Biological Functions of Lipids

Biological Functions of Lipids

• Energy Source: Lipids provide 9kcal of energy per gram, which is more than carbohydrates or proteins.
• Energy Storage: Triglycerides are stored in adipocytes (fat cells) for long-term energy reserves.
• Structural Component of Cell Membranes: Phosphoglycerides, steroids, and sphingolipids are essential components of cell membranes.
• Hormones: Steroids are vital intercellular messengers regulating various bodily functions.
• Lipid-Soluble Vitamins: Vitamins A, E, D, and K are fat-soluble and require dietary fat for absorption.
• Shock Absorption and Insulation: Lipids provide cushioning and insulation for organs and the body.

Classification of Lipids

• Fatty Acids: These are the building blocks of many lipids.
  • Saturated Fatty Acids: Have no double bonds between carbon atoms.
  • Unsaturated Fatty Acids: Have one or more double bonds between carbon atoms.
• Glycerides: Composed of glycerol and fatty acids.
• Nonglyceride Lipids: This category includes:
  • Sphingolipids: Essential for cell membrane structure.
  • Steroids: A class of compounds including cholesterol and hormones.
  • Waxes: Provide protective coatings for plants and animals.

Fatty Acids

• Structure: Unbranched, long, straight chains of an even number of carbon atoms with a carboxyl group (COOH).
• Length: Most common chains range from 10 to 20 carbons.
• Stearic Acid: A typical saturated fatty acid with 18 carbons.
• Oleic Acid: A typical unsaturated fatty acid with 18 carbons.

Saturated & Unsaturated Fatty Acids

• Saturated Fatty Acids: Have no double bonds between carbon atoms, resulting in more rigid structures that pack tightly.
• Unsaturated Fatty Acids: Have one or more double bonds between carbon atoms, resulting in less rigid structures that pack loosely.
• Cis configuration: In unsaturated fats, the presence of a double bond prevents tight packing, leading to lower melting temperatures.

Unsaturated Fatty Acids

• Double bonds: The first double bond is typically at the ninth carbon atom.
• Cis configuration: The double bonds are usually in the cis configuration, which causes a bend in the fatty acid chain.

Properties of Fatty Acids

• Melting Point: Increases with increasing carbon number.
• Melting Point Comparison: Saturated fatty acids have higher melting points than unsaturated fatty acids with the same number of carbons.
• Packing: Saturated fatty acids pack tightly due to the straight structure, while cis double bonds in unsaturated fatty acids prevent tight packing.
• Double Bonds Effect: Double bonds decrease melting point relative to saturated fatty acids.

Esterification of Fatty Acids

• Esterification: A reaction between a fatty acid and an alcohol, resulting in the formation of an ester and water.

Hydrolysis of Fatty Acids

• Acid Hydrolysis: The reverse of esterification, breaking down esters into fatty acids and alcohols.

Saponification of Fatty Acids

• Saponification: Base-catalyzed hydrolysis of an ester, producing:
  • An alcohol.
  • An ionized salt (soap).

Soap & Micelle Formation

• Soaps: Soaps have a long, uncharged hydrocarbon tail and a negatively charged carboxylate group at the end.
• Micelles: Soaps form micelles in water, encasing oil and dirt particles for removal.

Reactions at the Double Bond

• Hydrogenation: A chemical reaction that converts unsaturated fatty acids to saturated fatty acids by adding hydrogen.
• Food Industry: Hydrogenation is used to convert liquid vegetable oils into solids.
• Margarine: A common example of a hydrogenated fat.

Eicosanoids: Prostaglandins, Leukotrienes, & Thromboxanes

• Essential Fatty Acids: Cannot be synthesized by the body and must be obtained from the diet.
  • Linoleic Acid: An essential fatty acid needed to synthesize arachidonic acid.
• Arachidonic Acid: A 20-carbon unsaturated fatty acid that serves as the precursor for eicosanoids.
• Eicosanoids: A group of structurally similar compounds, including:
  • Prostaglandins: Potent biological molecules involved in inflammation and pain.
  • Leukotrienes: Involved in allergic and inflammatory responses.
  • Thromboxanes: Play a role in blood clotting.

Prostaglandins (PG)

• Production: Produced in most tissues, acting as "local hormones."
• Structure: Derived from 20-carbon unsaturated fatty acids and contain a 5-carbon ring.
• Effects: Exert their effects on nearby cells.

Aspirin & Prostaglandins

• Aspirin Mechanism: Inhibits prostaglandin synthesis by acetylating cyclooxygenase, an enzyme essential for prostaglandin production.