Polysaccharides in cell walls

While for some plant species the cell wall composition has been studied, the actual three-dimensional organization of the polysaccharides in the cell wall has not been fully elucidated. Moreover, it appears that while some aspects are constant for all walls (i.e., the presence of similar types of polysaccharides), the relative proportion and fine structure of the polysaccharides may be different, and thus the way they are organized in the wall may be different. Determination of the composition of polysaccharides in cell walls is commonly done by chemical analytical methods, but prior to determining the composition or structure of the polysaccharides in cell walls, the walls must first be isolated from the plant tissues and separated from the intracellular material. 

Several procedures have been used to hydrolyze polysaccharides in cell walls and cell wall fractions. For example, the noncellulosic polysaccharides can be hydrolyzed using 1 M sulfuric acid for 2 to 3 hr at 100°C (Selvendran and Ryden, 1990). One of the simplest procedures is that of Albersheim et al. (1967) in which hydrolysis of the noncellulosic polysaccharides is achieved by incubating in 2 M trifluoroacetic acid (TFA) at 121 °C for 1 hr. The advantage of the TFA procedure is that it is quick and the acid can be removed by evaporation in a gentle stream of air or nitrogen. However, neither the 1 M sulfuric acid or TFA procedures hydrolyze cellulose. Hydrolysis of cellulose can be achieved by an initial dispersion in 72% (w/w) sulfuric acid (Saeman et al., 1963 Selvendran et al., 1979 Fry, 1988 Harris et al., 1988 Selvendran and Ryden, 1990) followed by hydrolysis in 1 M sulfuric acid. 

These studies add to our understanding of the relationship between fine structure of polysaccharides in cell walls and their accessibility to enzymes. An important consideration in accessibility must obviously be the textural organization of crystallites and whether their packing provides interstices large enough to permit an enzyme to bind. 

Cellulose is an important part of woody plants, occurring in cell walls and making up part of the structural material of stems and trunks. Cotton and flax are almost pure cellulose. Chemically, cellulose is a polysaccharide—a polymer made by successive reaction of many glucose molecules giving a high molecular weight (molecular weight ->- 600,000). This polymer is not basically different from the polymers that were discussed in Section 18-6 ... 

Many plant products are very rich in cell wall materials. Cereal brans, seed hulls, various pulps (including beet pulp), citrus peels, apple pomace... are typical exemples of such by-products (1,2). They can be used after simple treatments as dietary fibres, functional fibres or bulking agents, depending on the nutritional claims (2). They can be used also eis sources of some polysaccharides. 

Polysaccharides occur (1) in cell walls, (2) extracellularly in capsules and gums, and (3) inside of bacterial cells. The first two have already been discussed. 

The origin and function of xylan in the cell wall are also not explained. Postulations that it is a plasticizer or is a reserve food are not fully substantiated. Its derivation from cellulose through the decarboxylation of an intermediary polyglucuronic acid seems very unlikely. There is evidence from a number of sources to indicate that the xylan polysaccharide is deposited along with cellulose in cell wall elaboration. 

The best-known aldopentose (1), D-ribose, is a component of RNA and of nucleotide coenzymes and is widely distributed. In these compounds, ribose always exists in the fura-nose form (see p. 34). Like ribose, D-xylose and L-arabinose are rarely found in free form. However, large amounts of both sugars are found as constituents of polysaccharides in the walls of plant cells (see p.42). 

The nature and amounts of low molecular weight phenolic constituents in cell walls of graminaceous plants (grasses and cereals) are reviewed and relationships discussed between these constituents and wall biodegradability. The formation in cell walls of 4,4 -dihydroxy-truxillic acid and other cyclodimers of p-coumaric and ferulic acid is suggested as an important mechanism for limiting the biodegradability of wall polysaccharides. 

The detailed distribution of polysaccharides within cell walls can be determined by immunolabeling sections of plant tissues with appropriate antibodies (Knox, 2008). Such studies also show the distribution of polysaccharides in the middle lamella (Figure 3.5), which develops from the cell plate, formed at cell division, and is responsible for cell-cell adhesion. Cell comers (tri-cellular junctions) and the comers of the intercellular spaces can be regarded as extensions of the middle lamella. They are where stresses that tend to separate plant cells are concentrated and have been referred to as reinforcing zones (Jarvis et al., 2003). These zones and the middle lamella are rich in pectic polysaccharides, but contain no cellulose microfibrils (Jarvis et al., 2003). 

Goldbach, H. and A. Amberger. 1986. Influence of boron nutrition on cell wall polysaccharides in cell cultures of Daucus carota L. Jour. Plant Physiol. 123 263-269. 

In the more abundant a chitin the chains in alternate sheets have opposite orientations,101102 possibly a result of hairpin folds in the strands. Native chitin exists as microfibrils of 7.25 nm diameter. These contain a 2.8-nm core consisting of 15-30 chitin chains surrounded by a sheath of 27-kDa protein subunits. The microfibrils pack in a hexagonal array, but the structure is not completely regular. Several proteins are present some of the glucosamine units of the polysaccharide are not acetylated and the chitin core is often calcified.103 The commercial product chitosan is a product of alkaline deacetylation of chitin but it also occurs naturally in some fungi.102 Chitin is also present in cell walls of yeasts and other fungi. It is covalently bonded to a P-l,3-linked glycan which may, in turn, be linked to a mannoprotein (see Section D,2)97... 

The so-called coated vesicles are an example of an enveloped membrane3). A phospholipid vesicle within a cell is coated by a polypeptide and resembles a foot-ball in a net. A comparable feature of membrane coating is found in cell walls of bacteria4 . These cell walls consist of polysaccharides which are cross-linked by oligopeptides. It is remarkable that this extreme stabilization by an exogenous support is found in bacteria. As parasites in foreign tissues, they have to be especially resistant. 

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