Nernst potential The simulator starts with two solutions separated by a membrane

Question

Nernst potential

The simulator starts with two solutions separated by a membrane (such as the plasma membrane of a nerve cell) that is permeable to potassium but not chloride or sodium ions (this is why the red “Potassium” section is highlighted in the left window). The internal solution (labeled “[K]i”) contains 100 mM KCl, while the external solution (labeled “[K]io) has 10 mM KCl.

1) Why do you think the simulator is set to begin with these parameters? (1 point)

2) Now change the value for the external concentration of potassium to 1 mM by dragging the box in the [Ko] line to the left until it reads “1”. What is the new value for the Nernst potential for potassium? Why does it increase or decrease? (1 point)

Explanation / Answer

Answer:

(Since no images, figures or windows (as mentioned in the text) are provided, the calculations and the answers have been given in general)

1. The equilibrium potential of K+ under normal conditions is calculated assuming an outside concentration, [K+]o, of 10mM and an inside concentration, [K+]i, of 100mM. So, the parameters are set to these values.

2. Nernst equation:

Vk =2.303* RT/+1F log [K+]o/[K+]in

1st Case:

[K+]o = 10mM, [K+]in = 100mM, Temperature = 298K

So, Vk = (8.314 * 298 * 2.303)/96485 * log (10/100)

= -1 * 0.059 = - 0.059 V = - 59 mV

2nd Case:

[K+]o = 1mM, [K+]in = 100mM, Temperature = 298K

Vk = (8.314 * 298 * 2.303)/96485 * log (1/100)

= -2 * 0.059 = -0.118 V = -118 mV

So, the new value for the Nerst value of potassium is -118 mV.

The Nernst potential has decreased from -59mV to -118mV. A decrease in the resting potential is called a hyperpolarization.

Hyperpolarization is a change in a cell's membrane potential that makes it more negative. It inhibits action potentials by increasing the stimulus required to move the membrane potential to the action potential threshold.

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