Which type of lipid is NOT a part of the plasma membrane?
Biological membrane formation is governed by amphipathic geometry where polar headgroup cross-section balances hydrophobic tail volume, satisfying packing parameter close to 1 for bilayer assembly and stability. Phospholipids such as phosphatidylserine, phosphatidylcholine, phosphatidylethanolamine, sphingolipids like sphingomyelin and cerebroside, and cholesterol as small amphiphile intercalated among chains all meet this criterion and readily incorporate into 5 nanometer fluid mosaic film with integral proteins diffusing laterally. Triglycerides, triacylglycerols composed of glycerol esterified to three fatty acids lacking phosphate, sugar, or charged moiety, are completely apolar with no amphipathic character, unable to orient with one end in water. In aqueous environment they coalesce into bulk oil phase, phase separating rather than forming lamellae. Cells store triglycerides in lipid droplet organelles comprising neutral hydrophobic core surrounded by phospholipid monolayer decorated with perilipins and seipin, located in cytosol or within ER lumen for VLDL assembly in liver. Adipocytes contain large droplets for energy reserve mobilized by hormone-sensitive lipase and lipolysis. Therefore plasma membrane excludes triglycerides, which would compromise barrier integrity and fluidity.
Ref: Lehninger Principles of Biochemistry, 8th ed., Chapter 10: Triglycerides vs Membrane Lipids.