Lipid Basics and Classifications

Overview

Lecture notes for Biochemistry Module 17 on lipids: definitions, classes, properties, biological significance, classifications, examples, tests, related diseases, and key compounds.

Lipids: Definition and Classes

• Lipids: collective term for fixed oils, fats, waxes; water-insoluble, non-polar, hydrocarbon-based.
• Soluble in non-polar solvents: ether, carbon tetrachloride (CCl4), carbon disulfide (CS2), benzene.
• Non-polar nature explains insolubility in polar water.
• Two major classes: saponifiable and non-saponifiable lipids.

Saponifiable vs Non-saponifiable

• Saponifiable: contain ester groups; hydrolyzed by acid, base, or enzymes; include simple and complex lipids.
• Non-saponifiable: no ester bonds; cannot be hydrolyzed into smaller molecules.

Biological Significance of Lipids

• Secondary energy source after carbohydrates; storage form during fasting.
• Insulation: prevents heat loss; cushions and protects internal organs.
• Structural components: cell membrane, mitochondria; transport fatty acids.
• Metabolic regulators: steroid hormones, prostaglandins.
• Surfactants/emulsifiers: amphipathic lipids aid dispersion.
• Shape and contour: body form influenced by fat distribution.
• Aid absorption of fat-soluble vitamins: A, D, E, K.
• Enhance taste and palatability of food.
• Dietary constituents: essential fatty acids, fat-soluble vitamins, micronutrients.
• Long-chain omega-3 fatty acids: suggested benefits in cardiovascular disease, rheumatoid arthritis, dementia.

Physical and Chemical Properties

• Greasy feel; lighter than water; float due to specific gravity < 1.
• Soluble in organic solvents; pure forms are colorless with bland odor/taste.
• Yellow fat color due to carotene.
• On strong heating: decompose to acidic, flammable vapors; burn with sooty flame; acridity from acrolein (propenal).
• Refractive index increases with longer chain length and higher unsaturation.
• Melting/solidification: occur over a range due to mixed fatty acids; higher with longer chains and more saturation.
• Viscosity: water < olive oil < honey; reflects resistance to flow.
• Optical rotation: rotate plane-polarized light; levorotatory (left) or dextrorotatory (right).

Classification: Simple Lipids (Fats and Fixed Oils)

• Esters of fatty acids with alcohol (often glycerol); termed glycerides.
• Fats: solid at room temperature; higher saturated fatty acids.
• Fixed oils: liquid vegetable oils at room temperature; higher unsaturated fatty acids.

Notable Exceptions

• Cocoa (cacao) butter: plant source; solid at room temperature; used in suppository bases.
• Cod liver oil: animal source; liquid; rich in vitamins.

Fixed Oils: Drying Behavior and Uses

• Drying behavior relates to iodine value and unsaturation.

Fixed Oils by Iodine Value

• Drying oils (e.g., linseed): polymerize with air to hard films; important in paint industry.
• General uses of fixed oils: emollients, vehicles in pharmaceuticals, soap manufacture, paints/varnishes, lubricants, dietary supplements.
• Selected pharmaceuticals: castor oil (laxative/cathartic); sclerosing agent to obliterate varicose veins.

Waxes

• Protective coatings in plants: outer epidermal tissues of fruits and leaves; reduce water loss; antimicrobial barrier.
• Make bird feathers pliable and soft; harden ointments and cosmetic creams.

Tests for Waxes

• Warm in alcohol: dissolves; crystallizes on cooling.
• Warm in ether: soluble.
• Acrolein test: heat with potassium bisulfate; acrid irritating odor indicates positive.

Composition Examples

• Beeswax: chiefly myricyl palmitate.
• Spermaceti: cetyl palmitate; from sperm whales.

Lipoproteins and Cholesterol Transport

• Cholesteryl ester transfer: from HDL to LDL via plasma proteins.
• HDL: “good” cholesterol; LDL: “bad” cholesterol.

