The other type of membrane in use consists of a polyvinyl chloride (PVC) disc, impregnated with a large organic molecule that can react with the ion. The binding must be weak enough for the ion to be passed from one molecule to another across the membrane under the driving force of a concentration gradient. For example, K ion selective membranes are made using the antibiotic valinomycin. This has a structure that accommodates the K ions and can pass them on from one molecule to another. The operation of this material mimics the way in which living cells transfer ions across the cell membrane. Unfortunately, such molecules are not usually completely specific for a single ion, and usually also channel chemically similar species. The potassium membrane, for example, can also pass lesser amounts of sodium ions