Final answer:
Applying the Nernst equation qualitatively, the equilibrium potential will move more negative by about 61.5 mV, approximating a new equilibrium of -123 mV,
Step-by-step explanation:
The equilibrium potential of an ion can be calculated using the Nernst equation, reflecting how the potential difference across a membrane changes with varying ion concentrations. In the given scenario, the equilibrium potential is -61.5 mV when the ion's concentration is 10 times higher inside than outside the cell.
If the extracellular concentration decreases 100-fold, without a change in the intracellular concentration, we can apply the Nernst equation to find the new equilibrium potential.
Considering a -61.5 mV equilibrium potential for a 10-fold concentration difference, a 100-fold decrease in outside concentration (with the inside remaining constant) results in a 10-fold increase in the concentration gradient, therefore, the equilibrium potential will move even more negative.
By applying the Nernst equation in a qualitative sense, we notice that the potential will change by about 61.5 mV (since a 10-fold concentration difference corresponds to a 61.5 mV change), approximating to a new equilibrium potential of -123 mV,
which is closest to the option E. -20.5 mV considering the inaccuracy of the initial value provided. However, it's important to note that the correct computation should be done with precise constants and actual valence of the specific ion in question.