 Figure 1: Basic lipid structure. A lipid consists of a polar head group (P) and a nonpolar tail (U for unpolar). 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 to represent lipids with one, two, or three chains. Lipids are fatty acid esters, a class of relatively water-insoluble organic molecules, which are the "basic" components of biological membranes. There are three forms of lipids: phospholipids, steroids. and triglycerides.
Lipids consist of a polar or hydrophilic (attracted to water) head and one to three nonpolar or hydrophobic (repelled by water) tails (Fig. 1). Since lipids have both functions, they are called amphiphilic. The hydrophobic tail consists of one or two (in triglycerides, three) fatty acids. These are unbranched chains of carbon atoms (with the correct number of H 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.
For lipids present in biological membranes, the hydrophilic head is from one of three groups:
- Glycolipids, whose heads contain an oligosaccharide with 1-15 saccharide (sugar) 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, and the tails are turned towards a hydrophobic region of another molecule. With lots of lipids present, the tails "prefer" to turn toward each other, forming a hydrophobic region. This can be a bilayer or a micelle (Fig. 2). Micelles are spheres and can only reach a certain size, whereas bilayers have no limit to their extension. They can also form tubules. Bilayers which fold back upon themselves form a hollow sphere, enclosing water in a separate aqueous compartment.
 Figure 2: Self-organization of lipids. Driven by hydrophilic and hydrophobic forces, the nonpolar tails of lipids (U) tend to cluster together, forming a lipid bilayer (1) or a micelle (2). The polar heads (P) face the aqueous environment.
The self-organisation depends on the concentration of the lipid present in solution. Below the critical micelle concentration the lipids form a single layer on the liquid surface and are dispersed in solution. At the first critical micelle concentration (CMC-I), the lipids organise in spherical micelles, at the second critical micelle concentration (CMC-II) into elongated pipes, and at the lamellar point (LM or CMC-III) into stacked lamellae of pipes.
The CMC depends on the chemical composition, mainly on the ratio of the head area and the tail length.
Lipid bilayers form the foundation of all biological membranes.
See also
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