Polysaccharides defined

Lactic acid bacteria synthesise a range of different polysaccharides, defined by their location in the cell. Some are located intracellularly and are used as energy or carbon sources others are cell wall components and some are located outside the cell wall. The latter are called extracellular polysaccharides (EPs) and are either associated with the cell wall as a slime capsule, or secreted into the environment. Many lactic acid bacteria such as Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus thermo-philus and Lactococcus lactis subsp. cremoris produce EPs. 

The side chains in the latter are flexible disaccharides on account of poor-quality diffraction patterns, their tentative molecular structures are known only from computer modeling.1" On the other hand, well-defined crystal structures are available for gellan and welan, and they can be correlated with the physical properties of the polysaccharides the details are presented here. Their conformation angles are listed in Table VI. 

Fig. 9 Conformation zoning of polysaccharide, a Conformation zones b Empirical plots for various polysaccharides of known conformation type used to form the zone plot. Measurement of s, ks and Ml for a target polysaccharide will then help define its conformation zone or type. Redrawn and based on [112,113]...

Changes in wall architecture occur during isodiametric cell expansion, cell elongation, and the thickening of a growing wall to its mature thickness, and so it is important to define whether the synthesis and secretion of particular cell-wall molecules correlates with differentiation events or with general cell expansion. Ceils are stimulated into producing wall polysaccharides upon subculture, so we used a non-inductive medium, in which the cells only expand, for comparison with inductive medium at all times. 

The primary walls of growing plant cells are composed of 90% carbohydrate and 10% protein (51). Carbohydrate in the primary wall is present predominantly as cellulose, hemicellulose, and pectin. The pectic polysaccharides, are defined as a group of cell wall polymers containing a-l,4-linked D-galactosyluronic acid residues (62,76). Pectic polysaccharides are a major component of the primary cell waU of dicots (22-35%), arc abundant in gymnosperms and non-graminaceous monocots, and are present in reduced amounts (-10%) in the primary walls of the graminaceae (27,62). 

Carbohydrate metabolism in the organism tissues encompasses enzymic processes leading either to the breakdown of carbohydrates (catabolic pathways), or to the synthesis thereof (anabolic pathways). Carbohydrate breakdown leads to energy release or intermediary products that are necessary for other biochemical processes. The carbohydrate synthesis serves for replenishment of polysaccharide reserve or for renewal of structural carbohydrates. The effectiveness of various routes of carbohydrate metabolism in tissues and organs is defined by the availability of appropriate enzymes in them. 

In this group we place mainly the neutral bacterial slimes and reserve carbohydrates. They are better defined products than those previously dealt with and as such may of course be regarded as true polysaccharides. Invariably, however, saponification methods are required to rid them of protein residues and to make them water-soluble. The more soluble mold polysaccharides appear to lose their protein constituents by autolytic processes during the longer periods required for mold metabolism. Mold slime production can, however, readily be demonstrated on a solid medium. It is proposed here to give briefly some of the types of structure known in the group. 

Dietary Fibre. An area of some contention is what should be included as dietary fibre. There are several different methods of measuring dietary fibre and all of them will give different results on the same sample. The EU favours the Association of Analytical Chemists (AOAC) method, which includes lignin and resistant starch, while the UK has preferred the Englyst method and has defined fibre as nonstarch polysaccharides from cell walls as the only substances that count as fibre. 

The trichloroacetimidate method has also found wide applications in the synthesis of various complex oligosaccharides. In the course of the development of chemically defined glycoconjugate vaccines against shigellosis, a decasaccharide, corresponding to two consecutive repeating units of the O-specifk polysaccharide of Shigella... 

Fig. 10.11 Carbon nanotubes have an absorption peak between 1,000 and 1,500 nm as defined by line (a) in this figure. A decrease in absorption is seen in lines (b) and (c) due to wrapping of polysaccharides around the tubes for water solubilization (Reprinted from Casey et al., 2005. With permission from Elsevier)...

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