What is the primary function of the cell membrane?

The primary function of the cell membrane is indeed to control the movement of substances in and out of the cell. It acts as a selective barrier, allowing essential nutrients and molecules to enter while keeping harmful substances out. This selective permeability is crucial for maintaining the internal environment of the cell, allowing it to function properly and maintain homeostasis.

Cell membranes are composed of a phospholipid bilayer with embedded proteins, which facilitate the transport of materials. By regulating what enters and exits, the cell membrane plays a crucial role in cellular processes, such as signaling, nutrient absorption, and waste removal.

The other functions mentioned—providing energy, maintaining shape, and producing proteins—are performed by other cellular structures. For example, energy is produced in the mitochondria, the cell's shape is maintained by the cytoskeleton, and proteins are synthesized in the ribosomes. Thus, the function of the cell membrane is distinct and fundamental to the overall operation of the cell.