Wood fiber primary wall

Figure 1.3e Microstructure of wood fiber cell wall P primary cell wall layer, SI, S2, and S3 are the inner, middle, and outer layers of the secondary wall, respectively. Reprinted with permission from Abdul Khalil etal. (2012]. Copyright 2012 Elsevier.

Fig. 1-19. Transverse section of a tension wood fiber in American beech (Fagus grandifolia), showing the middle lamella (M), primary wall (P), the outer (S,) and middle (S. ) layers of the secondary wall, the thick gelatinous layer (G), and the lumen (L). Transmission electron micrograph. Courtesy of Dr. T. E. Timell.

Fig. 7-3. UV absorbance (222 nm, 0.5 p,m section thickness) by various morphological regions of spruce fibers delignified to various lignin contents by the kraft and acid sulfite method (Wood and Goring, 1973). S, secondary wall P, primary wall CCP, primary wall at the cell corner.

Wood is composed of three major polymeric materials cellulose, hemicelluloses, and lignin. Cellulose is the common name used for the glucan present in wood, which constitutes about 42 percent of wood s dry weight. Cellulose is the primary component of the walls of cells making up wood fibers and is the main structural material of wood and other plants. Paper, paperboard, and other wood fiber products thus also are composed mostly of cellulose. The chemical structure of the cellulose macromolecule is shown in Fig. 28.4. In the plant the degree of polymerization (DP) of cellulose is approximately 14,000.5... 

During the division and enlargement phases of wood cell development, the cell wall is a thin, deformable, and extensible envelope of material referred to as the primary wall. Near the cessation of cell enlargement, however, a secondary wall may begin to be manufactured to the lumen side of the primary wall. Wood fibers, vessel elements, and certain other xylem or phloem elements that function in passive conduction and/or support normally develop a secondary wall (Figure 5). 

Figure 5. Cross-sectional view of fully differentiated wood fibers. (A) SE M of a Douglas-fir. Note the thick secondary walls (S) and the fiher lumen (L). (Reproduced from Ref 39. Copyright 1982, American Chemical Society.) (B) TEM of two adjacent fibers in white spruce. Key P, primary wau, S, secondary wall L, lumen and ML, middle lamella. (Reproduced with permission from Ref. 38. Copyright 1974, Forest Products...

Figure 16, Schematic of what is widely considered to be (at least in principle) the general wall architecture of normal wood fibers. Key ML, middle lamella F, primary wall and Sj, Sg, and So, layers of the secondary wall, (Adapted from Ref, i5.)...

Fibers. The morphology of a wood fiber is represented schematically in Figure 1. It consists of several layers of cell walls which contain aggregates of cellulosic chains called microfibrils which are embedded in a polymeric matrix composed of amorphous hemicelluloses, i.e. all types of polysaccharides other than cellulose, and of lignins 11 which are phenolic compounds acting as a glue between the wood components. The primary wall (PW) of the cell contains randomly organized cellulose... 

Both cotton and wood fibers have a thin primary wall that consists of a loose, random fibrillar network and surrounds the relatively thick secondary wall. The primary wall and adjacent intercellular substance between contiguous cells in wood is referred to as the compound middle lamella. In both wood and cotton the secondary wall usually consists of three layers designated SI, S2, and S3. The SI and S3 layers usually are... 

The distribution of constituents is quite simple in cotton. The secondary walls of cotton fibers consist almost entirely of highly crystalline cellulose. Almost all the hemicelluloses and extraneous materials (waxes, pectins, and certain nitrogenous substances) are contained in the cuticle and primary wall layers. In wood, on the other hand, the non-cellulosic materials are deposited in all regions of the cell walls from the lumen through the compound middle lamella. 

At the microscopic level, tension wood is much easier to identify when it is fully developed. Fiber cell walls are much thicker than normal, enclosing very small lumens. Secondary walls are loosely attached to the primary wall and thus are responsible for some of the differing mechanical properties. 

Figure 6.43 The cell walls of a tracheid or wood fiber have several layers, each with a different orientation of microfibrils (164). ML, middle lamella, composed of lignin P, primary wall S, Sj, S3, layers of the secondary wall VJ, warty layer. The lumen in the interior of the warty layer...

Fibrous and other highly differentiated cells, after their mature size and shape are fully attained, undergo formation of a massive inner or secondary wall. In a few instances, the wall is of a compound nature such as that shown in Fig. 2. Cellulose is the principal component of the secondary wall, but there is present in small amount the other components found in the primary wall. Especially in wood fibers, the secondary wall consists of three layers with the center layer the most massive. From superficial... 

The lignocellulosic cell wall may be further subdivided in four sublayers S3, S2, SI, and P. (See Fig. 1.) The S layers are secondary wall layers differing primarily in orientation of cellulose microfibrils. The S3 is adjacent to the cell cavity. The S2 layer occupies the greatest volume of the secondary wall. The SI layer is adjacent to the primary wall layer P, which lacks the fibrillar orientation of the secondary wall. The middle lamella consists primarily of lignin which bonds the individual fibers in wood. 

Homogenization has also been applied to various types of primary wall cellulose (e.g. sugar beet, potato tuber, banana rachis) where the microfibrils are more loosely organized than in the secondary wall of wood cells [28-31]. Hence, the disintegration requires less energy and the fibers are more easily delaminated. However, the material still mainly consists of bundles of microfibrils. 

Mention has already been made of two polymers that can be obtained naturally from living animals silk (from the silkworm) and wool (from sheep). They are proteins made of various amino acids both are used in textiles. Other biologically derived polymers are also familiar such as wood, starch, and some sugars. We will not cover these in detail here. However, certain cellulosics we will discuss briefly since they are compared to synthetic fibers later. Cellulose is the primary substance of which the walls of vegetable cells are constructed and is largely composed of glucose residues. It may be obtained from wood or derived in very high purity from cotton fibers, which are about 92% pure cellulose. 

Cellulose is the primary substance of which the walls of vegetable cells are constructed and is largely composed of glucose residues. It may be obtained from wood or derived in very high purity from cotton fibers, which are about 92% pure cellulose. 

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