A.
The membrane potential results from a thin (<1 nm) layer of ions close to the membrane, held in place by their electrical attraction to oppositely charged ions on the other side of the membrane.
B.
When there is an exact balance of charges on either side of the membrane, there is no membrane potential.
C.
When ions of one type cross the membrane, they establish a charge difference across the two sides of the membrane that creates a membrane potential. The number of ions that must move across the membrane to set up a membrane potential is a tiny fraction of all those present on either side.
D.
In the case of the plasma membrane in animal cells, for example, 6,000 K + ions crossing 1 μm 2 of membrane are enough to shift the membrane potential by about 100 mV; the number of K + ions in 1 μm 3 of cytosol is 70,000 times larger than this.