If the cell membrane were made of only polar substances, a cell would

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.

if the cell membrane was only made of polar substances then the membrane would be purely permeable which will affect the stability of the cell and afterwards result in the instability of the cell and will finally affect the existence  of that cell. This will lead to the improper functioning of the life processes of the organism.