When lipids are together they spontaneously form a lipid bilayer due to their amphiphatic nature. Lipids consist of a 'head' and a 'tail' section. The 'tail' is hydrophobic and will group together in the center of the bilayer and the hydrophilic heads will form the edges of the bilayer protecting the 'tails' from contact with water. There are 3 main classes of lipids used in the forming of a lipid bilayer for function a s a membrane: phospholipids; glycolipids and cholesterol. The lipid mostly used for membranes is the phospholipid. In all lipids the 'tail' is formed from hydrocarbon chains, varying in length and saturation. In phospholipids the 'head' is formed from a phosphate attached to an alcohol and a 'platform' either glycerol or sphingosine, giving the lipid it's hydrophillic properties.
The lipid bilayer is seen as the universal basis for membrane structure. It can be seen using electron microscopy. However, to reveal the details of its organisation, specialised techniques such as freeze-fracture electron microscopy and and x-ray diffraction are required .
All lipids are not soluble in water. Both triglycerides and phospholipids are lipids but the major difference between the two molecules is the number of fatty acids which varies between them with triglycerides possessing three fatty acids unlike phospholipids in which one of the three fatty acids is replaced by a phosphate group. The phosphate group is electrically charged and attracted to water thereby, forming the hydrophilic region(attracts water) on phospholipids while the two fatty acids form the hydrophobic region(repels water). These properties make the phospholipid molecules very essential in the cell membrane. The cell membrane is made up of a double layer of phospholipid molecules and this structure is as a result of the fact that a group of phospholipid molecules arranges itself into a bilayer when present in water, with the hydrophilic region (i.e head) facing outward, interacting with the watery environment while the hydrophobic region(i.e tail) faces inward thereby, avoiding the watery environment. The cell membrane is also made up of proteins, cholesterol molecules. glycolipids(phospholipids + short chains of carbohydrates) and glycoproteins( proteins + short chains of carbohydrates). The structure of the cell membrane is called "The Fluid Mosaic Model".
'Fluid' because of the motility of its components within their own layers, 'mosaic' because of the dotted nature of the protein molecules within the membrane and 'model' because, nobody has ever seen the membrane the way it is on diagrams due to the small nature of its components but the structure has been worked out because it explains the behaviour of the membrane .