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Phospholipid: A lipid that has an ester link between phosphoric acid and an alcohol (either glycerol or sphingosine). Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 1 • The glycolipids are also derived from sphingosine. They contain no phosphate group, but have an attached carbohydrate that is a monosaccharide or a short chain of monosaccharides. • The classes of membrane lipids overlap. • Glycolipids and sphingomyelins both contain sphingosine and are therefore classified as sphingolipids, • Glycerophospholipids and sphingomyelins both contain phosphate groups and are therefore classified as phospholipids. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 2 Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 3 Glycerophospholipids with a phosphate ester link to the amino alcohol choline are known as phosphatidylcholines, or lecithins. They are emulsifying agents, substances that surround droplets of nonpolar liquids and hold them in suspension in water. You will find lecithin listed as an ingredient in many foods where it is added to keep oils from separating out. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 4 Sphingomyelins are sphingosine derivatives with a phosphate ester group at C1 of sphingosine. The sphingomyelins are major components of the coating around nerve fibers (the myelin sheath) and are present in large quantities in brain tissue. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 5 Cerebrosides, glycolipids which contain a monosaccharide, are particularly abundant in nerve cell membranes in the brain, where the sugar is D-galactose. They are also found in other cell membranes, where the sugar unit is D-glucose. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 6 • The polar heads and hydrophobic tails of phospholipids and glycolipids allow them to form membrane bilayers that act as barriers separating the interior of cells from the environment. • Glycolipids extend their carbohydrate segments into the fluid surrounding the cells. They function as receptors that are essential for recognizing chemical messengers, other cells, pathogens, and drugs. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 7 23.6 Cell Membrane Lipids: Cholesterol • Animal cell membranes contain significant amounts of cholesterol. Cholesterol is a steroid, a member of the class of lipids that all contain the same four-ring system. In human biochemistry, the major functions of steroids other than cholesterol are as hormones and as the bile acids that are essential for the digestion of fats and oils in the diet. • The body of a 60 kg person contains about 175 g of cholesterol that serves two important functions: as a component of cell membranes and as the starting material for the synthesis of all other steroids. Some cholesterol is obtained from the diet, but cholesterol is also synthesized in the liver. An adult can make about 800 mg of cholesterol per day. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 8 Cholesterol is a nearly flat molecule. Except for its – OH group, cholesterol is hydrophobic. Within a cell membrane, cholesterol molecules are distributed among the hydrophobic tails of the phospholipids. Because the cholesterol molecules are more rigid than the hydrophobic tails, they help to maintain the structure of the membrane. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 9 23.7 Structure of Cell Membranes • Phospholipids provide the basic structure of cell membranes, where they aggregate in a closed, sheet-like structure the lipid bilayer. The bilayer is formed by two parallel layers of lipids oriented so that their ionic head groups protrude into the aqueous environments on either side of the bilayer. Their nonpolar tails cluster together in the middle of the bilayer where they can interact and avoid water. • When phospholipids are shaken vigorously with water, they spontaneously form liposomes, small spherical vesicles with a lipid bilayer surrounding an aqueous center. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 10 • Lipid bilayer: The basic structural unit of cell membranes; composed of two parallel sheets of membrane lipid molecules arranged tail to tail. • Liposome: A spherical structure in which a lipid bilayer surrounds a water droplet. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 11 • 20% or more of the weight of a membrane consists of protein molecules, many of them glycoproteins. • Peripheral proteins are associated with just one face of the bilayer. • Integral proteins extend completely through the cell membrane and may twist in and out of the membrane many times before ending on the outside with a hydrophilic sugar group. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 12 • The overall structure of cell membranes is represented by the fluid-mosaic model. • The membrane is described as fluid because it is not rigid and molecules can move around within it, and as a mosaic because it contains many kinds of molecules. Oil floating on water is an analogy for the fluid-mosaic cell membrane model. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 13 Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 14 23.8 Transport Across Cell Membranes • Active transport: Movement of substances across a cell membrane using energy. • Energy from the conversion of ATP to ADP is used to change the shape of an integral protein (the Na/K pump), simultaneously bringing two K+ ions into the cell and moving three Na+ ions out of the cell Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 15 • Passive transport: Movement of a substance across a cell membrane without the use of energy, from a region of higher concentration to a region of lower concentration. • Simple diffusion: Passive transport by the random motion of diffusion through the cell membrane or through channel proteins. Lipid soluble and small hydrophilic molecules move by simple diffusion. • Facilitated diffusion: Passive transport across a cell membrane with the assistance of a protein that changes shape. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 16 Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 17 23.9 Eicosanoids: Prostaglandins and Leukotrienes • The eicosanoids are a group of compounds derived from 20-carbon unsaturated fatty acids (eicosanoic acids) and synthesized throughout the body. They function as short-lived chemical messengers that act near their points of synthesis (“local hormones”). • The prostaglandins (named for their discovery in prostate cells) and the leukotrienes (named for their discovery in leukocytes) are two classes of eicosanoids that differ somewhat in their structure. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 18 • The prostaglandins all contain a five membered ring, which the leukotrienes lack. Prostaglandins and leukotrienes are synthesized in the body from the 20- carbon unsaturated fatty acid arachidonic acid. • Arachidonic acid, in turn, is synthesized from linolenic acid, helping to explain why linolenic is one of the two essential fatty acids. • The several dozen known prostaglandins have an extraordinary range of biological effects. They can lower blood pressure, influence platelet aggregation during blood clotting, stimulate uterine contractions, and lower the extent of gastric secretions. In addition, they are responsible for some of the pain and swelling that accompany inflammation. Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 19 Copyright © 2010 Pearson Education, Inc. Chapter Twenty Four 20