From WikiLectures

Phosphatidylcholine core

Phosphatidylcholines or lecithins form the most abundant group of phospholipids. They are derived from phosphatidic acid, to whose phosphate choline is bound.

Phosphatidylcholines are amphiphilic - part of their molecule (residues of higher fatty acids) is significantly non-polar, while the phosphate group and the rest of the choline are polar. This feature of phosphatidylcholines is significant for the structure of biological membranes (non-polar hydrophobic residues point to the center of the phospholipid bilayer, polar residues face the aquatic environment).

Most naturally occurring phospholipids have a saturated acyl residue in the sn-1 position and an unsaturated fatty acid in the sn-2 position of the glycerol.