Role of phosphatidic acid in the synthesis of glycerol

1. What is the role of phosphatidic acid in the synthesis of glycerol containing phospholipids? Is it limited to this role or does it also play a role in the synthesis of triacylglycerols? What about sphingosine containing lipids? What role does phosphatidic acid phosphatase play in this?
2. What is the source of the glycerol used in the synthesis of membrane containing phospholipids?

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