Lipids and membranes

Lipids belong to a large class of naturally occurring fat-soluble molecules: ranging from waxes, oils, fatty acids, sterols like cholesterol, fat-soluble vitamins e.g. A, D, E and K, (mono-, di-, tri-) glycerides, phospholipids etc. They are the main components of cell membranes and also function as energy storage and in some cases as vital signalling molecules in biochemical processes in the cell. Lipids are amphiphilic molecules, which means that they have a hydrophobic (water hating) moiety, composed of either carbon chains or aromatic rings, and a hydrophilic (water loving) moiety, composed of sugars, hydroxyl, amino or acids groups etc.

When lipids are placed in an aqueous environment the properties of both the hydrophilic and hydrophobic parts of the lipid on how the molecule will behave in solution. In pure water the individual water molecules are loosely connected in a network of hydrogen bonds. The hydrophobic part of lipid molecules is not able to form hydrogen bonds to the water molecules present, therefore the hydrogen bonding network in water suffers and the stability of the bonding network decreases. Because the lipid molecules have both a hydrophilic and a hydrophobic part the lipid molecules will clump together in such a way that their hydrophilic head groups will face the aqueous interface thus shielding their hydrophobic tails from the water molecules – a phenomenon called the hydrophobic effect. This clumping or self-assembly of lipid molecules in an aqueous environment forms a whole array of different lipid structures depending on the physical conditions present as shown below.

Several factors affect which lipid structures are formed: the overall shape of the lipid molecule, the concentration of lipid molecules present and in some cases the pH of the aqueous solution. Micelles are usually formed of conical shaped lipids or as inverted micelles if the solution is hydrophobic. If the lipid molecule is cylindrical liposomes and vesicles can be formed. These are made up of a double layer of lipid molecules referred to as a bilayer. Liposomes and vesicles can either be uni-lamellar composed of one bilayer or multi-lamellar composed of several bilayers stacked within each other. In order to study these structures fluorescent dyes can be added to the vesicles and liposomes before examining them in a confocal microscope.