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Chapter 17 Lipids 17.1 Lipids Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 1 LIPID Lipids: 1. Fatty acids 2. Neutral fats and oils 3. Waxes 4. Phospholipid 5. Sterols 6. Fat soluble vitamins Lipids Lipids are • Biomolecules that contain fatty acids or a steroid nucleus. • Soluble in organic solvents but not in water. • Named for the Greek word lipos, which means “fat.” • Extracted from cells using organic solvents. 3 Types of Lipids The types of lipids containing fatty acids are • Waxes. • Fats and oils (triacylglycerols). • Glycerophospholipids. • Prostaglandins. • Not steroids, as they do not contain fatty acids. 4 Structures of Lipids Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 5 Chapter 17 Lipids 17.2 Fatty Acids Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 6 Lipids are non-polar (hydrophobic) compounds, soluble in organic solvents. Most membrane lipids are amphipathic, having a non-polar end and a polar end. Fatty acids consist of a hydrocarbon chain with a carboxylic acid at one end. - A 16-C fatty acid: CH3(CH2)14 COO - Non-polar polar A 16-C fatty acid with one cis double bond between represented as 16:1 cis D9. C atoms 9-10 may be Fatty Acid Formulas The formulas for fatty acids are written as • Condensed formulas. • Line-bond formulas. • For example caprylic acid with 8 carbon atoms. CH3—(CH2)6—COOH CH3—CH2—CH2—CH2—CH2—CH2—CH2—COOH O OH 8 Fatty Acids O R C OH #1 Carbon O R C OH Acid Group Polar End - Hydrophilic End Non-polar End - Hydrophobic End (Fat-soluble tail) Fatty Acids Fatty acids • Are long-chain carboxylic acids. • Typically contain 1218 carbon atoms. • Are insoluble in water. • Can be saturated or unsaturated. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 10 Saturated Fatty Acids 8 7 CH3 CH2 O 5 3 4 6 2 1 CH2 CH2 CH2 CH2 CH2 C OH Octanoic Acid Unsaturated Fatty Acids 8 CH3 7 CH2 5 6 CH2 CH2 4 CH2 O 3 2 1 CH2 CH2 C OH 3 - Octenoic Acid 8 7 CH3 CH2 O 5 3 4 6 2 1 CH2 CH2 CH2 CH2 CH2 C OH 3, 6 - Octadienoic Acid Short hand: 8:1 (D3) 8:2 (D3,6) Cis And Trans Fatty Acids H H CH3 (CH2 )7 C C (CH2 )7 10 9 O C OH Cis 9 - Octadecenoic Acid (oleic) H CH3 (CH2 )7 O C C (CH2 )7 C OH H Trans 9 - Octadecenoic Acid (elaidic acid) Saturated Fatty Acids Saturated fatty acids have • Single C–C bonds. • Molecules that fit closely together in a regular pattern. • Strong attractions between fatty acid chains. • High melting points that make them solids at room temperature. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 14 Some Saturated Fatty Acids TABLE 17.1 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 15 Unsaturated Fatty Acids Unsaturated fatty acids • Have one or more double C=C bond • Typically contain cis double bonds. 16 Properties of Unsaturated Fatty Acids Unsaturated fatty acids • Have “kinks” in the fatty acid chains. • Do not pack closely. • Have few attractions between chains. • Have low melting points. • Are liquids at room temperature. “kinks” in chain Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 17 Polyunsaturated Fatty Acids Linoleic acid: Linolenic acid: Cis, cis, 9, 12 - Octadecadienoic acid Cis, cis, cis 9, 12, 15 - Octadecatrienoic acid Arachidonic acid: Cis, cis, cis, cis 5, 8, 11, 14 - Eicosatetraenoic acid Linoleic Acid Linolenic Acid Arachidonic Acid Double bonds in fatty acids usually have the cis configuration. Most naturally occurring fatty acids have an even number of carbon atoms. 4 O a 3 2 C 1 O fatty acid with a cis-D9 double bond Some fatty acids and their common names: 14:0 myristic acid; 16:0 palmitic acid; 18:0 stearic acid; 18:1 cisD9 oleic acid 18:2 cisD9,12 linoleic acid 18:3 cisD9,12,15 a-linonenic acid 20:4 cisD5,8,11,14 arachidonic acid Comparing Melting Points of Some Fatty Acids Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 20 Effects of Double Bonds on the Melting Points Fatty. Acids. 