Cell wall plasticity

Along the mung bean hypocotyl, the cell wall plasticity represents the limiting factor of cell growth potentials. Pectin molecules, known to control local cell wall pH s and to modulate phenolic cross-linking owing to the number of free acidic domains, were investigated. 

GOLDBERG etal. Control of Cell Wall Plasticity... 

Fig. 25.12. When a plastic foam is compressed beyond the linear region, the cell walls bend plastically, giving a long plateau exactly like that of Fig. 25.9.

Cellular materials can collapse by another mechanism. If the cell-wall material is plastic (as many polymers are) then the foam as a whole shows plastic behaviour. The stress-strain curve still looks like Fig. 25.9, but now the plateau is caused by plastic collapse. Plastic collapse occurs when the moment exerted on the cell walls exceeds its fully plastic moment, creating plastic hinges as shown in Fig. 25.12. Then the collapse stress (7 1 of the foam is related to the yield strength Gy of the wall by... 

The explanation is almost the same as that for the transverse modulus the cell walls bend like beams, and collapse occurs when these beams reach their plastic collapse load. As with the moduli, moisture and temperature influence the crushing... 

Fig. 6. Stress propagation across the wood cell walls, cell walls microbuckling and wood densification leading to the characteristic plastic deformation which wood undergoes during panel manufacture and hot-pressing.

Most rigid polyurethane foams have a closed cell structure. Closed cells form when the plastic cell walls remain intact during the expansion process and are not ruptured by the increasing cell pressure. Depending on the blowing process a small fraction (5-10%) of the cells remain open. Closed cell structures provide rigidity and obstruct gaseous or fluid diffusional processes. 

Ethylene is a colorless, flammable gas with a boiling point of-104 °C. More than 22 billion kilograms of ethylene are produced annually in the United States, making it one of the top five industrial chemicals. The manufacture of plastics (polyethylene is the most common example) consumes 75% of this output, and much of the rest is used to make antifreeze. Because ethylene stimulates the breakdown of cell walls. It Is used commercially to hasten the ripening of fruit, particularly bananas. 

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. 

The changes in fibre flexibility may affect not only the rigidity of the fibre but also the local plasticity of the cell wall, and this may be important in determining the ease with which the inter-fibre hydrogen bonds are formed during drying. Pulp types also differ in... 

Strains in the 80-90% region are found to cause some permanent buckling of the cell walls, but the majority of the deformation recovers with 24h. A criterion for the performance of the foam in protective helmets is proposed. With HDPE foam the performance can deteriorate by 30% in a single severe impact, but even so, this is better than the PS foam widely used. 8 refs. BXL PLASTICS LTD. COURTAULDS ADVANCED MATERIALS... 

Young, plastic walls were characterized both by a high level of branched and methylated rhamnogalacturonans which leads to swollen cell walls and by a low level of linear galacturonans which induces a low Cation Exchange Capacity (CEC). In contrast, stiff, mature cell walls are characterized by a low water content and a high CEC which favors cross-linking. 

Auxins produce many changes in cells, including increased wall plasticity, enhanced respiratory rate and alterations in nucleic acid metabolism. Although the primary mode of action of auxins is not fully understood the development of the chemistry has progressed rapidly. Early discoveries identified and exploited the herbicide potential of super-optimal concentrations of auxin (Chapter 2, Herbicides) but their... 

Myeloma proteins occur in animal tumors of various types. The synthesis of these proteins can be induced in some experimental animals by injection of mineral oil, or the implantation of plastic discs.161 Myeloma proteins were shown to possess antibody activity, with specificity for different types of substances.161 Some of these proteins were found to combine with such carbohydrates as bacterial, cell-wall polysaccharides, lipopolysaccharides, dextrans, levans, and galactans.162 Myeloma proteins that combine with galactans or dextrans possess anti-galactan activity,163,164 or anti-dextran activity,165-167 and have been studied most extensively. 

Keep the cell wall pellet in the plastic centrifuge bottle to minimize losses. Resuspend the pellet in 100 ml of 50 mM HEPES buffer, pH 6.7/20% ethanol. Stir for 6 hr at room temperature. Centrifuge at 9800 x g for 20 min at room temperature. Remove the supernatant as before and retain at 4°C. 

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