Ion Channels in the Plasma Membrane

The permeability of a membrane to a particular ion is a measure of the ease with which that ion can cross the membrane. It is a property of the membrane itself. Recall that ions cannot cross membranes through the lipid portion of the membrane; they must cross through aqueous pores or channels in the membrane. Thus, the ionic permeability of a membrane is determined by the properties of the ionic pores or channels in the membrane. The total permeability of a membrane to a particular ion is governed by the total number of membrane channels that allow that ion to cross and by the ease with which the ion can go through a single channel. Ion channels are protein molecules that are associated with the membrane, and thus an important function of membrane proteins is the regulation of ionic permeability of the cell membrane. In later chapters, we will discuss how specialized channels modulate ionic permeability in response to chemical or electrical signals and the role of such changes in permeability in the processing of signals in the nervous system.

Not all membrane channels allow all ions to cross with equal ease. Some channels allow only cations through, others only anions. Some channels are even more selective, allowing only K+ through but not Na+, or vice versa. Thus, it is possible for a membrane to have very different permeabilities to different ions, depending on the number of channels for each ion.

0 0

Post a comment