16:0 16:1 18:0 18:1 18:2 18:3 20:0 20:4 Melting point. P. (0C) 60 1 63 16 -5 -11 75 -50 Melting Points and Solubility in Water of Fatty Acids Melting Point Solubility in H O Chain Length 2 Learning Check Assign the melting points of –17°C, 13°C, and 69°C to the correct fatty acid. Explain. stearic acid (18 C) saturated oleic acid (18 C) one double bond linoleic acid (18 C) two double bonds 23 Solution Stearic acid is saturated and would have a higher melting point than the unsaturated fatty acids. Because linoleic has two double bonds, it would have a lower mp than oleic acid, which has one double bond. stearic acid mp 69°C saturated oleic acid mp 13°C linoleic acid mp -17°C most unsaturated 24 Omega-6 and Omega 3- Fatty Acids Fatty acids In vegetable oils are mostly omega-6 with the first C=C at C6. linoleic acid CH3─(CH2)4─CH=CH─CH2─CH=CH─(CH2)7─COOH 6 In fish oils are mostly omega-3 with the first C=C at C3. linolenic acid CH3─CH2─(CH=CH─CH2)3─(CH2)6─COOH 3 25 Some Omega-6 and Omega-3 Fatty Acids 26 Learning Check Write a fatty acid with 10 carbon atoms that is: A. saturated B. monounsaturated omega-3 C. monounsaturated omega-6 27 Solution Write a fatty acid with 10 carbon atoms that is: A. saturated CH3—CH2—CH2—CH2—CH2—CH2—CH2—CH2—CH2—COOH B. monounsaturated omega-3 CH3—CH2—CH=CH—CH2—CH2—CH2—CH2—CH2—COOH C. monounsaturated omega-6 CH3—CH2—CH2—CH2—CH2—CH=CH—CH2—CH2—COOH 28 Prostaglandin: One of a number of hormone-like substances that participate in a wide range of body functions such as the contraction and relaxation of smooth muscle, the dilation and constriction of blood vessels, control of blood pressure, and modulation of inflammation. Prostaglandins are derived from a chemical called arachidonic acid Prostaglandins Prostaglandins have • 20 carbon atoms in their fatty acid chains. • An OH on carbon 11 and 15. • A trans double bond at carbon 13. C 11 C 13 C 15 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 30 Prostaglandins in the Body Prostaglandins are • Produced by injured tissues. • Involved in pain, fever, and inflammation. • Not produced when anti-inflammatory drugs such as aspirin inhibit their synthesis. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 31 Chapter 17 Lipids 17.3 Waxes, Fats, and Oils Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 32 Waxes Waxes are • Esters of saturated fatty acids and long-chain alcohols. TABLE 17.2 • Coatings that prevent loss of water by leaves of plants. 33 Fats and Oils: Triacylglycerols Fats and oils are • Also called triacylglycerols. • Esters of glycerol. • Produced by esterification. • Formed when the hydroxyl groups of glycerol react with the carboxyl groups of fatty acids. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 34 Glycerophospholipids Glycerophospholipids (phosphoglycerides), are common constituents of cellular membranes. They have a glycerol backbone. Hydroxyls at C1 & C2 are esterified to fatty acids. An ester forms when a hydroxyl reacts with a carboxylic acid, with loss of H2O. CH2OH H C OH CH2OH glycerol Formation of an ester: O R'OH + HO-C-R" O R'-O-C-R'' + H2O FAT AND OILS Mostly Triglycerides: O HO C R O H2 C OH HC OH H2 C OH Glycerol + HO C R O HO C R 3 Fatty Acids O H2 C O C R O HC O C R O H2 C O C R + 3 H2O 37 Triacylglycerols In a triacylglycerol, • Glycerol forms ester bonds with three fatty acids. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 38 Formation of a Triacylglycerol glycerol + three fatty acids triacylglycerol O CH2 OH HO C O (CH2)14CH3 CH OH + HO C O (CH2)14CH3 HO C (CH2)14CH3 CH2 O C CH2 OH O (CH2)14CH3 O CH O C (CH2)14CH3 + 3H2O O CH2 O C (CH2)14CH3 39 GLYCERIDES O H2 C OH HC H2 C O C (CH 2 )16 CH3 OH O H2 C O C (CH2 )16 CH3 Monoglyceride (a - monostearin) H2 C HC H2 C HC OH O H2 C O C (CH 2 )16 CH3 Diglyceride (a, a' - distearin) O O C (CH2 )16 CH3 O O C (CH2 )14 CH3 O O C (CH2 )16 CH3 ( C18 ) (C16 ) (C18 ) Triglyceride ( - palmityl distearin) Olive Oil Olive oil • Contains a high percentage of oleic acid, which is a monounsaturated fatty acid with one cis double bond. