As I know-
You proceed from the 1. security check before entering to airport (In Turkey,all airports have those.)
At check-in, your luggage is controlled-not opening, by sticking a sticker.- and after it you check in it.
Then you proceed from passport control and 2. security control.
FOR FLIGHTS TO UK
You pass from a boarding pass-visa control,then you can enter to boarding lounge.
FOR FLIGHTS TO USA
You need to pass from a third security check.