That's a tough analogy. I can only think of two really good comparisons between a pie and a cell. The first is the cell membrane. The cell membrane is the outer boundary of a cell. It gives the cell its general shape, and controls what enters and leaves the cell. The cell membrane is similar to a pie's crust. Pie crust gives the pie its overall shape due to the support that it provides the pie. The crust also keeps all of the amazing goodness inside of the pie.
The next comparison is the cell cytoplasm. The cytoplasm is the the liquid inside of a cell. It contains all of the other cell parts (if the cell is eurkaryotic). The cytoplasm could loosely be defined as containing anything inside of the cell membrane. If the cell membrane is the pie crust, then the cell cytoplasm is going to be the fruit filling of the pie.