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According to the fluid mosaic model for cell membranes, they are composed of a phospholipid bilayer. This bilayer is amphoteric, meaning both hydrophobic and hydrophillic. The interior of the membrane is made of non-polar, hydrophobic lipids while the exterior is made of polar, hydrophillic phosphate groups. As a result, non-polar hydrophobic chemicals are able to diffuse through the membrane. But there are numerous proteins and glycoproteins that are embedded in the membrane and traverse from the outside of the cell to the inside. These proteins can allow polar, charged species like ions to traverse the membrane, though it is not natural diffusion but assisted transport.
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