Ion channels

Meaning: Ions are charged and cannot cross the hydrophobic core of the lipid bilayer on their own. A channel is a transmembrane protein whose hydrophilic pore lets them through. Transport is passive: ions move down their electrochemical gradient (concentration plus charge), so no ATP is needed. Selectivity comes from pore size and the charges lining it, so one channel may pass K⁺ but not Na⁺. Gating controls opening. Voltage-gated channels respond to membrane potential, ligand-gated channels to a bound molecule such as a neurotransmitter, and mechanically gated channels to stretch. Some leak channels stay open much of the time.

Equation: E = (RT/zF) ln([ion]out/[ion]in), where E is the equilibrium potential in volts, R is 8.314 J/(mol·K), T is temperature in kelvin, z is the ion's charge (unitless), F is 96,485 C/mol, and concentrations are in mol/L.

On the MCAT: Passages on the action potential or synaptic transmission describe voltage-gated Na⁺ channels opening during depolarization and K⁺ channels opening during repolarization. You may predict how a channel blocker, a mutation or a changed ion concentration alters membrane potential. Questions also ask you to classify a transport process as channel-mediated (passive) or pump-mediated (active).

Common confusion: Channels are not pumps. An open channel does not set the direction of flow. The electrochemical gradient does, and ions move toward their equilibrium potential. Pumps such as the Na⁺/K⁺ ATPase use ATP to move ions against their gradients and build the gradients that channels use.