LIPIDS

• Lipid


  An organic compound found in living organisms that is insoluble (or only sparingly soluble) in water but soluble in nonpolar organic solvents

  
• Lipids


  • Heterogeneous class of naturally occurring organic compounds classified together on the basis of common solubility properties:
  • Insoluble/immiscible in water (hydrophobic; nonpolar)
  • Soluble in organic solvents including diethyl ether, chloroform, and acetone

  
• Classification of Lipids (based on biochemical function)

  • Energy-storage lipids (triacylglycerols)
  • Membrane lipids (phospholipids, sphingoglycolipids, and cholesterol)
  • Emulsification lipids (bile acids)
  • Messenger lipids (steroid hormones and eicosanoids)
  • Protective-coating lipids (biological waxes)

  
• Classification of Lipids (based upon whether or not saponification occurs when a lipid is placed in basic aqueous solution; hydrolysis)


  • Saponifiable lipids (triacylglycerols, phospholipids, sphingoglycolipids, and biological waxes)
  • Nonsaponifiable lipids (cholesterol, steroid hormones, bile acids, and eicosanoids)

  
• Classification of Lipids (Structure)

  • Simple Lipids: Triglycerols, Fats and Oils, Waxes
  • Complex Lipids: Phospholipids, Glycolipids, Glycerophospholipids, Sphingophospholipids, Cerebrosides, Globosides, Gangliosides, Sulfatides

  
• Triacylglyceride or Triglycerol or TAG

  Glycerol with three fatty acid chains attached

  
• Fatty acids


  • Most frequently encountered lipid building block (structural unit)
  • A naturally occurring monocarboxylic acid
  • Has an acid group at one end and a methyl group at the other end
  • Made up of a long hydrophobic hydrocarbon chain (highly reduced; ranging from 4 to 36 carbons) and a carboxylic acid polar group
  • Amphipathic (containing both nonpolar and polar regions)
  • Hydrophobic (nonpolar)

  
• Classification of Fatty acids (based on the length of the hydrocarbon tails)

  • Long-chain (C12 to C26)
  • Medium-chain (C8 and C10)
  • Short-chain (C4 and C6)

  
• Classification of Fatty acids (based on degree of unsaturation or presence of double bond)

  • Saturated fatty acids (SFAs)
  • Unsaturated fatty acids: Monounsaturated fatty acids (MUFAs), Polyunsaturated fatty acids (PUFAs)

  
• Saturated fatty acid


  A fatty acid with a carbon chain in which all carbon–carbon bonds are single bonds

  
• Monounsaturated fatty acid (MUFA)


  A fatty acid with a carbon chain in which one carbon–carbon double bond is present

  
• Polyunsaturated fatty acid (PUFA)


  A fatty acid with a carbon chain in which two or more carbon–carbon double bonds are present

  
• Nomenclature of Fatty Acids

  • Specifies the chain length and number of double bonds, separated by a colon
  • The positions of any double bonds are specified by superscript numbers following (delta)

  
• Examples of fatty acid nomenclature

  • Butyric (butanoic acid): CH3(CH2)2COOH or C4:0
  • Caproic (hexanoic acid): CH3(CH2)4COOH or C6:0
  • Caprylic (octanoic acid): CH3(CH2)6COOH or C8:0
  • Capric (decanoic acid): CH3(CH2)8COOH or C10:0
  • Lauric (dodecanoic acid): CH3(CH2)10COOH or C12:0
  • Myristic (tetradecanoic acid): CH3(CH2)12COOH or C14:0
  • Palmitic (hexadecanoic acid): CH3(CH2)14COOH or C16:0
  • Stearic (octadecanoic acid): CH3(CH2)16COOH or C18:0
  • Arachidic (eicosanoic acid): CH3(CH2)18COOH or C20:0

  
• Nomenclature of unsaturated fatty acids

  • Unsaturated acids end in "-enoic"
  • If one double bond → octadecenoic acid
  • If two double bonds → octadecadienoic acid
  • If three double → octadecatrienoic acid

  
• Examples of unsaturated fatty acid nomenclature


  • cis-,cis-,cis-9,12,15-Octadecatrienoic acid (Linolenic acid)
  • cis-9-Octadecenoic acid (Oleic acid)

  
• Omega-3 fatty acid


  An unsaturated fatty acid with its endmost double bond three carbon atoms away from its methyl end

  
• Omega-6 fatty acid

  An unsaturated fatty acid with its endmost double bond six carbon atoms away from its methyl end

  
• Physical properties of fatty acids

  • Determined by the length of the hydrocarbon chain and the degree of unsaturation of the hydrocarbon chain
  • Solubility in water decreases as carbon chain length increases
  • Melting points increase with increasing molar mass (London forces), but the presence of double bonds (all cis-stereochemistry) lowers the melting point

