Cells don't treat them differently because they can't tell the difference. I don't think that was her question. It seems that Anuja wants to know WHY cells don't distinguish between isotopes. I guess the question makes sense, too, because it seem that with all their specificity, they'd be able to distinguish between two molecules with slightly different mlc.
Cells can distinguish between charge and stuff.
Usually when we use P32, we label only a part of a much larger molecule. A radiolabelled sugar has a mass of 181, or an increase of one-half of one percent. You may be right to be skeptical, but decades of experimental evidence argues that radiolabelling does little to perturb a biological process. Obviously you would use C14 instead of P32 for labelling glucose. The rest of the argument holds.