What would happen if the cell membrane were completely made of a polar substance?

A cell membrane has a lipid bilayer meaning that it is selectively permeable - allowing only certain ions to enter the cell. Thus maintaining the unique cellular composition which is different from the extracellular environment.

A cell membrane completely made of a polar substance would be completely permeable to all ions present in the extracellular environment, thus disrupting unique ionic composition within the cell.

Answer:done

The cell would be unable to maintain a stable internal environment.