Phospholipids are a class of lipids and are a major component of all cell membranes. Most
phospholipids contain a diglyceride, a phosphate group, and a simple organic molecule such
as choline. They are similar to fats, but have only two fatty-acids rather than three (see Fig 1-2).
The third hydroxyl group is joined to a phosphate group, which is negative in electrical charge
and is therefore soluble in water. Phospholipids are described as amphipathic (or amphiphilic)
molecules, having both a hydrophobic and a hydrophilic region. The two fatty acid tails which
consist of hydrocarbons are hydrophobic and are excluded from water. Their heads, however
which consist of the phosphate group and its attachments, are hydrophilic and have an affinity
for water. Since the two fatty-acid chains are insoluble in water (hydrophobic), they are thought
to project from the glycerol chain in a direction opposite to that taken by a polar group. When
many phospholipid molecules are placed in water, their hydrophilic heads tend to face water and
the hydrophobic tails are forced to stick together, forming a bilayer (see Fig 1-3)
The phospholipids differ among themselves in the identity of the fatty acids or of the
polar group or both. In phosphoglycerides, a principal class of phospholipids, glycerol forms the
backbone of the molecule, two fatty acid chains are esterified to two of the three hydroxyl groups
in glycerol, and the third hydroxyl group is esterified to phosphate (see Fig 1-5a). The phosphate
group can also be esterified to a hydroxyl group on another hydrophilic compound, such as
serine, ethanolamine, choline, glycerol, and the inositol. The structural formulas of phosphatidyl
choline and the other principal phosphoglycerides—namely, phosphatidyl ethanolamine,