Water conductivity potential, cell

When the water potential inside a cell differs from that outside, water is no longer in equilibrium and we can expect a net water movement from the region of higher water potential toward the region of lower water potential. This volume flux density of water, Jyw> is generally proportional to the difference in water potential (AY) across the membrane or membranes restricting the flow. The proportionality factor indicating the permeability to water flow at the cellular level is expressed by a water conductivity coefficient, L,1 ... 

When Equation 2.26 is applied to cells, Y° is the water potential in the external solution, and Y1 usually refers to the water potential in the vacuole. Lw then indicates the conductivity for water flow across the cell wall, the plasma membrane, and the tonoplast, all in series. For a group of barriers in series, the overall water conductivity coefficient of the pathway, L,., is related... 

When the value of the water conductivity coefficient is known, the water potential difference necessary to give an observed water flux can be calculated using Equation 2.26. For the internodal cells of Cham and Nitella, Am for water entry is about 7 x 10-13 m s-1 Pa-1. For convenience of calculation, we will consider cylindrical cells 100 mm long and 1 mm in diameter as an approximate model for such algal cells (see Fig. 3-13). The surface area across which the water flux occurs is 2nrl, where r is the cell radius and / is the cell length. Thus the area is... 

Water is conducted to and across the leaves in the xylem. It then moves to the individual leaf cells by flowing partly apoplastically in the cell walls and partly symplastically (only short distances are involved, because the xylem ramifies extensively in a leaf). The water potential is usually about the same in the vacuole, the cytosol, and the cell wall of a particular mesophyll cell (see values in Table 9-3). If this were not the case, water would redistribute by flowing energetically downhill toward lower water potentials. The water in the cell wall pores is in contact with air, where evaporation can take place, leading to a flow along the cell wall interstices to replace the lost water. This flow can be approximately described by Poiseuille s law (Eq. 9.11), which indicates that a (very small) hydrostatic pressure decrease exists across such cell walls. 

Bxtensive research is continuing to be conducted into ways to improve polymer membrane fuel cells (PBMFCs), as outlined in recent reviews. " One particular problem associated with PBMFCs is that the proton-conducting membranes require the use of aqueous electrolyte solutions to obtain high proton conductivity, which causes their proton conductivity to be affected by changes in temperature and humidity, limits their use to <100 °C, and requires the constant replacement of lost water. A potential benefit of using PILs as electrolytes in PBMFCs is that the solutions can be anhydrous and, hence, can be operated at temperatures in excess of 100 °C. 

Proton, that is, H+ ion, conductors are of importance as potential electrolytes in fuel cells. There are a number of hydroxides, zeolites, and other hydrated materials that conduct hydrogen ions, but these are not usually stable at moderate temperatures, when water or hydroxyl tends to be lost, and so have only limited applicability. 

For HCI electrolysis the cathodic reaction product is water, which is easily drained through the ODC without affecting the membrane water content. Consequently, the ODC can be attached directly to the membrane and pressure compensation is not necessary. The cell concept, which was developed in another co-operation with DeNora, could not be simpler - the basic cell principle is shown in Fig. 4.6. Initial laboratory tests conducted in 1994 at Bayer on the basis of old GE developments [4] demonstrated the feasibility of HCI electrolysis with ODC and the potential for a reduction of the cell voltage to about one-third of present values. 

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