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 41 Learning Check What are the fatty acids in the following triacylglycerol? O CH2 O C (CH2)16CH3 O CH CH2 O O C (CH2)7CH CH(CH2)7CH3 O C (CH2)12CH3 42 Solution O CH2 O C (CH2)16CH3 Stearic acid O CH CH2 O O C (CH2)7CH CH(CH2)7CH3 O C (CH2)12CH3 Oleic acid Myristic acid 43 MELTING POINTS OF TRIGLYCERIDES Triglyceride Melting Point (°C) C6 -15 C12 15 C14 33 C16 45 C18 55 C18:1 (cis) -32 C18:1 (trans) 15 Melting Points of Fats and Oils A triacylglycerol that is a fat • Is solid at room temperature. • Is prevalent in meats, whole milk, butter, and cheese. A triacylglycerol that is an oil • Is liquid at room temperature. • Is prevalent in plants such as olive and safflower. 45 Oils with Unsaturated Fatty Acids Oils • Have more unsaturated fats. • Have cis double bonds that cause “kinks” in the fatty acid chains. • Cannot pack triacylglycerol molecules as close together as in fats. • Have lower melting points than saturated fats. • Are liquids at room temperature. 46 Diagram of Triacylglycerol with Unsaturated Fatty Acids Unsaturated fatty acid chains have kinks that do not allow close packing. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 47 Saturated and Unsaturated Fatty Acids In Fats and Oils Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 48 Chapter 17 Lipids 17.4 Chemical Properties of Triacylglycerols Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 49 Chemical Properties of Triacylglycerols The chemical reactions of triacylglycerols are similar to those of alkenes and esters. • In hydrogenation, double bonds in unsaturated fatty acids react with H2 in the presence of a Ni or Pt catalyst. • In hydrolysis, ester bonds are split by water in the presence of an acid, a base, or an enzyme. 50 Hydrogenation of Oils The hydrogenation of oils • Adds hydrogen (H2) to the carbon atoms of double bonds. • Converts double bonds to single bonds. • Increases the melting point. • Produces solids such as margarine and shortening. 51 Hydrogenation O CH2 O C (CH2)5CH CH(CH2)7CH3 Ni O CH CH2 O O C (CH2)5CH O C + 3H2 CH(CH2)7CH3 (CH2)5CH CH(CH2)7CH3 glyceryl tripalmitoleate (tripalmitolean) O CH2 O C (CH2)14CH3 O CH O C (CH2)14CH3 O glyceryl tripalmitate CH2 (tripalmitin O C ) (CH2)14CH3 52 Olestra, A Fat Substitute Olestra is • Used in foods as an artificial fat. • Sucrose linked by ester bonds to several long-chain fatty chains. • Not broken down in the intestinal tract. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 53 Cis and Trans Fatty Acids Unsaturated fatty acids can be • Cis with bulky groups on same side of C=C. CH3─ (CH2)5 (CH2)7─ COOH cis C=C H H • Trans have bulky groups on opposite sides of C=C. CH3─ (CH2)5 H C=C H trans (CH2)7─ COOH 54 Trans Fatty Acids and Hydrogenation Trans fatty acids • Are formed during hydrogenation when cis double bonds are converted to trans double bonds. • In the body behave like saturated fatty acids. • Are estimated to make up 2-4% of our total Calories. • Are reported in several studies reported to raise LDL-cholesterol and lower HDL-cholesterol. 