Which part of the cell controls the passage of materials into and out of the cell?

• A. Golgi apparatus.
• B. Plasma membrane.
• C. Ribosome.
• D. Nuclear membrane.

Answer: (B)

The gate that controls what moves between the cell and its surroundings is the plasma membrane. This boundary is a phospholipid bilayer with embedded proteins that act as channels, carriers, and pumps, giving the membrane selective permeability. Small nonpolar molecules can diffuse through; other substances require specific transport proteins or energy to move in or out (active transport). For large particles, the cell can engulf or release them via endocytosis and exocytosis. This setup keeps internal conditions stable while still letting essential nutrients in and wastes out. The other organelles don’t regulate exchange with the outside environment: the Golgi apparatus packages and ships materials inside the cell, ribosomes build proteins, and the nuclear membrane controls traffic with the nucleus.