Answer:
The cell membrane, also known as the plasma membrane, is a thin, semipermeable membrane that surrounds the cytoplasm of a cell and separates it from the external environment. Its main function is to control the movement of substances into and out of the cell, as well as to maintain the cell's internal environment. The cell membrane is composed of a phospholipid bilayer, with proteins and carbohydrates embedded in it.
The cell membrane works by having molecules move from an area of higher concentration to an area of lower concentration. This movement is caused by the molecules having kinetic energy, which causes them to move around randomly. As they move, the molecules spread out, causing the concentration of the molecules to decrease in the area they are moving away from and to increase in the area they are moving towards. The cell membrane is semipermeable, meaning that it allows certain molecules to pass through it, while blocking other molecules. The molecules that are allowed to pass through the cell membrane must be small enough, and have the right type of charge, in order for them to fit through the phospholipid bilayer.