Cell wall. wood

Cellulose is a polysaccharide that is also made up of C6H10O5 units. Molecules of cellulose are huge, with molecular weights of around 400,000. The cellulose structure (Figure 3.5) is similar to that of starch. Cellulose is produced by plants and forms the structural material of plant cell walls. Wood is about 60% cellulose, and cotton contains over 90% of this material. Fibers of cellulose are extracted from wood and pressed together to make paper. 

Cassens DL and Feist WC (1991) Exterior wood in the south selection applications and finishes. USDA, Forest Service, Forest Products Laboratory, FPL-GTR-69 Cassens DL, Feist WC, Johnson BR and De Groot RC (1995) Selection and use of preservative-treated wood. Forest Products Society, Madison, Wisconsin Cave ID (1968) The anisotropic anisotropic elasticity of the plant cell wall. Wood Science and Technology, 2(4) 268-78... 

Furuno T, Imamura H and Kajita H (2004) The modification of wood by treatment with low molecular weight phenol-formaldehyde resin a properties enhancement with neutralized phenolic-resin and resin penetration into wood cell walls. Wood Science and Technology, 57 349-61... 

Gindl, W., Muller, U., and Teischinger, A., Transverse Compression Strength and Fracture of Spruce Wood Modified by Melamine-Formaldehyde Impregnation of Cell Walls, Wood and Fiber Science, Vol. 35, No. 2, 2003, pp. 239-246. 

Lignins are natural polymers occurring in plant cell walls - wood and other plants (878940) [a.447]. It has been shown that the structnres of lignins are very different from various sources, such as gymnosperm, dicotyledonous angiosperm and wheat straw (Scheme 32) among many others [a.223]. 

Hemicellulose [9034-32-6] is the least utilized component of the biomass triad comprising cellulose (qv), lignin (qv), and hemiceUulose. The term was origiaated by Schulze (1) and is used here to distinguish the nonceUulosic polysaccharides of plant cell walls from those that are not part of the wall stmcture. Confusion arises because other hemicellulose definitions based on solvent extraction are often used in the Hterature (2—4). The term polyose is used in Europe to describe these nonceUulosic polysaccharides from wood, whereas hemicellulose is used to describe the alkaline extracts from commercial pulps (4). The quantity of hemicellulose in different sources varies considerably as shown in Table 1. 

The common hemiceUulose components of arborescent plants are listed in Table 3. Xylans, arabinogalactans, and pectic substances are common to all while only traces (if at all) of glucomaimans are found in the cell walls of bamboo. Other polysaccharides are found in trace amounts in wood as well as in bark, growing tissues, and other specialized parts of trees. 

As a tree grows, the cells are produced in concentric lamella in the cambium layer, which is between the bark and the wood. In the spring, when moisture is plentiful and the tree is growing rapidly, the tracheid cell wall is thin (3—4 -lm) and the hoUow center or lumen is relatively large (26—43 p.m). 

Other distinct classes of wood in a tree include the portion formed in the first 10—12 years of a tree s growth, ie, juvenile wood, and the reaction wood formed when a tree s growth is distorted by external forces. Juvenile fibers from softwoods are slightly shorter and the cell walls thinner than mature wood fibers. Reaction wood is of two types because the two classes of trees react differentiy to externally applied stresses. Tension wood forms in hardwoods and compression wood forms in softwoods. Compression wood forms on the side of the tree subjected to compression, eg, the underside of a leaning tmnk or branch. Tension wood forms on the upper or tension side. Whereas in compression wood, the tracheid cell wall is thickened until the lumen essentially disappears, in tension wood, tme fiber lumens are filled with a gel layer of hemiceUulose. 

The anatomical stmcture of wood affects strength properties, appearance, resistance to penetration by water and chemicals, resistance to decay, pulp quabty, and the chemical reactivity of wood (5). To use wood most effectively requires a knowledge of not only the amounts of various substances that make up wood, but also how those substances are distributed ia the cell walls. 

Many mechanical properties of wood, such as bending and cmshing strength and hardness, depend upon the density of wood denser woods are generally stronger (6). Wood density is determined largely by the relative thickness of the cell wall and by the proportions of thick-walled and thin-walled cells present. 

Carbohydrates. Carbohydrates are the principal components of the cell wall, comprising 65—75% by weight of the dry wood. Total hydrolysis yields simple sugars, primarily glucose and xylose in hardwoods and glucose and mannose in softwoods. Minor amounts of galactose, arabinose, and rhamnose are present. 

Cellulose is the main component of the wood cell wall, typically 40—50% by weight of the dry wood. Pure cellulose is a polymer of glucose residues joined by 1,4-P-glucosidic bonds. The degree of polymerization (DP) is variable and may range from 700 to 10,000 DP or more. Wood cellulose is more resistant to dilute acid hydrolysis than hemiceUulose. X-ray diffraction indicates a partial crystalline stmcture for wood cellulose. The crystalline regions are more difficult to hydrolyze than the amorphous regions because removal of the easily hydrolyzed material has Htde effect on the diffraction pattern. 

In green wood, the cell walls are saturated, whereas some cell cavities are completely filled and others may be completely empty. Moisture ia the cell walls is called bound, hygroscopic, or adsorbed water. Moisture ia the cell cavities is called free or capillary water. The distiaction is made because, under ordinary conditions, the removal of the free water has Htde or no effect on many wood properties. On the other hand, the removal of the cell wall water has a pronounced effect. 

Resistance to Chemicals. Different species of wood vary in their resistance to chemical attack. The significant properties are beheved to be inherent to the wood stmcture, which governs the rate of ingress of the chemical and the composition of the cell wall, which affects the rate of action at the point of contact (56). 

Fiber-saturation point is the moisture content of celhilar materials (e.g., wood) at which the cell walls are completely saturated while the cavities are liquid-free. It may be defined as the equihbrium moisture content as the humidity of the surrounding atmosphere approaches saturation. 

Fig. 26.2. The microstructure of wood. Woods ore foams of relative densities between 0.07 and 0.5, with cell walls which ore fibre-reinforced. The properties ore very anisotropic, partly because of the cell shape and partly because the cell-wall fibres ore aligned near the axial direction.

---