LIPIDS

  • Definition
    • Lipids are a large and diverse group of naturally occurring organic compounds.
    • Soluble in non-polar organic solvents (e.g., ether, chloroform, acetone, & benzene).
    • Generally insoluble in water.

CLASSIFICATION OF LIPIDS

  • Classification by Chemical Nature

    • Lipids can be classified into three groups:
    • Simple lipids
    • Compound or complex lipids
    • Derived lipids or precursors

  • Classification by Biological Function

    • Lipids can be classified into two groups:
    • Storage lipids
    • Structural lipids

Simple Lipids

  • Definition

    • Simple lipids are esters of fatty acids with various alcohols.
    • Upon hydrolysis, they yield fatty acids and alcohols, with glycerol being the usual alcohol.

  • Examples

    • Fats: Esters of predominantly saturated fatty acids with glycerols (solid state).
    • Oils: Esters of predominantly unsaturated fatty acids with glycerols (liquid state).
    • Waxes: Esters of fatty acids with higher molecular weight monohydric alcohols.

Complex Lipids

  • Definition

    • Complex lipids are esters or amides of fatty acids with alcohol and other additional groups (besides alcohol and fatty acid).

  • Examples

    • Phospholipids: Contain fatty acids, alcohol, and a phosphoric acid residue.
    • Subcategories:
      • Glycerolphospholipids (alcohol is glycerol)
      • Sphingophospholipids (alcohol is sphingosine)
    • Glycolipids: Contain fatty acid, sphingosine, and carbohydrate.
    • Other complex lipids: Sulfolipids, aminolipids, lipoproteins.

Derived Lipids or Precursors

  • Examples
    • Steroids
    • Eicosanoids
    • Terpenes
    • Other alcohols, fatty aldehydes, ketone bodies
    • Hydrocarbons
    • Lipid-soluble vitamins
    • Hormones
    • Acyl glycerols
    • Cholesterol
    • Cholesteryl esters (uncharged – neutral lipids)

Storage Lipids

  • Storage Forms of Energy

    • Fats or oils, derivatives of fatty acids
    • Fatty acids serve as fuel molecules stored as triacylglycerols, which are uncharged esters of glycerol.
    • Cellular oxidation of fatty acids is highly exergonic.

  • Characteristics

    • Triacylglycerols are composed of fatty acids and glycerol.
    • Fatty acids are organic acids, glycerols are organic alcohols.

Fatty Acids

  • Definition

    • Fatty acids are organic acids that partially ionize.
    • Composed of a hydrocarbon chain with a carboxylic acid at one end; polar carboxylic acid end vs non-polar hydrocarbon chain.

  • Occurrence

    • Esters in natural fats and oils, free fatty acids as transport forms in plasma.
    • Natural fats are usually straight-chain derivatives containing an even number of carbon atoms.

  • Types

    • Saturated: No double bonds.
    • Unsaturated:
    • Monounsaturated (one double bond)
    • Polyunsaturated (multiple double bonds)
    • Unsaturated fatty acids usually exhibit a cis configuration.

  • Patterns of Unsaturation

    • General locations for double bonds: C-9 and C-10 in monounsaturated, Δ12 and Δ15 in polyunsaturated (with exceptions like arachidonic acid at Δ5,8,11,14).

  • Double Bond Configuration

    • Most naturally occurring unsaturated fatty acids exhibit cis configuration for double bonds.

Physical Properties of Fatty Acids

  1. Solubility

    • Fatty acids are insoluble in water but soluble in non-polar solvents.
    • Solubility decreases with increased chain length; polar carboxylic acid group allows for slight solubility of short-chain fatty acids.

  2. Melting Points

    • Influenced by chain length and degree of unsaturation.
    • Saturated fatty acids are waxy solids; unsaturated fatty acids are oily liquids at room temperature.
    • Saturated forms allow for flexibility and stable conformation due to free rotation around carbon-carbon bonds.

  3. Packing in Crystalline Arrays

    • Molecules can pack closely in fully saturated forms, reducing steric hindrance.

  4. Melting Points Summary

    SymbolCommon NameSystematic NameStructuremp (°C)
    12:0Lauric aciddodecanoic acidCH3(CH2)10COOH44.2
    14:0Myristic acidtetradecanoic acidCH3(CH2)12COOH52
    16:0Palmitic acidhexadecanoic acidCH3(CH2)14COOH63.1
    18:0Stearic acidoctadecanoic acidCH3(CH2)16COOH69.6
    20:0Arachidic acideicosanoic acidCH3(CH2)18COOH75.4

  5. Effect of Unsaturation

    • Unsaturated fatty acids have kinks caused by cis double bonds resulting in less tight packing and weaker interactions.
    • This leads to lower melting points in unsaturated compared to saturated fatty acids of the same chain length.

Nomenclature of Fatty Acids

  1. Naming Conventions

    • Types of naming:
      • Systematic naming
      • Common naming
      • Alphabetic notation.

