What cell structure controls the entry and exit of substances?

The cell membrane, also known as the plasma membrane, is the structure responsible for regulating what enters and exits a cell. It is a semi-permeable barrier made up of a lipid bilayer with embedded proteins that facilitate the movement of substances. This allows the cell to maintain homeostasis by controlling the internal environment, managing the exchange of ions, nutrients, and waste products crucial for its survival and function.

In contrast, the cytoplasm is the gel-like substance within the cell that supports the organelles but does not itself control substance movement. The nucleus serves as the control center of the cell, housing genetic material and overseeing cellular activities, but does not play a direct role in the transport of substances. The Golgi apparatus is involved in processing and packaging proteins and lipids for transport but does not regulate their entry or exit across the cell membrane. Thus, the cell membrane is essential for the selective permeability that sustains cellular integrity and function.