The Chemistry of Lipids

CHAPTER VI: THE CHEMISTRY OF LIPIDS

LEARNING OUTCOMES

• Define lipids.

• Draw the general structure of a triglyceride.

• Differentiate between the two general classes of lipids:

  • Saponifiable lipids

  • Non-saponifiable lipids

• Give examples of each class.

• Explain the meaning of essential fatty acids.

• Enumerate the chemical properties of simple lipids.

• Describe the cleansing action of soaps.

• Outline the distribution of lipids in the body and their functions.

• At the end of this unit, you should be able to:

LIPIDS

• Definition: Lipids are oily or waxy substances that are insoluble in water but may be extracted from tissues using nonpolar solvents.

• Building Blocks: The fundamental components of most lipids are fatty acids.

• Functions:

  • Form part of the structural components of cellular membranes.

  • Store energy for the cells.

CLASSIFICATION OF LIPIDS

• Lipids can be categorized into three main classes:

  • Simple lipids:

  • Composed of fatty acids and alcohols (e.g., triglycerides, waxes).

  • Compound lipids:

  • Composed of fatty acids, alcohols, and additional groups (e.g., phospholipids, glycolipids, lipoproteins).

  • Derived lipids:

  • Derived from simple or complex lipids (e.g., fatty acids, steroids, eicosanoids, ketone bodies).

SIMPLE LIPIDS

• Triglycerides (TAG/TG):

  • Composed of triacylglycerol which consists of three fatty acids esterified to glycerol.

  • They are referred to as neutral fats due to their uncharged nature.

  • Waxes: Esters of fatty acids with high molecular weight monohydric alcohols; important in industrial applications, cosmetics.

GENERAL STRUCTURE OF A TRIGLYCERIDE

• Triglycerides Structure;

  • Glycerol backbone with three fatty acids attached. Each fatty acid is attached through an ester bond.

COMPOUND LIPIDS

• Phospholipids:

  • Composed of fatty acids, alcohol (glycerol or sphingosine), phosphoric acid, and a nitrogenous base.

  • Examples include glycerophospholipids and sphingophospholipids.

GLYCEROPHOSPHOLIPID HEAD GROUPS

SPHINGOPHOSPHOLIPID HEAD GROUPS

SAPONIFIABLE vs NON-SAPONIFIABLE LIPIDS

Definitions

• Saponifiable Lipids: Composed of long-chain fatty acids forming esters.

• Non-saponifiable Lipids: Composed of ring structures that do not form esters.

Saponifiable Lipids

• Examples: Triacylglycerides, glycerophospholipids, sphingolipids, and waxes.

Non-saponifiable Lipids

• Examples: Terpenes (fat-soluble vitamins, carotenoids, coenzymes) and steroids (hormones, vitamins).

FATS AND OILS

• Neutral Lipids: Triglycerides or triacylglycerols; main form of fat storage in plants and adipose cells of vertebrates.

• Functions:

  • Insulation and conservation of body temperature.

  • Protection of internal organs against injury.

DIFFERENCE BETWEEN FATS AND OILS

FATTY ACIDS

• Definition: Long-chain carboxylic acids formed from hydrolysis of triglycerides.

• Characteristics:

  • Naturally occurring fatty acids typically have an even number of carbon atoms, generally unbranched.

  • Saturated Fatty Acids: Only single bonds between carbon atoms, waxy solids at room temperature.

  • Unsaturated Fatty Acids: One or more double bonds, usually liquids at room temperature.

COMMON FATTY ACIDS FORMULAE & MELTING POINTS

1. Saturated Fatty Acids:

   • Lauric Acid: $C{12}H{24}O_2$, 45 °C

   • Myristic Acid: $C{14}H{28}O_2$, 55 °C

   • Palmitic Acid: $C{16}H{32}O_2$, 63 °C

   • Stearic Acid: $C{18}H{36}O_2$, 69 °C

   • Arachidic Acid: $C{20}H{40}O_2$, 76 °C

2. Unsaturated Fatty Acids:

   • Palmitoleic Acid: $C{16}H{30}O_2$, 0 °C

   • Oleic Acid: $C{18}H{34}O_2$, 13 °C

   • Linoleic Acid: $C{18}H{32}O_2$, -5 °C

   • Linolenic Acid: $C{18}H{30}O_2$, -11 °C

   • Arachidonic Acid: $C{20}H{32}O_2$, -49 °C

ESSENTIAL FATTY ACIDS

• Certain fatty acids such as linoleic (C18:2), linolenic (C18:3), and arachidonic (C20:4) acids cannot be synthesized by the body and must be obtained through diet.

