User blog:Commander VB/Mass Effect 3 story understanding

just want to know, at the ending of mass effect 3 the child says that it is the catalyst and the citadel is a part of it, so the catalst controls the reapers, does it "the citadel" and if so, i can understand that but i dont understand is that sovereign was to bring about the next revolution of the cycle, and serve as the harbinger for the return of its kind by sending a signal to the keepers on the citadel. This would make the keepers open the citadel relay, a mass relay that leads to dark space, and usher the Reapers through. but if the catalyst controls the citadel why did it need sovereign to send the signal to the keepers to open it? and sence the protheans blocked that signal sovereign had to go and activate the citadel and turn it into a relay himself but why not just have the catalst do it?