Normalized coefficients of variation and normalized frequencies of change. (A) Normalized coefficient of variation (CV) and (B) normalized frequency of change (FC) for all model species in different volumes are illustrated by color coding (heat map). The CV describes the strength of noise and FC tells how many times the concentration changes on average in a given time period. The values for CV and FC were estimated using 100 second signals (simulated noise processes). The simulated volumes were equidistantly spaced between 5 × 10-16 l (comparable, for example, to the volume of the dentritic spine) and 10-13 l (comparable, for example, to the volume of a cell). The rows in the heat maps are sorted in an ascending order according to the initial concentrations of species. The CV and FC values are normalized by their maximum values. The intensities in the heat maps vary from values close to zero (dark blue) to 1 (dark red). The zero CV and FC values are mapped to gray points. The large network includes species that stay at zero concentration in all studied volumes (craf**deph, GqCaPLCbcomplex, GqPLC, GqCaPLC, G*GTP, cRaf1**) as well as a species remaining at a constant concentration (G*GDP). As supposed, the strength of noise seems to decrease and the frequency of change increase when the volume is increased.