Ion channels are transmembrane proteins that allow the passive movement of a specific ion across the membrane, down its electrochemical gradient. These channels contain pores for the ions to pass through. The channels can be gated or non-gated. Gated ion channels require a stimulus, such as ligand binding, voltage change, or mechanical stress, to open. Non-gated ion channels need no such stimulus, they can open and close at random. Ion channels have wide roles in living organisms. For example, in neurons, the opening and closing of sodium and potassium voltage-gated channels helps transmit nerve impulses. Likewise, the non-gated sodium and potassium channels play an essential role in maintaining the resting membrane potential of a cell, negative 70 millivolts. In plant leaves, the opening and closing of the stomata are triggered by the inward and outward flow of potassium ions through the specialized potassium channels. Ion channels are also widespread in bacteria, playing important roles in the influx of salts and nutrients and maintaining osmotic balance.