What structure in the cell is also known for regulating the passage of substances into and out of the cell?

The cell membrane, also known as the plasma membrane, plays a critical role in regulating the movement of substances into and out of the cell. This semi-permeable barrier is composed primarily of a phospholipid bilayer with embedded proteins that facilitate the transport of molecules. The selective nature of the cell membrane allows it to maintain homeostasis by controlling the internal environment of the cell, ensuring that essential nutrients can enter while waste products and harmful substances are kept out or expelled.

The proteins within the cell membrane can function as channels, carriers, or receptors, contributing to its regulatory capabilities. The membrane also has properties that enable it to respond to changes in the external environment, which further aids in the proper functioning of the cell.

In contrast, structures like the cytoplasm, ribosome, and nucleus serve different primary functions. The cytoplasm is the gel-like substance where various cellular components reside, ribosomes are responsible for protein synthesis, and the nucleus houses the genetic material and is involved in regulating gene expression. Each of these structures plays vital roles in cellular function, but none has the primary regulatory function of substance passage that the cell membrane does.