Scientist Solutions - Cell membrane electoconductivity

The membrane of a cell is a lipid bilayer. This barrier function is crucially important because it allows the cell to maintain concentrations of solutes in its cytosol that are different from those in the extracellular fluid and in each of the intracellular membrane-enclosed compartments. The lipid bilayer has charged polar head groups that face outwards into the cell cytoplasm and extracellular space. The fatty acid tails form a hydrophobic interior to the cell memebrane. Protein-free lipid bilayers are highly impermeable to ions because of the hydrophobic interior of the membrane. Given enough time, any molecule will diffuse across and pure lipid membrane, but the rate at which it does so, however, varies enormously, depending partly on the size of the molecule, but mostly on its relative solubility in oil. In general, the smaller the molcule the more soluble it is in oil (other factors are hyrophobicity and polarity). Small nonpolar molecules, such as Oxygen readily dissolve in lipid bilayers and therefore diffuse rapidly across them. Small uncharged polar molecules, such as water or urea, also diffuse across a bilayer, albeit much more slowly. By contrast, lipid bilayers are highly impermeable to charged molecules (ions), no matter how small: the charge and high degree of hydration of such molecules prevents them from entering the hydrocarbon phase of the bilayer.

Cell membranes allow water an non-polar molecues to cross by diffusion, but they contain special membrane transport proteins responsible for transferring such solutes as as ions, sugars, amino acids, nucleotides, and many cell metabolites. Each protein transports a particular class of molecule (such as ions, sugars, or amino acids) and often only certain molecular species of the class.

All membrane transport proteins that have been studied in detail have been found to be multipass transmembrane proteins-that is, their polypeptide chains traverse the lipid bilayer multiple times. By forming a continuous protein pathway across the membrane, these proteins enable specific hydrophilic solutes to cross the membrane without coming into direct contact with the hydrophobic interior of the lipid bilayer.

The number, class and properties of these membrane proteins in a cell membrane will determine its excitability. The activation of these proteins will depend on the presence of the appropriate target molecule or triggering event, be it a binding of a neurotransmitter, drug, hormone or change in voltage etc. The rate of activation of each protein type will also have a bearing on excitability. For example channels that respond to changes in the membrane potential will open and close more rapidly than a transpoter that requires the co- transport of ions in one direction while transporting its substrate in the other.

An excitable cell (e.g. a neuron) will probably express many more voltage dependent channels at its surface than say an epithelial cell. This makes the neuron inherently more "excitable".

I hope you have grasped the basic concept from this post. I tried not to be too technical. There are many basic Physiology, biology and membane bioogy text books out there from which you can get a more detailed answer. E.g. Alberts "Molecular biology of the Cell".