Fatty acids as the hydrocarbons in fossil fuels hydrocarbon derivatives at about the same low oxidation state Chapter 10 Lipids 10 1 Storage Lipids Two types of fatty acids o Triacylglycerols o Waxes Fatty Acids Are Hydrocarbon Derivatives Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons long o Some are unbranched and fully saturated o Others have one or more double bonds o A few have three carbon rings hydroxyl groups or methyl group branches Most common fatty acids have even numbers of carbon atoms in an unbranched chain of 12 to 24 carbons o Formed by successive condensations of acetate units In most monounsaturated fatty acids the double bond is between C 9 and C 10 o The other double bonds of polyunsaturated fatty acids are generally between 12 and 13 and between 15 and 16 o Double bonds of polyunsaturated fatty acids are almost never o In most naturally occurring unsaturated fatty acids the double bonds conjugated are in the cis configuration Humans cannot synthesize omega 3 PUFA linolenic acid and so must obtain it in the diet An imbalance of omega 3 and omega 6 PUFAS in the diet is associated with an increased risk of cardiovascular disease The longer the fatty acyl chain and the fewer the double bonds the lower the solubility in water consistency At room temperature saturated fatty acids from 12 0 to 24 0 have a waxy o Unsaturated fats of these lengths are liquids In vertebrates free fatty acids circulate in the blood bound noncovalently to a protein carrier serum albumin o Fatty acids are present in blood plasma mostly as carboxylic acid derivatives such as esters or amides Generally less soluble in water than free fatty acids Triacylglycerols Are Fatty Acid Esters of Glycerol Triacylglycerols simplest lipids constructed from fatty acids o Also called Triglycerides Fats Neutral fats glycerol o Composed of three fatty acids each in ester linkage with a single Simple triacylglycerols have the same kind of fatty acid in all three positions Most that are naturally occurring are mixed Triacylglycerols are nonpolar hydrophobic molecules essentially insoluble in water Lipids have lower specific gravities than water Triacylglycerols Provide Stored Energy and Insulation Adipocytes store large amount of triacylglycerols as fat droplets that nearly fill the cell o Triacylglycerols are also stored as oils in the seeds of many types of plants enzymes that catalyze the hydrolysis of stored triacylglycerols Lipases releasing fatty acids for export to sites where they are required as fuel o Found in adipocytes Two advantages to using triacylglycerols over polysaccharides for fuel o The carbon atoms of fatty acids are more reduced and oxidation of triacylglycerols yields more than twice as much energy as the oxidation of carbohydrates o Triacylglycerols are hydrophobic and therefore unhydrated so the organism that carries fat as fuel does not have to carry extra water weight Polysaccharides are advantageous because they are quick sources of metabolic energy and they are readily soluble in water Triacylglycerols can also serve as insulation Partial Hydrogenation of Cooking Oils Produces Trans Fatty Acids Rancidity occurs when a fat is exposed to oxygen and is when the double bonds are cleaved oxidatively higher volatility o Produces aldehydes and carboxylic acids of shorter chain length and Partial hydrogenation converts many of the cis double bonds to single bonds and increases the melting temperature of oils o Some cis bonds are converted to trans bonds which leads to a higher incidence of cardiovascular disease Waxes Serve as Energy Stores and Water Repellents Biological waxes are esters of long chain saturated and unsaturated fatty acids with long chain alcohols o Generally higher melting points Certain skin glands secrete waxes to protect hair and skin and keep it pliable lubricated and waterproof Some types of leaves are coated in waxes that prevent excessive evaporation of water and protects against parasites Lanolin beeswax carnauba wax and wax extracted from spermaceti oil are widely used in the manufacture of lotions ointments and polishes 10 2 Structural Lipids in Membranes In glycerophospholipids and some sphingolipids a polar head group is joined to the hydrophobic moiety by a phosphodiester linkage creating phospholipids oligosaccharide at their polar ends lack a phosphate but have a simple sugar or complex Glycolipids Glycerophospholipids Are Derivatives of Phosphatidic Acid Glycerophospholipids membrane lipids in which two fatty acids are attached in ester linkage to the first and second carbons of glycerol and a highly polar or charged group is attached through a phosphodiester linkage to the third carbon o Also called phosphoglycerides Glycerol is prochiral In glycerophospholipids the head group is joined to glycerol through a phosphodiester bond in which the phosphate group bears a negative charge at neutral pH o The polar alcohol may be negatively charged neutral or positively charged Some Glycerophospholipids Have Ether Linked Fatty Acids Ether lipids in ether rather than ester linkage lipids in which one of the two acyl chains is attached to glycerol o May be saturated or contain a double bond between C 1 and C 2 plasmalogens Vertebrate heart tissue halophilic bacteria ciliated protists and certain invertebrates are uniquely enriched in ether lipids Platelet activating factor a potent molecular signal and an ether lipid o Released from leukocytes and stimulates platelet aggregation and the release of serotonin from platelets Chloroplasts Contain Galactolipids and Sulfolipids the second group of membrane lipids that predominate in Galactolipids plant cells o One or two galactose residues are connected by a glycosidic linkage to C 3 of a 1 2 diacylglycerol o Localized in the thylakoid membranes Archaea Contain Unique Membrane Lipids Archaea have membrane lipids containing long chain branched hydrocarbons linked at each end to glycerol o Linkages are through ether bonds which are more stable to hydrolysis at low pH and high temperature the fourth large class of membrane lipids that have a polar when a fatty acid is attached in amide linkage to the NH2 on C 2 Sphingolipids Are Derivatives of Sphingosine Sphingolipids head group and two nonpolar tails o Contain no glycerol Ceramide of sphingosine Three subclasses of sphingolipids o Sphingomyelins as their polar head group contain phosphocholine or phosphoethanolamine Classified
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