function of hemicellulose
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Hemicelluloses are a significant component of the plant cell wall. In contrast to other plant-derived polysaccharides such as cellulose, starch, pectins, and gums, hemicelluloses do not have a comparable commercial value. However, they are a subject of importance since they can affect the extraction of cellulose and they also make significant contributions to wood and fibre quality. Some soluble hemicelluloses are important dietary constituents. Hemicelluloses are typically defined as components that can be precipitated by ethanol after extraction from the cell wall by dilute alkali. In such procedures they are extracted after depletion of the pectin content of the walls by aqueous solvents and calcium chelators. Use of alkali, however, makes it difficult to maintain the structural integrity of many polysaccharides during their extraction from the cell wall matrix. As an alternative to alkali extraction, enzymes can be used to dissociate hemicellulose fragments from the cell wall matrix. For example, xylanohydrolases and glucuronoxylanases can be used as probes to resolve glucuronoxylan and glucuronoarabinoxylan structures.1 As a consequence of such a chemical definition, the family of hemicelluloses include noncellulosic polysaccharides other than starch or fructans that are abundant in the ariel and normally lignified tissues of higher land plants. Such a definition has been extended to the soluble polymers of the endosperm and materials from the roots. Ultimately, however, they are best defined by their structures that make them distinct from the pectins. Here there is a fairly clear and widespread understanding of what constitutes a hemicellulose, and the contents of this chapter reflect this agreement. This chapter is primarily about the biosynthesis of hemicelluloses. However, some discussion of their structure and role is necessary to fully understand the considerable problems in elucidating their synthesis and assembly. Thus, it is important to know the full range of linkages present and their frequency in the nascent polysaccharide and any modifications that take place after the primary synthesis in order to understand the complexity of the biosynthetic process.
Hemicellulose(s)
Hemicellulose(s) are actually a class of materials. The plural form should be used to describe them generically, but the singular form should be used to describe a particular type such as the hardwood xylan hemicellulose. Physically, hemicelluloses are white solid materials that are rarely crystalline or fibrous in nature; they form some of the “flesh” that helps fill out the fiber. Hemicelluloses increase the strength of paper (especially tensile, burst, and fold) and the pulp yield but are not desired in dissolving pulps. (Dissolving pulps are relatively pure forms of cellulose used to make cellulose-based plastics.) Starch is often added to pulp to increase the strength of paper and probably has a very similar mechanism of effect as the hemicelluloses. Hemicelluloses chemically are a class of polymers of sugars, including the six-carbon sugars mannose, galactose, glucose, and 4-O-methyl-d-glucuronic acid and the five-carbon sugars xylose and arabinose. The structures of these monosaccharides are shown in Fig. 2.23. (Pectin, a related compound, occurs to a small degree in the middle lamella, especially in the pith and young tissue, and consists of polygalacturonic acid.) Hemicelluloses are condensation polymers with a molecule of water removed with every linkage. All of the monosaccharides that make up the hemicelluloses have the D configuration and occur in the six-member pyranoside forms, except arabinose, which has the L configuration and occurs as a five-member furanoside. The number average DP is about 100–200 sugar units per hemicellulose molecule.
Hemicelluloses are much more soluble and labile, that is, susceptible to chemical degradation, than is cellulose. They are soluble in 18.5% NaOH (which is the basis of their measurement in Tappi Test Method T203). Low-molecular-weight hemicelluloses become soluble in dilute alkali at elevated temperatures, such as in kraft cooking. Hemicelluloses are essentially linear polymers, except for single-sugar side chains and acetyl substituents.