What is the primary function of the cell membrane?

• A. It stores genetic information
• B. It powers the cell's activities
• C. It controls what enters and leaves the cell
• D. It organizes ribosomes

Answer: (C)

Regulating what enters and leaves the cell is the cell membrane’s main job. It acts as a selective barrier, made of a phosopholipid bilayer with proteins that form channels, carriers, and pumps. This setup lets in essential nutrients, allows wastes to exit, and enables signals to be received, while keeping harmful substances out. By controlling the internal environment, the membrane helps maintain homeostasis and supports communication with other cells through receptors and signaling molecules.

Storing genetic information is the job of DNA housed in the nucleus, not the membrane. The cell’s energy for activities comes primarily from mitochondria producing ATP, and ribosome organization occurs at the nucleolus (and on the rough endoplasmic reticulum), not at the cell membrane.