Lipid bi-layer

The lipid bi-layer provides a basic impermeable barrier around cells, and is approximately 5nm thick. They help to maintain different environments between cytosol and the extracellular environment/organelles, and also control the transport of solutes between compartments. The lipid bilayer consists of four major phospholipids which are; phopshatidylethanolamine, phosphatidylserine, phosphatidycholine and sphingomyelin. These are derived from glycerol apart from sphingomyelin, which is derived form serine. Phosphatidylserine is the only one negatively charged .

The fluidity of the membrane can be altered depending on the amount of cholesterol in the plasma membrane.  It slots in tightly between the hydrocarbon chains of the phoshoplipids and prevents them from packing together, so at high temperatures, the membrane is less fluid .