Zener diode- to protect- BJT

Could any body please help me with the following problem:
As shown in the attached picture I have 2 networks, network-1 and network-2. Network-1 charges the capacitor to positive voltage levels and network-2 charges it to negative voltage levels.
the network-1 must be seprated from the capacitor for the time in which the network-2 is working. I have selected a BJT as a switch to break the path when network-2 is working.
(Transistor must be off during the negative charge period)

Question:
1-since the capacitor is charged to positive voltages is a protecting zener diode necessary?when yes how should it be connected?
2- Or a zener diode is required because of negative voltages on the capacitor?when yes how should it be connected?

Note: care must be taken that the zener diode doesn't provide an unwanted current path while charging to negative voltages(no current path through this BJT).

How can I solve this problem? or any other idea?

thanks a lot in advance
Picture:http://www.4shared.com/photo/2zBEC-aJ/_2__question_zener.html
http://www.4shared.com/photo/h5FuYjVy/question_zener.html