Ion channels are found within the cell membranes of all biological cells. The channels are formed by proteins that create pores in the plasma membrane. The pores allow ions that cannot pass directly through the cell membrane to travel in and out of cells. Most channels are made up of more than one protein, and are actually complexes of proteins found within the membrane.
A type of atom, ions are differentiated by having an unequal number of electrons and protons. The ion has either a negative or positive charge depending on whether there are more electrons or protons, respectively. The phosopholipid bilayer that makes up a cell membrane has its own charge and actually acts as a barrier to ions passing through it freely. This is the reason that ion channels are necessary within cell membranes.
Ion channels act as electrical insulators and provide a pathway for ions to cross either into or out of a cell. Each ion channel acts as a catalyst in the process that moves the ion from one side of the cell membrane to the other. The channel can be either open or closed, with the transition between the two states occurring when an ion binds to the correct site on the ion channel.
The change between the open and closed state of an ion channel is referred to as gating. External factors play a role in whether the gate is open or closed. There are different groups of ion channels and they can be grouped according to what physical and chemical external factors play a role in opening and closing the gate.
Some ion channels are "ligand gated" and these open and close when a ligand, or a chemical messenger such as a neurotransmitter, binds to a receptor on the channel. Voltage gated ion channels are opened or closed when there are changes in the electrical potential difference — or voltage — surrounding the channel. This kind is most commonly found in nerve and muscle cells. Second messenger gated ion channels have their activity modulated by a group of proteins called G-proteins. Mechanosensitive channels are stimulated by external mechanical stimuli, like touch and changes in osmotic pressure.
As well as being activated by external modulators, ion channels can also be selective regarding which ions pass through the channel. The shape, size and charge of an ion can affect whether a channel will open or close and allow movement across the cell membrane. Common examples of ion specific channels include potassium channels, chloride channels, sodium channels and calcium channels.