What types of molecules make up biological membranes?
Both the cell surface membrane and the membranes surrounding certain organelles have the same basic structure. Much of the membrane is made up of a ‘sea’ of phospholipids with protein molecules ‘floating’ in between the phospholipids. Some of these proteins span the whole width of the membrane. Because the membrane is fluid, and because of the mosaic arrangement of the protein molecules, the structure of the membrane is called the fluid mosaic model. The phospholipids are arranged in two layers (a bilayer). The phosphate heads are polar molecules and so are water-soluble. The lipid tails are non-polar and therefore are not water-soluble. This means that the phospholipids are arranged with the heads in contact with the cytoplasm or extra-cellular fluid, both of which are watery environments. The tails are protected from this, by being as far from the cytoplasm and extra-cellular fluid as possible. The proteins in the membrane, line pores in the lipid bilayer. The polar groups of the pro
