Lipids II: TAGs and Phospholipids

Lipids-II: Chemistry of TAGs and Phospholipids

• Learning Objectives:
  • Describe the properties and reactions of triacylglycerols (TAGs).
  • List the main functions of glycolipids and phospholipids in the body.
  • Name the main eicosanoid derivatives and their functions.

Simple Lipids

• Definition: Simple lipids are composed of fatty acids and alcohol (e.g., TAGs).

Triacylglycerols (TAGs)

• Structure: TAGs are composed of a glycerol backbone with three fatty acids attached via ester linkages.
• Function: Storage form of lipids in adipose tissue.
• Properties: Insoluble in water (hydrophobic), nonpolar, not structural components of biological membranes.
• Physical properties depend on fatty acid components:
  • TAGs with high proportions of unsaturated fatty acids (FAs) are liquid at 20°C (oils).
  • TAGs with mainly saturated long-chain FAs are solid at ordinary temperatures (fats).
  • Fats are mainly of animal origin, while oils are often from plant sources.

TAG Hydrolysis

• Process: TAGs are hydrolyzed to glycerol and three fatty acids by the enzyme lipase.
• Significance: This process occurs during digestion of dietary fat and breakdown of TAGs in adipose tissue.

Saponification

• Process: The hydrolysis of TAGs by alkali to produce glycerol and soap.
• Reaction: TAG + 3 NaOH → Glycerol + 3 R-COONa (soaps)
• Importance: Creates soaps and glycerol, a triol alcohol.

Addition Reactions

• Process: Addition of hydrogen or halogen to double bonds of fatty acids.
• Example: Hydrogenation of oil leads to solidification and saturation.
• Significance: Important in the processing of fats and oils, can modify physical properties.

Rancidity

• Definition: Deterioration of fats and oils, resulting in an unpleasant taste and smell.
• Cause: Occurs when fats and oils are exposed to air, moisture, light, or bacteria.
• Susceptibility: Fats containing unsaturated fatty acids are more susceptible to rancidity.
• Prevention: Antioxidants can prevent oxidative rancidity.

Compound/Complex Lipids

• Definition: Lipids containing fatty acids, alcohol, and other molecules.
• Types:
  • Phospholipids: Important structural and functional components of biological membranes, amphipathic.
  • Glycolipids: Widely distributed in tissues, particularly nervous tissue; occur in the outer leaflet of cell membranes, important in cell surface carbohydrates (e.g., ABO blood group substances)
  • Sphingolipids: Important to cell membranes and nervous tissue, including sphingomyelins.

Phospholipids: Lecithin

• Component: Fatty acid attached to the β-carbon is a PUFA molecule.
• Abundance: Important and abundant component of cell membranes.
• Example: Di-palmitoyllecithin (lung surfactant) is found in large amounts in the extracellular liquid layer of lung alveoli, vital for lung function.

Sphingomyelins

• Structure: Composed of ceramide (sphingosine + fatty acid) + phosphorylcholine.
• Function: Major structural components of nerve cell membranes (myelin sheath).
• Presence: Also found in biological fluids like bile and amniotic fluid.

This quiz focuses on the chemistry of triacylglycerols (TAGs) and phospholipids, exploring their properties, functions, and reactions in the body. You'll learn about the structural components of simple lipids and the significance of eicosanoid derivatives. Test your knowledge on these essential macromolecules!