Chemistry of Lipids

Chemistry of Lipids

• Lipids are diverse compounds insoluble in water, but soluble in nonpolar solvents.
• Major functions include energy storage and membrane structure.

Classes of Lipids

• Simple Lipids: Composed of fatty acids and their derivatives.
  • Examples: fats, oils, waxes
• Complex Lipids: Contain additional components like phosphates, sugars, or amino groups.
  • Subtypes:
    • Phospholipids: Major membrane components, e.g., phosphatidylcholine
    • Sphingolipids: Important in nerve tissues and cell membranes
    • Glycolipids: Contain a sugar group; found in cell membranes, notably in the nervous system
• Steroids: Derived from a common four-ring structure.
  • Cholesterol: A crucial component in cell membranes and a precursor for steroid hormones.

Other Lipid Groups and Types

• Prostaglandins & Leukotrienes: Signaling molecules involved in inflammation, pain, fever, allergic reactions, and immune response.
• Steroid Hormones: Regulate metabolic processes and reproduction (e.g. testosterone, estrogen)

Classification Based on Solubility

• Saponifiable Lipids: Hydrolyzed by alkaline solutions (e.g., fats, oils, and waxes)
• Non-saponifiable Lipids: Cannot be hydrolyzed by alkaline solutions (e.g., steroids, prostaglandins)

Building Blocks of Lipids

• Glycerol: A three-carbon compound with three hydroxyl groups. Serves as a site for esterification with fatty acids.
• Fatty Acids: Carboxylic acids with long aliphatic chains. Classified as saturated or unsaturated based on the presence of double bonds.

Special Types of Lipids

• Waxes: Formed from long-chain fatty acids esterified with a monohydric alcohol. Used in various applications like ointments, polishes and creams.

Chemical Properties of Lipids

• Hydrolysis: Lipids break down into fatty acids and glycerol in the presence of specific enzymes or strong acids.
• Rancidity: Results from hydrolysis and oxidation of fats and oils, characterized by an unpleasant odor and taste.
• Hydrogenation: Conversion of oils (liquid) to fats (solid) by adding hydrogen to unsaturated fatty acids.
• Acrolein Formation: During hydrolysis, glycerol can break down into acrolein which is identifiable by a pungent odor. This occurs when fats or oils are burned.
• Reaction with Iodine: Unsaturated fatty acids react with iodine; the amount that reacts indicates the degree of unsaturation.

Tests for Lipids

• Liebermann-Burchard Reaction: Tests for cholesterol (color change from Pink → Lilac → Deep Green)
• Salkowski Reaction: Tests for cholesterol (observations in different layers of solution)
• Acrolein Reaction: Tests for glycerol (pungent odor/blackening)
• Iodine Reaction: Tests for saturation of lipids (disappearance of iodine = unsaturated; brown iodine = saturated).
• Solubility Test: Lipids are soluble in nonpolar solvents, but insoluble in polar ones (water).
• Emulsion Test: Formation of cloudy emulsions in a mixture of lipids, alcohol and water indicate lipid presence.
• Saponification Test: Alkaline hydrolysis of fats and oils to produce glycerol and soap.