Notes on Chapter 18: Lipids

Lipids Overview

Learning Objectives

• Describe four general characteristics of fatty acids.

• Draw structural formulas of triglycerides based on their component parts.

• Describe the structural similarities and differences between fats and oils.

• Write key reactions for fats and oils.

• Compare the structures of fats and waxes.

• Identify the structural characteristics of complex lipids.

• Identify classes of lipids found in cell membranes.

• Identify the structural characteristics of steroids and steroid hormones.

Classification of Lipids and Fatty Acids

• Lipids: Biological compounds soluble only in nonpolar solvents.

  • Functions:

    • Structural components of cellular membranes.

    • Storage forms of energy in plants and animals.

• Types of Lipids:

  • Saponifiable Lipids: Contain esters that can be hydrolyzed under basic conditions.

  • Nonsaponifiable Lipids: Do not contain esters and cannot be hydrolyzed.

    • Classification Basis: Based on the number of components in the structure.

    • Simple Lipids: Contain one or more fatty acids and an alcohol.

    • Complex Lipids: Contain more than two types of components (e.g., alcohol, fatty acids, and other molecules).

Major Lipid Types

• Simple Lipids: e.g., Waxes, Triglycerides.

• Complex Lipids: e.g., Phosphoglycerides, Sphingolipids, Steroids.

• Examples of Lipid Subtypes: Prostaglandins.

Fatty Acids

• Definition: Building blocks of lipids; long-chain carboxylic acids.

  • Structure:
    [ \text{General Formula: CH}3\text{-CH}2\text{-C(=O)OH} ]

• Characteristics:

  • Polarity: Polar, hydrophilic head (water-soluble) and long nonpolar tails (hydrophobic, water-insoluble).

  • Existence: Carboxyl group present, very hydrophilic at physiological pH (exists as -COO⁻).

Micelles

• Definition: Spherical clusters of molecules with:

  • Polar chains on the surface,

  • Nonpolar chains held together in the interior by weak dispersion forces.

• Importance: Micelle structure is crucial for biological functions, such as transporting insoluble lipids in blood.

Characteristics of Fatty Acids

• Typical Structure: Usually straight-chain carboxylic acids (no branching).

• Carbon Chain Length: Sizes typically range from 10-20 carbons (usually even numbers).

• Types:

  • Saturated: No double bonds (C=C).

  • Unsaturated: One or more C=C bonds, usually in the cis configuration.

Important Fatty Acids (Table 18.1)

Characteristics of Unsaturated Fatty Acids

• Cis unsaturation: Creates a long kink, preventing tight packing.

• Effects on Physical Properties:

  • Weaker intermolecular forces and lower melting points.

  • Fatty acids are typically liquids at room temperature.

• Melting Point Trends: Decreases as the number of double bonds increases.

Essential Fatty Acids

• Definition: Fatty acids not synthesized by the body; must be obtained through the diet.

  • include linoleic (ω-6) and linolenic (ω-3) acids.

• Function: Needed for producing hormonelike substances to regulate bodily functions:

  • Blood pressure, blood clotting.

  • Blood lipid levels.

  • Immune responses and inflammation.

Structure of Fats and Oils

• Definition: Esters containing:

  • An alcohol derived from glycerol.

  • An acidic portion from fatty acids.

• Esterification: Leads to the formation of triglycerides or triacylglycerols.

• Composition of Natural Triglycerides: Mixtures of different triglyceride molecules.

Example of Esterification Reaction

• Example Reaction:
  [ \text{Glycerol} + 3 \text{Fatty Acids} \rightarrow \text{Triglyceride} + 3 \text{H}_2 ext{O} ]

• Graphical Representation: Fatty acids attach to a glycerol molecule, forming triglycerides.

Differences Between Fats and Oils

• Fats:

  • Composed primarily of triglycerides of long-chain saturated fatty acids.

  • Derived from animal sources and solid at room temperature.

• Oils:

  • Derived from plant and fish sources.

  • Composed mostly of triglycerides containing unsaturated fatty acids.

  • Liquid at room temperature.

