The cell membrane has many important functions. One function is to act as a protective barrier for the cell. The cell membrane is because it controls movement of particles that move in or out of the cell. When particles move across the cell membrane without using energy this is called These particles move from an area of high concentration to an area of low concentration in an attempt to create an equilibrium (balance). This movement of particles is known as the from an area of high concentration to an area of low concentration this is called When all other particles/molecules move from an area of high concentration to an area of low concentration it is called