Ussing-Teorell equation

Similarly, the expected flux ratio is 0.20 for K+ and 0.000085 for Cl- (values given in Table 3-1, column 5). However, the observed influxes in the light equal the effluxes for each of these three ions (Table 3-1, columns 6 and 7). Equal influxes and effluxes are quite reasonable for mature cells of N. translucens, which are in a steady-state condition. On the other hand, if 7 n equals 7°ut, the flux ratios given by Equation 3.25 are not satisfied for K+, Na+, or Cl-. In fact, active transport of K+ and Cl- in and Na+ out accounts for the marked deviations from the Ussing-Teorell equation for N. translucens, as is summarized in Figure 3-13. 

For convenience, we have been discussing facilitated diffusion into a cell, but the same principles apply for exit and for fluxes at the organelle level. Let us assume that a transporter for K+ exists in the membrane of a certain cell and that it is used as a shuttle for facilitated diffusion. Not only does the carrier lead to an enhanced net flux density toward the side with the lower chemical potential, but also both the unidirectional fluxes and i ut can be increased over the values predicted for ordinary diffusion. This increase in the unidirectional fluxes by a carrier is often called exchange diffusion. In such a case, the molecules are interacting with a membrane component, namely, the carrier hence the Ussing-Teorell equation [Eq. 3.25 = c /(ctjeljFEM/RT)] is not obeyed because it does not consider... 

The term exchange diffusion has another usage in the literature—namely, to describe the carrier-mediated movement of some solute in one direction across a membrane in exchangefor a different solute being transported in the opposite direction. Again, the Ussing-Teorell equation is not obeyed. 

The net passive flux(J) of the ion is the ratio of two fluxes and and the ratio of these two fluxes is related to the difference in electrochemical potential by the equation (the Ussing-Teorell equation)... 

We begin by showing how active transport can directly affect membrane potentials. We then compare the temperature dependencies of metabolic reactions with those for diffusion processes across a barrier to show that a marked enhancement of solute influx caused by increasing the temperature does not necessarily indicate that active transport is taking place. Next we will consider a more reliable criterion for deciding whether fluxes are passive or not — namely, the Ussing-Teorell, or flux ratio, equation. We will then examine a specific case in which active transport is involved, calculate the energy required, and finally speculate on why K+ and Cl- are actively transported into plant cells and Na+ is actively transported out. 

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