Cellulosic fibrils

Proteia and starch stains are removed by proteases and amylases, respectively. Fats and oils are generally difficult to remove at low wash temperatures by conventional detergents. By usiag Upases, it is possible to improve the removal of fats/oils of animal and vegetable origin even at temperatures where the fatty material is ia a soUd form. Particulate soils can be difficult to remove, especially if the particle sise is small. Removal of particulate soil from cotton fabric can be improved by use of a ceUulase which removes cellulose fibrils from the surface of the yam. 

Strength increase (because the cellulose fibrils pack more closely). To prevent movement, wood should be dried to the value which is in equilibrium with the humidity where it will be used. In a centrally heated house (20°C, 65% humidity), for example, the equilibrium moisture content is 12%. Wood shows ordinary thermal expansion, of... 

Biofinishing, or biopolishing as it is more popularly known, is similar to denim washing in its use of cellulase enzymes, although the effects intended are quite different. The process is designed to eliminate, by dissolution, the cellulosic fibrils projecting from the surface of the fabric. This treatment results in [76] ... 

Figure 2. Hypothetical view of a fibre surface showing cellulose fibrils (C), lignin (L), hemicellulose (H) and reprecipitated xylan (RX).

Cellulose microfibrils make up the basic framework of the primary wall of young plant cells (3), where they form a complex network with other polysaccharides. The linking polysaccharides include hemicellulose, which is a mixture of predominantly neutral heterogly-cans (xylans, xyloglucans, arabinogalactans, etc.). Hemicellulose associates with the cellulose fibrils via noncovalent interactions. These complexes are connected by neutral and acidic pectins, which typically contain galac-turonic acid. Finally, a collagen-related protein, extensin, is also involved in the formation of primary walls. 

Estimation of the degree of polymerization of the D-glucan chains within the cellulose fibrils is complicated by the necessity of first solubilizing the D-glucans, and this process is likely to break the chains. One estimate put the degree of polymerization at 6000 to 7000 for cellulose chains derived from cotton fibers221 (see also, Ref. 217 and references cited therein). 

It is possible that the D-glucan chains of cellulose have no natural ends that is, once a chain is initiated, it never ends, except when a fibril is physically separated from its synthetic enzymes. This idea is supported by the electron-microscope observation that the cellulose fibrils do not appear to have natural termination-points.4,2174,218 It is also possible that the fibrils have an unlimited length, but that the individual D-glucan chains within the fibrils have a finite length the ends of the D-glucan chains may overlap, and thus result in fibrils of indeterminate length. 

The cellulose fibrils of secondary cell-walls have a considerably greater cross-sectional area than those of primary walls,4,223 It is possible that primary microfibrils aggregate to form secondary-wall fibrils. Hemicelluloses trapped between aggregating primary, cellulose microfibrils may constitute the origin of a major proportion of the non-D-glu-cosyl residues of cellulose obtained from secondary walls. 

Cellulose is present in both primary and secondary cell-walls. However, differences in both the degree of polymerization and the control of chain length between the celluloses from these two sources suggest that the two types of cellulose are formed by different mechanisms.289-291 Whereas the mechanisms responsible for formation and orientation of cellulose fibrils are not yet known, some information is available concerning the enzymes capable of catalyzing the formation of celluloselike, 0-(l—>4)-linked D-glucans (see also, Ref. 217). 

Significantly, the ordered pattern of fibril deposition in the secondary wall of Micrasterias367 was shown to be derived from the structure of the complexes located in the plasma membrane. The results indicated that the widest fibrils, those in the center of a band, are formed by the longest rows of rosettes, those in the center of arosette array. The shorter rows of rosettes within an array give rise to narrower fibrils. This proportionality between the width of a secondary cellulose fibril and the number of rosettes involved in its formation provides strong evidence that the rosette structure plays a significant role in the synthesis of cellulose fibrils. 

All of the available evidence suggests that the rosettes represent morphological equivalents of plasma-membrane-bound complexes of enzymes involved in the synthesis of cellulose fibrils in plant cells. 

The bacterial cellulose synthase from Acetobacter xylinum can be solubilized with detergents, and the resulting enzyme generates characteristic 1.7 ran cellulose fibrils (Fig. 20-4) from UDP-glucose.125/127-129 These are similar, but not identical, to the fibrils of cellulose I produced by intact bacteria.125 130 Each native fibril appears as a left-handed helix which may contain about nine parallel chains in a crystalline array. Three of these helices appear to coil together (Fig. 20-4) to form a larger 3.7-nm left-handed helical fibril. Similar fibrils are formed by plants. In both... 

The foregoing observations confirm the conclusions derived from former experiments with beechwood holocellulose (10) (1) A partial degradation of the hemicelluloses is imperative before the cellulose fibrils can be attacked. (2) The hemicelluloses seem to be deposited between the cellulose fibrils or even to be encrusting them. (3) The enzymatic hydrolysis of the cellulose is governed by the porosity of the tissue (enzyme diffusion), the impediment of the hemicelluloses, and the properties of the cellulose (e.g., crystallinity). 

The current observations confirm previous studies on beechwood and sprucewood holocellulose (7,10,19). The attack of the hemicellulose proceeds from the primary wall/Si as well as from the tertiary wall into S2 the pit chambers constitute preferred paths of enzyme diffusion into the walls. Also, substances of the middle lamella, especially in the cell corners, are removed by the xylanase and the mannanase treatments. Parallel to the removal of hemicelluloses, the fibrillar structure of the cellulose and its lamellar arrangement in transections of cell walls became obvious. In samples treated with cellulases, the cellulose fibrils were often completely hydrolyzed in the Si layer, occasionally accompanied by complete dissolution of cell-wall portions. This is also in conformity with the previous conclusion that the cellulases hydrolyze highly ordered zones of cellulose and remove hemicelluloses by hydrolysis or by detachment. 

Major obstacles in the hydrolysis of cellulose are the interference of lignin (which cements cellulosic fibers together) and the highly ordered crystalline structure of cellulose. These obstacles necessitate a costly pretreatment step in which elementary cellulosic fibrils are exposed and separated. 

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