  Cells in multicellular organisms adhere to the extracellular matrix through
two-dimensional clusters spanning a size range from very few to thousands of
adhesion bonds. For many common receptor-ligand systems, the ligands are
tethered to a surface via polymeric spacers with finite binding range, thus
adhesion cluster stability crucially depends on receptor-ligand distance. We
introduce a one-step master equation which incorporates the effect of
cooperative binding through a finite number of polymeric ligand tethers. We
also derive Fokker-Planck and mean field equations as continuum limits of the
master equation. Polymers are modeled either as harmonic springs or as
worm-like chains. In both cases, we find bistability between bound and unbound
states for intermediate values of receptor-ligand distance and calculate the
corresponding switching times. For small cluster sizes, stochastic effects
destabilize the clusters at large separation, as shown by a detailed analysis
of the stochastic potential resulting from the Fokker-Planck equation.