Chemical Properties of Fats and Oils

Hydrolysis

• Definition: Process important for the digestion of fats and oils, resulting in:

  • Formation of glycerol and free fatty acids.

  • Catalyzed by enzymes (lipases) in the digestive system.

Saponification

• Definition: Reaction of triglycerides with a strong base to produce glycerol and soaps (salts of fatty acids).

• Example Reaction:
  [ \text{Triglyceride} + 3 ext{NaOH} \rightarrow ext{Glycerol} + ext{Soaps} ]

• Types of Soaps:

  • Sodium salts: Found in cake soaps (hard soaps).

  • Potassium salts: Found in liquid soaps (soft soaps).

Hydrogenation

• Definition: Reaction converting fatty acid double bonds into single bonds.

• Effects:

  • Decreases unsaturation, increases melting point.

  • Complete hydrogenation yields a hard, waxy product; partial hydrogenation yields a smooth, creamy product.

  • Can cause isomerization of some cis fatty acids into trans fatty acids, raising cholesterol levels.

Waxes

• Definition: Esters of long-chain fatty acids and long-chain alcohols.

• Properties: Water-insoluble, not easily hydrolyzed.

• Uses: Protective coatings in nature (skin, fruits, leaves) and commercially in cosmetics, candles, ointments, and protective polishes.

Phosphoglycerides

• Definition: Complex lipids containing:

  • Glycerol,

  • Fatty acids,

  • Phosphoric acid,

  • An aminoalcohol component (phospholipids).

Types of Phosphoglycerides

• Lecithins: Contain choline; crucial for cell membranes, act as emulsifiers.

• Cephalins: Contain ethanolamine or serine; found in brain tissue and blood platelets, serve as emulsifiers.

Sphingolipids

• Definition: Complex lipids with sphingosine instead of glycerol.

• Types:

  • Sphingomyelin: Contains sphingosine, fatty acids, and phosphate with choline.

  • Glycolipids: Contains sphingosine, fatty acids, and carbohydrates.

Sphingomyelin

• Function: Protects nerve fibers, forms a component of myelin sheath.

• Structure: Contains amide linkage instead of an ester linkage.

Biological Membranes

Membrane Structure

• Composition: 60% lipid, 40% protein.

• Functions:

  • Acts as a selective barrier for cells.

  • Internal membranes support organelle function.

• Types of Membrane Lipids: Predominantly phosphoglycerides, sphingomyelin, and cholesterol.

• Fluid-Mosaic Model: Proteins embedded in a flexible lipid bilayer.

Membrane Characteristics

• Lipid Bilayer Properties: Hydrophobic chains face inward, hydrophilic heads face outward.

• Fluidity: Enhanced by unsaturated fatty acid chains, which allow for protein mobility.

Steroids

• Definition: Compounds with a complex ring structure (four fused rings).

• Primary Types: Cholesterol, bile salts.

• Cholesterol: Most abundant steroid in the human body, a crucial component of cell membranes, synthesized in the liver, correlated with atherosclerosis.

Steroid Hormones

Overview

• Function: Chemical messengers from glands to target tissues fueling specific responses.

• Categories:

  • Adrenocorticoids: Hormones affecting metabolism and ion concentrations.

  • Sex Hormones: Promote development of sexual characteristics.

Adrenocorticoid Hormones

• Functions:

  • Mineralocorticoids: Regulate ion concentrations (e.g., Aldosterone increases Na⁺ and Cl⁻ absorption).

  • Glucocorticoids: Enhance carbohydrate metabolism (e.g., Cortisol increases glucose concentration; anti-inflammatory effects).

Sex Hormones

• Types: Produced by testes (androgens) or ovaries (estrogens).

  • Androgens: Male hormones (e.g., testosterone).

  • Anabolic Steroids: Growth-promoting, synthetic derivatives of testosterone. Used in athletic performance but may cause adverse side effects such as testicular atrophy and reduced fertility.

Female Sex Hormones

• Estrogens: Estradiol and estrone involved in ovum development.

• Progesterone: Prepares the uterine wall for potential pregnancy.