  Cells measure concentrations of external ligands by capturing ligand
molecules with cell surface receptors. The numbers of molecules captured by
different receptors co-vary because they depend on the same extrinsic ligand
fluctuations. However, these numbers also counter-vary due to the intrinsic
stochasticity of chemical processes because a single molecule randomly captured
by a receptor cannot be captured by another. Such structure of receptor
correlations is generally believed to lead to an increase in information about
the external signal compared to the case of independent receptors. We analyze a
solvable model of two molecular receptors and show that, contrary to this
widespread expectation, the correlations have a small and negative effect on
the information about the ligand concentration. Further, we show that
measurements that average over multiple receptors are almost as informative as
those that track the states of every individual one.
