  We introduce a phase field approach for diffusion inside and outside a closed
cell with damping and with source terms at the interface. The method is
compared to exact solutions (where possible) and the more traditional finite
element method. It is shown to be very accurate, easy to implement and
computationally inexpensive. We apply our method to a recently introduced model
for chemotaxis by Rappel et al. [Biophys. J. 83, 1361 (2002)].
