Fosso lipids are the major component of plasma membranes. Fosso lipids are created when a phosphate group links two fatty acid tails to a non-lippid head. Don't forget that fatty acids are lipids or fats and therefore non-polar so they don't like water, where the polar head does like water. A biiler is made of two layers of these phospholipids. Each layer has a non-lippid or water loving head on the outside and the lipid or water fearing tails on the inside. This inside layer of tails will not associate with water or charge molecules so the membrane acts as a selectively permeable barrier to prevent water flow or ion movement. However, molecules that can be dissolved in lipids like oxygen and carbon dioxide can pass through. This membrane is what allows the extracellular and intracellular environments to be so different even though they are both mostly water.