55 Learning Check (1) True or (2) False A. There are more unsaturated fats in vegetable oils. B. Vegetable oils have higher melting points than fats. C. Hydrogenation of oils converts some cisdouble bonds to trans- double bonds. D. Animal fats have more saturated fats. 56 Solution (1) True or (2) False A. T There are more unsaturated fats in vegetable oils. B. F Vegetable oils have higher melting points than fats. C. T Hydrogenation of oils converts some cisdouble bonds to trans- double bonds. D. T Animal fats have more saturated fats. 57 Hydrolysis In hydrolysis, • Triacylglycerols split into glycerol and three fatty acids. • An acid or enzyme catalyst is required. O CH2 CH CH2 O O O C O (CH2)14CH3 C (CH2)14CH3 O C (CH2)14CH3 +3 H2O H+ +3 CH2 OH O CH OH HO C (CH2)14CH3 58 2. Saponification Value Saponification - hydrolysis of ester under alkaline condition. O H2 C O C R O HC O C R O H2 C O C R H2 C OH + 3 KOH HC OH H2 C OH + O 3 R C OK Saponification and Soap Saponification • Is the reaction of a fat with a strong base. • Splits triacylglycerols into glycerol and the salts of fatty acids. • Is the process of forming “soaps” (salts of fatty acids). • With KOH gives softer soaps. 60 Saponification O CH2 O C (CH2)14CH3 O O CH O C Na+ -O C (CH2)14CH3 + 3NaOH O CH2 O C (CH2)14CH3 CH2 OH (CH2)14CH3 CH OH + 3 CH2 OH “soap” 61 Saponification Value of Fats and Oils Fat Saponification # Milk Fat 210-233 Coconut Oil 250-264 Cotton Seed Oil 189-198 Soybean Oil 189-195 Lard 190-202 2. Saponification Value Determination Saponification # --mgs of KOH required to saponify 1 g of fat. 1. 5 g in 250 ml Erlenmeyer. 2. 50 ml KOH in Erlenmeyer. 3. Boil for saponification. 4. Titrate with HCl using phenolphthalein. 5. Conduct blank determination. SP# = 56.1(B - S) x N of HCl Gram of Sample B - ml of HCl required by Blank. S - ml of HCl required by Sample. 3. Iodine Number Number of iodine (g) absorbed by 100 g of oil. Molecular weight and iodine number can calculate the number of double bonds. 1 g of fat adsorbed 1.5 g of iodine value = 150. Iodine Value Determination Iodine Value = (ml of Na2S2O3 volume for blank - ml of Na2S2O3 volume for sample) N of Na2S2O3 0.127g/meq 100 Weight of Sample (g) CH CH CH Cl + ICl Iodine chloride Excess unreacted ICl ICl I2 + + KI 2 Na2 S2 O3 KCl + Na2 S4 O6 I2 + 2 NaI CH I Iodine Numbers of Triglycerides Fatty Acids # of Double-bonds Iodine # Palmitoleic Acid 1 95 Oleic Acid 1 86 Linoleic Acid 2 173 Linolenic Acid 3 261 Arachidonic Acid 4 320 Learning Check What products are obtained from the complete hydrolysis of glyceryl trioleate? 1. Glycerol and 3 oleic acids 2. Glyceryl tristearate 3. Glycerol and 3 stearic acids 67 Solution What products are obtained from the complete hydrolysis of glyceryl trioleate? 1. Glycerol and 3 oleic acids 68 Learning Check Write the product of the following reaction. O CH2 CH CH2 O O O C O (CH2)5CH CH(CH2)7CH3 C (CH2)5CH O C (CH2)5CH CH(CH2)7CH3 + 3 H2 Ni CH(CH2)7CH3 69 Solution O CH2 O C (CH2)14CH3 O CH O C (CH2)14CH3 O CH2 O C (CH2)14CH3 70 Chapter 17 Lipids 17.5 Glycerophospholipids Fatty acid Glycerol Fatty acid PO4 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Amino alcohol 71 PHOSPHOLIPID Lecithin (phosphatidyl choline) O O R C H2 C O O C R CH CH3 O H2 C O P O_ Phosphatidic Acid O CH2 CH 2 + N CH 3 CH3 Choline Glycerophospholipids Glycerophospholipids are • The most abundant lipids in cell membranes. • Composed of glycerol, two fatty acids, phosphate and an amino alcohol. Fatty acid Fatty acid Glycerol PO4 Amino alcohol 73 Glycerophospholipids Are Polar A glycerophospholipid has • Two nonpolar fatty acid chains. • A phosphate group and a polar amino alcohol. CH3 │+ + HO−CH2−CH2−N−CH3 HO−CH2−CH2−NH3 │ choline CH3 ethanolamine + NH3 │ HO−CH2−CH−COO− Amino alcohols serine 74 Structure and Polarity of A Glycerophospholipid Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 75 Lecithin and Cephalin Lecithin and cephalin are glycerophospholipids • Abundant in brain and nerve tissues. • Found in egg yolk, wheat germ, and yeast. Copyright © 2007 by Pearson Education, Inc Publishing as Benjamin Cummings 76 Lecithin • Abundant in brain and nerve tissues • Found in egg yolk, what germ, yeast O CH2 O C (CH2)14CH3 O CH O CH2 C O (CH2)14CH3 + O P O CH2CH2N(CH3)3 - O choline, an amino alcohol 77 Chapter 17 Lipids 17.6 Sphingolipids Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 78 Sphingolipids Sphingolipids • Are similar to phospholipids. • Contain sphingosine (a long-chain amino alcohol), a fatty acid, phosphate, and a small amino alcohol. fatty acid • Have polar and nonpolar regions. sphingosine PO4 amino alcohol 79 Sphingosine Sphingosine is a long-chain unsaturated amino alcohol. CH3−(CH2)12 −CH=CH−CH−OH │ CH−NH2 │ CH2−OH sphingosine 80 Sphingolipids In sphingomyelin, a sphingolipid found in nerve cells • There is an amide bond between a fatty acid and sphingosine, an 18-carbon alcohol. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 81 Glycosphingolipids • Glycosphingolipi ds contain monosaccharide s bonded to the –OH of sphingosine by a glycosidic bond. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 82 Glycosphingolipids and Cerebrosides Glycosphingolipids • Are sphingolipids that contain monosaccharides. • Can be a cerebroside with galactose. Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 83 Steroid Nucleus A steroid nucleus consists of • 3 cyclohexane rings. • 1 cyclopentane ring. • No fatty acids. steroid nucleus 84 Chapter 17 Lipids 17.7 Steroids: Cholesterol, Bile Salts, and Steroid Hormones CH3 CH3 CH3 CH3 CH3 HO Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 85 STEROLS Male & female sex hormones Bile acids Vitamin D Adrenal corticosteroids 21 H3C 18 Cholesterol H3C HO 22 CH3 20 12 19 17 11 13 H3C 14 10 1 9 2 8 3 6 7 4 5 16 15 CH3 Cholesterol Cholesterol • Is the most abundant steroid in the body. • Has methyl CH3- groups, alkyl chain, and -OH attached to the steroid CH3 nucleus. CH3 CH3 CH3 HO CH3 87 Cholesterol, an important constituent of cell membranes, has a rigid ring system and a short branched HO hydrocarbon tail. Cholesterol Cholesterol is largely hydrophobic. But it has one polar group, a hydroxyl, making it amphipathic. PDB 1N83 cholesterol HO Cholesterol Cholesterol in membrane Cholesterol inserts into bilayer membranes with its hydroxyl group oriented toward the aqueous phase & its hydrophobic ring system adjacent to fatty acid chains of phospholipids. The OH group of cholesterol forms hydrogen bonds with polar phospholipid head groups. Cholesterol in the Body • Cellular membranes • Myelin sheath, brain, and nerve tissue • Bile salts • Hormones • Vitamin D 90 Cholesterol in the Body Cholesterol in the body • Is obtained from meats, milk, and eggs. • Is synthesized in the liver. • Is needed for cell membranes, brain and nerve tissue, steroid hormones, and Vitamin D. • Clogs arteries when high levels form plaque. A normal, open artery. An artery clogged by cholesterol plaque Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 91 Cholesterol in Foods Cholesterol is • Synthesized in the liver. • Obtained from foods. • Considered elevated if plasma cholesterol exceeds 200 mg/dL. TABLE 17.4 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 92 FAT SOLUBLE VITAMINS Vitamin A: H3 C CH3 CH3 5 8 9 CH3 CH3 7 6 4 3 CH2 OH 2 1 CH3 H 3C Vitamin D2: H 3C H CH3 CH 3 H CH 2 Vitamin E: R1 R2 HO R3 HO CH3 CH 3 O (CH2 CH2 CH2 CH2 )2 CH2 CH2CH2 CH(CH3 )2 Bile Salts Bile salts • Are synthesized in the liver from cholesterol. • Are stored in the gallbladder. • Are secreted into the small intestine. • Have a polar and a nonpolar region • Mix with fats to break them part. • Emulsify fat particles to provide large surface area. 95 Bile Salts glycine, an amino acid cholic acid, a bile acid CH3 OH CH3 O C CH2 N COO- Na+ Polar region H CH3 HO Nonpolar region OH sodium glycocholate, a bile salt 96 Bile Salts • Synthesized in the liver from cholesterol • Stored in the gallbladder • Secreted into small intestine • Mix with fats to break them part • Emsulsify fat particles 97 Sex Hormones CH3 OH CH3 O CH3 OH CH3 HO t e st ost e r on e e st r ad iol 98 Lipoproteins Lipoproteins • Combine lipids with proteins and phospholipids. • Are soluble in water because the surface consists of polar lipids. 