The membrane of Streptomyces lividans cells lets potassium ions in but not sodium via a potassium-specific channel.
“The researchers cracked the structure of the through a technique called X-ray crystallography, and it then became clear how the channel would admit potassium ions only, even though closely-related sodium ions were even smaller. The structural analysis showed the channel is lined by oxygen atoms that mimic the water cluster normally surrounding a potassium ion in aqueous solution. Sodium has a slightly different water shell, and so cannot fit through the channel. The analysis also revealed parts of the protein that receive ‘on’ and ‘off’ chemical signals.” (Coghlan 2003)
