Thus, both of them are similar as they maintain “order” in the cell/airport, by controlling what goes in and out of the cell/airport.

Another similarity is that they only allow 'authorized' substances into the cell/airport. The cell membrane only allows certain proteins and ions to enter the cell, blocking harmful substances like bacteria from entering. Similarly, the security guards and airplane ticket checkers only allow the people who have tickets inside the terminal, and have scans to ensure that no harmful items and terrorists enter the terminal.

Also, no matter what precautions are taken, the cell membrane is sometimes unable to identify and successfully block bacteria from entering due to the way the bacteria camouflage themselves. Similarly, the security guards and airplane ticket checkers are unable to recognize or identify terrorists and block them. Both of these lead to the endangerment of the cell.

(EXTRA SIMILARITY: both the cell membrane and the security guards/ticket checkers are made of lipids, phosphates and proteins!)




DIFFERENCES! 


[Significance of this picture: the glass entrance gates at the Airport departure gate acts as the "cell membrane" in this case, allowing only certain people who are leaving the country to enter while the others have to remain outside; controlling the "substances" entering the departure gate (cell)]

However, there are differences between the cell membrane and the security guards/ticket checkers/entrance-exit gates as well.

One such difference is that although the cell membrane is fluid and flexible, the ticket checkers and entrance-exit gates are not. The entrance-exit gates are fixed at one spot, and the ticket checkers are positioned around them, hardly moving anywhere else.

Another difference is that although the cell membrane has two layers, security guards, ticket checkers and entrance-exit gates do not.

Also, the cell membrane allows substances to enter the cell from any area. However, the entrance-exit gates are positioned at one fixed spot, and everyone has to enter by that area and not anywhere else.