What will happen to a cell if its cytoplasmic mambrana loses the property of selective permeability?

The cell is not permeable to all substances, but only to those that it needs at the moment. Some (small, non-polar molecules) move freely along the concentration gradient, while others require special carrier proteins to transport.

If the cell loses its selective permeability, then all substances in a row will begin to enter it, including harmful and metabolic products that should be removed from it. In the cell, biochemical processes and balance of ions, impulse conduction (if we are talking about nerve cells) are disrupted. Due to excess water flow, it will burst.

Ultimately, the cell will die.