What enclosures the protoplasm of a cell and regulates substance entry and exit?

The cell membrane, also known as the plasma membrane, is a crucial structure that encloses the protoplasm of a cell. It plays an essential role in maintaining the integrity of the cell by creating a barrier that separates the internal environment from the external surroundings. This semi-permeable layer is composed of a phospholipid bilayer interspersed with proteins, which facilitates the selective entry and exit of substances, such as nutrients, ions, and waste products.

By regulating what enters and exits the cell, the cell membrane helps maintain homeostasis, ensuring that the internal conditions remain stable and conducive to cellular functions. Additionally, it is involved in cell signaling and communication with other cells. The presence of various proteins within the membrane also allows for specific transport mechanisms, which assist in the movement of larger or charged molecules that cannot simply diffuse through the lipid bilayer.

Other structures mentioned, such as the nucleus, cytoplasm, and Golgi apparatus, serve different functions within the cell, such as housing genetic material, providing a medium for chemical reactions, or processing and packaging proteins, but they do not perform the critical role that the cell membrane does in regulating movement in and out of the cell.