Answer:
The job of the cell membrane in a eukaryotic or prokaryotic cell is to act like a "screen door" that allows certain molecules & substances to pass through it more easily than others. We call this feature "selectively permeable." The cell membrane is made up of mainly a bilayer of phospholipids. Typically, due to the very hydrophobic interior of a cell membrane, hydrophobic, small molecules can pass through very easily. Hydrophilic but small molecules such as water, however, can also pass through with relative ease.
