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Chapter 28

Chapter 28. Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena. Lipids. Polar bears have a large reserve of lipids. 28.1 Lipids: Hydrophobic Molecules 28.2 Classification of Lipids 28.3 Simple Lipids

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Chapter 28

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  1. Chapter 28 Introduction to General, Organic, and Biochemistry, 10e John Wiley & Sons, Inc Morris Hein, Scott Pattison, and Susan Arena Lipids Polar bears have a large reserve of lipids.

  2. 28.1Lipids: Hydrophobic Molecules 28.2Classification of Lipids 28.3Simple Lipids 28.4Fats in Metabolism 28.5Compound Lipids 28.6Steroids 28.7Hydrophobic Lipids and Biology Chapter 28 Summary Course Outline 2

  3. Lipids: Hydrophobic Molecules Lipids are water-insoluble substances that have several important biological functions that are related to how lipids are classified. Lipids interact weakly with water molecules because they are composed primarily of nonpolar alkyl groups. Lipids are classified as hydrophobic(“water fearing”) to designate their inability to interact effectively with water or their strong tendency to move away from water. 3

  4. Lipids: Hydrophobic Molecules Fatty acids are common components of lipids. As fatty acids get larger, the water solubility of the fatty acid decreases dramatically as shown on the next slide . . . 4

  5. Lipids: Hydrophobic Molecules 5

  6. Your Turn! What characteristic do all lipids have in common? 6

  7. Your Turn! What characteristic do all lipids have in common? All lipids are insoluble in water. 7

  8. Your Turn! Which fatty acid would you predict to have the lowest melting point? 8

  9. Your Turn! Which fatty acid would you predict to have the lowest melting point? Both fatty acids are saturated. The fatty acid with the fewest number of carbon atoms should have the lowest melting point. 9

  10. Classification of Lipids • Lipids molecules are relatively large and nonpolar. Yet, within this broad description, lipid structures vary markedly. Lipids can be classified in four categories which recognize major structural similarities. • Simple lipids • Compound lipids • Steroids • Miscellaneous lipids 10

  11. Simple Lipids The simple lipids include fats,oils, and waxes. These simple lipids are derivatives of lipid-like substances call fatty acids. Fatty acids are long-chain carboxylic acids (generally greater than about 12 carbons) that have no solubility in water. The hydrophilic -COOH group is referred to as a polar head and the hydrophobic hydrocarbon portion is referred to as a nonpolar tail. 11

  12. Simple Lipids Fats and waxes are solids due to a higher composition of saturated fatty acids while oils are liquids due to a higher composition of unsaturated fatty acids. Table 28.1 on the next slide lists properties of saturated and unsaturated fatty acids separately. Notice how unsaturated fatty acids generally have lower melting points than saturated fatty acids. Lipids prepared from fatty acids with lower melting points tend to be liquids instead of solids at room temperature. 12

  13. Simple Lipids 13

  14. Simple Lipids Unsaturated fatty acids exist as cis and trans isomers. The cis isomer is more prevalent in nature. The cis and trans isomers of oleic acid are shown on the next slide. Oleic acid is an unsaturated fatty acid. The cis isomer has a bent structure which prevents close stacking resulting in a compound that resists solidification. 14

  15. Simple Lipids 15

  16. Your Turn! Draw the cis and trans isomers of palmitoleic acid. CH3(CH2)5CH=CH(CH2)7COOH. 16

  17. Your Turn! Draw the cis and trans isomers of palmitoleic acid. CH3(CH2)5CH=CH(CH2)7COOH. 17

  18. Simple Lipids Certain fatty acids, as well as other lipids, are biochemical precursors of several classes of hormones. For example, ω-6(omega-6) and ω-3 fatty acids are used to make hormones, the most common of which are the eicosanoids. These hormones are derived from fatty acids with 20 carbon atoms, either the ω-6 (arachidonic acid) or the ω-3 (eicosapentaenoic acid). 18

  19. Simple Lipids Omega (ω) is the last letter in the Greek alphabet. Correspondingly, the last carbon atom in a carbon chain of a compound is often referred to as the omega carbon. In reference to unsaturated carboxylic acids, omega plus a number (e.g., ω-3) indicates the location of the first carbon–carbon double bond, counting from the omega carbon. 19

  20. Simple Lipids So for example arachidonic acid is an ω-6 fatty acid because the first double bond from the last carbon atom is on the sixth carbon atom. 20

  21. Simple Lipids • Examples of eicosanoids are: • Thromboxanes • Prostacyclins • Prostaglandins • Leukotrienenes • The synthesis of eicosanoids from arachidonic acid is shown on the next slide . . . 21

  22. 22

  23. Simple Lipids • Eicosanoids are hormones and coordinate various cellular responses. • Some are involved with blood clotting as they can cause platelet aggregation while others trigger an increase in body temperature. • Some eicosanoids coordinate HCl and mucous secretion by the stomach lining and constriction of the bronchial tubes in the lungs. 23

  24. Simple Lipids • Some eicosanoids stimulate and attract white cells, while other eicosanoids cause the white cells to disperse. • Eicosanoids can also cause vasodilation as well as vasoconstriction. 24

  25. Simple Lipids • Many drugs control one or more of the physiological effects produced by the eicosanoids. • For example, aspirin is a nonsteroidal anti-inflammatory drug (and NSAID) that blocks the oxidation of arachidonic acid which in turn prevents the formation of prostaglandins and thromboxanes. 25

