Illustration of a KcsA potassium ion channel from the soil bacterium Streptomyces lividans in complex with barium (green sphere at centre). Ion channels are membrane-spanning proteins that form pores in cell membranes and act as pathways for the movement of inorganic ions into and out of the cell. KcsA is specific to potassium ions (K+) and open and closes in response to changes in pH. Binding of barium to the channel permanently blocks it.