Purpose
The cell membrane surrounds the cell, letting some things in and keeping others out (the process of diffusion). It also gives the cell some structure, and holds it in place.
Function
The cell membrane is a thing membrane surrounding the cell. It's purpose is much like that of the case for your computer: to keep somethings out, let other things in, and give some structure to the cell. (The other part of the cell responsible for structure is the cell wall.)
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