IN PLASMA, TWO THIRD OF PHOSPHORUS OCCUR AS PHOSPHOLIPIDS. PHOSPHOLIPIDS ARE THE MAIN LIPID CONSTITUENTS OF MEMBRANES. THE PHOSPHOLIPIDS INCLUDE THE FOLLOWING: (1) PHOSPHATIDIC ACID AND PHOSPHATIDYLGLYCEROL, (2) PHOS-
PHATIDYLCHOLINE, (3) PHOSPHATIDYLETHANOLAMINE, (4) PHOSPHATIDYLINOSITOL, (5) PHOSPHATIDYLSERINE, (6) LYSOPHOSPHOLIPIDS, (7) PLASMALOGENS, AND (8) SPHIN-
GOMYELINS.
PHOSPHATIDYLCHOLINES (LECITHINS) OCCUR IN CELL MEMBRANES.
THESE ARE PHOSPHOACYLGLYCEROLS CONTAINING CHOLINE. THEY ARE THE MOST ABUNDANT PHOSPHOLIPIDS OF THE CELL MEMBRANE AND REPRESENT A LARGE
PROPORTION OF THE BODY'S STORE OF CHOLINE. CHOLINE IS IMPORTANT IN NERVOUS TRANSMISSION, AS ACETYLCHOLINE, AND AS A STORE OF LABILE METHYL GROUPS. DIPALMITOYL LECITHIN IS A VERY EFFECTIVE SURFACE-ACTIVE AGENT AND A MAJOR CONSTITUENT OF THE SURFACTANT PREVENTING ADHERENCE, DUE TO SURFACE TENSION, OF THE INNER SURFACES OF THE LUNGS. ITS ABSENCE FROM THE LUNGS OF PREMATURE INFANTS CAUSES RESPIRATORY DISTRESS SYNDROME. IT IS TO BE NOTED THAT MOST PHOSPHOLIPIDS HAVE A SATURATED ACYL RADICAL IN THE SN-1 POSITION BUT AN UNSATURATED RADICAL IN THE SN-I POSITION OF GLYCEROL.