As we saw in Chapter 20, fats are esters of glycerol, with long-chain
fatty acids having 10 to 20 carbon atoms in their chains. Tristearin,
the most common animal fat, is shown to the right. The three chains
on one glycerol molecule do not have to be identical; mixed fats
are possible. Vegetable fats from plants generally have short chains
that are unsaturated (with C=C double bonds) or polyunsaturated
(with several C=C bonds). The double bonds, which almost invariably
are in the cis configuration, introduce kinks into the chains. Because
these short, bent chains do not pack together well in a solid, vegetable
fats tend to be oily liquids. In contrast, animal fats have longer,
saturated fatty acid chains that pack efficiently to form waxy solids
such as lard and tallow. Oily vegetable fats can be "hardened"
by hydrogenation, that is, by adding H2, across
the double bonds with a catalyst, thereby producing saturated chains
with the kinks straightened out.
It was mentioned in Chapter 20 that an alcohol can be esterified
with an inorganic acid just as well as with a carboxylic acid. If
glycerol is esterified with two fatty-acid molecules and one molecule
of phosphoric acid, the result is phosphatidic acid (centre button),
the simplest phospholipid. This can be thought of as a
"super fatty acid." Long-chain alcohols are partly hydrophobic
and partly polar but uncharged (left button), fatty acids are half
hydrophobic and half negatively charged, and phosphatidic acid has
twice the hydrocarbon chains and twice the negative charge per molecule.
These phospholipids are the raw materials of membrane structures.