Why do cells have a plasma membrane?

to separate proteins from the nucleus

to distinguish plants from animals

to define the size of the cell

to separate the inside of the cell from its environment

To separate the inside of the cell from its environment.

The cell membrane is primarily made up of lipids and proteins. It provides protection to the cell by keeping the interior of the cell separated from its external environment.  

It is a selectively permeable membrane meaning that it only allows certain substances to pass through it. It is involved in various cellular processes like cell signaling, cell adhesion, endocytosis, exocytosis etc.

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