The biosynthesis of membrane phospholipids occurs through distinct pathways in mammals and bacteria. In the mammalian pathway for the synthesis of phosphatidylethanolamine, ethanolamine is first phosphorylated, activated to form CDP-ethanolamine and then reacted with diacylglycerol to form phosphatidylethanolamine. Phosphatidylcholine is synthesized in mammals by a similar route. Phosphatidylserine is synthesized through exchange of the polar head group with phosphatidylethanolamine. By contrast, in the E.coli pathway, the three main phospholipids are formed from each other in sequence starting from phosphatidylserine, then progressing to phosphatidylethanolamine and phosphatidylcholine. Phosphatidylserine is decarboxylated to form phosphatidylethanolamine, to which three methyl groups are added to form phosphatidylcholine.