The number of channel molecules present means that, in normal circumstances, about 70% of the free-floating outer channels are lined up with tethered inner ones at any given time, with each dual channel letting through about 1m sodium ions per second.

ECONOMIST: Medical diagnostics

This channel is then unable to move around and line up with an inner channel, so it cannot transmit sodium ions.

ECONOMIST: Medical diagnostics

But the right circumstances depend on what is happening in the lipid layers because, by itself, a single gramicidin channel is not long enough to span the membrane and thus let any sodium ions through.

ECONOMIST: Medical diagnostics