Membrane transport of small molecules and the electrical. Lipid bilayers are naturally impermeable to ions. Small, hydrophobic or nonpolar molecules can diffuse across a lipid bilayer. Membrane transport proteins are specialized in transporting solutes across cell membranes. Each protein often transports only a specific molecular species or a class of molecules (such as ions, sugars, or amino acids). Transporters and channels are the two major classes of membrane transport proteins. Transporters (also called carriers or permeases) bind the solute to be transported and undergo conformational changes to transfer the solute across the membrane. Channels interact weakly with the solute and form continuous pores that extend across the lipid bilayer, which allow specific solutes to pass through them. Transport through channels occurs at a much faster rate than transport mediated by transporters. Passive transport does not require energy and occurs in the direction of a concentration gradient.