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The Functions of Proteins in Cell Membranes The plasma membrane of a cell is composed mostly of a lipid bilayer interspersed with cholesterol and proteins. According to the fluid mosaic model of the cell membrane, it is a dynamic structure of phospholipids free to move laterally, with proteins moving or fixed in the structure. The proportions of protein to lipid vary according to the membrane. For example in myelin, the lipids make up 76% and the proteins 18%, while in a mitochondrial inner membrane it is 24% lipid to 76% protein. While the lipids in a bilayer form a regular arrangement, the proteins do not, and different membranes have different varieties of protein molecules in different ratios. This means that each membrane can be specialised to the role it performs. The protein molecules in the membrane perform several different functions, such as cell surface receptor sites for hormones, enzymes involved in chemical...

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