6.13: Ion Channels
The movement of ions like sodium, potassium, and calcium into and out of the cell is essential to maintain the electrochemical gradient in living cells. The ion channels—a class of membrane transport proteins—help maintain this ionic gradient for the smooth functioning of physiological activities such as maintaining cell size and volume, conducting nerve impulses, and gas and nutrient exchange.
Ion channels are specialized integral membrane proteins on the plasma membrane that allow specific ions to pass down their electrochemical gradient. Their main function is to maintain the membrane potential critical for cell viability. Two types of channels traverse the plasma membrane, the gated and non-gated channels, that can transport more than a thousand ions within milliseconds.
For most cells, especially excitable ones, there is a more negative charge in the interior of the cell than the exterior due to a greater number of negative ions than positive ions. For excitable cells, like firing neurons, contracting muscle cells, or sensory touch cells, the membrane potential must be able to change rapidly, moving from a negative membrane potential to one that is more positive. To achieve this, cells rely on two types of gated ion channels: ligand-gated and voltage-gated ion channels.
Ion channels may play a role in migraine headaches. The dura mater is a protective covering for the brain. It is innervated by several cranial nerves. It is hypothesized that migraines originate in these nerves. Both ligand- and voltage-gated ion channels in the dura mater may potentiate pain signals by altering membrane potentials.
Ion channels are found in plants. For instance, in plant leaves, the opening of the stomata by the surrounding guard cells requires potassium ion (K+) gradients. The active transport of hydrogen ions (H+) out of the guard cell creates a membrane potential that drives the inward movement of K+. This uptake of K+ by guard cells through the potassium channels triggers water movement into the cells. This osmotic influx causes the guard cells to expand, opening the stomata. When the potassium leaves the guard cells, water follows via osmosis. The now flaccid guard cells close the stoma.
Bacteria require ion gradients for several cellular functions maintained by different ion channels. Porins in prokaryotes allow the passive diffusion of nutrients, salts, and waste across the membrane and down the concentration gradient without the expenditure of ATP. Also, gated mechanosensitive channels play an important role during osmotic shock. When the bacteria move from a region of high osmolarity to a region of low osmolarity, these channels, acting as emergency release valves, open to release the solutes out into the cytoplasm to maintain the imbalance between the external and internal environments.