Phospholipids are similar to the triglycerides with a couple
of exceptions. Phospholglycerides are esters of only two fatty
acids, phosphoric acid and a trifunctional alcohol - glycerol
(IUPAC name is 1,2,3-propantriol). The fatty acids are attached
to the glycerol at the 1 and 2 positions on glycerol through
ester bonds. There may be a variety of fatty acids, both saturated
and unsatured, in the phospholipids.
The third oxygen on glycerol is bonded to phosphoric acid
through a phosphate ester bond ( oxygen-phosphorus double
bond oxygen). In addition, there is usually a complex amino alcohol
also attached to the phosphate through a second phosphate ester
bond. The complex amino alcohols include choline, ethanolamine,
and the amino acid-serine.
The properties of a phospholipid are characterized by the
properties of the fatty acid chain and the phosphate/amino alcohol.
The long hydrocarbon chains of the fatty acids are of course
non-polar. The phosphate group has a negatively charged oxygen
and a positively charged nitrogen to make this group ionic. In
addition there are other oxygen of the ester groups, which make
on whole end of the molecule strongly ionic and polar.
Phospholipids are major components in the lipid
bilayers of cell membranes.
There are two common phospholipids:
Lecithin contains the amino alcohol, choline.
Cephalins contain the amino alcohols serine or ethanolamine.