Diffusion and equilibrium potentials

- A diffusion potential is the potential difference generated across a membrane because of a concentration difference of an ion.

- A diffusion potential can be generated only if the membrane is permeable to the ion.

- The size of the diffusion potential depends on the size of the concentration gradient.

- The sign of the diffusion potential depends on whether the diffusing ion is positively or negatively charged.

- Diffusion potentials are created by the diffusion of very few ions. Concentrations in the bulk solutions do not change as a result of this diffusion.

- The equilibrium potential is the diffusion potential that exactly balances (opposes) the tendency for diffusion caused by a concentration difference. At electrochemical equilibrium, the chemical and electrical driving forces that act on an ion are equal and opposite, and no more net diffusion of the ion occurs.

1. Example of a Na+ diffusion potential (Figure 1-4)

a. Two solutions of NaCl are separated by a membrane that is permeable to Na+ but not to Cl\ The NaCl concentration of solution 1 is higher than that of solution 2.

b. Because the membrane is permeable to Na+, Na+ will diffuse from solution 1 to solution 2 down its concentration gradient. CI" is impermeable and therefore will not accompany Na+.

c. As a result, a diffusion potential will develop and solution 1 will become negative with respect to solution 2.

d. Eventually, the potential difference will become large enough to oppose further net diffusion of Na+. The potential difference that exactly counterbalances the diffusion of Na+ down its concentration gradient is the Na+ equilibrium potential. At electrochemical equilibrium, the chemical and electrical driving forces on Na+ are equal and opposite, and there is no net diffusion of Na+.

2. Example of a CI" diffusion potential (Figure 1-5)

a. Two solutions identical to those shown in Figure 1-4 are now separated by a membrane that is permeable to CI" rather than to Na+.

b. CI" will diffuse from solution 1 to solution 2 down its concentration gradient. Na+ is impermeable and therefore will not accompany CI".

c. A diffusion potential will be established such that solution 1 will become positive with respect to solution 2. The potential difference that exactly counterbalances the diffusion of CI" down its concentration gradient is the CI" equilibrium potential. At electrochemical equilibrium, the chemical and electrical driving forces on CI" are equal and opposite, and there is no net diffusion of CI".

Figure 1-4. Generation of a Na+ diffusion potential across a Na*-selective membrane.

CI -selective membrane

Na+

Na+

cr^

^^cr

Na+

+

- Na+

cn

+

-

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Figure 1-5. Generation of a CI" diffusion potential across a CI"-selective membrane.

3. Using the Nernst equation to calculate equilibrium potentials a. The Nernst equation is used to calculate the equilibrium potential at a given concentration difference of a permeable ion across a cell membrane. It tells us what potential would exactly balance the tendency for diffusion down the concentration gradient; in other words, at what potential would the ion be at electrochemical equilibrium?

equilibrium potential (mV) constants (60 mV at 37°C)

charge on the ion (+1 for Na+; +2 for Ca2+; -1 for CI") intracellular concentration (mM) extracellular concentration (mM)

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