• Consequences of Deficiency: Infants lacking these fatty acids may experience weight loss and the development of eczema.

• Functions: These acids are precursors to prostaglandins, which are significant in various physiological processes.

CHEMICAL PROPERTIES OF SIMPLE LIPIDS

Hydrolysis

• Heating fats can lead to the hydrolysis of triglycerides, producing glycerol and fatty acids.

• Formation of Acrolein: Upon hydrolysis and heating, glycerol can be converted into acrolein, which is responsible for the unpleasant odor of burning fats and can irritate the digestive tract.

• Hydrolysis Conditions: Can occur via superheated steam, hot mineral acids, or specific enzymes, resulting in the production of glycerol and three fatty acids.

IODINE NUMBER

• The iodine number measures the degree of unsaturation in fatty acids and is defined as the grams of iodine that react with 100 grams of fat or oil.

• General ranges: Fats typically have an iodine number below 70, while oils have an iodine number above 70.

HYDROGENATION

• Process adding hydrogen to unsaturated fatty acid double bonds, converting oils to solid fats.

• Common applications: Production of vegetable shortenings and margarine.

RANCIDITY

• Caused by hydrolysis and oxidation leading to disagreeable odors and tastes in fats and oils.

• Preventive Measures: Antioxidants are used during food preparation to delay oxidation.

SAPONIFICATION

• Definition: The hydrolysis of fats in the presence of strong bases (e.g., sodium hydroxide) resulting in glycerol and sodium salts of fatty acids.

CLEANSING ACTION OF SOAPS AND DETERGENTS

• Soap Structure: Contains a hydrophobic hydrocarbon end and a hydrophilic ionic polar end.

• Mechanism: The non-polar ends surround oily substances while the polar ends solubilize the entire droplet, creating a micelle that can dissolve in water.

Soap Micelle Structure

• The hydrophobic ends of soap molecules are attracted to the oil droplets, while the hydrophilic ends are solubilized in water, making the micelle hydrophilic overall.

PHOSPHATIDES

• Example: Lecithin, which consists of choline and is used as an emulsifying agent in many products.

• Plays a critical role in fat metabolism, transport, and tissue formation.

LECITHIN USAGE

• Found in products such as chocolate and margarine; also serves as a natural emulsifier in making mayonnaise.

• Lysolecithin: Formed by removing a fatty acid from lecithin and can cause hemolysis of red blood cells.

CEPHALIN

• Another phosphatide with ethanolamine, important for blood clotting and tissue formation.

SPHINGOLIPIDS

• Found in nervous tissue in the form of sphingomyelin, contributing to myelin sheath stability and function.

• Deficiencies lead to disorders like Niemann-Pick disease and multiple sclerosis.

GLYCOLIPIDS

• Composed of sugar (often galactose) and can be found in nerve cells, with cerebrosides being a prime example.

LIPOPROTEINS

• These are complex molecules made up of lipid and protein components, categorized based on density:

  • Chylomicrons: Transport absorbed lipids from intestines.

  • VLDL: Transport triglycerides from the liver.

  • LDL: Transport cholesterol, higher levels linked to cardiovascular disease risk.

  • HDL: Carry cholesterol back to the liver, aiding in cholesterol management.

DERIVED LIPIDS

• Include fatty acids, steroids, eicosanoids derived from arachidonic acid, and ketone bodies derived from Acetyl CoA.

NON-SAPONIFIABLE LIPIDS

• Comprise steroids characterized by a complex four-ring structure important in various biological functions including hormones and bile acids.

CHOLESTEROL

• A vital component of cell membranes, precursor to steroid synthesis, synthesized in the liver from acetyl CoA, and also obtained from dietary sources.

• Hypercholesterolemia: Condition from high levels of LDL leading to plaque formation and atherosclerosis.

PROSTAGLANDINS

• Fatty acid derivatives that imitate hormones; synthesized from unsaturated fatty acids.

• Effects: Influence inflammation, fevers, and various physiological functions; targets for anti-inflammatory drugs.

LIPOXINS AND LEUKOTRIENES

• Eicosanoids with distinct effects, involved in inflammatory response.

MISCELLANEOUS LIPIDS

• Squalene: Intermediary in cholesterol metabolism.

• Carotenoids: Plant pigments with antioxidant properties, important for immune function. Examples include lycopene and beta-carotene.