Topic 8: Lipids

8.1 Introduction

• Definition: Naturally occurring esters of fatty acids and alcohols.
• Types: Fats, waxes, fat-soluble vitamins (A, D, E, K), glycerides, phospholipids, steroids (lipids with a ring structure).
• Characteristics:
  • Water-insoluble
  • Extracted using organic solvents (ether, chloroform, benzene).
• Biological Functions:
  • Energy storage
  • Cell signaling
  • Structural support of cell membranes
  • Heat and electrical insulation
  • Buoyancy
  • Waterproofing
  • Production of metabolic water
  • Protection of internal organs

8.2 Fatty Acids

• Structure: Hydrocarbon chain with a terminal carboxylic acid group [CH₃(CH₂)ₓCOOH].
• Properties: Amphipathic with a polar head (hydrophilic) and nonpolar tail (hydrophobic).

8.2.1 Saturated Fatty Acids

• Characteristics: No carbon-carbon double bonds, only single bonds (C-C).
• Properties: Rigid, waxy solids at room temperature.
• Example: Stearic acid (CH₃(CH₂)₁₆COOH).
  • Found in: Butter, cheese, coconut oil, meat, margarine, fried foods.
  • Uses: Detergents, soaps, shampoos, shaving creams.

8.2.2 Unsaturated Fatty Acids

• Characteristics: One or more carbon-carbon double bonds (C=C).
• Types:
  • Monounsaturated: One double bond.
  • Polyunsaturated: Two or more double bonds.
• Properties: Flexible, lower melting points than saturated fatty acids.
• Example: Oleic acid (CH₃(CH₂)₇CH=CH(CH₂)₇COOH).
  • Found in: Olive oil, avocado, soybean oil, canola oil, sunflower oil, walnuts.

8.2.2.1 Cis and Trans Unsaturated Fatty Acids

• Cis: Hydrogen atoms on the same side of the double bond, more flexible.
• Trans: Hydrogen atoms on opposite sides of the double bond.

8.2.3 Glycerol

• Structure: Three-carbon poly alcohol.
• Properties: Colorless, viscous, sweet-tasting, water-soluble.
• Uses: Solvent, sweetener, preservative, additive in food, pharmaceuticals, cosmetics.

8.2.4 Glycerides

• Definition: Ester compounds of glycerol and fatty acids.
• Formation: Combines through condensation, releasing water.
• Types:
  • Monoglyceride: One fatty acid chain.
  • Diglyceride: Two fatty acid chains.
  • Triglyceride: Three fatty acid chains.
• Properties: Hydrophobic tails, insoluble in water.
• Classification:
  • Fats: Solid at 20°C.
  • Oils: Liquid at 20°C.

8.2.5 Phospholipids

• Structure: Modified triglycerides with one fatty acid replaced by a phosphate group.
• Properties: Forms micelles or liposomes in water due to hydrophobic interactions.
• Function: Found in cell membranes, forming a phospholipid bilayer.

8.3.6 Waxes

• Structure: Esters of fatty acids with long-chain alcohols.
• Functions: Waterproofing in plants and animals.
• Examples:
  • Beeswax: Used in honeycombs.
  • Carnauba wax: Used in polishes, dental floss, food products.

8.3.7 Steroids

• Structure: Lipid compounds with four rings.
• Properties: Non-polar due to CH groups.
• Examples:
  • Cholesterol: Found in animal cell membranes, precursor to vitamins and hormones.