<P> By convention, structures of these compounds show the 3 glycerol carbon atoms vertically with the phosphate attached to carbon atom number three (at the bottom). Plasmalogens and phosphatidates are examples . </P> <P> In general, glycerophospholipids use a "sn" notation, which stands for stereospecific numbering . When the letters "sn" appear in the nomenclature, by convention the hydroxyl group of the second carbon of glycerol (sn - 2) is on the left on a Fischer projection . The numbering follows the one of Fischer's projections, being sn - 1 the carbon at the top and sn - 3 the one at the bottom . </P> <P> The advantage of this particular notation is that the spatial conformation (R or L) of the glycero - molecule is determined intuitively by the residues on the positions sn - 1 and sn - 3 . </P> <P> For example sn - glycero - 3 - phosphoric acid and sn - glycero - 1 - phosphoric acid are enantiomers . </P>

Which of the following is not part of a glycerophospholipid