## 2 Wood and Its Petocellulosic Cell Wall Organization  
Lignocellulosic materials such as wood are complex materials, composed of a variety of cells with very different functions. All these cells have in common a pectocellulosic wall, composed mainly of cellulose, hemicellulose, lignin, and pectins, organized to protect the cell [22]. _Cellulose_ is a homopolymer of \(\beta\)-D-glucopyranose units, linked by \(\beta\)-(1-4) glycosidic bonds [23; 24; 25; 26; 27]. It is thus formed by long linear chains that can reach a degree of polymerization of 15,000 [24; 28]. As each unit has alcohol functions, the linear chains come together through hydrogen bonds and Van der Waals forces to form cellulose microfibrils [25; 29; 30; 31]. When the microfibrils are perfectly ordered, they are referred to as crystalline cellulose or non-accessible cellulose [23; 24; 25; 29]. The cellulose microfibrils can then combine to form a cellulose fiber or macrofibril. The cellulose, arranged in microfibrils and macrofibrils, thus provides mechanical strength to the cell wall, enabling the plant to expand in height [28; 29]. _Hemicelluloses_ are heteropolymers of pentoses (xylose and arabinose), hexoses (mannose, glucose, and galactose) and acid oes (uronic acids), linked by \(\beta\)-(1-4) osidic bonds, with degrees of polymerization ranging from 100 to 300 [23; 24; 32]. There are four types of hemicellulose [33]:  
* Xyloglucans: backbone of \(\beta\)-(1-4)-linked glucoses, with branches of xylose, galactose, and fucose;
* Glucuronoxylans and glucuronoarabinoxylans: skeleton of xyloses linked by \(\beta\)-(1-4) bonds, with glucuronic acid and arabinose branches;
* Mannans and glucomannans: skeleton made up entirely of mannose (for mannans and galactomannans) or glucose and mannose (for mannans and galactoglucomannans), linked by \(\beta\)-(1-4) bonds;
* \(\beta\)-(1-3,1-4)-glucanas: backbone of glucoses linked by \(\beta\)-(1-3) or \(\beta\)-(1-4). The type of hemicelluloses and their quantities depend on the organism studied. In fact, xyloglucans are found in greater quantities in the primary cell walls of plants, particularly hardwoods. Glucuronoxylans are mainly found in the secondary cell walls of hardwoods. As for galactoglucomannans, they are mostly found in the secondary walls of softwood [33]. Hemicelluloses bind to cellulose microfibrils via hydrogen bonds. They thus play a bridging role between cellulose microfibrils, but also with other wall components. Hemicelluloses also bind to lignin via covalent bonds. In this way, hemicelluloses reinforce the cell wall [23; 24; 28; 33]. _Lignin_ is an amorphous, three-dimensional heteropolymer made up of three monomers called monolignols (phenylpropanoids): \(p\)-coumaryl alcohol (H unit), coniferyl alcohol (G unit), and sinapyl alcohol (S unit) [24; 25; 34; 35]. Each unit is found in different quantities depending on the plant type: hardwood lignin is predominantly composed of S and G units, while softwood lignin is predominantly constituted of G units. All these monolignols are linked together by various ether bonds of different strengths [35]. Lignin therefore has a highly complex structure. Lignin gives cells rigidity, enabling woody plants to grow vertically. Lignin also helps protect cells from microbial and fungal attack by rendering wall polysaccharides inaccessible [36; 37]. _Pectins_ are polysaccharides found mainly in the middle lamella and primary wall. There are different types of pectin [38; 39]:Homogalacturonans: linear chains of uronic acids linked by \(\alpha\)-(1-4) bonds, which account for around 65% of pectin. The homogalacturonans are often methylated and sometimes acetylated. * Rhamogalacturonans I: homogalacturonan and rhamnose skeletons, with arabinose and galactose branches on the rhamnose units. The uronic acid units of the backbone can be methylated and sometimes acetylated. Rhamnogalacturonanes I account for between 20 and 35% of pectin. * Rhamnogalacturonans II: branched homogalacturonan skeletons with more than 12 types of sugar and 20 different types of bonds.