Which cellular structure is selectively permeable and regulates the entry and exit of substances?

The plasma membrane is the structure that is selectively permeable and is crucial for regulating the entry and exit of substances in and out of the cell. It is primarily composed of a phospholipid bilayer with embedded proteins, which allows it to control the movement of ions, nutrients, and waste products. This selective permeability is essential for maintaining the cell's internal environment, or homeostasis, and it enables the cell to respond to changes in its external environment.

The other structures mentioned do not perform this function. The nucleus, for example, is surrounded by a nuclear envelope that regulates the passage of materials but not in the same manner as the plasma membrane does for the entire cell. The cell wall, found in plants, fungi, and some bacteria, provides structural support and protection but is not selectively permeable in the way the plasma membrane is. Cytoplasm refers to the jelly-like substance within the cell where organelles are located and does not play a direct role in the selective regulation of substance movement.