A typical cell has a layer of negative charge on the inner surface of the cell wall and a layer of positive charg on the wall outside surface, thus making the cell wall a capacitor.

What is the capacitance of a 50 um diameter cell with a 7.o nm thick cell wall whose dielectric constant is 9.0?

Because the cell's diameter is much larger than the wall thickness, it is reasonable to ignore the curvature of the cell and think of it as a parallel-plate capacitor.