  In biological cells and novel diagnostic devices biochemical receptors need
to be sensitive to extremely small concentration changes of signaling
molecules. The accuracy of such molecular signaling is ultimately limited by
the counting noise imposed by the thermal diffusion of molecules. Many
macromolecules and organelles transiently bind to molecular motors and are then
actively transported. We here show that a random albeit directed delivery of
signaling molecules to within a typical diffusion distance to the receptor
reduces the correlation time of the counting noise, effecting an improved
sensing precision. The conditions for this active focusing are indeed
compatible with observations in living cells. Our results are relevant for a
better understanding of molecular cellular signaling and the design of novel
diagnostic devices.