Diseases Related to Lipid Storage/Metabolism

• Schüller-Christian disease (Langerhans cell granulomatosis): high cholesteryl ester deposits; triad—diabetes insipidus, exophthalmos, osteolytic bone lesions.
• Niemann-Pick disease (sphingomyelin lipidosis): sphingomyelinase deficiency; cherry-red macula, hepatosplenomegaly, severe mental retardation, foamy macrophages (zebra bodies), early death.
• Gaucher disease (glucocerebrosidosis): glucocerebrosidase deficiency; hepatosplenomegaly, abdominal distention, bone fractures and avascular necrosis, anemia, thrombocytopenia.
• Krabbe disease (galactocerebrosidosis): galactocerebrosidase deficiency; demyelination, feeding difficulty, irritability, fever, developmental delay/regression, muscle spasms, poor head control, vomiting.
• Tay-Sachs disease (gangliosidosis): gangliosidase deficiency; cherry-red macula, blindness, motor retardation, startle reflex, death by ~2 years.

Complex Lipids: Phospholipids

• Lipids with fatty acids, alcohol, and phosphoric acid; often contain nitrogenous bases.
• Glycerophospholipids: alcohol is glycerol.
• Sphingophospholipids: alcohol is sphingosine.

Lecithin (Phosphatidylcholine)

• Composition: glycerol + two fatty acids + H3PO4 + choline.
• Uses: emulsifier; mobilizes cholesterol; aids fatty acid absorption; used in memory disorders (dementia, Alzheimer’s).
• Industrial: provides creamy consistency to margarine; prevents storage tank corrosion due to lead in gasoline.

Cephalin (Phosphatidylethanolamine)

• Thromboplastic substance initiating blood coagulation.
• One large egg contains ~0.3 g cephalin.

Glycolipids (Glycosphingolipids)

• Lipids with attached carbohydrate chains; abundant in nervous tissue and outer plasma membrane (glycocalyx).
• Major types in animals: glycosphingolipids.

Key Glycolipids

Derived Lipids: Fatty Acids and Alcohols

• Fatty acids: carboxylic acids with hydrocarbon chains (C4–C36); mainly esterified in fats/oils; free fatty acids circulate in plasma for transport.

Saturated Fatty Acids

• Low molecular weight: liquids at room temperature, low melting points, volatile.
• Higher molecular weight: solids, higher melting points, nonvolatile.

Unsaturated Fatty Acids

• Oleic acid: most abundant; ~50% of natural fatty acids; in fats and phospholipids.
• Linolenic acid: chief in linseed oil; key drying oil in paints.
• Omega-3 types:
  • ALA (alpha-linolenic acid): plant oils.
  • EPA (eicosapentaenoic acid): marine oils.
  • DHA (docosahexaenoic acid): marine oils.
• Properties: reactive due to double bonds; cis/trans isomerism (cis less stable; trans more stable); generally liquid, nonvolatile.

Hydroxy and Cyclic Acids

• Chaulmoogra oil: ethyl esters/sodium salts used in leprosy treatment.
• Thalic acid: saturated cyclic acid; liquid at room temperature; associated with epithelial giant cell proliferation in TB.

Alcohols and Sterols

• Low molecular weight alcohols: glycerol; sphingosine.
• Sterols:
  • Cholesterol: C27H45O; steroid nucleus; OH at C3; double bond C5–C6; methyl groups at C10, C19; precursor of bile acids, adrenal cortical hormones, sex hormones, vitamin D, cardiac glycosides; synthesized mainly in liver.
  • Ergosterol: plant sterol; more unsaturated; precursor of vitamin D.
• Hormones: progesterone (female), testosterone (male).
• Vitamin D:
  • D3 (cholecalciferol): from irradiated 7-dehydrocholesterol.
  • D2 (ergocalciferol): from irradiated ergosterol.

Key Terms & Definitions

• Saponification: hydrolysis of esters in lipids, typically by base, yielding glycerol and fatty acid salts.
• Specific gravity: ratio to water; fats < 1; float on water.
• Refractive index: measure of light bending; increases with chain length/unsaturation.
• Viscosity: resistance to flow; higher values flow more slowly.
• Levorotatory/Dextrorotatory: rotate plane-polarized light left/right.
• Amphipathic: molecules with both hydrophilic and hydrophobic parts.
• Glycocalyx: carbohydrate-rich cell surface layer formed by glycolipids and glycoproteins.

Action Items / Next Steps

• Review iodine value categories and examples for non-/semi-/drying oils.
• Memorize key lipid storage diseases, deficient enzymes, and hallmark symptoms.
• Understand structures and functions of lecithin and cephalin.
• Differentiate saponifiable vs non-saponifiable lipids and their properties.
• Revisit physical properties: melting points, viscosity, refractive index, and specific gravity.