Practice question

Membrane lipid synthesis predominantly occurs in endoplasmic reticulum where fatty acid activation, glycerol-three-phosphate acyltransferase and phospholipid synthases reside on cytosolic face of ER membranes. Phosphatidylcholine formed via Kennedy pathway using choline, phosphatidylethanolamine, phosphatidylserine interconverted by decarboxylase at mitochondria associated membranes, phosphatidylinositol synthesized from CDP-DAG. These lipids transfer to growing membranes via vesicular transport and lipid transfer proteins at contact sites. Golgi then converts PC to sphingomyelin and synthesizes complex glycosphingolipids. Nucleolus is membrane-less condensate containing fibrillarin, nucleolin and rDNA repeats dedicated to ribosome production, lacking acyltransferases, absent from lipid synthesis pathways, nucleoplasm similarly lacks lipid anabolic enzymes confined to cytosolic leaflet of ER. Radiolabeled glycerol first incorporates into ER lipids then later Golgi plasma membrane, proving precursor-product relationship. Therefore assigning phospholipid synthesis to nucleolus misplaces metabolism; true site is ER network providing bulk phospholipids for expansion during cell growth and division, regulated by lipin and CCT enzymes. Integration with cell cycle kinases, calcium signaling and mechanical cues ensures coordinated remodeling during growth, migration and differentiation.