  
• In nearly all naturally occurring unsaturated fatty acids, the double bonds are in the cis-configuration

  
• Trans-fatty acids are produced during hydrogenation of fish or vegetable oils and are generally recommended to be avoided due to their correlation with increased LDL (bad cholesterol) and decreased HDL (good cholesterol)

  
• Characteristics of fatty acids: long straight-chain (no branching) monocarboxylic acids, have an even number of carbon atoms (usually 12 to 24), either saturated or unsaturated, the double bonds are almost always in the cis configuration, rarely found free in nature but rather occur as part of the structure of more complex lipid molecules

  
• Fast food items like fries, doughnuts, and cookies tend to be high in trans fatty acids
• Characteristics of fatty acids

  • Long straight-chain (no branching) monocarboxylic acids
  • Have an even number of carbon atoms (usually 12 to 24)
  • Either saturated (no double bonds) or unsaturated (contain one or more carbon-carbon double bonds, the first is usually at the 9th carbon)
  • The double bonds are almost always in the cis configuration
  • The cis double bonds result in a bent chain which does not allow fatty acids to pack as close together and consequently lower the melting point
  • Rarely found free in nature but rather occur as part of the structure of more complex lipid molecules

  
• Essential fatty acid


  A fatty acid needed in the human body that must be obtained from dietary sources because it cannot be synthesized within the body, in adequate amounts, from other substances
• Essential fatty acids


  • Linoleic acid 18:2 9,12 or 18:2 6
  • Linolenic acid 18:3 9,12,15 or 18:3 3

  
• Essential fatty acids (PUFAs)

  • Found in plant and fish oils
  • Needed for proper membrane structure
  • Serve as starting materials for the production of several nutritionally important longer chain omega-6 and omega-3 acids
  • When these two acids are missing from the diet, the skin reddens and becomes irritated, infections and dehydration are likely to occur, and the liver may develop abnormalities
  • If the fatty acids are restored, then the conditions reverse themselves
  • Infants are especially in need of these acids for their growth
  • Human breast milk has a much higher percentage of the essential fatty acids than cow's milk
• Linoleic acid


  The starting material for the biosynthesis of arachidonic acid, which is the major starting material for eicosanoids, substances that help regulate blood pressure, clotting, and several other important body functions

  
• Linolenic acid


  The starting material for the biosynthesis of two additional omega-3 fatty acids, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), which are important constituents of the communication membranes of the brain and are necessary for normal brain development, and are also active in the retina of the eye

  
• Classification of lipids (based on biochemical function)

  • Energy-storage lipids (triacylglycerols)
  • Membrane lipids (phospholipids, sphingoglycolipids, and cholesterol)
  • Emulsification lipids (bile acids)
  • Messenger lipids (steroid hormones and eicosanoids)
  • Protective-coating lipids (biological waxes)

  
• Triacylglycerol or triglycerides


  A lipid formed by esterification of three fatty acids to a glycerol molecule, which serves as energy storage in adipose cells

  
• Fats


  • A mixture of triacylglycerols that is solid because it contains a high proportion of saturated fatty acids
  • Straight chain fatty acids
  • Saturated or trans-fatty acids (more linear than cis-fatty acids) pack closely together, have attractive forces, and have higher melting points

  
• Oils


  • A mixture of triacylglycerols that is liquid because it contains a high proportion of unsaturated (mono- or polyunsaturated) fatty acids
  • Unsaturated fatty acids cannot pack closely together because of cis double bonds, which causes kinks
  • Intermolecular forces are weaker, less energy is needed to separate molecules, and they have lower melting points

  
• Cell membrane


  A lipid-based structure that separates a cell's aqueous-based interior from the aqueous environment surrounding the cell, controlling the movement of substances into and out of the cell
• Membrane lipids


  • Up to 80% of the mass of a cell membrane is lipid material consisting primarily of phospholipids, glycolipids, and cholesterol

  
• Phospholipids


  The most abundant lipids in all biological cellular membranes, with a bipolar nature central to the structure and function of cell membranes, acting as emulsifying agents at cell membrane surfaces since they contain both a polar and nonpolar component

  
• Glycerophospholipids


  A lipid that contains two fatty acids and a phosphate group esterified to a glycerol molecule and an alcohol esterified to the phosphate group

  
• Alcohols esterified to the phosphate group in glycerophospholipids

  • Choline (phosphatidylcholine)
  • Ethanolamine (phosphatidylethanolamine)
  • Serine (phosphatidylserine)
• Sphingophospholipids


  A lipid that contains one fatty acid and one phosphate group attached to a sphingosine molecule and an alcohol attached to the phosphate group

  
• Sphingomyelins


  Sphingophospholipids in which the alcohol esterified to the phosphate group is choline, found in all cell membranes and important structural components of the myelin sheath