The cell. Cell membrane.
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Lipids are in charge of the physical properties of cell membranes, but also play a role in chemical processes.
Functions: determine cell membrane structure and fluidity, help with electrochemical gradients, may work as second messengers, participate in generating membrane heterogeneity and lateral segregation of proteins.
They are about 50 % of the plasma membrane weight.
Types: glycerophospholipids, sphingolipids and sterols.
Cell membrane organization mostly depends on lipids. Proteins, however, are not much involved in structural functions but in other membrane functions, which may be regulated by the interactions with lipids. Lipids are in charge of physical features of membranes. Length and saturation of their fatty acids regulate fluidity and thickness of membranes. Electric charges located in the lipid heads help to generate the electrical gradient between the external and internal monolayers, modulating the membrane potential. Furthermore, lipids may work as second messengers, leaving membranes to diffuse to intracellular compartments and trigger cellular responses. The lateral heterogeneity of cell membranes are thought to be caused by lipid-lipid interactions, creating small domains of segregated lipids with higher density and containing a higher proportion of certain type of lipids and proteins. These regions are known as lipid rafts.
Lipids make up around 50 % of the plasma membrane weight, about 5 millions of lipids per µm2 of membrane. There are more than 1000 types of lipids distributed trough the different membranes of a eukaryotic cell, with particular proportions depending on the membrane. About 5 % of the genes of a cell are related to lipid synthesis. Below, we will deal with the more abundant lipids.
Glycerophospholipids or phosphoglycerides
Glycerophospholipids are the more abundant type of lipids in cell membranes. Structurally, they contain 3 domains: two fatty acid chains, glycerol, and phosphoric acid. Fatty acid chains are 13 to 19 carbon atoms in length. Most of the carbon-carbon bonds are simple, which are referred as saturated bounds. More than half of the fatty acids contain at least one doble carbon-carbon bond, which is referred as unsaturated. The double bounds make fatty acid chain to be bent and, although rotation of these chains is restricted, the increase of unsaturated fatty acids makes membranes more fluid, because the lipids are more separated between each other. Fatty acids constitute the hydrophobic (water phobia or water flee) part of membranes. Glycerol links the two fatty acid chains with the hydrophilic (affinity for water) part of the lipid, which may be ethanolamine, choline, serin, glycerol, inositol 4,5-bisphosphate. The different name of glycerophospholipids depends of these head molecules. For example, phosphatidylethanolamine contains ethanolamine and is up to 50 % of the glycerophospholipids of the eukaryotic membranes.
Sphingolipids contain a molecule of sphingosine, a nitrogenated alcohol with a large carbon chain, to which a fatty acid chain is bound. These two form a basic molecule known as ceramide (see figure), which shows a molecular structure similar to glycerophospholipids: two hydrophobic chains and one hydrophilic head. Most of glycolipids, which are those containing sugar bound to the hydrophilic domain, are sphingolipids. Sphingomyelin, other type of sphingolipid, contains one ethanolamine or one phosphorylated choline in the hydrophilic head. Sphingolipids are more abundant in the plasma membrane than in organelles, and are proposed, together with cholesterol, as main players in segregating membrane molecules into domains such as lipid rafts.
Cholesterol is the most important sterol in animal cell membranes, and the third most abundant type of lipids in plasma membraneof animal cells , whereas it is scarce in organelle membranes. Cholesterol is not present in plant cell membranes, in some unicellular eukaryotes, nor in bacteria. However, these cells contain other types of sterols. Cholesterol is located among the fatty acid chains of other lipids and is important for the organization of membranes because, together with sphingolipids, contributes to create membrane domains (lipid rafts). Furthermore, changes in the concentration of cholesterol modulate membrane fluidity. It also participates in metabolic processes such as synthesis of steroid hormones and bile salts.
Janmey PA, Kinnunen PKJ. 2006. Byophisical properties of lipids and dynamic membranes. Trends in cell biology. 16:538-546.
van Meer G, Voelker DR, Feigenson GW. 2008. Membrane lipids: where are they a how they behave. Nature reviews in molecular cell biology. 9:112-124.
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Updated: 2016-04-01. 13:14
Atlas of Plant and Animal Histology
Dep. of Functional Biology and Health Sciences.
Faculty of Biology.
University of Vigo