Cell-wall polysaccharides location

To examine relative roles of hexose and uronic acid as AsA precursors, the possibility of generating labeled GlcUA from [2-3H]- or [2-14C]MI in situ in strawberry fruit was tested (Loewus, 1965 Loewus et al., 1962). In [2-3H] MI-labeled strawberry fruit, 40% of the 3H was recovered in free D-xylose (Xyl) and uronosyl and pentosyl residues of pectin. In [2-14C] Mi-labeled strawberry fruit, 33% of the 14C was recovered in these products. Only trace amounts of 3H or 14C appeared in AsA (Loewus et al., 1962). D-Galacturonosyl, D-xylo-syl, and L-arabinosyl residues of cell wall polysaccharides as well as free Xyl were degraded to establish the location of the radiolabel. In each instance, it was at carbon 5 (Loewus and Kelly, 1963). 

Nantes was chosen as the location because of its INRA Research Centre, which is renowned for its basic and applied research on plant biopolymers, starch, proteins and cell wall polysaccharides. The main objectives of the Nantes Centre in Plant Science first of all concerns the biosynthesis of macromolecules and assemblies in planta, secondly their structural characteristics and related physico-chemical and functional properties, and thirdly with their behaviour in multiphasic systems in relation to end-uses in food and non-food applications. In addition, human nutrition is also considered. 

A number of polysaccharides, of microbial origin, contain nonsugar substituents, such as ketals of pyruvic acid, that are stable under the conditions of methylation analysis but are acid labile. Unless precautions are taken, such groups can easily lead to the misidentification of branchpoint residues. However, most plant cell wall polysaccharides contain noncarbohydrate substituents that are either alkali labile (e.g., 0-acetyl and phenolic esters) or stable to base and acid hydrolysis (e.g., 0-methyl ethers). The detection and location of 0-methyl ethers can be achieved by performing the methylation with deuterated methyl iodide (Ring and Selvendran, 1980 Selvendran, 1983b) or ethyl iodide. 

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. 

Pectin. Pectin [9000-69-5] is a generic term for a group of polysaccharides, mainly partially methoxylated polygalacturonic acids, which are located in the cell walls of all plant tissues. The main commercial sources of pectin are citms peel and apple pomace, where it represents 20—40% and 10—20% of the dry weight respectively. The pectin is extracted, the extract purified, and the pectin precipitated (50) increased extraction times lead to the production of low methoxyl pectins. 

Many different types of carbohydrate-containing molecules are located on the surface of microbial cells. Some of these are components of die microbial cell wall and are limited to certain types of micro-organisms such as bacterial peptidoglycan, lipopolysaccharides, techoic adds and yeast mannans. Other polysaccharides are not... 

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. 

The pectin network.-The second polysaccharide network present in primary cell walls is composed of pectic polysaccharides. The pectin network appears to coexist with the cellulose/hemicellulose network, that is, both networks appear to be able to share the same space [16-19]. However, the proportions of the two networks appear to vary from location to location within a single cell wall as well as from the primary wall of one type of cell to the primary wall of a another type of cell [9,20-22]. 

Feruloylated pectins have been found in the parenchymatous cell walls of many Dicotyledons (mainly in the Centrospermae and Solanaceae), but UV-fluorescence microscopy suggests that at least the epidermal cell walls of all Dicotyledons contain phenolic residues it remains to be seen whether these phenolic residues are attached to polysaccharides or to cutin, but location of even a small quantity of, say, feruloyl-pectin in the epidermal wall would be particularly significant in the control of growth because the extensibility of the epidermis controls the expansion of whole stems (23) and leaves (Fry, unpublished observations). The extensins, as already mentioned, are rich in the phenolic amino acid tyrosine (2). 

Figure 3.5 A simplified model of the molecular architecture of a primary cell wall rich in pectic polysaccharides, such as a potato cell wall. Two co-extensive, but independent polysaccharide networks are shown a cellulose-xyloglucan network and a pectic-polysaccharide network. The middle lamella is located between the primary cell walls of adjacent cells and is responsible for cell-cell adhesion. Reprinted with permission from McCann and Roberts (1991).

Principles to stabilize lipid bilayers by polymerization have been outlined schematically in Fig. 4a-d. Mother Nature — unfamiliar with the radically initiated polymerization of unsaturated compounds — uses other methods to-stabilize biomembranes. Polypeptides and polysaccharide derivatives act as a type of net which supports the biomembrane. Typical examples are spectrin, located at the inner surface of the erythrocyte membrane, clathrin, which is the major constituent of the coat structure in coated vesicles, and murein (peptidoglycan) a macromolecule coating the bacterial membrane as a component of the cell wall. Is it possible to mimic Nature and stabilize synthetic lipid bilayers by coating the liposome with a polymeric network without any covalent linkage between the vesicle and the polymer One can imagine different ways for the coating of liposomes with a polymer. This is illustrated below in Fig. 53. 

Differences in compositions of the cell walls of higher plants occur within phyla, classes, families, and genera of higher plants, with location within a given plant (because different cells have different functions and exist in different environments), and with stage of development [15]. Nevertheless, some general features can be described. Polysaccharides are the primary constituents of plant cell walls. 

Hemicelluloses are also polysaccharides that are structural components of plant cell walls. However, unlike what their name implies, they are unrelated to cellulose. They are polymers that are made up of a variety of sugar monomers that include glucose, galactose, mannose, arabinose, and xylose, as well as acidic forms of these monosaccharides. Xylose is the monosaccharide that is most abundant. Hemicelluloses have a random, amorphous structure that is suitable for their location in the plant cell wall matrix. Depending on their molecular structure, hemicelluloses are partially digestible. 

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