Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Membranes Biological membranes surround cells and serve to keep the insides separated from the outsides. They are formed of phospholipid bilayer1 s, which by definition are a double layer of fatty acid2 molecules (mostly phospholipid3 s, lipids containing lots of phosphorus). Proteins4 serve very important functions in cellular membranes. They are active transports in and out of the cell, acting as gatekeepers. They relay signals in and out of the cell. Proteins are the site of many enzymatic reactions in the cell, and play a role in regulation of cellular processes.


Question: What is it that surrounds cells and serve to keep the insides separated from the outsides?
biological membranes
Biological membranes.