The membrane is like a filter because a filter has to let all the bad things go away, and leave all the good things. So, if you think about it, the cell membrane controls what goes in and out of cell. So, no bad stings can go in, and no good things can leave. Think of the cell membrane like the people at the front desk at your school, that let good people in, and keep all the bad people out. Hope this helps ((:
