. Biophysics: concepts and mechanisms. Biophysics. 186 A CONCEPTUAL INTRODUCTION TO BIOENERGETICS solutions of different concentrations separated by a suitable membrane. Fig- ure 7-7) are allowed to interact, the difference in free enegery, AF, can be manifested by transport or movement of molecules or ions. By a rather neat argument involving the dependence of electrical potential upon concentra- tion of ions, it can be shown that the A/7 can also be manifested as a poten- tial difference in such a system. With suitable electrodes the value can be measured. A form of the Nernst equation relat
. Biophysics: concepts and mechanisms. Biophysics. 186 A CONCEPTUAL INTRODUCTION TO BIOENERGETICS solutions of different concentrations separated by a suitable membrane. Fig- ure 7-7) are allowed to interact, the difference in free enegery, AF, can be manifested by transport or movement of molecules or ions. By a rather neat argument involving the dependence of electrical potential upon concentra- tion of ions, it can be shown that the A/7 can also be manifested as a poten- tial difference in such a system. With suitable electrodes the value can be measured. A form of the Nernst equation relates the emf of this concentra- tion cell to the ratio of the salt activities. Thus log (a,/a,; This equation shows the relationship between the potential and the activity ratio for condition of no transport across the interface. For example, for a cell composed of IN - - NaCl, in which the activity ratio is about 10, the value of £conc = v ( = 60 mv). I I salt in J water) membrane dif f use interface. ai greater than o 2 Figure 7-7. Concentration Cell (left); with Transport (right). If flow or transport of ions or water occurs, and it usually does to some extent across living membranes, the value observed, E, differs from E by a "diffusion potential," Em, which can be approximated by either the Hen- derson (1911) or Planck (1915) equations, and measured, approximately, under certain rigorous experimental conditions. Thus, E = £ 'diff Values 50 to 100 mv are found routinely in living systems, across the mem- branes of nerve cells and red blood cells, for example (see Table 7-6). These values are due principally to potassium chloride concentration differences across the membranes. It is interesting to note that in the electric eel, simi- lar cells are arranged in series, and potential differences of 200 to 1000 v are usually observed! In nerve, the stationary values of about 80 mv are modi- fied rapidly with passage of a stimulus, due to a change
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