Structure and Classification of Lipids

1. Structure and Classification of Lipids • Lipid: A naturally occurring molecule from a plant or animal soluble in nonpolar organic solvents. • Fatty acid: A long-chain carboxylic acid; those in animal fats and vegetable oils often have 12–22 carbon atoms. • Waxesare carboxylic acid esters, RCOOR’,with long, straight hydrocarbon chains in both R groups; they are secreted by sebaceous glands in the skin of animals and perform mostly external protective functions. Chapter Twenty Four

2. Triacylglycerols are carboxylic acid triesters of glycerol, a three-carbon trialcohol. They make up the fats stored in our bodies and most dietary fats and oils. They are a major source of biochemical energy. Chapter Twenty Four

3. Glycerophospholipids are triesters of glycerol that contain charged phosphate diester groups and are abundant in cell membranes. Together with other lipids, they help to control the flow of molecules into and out of cells. • Sphingomyelins, amides derived from an amino alcohol (sphingosine), also contain charged phosphate diester groups; they are essential to the structure of cell membranes and are abundant in nerve cell membranes. • Glycolipids, different amides derived from sphingosine, contain polar carbohydrate groups; on cell surfaces the carbohydrate portion is recognized by and connects to intracellular messengers. Chapter Twenty Four

4. The previously listed groups of lipids are all esters or amides of fatty acids, the next two groups of lipids are not: the steroids and the eicosanoids. • Steroids are tetracyclic molecules that act as hormones and contribute to the structure of cell membranes. • The eicosanoids are carboxylic acids that are a special type of intracellular chemical messenger. Chapter Twenty Four

5. A few representative lipid structures are shown below. Chapter Twenty Four

6. Fatty Acids and Their Esters • The naturally occurring fats and oils are triesters formed between glycerol and fatty acids. • Saturated fatty acid: A long-chain carboxylic acid containing only carbon–carbon single bonds. • Unsaturated fatty acid: A long-chain carboxylic acid containing one or more carbon–carbon double bonds. • If double bonds are present in naturally occurring fats and oils, the double bonds are usually cis rather than trans. Chapter Twenty Four

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8. Polyunsaturated fatty acids have more than one C=C double bond. Linoleic and linolenic acids are essential in the human diet because the body does not synthesize them and they are needed for the synthesis of other lipids. Chapter Twenty Four

9. The simplest fatty acid esters in nature are waxes. A wax is a mixture of fatty acid—long-chain alcohol esters. The acids usually have an even number from 16 to 36 carbons, whereas the alcohols have an even number from 24 to 36 carbons. • A component in beeswax is the ester formed from a 30-C alcohol (triacontanol) and a 16-C acid (palmitic acid). Chapter Twenty Four

10. Animal fats and vegetable oils are the most plentiful lipids in nature. All fats and oils are composed of triesters of glycerol (1,2,3-propanetriol, also known as glycerine) with three fatty acids. They are named chemically as triacylglycerols, but are often called triglycerides. Chapter Twenty Four

11. The three fatty acids of any specific triacylglycerol are not necessarily the same. The fat or oil from a given natural source is a complex mixture of many different triacylglycerols. • Vegetable oils consist almost entirely of unsaturated fatty acids, whereas animal fats contain a much larger percentage of saturated fatty acids. This difference in composition is the primary reason for the different melting points of fats and oils. Chapter Twenty Four

12. Properties of Fats and Oils • Triacylglycerols in natural fats and oils are nonpolar, hydrophobic molecules with no ionic charges. • Oil: A mixture of triacylglycerols that is liquid because it contains a high proportion of unsaturated fatty acids. • Fat: A mixture of triacylglycerols that is solid because it contains a high proportion of saturated fatty acids. Chapter Twenty Four

13. The hydrocarbon chains in saturated acids are flexible and uniform in shape, allowing them to nestle together. By contrast, the carbon chains in unsaturated acids have rigid kinks wherever they contain cis double bonds. The kinks make it difficult for such chains to fit next to each other in the orderly fashion necessary to form a solid. Chapter Twenty Four

14. The more double bonds there are in a triacylglycerol, the harder it is for it to solidify. Chapter Twenty Four

15. Chemical Reactions of Triglycerols • The C=C double bonds in vegetable oils can be hydrogenated to yield saturated fats in the same way that any alkene can react with H2 to yield an alkane. • Margarine and shortening are produced by hydrogenation of vegetable oils to give a product chemically similar to that found in animal fats. Chapter Twenty Four

16. Hydrolysis of fats and oils carried out by strong aqueous bases to form soaps is called saponification. Chapter Twenty Four

17. Micelle: A spherical cluster formed by the aggregation of soap or detergent molecules so that their hydrophobic ends are in the center and their hydrophilic ends are on the surface. Chapter Twenty Four

18. Cell Membrane Lipids: Phospholipids and Glycolipids Phospholipid: A lipid that has an ester link between phosphoric acid and an alcohol (either glycerol or sphingosine). Chapter Twenty Four

19. 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. Chapter Twenty Four

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22. 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. Chapter Twenty Four

23. 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. Chapter Twenty Four

24. 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. Chapter Twenty Four

25. 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. Chapter Twenty Four

26. 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. Chapter Twenty Four

27. 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. Chapter Twenty Four

28. 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. Chapter Twenty Four

29. 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. Chapter Twenty Four

30. 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. Chapter Twenty Four

31. 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. Chapter Twenty Four

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33. 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 Chapter Twenty Four

34. 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. Chapter Twenty Four

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36. 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. Chapter Twenty Four

37. 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. Chapter Twenty Four

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