Figure 19.2 schematically depicts the primary pathways of prokaryotic and eukaryotic glycerophospholipid biosynthesis. Note that the center pathway shown in purple occurs in both prokaryotic and eukaryotic cells. Phosphatidic acid, the branch point between the synthesis of fats and other glycerophospholipids, can be made via three different pathways in eukaryotes-from glycerol-3-phosphate (Figure 19.3), from diacylglycerol (Figure 19.2), and from dihydroxyacetone phosphate (DHAP) (see here).

DHAP can also give rise to the ether phospholipids (Figure 19.2), which include a class of molecules called plasmalogens (Figure 19.12) and platelet-activating factor. Plasmalogens are abundant in heart tissue and their deficiency leads to problems with brain, liver, kidney, and an early death. Platelet activating factor (see here) at very low concentration exerts numerous physiological effects, including inflammation, reduction of blood pressure, and stimulation of uterine contraction.

Phosphatidylcholine (PC) can be synthesized in eukaryotes starting from choline (Figure 19.5) or from phosphatidic acid via CDP-diacylglycerol, phosphatidyl serine (PS) and phosphatidylethanolamine (PE). The interconversion of PS, PE, and PC is shown here.

Phospholipid fatty acids may be redistributed, after they are synthesized, in response to varying environmental conditions or needs. Lung surfactant is one such phospholipid with redistributed fatty acids. Lung surfactant contains some 50% to 60% of dipalmitoylphosphatidylcholine, a form of phosphatidylcholine in which palmitoyl chains occupy both positions 1 and 2 on the glycerol. Phospholipids are usually synthesized with an unsaturated fatty acid on position 2, so some modification has likely occurred. Phospholipases may be involved in the modification, because they can remove fatty acids from phospholipids.

Lysolecithin is another name for 1-acylglycerophosphorylcholine, which is phosphatidylcholine with the fatty acid at position 2 removed. Lysolecithin is an excellent detergent, capable of solubilizing membranes, and causing cells (such as erythrocytes) to lyse.

Finally, Figure 19.2 shows that CDP-diacylglycerol serves in eukaryotes as a branch point in the synthesis of phosphatidyl-inositol, yet another major phospholipid found in membranes.