Chapter 10 – Lipids10.1 Storage Lipids- Fatty acids – hydrocarbon derivatives at about the same low oxidation state as the hydrocarbons in fossil fuels- Two types of fatty acids:o Triacylglycerolso WaxesFatty Acids Are Hydrocarbon Derivatives- Fatty acids are carboxylic acids with hydrocarbon chains ranging from 4 to 36 carbons longo Some are unbranched and fully saturatedo Others have one or more double bondso 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 carbonso Formed by successive condensations of acetate units- In most monounsaturated fatty acids the double bond is between C-9 and C-10o The other double bonds of polyunsaturated fatty acids are generally between 12 and 13 and between 15 and 16o Double bonds of polyunsaturated fatty acids are almost never conjugatedo In most naturally occurring unsaturated fatty acids, the double bonds 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- At room temperature, saturated fatty acids from 12:0 to 24:0 have a waxy consistencyo 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 acidsTriacylglycerols Are Fatty Acid Esters of Glycerol- Triacylglycerols – simplest lipids constructed from fatty acidso Also called: Triglycerides Fats Neutral fatso Composed of three fatty acids each in ester linkage with a single glycerol 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 waterTriacylglycerols Provide Stored Energy and Insulation- Adipocytes store large amount of triacylglycerols as fat droplets that nearly fill the cello Triacylglycerols are also stored as oils in the seeds of many types of plants- Lipases – enzymes that catalyze the hydrolysis of stored triacylglycerols, releasing fatty acids for export to sites where they are required as fuelo 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 carbohydrateso 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 insulationPartial 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 oxidativelyo Produces aldehydes and carboxylic acids of shorter chain length and higher volatility- Partial hydrogenation converts many of the cis double bonds to single bonds and increases the melting temperature of oilso Some cis bonds are converted to trans bonds which leads to a higher incidence of cardiovascular diseaseWaxes Serve as Energy Stores and Water Repellents- Biological waxes are esters of long-chain saturated and unsaturated fatty acids with long-chain alcoholso 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 polishes10.2 Structural Lipids in Membranes- In glycerophospholipids and some sphingolipids, a polar head group is joinedto the hydrophobic moiety by a phosphodiester linkage creating phospholipids- Glycolipids – lack a phosphate but have a simple sugar or complex oligosaccharide at their polar endsGlycerophospholipids 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 carbono 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 pHo The polar alcohol may be negatively charged, neutral, or positively charged.Some Glycerophospholipids Have Ether-Linked Fatty Acids- Ether lipids – lipids in which one of the two acyl chains is attached to glycerolin ether, rather than ester, linkageo 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 lipido Released from leukocytes and stimulates platelet aggregation and the release of serotonin from plateletsChloroplasts Contain Galactolipids and Sulfolipids- Galactolipids – the second group of membrane lipids that predominate in plant cellso One or two galactose residues are connected by a glycosidic linkage toC-3 of a 1,2-diacylglycerolo Localized in the thylakoid membranesArchaea Contain Unique Membrane Lipids- Archaea have membrane lipids containing long-chain branched hydrocarbons linked at each end to glycerolo Linkages are through ether bonds which are more stable to hydrolysisat low pH and high temperatureSphingolipids Are Derivatives of Sphingosine- Sphingolipids – the fourth large class of membrane lipids that have a polar head group and two nonpolar tailso Contain no glycerol- Ceramide – when a fatty acid is attached in amide linkage to the –NH2 on C-2 of sphingosine- Three
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