The cell membrane is often referred to as a “”? because of the two layers of phospholipids.

A Phospholipid Bilayer
The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer. As shown in the Figure below, each phospholipid molecule has a head and two tails

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