Most cells store an excess of lipids in lipid droplets, from where they can be retrieved as building blocks for membrane synthesis or as a food source. Fat cells, or adipocytes, are specialized for lipid storage. They contain a giant lipid droplet that fills up most of their cytoplasm. Most other cells have many smaller lipid droplets, the number and size varying with the cell’s metabolic state. Fatty acids can be liberated from lipid droplets on demand and exported to other cells through the bloodstream. Lipid droplets store neutral lipids, such as triacylglycerols and cholesterol esters, which are synthesized from fatty acids and cholesterol by enzymes in the endoplasmic reticulum membrane. Because these lipids do not contain hydrophilic head groups, they are exclusively hydrophobic molecules, and therefore aggregate into three-dimensional droplets rather than into bilayers.
Lipid droplets are unique organelles in that they are surrounded by a single monolayer of phospholipids, which contains a large variety of proteins. Some of the proteins are enzymes involved in lipid metabolism, but the functions of most are unknown. Lipid droplets form rapidly when cells are exposed to high concentrations of fatty acids. They are thought to form from discrete regions of the endoplasmic reticulum membrane where many enzymes of lipid metabolism are concentrated. Figure 10–14 shows one model of how lipid droplets may form and acquire their surrounding monolayer of phospholipids and proteins.
the Asymmetry of the lipid Bilayer Is Functionally Important
The lipid compositions of the two monolayers of the lipid bilayer in many membranes are strikingly different. In the human red blood cell (erythrocyte) membrane, for example, almost all of the phospholipid molecules that have cho-line—(CH3)3N+CH2CH2OH—in their head group (phosphatidylcholine and
Figure 10–13 a model of a raft domain.
Weak protein–protein, protein–lipid, and lipid–lipid interactions reinforce one another to partition the interacting components into raft domains. cholesterol, sphingolipids, glycolipids, glycosylphosphatidylinositol (gpI)-anchored proteins, and some transmembrane proteins are enriched in these domains. note that because of their composition, raft domains have an increased membrane thickness.We discuss glycolipids, gpI-anchored proteins, and oligosaccharide linkers later. (Adapted from D. lingwood and K. Simons, Science 327:46–50, 2010.)
Figure 10–14 a model for the formation of lipid droplets. neutral lipids are deposited between the two monolayers of the endoplasmic reticulum membrane. there, they aggregate into a three-dimensional droplet, which buds and pinches off from the endoplasmic reticulum membrane as a unique organelle, surrounded by a single monolayer of phospholipids and associated proteins. (Adapted from S. Martin and r.g. parton, Nat. Rev. Mol. Cell Biol. 7:373–378, 2006. With permission from Macmillan publishers ltd.) sphingomyelin) are in the outer monolayer, whereas almost all that contain a terminal primary amino group (phosphatidylethanolamine and phosphatidylserine) are in the inner monolayer (Figure 10–15). Because the negatively charged phosphatidylserine is located in the inner monolayer, there is a significant difference in charge between the two halves of the bilayer. We discuss in Chapter 12 how membrane-bound phospholipid translocators generate and maintain lipid asymmetry.