What cellular structure acts as a barrier, allowing only certain molecules to enter or exit the cell?

The plasma membrane is crucial in maintaining the cell's internal environment while allowing for communication and interaction with the external surroundings. It is a selectively permeable barrier composed primarily of phospholipids and proteins. This structure facilitates the transport of necessary molecules into the cell, such as nutrients and ions, while keeping out potentially harmful substances. The selective permeability is achieved through various mechanisms, including passive transport, active transport, and endocytosis or exocytosis, depending on the size and charge of the molecules involved.

In contrast, the cell wall plays a protective role in plant cells, fungi, and some bacteria, providing structural support but is not involved in selective transport. The nuclear envelope surrounds the nucleus and regulates the entry and exit of materials between the nucleus and the cytoplasm, but it is not the primary barrier protecting the entire cell. The cytoskeleton, while important for maintaining cell shape and aiding in intracellular transport, does not function as a permeability barrier. Thus, the plasma membrane is the primary structure responsible for the selective exchange of substances, making it essential for cellular homeostasis and function.