PhosphatidyicholineThe two hydrocarbon lipid tails, here both eighteen carbons long, may
be of varying lengths. The head group contains a phosphorus atom,
multiple oxygen atoms, and a nitrogen atom that carries a positive
charge. The positive charge comes from an electron deficit arising
from the nitrogen having four bonds with other atoms, rather than its
customary three. This configuration is called a quaternary amine.
The peculiar structure of phospholipids, with their distinct
hydrophilic and hydrophobic portions, gives rise to a remarkable
phenomenon when these lipids are present in an aqueous (water)
environment. When surrounded by water, phospholipids will cluster
together such that the hydrophobic/lipophilic tail groups associate
with one another and the hydrophilic/lipophobic head groups asso-
ciate with one another. Moreover, the hydrophilic head groups want
to be in contact with the water environment and the hydrophobic tail
groups want to be protected from contact with the water environ-
ment. To accomplish this, they form a double layer of phospholipid
molecules (Fig. 3.2): the hydrophilic head groups form the exterior
surfaces (in contact with one another and in contact with the water
environment), and the hydrophobic tail groups are inside the layers
(in contact with one another and shielded from contact with the
water environment by the hydrophilic heads).