What is the function of a cell’s selectively permeable plasma membrane?

the function of plasma membrane or cell membrane is to protect the internal cell contents from external environment and to maitain the balance between cell & their surrounding.

Explanation:palsma membran also known as selectively permeable membrane is made of phospholipid bilayer in which protein molecules are embeded and these proteins molecules act as receptors,pumps and as channels. and through these channels certain ions, molecules ,enzymes are passing throug it by the process of osmosis and diffusion.So, plasma membrane is not only working to protect the cell from their external surroundings but also keeps checks & balance on the molecules movement inside & outside of the cell.