As is the case for the assembly of protein components of the membranes in animal 
mitochondria, the bilayer phospholipids arise from a complicated interplay of 
intra- and extra-mitochondrial reactions. Our early studies indicated that the 
bulk of mitochondrial phospholipids (typified by phosphatidylcholine) had their 
origin in the endoplasmic reticulum and were transported to the mitochondria as 
complexes with phospholipid-exchange proteins. The polyglycerophosphatides 
(typified by diphosphatidylglycerol) were apparently synthesized in situ by 
intramitochondrial membrane-bound enzymes using CDP-diglycerides as 
intermediates. The case for the precursors in the latter pathway is less clear, 
although evidence has been presented for dual localization of enzymes for 
glycerophosphate acylation and CTP:phosphatidate cytidylyl transfer in both 
mitochondria and microsomes. Phosphatidylethanolamine also shows evidence for 
two sites of origin: by translocation from its site of synthesis in the 
endoplasmic reticulum and by translocation of phosphatidylserine followed by 
decarboxylation within the mitochondria. In the latter case mitochondrial 
phosphatidylserine decarboxylase may play an important role in the regulation of 
phospholipid metabolism throughout the cell.