Question

You are studying neurons in the lab by growing them in culture (in a petri dish outside the body). You grow them in an incubator at 37 degrees C. With this experimental setup, you can easily change the extracellular ion concentrations by switching the cell culture medium (the fluid that surrounds the cells).

	Intracellular concentration	Medium 1 (extracellular concentration)	Medium 2 (extracellular concentration)
K+	140 mM	4.5 mM	50 mM
Na+	10 mM	135 mM	100 mM
Cl-	12 mM	125 mM	120 mM

a.If you switch from Medium 1 to Medium 2, will the intracellular concentrations change? Briefly explain your answer.

d. In Medium 1, which ion has the largest driving force at rest, and what is its driving force?

e.  What do you predict will happen to the membrane potential if the relative permeability of potassium:sodium:chloride changes to 100:200:5? No calculation needed, just explain your thinking.

f.  In Medium 1, if we voltage clamp the cell at -100 mV, which direction will each ion move if the cell becomes suddenly highly permeable to that ion? For each ion, write 1 sentence to explain what is happening.

Answer 1

Answer 2

The entry rate of the substance is given in percentage of substance taken in. So the maximum entry rate is 100% and not the total concentration inside the cell (120mM).

So, half the maximum rate is 50%. We need to find at what external concentration of the substance we get 50% of the substance inside the cell. From looking at the table, it is clear that the external concentration is an increment of 10 mM, but the increment on the right side is not proportional. To every increment in the external concentration, the rate of entry varies. Hence, we cannot directly cross multipy to check the concentration to get 50% entry rate.

So, we need to cross multiply the nearest values to 50%

That is, 30mM= 45%,

?mM =50%

=> 30 X 50 / 45

= 33.333 => 34 mM

Likewise, 40mM = 65%

?mM =50%

=> 40 X 50 / 65

   = 30.76 => 31 mM

So we get a range of concentration, 31-34 mM for an entry rate of 50%