The cell membrane performs several important functions in addition to separating the cytoplasm from the outside environment. It controls which molecules enter and leave the cell. The cell membrane is “selectively permeable,” which means it allows certain substances through and not others. One way that substances enter and leave the cell is through diffusion, the movement of particles from higher concentration to lower concentration. For example, if the concentration of salt molecules is higher inside the cell than outside, salt particles will move out of the cell. Diffusion is a type of passive transport, a movement of particles that does not require energy to occur. Facilitated diffusion is a kind of passive transport where proteins in the cell membrane help particles move from high to low concentration. The cell membrane functions most like a?