Structure of the cell membrane

All living cells are surrounded by a plasma membrane composed of lipids and proteins, whose main function is to control the passage of substances into and out of the cell. In general, the role of the lipids is to furnish a continuous matrix that is impermeable even to the smallest ions, in which proteins are embedded to provide selective pathways for the transport of ions and organic molecules both down and against the prevailing gradients of chemical activity. The ease with which a molecule can cross a cell membrane depends to some extent on its size, but more importantly on its charge and lipid solubility. Hence the lipid matrix can exclude completely all large water-soluble molecules and also small charged molecules and ions, but is permeable to water and small uncharged molecules like urea. The nature of the transport pathways is dependent on the specific function of the cell under consideration. In the case of nerve and muscle, the pathways that are functionally important in connection with the conduction mechanism are: (1) the voltage-sensitive sodium and potassium channels peculiar to electrically excitable membranes, (2) the ligand-gated channels at synapses that transfer excitation onwards from the nerve terminal, and (3) the ubiquitous sodium pump, which is responsible in all types of cell for the extrusion of sodium ions from the interior.

The essential feature of membrane lipids that enables them to provide a structure with electrically insulating properties, i.e. to act as a barrier to the free passage of ions, is their possession of hydrophilic (polar) head groups and hydrophobic (non-polar) tails.