The main purpose of the cell membrane is to function as a selective barrier that regulates the movement of substances in and out of the cell. Comprised primarily of a phospholipid billyer, proteins, and carbohydrates, it maintains the homeostasis of the cell environment. By controlling the flow of ions, nutrients, and waste products, it facilitates communication between cells and their surroundings. This selective permeability is essential for processes like signal transduction and energy production, ensuring that the cell maintains its essential functions.