Structural and Molecular Biology for Chemists - Biological Inorganic Chemistry - page 57

FIGURE 3.25 An illustration of how the configuration of glycosidic bonds determine polysaccharide structure and function. The b -1 e 4
linkages in cellulose favourise straight chains, optimal for structural purposes, whereas the a -1 e 4 linkages are favourable to bent structures,
better adapted to storage in a hydrated form.

Lipids and Biological Membranes
Lipids 11 are fat-soluble molecules, unlike the other biological macromolecules we have described up to now,
which are all water-soluble in addition, they are not macromolecules, but of relatively low molecular weight.

With the exception of a class of lipids (for reasons of Commissions of Nomenclature 12 known as simple lipids,
despite the organic complexity of their structures

e

they include steroids, isoprenoids, etc.), they are complex
lipids. By definition, they are saponifiable; in other words, upon treatment with an alkali, they produce a soap,
containing an alcohol, classically glycerol, and other products, usually fatty acids. Fatty acids typically have an
even number of carbon atoms (reflecting their synthesis from acetyl CoA, Chapter 5), often 16 or 18 and may be
saturated, like stearic acid (mp 69.6 C) or have one (or more) double bonds, like oleic acid (mp 13.4 C)
( Figure 3.26 a ). However, the important message to get across here is that the properties of the lipid will be
determined by the nature of the fatty acids that it contains. A simple example from everyday life is the difference
between lard (animal fat) and olive oil. Lard, at 20 C is solid, yet olive oil at the same temperature, is liquid

e

e
why? Both are triglycerides ( Figure 3.26 b), but whereas the former consists of glycerol esterified to three
molecules of saturated fatty acids (typically stearic acid), in the latter, glycerol is esterified to three molecules of
oleic acid, with a single double bond. Triglycerides are a very efficient reservoir of energy resources, not only
because they can produce twice as much energy/g than carbohydrates or proteins, but also because they can be
stored in an anhydrous form. This is in marked contrast to glycogen, which binds about twice its weight in water.
Animals have cells which are specialised in the synthesis and storage of triglycerides, adipocytes, with the fat
globules occupying almost the entire cell.
Having established that the simplest 'complex' lipid is a triglyceride, we pass to the biological properties of
phospholipids which combine the fat-soluble properties of the triglyceride with the addition of a polar, charged
group on one of the glycerol hydroxyl functions ( Figure 3.26 c ). Several alcohols are found in phospholipids, but we
consider here the glycerophospholipids. The polar head group is made up of a phosphate group, attached to

11. From Greek lipos e fat.
12. There are two classes of sins e those of omission and of comission. When it comes to scientific nomenclature, these are clearly sins of
Commission!