What are the functions of a cell membrane?
Overall, the cell membrane allows division of the intracellular fluid(inside the cell) and the extracellular fluid(outside the cell).
Hello guy from the Philippines. I'm also from Philippines. Anyhow, let us proceed to your question.
It functions actually as a semipermeable barrier of the cell. What I mean by semipermeable is it allows some molecules to enter/exit the cell. Take note of the word some.
Think of the cell membrane as a fishing net. The fishing net has holes in order to let the small fishes get out/in and the big fishes to not get out. In this illustration, the big fishes are the substances not allowed to get into the cell and the small fishes are the substances needed by the cell and thus is allowed to go in/out.
Here are a couple of videos to help with this concept.
Hope this helped! :)