  26. Simple Lipids Fats and oils are triester derivatives of glycerol and three fatty acid molecules as shown below. Because there are three ester groups per glycerol, these lipids are called triacylglycerols or triglycerides. The fatty acids have carbon chains with 14-18 carbon atoms. 26

  27. Simple Lipids Triglycerides are prepared by the reaction of a glycerol molecule and three fatty acid molecules. The fatty acid molecules can be saturated or unsaturated. 27

  28. Simple Lipids Waxes are esters of high-molar-mass fatty acids and high-molar-mass alcohols. They have the general formula shown below in which the alcohol (ROH) contributes up to about 30 carbons. 28

  29. Simple Lipids Waxes are very large molecules with almost no polar groups. They represent one of the most hydrophobic classes of lipids. Their extreme water insolubility allows waxes to serve a protective function. Leaves, feathers, fruit, and fur are often naturally coated with wax. 29

  30. Simple Lipids An important biological function of a wax is to act as a protective coating.The “shine” on these leaves is due to a thick protective wax coating. 30

  31. Your Turn! Write the formula for a wax formed from palmitic acid [CH3(CH2)14COOH] and 1-hexacosanol [CH3(CH2)24CH2OH]. 31

  32. Your Turn! Write the formula for a wax formed from palmitic acid [CH3(CH2)14COOH] and 1-hexacosanol [CH3(CH2)24CH2OH]. As with all waxes, this wax is an ester. 32

  33. Your Turn! • What is the correct classification of the fatty acid shown below? • CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH • ω-3 • ω-6 33

  34. Your Turn! • What is the correct classification of the fatty acid shown below? • CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH • ω-3 • ω-6 The double bond is attached to the sixth carbon from the end of the molecule furthest from the carboxyl group. This is an ω-6 fatty acid. 34

  35. Fats in Metabolism Fats are an important food source for humans and normally account for about 25–50% of our caloric intake. Fats are an especially good source of metabolic energy. Most metabolic energy is derived from carbon oxidation. When oxidized to carbon dioxide and water, fats supply about 40 kJ per gram (9.5 kcal/g), which is more than twice the amount obtained from carbohydrates or proteins. 35

  36. Fats in Metabolism Fats are what our bodies prefer when storing energy reserves. These reserves are in the form of triacylglycereols in fatty tissue. On average, this tissue stores about two to three weeks’ worth of energy. 36

  37. Your Turn! The carbohydrate glucose (C6H12O6) and the fatty acid capric acid (C9H19COOH) have similar molecular weights. Recall that most metabolic energy is derived from carbon oxidation. Using these two molecules as examples, why does fat supply more energythan an equivalent amount of carbohydrate? 37

  38. Your Turn! The carbohydrate glucose (C6H12O6) and the fatty acid capric acid (C9H19COOH) have similar molecular weights. Recall that most metabolic energy is derived from carbon oxidation. Using these two molecules as examples, why does fat supply more energythan an equivalent amount of carbohydrate? Fats are about 75% carbon by mass as compared to about 40% for carbohydrates. In addition fats have a lower degree of carbon oxidation when compared to carbohydrates which results in higher energy yields when fats are oxidized. 38

  39. Compound Lipids • There are three broad categories of compound lipids. • Phospholipids • Sphingolipids • Glycolipids 39

  40. Compound Lipids The phospholipidsare a group of compounds that yield one or more fatty acid molecules, a phosphate group, and usually a nitrogenous base upon hydrolysis. In contrast to the triacylglycerols, phospholipids have a hydrophilic end that interacts with water. 40

  41. Compound Lipids Phospholipids are one of the most important membrane components. They are also involved in the metabolism of other lipids and nonlipids. Three categories of phospholipids are phosphatidic acids, phosphatidylcholines, and phosphatidylethanolamines. 41

  42. Compound Lipids Phosphatidic acids are glyceryl esters of fatty acids and phosphoric acid. The phosphatidic acids are important intermediates in the synthesis of triacylglycerols and other phospholipids. 42

  43. Compound Lipids Phosphatidylcholines (lecithins) are glyceryl esters of fatty acids, phosphoric acid, and choline. 43

  44. Compound Lipids The single most important biological function for phosphatidylcholine is as a membrane component which makes up between 10 and 20% of many membranes. 44

  45. Compound Lipids Another important constituent of biological membranes is the phosphatidylethanolamines (cephalins). These lipids are glyceryl esters of fatty acids, phosphoric acid, and ethanolamine (HOCH2CH2NH2). 45

  46. Compound Lipids • Sphingolipids are another type of compound lipid that, when hydrolyzed, yield: • a hydrophilic group (either phosphate and choline or a carbohydrate) • a long-chain fatty acid (18–26 carbons) • sphingosine (an unsaturated amino alcohol) 46

  47. Compound Lipids The similarities between sphingosine and glycerol are shown in the structures below. 47

  48. Compound Lipids Sphingolipids are common membrane components because they have both hydrophobic and hydrophilic character. Sphingomyelins are found in the myelin sheath membranes that surround nerves. 48

  49. Compound Lipids Glycolipids are a third type of compound lipid. These compounds that contain a carbohydrate group. The two most important classes of glycolipids are cerebrosides and gangliosides. These substances are found mainly in cell membranes of nerve and brain tissue. 49

  50. Compound Lipids A cerebroside may contain either D-galactose or D-glucose. The following formula of a galactocerebroside shows the typical structure of cerebrosides. 50

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