Ions are charged molecules and cannot easily pass through the cell membrane. As we know, cell membranes are basically made up of a double layer of phospholipids. By their nature, ions cannot therefore pass through these membranes. That’s why ions can only get in and out of cells through ion channels.
These channels are protein molecules inserted through the membranes. They have an inner core that is hydrophilic through which the ions can pass and an outer hydrophobic region that interacts with the phospholipid membrane.
There are two major kinds of ion channels: leaky channels and gated channels. As the name implies, leaky channels are open all the time and ions can leak through if there is a concentration difference across the membrane. However, there are only few leaky channels. The gated channels are more numerous and different factors can swing open or close their gates.
For example, some gated channels swing open or close depending on changes in voltage across the cell membrane. These voltage-gated channels usually participate in conducting electrical signals.
Other gated channels swing or close when specific molecules bind to receptors associated with them. They are therefore called ligand-gated channels.
There are also volume-gated channels that swing or close when there are changes in the volume in and around cells. And mechanical-gated channels like those associated with touch receptors, swing or close when there is mechanical change around them.
Opening and closing of these gated channels usually produce specific physiological changes in the cell. Changes in ion concentrations inside cells are signals for specific physiological activity. Examples of physiological activity are: nerve impulse conduction, beating of the heart, activation of visual receptors, touch receptors and other sensory receptors and many more.
Many disorders are associated with ion channel malfunction. Many toxins produce their effct by blocking ion channels. For example, the reason why red tide can poison people is due to blockage in sodium channels.