Cell membrane does maintain the shape of a cell...but this is not its major function! a cell membrane protects the interior contents present in the cell from spilling out into the external environment! it also protects the cell from harmful substances present in the external environment from entering into the cell. Amoeba forms different shapes while feeding...you might have noticed this...without a cell membrane it would become clumpsy n like a fluid without any purpose in its living! cell membrane maintains shape, rigidity n also protects the cell thereby helping it in its survival!! hope ur doubt has got cleared nw!
So, the cell membrane major function is it protects the external structure of the cell.