Physics looks at cell membranes
The diffusion of potassium ions across cell membranes underlies many fundamental process in biology, including electrical signalling in the central nervous system. The ions move through proteins known as K+ channels: however, sodium ions, which are smaller than potassium ions, cannot pass through.
Researchers at Rockefeller University in New York have used a variety of techniques, including synchrotron radiation, to establish how these K+ channels work. They found that four identical subunits create an inverted teepee. This teepee cradles a selectivity filter that is 12 Å long and contains two K+ ions (green dots) about 7.5 Å apart. For full details of this experiment see D A Doyle et al. 1998 Science 280 69-77.