Plastic extension, cell wall

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. 

Before concluding this discussion of cell walls, we note that the case of elasticity or reversible deformability is only one extreme of stress-strain behavior. At the opposite extreme is plastic (irreversible) extension. If the amount of strain is directly proportional to the time that a certain stress is applied, and if the strain persists when the stress is removed, we have viscous flow. The cell wall exhibits intermediate properties and is said to be viscoelastic. When a stress is applied to a viscoelastic material, the resulting strain is approximately proportional to the logarithm of time. Such extension is partly elastic (reversible) and partly plastic (irreversible). Underlying the viscoelastic behavior of the cell wall are the crosslinks between the various polymers. For example, if a bond from one cellulose microfibril to another is broken while the cell wall is under tension, a new bond may form in a less strained configuration, leading to an irreversible or plastic extension of the cell wall. The quantity responsible for the tension in the cell wall — which in turn leads to such viscoelastic extension — is the hydrostatic pressure within the cell. 

Cells that have been formed recently at the vascular cambium have only a very thin primary cell wall. Even in the fully lignified cell the primary wall is very thin (0.1 j,m) and can be hard to distinguish from or isolate from the middle lamella many studies analyse the two together (ML+P) and relate results to the compound middle lamella (CML), a term which embraces both middle lamella and primary wall. The primary wall displays both elasticity and plasticity (permanent extension) during early cell growth and extension - at this stage in tracheid cell development... 

The extensins are highly insoluble components of cell walls of terrestrial plants and are considered to be important in the plastic extension of the typical cell wall of the land plants (Lamport, 1970, 1973). However, some of the related arabinoproteins are found elsewhere in the cell, and one of the best studied is the readily soluble lectin of the potato, Solanum tuberosum (Allen and Neuberger, 1973), which has a high content of carbohydrate. 

More recent hypotheses have tended to the view that the plastic extension of the cell wall requires the making and breaking of covalent linkages and, under physiological conditions, cellular extension is accompanied by a net synthesis of new wall material. Auxins can induce such syntheses, both of cellulose and of the non-cellulose components, and different mechanisms are involved in each of these (Baker and Ray, 1965a, b Ray and Baker, 1965). 

Determinations of wall rheological properties have shown marked differences between mutant and wild type. Extensibility (mm/g applied load) was measured as 0.11 for normals and 0.33 for slender seedlings whilst load/relaxation hysteresis values (under 10 g applied load) were respectively 3.3 X 10 and 8.4 X 10 J. Thus, there are marked differences in the apparent plasticity of the mutant cell wall, but these were abolished when measurements were made with killed tissue, suggesting that the differences were mediated by dynamic wall-based processes. 

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