The plasma membrane is selectively permeable. This means it allows some molecules to enter while keeping others out. An example of this can be seen in examining water and calcium in the cell. Water molecules are allowed to freely enter cells through the plasma membrane. Calcium molecules are only allowed into the cell at certain times and in certain amounts to prevent problems resulting from a buildup of calcium molecules. The plasma membrane uses the proteins embedded in its lipid bilayer to allow or block materials from entering the cell.