Here's why:

* Function: The plasma membrane acts as a barrier, controlling what enters and exits the cell. It's crucial for maintaining the cell's internal environment and carrying out essential functions.

* Structure: It's composed of a phospholipid bilayer, with embedded proteins and other molecules. This structure allows for selective permeability, meaning only certain substances can pass through.

* Location: It's the outermost layer of the cytoplasm, directly interacting with the cell's surroundings.

While some cells may have additional layers outside the plasma membrane (like cell walls in plants), these are not considered part of the cell's "living" structure.