Time dependent processes in biophysics generally involve molecular or ionic diffusion, and this generally occurs in systems with complex geometries. The underlying physics of diffusion is well understood, but the extra complications of a complex shape are not easy to analyze. Even the apparently simple problem of diffusion in a tube with constrictions and bends is surprisingly difficult. Another common problem involves diffusion past a random array of obstacles. Considerable progress is being made on finding mathematical methods for handling such problems.
