Initially there are 20 A molecules (red) diffusing rapidly on a 2D surface compartment. 5 of these molecules have been tagged with GFP (green). When two A molecules collide they form two As molecules with probability 0.02. As diffuses 100 times slower than A. As can also return back to the A form with a of rate 0.1/s.
The right panel shows the results of microscopy simulation using the Spatiocyte visualizer. It shows the GFP tagged molecules when they are in the slower diffusion state.