 <unk> proteins can also serve as ligand transport proteins that alter the <unk> of the cell membrane to small molecules and ions . The membrane alone has a hydrophobic core through which polar or charged molecules cannot diffuse . Membrane proteins contain internal channels that allow such molecules to enter and exit the cell . Many ion channel proteins are specialized to select for only a particular ion ; for example , potassium and sodium channels often discriminate for only one of the two ions . 
