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These transmembrane proteins have regions that easily associate with water (i.e.

However, many other proteins extend their structures completely though the bilayer, crossing from one side to another. They may be attached by way of carbohydrate links, or be complexed with other proteins already embedded in the hydrophobic container. Some proteins associated with the cell membrane simply connect with one surface or other of the lipid bilayer. Proteins associated with a cell membrane, therefore, must be able to interact with both an aqueous, hydrophilic environment, and with the lipid, hydrophobic environment of the inner parts of the membrane. It would be useless to make a container of something that easily dissolved in water! Hydrocarbon molecules are strongly hydrophobic ("water fearing"), and it is this strongly hydrophobic layer of material that gives the cell membrane its "water proof" nature and allows it to act as a container for the cell and its contents. The bilayer of molecules that surround cells, however, is mostly made up of phospholipids arranged in such a way that their hydrocarbon "tails" are all pointing into the center of the structure. Proteins must therefore be hydrophilic ("water loving") in order to be suspended in this environment. The interior and the exterior of cells is liquid, usually a solution or suspension of ions, small molecules and large molecules dissolved in water. Highly specialized membranes, such as those found surrounding a mitochondrion, are more than 70% protein, whereas a human nerve cell in the arm or brain, has only slightly more than 20% protein in its structure. The quantity of protein in the average cell membrane varies considerably. This is the cell membrane.Īlthough the major component of the membrane, and that which gives it many of its properties, is a double layer of phospholipid molecules, the lipid bilayer, almost all the highly specific functions and properties of membranes are the result of actions and properties of proteins. All living cells are surrounded by a thin, complicated, flexible, waterproof, sensitive, and selfrepairing container that holds the cell together, allows it to grow, feeds it information and stops the contents from escaping.