  This work reports on two results. At first we revisit the Berg and Purcell
calculation that provides a lower bound to the error in concentration
measurement by cells, by considering the realistic case when the cell starts
measuring the moment it comes in contact with the chemoattractants, instead of
measuring after equilibrating with the chemotactic concentration as done in the
classic Berg and Purcell paper. We find that the error in concentration
measurement is still the same as evaluated by Berg and Purcell. We next derive
a lower bound on measurement time below which it is not possible for the cell
to discern extra-cellular chemotactic gradients through spatial sensing
mechanisms. This bound is independent of diffusion rate and concentration of
the chemoattracts and is instead set by detachment rate of ligands from the
cell receptors. The result could help explain experimental observations.
