Some substances but not other can cross the (blank) membrane of a cell

The correct answer is :

Some substances but not other can cross the (blank ) membrane of a cell. The   blank is to be filled with either cell or plasma. Cell membrane or plasma membrane is the living boundary of a cell which is made of phospholipid. It is a semi-permeable or differentially permeable membrane. It only allows small and non-polar molecules to pass through it. It prevents polar ions or molecules to pass through it  

yo