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Cellular membranes are composed of a phospholipid bilayer. The phospholipids are monomers that are composed of a phosphate head and a lipid (hydrocarbon chain) tail. The head is a very polar region while the tail is a very non-polar region. These monomeric units bind together to form a polymeric layer. A cell membrane is composed of two of these phospholipid layers arranged such that the tail sections form the interior of the membrane and the head sections form the exterior of the membrane. This arrangement makes sense for a couple of reasons. One is that the lipid chains will have a natural hydrophobic interaction that will attract them together. Also, it makes sense that the polar regions would be on the exterior of the membrane since blood plasma is mostly water which is a highly polar substance as well.
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