Abacterium will construct different proteins to metabolize the sugars lactose or glucose, depending on which one it detects in the outside environment. what part of the bacterium allows it to recognize different substances in the outside environment?

Just like other cells, the part of the bacterium that is able to interact with its outside environment is its cell membrane. Generally, the main function of the cell membrane is for barrier and protection. This decides which molecules will be allowed to permeate or not. Hope this helps.