What structure in a cell acts as a barrier and regulates entry and exit of substances?

The correct answer is the cell membrane, which is crucial for maintaining the integrity and function of the cell. The cell membrane, also known as the plasma membrane, serves as a selective barrier that regulates the movement of substances into and out of the cell. It is composed of a phospholipid bilayer with embedded proteins that facilitate various functions, including transport, signaling, and communication.

This selective permeability allows the cell to maintain a stable internal environment, or homeostasis, by controlling which molecules can enter or exit, thus ensuring that essential nutrients can enter and waste products can be removed. Additionally, the cell membrane plays a role in protecting the cell from harmful substances in the external environment.

In contrast, the nucleus primarily functions as the control center of the cell, housing genetic material and regulating cellular activities. The cell wall provides structure and support in plant cells and some microorganisms but does not selectively regulate the passage of substances like the cell membrane. The cytoplasm is the gel-like substance inside the cell where various cellular processes occur, but it does not act as a barrier controlling substance movement.