Lipids Lipids are a heterogeneous class of naturally occurring organic compounds classified together on the basis of common solubility properties they are insoluble in water, but soluble in aprotic organic solvents, including diethyl ether, methylene chloride, and acetone Lipids include triglycerides, phospholipids, prostaglandins, prostacyclins, and fat-soluble vitamins cholesterol, steroid hormones, and bile acids 3 3

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Fig 20.1, p.501

Fatty Acids Fatty acid: a long, unbranched chain carboxylic acid, most commonly of 12 to 20 carbons, derived from hydrolysis of animal fats, vegetable oils, or the phospholipids of biological membranes 5 5

Table 20.1, p.496

Fatty Acids Among the fatty acids most abundant in plants and animals nearly all have an even number of carbon atoms, most between 12 and 20, in an unbranched chain the three most abundant are palmitic, stearic acid, and oleic acid in most unsaturated fatty acids, the cis isomer predominates; the trans isomer is rare unsaturated fatty acids have lower melting points than their saturated counterparts; the greater the degree of unsaturation, the lower the melting point 7 7

Fig. 21.UN, p.496

Fig. 21.UN, p.496

Triglycerides Triglyceride: an ester of glycerol with three fatty acids 4 4

Fig. 21.UN, p.497

Triglycerides Physical properties depend on the fatty acid components melting point increases as the number of carbons in its hydrocarbon chains increases and as the number of double bonds decreases triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oils triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fats 8 8

Triglycerides The lower melting points of triglycerides rich in unsaturated fatty acids are related to differences in their three-dimensional shape hydrocarbon chains of saturated fatty acids can lie parallel, there are strong dispersion forces between their chains; these triglycerides pack into well-ordered, compact crystalline forms and have melting points above room temperature because of the cis configuration of the double bonds in unsaturated fatty acids, their hydrocarbon chains have a less ordered structure and dispersion forces between hydrocarbon chains are weaker; these triglycerides have melting points below room temperature 9 9

Table 20.2, p.498

Soaps and Detergents Natural soaps are prepared by boiling lard or other animal fat with NaOH, in a reaction called saponification (Latin, sapo, soap) 10 10

Soaps and Detergents Soaps clean by acting as emulsifying agents the long hydrophobic hydrocarbon chains of soaps are insoluble in water and tend to cluster in such a way as to minimize their contact with water the polar hydrophilic carboxylate groups, on the other hand, tend to remain in contact with the surrounding water molecules driven by these two forces, soap molecules spontaneously cluster into micelles 11 11

Chem Connect 20C, p.500

Soaps and Detergents Micelle: a spherical arrangement of organic molecules in water clustered so that their hydrophobic parts are buried inside the sphere and their hydrophilic parts are on the surface of the sphere and in contact with water When soap is mixed with water-insoluble grease, oil, and fat stains, the nonpolar parts of the soap micelles “dissolve” these nonpolar dirt molecules and they are carried away in the polar wash water 12 12

Soaps and Detergents Soaps form water-insoluble salts when used in water containing Ca(II), Mg(II), and Fe(III) ions (hard water) 13 13

Synthetic Detergents The design criteria for a good detergent are a long hydrocarbon tail of 12 to 20 carbons a polar head group that does not form insoluble salts with Ca(II), Mg(II), or Fe(III) ions The most widely used synthetic detergents are the linear alkylbenzene sulfonates (LAS) 14 14

Chem Connect 20C, p.500

Phospholipids Phospholipids are the second most abundant group of naturally occurring lipids they are found almost exclusively in plant and animal membranes, which typically consist of 40% -50% phospholipids and 50% - 60% proteins the most abundant phospholipids are derived from phosphatidic acid, a molecule in which glycerol is esterified with two molecules of fatty acid and one of phosphoric acid the three most abundant fatty acids in phosphatidic acids are palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1) 44 44

Phospholipids A phosphatidic acid further esterification with a low-molecular weight alcohol gives a phospholipid among the most common of these low-molecular-weight alcohols are 45 45

Phospholipids 46 46

Fig 20.UN, p.504

Phospholipids A lecithin In aqueous solution, phospholipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers 47 47

Fig 20.3, p.504

Biological Membranes Fluid mosaic model: a biological membrane consists of a phospholipid bilayer with proteins, carbohydrates, and other lipids embedded on the surface and in the bilayer fluid signifies that the protein components of membranes “float”in the bilayer and can move freely along the plane of the membrane mosaic signifies that the various components of the membrane exist side-by-side, as discrete units rather than combining to form new molecules and ions 48 48

Fig 20.2, p.503

Chem Connect 20D, p.502

Go to gob – Ch 20 – complex lipids tutorial D:\GOB.exe

Fig 20.UN, p.505

Fig 20.UN, p.505

Chem Connect 20E, p.506

Steroids Steroids: a group of plant and animal lipids that have this tetracyclic ring structure The features common to the ring system of most naturally occurring steroids are illustrated on the next screen 23 23

Steroids 24 24

Cholesterol 26 26

Fig 20.6, p.513

Androgens Androgens - male sex hormones synthesized in the testes responsible for the development of male secondary sex characteristics 27 27

Anabolic Steroids Among the synthetic anabolic steroids are 28 28

Estrogens Estrogens - female sex hormones synthesized in the ovaries responsible for the development of female secondary sex characteristics and control of the menstrual cycle 29 29

Synthetic Estrogens Progesterone-like analogs are used in oral contraceptives 30 30

Glucorticoid Hormones synthesized in the adrenal cortex regulate metabolism of carbohydrates decrease inflammation involved in the reaction to stress 31 31

Prostaglandins Prostaglandins: a family of compounds that have the 20-carbon skeleton of prostanoic acid 16 16

Chem Connect 20I, p.519

Fig 20.UN, p.520

Prostaglandins Prostaglandins are not stored in tissues as such, but are synthesized from membrane-bound 20-carbon polyunsaturated fatty acids in response to specific physiological triggers one such polyunsaturated fatty acid is arachidonic acid 17 17

Prostaglandins Among the prostaglandins synthesized biochemically from arachidonic acid are 18 18

Eicosanoids The prostaglandins are members of an even larger family of compounds called eicosanoids, all of which contain 20 carbons and are derived from polyunsaturated fatty acids 21 21