On the cellular scale, pH controls the extent of ionization of the protein molecules that are embedded in the cell walls. As a result, cellular surfaces are generally charged and surrounded by diffuse double layers, the thickness of which is controlled by the overall electrolyte concentration. Interaction of these ionic space-charge regions with those that also surround small particles of mineral phases can strongly affect the motion of the cells through a natural porous medium. The effect of pH on the surface charge of a protein depends on the relative numbers of acidic and basic groups in the side chains. Protein molecules are often characterized by a pH called the isoelectric point, at which the positive and negative charges resulting fro
ionization of side chains are balanced.