What structure selectively allows substances to pass into and out of the cell?

The cell membrane, also known as the plasma membrane, plays a critical role in maintaining the internal environment of the cell by selectively allowing substances to pass in and out. This structure is primarily composed of a phospholipid bilayer embedded with proteins that facilitate various functions, including transport. The selective permeability of the cell membrane is essential for regulating the movement of ions, nutrients, and waste products, ensuring that the cell maintains homeostasis.

The proteins within the cell membrane serve as channels or carriers that facilitate the movement of specific molecules, while the lipid bilayer acts as a barrier to most water-soluble substances. This selective nature ensures that essential compounds can enter the cell while preventing harmful substances from entering and allowing waste products to exit efficiently.

In contrast, the cell wall, found in plants, fungi, and bacteria, provides structural support and protection but is not involved in selective transport. The nucleus houses genetic material and is not involved in transport functions. Ribosomes are responsible for protein synthesis and also do not play a role in substance transport across the cell membrane. Thus, the cell membrane is the correct answer as it is responsible for the selective passage of substances in and out of the cell.