Primary cell wall cellulose fibrils

Figure 4-14 Tentative structure of the walls of suspension-cultured sycamore cells. The wall components are in approximately proper proportions but the distance between cellulose elementary fibrils is expanded to allow room to present the interconnecting structure. There are probably between 10 and 100 cellulose elementary fibrils across a single primary cell wall. From Albersheim et al.n6...

Fractionation and Chemistry of Citrus Pectic Polysaccharides. Pectic polysaccharides, commonly known as pectin, appear early in plant cell-wall formation. A series of complex biochemical steps results in the formation of cell plates followed first by its growth in area (primary cell wall) then in thickness (secondary cell wa.ll). Exclusive of randomly oriented cellulose fibrils, primary cell wall is composed mainly of pectic polysaccharides (34). These pectic polysaccharides are rich in D-galacturonic acid, D-galactose and L-arabinose residues. With growth in thickness of cell wall (secondary cell wall),there appears to be a replacement of pectic polysaccharide deposition with polysaccharides rich in D-glucuronic acid or 4-0-methyl-D-glucuronic acid,... 

The polymer is about 0.8 nm in its maximum width and 0.33 nm2 in cross-sectional area, and can contain about 10,000 glucose residues with their rings in the same plane. In the cell wall these polymers are organized into micro-fibrils that can be 5 nm by 9 nm in cross section. These microfibrils apparently consist of an inner core of about 50 parallel chains of cellulose arranged in a crystalline array surrounded by a similar number of cellulose and other polymers in a paracrystalline array. Microfibrils are the basic unit of the cell wall and are readily observed in electron micrographs. Although great variation exists, they tend to be interwoven in the primary cell wall and parallel to each other in the secondary cell wall (Fig. 1-13). 

The secondary cell wall contains about 94% cellulose and only grows when the primary cell wall is formed. In it, the cellulose molecules are highly oriented. With decreasing diameter, the following structural types can be distinguished fibers (0.06-0.28 mm), cell walls, macrofibrils (400 nm), microfibrils (20-30 nm), and elementary fibrils (3.5 nm). Lignin deposits as a cement in interfibrillar spaces of 5-10 nm in width. In fact, the inter-micellar spaces of 1 nm in width that occur between the elementary fibrils are accessible to H2O, ZnCl2, or I2, but not to dyestuffs. 

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. 

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). 

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. 

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