What cell structure is responsible for regulating the passage of substances in and out of the cell?

The cell structure responsible for regulating the passage of substances in and out of the cell is the cell membrane. This membrane is a selectively permeable barrier that controls the entry and exit of various molecules, ions, and nutrients, thus maintaining the internal environment of the cell. It consists of a phospholipid bilayer embedded with proteins that facilitate transport processes such as diffusion, osmosis, and active transport.

The nucleus, while important for storing genetic information and coordinating activities such as growth and reproduction, does not directly regulate substance passage. The cytoplasm is the jelly-like fluid that contains the organelles and is where many metabolic processes occur, but it does not control what enters or exits the cell. Similarly, ribosomes are responsible for protein synthesis, playing a vital role in translating genetic information into proteins, but they do not participate in the regulation of substance movement across the cell membrane. Thus, the function of the cell membrane is essential for maintaining homeostasis within the cell.