Cell Membrane and Security
The cell membrane of a cell is like the security of an airport.
The cell membrane is a membrane separating the interior of the cell from the outside environment. It’s role is to control what goes in and out of the cell. The cell membrane allows some substances, like oxygen and water molecules, to enter while keeping larger molecules, like proteins, out. Therefore, it is called a “semi-permeable membrane”. The basic function of the cell membrane is to protect the cell from its surroundings. It supports, defends and holds the cell’s shape.
The security of an airport is similar to the cell membrane of a cell. It only allows people that have tickets to enter inside the terminal. Just like the cell membrane tries to prevent any harmful substances from entering the cell, airport security has scanners and detectors to prevent dangerous items and terrorists from entering the terminal. The airport security has a similar purpose as the cell membrane: it works to protect the airport from threatening events. Sometimes the airport security and cell membrane are unable to identify the substances and might allow harmful substances or items to enter the cell/airport.
So, the security of an airport is alike the cell membrane of a cell.