Bundled cellulose microfibrils

In tobacco primary cell wall the cellulose microfibrils observed individually or associated with bundles were also triple-stranded and left-hand helical. These observations are shown in Figure 10. Since cellulose is only 19% of the tobacco cell wall (17), the task of finding and identifying cellulose was complicated. For this reason A. xylinum which produces a pure ribbon of cellulose was used for studying cellulose structure. 

Twisting of bundles of bacterial cellulose microfibrils. J. Polymer Sd. 49,... 

Pairs of unbranched cellulose molecules, which may contain as many as 12,000 glucose units each, are held together by hydrogen bonding to form sheetlike strips called microfibrils (Figure 7.34). Each bundle of microfibrils contains approximately 40 of these pairs. These structures are found in both plant primary and secondary cell walls, where they provide a structural framework that both protects and supports cells. 

In electron micrographs, the native cellulose microfibrils are usually seen as bundles of lamellae containing an indefinite number of fibrillar units. A schematic representation of the cross section of a small lamellae of microfibrils is shown in Figure 3 (14). Two structural features should be pointed out. The cellulose lattice extends through the whole cross section of the microfibrils but the surface layers are supposed to be disordered to some extent because they represent a discontinuity. The microfibrils show a preferred orientation with their lattice planes 101 well aligned parallel with the flat surface of the lamellae—i.e., the tan-... 

Within each layer of the secondary wall, the cellulose and other cell-wall constituents are aggregated into long slender bundles called microfibrils. The microfibrils are distinct entities in that few cellulose... 

Figure 12-30. Structures of cellulose microfibrils synthesized by distinct TCs. Note cross sectional views of cellulose microfibrils (oblique lines) and particle arrangement of TCs on the fractured face of the plasma membrane. (A) A bundle consisting of 2-nm fine fibrils, which is synthesized by a dinoflagellate TC (a). (B) A thin, ribbon-like microfibril synthesized by each of phaeophycean and eustigmatophycean TCs (b), and rhodophycean (c), xanthophycean (d), and phaeothamniophycean TCs (e). (C) A large microfibril synthesized by each of ulvophycean (f), chlorophycean (g), and glau-cophycean TCs (h). (D) A 3.5-nm microfibril synthesized by a rosette TC (i). (E) A microfibril with a parallelogrammic section synthesized by a tunicate TC (j).

Figure 13-7. A schematic illustration showing the step-wise involvement of glomerulocytes in the formation of a cellulose network in the hemocoel (1) glomerulocytes are transferred into hemocoel (2) bundles of cellulose skeleton are released in the hemocoel (3) cellulose microfibrils of the skeleton are untied to make cellulose network. Tu = tunic, ep = epidermis, gl = glomerulocyte, ae =atrial epitherium, b = blood cell (See Color Plate of this figure beginning on page 355)...

Figure 13-10. Ultrastructure of the inlet filter in the house of O. rufescens. (a) SEM image of the inlet filter shows a highly ordered meshwork structure like an elaborately woven textile. A stitch of the meshwork circled line in a) is composed of orthogonally arranged bundles of cellulose microfibrils, (b) Cellulose microfibrils in the house are bundled, and (c) consist of highly crystalline cellulose Ip similar to that of ascidians inset in c). (Figure 2 from Kimura, S., Ohshima, C., Hirose, E., Nishikawa, J., and Itoh, T. 2001. Cellulose in the house of the appendicularian Oikopleura rufescens. Protoplasma 216 71-74. Reproduced with kind permission of Springer Science and Business Media).

Fig. 1 Stmctural disintegration of cellulose, a a cellulose chain b an elementary fibril containing bundles of cellulose chains c parallel elementary fibrils d four microfibrils held together by hemicellulose and Ugnin, internal structure of a cellulose microfibril (Adapted from Ramos 2003)...

Overall, cellulose I is mainly responsible for the mechanical properties of reinforced polymer composites due to its high elastic modulus and crystallinity. The elastic modulus of perfect cellulose crystals has been calculated and estimated between 130 GPa to 250 GPa, whereas the tensile strength is approximately between 0.8 GPa to 10 GPa [28]. In previous studies cellulose has already been processed into films, gels, fibers, microfibers, nanofibers and nanocrystals for different applications [29-32]. Actually, cellulose fiber is the bundle of microfibrils comprising nanocrystalline domains linking through amorphous domains [33]. 

Microfibrillar cellulose refers to isolated cellulose microfibrils or microfibril bundles that are derived from a cellulose raw material. Microfibrils typically have a high aspect ratio. The niunber average diameter is typically below 200 nm. The smallest microfibrils are similar to so-called elementary fibrils, which are typically 2-12 nm in diameter. 

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