Fibres microfibrils

Fig 1. Electron micrograph of a platinum/carbon replica prepared by the fast-freeze, deep-etch, rotary-shadow replica technique printed in reverse contrast. Cell walls of onion parenchyma have an elaborate structure with many thin fibres bridging between thicker cellulosic microfibrils. Scale bar represents 200nm. 

Replicas of the tomato cell walls are very similar to those of onion parench5una cell walls but replicas of the DCB-adapted walls did not show the structure of the walls clearly. The principle components of the adapted walls are shorter thinner fibres which seemed to form a gel-like structure with little evidence of long cellulosic microfibrils characteristic of the unadapted cells. It is possible that such a gel will bind water more strongly and reduce the amount of etching that takes place, resulting in a less well-defined replica (2). 

In each of the three polymorphs some weak reflections could be indexed only by doubling the a-axis, while some meridional reflections did not fit in the structure anyway. The structure of polymorph II was confirmed by fibre patterns from stacked films of the precipitated glucan. Electron micrographs showed short microfibrils with a beaded... 

In the S2 layer (1-5 pm), which constitutes 90% of the weight of the cell wall, the angle of the microfibrils is between 10 and 30°. The shorter this angle, the higher the modulus and the strength of the fibre (and the lower the elongation at break). 

Barber and Meylan (1964) developed a model based on matrix-micro fibril interaction. In its simplest form their model ignores the different characteristics of the various layers in the cell wall. Instead it assumes that the behaviour of the wood is determined by the thick, dominant S2 layer, where the microfibrils can be inclined to the fibre axis at some angle between 5° and 50° (Figure 4.4). 

Cave ID (1997a) Theory of X-ray measurement of microfibril angle in wood. Part 1. The condition for reflection X-ray diffraction by materials with fibre type s raimetty. Wood Science and Technology, 31(2) 143-52... 

An important component of cuticle is 18 - methyl - eicosanoic acid [40]. Fatty acid is bound to a protein matrix, forming a layer in the epicuticle [41,42], and this layer is referred to as F - layer [43]. The F - layer can be removed by treatment with alcoholic alkaline chlorine solution in order to enhance wettability. The cuticle and epicuticle control the rate of diffusion of dyes and other molecules onto the fibre [44]. The cortex, however, controls the bulk properties of wool and has a bilateral structure composed of two types of cells referred to as ortho and para [45,46]. The cortical cells of both are enclosed by membranes of at least three distinct layers within which the microfibrils fit. Cells of intermediate appearance and reactivity designated meso - cortical have also been reported [47]. Cortical cells on the ortho side are denti-cuticle and thin, those on the para side are polygonal and thick [47]. Fig. 1-7 illustrates the bilateral structure which is responsible for the crimp of the... 

The cellulase complex diffuses through the pore system to the microfibrils, attacks the cellulose chains and hydrolyses each chain to the end. The diflerences in the efficacy of cellulases on various fibres are dependent on number of factors such as the amounts of non-cellulosic wood pulp-derived matter, the degree ol polymerisation, the type and degree of crystallinity, and the type and number of chemical substitutions to the cellulose [27-30]. Key features for the cellulose substrate are crystallinity, accessible surface area and pore dimensions [31 ]. Variation of any of these factors, e.g., structural changes of cellulose substrate by pre-treatments, will influence the course of the entire degradation process [32, 33]. 

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