Microfibrillated appearance

Fig. 42. Dif action-contrast transmission electron microscopy of a fragment of Valonia ventricosa cell-wall cross-sectioned perpendicular to one of the main microfibrillar directions. The pictme is printed in reverse contrast, so that the cross-sectioned microfibrils appear as white squares. (See Color Plate 17.)...

The shape of the fibrils and microfibrils appears to be flat or tape-like and some twisting is also observed. The images suggest that the smallest nucrofibrils are < 30 nm wide and < 5 nm thick for the lyotropic and thermotropic fibers, measured using the same techniques. 

The solvent-etched specimen was first polished. The spherical holes indicate the presence of spherical polystyrene particles (Fig. 11.17(a)). The fracture surface contains many small spherical particles which consist of polystyrene. The polyethylene component had undergone some plastic deformation which is seen in the microfibrils appearing in the micrograph (Fig. 11.17(b)). 

There are a number of possible locations within the cell wall for the pectin further away from cellulose. If there are covalent links between pectins and xyloglucans (16), then pectic chain segments close to these links would appear in the region sharing the same mean mobility characteristics as cellulose. The majority of the pectic molecule, diverging from the microfibrils would appear in the region with greater mean mobility. 

Consolidation of linear terminal complexes. The correlation between linear consolidation of terminal complexes and microfibril width does not appear to be as consistent for linear terminal complexes, although this correlation... 

Thus, linear terminal complex consolidation appears to be a manifestation of the stage of cell wall development rather than a significant factor in the determination of microfibril dimensions. 

Synthesis of the left-hand helical submicrofibril appears to be the driving force for self-assembly of a left-hand helical microfibril from three submicrofibrils. 

Submicrofibril and triple-stranded left-hand helical microfibrils are found in tobacco primary cell wall and bacterial A. xylinum cellulose. We suspect from our results and the literature survey outlined in reference (1) that the triple stranded structures are prominent in the primary plant cell wall. The highly crystalline cellulose of plant and algae secondary cell wall appears by X-ray fiber diffraction (18,19) and TEM lattice imaging (20-23) to be largely crystalline arrays of planar straight chains of (l-4)-/3-D-glucan chains. 

Figure 8. Since the submicrofibrils exit the cell wall of A. xylinum through pores (4), the self-assembly of a triple-stranded microfibril has occurred at the exterior surface of the cell (1). Submicrofibrils 1 and 2 appeared super-twisted on the cell surface. At their junction submicrofibril 2 crossed 1 in a left-handed manner which is only visible with stereo-micrographs (not shown). Submicrofibril 3, which was also left-hand super-twisted, joined and crossed the double fiber in a left-handed manner. This specimen was coated with 16.4A of Pt-C. (Reproduced with permission from Ref. 1. Elsevier Science Publishers B. V.)...

The model in Figure 9 predicts that each microfibril would rotate in the process of cellulose ribbon formation. If the A. xylinum cell were held stationary, then the ribbon would be left-hand twisted (2-5) however, if the ribbon were held stationary, then the cell would rotate (32). The latter case explains why ribbons appear untwisted in the pellicle of ribbons shown in Figure 1. Moreover, it has been demonstrated that an A. xylinum cell ceased rotation when Calcofluor (> 0.1 mM) was added to the solution... 

Purified, crystalline cellulose isolated from secondary walls appears to contain minor proportions of D-glycosyl residues other than D-glucosyl, in hemicellulosic chains of paracrystalline regions within the microfibril structure.223 These hemicelluloses contain xylose and probably lesser proportions of arabinose, mannose, and fucose. It was conceived... 

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