2 Nanomachines in nature
Cell biology gives us endless examples of sophisticated nano-scale machines. This biological nanomachine, for example, can be activated by a membrane voltage or a signalling molecule, which switches a potassium "ion channel" from a closed (left) to an open state (right). Potassium ions can therefore be selectively allowed to cross the membrane (shown as a yellow bar). Credit: David S Goodsell, Scripps Research Institute/RCSB Protein Data Bank.