Ion Channel Kinetics: A Fractal Time Sequence of Conformational States

Abstract

Ions such as sodium, potassium, and chloride can move freely through water but cannot cross the hydrophobic lipids that form the cell membrane. However, these ions can interact with proteins in the cell membrane that transport them across the cell membrane. Three types of proteins are involved in ion transport: 1) Pumps, such as the sodium-potassium ATPase, bind tightly to a few ions at a time and use energy from ATP to move these ions against their electrochemical gradient. 2) Carriers, such as the sodium-potassium-chloride cotransporter, bind tightly to a few ions at a time and help them move down their electrochemical gradient. 3) Channels, such as the sodium channel, bind weakly to many ions at a time and allow them to move down their electrochemical gradient.

These equations can be given a physical basis if we assume that potassium can only cross the membrane when four similar particles occupy a certain region of the membrane...and if the sodium conductance is assumed to be proportional to the number of sites on the inside of the membrane which are occupied simultaneously by three activating molecules but are not blocked by an inactivating molecule.

Hodgkin and Huxley (1952)