[Computational-biology] Implimenting Periodic boundary comdition

[Mcirc]

Hi
I am trying to impliment finite difference technique in matlab for
solving a 2D parabolic heat equation.
I am interested in using Periodic boundary condition. Can any one
please help me on this regard,In fact, I need to know how to set the
boundary condition while writting the matlab code for all the grid
points. for internal grid points (say a grid of 20x20) no problems, so
when I reach at the boundary how should my equation look like. or what
should I check for all boundaries.
THE 2D time dependent PDE looks like this:
dC/dt = dC/dx^2  + dc/dy^2
I look for a neumerical solution for C.

best wishes 
ddhr