The membrane of Streptomyces lividans cells lets potassium ions in but not sodium via a potassium-specific channel.

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"The researchers cracked the structure of the protein 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)

Journal article
Chemistry Nobel winner: My work is not doneNew ScientistOctober 18, 2010
Andy Coghlan

Journal article
Chemistry Nobel rewards crucial cell membrane workAndy Coghlan

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