Nernst-Goldman simulator: http://www.nernstgoldman.physiology.arizona.edu/ Potas

Question

Nernst-Goldman simulator: http://www.nernstgoldman.physiology.arizona.edu/

Potassium: 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.

Why do you think the simulator is set to begin with these parameters? (1 point) 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) Determine what the Nernst equilibrium potential is for values of 1, 10, 100, and 1000 (or close, if you cannot get the dial on the exact number) mM extracellular potassium, leaving the value for internal potassium at 100. Produce a neat, well-labeled figure and insert it into this document. Plot the value of the trans-membrane potential vs. the concentration of extracellular potassium (hint: the X axis, which should show just the extracellular potassium concentration, should be on a logarithmic scale). Describe the nature of the mathematical relationship between changes in extracellular potassium and the voltage across the membrane (2 points)

Explanation / Answer

1. Simulator starts at external concentration of 10mM and internal concentration of 100mM because it is the physiological condition or condition of real cell of human body.

2. When external concentration is changed to 1mM from 10mM, the potential changed from -61.5mV to -123 mV. It decrease because of less concentration of potassium.

3. E = - RT/zF x ln Kin/Kout

E = 8.314 x 310 / 1 x 96485 x ln 100/1 = - 123 mV

At 1mM, Potential is -123 mV

E = 8.314 x 310 / 1 x 96485 x ln 100/10 = - 61.5 mV

At 10 mM, potential is -61.5 mV

E = 8.314 x 310 / 1 x 96485 x ln 100/100 = 0 mV

At 100 mM, potential is 0mV

E = 8.314 x 310 / 1 x 96485 x ln 100/1000 = + 61.4 mV

At 1000 mM, potential is +61.4 mV

4. Mathematical relationship between changes in extracellular potassium and the voltage across the membrane

E = - RT/zF x ln Kin/Kout

When external concentration of potassium is lower than internal concentration, potential is lower and this allows movement of potassium to outside. And when external and internal concentration is equal, potential is zero so there will be no movement. When external concentration is higher than internal, potential is higher.

External concentration is directly proportional to change in voltage.

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