Although lipid membranes exhibit some permeability to the weak base NH₃, organisms have developed specialized proteins that increase and regulate the NH₃ fluxes across cellular membranes. In humans, the Rh glycoproteins, such as the erythrocyte-specific RhAG and the liver and kidney homologs RhBG and RhCG, are involved in the passage of NH₃. Rh glycoproteins have distant relatives, called ammonium transporters (AMTs), in archae and bacteria. The crystal structures of AMTs show that the proteins are homo-trimers and that the center of each monomer forms a pore. AMT/Rh proteins have also been identified in plants. In contrast to the human Rh glycoproteins, these AMTs specifically transport NH₄⁺ or co-transport NH₃/H⁺. Hence, they can transport against NH₃ gradients. The molecular determinants for the different transport mechanisms within proteins of the same family are currently unclear. The functional differences between AMT/Rh transporters are likely to be an evolutionary adaptation to different ammonium and nitrogen requirements in bacteria, plants and animals.