Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Lipid Biosynthesis
Polar lipids integrate into cell membranes

Polar Lipids, which include the phosphoglycerols, Sphingolipids, and Glycolipids just discussed, are not stored in fat Cells but are instead incorporated into cellular membranes in strictly defined proportions. Phosphoglycerols, synthesized by Endoplasmic reticulum Enzymes, are embedded primarily into Cell/29.html">The Lipid Bilayer of the reticulum. The total surface area of The endoplasmic reticulum is particularly large in Liver and pancreatic cells. The membranes of the endoplasmic reticulum serve as precursors for Golgi apparatus membranes. Membrane vesicles continuously bud off from the Golgi apparatus, transporting secretory products to The Plasma Membrane (Fig. 21-19). These vesicles frequently fuse with the plasma membrane. Phosphoglycerols can also be transferred from the endoplasmic reticulum to Mitochondria via transport Proteins. Thus, a continuous flow of newly synthesized polar lipids is directed toward various types of cellular membranes within The Cell.

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Fig. 21-19. Following the completion of their synthesis, molecules of various polar lipids are incorporated into the lipid bilayer of cellular membranes in specific ratios. The bulk of polar lipids is integrated into the bilayer of endoplasmic reticulum membranes. These lipids are subsequently routed through the membranes of the Golgi apparatus, secretory vesicles, and the plasma membrane. With the aid of specific proteins, lipids from the endoplasmic reticulum are transported across the Cytosol and integrated into mitochondrial membranes. The pathway of Membrane Lipids is highlighted in red.