Question

A cell biologist interested in the transport of calcium ions (Ca2+) across the plasma membrane of bacterial cells made the following measurements on bacterial cells at 25°C:

Vm = −150 mV [Ca2+] outside = 20 mM [Ca2+] inside = 0.4 mM
Assuming no change in either the internal or external calcium ion concentration, at what membrane potential would there be no net movement of calcium across the membrane?
A) −25 mV
B) −50 mV
C) 0 mV
D) +25 mV
E) +50 mV

Answer 1

Using the Nernst equation, the equilibrium potential for calcium ions across the membrane would be approximately +123 mV, which is not listed in the options provided.

Step-by-step explanation:

To determine at what membrane potential there would be no net movement of calcium across the membrane, we need to apply the Nernst equation, which calculates the equilibrium potential for a particular ion species across a membrane:

The Nernst equation is given by:

E = (RT/zF) * ln([ion outside]/[ion inside])

Where:

E is the equilibrium potential,

R is the gas constant (8.314 J/(mol*K)),

T is the temperature in Kelvin,

z is the charge of the ion (+2 for Ca2+), and

F is the Faraday constant (96485 C/mol).

Plugging in the values for calcium ions at 25°C (which is 298K), we get:

E = (8.314*298)/(2*96485) * ln(20/0.4)

After calculations, the equilibrium potential E for calcium ions is approximately +123 mV, indicating that no net movement of Ca2+ ions will occur at this potential. However, this is not one of the options provided. Given that the options are likely referring to a range within which we could expect no net movement, we can say that no option is strictly correct. It might be necessary to re-evaluate the question or the provided options.