 Transmembrane proteins can also serve as ligand transport proteins that alter the permeability 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 . 
 Transmembrane proteins can also serve as ligand transport proteins <a href='null_relcl' title='1'><mark>that</mark></a>alter the permeability of the cell membrane to small molecules and ions . The membrane alone <a href='got' title='2'><mark>has</mark></a> a hydrophobic core through which polar or charged molecules cannot diffuse . Membrane proteins contain internal channels <a href='null_relcl' title='3'><mark>that</mark></a>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 . 
Transmembrane proteins can also serve as ligand transport proteins alter the permeability of da cell membrane to small molecules and ions . Tdamembrane alone got a hydrophobic core through which polar or charged molecules cannot diffuse . Membrane proteins contain internal channels allow such molecules to enter and exit the cell . Many ion channel proteins are specialized to select fir only a particular ion ; for example , potassium and sodium channels often discriminate for only one of the two ions .