The cell boundary is shown in green as an isosceles triangle 25.6μm wide and 5μm tall. The actin islands are shown as black circles with 1μm radii. As discussed in the text, unbound Rac1 molecules are free to diffuse throughout the cell and reflect off the cell boundary. Bound Rac1 molecules diffuse slower and are restricted to the inside of the island in which they are bound. We can individually set the KD of each island thereby setting, on average, the percent of the total Rac1 population bound to each island. The 256 magenta boxes represent the (0.1μm)2 bins used to generate the intensity carpet; these bins do not affect the behavior of the molecules.