  Chemotactic cells of eukaryotic organisms are able to accurately sense
shallow chemical concentration gradients using cell-surface receptors. This
sensing ability is remarkable as cells must be able to spatially resolve small
fractional differences in the numbers of particles randomly arriving at
cell-surface receptors by diffusion. An additional challenge and source of
uncertainty is that particles, once bound and released, may rebind the same or
a different receptor, which adds to noise without providing any new information
about the environment. We recently derived the fundamental physical limits of
gradient sensing using a simple spherical-cell model, but not including
explicit particle-receptor kinetics. Here, we use a method based on the
fluctuation-dissipation theorem (FDT) to calculate the accuracy of gradient
sensing by realistic receptors. We derive analytical results for two receptors,
as well as two coaxial rings of receptors, e.g. one at each cell pole. For
realistic receptors, we find that particle rebinding lowers the accuracy of
gradient sensing, in line with our previous results.
