Plasma membrane: the membrane that surrounds the cell. Other membranes around: nucleus, mitochondria, golgi apparatus. Membranes are selectively permeable some things can go in and some things can go out . Lipids and proteins are in the membrane (they also contain carbohydrates). Lipids and proteins are bond together in a way that makes up a fluid mosaic model; because it is a fluid model it is flexible. It is very thin 8 x 10-9 (8,000 membranes = thickness of one paper page). Lipids = phospholipids that have a hydrophilic end and a hydrophobic end. Some glycoproteins serve as identification tags that are specifically recognized by membrane proteins of other cells. This type of cell-to-cell binding is usually short-lived compared to that shown in intercellular joining: intercellular joining: Membrane proteins of adjacent cells may hook together in various kinds of junctions (see textbook figure 6. 32).