Cell walls hemicelluloses

The predominant hemicelluloses in secondary cell walls of both monocotyledons and dicotyledons are xylans. These polysaccharides have a xylan main chain that is decorated with 4-0-methyl-D-glucopyranosyluronic acid, D-glucopyranosyluronic acid, L-arabinofuranosyl, and/or L-arabinopyranosyl units. These related polysaccharides are usually acetylated to various degrees. Gymnosperms contain glucomannans, also acetylated, as the principal secondary cell-wall hemicellulose [26]. 

Suspension-cultured tissues of cereal endosperm Cells containing mainly primary cell walls Hemicelluloses (mainly acidic arabinoxylans and p-D-glucans) cellulose, some phenolics and phenolic esters, and proteoglycans... 

In the cell wall, hemicelluloses are thought to surround and sheath microfibrils, forming a chemically bonded cellulose-hemicellulose network. Whereas the composition of cellulose is consistent for softwoods and hardwoods, the chemistry and structure of softwood and hardwood hemicelluloses differ. Even within a single tree, different parts will contain varying amounts of certain hemicellulose species (9). Given this variability, hemicellulose contents for hardwoods and softwoods are generally reported as ratios, rather than as exact quantitative measurements. 

Cellulose microfibrils are usually embedded in a continuous phase of lignin, pectin, and hemicellulose hemicellulose usually predominates (Whistler and Richards, 1970). Most of the hemicellulose is found mixed with cellulose in both the primary and secondary cell walls. Hemicellulose is now known not be a precursor to cellulose. In general, hemicellu-loses are the cell wall polysaccharides other than cellulose and pectin. These polymers are classified on the basis of the type of sugar residues present. For example, o-xylan is made up of D-xylose, D-mannan of D-mannose, and D-galactan of D-galactose units (Whistler and Richards, 1970). In practice, few hemicelluloses contain only one sugar, most contain two to four. Furthermore most hemicelluloses have branched structures. The major types of hemicellulose and their distribution have been reviewed (Whistler and Richards, 1970). Methods for the analysis of mannans have been reviewed (Matheson, 1990). 

Hemicelluloses are non-cellulosic and short-branched chain heteropolysaccharides, which consist of various different sugar units. They can be arranged in different proportions and with different substituents. Large amounts of hemicelluloses with a wide variation in content and chemical structure are found in plant cell walls. Hemicelluloses generally consist of several populations of polysaccharide molecules, varying in structural characteristics. Several fractionation techniques have been employed in order to obtain more homogeneous fractions as well as exploring structure-property relationships for the hemicellulosic polymers. ... 

Hemicellulose, which is a highly branched polysaccharide in contrast to linear cellulose, is located attached to the cellulose in plant cell walls. Hemicellulose, such as ara-binoxylans, is not a form of cellulose but is another group of polysaccharides [3]. Being... 

Plant cell wall polysaccharides are the main organic compounds found in nature. They are divided into three groups cellulose, hemicelluloses, and pectic substances (IJ)- Cellulose is a linear and long homopolymer consisting of 1,4-linked P-D-glucopyranosyl residues. Its main function is to ensure the rigidity of the plant cell wall. Hemicelluloses constitute the second most abundant plant material after cellulose. They are highly hydroscopic and have an influence on the flexibility of cell walls. 

Hemicellulose is a collective name for the non-cellulosic polysaccharides, which are a large constituent in the structure of the wooden cell wall. Hemicelluloses are generally considered to cover the elementary cellulose fibrils and function as an amorphous glue binding the cellulose fibrils together. Hemicelluloses comprise typically 25-35% of the wood, and are thus the second most abundant component group in wood. Hemicelluloses have only a very weak surface activity but are, nevertheless, of scientific and technical interest because of their ability to function as emulsion stabilizers through a steric stabilization mechanism. 

C.B.S. Tong, K.C. Gross. Glycosyl-linkage composition of tomato fruit cell wall hemicellulosic fractions during ripening. Physiol Plant, 1988, 74, 365-370... 

Hemicellulose [9034-32-6] is the least utilized component of the biomass triad comprising cellulose (qv), lignin (qv), and hemiceUulose. The term was origiaated by Schulze (1) and is used here to distinguish the nonceUulosic polysaccharides of plant cell walls from those that are not part of the wall stmcture. Confusion arises because other hemicellulose definitions based on solvent extraction are often used in the Hterature (2—4). The term polyose is used in Europe to describe these nonceUulosic polysaccharides from wood, whereas hemicellulose is used to describe the alkaline extracts from commercial pulps (4). The quantity of hemicellulose in different sources varies considerably as shown in Table 1. 

