Cell walls food reserve

The origin and function of xylan in the cell wall are also not explained. Postulations that it is a plasticizer or is a reserve food are not fully substantiated. Its derivation from cellulose through the decarboxylation of an intermediary polyglucuronic acid seems very unlikely. There is evidence from a number of sources to indicate that the xylan polysaccharide is deposited along with cellulose in cell wall elaboration. 

Carbohydrates are among the most abundant constituents of plants, animals, and microorganisms. Polymeric carbohydrates function as important food reserves, and as stmctural components in cell walls. Animals and most microorganisms are dependent upon the carbohydrates produced by plants for their very existence. Carbohydrates are the first products formed in photosynthesis, and are the products from which plants S5mthesize their own food reserves, as well as other chemical constituents. These materials then become the foodstuffs of other organisms. The main pathways of carbohydrate biosynthesis and degradation comprise an important component of... 

Generally, reinforcing, cell-wall polysaccharides are least soluble while emollients, mucilaginous, and food reserve polysaccharides represent the most soluble group. Exceptions to the generalization that reserve food polysaccharides are easily soluble occur in starch amylose and seed mannan. Starch amylose is readily dispersible in most of its natural forms since it occurs mixed with easily soluble amylopectin which facilitates the dissolution of the amylose. 

Like mosses and higher plants, liverworts use chlorophyll-a, chlorophyll-b, and carotenoids as photosynthetic pigments and store their food reserves as starch. As in mosses and higher plants, their cell walls are composed of cellulose. 

When wood is laid down by the cambium of a living tree, two major types of wood cells are formed—thick-walled fiber cells that make wood strong and thin-walled parenchyma cells in which reserve foods are stored. Wood fiber cells die a few days or weeks after they are formed and lose their cytoplasmic contents as soon as they become functional in water transport. Thus, mature wood fiber cells consist almost entirely of cell wall polymers—cellulose, hemicellu-loses, and lignin. For this reason, wood fiber cells can be degraded only by organisms that have the ability to decompose these structurally complex high-polymeric materials. 

By contrast, wood-storage cells remain alive for many years and only lose their cytoplasmic contents when sapwood is transformed into heartwood. The sugars, starch, amino acids, and proteins in the wood-storage cells make sapwood highly susceptible to invasion by a large number of fungi and bacteria that can use the reserve food materials but cannot attack the more complicated cell wall polymers. 

Insofar as the monosaccharides do occur as such in nature, it is more common to find the sugars occurring naturally in pairs (disaccharides) or in threes (trisaccharides) and, more likely, as the high-molecular-weight polysaccharides (Table 3.7). It is the polysaccharides which most probably contribute to the source material, especially the two well-known polysaccharides cellulose and starch. The fibrous tissue in the cell wall of plants and trees contains cellulose and starch also occurs throughout the plant kingdom in various forms but usually as a food reserve. The chemical composition of starch vanes with the source but in any one starch there are two structurally different polysaccharides. Both usually consist entirely of glucose units but one is a linear structure (amylose) whereas the other is a branched structure (amylopectin). 

Mannans. A variety of other polysaccharides may occur in the cell walls of land plants. Among these are mannans, galactans, and pectic substances. In general, softwoods contain approximately 11% combined D-mannose, whereas hardwoods contain only about 1 %. A linear mannan is also the chief constituent of the thickened cell walls of palm seeds, where it occurs as a food reserve and disappears on germination. A rich source is the endosperm of the tagua palm which is known as vegetable ivory and from which buttons have been made. This mannan is a linear chain of D-mannopyranose... 

Plants store polysaccharides as food reserve in all types of cell but primarily in special storage cells or organs such as parenchymous cells or roots, tubers, and pith. The most important of these food reserves are starches, fructans, mannans, and galactomannans. The mannans are sometimes food reserves and sometimes structural material in the plant cell wall. In animals the chief reserve polysaccharide is glycogen, which in chemical structure is much like one of the components of starch (see Part II of this chapter.). 

Fig. 2.7A-C. Histology of food reserves 2. (A) Electron micrograph of castor bean (Ricinus communis) endosperm cell. From Vigil, 1970 [72]. (B) Electron micrograph of cotyledon cell of Sinapis alba. From Rest and Vaughan, 1972 [62]. (C) Electron micrograph of cotyledon cell of Sinapis alba. From Werker and Vaughan, 1974 [73]. AG,alg aleurone grains (protein bodies), G globoid, mg my rosin grains (presumed to contain the enzyme myrosi-nase), PB protein body, S, s oil bodies, CIV cell wall...

Many polysaccharides contain branched structures and are chemically modified by the addition of other molecules. Their monomeric or repeat units are often made up of more than one sugar molecule and, consequently, can be quite complex. They form protective capsules of some of the most virulent microorganisms, capsules that, nevertheless, carry information that activate mammalian defenses the immune, interferon, and properdin systems [9, 136]. They are found as key portions of the exoskeletons of insects and arthropods and cell walls of plants and microbes and perform as reserve foodstuffs and important components of intercellular, mucous secretions, synovial and ocular fluids, and blood serum in many organisms. Food Applications compiles recent data on the food applications of marine polysaccharides from such various sources as fishery products, microorganisms, seaweeds, microalgae, and corals [137, 138]. One of the applications of this biopolymer relates to a method for protecting against diseases induced by Streptococcus pneumoniae infections, which comprises mucosal administration of a S. pneumoniae capsular polysaccharide to a patient in need. 

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