Ion channels have three important properties:
- They are selective to specific ions
- They open and close to specific stimuli such as electrical, chemical, or mechanical signals
- They conduct ions across the membrane.
The fastest flow of ions across channels is comparable to the fastest turnover of enzymes such as carbonic anhydrase or catalase. The turnover rate of most enzymes is 10 to 1000 per second. Such extraordinary rates of ion flow are selective to ions through permeability. For example, the negative resting membrane potential of neurons is dependent on the class of K+ channels that are 100-fold more permeable to K+ ions than Na+ ions. Whereas, during an action potential, the class of Na+ channels activated are just 10 to 20 fold more permeable to Na+ than to K+ ions.
The plasma membrane of the neuron, like other cells, is 6 to 8 nanometer-thin and is a mosaic of lipids and proteins. The lipids of the membrane are hydrophobic. In contrast, the ions inside and outside the cell are hydrophilic and attract water. Ions attract water because water molecules are dipolar; even though the net charge on a water molecule is zero, the charge is separated within the molecule. The oxygen atoms in the water molecule tend to electrons and bear a partial negative charge. Whereas the hydrogen in the water molecule tends to lose electrons and bears a partial positive charge. Due to this unequal distribution charge or charge separation, the positively charged ions (cations) are attracted to the oxygen atoms of the water molecule, and negatively charged ions (anions) are attracted to the hydrogen atom of the water molecule. Similarly, water molecules are attracted to ions as well. Hence, the electrostatically bound water molecules around an ion are called waters of hydration.
Before ions try to pass through the ion channel, it is indeed bound by water molecules. But a large amount of energy should be expended to overcome the electrostatic forces between the ion and the surrounding water molecules. In this way, energetics favor ion movement.
Ion channels are not simple holes in the membrane. They are distinct protein structures spanning the lipid bilayer. Hence, how does an ion channel allows potassium ions to pass while excludes sodium ions? Here, selectivity cannot be solely based on the size or diameter of ions because the K+ ion has a crystal radius of 0.133 nm which is larger than the Na+ ion (0.095 nm). But still, potassium ions are more favorable. Why? The answer is more interesting than we think.
The smaller an ion is, the more localized is its charge. This results in a strong electric field. Hence, smaller ions attract water more strongly. As Na+ ions move through the solution, their strong electrostatic attraction for water causes them to have a large water shell around them. Relatively, potassium ions have a smaller water shell around them. Due to the large water shell, Na+ ions behave larger than K+ ions. Thus, potassium ions are selectively more permeable than sodium ions simply based on their interaction with water in a water-filled channel.