Question

If the outside of a cell has a potassium concentration of 12 mM what must be the inside concentration of potassium for the condition of no net flux of potassium across the membrane at Vm = - 75 mV

Answer 1

In normal resting condition cell has a resting membrane potential which is given( -75mV.)

A small part of this is contributed by Na K atpase pump which operrates during resting stage and responsible for sending 3 Na ions out of and 2K ions into the cell making inside of cell relatively electro negative.but contributes say (-5mV).

During resting stage celk is impermeable to Na ions bcoz all thee Na channels aee closed but permeable to K ions.Thus in resting stage K+ ions constantly leaks out of cell along the concentration gradient.(k+inside cell is higher than outside).This contributes major part of RMP making inside more electronegative. Movement if ion depends on _concentration gradient across cell and electrical gradient across cell.For K+ both are in opposite direction to each other.That is cocentration gradiant is towards out of cell (inside has higher k+ concentration) supports leaking of k+ ions out of cell.

Electrical gradient is towards into the cell becoz inside of cell is relatively electro negative hence tries to keep the positive ions inside.But it is weaker than concentration gradient so k+ions leaks out even in the presence of electrical gradient.

This leakege will stop when these 2 forces(concentration gradient of k+ and electrical gradient)will be equal and opposite.

Acc to this question RMP=-75mv,

Let say contributed by K+ ions=(-70mV)=electrical gradient

Thus concentration gradient must be 70mmol.

Which is( inside k+ concentration--ouside k+ concentration)=70mmol.

Out side concentration = 12mmol.

Thus inside cell k+ concentration must be 70+12=82mmol.

Thus when inside cell k+ concentration will be 82 mmol ,there will be no net flux of k+ out of the cell.