How is a cell set off as a separate unit from it's environment?

The structure of the cells reveal the presence of the fluid called cytosol or cytoplasm separated by a membrane known as cell membrane or plasma membrane.

The cytoplasm have a different environment as compared to outside. The different environment inside the cell is maintained by the presence of the plasma membrane, the plasma membrane does not allow the mixing of inside and outside environment of the cell, hence, maintain the different environment.

At the same time the plasma membrane is semipermeable and allow few molecules to pass in the cell, in order to support the proper functioning of the cell.