To an electrophysiologist, a cell is basically a circuit. For each component that a circuit can be made up of, there is an equivalent in these microscopic organisms that serve as the building blocks of life. The cell membrane is the semipermeable layer that protects the cell from external elements. It is mainly composed of a double layer of lipids, but also contains proteins that allow for the passage of molecules into and out of the internal cytoplasm. Cell membranes are unique in that the act as both a resistor and a capacitor in the overarching circuit of the cell. Resistors in a circuit hinder current and create heat, which allows them to carry out a specific function such as light up or move. Capacitors in a circuit are filled with insulators and store charge from the current. Current is the flow of charge through a circuit, and this flow is caused by the voltage, or potential, of the battery.
Although lipids are exceptional insulators, the proteins that are present in the cell membrane allow current to pass through them, decreasing the membrane’s otherwise extremely high resistance. This has shown to have significant effects because, according to Ohm’s Law, a lower resistance allows for a higher voltage to be applied to the membrane. Because the membrane is primarily made of insulators, it also functions as a capacitor which divides charges inside and outside of the cell. Every capacitor has a capacitance, the ability for the capacitor to store charge. The capacitance for a cell membrane is significant to the cell in that it is inversely related to the voltage change across the membrane. A smaller capacitance allows for a higher voltage change and vice versa.