Polysaccharides occurence

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. 

Evidence for Intracellular Feruloylation Reactions. The reactions by which feruloyl residues are added to polysaccharides occur intracellularly. This was established for the Fer-Ara2 units of the pectic polysaccharides of culture spinach cells by administration of [3H]arabinose so that the careers of polymer-bound pentose residues could be followed from their intracellular incorporation into nascent polysaccharides and glycoproteins, via their secretion through the plasma membrane, to their ultimate fate within the wall (28). The cells were fed [3H]arabinose for various defined periods of time, after which two distinct analyses were performed (Fig. 3) ... 

The analysis of monosaccharide mixtures as the permethylated derivatives was proposed early in the application of gas-liquid chromatography to carbohydrates, but the method has now been superseded by more convenient procedures.230,231 There are, however, situations in which this method is useful, such as during a structural study of a polysaccharide by the methylation technique. The mixture of partially methylated monosaccharides obtained by methanolysis may then be fully methylated, and the proportions of the various monosaccharides determined. This approach has been used, for example, in studies on a galactomannan392 and on tamarind-kernel polysaccharide.393 Such an analysis also constitutes a useful check to ensure that no significant change in the composition of the polysaccharide occurred during methylation. 

CELLULOSE AND DERIVATIVES Cellulose, (C6HtoOB)n mw (162.14), d 1.27 -1.61 a white, polysaccharide occurring in all vegetable tissues fibers. Its fundamental... 

Many monosaccharides and their derivatives occur naturally in a form in which one or more of the hydroxyl groups has been substituted by a phosphate or a sulfate group. These are known as esters. In general, the phosphate esters are found as components of metabolic pathways within cells, whereas the sulfate esters are found in oligosaccharides and polysaccharides occurring outside cells. 

Closely associated with cellulose in the wood structure and paper products are other polysaccharides called hemicelluloses, which often have been labeled as the matrix material of wood. In hardwoods the primary hemi-cellulose is a xylan (polymer of xylose), whereas in softwoods the primary hemi-cellulose is a glucomannan, although both of these polysaccharides occur to some extent in both types of wood. The DP of the hemicelluloses is much less than that of cellulose, in the range of 100-200. 

Hemicelluloses in reaction woods are quite different from those in the normal woods, namely, galactan and P-(l-3)-gIucan in compression wood and galac-tan in tension wood. It is also well known that a remarkable amount of a water-soluble polysaccharide, arabinogalactan, is contained in the heartwood of larch. Since this polysaccharide occurs mainly in the lumen of tracheids and is not a cell wall component, it may not be included in hemicelluloses. Although structures and distributions of hemicelluloses have been comprehensively studied in the last 20 years, their physiologic meanings in a cell wall are not known yet. This must be the most important point for the future study of hemicelluloses. 

Circumventing the lignin barrier to enzymatic digestion of the polysaccharides occurs when wood is finely ground. Below a certain wood particle size, the polysaccharides (celluloses and hemicellu-... 

Polysaccharides are T-independent antigens [26] and as a general rule the response to polysaccharides occurs 3-18 months after birth in humans, but it depends on the CPS structure generally, children less than 2 years of age respond poorly. Immunological memory is not induced and as a result no boosting of the antibody response takes place on reimmunization. The affinity maturation and isot) e switching of the antibody repertoire is absent. 

Pectin is a partially esterified acidic polysaccharide occurring in a variety of plants. The ester groups are subject to hydroxyl ion-catalyzed hydrolysis which may be represented schematically by... 

STARCH Starch, the energy reservoir of plant cells, is a significant source of carbohydrate in the human diet. Much of the nutritional value of the world s major foodstuffs (e.g., potatoes, rice, com, and wheat) comes from starch. Two polysaccharides occur together in starch amylose and amylopectin. 

Koll and Metzger (1978) report on the use of supercritical acetone as the reaction medium for the thermal degradation of cellulose and chitin. Since the pyrolysis of these polysaccharides occurs at such high temperatures, it is necessary to remove the primary products from the reaction zone as soon as they are formed to avoid degradation of the products into coke. The high operating temperature also adversely affects both yield and product distribution. It is possible to reduce the carbon formation by carrying out the pyrolysis under vacuum but the reaction rate is also reduced because of the poor heat transfer to the reactants. 

A recent classification (1 ) describes the pectic polysaccharides as those polymers found in covalent association with galactu-ronosyl-containing polysaccharides. The hemicelluloses are those carbohydrate polymers which are noncovalently associated with cellulose. Diverse categories of pectic polysaccharides occur not only among plant sources, but among tissues in a given source. 

The nucleotide sugar compounds are hydrophilic and it is necessary to transfer them to the sites of the synthases within the endomembrane system. Their transport across the membrane is an obvious control point. Not only is the rate of polysaccharide synthesis controlled by this mechanism but in addition the qualitative nature of the polymer found in the membrane compartment may be determined. Thus although the synthases may be present in a particular part of the membrane system e.g. endoplasmic reticulum, the assembly of the polysaccharide does not occur at this level if the nucleotide sugar cannot enter the lumen of the compartment (13). The main assembly of the polysaccharides occurs at the Golgi apparatus. Figure 1. 

The most important storage polysaccharides are starch in plant cells and glycogen in animal cells. Both polysaccharides occur intracellularly as large clusters or granules (Fig. 7-14). Starch and glycogen molecules are heavily hydrated, because they have many exposed hydroxyl groups available to hydrogen-bond with water. Most plant cells have the ability to form starch, but it is... 

The reaction of monochloroacetic acid with polysaccharides occurs first with the secondary hydroxyl groups, particularly at the beginning of the reaction. However, as in the etherifications already discussed, digestion of 0-(carboxymethyl)cellulose with eellulase reveals that substitution along the chain is non-uniform, accessibility playing an important part. 

Acetobacter xylinum produces two forms of cellulose (1) cellulose I, the rib-bon-like polymer, and (2) cellulose II, the thermodynamically more stable amorphous polymer [9]. They can be divided according to their morphological localization as intracellular polysaccharides located inside, or as part of the cytoplasmic membrane cell wall polysaccharides forming a structural part of the cell wall and extracellular polysaccharides located outside the cell wall. Extracellular polysaccharides occur in two forms loose slime, which is non-adherent to the cell and imparts a sticky consistency to bacterial growth on a solid medium or an increased viscosity in a liquid medium and microcapsules or capsules, which adhere to the ceU wall. 

Barley contains relatively high amounts of soluble fiber (P-glucans). Beta-glucans are a diverse group of polysaccharides occurring as cell wall components in nature (Back, Stone, and Fincher 2009). They vary in molecular weight, solubility, and viscosity and, as discussed in Chapter 14, have well-established health benefits when included in the diet. 

Starch is one of the most abundant polysaccharides occurring in nature. All organs of higher plants contain starch. Starch granules can be found in pollen, leaves, stems, woody tissues, roots, tubers, fruits, flowers, bulbs, and seeds. Starch can also be found in mosses, ferns, algae, and bacteria. 

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