Ion Diffusion

Although the phospholipid bilayer portions of the cell membrane are impermeable to ions, ion channels formed by transmembrane proteins render the cell membrane selectively permeable to Na+, K+, Ca2+ and Cl-. The walls of these channels are formed by polypeptide subunits, which may number up to 12 per channel (Figure PG.13). Rapid changes in ion concentrations (such as in the production of action potentials) are produced by opening and closing the channels (gating) and consequently producing dramatic changes in permeability to a given ion. The gating of ion channels may be controlled by chemical messengers (ligand gating, e.g. acetyl choline receptors) binding to the subunits, changes in membrane potential (voltage gating, e.g. Na+ channel) and stretching of the membrane (mechanical gating). Factors determining permeability of a cell membrane to a given ion are shown below:

• Chemical messenger concentration

• Membrane potential

• Membrane conformation

• Density of specific ion channels ion cfiorncl ion cfiorncl

Figure PG.13 Ion channel i^ri chonwl OOSi i.ixliijr

Figure PG.13 Ion channel

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