Freezing membrane

Valli et al. (88) suggested that intracellular nitroreductases may be involved in 2,4-DNT reduction by R chrysosporium. Even though intracellular nitroreductases of R chrysosporium probably exist, there is no current evidence that supports their involvement in TNT reduction. Conversely, there are a number of experiments which suggest that TNT is reduced by a plasma membrane redox system in R chrysosporium (81). TNT reduction requires live, intact mycelia. Any condition that disrupts the integrity of the plasma membranes (freeze-thawing or grinding) destroys the reduction activity. Also, no significant reduction is observed with either supplemented (NADPH, NADH, or ATP) or unsupplemented extracellular or intracellular fractions of the culture, under aerobic or anaerobic conditions (81). 

Moisture pickup and freeze—thaw resistance of various insulations and the effect of moisture on the thermal performance of these insulations has been reported (207). In protected membrane roofing appHcations the order of preference for minimizing moisture pickup is... 

Specifications for gas turbine fuels prescribe test limits that must be met by the refiner who manufactures fuel however, it is customary for fuel users to define quality control limits for fuel at the point of delivery or of custody transfer. These limits must be met by third parties who distribute and handle fuels on or near the airport. Tests on receipt at airport depots include appearance, distfllation, flash point (or vapor pressure), density, freezing point, smoke point, corrosion, existing gum, water reaction, and water separation. Tests on delivery to the aircraft include appearance, particulates, membrane color, free water, and electrical conductivity. 

Physical methods such as osmotic shock, in which the cells are exposed to high salt concentrations to generate an osmotic pressure difference across the membrane, can lead to cell-wall disruption. Similar disruption can be obtained by subjecting the cells to freeze/thaw cycles, or by pressuriziug the cells with an inert gas (e.g., nitrogen) followed by a rapid depressurization. These methods are not typically used for large-scale operations. 

A number of recent studies consider more complex systems, such as freezing vesicles [246] (freezing can be induced by reducing the tether length) or mixed membranes which contain more than one component [247,248]. The possibility that a membrane may break up and form pores has also been considered [249]. 

Although freeze-fracture experiments have demonstrated that monomers are assembled into stable tetramers in the membranes, radiation inactivation studies and, later, expression studies revealed that each monomer is a functional water channel (Fig. lc). 

The thermodynamic aspect of osmotic pressure is to be sought in the expenditure of work required to separate solvent from solute. The separation may be carried out in other ways than by osmotic processes thus, if we have a solution of ether in benzene, we can separate the ether through a membrane permeable to it, or we may separate it by fractional distillation, or by freezing out benzene, or lastly by extracting the mixture with water. These different processes will involve the expenditure of work in different ways, but, provided the initial and final states are the same in each case, and all the processes are carried out isothermally and reversibly, the quantities of work are equal. This gives a number of relations between the different properties, such as vapour pressure and freezing-point, to which we now turn our attention. 

To survive freezing, a cell must be cooled in such a way that it contains little or no freezable water by the time it reaches the temperature at which internal ice formation becomes possible. Above that temperature, the plasma membrane is a barrier to the movement of ice crystals into the cytoplasm. The critical factor is the cooling rate. Even in the presence of external ice, most cells remain unfrozen, and hence, supercooled, 10 to 30 degrees below their actual freezing point (-0.5 °C in mammalian cells). Supercooled cell water has a higher chemical potential than that of the water and ice in the external medium, and as a consequence, it tends to flow out of the cells osmotically and freeze externally (Figure 1). 

Why should contact between shrunken cells be damaging One possibility is that the contacts induce damaging membrane-membrane fusion, for close contact between shnmken cells has been found to induce membrane fusion above 0 °C. Moreover, freezing has been shown to induce fusion in model membranes (Anchor-doguy et al., 1987) and to produce changes in fungal hyphae that are symptomatic of fusion (Fujikawa and Miura, 1986). 

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