It's the cell membrane so basically enclosing the entire cell. And everything found inside that cell is a semipermeable phospholipid bilayer, a cell membrane that plays a role in cell transport. It basically allows things into the cell and out of the cell. It also functions in cell communication as well as protecting the cell from outside sources of harm. Now, other types of eukaryotic cells, not all, but some eukaryotic cells, for example, plant cells, also contain an additional second protecting barrier known as the cell wall. The cell wall protects plant cells and it also gives them the shape and structure.