Question

why plasma membrane must be bilayer​

Answer 1

Plasma membrane is a selectively permeable membrane and it is not bilayer I think so

1 plasma membrane above it plants have cell wall which animals don't have..

Answer 2

the primary function of the plasma membrane is to protect the cell from its surroundings composed of a phospholipid bilayer with embedded proteins with embedded proteins the plasma membrane is selectively permeable to ions and organic molecules and regulate the movement of substances in or out of cells.