99 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Types of Lipoproteins Lipoproteins • Differ in density, composition, and function. • Include low-density lipoprotein (LDLs) and high-density lipoprotein (HDLs). TABLE 17.5 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 100 Steroid Hormones Steroid hormones • Are chemical messengers in cells. • Are produced from cholesterol. • Include sex hormones such as androgens (testosterone) in males and estrogens (estradiol) in females. 101 Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings Adrenal Corticosteroids Adrenal corticosteroids are steroid hormones that • Are produced by the adrenal glands located on the top of each kidney. • Include aldosterone, which regulates electrolytes and water balance by the kidneys. • Include cortisone, a glucocorticoid, which increases blood glucose level and stimulates the synthesis of glycogen in the liver. 102 Adrenal Corticosteroids 103 Soxhlet Method. Learning Check Identify each as a 1. fatty acid 2. steroid 3. triacylglycerol 4. phospholipid 5. sphingolipid A. cholesterol B. glycerol, 2 fatty acids, phosphate, and choline C. glyceryl tristearate D. sphingosine, fatty acid, phosphate, and choline E. estradiol F. bile salts G. lipids in plasma membranes 105 Solution A. cholesterol 2 steroid B. glycerol, 2 fatty acids, phosphate, and choline 4 phospholipid C. glyceryl tristearate 3 triacylglycerol D. sphingosine, fatty acid, phosphate, and choline 5 sphingolipid E. estradiol 2 steroid F. bile salts 2 steroid G. lipids in plasma membranes 4 phospholipid, 5 sphingolipid . 106 Chapter 17 Lipids 17.8 Cell Membranes 107 Plasma Membranes • Surround cells • Lipid bilayer pf phospholipids • Nonpolar hydrocarbon tails in center • Polar (hydrophilic) sections on outside • Some unsaturated fatty acids give flexibility • Keep aqueous contents inside • Allow certain biochemicals to pass through 108 Cell Membranes Cell membranes • Separate cellular contents from the external environment. • Consist of a lipid bilayer made of two rows of phospholipids. • Have an inner portion made of the nonpolar tails of phospholipids with the polar heads at the outer and inner surfaces. 109 Fluid Mosaic Model of Cell Membranes The lipid bilayer • Contains proteins, carbohydrates, and cholesterol. • Has unsaturated fatty acids that make cell membranes fluid-like rather than rigid. • Has proteins and carbohydrates on the surface that communicate with hormones and neurotransmitters. 110 Transport Through Cell Membranes The transport of substances through cell Membranes involves • Diffusion (passive transport), which moves particles from a higher to a lower concentration. • Facilitated transport, which uses protein channels to increase the rate of diffusion. • Active transport, which moves ions against a concentration gradient. 111 Transport Pathways Through Cell Membranes Copyright © 2007 by Pearson Education, Inc. Publishing as Benjamin Cummings 112 Other Structures in Membranes • Cholesterol molecules add strength • Glycolipids on surface communicate with other cells • Proteins embedded in bilayer move certain substances (water, K+, Cl-, HCO3-) across the cell membrane • Proteins on surface act as receptors for hormones and other biochemicals 113 Diagram of a Plasma Membrane Polar sections Nonpolar tails 114 Learning Check The transport of particles across a cell membrane from high concentration to low concentration is called 1. Facilitated transport 2. Diffusion 3. Active transport 115 Solution The transport of particles across a cell membrane from high concentration to low concentration is called 2. Diffusion 116