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,