They are single cell organisms and they had limited source of energy. So, it is very important for them to use the limited energy in very efficient manner. They can’t take the risk of wasting energy running constitutive gene expression which is quite not necessary at particular set of environmental conditions.
If we talk about, lac operon. let's say, in first set of environmental condition, i am getting glucose in the medium directly, so it will be total waste of my energy if i turn the operon on ( In the presence of glucose, the halts production of the enzymes encoded by the lac operon, the (CAP), required for production of the enzymes, remains inactive, and shuts down lactose permease to prevent transport of lactose into the cell). In the second case, when there is no direct availability of preferable energy sources, like glucose, and there is only lactose in the medium, the lac operon gets turn on. The three genes of the lac operon can be expressed and their subsequent proteins translated: lacZ, lacY, and lacA. The gene product of lacZ is which cleaves lactose, a disaccharide, into and . lacY encodes , a protein which becomes embedded in the cytoplasmic membrane to enable transport of lactose into the cell. Finally, lacA encodes