicelluloses represent the second most abundant constituent of plant fiber. Hemicelluloses are polysaccharides found in lignocelluloses alongside cellulose and pectin. Hemicelluloses, unlike cellulose, are composed of several sugars that form short chains with ramifications. The sugars present can be divided into different groups: pentoses (xylose, arabinose), hexoses (glucose, mannose, galactose), hexo-uronic acids (glucuronic acid and methyl-glucuronic acid) and L-deoxyhexoses (rhamnose and fucose). Hemicelluloses are, by definition, water-soluble polysaccharides that can be extracted from the plant cell walls using alkaline solutions. They are the most hydrophilic biopolymers in the cell wall that promote moisture absorption. In their natural state, they have a degree of polymerization that varies from 200 to 300, and their structure depends on the plant species. The best-studied class of hemicelluloses are xyloglucans. They have a bridging role between cellulose microfibrils in order to strengthen the cell wall by interaction with cellulose and, in some cell walls, with lignin. They consist of a glucose chain and short side chains of xylose, galactose and fructose.

Lignin together with cellulose and hemicelluloses is part of the wood industry. Its proportion in wood varies between 15 and 30% [43]. Lignin or 'lignins' are threedimensional polymers from the radical polymerization of three phenylpropenoic alcohols: coniferryl alcohol, sinapyl alcohol and p-coumaryl alcohol [44]. Lignin contributes to the rigidity of cell walls, and thus to the erect port of terrestrial higher plants. Lignin also offers a protective barrier against the microbial attack of plants. Indeed, due to its chemical nature, lignin is very resistant to various chemical agents and biological degradation. To sum up, lignin polymers make the cell wall rigid and impermeable, allowing the transport of water and nutrients through the vascular system by protecting plants from microbial invasion. Lignin is totally amorphous and hydrophobic. It is not hydrolysed by acids, but hot soluble in soda, easily oxidized and also condensable with phenol.

Pectins are polymers of acidic polysaccharides, composed of a main chain of uronic acid bound in 1–4. Regularly, rhamnose molecules are interspersed between these monomers by bonds 1–2 and 1–4. Some of these rhamnose units carry side chains composed of neutral oses among which galactose and arabinose are the most abundant. The type of bond between uronic acid and rhamnose molecules forms elbows. The pectin macromolecule appears like a zigzag. This arrangement contributes to its special properties and provides some flexibility to plants. Pectins are extracted from the fiber by a chemical method either by boiling water or by ethylene diamine tetraacetic acid.
