Natural fiber cell wall

The plant cell wall is the most important part of lignocellulosic natural fibers. Figure 1.4(c) shows the schematic representation of the natural plant cell wall [19]. The cell wall of lignocellulosic natural fibers primarily consists of a hollow tube with four different layers [19]. The first layer is called the primary cell wall, the other three, the secondary cell walls, while an open channel in the center of the microfibrils is called the lumen... 

Fibers for commercial and domestic use are broadly classified as natural or synthetic. The natural fibers are vegetable, animal, or mineral ia origin. Vegetable fibers, as the name implies, are derived from plants. The principal chemical component ia plants is cellulose, and therefore they are also referred to as ceUulosic fibers. The fibers are usually bound by a natural phenoHc polymer, lignin, which also is frequentiy present ia the cell wall of the fiber thus vegetable fibers are also often referred to as lignocellulosic fibers, except for cotton which does not contain lignin. 

The filaments of all plant fibers consist of several cells. These cells form crystalline microfibrils (cellulose), which are connected together into a complete layer by amorphous lignin and hemi-cellulose. Multiple layers stick together to form multiple layer composites, filaments. A single cell is subdivided into several concentric layers, one primary and three secondary layers. Figure 5 shows a jute cell. The cell walls differ in their composition and in the orientation of the cellulose microfibrils whereby the characteristic values change from one natural fiber to another. 

Cellulose, a linear homoglycan of pi 4-linked glucose residues, is the most abundant organic substance in nature. Almost half of the total biomass consists of cellulose. Some 40-50% of plant cell walls are formed by cellulose. The proportion of cellulose in cotton fibers, an important raw material, is 98%. Cellulose molecules can contain more than 10" glucose residues (mass 1-2 10 Da) and can reach lengths of 6-8 pm. 

T ine structural studies on woody cell walls attacked by ectoenzymes of fungi in situ are numerous (cf. 1,2). In contrast, investigations on the selective degradation of cell walls by enzymes isolated from fungi are few. Jutte and Wardrop (3) attempted the use of crude commercial cellu-lase preparations to determine the degradation pattern of Valonia cellulose and beechwood fibers. Similar use of commercial preparations of enzymes was made by Reis and Roland (4) to evaluate the nature of diverse cell walls and to show the distribution of polysaccharides. An endo-/ -l,4-xylanase with specific xylanolytic activities was isolated from a commercial cellulase preparation using chromatographic methods and... 

Among the natural fibers are cellulose, the primary structural component of plants and bacterial cell walls animal fibers such as wool and silk and biochemical fibers. Plant fibers are composed of cellulose (see Figure 1), lignin (see Figure 2), or similar compounds animal fibers are composed of protein (see Figure 3). 

So what effect do these differences in stereochemistry and extent of branching have on the uses of these polysaccharides Because of the extended nature of its chains, cellulose is the most common structural component of plants. Cellulose makes up the main component of the cell wall of plants. Wood is approximately 50% cellulose, while fiber-producing plants such as flax, jute, and hemp are 65% to 80%. The seed hairs of cotton are virtually pure cellulose. The long fibers from cottonseed (up to 5 cm long and 9 to 25 jxm in diameter [Franz 1986)) are spun into thread that is then woven into fabric for clothing. Short fibers, called linters, are used as the raw materials for chemical derivatives such as cellulose acetate (see Chapter 4). 

The middle layer (S2) forms the main portion of the cell wall. Its thickness in softwood tracheids varies between 1 (earlywood) and 5 (latewood) jiim and it may thus contain 30-40 lamellae or more than 150 lamellae. The thickness naturally varies with the cell types. The microfibrillar angle (Fig. 1 -16) varies between 10° (earlywood) and 20-30° (latewood). It decreases in a regular fashion with increasing fiber length. The characteristics of the S2 layer (thickness, microfibrillar angle, etc.) have a decisive influence on the fiber stiffness as well as on other papermaking properties. 

Cellulose is a polysaccharide found in plant cell walls. Cellulose forms the fibrous part of the plant cell wall. In terms of human diets, cellulose is indigestible, and thus forms an important, easily obtained part of dietary fiber. As compared to starch and glycogen, which are each made up of mixtures of a and (3 glucoses, cellulose (and the animal structural polysaccharide chitin) are made up of only (3 glucoses. The three-dimensional structure of the structural polysaccharides is thus constrained into straight microfibrils by the uniform nature of the glucoses, which resist the actions of enzymes (such as amylase) that breakdown storage polysaccharides (such a starch). 

It is found as a component of fungal and bacterial cell-walls, in insect cuticles, and as the shell of crustaceans. Being so similar to cellulose in chemical composition, its structure is important, if for no other reason than that comparison of the two structures might aid in our understanding of each. The similar fibrillar fine-structure (see Fig. 12) of these two polysaccharides is noteworthy, as the lateral forces between molecules are different. Although chitin does not occur in Nature specifically as a fiber, it is frequently found well-oriented in bristles and as tendon material. Samples from invertebrates are usually admixed with protein and carbonate, both of which must be removed before x-ray diagrams of high quality can be obtained. 

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