Cell wall elasticity

Zimmerman, U., Steudle, E. Lelkes, P.I. (1976). Turgor pressure regulation in Valonia utricularis. Effect of cell wall elasticity and auxin. Plant Physiology, 58, 608-13. 

Some wild species have larger capacities for osmotic adjustment, a trait which may improve yield during drought (Table 3, Turner, 1986). Interesting examples of this are Dubautia species from Hawaii which differ in osmotic adjustment mainly as a result of differences in cell wall elasticity. Interspecific hybrids can be made which have intermediate properties (Robichaux, Holsinger Morse, 1986). Material such as this could make a basis for the molecular study of differences in cell wall elasticity. 

Its first function is to protect the cell. Without its wall, the cell would burst under the internal osmotic pressure, determined by the composition of the cell s enviromnent. Protoplasts placed in pure water are immediately lysed in this manner. Cell wall elasticity can be demonstrated by placing yeasts, taken during their log phase, in a hypertonic (NaCl) solution. Their cellular volume decreases by approximately 50%. The cell wall appears thicker and is almost in contact with the membrane. The cells regain their initial form after being placed back into an isotonic medium. 

Changed membrane permeabilities and cell wall elasticities since the structure and function of membranes and cell walls depend on their ionic micro-environment. 

BrmSNICH, D.J., L.O. ENIVISIjE, and T.F. NEAIES. 1987. Acid-induced stomatal opaiing in Vicia faba L. and the role of guard cell wall elasticity. Plant Physiol. 85, 554-557. 

When a foam is compressed, the stress-strain curve shows three regions (Fig. 25.9). At small strains the foam deforms in a linear-elastic way there is then a plateau of deformation at almost constant stress and finally there is a region of densification as the cell walls crush together. 

Fig. 25.10. Cell wall bending gives the linear-elastic portion of the stress-strain curve.

Fig. 25.11. When on elastomeric foam is compressed beyond the I inear region, the cell walls buckle elastically, giving the long plateau shown in Fig. 25.9.

The variation in wall thickness and the development of cell wall rigidity (stiffness) with time have significant consequences when considering the flow sensitivity of biomaterials in suspension. For an elastic material, stiffness can be characterised by an elastic constant, for example, by Young s modulus of elasticity (E) or shear modulus of elasticity (G). For a material that obeys Hooke s law,for example, a simple linear relationship exists between stress, , and strain, a, and the ratio of the two uniquely determines the value of the Young s modulus of the material. Furthermore, the (strain) energy associated with elastic de-... 

For a Hookian material, the concept of minimum strain energy states that a material fails, for example cell wall disruption occurs, when the total strain energy per unit volume attains a critical value. Such an approach has been used in the past to describe a number of experimental observations on the breakage of filamentous micro-organisms [78,79]. Unfortunately, little direct experimental data are available on the Young s modulus of elasticity, E, or shear modulus of elasticity G representing the wall properties of biomaterial. Few (natural) materials behave in an ideal Hookian manner and in the absence of any other information, it is not unreasonable to assume that the mechanical properties of the external walls of biomaterials will be anisotropic and anelastic. 

This apparent time dependent cell disruption is caused because of the statistically random distribution of the orientation of the cells within a flow field and the random changes in that distribution as a function of time, the latter is caused as the cells spin in the flow field in response to the forces that act on them. In the present discussion this is referred to as apparent time dependency in order to distinguish it from true time-dependent disruption arising from anelastic behaviour of the cell walls. Anelastic behaviour, or time-dependent elasticity, is thought to arise from a restructuring of the fabric of the cell wall material at a molecular level. Anelasticity is stress induced and requires energy which is dissipated as heat, and if it is excessive it can weaken the structure and cause its breakage. 

Concerning bread, characteristics such as cell wall thickness, cell size, and uniformity of cell size affect the texture of bread crumb (Kamman, 1970) and also the appearance, taste perception, and stability of the final product (Autio and Laurikainen, 1997). Crumb elasticity can be predicted from its specific volume and is strongly affected by the amylose-rich regions joining partially gelatinized starch granules in the crumb cell walls (Scanlon and Liu, 2003). 

An analytical elastic membrane model was developed by Feng and Yang (1973) to model the compression of an inflated, non-linear elastic, spherical membrane between two parallel surfaces where the internal contents of the cell were taken to be a gas. This model was extended by Lardner and Pujara (1980) to represent the interior of the cell as an incompressible liquid. This latter assumption obviously makes the model more representative of biological cells. Importantly, this model also does not assume that the cell wall tensions are isotropic. The model is based on a choice of cell wall material constitutive relationships (e.g., linear-elastic, Mooney-Rivlin) and governing equations, which link the constitutive equations to the geometry of the cell during compression. 

The elastic membrane model assumes that the cell is a thin-walled sphere filled with incompressible fluid. Because the wall is thin, it may be treated as a mechanical membrane. It can be presumed that the wall cannot support out-of-plane shear stresses or bending moments. This situation is described as plane stress, as the only non-zero stresses are in the plane of the cell wall. Furthermore, the stresses can be expressed as... 

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