Surface tension, water cell wall

One might anticipate that if the water in the cell wall were to be frozen and then the ice were to be subliminated off there should be no liquid capillary tension, no cell wall shrinkage and it should be possible to create a porous cell wall. However, sublimating the water molecules from the cell wall at -20°C does not prevent collapse of the internal pore structure (Merchant, 1957). This implies that the cell wall water is not actually frozen at this temperature the cell wall still shrinks and very little internal surface is created. Indeed there is evidence (Tarkow, 1971) that at least some adsorbed water does not lose the mobility characteristic of the liquid phase until very low temperatures (<-80°C). Of course water in the lumens behaves like bulk water and freezes at a temperature between -0.1°C and -2.0°C, depending on the concentration of dissolved sugars in the sap. 

A loss of water from plant shoots—indeed, sometimes even an uptake — occurs at cell-air interfaces. As we would expect, the chemical potential of water in cells compared with that in the adjacent air determines the direction for net water movement at such locations. Thus we must obtain an expression for the water potential in a vapor phase and then relate this P to for the liquid phases in a cell. We will specifically consider the factors influencing the water potential at the plant cell-air interface, namely, in the cell wall. We will find that vFcel1 wal1 is dominated by a negative hydrostatic pressure resulting from surface tension effects in the cell wall pores. 

Plant cells come into contact with air where the cell walls are adjacent to the intercellular air spaces (see Fig. 1-2). Thus, the water potential in the cell walls must be considered with respect to T 1W in the adjacent gas phase. The main contributing term for T in cell wall water is usually the negative hydrostatic pressure arising from surface tension at the numerous ail-liquid interfaces of the cell wall interstices near the cell surface. In turn, Z 11 wal1 can be related to the geometry of the cell wall pores and the contact angles. 

The strong water-wall adhesive forces, which are transmitted throughout the cell wall interstices by water-water hydrogen bonding, can lead to very negative hydrostatic pressures in the cell wall. At 20°C the surface tension of water is 7.28 x 1CT8 MPa m (Appendix I), the voids between the microfibrils in the cell wall are often about 10 nm across (r = 5 nm), and cos a can equal 1 for wettable walls. For water in such cylindrical pores, Equation 2.25 indicates that when the contact angle is zero P would be... 

In the toxic action of tensides, besides the toxicity which follows from the chemical structure of tensides, its physico-chemical effect is important, since it leads to the hydration and swelling of cells. The cell swelling is increased on decreasing the water surface tension (with increasing concentration of tensides), and thus, metabolic processes in cells are suppressed. With a long-term burden or excess swelling the cells are destroyed. Bacteria, moulds and yeast cells, which have firm cellular walls are more sensitive to tenside effects than, for example, protozoa, which have elastic cell walls, which allow limited changes of the cell shape [38]. 

Second, the strength of the cell walls has also been reduced, and they are therefore less resistant to drying stresses. In particular, cell walls may have become so thin and weak that they can no longer resist the surface tension of receding free-water columns in the cell cavities and thus they collapse. The effect is most pronounced in the tangential direction, where shrinkage of as much as 65% has been observed in waterlogged beech (IT). 

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