The biochemistry that takes place inside cells depends on various elements, such as sodium, calcium and potassium that are dissolved in water as ions. These ions enter cells during narrow pores in the cell membrane known as ion channels. Each ion channel, which is formed from a specialized protein molecule, is selective for one kind of ion. Measurements with microelectrodes have shown that a 0.30-nm-diameter potassium ion (K+) channel carries a current of 1.8 pA.
What is the current density in the ion channel?