Final answer:
The outer layer of a cell which acts as a gatekeeper to the cell is called the Cell membrane. It forms the physical boundary between the cell and its environment, protects and supports the cell, and regulates the passage of materials into and out of the cell.
Step-by-step explanation:
The outer layer of a cell which acts as a gatekeeper to the cell is called the Cell membrane. All living cells are surrounded by a cell membrane, which forms the physical boundary between the cell and its environment. It protects and supports the cell and regulates the passage of materials into and out of the cell through specialized proteins and lipid bilayers.