The lipid composition of the membrane also restricts what kinds of chemical compounds can easily pass across the membrane, which is therefore said to be semi-permeable. Every cell membrane has many different types of proteins and these different types of membrane protein serve different functions. Each type of receptor can recognise only a very restricted range of molecular stimuli, which requires that they bind or attach to each other. The stimulus molecules are called O ligands . The actions of nerve and muscle cells are regulated by this kind of ligand-receptor binding and hormones act as ligands in influencing the actions of their target cells.
Many therapeutic drugs exert their effects by binding to receptor proteins in cell membranes.
This allows small polar molecules, like ions, to pass across the membrane without having to penetrate the non-polar environment of the lipid bilayer. Also, channels may be permanently open, like the potassium channels of most cells, which are called leak channels . Or they may be gated, only opening or closing in certain circumstances. Some are controlled by binding with specific molecules, so they are called ligand-gated channels. Others, called voltage-gated channels, open or close according to changes in the local electrical environment.