What cellular structure controls the movement of substances in and out of the cell?

The cell membrane, also known as the plasma membrane, is the structure responsible for controlling the movement of substances into and out of the cell. It is composed of a phospholipid bilayer with embedded proteins that facilitate the transport of specific molecules, thereby maintaining the appropriate internal environment of the cell. This selectively permeable membrane allows essential nutrients to enter the cell while keeping out harmful substances, and it also plays a crucial role in cellular communication and signaling.

The other structures listed serve different functions: the nucleus houses the cell’s genetic material and controls cellular activities, ribosomes are involved in protein synthesis, and the cell wall provides structural support and protection in plant cells but does not regulate transport. Therefore, the cell membrane is essential for maintaining homeostasis within the cell and is rightly identified as the control point for substance movement.