Chemical Constituents of Cell Wall. Variation in chemical composition across the cell wall is also shown in Figure 6. The principal constituents of cellulose, hemicellulose, and lignin are present throughout the cell wall but in different proportions. Cellulose is not present in the interfiber middle lamella, which is virtually all lignin. The layer is essentially all carbohydrates (qv), especially hemiceUuloses, having Uttie or no lignin. 

Fig. 26.3. The molecular structure of a cell wall. It is a fibre-reinforced composite (cellulose fibres in o matrix of hemicellulose and lignin).

Based on the current stage of knowledge, hemicelluloses can be divided into four general classes of structurally different cell-wall polysaccharide types, i.e.,... 

Xyloglucans are classified as gum when they are extractable with hot water from seed endosperm cell walls, such as the tamarind seed xyloglucan, and as hemicelluloses because they are alkali-extractable from the cell walls of vegetative plant tissues where they are closely associated with cellulose [2]. Also /3-glucans with mixed linkages appear under the name gum as well as hemicellulose in the literature. 

Within the scope of this review, the contributions of the last decade concerning cell-wall polysaccharides isolated from woody and other plant tissues will be reviewed according to the above-proposed classification of hemicelluloses including larch arabinogalactans. The present review article updates and extends previous reviews [3-5] and will focus in particular on new investigated plant sources, isolation methods, structural features, physicochemical and various functional properties of hemicelluloses. Attention will also be paid to the modification of isolated hemicelluloses or hemicellulosic materials and the appHcation possibiUties of hemicelluloses and their derivatives, including their use for the production of composite materials and other biomaterials. 

Xylan-type polysaccharides are the main hemicellulose components of secondary cell walls constituting about 20-30% of the biomass of dicotyl plants (hardwoods and herbaceous plants). In some tissues of monocotyl plants (grasses and cereals) xylans occur up to 50% [6j. Xylans are thus available in huge and replenishable amoimts as by-products from forestry, the agriculture, wood, and pulp and paper industries. Nowadays, xylans of some seaweed represent a novel biopolymer resource [4j. The diversity and complexity of xylans suggest that many useful by-products can be potentially produced and, therefore, these polysaccharides are considered as possible biopolymer raw materials for various exploitations. As a renewable resource, xylans are... 

AGX are also the dominant hemicelluloses in the cell walls of hgnified supporting tissues of grasses and cereals. They were isolated from sisal, corncobs and the straw from various wheat species [4]. A more recent study on corncob xylans [30] showed the presence of a hnear, water-insoluble polymer... 

From recent literature it is known that the disintegration of lignified cell walls can be achieved by steam explosion treatments resulting in solubilization of partially depolymerized hemicelluloses [91,92]. The application of this method on wheat bran yielded feruloylated GAX with different feruUc acid content [93]. Partly depolymerized water-soluble, acetylated AGX was obtained from spruce wood by employing microwave treatment [94]. 

Glucomannans (GM) and galactoglucomannans (GGM), common constituents of plant cell walls, are the major hemicellulosic components of the secondary cell walls of softwoods, whereas in the secondary cell walls of hardwoods they occur in minor amounts. They are suggested to be present together with xylan and fucogalactoxyloglucan in the primary cell walls of higher plants [192]. These polysaccharides were extensively studied in the 1960s [6,193]. 

The difficulties in the extraction of XG from the cell walls as well as its separation from the other cell-wall polymers have been interpreted by various suggestions [260]. In addition to the existence of strong hydrogen bonds with cellulose and some hemicelluloses, various covalent bonds have been considered to fix the XG in the cell walls [261] such as esters with the COOH groups... 

AG type II is most abundant in the heartwood of the genus Larix and occurs as minor, water-soluble components in softwoods. Certain tree parts of western larch (I. occidentalis) were reported to contain up to 35% AG [378]. The polysaccharide is located in the lumen of the tracheids and ray cells. Consequently, it is not a cell-wall component and, by definition, not a true hemicellulose. However, it is commonly classified as such in the field of wood and pulping research. This motivated us to include the larch AG in the review. 

Kacurakova, M., Capek, P., Sasinkova, V., WeUner, N. Ebringerova, A. (2000). FT-IR study of plant cell wall model compounds pectic polysaccharides and hemicelluloses. Carbohydrate Polymers, Vol. 43,2, (October 2000), pp. (195-203), ISSN 0144-8617... 

The ability of PO to interact with the acetyl residues of chitin allows us to compare them with monovalent lectins (i.e. extensins) which when binding with hemicellulose are only affected in a medium with a high ionic strength (Brownleader et al., 2006). As a rule, POs are bound with the plant cell wall and act as its modifiers. Some POs can form complexes with an extensin of cell walls (Brownleader et al., 2006). Consequently, chitin-specific sites that are capable of interacting with polysaccharides exist in the molecules of PO, and these sites can resemble the membrane receptor binding sites or else be similar to the domains of heparinbinding proteins (Kim et al., 2001). 

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