Limiting accuracy of molecule detection. (A) Uncertainty δc/c for the detection of resact versus the receptor number N_R for different resact concentrations. Because the effective radius s of the binding site is not known, the radii of the globular extracellular domain and of the ligand were used as an upper and lower estimate, respectively. (B) The resact structure was modeled using PEP-FOLD (Maupetit et al., 2009, 2010; Thévenet et al., 2012) with a disulfide bridge between C1 and C8. The model was visualized using RasTop, a tool based on RasMol 2.7. (C) Schematic distribution of a GC and CNGK on the flagellum. The scheme depicts the intracellular site of the flagellar membrane. An active GC dimer is depicted in red, an inactive GC dimer is shown in gray, CNGK is depicted in green, and the gradient of cGMP concentration is given in shades of magenta. The GC and CNGK densities are shown to scale. The GC dimer/CNGK ratio is 10:1.