  Many cell signaling pathways involve the diffusion of messengers that
bind/unbind to intracellular components. Quantifying their net transport rate
under different conditions, then requires having separate estimates of their
free diffusion coefficient and binding/unbinding rates. In this paper, we show
how performing sets of Fluorescence Correlation Spectroscopy (FCS) experiments
under different conditions, it is possible to quantify free diffusion
coefficients and on and off rates of reaction-diffusion systems. We develop the
theory and present a practical implementation for the case of the universal
second messenger, calcium (Ca$^{2+}$) and single-wavelength dyes that increase
their fluorescence upon Ca$^{2+}$ binding. We validate the approach with
experiments performed in aqueous solutions containing Ca$^{2+}$ and Fluo4
dextran (both in its High and Low Affinity versions). Performing FCS
experiments with tetramethylrhodamine-dextran in Xenopus laevis oocytes, we
infer the corresponding free diffusion coefficients in the cytosol of these
cells. Our approach can be extended to other physiologically relevant
reaction-diffusion systems to quantify biophysical parameters that determine
the dynamics of various variables of interest.
