Explain what the equilibrium potential means with respect to the charge on the membrane.
The correct answer and explanation is:
The equilibrium potential refers to the electrical potential difference across the membrane that exactly balances the concentration gradient of a particular ion. It is the voltage at which the net movement of that ion across the membrane is zero, meaning the rate at which the ion flows into the cell is equal to the rate at which it flows out. Each ion has its own specific equilibrium potential, which is determined by its concentration gradient and the charge of the ion.
For a charged particle like an ion, there are two driving forces at play: the concentration gradient and the electrical potential. The concentration gradient is the difference in ion concentration across the membrane, while the electrical potential is created by the distribution of charges. When an ion moves down its concentration gradient, it creates an electrical potential, which opposes further movement.
The equilibrium potential for an ion is calculated using the Nernst equation, which takes into account the ion’s charge, the concentrations inside and outside the cell, and the temperature. The formula is as follows: Eion=RTzFln([ion]out[ion]in)E_{\text{ion}} = \frac{RT}{zF} \ln \left(\frac{[ion]_{\text{out}}}{[ion]_{\text{in}}}\right)
Where:
- EionE_{\text{ion}} is the equilibrium potential for the ion.
- RR is the gas constant.
- TT is the temperature in Kelvin.
- zz is the charge of the ion.
- FF is the Faraday constant.
- [ion]out[ion]_{\text{out}} and [ion]in[ion]_{\text{in}} are the concentrations of the ion outside and inside the cell, respectively.
At the equilibrium potential, the electrochemical forces driving the ion’s movement are balanced. This means no net movement of the ion occurs at that specific voltage. For example, the equilibrium potential for potassium (K+K^+) is typically negative because there is a higher concentration of potassium inside the cell than outside. On the other hand, for sodium (Na+Na^+), the equilibrium potential is positive due to a higher concentration outside the cell compared to the inside.