Cell shape, effect

Density and polymer composition have a large effect on compressive strength and modulus (Fig. 3). The dependence of compressive properties on cell size has been discussed (22). The cell shape or geometry has also been shown important in determining the compressive properties (22,59,60,153,154). In fact, the foam cell stmcture is controlled in some cases to optimize certain physical properties of rigid cellular polymers. 

Mechanical Properties and Structural Performance. As a result of the manufacturing process, some cellular plastics have an elongated cell shape and thus exhibit anisotropy in mechanical, thermal, and expansion properties (35,36). Efforts are underway to develop manufacturing techniques that reduce such anisotropy and its effects. In general, higher strengths occur for the paraHel-to-rise direction than in the perpendicular-to-rise orientation. Properties of these materials show variabiUty due to specimen form and position in the bulk material and to uncertainty in the axes with respect to direction of foam rise. Expanded and molded bead products exhibit Httie anisotropy. 

Mentzer Jr., W.C., and Lubin, B.H. (1979) The effect of cross-linking agents on red-cell shape. Semin. Hematol. 16, 115-127. 

Other examples of important insight obtained by biochemical engineers are the effects that physiological values of fluid stress have on cells. Cells that line blood vessels are normally exposed to blood flow. This flow places a mechanical stress on a cell that not only alters cell shape but also its function. The mechanical stresses alter expression of genes. The mechanisms by which this occurs in cells are incompletely understood and a matter of active research. 

Pahlow et al. [67] have also carefully examined the role of cell shape on nutrient uptake by diatoms in the presence of advection, under the assumption that nonspherical cells will rotate intermittently. In that case, the relative increase in transport capacity is small for solitary, nonspherical cells with respect to spheres [67], These authors found that advection could significantly increase transport for cell chains, even though it did not compensate for the loss in diffusive flux when calculations were made on a cell surface area basis. Therefore, although advection has a much greater effect on the supply of solute to chains than it does to solitary cells, solitary cells will always maintain an advantage over chains. 

Gorton, H.L. and Vogelmann, T.C., Effects of epidermal cell shape and pigmentation on optical properties of Antirrhinum petals at visible and ultraviolet wavelengths, Plant Physiol, 112, 879, 1996. 

Color perception can also be influenced by the physical properties of the cells where anthocyanins accumulate. An example is provided by the effect of the Mixta gene in snapdragon, which controls conical cell shape. Petal cells lacking Mixta function are palely pigmented because of the way light reflects, which could have significant consequences in the ways pollinators are attracted.68... 

In every tissue of the body cells are cemented into place by a variety of proteins and proteoglycans which constitute the extracellular matrix (ECM). This protein mat provides a secure foundation for the cells, but its components also have effects on cell function and differentiation. Cellular attachment is mediated by a family of matrix receptors found on the cell surface called integrins. Integrins secure the cell to the matrix, and determine cell shape, migration and spread. 

As with cell shapes of a real foam, cell sizes in this material can also be characterized only by nominal (effective) values. The actual effective values depend, first, on the observation method (whether direct — macroscopic, or indirect — adsorption, volumetric, picnometric, etc.). Secondly, they depend on the particular simplified model of the structure and cell shape and thirdly on the method of processing the measured data. 

Rozanska et al. compared plane wave LDA and GGA calculations for the chemisorption of propylene in chabazite.282 They concluded that allowing zeolite atoms to relax upon chemisorption made a significant impact on the computed results, but relaxation of the zeolite s unit cell shape and volume had considerably less effect. The same group performed plane wave GGA calculations for the chemisorption of isobutene in three different zeolites, chabazite, ZSM-22 (TON), and mordenite.283... 

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