The net driving force tending to move a charged solute across a cell membrane—its electrochemical gradient—is the sum of a force from the concentration gradient of the solute and a force from the membrane potential.

The cytosolic side of the plasma membrane is usually at a negative potential relative to the extracellular side, so the membrane potential tends to pull positively charged ions and molecules into the cell and drive negatively charged solutes out.

For some ions, the voltage and concentration gradients work in the same direction, creating a relatively steep electrochemical gradient. This is the case for Na + , which is positively charged and at a higher concentration outside cells than inside. Na + therefore tends to enter cells when given an opportunity.

If the voltage and concentration gradients have opposing effects, the resulting electrochemical gradient can be small. This is the case for K + , which is present at a much higher concentration inside cells, where the resting membrane potential is negative. Because its electrochemical gradient across the plasma membrane of resting cells is small, there is little net movement of K + across the membrane even when K + channels are open.