  2. Systematic Naming

    • Derived from the parent hydrocarbon's name by replacing the final 'e' with 'oic'.
    • Example: C18 saturated fatty acid becomes octadecanoic acid.

  3. Symbolic Notation

    • 18:0 denotes C18 fatty acid with no double bond.
    • 18:2 denotes C18 fatty acid with two double bonds.
    • Δ symbol used to indicate double bond positions.
    • Example: cis-Δ9 indicates a cis double bond between carbons 9 and 10.

  4. Counting from Distal End

    • Position of double bonds can also be designated by counting from the ω carbon (distal end):
      • ω-3, ω-6, ω-9 indicate double bonds at specific counts from the ω end.

  5. Fatty Acid Examples

    • 14:0: Myristic acid
    • 16:0: Palmitic acid
    • 18:0: Stearic acid
    • 18:1: cisΔ9 Oleic acid
    • 18:2: cis Δ9,12 Linoleic acid
    • 18:3: cis Δ9,12,15 Linolenic acid
    • 20:4: cis Δ5,8,11,14 Arachidonic acid

Functional Significance of Some Fatty Acids

Common NameShort FormFunctional Significance
Butyric acid4:0Found in significant amounts in milk
Capric acid10:0
Palmitic acid16:0Structural lipids and triacylglycerols contain fatty acids with at least 16 carbons
Palmitoleic acid16:1D9
Linoleic acid18:2D9,12Omega-6 fatty acid; essential in diet
Linolenic acid18:3D9,12,15Omega-3 fatty acid; essential in diet
Arachidonic acid20:4D5,8,11,14Precursor of prostaglandins

Triacylglycerols (Triglycerides)

  • Structure

    • Triacylglycerols are nonpolar, hydrophobic molecules formed from glycerol and fatty acids.
    • Glycerol's hydroxyl groups link to fatty acids via ester linkages, resulting in a nonpolar structure.

  • Types

    • Simple triacylglycerols: Same fatty acid in all three positions; named by the fatty acid contained.
    • Mixed triacylglycerols: Different fatty acids in different positions; common in natural fats.

  • Functions

    • Advantages of using triacylglycerols as stored fuels:
    • More energy yield per gram when oxidized than carbohydrates due to a more reduced state of carbon atoms in fatty acids.
    • Hydrophobic nature means they lack additional weight associated with water from polysaccharides.
    • Energy storage: Triacylglycerols can store more energy than muscle glycogen, serving as long-term energy sources.

Structural Lipids in Biological Membranes

  • Major Classes
    • Glycerophospholipids
    • Sphingolipids
    • Cholesterol

  1. Glycerophospholipids

    • Derivatives of phosphatidic acid.
    • Major lipid components of biological membranes.
    • Composed of sn-glycerol-3-phosphate with two fatty acids and a polar/charged group.
    • Amphiphilic molecules (both hydrophilic and hydrophobic parts).

  2. Sphingolipids

    • Derivatives of sphingosine (long-chain amino alcohol).
    • Comprises a polar head group and two nonpolar tails.
    • Types include Sphingomyelins, Glycosphingolipids, and Gangliosides.

  3. Cholesterol

    • Major sterol in animal tissues.
    • Characterized by a polar head group and a nonpolar hydrocarbon body.
    • Important in plasma membranes and precedes steroid hormones and bile acids.

Physicochemical Properties of Fats/Oils

  • Physical State
    • Saturated fatty acids are solid, while unsaturated are liquid at room temperature.
  • Taste and Odor
    • Pure fats are tasteless and odorless.
  • Solubility
    • Insoluble in water; solubility in organic solvents decreases as chain length increases.
  • Melting Point
    • Depends on chain length and degree of unsaturation.
  • Specific Gravity
    • Less than 1, allowing fats to float on water.
  • Geometric Isomerism
    • Includes cis and trans forms.
  • Emulsification
    • Process of converting lipid masses into smaller droplets.

Chemical Properties of Fats

  • Hydrogenation
    • Addition of hydrogen to double bonds.
  • Halogenation
    • Addition of halogens to double bonds.
  • Oxidation
    • Unsaturated fatty acids are susceptible to oxidation.
  • Rancidity
    • Hydrolytic rancidity: triglycerides hydrolyzed to fatty acids and glycerol.
    • Oxidative rancidity: unsaturated fatty acids oxidized by atmospheric oxygen.

Quantitative Tests for Fats

  • Acid Value
    • Milligrams of KOH needed to neutralize free fatty acids in 1 g of fat.
  • Saponification Value
    • Milligrams of KOH needed to saponify 1 g of fat.
  • Iodine Value
    • Grams of iodine absorbed by 100 g of fat.
  • Polenske Value
    • Milliliters of 0.1N KOH needed to neutralize insoluble fatty acids from 5 g of fat.
  • Reichert-Meissl Number
    • Milliliters of 0.1N KOH needed to neutralize soluble, volatile fatty acids from 5 g of fat.
  • Acetyl Number
    • Milligrams of KOH needed to neutralize acetic acid from saponification of acetylated fat.

The End