Structure and Permeability

Cell Structure and Permeability. Table 33 shows the values of water absorption and water-vapor transmission (1). 

Water-vapor transmission Qosed-cell content (corrected) Cell size (freezer-wall sample) 

Dimensional Stability. The dimensional changes of a pyranyl foam after a 28-day test period are shown in Table 34. 

The data show the excellent stability of the foam over a wide range of conditions. 

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Wertz PW, Swartzendruber DC, Squier CA (1993) Regional variation in the structure and permeability of oral mucosa and skin. Adv Drug Del Rev 12 1-12... 

The major secondary events are changes in membrane structure and permeability, changes in the cytoskeleton, mitochondrial damage, depletion of ATP and other cofactors, changes in Ca2+ concentration, DNA damage and poly ADP-ribosylation, lysosomal destabilization, stimulation of apoptosis, and damage to the endoplasmic reticulum. 

Van den Bergh, B.A., et al. 1999. Elasticity of vesicles affects hairless mouse skin structure and permeability. J Control Release 62 367. 

In this chapter membrane preparation techniques are organized by membrane structure isotropic membranes, anisotropic membranes, ceramic and metal membranes, and liquid membranes. Isotropic membranes have a uniform composition and structure throughout such membranes can be porous or dense. Anisotropic (or asymmetric) membranes, on the other hand, consist of a number of layers each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense, thin surface layer supported on an open, much thicker micro-porous substrate. The surface layer performs the separation and is the principal barrier to flow through the membrane. The open support layer provides mechanical strength. Ceramic and metal membranes can be either isotropic or anisotropic. 

Lipids have several important functions in animal cells, which include serving as structural components of membranes and as a stored source of metabolic fuel (Griner et al., 1993). Eukaryotic cell membranes are composed of a complex array of proteins, phospholipids, sphingolipids, and cholesterol. The relative proportions and fatty acid composition of these components dictate the physical properties of membranes, such as fluidity, surface potential, microdomain structure, and permeability. This in turn regulates the localization and activity of membrane-associated proteins. Assembly of membranes necessitates the coordinate synthesis and catabolism of phospholipids, sterols, and sphingolipids to create the unique properties of a given cellular membrane. This must be an extremely complex process that requires coordination of multiple biosynthetic and degradative enzymes and lipid transport activities. 

Regulates structure and permeability of membranes, root growth Enzyme catalyst Component of chlorophyll Activates enzymes... 

Hawley (64) first demonstrated the complex nature of moisture flow through wood above the fiber-saturation point resulting from capillary forces associated with air bubbles and pores of variable radii interconnecting cells. Using Comstock s (65) simplified structural model for softwoods, Spolek and Plumb (66), however, were able to predict the capillary pressures in southern yellow pine as a function of percent of water saturation of the cell cavities. Such a quantitative analysis would be more difficult to implement in the case of woods other than southern yellow pine because their structures and permeabilities are more variable in most cases. However, computer modeling techniques are developing to the point where more general models may become feasible. 

Species emitted to snowpack interstitial air must be transported out of the snow into the atmosphere to have an atmospheric impact. The most efficient mechanism is ventilation by wind, which depends on wind speed, snow surface structure and permeability. Hansen et al." predict little surface wind change in polar regions. Snow surface structure depends mostly on wind, but speculating on its evolution is beyond our scope. We discussed that snow permeability will in general decrease with warming. The residence time of species produced in interstitial air will therefore increase under a warmer climate, reducing the chances of reactive species to escape before they react in the snowpack. 

Petty JA and Puritch GS (1970) The effects of drying on the structure and permeability of the viooAof Abies grandis. Wood Science and Technology, 4(2) 140-54 Petty JA (1972) The aspiration of bordered pits in conifer wood. Proceedings of the Royal Society, London, B 757(1065) 395-406... 

The buffer in this experiment is made of bentonite blocks. Because there are gaps of up to 30 mm at the upper contact to the host rock the swelling of the blocks is not hindered at these blocks. The differences in back pressure at each block leads to different changes of the pore structure and permeability at each block. 

V. Hoffmann, D. FrOhlich, U. Hcllstcm, S. Kohlhage, W. Nahm, K. NuBbaumer, D. Oelkrug, Spectroscopical examination of photopolymerization, structure and permeability of thin polymer films, J. Mol. Struct. 293 (1993) 253-256. 

Lill3 hite, H. B., and Maderson, P. F. A., 1982, Skin structure and permeability, "Biology of the Reptilia, Volume 12, Physiology C, Physiological Ecology," C. Cans, and F. H. Pough, eds.. Academic Press, New York. 

Fig, 1 Schematic of the sample configuration FeGaB/Si/PMN-PT hetero-structure and permeability... 

Breathers Typically, the entire set-up is covered with a layer of breather fabric. A breather consists of an open fabric or fleece textile which is sufficiently rigid to resist crushing under pressure, thus maintaining its open structure and permeability to air flow under all process conditions. The presence of the breather will allow for a homogeneous pressure distribution, and thereby a homogeneous compaction of the prepreg stack, over the entire surface of the set-up. 

There is little doubt that fertility- and productivity-affecting features of soils, such as liquid and electrolyte household management, i.e. the capacity to take up, hold, and pass liquids, ventilation, sorptional capacity, and the transport of dissolved substances, are highly dependent upon the structure and permeability of soil component colloid aggregates [1, 2]. Accordingly, the structure and permeability of mineral component clays are important factors [3]. 

Robards A W, Clarkson D T, Sanderson J 1979 Structure and permeability of the epider-mal/hypodermal layers of the sand sedge Carex arenaria L.). Protoplasma 101 331-347 Robards A W, Jackson S M, Clarkson D T, Sanderson J 1973 The structure of barley roots in relation to the transport of ions into the stele. Protoplasma 77 291-311 381a Roberts E, Kutchan T, Kolattukudy P E 1988 Cloning and sequencing of cDNA for a highly anionic peroxidase from potato and the induction of its mRNA in suberizing potato tubers and tomato fruits. Plant Mol Biol 11 15-26... 

Many technologists are interested not only in the permeability of a powder system but also in the actual structure of the pores in the powder system. Sandstone or porous ceramics also have pore structures and permeabilities which are important in determining the physical properties of the system. In the pharmaceutical industry the porosity and permeability of tablets and in what are known as controlled release drug systems have physical properties determined by these pore structures. It should be noticed that the term pore can refer to either a connected or closed system. Thus the permeability of a porous structure is related to the interconnectiveness of pores whereas the activity of a catalyst may also be a function not only of the permeability of the system but also on the presence of pores in the individual particles which are said to be blind pores, i. e. not interconnected. The flow of oil through sandstone reservoirs, and of ground water pollution through a sedimentary rocks and soil, is also related to the pore structure of the system. 

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