Lipids. Classifying Lipids

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1 (Woods) Chem-131 Lec Lipids 1 Lipids Triacylglycerols (triglycerides): a storage form of energy not required for immediate use. Phospholipids, sphingolipids, and cholesterol (together with proteins) are the primary structural components of the membranes that surround all cells and organelles. Steroid hormones: including the sex hormones act as chemical messengers, initiating or altering activity in specific cells. Fat-soluble vitamins A, D, E, and K are required for a variety of physiological functions. Bile salts are needed for the digestion of lipids in the intestinal tract. Classifying Lipids The classification of a compound as a lipid is based on its solubility behavior rather than the presence of a common functional group. Compounds that dissolve in a nonpolar solvent such as toluene or carbon tetrachloride are classified as lipids. Lipids: Hydrolyzable (saponifiable): Undergo hydrolytic cleavage in the presence of an acid, a base, or a digestive enzyme. Nonhydrolyzable (non-saponifiable): Do not undergo hydrolytic cleavage due to lack of an ester, amine, phosphate, or acetal groups. 1

2 (Woods) Chem-131 Lec Lipids 2 Classifying Lipids Cholesterol: a Steroid 2

3 (Woods) Chem-131 Lec Lipids 3 Fatty Acids Fatty Acids Saturated fatty acids: contain no C=C bonds. Palmitic (C 16 ) and Stearic (C 18 ) acids are the most common. Unsaturated fatty acids: contain one or more C=C bonds (monounsaturated and polyunsaturated fatty acids). Oleic (C 18, one C=C) is the most common. Almost all natural unsaturated fatty acids contain cis double bonds. Linoleic and Linolenic acids are essential fatty acids and are synthesized only by plants. All other fatty acids are non-essential. Omega number (ω-). The ω-1 1 carbon is the methyl group farthest from the carbonyl carbon. Linoleic acid ω-6 Linolenic acid ω-3 3

4 (Woods) Chem-131 Lec Lipids 4 Fatty Acids The length of the hydrocarbon chain and the number of double bonds affect the physical properties of fatty acids. Water solubility decreases as the number of carbon atoms increases: Lauric acid (C 12 ): Stearic acid (C 18 ): Glucose: g/l at 30 o C g/l at 30 o C 1100 g/l at 30 0 C (MW Lauric acid) Benzene solubility increases as the number of carbon atoms increases: Lauric acid (C 12 ): Stearic acid (C 18 ): 124 g/l at 30 o C 2600 g/l at 30 o C Melting Points Increase as the number of carbon atoms increases. Decrease as the number of double bonds increases. O HO O HO O HO Stearic Acid MP: 70 C O Lauric Acid MP: 44 C Oleic Acid MP: 13 C HO HO O Linoleic Acid (Omega-6) MP: -5 C Linolenic Acid (Omega-3) MP: -11 C Cis Isomers TRANS vs. CIS R R 4

5 (Woods) Chem-131 Lec Lipids 5 Lauric acid (C12:0): Palm kernel oil CH 3 (CH 2 ) 10 COOH Myristic acid (C14:0): Oil of nutmeg CH 3 (CH 2 ) 12 COOH Palmitic acid (C16:0): Palm oil CH 3 (CH 2 ) 14 COOH Stearic acid (C18:0): Beef tallow CH 3 (CH 2 ) 16 COOH Oleic acid (C18:1 cis Δ 9 ): Olive oil Some Common Fatty Acids (Counting from the carboxyl group) CH 3 (CH 2 ) 7 CH=CH(CHCH(CH 2 ) 7 COOH Linoleic acid (C18:2 cis Δ 9, 12 ): Soybean oil (ω-6) CH 3 (CH 2 ) 4 CH=CHCH 2 CH=CH(CH 2 ) 7 COOH Linolenic acid (C18:3 cis Δ 9, 12, 15 ): Fish oil (ω-3) CH 3 CH 2 CH=CHCH 2 CH=CHCH 2 CH=CH(CH 2 ) 7 COOH Triacylglycerols (Triglycerides) Triglycerides are triesters of glycerol: Simple triglycerides : R 1 =R 2 =R 3 Complex or mixed triglycerides: R 1, R 2, and R 3 are different. The percentage of unsaturated fatty acids varies with the source: Animal sources other than fish: 40-60% Fish (high % of polyunsaturation, ω-3): 75-80% Plant sources: 85-90% 5

6 (Woods) Chem-131 Lec Lipids 6 Triglycerides Coconut oils are unusually high in saturated fatty acids (92%). The high content of double bonds in plant triglycerides results in low melting points. Plant triglycerides are liquid at room temperature Animal triglycerides are solid. The terms fats & oils are used to indicate solid & liquid triglycerides. All triglycerides are water insoluble Thomson - Wadsworth 6

7 (Woods) Chem-131 Lec Lipids 7 Triglycerides: Hydrolysis Acid- or based-catalyzed hydrolysis at the ester groups. Saponification: Basic hydrolysis produces soaps. Digestion and Storage of Triglycerides Triglycerides are too large to diffuse through intestinal membranes. They are first digested in the small intestine (basic ph) with enzymes called lipases with the help of bile salts. The digestion in the intestines produces a mixture of primarily monoglycerides and fatty acids with some diglycerides and glycerol. The intestinal cells then rebuild the triglycerides and combine them with proteins into particles called chylomicrons. Chylomicrons are transported via the lymphatic system to the bloodstream where they are carried to various tissues. Fatty acids not needed for energy are reconverted to triglycerides and stored in adipose cells (fat cells) as fat droplets 7

