Which organelle's main function is to control what comes into and out of the cell?

• A. Nucleus
• B. Cytoplasm
• C. Cell Membrane
• D. Ribosome

Answer: (C)

Movement of substances into and out of the cell is controlled by the cell membrane. It acts as a boundary that decides which molecules can pass through and which cannot. The membrane is a phospholipid bilayer with proteins that form channels and pumps, giving it selective permeability. This setup lets small, nonpolar molecules diffuse through more easily, while ions and larger molecules need special transport proteins or energy to cross. By regulating traffic, the cell membrane helps maintain a stable internal environment and communication with its surroundings. The other organelles have different roles—like the nucleus storing DNA and directing activities, or ribosomes making proteins—so they don’t control what crosses the cell’s boundary.