Lipids and Their Classification

Lipids

• Lipids are insoluble in water but soluble in fat solvents (non-polar solvents) like benzene, chloroform, acetone, and ether.
• Lipids are physiologically important for humans and have four major functions.
• Lipids serve as structural components of biological membranes.
• Lipids provide energy reserves, mainly in the form of triacylglycerols.
• Lipids and lipid derivatives function as vitamins and hormones.
• Bile acids and bile salts aid in fat emulsification.

Lipid Classification

• Lipids are classified into simple lipids (fatty acid + glycerol), compound lipids (fatty acid + alcohol + other group), and derived lipids (hydrolysis products of simple or compound lipids).
• Simple lipids include neutral fats (TAGs) and waxes.
• Compound lipids include phospholipids and glycolipids.
• Derived lipids include steroids, carotenoids, and fat-soluble vitamins.

Fatty Acids

• Fatty acids are long-chain hydrocarbon molecules with a carboxylic acid group (-COOH) at one end.
• The hydrocarbon chain (R) makes fatty acids hydrophobic.
• The -COOH group gives fatty acids hydrophilic properties.
• Fatty acids have an amphipathic nature, meaning they have both hydrophobic and hydrophilic regions.

Fatty Acid Classification

• Fatty acids are classified as saturated or unsaturated based on the presence or absence of double bonds.
• Saturated fatty acids have no double bonds.
• Unsaturated fatty acids have one or more double bonds.

Examples of Saturated Fatty Acids

• Butyric acid (C4) - found in butter
• Palmitic acid (C16) - found in animal fats
• Stearic acid (C18) - found in animal fats

Unsaturated Fatty Acids

• Unsaturated fatty acids exist, but their structures are not listed

Examples of Unsaturated Fatty Acids

• Oleic acid (C18:1)

• Linoleic acid (C18:2)

• Linolenic acid (C18:3)

• Arachidonic acid (C20:4)

• Polyunsaturated fatty acids (PUFAs) have two or more double bonds.

• Linoleic and linolenic acids are essential fatty acids because the body cannot make them.

Eicosanoids

• Eicosanoids are hormone-like molecules produced by most mammalian tissues.
• They're derived from polyunsaturated fatty acids, specifically those with 20 carbon atoms (eicosanoids).
• Examples include arachidonic acid.
• Eicosanoids act locally within the same cells or adjacent cells.

Eicosanoid Classification

• Eicosanoids are classified into cyclic and acyclic compounds.
  • Cyclic compounds include prostaglandins, prostacyclins, and thromboxanes.
  • Acyclic compounds include leukotrienes and lipoxins.

Prostaglandins

• Prostaglandins are cyclic eicosanoids produced by most tissues.
• They play roles in inflammation, uterine contractions, and gastric secretions, and some produce vasodilation or vasoconstriction.

Prostacyclins

• Prostacyclins are cyclic eicosanoids produced by the endothelium of blood vessels.
• Prostacyclins cause vasodilation and inhibit platelet aggregation.

Thromboxanes

• Thromboxanes are cyclic eicosanoids produced by platelets.
• Thromboxanes cause vasoconstriction and promote platelet aggregation.

Leukotrienes

• Leukotrienes are acyclic eicosanoids produced by leukocytes, platelets, and mast cells.
• They are released during allergic reactions; and increase vascular permeability, and cause bronchospasm.