Chapter 10: Lipid bilayers contain: 1) Phospholipids a. Phosphoglycerides b. Sphingolipids c. Inositol phospholipids 2) Glycolipids 3) Cholesterol Phospholipids are the most abundant membrane lipids The main phospholipids in eukaryotes are phosphoglycerides (3C-backbone and two long-chain fatty acid chains) Many different phosphoglycerides can be made by combining several different fatty acids and head groups. Most abundant ones are phosphatidylethanolamine, phosphatidylserineand phosphatidylcholine in mammalian cell membranes. Another class of phospholipids are sphingolipids (sphingosine i.o. glycerol. NH2 and twohydroxyl groups at one end). Sphingomyelin is the most abundant in mammals. A minor class of phospholipids are inositol phospholipids. In the cytosolic leaflet of the plasma membrane they play an important part in cell signalling: in response to extracellularsignals, specific lipid kinases phosphorylate the head groups of these lipids to form dockingsites for cytosolic signalling proteins. Cholesterol is a sterol. It contains a rigid ring structure, to which a single polar hydroxylgroup and a short nonpolar hydrocarbon chain are attached. Glycolipids resemble sphingolipids, but i.o. a phosphate-linked head group they havesugars attached. Bilayers are spontaneously formed by phospholipids since the hydrophobic tails pointinwards. Another possibility is that spherical micelles are formed lOMoAR cPSD|5961093 ALW Liposomes: Spherical lipid bilayer vesicles Since individual phospholipids are confined to their own monolayer, newly made phospholipids from the cytosolic monolayer of the ER membrane would not be able to migrate. This is solved by phospholipid translocators, or flippases, which catalyse the flip-flop of phospholipids from one monolayer to the other Lipid rafts: Small region of a membrane enriched in sphingolipids and cholesterol The lipid bilayer is asymmetrically charged. This is exploited to distinguish between live anddead cells, as ell as converting extracellular signals into intracellular ones. Glycolipids havethe most extreme asymmetry, as they are only found in the monolayer facing away from thecytosol. Gangliosides are the most complex of the glycolipids. Their presence alters the electrical field across the membrane and the concentrations of ions at the membrane surface. Besides, they function in cell-recognition processes. Lumen: The space inside a hollow structure Carbohydrate layer: The carbohydrate-rich zone on the eukaryotic cell surface attributableto glycoproteins, glycolipids, and proteoglycans of the plasma membrane. Lectin: Protein that binds tightly to a specific sugar. Abundant lectins from plant seeds areused as affinity reagents to purify glycoproteins or to detect them on the surface of cells. By its affinity for lectins, the carbohydrate layer can be visualized.