A cell is separated from its environment by a selectively permeable plasma membrane. The plasma membrane is commonly described as a fluid mosaic. It is like a mosaic and having diverse protein molecules embedded in a matrix of phospholipids. Its component proteins and phospholipid molecules can move about, so it is quite fluid and flexible. The phospholipids in a membrane form a two-layer framework called a phospholipid bilayer. The steroid cholesterol helps stabilize the phospholipids. Carpohydrate molecules are attached to some of the molecules that make up the membrane. Review membrane structure by labeling this diagram. Click on each box to select from a pull-down menu. The phospholipid bilayer is a two-layer framework called a phospholipid bilayer. The phospholipid bilayer is a two-layer framework called a phospholipid bilayer.