Final answer:
The plasma membrane is the structure that forms the boundary between the inside of the cell and the outer environment, functioning as a selective barrier.
Step-by-step explanation:
The structure that forms the boundary between the inside of the cell and the outside environment is the plasma membrane, also called the cell membrane. This phospholipid bilayer acts like the "skin" of the cell, serving as a protective barrier that controls entry and exit of substances, thus maintaining the cell's integrity and composition. It is crucial for selective permeability, allowing certain molecules to pass through while keeping others in or out, ensuring that the cell's environment is selectively regulated. The plasma membrane's structure is essential to its function, comprised not only of phospholipids but also of cholesterol, which contributes to its fluidity, and proteins that have a variety of functions and can be found on the membrane's surface or embedded within it.