What structure serves as the boundary between the cell and its external environment?

The cell membrane, also known as the plasma membrane, serves as the boundary between the cell and its external environment by providing a selective barrier. It is composed of a phospholipid bilayer with embedded proteins, which allows it to regulate what enters and exits the cell. This selective permeability is crucial for maintaining homeostasis, allowing essential nutrients to enter while keeping harmful substances out. Additionally, the cell membrane plays a key role in communication and signaling with other cells through receptor proteins, aiding in cellular responses to the external environment.

In contrast, other structures mentioned in the choices serve different functions. The nucleus is responsible for storing genetic material and controlling cellular activities, while the cell wall is a rigid structure found in plant cells, providing support and protection but not directly functioning as the boundary for all cells. The cytoplasm is the gel-like substance within the cell that contains organelles and is involved in cellular processes but does not act as a boundary.