Cell wall monomers

This is a very widely available polymer, since it is the main component of the cell walls of all plants. It is a carbohydrate of molecular formula (C5H q05), where n runs to thousands. The cellulose monomer is D-glucose, and the cellulose molecules are built up from this substance, effectively by condensation and removal of the elements of water. 

Isolation from sugar beet cell walls of arabinan oligosaccharides esterified by two ferulic acid monomers / / Plant Physiology. V. 134. P. 1173-1180. 

The largest and most complex carbohydrates are the polysaccharides. They are polymers, long chains of repeating chemical units. Each individual unit is called a monomer. The monomer unit of polysaccharides is the monosaccharide, normally glucose. A typical polysaccharide contains several hundred individual monomers. Examples of common polysaccharides are starches, plant products that are major macronutrients in the human diet, and cellulose, found in plant cell walls. In the human diet, cellulose is referred to as fiber, indigestible but beneficial for normal intestinal motility. More than half of the Earth s total carbon is stored in these two polysaccharides. 

Monomer (or oligomer) impregnation, with subsequent polymerization within the cell wall. 

A solution of styrene in methanol to impregnate wood samples, followed by polymerization, was used by Furuno and Goto (1979). Penetration of the monomer into the cell wall was determined by solvent extraction of samples after polymerization. This removed lumen located polymer, whilst leaving the cell wall bound polymer in place. This showed that the concentration of cell wall bound polymer increased in proportion to the monomer content in methanol, up to a maximum of 80% of the monomer in the solvent. No cell wall penetration was observed for treatment with neat monomer. This was also found for bulking of the wood, as determined from external dimensions of the samples. Improvements in ASE were obtained as a result of the presence of cell wall bound polymer. To achieve similar ASE values with lumen located polymer required very high polymer loadings. 

Shiraishi, N., Murata, M. and Yokota, T. (1972). Polymerization of vinyl monomer within the cell wall of wood. II. Polymerization of methyl methacrylate in the presence of wood, water and carbon tetrachloride. Mokuzai Gakkaishi, 18(6), 299-306. 

One alternative is to select precursors which form a gas as a reaction product in situ during the network formation of thermosets. However this approach is restricted to a very limited number of precursors reacting via a polycondensation mechanism to split off a gas. For example, flexible polyurethane foams are commercially produced using CO2 that is liberated as a reaction product of the isocyanate monomer with water [5]. Very recently, Macosko and coworkers studied the macroscopic cell opening mechanism in polyurethane foams and unraveled a microphase separation occurring in the cell walls. This leads to nanosized domains, which are considered as hard segments and responsible for a rise in modulus after the cell opening [6]. 

The cell wall is formed from materials within the cytoplasm which are subsequently transported either as monomers or polymers to the outside of the cell. During growth and differentiation of the cell its composition and structure changes, and it can also alter in response to environmental factors. There is therefore a dialogue between the outside of the cell and the synthetic and transport systems at the inside of the cell so that the changes in the wall are brought about in an ordered manner at particular stages of its development (1). 

Unfortunately, our knowledge of the transport mechanism is currently insufficient for a detailed discussion. The primary mechanism can probably be explained in terms of the endomembrane theory (16,17), i.e., phenylpropanoid monomers are transported via vesicles to the plasma membrane, and are then released into the cell wall following membrane fusion. However, more extensive experimental evidence is needed to unambiguously establish the detailed mechanism(s) involved. 

As regards the second topic, namely that of phenylpropanoid reactions within plant cell walls, a more comprehensive discussion is possible and also timely, due to the recent increase in interest in this area. For the purpose of this review, the phenylpropanoids present in plant cell walls are first classified according to structural complexity (monomers, dimers, polymers, etc.), following which their main reactions are discussed. 

The second group of phenylpropanoids, which is the main emphasis of this chapter, consists of those components which are integrated into the cell wall framework. This group can be subdivided into three categories monomers, such as hydroxycinnamic acids, dimers, such as didehydrofer-ulic and 4,4 -dihydroxytruxillic acids, and polymers, such as lignins and suberins. It is important to emphasize, at this juncture, that the dimers (4,5) and polymers (8,9) discussed in this chapter are considered to be formed within the cell walls from their corresponding monomers. 

Vascular plant cell walls contain a wide variety of phenylpropanoids, such as monomers, dimers and polymers. Of these, the polymers (i.e., lignins and suberins) are the most abundant. According to our current knowledge, all cell-wall phenylpropanoids are derived from monomers synthesized in the cytoplasm. Following their excretion into the plant cell wall, these monomers can then be either photochemically or biochemically modified within the cell wall. 

The only biological function which has been repeatedly confirmed is the role of peroxidases in lignin monomer polymerization (1). But even in this case, the role of the various isozymes is not yet clear, although anionic, cell wall bound peroxidases generally seem to be involved in lignification... 

In plant cell walls, lignin monomers seem to be present in vivo in the form of cinnamyl alcohols. In vitro, their acid precursors can also be oxidized by peroxidases (3). In order to gain further insight into the possible... 

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