Phoe is synthesized under phosphate-limited conditions and is specific for anions. The pore size of porins varies depending on the species,and is regulated by the molecular arrangement around it. Nutrients with a higher molecular weight cross the OM through receptor protein. The speed of diffusion across the OM through non-specific porins and receptors is dependent on the concentration gradient,but the speed increases only up to the saturation concentration through the porins and receptor specific for that nutrient.