What is a Lipid? Tuesday, Oct 25 2011
For most people who have heard it, the word lipid sounds like the scientific word for fat. Although scientists tend not to use the word fat, and do tend to use the word lipid, the two do not quite match-up. When we say fat in the context of food, we think of cooking or olive oil, butter, grease and possibly cream. Perhaps confusingly, all of these have lipids in them, but none of them are pure lipids.
The confusion about these strange molecules is probably related to the fact that they are hard to pin down. There is virtually an art form in isolating them. Only in the last 50 years have we really started to understand them, or their role in biology. The thing that makes lipids what they are, is that they are what is known as amphiphilic. This is from the Greek ‘amphi-’, meaning ‘both’, and ‘‑philic’, meaning ‘loving’. Although this sounds like an intellectual’s way of referring to a bisexual person, here the ‘both’ refers to water and grease. As we know, water and grease do not mix: if we pour oil onto water, the two just sit there. Lipids are special because they can dissolve in both of them. This property can even be used as a way of mixing grease and water together. Homogenous mixtures of grease and water with another agent are known as emulsions. Examples include milk, cake batter and margarine. The emulsion as a construct is useful in the manufacture of foods, giving rise to a plethora of agents that are able to perform this role, namely emulsifiers. Although in theory all lipids can be emulsifiers, not emulsifiers all are lipids.
We can see the roots of the amphiphilic behaviour in the structure of the lipid molecules. Moreover, structural analysis of molecules can help us determine whether or not a molecule is a lipid at all, and if so, which sort. Figure 1 shows an ordinary lipid with the head (water-loving) and tail (grease-loving) regions marked out.
The structural approach has been used by several institutions in recent years, including Lipid Maps. This has given rise to a broader definition of the term lipid. Thus, a variety of compounds not traditionally referred to as lipids are included, even if they are not measurably amphiphilic. However, when these ‘sort-of lipids’ are inserted into lipid systems whose physical behaviour is well understood, their influence on the character of the system is measurable. This influence of the added component is also described as concentration-dependent, i.e., the more of the molecule that is put in (the higher the concentration), the stronger the given effect.
Some of the most amazing and unexpected behaviour of lipids occurs when they are exposed to water. As part of the molecule likes water, and part of it does not, you can imagine that this situation is not going to be simple for the poor little lipid. We as scientists describe the stresses and strains in systems like these according to the laws of thermodynamics. Those of a certain generation may like to remind themselves of the Flanders and Swann song at this point. Either way, we have put an unsuspecting lipid into a system with water and so what does it do? Generally, lipids will self-assemble. The exact manner in which this occurs varies between types of lipid, but the principle is observable across all of them.
The principle of self-assembly can be explained in terms of two competing priorities. Thermodynamics means that the grease-loving (lipophilic) part of the lipid molecule wants to be shielded from the water, but at the same time, the water-loving (hydrophilic) part wants to be in touch with the water. As our lipid cannot bear to disobey the laws of thermodynamics (who would?), it arranges itself in order to compromise between these competing forces. This compromise can also be described in terms of energy. In order to adopt a position in which the greasy section is exposed to water requires a lot of energy. When the energy that is available to the system is insufficient for this, exposing the greasy section of the lipid to the water is described as energetically unfavourable. This is the thermodynamic law I refer to and is also known as the hydrophobic effect. The latter term, as you might well guess, is from the Greek ‘hydro-’ meaning ‘water’ and ‘‑phobic’ meaning ‘hating’.
The hydrophobic effect may sound obscure, but the forces involved in it are what hold the membranes of cells together. This means it has a huge and unsung part to play in understanding biology.
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