What structure is primarily responsible for controlling the movement of substances in and out of a cell?

The cell membrane, also known as the plasma membrane, plays a crucial role in regulating the movement of substances into and out of the cell. It is composed of a phospholipid bilayer with embedded proteins, which create a semi-permeable barrier that allows selective passage of molecules. This means that certain substances can cross the membrane while others are kept out, thereby maintaining the internal environment of the cell.

The cell membrane is also involved in various transport mechanisms, such as passive transport (diffusion and osmosis) and active transport, which require energy. Additionally, it has receptor sites that allow the cell to respond to external signals, facilitating communication between the cell and its environment.

In contrast, while the nucleus is responsible for storing genetic information and coordinating activities such as growth and reproduction, the cell wall is primarily found in plant cells and provides structural support, rather than controlling substance movement. The Golgi apparatus is involved in modifying, sorting, and packaging proteins and lipids for secretion or delivery to various destinations in the cell, but it does not directly control the movement of substances across the membrane.