Unfortunately I think we will need some more details.First of all, from what I can tell from your link, the XBEE module does not have an I2C interface. From what I read (between the lines) it appears that these two modules will be connected to a host micro.It also seems that this micro will chose to when top read the temperature and when to transmit the data. However the XBEE blurb says that you can program the on-board micro, so perhaps this is where you are going. If you can clarify this, it would be easier to suggest a solution.As I understand it, all you are looking for is how to turn the sensor and the Xbee on and off. The two URLs you provide do not detail the current draw of each device. I would think that you would want to turn each one of these peripherals on independently (or if the XBEE has a sleep function then you only need to control the temperature sensor). The simplest would be to control the power to the temp. sensor through a PNP transistor, but we still need to know the temp. sensor's current in order to be more specific. If you don't have a sleep mode on the XBEE or if you are using an external micro will affect the rest of the circuit. Let us know.