8 (Woods) Chem-131 Lec Lipids 8 Absorption of Lipids Glycerol and Short-Medium Chain (6-10) Fatty Acids diffuse and are absorbed directly into the bloodstream. Monoglycerides and Long Chain (12-24) Fatty Acids form micelles, are absorbed, and are reformed into new triglycerides. With protein they are transported by Chylomicrons. Bonds break Bonds break Triglyceride The triglyceride and two molecules of water are split to give two fatty acids and a monoglyceride. Monoglyceride + 2 fatty acids Fatty acids, monoglycerides, and glycerol are absorbed into intestinal cells. 8

9 (Woods) Chem-131 Lec Lipids 9 9

10 (Woods) Chem-131 Lec Lipids 10 Triglycerides Triglycerides are the primary energy storage form in animals. Triglycerides: Glycogen: 9.2 kcal/g kcal/g Triglycerides are more efficient for storing energy because they are more highly reduced than glycogen (more O 2 can be added, therefore more energy can be generated). Normal liver and muscle glycogen levels can fill your energy needs for approximately 12 hours. Adipose tissue triglycerides can fill your energy needs for several weeks to a couple of months. Hibernating animals live off of their accumulated body fat during their entire period of hibernation. Catalytic Hydrogenation Catalytic (Ni or Pt) hydrogenation:addition of H 2 to alkene double bonds. Conversion of plant oil into margarine and other products. Double bonds in vegetable oils are hydrogenated in order to convert them into a solid, more palatable form. Vegetable oils contain a much higher percentage of unsaturated fatty acids and almost no cholesterol. 10

11 (Woods) Chem-131 Lec Lipids 11 Catalytic Hydrogenation Catalysts: some trans vs. cis double bonds. Evidence suggests that trans double bonds raise blood cholesterol levels more than cis double bonds and perhaps more than saturated fatty acids. Decreasing double bonds of vegetable oils also increases their shelf life (oxidation of the double bonds causes rancidity). Air oxidation of alkene double bonds: Air oxidation of double bonds generates two fragments, each carbon of the double bond being converted into a COOH group. This generates low molecular weight, volatile and offensive-smelling smelling carboxylic and dicarboxylic acids. Rancidity reactions are slowed by refrigeration. Antioxidants such as BHA (butylated hydroxyanisole) or BHT (butylated hydroxytoluene) are often added to vegetable oils. Waxes Waxes have a variety of protective functions in plants and animals: Coating on fruits, vegetables, and plant leaves to protect against parasites, prevent mechanical damage, and prevent water loss. Coating on hair, furs, feathers, and skin keeping them lubricated, pliable, and waterproof. Plankton and some other marine organism use waxes instead of triacylglycerols for energy storage. 11

12 (Woods) Chem-131 Lec Lipids 12 Phospholipids & Sphingolipids Cell-membrane: Glycerophospholipids (phospholipids): based on glycerol. Sphingolipids: based on sphingosine. Unlike the triglycerides, the glycerophospholipids and sphingolipids have one highly hydrophilic group. The hydrophilic group is responsible for the amphipathic nature of these lipids, which allows their assembly into cell membranes. Lecithin (phosphatidylcholine) 12

13 (Woods) Chem-131 Lec Lipids 13 Membranes Membrane lipids are organized into lipid bilayers as shown in the fluid mosaic model: Passive Transport of Two Types of Molecule Equilibrium Nonpolar vs. Polar O 2 /CO 2 vs. Na + 13

14 (Woods) Chem-131 Lec Lipids 14 Osmosis How Animal and Plant Cells Behave Isotonic solution Hypotonic solution Hypertonic solution H H 2 O H 2 O 2 O H 2 O Animal cell (1) Normal (2) Lyse (3) Shrivel (crenate) H 2 O H 2 O H 2 O Plasma membrane H 2 O Plant cell (4) Flaccid (5) Turgid (6) Shrivel (plasmolyze) 14

15 (Woods) Chem-131 Lec Lipids 15 Transport of a Solute Across a Membrane Active Transport of a Solute Across a Membrane Transport protein Solute ATP P ADP Protein changes shape P Phosphate detaches 1 Solute binding 2 Phosphorylation 3 Transport 4 Protein reversion P 15

16 (Woods) Chem-131 Lec Lipids 16 Catabolism of Various Food Molecules (The reverse occurs when excess energy is available to the body) Starch & Glycogen Hydrolyzed to Glucose Triglycerides Hydrolyzed to Glycerol & Fatty Acids Proteins Hydrolyzed to Amino Acids (No Storage) Fatty Acids Hydrolyzed to Acetyl CoA Carbohydrates: Quick Energy Fats: Energy Storage 16

17 (Woods) Chem-131 Lec Lipids 17 Compounds in pink are Glucogenic, as they can eventually be converted to glucose. Compounds in blue are Ketogenic, as they can be converted to acetyl-coa (ketone bodies). Fatty Acids CAN NOT be converted to glucose Amino acids are a good source of glucose when carbohydrate is not available (Protein Sparing). Urea Formation & Excretion Ammonia Urea: a much less toxic compound, formed in the liver. Urea is excreted through the kidneys to rid the body of unused nitrogen. 17

18 (Woods) Chem-131 Lec Lipids 18 Fat vs. Carbohydrate Energy Excess Energy Excess Protein is converted to fat but this is inefficient and indirect. Its priority is other roles. Excess Carbohydrate is converted to fat but this is inefficient and indirect. Its priority is glycogen stores. Excess Fat is EFFICIENTLY converted to fat. 18