What is the main function of the cell membrane?

The main function of the cell membrane is to regulate the movement of substances in and out of the cell. This selective permeability is crucial, as it allows the cell to maintain a stable internal environment, also known as homeostasis. The cell membrane is composed of a phospholipid bilayer embedded with proteins, cholesterol, and carbohydrates, which all contribute to its ability to control what enters and exits the cell.

Through various mechanisms, such as passive transport, active transport, and facilitated diffusion, the cell membrane selectively allows nutrients, ions, and waste products to move while preventing harmful substances from entering. This function is essential for maintaining the cell's metabolic processes, ensuring that it has the right balance of ions and molecules to function effectively.

Other functions mentioned in the options, such as energy production, protein synthesis, or storage of genetic information, do not pertain to the cell membrane itself. These processes occur in specific organelles or structures within the cell, rather than being a primary role of the cell membrane. Thus, regulating substance movement truly captures the essence of what the cell membrane does.