Lipids are fatty acid esters, a class of water-insoluble organic molecules, that are used as basic building blocks of biological membranes as well as for energy storage (e.g., triglycerides). Lipids consist of a polar or hydrophilic (attracted by water) head and one to three unpolar or hydrophobic (repelled by water) tails (Fig. 1). Since lipids have both functions, they are called amphipathic. The hydrophobic tail consists of one or two (triglycerides: three) fatty acids. These are chains of carbon atoms, which are connected by single bonds alone (saturated fatty acids) or by both single and double bonds (unsaturated fatty acids). The chains are usually 14-24 carbon groups long.
Figure 1 : Basic lipid structure. A lipid consists of a polar head group (P) and an unpolar tail (U). The lipid shown is a phospholipid (two tails). The image on the left is a zoomed version of the more schematic image on the right, which will be used from now on. The schematic image will represent lipids with one, two and three chains.
For lipids present in biological membranes, the hydrophilic head is from one of three groups. They are :
- Glycolipids, whose heads contain an oligosaccharide (sugar) with 1-15 saccharide residues.
- Phospholipids, whose heads contain a positively charged group that is linked to the tail by a negatively charged phosphate group.
- Sterols, whose heads contain a planar steroid ring, for example cholesterol (only in animals).
In an aqueous environment, the heads of lipids are turned towards the environment, whereas the tails are turned towards a hydrophobic region of another molecule. With lots of lipids present, it is likely for the tails to turn towards each other, forming a hydrophobic region where they touch. This can be a bilayer or a micelle (Fig. 2). Micelles have the shape of a sphere and can only reach a certain size, whereas bilayers have no extension limit.
Figure 2 : Self-organization of lipids.
Driven by hydrophilic and hydrophobic forces, the unpolar tails of lipids (U) tend to cluster together, forming a lipid bilayer (1) or a micelle (2). The polar heads (P) are exposed to the aqueous environment.
Biological membranes, such as the plasma membrane, are constructed from lipid bilayers. Membranes separate two aqueous environments by creating a hydrophobic border between them.They conceal the interior of the cell from its environment, as well as separating organelles (specialized compartments in eukaryotes) from the cytosol (the cell's internal fluid). Liposomes (small hollow spheres made of a lipid bilayer) are used by organisms to transport water-soluble molecules. Due to this property of liposomes, they have been evaluated as delivery systems for drugs, vitamins and cosmetic materials.